Patent Publication Number: US-2022229336-A1

Title: Liquid crystal display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/110,583, filed Dec. 3, 2020, now allowed, which is a continuation of U.S. application Ser. No. 16/014,060, filed Jun. 21, 2018, now abandoned, which is a continuation of U.S. application Ser. No. 15/585,221, filed May 3, 2017, now U.S. Pat. No. 10,012,880, which is a continuation of U.S. application Ser. No. 14/945,651, filed Nov. 19, 2015, now U.S. Pat. No. 9,645,461, which is a continuation of U.S. application Ser. No. 14/317,286, filed Jun. 27, 2014, now U.S. Pat. No. 9,360,722, which is a continuation of U.S. application Ser. No. 13/451,619, filed Apr. 20, 2012, now U.S. Pat. No. 8,767,159, which is a continuation of U.S. application Ser. No. 12/115,319, filed May 5, 2008, now U.S. Pat. No. 8,253,911, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2007-133533 on May 18, 2007, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an object, a method, or a method for producing an object. In particular, the present invention relates to a display device or a semiconductor device, particularly relates to a display device. Specifically, the present invention relates to an active matrix liquid crystal display device. 
     2. Description of the Related Art 
     In recent years, a liquid crystal display device and an EL display device have been actively developed as a display device. In particular, a liquid crystal display device has been remarkably spread. In a liquid crystal display device, high contrast, high-speed response, a wide viewing angle, and the like are necessary. Further, in a liquid crystal display device which is mounted on a portable electronic device, reduction in power consumption, weight, and size is also an important object. 
     In order to widen the viewing angle of a liquid crystal display device, various techniques have been developed. Examples of techniques for widening the viewing angle are an MVA (multi-vertical domain (hereinafter referred to as MVA)) mode, a PVA (patterned vertical alignment (hereinafter referred to as PVA)) mode, and a CPA (continuous pinwheel alignment) mode. With such a technique, the viewing angle has been widened compared to that of a conventional liquid crystal display device; however, the widened viewing angle has been insufficient. Therefore, a technique has been developed in which one pixel is divided into two subpixels to vary alignment of liquid crystals and inclined angles of liquid crystal molecules are averaged from appearance to cause a false sense of uniform display from any direction, so that viewing angle characteristics are improved (e.g., Reference  1 : Japanese Published Patent Application No. 2006-276582). 
     SUMMARY OF THE INVENTION 
     In a liquid crystal display device, when a pixel is provided with subpixels so as to have a plurality of alignment, viewing angle characteristics can be improved. However, it cannot be said that viewing angle characteristics are sufficient, and there is a possibility that the viewing angle characteristics can be improved when subpixels are additionally provided. 
     However, when the number of subpixels is simply increased, disadvantages such as decrease in the aperture ratio and increase of driver circuits occur to increase manufacturing cost and cause an adverse effect such as decrease in performance as a display device itself. Specifically, when the aperture ratio is decreased, luminance and contrast are decreased, so that power consumption is increased. Alternatively, layout density of pixels is increased, so that manufacturing yield is decreased and cost is increased. Further alternatively, since the number of subpixels is increased, the number of image signals which should be input is also increased. Therefore, the number of connections between a glass substrate and an external driver circuit is increased. Accordingly, reliability is decreased due to a connection defect or the like. 
     It is an object of the present invention to provide a display device which maintains performance as a display device and has excellent viewing angle characteristics. Alternatively, it is an object of the present invention to provide a highly reliable display device. Alternatively, it is an object of the present invention to provide a display device having high contrast. Alternatively, it is an object of the present invention to provide a lightweight display device. Alternatively, it is an object of the present invention to provide a small display device. Alternatively, it is an object of the present invention to provide a display device having high luminance. Alternatively, it is an object of the present invention to provide a display device with low power consumption. Alternatively, it is an object of the present invention to provide a display device having a high aperture ratio. Alternatively, it is an object of the present invention to provide a display device with low manufacturing cost. 
     One aspect of the present invention is a liquid crystal display device in which one pixel is provided with three or more liquid crystal elements and the level of voltage which is applied is varied between the liquid crystal elements. In order to vary the level of the voltage which is applied to the liquid crystal elements, an element which divides the applied voltage is provided. Alternatively, an element which converts current into voltage or an element which converts voltage into current is provided. For example, a capacitor, a resistor, a non-linear element, a switch, a transistor, a diode-connected transistor, a diode (e.g., a PIN diode, a PN diode, a Schottky diode, an MIM diode, or an MIS diode), an inductor, or the like is provided. 
     Note that various types of switches can be used as a switch. An electrical switch, a mechanical switch, and the like are given as examples. That is, any element can be used as long as it can control a current flow, without limiting to a certain element. For example, a transistor (e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky diode, an MIM (metal insulator metal) diode, an MIS (metal insulator semiconductor) diode, or a diode-connected transistor), a thyristor, or the like can be used as a switch. Alternatively, a logic circuit combining such elements can be used as a switch. 
     An example of a mechanical switch is a switch formed using MEMS (micro electro mechanical system) technology, such as a digital micromirror device (DMD). Such a switch includes an electrode which can be moved mechanically, and operates by controlling connection and non-connection based on movement of the electrode. 
     In the case of using a transistor as a switch, polarity (a conductivity type) of the transistor is not particularly limited because it operates just as a switch. However, a transistor of polarity with smaller off-current is preferably used when off-current is to be suppressed. Examples of a transistor with smaller off-current are a transistor provided with an LDD region, a transistor with a multi-gate structure, and the like. In addition, it is preferable that an N-channel transistor be used when a potential of a source terminal is closer to a potential of a low-potential-side power supply (e.g., V ss , GND, or 0 V), while a P-channel transistor be used when the potential of the source terminal is closer to a potential of a high-potential-side power supply (e.g., V dd ). This is because the absolute value of gate-source voltage can be increased when the potential of the source terminal is closer to a potential of a low-potential-side power supply in an N-channel transistor and when the potential of the source terminal is closer to a potential of a high-potential-side power supply in a P-channel transistor, so that the transistor can be more precisely operated as a switch. This is also because the transistor does not often perform a source follower operation, so that reduction in output voltage does not often occur. 
     Note that a CMOS switch may be employed as a switch by using both N-channel and P-channel transistors. When a CMOS switch is employed, the switch can more precisely operate as a switch because current can flow when either the P-channel transistor or the N-channel transistor is turned on. For example, voltage can be appropriately output regardless of whether voltage of an input signal to the switch is high or low. In addition, since a voltage amplitude value of a signal for turning on or off the switch can be made smaller, power consumption can be reduced. 
     Note that when a transistor is used as a switch, the switch includes an input terminal (one of a source terminal and a drain terminal), an output terminal (the other of the source terminal and the drain terminal), and a terminal for controlling conduction (a gate terminal). On the other hand, when a diode is used as a switch, the switch does not have a terminal for controlling conduction in some cases. Therefore, when a diode is used as a switch, the number of wirings for controlling terminals can be reduced compared to the case of using a transistor as a switch. 
     Note that when it is explicitly described that “A and B are connected”, the case where A and B are electrically connected, the case where A and B are functionally connected, and the case where A and B are directly connected are included therein. Here, each of A and B corresponds to an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). Accordingly, another element may be interposed between elements having a connection relation shown in drawings and texts, without limiting to a predetermined connection relation, for example, the connection relation shown in the drawings and the texts. 
     For example, in the case where A and B are electrically connected, one or more elements which enable electric connection between A and B (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, and/or a diode) may be provided between A and B. In addition, in the case where A and B are functionally connected, one or more circuits which enable functional connection between A and B (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit, a signal converter circuit such as a DA converter circuit, an AD converter circuit, or a gamma correction circuit, a potential level converter circuit such as a power supply circuit (e.g., a boosting circuit or a voltage lower control circuit) or a level shifter circuit for changing a potential level of a signal, a voltage source, a current source, a switching circuit, or an amplifier circuit such as a circuit which can increase signal amplitude, the amount of current, or the like (e.g., an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit), a signal generating circuit, a memory circuit, and/or a control circuit) may be provided between A and B. Alternatively, in the case where A and B are directly connected, A and B may be directly connected without interposing another element or another circuit therebetween. 
     Note that when it is explicitly described that “A and B are directly connected”, the case where A and B are directly connected (i.e., the case where A and B are connected without interposing another element or another circuit therebetween) and the case where A and B are electrically connected (i.e., the case where A and B are connected by interposing another element or another circuit therebetween) are included therein. 
     Note that when it is explicitly described that “A and B are electrically connected”, the case where A and B are electrically connected (i.e., the case where A and B are connected by interposing another element or another circuit therebetween), the case where A and B are functionally connected (i.e., the case where A and B are functionally connected by interposing another circuit therebetween), and the case where A and B are directly connected (i.e., the case where A and B are connected without interposing another element or another circuit therebetween) are included therein. That is, when it is explicitly described that “A and B are electrically connected”, the description is the same as the case where it is explicitly only described that “A and B are connected”. 
     Note that a display element, a display device which is a device having a display element, a light-emitting element, and a light-emitting device which is a device having a light-emitting element can use various types and can include various elements. For example, a display medium, whose contrast, luminance, reflectivity, transmittivity, or the like changes by an electromagnetic action, such as an EL element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an electron emitter, a liquid crystal element, electronic ink, an electrophoresis element, a grating light valve (GLV), a plasma display panel (PDP), a digital micromirror device (DMD), a piezoelectric ceramic display, or a carbon nanotube can be used as a display element, a display device, a light-emitting element, or a light-emitting device. Note that display devices using an EL element include an EL display; display devices using an electron emitter include a field emission display (FED), an SED-type flat panel display (SED: surface-conduction electron-emitter display), and the like; display devices using a liquid crystal element include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display); and display devices using electronic ink or an electrophoresis element include electronic paper. 
     Note that an EL element is an element having an anode, a cathode, and an EL layer interposed between the anode and the cathode. Note that as an EL layer, a layer utilizing light emission (fluorescence) from a singlet exciton, a layer utilizing light emission (phosphorescence) from a triplet exciton, a layer utilizing light emission (fluorescence) from a singlet exciton and light emission (phosphorescence) from a triplet exciton, a layer formed using an organic material, a layer formed using an inorganic material, a layer formed using an organic material and an inorganic material, a layer including a high-molecular material, a layer including a low molecular material, a layer including a low-molecular material and a high-molecular material, or the like can be used. Note that the present invention is not limited to this, and various EL elements can be used as an EL element. 
     Note that an electron emitter is an element in which electrons are extracted by high electric field concentration on a pointed cathode. For example, as an electron emitter, a Spindt type, a carbon nanotube (CNT) type, a metal-insulator-metal (MIM) type in which a metal, an insulator, and a metal are stacked, a metal-insulator-semiconductor (MIS) type in which a metal, an insulator, and a semiconductor are stacked, a MOS type, a silicon type, a thin film diode type, a diamond type, a surface conduction emitter SCD type, a thin film type in which a metal, an insulator, a semiconductor, and a metal are stacked, an HEED type, an EL type, a porous silicon type, a surface-conduction (SED) type, or the like can be used. However, the present invention is not limited to this, and various elements can be used as an electron emitter. 
     Note that a liquid crystal element is an element which controls transmission or non-transmission of light by optical modulation action of a liquid crystal and includes a pair of electrodes and a liquid crystal. Note that optical modulation action of a liquid crystal is controlled by an electric filed applied to the liquid crystal (including a horizontal electric field, a vertical electric field, and an oblique electric field). Note that the following can be used for a liquid crystal element: a nematic liquid crystal, a cholesteric liquid crystal, a smectic liquid crystal, a discotic liquid crystal, a thermotropic liquid crystal, a lyotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, a main-chain liquid crystal, a side-chain high-molecular liquid crystal, a plasma addressed liquid crystal (PALC), a banana-shaped liquid crystal, and the like. In addition, the following can be used as a diving method of a liquid crystal: a TN (twisted nematic) mode, an STN (super twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA (multi-domain vertical alignment) mode, a PVA (patterned vertical alignment) mode, an ASV (advanced super view) mode, an ASM (axially symmetric aligned microcell) mode, an OCB (optical compensated birefringence) mode, an ECB (electrically controlled birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC (anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersed liquid crystal) mode, a guest-host mode, and the like. Note that the present invention is not limited to this, and various liquid crystal elements and driving methods can be used as a liquid crystal element and a driving method thereof. 
     Note that electronic paper corresponds to a device which displays an image by molecules which utilize optical anisotropy, dye molecular orientation, or the like; a device which displays an image by particles which utilize electrophoresis, particle movement, particle rotation, phase change, or the like; a device which displays an image by moving one end of a film; a device which displays an image by using coloring properties or phase change of molecules; a device which displays an image by using optical absorption by molecules; and a device which displays an image by using self-light emission by bonding electrons and holes. For example, the following can be used for a display method of electronic paper: microcapsule electrophoresis, horizontal electrophoresis, vertical electrophoresis, a spherical twisting ball, a magnetic twisting ball, a columnar twisting ball, a charged toner, electro liquid powder, magnetic electrophoresis, a magnetic thermosensitive type, an electrowetting type, a light-scattering (transparent-opaque change) type, a cholesteric liquid crystal and a photoconductive layer, a cholesteric liquid crystal device, a bistable nematic liquid crystal, a ferroelectric liquid crystal, a liquid crystal dispersed type with a dichroic dye, a movable film, coloring and decoloring properties of a leuco dye, a photochromic material, an electrochromic material, an electrodeposition material, flexible organic EL, and the like. Note that the present invention is not limited to this, and various electronic paper and display methods can be used as electronic paper and a display method thereof. Here, when microcapsule electrophoresis is used, defects of electrophoresis, which are aggregation and precipitation of phoresis particles, can be solved. Electro liquid powder has advantages such as high-speed response, high reflectivity, wide viewing angle, low power consumption, and memory properties. 
     Note that a plasma display panel has a structure in which a substrate having a surface provided with an electrode and a substrate having a surface provided with an electrode and a minute groove in which a phosphor layer is formed face each other at a narrow interval and a rare gas is sealed therein. Note that display can be performed by applying voltage between the electrodes to generate an ultraviolet ray so that a phosphor emits light. Note that the plasma display panel may be a DC-type PDP or an AC-type PDP. As a driving method of the plasma display panel, AWS (address while sustain) driving, ADS (address display separated) driving in which a subframe is divided into a reset period, an address period, and a sustain period, CLEAR (high-contrast, low energy address and reduction of false contour sequence) driving, ALIS (alternate lighting of surfaces) method, TERES (technology of reciprocal sustainer) driving, or the like can be used. Note that the present invention is not limited to this, and various driving methods can be used as a driving method of a plasma display panel. 
     Note that electroluminescence, a cold cathode fluorescent lamp, a hot cathode fluorescent lamp, an LED, a laser light source, a mercury lamp, or the like can be used as a light source of a display device in which a light source is necessary, such as a liquid crystal display (a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display), a display device using a grating light valve (GLV), or a display device using a digital micromirror device (DMD). Note that the present invention is not limited to this, and various light sources can be used as a light source. 
     Note that various types of transistors can be used as a transistor, without limiting to a certain type. For example, a thin film transistor (TFT) including a non-single crystal semiconductor film typified by amorphous silicon, polycrystalline silicon, microcrystalline (also referred to as semi-amorphous) silicon, or the like can be used. In the case of using the TFT, there are various advantages. For example, since the TFT can be formed at temperature lower than that of the case of using single-crystal silicon, manufacturing cost can be reduced or a manufacturing apparatus can be made larger. Since the manufacturing apparatus is made larger, the TFT can be formed using a large substrate. Therefore, many display devices can be formed at the same time at low cost. In addition, a substrate having low heat resistance can be used because of low manufacturing temperature. Therefore, the transistor can be formed using a light-transmitting substrate. Accordingly, transmission of light in a display element can be controlled by using the transistor formed using the light-transmitting substrate. Alternatively, part of a film which forms the transistor can transmit light because the film thickness of the transistor is thin. Therefore, the aperture ratio can be improved. 
     Note that when a catalyst (e.g., nickel) is used in the case of forming polycrystalline silicon, crystallinity can be further improved and a transistor having excellent electric characteristics can be formed. Accordingly, a gate driver circuit (e.g., a scan line driver circuit), a source driver circuit (e.g., a signal line driver circuit), and/or a signal processing circuit (e.g., a signal generation circuit, a gamma correction circuit, or a DA converter circuit) can be formed over the same substrate as a pixel portion. 
     Note that when a catalyst (e.g., nickel) is used in the case of forming microcrystalline silicon, crystallinity can be further improved and a transistor having excellent electric characteristics can be formed. At this time, crystallinity can be improved by just performing heat treatment without performing laser irradiation. Accordingly, a gate driver circuit (e.g., a scan line driver circuit) and part of a source driver circuit (e.g., an analog switch) can be formed over the same substrate. In addition, in the case of not performing laser irradiation for crystallization, crystallinity unevenness of silicon can be suppressed. Therefore, a clear image can be displayed. 
     Note that polycrystalline silicon and microcrystalline silicon can be formed without using a catalyst (e.g., nickel). 
     Note that it is preferable that crystallinity of silicon be improved to polycrystal, microcrystal, or the like in the whole panel; however, the present invention is not limited to this. Crystallinity of silicon may be improved only in part of the panel. Selective increase in crystallinity can be achieved by selective laser irradiation or the like. For example, only a peripheral driver circuit region excluding pixels may be irradiated with laser light. Alternatively, only a region of a gate driver circuit, a source driver circuit, or the like may be irradiated with laser light. Further alternatively, only part of a source driver circuit (e.g., an analog switch) may be irradiated with laser light. Accordingly, crystallinity of silicon can be improved only in a region in which a circuit needs to be operated at high speed. Since a pixel region is not particularly needed to be operated at high speed, even if crystallinity is not improved, the pixel circuit can be operated without problems. Since a region, crystallinity of which is improved, is small, manufacturing steps can be decreased, throughput can be increased, and manufacturing cost can be reduced. Since the number of necessary manufacturing apparatus is small, manufacturing cost can be reduced. 
     A transistor can be formed by using a semiconductor substrate, an SOI substrate, or the like. Thus, a transistor with few variations in characteristics, sizes, shapes, or the like, with high current supply capacity, and with a small size can be formed. When such a transistor is used, power consumption of a circuit can be reduced or a circuit can be highly integrated. 
     A transistor including a compound semiconductor or an oxide semiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO, or SnO, a thin film transistor obtained by thinning such a compound semiconductor or a oxide semiconductor, or the like can be used. Thus, manufacturing temperature can be lowered and for example, such a transistor can be formed at room temperature. Accordingly, the transistor can be formed directly on a substrate having low heat resistance, such as a plastic substrate or a film substrate. Note that such a compound semiconductor or an oxide semiconductor can be used for not only a channel portion of the transistor but also other applications. For example, such a compound semiconductor or an oxide semiconductor can be used as a resistor, a pixel electrode, or a light-transmitting electrode. Further, since such an element can be formed at the same time as the transistor, cost can be reduced. 
     A transistor formed by using an inkjet method or a printing method, or the like can be used. Accordingly, a transistor can be formed at room temperature, can be formed at a low vacuum, or can be formed using a large substrate. In addition, since the transistor can be formed without using a mask (a reticle), a layout of the transistor can be easily changed. Further, since it is not necessary to use a resist, material cost is reduced and the number of steps can be reduced. Furthermore, since a film is formed only in a necessary portion, a material is not wasted compared with a manufacturing method in which etching is performed after the film is formed over the entire surface, so that cost can be reduced. 
     A transistor including an organic semiconductor or a carbon nanotube, or the like can be used. Accordingly, such a transistor can be formed using a substrate which can be bent. Therefore, a device using a transistor including an organic semiconductor or a carbon nanotube, or the like can resist a shock. 
     Further, transistors with various structures can be used. For example, a MOS transistor, a junction transistor, a bipolar transistor, or the like can be used as a transistor. When a MOS transistor is used, the size of the transistor can be reduced. Thus, a large number of transistors can be mounted. When a bipolar transistor is used, large current can flow. Thus, a circuit can be operated at high speed. 
     Note that a MOS transistor, a bipolar transistor, and the like may be formed over one substrate. Thus, reduction in power consumption, reduction in size, high speed operation, and the like can be realized. 
     Furthermore, various transistors can be used. 
     Note that a transistor can be formed using various types of substrates without limiting to a certain type. For example, a single-crystal semiconductor substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), or the like), a leather substrate, a rubber substrate, a stainless steel substrate, a substrate including a stainless steel foil, or the like can be used as a substrate. Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissue of an animal such as a human being can be used as a substrate. Further alternatively, the transistor may be formed using one substrate, and then, the transistor may be transferred to another substrate. A single-crystal semiconductor substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), or the like), a leather substrate, a rubber substrate, a stainless steel substrate, a substrate including a stainless steel foil, or the like can be used as a substrate to which the transistor is transferred. Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissue of an animal such as a human being can be used as a substrate to which the transistor is transferred. Further alternatively, the transistor may be formed using one substrate and the substrate may be thinned by polishing. A single-crystal semiconductor substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), or the like), a leather substrate, a rubber substrate, a stainless steel substrate, a substrate including a stainless steel foil, or the like can be used as a substrate to be polished. Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissue of an animal such as a human being can be used as a substrate to be polished. When such a substrate is used, a transistor with excellent properties or a transistor with low power consumption can be formed, a device with high durability, high heat resistance can be provided, or reduction in weight or thickness can be achieved. 
     Note that a structure of a transistor can be various modes without limiting to a certain structure. For example, a multi-gate structure having two or more gate electrodes may be used. When the multi-gate structure is used, a structure where a plurality of transistors are connected in series is provided because channel regions are connected in series. With the multi-gate structure, off-current can be reduced or the withstand voltage of the transistor can be increased to improve reliability. Alternatively, with the multi-gate structure, drain-source current does not fluctuate very much even if drain-source voltage fluctuates when the transistor operates in a saturation region, so that a flat slope of voltage-current characteristics can be obtained. When the flat slope of the voltage-current characteristics is utilized, an ideal current source circuit or an active load having an extremely high resistance value can be realized. Accordingly, a differential circuit or a current mirror circuit having excellent properties can be realized. As another example, a structure where gate electrodes are formed above and below a channel may be used. When the structure where gate electrodes are formed above and below the channel is used, a channel region is increased, so that the amount of current flowing therethrough can be increased or a depletion layer can be easily formed to decrease subthreshold swing. When the gate electrodes are formed above and below the channel, a structure where a plurality of transistors are connected in parallel is provided. 
     Alternatively, a structure where a gate electrode is formed above a channel region, a structure where a gate electrode is formed below a channel region, a staggered structure, an inversely staggered structure, a structure where a channel region is divided into a plurality of regions, or a structure where channel regions are connected in parallel or in series can be used. Further alternatively, a source electrode or a drain electrode may overlap with a channel region (or part of it). When the structure where the source electrode or the drain electrode may overlap with the channel region (or part of it) is used, the case can be prevented in which electric charges are accumulated in part of the channel region, which would result in an unstable operation. Further alternatively, an LDD region may be provided. When the LDD region is provided, off-current can be reduced or the withstand voltage of the transistor can be increased to improve reliability. Further, when the LDD region is provided, drain-source current does not fluctuate very much even if drain-source voltage fluctuates when the transistor operates in the saturation region, so that a flat slope of voltage-current characteristics can be obtained. 
     Note that various types of transistors can be used as a transistor and the transistor can be formed using various types of substrates. Accordingly, all the circuits that are necessary to realize a predetermined function may be formed using the same substrate. For example, all the circuits that are necessary to realize the predetermined function may be formed using a glass substrate, a plastic substrate, a single-crystal semiconductor substrate, an SOI substrate, or any other substrate. When all the circuits that are necessary to realize the predetermined function are formed using the same substrate, cost can be reduced by reduction in the number of component parts or reliability can be improved by reduction in the number of connections to circuit components. Alternatively, part of the circuits which are necessary to realize the predetermined function may be formed using one substrate and another part of the circuits which are necessary to realize the predetermined function may be formed using another substrate. That is, not all the circuits that are necessary to realize the predetermined function are required to be formed using the same substrate. For example, part of the circuits which are necessary to realize the predetermined function may be formed by transistors using a glass substrate and another part of the circuits which are necessary to realize the predetermined function may be formed using a single-crystal semiconductor substrate, so that an IC chip formed by a transistor using the single-crystal semiconductor substrate may be connected to the glass substrate by COG (chip on glass) and the IC chip may be provided over the glass substrate. Alternatively, the IC chip may be connected to the glass substrate by TAB (tape automated bonding) or a printed wiring board. When part of the circuits are formed using the same substrate in this manner, cost can be reduced by reduction in the number of component parts or reliability can be improved by reduction in the number of connections to circuit components. Further alternatively, when circuits with high driving voltage and high driving frequency, which consume large power, are formed using a single-crystal semiconductor substrate instead of forming such circuits using the same substrate and an IC chip formed by the circuit is used, increase in power consumption can be prevented. 
     Note that one pixel corresponds to one element whose brightness can be controlled. Therefore, for example, one pixel corresponds to one color element and brightness is expressed with the one color element. Accordingly, in the case of a color display device having color elements of R (red), G (green), and B (blue), a minimum unit of an image is formed of three pixels of an R pixel, a G pixel, and a B pixel. Note that the color elements are not limited to three colors, and color elements of more than three colors may be used or a color other than RGB may be used. For example, RGBW (W corresponds to white) may be used by adding white. Alternatively, one or more colors of yellow, cyan, magenta emerald green, vermilion, and the like may be added to RGB. Further alternatively, a color similar to at least one of R, G, and B may be added to RGB. For example, R, G, B1, and B2 may be used. Although both B1 and B2 are blue, they have slightly different frequency. Similarly, R1, R2, G, and B may be used. When such color elements are used, display which is closer to the real object can be performed and power consumption can be reduced. As another example, in the case of controlling brightness of one color element by using a plurality of regions, one region may correspond to one pixel. Therefore, for example, in the case of performing area ratio gray scale display or the case of including a subpixel, a plurality of regions which control brightness are provided in each color element and gray scales are expressed with the whole regions. In this case, one region which controls brightness may correspond to one pixel. Thus, in that case, one color element includes a plurality of pixels. Alternatively, even when the plurality of regions which control brightness are provided in one color element, these regions may be collected as one pixel. Thus, in that case, one color element includes one pixel. In that case, one color element includes one pixel. Further alternatively, in the case where brightness is controlled in a plurality of regions in each color element, regions which contribute to display have different area dimensions depending on pixels in some cases. Further alternatively, in the plurality of regions which control brightness in each color element, signals supplied to each of the plurality of regions may be slightly varied to widen a viewing angle. That is, potentials of pixel electrodes included in the plurality of regions provided in each color element may be different from each other. Accordingly, voltage applied to liquid crystal molecules are varied depending on the pixel electrodes. Therefore, the viewing angle can be widened. 
     Note that explicit description “one pixel (for three colors)” corresponds to the case where three pixels of R, G, and B are considered as one pixel. Meanwhile, explicit description “one pixel (for one color)” corresponds to the case where the plurality of regions are provided in each color element and collectively considered as one pixel. 
     Note that pixels are provided (arranged) in matrix in some cases. Here, description that pixels are provided (arranged) in matrix includes the case where the pixels are arranged in a straight line and the case where the pixels are arranged in a jagged line, in a longitudinal direction or a lateral direction. Thus, for example, in the case of performing full color display with three color elements (e.g., RGB), the following cases are included therein: the case where the pixels are arranged in stripes and the case where dots of the three color elements are arranged in a delta pattern. In addition, the case is also included therein in which dots of the three color elements are provided in Bayer arrangement. Note that the color elements are not limited to three colors, and color elements of more than three colors may be used. For example, RGBW (W corresponds to white), RGB plus one or more of yellow, cyan, and magenta, or the like may be used. Further, the sizes of display regions may be different between respective dots of color elements. Thus, power consumption can be reduced or the life of a display element can be prolonged. 
     Note that an active matrix method in which an active element is included in a pixel or a passive matrix method in which an active element is not included in a pixel can be used. 
     In an active matrix method, as an active element (a non-linear element), not only a transistor but also various active elements (non-linear elements) can be used. For example, an MIM (metal insulator metal), a TFD (thin film diode), or the like can also be used. Since such an element has few number of manufacturing steps, manufacturing cost can be reduced or yield can be improved. Further, since the size of the element is small, the aperture ratio can be improved, so that power consumption can be reduced or high luminance can be achieved. 
     Note that as a method other than an active matrix method, a passive matrix method in which an active element (a non-linear element) is not used can also be used. Since an active element (a non-linear element) is not used, manufacturing steps is few, so that manufacturing cost can be reduced or yield can be improved. Further, since an active element (a non-linear element) is not used, the aperture ratio can be improved, so that power consumption can be reduced or high luminance can be achieved. 
     Note that a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor has a channel region between a drain region and a source region, and current can flow through the drain region, the channel region, and the source region. Here, since the source and the drain of the transistor change depending on the structure, the operating condition, and the like of the transistor, it is difficult to define which is a source or a drain. Therefore, in this document, a region functioning as a source and a drain may not be called the source or the drain. In such a case, one of the source and the drain may be referred to as a first terminal and the other thereof may be referred to as a second terminal, for example. Alternatively, one of the source and the drain may be referred to as a first electrode and the other thereof may be referred to as a second electrode. Further alternatively, one of the source and the drain may be referred to as a source region and the other thereof may be called a drain region. 
     Note that a transistor may be an element having at least three terminals of a base, an emitter, and a collector. In this case, one of the emitter and the collector may be similarly referred to as a first terminal and the other terminal may be referred to as a second terminal. 
     Note that a gate corresponds to all or part of a gate electrode and a gate wiring (also referred to as a gate line, a gate signal line, a scan line, a scan signal line, or the like). A gate electrode corresponds to a conductive film which overlaps with a semiconductor which forms a channel region with a gate insulating film interposed therebetween. Note that part of the gate electrode overlaps with an LDD (lightly doped drain) region or the source region (or the drain region) with the gate insulating film interposed therebetween in some cases. A gate wiring corresponds to a wiring for connecting a gate electrode of each transistor to each other, a wiring for connecting a gate electrode of each pixel to each other, or a wiring for connecting a gate electrode to another wiring. 
     However, there is a portion (a region, a conductive film, a wiring, or the like) which functions as both a gate electrode and a gate wiring. Such a portion (a region, a conductive film, a wiring, or the like) may be referred to as either a gate electrode or a gate wiring. That is, there is a region where a gate electrode and a gate wiring cannot be clearly distinguished from each other. For example, in the case where a channel region overlaps with part of an extended gate wiring, the overlapped portion (region, conductive film, wiring, or the like) functions as both a gate wiring and a gate electrode. Accordingly, such a portion (a region, a conductive film, a wiring, or the like) may be referred to as either a gate electrode or a gate wiring. 
     Note that a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a gate electrode, forms the same island as the gate electrode, and is connected to the gate electrode may also be referred to as a gate electrode. Similarly, a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a gate wiring, forms the same island as the gate wiring, and is connected to the gate wiring may also be referred to as a gate wiring. In a strict detect, such a portion (a region, a conductive film, a wiring, or the like) does not overlap with a channel region or does not have a function of connecting the gate electrode to another gate electrode in some cases. However, there is a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a gate electrode or a gate wiring, forms the same island as the gate electrode or the gate wiring, and is connected to the gate electrode or the gate wiring because of specifications or the like in manufacturing. Thus, such a portion (a region, a conductive film, a wiring, or the like) may also be referred to as either a gate electrode or a gate wiring. 
     Note that in a multi-gate transistor, for example, a gate electrode is often connected to another gate electrode by using a conductive film which is formed using the same material as the gate electrode. Since such a portion (a region, a conductive film, a wiring, or the like) is a portion (a region, a conductive film, a wiring, or the like) for connecting the gate electrode to another gate electrode, it may be referred to as a gate wiring, and it may also be referred to as a gate electrode because a multi-gate transistor can be considered as one transistor. That is, a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a gate electrode or a gate wiring, forms the same island as the gate electrode or the gate wiring, and is connected to the gate electrode or the gate wiring may be referred to as either a gate electrode or a gate wiring. In addition, for example, part of a conductive film which connects the gate electrode and the gate wiring and is formed using a material which is different from that of the gate electrode or the gate wiring may also be referred to as either a gate electrode or a gate wiring. 
     Note that a gate terminal corresponds to part of a portion (a region, a conductive film, a wiring, or the like) of a gate electrode or a portion (a region, a conductive film, a wiring, or the like) which is electrically connected to the gate electrode. 
     Note that when a wiring is referred to as a gate wiring, a gate line, a gate signal line, a scan line, a scan signal line, there is the case in which a gate of a transistor is not connected to a wiring. In this case, the gate wiring, the gate line, the gate signal line, the scan line, or the scan signal line corresponds to a wiring formed in the same layer as the gate of the transistor, a wiring formed using the same material of the gate of the transistor, or a wiring formed at the same time as the gate of the transistor in some cases. As examples, there are a wiring for a storage capacitor, a power supply line, a reference potential supply line, and the like. 
     Note that a source corresponds to all or part of a source region, a source electrode, and a source wiring (also referred to as a source line, a source signal line, a data line, a data signal line, or the like). A source region corresponds to a semiconductor region including a large amount of p-type impurities (e.g., boron or gallium) or n-type impurities (e.g., phosphorus or arsenic). Therefore, a region including a small amount of p-type impurities or n-type impurities, namely, an LDD (lightly doped drain) region is not included in the source region. A source electrode is part of a conductive layer which is formed using a material different from that of a source region and is electrically connected to the source region. However, there is the case where a source electrode and a source region are collectively referred to as a source electrode. A source wiring is a wiring for connecting a source electrode of each transistor to each other, a wiring for connecting a source electrode of each pixel to each other, or a wiring for connecting a source electrode to another wiring. 
     However, there is a portion (a region, a conductive film, a wiring, or the like) functioning as both a source electrode and a source wiring. Such a portion (a region, a conductive film, a wiring, or the like) may be referred to as either a source electrode or a source wiring. That is, there is a region where a source electrode and a source wiring cannot be clearly distinguished from each other. For example, in the case where a source region overlaps with part of an extended source wiring, the overlapped portion (region, conductive film, wiring, or the like) functions as both a source wiring and a source electrode. Accordingly, such a portion (a region, a conductive film, a wiring, or the like) may be referred to as either a source electrode or a source wiring. 
     Note that a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a source electrode, forms the same island as the source electrode, and is connected to the source electrode, or a portion (a region, a conductive film, a wiring, or the like) which connects a source electrode and another source electrode may also be referred to as a source electrode. Further, a portion which overlaps with a source region may be referred to as a source electrode. Similarly, a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a source wiring, forms the same island as the source wiring, and is connected to the source wiring may also be referred to as a source wiring. In a strict sense, such a portion (a region, a conductive film, a wiring, or the like) does not have a function of connecting the source electrode to another source electrode in some cases. However, there is a portion (a region, a conductive film, a wiring, or the like) which is formed using the same material as a source electrode or a source wiring, forms the same island as the source electrode or the source wiring, and is connected to the source electrode or the source wiring because of specifications or the like in manufacturing. Thus, such a portion (a region, a conductive film, a wiring, or the like) may also be referred to as either a source electrode or a source wiring. 
     For example, part of a conductive film which connects a source electrode and a source wiring and is formed using a material which is different from that of the source electrode or the source wiring may be referred to as either a source electrode or a source wiring. 
     Note that a source terminal corresponds to part of a source region, a source electrode, or a portion (a region, a conductive film, a wiring, or the like) which is electrically connected to the source electrode. 
     Note that when a wiring is referred to as a source wiring, a source line, a source signal line, a data line, a data signal line, there is the case in which a source (a drain) of a transistor is not connected to a wiring. In this case, the source wiring, the source line, the source signal line, the data line, or the data signal line corresponds to a wiring formed in the same layer as the source (the drain) of the transistor, a wiring formed using the same material of the source (the drain) of the transistor, or a wiring formed at the same time as the source (the drain) of the transistor in some cases. As examples, there are a wiring for a storage capacitor, a power supply line, a reference potential supply line, and the like. 
     Note that the same can be said for a drain. 
     Note that a semiconductor device corresponds to a device having a circuit including a semiconductor element (e.g., a transistor, a diode, or a thyristor). The semiconductor device may also include all devices that can function by utilizing semiconductor characteristics. In addition, the semiconductor device corresponds to a device having a semiconductor material. 
     Note that a display element corresponds to an optical modulation element, a liquid crystal element, a light-emitting element, an EL element (an organic EL element, an inorganic EL element, or an EL element including organic and inorganic materials), an electron emitter, an electrophoresis element, a discharging element, a light-reflective element, a light diffraction element, a digital micromirror device (DMD), or the like. Note that the present invention is not limited to this. 
     Note that a display device corresponds to a device having a display element. The display device may include a plurality of pixels each having a display element. Note that that the display device may also include a peripheral driver circuit for driving the plurality of pixels. The peripheral driver circuit for driving the plurality of pixels may be formed over the same substrate as the plurality of pixels. The display device may also include a peripheral driver circuit provided over a substrate by wire bonding or bump bonding, namely, an IC chip connected by chip on glass (COG) or an IC chip connected by TAB or the like. Further, the display device may also include a flexible printed circuit (FPC) to which an IC chip, a resistor, a capacitor, an inductor, a transistor, or the like is attached. Note that the display device includes a printed wiring board (PWB) which is connected through a flexible printed circuit (FPC) and to which an IC chip, a resistor, a capacitor, an inductor, a transistor, or the like is attached. The display device may also include an optical sheet such as a polarizing plate or a retardation plate. The display device may also include a lighting device, a housing, an audio input and output device, a light sensor, or the like. Here, a lighting device such as a backlight unit may include a light guide plate, a prism sheet, a diffusion sheet, a reflective sheet, a light source (e.g., an LED or a cold cathode fluorescent lamp), a cooling device (e.g., a water cooling device or an air cooling device), or the like. 
     Note that a lighting device corresponds to a device having a backlight unit, a light guide plate, a prism sheet, a diffusion sheet, a reflective sheet, or a light source (e.g., an LED, a cold cathode fluorescent lamp, or a hot cathode fluorescent lamp), a cooling device, or the like. 
     Note that a light-emitting device corresponds to a device having a light-emitting element and the like. In the case of including a light-emitting element as a display element, the light-emitting device is one of specific examples of a display device. 
     Note that a reflective device corresponds to a device having a light-reflective element, a light diffraction element, light-reflective electrode, or the like. 
     Note that a liquid crystal display device corresponds to a display device including a liquid crystal element. Liquid crystal display devices include a direct-view liquid crystal display, a projection liquid crystal display, a transmissive liquid crystal display, a reflective liquid crystal display, a transflective liquid crystal display, and the like. 
     Note that a driving device corresponds to a device having a semiconductor element, an electric circuit, or an electronic circuit. For example, a transistor which controls input of a signal from a source signal line to a pixel (also referred to as a selection transistor, a switching transistor, or the like), a transistor which supplies voltage or current to a pixel electrode, a transistor which supplies voltage or current to a light-emitting element, and the like are examples of the driving device. A circuit which supplies a signal to a gate signal line (also referred to as a gate driver, a gate line driver circuit, or the like), a circuit which supplies a signal to a source signal line (also referred to as a source driver, a source line driver circuit, or the like) are also examples of the driving device. 
     Note that a display device, a semiconductor device, a lighting device, a cooling device, a light-emitting device, a reflective device, a driving device, and the like overlap with each other in some cases. For example, a display device includes a semiconductor device and a light-emitting device in some cases. Alternatively, a semiconductor device includes a display device and a driving device in some cases. 
     Note that when it is explicitly described that “B is formed on A” or “B is formed over A”, it does not necessarily mean that B is formed in direct contact with A. The description includes the case where A and B are not in direct contact with each other, i.e., the case where another object is interposed between A and B. Here, each of A and B corresponds to an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). 
     Accordingly, for example, when it is explicitly described that “a layer B is formed on (or over) a layer A”, it includes both the case where the layer B is formed in direct contact with the layer A, and the case where another layer (e.g., a layer C or a layer D) is formed in direct contact with the layer A and the layer B is formed in direct contact with the layer C or D. Note that another layer (e.g., a layer C or a layer D) may be a single layer or a plurality of layers. 
     Similarly, when it is explicitly described that “B is formed above A”, it does not necessarily mean that B is formed in direct contact with A, and another object may be interposed therebetween. Thus, for example, when it is described that “a layer B is formed above a layer A”, it includes both the case where the layer B is formed in direct contact with the layer A, and the case where another layer (e.g., a layer C or a layer D) is formed in direct contact with the layer A and the layer B is formed in direct contact with the layer C or D. Note that another layer (e.g., a layer C or a layer D) may be a single layer or a plurality of layers. 
     Note that when it is explicitly described that “B is formed in direct contact with A”, it includes not the case where another object is interposed between A and B but the case where B is formed in direct contact with A. 
     Note that the same can be said when it is described that B is formed below or under A. 
     Note that when an object is explicitly described in a singular form, the object is preferably singular. Note that the present invention is not limited to this, and the object can be plural. Similarly, when an object is explicitly described in a plural form, the object is preferably plural. Note that the present invention is not limited to this, and the object can be singular. 
     In accordance with the present invention, performance as a display device can be maintained and viewing angle characteristics can be improved compared to that of a conventional display device. Alternatively, in accordance with the present invention, a highly reliable display device can be provided. Alternatively, in accordance with the present invention, a display device having high contrast can be provided. Alternatively, in accordance with the present invention, a lightweight display device can be provided. Alternatively, in accordance with the present invention, a small display device can be provided. Alternatively, in accordance with the present invention, a display device having high luminance can be provided. Alternatively, in accordance with the present invention, a display device with low power consumption can be provided. Alternatively, in accordance with the present invention, a display device having a high aperture ratio can be provided. Alternatively, in accordance with the present invention, a display device with low manufacturing cost can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1C  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 2A and 2B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 3A and 3B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 4A and 4B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 5A and 5B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 6A and 6B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 7A and 7B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 8A and 8B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 9A and 9B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 10A and 10B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 11A and 11B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 12A and 12B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 13A and 13B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 14A and 14B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 15A and 15B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 16A and 16B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 17A and 17B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 18A and 18B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 19A and 19B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 20A and 20B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 21A and 21B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 22A and 22B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 23A and 23B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 24A and 24B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 25A and 25B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 26A and 26B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 27A and 27B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 28A and 28B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 29A and 29B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 30A to 30T  each illustrate a divider element included in a pixel circuit of a display device of the present invention; 
         FIG. 31  illustrates a display device of the present invention; 
         FIG. 32  illustrates an example of a top surface layout of a pixel included in a display device of the present invention; 
         FIG. 33  illustrates a pixel circuit of a display device of the present invention; 
         FIG. 34  illustrates an example of a top surface layout of a pixel included in a display device of the present invention; 
         FIG. 35  illustrates a pixel circuit of a display device of the present invention; 
         FIGS. 36A and 36B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 37A and 37B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 38A to 38C  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 39A and 39B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 40A and 40B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 41A and 41B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 42A and 42B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 43A and 43B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 44A and 44B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 45A and 45B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 46A and 46B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 47A and 47B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 48A and 48B  each illustrate a pixel circuit of a display device of the present invention; 
         FIG. 49  illustrates a pixel circuit of a display device of the present invention; 
         FIGS. 50A and 50B  each illustrate a pixel circuit of a display device of the present invention; 
         FIGS. 51A to 51G  illustrate the present invention; 
         FIG. 52  illustrates the present invention; 
         FIG. 53  illustrates the present invention; 
         FIG. 54  illustrates the present invention; 
         FIG. 55  illustrates the present invention; 
         FIGS. 56A to 56C  illustrate the present invention; 
         FIGS. 57A to 57D  illustrate the present invention; 
         FIGS. 58A to 58C  illustrate the present invention; 
         FIGS. 59A to 59D  illustrate the present invention; 
         FIGS. 60A to 60D  illustrate the present invention; 
         FIGS. 61A to 61C  each illustrate the present invention; 
         FIGS. 62A and 62B  each illustrate the present invention; 
         FIG. 63  illustrates the present invention; 
         FIGS. 64A and 64B  each illustrate the present invention; 
         FIG. 65  illustrates the present invention; 
         FIG. 66  illustrates the present invention; 
         FIG. 67  illustrates the present invention; 
         FIG. 68  illustrates the present invention; 
         FIG. 69  illustrates the present invention; 
         FIG. 70  illustrates the present invention; 
         FIGS. 71A to 71C  each illustrate the present invention; 
         FIGS. 72A to 72E  each illustrate the present invention; 
         FIGS. 73A and 73B  each illustrate the present invention; 
         FIGS. 74A to 74D  each illustrate the present invention; 
         FIG. 75  illustrates the present invention; 
         FIGS. 76A to 76D  each illustrate the present invention; 
         FIG. 77  illustrates the present invention; 
         FIGS. 78A to 78C  each illustrate the present invention; 
         FIGS. 79A and 79B  each illustrate the present invention; 
         FIGS. 80A to 80E  each illustrate the present invention; 
         FIGS. 81A and 81B  each illustrate the present invention; 
         FIGS. 82A to 82C  each illustrate the present invention; 
         FIGS. 83A to 83C  each illustrate the present invention; 
         FIGS. 84A to 84C  each illustrate the present invention; 
         FIG. 85  illustrates the present invention; 
         FIGS. 86A and 86B  each illustrate the present invention; 
         FIGS. 87A and 87B  each illustrate the present invention; 
         FIG. 88  illustrates the present invention; 
         FIGS. 89A and 89B  each illustrate the present invention; 
         FIGS. 90A and 90B  each illustrate the present invention; 
         FIGS. 91A to 91E  illustrate the present invention; 
         FIGS. 92A to 92C  illustrate the present invention; 
         FIGS. 93A to 93D  illustrate the present invention; 
         FIGS. 94A to 94C  illustrate the present invention; 
         FIGS. 95A and 95B  illustrate the present invention; 
         FIGS. 96A and 96B  illustrate the present invention; 
         FIG. 97  illustrates the present invention; 
         FIG. 98  illustrates the present invention; 
         FIG. 99  illustrates the present invention; 
         FIG. 100  illustrates the present invention; 
         FIG. 101  illustrates the present invention; 
         FIGS. 102A and 102B  illustrate the present invention; 
         FIGS. 103A and 103B  illustrate the present invention; 
         FIGS. 104A and 104B  illustrate the present invention; 
         FIGS. 105A and 105E  each illustrate the present invention; 
         FIG. 106  illustrates the present invention; 
         FIG. 107  illustrates the present invention; 
         FIGS. 108A to 108C  each illustrate the present invention; 
         FIGS. 109A to 109C  each illustrate the present invention; 
         FIGS. 110A and 110B  illustrate the present invention; 
         FIG. 111  illustrates the present invention; 
         FIG. 112  illustrates the present invention; 
         FIG. 113  illustrates the present invention; 
         FIGS. 114A to 114C  each illustrate the present invention; 
         FIG. 115  illustrates the present invention; 
         FIG. 116  illustrates the present invention; 
         FIGS. 117A and 117B  each illustrate the present invention; 
         FIGS. 118A and 118B  each illustrate the present invention; 
         FIG. 119  illustrates the present invention; 
         FIG. 120  illustrates the present invention; 
         FIGS. 121A to 121C  each illustrate the present invention; 
         FIG. 122  illustrates the present invention; 
         FIG. 123  illustrates the present invention; 
         FIG. 124  illustrates the present invention; 
         FIG. 125  illustrates the present invention; 
         FIGS. 126A and 126B  illustrate the present invention; 
         FIGS. 127A and 127B  illustrate the present invention; 
         FIGS. 128A to 128C  each illustrate the present invention; 
         FIGS. 129A and 129B  each illustrate the present invention; 
         FIG. 130  illustrates the present invention; 
         FIGS. 131A and 131B  each illustrate the present invention; 
         FIG. 132  illustrates the present invention; 
         FIG. 133  illustrates the present invention; 
         FIGS. 134A and 134B  each illustrate the present invention; 
         FIGS. 135A to 135D  each illustrate the present invention; 
         FIGS. 136A to 136D  each illustrate the present invention; 
         FIGS. 137A to 137D  each illustrate the present invention; 
         FIG. 138  illustrates the present invention; 
         FIGS. 139A to 139D  each illustrate the present invention; and 
         FIGS. 140A to 140D  each illustrate the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described by way of embodiment modes with reference to the drawings. Note that the present invention can be implemented in various different ways and it will be readily appreciated by those skilled in the art that various changes and modifications are possible without departing from the spirit and the scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiment modes of the present invention. Note that in structures of the present invention described hereinafter, like portions or portions having similar functions are denoted by common reference numerals in different drawings, and detailed description thereof is omitted. 
     Hereinafter, embodiment modes will be described with reference to various drawings. In that case, in embodiment mode, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part in a drawing described in embodiment mode is combined with another part in the above-described drawing. 
     Similarly, the contents (or may be part of the contents) described in each drawing of embodiment mode or a plurality of embodiment modes can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing of another embodiment mode or a plurality of other embodiment modes. Further, even more drawings can be formed when each part in the drawing of embodiment mode or a plurality of embodiment modes is combined with part of another embodiment mode or a plurality of other embodiment modes. 
     Note that the contents (or may be part of the contents) described in embodiment mode will show an example of an embodied case of other contents (or may be part of the contents) described in the embodiment mode, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents (or may be part of the contents) described in embodiment mode can be freely applied to, combined with, or replaced with other contents (or may be part of the contents) described in the embodiment mode. 
     Note that the contents (or may be part of the contents) described in embodiment mode or a plurality of embodiment modes will show an example of an embodied case of the contents (or may be part of the contents) described in the embodiment mode or the plurality of embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents (or may be part of the contents) described in another embodiment mode can be freely applied to, combined with, or replaced with other contents (or may be part of the contents) described in another embodiment mode or a plurality of other embodiment modes. 
     Embodiment Mode 1 
     In this embodiment mode, structures and operations of a pixel circuit included in a liquid crystal display device of the present invention are described with reference to the drawings. The pixel circuit of the liquid crystal display device of the present invention has a structure in which one pixel is provided with a plurality of liquid crystal elements and voltage which is applied is varied between the liquid crystal elements. Specifically, one of or both a capacitor and a resistor connected to a liquid crystal element are provided to vary voltage applied to the liquid crystal element. 
     Note that a display element is not limited to a liquid crystal element, and various display elements (e.g., a light-emitting element (an EL element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element) or an electron emitter), an electrophoresis element, and the like) can be used. 
     There are various operation modes of liquid crystals to which this embodiment mode can be applied. For example, there are a TN (twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA (multi-domain vertical alignment) mode, a PVA (patterned vertical alignment) mode, a CPA (continuous pinwheel alignment) mode, an ASM (axially symmetric aligned micro-cell) mode, an OCB (optical compensated birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode, and the like. Note that the present invention is not limited to this. Note that a liquid crystal to which a CPA mode is applied is often referred to as an ASV (advanced super view) liquid crystal. 
       FIG. 1A  shows an example of the structure of a pixel included in a liquid crystal display device of the present invention. A pixel  100  includes a first switch  101 , a second switch  102 , a first liquid crystal element  103 , a second liquid crystal element  104 , a third liquid crystal element  105 , a first capacitor  106 , and a second capacitor  107 . 
     A first wiring  108  is connected to a first electrode of the first liquid crystal element  103  and a first electrode (also referred to as a first terminal) of the first capacitor  106  through the first switch  101 . A second wiring  109  is connected to a first electrode of the second liquid crystal element  104  and a first electrode of the second capacitor  107  through the second switch  102 . A second electrode (also referred to as a second terminal) of the first capacitor  106  is connected to a second electrode of the second capacitor  107  and a first electrode of the third liquid crystal element  105 . 
     Second electrodes of the first liquid crystal element  103 , the second liquid crystal element  104 , and the third liquid crystal element  105  are connected to a common electrode  111 . 
     Each of the first wiring  108  and the second wiring  109  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  108  and the second wiring  109 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. 
     Each of the first switch  101  and the second switch  102  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. The case where a transistor is used as each of the first switch  101  and the second switch  102  is described below (see  FIG. 1B ). In the case of using a transistor, the transistor may be either a P-channel transistor or an N-channel transistor. For example, in an N-channel transistor, when gate-source voltage (V gs ) exceeds the threshold voltage (Vth), a source and a drain are conducted. Note that drain-source voltage of the transistor is denoted by V ds . 
       FIG. 1B  shows the case where an N-channel transistor is used as a switch, and  FIG. 1C  shows the case where a P-channel transistor is used as a switch. In  FIGS. 1B and 1C , gates of a first switch  101 N (or a first switch  101 P) and a second switch  102 N (or a second switch  102 P) are connected to a third wiring  110 . The third wiring  110  functions as a scan line. 
     Note that the number of scan lines may be two, as shown in  FIG. 49 . A circuit shown in  FIG. 49  is similar to a circuit where two signal lines are provided in a circuit in  FIG. 8B . 
     Although the case where a P-channel transistor is used as a switch is only shown in  FIG. 1C , the present invention is not limited to this. In other drawings, at least one transistor can be replaced with a P-channel transistor. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     A video signal is input to the first wiring  108  and the second wiring  109 . A scan signal is input to the third wiring  110 . The scan signal is an H-level or L-level digital voltage signal. In the case where the first switch  101  is an N-channel transistor, an H level of the scan signal is a potential which can turn on the first switch  101  and the second switch  102 , and an L level of the scan signal is a potential which can turn off the first switch  101  and the second switch  102 . Alternatively, in the case where the first switch  101  and the second switch  102  are P-channel transistors, an H level of the scan signal is a potential which can turn off the first switch  101  and the second switch  102 , and an L level of the scan signal is a potential which can turn on the first switch  101  and the second switch  102 . Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current, which may be either analog or digital. It is preferable that a potential of the video signal be lower than the H level of the scan signal and higher than the L level of the scan signal. 
     Operations of the pixel  100  are described by dividing the whole operations into the case where the first switch  101  and the second switch  102  are on and the case where the first switch  101  and the second switch  102  are off. 
     In the case where the first switch  101  is on, the first wiring  108  is electrically connected to the first electrode (a pixel electrode) of the first liquid crystal element  103  and the first electrode of the first capacitor  106 . In the case where the second switch  102  is on, the second wiring  109  is electrically connected to the first electrode (a pixel electrode) of the second liquid crystal element  104  and the first electrode of the second capacitor  107 . Therefore, a video signal is input from the first wiring  108  to the first electrode (the pixel electrode) of the first liquid crystal element  103  and the first electrode of the first capacitor  106 . Alternatively, a video signal is input from the second wiring  109  to the first electrode (the pixel electrode) of the second liquid crystal element  104  and the first electrode of the second capacitor  107 . Therefore, a potential V 103  of a signal input to the first liquid crystal element  103  is almost equal to a potential input from the first wiring  108 , and a potential V 104  of a signal input to the second liquid crystal element  104  is almost equal to a potential input from the second wiring  109 . In addition, a potential V 105  of the first electrode of the third liquid crystal element  105  has a value which is divided by voltage of the first capacitor  106  and voltage of the second capacitor  107 . Here, a capacitance value of the first capacitor  106  is denoted by C 106  and a capacitance value of the second capacitor  107  is denoted by C 107 . Then, V 105 =ΔV×C 107 /(C 106 +C 107 )+V 103  is satisfied, where ΔV=V 104 −V 103  and no initial charge is accumulated in each capacitor. Here, when the values of C 106  and C 107  are the same, V 105  is half the sum of V 103  and V 104 . Here, when a potential of the common electrode is 0, voltage applied to the first liquid crystal element is represented by V 103 , voltage applied to the second liquid crystal element is represented by V 104 , and voltage applied to the third liquid crystal element is represented by V 105 =(V 103 +V 104 )/2. When a potential of the signal input from the first wiring  108  and a potential of the signal input from the second wiring  109  are varied, voltage which is applied is varied between the liquid crystal elements can be varied, so that the liquid crystal elements can be aligned differently. Therefore, it is preferable that the potential of the signal input from the first wiring  108  and the potential of the signal input from the second wiring  109  be different from each other. 
     When two signals having different potentials are supplied and capacitors are used in this manner, voltage is divided in a pixel, so that intermediate voltage (third voltage) of the two signals can be produced. Then, when the third voltage is applied to the third liquid crystal element  105 , liquid crystals can be easily controlled. Further, the third voltage is voltage between voltage applied to the first liquid crystal element  103  and voltage applied to the second liquid crystal element  104 . Therefore, even when any gray scale is to be displayed, an adequate gray scale can be displayed. In addition, even when polarity of the image signal is positive (i.e., the image signal is higher than that of the common electrode) or polarity of the image signal is negative (i.e., the image signal is lower than that of the common electrode), an adequate gray scale can be displayed. 
     In addition, increase in number of scan lines, signal lines, transistors, and the like is suppressed and the third voltage is produced, so that the third liquid crystal element  105  can be controlled. Thus, the aperture ratio can be improved and power consumption can be reduced. In addition, since pixels can be arranged having a margin of layout, a defect such as short circuit which would occur due to dust or the like generated in manufacturing steps can be reduced, so that yield can be improved. Accordingly, manufacturing cost can be reduced. Further, since the third liquid crystal element  105  can be controlled without additionally providing a wiring functioning as a signal line for controlling the third liquid crystal element  105 , the number of connections between a glass substrate and an external driver circuit is not increased. Accordingly, high reliability can be maintained. 
     Note that it is preferable that the capacitance value of the first capacitor  106  and the capacitance value of the second capacitor  107  be almost equal. When the capacitance values of the two capacitors are almost equal, the divided potential has an intermediate value of a potential supplied to the two capacitors. If there is difference in the capacitance values, the potential is biased on one of potentials, so that the liquid crystal elements cannot be controlled uniformly. Therefore, it is preferable that the capacitance value of the first capacitor  106  and the capacitance value of the second capacitor  107  be almost equal. Note that the present invention is not limited to this. 
     In the case where the first switch  101  is off, the first wiring  108  is electrically disconnected to the first electrode (the pixel electrode) of the first liquid crystal element  103  and the first electrode of the first capacitor  106 . In the case where the second switch  102  is off, the second wiring  109  is electrically disconnected to the first electrode (the pixel electrode) of the second liquid crystal element  104  and the first electrode of the second capacitor  107 . Therefore, each of the first electrode of the first liquid crystal element  103 , the first electrode of the first capacitor  106 , the first electrode of the second liquid crystal element  104 , and the first electrode of the second capacitor  107  is set in a floating state. In addition, the third liquid crystal element  105  is connected to the first liquid crystal element  103  through the first capacitor  106 . However, because of principle of conservation of charge, electric charge conserved in the third liquid crystal element  105  does not leak toward the first liquid crystal element  103 . Similarly, the third liquid crystal element  105  is connected to the second liquid crystal element  104  through the second capacitor  107 . However, because of principle of conservation of charge, the electric charge conserved in the third liquid crystal element  105  does not leak toward the second liquid crystal element  104 . Therefore, a potential of a signal which is input just before is held in each of the first to third liquid crystal elements. 
     Note that each of the first liquid crystal element  103 , the second liquid crystal element  104 , and the third liquid crystal element  105  has transmittivity in accordance with a video signal. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that each of the liquid crystal elements may be divided into a plurality of elements. For example,  FIGS. 11A and 11B  each show the case where the third liquid crystal element  105  is divided into two elements of a third liquid crystal element  105   a  and a fourth liquid crystal element  105   b . Similarly, each of the first liquid crystal element  103  and the second liquid crystal element  104  may be divided into a plurality of elements. Note that the same can be said for drawings other than  FIGS. 1A to 1C . 
     Note that in  FIGS. 1A to 1C  and  FIGS. 11A and 11B , when the first switch  101  and the second switch  102  are transistors, gates of the switches are connected to the third wiring  110 . However, the present invention is not limited to this. The gate of the first switch  101  and the gate of the second switch  102  may be connected to different wirings (see  FIG. 49 ). The same can be said for drawings other than  FIGS. 1A to 1C  and  FIGS. 11A and 11B . 
     Note that although the first switch  101  and the second switch  102  are connected to different signal lines in  FIGS. 1A to 1C  and  FIGS. 11A and 11B , the present invention is not limited to this. As shown in  FIGS. 8A and 8B  and  FIGS. 17A and 17B , the first switch  101  and the second switch  102  may be connected to the same wiring. The same can be said for drawings other than  FIGS. 1A to 1C  and  FIGS. 11A and 11B . 
     Note that although a liquid crystal element exhibits voltage holding properties, the retention rate thereof is not 100%. Therefore, in  FIGS. 1A to 1C  and  FIGS. 11A and 11B , voltage may be held by providing a capacitor serving as a storage capacitor (hereinafter simply referred to as a storage capacitor) for each of the liquid crystal elements. Storage capacitors may be provided for all the liquid crystal elements, or may be provided for only part of the liquid crystal elements. Storage capacitors are provided between the respective pixel electrodes and a capacitor line connected to the respective pixel electrodes. The storage capacitors may be connected to different capacitor lines, or may be connected to the same capacitor line. Alternatively, part of the storage capacitors may be connected to the same capacitor line and other storage capacitors may be connected to different storage capacitor lines. In addition, a capacitor line may be shared with another pixel. For example, a capacitor line can be shared with a pixel in the previous row or a pixel in the next row. When a capacitor line is shared between different pixels, the number of wirings can be reduced and the aperture ratio can be improved. Alternatively, a capacitor line may be shared with a scan line. When a capacitor line is shared with a scan line, the number of wirings can be reduced and the aperture ratio can be improved. When a capacitor line is shared with a scan line, a scan line of the pixel in the adjacent row (the pixel in the previous row) is preferably used. This is because selection of signals has been already finished in an (i−1)th row (the previous row) when the pixel in an i-th row is selected. Note that in the case where liquid crystals are IPS mode, an FFS mode, or the like, the common electrode is provided over a substrate over which a transistor is formed. Therefore, a capacitor line is shared with the common electrode. When a capacitor line is shared with the common electrode, the number of wirings can be reduced and the aperture ratio can be improved. Note that the storage capacitor may be divided into a plurality of elements, in a similar manner that in the liquid crystal elements in  FIGS. 11A and 11B . The same can be said for drawings other than  FIGS. 1A to 1C  and  FIGS. 11A and 11B . 
     Next, a display device including the pixel  100  in  FIGS. 1A to 1C  is described with reference to  FIG. 31 . 
     The display device includes a signal line driver circuit  1911 , a scan line driver circuit  1912 , and a pixel portion  1913 . The pixel portion  1913  includes first wirings S 1 _ 1  to S m _ 1  and second wirings S 1 _ 2  to S m _ 2  which extend from the signal line driver circuit  1911  in a column direction; third wirings G 1  to G n  which extend from the scan line driver circuit  1912  in a row direction; and pixels  1914  which are arranged in matrix. The first and second wirings function as signal lines. The third wirings function as scan lines. In addition, each of the pixels  1914  is connected to a first wiring S j _ 1  (any one of the signal lines S 1 _ 1  to S m _ 1 ), a second wiring S j _ 2  (any one of the signal lines S 1 _ 2  to S m _ 2 ), and a third wiring G i  (any one of the scan lines G 1  to G n ). 
     Note that the first wiring Sj_ 1 , the second wiring Sj_ 2 , and the third scan line Gi correspond to the first wiring  108 , the second wiring  109 , the third wiring  110  in  FIGS. 1A to 1C , respectively. 
     When a row of pixels to be operated is selected by a signal output from the scan line driver circuit  1912 , pixels in the same row are selected at the same time. A video signal output from the signal line driver circuit  1911  is written to the pixels in the selected row. At this time, a potential in accordance with luminance data of each pixel is supplied to the first wirings S 1 _ 1  to S m _ 1  and second wirings S 1 _ 2  to S m _ 2 . 
     For example, when a data writing period in the i-th row is finished, writing of a signal to pixels in an (i+1)th row is performed. Then, a pixel which finishes the data writing period in the i-th row has transmittivity in accordance with the signal. 
     Note that a plurality of signal line driver circuits  1911  or a plurality of scan line driver circuits  1912  may be provided. For example, the first wiring S j _ 1  (any one of the signal lines S 1 _ 1  to S m _ 1 ) may be driven by a first signal line driver circuit and the second wiring S j _ 2  (any one of the signal lines S 1 _ 2  to S m _ 2 ) may be driven by a second signal line driver circuit. In that case, the first signal line driver circuit and the second signal line driver circuit may be provided above and below the pixel portion  1913 . For example, the first signal line driver circuit may be provided on one side over a main surface of a substrate, the second signal line driver circuit may be provided on an opposite side, and the pixel portion  1913  may be provided in a region sandwitched by the two signal line driver circuits. 
     Note that in order to suppress display unevenness such as deterioration in a liquid crystal material and flickers, inversion driving is preferably used in which driving is performed with polarity of voltage which is applied to a pixel electrode inverted every certain period with respect to a potential (a common potential) of a common electrode in liquid crystal capacitance. In this specification, when a potential of a pixel electrode is higher than a potential of a common electrode, description that “positive voltage is applied to liquid crystal capacitance” is used, and when the potential of the common electrode is higher than the potential of the pixel electrode, negative voltage is applied to the liquid crystal capacitance. In addition, an image signal which is input from a signal line when the positive voltage is applied to the liquid crystal capacitance is referred to as a positive signal, and an image signal which is input from the signal line when the negative voltage is applied to the liquid crystal capacitance is referred to as a negative signal. Note that examples of inversion driving are frame inversion driving, source line inversion driving, gate line inversion driving, dot inversion driving, and the like. 
     Frame inversion driving is a driving method in which polarity of voltage which is input to liquid crystal capacitance is inverted every one frame period. Note that one frame period corresponds to a period for displaying an image for one screen. Although one frame period is not particularly limited to a certain period, it is at least preferable that one frame period be 1/60 second or less so that a person viewing an image does not perceive flickers. 
     Source line inversion driving is a driving method in which polarity of voltage which is applied to liquid crystal capacitance in pixels connected to the same signal line is inverted with respect to polarity of voltage which is applied to liquid crystal capacitance in pixels connected to an adjacent signal line, and further frame inversion is performed on each pixel. On the other hand, gate line inversion driving is a driving method in which polarity of voltage which is applied to liquid crystal capacitance in pixels connected to the same wiring functioning as a scan line is inverted with respect to polarity of voltage which is applied to liquid crystal capacitance in pixels connected to an adjacent scan line, and further frame inversion is performed on each pixel. 
     Dot inversion driving is a driving method in which polarity of voltage which is applied to liquid crystal capacitance between adjacent pixels is inverted, and source line inversion driving and gate line inversion driving are combined. 
     In the case where the above-described frame inversion driving, source line inversion driving, gate line inversion driving, dot inversion driving, or the like is employed, the width of a potential which is necessary for an image signal written to a signal line is twice as wide as the width of a potential in the case of not performing inversion driving. Therefore, in order to solve this problem, in the case of frame inversion driving or gate line inversion driving, common inversion driving in which a potential of a counter electrode is inverted is also employed in some cases. 
     Common inversion driving is a driving method in which a potential of a common electrode is changed in synchronization with inversion of polarity of voltage which is applied to liquid crystal capacitance. When common inversion driving is performed, the width of a potential which is necessary for an image signal written to a signal line can be decreased. 
     Further, one pixel may include a plurality of above-described pixel structures. For example, one pixel may include a plurality of subpixels and gray scales of one pixel may be displayed by using the plurality of subpixels. A signal line connected to different subpixels may be shared between the subpixels. Note that when different potentials are supplied to capacitor lines connected to the subpixels, different voltage can also be applied to liquid crystal capacitance in the subpixels. When difference in alignment of liquid crystals in the respective subpixels is utilized in this manner, the viewing angle can be further improved. 
     Note that although storage capacitors are not shown in  FIGS. 1A to 1C , it is preferable to provide storage capacitors as described above. When storage capacitors are provided, adverse effects of leakage current of the liquid crystal elements can be reduced and potentials can be easily held. In addition, adverse effects of switching noise such as feed through can be reduced. Then,  FIGS. 16A and 16B  show the case where storage capacitors are provided for the circuits in  FIGS. 1A and 1B  as an example of the case of illustrating storage capacitors. 
     In  FIG. 16A , a pixel  400  includes a first switch  401 , a second switch  402 , a first liquid crystal element  403 , a second liquid crystal element  404 , a third liquid crystal element  405 , a first capacitor  406 , a second capacitor  407 , a third capacitor  408 , a fourth capacitor  409 , and a fifth capacitor  417 . 
     A first wiring  410  is connected to a first electrode of the first liquid crystal element  403 , a first electrode of the first capacitor  406 , and a first electrode of the second capacitor  407  through the first switch  401 . A second wiring  411  is connected to a first electrode of the second liquid crystal element  404 , a first electrode of the third capacitor  408 , and a first electrode of the fourth capacitor  409  through the second switch  402 . Second electrodes of the first capacitor  406  and the third capacitor  408  are connected to a first electrode of the third liquid crystal element  405  and a first electrode of the fifth capacitor  417 . A second electrode of the second capacitor  407  is connected to a fourth wiring  413 . A second electrode of the fourth capacitor  409  is connected to a fifth wiring  414 . A second electrode of the fifth capacitor  417  is connected to a sixth wiring  415 . 
     Second electrodes of the first liquid crystal element  403 , the second liquid crystal element  404 , and the third liquid crystal element  405  are connected to a common electrode  416 . 
     Each of the first wiring  410  and the second wiring  411  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  410  and the second wiring  411 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. Each of the fourth wiring  413 , the fifth wiring  414 , and the sixth wiring  415  functions as a capacitor line. 
     Each of the first switch  401  and the second switch  402  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. The case where a transistor is used as each of the first switch  401  and the second switch  402  is described below. In the case of using a transistor, the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 16B  shows the case where an N-channel transistor is used as a switch. In  FIG. 16B , gates of a first switch  401 N and a second switch  402 N are connected to the third wiring  412 . The third wiring  412  functions as a scan line. 
     Note that although storage capacitors may be provided for all the liquid crystal elements as shown in  FIGS. 16A and 16B , the present invention is not limited to this. For example, as shown in  FIGS. 7A and 7B , storage capacitors may be provided for only part of the liquid crystal elements. Note that the storage capacitors may be connected to different capacitor lines, or may be connected to the same capacitor line. Alternatively, part of the storage capacitors may be connected to the same capacitor line and other storage capacitors may be connected to different storage capacitor lines. In addition, a capacitor line may be shared with another pixel. For example, a capacitor line can be shared with a pixel in the previous row or a pixel in the next row. When a capacitor line is shared between different pixels, the number of wirings can be reduced and the aperture ratio can be improved. Alternatively, a capacitor line may be shared with a scan line. When a capacitor line is shared with a scan line, the number of wirings can be reduced and the aperture ratio can be improved. When a capacitor line is shared with a scan line, a scan line of the adjacent pixel (the pixel in the previous row) is preferably used. This is because selection of signals has been already finished in an (i−1)th row (the previous row) when a pixel in an i-th row is selected. Note that in the case where liquid crystals are IPS mode, an FFS mode, or the like, the common electrode is provided over a substrate over which a transistor is formed. Therefore, a capacitor line is shared with the common electrode. When a capacitor line is shared with the common electrode, the number of wirings can be reduced and the aperture ratio can be improved. 
     Note that constant potential is preferably supplied to the capacitor lines. Note that the present invention is not limited to this. For example, in  FIGS. 7A and 7B , a signal which periodically varies a plurality of times may be supplied to each of the capacitor lines, i.e., the fourth wiring  413  and the fifth wiring  414  in one frame period. Further, signals which are inverted with respect to each other may be supplied to the capacitor lines, i.e., the fourth wiring  413  and the fifth wiring  414 . Accordingly, effective voltage applied to the first liquid crystal element  404 , the second liquid crystal element  403 , and the like can be made different. 
     Note that although three wirings functioning as capacitor lines are included in  FIGS. 16A and 16B , the present invention is not limited to this. The capacitor lines can be put into one capacitor line. Further, the common electrode and the capacitor line can be shared. This is because the common electrode and the capacitor line are not particularly limited to certain types except that potentials of the common electrode and the capacitor line need to be held constant.  FIGS. 50A and 50B  show the case where capacitor lines is put into one capacitor line and a common electrode and the capacitor line are shared.  FIGS. 50A and 50B  have similar advantages to  FIGS. 16A and 16B . 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that although the transistors which are used as the first switch or the second switch in drawings other than  FIGS. 1A to 1C  and the like used for the above description are connected to different signal lines, the present invention is not limited to this. These switches may be connected to the same signal line. For example,  FIG. 8B  shows an example of the case where the number of signal lines is one unlike the case where the number of signal lines is two in  FIGS. 1A to 1C  and a plurality of scan lines are provided. In addition,  FIG. 17B  shows the case where the scan lines in  FIG. 8B  is put into one wiring. 
     Note that in  FIGS. 8A and 8B and 17A and 17B , storage capacitors can be provided for different liquid crystal elements, as shown in  FIGS. 7A and 7B  and  FIGS. 16A and 16B . Then, for example,  FIGS. 18A and 18B  and  FIGS. 19A and 19B  each show an example where storage capacitors are provided for the first and second liquid crystal elements, in a similar manner that in  FIGS. 7A and 7B . 
     Therefore, the contents described in  FIGS. 1A to 1C  and  FIGS. 7A and 7B  can also be applied to  FIGS. 8A and 8B ,  FIGS. 16A and 16B ,  FIGS. 17A and 17B , and  FIGS. 18A and 18B . 
     In  FIG. 8A , a pixel  450  includes a first switch  451 , a second switch  452 , a first liquid crystal element  453 , a second liquid crystal element  454 , a third liquid crystal element  455 , a first capacitor  456 , and a second capacitor  407 . 
     A first wiring  458  is connected to a first electrode of the first liquid crystal element  453  and a first electrode of the first capacitor  456  through the first switch  451 . Further, the first wiring  458  is connected to a first electrode of the second liquid crystal element  454  and a first electrode of the second capacitor  457  through the second switch  452 . Second electrodes of the first capacitor  456  and the second capacitor  457  are connected to a first electrode of the third liquid crystal element  455 . 
     Note that a transistor can be used as a switch. A gate of a first switch  451 N is connected to a second wiring  459 . A gate of a second switch  452 N is connected to a third wiring  460 . 
     Second electrodes of the first liquid crystal element  453 , the second liquid crystal element  454 , and the third liquid crystal element  455  are connected to a common electrode  461 . 
     The first wiring  458  functions as a signal line. Therefore, an image signal is usually supplied to the first wiring  458 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. Each of the second wiring  459  and the third wiring  460  functions as a scan line. 
     Operations in  FIGS. 8A and 8B  and  FIGS. 18A and 18B  are described. First, an active signal is supplied to the third wiring  460 , so that the second switch  452  or the second switch  452 N is turned on. Here, an active signal corresponds to a signal which can turn on the second switch  452  or the second switch  452 N. When the second switch  452  or the second switch  452 N is turned on, a video signal is supplied from the first wiring  458  to the first electrode (a pixel electrode) of the second liquid crystal element  454  and the first electrode of the second capacitor  457 . 
     Next, the second switch  452  or the second switch  452 N is turned off and an active signal is supplied to the second wiring  459 , so that the first switch  451  or the first switch  451 N is turned on. Here, an active signal corresponds to a signal which can turn on the first switch  451  or the first switch  451 N. Then, a video signal is supplied from the first wiring  458  to the first electrode (a pixel electrode) of the first liquid crystal element  453  and the first electrode of the first capacitor  456 . The video signal supplied at this time preferably has a potential which is different from the potential when the second switch  452  or the second switch  452 N is turned on. Since the potentials are different, different voltage can be applied to the liquid crystal elements. Therefore, the viewing angle can be improved. 
     Note that when the second switch  452  or the second switch  452 N is on, the third liquid crystal element  455  is capacitively coupled to the pixel electrode of the first liquid crystal element  453  through the first capacitor  456 . Therefore, a potential of a pixel electrode of the third liquid crystal element  455  is changed in accordance with the voltage applied from the first wiring  458  when the second switch  452  or the second switch  452 N is on. 
     Similarly, when the first switch  451  or the first switch  451 N is on, the second liquid crystal element  454  is capacitively coupled to the pixel electrode of the first liquid crystal element  456  through the first capacitor  456  and the second capacitor  457 . Therefore, a potential of the pixel electrode of the second liquid crystal element  454  is changed in accordance with the voltage applied from the first wiring  458  when the first switch  451  or the first switch  451 N is on. 
     Next, the first switch  451  or the first switch  451 N is turned off, so that the potential of each of the liquid crystal elements is held. With such operations, the voltage which is applied can be varied between the liquid crystal elements. Accordingly, the viewing angle can be widened. Note that the driving method is not limited to this. Driving can be performed by using a variety of timing for turning on/off each transistor, potentials of a signal line, and the like. 
     Note that in  FIGS. 18A and 18B , a constant potential is preferably supplied to each of the capacitor lines. Note that the present invention is not limited to this. For example, a signal which periodically varies a plurality of times may be supplied to the capacitor lines, i.e., the first wiring and the second wiring in one frame period. Further, signals which are inverted with respect to each other may be supplied to the capacitor lines, i.e., the first wiring and the second wiring. Accordingly, effective voltage applied to the first liquid crystal element  453 , the second liquid crystal element  454 , and the like can be made different. With such operations, the potentials of the liquid crystal elements can be varied. Accordingly, the viewing angle can be widened. 
     Next, operations in  FIGS. 17A and 19A  are described. 
     An active signal is supplied to the second wiring  459 , so that the first switch  451  and the second switch  452  are turned on. Then, a video signal is supplied from the first wiring  458  to the first electrode (the pixel electrode) of the first liquid crystal element  453 , the first electrode of the first capacitor  456 , the first electrode (the pixel electrode) of the second liquid crystal element  454 , and the first electrode of the second capacitor  457 . 
     At this time, when transistors are used as the first switch  451  and the second switch  452 , on resistance is generated. On resistance of the first switch  451  is preferably higher than on resistance of the second switch  452 . High on resistance of a transistor corresponds to a small ratio of the channel width W to the channel length L (W/L). When the on resistance of the transistor is increased in this manner, the potential of the pixel electrode of each of the liquid crystal elements is determined by balance of leakage current or the like of each capacitor, each storage capacitor, or the like. Then, different voltage can be applied to the liquid crystal elements, so that the viewing angle can be improved. Note that the present invention is not limited to this, and the on resistance of the first switch  451  and the on resistance of the second switch  452  can be almost equal. 
     Next, the first switch  451  and the second switch  452  are turned off, so that the potential of each of the liquid crystal elements is held. 
     With such operations, the voltage which is applied can be varied between the liquid crystal elements. Accordingly, the viewing angle can be widened. Note that the driving method is not limited to this. Driving can be performed by using a variety of timing for turning on/off each transistor, potentials of a signal line, and the like. 
     Note that in  FIGS. 19A and 19B , a constant potential is preferably supplied to the capacitor lines. Note that the present invention is not limited to this. For example, a signal which periodically varies a plurality of times may be supplied to the capacitor lines, i.e., the first wiring  463  and the second wiring  465  in one frame period. Alternatively, signals which are inverted with respect to each other may be supplied to the capacitor lines, i.e., the first wiring  463  and the second wiring  465 . Accordingly, effective voltage applied to the first liquid crystal element  453 , the second liquid crystal element  454 , and the like can be made different. With such operations, the voltage which is applied can be varied between the liquid crystal elements. Accordingly, the viewing angle can be widened. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
       FIG. 2A  shows an example of the structure of a pixel circuit included in a liquid crystal display device of the present invention, which is different from that of  FIG. 1A . A pixel  150  includes a first switch  151 , a second switch  152 , a first liquid crystal element  153 , a second liquid crystal element  154 , a third liquid crystal element  155 , a first capacitor  156 , a second capacitor  157 , and a third capacitor  161 . 
     A first wiring  158  is connected to a first electrode of the first liquid crystal element  153  and a first electrode of the first capacitor  156  through the first switch  151 . A second wiring  159  is connected to a first electrode of the second liquid crystal element  154  and a first electrode of the second capacitor  157  through the second switch  152 . A second electrode of the first capacitor  156  is connected to a second electrode of the second capacitor  157  and a first electrode of the third capacitor  161 . A second electrode of the third capacitor  161  is connected to a first electrode of the third liquid crystal element  155 . 
     Second electrodes of the first liquid crystal element  153 , the second liquid crystal element  154 , and the third liquid crystal element  155  are connected to a common electrode. 
     Each of the first wiring  158  and the second wiring  159  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  158  and the second wiring  159 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  160  functions as a scan line. 
     Each of the first switch  151  and the second switch  152  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. The case where a transistor is used as each of the first switch  151  and the second switch  152  is described below. In the case of using a transistor, the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 2B  shows the case where an N-channel transistor is used as a switch. In  FIG. 2B , gates of a first switch  151 N and a second switch  152 N are connected to the third wiring  160 . The third wiring  160  functions as a scan line. 
     Note that in  FIGS. 2A and 2B , the number of scan lines may be two in a similar manner that in  FIGS. 1A to 1B , as shown in  FIG. 49 . 
     Note that a P-channel transistor can be used as a switch. 
     A video signal is input to the first wiring  158  and the second wiring  159 . A scan signal is input to the third wiring  160 . The scan signal is an H-level or L-level digital voltage signal. In the case where each of the first switch  151  and the second switch  152  is an N-channel transistor, an H level of the scan signal is a potential which can turn on the first switch  151  and the second switch  152 , and an L level of the scan signal is a potential which can turn off the first switch  151  and the second switch  152 . Alternatively, in the case where each of the first switch  151  and the second switch  152  is a P-channel transistor, an H level of the scan signal is a potential which can turn off the first switch  151  and the second switch  152 , and an L level of the scan signal is a potential which can turn on the first switch  151  and the second switch  152 . Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current. In addition, current of the video signal may be either analog or digital. A potential of the video signal is lower than the H level of the scan signal and higher than the L level of the scan signal. 
     Operations of the pixel  150  in  FIG. 2A  are described by dividing the whole operations into the case where the first switch  151  and the second switch  152  are on and the case where the first switch  151  and the second switch  152  are off. 
     In the case where the first switch  151  is on, the first wiring  158  is electrically connected to the first electrode (a pixel electrode) of the first liquid crystal element  153  and the first electrode of the first capacitor  156 . In the case where the second switch  152  is on, the second wiring  159  is electrically connected to the first electrode (a pixel electrode) of the second liquid crystal element  154  and the first electrode of the second capacitor  157 . Therefore, a video signal is input from the first wiring  158  to the first electrode (the pixel electrode) of the first liquid crystal element  153  and the first electrode of the first capacitor  156 , and a video signal is input from the second wiring  159  to the first electrode (the pixel electrode) of the second liquid crystal element  154  and the first electrode of the second capacitor  157 . Therefore, a potential V 153  of a signal input to the first liquid crystal element  153  is almost equal to a potential input from the first wiring  158 , and a potential V 154  of a signal input from the second liquid crystal element  154  is almost equal to a potential input to the second wiring  159 . In addition, a potential V 161  of the first electrode of the third liquid crystal element  161  is almost similar to the potential V 105  of the first electrode of the third liquid crystal element  105  in  FIGS. 1A to 1C , and when the values of C 156  and C 157  are the same, V 161  is almost half the sum of V 153  and V 154 . Note that a potential of a first electrode of third liquid crystal element  155  is denoted by V 155 . Here, when a potential of the common electrode is 0, voltage applied to the third liquid crystal element  155  is denoted by V 155 . The voltage V 155  has a value which is divided by voltage of the third capacitor  161  and voltage of the third liquid crystal element  155 . When the capacitors are used in this manner, different voltage can be further applied to the liquid crystal elements. The voltage which is applied can be varied between the liquid crystal elements in this manner, so that the liquid crystal elements can be aligned differently. 
     When two signals having different potentials are supplied and capacitors are used in this manner, voltage is divided in a pixel, so that third voltage can be produced. Then, when the third voltage is applied to the third liquid crystal element  155 , liquid crystals can be easily controlled. Further, the third voltage is voltage between voltage applied to the first liquid crystal element  153  and voltage applied to the second liquid crystal element  154 . Therefore, even when any gray scale is to be displayed, an adequate gray scale can be displayed. In addition, even when polarity of the image signal is positive (i.e., the image signal is higher than that of the common electrode) or polarity of the image signal is negative (i.e., the image signal is lower than that of the common electrode), an adequate gray scale can be displayed. 
     In addition, increase in number of scan lines, signal lines, transistors, and the like is suppressed and the third voltage is produced, so that the third liquid crystal element  155  can be controlled. Thus, the aperture ratio can be improved and power consumption can be reduced. In addition, since pixels can be arranged having a margin of layout, a defect such as short circuit due to dust or the like generated in manufacturing steps can be reduced, so that yield can be improved. Accordingly, manufacturing cost can be reduced. Further, since the third liquid crystal element  155  can be controlled without additionally providing a signal line, the number of connections between a glass substrate and an external driver circuit is not increased. Accordingly, high reliability can be maintained. 
     In the case where the first switch  151  is off, the first wiring  158  is electrically disconnected to the first electrode (the pixel electrode) of the first liquid crystal element  153  and the first electrode of the first capacitor  156 . In the case where the second switch  152  is off, the second wiring  159  is electrically disconnected to the first electrode (the pixel electrode) of the second liquid crystal element  154  and the first electrode of the second capacitor  157 . Therefore, each of the first electrode of the first liquid crystal element  153 , the first electrode of the first capacitor  156 , the first electrode of the second liquid crystal element  154 , and the first electrode of the second capacitor  157  is set in a floating state. In addition, the third liquid crystal element  155  is connected to the first liquid crystal element  153  through the first capacitor  156  and the third capacitor  161 . However, because of principle of conservation of charge, electric charge conserved in the third liquid crystal element  155  does not leak toward the first liquid crystal element  153 . The third liquid crystal element  155  is connected to the first liquid crystal element  153  through the second capacitor  157 . However, because of principle of conservation of charge, the electric charge conserved in the third liquid crystal element  155  does not leak toward the second liquid crystal element  154 . Therefore, a potential of a signal which is input just before is held in each of the first to third liquid crystal elements. 
     Note that each of the first liquid crystal element  153 , the second liquid crystal element  154 , and the third liquid crystal element  155  has transmittivity in accordance with a video signal. 
     That is, when  FIGS. 2A and 2B  are compared to  FIGS. 1A to 1B ,  FIGS. 2A and 2B  correspond to the case where the third liquid crystal element  105  in  FIGS. 1A to 1C  is replaced with the third capacitor  161  and the third liquid crystal element  155  in  FIGS. 2A and 2B  which are connected in series. Therefore, the contents described in  FIGS. 1A to 1C  can also be applied to  FIGS. 2A and 2B . For example, as shown in  FIGS. 15A and 15B , the third capacitor  161  and the third liquid crystal element  155  which are connected in series may be divided into a plurality of elements. Alternatively, as shown in  FIGS. 12A and 12B , the capacitor may be eliminated and only the liquid crystal element may be divided into a plurality of elements. 
     Note that although the third liquid crystal element  105  in  FIGS. 1A to 1C  is replaced with the third capacitor  161  and the third liquid crystal element  155  which are connected in series in  FIGS. 2A and 2B , the present invention is not limited to this. Another liquid crystal element may be replaced with a capacitor and a liquid crystal element which are connected in series. For example,  FIGS. 13A and 13B  show the case where the first liquid crystal element  153  is replaced with a capacitor and a liquid crystal element which are connected in series. In this case, in a similar manner that in  FIGS. 12A and 12B , the liquid crystal element may be divided into a plurality of elements as shown in  FIGS. 14A and 14B . 
     Since  FIGS. 2A and 2B  show the case where the third liquid crystal element  105  in  FIGS. 1A to 1C  is replaced with the third capacitor  161  and the third liquid crystal element  155  in  FIGS. 2A and 2B  which are connected in series, transformation which is similar to transformation in  FIGS. 1A to 1C  can be performed. That is, a storage capacitor may be added to part of the liquid crystal elements as shown in FIGS.  7 A and  7 B, or storage capacitors may be added to all the liquid crystal elements as shown in  FIGS. 16A and 16B . In addition, the number of scan lines may be two and the signal lines may be put into one signal line, as shown in  FIGS. 8A and 8B  or  FIGS. 18A and 18B . Alternatively, the scan lines and the signal lines may be put into one scan line and one signal line, as shown in  FIGS. 17A and 17B  and  FIGS. 19A and 19B . 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
       FIG. 3A  shows an example of the structure of a pixel circuit included in a liquid crystal display device of the present invention, which is different from other examples. A pixel  200  includes a first switch  201 , a second switch  202 , a transistor  203 , a first liquid crystal element  204 , a second liquid crystal element  205 , a third liquid crystal element  206 , a first capacitor  207 , and a second capacitor  208 . 
     A first wiring  209  is connected to a first electrode of the first liquid crystal element  204  and a first electrode of the first capacitor  207  through the first switch  201 . A second wiring  210  is connected to a first electrode of the second liquid crystal element  205  and a first electrode of the second capacitor  208  through the second switch  202 . Further, the second wiring  210  is connected to a first electrode of the third liquid crystal element  206  through the transistor  203 . Gates of the first switch  201 , the second switch  202 , and the transistor  203  are connected to a third wiring  211 . A second electrode of the first capacitor  207  is connected to a second electrode of the second capacitor  208  and the first electrode of the third liquid crystal element  206 . 
     Note that the transistor  203  is operated as a switch having higher on resistance than on resistance of the first switch  201  and the second switch  202 . That is, the transistor  203  can be handled in a similar manner that in a switch to which a resistor is connected in series. However, the present invention is not limited to this. The on resistance of the transistor  203  may be lower than the on resistance of the first switch  201  and the on resistance of the second switch  202 . 
     Note that although the transistor  203  is an N-channel transistor in  FIGS. 3A and 3B , the present invention is not limited to this. That is, the transistor  203  may be a P-channel transistor. 
     Second electrodes of the first liquid crystal element  204 , the second liquid crystal element  205 , and the third liquid crystal element  206  are connected to a common electrode. 
     Each of the first wiring  209  and the second wiring  210  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  209  and the second wiring  210 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  211  functions as a scan line. 
     Each of the first switch  201  and the second switch  202  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. The case where a transistor is used as each of the first switch  201  and the second switch  202  is described below. In the case of using a transistor, the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 3B  shows the case where an N-channel transistor is used as a switch. In  FIG. 3B , gates of a first switch  201 N and a second switch  202 N are connected to a third wiring  211 A. The third wiring  211 A functions as a scan line. 
     Note that in  FIGS. 3A and 3B , the number of scan lines may be two in a similar manner that in  FIGS. 1A to 1C , as shown in  FIG. 49 . 
     Note that a P-channel transistor can be used as a switch. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     A video signal is input to the first wiring  209  and the second wiring  210 . A scan signal is input to the third wiring  211 . The scan signal is an H-level or L-level digital voltage signal. In the case where each of the first and second switches and the transistor  203  is an N-channel transistor, an H level of the scan signal is a potential which can turn on the first and second switches and the transistor  203  and an L level of the scan signal is a potential which can turn off the first and second switches and the transistor  203 . Alternatively, in the case where each of the first and second switches and the transistor  203  is a P-channel transistor, an H level of the scan signal is a potential which can turn off the first and second switches and the transistor  203 , and an L level of the scan signal is a potential which can turn on the first and second switches and the transistor  203 . Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current. In addition, current of the video signal may be either analog or digital. A potential of the video signal is lower than the H level of the scan signal and higher than the L level of the scan signal. 
     That is, when  FIGS. 3A and 3B  are compared to  FIGS. 1A to 1B , it can be said that  FIGS. 3A and 3B  correspond to the case where the transistor  203  which connects a pixel electrode of the third liquid crystal element  206  and the second wiring  210  are added to  FIGS. 1A to 1C . In the case of  FIGS. 1A to 1C , when some noise or leakage current enters a point where the first capacitor  207  and the second capacitor  208  are connected, electric charge is accumulated therein. Accordingly, there is a possibility that voltage applied to the liquid crystal elements is adversely affected, so that image quality is decreased. However, as shown in  FIGS. 3A and 3B , when the transistor  203  is added, the accumulated electric charge can be extracted. Accordingly, defects in the image quality such as burn-in can be reduced. 
     Note that as described above, the on resistance of the transistor  203  is preferably higher than the on resistance of the first switch  201  and the on resistance of the second switch  202 . High on resistance of a transistor corresponds to a small ratio of the channel width W to the channel length L (W/L). When the on resistance of the transistor is increased in this manner, a potential of a point where the first capacitor  207  and the second capacitor  208  are connected is determined by balance of leakage current or the like of each capacitor, each storage capacitor, or the like. Note that the present invention is not limited to this, and the first to third transistors may be formed with almost the same size and a resistor may be connected to the third transistor  203  in series. 
     Therefore, the contents described in  FIGS. 1A to 1C ,  FIGS. 2A and 2B , and the like can also be applied to  FIGS. 3A and 3B . For example,  FIGS. 4A and 4B  show the case where the contents described in  FIGS. 2A and 2B  are applied to  FIGS. 3A and 3B . 
     Note that although the first switch  201 N (or a first switch  251 N), the second switch  202 N (or a second switch  252 N), and the transistor  203  (or a transistor  253 ) are controlled by the third wiring  211  (or a third wiring  262 ) in  FIGS. 3A and 3B ,  FIGS. 4A and 4B , and the like, the present invention is not limited to this. They may be connected to different wirings and controlled differently. Alternatively, part of them may be connected to another wiring. 
     Note that although the transistor  203  is connected to the second wiring  210  in  FIGS. 3A and 3B , the transistor  203  may be connected to the first wiring  209 . The same can be said for the case where the third transistor  203  is connected to the first wiring  209 . Although the transistor  253  is connected to a second wiring  261  in  FIGS. 4A and 4B  in a similar manner that in  FIGS. 3A and 3B , the transistor  253  may be connected to a first wiring  260 . 
     Alternatively, another wiring for connecting the transistor may be provided.  FIGS. 5A and 5B , each show such a case. In  FIG. 5B , the number of scan lines is two, and a scan line for controlling a first switch  301 N and a second switch  302 N is different from a scan line for controlling a transistor  303 ; however, the present invention is not limited to this. The first switch  301 N, the second switch  302 N, and the transistor  303  may be connected to the same scan line. Therefore, the contents described in drawings other than  FIGS. 1A to 1C  and the like can also be applied to  FIG. 5B . For example,  FIGS. 6A and 6B  show the case where the contents described in  FIG. 5B  are applied to  FIGS. 2A and 2B . 
     Note that although the transistor  303  is preferably turned on when a first switch  301  or a second switch  302  is off in  FIG. 5A , the present invention is not limited to this. The transistor  303  may be turned on when the first switch  301  or the second switch  302  is on or in part of a period (preferably the first half of the period) during which the first switch  301  or the second switch  302  is on. 
     Note that although it is preferable that a potential of a fifth wiring  313  be almost equal to a potential of a common electrode, the present invention is not limited to this. The potential of the fifth wiring  313  can be almost equal to a potential of a first wiring  309  or a second wiring  310 . 
     Note that the fifth wiring  313  can be shared with another wiring. For example, the fifth wiring  313  can be shared with a capacitor line, a scan line, or the like. Note that a wiring with which the fifth wiring  313  is shared may be a wiring in another pixel. Thus, the aperture ratio can be improved. Note that the contents described in drawings other than  FIGS. 1A to 1C  and the like can also be applied to  FIGS. 5A and 5B . That is, at least one transistor may be a P-channel transistor, or liquid crystal elements may be divided into a plurality of elements. 
     Note that a transistor  353  is connected to a third capacitor  359  in  FIGS. 6A and 6B , the present invention is not limited to this. The transistor  353  may be connected between a fifth wiring  364  and a contact point between the third capacitor  359  and a third liquid crystal element  356 . Note that the contents described in drawings other than  FIGS. 1A to 1C  and the like can also be applied to  FIGS. 6A and 6B . 
     Note that each of the first to third liquid crystal elements has transmittivity in accordance with a video signal. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that the case where the number of capacitors connected between the signal lines through the switch is two has been described heretofore, the present invention is not limited to this. Much more capacitors can be provided. When a capacitor is added, voltage applied to the liquid crystal elements can be further varied. In addition, when the voltage is applied to each of the liquid crystal elements, much more liquid crystal elements having different applied voltage can be provided. Accordingly, the viewing angle can be widened. 
     Then,  FIGS. 9A and 9B  show an example of the case where a capacitor and a liquid crystal element are further added to  FIGS. 1A to 1C . In addition,  FIGS. 20A and 20B  show an example of the case where a capacitor and a liquid crystal element are further added to  FIGS. 3A and 3B . Much more liquid crystal elements may be added. Further, similarly, a first liquid crystal  503  may be connected to a third liquid crystal element  505 . Similarly, in the circuits shown in other drawings, a capacitor and a liquid crystal element can be added. Note that the contents described in other drawings can also be applied to  FIGS. 9A and 9B  and  FIGS. 20A and 20B . 
     In  FIG. 9A , a pixel  500  includes a first switch  501 , a second switch  502 , a first liquid crystal element  503 , a second liquid crystal element  504 , a third liquid crystal element  505 , a fourth liquid crystal element  506 , a first capacitor  507 , a second capacitor  508 , a third capacitor  509 , a first wiring  510 , a second wiring  511 , and a third wiring  512 . 
     A first wiring  510  is connected to a first electrode of the first liquid crystal element  503  and a first electrode of the first capacitor  507  through the first switch  501 . A second wiring  511  is connected to a first electrode of the second liquid crystal element  504  and a first electrode of the third capacitor  509  through the second switch  502 . A second electrode of the first capacitor  507  is connected to a first electrode of the second capacitor  508  and a first electrode of the third liquid crystal element  505 . A second electrode of the second capacitor  508  is connected to a second electrode of the third capacitor  509  and a first electrode of the fourth liquid crystal element  506 . 
     Second electrodes of the first liquid crystal element  503 , the second liquid crystal element  504 , the third liquid crystal element  505 , and the fourth liquid crystal element  506  are connected to a common electrode. 
     Each of the first wiring  510  and the second wiring  511  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  510  and the second wiring  511 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  512  functions as a scan line. 
     Each of the first switch  501  and the second switch  502  is not particularly limited to a certain type as long as it functions as a switch. For example, in the case of using a transistor, the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 9B  shows the case where an N-channel transistor is used as a switch. In  FIG. 9B , gates of a first switch  501 N and a second switch  502 N are connected to the third wiring  512 . The third wiring  512  functions as a scan line. 
     Note that in  FIGS. 9A and 9B , the number of scan lines may be two in a similar manner that in  FIGS. 1A to 1C , as shown in  FIG. 49 . 
     Note that a P-channel transistor can be used as a switch. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     Further, the liquid crystal elements may be divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that each of the first liquid crystal element  503 , the second liquid crystal element  504 , the third liquid crystal element  505 , and the fourth liquid crystal element  506  has transmittivity in accordance with a video signal. 
     As described above, the number of liquid crystal elements in each pixel can be four and the number of liquid crystal elements in each pixel can be further increased. When the number of liquid crystal elements in each pixel is increased, liquid crystal elements can be aligned differently, so that a liquid crystal display device having a wider viewing angle can be provided. 
     Note that in  FIGS. 9A and 9B  and  FIGS. 20A and 20B , the case is described in which a liquid crystal element is added by adding a capacitor. Note that the present invention is not limited to this. When the number of transistors, signal lines, and the like is increased, the number of liquid crystal elements provided in one pixel can be increased. Thus, for example,  FIGS. 10A and 10B  show the case where a liquid crystal element is added to the circuits in  FIGS. 1A to 1C  by increasing the number of transistors and signal lines. Note that the present invention is not limited to this structure. Although a signal line is added without adding a scan line in  FIGS. 10A and 10B , a scan line can be added without adding a signal line.  FIGS. 21A and 21B  show the case where a capacitor  566  is added without adding a signal line and is provided between a fourth liquid crystal element  557  and a signal line, so that a potential supplied from the signal line is divided.  FIGS. 22A and 22B  show the case where a capacitor is added without adding a signal line and a capacitor  572  is added between a signal line and a first liquid crystal element  554 , so that a potential supplied from the signal line is divided. With the structures shown in  FIGS. 21A and 21B  and  FIGS. 22A and 22B , different voltage can be applied to four liquid crystal elements without adding a signal line. 
     Note that although the fourth liquid crystal element  557  is connected to a first wiring  560  in  FIGS. 21A and 21B  and  FIGS. 22A and 22B , the fourth liquid crystal element  557  may be connected to the second wiring  561 . 
     Note that in a similar manner that in the case in  FIGS. 1A to 1C , a liquid crystal element may be added to the circuits shown in other drawings. Note that the contents described in other drawings can also be applied to  FIGS. 10A and 10B . That is, P-channel transistors may be used as the transistors, or the liquid crystal element may be divided into a plurality of elements. 
     In  FIG. 10A , a pixel  550  includes a first switch  551 , a second switch  552 , a third switch  553 , a first liquid crystal element  554 , a second liquid crystal element  555 , a third liquid crystal element  556 , a fourth liquid crystal element  557 , a first capacitor  558 , and a second capacitor  559 . 
     The first wiring  560  is connected to a first electrode of the first liquid crystal element  554  and a first electrode of the first capacitor  558  through the first switch  551 . A second wiring  561  is connected to a first electrode of the second liquid crystal element  555  and a first electrode of the second capacitor  559 . A third wiring  562  is connected to a first electrode of the fourth liquid crystal element  557  through the third switch  553 . A second electrode of the first capacitor  558  is connected to one of a second electrode of the second capacitor  559  and a first electrode of the third liquid crystal element  556 . 
       FIG. 10B  shows the case where an N-channel transistor is used as a switch. In  FIG. 10B , gates of a first switch  551 N and a second switch  552 N are connected to a fourth wiring  563 . The fourth wiring  563  functions as a scan line. 
     Note that in  FIGS. 10A and 10B , the number of scan lines may be two in a similar manner that in  FIGS. 1A to 1C , as shown in  FIG. 49 . 
     Note that a P-channel transistor can be used as a switch. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     Further, the liquid crystal element may be divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Second electrodes of the first liquid crystal element  554 , the second liquid crystal element  555 , the third liquid crystal element  556 , and the fourth liquid crystal element  557  are connected to a common electrode. 
     Each of the first wiring  560 , the second wiring  561 , and the third wiring  562  functions as a signal line. Therefore, an image signal is usually supplied to each of first wiring  560 , the second wiring  561 , and the third wiring  562 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The fourth wiring  563  functions as a scan line. 
     Note that a capacitor may be provided between the liquid crystal element and the wiring functioning as a signal line. When a capacitor  566  is provided as shown in  FIGS. 21A and 21B , voltage applied to the liquid crystal elements can be varied. Therefore, the first wiring  560  and the third wiring  562  in  FIGS. 10A and 10B  can be put into one wiring. 
     Note that the position to which a capacitor is added is not limited to the position between the fourth liquid crystal element and the signal line, and as shown in  FIGS. 22A and 22B , a capacitor (e.g., a capacitor  565 ) may be provided between another liquid crystal element and a signal line. In this case, a plurality of signal lines can be put into one wiring. 
     As described above, the number of liquid crystal elements in each pixel can be four and the number of liquid crystal elements in each pixel can be further increased. When the number of liquid crystal elements in each pixel is increased, liquid crystal elements can be aligned differently, so that a liquid crystal display device having a wider viewing angle can be provided. 
       FIG. 32  shows an example of a top view of a pixel of a liquid crystal display device to which the present invention is applied. In addition,  FIG. 33  is a circuit diagram of  FIG. 32 . Note that corresponding portions between  FIGS. 32 and 33  are denoted by the same reference numerals. 
     In a pixel  1000  shown in  FIG. 32 , a first insulating film (not shown) is provided over a first conductive layer (shown by a hatch pattern of a third wiring  1013 ) serving as a scan line and a capacitor line; a semiconductor film is provided over the first insulating film; a second conductive layer (shown by a hatch pattern of a first wiring  1011 ) is provided over the semiconductor film; a second insulating film (not shown) is provided over the second conductive layer; and a third conductive layer (shown by a hatch pattern of a first liquid crystal element  1003 ) is provided over the second insulating film. 
     In  FIG. 33 , the pixel  1000  includes a first transistor  1001 , a second transistor  1002 , a first liquid crystal element  1003 , a second liquid crystal element  1004 , a third liquid crystal element  1005 , a first capacitor  1007 , a second capacitor  1008 , a third capacitor  1009 , a fourth capacitor  1010 , a fifth capacitor  1016 , and a sixth capacitor  1017 . 
     The first wiring  1011  is connected to a first electrode of the fourth liquid crystal element  1006  and first electrodes of the first capacitor  1007  and the second capacitor  1008  through the first transistor  1001 . A second wiring  1012  is connected to a first electrode of the first liquid crystal element  1003  and first electrodes of the fourth capacitor  1010  and the third capacitor  1009  through the second transistor  1002 . A second electrode of the second capacitor  1008  is connected to a second electrode of the third capacitor  1009 , a first electrodes of the fifth capacitor  1016 , a first electrode of the second liquid crystal element  1004 , a first electrodes of the sixth capacitor  1017 , and a first electrode of the third liquid crystal element  1005 . A second electrode of the first capacitor  1007  and a second electrode of the sixth capacitor  1017  are connected to a fifth wiring  1015 . A second electrode of the fifth capacitor  1016  and a second electrode of the fourth capacitor  1010  are connected to a fourth wiring  1014 . 
     Note that  FIG. 33  shows the case where each of the liquid crystal elements in  FIG. 11B  are provided with a storage capacitor. That is,  FIG. 33  shows the case where the contents described in  FIGS. 11B and 16B  are combined. Therefore, structures which are similar to the structures in  FIGS. 1A to 1C  can be applied to  FIG. 33 . In other words, a wiring functioning as a capacitor line may be shared with a common electrode as shown in  FIGS. 50A and 50B , the switches can be replaced with transistors, and either N-channel transistors or P-channel transistors may be used as the transistors. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     Each of the first wiring  1011  and the second wiring  1012  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  1011  and the second wiring  1012 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  1013  functions as a scan line. Each of the fourth wiring  1014  and the fifth wiring  1015  functions as a capacitor line. 
     When a pixel like the pixel shown in the top view in  FIG. 32  is provided, liquid crystal elements can be aligned differently, so that a liquid crystal display device having a wider viewing angle can be provided. 
     Note that although the case in which all the transistors provided in one pixel have the same conductivity type is only described in this embodiment mode, the present invention is not limited to this. That is, the transistors provided in one pixel may have different conductivity types. 
     Further, various types of transistors can be used as the transistor in this embodiment mode, without particularly limiting to a certain type. Therefore, a thin film transistor (TFT) formed by using a crystalline semiconductor film, a thin film transistor formed by using a non-single crystal semiconductor film typified by amorphous silicon or polycrystalline silicon, a transistor formed by using a semiconductor substrate or an SOI substrate, a MOS transistor, a junction transistor, a bipolar transistor, a transistor formed by using a compound semiconductor such as ZnO or a-InGaZnO, a transistor formed by using an organic semiconductor or carbon nanotube, or other transistors can be employed. However, a transistor with smaller off-current is preferably used. Examples of a transistor with smaller off-current are a transistor provided with an LDD region, a transistor with a multi-gate structure, and the like. Alternatively a CMOS switch may be employed by using both N-channel and P-channel transistors. 
     Note that although this embodiment mode is described with reference to various drawings, part of or all the contents described in each drawing can be freely applied to, combined with, or replaced with part of or all the contents described in another drawing. Further, even more structures are possible when each part is combined with another part in the above-described drawings, and the description of this embodiment mode does not impede this. 
     Similarly, part of or all the contents described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with part of or all the contents described in a drawing in another embodiment mode. Further, even more drawings are possible when each part is combined with part of another embodiment mode in the drawings of this embodiment mode, and the description of this embodiment mode does not impede this. 
     Note that this embodiment mode shows an example of an embodied case of part of or all the contents described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, or an example of related part thereof. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 2 
     In Embodiment Mode 1, new voltage is produced by voltage division by using a capacitor and is supplied to a liquid crystal element. Note that an element for producing new voltage is not limited to a capacitor. Various elements such as a divider element, an element which converts current into voltage, a non-linear element, an element having a resistance component, an element having a capacitance component, an inductor, a diode, a transistor, a resistor, and a switch can be used. In addition, when these elements are connected in series or in parallel in combination, a desired circuit can be realized. Such an element is referred to as a divider element. 
       FIGS. 23A and 23B  show the case where the capacitors in  FIGS. 1A to 1C  are generalized as divider elements. Therefore, the contents described in Embodiment Mode 1 can also be applied to  FIGS. 23A and 23B . 
       FIG. 23A  shows an example of a structure of a pixel circuit included in a liquid crystal display device of the present invention. A pixel  600  includes a first switch  601 , a second switch  602 , a first liquid crystal element  603 , a second liquid crystal element  604 , a third liquid crystal element  605 , a first divider element  606 , and a second divider element  607 . 
     A first wiring  608  is connected to a first electrode of the first liquid crystal element  603  and one electrode of the first divider element  606  through the first switch  601 . A second wiring  609  is connected to a first electrode of the second liquid crystal element  604  and one electrode of the second divider element  607  through the second switch  602 . The first divider element  606  and the second divider element  607  are connected in series. A first electrode of the third liquid crystal element  605  is connected between the first divider element  606  and the second divider element  607 . 
     Second electrodes of the first liquid crystal element  603 , the second liquid crystal element  604 , and the third liquid crystal element  605  are connected to a common electrode. 
       FIG. 23B  shows the case where an N-channel transistor is used as a switch. In  FIG. 23B , gates of a first switch  601 N and a second switch  602 N are connected to a third wiring  610 . The third wiring  610  functions as a scan line. 
     Note that in  FIGS. 23A and 23B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     Each of the first wiring  608  and the second wiring  609  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  608  and the second wiring  609 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  610  functions as a scan line. 
     Note that each of the first liquid crystal element  603 , the second liquid crystal element  604 , and the third liquid crystal element  605  has transmittivity in accordance with a video signal. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that as the first divider element  606  and the second divider element  607 , various elements as well as capacitors can be used. For example, any of a divider element, an element which converts current into voltage, a non-linear element, an  30  element having a resistance component, an element having a capacitance component, an inductor, a diode, a transistor, a resistor, and a switch can be used as the divider elements.  FIGS. 30A to 30T  show examples of divider elements. 
     First, as shown in  FIGS. 30J and 30K , an N-channel transistor and a P-channel transistor can be used. 
       FIG. 30A  shows a diode-connected N-channel transistor.  FIG. 30B  shows a diode-connected N-channel transistor shown in  FIG. 30A , the connection direction of which is reversed.  FIG. 30C  shows the case where the elements shown in  FIGS. 30A and 30B  are connected in parallel.  FIGS. 30D and 30E  show the case where the N-channel transistors shown in  FIGS. 30A and 30B  are replaced with P-channel transistors. The P-channel transistors may be connected in parallel, in a similar manner that in  FIG. 30C . Alternatively, a P-channel transistor and an N-channel transistor may be connected in parallel, as shown in  FIG. 30F . 
       FIGS. 30G and 30L  each show a divider element in which a resistor and a capacitor are connected in series or in parallel. 
     In  FIGS. 30H and 30L  a P-channel transistor or an N-channel transistor and a resistor are connected in series. 
     Note that wirings to which gates of transistors shown in  FIGS. 30H, 30I, 30J , and  30 K are connected are not particularly limited to certain wirings. The gates of the transistors shown in  FIGS. 30H, 30I, 30J, and 30K  may be connected to scan lines, capacitor lines, or signal lines. Alternatively, the gates of the transistors shown in  FIGS. 30H, 30I, 30J, and 30K  may be connected to scan lines or the like in a row which is adjacent to the pixel. When potentials of the gates are controlled, resistance values of the divider elements can be controlled. 
       FIGS. 30M and 30N  each show a diode. There are various kinds of diodes, and diodes which can be used as the divider elements are not particularly limited to certain types. For example, a PN diode, PIN diode, a Schottky diode, an MIM diode, an MIS diode, or the like can be used. Alternatively, as shown in  FIG. 30O , two diodes may be connected in parallel in a reverse direction. 
     Further alternatively, an inductor shown in  FIG. 30P  may be used, or a resistor may be used as shown in  FIG. 30Q . As a resistor, a resistor having a variable resistance value may be used, as shown in  FIG. 30R . 
     Therefore, in each of the structures described in Embodiment Mode 1, the capacitor is replaced with each of the divider elements shown in  FIGS. 30A to 30T , so that a new circuit can be formed. Thus, the contents described in Embodiment Mode 1 can also be applied to  FIGS. 23A and 23B  and the circuit in which the capacitor is replaced with the divider element. 
       FIGS. 36A to 48B  are circuit diagrams where the first divider element  606  and the second divider element  607  shown in  FIGS. 23A and 23B  are replaced with various elements shown in  FIGS. 30A to 30S . Therefore, structures which are similar to the structures in  FIG. 1A to 1C  can be applied to  FIGS. 36A to 48B . That is, as shown in  FIGS. 7A and 7B , the first electrodes of part of or all the liquid crystal elements may be connected to a capacitor line. The capacitor line may be shared with a common electrode, as shown in  FIGS. 50A and 50B . The switches can be replaced with transistors, and either N-channel transistors or P-channel transistors may be used as transistors. In the case of using transistors, a gate of each transistor may be connected to the same scan line, or may be connected to different scan lines. In addition, as shown in  FIGS. 11A and 11B , the liquid crystal element may be divided into a plurality of elements. The number of signal lines may be plural, or signal lines may be put into one signal line as shown in  FIGS. 8A and 8B . Further, as shown in  FIGS. 2A and 2B ,  FIGS. 12A and 12B , and the like, the divider elements may be provided in suitable positions as appropriate. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     Note that resistance values of the divider elements are not necessarily constant, and the resistance values may be set to be varied in accordance with time or a pixel. In order to vary the resistance values, the divider elements may include transistors. In the case of using transistors, potentials of gates of the transistors may be varied in accordance with time or a pixel. 
     Note that when the divider element is connected between the liquid crystal elements, electric charge leaks between the respective liquid crystal elements in some cases when the signal line and the liquid crystal element are not connected. In order to prevent leakage of electric charge, the divider element and the switch are connected in series so that they may be connected between the respective liquid crystal elements.  FIGS. 24A and 24B  show such a case. Note that the divider element and the switch may be connected in reverse. 
     Note that although one divider element and one switch are provided between the liquid crystal elements, the present invention is not limited to this. A plurality of divider elements and a plurality of divider elements may be provided. Note that the contents described in Embodiment Mode 1 and  FIGS. 23A and 23B  can also be applied to  FIGS. 24A and 24B . 
     A pixel  650  includes a first switch  651 , a second switch  652 , a first liquid crystal element  653 , a second liquid crystal element  654 , a third liquid crystal element  655 , a first divider element  656 , a second divider element  657 , a third switch  658 , and a fourth switch  659 . 
     A first wiring  660  is connected to a first electrode of the first liquid crystal element  653  and one electrode of the third switch  658  through the first switch  651 . A second wiring  661  is connected to a first electrode of the second liquid crystal element  654  and one electrode of the fourth switch  659 . The third switch  658  and the fourth switch  659  are connected in series. The first divider element  656  and the second divider element  657  which are connected in series are provided between the third switch  658  and the fourth switch  659 . A first electrode of the third liquid crystal element  655  is connected between the first divider element  656  and the second divider element  657 . 
     Second electrodes of the first liquid crystal element  653 , the second liquid crystal element  654 , and the third liquid crystal element  655  are connected to a common electrode. 
     Each of the first wiring  660  and the second wiring  661  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  660  and the second wiring  661 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. A third wiring  662  functions as a scan line. 
     Each of the first switch  651  and the second switch  652  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. In the case where a transistor is used as each of the first switch  651  and the second switch  652 , the transistor may be either a P-channel transistor or an N-channel transistor. 
     Each of the third switch  658  and the fourth switch  659  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. A transistor which is used as each of the third switch  658  and the fourth switch  659  may be either a P-channel transistor or an N-channel transistor. 
       FIG. 24B  shows the case where an N-channel transistor is used as a switch. In  FIG. 24B , gates of a first switch  651 N and a second switch  652 N are connected to the third wiring  662 . The third wiring  662  functions as a scan line. 
     Note that in  FIGS. 24A and 24B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further  30  divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     Note that each of the first liquid crystal element  653 , the second liquid crystal element  654 , and the third liquid crystal element  655  has transmittivity in accordance with a video signal. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Next, a specific example of the case where the divider elements shown in  FIGS. 30A to 30T  are applied to  FIGS. 23A and 23B  and  FIGS. 24A and 24B  is described. First, the case where one of the divider elements shown in  FIGS. 30A to 30T  is used is described with reference to  FIGS. 25A and 25B . Gates are connected to a scan line.  FIGS. 23A and 23B  and  FIGS. 24A and 24B  correspond to diagrams in which the first capacitor  106  and the second capacitor  107  in  FIGS. 1A to 1C  are replaced with transistors. Therefore, the contents described in Embodiment Mode 1,  FIGS. 23A and 23B , and  FIGS. 24A and 24B  can also be applied to  FIGS. 25A and 25B . 
     A pixel  700  includes a first switch  701 , a second switch  702 , a first liquid crystal element  703 , a second liquid crystal element  704 , a third liquid crystal element  705 , a first transistor  706 , and a second transistor  707 . 
     A first wiring  708  is connected to a first electrode of the first liquid crystal element  703  and one of a source and a drain of the first transistor  706  through the first switch  701 . A second wiring  709  is connected to a first electrode of the second liquid crystal element  704  and one of a source and a drain of the second transistor  707  through the second switch  702 . The other of the source and the drain of the first transistor  706  and the other of the source and the drain of the second transistor  707  are connected to a first electrode of the third liquid crystal element  705 . The first and second transistors are connected to a third wiring  710 . 
     Second electrodes of the first liquid crystal element  703 , the second liquid crystal element  704 , and the third liquid crystal element  705  are connected to a common electrode. 
     Each of the first wiring  708  and the second wiring  709  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  708  and the second wiring  709 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  710  functions as a scan line. 
     Each of the first switch  701  and the second switch  702  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. The case where a transistor is used as each of the first switch  701  and the second switch  702  is described below. In the case of using a transistor, the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 25B  shows the case where an N-channel transistor is used as a switch. In  FIG. 25B , gates of a first switch  701 N and a second switch  702 N are connected to the third wiring  710 . The third wiring  710  functions as a scan line. 
     Note that in  FIGS. 25A and 25B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     It is acceptable as long as each of the first transistor  706  and the second transistor  707  functions as a divider element, and each of the first transistor  706  and the second transistor  707  may be either a P-channel transistor or an N-channel transistor. 
     Next, operations of the pixel  700  are described. First, when the third wiring  710  is selected, the first switch  701  and the second switch  702  are turned on. Then, video signals are supplied from the first wiring  708  and the second wiring  709 . The first transistor  706  and the second transistor  707  are turned on at the same time as the first and second switches. Therefore, the first wiring  708  and the second wiring  709  are connected through the transistor. Then, since the transistors have resistance components (on resistance), voltage is divided in each transistor. At this time, when the on resistance of the first transistor  706  and the second transistor  707  is high, most of voltage is applied to the transistors. 
     Therefore, a potential which is almost equal to a potential of the first wiring  708  is applied to a pixel electrode of the first liquid crystal element  703 . More precisely, a potential which is obtained by subtracting a potential of voltage drop by the first switch  701  from the potential of the first wiring  708  is applied to the pixel electrode of the first liquid crystal element  703 . Similarly, a potential which is almost equal to a potential of the second wiring  709  is applied to a pixel electrode of the second liquid crystal element  704 . More precisely, a potential which is obtained by subtracting a potential of voltage drop by the second switch  702  from the potential of the second wiring  709  is applied to the pixel electrode of the second liquid crystal element  704 . 
     Then, the potential of the pixel electrode of the first liquid crystal element  703  and the potential of the pixel electrode of the second liquid crystal element  704  are divided by voltage of the first transistor  706  and voltage of the second transistor  707 , and supplied to a pixel electrode of the third liquid crystal element  705 . If the on resistance of the first transistor  706  is almost equal to the on resistance of the second transistor  707 , the potential of the pixel electrode of the third liquid crystal element  705  is an intermediate potential between the potential of the pixel electrode of the first liquid crystal element  703  and the potential of the pixel electrode of the second liquid crystal element  704 . 
     Note that when the on resistance of the first switch  701 , the second switch  702 , the first transistor  706 , the second transistor  707 , and the like is low, large current flows. Therefore, the on resistance of the first transistor  706  and the second transistor  707  for voltage division is preferably high. For example, the first switch  701  or the second switch  702  has preferably the smaller ratio of the channel width W to the channel length L (W/L) than that of the first transistor  706  or the second transistor  707 . For example, the first transistor  706  or the second transistor  707  may have the longer channel length L with a multi-gate structure. 
     Note that it is preferable that the on resistance of the first transistor  706  and the on resistance of the second transistor  707  be almost equal. When the on resistance of the two transistors is almost equal, divided voltage has an intermediate potential. If there is difference in the on resistance, the potential is biased on one of potentials, so that the liquid crystal elements cannot be controlled uniformly. For example, it is preferable that the ratio of the channel width W to the channel length L (W/L) of the first transistor  706  and the ratio of the channel width W to the channel length L (W/L) of the second transistor  707  be almost equal. Note that the present invention is not limited to this. 
     When the third wiring  710  is not selected, the first switch  701 , the second switch  702 , the first transistor  706 , and the second transistor  707  are turned off. Then, the voltage applied to each of the liquid crystal elements is held. With such operations, the voltage applied to each of the liquid crystal elements can be varied. Accordingly, the viewing angle can be widened. Note that the driving method is not limited to this. A variety of timing for turning on/off each transistor, potentials of a signal line, and the like can be controlled by using various methods. 
     Note that since the first transistor  706  and the second transistor  707  are turned off, electric charge does not leak between the respective liquid crystal elements. Therefore, it can also be said that each of the first transistor  706  and the second transistor  707  realizes the divider element and the switch in  FIGS. 24A and 24B  by one element. 
     Note that each of the first liquid crystal element  703 , the second liquid crystal element  704 , and the third liquid crystal element  705  has transmittivity in accordance with a video signal. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that the structures of the first transistor  706  and the second transistor  707  are not limited to the structures which are shown. For example, one of or both the first transistor  706  and the second transistor  707  may have a multi-gate structure. With a multi-gate structure, resistance values of the first transistor  706  and the second transistor  707  can be easily adjusted compared to the case of a single-gate structure. Further, on resistance of first transistor  706  and the second transistor  707  can be further increased compared to the case of a single-gate structure. 
     Note that the resistance values of the first transistor  706  and the second transistor  707  are not necessarily constant, and the resistance values may be set to be varied in accordance with time or a pixel. In order to vary the resistance values, potentials of gates of the first transistor  706  and the second transistor  707  which function as divider elements may be varied in accordance with time or a pixel. 
     Note that although storage capacitors are not shown in  FIGS. 23A to 25B , storage capacitors may be provided, as shown in  FIGS. 1A to 1C  and the like. As an example,  FIGS. 26A and 26B  show the case where a storage capacitor is provided for each of the liquid crystal elements in  FIGS. 25A and 25B . 
     In  FIG. 26A , a pixel  750  includes a first switch  751 , a second switch  752 , a first liquid crystal element  753 , a second liquid crystal element  754 , a third liquid crystal element  755 , a first transistor  756 , a second transistor  757 , a first capacitor  762 , a second capacitor  763 , and a third capacitor  764 . 
     A first wiring  758  is connected to a first electrode of the first liquid crystal element  753 , one of a source and a drain of the first transistor  756 , and a first electrode of the third capacitor  764  through the first switch  751 . A second wiring  759  is connected to a first electrode of the second liquid crystal element  754 , one of a source and a drain of the second transistor  757 , and a first electrode of the first capacitor  762 . The other of the source and the drain of the first transistor  756  and the other of the source and the drain of the second transistor  757  are connected to a first electrode of the third liquid crystal element  755  and a first electrode of the second capacitor  763 . The first and second switches and the first and second transistors are connected to a third wiring  760 . Second electrodes of the first capacitor  762 , the second capacitor  763 , and the third capacitor  764  are connected to a fourth wiring  761 . 
     Second electrodes of the first liquid crystal element  753 , the second liquid crystal element  754 , and the third liquid crystal element  755  are connected to a common electrode. 
     Each of the first wiring  758  and the second wiring  759  functions as a signal line. The third wiring  760  functions as a scan line. The fourth wiring  761  functions as a capacitor line. 
     Each of the first switch  751  and the second switch  752  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. In the case where a transistor is used as each of the first switch  751  and the second switch  752 , the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 26B  shows the case where an N-channel transistor is used as a switch. In  FIG. 26B , gates of a first switch  751 N and a second switch  752 N are connected to the third wiring  760 . The third wiring  760  functions as a scan line. 
     Note that in  FIGS. 26A and 26B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     It is acceptable as long as each of the first transistor  756  and the second transistor  757  functions as a divider element, and each of the first transistor  756  and the second transistor  757  may be either a P-channel transistor or an N-channel transistor. 
     Note that each of the first liquid crystal element  753 , the second liquid crystal element  754 , and the third liquid crystal element  755  has transmittivity in accordance with a video signal. 
     Note that resistance values of the first transistor  756  and the second transistor  757  are not necessarily constant, and the resistance values may be set to be varied in accordance with time or a pixel. In order to vary the resistance values, potentials of gates of the first transistor  756  and the second transistor  757  which function as resistors may be varied in accordance with time or a pixel. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that although the gates of the first and second transistors are connected to the scan line in  FIGS. 25A and 25B  and  FIGS. 26A and 26B , the present invention is not limited to this. Another wiring may be provided and the first and second transistors may be connected to the wiring. Alternatively, a plurality of different wirings may be provided and the gates of the first and second transistors may be connected to different wirings.  FIG. 27B  shows the case where gates of a first transistor and a second transistor are connected to a fourth wiring in  FIG. 27A . With such a structure, potentials of the gates of the first transistor and the second transistor can be controlled independently from first and second switches, so that on resistance of the first and second transistors can be easily controlled. For example, in the case of inputting a negative (a potential of a video signal is lower than a potential of a common electrode) video signal, gate-source voltage of the first and second transistors is extremely increased. Therefore, on resistance of the first and second transistors is decreased and much current flows, so that power consumption is increased in some cases. Then, when the first and second transistors are turned on to be divided, the potentials of the gates of the first and second transistors in the case of inputting a negative video signal are made lower than the potentials of the gates of the first and second transistors in the case of inputting a positive (a potential of a video signal is higher than a potential of the common electrode) video signal. Accordingly, much current can be prevented from flowing. 
     A pixel  800  includes a first switch  801 , a second switch  802 , a first transistor  803 , a second transistor  804 , a first liquid crystal element  805 , a second liquid crystal element  806 , and a third liquid crystal element  807 . 
     A first wiring  808  is connected to a first electrode of the first liquid crystal element  805  and one of a source and a drain of the first transistor  803  through the first switch  801 . A second wiring  809  is connected to a first electrode of the second liquid crystal element  806  and one of a source and a drain of the second transistor  804  through the second switch  802 . The other of the source and the drain of the first transistor  803  and the other of the source and the drain of the second transistor  804  are connected to a first electrode of the third liquid crystal element  807 . Gates of the first switch  801  and the second switch  802  are connected to a third wiring  810 . Gates of the first transistor  803  and the second transistor  804  are connected to a fourth wiring  811 . 
     Second electrodes of the first liquid crystal element  805 , the second liquid crystal element  806 , and the third liquid crystal element  807  are connected to a common electrode. 
     Each of the first wiring  808  and the second wiring  809  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  808  and the second wiring  809 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. Each of the third wiring  810  and the fourth wiring  811  functions as a scan line. 
     Each of the first switch  801  and the second switch  802  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. In the case where a transistor is used as each of the first switch  801  and the second switch  802 , the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 27B  shows the case where an N-channel transistor is used as a switch. In  FIG. 27B , gates of a first switch  801 N and a second switch  802 N are connected to the third wiring  810 . The third wiring  810  functions as a scan line. 
     Note that in  FIGS. 27A and 27B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     It is acceptable as long as each of the first transistor  803  and the second transistor  804  functions as a divider element, and each of the first transistor  803  and the second transistor  804  may be either a P-channel transistor or an N-channel transistor. 
     Note that when each of the first transistor  803  and the second transistor  804  is turned on to function as a divider element, each of the first transistor  803  and the second transistor  804  is preferably operated in a linear region. This is to have an appropriate value of on resistance in each of the first transistor  803  and the second transistor  804 . 
     Note that it is preferable that timing for turning on/off the first switch  801  and  30  the second switch  802  and timing for turning on/off the first transistor  803  and the second transistor  804  be almost the same. Note that the present invention is not limited to this. When the first switch  801  and the second switch  802  are turned on, the first transistor  803  and the second transistor  804  may be turned on a bit late. Thus, a period during which the first wiring  808  and the second wiring  809  are connected can be shortened. Therefore, electric charge can be easily input to the first liquid crystal element  805  and the second liquid crystal element  806 . 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Next, an example is described in which the contents described in Embodiment Mode 1 is applied to  FIGS. 25A and 25B . An example of a circuit is described in which the capacitors are replaced with the divider elements shown in  FIGS. 30A to 30T .  FIGS. 28A and 28B  show the case where the first capacitor and the second capacitor in  FIGS. 2A and 2B  are replaced with the divider elements shown in  FIG. 30J . At this time, gates of transistor of the divider elements are connected to a scan line. Note that the present invention is not limited to this. Therefore, the contents described in Embodiment Mode 1 can also be applied to  FIGS. 28A and 28B . 
     A pixel  850  includes a first switch  851 , a second switch  852 , a first liquid crystal element  853 , a second liquid crystal element  854 , a third liquid crystal element  855 , a first transistor  856 , a second transistor  857 , and a capacitor  861 . 
     A first wiring  858  is connected to a first electrode of the first liquid crystal element  853  and one of a source and a drain of the first transistor  856  through the first switch  851 . A second wiring  859  is connected to a first electrode of the second liquid crystal element  854  and one of a source and a drain of the second transistor  857 . The other of the source and the drain of the first transistor  856  and the other of the source and the drain of the second transistor  857  are connected to a first electrode of the capacitor  861 . A second electrode of the capacitor  861  is connected to a first electrode of the third liquid crystal element  855 . The first and second transistors are connected to a third wiring  860 . 
     Second electrodes of the first liquid crystal element  853 , the second liquid crystal element  854 , and the third liquid crystal element  855  are connected to a common electrode. 
     Each of the first wiring  858  and the second wiring  859  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  858  and the second wiring  859 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  860  functions as a scan line. 
     Each of the first switch  851  and the second switch  852  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. In the case where a transistor is used as each of the first switch  851  and the second switch  852 , the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 28B  shows the case where an N-channel transistor is used as a switch. In  FIG. 28B , gates of a first switch  851 N and a second switch  852 N are connected to the third wiring  860 . The third wiring  860  functions as a scan line. 
     Note that in  FIGS. 28A and 28B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     It is acceptable as long as each of the first transistor  856  and the second transistor  857  functions as a divider element, and each of the first transistor  856  and the second transistor  857  may be either a P-channel transistor or an N-channel transistor. 
     When the circuit structures shown in  FIGS. 28A and 28B  are used, a potential of the first electrode of the third liquid crystal element  855  can be lowered by a potential of the capacitor  861 , in a similar manner that in  FIGS. 2A and 2B  and the like. 
     Note that the structures of the first transistor  856  and the second transistor  857  are not limited to the structures which are shown. For example, one of or both the first transistor  856  and the second transistor  857  may have a multi-gate structure. 
     Note that resistance values of the first transistor  856  and the second transistor  857  are not necessarily constant, and the resistance values may be set to be varied in accordance with time or a pixel. In order to vary the resistance values, potentials of gates of the first transistor  856  and the second transistor  857  functioning as resistors may be varied in accordance with time or a pixel. 
     Note that the structures of the first transistor  856  and the second transistor  857  are not limited to the structures which are shown. For example, one of or both the first transistor  856  and the second transistor  857  may have a multi-gate structure. With a multi-gate structure, on resistance of first transistor  856  and the second transistor  857  can be further increased compared to the case of a single-gate structure. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that although the case in which two divider elements are used is described in  FIGS. 23A to 28B , the present invention is not limited to this. Much more divider elements are used so that viewing angle characteristics can be further improved. As an example,  FIGS. 29A and 29B  show an example of a circuit in the case where a divider element is added to the structures in  FIGS. 25A and 25B  or in the case where the capacitors in  FIGS. 9A and 9B  are replaced with two divider elements shown in  FIG. 30J , which are connected in series. 
     In  FIG. 29A , a pixel  900  includes a first switch  901 , a second switch  902 , a first liquid crystal element  903 , a second liquid crystal element  904 , a third liquid crystal element  905 , a fourth liquid crystal element  906 , a first transistor  907 , a second transistor  908 , and a third transistor  909 . 
     A first wiring  910  is connected to a first electrode of the first liquid crystal element  903  and one of a source and a drain of the first transistor  907  through the first switch  901 . A second wiring  911  is connected to a first electrode of the second liquid crystal element  904  and one of a source and a drain of the third transistor  909  through the second switch  902 . The other of the source and the drain of the first transistor  907  is connected to a first electrode of the third liquid crystal element  905  and one of a source and a drain of the second transistor  908 . The other of the source and the drain of the third transistor  909  is connected to a first electrode of the fourth liquid crystal element  906  and the other of the source and the drain of the second transistor  908 . Gates of the first and second switches  901  and  902  and the first transistor and second transistors are connected to a third wiring  912 . 
     Second electrodes of the first liquid crystal element  903 , the second liquid crystal element  904 , and the third liquid crystal element  905  are connected to a common electrode. 
     Each of the first wiring  910  and the second wiring  911  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  910  and the second wiring  911 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  912  functions as a scan line. 
     Each of the first switch  901  and the second switch  902  is not particularly limited to a certain type as long as it functions as a switch. For example, a transistor can be used. In the case where a transistor is used as each of the first switch  901  and the second switch  902 , the transistor may be either a P-channel transistor or an N-channel transistor. 
       FIG. 29B  shows the case where an N-channel transistor is used as a switch. In  FIG. 29B , gates of a first switch  901 N and a second switch  902 N are connected to the third wiring  912 . The third wiring  912  functions as a scan line. 
     Note that in  FIGS. 29A and 29B , in a similar manner that in  FIGS. 1A to 1C  and the like, the number of scan lines may be two as shown in  FIG. 49  and a P-channel transistor can be used as a switch. In addition, a liquid crystal element may be further divided into a plurality of elements, as shown in  FIGS. 11A and 11B  and the like. 
     Note that a switch is not limited to a transistor. Various elements such as diodes can be used as a switch. 
     It is acceptable as long as each of the first to third transistors functions as a divider element, and each of the first to third transistor may be either a P-channel transistor or an N-channel transistor. In  FIGS. 28A and 28B , an N-channel transistor is used. 
     As shown in  FIGS. 29A and 29B , only one of the first and second transistors may have a multi-gate structure in  FIGS. 25A and 25B . 
     Note that although gates of the first to third transistors are connected to the third wiring which controls the first and second switches, the present invention is not limited to this. As described with reference to  FIGS. 27A and 27B , the gates of the first to third transistors may be connected to a wiring which is different from the third wiring which controls the first and second switches. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
     Note that resistance values of the first transistor  907 , the second transistor  908 , and the third transistor  909  are not necessarily constant, and the resistance values may be set to be varied in accordance with time or a pixel. In order to vary the resistance values, potentials of gates of the third to fifth transistors which function as resistors may be varied in accordance with time or a pixel. Note that the structures of the first transistor  907  and the second transistor  908  are not limited to the structures which are shown. 
     As described above, when liquid crystal elements are aligned differently, the viewing angle can be widened. 
       FIG. 34  shows an example of a top view of a pixel of a liquid crystal display device to which the present invention is applied. In addition,  FIG. 35  is a circuit diagram of  FIG. 34 . Note that corresponding portions between  FIGS. 34 and 35  are denoted by the same reference numerals. 
     In a pixel  1020  shown in  FIG. 34 , a first insulating film (not shown) is provided over a first conductive layer (shown by a hatch pattern of a third wiring  1033 ) serving as a scan line and a capacitor line; a semiconductor film is provided over the first insulating film, a second conductive film (shown by a hatch pattern of a first wiring  1031 ) is provided over the semiconductor film; a second insulating film (not shown) is provided over the second conductive layer; and a third conductive layer (shown by a hatch pattern of a first liquid crystal element  1023 ) is provided over the second insulating film. 
     In  FIG. 35 , the pixel  1020  includes a first transistor  1021  serving as a first switch, a second transistor  1022  serving as a second switch, the first liquid crystal element  1023 , a second liquid crystal element  1024 , a third liquid crystal element  1025 , a fourth liquid crystal element  1026 , a first capacitor  1027 , a second capacitor  1030 , a third capacitor  1036 , a fourth capacitor  1037 , a third transistor  1028 , a fourth transistor  1029 , and a fifth transistor  1039 . 
     The first wiring  1031  is connected to a second wiring  1032  through the first to fifth transistors connected in series. First electrodes of the first to fourth liquid crystal elements are connected between the respective first to fifth transistors. The first to fourth liquid crystal elements are connected to first electrodes of the capacitors, second electrodes of which are connected to a fourth wiring  1034  or a fifth wiring  1035 . Gates of the first to fifth liquid transistors are connected to the third wiring  1033 . 
     Note that  FIG. 35  shows the case where all the capacitors which function as divider elements in  FIG. 9B  are replaced with transistors and all the capacitors are provided with storage capacitors. That is,  FIG. 35  shows the case where the contents described in  FIGS. 9B and 16B  are combined. Therefore, structures which are similar to the structures in  FIGS. 1A to 1C  can be applied to  FIG. 35 . In other words, a wiring which functions as a capacitor line may be shared with a common electrode as shown in  FIGS. 50A and 50B , the switches can be replaced with transistors, and either N-channel transistors or P-channel transistors may be used as the transistors. 
     Each of the first wiring  1031  and the second wiring  1032  functions as a signal line. Therefore, an image signal is usually supplied to each of the first wiring  1031  and the second wiring  1032 . Note that the present invention is not limited to this. A certain signal may be supplied regardless of an image. The third wiring  1033  functions as a scan line. Each of the fourth wiring  1034  and the fifth wiring  1035  functions as a capacitor line. 
     When a pixel like the pixel shown in the top view in  FIG. 34  is provided, alignment of liquid crystal elements can be varied, so that a liquid crystal display device having a wider viewing angle can be provided. 
     Note that although this embodiment mode is described with reference to various drawings, part of or all the contents described in each drawing can be freely applied to, combined with, or replaced with part of or all the contents described in another drawing. Further, even more structures are possible when each part is combined with another part in the above-described drawings, and the description of this embodiment mode does not impede this. 
     Similarly, part of or all the contents described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with part of or all the contents described in a drawing in another embodiment mode. Further, even more drawings are possible when each part is combined with part of another embodiment mode in the drawings of this embodiment mode, and the description of this embodiment mode does not impede this. 
     Note that this embodiment mode shows an example of an embodied case of part of or all the contents described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, or an example of related part thereof. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 3 
     In this embodiment mode, a structure and a manufacturing method of a transistor are described. 
       FIGS. 51A to 51G  are cross-sectional views showing examples of a structure and a manufacturing method of a transistor.  FIG. 51A  is a cross-sectional view showing a structural example of the transistor.  FIGS. 51B to 51G  are cross-sectional views showing an example of a manufacturing method of the transistor. 
     Note that the structure and the manufacturing method of the transistor are not limited to those shown in  FIGS. 51A to 51Q  and various structures and manufacturing methods can be used. 
     First, structural examples of transistors are described with reference to  FIG. 51A .  FIG. 51A  is a cross-sectional view of a plurality of transistors each having a different structure. Here, in  FIG. 51A , the plurality of transistors each having a different structure are arranged, which is for describing the structures of the transistors. Accordingly, it is not necessary to arrange the transistors actually as shown in  FIG. 56A  and can be separately formed as necessary. 
     Next, characteristics of each layer included in the transistor are described. 
     As a substrate  110111 , a glass substrate such as a barium borosilicate glass substrate or an aluminoborosilicate glass substrate, a quartz substrate, a ceramic substrate, or a metal substrate including stainless steel, or the like can be used. Alternatively, a substrate formed using a flexible synthetic resin such as acrylic or plastic typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyethersulfone (PES) can be used. When such a flexible substrate is used, a semiconductor device which can be bent can be formed. Since a flexible substrate has no limitations on the area and the shape of a substrate, when a rectangular substrate with a side of one meter or more is used as the substrate  110111 , for example, productivity can be significantly improved. Such a merit is greatly advantageous over the case of using a circular silicon substrate. 
     An insulating film  110112  functions as a base film. The insulating film  110112  is provided to prevent alkali metal such as Na or alkaline earth metal from the substrate  110111  from adversely affecting characteristics of a semiconductor element. The insulating film  110112  can have a single-layer structure or a stacked-layer structure of an insulating film including oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y , x&gt;y), or silicon nitride oxide (SiN x O y , x&gt;y). For example, when the insulating film  110112  is provided to have a two-layer structure, it is preferable that a silicon nitride oxide film be used as a first insulating film and a silicon oxynitride film be used as a second insulating film. As another example, when the insulating film  110112  is provided to have a three-layer structure, it is preferable that a silicon oxynitride film be used as a first insulating film, a silicon nitride oxide film be used as a second insulating film, and a silicon oxynitride film be used as a third insulating film. 
     Semiconductor layers  1101143 ,  110114 , and  110115  can be formed by using an amorphous semiconductor or a semi-amorphous semiconductor (SAS). Alternatively, a polycrystalline semiconductor film may be used. SAS is a semiconductor having an intermediate structure between amorphous and crystalline (including single-crystal and polycrystalline) structures and having a third state which is stable in free energy. Moreover, SAS includes a crystalline region with a short range order and lattice distortion. A crystalline region of 0.5 to 20 nm can be observed at least in part of an SAS film. When silicon is contained as a main component, Raman spectrum shifts to a wave number side lower than 520 cm −1 . The diffraction peaks of (111) and (220) which are thought to be derived from a silicon crystalline lattice are observed by X-ray diffraction. SAS contains hydrogen or halogen of at least 1 at. % or more to terminate dangling bonds. SAS is formed by glow discharge decomposition (plasma CVD) of a material gas. As the material gas, Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , SiF 4 , or the like as well as SiH 4  can be used. Further, GeF 4  may be mixed. Alternatively, the material gas may be diluted with H 2 , or H 2  and one or more kinds of rare gas elements selected from He, Ar, Kr, or Ne. The dilution ratio may be in the range of 2 to 1000 times, pressure may be in the range of approximately 0.1 to 133 Pa, a power supply frequency may be 1 to 120 MHz and preferably 13 to 60 MHz, and a substrate heating temperature may be 300° C. or lower. A concentration of impurities in atmospheric components such as oxygen, nitrogen, and carbon is preferably 1×10 20  cm −1  or less as impurity elements in the film. In particular, an oxygen concentration is 5×10 19 /cm 3  or less, and preferably 1×10 19 /cm 3  or less. Here, an amorphous silicon film is formed using a material including silicon (Si) as a main component (e.g., Si x Ge 1-x ) by sputtering, LPCVD, plasma CVD, or the like. Then, the amorphous silicon film is crystallized by a crystallization method such as a laser crystallization method, a thermal crystallization method using RTA or an annealing furnace, or a thermal crystallization method using a metal element which promotes crystallization. 
     An insulating film  110116  can have a single-layer structure or a stacked-layer structure of an insulating film including oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y , x&gt;y), or silicon nitride oxide (SiN x O y , x&gt;y). 
     A gate electrode  110117  can have a single-layer structure of a conductive film or a stacked-layer structure of two or three conductive films. As a material for the gate electrode  110117 , a conductive film can be used. For example, a film of an element such as tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), chromium (Cr), or silicon (Si); a nitride film including the element (typically a tantalum nitride film, a tungsten nitride film, or a titanium nitride film); an alloy film in which the elements are combined (typically a Mo—W alloy or a Mo—Ta alloy); a silicide film including the element (typically a tungsten silicide film or a titanium silicide film); and the like can be used. Note that the above-described film of such an element, nitride film, alloy film, silicide film, and the like can have a single-layer structure or a stacked-layer structure. 
     An insulating film  110118  can have a single-layer structure or a stacked-layer structure of an insulating film including oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y , x&gt;y), or silicon nitride oxide (SiN x O y , x&gt;y); or a film including carbon, such as a DLC (diamond like carbon), by sputtering, plasma CVD, or the like. 
     An insulating film  110119  can have a single-layer structure or a stacked-layer structure of a siloxane resin; an insulating film including oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y , x&gt;y), or silicon nitride oxide (SiN x O y , x&gt;y); or a film including carbon, such as a DLC (diamond like carbon); an organic material such as epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic. Note that a siloxane resin corresponds to a resin having Si—O—Si bonds. Siloxane includes a skeleton structure of a bond of silicon (Si) and oxygen (O). As a substituent, an organic group including at least hydrogen (e.g., an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group, or a fluoro group and an organic group including at least hydrogen can be used as a substituent. Note that the insulating film  110119  can be directly provided so as to cover the gate electrode  110117  without providing the insulating film  110118 . 
     As a conductive film  110123 , a film of an element such as Al, Ni, C, W, Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film including the element, an alloy film in which the elements are combined, a silicide film including the element, or the like can be used. For example, as an alloy including some of such elements, an Al alloy including C and Ti, an Al alloy including Ni, an Al alloy including C and Ni, an Al alloy including C and Mn, or the like can be used. In the case of a stacked-layer structure, for example, a structure can be such that Al is interposed between Mo, Ti, or the like, so that resistance of Al to heat and chemical reaction can be improved. 
     Next, characteristics of each structure is described with reference to the cross-sectional view of the plurality of transistors each having a different structure in  FIG. 51A . 
     A transistor  110101  is a single-drain transistor. Since the transistor  110101  can be formed by a simple method, it is advantageous in low manufacturing cost and high yield. Here, semiconductor layers  110113  and  110115  have different concentration of impurities, and the semiconductor layer  110113  is used as a channel region and the semiconductor layers  110115  are used as a source region and a drain region. When the amount of impurities is controlled in this manner, resistivity of the semiconductor layer can be controlled. Further, an electric connection state between the semiconductor layer and the conductive film  110123  can be closer to ohmic contact. Note that as a method for separately forming the semiconductor layers including different amount of impurities, a method where impurities are added to the semiconductor layer by using the gate electrode  110117  as a mask can be used. 
     A transistor  110102  is a transistor in which the gate electrode  110117  has a certain tapered angle or more. Since the transistor  110102  can be formed by a simple method, it is advantageous in low manufacturing cost and high yield. Here, the semiconductor layers  110113 ,  110114 , and  10115  have different concentration of impurities. The semiconductor layer  110113  is used as a channel region, the semiconductor layers  110114  are used as lightly doped drain (LDD) regions, and the semiconductor layers  110115  are used as a source region and a drain region. When the amount of impurities is controlled in this manner, resistivity of the semiconductor layer can be controlled. Further, an electric connection state between the semiconductor layer and the conductive film  110123  can be closer to ohmic contact. Moreover, since the transistor includes the LDD region, high electric field is not easily applied to the transistor, deterioration of the element due to hot carriers can be suppressed. Note that as a method for separately forming the semiconductor layers including different amount of impurities, a method where impurities are added to the semiconductor layer by using the gate electrode  110117  as a mask can be used. In the transistor  110102 , since the gate electrode  110117  has a certain tapered angle or more, gradient of the concentration of impurities added to the semiconductor layer through the gate electrode  110117  can be provided, and the LDD region can be easily formed. 
     A transistor  110103  is a transistor in which the gate electrode  110117  includes at least two layers and a lower gate electrode is longer than an upper gate electrode. In this specification, the shape of the upper gate electrode and the lower gate electrode is referred to as a hat shape. When the gate electrode  110117  has a hat shape, an LDD region can be formed without adding a photomask. Note that a structure where the LDD region overlaps with the gate electrode  110117 , like the transistor  110103 , is particularly referred to as a GOLD (gate overlapped LDD) structure. As a method for forming the gate electrode  110117  with a hat shape, the following method may be used. 
     First, when the gate electrode  110117  is patterned, the lower and upper gate electrodes are etched by dry etching so that side surfaces thereof are inclined (tapered). Then, the inclination of the upper gate electrode is processed to be almost perpendicular by anisotropic etching. Thus, the gate electrode is formed such that the cross section is hat-shaped. Then, doping of impurity elements is performed twice, so that the semiconductor layer  110113  used as a channel region, the semiconductor layers  110114  used as LDD regions, and the semiconductor layers  110115  used as a source electrode and a drain electrode are formed. 
     Note that a portion of the LDD region, which overlaps with the gate electrode  110117 , is referred to as an L ov  region, and a portion of the LDD region, which does not overlap with the gate electrode  110117 , is referred to as an L off  region. The L off  region is highly effective in suppressing an off-current value, whereas it is not very effective in preventing deterioration in an on-current value due to hot carriers by relieving an electric field in the vicinity of the drain. On the other hand, the L ov  region is highly effective in preventing deterioration in the on-current value by relieving the electric field in the vicinity of the drain, whereas it is not very effective in suppressing the off-current value. Thus, it is preferable to form a transistor having a structure corresponding to characteristics necessary for each of various circuits. For example, when the semiconductor device is used for a display device, a transistor having an L off  region is preferably used as a pixel transistor in order to suppress the off-current value. On the other hand, as a transistor in a peripheral circuit, a transistor having an L ov  region is preferably used in order to prevent deterioration in the on-current value by relieving the electric field in the vicinity of the drain. 
     A transistor  110104  is a transistor including a sidewall  110121  in contact with a side surface of the gate electrode  110117 . When the transistor includes the sidewall  110121 , a region overlapping with the sidewall  110121  can be formed as an LDD region. 
     A transistor  110105  is a transistor in which an LDD (L off ) region is formed by doping the semiconductor layer with an impurity element by using a mask  110122 . Thus, the LDD region can be surely formed, and an off-current value of the transistor can be reduced. 
     A transistor  110106  is a transistor in which an LDD (L ov ) region is formed by doping in the semiconductor layer by using a mask. Thus, the LDD region can be surely formed, and deterioration in an on-current value can be prevented by relieving the electric field in the vicinity of the drain of the transistor. 
     Next, an example of a manufacturing method of a transistor is described with reference to  FIGS. 51B to 51G . 
     Note that a structure and a manufacturing method of a transistor are not limited to those in  FIGS. 51A to 51G  and various structures and manufacturing methods can be used. 
     In this embodiment mode, a surface of the substrate  110111 , a surface of the insulating film  110112 , a surface of the semiconductor layer  110113 , a surface of the semiconductor layer  110114 , a surface of the semiconductor layer  110115 , a surface of the insulating film  110116 , a surface of the insulating film  110118 , or a surface of the insulating film  110119  is oxidized or nitrided by using plasma treatment, so that the semiconductor layer or the insulating film can be oxidized or nitrided. When the semiconductor layer or the insulating film is oxidized or nitrided by plasma treatment in such a manner, the surface of the semiconductor layer or the insulating film is modified, and the insulating film can be formed to be denser than an insulating film formed by CVD or sputtering. Thus, a defect such as a pinhole can be suppressed, and characteristics and the like of the semiconductor device can be improved. 
     First, the surface of the substrate  110111  is washed by using hydrofluoric acid (HF), alkaline, or pure water. As the substrate  110111 , a glass substrate such as a barium borosilicate glass substrate or an aluminoborosilicate glass substrate, a quartz substrate, a ceramic substrate, a metal substrate including stainless steel, or the like can be used. Alternatively, a substrate formed using plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or polyethersulfone (PES), or a substrate formed using a flexible synthetic resin such as acrylic can be used. Here, the case where a glass substrate is used as the substrate  110111  is shown. 
     Here, an oxide film or a nitride film may be formed on the surface of the substrate  110111  by oxidizing or nitriding the surface of the substrate  110111  by plasma treatment ( FIG. 51B ). Hereinafter, an insulating film such as an oxide film or a nitride film formed by performing plasma treatment on the surface is also referred to as a plasma-treated insulating film. In  FIG. 51B , an insulating film  110131  is a plasma-treated insulating film. In general, when a semiconductor element such as a thin film transistor is provided over a substrate formed of glass, plastic, or the like, an impurity element such as alkali metal (e.g., Na) or alkaline earth metal included in glass, plastic, or the like might be mixed into the semiconductor element so that the semiconductor element is contaminated; thus, characteristics of the semiconductor element may be adversely affected in some cases. However, nitridation of a surface of the substrate formed of glass, plastic, or the like can prevent an impurity element such as alkali metal (e.g., Na) or alkaline earth metal included in the substrate from being mixed into the semiconductor element. 
     When the surface is oxidized by plasma treatment, the plasma treatment is performed in an oxygen atmosphere (e.g., in an atmosphere of oxygen (O 2 ) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe), in an atmosphere of oxygen, hydrogen (H 2 ), and a rare gas, or in an atmosphere of dinitrogen monoxide and a rare gas). On the other hand, when the semiconductor layer is nitrided by plasma treatment, the plasma treatment is performed in a nitrogen atmosphere (e.g., in an atmosphere of nitrogen (N 2 ) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe), in an atmosphere of nitrogen, hydrogen, and a rare gas, or in an atmosphere of NH 3  and a rare gas). As a rare gas, Ar can be used, for example. Alternatively, a gas in which Ar and Kr are mixed may be used. Accordingly, the plasma-treated insulating film contains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. For example, the plasma-treated insulating film contains Ar when Ar is used. 
     It is preferable to perform plasma treatment in the atmosphere containing the aforementioned gas, with conditions of an electron density in the range of 1×10 11  to 1×10 13  cm −3  and a plasma electron temperature in the range of 0.5 to 1.5 eV. Since the plasma electron density is high and the electron temperature in the vicinity of an object to be treated is low, damage by plasma to the object to be treated can be prevented. Since the plasma electron density is as high as 1×10 11  cm −3  or more, an oxide film or a nitride film formed by oxidizing or nitriding the object to be treated by plasma treatment is superior in its uniformity of thickness and the like as well as being dense, as compared to a film formed by CVD, sputtering, or the like. Alternatively, since the plasma electron temperature is as low as 1 eV or less, oxidation or nitridation can be performed at a lower temperature as compared to conventional plasma treatment or thermal oxidation. For example, oxidation or nitridation can be performed sufficiently even when plasma treatment is performed at a temperature lower than a strain point of a glass substrate by 100 degrees or more. Note that as frequency for generating plasma, high frequency waves such as microwaves (2.45 GHz) can be used. Note that hereinafter, plasma treatment is performed by using the aforementioned conditions unless otherwise specified. 
     Note that although  FIG. 51B  shows the case where the plasma-treated insulating film is formed by performing plasma treatment on the surface of the substrate  110111 , this embodiment mode includes the case where a plasma-treated insulating film is not formed on the surface of the substrate  110111 . 
     Note that although a plasma-treated insulating film formed by performing plasma treatment on the surface of the object to be treated is not shown in  FIGS. 51C to 51Q  this embodiment mode includes the case where a plasma-treated insulating film formed by plasma treatment exists on the surface of the substrate  110111 , the insulating film  110112 , the semiconductor layers  110113 , the semiconductor layer  110114 , the semiconductor layer  110115 , the insulating film  110116 , the insulating film  110118 , or the insulating film  110119 . 
     Next, the insulating film  110112  is formed over the substrate  110111  by sputtering, LPCVD, plasma CVD, or the like ( FIG. 51C ). For the insulating film  110112 , silicon oxide (SiO x ) or silicon oxynitride (SiO x N y ) (x&gt;y) can be used. 
     Here, a plasma-treated insulating film may be formed on the surface of the insulating film  110112  by oxidizing or nitriding the surface of the insulating film  110112  by plasma treatment. By oxidizing the surface of the insulating film  110112 , the surface of the insulating film  110112  is modified, and a dense film with fewer defects such as a pinhole can be obtained. Further, by oxidizing the surface of the insulating film  110112 , the plasma-treated insulating film containing a little amount of N atoms can be formed; thus, interface characteristics of the plasma-treated insulating film and a semiconductor layer are improved when the semiconductor layer is provided over the plasma-treated insulating film. The plasma-treated insulating film contains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. Note that the plasma treatment can be performed in a similar manner under the aforementioned conditions. 
     Next, the island-shaped semiconductor layers  110113  and  110114  are formed over the insulating film  110112  ( FIG. 51D ). The island-shaped semiconductor layers  110113  and  110114  can be formed in such a manner that an amorphous semiconductor layer is formed over the insulating film  110112  by using a material containing silicon (Si) as its main component (e.g., Si x Ge 1-x ) or the like by sputtering, LPCVD, plasma CVD, or the like, the amorphous semiconductor layer is crystallized, and the semiconductor layer is selectively etched. Note that crystallization of the amorphous semiconductor layer can be performed by a known crystallization method such as a laser crystallization method, a thermal crystallization method using RTA or an annealing furnace, a thermal crystallization method using a metal element which promotes crystallization, or a method in which these methods are combined. Here, end portions of the island-shaped semiconductor layers are provided with an angle of about 900 (θ=85 to 100°). Alternatively, the semiconductor layer  110114  to be a low concentration drain region may be formed by doping impurities with the use of a mask. 
     Here, a plasma-treated insulating film may be formed on the surfaces of the semiconductor layers  110113  and  110114  by oxidizing or nitriding the surfaces of the semiconductor layers  110113  and  110114  by plasma treatment. For example, when Si is used for the semiconductor layers  110113  and  110114 , silicon oxide (SiO x ) or silicon nitride (SiN x ) is formed as the plasma-treated insulating film. Alternatively, after being oxidized by plasma treatment, the semiconductor layers  110113  and  110114  may be nitrided by performing plasma treatment again. In this case, silicon oxide (SiO z ) is formed in contact with the semiconductor layers  110113  and  110114 , and silicon nitride oxide (SiN x O y ) (x&gt;y) is formed on the surface of the silicon oxide. Note that when the semiconductor layer is oxidized by plasma treatment, the plasma treatment is performed in an oxygen atmosphere (e.g., in an atmosphere of oxygen (O 2 ) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe), in an atmosphere of oxygen, hydrogen (H 2 ), and a rare gas, or in an atmosphere of dinitrogen monoxide and a rare gas). On the other hand, when the semiconductor layer is nitrided by plasma treatment, the plasma treatment is performed in a nitrogen atmosphere (e.g., in an atmosphere of nitrogen (N 2 ) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe), in an atmosphere of nitrogen, hydrogen, and a rare gas, or in an atmosphere of NH 3  and a rare gas). As a rare gas, Ar can be used, for example. Alternatively, a gas in which Ar and Kr are mixed may be used. Accordingly, the plasma-treated insulating film contains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. For example, the plasma-treated insulating film contains Ar when Ar is used. 
     Next, the insulating film  110116  is formed ( FIG. 51E ). The insulating film  110116  can have a single-layer structure or a stacked-layer structure of an insulating film containing oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ) (x&gt;y), or silicon nitride oxide (SiN x O y ) (x&gt;y), by sputtering, LPCVD, plasma CVD, or the like. Note that when the plasma-treated insulating film is formed on the surfaces of the semiconductor layers  110113  and  110114  by performing plasma treatment on the surfaces of the semiconductor layers  110113  and  110114 , the plasma-treated insulating film can be used as the insulating film  110116 . 
     Here, the surface of the insulating film  110116  may be oxidized or nitrided by plasma treatment, so that a plasma-treated insulating film is formed on the surface of the insulating film  110116 . Note that the plasma-treated insulating film contains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. The plasma treatment can be performed in a similar manner under the aforementioned conditions. 
     Alternatively, after the insulating film  110116  is oxidized by performing plasma treatment once in an oxygen atmosphere, the insulating film  110116  may be nitrided by performing plasma treatment again in a nitrogen atmosphere. By oxidizing or nitriding the surface of the insulating film  110116  by plasma treatment in such a manner, the surface of the insulating film  110116  is modified, and a dense film can be formed. An insulating film obtained by plasma treatment is denser and has fewer defects such as a pinhole, as compared with an insulating film formed by CVD or sputtering. Thus, characteristics of a thin film transistor can be improved. 
     Next, the gate electrode  110117  is formed ( FIG. 51F ). The gate electrode  110117  can be formed by a sputtering, LPCVD, plasma CVD, or the like. 
     In the transistor  110101 , the semiconductor layers  110115  used as the source region and the drain region can be formed by doping impurities after the gate electrode  110117  is formed. 
     In the transistor  110102 , the semiconductor layers  110114  used as the LDD regions and the semiconductor layers  110115  used as the source region and the drain region can be formed by doping impurities after the gate electrode  110117  is formed. 
     In the transistor  110103 , the semiconductor layers  110114  used as the LDD regions and the semiconductor layers  110115  used as the source region and the drain region can be formed by doping impurities after the gate electrode  110117  is formed. 
     In the transistor  110104 , the semiconductor layers  110114  used as the LDD regions and the semiconductor layers  110115  used as the source region and the drain region can be formed by doping impurities after the sidewall  110121  is formed on the side surface of the gate electrode  110117 . 
     Note that silicon oxide (SiO x ) or silicon nitride (SiN x ) can be used for the sidewall  110121 . As a method for forming the sidewall  110121  on the side surface of the gate electrode  110117 , a method can be used, for example, in which a silicon oxide (SiO x ) film or a silicon nitride (SiN x ) film is formed by a known method after the gate electrode  110117  is formed, and then, the silicon oxide (SiO x ) film or the silicon nitride (SiN x ) film is etched by anisotropic etching. Thus, the silicon oxide (SiO x ) film or the silicon nitride (SiN x ) film remains only on the side surface of the gate electrode  110117 , so that the sidewall  110121  can be formed on the side surface of the gate electrode  110117 . 
     In the transistor  110105 , the semiconductor layers  110114  used as the LDD (L off ) regions and the semiconductor layer  110115  used as the source region and the drain region can be formed by doping impurities after a mask  110122  is formed to cover the gate electrode  110117 . 
     In the transistor  110106 , the semiconductor layers  110114  used as the LDD (L ov ) regions and the semiconductor layers  110115  used as the source region and the drain region can be formed by doping impurities after the gate electrode  110117  is formed. 
     Next, the insulating film  110118  is formed ( FIG. 51G ). The insulating film  110118  can have a single-layer structure or a stacked-layer structure of an insulating film containing oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ) (x&gt;y), or silicon nitride oxide (SiN x O y ) (x&gt;y); or a film containing carbon, such as a DLC (diamond-like carbon), by sputtering, plasma CVD, or the like. 
     Here, the surface of the insulating film  110118  may be oxidized or nitrided by plasma treatment, so that a plasma-treated insulating film is formed on the surface of the insulating film  110118 . Note that the plasma-treated insulating film contains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. The plasma treatment can be performed in a similar manner under the aforementioned conditions. 
     Next, the insulating film  110119  is formed. The insulating film  110119  can have a single-layer structure or a stacked-layer structure of an organic material such as epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic; or a siloxane resin, in addition to an insulating film containing oxygen or nitrogen, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ) (x&gt;y), or silicon nitride oxide (SiN x O y ) (x&gt;y); or a film containing carbon, such as a DLC (diamond-like carbon), by sputtering, plasma CVD, or the like. Note that a siloxane resin corresponds to a resin having Si—O—Si bonds. Siloxane includes a skeleton structure of a bond of silicon (Si) and oxygen (O). As a substituent, an organic group containing at least hydrogen (such as an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group, or a fluoro group and an organic group containing at least hydrogen can be used as a substituent. Note that the plasma-treated insulating film contains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. For example, the plasma-treated insulating film contains Ar when Ar is used. 
     When an organic material such as polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic, a siloxane resin, or the like is used for the insulating film  110119 , the surface of the insulating film  110119  can be modified by oxidizing or nitriding the surface of the insulating film by plasma treatment. Modification of the surface improves strength of the insulating film  110119 , and physical damage such as a crack generated when an opening is formed, for example, or film reduction in etching can be reduced. When the conductive film  110123  is formed over the insulating film  110119 , modification of the surface of the insulating film  110119  improves adhesion to the conductive film. For example, when a siloxane resin is used for the insulating film  110119  and nitrided by plasma treatment, a plasma-treated insulating film containing nitrogen or a rare gas is formed by nitriding a surface of the siloxane resin, and physical strength is improved. 
     Next, contact holes are formed in the insulating films  110119 ,  110118 , and  110116  in order to form the conductive film  110123  which is electrically connected to the semiconductor layer  110115 . Note that the contact holes may have a tapered shape. Thus, coverage with the conductive film  110123  can be improved. 
       FIG. 55  shows cross-sectional structures of a bottom-gate transistor and a capacitor. 
     A first insulating film (an insulating film  110502 ) is formed over the entire surface of a substrate  110501 . The first insulating film can prevent impurities from the substrate from adversely affecting a semiconductor layer and changing properties of a transistor. That is, the first insulating film functions as a base film. Thus, a transistor with high reliability can be formed. As the first insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     A first conductive layer (conductive layers  110503 A and  110503 B) is formed over the first insulating film. The conductive layer  110503 A includes a portion functioning as a gate electrode of a transistor  110520 . The conductive layer  110503 B includes a portion functioning as a first electrode of a capacitor  110521 . As the first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     A second insulating film (an insulating film  110504 ) is formed so as to cover at least the first conductive layer. The second insulating film functions as a gate insulating film. As the second insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that for a portion of the second insulating film, which is in contact with the semiconductor layer, a silicon oxide film is preferably used. This is because the trap level at the interface between the semiconductor layer and the second insulating film is lowered. 
     When the second insulating film is in contact with Mo, a silicon oxide film is preferably used for a portion of the second insulating film in contact with Mo. This is because the silicon oxide film does not oxidize Mo. 
     A semiconductor layer is formed in part of a portion over the second insulating film, which overlaps with the first conductive layer, by photolithography, an inkjet method, a printing method, or the like. Part of the semiconductor layer extends to a portion over the second insulating film, which does not overlap with the first conductive layer. The semiconductor layer includes a channel formation region (a channel formation region  110510 ), an LDD region (LDD regions  110508  and  110509 ), and an impurity region (impurity regions  110505 ,  110506 , and  110507 ). The channel formation region  110510  functions as a channel formation region of the transistor  110520 . The LDD regions  110508  and  110509  function as LDD regions of the transistor  110520 . Note that the LDD regions  110508  and  110509  are not necessarily formed. The impurity region  110505  includes a portion functioning as one of a source electrode and a drain electrode of the transistor  110520 . The impurity region  110506  includes a portion functioning as the other of the source electrode and the drain electrode of the transistor  110520 . The impurity region  110507  includes a portion functioning as a second electrode of the capacitor  110521 . 
     A third insulating film (an insulating film  110511 ) is formed over the entire surface. A contact hole is selectively formed in part of the third insulating film. The insulating film  110511  functions as an interlayer film. As the third insulating film, an inorganic material (e.g., silicon oxide, silicon nitride, or silicon oxynitride), an organic compound material having a low dielectric constant (e.g., a photosensitive or nonphotosensitive organic resin material), or the like can be used. Alternatively, a material containing siloxane may be used. Note that siloxane is a material in which a skeleton structure is formed by a bond of silicon (Si) and oxygen (O). As a substitute, an organic group containing at least hydrogen (such as an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group, or a fluoro group and an organic group containing at least hydrogen may be used as a substituent. 
     A second conductive layer (conductive layers  110512  and  110513 ) is formed over the third insulating film. The conductive layer  110512  is connected to the other of the source electrode and the drain electrode of the transistor  110520  through the contact hole formed in the third insulating film. Thus, the conductive layer  110512  includes a portion functioning as the other of the source electrode and the drain electrode of the transistor  110520 . The conductive layer  110513  includes a portion functioning as the first electrode of the capacitor  110521 . As the second conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     Note that in steps after forming the second conductive layer, various insulating films or various conductive films may be formed. 
     Next, structures of a transistor and a capacitor are described in the case where an amorphous silicon (a-Si:H) film is used as a semiconductor layer of the transistor. 
       FIG. 52  shows cross-sectional structures of a top-gate transistor and a capacitor. 
     A first insulating film (an insulating film  110202 ) is formed over the entire surface of a substrate  110201 . The first insulating film can prevent impurities from the substrate from adversely affecting a semiconductor layer and changing properties of a transistor. That is, the first insulating film functions as a base film. Thus, a transistor with high reliability can be formed. As the first insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that the first insulating film is not necessarily formed. When the first insulating film is not formed, reduction in the number of steps and reduction in manufacturing cost can be realized. Further, since the structure can be simplified, yield can be improved. 
     A first conductive layer (conductive layers  110203 ,  110204 , and  110205 ) is formed over the first insulating film. The conductive layer  110203  includes a portion functioning as one of a source electrode and a drain electrode of a transistor  110220 . The conductive layer  110204  includes a portion functioning as the other of the source electrode and the drain electrode of the transistor  110220 . The conductive layer  110205  includes a portion functioning as a first electrode of a capacitor  110221 . As the first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     A first semiconductor layer (semiconductor layers  110206  and  110207 ) is formed above the conductive layers  110203  and  110204 . The semiconductor layer  110206  includes a portion functioning as one of the source electrode and the drain electrode. The semiconductor layer  110207  includes a portion functioning as the other of the source electrode and the drain electrode. As the first semiconductor layer, silicon containing phosphorus or the like can be used, for example. 
     A second semiconductor layer (a semiconductor layer  110208 ) is formed over the first insulating film and between the conductive layer  110203  and the conductive layer  110204 . Part of the semiconductor layer  110208  extends over the conductive layers  110203  and  110204 . The semiconductor layer  110208  includes a portion functioning as a channel formation region of the transistor  110220 . As the second semiconductor layer, a semiconductor layer having no crystallinity such as an amorphous silicon (a-Si:H) layer, a semiconductor layer such as a microcrystalline semiconductor (μ-Si:H) layer, or the like can be used. 
     A second insulating film (insulating films  110209  and  110210 ) is formed so as to cover at least the semiconductor layer  110208  and the conductive layer  110205 . The second insulating film functions as a gate insulating film. As the second insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that for a portion of the second insulating film, which is in contact with the second semiconductor layer, a silicon oxide film is preferably used. This is because the trap level at the interface between the second semiconductor layer and the second insulating film is lowered. 
     When the second insulating film is in contact with Mo, a silicon oxide film is preferably used for a portion of the second insulating film in contact with Mo. This is because the silicon oxide film does not oxidize Mo. 
     A second conductive layer (conductive layers  110211  and  110212 ) is formed over the second insulating film. The conductive layer  110211  includes a portion functioning as a gate electrode of the transistor  110220 . The conductive layer  110212  functions as a second electrode of the capacitor  110221  or a wiring. As the second conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     Note that in steps after forming the second conductive layer, various insulating films or various conductive films may be formed. 
       FIG. 53  shows cross-sectional structures of an inversely staggered (bottom gate) transistor and a capacitor. In particular, the transistor shown in  FIG. 53  has a channel-etched structure. 
     A first insulating film (an insulating film  110302 ) is formed over the entire surface of a substrate  110301 . The first insulating film can prevent impurities from the substrate from adversely affecting a semiconductor layer and changing properties of a transistor. That is, the first insulating film functions as a base film. Thus, a transistor with high reliability can be formed. As the first insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that the first insulating film is not necessarily formed. When the first insulating film is not formed, reduction in the number of steps and reduction in manufacturing cost can be realized. Further, since the structure can be simplified, yield can be improved. 
     A first conductive layer (conductive layers  110303  and  110304 ) is formed over the first insulating film. The conductive layer  110303  includes a portion functioning as a gate electrode of a transistor  110320 . The conductive layer  110304  includes a portion functioning as a first electrode of a capacitor  110321 . As the first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     A second insulating film (an insulating film  110305 ) is formed so as to cover at least the first conductive layer. The second insulating film functions as a gate insulating film. As the second insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that for a portion of the second insulating film, which is in contact with the semiconductor layer, a silicon oxide film is preferably used. This is because the trap level at the interface between the semiconductor layer and the second insulating film is lowered. 
     When the second insulating film is in contact with Mo, a silicon oxide film is preferably used for a portion of the second insulating film in contact with Mo. This is because the silicon oxide film does not oxidize Mo. 
     A first semiconductor layer (a semiconductor layer  110306 ) is formed in part of a portion over the second insulating film, which overlaps with the first conductive layer, by photolithography, an inkjet method, a printing method, or the like. Part of the semiconductor layer  110306  extends to a portion over the second insulating film, which does not overlap with the first conductive layer. The semiconductor layer  110306  includes a portion functioning as a channel formation region of the transistor  110320 . As the semiconductor layer  110306 , a semiconductor layer having no crystallinity such as an amorphous silicon (a-Si:H) layer, a semiconductor layer such as a microcrystalline semiconductor (μ-Si:H) layer, or the like can be used. 
     A second semiconductor layer (semiconductor layers  110307  and  110308 ) is formed over part of the first semiconductor layer. The semiconductor layer  110307  includes a portion functioning as one of a source electrode and a drain electrode. The semiconductor layer  110308  includes a portion functioning as the other of the source electrode and the drain electrode. As the second semiconductor layer, silicon containing phosphorus or the like can be used, for example. 
     A second conductive layer (conductive layers  110309 ,  110310 , and  110311 ) is formed over the second semiconductor layer and the second insulating film. The conductive layer  110309  includes a portion functioning as one of the source electrode and the drain electrode of the transistor  110320 . The conductive layer  110310  includes a portion functioning as the other of the source electrode and the drain electrode of the transistor  110320 . The conductive layer  110311  includes a portion functioning as a second electrode of the capacitor  110321 . As the second conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     Note that in steps after forming the second conductive layer, various insulating films or various conductive films may be formed. 
     Here, an example of a step which is characteristic of the channel-etched type transistor is described. The first semiconductor layer and the second semiconductor layer can be formed using the same mask. Specifically, the first semiconductor layer and the second semiconductor layer are continuously formed. Further, the first semiconductor layer and the second semiconductor layer are formed using the same mask. 
     Another example of a step which is characteristic of the channel-etched type transistor is described. The channel region of the transistor can be formed without using an additional mask. Specifically, after the second conductive layer is formed, part of the second semiconductor layer is removed using the second conductive layer as a mask. Alternatively, part of the second semiconductor layer is removed by using the same mask as the second conductive layer. The first semiconductor layer below the removed second semiconductor layer serves as the channel formation region of the transistor. 
       FIG. 54  shows cross-sectional structures of an inversely staggered (bottom gate) transistor and a capacitor. In particular, the transistor shown in  FIG. 54  has a channel protection (channel stop) structure. 
     A first insulating film (an insulating film  110402 ) is formed over the entire surface of a substrate  110401 . The first insulating film can prevent impurities from the substrate from adversely affecting a semiconductor layer and changing properties of a transistor. That is, the first insulating film functions as a base film. Thus, a transistor with high reliability can be formed. As the first insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that the first insulating film is not necessarily formed. When the first insulating film is not formed, reduction in the number of steps and reduction in manufacturing cost can be realized. Further, since the structure can be simplified, yield can be improved. 
     A first conductive layer (conductive layers  110403  and  110404 ) is formed over the first insulating film. The conductive layer  110403  includes a portion functioning as a gate electrode of a transistor  110420 . The conductive layer  110404  includes a portion functioning as a first electrode of a capacitor  110421 . As the first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     A second insulating film (an insulating film  110405 ) is formed so as to cover at least the first conductive layer. The second insulating film functions as a gate insulating film. As the second insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     Note that for a portion of the second insulating film, which is in contact with the semiconductor layer, a silicon oxide film is preferably used. This is because the trap level at the interface between the semiconductor layer and the second insulating film is lowered. 
     When the second insulating film is in contact with Mo, a silicon oxide film is preferably used for a portion of the second insulating film in contact with Mo. This is because the silicon oxide film does not oxidize Mo. 
     A first semiconductor layer (a semiconductor layer  110406 ) is formed in part of a portion over the second insulating film, which overlaps with the first conductive layer, by photolithography, an inkjet method, a printing method, or the like. Part of the semiconductor layer  110406  extends to a portion over the second insulating film, which does not overlap with the first conductive layer. The semiconductor layer  110406  includes a portion functioning as a channel formation region of the transistor  110420 . As the semiconductor layer  110406 , a semiconductor layer having no crystallinity such as an amorphous silicon (a-Si:H) layer, a semiconductor layer such as a microcrystalline semiconductor (μ-Si:H) layer, or the like can be used. 
     A third insulating film (an insulating film  110412 ) is formed over part of the first semiconductor layer. The insulating film  110412  prevents the channel region of the transistor  110420  from being removed by etching. That is, the insulating film  110412  functions as a channel protection film (a channel stop film). As the third insulating film, a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film (SiO x N y ), or the like can be used. 
     A second semiconductor layer (semiconductor layers  110407  and  110408 ) is formed over part of the first semiconductor layer and part of the third insulating film. The semiconductor layer  110407  includes a portion functioning as one of a source electrode and a drain electrode. The semiconductor layer  110408  includes a portion functioning as the other of the source electrode and the drain electrode. As the second semiconductor layer, silicon containing phosphorus or the like can be used, for example. 
     A second conductive layer (conductive layers  110409 ,  110410 , and  110411 ) is formed over the second semiconductor layer. The conductive layer  110409  includes a portion functioning as one of the source electrode and the drain electrode of the transistor  110420 . The conductive layer  110410  includes a portion functioning as the other of the source electrode and the drain electrode of the transistor  110420 . The conductive layer  110411  includes a portion functioning as a second electrode of the capacitor  110421 . As the second conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively, a stacked layer of these elements (including the alloy thereof) can be used. 
     Note that in steps after forming the second conductive layer, various insulating films or various conductive films may be formed. 
     Here, an example of a step which is characteristic of the channel protection type transistor is described. The first semiconductor layer, the second semiconductor layer, and the second conductive layer can be formed using the same mask. At the same time, the channel formation region can be formed. Specifically, the first semiconductor layer is formed, and then, the third insulating film (i.e., the channel protection film or the channel stop film) is patterned using a mask. Next, the second semiconductor layer and the second conductive layer are continuously formed. Then, after the second conductive layer is formed, the first semiconductor layer, the second semiconductor layer, and the second conductive film are patterned using the same mask. Note that part of the first semiconductor layer below the third insulating film is protected by the third insulating film, and thus is not removed by etching. This part (a part of the first semiconductor layer over which the third insulating film is formed) serves as the channel region. 
     Next, an example where a semiconductor substrate is used as a substrate for a transistor is described. Since a transistor formed using a semiconductor substrate has high mobility, the size of the transistor can be decreased. Accordingly, the number of transistors per unit area can be increased (the degree of integration can be improved), and the size of the substrate can be decreased as the degree of integration is increased in the case of the same circuit structure. Thus, manufacturing cost can be reduced. Further, since the circuit scale can be increased as the degree of integration is increased in the case of the same substrate size, more advanced functions can be provided without increase in manufacturing cost. Moreover, reduction in variations in characteristics can improve manufacturing yield. Reduction in operating voltage can reduce power consumption. High mobility can realize high-speed operation. 
     When a circuit which is formed by integrating transistors formed using a semiconductor substrate is mounted on a device in the form of an IC chip or the like, the device can be provided with a variety of functions. For example, when a peripheral driver circuit (e.g., a data driver (a source driver), a scan driver (a gate driver), a timing controller, an image processing circuit, an interface circuit, a power supply circuit, or an oscillation circuit) of a display device is formed by integrating transistors formed using a semiconductor substrate, a small peripheral circuit which can be operated with low power consumption and at high speed can be formed at low cost in high yield. Note that a circuit which is formed by integrating transistors formed using a semiconductor substrate may include a unipolar transistor. Thus, a manufacturing process can be simplified, so that manufacturing cost can be reduced. 
     A circuit which is formed by integrating transistors formed using a semiconductor substrate may also be used for a display panel, for example. More specifically, the circuit can be used for a reflective liquid crystal panel such as a liquid crystal on silicon (LCOS) device, a digital micromirror device (DMD) in which micromirrors are integrated, an EL panel, and the like. When such a display panel is formed using a semiconductor substrate, a small display panel which can be operated with low power consumption and at high speed can be formed at low cost in high yield. Note that the display panel may be formed over an element having a function other than a function of driving the display panel, such as a large-scale integration (LSI). 
     Hereinafter, a method for forming a transistor using a semiconductor substrate is described. 
     First, element isolation regions  110604  and  110606  (hereinafter, referred to as regions  110604  and  110606 ) are formed on a semiconductor substrate  110600  (see  FIG. 56A ). The regions  110604  and  110606  provided in the semiconductor substrate  110600  are isolated from each other by an insulating film  110602 . The example shown here is the case where a single crystal Si substrate having n-type conductivity is used as the semiconductor substrate  110600 , and a p-well  110607  is provided in the region  110606  of the semiconductor substrate  110600 . 
     Any substrate can be used as the substrate  110600  as long as it is a semiconductor substrate. For example, a single crystal Si substrate having n-type or p-type conductivity, a compound semiconductor substrate (e.g., a GaAs substrate, an InP substrate, a GaN substrate, a SiC substrate, a sapphire substrate, or a ZnSe substrate), an SOI (silicon on insulator) substrate formed by a bonding method or a SIMOX (separation by implanted oxygen) method, or the like can be used. 
     The regions  110604  and  110606  can be formed by a LOCOS (local oxidation of silicon) method, a trench isolation method, or the like as appropriate. 
     The p-well formed in the region  110606  of the semiconductor substrate  110600  can be formed by selective doping of the semiconductor substrate  110600  with a p-type impurity element. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. 
     Note that in this embodiment mode, although the region  110604  is not doped with an impurity element because a semiconductor substrate having n-type conductivity is used as the semiconductor substrate  110600 , an n-well may be formed in the region  110604  by introduction of an n-type impurity element. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. In contrast, when a semiconductor substrate having p-type conductivity is used, the region  110604  may be doped with an n-type impurity element to form an n-well, whereas the region  110606  may be doped with no impurity element. 
     Next, insulating films  110632  and  110634  are formed so as to cover the regions  110604  and  110606 , respectively (see  FIG. 56B ). 
     For example, surfaces of the regions  110604  and  110606  provided in the semiconductor substrate  110600  are oxidized by heat treatment, so that the insulating films  110632  and  110634  can be formed of silicon oxide films. Alternatively, the insulating films  110632  and  110634  may be formed to have a stacked-layer structure of a silicon oxide film and a film containing oxygen and nitrogen (a silicon oxynitride film) by forming a silicon oxide film by a thermal oxidation method and then nitriding the surface of the silicon oxide film by nitridation treatment. 
     Further alternatively, the insulating films  110632  and  110634  may be formed by plasma treatment as described above. For example, the insulating films  110632  and  110634  can be formed using a silicon oxide (SiO x ) film or a silicon nitride (SiN x ) film obtained by application of high-density plasma oxidation treatment or high-density plasma nitridation treatment to the surfaces of the regions  110604  and  110606  provided in the semiconductor substrate  110600 . As another example, after application of high-density plasma oxidation treatment to the surfaces of the regions  110604  and  110606 , high-density plasma nitridation treatment may be performed. In that case, silicon oxide films are formed on the surfaces of the regions  110604  and  110606 , and then silicon oxynitride films are formed on the silicon oxide films. Thus, each of the insulating films  110632  and  110634  is formed to have a stacked-layer structure of the silicon oxide film and the silicon oxynitride film. As another example, after silicon oxide films are formed on the surfaces of the regions  110604  and  110606  by a thermal oxidation method, high-density plasma oxidation treatment or high-density nitridation treatment may be applied to the silicon oxide films. 
     The insulating films  110632  and  110634  formed over the regions  110604  and  110606  of the semiconductor substrate  110600  function as the gate insulating films of transistors which are completed later. 
     Next, a conductive film is formed so as to cover the insulating films  110632  and  110634  which are formed over the regions  110604  and  110606 , respectively (see  FIG. 56C ). Here, an example is shown in which the conductive film is formed by sequentially stacking conductive films  110636  and  110638 . Needless to say, the conductive film may be formed using a single-layer structure or a stacked-layer structure of three or more layers. 
     As a material of the conductive films  110636  and  110638 , an element selected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and the like, or an alloy material or a compound material containing such an element as its main component can be used. Alternatively, a metal nitride film obtained by nitridation of the above element can be used. Further alternatively, a semiconductor material typified by polycrystalline silicon doped with an impurity element such as phosphorus or silicide in which a metal material is introduced can be used. 
     In this case, a stacked-layer structure is employed in which tantalum nitride is used for the conductive film  110636  and tungsten is used for the conductive film  110638 . Alternatively, it is also possible to form the conductive film  110636  using a single-layer film or a stacked-layer film of tungsten nitride, molybdenum nitride, and/or titanium nitride. For the conductive film  110638 , it is possible to use a single-layer film or a stacked-layer film of tantalum, molybdenum, and/or titanium. 
     Next, the stacked conductive films  110636  and  110638  are selectively removed by etching, so that the conductive films  110636  and  110638  remain above part of the regions  110604  and  110606 , respectively. Thus, gate electrodes  110640  and  110642  are formed (see  FIG. 57A ). 
     Next, a resist mask  110648  is selectively formed so as to cover the region  110604 , and the region  110606  is doped with an impurity element by using the resist mask  110648  and the gate electrode  110642  as masks; thus, impurity regions  110652  are formed (see  FIG. 57B ). As an impurity element, an n-type impurity element or a p-type impurity element is used. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. Here, phosphorus (P) is used as the impurity element. Note that after the impurity element is introduced, heat treatment may be performed in order to disperse the impurity element and to recover the crystalline structure. 
     In  FIG. 57B , by introduction of an impurity element, impurity regions  110652  which form source and drain regions and a channel formation region  110650  are formed in the region  110606 . 
     Next, a resist mask  110666  is selectively formed so as to cover the region  110606 , and the region  110604  is doped with an impurity element by using the resist mask  110666  and the gate electrode  110640  as masks; thus, impurity regions  110670  are formed (see  FIG. 57C ). As the impurity element, an n-type impurity element or a p-type impurity element is used. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. At this time, an impurity element (e.g., boron (B)) of a conductivity type different from that of the impurity element introduced into the region  110606  in  FIG. 57B  is used. As a result, the impurity regions  110670  which form source and drain regions and a channel formation region  110668  are formed in the region  110604 . Note that after the impurity element is introduced, heat treatment may be performed in order to disperse the impurity element and to recover the crystalline structure. 
     Next, a second insulating film  110672  is formed so as to cover the insulating films  110632  and  110634  and the gate electrodes  110640  and  110642 . Further, wirings  110674  which are electrically connected to the impurity regions  110652  and  110670  formed in the regions  110606  and  110604  respectively are formed (see  FIG. 57D ). 
     The second insulating film  110672  can be formed to have a single-layer structure or a stacked-layer structure of an insulating film containing oxygen and/or nitrogen such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ) (x&gt;y), or silicon nitride oxide (SiN x O y ) (x&gt;y); a film containing carbon such as DLC (diamond-like carbon); an organic material such as epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic; or a siloxane material such as a siloxane resin by CVD, sputtering, or the like. A siloxane material corresponds to a material having a bond of Si—O—Si. Siloxane has a skeleton structure with the bond of silicon (Si) and oxygen (O). As a substituent of siloxane, an organic group containing at least hydrogen (e.g., an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group, or both a fluoro group and an organic group containing at least hydrogen may be used as the substituent. 
     The wirings  110674  are formed with a single layer or a stacked layer of an element selected from aluminum (Al), tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), neodymium (Nd), carbon (C), and silicon (Si), or an alloy material or a compound material containing such an element as its main component by CVD, sputtering, or the like. An alloy material containing aluminum as its main component corresponds to, for example, a material which contains aluminum as its main component and also contains nickel, or a material which contains aluminum as its main component and also contains nickel and one or both of carbon and silicon. The wirings  110674  are preferably formed to have a stacked-layer structure of a barrier film, an aluminum-silicon (Al—Si) film, and a barrier film or a stacked-layer structure of a barrier film, an aluminum-silicon (Al—Si) film, a titanium nitride film, and a barrier film. Note that the barrier film corresponds to a thin film formed of titanium, titanium nitride, molybdenum, or molybdenum nitride. Aluminum and aluminum silicon are suitable materials for forming the wirings  110674  because they have high resistance values and are inexpensive. For example, when barrier layers are provided as the top layer and the bottom layer, generation of hillocks of aluminum or aluminum silicon can be prevented. For example, when a barrier film is formed of titanium which is an element having a high reducing property, even if a thin natural oxide film is formed on a crystalline semiconductor film, the natural oxide film can be reduced. As a result, the wirings  110674  can be connected to the crystalline semiconductor in an electrically and physically favorable condition. 
     Note that the structure of a transistor is not limited to that shown in the drawing. For example, a transistor with an inversely staggered structure, a FinFET structure, or the like can be used. A FinFET structure is preferable because it can suppress a short channel effect which occurs along with reduction in transistor size. 
     Next, another example in which a semiconductor substrate is used as a substrate for forming a transistor is described. 
     First, an insulating film is formed on a substrate  110800 . Here, a single crystal Si having n-type conductivity is used for the substrate  110800 , and insulating films  110802  and  110804  are formed on the substrate  110800  (see  FIG. 58A ). For example, silicon oxide (SiO x ) is formed for the insulating film  110802  by performing heat treatment on the substrate  110800 . Moreover, silicon nitride (SiN x ) is formed by CVD or the like. 
     Any substrate can be used as the substrate  110800  as long as it is a semiconductor substrate. For example, a single-crystal Si substrate having n-type or p-type conductivity, a compound semiconductor substrate (e.g., a GaAs substrate, an InP substrate, a GaN substrate, a SiC substrate, a sapphire substrate, or a ZnSe substrate), an SOI (silicon on insulator) substrate formed by a bonding method or a SIMOX (separation by implanted oxygen) method, or the like can be used. 
     The insulating film  110804  may be provided by forming the insulating film  110802  and then nitriding the insulating film  110802  by high-density plasma treatment. Note that the insulating film may have a single-layer structure or a stacked-layer structure of three or more layers. 
     Next, a pattern of a resist mask  110806  is selectively formed. Then, etching is selectively performed using the resist mask  110806  as a mask, whereby depressed portions  110808  are selectively formed in the substrate  110800  (see  FIG. 58B ). The substrate  110800  and the insulating films  110802  and  110804  can be etched by dry etching using plasma. 
     Next, after the pattern of the resist mask  110806  is removed, an insulating film  110810  is formed so as to fill the depressed portions  110808  formed in the substrate  110800  (see  FIG. 58C ). 
     The insulating film  110810  is formed using an insulating material such as silicon oxide, silicon nitride, silicon oxynitride (SiO x N y ) (x&gt;y&gt;0), or silicon nitride oxide (SiN z O y ) (x&gt;y&gt;0) by CVD, sputtering, or the like. Here, as the insulating film  110810 , a silicon oxide film is formed using a tetraethyl orthosilicate (TEOS) gas by atmospheric pressure CVD or low pressure CVD. 
     Next, a surface of the substrate  110800  is exposed when grinding treatment polishing treatment, or chemical mechanical polishing (CMP) treatment is performed. Then, the surface of the substrate  110800  is separated by insulating films  110810  formed in the depressed portions  110808  of the substrate  110800 . Here, the separated regions are referred to as regions  110812  and  110813  (see  FIG. 59A ). Note that the insulating films  110810  are obtained by partial removal of the insulating films  110810  by grinding treatment, polishing treatment, or CMP treatment. 
     Subsequently, the p-well can be formed in the region  110813  of the semiconductor substrate  110800  by selective introduction of an impurity element having p-type conductivity. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. Here, as the impurity element, boron (B) is introduced into the region  110813 . Note that after the impurity element is introduced, heat treatment may be performed in order to disperse the impurity element and to recover the crystalline structure. 
     Note that although an impurity element is not necessarily introduced into the region  110812  when a semiconductor substrate having n-type conductivity is used as the semiconductor substrate  110800 , an n-well may be formed in the region  110812  by introduction of an n-type impurity element. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. 
     Meanwhile, when a semiconductor substrate having p-type conductivity is used, the region  110812  may be doped with an n-type impurity element to form an n-well, whereas the region  110813  may be doped with no impurity element. 
     Next, insulating films  110832  and  110834  are formed, respectively, on the surfaces of the regions  110812  and  110813  of the substrate  110800  (see  FIG. 59B ). 
     For example, the surfaces of the regions  110812  and  110813  provided in the semiconductor substrate  110800  are oxidized by heat treatment, so that the insulating films  110832  and  110834  can be formed of silicon oxide films. Alternatively, the insulating films  110832  and  110834  may be formed to have a stacked-layer structure of a silicon oxide film and a film containing oxygen and nitrogen (a silicon oxynitride film) by the forming a silicon oxide film by a thermal oxidation method and then nitriding the surface of the silicon oxide film by nitridation treatment. 
     Further alternatively, the insulating films  110832  and  110834  may be formed by plasma treatment as described above. For example, the insulating films  110832  and  110834  can be formed using a silicon oxide (SiO x ) film or a silicon nitride (SiN x ) film obtained by application of high-density plasma oxidation treatment or high-density plasma nitridation treatment to the surfaces of the regions  110812  and  110813  provided in the substrate  110800 . As another example, after application of high-density plasma oxidation treatment to the surfaces of the regions  110812  and  110813 , high-density plasma nitridation treatment may be performed. In that case, silicon oxide films are formed on the surfaces of the regions  110812  and  110813 , and then silicon oxynitride films are formed on the silicon oxide films. Thus, each of the insulating films  110832  and  110834  is formed to have a stacked-layer structure of the silicon oxide film and the silicon oxynitride film. As another example, after silicon oxide films are formed on the surfaces of the regions  110812  and  110813  by a thermal oxidation method, high-density plasma oxidation treatment or high-density nitridation treatment may be applied to the silicon oxide films. 
     The insulating films  110832  and  110834  formed over the regions  110812  and  110813  of the semiconductor substrate  110800  function as the gate insulating films of transistors which are completed later. 
     Next, a conductive film is formed so as to cover the insulating films  110832  and  110834  which are formed over the regions  110812  and  110813 , respectively, provided in the substrate  110800  (see  FIG. 59C ). Here, an example is shown in which the conductive film is formed by sequentially stacking conductive films  110836  and  110838 . It is needless to say that the conductive film may be formed using a single-layer structure or a stacked-layer structure of three or more layers. 
     For the conductive films  110836  and  110838 , an element selected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and the like, or an alloy material or a compound material containing such an element as its main component can be used. Alternatively, a metal nitride film obtained by nitridation of the above element can be used. Further alternatively, a semiconductor material typified by polycrystalline silicon doped with an impurity element such as phosphorus or silicide in which a metal material is introduced can be used. 
     In this case, a stacked-layer structure is employed in which tantalum nitride is used for the conductive film  110836  and tungsten is used for the conductive film  110838 . Alternatively, it is also possible to form the conductive film  110836  using a single-layer film or a stacked-layer film of tantalum nitride, tungsten nitride, molybdenum nitride, and/or titanium nitride. For the conductive film  110838 , it is possible to use a single-layer film or a stacked-layer film of tungsten, tantalum, molybdenum, and/or titanium. 
     Next, the stacked conductive films  110836  and  110838  are selectively removed by etching, so that the conductive films  110836  and  110838  remain above part of the regions  110812  and  110813  of the substrate  110800 , respectively. Thus, conductive films  110840  and  110842  functioning as gate electrodes are formed (see  FIG. 59D ). Here, the surface of the substrate  110800  is made to be exposed in the region which does not overlap with the conductive films  110840  and  110842 . 
     Specifically, in the region  110812  of the substrate  110800 , a portion of the insulating film  110832  which does not overlap with the conductive film  110840  is selectively removed, and an end portion of the conductive film  110840  and an end portion of the insulating film  110832  are made to roughly match. Further, in the region  110813  of the substrate  110800 , part of the insulating film  110834  which does not overlap with the conductive film  110842  is selectively removed, and an end portion of the conductive film  110842  and an end portion of the insulating film  110834  are made to roughly match. 
     In this case, insulating films and the like of the portions which do not overlap with the conductive films  110840  and  110842  may be removed at the same time as formation of the conductive films  110840  and  110842 . Alternatively, the insulating films and the like of the portions which do not overlap may be removed using the resist mask, which is left after the conductive films  110840  and  110842  are formed, or the conductive films  110840  and  110842  as masks. 
     Next, an impurity element is selectively introduced into the regions  110812  and  110813  of the substrate  110800  (see  FIG. 30A ). Here, an n-type impurity element having a low concentration is selectively introduced into the region  110813  at a low concentration by using the conductive film  110842  as a mask. On the other hand, a p-type impurity element is selectively introduced into the region  110812  at a low concentration by using the conductive film  110840  as a mask. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. Note that after the impurity element is introduced, heat treatment may be performed in order to disperse the impurity element and to recover the crystalline structure. 
     Next, sidewalls  110854  which are in contact with side surfaces of the conductive films  110840  and  110842  are formed. Specifically, the sidewalls are formed to have a single-layer structure or a stacked-layer structure of a film containing an inorganic material such as silicon, oxide of silicon, or nitride of silicon, or a film containing an organic material such as an organic resin by plasma CVD, sputtering, or the like. Then, the insulating films are selectively etched by anisotropic etching mainly in a perpendicular direction, so that the sidewalls are formed in contact with the side surfaces of the conductive films  110840  and  110842 . Note that the sidewalls  110854  are used as masks for doping in forming LDD (lightly doped drain) regions. Here, the sidewalls  110854  are formed to be also in contact with side surfaces of the insulating films or floating gate electrodes formed under the conductive films  110840  and  110842 . 
     Subsequently, an impurity element is introduced into the regions  110812  and  110813  of the substrate  110800 , using the sidewalls  110854  and the conductive films  110840  and  110842  as masks; thus, impurity regions functioning as source and drain regions are formed (see  FIG. 60B ). Here, an n-type impurity element is introduced into the region  110813  of the substrate  110800  at a high concentration by using the sidewalls  110854  and the conductive film  110842  as masks, and a p-type impurity element is introduced into the region  110812  at a high concentration by using the sidewalls  110854  and the conductive film  110840  as masks. 
     As a result, in the region  110812  of the substrate  110800 , an impurity region  110858  forming a source or drain region, a low-concentration impurity region  110860  forming an LDD region, and a channel formation region  110856  are formed. Moreover, in the region  110813  of the substrate  110800 , an impurity region  110864  forming a source or drain region, a low-concentration impurity region  110866  forming an LDD region, and a channel formation region  110862  are formed. 
     Note that although the example in which the LDD regions are formed using the sidewalls is described, the present invention is not limited to this. The LDD regions may be formed using a mask or the like without the use of the sidewalls, or is not necessarily formed. When the LDD regions are not formed, a manufacturing process can be simplified, so that manufacturing cost can be reduced. 
     Note that in this embodiment mode, impurity elements are introduced in a state where the surface of the substrate  110800  is exposed in the region which does not overlap with the conductive films  110840  and  110842 . Accordingly, the channel formation regions  110856  and  110862  formed in the regions  110812  and  110813  respectively of the substrate  110800  can be formed in a self-aligned manner with the conductive films  110840  and  110842 , respectively. 
     Next, a second insulating film  110877  is formed so as to cover the insulating films, conductive films, and the like provided over the regions  110812  and  110813  of the substrate  110800 , and openings  110878  are formed in the insulating film  110877  (see  FIG. 60C ). 
     The second insulating film  110877  can be formed to have a single-layer structure or a stacked-layer structure of an insulating film containing oxygen and/or nitrogen such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ) (x&gt;y), or silicon nitride oxide (SiN x O y ) (x&gt;y); a film containing carbon such as diamond-like carbon (DLC); an organic material such as epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic; or a siloxane material such as a siloxane resin by CVD, sputtering, or the like. A siloxane material corresponds to a material having a bond of Si—O—Si. Siloxane has a skeleton structure with the bond of silicon (Si) and oxygen (O). As a substituent of siloxane, an organic group containing at least hydrogen (for example, an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group, or both a fluoro group and an organic group containing at least hydrogen may be used as the substituent. 
     Next, a conductive film  110880  is formed in each of the openings  110878  by CVD, and conductive films  110882   a  to  110882   d  are selectively formed over the insulating film  110877  so as to be electrically connected to the conductive films  110880  (see  FIG. 60D ). 
     The conductive films  110880  and  110882   a  to  110882   d  are formed to have a single-layer structure or a stacked-layer structure of an element selected from aluminum (Al), tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), neodymium (Nd), carbon (C), and silicon (Si), or an alloy material or a compound material containing such an element as its main component by CVD, sputtering, or the like. An alloy material containing aluminum as its main component corresponds to, for example, a material which contains aluminum as its main component and also contains nickel, or a material which contains aluminum as its main component and also contains nickel and one or both of carbon and silicon. The conductive films  110880  and  110882   a  to  110882   d  are preferably formed to have a stacked-layer structure of a barrier film, an aluminum-silicon (Al—Si) film, and a barrier film or a stacked structure of a barrier film, an aluminum-silicon (Al—Si) film, a titanium nitride film, and a barrier film. Note that the barrier film corresponds to a thin film formed of titanium, titanium nitride, molybdenum, or molybdenum nitride. Aluminum and aluminum silicon are suitable materials for forming the conductive film  110880  because they have high resistance values and are inexpensive. For example, when barrier layers are provided as the top layer and the bottom layer, generation of hillocks of aluminum or aluminum silicon can be prevented. For example, when a barrier film is formed of titanium which is an element having a high reducing property, even if a thin natural oxide film is formed on the crystalline semiconductor film, the natural oxide film can be reduced, and a favorable contact between the conductive film and the crystalline semiconductor film can be obtained. Here, the conductive films  110880  can be formed by selective growth of tungsten (W) by CVD. 
     By the steps described above, a p-channel transistor formed in the region  110812  of the substrate  110800  and an n-channel transistor formed in the region  110813  of the substrate  1300  can be obtained. 
     Note that the structure of a transistor of the present invention is not limited to that shown in the drawing. For example, a transistor with an inversely staggered structure, a FinFET structure, or the like can be used. A FinFET structure is preferable because it can suppress a short channel effect which occurs along with reduction in transistor size. 
     Heretofore, the structures and the manufacturing methods of transistors have been described. In this embodiment mode, a wiring, an electrode, a conductive layer, a conductive film, a terminal, a via, a plug, and the like are preferably formed of one or more elements selected from aluminum (Al), tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd), chromium (Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag), copper (Cu), magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn), niobium (Nb), silicon (Si), phosphorus (P), boron (B), arsenic (As), gallium (Ga), indium (In), tin (Sn), and oxygen (O); or a compound or an alloy material including one or more of the aforementioned elements (e.g., indium tin oxide (ITO), indium zinc oxide (IZO), indium tin oxide containing silicon oxide (ITSO), zinc oxide (ZnO), tin oxide (SnO), cadmium tin oxide (CTO), aluminum neodymium (Al—Nd), magnesium silver (Mg—Ag), or molybdenum-niobium (Mo—Nb)); a substance in which these compounds are combined; or the like. Alternatively, they are preferably formed to contain a substance including a compound (silicide) of silicon and one or more of the aforementioned elements (e.g., aluminum silicon, molybdenum silicon, or nickel silicide); or a compound of nitrogen and one or more of the aforementioned elements (e.g., titanium nitride, tantalum nitride, or molybdenum nitride). 
     Note that silicon (Si) may contain an n-type impurity (such as phosphorus) or a p-type impurity (such as boron). When silicon contains the impurity, the conductivity is increased, and a function similar to a general conductor can be realized. Accordingly, such silicon can be utilized easily as a wiring, an electrode, or the like. 
     In addition, silicon having a variety of crystallinity, such as single-crystal silicon, polycrystalline silicon, or microcrystalline silicon can be used. Alternatively, silicon having no crystallinity, such as amorphous silicon can be used. When single-crystal silicon or polycrystalline silicon is used, resistance of a wiring, an electrode, a conductive layer, a conductive film, a terminal, or the like can be reduced. When amorphous silicon or microcrystalline silicon is used, a wiring or the like can be formed by a simple process. 
     Aluminum and silver have high conductivity, and thus can reduce signal delay. Moreover, since aluminum and silver can be easily etched, they are easily patterned and can be minutely processed. 
     Copper has high conductivity, and thus can reduce signal delay. When copper is used, a stacked-layer structure is preferably employed to improve adhesion. 
     Molybdenum and titanium are preferable because even if molybdenum or titanium is in contact with an oxide semiconductor (e.g., ITO or IZO) or silicon, molybdenum or titanium does not cause defects. Moreover, molybdenum and titanium are preferable because they are easily etched and has high heat resistance. 
     Tungsten is preferable because it has advantages such as high heat resistance. 
     Neodymium is preferable because it has advantages such as high heat resistance. In particular, an alloy of neodymium and aluminum is preferable because heat resistance is increased and aluminum dose not easily cause hillocks. 
     Silicon is preferable because it can be formed at the same time as a semiconductor layer included in a transistor and has high heat resistance. 
     Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO), and cadmium tin oxide (CTO) have light-transmitting properties, they can be used for a portion which transmits light. For example, they can be used for a pixel electrode or a common electrode. 
     IZO is preferable because it is easily etched and processed. In etching IZO, a residue is hardly left. Accordingly, when IZO is used for a pixel electrode, defects (such as short circuit or orientation disorder) of a liquid crystal element or a light-emitting element can be reduced. 
     A wiring, an electrode, a conductive layer, a conductive film, a terminal, a via, a plug, or the like may have a single-layer structure or a multi-layer structure. By employing a single-layer structure, each manufacturing process of a wiring, an electrode, a conductive layer, a conductive film, a terminal, or the like can be simplified, the number of days for a process can be reduced, and cost can be reduced. Alternatively, by employing a multi-layer structure, a wiring, an electrode, and the like with high quality can be formed while an advantage of each material is utilized and a disadvantage thereof is reduced. For example, when a low-resistant material (e.g., aluminum) is included in a multi-layer structure, reduction in resistance of a wiring can be realized. As another example, when a stacked-layer structure in which a low heat-resistant material is interposed between high heat-resistant materials is employed, heat resistance of a wiring, an electrode, and the like can be increased, utilizing advantages of the low heat-resistance material. For example, it is preferable to employ a stacked-layer structure in which a layer containing aluminum is interposed between layers containing molybdenum, titanium, neodymium, or the like. 
     When wirings, electrodes, or the like are in direct contact with each other, they adversely affect each other in some cases. For example, one wiring or one electrode is mixed into a material of another wiring or another electrode and changes its properties, and thus, an intended function cannot be obtained in some cases. As another example, when a high-resistant portion is formed, a problem may occur so that it cannot be normally formed. In such cases, a reactive material is preferably interposed by or covered with a non-reactive material in a stacked-layer structure. For example, when ITO and aluminum are connected, titanium, molybdenum, or an alloy of neodymium is preferably interposed between ITO and aluminum. As another example, when silicon and aluminum are connected, titanium, molybdenum, or an alloy of neodymium is preferably interposed between silicon and aluminum. 
     Note that a wiring refers to a portion including a conductor. A wiring may extend linearly or be made to be short without extension. Therefore, an electrode is included in a wiring. 
     Note that a carbon nanotube may be used for a wiring, an electrode, a conductive layer, a conductive film, a terminal, a via, a plug, or the like. Since a carbon nanotube has a light-transmitting property, it can be used for a portion which transmits light. For example, a carbon nanotube can be used for a pixel electrode or a common electrode. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 4 
     In this embodiment mode, a structure of a display device is described. 
     A structure of a display device is described with reference to  FIG. 61A .  FIG. 61A  is a top view of the display device. 
     A pixel portion  170101 , a scan line input terminal  170103 , and a signal line input terminal  170104  are formed over a substrate  170100 . Scan lines extending in a row direction from the scan line input terminal  170103  are formed over the substrate  170100 , and signal lines extending in a column direction from the signal line input terminal  170104  are formed over the substrate  170100 . Pixels  170102  are arranged in matrix at each intersection of the scan lines and the signal lines in the pixel portion  170101 . 
     The scan line side input terminal  170103  is formed on both sides of the row direction of the substrate  170100 . Further, a scan line extending from one scan line side input terminal  170103  and a scan line extending from the other scan line side input terminal  170103  are alternately formed. In this case, since the pixels  170102  can be arranged with high density, a high-definition display device can be obtained. Note that the present invention is not limited to this, and the scan line side input terminal  170103  may be formed only on one side of the row direction of the substrate  170100 . In this case, a frame of the display device can be made smaller. Moreover, the area of the pixel portion  170101  can be increased. As another example, the scan line extending from one scan line side input terminal  170103  and the scan line extending from the other scan line side input terminal  170103  may be used in common. In this case, the structure is suitable for display devices in which a load on a scan line is large, such as large-scale display devices. Note that signals are input from an external driver circuit to the scan line through the scan line side input terminal  170103 . 
     The signal line side input terminal  170104  is formed on one side of the column direction of the substrate  170100 . In this case, the frame of the display device can be made smaller. Moreover, the area of the pixel portion  170101  can be increased. Note that the present invention is not limited to this, and the signal line side input terminal  170104  may be formed on both sides of the column direction of the substrate  170100 . In this case, the pixels  170102  are arranged with high density. Note that signals are input from an external driver circuit to the scan line through the signal line side input terminal  170104 . 
     The pixel  170102  includes a switching element and a pixel electrode. In each pixel  170102 , a first terminal of the switching element is connected to the signal line, and a second terminal of the switching element is connected to the pixel electrode. On/off of the switching element is controlled by the scan line. Note that the present invention is not limited to this structure, and a variety of structures can be employed. For example, the pixel  170102  may include a capacitor. In this case, a capacitor line is preferably formed over the substrate  170100 . As another example, the pixel  170102  may include a current source such as a driving transistor. In this case, a power supply line is preferably formed over the substrate  170100 . 
     As the substrate  170100 , a single-crystal substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), and a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester)), a leather substrate, a rubber substrate, a stainless steel substrate, a substrate including a stainless steel foil, or the like can be used. Alternatively, a skin (e.g., surfaces of the skin or corium) or hypodermal tissue of an animal such as a human can be used as the substrate. Note that the substrate  170100  is not limited to those described above, and a variety of substrates can be used. 
     As the switching element included in the pixel  170102 , a transistor (e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky diode, an MIM (metal insulator metal) diode, an MIS (metal insulator semiconductor) diode, or a diode-connected transistor), a thyristor, or the like can be used. Note that the switching element is not limited to those described above, and a variety of switching elements can be used. Note that when a MOS transistor is used as the switching element included in the pixel  170102 , a gate electrode is connected to the scan line, a first terminal is connected to the signal line, and a second terminal is connected to the pixel electrode. 
     Heretofore, the case in which a signal is input from an external driver circuit has been described. However, the present invention is not limited to this, and an IC chip can be mounted on a display device. 
     For example, as shown in  FIG. 62A , an IC chip  170111  can be mounted on the substrate  170100  by a COG (chip on glass) method. In this case, the IC chip  170111  can be examined before being mounted on the substrate  170100 , so that improvement in yield and reliability of the display device can be realized. Note that portions which are common to those in  FIG. 61A  are denoted by common reference numerals, and description thereof is omitted. 
     As another example, as shown in  FIG. 62B , an IC chip  170201  can be mounted on an FPC (flexible printed circuit)  170200  by a TAB (tape automated bonding) method. In this case, the IC chip  170111  can be examined before being mounted on the FPC  170200 , so that improvement in yield and reliability of the display device can be realized. Note that portions which are common to those in  FIG. 61A  are denoted by common reference numerals, and description thereof is omitted. 
     Not only the IC chip can be mounted on the substrate  170100 , but also a driver circuit can be formed over the substrate  170100 . 
     For example, as shown in  FIG. 61B , a scan line driver circuit  170105  can be formed over the substrate  170100 . In this case, cost can be reduced by reduction in number of components. Further, reliability can be improved by reduction in number of connection points between components. Since the driving frequency of the scan line driver circuit  170105  is low, the scan line driver circuit  170105  can be easily formed by using amorphous silicon or microcrystalline silicon as a semiconductor layer of a transistor. Note that an IC chip for outputting a signal to the signal line may be mounted on the substrate  170100  by a COG method. Alternatively, an FPC on which an IC chip for outputting a signal to the signal line is mounted by a TAB method may be provided on the substrate  170100 . In addition, an IC chip for controlling the scan line driver circuit  170105  may be mounted on the substrate  170100  by COG. Alternatively, an FPC on which an IC chip for controlling the scan line driver circuit  170105  is mounted by a TAB method may be provided on the substrate  170100 . Note that portions which are common to those in  FIG. 61A  are denoted by common reference numerals, and description thereof is omitted. 
     As another example, as shown in  FIG. 61C , the scan line driver circuit  170105  and a signal line driver circuit  170106  can be formed over the substrate  170100 . Thus, cost can be reduced by reduction in number of components. Further, reliability can be improved by reduction in number of connection points between components. Note that an IC chip for controlling the scan line driver circuit  170105  may be mounted on the substrate  170100  by COG Alternatively, an FPC on which an IC chip for controlling the scan line driver circuit  170105  is mounted by a TAB method may be provided on the substrate  170100 . In addition, an IC chip for controlling the signal line driver circuit  170106  may be mounted on the substrate  170100  by COG. Alternatively, an FPC on which an IC chip for controlling the signal line driver circuit  170106  is mounted by a TAB method may be provided on the substrate  170100 . Note that portions which are common to those in  FIG. 61A  are denoted by common reference numerals, and description thereof is omitted. 
     Next, another structure of a display device is described with reference to  FIG. 63 . Specifically, the display device includes a TFT substrate, a counter substrate, and a display layer interposed between the TFT substrate and the counter substrate.  FIG. 63  is a top view of the display device. 
     A pixel portion  170301 , a scan line driver circuit  170302   a , a scan line driver circuit  170302   b , and a signal line driver circuit  170303  are formed over a substrate  170300 . The scan line driver circuits  170302   a  and  170302   b  and the signal line driver circuit  170303  are sealed between the substrate  170300  and a substrate  170310  with a sealant  170321 . 
     Further, an FPC  107320  is arranged on the substrate  170300 . Moreover, an IC chip  107321  is mounted on the FPC  170320  by a TAB method. 
     A plurality of pixels are arranged in matrix in the pixel portion  170301 . A scan line extending in the column direction from the scan line driver circuit  170302   a  is formed over the substrate  170300 . A scan line extending in the row direction from the scan line driver circuit  170302   b  is formed over the substrate  170300 . A signal line extending in the column direction from the signal line driver circuit  170303  is formed over the substrate  170300 . 
     The scan line driver circuit  170302   a  is formed on one side of the row direction of the substrate  170300 . The scan line driver circuit  170302   b  is formed on the other side of the row direction of the substrate  170300 . Further, the scan line extending from the scan line driver circuit  170302   a  and the scan line extending from the scan line driver circuit  170302   b  are alternately formed. Accordingly, a high-definition display device can be obtained. Note that the present invention is not limited to this, and only one of the scan line driver circuits  170302   a  and  170302   b  may be formed over the substrate  170300 . In this case, the frame of the display device can be made smaller. Moreover, the area of the pixel portion  170301  can be increased. As another example, the scan line extending from the scan line driver circuit  170302   a  and the scan line extending from the scan line driver circuit  170302   b  may be used in common. In this case, the structure is suitable for display devices in which a load on a scan line is large, such as large-scale display devices. 
     The signal line driver circuit  170303  is formed on one side of the column direction of the substrate  170300 . Accordingly, the frame of the display device can be made smaller. Further, the area of the pixel portion  170301  can be increased. Note that the present invention is not limited to this, and the signal line driver circuit  170303  may be formed on both sides of the column direction of the substrate  170300 . In this case, a high-definition display device can be obtained. 
     As the substrate  170300 , a single-crystal substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), and a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester)), a leather substrate, a rubber substrate, a stainless steel substrate, a substrate including a stainless steel foil, or the like can be used. Alternatively, a skin (e.g., surfaces of the skin or corium) or hypodermal tissue of an animal such as a human can be used as the substrate. Note that the substrate  170300  is not limited to those described above, and a variety of substrates can be used. 
     As the switching element included in the display device, a transistor (e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky diode, an MIM (metal insulator metal) diode, an MIS (metal insulator semiconductor) diode, or a diode-connected transistor), a thyristor, or the like can be used. Note that the switching element is not limited to those described above, and a variety of switching elements can be used. 
     Heretofore, the case in which a driver circuit and a pixel portion are formed over the same substrate has been described. However, the present invention is not limited to this case, and another substrate over which the driver circuit is partially or entirely formed may be made to be an IC chip so that the substrate is mounted on the substrate over which the pixel portion is formed. 
     For example, as shown in  FIG. 64A , an IC chip  170401  instead of the signal line driver circuit can be mounted on the substrate  170300  by COG In this case, increase in power consumption can be prevented by mounting of the IC chip  170401  instead of the signal line driver circuit on the substrate  170300  by COG This is because the drive frequency of the signal line driver circuit is high and thus power consumption is increased. Since the IC chip  170401  can be examined before it is mounted on the substrate  170300 , yield of a display device can be improved. Moreover, reliability can be improved. Since the drive frequency of the scan line driver circuits  170302   a  and  170302   b  is low, the scan line driver circuits  170302   a  and  170302   b  can be easily formed using amorphous silicon or microcrystalline silicon for a semiconductor layer of a transistor. Accordingly, a display device can be formed using a large substrate. Note that portions which are common to those in the structure of  FIG. 63  are denoted by common reference numerals, and the description thereof is omitted. 
     As another example, as shown in  FIG. 64B , the IC chip  170401  instead of the signal line driver circuit may be mounted on the substrate  170300  by COG, an IC chip  170501   a  instead of the scan line driver circuit  170302   a  may be mounted on the substrate  170300  by COG, and an IC chip  170501   b  instead of the scan line driver circuit  170302   b  may be mounted on the substrate  170300  by COG In this case, since the IC chips  170401 ,  170501   a , and  170501   b  can be examined before they are mounted on the substrate  170300 , yield of a display device can be improved. Moreover, reliability can be improved. Amorphous silicon or microcrystalline silicon can be easily used for a semiconductor layer of a transistor to be formed over the substrate  170300 . Accordingly, a display device can be formed using a large substrate. Note that portions which are common to those in the structure of  FIG. 63  are denoted by common reference numerals, and the description thereof is omitted. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 5 
     In this embodiment mode, operations of a display device are described. 
       FIG. 65  shows a structural example of a display device. 
     A display device  180100  includes a pixel portion  180101 , a signal line driver circuit  180103 , and a scan line driver circuit  180104 . In the pixel portion  180101 , a plurality of signal lines S 1  to S n  extend from the signal line driver circuit  180103  in a column direction. In the pixel portion  180101 , a plurality of scan lines G 1  to G m  extend from the scan line driver circuit  180104  in a row direction. Pixels  180102  are arranged in matrix at each intersection of the plurality of signal lines S 1  to S n  and the plurality of scan lines G 1  to G m . 
     The signal line driver circuit  180103  has a function of outputting a signal to each of the signal lines S 1  to S n . This signal may be referred to as a video signal. The scan line driver circuit  180104  has a function of outputting a signal to each of the scan lines G 1  to G m . This signal may be referred to as a scan signal. 
     Note that the pixel  180102  includes at least a switching element connected to the signal line. On/off of the switching element is controlled by a potential of the scan line (a scan signal). When the switching element is on, the pixel  180102  is selected. On the other hand, when the switching element is off, the pixel  180102  is not selected. 
     When the pixel  180102  is selected (in a selection state), a video signal is input to the pixel  180102  from the signal line. The state (e.g., luminance, transmittivity, or voltage of a storage capacitor) of the pixel  180102  is changed in accordance with the input video signal. 
     When the pixel  180102  is not selected (in a non-selection state), the video signal is not input to the pixel  180102 . Note that since the pixel  180102  holds a potential corresponding to the video signal which is input when selected, the pixel  180102  maintains the state (e.g., luminance, transmittivity, or voltage of a storage capacitor) in accordance with the video signal. 
     Note that the structure of the display device is not limited to that shown in  FIG. 65 . For example, a wiring (e.g., a scan line, a signal line, a power supply line, a capacitor line, or a common line) may be added in accordance with the structure of the pixel  180102 . As another example, a circuit having various functions may be added. 
       FIG. 66  shows an example of a timing chart for describing operations of a display device. 
     The timing chart in  FIG. 66  shows one frame period corresponding to a period for displaying an image for one screen. Although one frame period is not particularly limited to a certain period, it is at least preferable that one frame period be 1/60 second or less so that a person viewing an image does not perceive flickers. 
     The timing chart in  FIG. 66  shows timing of selecting the scan line G 1  in a first row, the scan line G i  (one of the scan lines G 1  to G m ) in an i-th row, the scan line G i +1 in an (i+1)th row, and the scan line G m  in an m-th row. 
     At the same time as the scan line is selected, the pixel  180102  connected to the scan line is also selected. For example, when the scan line G i  in the i-th row is selected, the pixel  180102  connected to the scan line G i  in the i-th row is also selected. 
     The scan lines G 1  to G m  are sequentially selected (hereinafter also referred to as scanned) from the scan line G 1  in the first row to the scan line G m  in the m-th row. For example, while the scan line G i  in the i-th row is selected, the scan lines (G 1  to G i −1 and G i +1 to G m ) other than the scan line G i  in the i-th row are not selected. Then, during the next period, the scan line G i +1 in the (i+1)th row is selected. Note that a period during which one scan line is selected is referred to as one gate selection period. 
     Accordingly, when a scan line in a certain row is selected, video signals from the signal lines S 1  to S n  are input to a plurality of pixels  180102  connected to the scan line, respectively. For example, while the scan line G i  in the i-th row is selected, given video signals are input from the signal lines S 1  to S n  to a plurality of pixels  180102  connected to the scan line G i  in the i-th row, respectively. Thus, each of the plurality of pixels  180102  can be controlled individually by the scan signal and the video signal. 
     Next, the case where one gate selection period is divided into a plurality of subgate selection periods is described.  FIG. 67  is a timing chart in the case where one gate selection period is divided into two subgate selection periods (a first subgate selection period and a second subgate selection period). 
     Note that one gate selection period may be divided into three or more subgate selection periods. 
     The timing chart in  FIG. 67  shows one frame period corresponding to a period for displaying an image for one screen. Although one frame period is not particularly limited to a certain period, it is at least preferable that one frame period be 1/60 second or less so that a person viewing an image does not perceive flickers. 
     Note that one frame is divided into two subframes (a first subframe and a second subframe). 
     The timing chart of  FIG. 67  shows timing of selecting the scan line G i  in the i-th row, the scan line G i +1 in the (i+1)th row, the scan line G j  (one of the scan lines G i +1 to G m ) in the j-th row, and the scan line G i +1 (one of the scan lines G i +1 to G m ) in the (j+1)th row. 
     At the same time as the scan line is selected, the pixel  180102  connected to the scan line is also selected. For example, when the scan line G i  in the i-th row is selected, the pixel  180102  connected to the scan line G i  in the i-th row is also selected. 
     The scan lines G 1  to G m  are sequentially scanned in each subgate selection period. For example, in one gate selection period, the scan line G i  in the i-th row is selected in the first subgate selection period, and the scan line G j  in the j-th row is selected in the second subgate selection period. Thus, in one gate selection period, an operation can be performed as if scan signals for two rows are selected. At this time, different video signals are input to the signal lines S 1  to S n  in the first subgate selection period and the second subgate selection period. Therefore, different video signals can be input to a plurality of pixels  180102  connected to the i-th row and a plurality of pixels  180102  connected to the j-th row. 
     Next, a driving method for converting a frame rate of image data to be input (also referred to as input frame rate) and a frame rate of display (also referred to as a display frame rate) is described. Note that the frame rate is the number of frames per second, and its unit is Hz. 
     In this embodiment mode, the input frame rate does not necessarily correspond to the display frame rate. When the input frame rate and the display frame rate are different from each other, the frame rate can be converted by a circuit which converts a frame rate of image data (a frame rate conversion circuit). In such a manner, even when the input frame rate and the display frame rate are different from each other, display can be performed at a variety of display frame rates. 
     When the input frame rate is higher than the display frame rate, part of the image data to be input is discarded and the input frame rate is converted so that display is performed at a variety of display frame rates. In this case, the display frame rate can be reduced; thus, operating frequency of a driver circuit used for display can be reduced, and power consumption can be reduced. On the other hand, when the input frame rate is lower than the display frame rate, display can be performed at a variety of converted display frame rates by a method such as a method in which all or part of the image data to be input is displayed more than once, a method in which another image is generated from the image data to be input, or a method in which an image having no relation to the image data to be input is generated. In this case, quality of moving images can be improved by the display frame rate being increased. 
     In this embodiment mode, a frame rate conversion method in the case where the input frame rate is lower than the display frame rate is described in detail. Note that a frame rate conversion method in the case where the input frame rate is higher than the display frame rate can be realized by performing the frame rate conversion method in the case where the input frame rate is lower than the display frame rate in reverse order. 
     In this embodiment mode, an image displayed at the same frame rate as the input frame rate is referred to as a basic image. An image which is displayed at a frame rate different from that of the basic image and displayed to ensure that the input frame rate and the display frame rate are consistent to each other is referred to as an interpolation image. As the basic image, the same image as that of the image data to be input can be used. As the interpolation image, the same image as the basic image can be used. Further, an image different from the basic image can be generated, and the generated image can be used as the interpolation image. 
     In order to generate the interpolation image, the following methods can be used, for example: a method in which time change (movement of images) of the image data to be input is detected and an image in an intermediate state between the images is employed as the interpolation image, a method in which an image obtained by multiplication of luminance of the basic image by a coefficient is employed as the interpolation image, and a method in which a plurality of different images are generated from the image data to be input and the plurality of images are continuously displayed (one of the plurality of images is employed as the basic image and the other images are employed as interpolation images) so as to allow a viewer to perceive an image corresponding to the image data to be input. Examples of the method in which a plurality of different images are generated from the image data to be input include a method in which a gamma value of the image data to be input is converted and a method in which a gray scale value included in the image data to be input is divided. 
     Note that an image in an intermediate state (an intermediate image) refers to an image obtained by detection of time change (movement of images) of the image data to be input and interpolation of the detected movement. Obtaining an intermediate image by such a method is referred to as motion compensation. 
     Next, a specific example of a frame rate conversion method is described. With this method, frame rate conversion multiplied by a given rational number (n/m) can be realized. Here, each of n and m is an integer equal to or more than 1. A frame rate conversion method in this embodiment mode can be handled as being divided into a first step and a second step. The first step is a step in which a frame rate is converted by being multiplied by the given rational number (n/m). As the interpolation image, the intermediate image obtained by motion compensation or the basic image may be used. The second step is a step in which a plurality of different images (sub-images) are generated from the image data to be input or from images each of which frame rate is converted in the first step and the plurality of sub-images are continuously displayed. When a method of the second step is used, human eyes can be made to perceive display such that the display appears to be an original image, despite the fact that a plurality of different images are displayed. 
     Note that in the frame rate conversion method in this embodiment mode, both the first and second steps may be used, only the second step may be used with the first step omitted, or only the first step may be used with the second step omitted. 
     First, as the first step, frame rate conversion multiplied by the given rational number (n/m) is described with reference to  FIG. 68 . In  FIG. 68 , the horizontal axis represents time, and the vertical axis represents cases for various combinations of n and m. Each pattern in  FIG. 68  is a schematic diagram of an image to be displayed, and a horizontal position of the pattern represents timing of display. A dot in the pattern schematically represents movement of an image. Note that each of these images is an example for explanation, and an image to be displayed is not limited to one of these images. This method can be applied to a variety of images. 
     A period T in  represents a cycle of input image data. The cycle of input image data corresponds to an input frame rate. For example, when the input frame rate is 60 Hz, the cycle of input image data is 1/60 seconds. Similarly, when the input frame rate is 50 Hz, the cycle of input image data is 1/50 seconds. Accordingly, the cycle (unit: second) of input image data is an inverse number of the input frame rate (unit: Hz). Note that a variety of input frame rates such as 24 Hz, 50 Hz, 60 Hz, 70 Hz, 48 Hz, 100 Hz, 120 Hz, and 140 Hz can be used. 24 Hz is a frame rate for movies on film, for example. 50 Hz is a frame rate for a video signal of the PAL standard, for example. 60 Hz is a frame rate for an image signal of the NTSC standard, for example. 70 Hz is a frame rate of a display input signal of a personal computer, for example. 48 Hz, 100 Hz, 120 Hz, and 140 Hz are twice as high as 24 Hz, 50 Hz, 60 Hz, and 70 Hz, respectively. Note that the frame rate can not only be doubled but also be multiplied by a variety of numbers. As described above, with the method shown in this embodiment mode, a frame rate can be converted with respect to an input signal of various standards. 
     Procedures of frame rate conversion multiplied by the given rational number (n/m) times in the first step are as follows. As a procedure 1, display timing of a k-th interpolation image (k is an integer equal to or more than 1, where the initial value is 1) with respect to a first basic image is determined. The display timing of the k-th interpolation image is at the timing of passage of a period obtained by multiplication of the cycle of input image data by k(m/n) after the first basic image is displayed. As a procedure 2, whether the coefficient k(m/n) used for deciding the display timing of the k-th interpolation image is an integer or not is determined. When the coefficient k is an integer, a (k(m/n)+1)th basic image is displayed at the display timing of the k-th interpolation image, and the first step is finished. When the coefficient k is not an integer, the operation proceeds to a procedure 3. As the procedure 3, an image used as the k-th interpolation image is determined. Specifically, the coefficient k(m/n) used for deciding the display timing of the k-th interpolation image is converted into the form (x+(y/n)). Each of x and y is an integer, and y is smaller than n. When an intermediate image obtained by motion compensation is employed as the k-th interpolation image, an intermediate image which is an image corresponding to movement obtained by multiplication of the amount of movement from an (x+1)th basic image to an (x+2)th basic image by (y/n) is employed as the k-th interpolation image. When the k-th interpolation image is the same image as the basic image, the (x+1)th basic image can be used. Note that a method for obtaining an intermediate image as an image corresponding to movement obtained by multiplication of the amount of movement of the image by (y/n) is described in detail later. As a procedure 4, a next interpolation image is set to be the objective interpolation image. Specifically, the value of k is increased by one, and the operation returns to the procedure 1. 
     Next, the procedures in the first step are described in detail using specific values of n and m. 
     Note that a mechanism for performing the procedures in the first step may be mounted on a device or determined in the design phase of the device in advance. When the mechanism for performing the procedures in the first step is mounted on the device, a driving method can be switched so that optimal operations depending on circumstances can be performed. Note that the circumstances here include contents of image data, environment inside and outside the device (e.g., temperature, humidity, barometric pressure, light, sound, electric field, the amount of radiation, altitude, acceleration, or movement speed), user settings, software version, and the like. On the other hand, when the mechanism for performing the procedures in the first step is determined in the design phase of the device in advance, driver circuits optimal for respective driving methods can be used. Moreover, since the mechanism is determined, reduction in manufacturing cost due to efficiency of mass production can be expected. 
     When n=1 and m=1, that is, when a conversion ratio (n/m) is 1 (where n=1 and m=1 in  FIG. 68 ), an operation in the first step is as follows. When k=1, in the procedure 1, display timing of a first interpolation image with respect to the first basic image is determined. The display timing of the first interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, 1 after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the first interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is 1, which is an integer. Consequently, the (k(m/n)+1)th basic image, that is, a second basic image is displayed at the display timing of the first interpolation image, and the first step is finished. 
     In other words, when the conversion ratio is 1, the k-th image is a basic image, the (k+1)th image is a basic image, and an image display cycle is equal to the cycle of input image data. 
     Specifically, in a driving method of a display device in which, when the conversion ratio is 1 (n/m=1), i-th image data (i is a positive integer) and (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer) and the (k+1)th image are sequentially displayed at an interval equal to the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, and the (k+1)th image is displayed in accordance with the (i+1)th image data. 
     Since the frame rate conversion circuit can be omitted when the conversion ratio is 1, manufacturing cost can be reduced. Further, when the conversion ratio is 1, quality of moving images can be improved compared to the case where the conversion ratio is less than 1. Moreover, when the conversion ratio is 1, power consumption and manufacturing cost can be reduced compared to the case where the conversion ratio is more than 1. 
     When n=2 and m=1, that is, when the conversion ratio (n/m) is 2 (where n=2 and m=1 in  FIG. 68 ), an operation in the first step is as follows. When k=1, in the procedure 1, display timing of the first interpolation image with respect to the first basic image is determined. The display timing of the first interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, ½ after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the first interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is ½, which is not an integer. Consequently, the operation proceeds to the procedure 3. 
     In the procedure 3, an image used as the first interpolation image is determined. In order to decide the image, the coefficient ½ is converted into the form (x+(y/n)). In the case of the coefficient ½, x=0 and y=1. When an intermediate image obtained by motion compensation is employed as the first interpolation image, an intermediate image corresponding to movement obtained by multiplication of the amount of movement from the (x+1)th basic image, that is, the first basic image to the (x+2)th basic image, that is, the second basic image by (y/n), that is, ½ is employed as the first interpolation image. When the first interpolation image is the same image as the basic image, the (x+1)th basic image, that is, the first basic image can be used. 
     According to the procedures performed up to this point, the display timing of the first interpolation image and the image displayed as the first interpolation image can be determined. Next, in the procedure 4, the objective interpolation image is shifted from the first interpolation image to a second interpolation image. That is, k is changed from 1 to 2, and the operation returns to the procedure 1. 
     When k=2, in the procedure 1, display timing of the second interpolation image with respect to the first basic image is determined. The display timing of the second interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, 1 after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the second interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is 1, which is an integer. Consequently, the (k(m/n)+1)th basic image, that is, the second basic image is displayed at the display timing of the second interpolation image, and the first step is finished. 
     In other words, when the conversion ratio is 2 (n/m=2), the k-th image is a basic image, the (k+1)th image is an interpolation image, a (k+2)th image is a basic image, and an image display cycle is half the cycle of input image data. 
     Specifically, in a driving method of a display device in which, when the conversion ratio is 2 (n/m=2), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, and the (k+2)th image are sequentially displayed at an interval which is half the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ½, and the (k+2)th image is displayed in accordance with the (i+1)th image data. 
     Even specifically, in a driving method of a display device in which, when the conversion ratio is 2 (n/m=2); the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k Is a positive integer), the (k+1)th image, and the (k+2)th image are sequentially displayed at an interval which is half the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with the i-th image data, and the (k+2)th image is displayed in accordance with the (i+1)th image data. 
     Specifically, when the conversion ratio is 2, driving is also referred to as double-frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 120 Hz (120 Hz driving). Accordingly, two images are continuously displayed with respect to one input image. At this time, when an interpolation image is an intermediate image obtained by motion compensation, the movement of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Further, quality of moving images can be significantly improved particularly when the display device is an active matrix liquid crystal display device. This is related to a problem of lack of writing voltage due to change in the electrostatic capacity of a liquid crystal element by applied voltage, so-called dynamic capacitance. That is, when the display frame rate is made higher than the input frame rate, the frequency of a writing operation of image data can be increased; thus, defects such as an afterimage and a phenomenon of a moving image in which traces are seen due to lack of writing voltage because of dynamic capacitance can be reduced. Moreover, a combination of 120 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 120 Hz and frequency of alternating-current driving is an integer multiple of 120 Hz or a unit fraction of 120 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     When n=3 and m=1, that is, when the conversion ratio (n/m) is 3 (where n=3 and m=1 in  FIG. 68 ), an operation in the first step is as follows. First, when k=1, in the procedure 1, display timing of the first interpolation image with respect to the first basic image is determined. The display timing of the first interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, ⅓ after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the first interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is ⅓, which is not an integer. Consequently, the operation proceeds to the procedure 3. 
     In the procedure 3, an image used as the first interpolation image is determined. In order to decide the image, the coefficient ⅓ is converted into the form (x+(y/n)). In the case of the coefficient ⅓, x=0 and y=1. When an intermediate image obtained by motion compensation is employed as the first interpolation image, an intermediate image corresponding to movement obtained by multiplication of the amount of movement from the (x+1)th basic image, that is, the first basic image to the (x+2)th basic image, that is, the second basic image by (y/n), that is, ⅓ is employed as the first interpolation image. When the first interpolation image is the same image as the basic image, the (x+1)th basic image, that is, the first basic image can be used. 
     According to the procedures performed up to this point, the display timing of the first interpolation image and the image displayed as the first interpolation image can be determined. Next, in the procedure 4, the objective interpolation image is shifted from the first interpolation image to the second interpolation image. That is, k is changed from 1 to 2, and the operation returns to the procedure 1. 
     When k=2, in the procedure 1, display timing of the second interpolation image with respect to the first basic image is determined. The display timing of the second interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, ⅔ after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the second interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is ⅔, which is not an integer. Consequently, the operation proceeds to the procedure 3. 
     In the procedure 3, an image used as the second interpolation image is determined. In order to decide the image, the coefficient ⅔ is converted into the form (x+(y/n)). In the case of the coefficient ⅔, x=0 and y=2. When an intermediate image obtained by motion compensation is employed as the second interpolation image, an intermediate image corresponding to movement obtained by multiplication of the amount of movement from the (x+1)th basic image, that is, the first basic image to the (x+2)th basic image, that is, the second basic image by (y/n), that is, ⅔ is employed as the second interpolation image. When the second interpolation image is the same image as the basic image, the (x+1)th basic image, that is, the first basic image can be used. 
     According to the procedures performed up to this point, the display timing of the second interpolation image and the image displayed as the second interpolation image can be determined. Next, in the procedure 4, the objective interpolation image is shifted from the second interpolation image to a third interpolation image. That is, k is changed from 2 to 3, and the operation returns to the procedure 1. 
     When k=3, in the procedure 1, display timing of the third interpolation image with respect to the first basic image is determined. The display timing of the third interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, 1 after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the third interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is 1, which is an integer. Consequently, the (k(m/n)+1)th basic image, that is, the second basic image is displayed at the display timing of the third interpolation image, and the first step is finished. 
     In other words, when the conversion ratio is 3 (n/m=3), the k-th image is a basic image, the (k+1)th image is an interpolation image, the (k+2)th image is an interpolation image, a (k+3)th image is a basic image, and an image display cycle is ⅓ times the cycle of input image data. 
     Specifically, in a driving method of a display device in which, when the conversion ratio is 3 (n/m=3), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, and the (k+3)th image are sequentially displayed at an interval which is ⅓ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ⅓, the (k+2)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ⅔, and the (k+3)th image is displayed in accordance with the (i+1)th image data. 
     Even specifically, in a driving method of a display device in which, when the conversion ratio is 3 (n/m=3), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, and the (k+3)th image are sequentially displayed at an interval which is ⅓ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with the i-th image data, the (k+2)th image is displayed in accordance with the i-th image data, and the (k+3)th image is displayed in accordance with the (i+1)th image data. 
     When the conversion ratio is 3, quality of moving images can be improved compared to the case where the conversion ratio is less than 3. Moreover, when the conversion ratio is 3, power consumption and manufacturing cost can be reduced compared to the case where the conversion ratio is more than 3. 
     Specifically, when the conversion ratio is 3, driving is also referred to as triple-frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 180 Hz (180 Hz driving). Accordingly, three images are continuously displayed with respect to one input image. At this time, when an interpolation image is an intermediate image obtained by motion compensation, the movement of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Further, when the display device is an active matrix liquid crystal display device, a problem of lack of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved, in particular with respect to defects such as an afterimage and a phenomenon of a moving image in which traces are seen. Moreover, a combination of 180 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 180 Hz and frequency of alternating-current driving is an integer multiple of 180 Hz or a unit fraction of 180 Hz (e.g., 45 Hz, 90 Hz, 180 Hz, or 360 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     When n=3 and m=2, that is, when the conversion ratio (n/m) is 3/2 (where n=3 and m=2 in  FIG. 68 ), an operation in the first step is as follows. When k=1, in the procedure 1, the display timing of the first interpolation image with respect to the first basic image is determined. The display timing of the first interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, ⅔ after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the first interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is ⅔, which is not an integer. Consequently, the operation proceeds to the procedure 3. 
     In the procedure 3, an image used as the first interpolation image is determined. In order to decide the image, the coefficient ⅔ is converted into the form (x+(y/n)). In the case of the coefficient ⅔, x=0 and y=2. When an intermediate image obtained by motion compensation is employed as the first interpolation image, an intermediate image corresponding to movement obtained by multiplication of the amount of movement from the (x+1)th basic image, that is, the first basic image to the (x+2)th basic image, that is, the second basic image by (y/n), that is, ⅔ is employed as the first interpolation image. When the first interpolation image is the same image as the basic image, the (x+1)th basic image, that is, the first basic image can be used. 
     According to the procedures performed up to this point, the display timing of the first interpolation image and the image displayed as the first interpolation image can be determined. Next, in the procedure 4, the objective interpolation image is shifted from the first interpolation image to the second interpolation image. That is, k is changed from 1 to 2, and the operation returns to the procedure 1. 
     When k=2, in the procedure 1, the display timing of the second interpolation image with respect to the first basic image is determined. The display timing of the second interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, 4/3 after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the second interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is 4/3, which is not an integer. Consequently, the operation proceeds to the procedure 3. 
     In the procedure 3, an image used as the second interpolation image is determined. In order to decide the image, the coefficient 4/3 is converted into the form (x+(y/n)). In the case of the coefficient 4/3, x=1 and y=1. When an intermediate image obtained by motion compensation is employed as the second interpolation image, an intermediate image corresponding to movement obtained by multiplication of the amount of movement from the (x+1)th basic image, that is, the second basic image to the (x+2)th basic image, that is, a third basic image by (y/n), that is, ⅓ is employed as the second interpolation image. When the second interpolation image is the same image as the basic image, the (x+1)th basic image, that is, the second basic image can be used. 
     According to the procedures performed up to this point, the display timing of the second interpolation image and the image displayed as the second interpolation image can be determined. Next, in the procedure 4, the objective interpolation image is shifted from the second interpolation image to the third interpolation image. That is, k is changed from 2 to 3, and the operation returns to the procedure 1. 
     When k=3, in the procedure 1, the display timing of the third interpolation image with respect to the first basic image is determined. The display timing of the third interpolation image is at the timing of passage of a period obtained by multiplication of the length of the cycle of input image data by k(m/n), that is, 2 after the first basic image is displayed. 
     Next, in the procedure 2, whether the coefficient k(m/n) used for determining the display timing of the third interpolation image is an integer or not is judged. Here, the coefficient k(m/n) is 2, which is an integer. Consequently, the (k(m/n)+1)th basic image, that is, the third basic image is displayed at the display timing of the third interpolation image, and the first step is finished. 
     In other words, when the conversion ratio is 3/2 (n/m=3/2), the k-th image is a basic image, the (k+1)th image is an interpolation image, the (k+2)th image is an interpolation image, the (k+3)th image is a basic image, and an image display cycle is ⅔ times the cycle of input image data. 
     Specifically, in a driving method of a display device in which, when the conversion ratio is 3/2 (n/m=3/2), the i-th image data (i is a positive integer), the (i+1)th image data, and (i+2)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, and the (k+3)th image are sequentially displayed at an interval which is ⅔ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ⅔, the (k+2)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the (i+1)th image data to the (i+2)th image data by ⅓, and the (k+3)th image is displayed in accordance with the (i+2)th image data. 
     Even specifically, in a driving method of a display device in which, when the conversion ratio is 3/2 (n/m=3/2), the i-th image data (i is a positive integer), the (i+1)th image data, and the (i+2)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, and the (k+3)th image are sequentially displayed at an interval which is ⅔ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with the i-th image data, the (k+2)th image is displayed in accordance with the (i+1)th image data, and the (k+3)th image is displayed in accordance with the (i+2)th image data. 
     When the conversion ratio is 3/2, quality of moving images can be improved compared with the case where the conversion ratio is less than 3/2. Moreover, when the conversion ratio is 3/2, power consumption and manufacturing cost can be reduced compared with the case where the conversion ratio is more than 3/2. 
     Specifically, when the conversion ratio is 3/2, driving is also referred to as 3/2-fold frame rate driving or 1.5-fold frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 90 Hz (90 Hz driving). Accordingly, three images are continuously displayed with respect to two input images. At this time, when an interpolation image is an intermediate image obtained by motion compensation, the movement of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Moreover, operating frequency of a circuit used for obtaining an intermediate image by motion compensation can be reduced, in particular, compared with a driving method with high driving frequency, such as 120 Hz driving (double-frame rate driving) or 180 Hz driving (triple-frame rate driving); thus, an inexpensive circuit can be used, and manufacturing cost and power consumption can be reduced. Further, when the display device is an active matrix liquid crystal display device, a problem of lack of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved, in particular with respect to defects such as an afterimage and a phenomenon of a moving image in which traces are seen. Moreover, a combination of 90 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 90 Hz and frequency of alternating-current driving is an integer multiple of 90 Hz or a unit fraction of 90 Hz (e.g., 30 Hz, 45 Hz, 90 Hz, or 180 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     Detailed description of procedures for positive integers n and m other than those described above is omitted. A conversion ratio can be set as a given rational number (n/m) in accordance with the procedures of frame rate conversion in the first step. Note that among combinations of the positive integers n and m, a combination in which a conversion ratio (n/m) can be reduced to its lowest term can be treated the same as a conversion ratio that is already reduced to its lowest term. 
     For example, when n=4 and m=1, that is, when the conversion ratio (n/m) is 4 (where n=4 and m=1 in  FIG. 68 ), the k-th image is a basic image, the (k+1)th image is an interpolation image, the (k+2)th image is an interpolation image, the (k+3)th image is an interpolation image, a (k+4)th image is a basic image, and an image display cycle is ¼ times the cycle of input image data. 
     Specifically, in a driving method of a display device in which, when the conversion ratio is 4 (n/m=4), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, and the (k+4)th image are sequentially displayed at an interval which is ¼ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ¼, the (k+2)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ½, the (k+3)th image is displayed in accordance with image data corresponding to movement obtained by multiplication of the amount of movement from the i-th image data to the (i+1)th image data by ¾, and the (k+4)th image is displayed in accordance with the (i+1)th image data. 
     Even specifically, in a driving method of a display device in which, when the conversion ratio is 4 (n/m=4), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, and the (k+4)th image are sequentially displayed at an interval which is ¼ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with the i-th image data, the (k+2)th image is displayed in accordance with the i-th image data, the (k+3)th image is displayed in accordance with the i-th image data, and the (k+4)th image is displayed in accordance with the (i+1)th image data. 
     When the conversion ratio is 4, quality of moving images can be improved compared with the case where the conversion ratio is less than 4. Moreover, when the conversion ratio is 4, power consumption and manufacturing cost can be reduced compared with the case where the conversion ratio is more than 4. 
     Specifically, when the conversion ratio is 4, driving is also referred to as quadruple-frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 240 Hz (240 Hz driving). Accordingly, four images are continuously displayed with respect to one input image. At this time, when an interpolation image is an intermediate image obtained by motion compensation, the movement of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Moreover, an interpolation image obtained by more accurate motion compensation can be used, in particular, compared with a driving method with low driving frequency, such as 120 Hz driving (double-frame rate driving) or 180 Hz driving (triple-frame rate driving); thus, the movement of moving images can be made smoother, and quality of the moving image can be significantly improved. Further, when the display device is an active matrix liquid crystal display device, a problem of lack of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved, in particular with respect to defects such as an afterimage and a phenomenon of a moving image in which traces are seen. Moreover, a combination of 240 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 240 Hz and frequency of alternating-current driving is an integer multiple of 240 Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 40 Hz, 60 Hz, or 120 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     Moreover, when n=4 and m=3, that is, when the conversion ratio (n/m) is 4/3 (where n=4 and m=3 in  FIG. 68 ), the k-th image is a basic image, the (k+1)th image is an interpolation image, the (k+2)th image is an interpolation image, the (k+3)th image is an interpolation image, the (k+4)th image is a basic image, and the length of an image display cycle is ¾ times the cycle of input image data. 
     As further specific description, in a driving method of a display device in which when the conversion ratio is 4/3 (n/m=4/3), the i-th image data (i is a positive integer), the (i+1)th image data, the (i+2)th image data, and the (i+3)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, and the (k+4)th image are sequentially displayed at an interval which is ¾ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ¾, the (k+2)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the (i+1)th image data to the (i+2)th image data by ½, the (k+3)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the (i+2)th image data to the (i+3)th image data by ¼, and the (k+4)th image is displayed in accordance with the (i+3)th image data. 
     As further specific description, in a driving method of a display device in which when the conversion ratio is 4/3 (n/m=4/3), the i&#39;th image data (i is a positive integer), the (i+1)th image data, the (i+2)th image data, and the (i+3)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, and the (k+4)th image are sequentially displayed at an interval which is ¾ times the cycle of the input image data, the k-th image is displayed in accordance with the i&#39;th image data, the (k+1)th image is displayed in accordance with the i&#39;th image data, the (k+2)th image is displayed in accordance with the (i+1)th image data, the (k+3)th image is displayed in accordance with the (i+2)th image data, and the (k+4)th image is displayed in accordance with the (i+3)th image data. 
     When the conversion ratio is 4/3, quality of moving images can be improved compared to the case where the conversion ratio is less than 4/3. Moreover, when the conversion ratio is 4/3, power consumption and manufacturing cost can be reduced compared to the case where the conversion ratio is more than 4/3. 
     Specifically, when the conversion ratio is 4/3, driving is also referred to as 4/3-fold frame rate driving or 1.25-fold frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 80 Hz (80 Hz driving). Four images are continuously displayed with respect to three input images. At this time, when an interpolation image is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Moreover, operating frequency of a circuit for obtaining an intermediate image by motion compensation can be reduced particularly as compared with a driving method with high driving frequency, such as 120 Hz driving (double-frame rate driving) or 180 Hz driving (triple-frame rate driving); thus, an inexpensive circuit can be used, and manufacturing cost and power consumption can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved particularly with respect to defects such as traces and afterimages of a moving image. Moreover, a combination of 80 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 80 Hz and frequency of alternating-current driving is an integer multiple of 80 Hz or a unit fraction of 80 Hz (e.g., 40 Hz, 80 Hz, 160 Hz, or 240 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     Moreover, when n=5 and m=1, that is, when the conversion ratio (n/m) is 5 (where n=5 and m=1 in  FIG. 68 ), the k-th image is a basic image, the (k+1)th image is an interpolation image, the (k+2)th image is an interpolation image, the (k+3)th image is an interpolation image, a (k+4)th image is an interpolation image, a (k+5)th image is a basic image, and the length of an image display cycle is ⅕ times the cycle of input image data. 
     As further specific description, in a driving method of a display device in which when the conversion ratio is 5 (n/m=5), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, the (k+4)th image, and the (k+5)th image are sequentially displayed at an interval whose length is ⅕ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ⅕, the (k+2)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ⅖, the (k+3)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ⅗, the (k+4)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ⅘, and the (k+5)th image is displayed in accordance with the (i+1)th image data. 
     As further specific description, in a driving method of a display device in which when the conversion ratio is 5 (n/m=5), the i-th image data (i is a positive integer) and the (i+1)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, the (k+4)th image, and the (k+5)th image are sequentially displayed at an interval whose length is ⅕ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with the i-th image data, the (k+2)th image is displayed in accordance with the i-th image data, the (k+3)th image is displayed in accordance with the i-th image data, the (k+4)th image is displayed in accordance with the i-th image data, and the (k+5)th image is displayed in accordance with the (i+1)th image data. 
     When the conversion ratio is 5, quality of moving images can be improved compared to the case where the conversion ratio is less than 5. Moreover, when the conversion ratio is 5, power consumption and manufacturing cost can be reduced compared to the case where the conversion ratio is more than 5. 
     Specifically, when the conversion ratio is 5, driving is also referred to as 5-fold frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 300 Hz (300 Hz driving). Five images are continuously displayed with respect to one input image. At this time, when an interpolation image is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Moreover, an intermediate image obtained by more accurate motion compensation can be used as the interpolation image particularly as compared with a driving method with low driving frequency, such as 120 Hz driving (double-frame rate driving) or 180 Hz driving (triple-frame rate driving); thus, motion of moving images can be made smoother, and quality of the moving image can be significantly improved. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved particularly with respect to defects such as traces and afterimages of a moving image. Moreover, a combination of 300 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 300 Hz and frequency of alternating-current driving is an integer multiple of 300 Hz or a unit fraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     Moreover, when n=5 and m=2, that is, when the conversion ratio (n/m) is 5/2 (where n=5 and m=2 in  FIG. 68 ), the k-th image is a basic image, the (k+1)th image is an interpolation image, the (k+2)th image is an interpolation image, the (k+3)th image is an interpolation image, a (k+4)th image is an interpolation image, the (k+5)th image is a basic image, and the length of an image display cycle is ⅖ times the cycle of input image data. 
     As further specific description, in a driving method of a display device in which when the conversion ratio is 5/2 (n/m=5/2), the i-th image data (i is a positive integer), the (i+1)th image data, and the (i+2)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, the (k+4)th image, and the (k+5)th image are sequentially displayed at an interval whose length is ⅖ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ⅖, the (k+2)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the i-th image data to the (i+1)th image data by ⅘, the (k+3)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the (i+1)th image data to the (i+2)th image data by ⅕, the (k+4)th image is displayed in accordance with image data corresponding to movement obtained by multiplying the amount of movement from the (i+1)th image data to the (i+2)th image data by ⅗, and the (k+5)th image is displayed in accordance with the (i+2)th image data. 
     As further specific description, in a driving method of a display device in which when the conversion ratio is 5/2 (n/m=5/2), the i-th image data (i is a positive integer), the (i+1)th image data, the (i+2)th image data, and the (i+3)th image data are sequentially input as input image data in a certain cycle and the k-th image (k is a positive integer), the (k+1)th image, the (k+2)th image, the (k+3)th image, the (k+4)th image, and the (k+5)th image are sequentially displayed at an interval whose length is ⅖ times the cycle of the input image data, the k-th image is displayed in accordance with the i-th image data, the (k+1)th image is displayed in accordance with the i-th image data, the (k+2)th image is displayed in accordance with the i-th image data, the (k+3)th image is displayed in accordance with the (i+1)th image data, the (k+4)th image is displayed in accordance with the (i+1)th image data, and the (k+5)th image is displayed in accordance with the (i+2)th image data. 
     When the conversion ratio is 5/2, quality of moving images can be improved compared with the case where the conversion ratio is less than 5/2. Moreover, when the conversion ratio is 5/2, power consumption and manufacturing cost can be reduced compared with the case where the conversion ratio is more than 5/2. 
     Specifically, when the conversion ratio is 5/2, driving is also referred to as 5/2-fold frame rate driving or 2.5-fold frame rate driving. For example, when the input frame rate is 60 Hz, the display frame rate is 150 Hz (150 Hz driving). Five images are continuously displayed with respect to two input images. At this time, when an interpolation image is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of the moving image can be significantly improved. Moreover, an intermediate image obtained by more accurate motion compensation can be used as the interpolation image particularly as compared with a driving method with low driving frequency, such as 120 Hz driving (double-frame rate driving); thus, motion of moving images can be made smoother, and quality of the moving image can be significantly improved. Further, operating frequency of a circuit for obtaining an intermediate image by motion compensation can be reduced particularly as compared with a driving method with high driving frequency, such as 180 Hz driving (triple-frame rate driving); thus, an inexpensive circuit can be used, and manufacturing cost and power consumption can be reduced. Furthermore, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved particularly with respect to defects such as traces and afterimages of a moving image. Moreover, a combination of 150 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when driving frequency of the liquid crystal display device is 150 Hz and frequency of alternating-current driving is an integer multiple of 150 Hz or a unit fraction of 150 Hz (e.g., 30 Hz, 50 Hz, 75 Hz, or 150 Hz), flickers which appear in alternating-current driving can be reduced so as not to be perceived by human eyes. 
     In this manner, by setting positive integers n and m to be various numbers, the conversion ratio can be set to be a given rational number (n/m). Although detailed description is omitted, when n is 10 or less, combinations listed below can be possible: n=1, m=1, that is, the conversion ratio is (n/m)=1 (one-times frame rate driving, 60 Hz), n=2, m=1, that is, the conversion ratio is (n/m)=2 (double-frame rate driving, 120 Hz), n=3, m=1, that is, the conversion ratio is (n/m)=3 (triple-frame rate driving, 180 Hz), n=3, m=2, that is, the conversion ratio is (n/m)=3/2 (3/2-fold frame rate driving, 90 Hz), n=4, m=1, that is, the conversion ratio is (n/m)=4 (quadruple-frame rate driving, 240 Hz), n=4, m=3, that is, the conversion ratio is (n/m)=4/3 (4/3-fold frame rate driving, 80 Hz), n=5, m=1, that is, the conversion ratio is (n/m)=5/1 (5-fold frame rate driving, 300 Hz), n=5, m=2, that is, the conversion ratio is (n/m)=5/2 (5/2-fold frame rate driving, 150 Hz), n=5, m=3, that is, the conversion ratio is (n/m)=5/3 (5/3-fold frame rate driving, 100 Hz), n=5, m=4, that is, the conversion ratio is (n/m)=5/4 (5/4-fold frame rate driving, 75 Hz), n=6, m=1, that is, the conversion ratio is (n/m)=6 (6-fold frame rate driving, 360 Hz), n=6, m=5, that is, the conversion ratio is (n/m)=6/5 (6/5-fold frame rate driving, 72 Hz), n=7, m=1, that is, the conversion ratio is (n/m)=7 (7-fold frame rate driving, 420 Hz), n=7, m=2, that is, the conversion ratio is (n/m)=7/2 (7/2-fold frame rate driving, 210 Hz), n=7, m=3, that is, the conversion ratio is (n/m)=7/3 (7/3-fold frame rate driving, 140 Hz), n=7, m=4, that is, the conversion ratio is (n/m)=7/4 (7/4-fold frame rate driving, 105 Hz), n=7, m=5, that is, the conversion ratio is (n/m)=7/5 (7/5-fold frame rate driving, 84 Hz), n=7, m=6, that is, the conversion ratio is (n/m)=7/6 (7/6-fold frame rate driving, 70 Hz), n=8, m=1, that is, the conversion ratio is (n/m)=8 (8-fold frame rate driving, 480 Hz), n=8, m=3, that is, the conversion ratio is (n/m)=8/3 (8/3-fold frame rate driving, 160 Hz), n=8, m=5, that is, the conversion ratio is (n/m)=8/5 (8/5-fold frame rate driving, 96 Hz), n=8, m=7, that is, the conversion ratio is (n/m)=8/7 (8/7-fold frame rate driving, 68.6 Hz), n=9, m=1, that is, the conversion ratio is (n/m)=9 (9-fold frame rate driving, 540 Hz), n=9, m=2, that is, the conversion ratio is (n/m)=9/2 (9/2-fold frame rate driving, 270 Hz), n=9, m=4, that is, the conversion ratio is (n/m)=9/4 (9/4-fold frame rate driving, 135 Hz), n=9, m=5, that is, the conversion ratio is (n/m)=9/5 (9/5-fold frame rate driving, 108 Hz), n=9, m=7, that is, the conversion ratio is (n/m)=9/7 (9/7-fold frame rate driving, 77.1 Hz), n=9, m=8, that is, the conversion ratio is (n/m)=9/8 (9/8-fold frame rate driving, 67.5 Hz), n=10, m=1, that is, the conversion ratio is (n/m)=10 (10-fold frame rate driving, 600 Hz), n=10, m=3, that is, the conversion ratio is (n/m)=10/3 (10/3-fold frame rate driving, 200 Hz), n=10, m=7, that is, the conversion ratio is (n/m)=10/7 (10/7-fold frame rate driving, 85.7 Hz), and n=10, m=9, that is, the conversion ratio is (n/m)=10/9 (10/9-fold frame rate driving, 66.7 Hz). Note that these frequencies are examples in the case where the input frame rate is 60 Hz. With regard to other frame rates, a product obtained by multiplication of each conversion ratio and an input frame rate can be a driving frequency. 
     In the case where n is an integer more than 10, although specific numbers for n and m are not stated here, the procedure of frame rate conversion in the first step can be obviously applied to various n and m. 
     Note that depending on how many images which can be displayed without motion compensation to the input image data are included in the displayed images, the conversion ratio can be determined. Specifically, the smaller m becomes, the higher the proportion of images which can be displayed without motion compensation to the input image data becomes. When motion compensation is performed less frequently, power consumption can be reduced because a circuit which performs motion compensation operates less frequently. In addition, the likelihood of generation of an image (an intermediate image which does not correctly reflect motion of an image) including an error by motion compensation can be decreased, so that image quality can be improved. For example, as such a conversion ratio, in the case where n is 10 or less, 1, 2, 3, 3/2, 4, 5, 5/2, 6, 7, 7/2, 8, 9, 9/2, or 10 is possible. By employing such a conversion ratio, especially when an intermediate image obtained by motion compensation is used as an interpolation image, the image quality can be improved and power consumption can be reduced because the number (half the total number of images input) of images, which can be displayed without motion compensation to the input image data, is comparatively large and motion compensation is performed less frequently in the case where m is 2; and because the number (equal to the total number of images input) of images which can be displayed without motion compensation to the input image data is large and motion compensation cannot be performed in the case where m is 1. On the other hand, the larger m becomes, the smoother motion of images can be made because an intermediate image which is generated by motion compensation with high accuracy is used. 
     Note that in the case where a display device is a liquid crystal display device, the conversion ratio can be determined in accordance with a response time of a liquid crystal element. Here, the response time of the liquid crystal element is the time from when a voltage applied to the liquid crystal element is changed until when the liquid crystal element responds. When the response time of the liquid crystal element differs depending on the amount of change of the voltage applied to the liquid crystal element, an average of the response times of plural typical voltage changes can be used. Alternatively, the response time of the liquid crystal element can be defined as MRPT (moving picture response time). Then, by frame rate conversion, the conversion ratio which enables the length of the image display cycle to be near the response time of the liquid crystal element can be determined. Specifically, the response time of the liquid crystal element is preferably the time from the value obtained by multiplication of the cycle of input image data and the inverse number of the conversion ratio, to approximately half that value. In this manner, the image display cycle can be made to correspond to the response time of the liquid crystal element, so that the image quality is improved. For example, when the response time of the liquid crystal element is more than or equal to 4 milliseconds and less than or equal to 8 milliseconds, double-frame rate driving (120 Hz driving) can be employed. This is because the image display cycle of 120 Hz driving is approximately 8 milliseconds and the half of the image display cycle of 120 Hz driving is approximately 4 milliseconds. Similarly, for example, when the response time of the liquid crystal element is more than or equal to 3 milliseconds and less than or equal to 6 milliseconds, triple-frame rate driving (180 Hz driving) can be employed; when the response time of the liquid crystal element is more than or equal to 5 milliseconds and less than or equal to 11 milliseconds, 1.5-fold frame rate driving (90 Hz driving) can be employed; when the response time of the liquid crystal element is more than or equal to 2 milliseconds and less than or equal to 4 milliseconds, quadruple-frame rate driving (240 Hz driving) can be employed; and when the response time of the liquid crystal element is more than or equal to 6 milliseconds and less than or equal to 12 milliseconds, 1.25-fold frame rate driving (80 Hz driving) can be employed. Note that this is similar to the case of other driving frequencies. 
     Note that the conversion ratio can also be determined by a tradeoff between the quality of the moving image, and power consumption and manufacturing cost. That is, the quality of the moving image can be improved by increasing the conversion ratio while power consumption and manufacturing cost can be reduced by decreasing the conversion ratio. Therefore, when n is 10 or less, each conversion ratio has an advantage described below. 
     When the conversion ratio is 1, the quality of the moving image can be improved compared to the case where the conversion ratio is less than 1, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 1. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 1 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 1 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 2, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 2, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 2. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 2 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ½ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 3, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 3, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 3. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 3 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅓ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 3/2, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 3/2, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 3/2. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 3/2 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅔ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 4, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 4, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 4. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 4 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ¼ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 4/3, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 4/3, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 4/3. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 4/3 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ¾ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 5, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 5, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 5. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 5 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅕ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 5/2, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 5/2, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 5/2. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 5/2 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅖ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 5/3, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 5/3, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 5/3. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 5/3 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅗ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 5/4, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 5/4, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 5/4. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 5/4 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅘ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 6, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 6, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 6. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 6 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅙ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 6/5, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 6/5, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 6/5. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 6/5 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅚ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 7, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 7, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 7. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 7 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 1/7 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 7/2, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 7/2, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 7/2. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 7/2 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 2/7 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 7/3, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 7/3, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 7/3. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 7/3 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 3/7 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 7/4, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 7/4, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 7/4. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 7/4 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 4/7 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 7/5, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 7/5, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 7/5. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 7/5 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 5/7 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 7/6, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 7/6, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 7/6. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 7/6 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 6/7 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 8, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 8, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 8. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 8 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅛ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 8/3, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 8/3, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 8/3. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 8/3 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅜ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 8/5, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 8/5, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 8/5. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 8/5 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅝ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 8/7, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 8/7, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 8/7. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 8/7 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately ⅞ times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 9, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 9, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 9. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 9 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 1/9 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 9/2, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 9/2, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 9/2. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 9/2 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 2/9 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 9/4, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 9/4, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 9/4. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 9/4 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 4/9 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 9/5, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 9/5, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 9/5. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 9/5 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 5/9 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 9/7, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 9/7, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 9/7. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 9/7 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 7/9 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 9/8, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 9/8, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 9/8. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 9/8 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 8/9 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 10, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 10, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 10. Moreover, since m is small, power consumption can be reduced while high image quality is obtained. Further, by applying the conversion ratio of 10 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 1/10 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 10/3, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 10/3, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 10/3. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 10/3 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 3/10 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 10/7, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 10/7, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 10/7. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 10/7 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 7/10 times the cycle of input image data, the image quality can be improved. 
     When the conversion ratio is 10/9, the quality of the moving image can be further improved compared to the case where the conversion ratio is less than 10/9, and power consumption and manufacturing cost can be further reduced compared to the case where the conversion ratio is more than 10/9. Moreover, since m is large, motion of the image can be made smoother. Further, by applying the conversion ratio of 10/9 to a liquid crystal display device in which the response time of the liquid crystal elements is approximately 9/10 times the cycle of input image data, the image quality can be improved. 
     Note that it is obvious that each conversion ratio where n is more than 10 also has a similar advantage. 
     Next, as the second step, a method is described in which a plurality of different images (sub-images) are generated from an image based on input image data or each image (hereinafter referred to as an original image) whose frame rate is converted by a given rational number (n/m) times in the first step, and the plurality of sub-images are displayed in temporal succession. In this manner, a method of the second step can make human eyes perceive as if one original image were displayed in appearance, despite the fact that a plurality of different images are displayed. 
     Here, among the sub-images generated from one original image, a sub-image which is displayed first is referred to as a first sub-image. The timing when the first sub-image is displayed is the same as the timing when the original image determined in the first step is displayed. On the other hand, a sub-image which is displayed after that is referred to as a second sub-image. The timing when the second sub-image is displayed can be determined as appropriate regardless of the timing when the original image determined in the first step is displayed. Note that an image which is actually displayed is an image generated from the original image by a method in the second step. Various images can be used for the original image for generating sub-images. The number of sub-images is not limited to two and more than two sub-images are also possible. In the second step, the number of sub-images is represented as J (J is an integer of 2 or more). At this time, a sub-image which is displayed at the same timing as the timing when the original image determined in the first step is displayed is referred to as a first sub-image. Sub-images which are sequentially displayed are referred to as a second sub-image, a third sub image . . . and J-th sub-image in order from a sub-image which is displayed. 
     There are many methods for generating a plurality of sub-images from one original image. As main ones, the following methods can be given. The first one is a method in which the original image is used as it is as the sub-image. The second one is a method in which brightness of the original image is distributed to the plurality of sub-images. The third one is a method in which an intermediate image obtained by motion compensation is used as the sub-image. 
     Here, a method for distributing brightness of the original image to the plurality of sub-images can be further divided into some methods. As main ones, the following methods can be given. The first one is a method in which at least one sub-image is a black image (hereinafter referred to as black data insertion). The second one is a method in which the brightness of the original image is distributed to a plurality of ranges and just one sub-image among all the sub-images is used to control the brightness in the ranges (hereinafter referred to as time-division gray scale control). The third one is a method in which one sub-image is a bright image which is made by changing a gamma value of the original image, and the other sub-image is a dark image which is made by changing the gamma value of the original image (hereinafter referred to as gamma complement). 
     Some of the methods described above are briefly described. In the method in which the original image is used as it is as the sub-image, the original image is used as it is as the first sub-image. Further, the original image is used as it is as the second sub-image. By using this method, a circuit which newly generates a sub-image does not need to operate, or the circuit itself is not necessary, so that power consumption and manufacturing cost can be reduced. Particularly in a liquid crystal display device, this method is preferably used after frame rate conversion using an intermediate image obtained by motion compensation in the first step as an interpolation image. This is because defects such as traces and afterimages of a moving image attributed to shortage of writing voltage due to dynamic capacitance of the liquid crystal elements can be reduced by using the intermediate image obtained by motion compensation as the interpolation image to make motion of the moving image smooth and displaying the same image repeatedly. 
     Next, in the method in which the brightness of the original image is distributed to the plurality of sub-images, a method for setting the brightness of the image and the length of a period when the sub-images are displayed is specifically described. Note that J is the number of sub-images, and an integer of 2 or more. The lower case j and capital J are distinguished. The lower case j is an integer of more than or equal to 1 and less than or equal to J. The brightness of a pixel in normal hold driving is L, the cycle of original image data is T, the brightness of a pixel in a j-th sub-image is L j , and the length of a period when the j-th sub-image is displayed is T j . The total sum of products of L j  and T j  where j=1 to where j=J (L1T1+L2T2+ . . . +LJTJ) is preferably equal to a product of L and T (LT) (brightness is unchangeable). Further, the total sum of T where j=1 to where j=J is preferably equal to T (a display cycle of the original image is maintained). Here, unchangeableness of brightness and maintenance of the display cycle of the original image is referred to as sub-image distribution condition. 
     In the methods for distributing brightness of the original image to a plurality of sub-images, black data insertion is a method in which at least one sub-image is made a black image. In this manner, a display method can be made close to pseudo impulse display so that deterioration of quality of moving image due to hold-type display method can be prevented. In order to prevent decrease in brightness due to black data insertion, sub-image distribution condition is preferably satisfied. However, in the situation that decrease in brightness of the displayed image is acceptable (dark surrounding or the like) or in the case where decrease in brightness of the displayed image is set to be acceptable by the user, sub-image distribution condition is not necessarily satisfied. For example, one sub-image may be the same as the original image and the other sub-image can be a black image. In this case, power consumption can be reduced compared to the case where sub-image distribution condition is satisfied. Further, in a liquid crystal display device, when one sub-image is made by increasing the whole brightness of the original image without limitation of the maximum brightness, sub-image distribution condition can be satisfied by increasing brightness of a backlight. In this case, since sub-image distribution condition can be satisfied without controlling the voltage value which is applied to a pixel, operation of an image processing circuit can be omitted, so that power consumption can be reduced. 
     Note that a feature of black data insertion is to make L j  of all pixels 0 in any one of sub-images. In this manner, a display method can be made close to pseudo impulse display, so that deterioration of quality of a moving image due to a hold-type display method can be prevented. 
     In the methods for distributing the brightness of the original image to a plurality of sub-images, time-division gray scale control is a method in which brightness of the original image is divided into a plurality of ranges and brightness in that range is controlled by just one sub-image among all sub-images. In this manner, a display method can be made close to pseudo impulse display without decrease in brightness. Therefore, deterioration of quality of moving image due to a hold-type display method can be prevented. 
     As a method for dividing the brightness of the original image into a plurality of ranges, a method in which the maximum brightness (L max ) is divided into the number of sub-images can be given. This method is described with a display device which can adjust brightness of 0 to L max  by 256 grades (from the grade 0 to 255) in the case where two sub-images are provided. When the grade 0 to 127 is displayed, brightness of one sub-image is adjusted in a range of the grade 0 to 255 while brightness of the other sub-image is set to be the grade 0. When the grade 128 to 255 is displayed, the brightness of on sub-image is set to be 255 while brightness of the other sub-image is adjusted in a range of the grade 0 to 255. In this manner, this method can make human eyes perceive as if an original image is displayed and make a display method close to pseudo impulse display, so that deterioration of quality of an moving image due to a hold-type display method can be prevented. Note that more than two sub-images can be provided. For example, if three sub-images are provided, the grade (grade 0 to 255) of brightness of an original image is divided into three. In some cases, the number of grades of brightness is not divisible by the number of sub-images, depending on the number of grades of brightness of the original image and the number of sub-images; however, the number of grades of brightness which is included in a range of each divided brightness can be distributed as appropriate even if the number of grades of brightness is not just the same as the number of sub-images. 
     In the case of time-division gray scale control, by satisfying sub-image distribution condition, the same image as the original image can be displayed without decrease in brightness or the like, which is preferable. 
     In the methods for distributing brightness of the original image to a plurality of sub-images, gamma complement is a method in which one sub-image is made a bright image by changing the gamma characteristic of the original image while the other sub-image is made a dark image by changing the gamma characteristic of the original image. In this manner, a display method can be made close to pseudo impulse display without a decrease in brightness. Therefore, deterioration of quality of moving image due to a hold-type display method can be prevented. Here, a gamma characteristic is a degree of brightness with respect to a grade (gray scale) of brightness. In general, a line of the gamma characteristic is adjusted so as to be close to a linear shape. This is because a smooth gray scale can be obtained if change in brightness is proportion to one gray scale in the grade of brightness. In gamma complement, the curve of the gamma characteristic of one sub-image is deviated from the linear shape so that the one sub-image is brighter than a sub-image in the linear shape in a region of intermediate brightness (halftone) (the image in halftone is brighter than as it usually is). Further, a line of the gamma characteristic of the other sub-image is also deviated from the linear shape so that the other sub-image is darker than the sub-image in the linear shape in a region of intermediate brightness (the image in halftone is darker than as it usually is). Here, the amount of change for brightening the one sub-image than that in the linear shape, and the amount of change for darkening the other sub-image than the sub-image in the linear shape, are preferably almost the same. This method can make human eyes perceive as if an original image is displayed and a decrease in quality of a moving image due to a hold-type display method can be prevented. Note that more than two sub-images can be provided. For example, if three sub-images are provided, each gamma characteristic of three sub-images are adjusted and the sum of the amounts of change for brightening sub-images, and the sum of the amounts of change for darkening sub-images are almost the same. 
     Note that also in the case of gamma complement, by satisfying sub-image distribution condition, the same image as the original image can be displayed without decrease in brightness or the like, which is preferable. Further, in gamma complement, since change in brightness L j  of each sub-image with respect to gray scale follows a gamma curve, the gray scale of each sub-image can be displayed smoothly by itself. Therefore, there is an advantage that image quality to be perceived by human eyes is improved. 
     A method in which an intermediate image obtained by motion compensation is used as a sub-image is a method in which one sub-image is an intermediate image obtained by motion compensation using previous and next images. In this manner, motion of images can be made smooth and quality of a moving image can be improved. 
     The relation between the timing when a sub-image is displayed and a method of generating a sub-image is described. Although the timing when the first sub-image is displayed is the same as that when the original image determined in the first step is displayed, and the timing when the second sub-image is displayed can be decided as appropriate regardless of the timing when the original image determined in the first step is displayed, the sub-image itself may be changed in accordance with the timing when the second sub-image is displayed. In this manner, even if the timing when the second sub-image is displayed is changed variously, human eyes can be made to perceive as if the original image is displayed. Specifically, if the timing when the second sub-image is displayed is earlier, the first sub-image can be brighter and the second sub-image can be darker. Further, if the timing when the second sub-image is displayed is later, the first sub-image may be darker and the second sub-image may be brighter. This is because brightness perceived by human eyes changes in accordance with the length of a period when an image is displayed. More specifically, the longer the length of the period when an image is displayed becomes, the higher brightness perceived by human eyes becomes while the shorter the length of the period when an image is displayed becomes, the lower brightness perceived by human eyes becomes. That is, by making the timing when the second sub-image is displayed earlier, the length of the period when the first sub-image is displayed becomes shorter and the length of period when the second sub-image is displayed becomes longer. This means human eyes perceive as if the first sub-image is dark and the second sub-image is bright. As a result, a different image from the original image is perceived by human eyes. In order to prevent this, the first sub-image can be made much brighter and the second sub-image can be made much darker. Similarly, by making the timing when the second sub-image is displayed later, the length of the period when the first sub-image is displayed becomes longer, and the length of the period when the second sub-image is displayed becomes shorter; in such a case, the first sub-image can be made much darker and the second sub-image can be made much brighter. 
     In accordance with the above description, procedures in the second step is shown below. As a procedure 1, a method for generating a plurality of sub-images from one original image is decided. More specifically, a method for generating a plurality of sub-images can be selected from a method in which an original image is used as it is as a sub-image, a method in which brightness of an original image is distributed to a plurality of sub-images, and a method in which an intermediate image obtained by motion compensation is used as a sub-image. As a procedure 2, the number J of sub-images is decided. Note that J is an integer of 2 or more. As a procedure 3, the brightness L j  of a pixel in j-th sub-image and the length of the period T j  when the j-th sub-image is displayed are decided in accordance with the method shown in the procedure 1. Through the procedure 3, the length of a period when each sub-image is displayed and the brightness of each pixel included in each sub-image are specifically decided. As a procedure 4, the original image is processed in accordance with what decided in respective procedures 1 to 3 to actually perform display. As a procedure 5, the objective original image is shifted to the next original image and the operation returns to the procedure 1. 
     Note that a mechanism for performing the procedures in the second step may be mounted on a device or decided in the design phase of the device in advance. When the mechanism for performing the procedures in the second step is mounted on the device, a driving method can be switched so that an optimal operation depending on circumstances can be performed. Note that the circumstances here include contents of image data, environment inside and outside the device (e.g., temperature, humidity, barometric pressure, light, sound, an electromagnetic field, an electric field, radiation quantity, an altitude, acceleration, or movement speed), user setting, a software version, and the like. On the other hand, when the mechanism for performing the procedures in the second step is decided in the design phase of the device in advance, driver circuits optimal for respective driving methods can be used. Further, since the mechanism is determined, reduction in manufacturing cost due to efficiency of mass production can be expected. 
     Next, various driving methods are employed depending on the procedures in the second step and are described in detail, specifically showing values of n and m in the first step. 
     In the procedure 1 in the second step, in the case where a method using an original image as it is as a sub-image is selected, the driving method is as follows. 
     One feature of a driving method of the display device is that i-th (i is a positive integer) image data and (i+1)th image data are sequentially prepared in a constant cycle T. The cycle T is divided into J (J is an integer equal to or more than 2) sub-image display periods. The i-th image data is data which can make each of a plurality of pixels have unique brightness L. The j-th (f is an integer equal to or more than 1, and equal to or less than J) sub-image is formed by arranging the plurality of pixels each having unique brightness L j , and is an image displayed only during the j-th sub-image display period T j . The aforementioned L, T, L j , and T j  satisfy the sub-image distribution condition. In all values of j, the brightness L j  of each pixel which is included in the j-th sub-image is equal to L. Here, as image data which are prepared sequentially in a constant cycle T, the original image data which is formed in the first step can be used. That is, all display patterns given in the description of the first step can be combined with the above mentioned driving method. 
     Then, in the case where the number of sub-images J is determined to be 2 in the procedure 2 in the second step, and it is determined that T j =T 2 =T/2 in the procedure 3, the above-mentioned driving method is as shown in  FIG. 69 . In  FIG. 69 , the horizontal axis indicates time, and the vertical axis indicates cases which are classified with respect to various values of n and m used in the first step. 
     For example, in the first step, in the case of n=1 and m=1, that is, when the conversion ratio (n/m) is 1, a driving method as shown in the case of n=1 and m=1 in  FIG. 69  is employed. At this time, the display frame rate is twice (double-frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 120 Hz (120 Hz driving). Then, two images are continuously displayed with respect to one piece of input image data. Here, in the case of double-frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the double-frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the double-frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 120 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 120 Hz and the frequency of alternating-current driving is an integer multiple of 120 Hz or a unit fraction of 120 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately half the cycle of input image data. 
     Further, for example, in the first step, in the case of n=2 and m=1, that is, when the conversion ratio (n/m) is 2, a driving method as shown in the case of n=2 and m=1 in  FIG. 69  is employed. At this time, the display frame rate is 4-fold (quadruple-frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 240 Hz (240 Hz driving). Then, four images are continuously displayed with respect to one piece of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of quadruple-frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the quadruple-frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the quadruple-frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particularly, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 240 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 240 Hz and the frequency of alternating-current driving is an integer multiple of 240 Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately quarter the cycle of input image data. 
     Further, for example, in the first step, in the case of n=3 and m=1, that is, when the conversion ratio (n/m) is 3, a driving method as shown in the case of n=3 and m=1 in  FIG. 69  is employed. At this time, the display frame rate is 6-fold (6-fold frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 360 Hz (360 Hz driving). Then, six images are continuously displayed with respect to one piece of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of 6-fold frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the 6-fold frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the 6-fold frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 360 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 360 Hz and the frequency of alternating-current driving is an integer multiple of 360 Hz or a unit fraction of 360 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately ⅙ times the cycle of input image data. 
     Further, for example, in the first step, in the case of n=3 and m=2, that is, when the conversion ratio (n/m) is 3/2, a driving method as shown in the case of n=3 and m=2 in  FIG. 69  is employed. At this time, the display frame rate is triple (triple frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 180 Hz (180 Hz driving). Then, three images are continuously displayed with respect to one piece of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of triple frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the triple frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the triple frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 180 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 180 Hz and the frequency of alternating-current driving is an integer multiple of 180 Hz or a unit fraction of 180 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately ⅓ times the cycle of input image data. 
     Further, for example, in the first step, in the case of n=4 and m=1, that is, when the conversion ratio (n/m) is 4, a driving method as shown in the case of n=4 and m=1 in  FIG. 69  is employed. At this time, the display frame rate is 8-fold (8-fold frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 480 Hz (480 Hz driving). Then, eight images are continuously displayed with respect to one piece of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of 8-fold frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the 8-fold frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the 8-fold frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 480 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 480 Hz and the frequency of alternating-current driving is an integer multiple of 480 Hz or a unit fraction of 480 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately ⅛ times the cycle of input image data. 
     Further, for example, in the first step, in the case of n=4 and m=3, that is, when the conversion ratio (n/m) is 4/3, a driving method as shown in the case of n=4 and m=3 in  FIG. 69  is employed. At this time, the display frame rate is 8/3 times (8/3-fold frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 160 Hz (160 Hz driving). Then, eight images are continuously displayed with respect to three pieces of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of 8/3-fold frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the 8/3-fold frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the 8/3-fold frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 160 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 160 Hz and the frequency of alternating-current driving is an integer multiple of 160 Hz or a unit fraction of 160 Hz (e.g., 40 Hz, 80 Hz, 160 Hz, or 320 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately ⅜ times the cycle of input image data. 
     Further, for example, in the first step, in the case of n=5 and m=1, that is, when the conversion ratio (n/m) is 5, a driving method as shown in the case of n=5 and m=1 in  FIG. 69  is employed. At this time, the display frame rate is 10-fold (10-fold frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 600 Hz (600 Hz driving). Then, ten images are continuously displayed with respect to one piece of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of 10-fold frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the 10-fold frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the 10-fold frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 600 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 600 Hz and the frequency of alternating-current driving is an integer multiple of 600 Hz or a unit fraction of 600 Hz (e.g., 30 Hz, 60 Hz, 100 Hz, or 120 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately 1/10 times the cycle of input image data. 
     Further, for example, in the first step, in the case of n=5 and m=2, that is, when the conversion ratio (n/m) is 5/2, a driving method as shown in the case of n=5 and m=2 in  FIG. 69  is employed. At this time, the display frame rate is 5-fold (5-fold frame rate driving) as high as the frame rate of input image data. Specifically, for example, when the input frame rate is 60 Hz, the display frame rate is 300 Hz (300 Hz driving). Then, five images are continuously displayed with respect to one piece of input image data. At this time, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. In the case of 5-fold frame rate driving, quality of moving images can be improved compared to the case where the frame rate is lower than that of the 5-fold frame rate driving, and power consumption and manufacturing cost can be reduced compared to the case where the frame rate is higher than that of the 5-fold frame rate driving. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Moreover, a combination of 300 Hz driving and alternating-current driving of a liquid crystal display device is effective. That is, when the driving frequency of the liquid crystal display device is 300 Hz and the frequency of alternating-current driving is an integer multiple of 300 Hz or a unit fraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz), flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately ⅕ times the cycle of input image data. 
     As described above, when a method in which an original image is used as it is as a sub-image is selected the procedure 1 in the second step; the number of sub-images is determined to be 2 in the procedure 2 in the second step; when it is determined that T 1 =T 2 =T/2 in the procedure 3 in the second step, the display frame rate can be double of the display frame rate obtained by the frame rate conversion using a conversion ratio determined by the values of n and m in the first step; thus, quality of moving images can be further improved. Further, the quality of moving images can be improved compared to the case where a display frame rate is lower than the display frame rate, and power consumption and manufacturing cost can be reduced compared to the case where a display frame rate is higher than the display frame rate. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Furthermore, when the driving frequency of the liquid crystal display device is made high and the frequency of alternating-current driving is an integer multiple or a unit fraction, flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(double the conversion ratio)) times the cycle of input image data. 
     Note that it is obvious that there are similar advantages in the case of using a conversion ratio than those described above, though detailed description is omitted. For example when n is 10 or less, the following combinations are possible in addition to the above mentioned cases: n=5, m=3, that is, the conversion ratio (n/m)=5/3 (10/3-fold frame rate driving, 200 Hz), n=5, m=4, that is, the conversion ratio (n/m)=5/4 (5/2-fold frame rate driving, 150 Hz), n=6, m=1, that is, the conversion ratio (n/m)=6 (12-fold frame rate driving, 720 Hz), n=6, m=5, that is, the conversion ratio (n/m)=6/5 (12/5-fold frame rate driving, 144 Hz), n=7, m=1, that is, the conversion ratio (n/m)=7 (14-fold frame rate driving, 840 Hz), n=7, m=2, that is, the conversion ratio (n/m)=7/2 (7-fold frame rate driving, 420 Hz), n=7, m=3, that is, the conversion ratio (n/m)=7/3 (14/3-fold frame rate driving, 280 Hz), n=7, m=4, that is, the conversion ratio (n/m)=7/4 (7/2-fold frame rate driving, 210 Hz), n=7, m=5, that is, the conversion ratio (n/m)=7/5 (14/5-fold frame rate driving, 168 Hz), n=7, m=6, that is, the conversion ratio (n/m)=7/6 (7/3-fold frame rate driving, 140 Hz), n=8, m=1, that is, the conversion ratio (n/m)=8 (16-fold frame rate driving, 960 Hz), n=8, m=3, that is, the conversion ratio (n/m)=8/3 (16/3-fold frame rate driving, 320 Hz), n=8, m=5, that is, the conversion ratio (n/m)=8/5 (16/5-fold frame rate driving, 192 Hz), n=8, m=7, that is, the conversion ratio (n/m)=8/7 (16/7-fold frame rate driving, 137 Hz), n=9, m=1, that is, the conversion ratio (n/m)=9 (18-fold frame rate driving, 1080 Hz), n=9, m=2, that is, the conversion ratio (n/m)=9/2 (9-fold frame rate driving, 540 Hz), n=9, m=4, that is, the conversion ratio (n/m)=9/4 (9/2-fold frame rate driving, 270 Hz), n=9, m=5, that is, the conversion ratio (n/m)=9/5 (18/5-fold frame rate driving, 216 Hz), n=9, m=7, that is, the conversion ratio (n/m)=9/7 (18/7-fold frame rate driving, 154 Hz), n=9, m=8, that is, the conversion ratio (n/m)=9/8 (9/4-fold frame rate driving, 135 Hz), n=10, m=1, that is, the conversion ratio (n/m)=10 (20-fold frame rate driving, 1200 Hz), n=10, m=3, that is, the conversion ratio (n/m)=10/3 (20/3-fold frame rate driving, 400 Hz), n=10, m=7, that is, the conversion ratio (n/m)=10/7 (20/7-fold frame rate driving, 171 Hz), and n=10, m=9, that is, the conversion ratio (n/m)=10/9 (20/9-fold frame rate driving, 133 Hz). Note that these frequencies are examples in the case where the input frame rate is 60 Hz. As for other frame rates, the product of an input frame rate multiplied by double of conversion ratio in each case is a driving frequency. 
     Although specific numbers for n and m in the case where n is an integer more than 10 are not described here, the procedure in the second step can be obviously applied to various values of n and m. 
     Note that in the case of J=2, it is particularly effective that the conversion ratio in the first step is larger than 2. This is because when the number of sub-images is comparatively smaller like J=2 in the second step, the conversion ratio in the first step can be higher. Such a conversion ratio includes 3, 4, 5, 5/2, 6, 7, 7/2, 7/3, 8, 8/3, 9, 9/2, 9/4, 10, and 10/3, when n is equal to or less than 10. When display frame rate after the first step is such a value, by setting the value of J at 3 or more balance between an advantage (e.g., reduction in power consumption and manufacturing cost) by the number of sub-images in the second step being small and an advantage (e.g., increase of moving image quality, reduction of flickers) by the final display frame rate being high can be achieved. 
     Note that although the case where the number of sub-images J is determined to be 2 in the procedure 2 and it is determined that T 1 =T 2 =T/2 in the procedure 3 has been described here, the present invention is not limited to this obviously. 
     For example, in the case where it is determined that T 1 &lt;T 2  in the procedure 3 in the second step, the first sub-image can be brightened and the second sub-image can be darkened. Further, in the case where it is determined that T 1 &gt;T 2  in the procedure 3 in the second step, the first sub-image can be darkened and the second sub-image can be brightened. Thus, display method can be made close to pseudo impulse driving, while the original image can be perceived by human eyes; therefore, quality of moving images can be improved. Note that when a method in which an original image is used as it is as a sub-image is selected in the procedure 1 as the case of the above-mentioned driving method, the sub-image can be directly displayed without changing the brightness of the sub-image. This is because an image which is used as a sub-image is the same in this case, and the original image can be displayed adequately regardless of display timing of the sub-image. 
     Further, it is obvious that the number of sub-images J may be another value instead of 2 in the procedure 2. In this case, the display frame rate can be J times as high as the display frame rate obtained by the frame rate conversion using a conversion ratio determined by the values of n and m in the first step; thus, quality of moving images can be further improved. Further, the quality of moving images can be improved compared to the case where a display frame rate is lower than the display frame rate, and power consumption and manufacturing cost can be reduced compared to the case where a display frame rate is higher than the display frame rate. Further, in the procedure 1 in the second step, when a method in which an original image is used as it is as a sub-image is selected, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Furthermore, when the driving frequency of the liquid crystal display device is made high and the frequency of alternating-current driving is an integer multiple or a unit fraction, flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(J times the conversion ratio)) of the cycle of input image data. 
     For example, in the case of J=3, particularly there is advantages that the quality of moving images can be improved compared to the case where the number of sub-images is smaller than 3, and that power consumption and manufacturing cost can be reduced compared to the case where the number of sub-images is larger than 3. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(three times the conversion ratio)) of the cycle of input image data. 
     For example, in the case of J=4, particularly there is advantages that the quality of moving images can be improved compared to the case where the number of sub-images is smaller than 4, and that power consumption and manufacturing cost can be reduced compared to the case where the number of sub-images is larger than 4. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(four times the conversion ratio)) of the cycle of input image data. 
     For example, in the case of J=5, particularly there is advantages that the quality of moving images can be improved compared to the case where the number of sub-images is smaller than 5, and that power consumption and manufacturing cost can be reduced compared to the case where the number of sub-images is larger than 5. Moreover, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(five times the conversion ratio)) of the cycle of input image data. 
     Furthermore, there are similar advantages even in the case where the number of J is any number other than the above mentioned numbers. 
     Note that in the case of J=3 or more, the conversion ratio in the first step can be various values. The case of J=3 or more is effective particularly when the conversion ratio in the first step is relatively small (equal to or less than 2). This is because when the display frame rate after the first step is relatively lower, J can be larger in the second step. Such a conversion ratio includes 1, 2, 3/2, 4/3, 5/3, 5/4, 6/5, 7/4, 7/5, 7/6, 8/7, 9/5, 9/7, 9/8, 10/7, and 10/9 when n is equal to or less than 10.  FIG. 72  shows the case where the conversion ratio is 1, 2, 3/2, 4/3, 5/3, and 5/4 among the above-described conversion ratios. As described above, when the display frame rate after the first step is a relatively small value, by setting the value of J at 3 or more balance between an advantage (e.g., reduction in power consumption and manufacturing cost) by the number of sub-images in the first step being small and an advantage (e.g., increase of moving image quality, reduction of flickers) by the final display frame rate being high can be achieved. 
     Next, another example of the driving method determined by the procedure in the second step is described. 
     In the procedure 1 in the second step, when black data insertion is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, the driving method is as follows. 
     One feature of a driving method of the display device is that i-th (i is a positive integer) image data and (i+1)th image data are sequentially prepared in a constant cycle T. The cycle T is divided into J (J is an integer equal to or more than 2) sub-image display periods. The i-th image data is data which can make each of a plurality of pixels have unique brightness L. The j-th (j is an integer equal to or more than 1, and equal to or less than J) sub-image is formed by arranging a plurality of pixels each having unique brightness L j , and is an image which is displayed only during the j-th sub-image display period T j . The aforementioned L, T, L j , and T j  satisfy the sub-image distribution condition. In at least one value of j, the brightness L j  of all pixels which are included in the j-th sub-image is equal to 0. Here, as image data which are prepared sequentially in a constant cycle T, the original image data which is formed in the first step can be used. That is, all display patterns given in the description of the first step can be combined with the above mentioned driving method. 
     It is obvious that the driving method can be implemented by combining various values of n and m which are used in the first step. 
     Then, when the number of sub-images J is determined to be 2 in the procedure 2 in the second step, and it is determined that T 1 =T 2 =T/2 in the procedure 3, the driving method can be as shown in  FIG. 69 . Since features and advantages of the driving method (display timing using various values of n and m) shown in  FIG. 69  have already been described, detailed description is omitted here. In the procedure 1 in the second step, even when black data insertion is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, it is obvious that similar advantages can be obtained. For example, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of a moving image can be made smooth; thus, quality of moving images can be significantly improved. The quality of moving images can be improved when the display frame rate is high, and power consumption and manufacturing cost can be reduced when the display frame rate is low. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. 
     In the procedure 1 in the second step, as a typical advantage of selecting black data insertion among methods in which brightness of the original image is distributed to a plurality of sub-images, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, the display method can be made close to pseudo impulse driving regardless of the gray scale value included in the image data; therefore, quality of a moving image can be improved. 
     Note that the case where the number of sub-images J is determined to be 2 in the procedure 2 and it is determined that T 1 =T 2 =T/2 in the procedure 3 has been described here, the present invention is not limited to this obviously. 
     For example, in the case where it is determined that T 1 &lt;T 2  in the procedure 3 in the second step, the first sub-image can be brightened and the second sub-image can be darkened. Further, in the case where it is determined that T 1 &gt;T 2  in the procedure 3 in the second step, the first sub-image can be darkened and the second sub-image can be brightened. Thus, the display method can be pseudo impulse driving, while the original image can be perceived by human eyes; therefore, quality of moving images can be improved. Note that as in the case of the above-mentioned driving method, when black data insertion is selected among methods in which brightness of the original image is distributed to a plurality of sub-images in the procedure 1, the sub-image may be directly displayed without changing the brightness of the sub-image. This is because when the brightness of the sub-image is not changed, the original image is merely displayed in such a manner that entire brightness of the original image is low. That is, when this method is positively used for controlling the brightness of the display device, brightness can be controlled and the quality of moving images increases at the same time. 
     Further, it is obvious that the number of sub-images J may be another value instead of 2 in the procedure 2. Since advantages in that case have been already described, detailed description is omitted here. In the procedure 1 in the second step, even when black data insertion is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, it is obvious that similar advantages can be obtained. For example, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(J times the conversion ratio)) of the cycle of input image data. 
     Next, another example of the driving method determined by the procedure in the second step is described. 
     In the procedure 1 in the second step, when a time ratio gray scale controlling method is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, the driving method is as follows. 
     One feature of a driving method of the display device is that i-th (i is a positive integer) image data and (i+1)th image data are sequentially prepared in a constant cycle T. The cycle T is divided into J (J is an integer equal to or more than 2) sub-image display periods. The i-th image data is data which can make each of a plurality of pixels have unique brightness L. The maximum value of the unique brightness L is L max . The j-th (j is an integer equal to or more than 1, and equal to or less than J) sub-image is formed by arranging a plurality of pixels each having unique brightness L j  and is an image which is displayed only during the j-th sub-image display period T j . The aforementioned L, T, L j , and T j  satisfy the sub-image distribution condition. When the unique brightness L is displayed, the brightness is adjusted in the range of from (j−1)×L max /J to J×L max /J by adjusting brightness in only one sub-image display period among the J sub-image display periods. Here, as image data which are prepared sequentially in a constant cycle T, the original image data which is formed in the first step can be used. That is, all display patterns given in the description of the first step can be combined with the above mentioned driving method. 
     It is obvious that the driving method can be implemented by combining various values of n and m which are used in the first step. 
     Then, when the number of sub-images J is determined to be 2 in the procedure 2 in the second step, and it is determined that T 1 =T 2 =T/2 in the procedure 3, the driving method can be as shown in  FIG. 69 . Since features and advantages of the driving method (display timing using various values of n and m) shown in  FIG. 69  have already been described, detailed description is omitted here. In the procedure 1 in the second step, even when the time ratio gray scale controlling method is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, it is obvious similar advantages can be obtained. For example, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of a moving image can be made smooth; thus, quality of moving images can be significantly improved. The quality of moving images can be improved when the display frame rate is high, and power consumption and manufacturing cost can be reduced when the display frame rate is low. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. 
     In the procedure 1 in the second step, as a typical advantage of selecting the time ratio gray scale controlling method among methods in which brightness of the original image is distributed to a plurality of sub-images, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, since the display method can be pseudo impulse driving, quality of a moving image can be improved, and since brightness of the display device does not become lower, power consumption can be further reduced. 
     Note that although the case where the number of sub-images J is determined to be 2 in the procedure 2 and it is determined that T 1 =T 2 =T/2 in the procedure 3 has been described here, the present invention is not limited to this obviously. 
     For example, in the case where it is determined that T 1 &lt;T 2  in the procedure 3 in the second step, the first sub-image can be brightened and the second sub-image can be darkened. Further, in the case where it is determined that T 1 &gt;T 2  in the procedure 3 in the second step, the first sub-image can be darkened and the second sub-image can be brightened. Thus, the display method can be made close to pseudo impulse driving, while the original image can be perceived by human eyes; therefore, quality of moving image can be improved. 
     Further, it is obvious that the number of sub-images J may be another value instead of 2 in the procedure 2. Since advantages in that case have been already described, detailed description is omitted here. In the procedure 1 in the second step, even when the time ratio gray scale controlling method is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, it is obvious similar advantages can be obtained. For example, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(J times the conversion ratio)) of the cycle of input image data. 
     Next, another example of the driving method determined by the procedure in the second step is described. 
     In the procedure 1 in the second step, when gamma complement is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, the driving method is as follows. 
     One feature of a driving method of the display device is that i-th (i is a positive integer) image data and (i+1)th image data are sequentially prepared in a constant cycle T. The cycle T is divided into J (J is an integer equal to or more than 2) sub-image display periods. The i-th image data is data which can make each of a plurality of pixels have unique brightness L. The j-th (j is an integer equal to or more than 1, and equal to or less than J) sub-image is formed by arranging a plurality of pixels each having unique brightness L j , and is an image which is displayed only during the j-th sub-image display period T j . The aforementioned L, T, L j , and T j  satisfy the sub-image distribution condition. In each sub-image, characteristics of a change of brightness with respect to the gray scale is changed from the linear shape, and total amount of brightness which is changed to a blighter area from the linear shape and the total amount of brightness which is changed to a darker area from the linear shape are almost the same in all gray scale. Here, as image data which are prepared sequentially in a constant cycle T, the original image data which is formed in the first step can be used. That is, all display patterns given in the description of the first step can be combined with the above-mentioned driving method. 
     It is obvious that the driving method can be implemented by combining various values of n and m which are used in the first step. 
     Then, when the number of sub-images J is determined to be 2 in the procedure 2 in the second step, and it is determined that T 1 =T 2 =T/2 in the procedure 3, the driving method can be as shown in  FIG. 69 . Since features and advantages of the driving method (display timing using various values of n and m) shown in  FIG. 69  have already been described, detailed description is omitted here. In the procedure 1 in the second step, even when gamma complement is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, it is obvious similar advantages can be obtained. For example, when an interpolated image in the first step is an intermediate image obtained by motion compensation, motion of moving images can be made smooth; thus, quality of moving images can be significantly improved. The quality of moving images can be improved when the display frame rate is high, and power consumption and manufacturing cost can be reduced when the display frame rate is low. Further, when a display device is an active matrix liquid crystal display device, a problem of shortage of writing voltage due to dynamic capacitance can be avoided; thus, quality of moving images can be significantly improved while defects, in particular, such as a phenomenon of a moving image in which traces are seen and an afterimage are reduced. Flickers which appear by alternating-current driving can be reduced so as not to be perceived by human eyes. 
     In the procedure 1 in the second step, as a typical advantage of selecting gamma complement among methods in which brightness of the original image is distributed to a plurality of sub-images, a circuit operation which produces an intermediate image by motion compensation can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, since the display method can be made close to pseudo impulse driving regardless of the gray scale value included in the image data, quality of a moving image can be improved. Moreover, image data may be directly subjected to gamma conversion to obtain a sub-image. In this case, there is an advantage in that the gamma value can be controlled variously by the amount of movement of a moving image. Further, without the image data being directly subjected to gamma conversion, a sub-image whose gamma value is changed may be obtained by change of the reference voltage of a digital-to-analog converter circuit (DAC). In this case, since the image data is not directly subjected to gamma conversion, a circuit operation for gamma conversion can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. Further, in gamma complement, since the change of the brightness L j  of each sub-image with respect to gray scale follows a gamma curve, the gray scale of each sub-image can be displayed smoothly by itself; therefore, there is an advantage in that image quality to be perceived in the end by human eyes is improved. 
     Note that although the case where the number of sub-images J is determined to be 2 in the procedure 2 and it is determined that T 1 =T 2 =T/2 in the procedure 3 has been described here, the present invention is not limited to this obviously. 
     For example, in the case where it is determined that T 1 &lt;T 2  in the procedure 3 in the second step, the first sub-image can be brightened and the second sub-image can be darkened. Further, in the case where it is determined that T 1 &gt;T 2  in the procedure 3 in the second step, the first sub-image can be darkened and the second sub-image can be brightened. Thus, the display method can be made close to pseudo impulse driving, while the original image can be perceived by human eyes; therefore, quality of moving images can be improved. In the procedure 1, when gamma complement is selected among methods in which brightness of the original image is distributed to a plurality of sub-images as in the case of the above-mentioned driving method, the gamma value may be changed in the case where brightness of the sub-image is changed. That is, the gamma value may be determined in accordance with display timing of the second sub-image. Accordingly, the operation of a circuit for changing brightness of the entire image can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. 
     Further, it is obvious that the number of sub-images J may be another value instead of 2 in the procedure 2. Since advantages in that case have been already described, detailed description is omitted here. In the procedure 1 in the second step, even when gamma complement is selected among methods in which brightness of the original image is distributed to a plurality of sub-images, it is obvious similar advantages can be obtained. For example, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(J times the conversion ratio)) of the cycle of input image data. 
     Next, another example of the driving method determined by the procedure in the second step is described in detail. 
     When a method in which an intermediate image obtained by motion compensation is used as a sub-image is selected in the procedure 1 in the second step; when the number of sub-images is determined to be 2 in the procedure 2 in the second step; and when it is determined that T 1 =T 2 =T/2 in the procedure 3 in the second step, the driving method determined by the procedures in the second step can be as follows. 
     One feature of a driving method of the display device is that i-th (i is a positive integer) image data and (i+1)th image data are sequentially prepared in a constant cycle T. A k-th (k is a positive integer) image, a (k+1)th image, and a (k+2)th image are sequentially displayed at half interval of the period of the original image data. The k-th image is displayed in accordance with the i-th image data. The (k+1)th image is displayed in accordance with the image data which corresponds to half amount of the movement of from the i-th image data to the (i+1)th image data. The (k+2)th image is displayed in accordance with the (i+1)th image data. Here, as the image data which are prepared sequentially in a constant cycle T, the original image data which is formed in the first step can be used. That is, all display patterns given in the description of the first step can be combined with the above-mentioned driving method. 
     It is obvious that the driving method can be implemented by combining various values of n and m which are used in the first step. 
     In the procedure 1 in the second step, a typical advantage of selecting a method in which an intermediate image obtained by motion compensation is used as a sub-image is that a method for obtaining an intermediate image employed in the first step can be similarly used in the second step when an intermediate image obtained by motion compensation is an interpolated image. That is, a circuit for obtaining an intermediate image by motion compensation can be used not only in the first step, but also in the second step, whereby the circuit can be used efficiently and treatment efficiency can be increased. In addition, motion of moving images can be made further smooth; thus, quality of moving images can be further improved. 
     Note that although the case where the number of sub-images J is determined to be 2 in the procedure 2 and it is determined that T 1 =T 2 =T/2 in the procedure 3 has been described here, the present invention is not limited to this obviously. 
     For example, in the case where it is determined that T 1 &lt;T 2  in the procedure 3 in the second step, the first sub-image can be brightened and the second sub-image can be darkened. Further, in the case where it is determined that T 1 &gt;T 2  in the procedure 3 in the second step, the first sub-image can be darkened and the second sub-image can be brightened. Thus, the display method can be made close to pseudo impulse driving, while the original image can be perceived by human eyes; therefore, quality of moving images can be improved. Note that as in the case of the above-mentioned driving method, when a method in which an intermediate image obtained by motion compensation is used as a sub-image is selected in the procedure 2, it is not necessary that brightness of the sub-image is changed. This is because the image in an intermediate state is completed as an image in itself, and even when display timing of the second sub-image is changed, the image which is perceived by human eyes is not changed. In this case, the operation of a circuit for changing brightness of the entire image can be stopped, or the circuit itself can be omitted from the device, whereby power consumption and manufacturing cost of the device can be reduced. 
     Further, it is obvious that the number of sub-images J may be another value instead of 2 in the procedure 2. Since advantages in that case have been already described, detailed description is omitted here. In the procedure 1 in the second step, even when a method in which an intermediate image obtained by motion compensation is used as a sub-image is selected, it is obvious similar advantages can be obtained. For example, image quality can be improved by applying the driving method to the liquid crystal display device in which the response time of the liquid crystal element is approximately (1/(J times the conversion ratio)) of the cycle of input image data. 
     Next, specific examples of a method for converting the frame rate when the input frame rate and the display frame rate are different are described with reference to  FIGS. 71A to 71C . In methods shown in  FIGS. 71A to 71C , circular regions in images are changed from frame to frame, and triangle regions in the images are hardly changed from frame to frame. Note that the images are just examples for explanation, and the images to be displayed are not limited to these examples. The methods shown in  FIGS. 71A to 71C  can be applied to various images. 
       FIG. 71A  shows the case where the display frame rate is twice as high as the input frame rate (the conversion ratio is 2). When the conversion ratio is 2, there is an advantage in that quality of moving images can be improved compared to the case where the conversion ratio is less than 2. Further, when the conversion ratio is 2, there is an advantage in that power consumption and manufacturing cost can be reduced compared to the case where the conversion ratio is more than 2.  FIG. 71A  schematically shows time change in images to be displayed with time represented by the horizontal axis. Here, a focused image is referred to as a p-th image (p is a positive integer). An image displayed after the focused image is referred to as a (p+1)th image, and an image displayed before the focused image is referred to as a (p−1)th image, for example. Thus, how far an image to be displayed is apart from the focused image is described for convenience. An image  180701  is the p-th image; an image  180702  is the (p+1)th image; an image  180703  is a (p+2)th image; an image  180704  is a (p+3)th image; and an image  180705  is a (p+4)th image. The period T in  shows a cycle of input image data. Note that since  FIG. 71A  shows the case where the conversion ratio is 2, the period T in  is twice as long as a period after the p-th image is displayed until the (p+1)th image is displayed. 
     Here, the (p+1)th image  180702  may be an image which is made to be in an intermediate state between the p-th image  180701  and the (p+2)th image  180703  by detecting the amount of change in the images from the p-th image  180701  to the (p+2)th image  180703 .  FIG. 71A  shows an image in an intermediate state by a region whose position is changed from frame to frame (the circular region) and a region whose position is hardly changed from frame to frame (the triangle region). In other words, the position of the circular region in the (p+1)th image  180702  is an intermediate position between the positions of the circular regions in the p-th image  180701  and the (p+2)th image  180703 . That is, as for the (p+1)th image  180702 , image data is interpolated by motion compensation. When motion compensation is performed on a moving object on the image in this manner to interpolate the image data, smooth display can be performed. 
     Further, the (p+1)th image  180702  may be an image which is made to be in an intermediate state between the p-th image  180701  and the (p+2)th image  180703  and may be an image, luminance of which is controlled by a certain rule. As the certain rule, for example, L&gt;L c  may be satisfied when typical luminance of the p-th image  180701  is denoted by L and typical luminance of the (p+1)th image  180702  is denoted by L c , as shown in  FIG. 71A . Preferably, 0.1L&lt;L c &lt;0.8L is satisfied, and more preferably 0.2L&lt;L c &lt;0.5L is satisfied. Alternatively, L&lt;L c  may be satisfied, preferably 0.1L c &lt;L&lt;0.8L c  is satisfied, and more preferably 0.2L c &lt;L&lt;0.5L c  is satisfied. In this manner, display can be made close to pseudo impulse display, so that an afterimage perceived by human eyes can be suppressed. 
     Note that typical luminance of the images is described later in detail with reference to  FIGS. 72A to 72E . 
     When two different causes of motion blur (non-smoothness in movement of images and an afterimage perceived by human eyes) are removed at the same time in this manner, motion blur can be considerably reduced. 
     Moreover, the (p+3)th image  180704  may also be formed from the (p+2)th image  180703  and the (p+4)th image  180705  by using a similar method. That is, the (p+3)th image  180704  may be an image which is made to be in an intermediate state between the (p+2)th image  180703  and the (p+4)th image  180705  by detecting the amount of change in the images from the (p+2)th image  180703  to the (p+4)th image  180705  and may be an image, luminance of which is controlled by a certain rule. 
       FIG. 71B  shows the case where the display frame rate is three times as high as the input frame rate (the conversion ratio is 3).  FIG. 71B  schematically shows time change in images to be displayed with time represented by the horizontal axis. An image  180711  is the p-th image; an image  180712  is the (p+1)th image; an image  180713  is a (p+2)th image; an image  180714  is a (p+3)th image; an image  180715  is a (p+4)th image; an image  180716  is a (p+5)th image; and an image  180717  is a (p+6)th image. The period T in  shows a cycle of input image data. Note that since  FIG. 71B  shows the case where the conversion ratio is 3, the period T in  is three times as long as a period after the p-th image is displayed until the (p+1)th image is displayed. 
     Here, each of the (p+1)th image  180712  and the (p+2)th image  180713  may be an image which is made to be in an intermediate state between the p-th image  180711  and the (p+3)th image  180714  by detecting the amount of change in the images from the p-th image  180711  to the (p+3)th image  180714 .  FIG. 71B  shows an image in an intermediate state by a region whose position is changed from frame to frame (the circular region) and a region whose position is hardly changed from frame to frame (the triangle region). That is, the position of the circular region in each of the (p+1)th image  180712  and the (p+2)th image  180713  is an intermediate position between the positions of the circular regions in the p-th image  180711  and the (p+3)th image  180714 . Specifically, when the amount of movement of the circular regions detected from the p-th image  180711  and the (p+3)th image  180714  is denoted by X, the position of the circular region in the (p+1)th image  180712  may be displaced by approximately (⅓)X from the position of the circular region in the p-th image  180711 . Further, the position of the circular region in the (p+2)th image  180713  may be displaced by approximately (⅔)X from the position of the circular region in the p-th image  180711 . That is, as for each of the (p+1)th image  180712  and the (p+2)th image  180713 , image data is interpolated by motion compensation. When motion compensation is performed on a moving object on the image in this manner to interpolate the image data, smooth display can be performed. 
     Further, each of the (p+1)th image  180712  and the (p+2)th image  180713  may be an image which is made to be in an intermediate state between the p-th image  180711  and the (p+3)th image  180714  and may be an image, luminance of which is controlled by a certain rule. As the certain rule, for example, L&gt;L c   1 , L&gt;L c   2 , or L c   1 =L c   2  may be satisfied when typical luminance of the p-th image  180711  is denoted by L, typical luminance of the (p+1)th image  180712  is denoted by L c   1 , and typical luminance of the (p+2)th image  180713  is denoted by L c   2 , as shown in  FIG. 71B . Preferably, 0.1L&lt;L c   1 =L c   2 &lt;0.8L is satisfied, and more preferably 0.2L&lt;L c   1 =L c   2 &lt;0.5L is satisfied. Alternatively, L&lt;L c   1 , L&lt;L c   2 , or L1=L c   2  may be satisfied, preferably 0.1L c   1 =0.1L c   2 &lt;L&lt;0.8L c   1 =0.8L c   2  is satisfied, and more preferably 0.2L c   1 =0.2L c   2  &lt;L&lt;0.5L c   1 =0.5L c   2  is satisfied. In this manner, display can be made close to pseudo impulse display, so that an afterimage perceived by human eyes can be suppressed. Alternatively, images, luminance of which is changed, may be made to appear alternately. In this manner, a cycle of luminance change can be shortened, so that flickers can be reduced. 
     When two different causes of motion blur (non-smoothness in movement of images and an afterimage perceived by human eyes) are removed at the same time in this manner, motion blur can be considerably reduced. 
     Moreover, each of the (p+4)th image  180715  and the (p+5)th image  180716  may also be formed from the (p+3)th image  180714  and the (p+6)th image  180717  by using a similar method. That is, each of the (p+4)th image  180715  and the (p+5)th image  180716  may be an image which is made to be in an intermediate state between the (p+3)th image  180714  and the (p+6)th image  180717  by detecting the amount of change in the images from the (p+3)th image  180714  to the (p+6)th image  180717  and may be an image, luminance of which is controlled by a certain rule. 
     Note that when the method shown in  FIG. 71B  is used, the display frame rate is so high that movement of the image can follow movement of human eyes, so that movement of the image can be displayed smoothly. Therefore, motion blur can be considerably reduced. 
       FIG. 71C  shows the case where the display frame rate is 1.5 times as high as the input frame rate (the conversion ratio is 1.5).  FIG. 71C  schematically shows time change in images to be displayed with time represented by the horizontal axis. An image  180721  is the p-th image; an image  180722  is the (p+1)th image; an image  180723  is the (p+2)th image; and an image  180724  is the (p+3)th image. Note that although not necessarily displayed actually, an image  180725 , which is input image data, may be used to form the (p+1)th image  180722  and the (p+2)th image  180723 . The period T 1 , shows a cycle of input image data. Note that since  FIG. 71C  shows the case where the conversion ratio is 1.5, the period T in  is 1.5 times as long as a period after the p-th image is displayed until the (p+1)th image is displayed. 
     Here, each of the (p+1)th image  180722  and the (p+2)th image  180723  may be an image which is made to be in an intermediate state between the p-th image  180721  and the (p+3)th image  180724  by detecting the amount of change in the images from the p-th image  180721  to the (p+3)th image  180724  via the image  180725 .  FIG. 71C  shows an image in an intermediate state by a region whose position is changed from frame to frame (the circular region) and a region whose position is hardly changed from frame to frame (the triangle region). That is, the position of the circular region in each of the (p+1)th image  180722  and the (p+2)th image  180723  is an intermediate position between the positions of the circular regions in the p-th image  180721  and the (p+3)th image  180724 . That is, as for each of the (p+1)th image  180722  and the (p+2)th image  180723 , image data is interpolated by motion compensation. When motion compensation is performed on a moving object on the image in this manner to interpolate the image data, smooth display can be performed. 
     Further, each of the (p+1)th image  180722  and the (p+2)th image  180723  may be an image which is made to be in an intermediate state between the p-th image and the (p+3)th image  180724  and may be an image, luminance of which is controlled by a certain rule. As the certain rule, for example, L&gt;L c   1 , L&gt;L c   2 , or L c   1 =L c   2  is satisfied when typical luminance of the p-th image  180721  is denoted by L, typical luminance of the (p+1)th image  180722  is denoted by L c   1 , and typical luminance of the (p+2)th image  180723  is denoted by L c   2 , as shown in  FIG. 71C . Preferably, 0.1L&lt;L c   1 =L c   2 &lt;0.8L is satisfied, and more preferably 0.2L&lt;L c   1 =L c   2 &lt;0.5L is satisfied. Alternatively, L&lt;L c   1 , L&lt;L c   2 , or L c =L c   2  may be satisfied, preferably 0.1L c   1 =0.1L c   2 &lt;L&lt;0.8L c   1 =0.8L c   2  is satisfied, and more preferably 0.2L c   1 =0.2L c   2  &lt;L&lt;0.5L c   1 =0.5L c   2  is satisfied. In this manner, display can be made close to pseudo impulse display, so that an afterimage perceived by human eyes can be suppressed. Alternatively, images, luminance of which is changed, may be made to appear alternately. In this manner, a cycle of luminance change can be shortened, so that flickers can be reduced. 
     When two different causes of motion blur (non-smoothness in movement of images and an afterimage perceived by human eyes) are removed at the same time in this manner, motion blur can be considerably reduced. 
     Note that when the method shown in  FIG. 71C  is used, the display frame rate is so low that time for writing a signal to a display device can be increased. Therefore, clock frequency of the display device can be made lower, so that power consumption can be reduced. Further, processing speed of motion compensation can be decreased, so that power consumption can be reduced. 
     Next, typical luminance of images is described with reference to  FIGS. 72A to 72E .  FIGS. 72A to 72D  each schematically show time change in images to be displayed with time represented by the horizontal axis.  FIG. 72E  shows an example of a method for measuring luminance of an image in a certain region. 
     An example of a method for measuring luminance of an image is a method for individually measuring luminance of each pixel which forms the image. With this method, luminance in every detail of the image can be strictly measured. 
     Note that since a method for individually measuring luminance of each pixel which forms the image needs much energy, another method may be used. An example of another method for measuring luminance of an image is a method for measuring average luminance of a region in an image, which is focused. With this method, luminance of an image can be easily measured. In this embodiment mode, luminance measured by a method for measuring average luminance of a region in an image is referred to as typical luminance of an image for convenience. 
     Then, which region in an image is focused in order to measure typical luminance of the image is described below. 
       FIG. 72A  shows an example of a measuring method in which luminance of a region whose position is hardly changed with respect to change in an image (the triangle region) is typical luminance of the image. The period T in  shows a cycle of input image data; an image  180801  is the p-th image; an image  180802  is the (p+1)th image; an image  180803  is the (p+2)th image; a first region  180804  is a luminance measurement region in the p-th image  180801 ; a second region  180805  is a luminance measurement region in the (p+1)th image  180802 ; and a third region  180806  is a luminance measurement region in the (p+2)th image  180803 . Here, the first to third regions may be provided in almost the same spatial positions in a device. That is, when typical luminance of the images is measured in the first to third regions, time change in typical luminance of the images can be calculated. 
     When the typical luminance of the images is measured, whether display is made close to pseudo impulse display or not can be judged. For example, if L c &lt;L is satisfied when luminance measured in the first region  180804  is denoted by L and luminance measured in the second region  180805  is denoted by L c , it can be said that display is made close to pseudo impulse display. At that time, it can be said that quality of moving images is improved. 
     Note that when the amount of change in typical luminance of the images with respect to time change (relative luminance) in the luminance measurement regions is in the following range, image quality can be improved. As for relative luminance, for example, relative luminance between the first region  180804  and the second region  180805  can be the ratio of lower luminance to higher luminance; relative luminance between the second region  180805  and the third region  180806  can be the ratio of lower luminance to higher luminance; and relative luminance between the first region  180804  and the third region  180806  can be the ratio of lower luminance to higher luminance. That is, when the amount of change in typical luminance of the images with respect to time change (relative luminance) is 0, relative luminance is 100%. When the relative luminance is less than or equal to 80%, quality of moving images can be improved. In particular, when the relative luminance is less than or equal to 50%, quality of moving images can be significantly improved. Further, when the relative luminance is more than or equal to 10%, power consumption and flickers can be reduced. In particular, when the relative luminance is more than or equal to 20%, power consumption and flickers can be significantly reduced. That is, when the relative luminance is more than or equal to 10% and less than or equal to 80%, quality of moving images can be improved and power consumption and flickers can be reduced. Further, when the relative luminance is more than or equal to 20% and less than or equal to 50%, quality of moving images can be significantly improved and power consumption and flickers can be significantly reduced. 
       FIG. 72B  shows an example of a method in which luminance of regions which are divided into tiled shapes is measured and an average value thereof is typical luminance of an image. The period T in  shows a cycle of input image data; an image  180811  is the p-th image; an image  180812  is the (p+1)th image; an image  180813  is the (p+2)th image; a first region  180814  is a luminance measurement region in the p-th image  180811 ; a second region  180815  is a luminance measurement region in the (p+1)th image  180812 ; and a third region  180816  is a luminance measurement region in the (p+2)th image  180813 . Here, the first to third regions may be provided in almost the same spatial positions in a device. That is, when typical luminance of the images is measured in the first to third regions, time change in typical luminance of the images can be measured. 
     When the typical luminance of the images is measured, whether display is made close to pseudo impulse display or not can be judged. For example, if L c &lt;L is satisfied when luminance measured in the first region  180814  is denoted by L and luminance measured in the second region  180815  is denoted by Lc, it can be said that display is made close to pseudo impulse display. At that time, it can be said that quality of moving images is improved. 
     Note that when the amount of change in typical luminance of the images with respect to time change (relative luminance) in the luminance measurement regions is in the following range, image quality can be improved. As for relative luminance, for example, relative luminance between the first region  180814  and the second region  180815  can be the ratio of lower luminance to higher luminance; relative luminance between the second region  180815  and the third region  180816  can be the ratio of lower luminance to higher luminance; and relative luminance between the first region  180814  and the third region  180816  can be the ratio of lower luminance to higher luminance. That is, when the amount of change in typical luminance of the images with respect to time change (relative luminance) is 0, relative luminance is 100%. When the relative luminance is less than or equal to 80%, quality of moving images can be improved. In particular, when the relative luminance is less than or equal to 50%, quality of moving images can be significantly improved. Further, when the relative luminance is more than or equal to 10%, power consumption and flickers can be reduced. In particular, when the relative luminance is more than or equal to 20%, power consumption and flickers can be significantly reduced. That is, when the relative luminance is more than or equal to 10% and less than or equal to 80%, quality of moving images can be improved and power consumption and flickers can be reduced. Further, when the relative luminance is more than or equal to 20% and less than or equal to 50%, quality of moving images can be significantly improved and power consumption and flickers can be significantly reduced. 
       FIG. 72C  shows an example of a method in which luminance of a center region in an image is measured and an average value thereof is typical luminance of the image. The period T in  shows a cycle of input image data; an image  180821  is the p-th image; an image  180822  is the (p+1)th image; an image  180823  is the (p+2)th image; a first region  180824  is a luminance measurement region in the p-th image  180821 ; a second region  180825  is a luminance measurement region in the (p+1)th image  180822 ; and a third region  180826  is a luminance measurement region in the (p+2)th image  180823 . 
     When the typical luminance of the images is measured, whether display is made close to pseudo impulse display or not can be judged. For example, if L c &lt;L is satisfied when luminance measured in the first region  180824  is denoted by L and luminance measured in the second region  180825  is denoted by L c , it can be said that display is made close to pseudo impulse display. At that time, it can be said that quality of moving images is improved. 
     Note that when the amount of change in typical luminance of the images with respect to time change (relative luminance) in the luminance measurement regions is in the following range, image quality can be improved. As for relative luminance, for example, relative luminance between the first region  180824  and the second region  180825  can be the ratio of lower luminance to higher luminance; relative luminance between the second region  180825  and the third region  180826  can be the ratio of lower luminance to higher luminance; and relative luminance between the first region  180824  and the third region  180826  can be the ratio of lower luminance to higher luminance. That is, when the amount of change in typical luminance of the images with respect to time change (relative luminance) is 0, relative luminance is 100%. When the relative luminance is less than or equal to 80%, quality of moving images can be improved. In particular, when the relative luminance is less than or equal to 50%, quality of moving images can be significantly improved. Further, when the relative luminance is more than or equal to 10%, power consumption and flickers can be reduced. In particular, when the relative luminance is more than or equal to 20%, power consumption and flickers can be significantly reduced. That is, when the relative luminance is more than or equal to 10% and less than or equal to 80%, quality of moving images can be improved and power consumption and flickers can be reduced. Further, when the relative luminance is more than or equal to 20% and less than or equal to 50%, quality of moving images can be significantly improved and power consumption and flickers can be significantly reduced. 
       FIG. 72D  shows an example of a method in which luminance of a plurality of points sampled from the entire image is measured and an average value thereof is typical luminance of the image. The period T in  shows a cycle of input image data; an image  180831  is the p-th image; an image  180832  is the (p+1)th image; an image  180833  is the (p+2)th image; a first region  180834  is a luminance measurement region in the p-th image  180831 ; a second region  180835  is a luminance measurement region in the (p+1)th image  180832 ; and a third region  180836  is a luminance measurement region in the (p+2)th image  180833 . 
     When the typical luminance of the images is measured, whether display is made close to pseudo impulse display or not can be judged. For example, if L c &lt;L is satisfied when luminance measured in the first region  180834  is denoted by L and luminance measured in the second region  180835  is denoted by L c , it can be said that display is made close to pseudo impulse display. At that time, it can be said that quality of moving images is improved. 
     Note that when the amount of change in typical luminance of the images with respect to time change (relative luminance) in the luminance measurement regions is in the following range, image quality can be improved. As for relative luminance, for example, relative luminance between the first region  180834  and the second region  180835  can be the ratio of lower luminance to higher luminance; relative luminance between the second region  180835  and the third region  180836  can be the ratio of lower luminance to higher luminance; and relative luminance between the first region  180834  and the third region  180836  can be the ratio of lower luminance to higher luminance. That is, when the amount of change in typical luminance of the images with respect to time change (relative luminance) is 0, relative luminance is 100%. When the relative luminance is less than or equal to 80%, quality of moving images can be improved. In particular, when the relative luminance is less than or equal to 50%, quality of moving images can be significantly improved. Further, when the relative luminance is more than or equal to 10%, power consumption and flickers can be reduced. In particular, when the relative luminance is more than or equal to 20%, power consumption and flickers can be significantly reduced. That is, when the relative luminance is more than or equal to 10% and less than or equal to 80%, quality of moving images can be improved and power consumption and flickers can be reduced. Further, when the relative luminance is more than or equal to 20% and less than or equal to 50%, quality of moving images can be significantly improved and power consumption and flickers can be significantly reduced. 
       FIG. 72E  shows a measurement method in the luminance measurement regions shown in  FIGS. 72A to 72D . A region  180841  is a focused luminance measurement region, and a point  180842  is a luminance measurement point in the region  180841 . In a luminance measurement apparatus having high time resolution, a measurement range thereof is small in some cases. Therefore, in the case where the region  180841  is large, unlike the case of measuring the whole region, a plurality of points in the region  180841  may be measured uniformly by dots and an average value thereof may be the luminance of the region  18084 , as shown in  FIG. 72E . 
     Note that in the case where the image is formed using combination of three primary colors of R, G, and B, luminance to be measured may be luminance of R, G, and B, luminance of R and G, luminance of G and B, luminance of B and R, or each luminance of R, G, and B. 
     Next, a method for producing an image in an intermediate state by detecting movement of an image, which is included in input image data, and a method for controlling a driving method in accordance with movement of an image, which is included in input image data, or the like are described. 
     A method for producing an image in an intermediate state by detecting movement of an image, which is included in input image data, is described with reference to  FIGS. 73A and 73B .  FIG. 73A  shows the case where the display frame rate is twice as high as the input frame rate (the conversion ratio is 2).  FIG. 73A  schematically shows a method for detecting movement of an image with time represented by the horizontal axis. The period T in  shows a cycle of input image data; an image  180901  is the p-th image; an image  180902  is the (p+1)th image; and an image  180903  is the (p+2)th image. Further, as regions which are independent of time, a first region  180904 , a second region  180905 , and a third region  180906  are provided in images. 
     First, in the (p+2)th image  180903 , the image is divided into a plurality of tiled regions, and image data in the third region  180906  which is one of the regions is focused. 
     Next, in the p-th image  180901 , a region which uses the third region  180906  as the center and is larger than the third region  180906  is focused. Here, the region which uses the third region  180906  as the center and is larger than the third region  180906  corresponds to a data retrieval region. In the data retrieval region, a range in a horizontal direction (an X direction) is denoted by  180907  and a range in a perpendicular direction (a Y direction) is denoted by  180908 . Note that the range in the horizontal direction  180907  and the range in the perpendicular direction  180908  may be ranges in which each of a range in a horizontal direction and a range in a perpendicular direction of the third region  180906  is enlarged by approximately 15 pixels. 
     Then, in the data retrieval region, a region having image data which is most similar to the image data in the third region  180906  is retrieved. As a retrieval method, a least-squares method or the like can be used. As a result of retrieval, it is assumed that the first region  180904  be derived as the region having the most similar image data. 
     Next, as an amount which shows positional difference between the derived first region  180904  and the third region  180906 , a vector  180909  is derived. Note that the vector  180909  is referred to as a motion vector. 
     Then, in the (p+1)th image  180902 , the second region  180905  is formed by a vector calculated from the motion vector  180909 , the image data in the third region  180906  in the (p+2)th image  180903 , and image data in the first region  180904  in the p-th image  180901 . 
     Here, the vector calculated from the motion vector  180909  is referred to as a displacement vector  180910 . The displacement vector  180910  has a function of determining a position in which the second region  180905  is formed. The second region  180905  is formed in a position which is apart from the third region  180906  by the displacement vector  180910 . Note that the amount of the displacement vector  180910  may be an amount which is obtained by multiplying the motion vector  180909  by a coefficient (½). 
     Image data in the second region  180905  in the (p+1)th image  180902  may be determined by the image data in the third region  180906  in the (p+2)th image  180903  and the image data in the first region  180904  in the p-th image  180901 . For example, the image data in the second region  180905  in the (p+1)th image  180902  may be an average value between the image data in the third region  180906  in the (p+2)th image  180903  and the image data in the first region  180904  in the p-th image  180901 . 
     In this manner, the second region  180905  in the (p+1)th image  180902 , which corresponds to the third region  180906  in the (p+2)th image  180903 , can be formed. Note that when the above-described treatment is also performed on other regions in the (p+2)th image  180903 , the (p+1)th image  180902  which is made to be in an intermediate state between the (p+2)th image  180903  and the p-th image  180901  can be formed. 
       FIG. 73B  shows the case where the display frame rate is three times as high as the input frame rate (the conversion ratio is 3).  FIG. 73B  schematically shows a method for detecting movement of an image with time represented by the horizontal axis. The period T in  shows a cycle of input image data; an image  180911  is the p-th image; an image  180912  is the (p+1)th image; an image  180913  is the (p+2)th image; and an image  180914  is the (p+3)th image. Further, as regions which are independent of time, a first region  180915 , a second region  180916 , a third region  180917 , and a fourth region  180918  are provided in images. 
     First, in the (p+3)th image  180914 , the image is divided into a plurality of tiled regions, and image data in the fourth region  180918  which is one of the regions is focused. 
     Next, in the p-th image  180911 , a region which uses the fourth region  180918  as the center and is larger than the fourth region  180918  is focused. Here, the region which uses the fourth region  180911  as the center and is larger than the fourth region  180918  corresponds to a data retrieval region. In the data retrieval region, a range in a horizontal direction (an X direction) is denoted by  180919  and a range in a perpendicular direction (a Y direction) is denoted by  180920 . Note that the region in the horizontal direction  180919  and the range in the perpendicular direction  180920  may be ranges in which each of a range in a horizontal direction and a range in a perpendicular direction of the fourth region  180918  is enlarged by approximately 15 pixels. 
     Then, in the data retrieval region, a region having image data which is most similar to the image data in the fourth region  180918  is retrieved. As a retrieval method, a least-squares method or the like can be used. As a result of retrieval, it is assumed that the first region  180915  be derived as the region having the most similar image data. 
     Next, as an amount which shows positional difference between the derived first region  180915  and the fourth region  180918 , a vector is derived. Note that the vector is referred to as a motion vector  180921 . 
     Then, in each of the (p+1)th image  180912  and the (p+2)th image  180913 , the second region  1809016  and the third region  180917  are formed by a first vector and a second vector calculated from the motion vector  180921 , the image data in the fourth region  180918  in the (p+3)th image  180914 , and image data in the first region  180915  in the p-th image  180911 . 
     Here, the first vector calculated from the motion vector  180921  is referred to as a first displacement vector  180922 . In addition, the second vector is referred to as a second displacement vector  180923 . The first displacement vector  180922  has a function of determining a position in which the second region  180916  is formed. The second region  180916  is formed in a position which is apart from the fourth region  180918  by the first displacement vector  180922 . Note that the first displacement vector  180922  may be an amount which is obtained by multiplying the motion vector  180921  by a coefficient (⅓). Further, the second displacement vector  180923  has a function of determining a position in which the third region  180917  is formed. The third region  180917  is formed in a position which is apart from the fourth region  180918  by the second displacement vector  180923 . Note that the second displacement vector  180923  may be an amount which is obtained by multiplying the motion vector  180921  by a coefficient (⅔). 
     Image data in the second region  180916  in the (p+1)th image  180912  may be determined by the image data in the fourth region  180918  in the (p+3)th image  180914  and the image data in the first region  180915  in the p-th image  180911 . For example, the image data in the second region  180916  in the (p+1)th image  180912  may be an average value between the image data in the fourth region  180918  in the (p+3)th image  180914  and the image data in the first region  180915  in the p-th image  180911 . 
     Image data in the third region  180917  in the (p+2)th image  180913  may be determined by the image data in the fourth region  180918  in the (p+3)th image  180914  and the image data in the first region  180915  in the p-th image  180911 . For example, the image data in the third region  180917  in the (p+2)th image  180913  may be an average value between the image data in the fourth region  180918  in the (p+3)th image  180914  and the image data in the first region  180915  in the p-th image  180911 . 
     In this manner, the second region  180916  in the (p+1)th image  180912  and the third region  180917  in the (p+2)th image  180913  which correspond to the fourth region  180918  in the (p+3)th image  180914  can be formed. Note that when the above-described treatment is also performed on other regions in the (p+3)th image  180914 , the (p+1)th image  180912  and the (p+2)th image  180913  which are made to be in an intermediate state between the (p+3)th image  180914  and the p-th image  180911  can be formed. 
     Next, an example of a circuit which produces an image in an intermediate state by detecting movement of an image, which is included in input image data, is described with reference to  FIGS. 74A to 74D .  FIG. 74A  shows a connection relation between a peripheral driver circuit including a source driver and a gate driver for displaying an image on a display region, and a control circuit for controlling the peripheral driver circuit.  FIG. 74B  shows an example of a specific circuit structure of the control circuit.  FIG. 74C  shows an example of a specific circuit structure of an image processing circuit included in the control circuit.  FIG. 74D  shows another example of the specific circuit structure of the image processing circuit included in the control circuit. 
     As shown in  FIG. 74A , a device in this embodiment mode may include a control circuit  181011 , a source driver  181012 , a gate driver  181013 , and a display region  181014 . 
     Note that the control circuit  181011 , the source driver  181012 , and the gate driver  181013  may be formed over the same substrate as the display region  181014 . 
     Note that part of the control circuit  181011 , the source driver  181012 , and the gate driver  181013  may be formed over the same substrate as the display region  181014 , and other circuits may be formed over a different substrate from that of the display region  181014 . For example, the source driver  181012  and the gate driver  181013  may be formed over the same substrate as the display region  181014 , and the control circuit  181011  may be formed over a different substrate as an external IC. Similarly, the gate driver  181013  may be formed over the same substrate as the display region  181014 , and other circuits may be formed over a different substrate as an external IC. Similarly, part of the source driver  181012 , the gate driver  181013 , and the control circuit  181011  may be formed over the same substrate as the display region  181014 , and other circuits may be formed over a different substrate as an external IC. 
     The control circuit  181011  may have a structure to which an external image signal  181000 , a horizontal synchronization signal  181001 , and a vertical synchronization signal  181002  are input and an image signal  181003 , a source start pulse  181004 , a source clock  181005 , a gate start pulse  181006 , and a gate clock  181007  are output. 
     The source driver  181012  may have a structure in which the image signal  181003 , the source start pulse  181004 , and the source clock  181005  are input and voltage or current in accordance with the image signal  181003  is output to the display region  181014 . 
     The gate driver  181013  may have a structure to which the gate start pulse  181006  and the gate clock  181007  are input and a signal which specifies timing for writing a signal output from the source driver  181012  to the display region  181014  is output. 
     In the case where frequency of the external image signal  181000  is different from frequency of the image signal  181003 , a signal for controlling timing for driving the source driver  181012  and the gate driver  181013  is also different from frequency of the horizontal synchronization signal  181001  and the vertical synchronization signal  181002  which are input. Therefore, in addition to processing of the image signal  181003 , it is necessary to process the signal for controlling timing for driving the source driver  181012  and the gate driver  181013 . The control circuit  181011  may have a function of processing the signal for controlling timing for driving the source driver  181012  and the gate driver  181013 . For example, in the case where the frequency of the image signal  181003  is twice as high as the frequency of the external image signal  181000 , the control circuit  181011  generates the image signal  181003  having twice frequency by interpolating an image signal included in the external image signal  181000  and controls the signal for controlling timing so that the signal also has twice frequency. 
     Further, as shown in  FIG. 74B , the control circuit  181011  may include an image processing circuit  181015  and a timing generation circuit  181016 . 
     The image processing circuit  181015  may have a structure to which the external image signal  181000  and a frequency control signal  181008  are input and the image signal  181003  is output. 
     The timing generation circuit  181016  may have a structure to which the horizontal synchronization signal  181001  and the vertical synchronization signal  181002  are input, and the source start pulse  181004 , the source clock  181005 , the gate start pulse  181006 , the gate clock  181007 , and the frequency control signal  181008  are output. Note that the timing generation circuit  181016  may have a memory, a register, or the like for holding data for specifying the state of the frequency control signal  181008 . Alternatively, the timing generation circuit  181016  may have a structure to which a signal for specifying the state of the frequency control signal  181008  is input from outside. 
     As shown in  FIG. 74C , the image processing circuit  181015  may include a motion detection circuit  181020 , a first memory  181021 , a second memory  181022 , a third memory  181023 , a luminance control circuit  181024 , and a high-speed processing circuit  181025 . 
     The motion detection circuit  181020  may have a structure in which a plurality of pieces of image data are input, movement of an image is detected, and image data which is in an intermediate state of the plurality of pieces of image data is output. 
     The first memory  181021  may have a structure in which the external image signal  181000  is input, the external image signal  181000  is held for a certain period, and the external image signal  181000  is output to the motion detection circuit  181020  and the second memory  181022 . 
     The second memory  181022  may have a structure in which image data output from the first memory  181021  is input, the image data is held for a certain period, and the image data is output to the motion detection circuit  181020  and the high-speed processing circuit  181025 . 
     The third memory  181023  may have a structure in which image data output from the motion detection circuit  181020  is input, the image data is held for a certain period, and the image data is output to the luminance control circuit  181024 . 
     The high-speed processing circuit  181025  may have a structure in which image data output from the second memory  181022 , image data output from the luminance control circuit  181024 , and a frequency control signal  181008  are input and the image data is output as the image signal  181003 . 
     In the case where the frequency of the external image signal  181000  is different from the frequency of the image signal  181003 , the image signal  181003  may be generated by interpolating the image signal included in the external image signal  181000  by the image processing circuit  181015 . The input external image signal  181000  is once held in the first memory  181021 . At that time, image data which is input in the previous frame is held in the second memory  181022 . The motion detection circuit  181020  may read the image data held in the first memory  181021  and the second memory  181022  as appropriate to detect a motion vector by difference between the both pieces of image data and to generate image data in an intermediate state. The generated image data in an intermediate state is held in the third memory  181023 . 
     When the motion detection circuit  181020  generates the image data in an intermediate state, the high-speed processing circuit  181025  outputs the image data held in the second memory  181022  as the image signal  181003 . After that, the image data held in the third memory  181023  is output through the luminance control circuit  181024  as the image signal  181003 . At this time, frequency which is updated by the second memory  181022  and the third memory  181023  is the same as the external image signal  181000 ; however, the frequency of the image signal  181003  which is output through the high-speed processing circuit  181025  may be different from the frequency of the external image signal  181000 . Specifically, for example, the frequency of the image signal  181003  is 1.5 times, twice, or three times as high as the frequency of the external image signal  181000 . However, the present invention is not limited to this, and a variety of frequency can be used. Note that the frequency of the image signal  181003  may be specified by the frequency control signal  181008 . 
     The structure of the image processing circuit  181015  shown in  FIG. 74D  is obtained by adding a fourth memory  181026  to the structure of the image processing circuit  181015  shown in  FIG. 74C . When image data output from the fourth memory  181026  is also output to the motion detection circuit  181020  in addition to the image data output from the first memory  181021  and the image data output from the second memory  181022  in this manner, movement of an image can be detected adequately. 
     Note that in the case where image data to be input has already included a motion vector for data compression or the like, for example, the image data to be input is image data which is based on an MPEG (moving picture expert group) standard, an image in an intermediate state may be generated as an interpolated image by using this image data. At this time, a portion which generates a motion vector included in the motion detection circuit  181020  is not necessary. Further, since encoding and decoding processing of the image signal  181003  is simplified, power consumption can be reduced. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 6 
     In this embodiment mode, a peripheral portion of a liquid crystal panel is described. 
       FIG. 75  shows an example of a liquid crystal display device including a so-called edge-light type backlight unit  20101  and a liquid crystal panel  20107 . An edge-light type corresponds to a type in which a light source is provided at an end of a backlight unit and fluorescence of the light source is emitted from the entire light-emitting surface. The edge-light type backlight unit is thin and can save power. 
     The backlight unit  20101  includes a diffusion plate  20102 , a light guide plate  20103 , a reflection plate  20104 , a lamp reflector  20105 , and a light source  20106 . 
     The light source  20106  has a function of emitting light as necessary. For example, as the light source  20106 , a cold cathode fluorescent lamp, a hot cathode fluorescent lamp, a light-emitting diode, an inorganic EL element, an organic EL element, or the like is used. The lamp reflector  20105  has a function of efficiently guiding fluorescence from the light source  20106  to the light guide plate  20103 . The light guide plate  20103  has a function of guiding light to the entire surface by total reflection of fluorescence. The diffusion plate  20102  has a function of reducing variations in brightness. The reflection plate  20104  has a function of reflecting light which is leaked from the light guide plate  20103  downward (a direction which is opposite to the liquid crystal panel  20107 ) to be reused. 
     Note that a control circuit for controlling luminance of the light source  20106  is connected to the backlight unit  20101 . When this control circuit is used, luminance of the light source  20106  can be controlled. 
       FIGS. 76A to 76D  each show a detailed structure of the edge-light type backlight unit. Note that description of a diffusion plate, a light guide plate, a reflection plate, and the like is omitted. 
     A backlight unit  20201  shown in  FIG. 76A  has a structure in which a cold cathode fluorescent lamp  20203  is used as a light source. In addition, a lamp reflector  20202  is provided to efficiently reflect light from the cold cathode fluorescent lamp  20203 . Such a structure is often used for a large display device because luminance of light from the cold cathode fluorescent lamp  20203  is high. 
     A backlight unit  20211  shown in  FIG. 76B  has a structure in which light-emitting diodes (LEDs)  20213  are used as light sources. For example, the light-emitting diodes (LEDs)  20213  which emit white light are provided at a predetermined interval. In addition, a lamp reflector  20212  is provided to efficiently reflect light from the light-emitting diodes (LEDs)  20213 . 
     Since luminance of light-emitting diodes is high, a structure using light-emitting diodes is suitable for a large display device. Since light-emitting diodes are superior in color reproductivity, an image which is closer to the real object can be displayed. Since the size of chips of LEDs is small, the arrangement area can be reduced. Therefore, a frame of a display device can be narrowed. 
     Note that in the case where light-emitting diodes are mounted on a large display device, the light-emitting diodes can be provided on a back side of the substrate. The light-emitting diodes of R, Q and B are sequentially provided at a predetermined interval. When the light-emitting diodes are provided, color reproductivity can be improved. 
     A backlight unit  20221  shown in  FIG. 76C  has a structure in which light-emitting diodes (LEDs)  20223 , light-emitting diodes (LEDs)  20224 , and light-emitting diodes (LEDs)  20225  of R, Q and B are used as light sources. The light-emitting diodes (LEDs)  20223 , the light-emitting diodes (LEDs)  20224 , and the light-emitting diodes (LEDs)  20225  of R, Q and B are each provided at a predetermined interval. When the light-emitting diodes (LEDs)  20223  are used, the light-emitting diodes (LEDs)  20224 , and the light-emitting diodes (LEDs)  20225  of R, G and B, color reproductivity can be improved. In addition, a lamp reflector  20222  is provided to efficiently reflect light from the light-emitting diodes. 
     Since luminance of light-emitting diodes is high, a structure in which light-emitting diodes of R, G and B are used as light sources is suitable for a large display device. Since light-emitting diodes are superior in color reproductivity, an image which is closer to the real object can be displayed. Since the size of chips of LEDs is small, the arrangement area can be reduced. Therefore, a frame of a display device can be narrowed. 
     When the light-emitting diodes of R, Q and B are made sequentially emit light in accordance with time, color display can be performed. This is a so-called field sequential mode. 
     Note that a light-emitting diode which emits white light can be combined with the light-emitting diodes (LEDs)  20223 , the light-emitting diodes (LEDs)  20224 , and the light-emitting diodes (LEDs)  20225  of R, G and B. 
     Note that in the case where light-emitting diodes are mounted on a large display device, the light-emitting diodes can be provided on a back side of the substrate. The light-emitting diodes of R, G and B are sequentially provided at a predetermined interval. When the light-emitting diodes are provided, color reproductivity can be improved. 
     A backlight unit  20231  shown in  FIG. 76D  has a structure in which light-emitting diodes (LEDs)  20233 , light-emitting diodes (LEDs)  20234 , and light-emitting diodes (LEDs)  20235  of R, G, and B are used as light sources. For example, among the light-emitting diodes (LEDs)  20233 , the light-emitting diodes (LEDs)  20234 , and the light-emitting diodes (LEDs)  20235  of R, G and B, a plurality of the light-emitting diodes of a color with low emission intensity (e.g., green) are provided. By using the light-emitting diodes (LEDs)  20233 , the light-emitting diodes (LEDs)  20234 , and the light-emitting diodes (LEDs)  20235  of R,  4  and B, color reproductivity can be improved. In addition, a lamp reflector  20232  is provided to efficiently reflect light from the light-emitting diodes. 
     Since luminance of light-emitting diodes is high, a structure in which light-emitting diodes of R, Q and B are used as light sources is suitable for a large display device. Since light-emitting diodes are superior in color reproductivity, an image which is closer to the real object can be displayed. Since the size of chips of LEDs is small, the arrangement area can be reduced. Therefore, a frame of a display device can be narrowed. 
     When the light-emitting diodes of R, G, and B are made sequentially emit light in accordance with time, color display can be performed. This is a so-called field sequential mode. 
     Note that a light-emitting diode which emits white light can be combined with the light-emitting diodes (LEDs)  20233 , the light-emitting diodes (LEDs)  20234 , and the light-emitting diodes (LEDs)  20235  of R, G and B. 
     Note that in the case where light-emitting diodes are mounted on a large display device, the light-emitting diodes can be provided on a back side of the substrate. The light-emitting diodes of R, G and B are sequentially provided at a predetermined interval. When the light-emitting diodes are provided, color reproductivity can be improved. 
       FIG. 79A  shows an example of a liquid crystal display device including a so-called direct-type backlight unit and a liquid crystal panel. A direct type corresponds to a type in which a light source is provided directly under a light-emitting surface and fluorescence of the light source is emitted from the entire light-emitting surface. The direct-type backlight unit can efficiently utilize the amount of emitted light. 
     A backlight unit  20500  includes a diffusion plate  20501 , a light-shielding plate  20502 , a lamp reflector  20503 , and a light source  20504 . 
     Light emitted from the light source  20504  is gathered on one surface of the backlight unit  20500  by the lamp reflector  20503 . That is, the backlight unit has a surface on which light is emitted intensely and a surface on which light is hardly emitted. At this time, when a liquid crystal panel  20505  is provided on the side of the surface of the backlight unit  20500 , on which light is emitted intensely, light emitted from the light source  20504  can be efficiently delivered to the liquid crystal panel  20505 . 
     The light source  20504  has a function of emitting light as necessary. For example, as the light source  20504 , a cold cathode fluorescent lamp, a hot cathode fluorescent lamp, a light-emitting diode, an inorganic EL element, an organic EL element, or the like is used. The lamp reflector  20503  has a function of efficiently guiding fluorescence from the light source  20504  to the diffusion plate  20501  and the light-shielding plate  20502 . The light-shielding plate  20502  has a function of reducing variations in brightness by shielding much light as light becomes intenser in accordance with provision of the light source  20504 . The diffusion plate  20501  also has a function of reducing variations in brightness. 
     A control circuit for controlling luminance of the light source  20504  is connected to the backlight unit  20500 . When this control circuit is used, luminance of the light source  20504  can be controlled. 
       FIG. 79B  shows an example of a liquid crystal display device including a so-called direct-type backlight unit and a liquid crystal panel. A direct type corresponds to a type in which a light source is provided directly under a light-emitting surface and fluorescence of the light source is emitted from the entire light-emitting surface. The direct-type backlight unit can efficiently utilize the amount of emitted light. 
     A backlight unit  20510  includes a diffusion plate  20511 ; a light-shielding plate  20512 ; a lamp reflector  20513 ; and a light source (R)  20514   a , a light source (G)  20514   b , and a light source (B)  20514   c  of R, G, and B. 
     Light emitted from the light source (R)  20514   a , the light source (G)  20514   b , and the light source (B)  20514   c  is gathered on one surface of the backlight unit  20510  by the lamp reflector  20513 . That is, the backlight unit has a surface on which light is emitted intensely and a surface on which light is hardly emitted. At this time, when a liquid crystal panel  20515  is provided on the side of the surface of the backlight unit  20510 , on which light is emitted intensely, light emitted from the light source (R)  20514   a , the light source (G)  20514   b , and the light source (B)  20514   c  can be efficiently delivered to the liquid crystal panel  20515 . 
     Each of the light source (R)  20514   a , the light source (G)  20514   b , and the light source (B)  20514   c  of R, G, and B has a function of emitting light as necessary. For example, as each of the light source (R)  20514   a , the light source (G)  20514   b , and the light source (B)  20514   c , a cold cathode fluorescent lamp, a hot cathode fluorescent lamp, a light-emitting diode, an inorganic EL element, an organic EL element, or the like is used. The lamp reflector  20513  has a function of efficiently guiding fluorescence from the light sources  20514   a  to  20514   c  to the diffusion plate  20511  and the light-shielding plate  20512 . The light-shielding plate  20512  has a function of reducing variations in brightness by shielding much light as light becomes intenser in accordance with provision of the light sources  20514   a  to  20514   c . The diffusion plate  20511  also has a function of reducing variations in brightness. 
     A control circuit for controlling luminance of the light source (R)  20514   a , the light source (G)  20514   b , and the light source (B)  20514   c  of R, Q and B is connected to the backlight unit  20510 . When this control circuit is used, luminance of the light source (R)  20514   a , the light source (G)  20514   b , and the light source (B)  20514   c  of R, G, and B can be controlled. 
       FIG. 77  shows an example of a structure of a polarizing plate (also referred to as a polarizing film). 
     A polarizing film  20300  includes a protective film  20301 , a substrate film  20302 , a PVA polarizing film  20303 , a substrate film  20304 , an adhesive layer  20305 , and a mold release film  20306 . 
     The PVA polarizing film  20303  has a function of generating light in only a certain vibration direction (linear polarized light). Specifically, the PVA polarizing film  20303  includes molecules (polarizers) in which lengthwise electron density and widthwise electron density are greatly different from each other. The PVA polarizing film  20303  can generate linear polarized light by uniforming directions of the molecules in which lengthwise electron density and widthwise electron density are greatly different from each other. 
     For example, a high molecular film of poly vinyl alcohol is doped with an iodine compound and a PVA film is pulled in a certain direction, so that a film in which iodine molecules are aligned in a certain direction can be obtained as the PVA polarizing film  20303 . Then, light which is parallel to a major axis of the iodine molecule is absorbed by the iodine molecule. Note that a dichroic dye may be used instead of iodine for high durability use and high heat resistance use. Note that it is preferable that the dye be used for a liquid crystal display device which needs to have durability and heat resistance, such as an in-car LCD or an LCD for a projector. 
     When the PVA polarizing film  20303  is sandwiched by films to be base materials (the substrate film  20302  and the substrate film  20304 ) from both sides, reliability can be improved. Note that the PVA polarizing film  20303  may be sandwiched by triacetylcellulose (TAC) films with high light-transmitting properties and high durability. Note that each of the substrate films and the TAC films function as protective films of polarizer included in the PVA polarizing film  20303 . 
     The adhesive layer  20305  which is to be attached to a glass substrate of the liquid crystal panel is attached to one of the substrate films (the substrate film  20304 ). Note that the adhesive layer  20305  is formed by applying an adhesive to one of the substrate films (the substrate film  20304 ). The mold release film  20306  (a separate film) is provided to the adhesive layer  20305 . 
     The protective film  20301  is provided to the other of the substrates films (the substrate film  20302 ). 
     A hard coating scattering layer (an anti-glare layer) may be provided on a surface of the polarizing film  20300 . Since the surface of the hard coating scattering layer has minute unevenness formed by AG treatment and has an anti-glare function which scatters external light, reflection of external light in the liquid crystal panel can be prevented. Surface reflection can also be prevented. 
     Note that treatment in which plurality of optical thin film layers having different refractive indexes are layered (also referred to as anti-reflection treatment or AR treatment) may be performed on the surface of the polarizing film  20300 . The plurality of layered optical thin film layers having different refractive indexes can reduce reflectivity on the surface by an interference effect of light. 
       FIGS. 78A to 78C  each show an example of a system block of the liquid crystal display device. 
     In a pixel portion  20405 , signal lines  20412  which extend from a signal line driver circuit  20403  are provided. In addition, in the pixel portion  20405 , scan lines  20410  which extend from a scan line driver circuit  20404  are also provided. In addition, a plurality of pixels are arranged in matrix in cross regions of the signal lines  20412  and the scan lines  20410 . Note that each of the plurality of pixels includes a switching element. Therefore, voltage for controlling inclination of liquid crystal molecules can be separately input to each of the plurality of pixels. A structure in which a switching element is provided in each cross region in this manner is referred to as an active matrix type. Note that the present invention is not limited to such an active matrix type and a structure of a passive matrix type may be used. Since the passive matrix type does not have a switching element in each pixel, a process is simple. 
     A driver circuit portion  20408  includes a control circuit  20402 , the signal line driver circuit  20403 , and the scan line driver circuit  20404 . An image signal  20401  is input to the control circuit  20402 . The signal line driver circuit  20403  and the scan line driver circuit  20404  are controlled by the control circuit  20402  in accordance with this image signal  20401 . That is, the control circuit  20402  inputs a control signal to each of the signal line driver circuit  20403  and the scan line driver circuit  20404 . Then, in accordance with this control signal, the signal line driver circuit  20403  inputs a video signal to each of the signal lines  20412  and the scan line driver circuit  20404  inputs a scan signal to each of the scan lines  20410 . Then, the switching element included in the pixel is selected in accordance with the scan signal and the video signal is input to a pixel electrode of the pixel. 
     Note that the control circuit  20402  also controls a power source  20407  in accordance with the image signal  20401 . The power source  20407  includes a unit for supplying power to a lighting unit  20406 . As the lighting unit  20406 , an edge-light type backlight unit or a direct-type backlight unit can be used. Note that a front light may be used as the lighting unit  20406 . A front light corresponds to a plate-like lighting unit including a luminous body and a light conducting body, which is attached to the front surface side of a pixel portion and illuminates the whole area. When such a lighting unit is used, the pixel portion can be uniformly illuminated at low power consumption. 
     As shown in  FIG. 78B , the scan line driver circuit  20404  includes a shift register  20441 , a level shifter  20442 , and a circuit functioning as a buffer  20443 . A signal such as a gate start pulse (GSP) or a gate clock signal (GCK) is input to the shift register  20441 . 
     As shown in  FIG. 78C , the signal line driver circuit  20403  includes a shift register  20431 , a first latch  20432 , a second latch  20433 , a level shifter  20434 , and a circuit functioning as a buffer  20435 . The circuit functioning as the buffer  20435  corresponds to a circuit which has a function of amplifying a weak signal and includes an operational amplifier or the like. A signal such as a source start pulse (SSP) is input to the level shifter  20434  and data (DATA) such as a video signal is input to the first latch  20432 . A latch (LAT) signal can be temporally held in the second latch  20433  and is simultaneously input to the pixel portion  20405 . This is referred to as line sequential driving. Therefore, when a pixel is used in which not line sequential driving but dot sequential driving is performed, the second latch can be omitted. 
     Note that in this embodiment mode, a known liquid crystal panel can be used for the liquid crystal panel. For example, a structure in which a liquid crystal layer is sealed between two substrates can be used as the liquid crystal panel. A transistor, a capacitor, a pixel electrode, an alignment film, or the like is formed over one of the substrates. A polarizing plate, a retardation plate, or a prism sheet may be provided on the surface opposite to a top surface of the one of the substrates. A color filter, a black matrix, a counter electrode, an alignment film, or the like is provided on the other of the substrates. A polarizing plate or a retardation plate may be provided on the surface opposite to a top surface of the other of the substrates. The color filter and the black matrix may be formed over the top surface of the one of the substrates. Note that three-dimensional display can be performed by providing a slit (a grid) on the top surface side of the one of the substrates or the surface opposite to the top surface side of the one of the substrates. 
     Each of the polarizing plate, the retardation plate, and the prism sheet can be provided between the two substrates. Alternatively, each of the polarizing plate, the retardation plate, and the prism sheet can be integrated with one of the two substrates. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 7 
     In this embodiment mode, a pixel structure and an operation of a pixel which can be applied to a liquid crystal display device are described. 
     In this embodiment mode, as an operation mode of a liquid crystal element, a TN (twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA (multi-domain vertical alignment) mode, a PVA (patterned vertical alignment) mode, an ASM (axially symmetric aligned micro-cell) mode, an OCB (optical compensated birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode, or the like can be used. 
       FIG. 131A  shows an example of a pixel structure which can be applied to the liquid crystal display device. 
     A pixel  40100  includes a transistor  40101 , a liquid crystal element  40102 , and a capacitor  40103 . A gate of the transistor  40101  is connected to a wiring  40105 . A first terminal of the transistor  40101  is connected to a wiring  40104 . A second terminal of the transistor  40101  is connected to a first electrode of the liquid crystal element  40102  and a first electrode of the capacitor  40103 . A second electrode of the liquid crystal element  40102  corresponds to a counter electrode  40107 . A second electrode of the capacitor  40103  is connected to a wiring  40106 . 
     The wiring  40104  functions as a signal line. The wiring  40105  functions as a scan line. The wiring  40106  functions as a capacitor line. The transistor  40101  functions as a switch. The capacitor  40103  functions as a storage capacitor. 
     It is acceptable as long as the transistor  40101  functions as a switch, and the transistor  40101  may be either a P-channel transistor or an N-channel transistor. 
     A video signal is input to the wiring  40104 . A scan signal is input to the wiring  40105 . A constant potential is supplied to the wiring  40106 . Note that the scan signal is an H-level or L-level digital voltage signal. In the case where the transistor  40101  is an N-channel transistor, an H level of the scan signal is a potential which can turn on the transistor  40101  and an L level of the scan signal is a potential which can turn off the transistor  40101 . Alternatively, in the case where the transistor  40101  is a P-channel transistor, the H level of the scan signal is a potential which can turn off the transistor  40101  and the L level of the scan signal is a potential which can turn on the transistor  40101 . Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current. In addition, current of the video signal may be either analog or digital. The video signal has a potential which is lower than the H level of the scan signal and higher than the L level of the scan signal. Note that the constant potential supplied to the wiring  40106  is preferably equal to a potential of the counter electrode  40107 . 
     Operations of the pixel  40100  are described by dividing the whole operations into the case where the transistor  40101  is on and the case where the transistor  40101  is off. 
     In the case where the transistor  40101  is on, the wiring  40104  is electrically connected to the first electrode (a pixel electrode) of the liquid crystal element  40102  and the first electrode of the capacitor  40103 . Therefore, the video signal is input to the first electrode (the pixel electrode) of the liquid crystal element  40102  and the first electrode of the capacitor  40103  from the wiring  40104  through the transistor  40101 . In addition, the capacitor  40103  holds a potential difference between a potential of the video signal and the potential supplied to the wiring  40106 . 
     In the case where the transistor  40101  is off, the wiring  40104  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40102  and the first electrode of the capacitor  40103 . Therefore, each of the first electrode of the liquid crystal element  40102  and the first electrode of the capacitor  40103  is set in a floating state. Since the capacitor  40103  holds the potential difference between the potential of the video signal and the potential supplied to the wiring  40106 , each of the first electrode of the liquid crystal element  40102  and the first electrode of the capacitor  40103  holds a potential which is the same as (corresponds to) the video signal. Note that the liquid crystal element  40102  has transmittivity in accordance with the video signal. 
       FIG. 131B  shows an example of a pixel structure which can be applied to the liquid crystal display device. In particular,  FIG. 131B  shows an example of a pixel structure which can be applied to a liquid crystal display device suitable for a horizontal electric field mode (including an IPS mode and an FFS mode). 
     A pixel  40110  includes a transistor  40111 , a liquid crystal element  40112 , and a capacitor  40113 . A gate of the transistor  40111  is connected to a wiring  40115 . A first terminal of the transistor  40111  is connected to a wiring  40114 . A second terminal of the transistor  40111  is connected to a first electrode of the liquid crystal element  40112  and a first electrode of the capacitor  40113 . A second electrode of the liquid crystal element  40112  is connected to a wiring  40116 . A second electrode of the capacitor  40103  is connected to the wiring  40116 . 
     The wiring  40114  functions as a signal line. The wiring  40115  functions as a scan line. The wiring  40116  functions as a capacitor line. The transistor  40111  functions as a switch. The capacitor  40113  functions as a storage capacitor. 
     It is acceptable as long as the transistor  40111  functions as a switch, and the transistor  40111  may be either a P-channel transistor or an N-channel transistor. 
     A video signal is input to the wiring  40114 . A scan signal is input to the wiring  40115 . A constant potential is supplied to the wiring  40116 . Note that the scan signal is an H-level or L-level digital voltage signal. In the case where the transistor  40111  is an N-channel transistor, an H level of the scan signal is a potential which can turn on the transistor  40111  and an L level of the scan signal is a potential which can turn off the transistor  40111 . Alternatively, in the case where the transistor  40111  is a P-channel transistor, the H level of the scan signal is a potential which can turn off the transistor  40111  and the L level of the scan signal is a potential which can turn on the transistor  40111 . Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current. In addition, current of the video signal may be either analog or digital. The video signal has a potential which is lower than the H level of the scan signal and higher than the L level of the scan signal. 
     Operations of the pixel  40110  are described by dividing the whole operations into the case where the transistor  40111  is on and the case where the transistor  40111  is off. 
     In the case where the transistor  40111  is on, the wiring  40114  is electrically connected to the first electrode (a pixel electrode) of the liquid crystal element  40112  and the first electrode of the capacitor  40113 . Therefore, the video signal is input to the first electrode (the pixel electrode) of the liquid crystal element  40112  and the first electrode of the capacitor  40113  from the wiring  40114  through the transistor  40111 . In addition, the capacitor  40113  holds a potential difference between a potential of the video signal and the potential supplied to the wiring  40116 . 
     In the case where the transistor  40111  is off, the wiring  40114  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40112  and the first electrode of the capacitor  40113 . Therefore, each of the first electrode of the liquid crystal element  40112  and the first electrode of the capacitor  40113  is set in a floating state. Since the capacitor  40113  holds the potential difference between the potential of the video signal and the potential supplied to the wiring  40116 , each of the first electrode of the liquid crystal element  40112  and the first electrode of the capacitor  40113  holds a potential which is the same as (corresponds to) the video signal. Note that the liquid crystal element  40112  has transmittivity in accordance with the video signal. 
       FIG. 132  shows an example of a pixel structure which can be applied to the liquid crystal display device. In particular,  FIG. 132  shows an example of a pixel structure in which the aperture ratio of a pixel can be increased by reducing the number of wirings. 
       FIG. 132  shows two pixels which are provided in the same column direction (a pixel  40200  and a pixel  40210 ). For example, when the pixel  40200  is provided in an N-th row, the pixel  40210  is provided in an (N+1)th row. 
     A pixel  40200  includes a transistor  40201 , a liquid crystal element  40202 , and a capacitor  40203 . A gate of the transistor  40201  is connected to a wiring  40205 . A first terminal of the transistor  40201  is connected to a wiring  40204 . A second terminal of the transistor  40201  is connected to a first electrode of the liquid crystal element  40202  and a first electrode of the capacitor  40203 . A second electrode of the liquid crystal element  40202  corresponds to a counter electrode  40207 . A second electrode of the capacitor  40203  is connected to a wiring which is the same as a wiring connected to a gate of a transistor of the previous row. 
     A pixel  40210  includes a transistor  40211 , a liquid crystal element  40212 , and a capacitor  40213 . A gate of the transistor  40211  is connected to a wiring  40215 . A first terminal of the transistor  40211  is connected to the wiring  40204 . A second terminal of the transistor  40211  is connected to a first electrode of the liquid crystal element  40212  and a first electrode of the capacitor  40213 . A second electrode of the liquid crystal element  40212  corresponds to a counter electrode  40217 . A second electrode of the capacitor  40213  is connected to the wiring which is the same as the wiring connected to the gate of the transistor of the previous row (the wiring  40205 ). 
     The wiring  40204  functions as a signal line. The wiring  40205  functions as a scan line of the N-th row. The wiring  40205  also functions as a scan line of the (N+1)th row. The transistor  40201  functions as a switch. The capacitor  40203  functions as a storage capacitor. 
     The wiring  40215  functions as a scan line of the (N+1)th row. The wiring  40215  also functions as a scan line of the (N+2)th row. The transistor  40211  functions as a switch. The capacitor  40213  functions as a storage capacitor. 
     It is acceptable as long as each of the transistor  40201  and the transistor  40211  functions as a switch, and each of the transistor  40201  and the transistor  40211  may be either a P-channel transistor or an N-channel transistor. 
     A video signal is input to the wiring  40204 . A scan signal (of an N-th row) is input to the wiring  40205 . A scan signal (of an (N+1)th row) is input to the wiring  40215 . 
     The scan signal is an H-level or L-level digital voltage signal. In the case where the transistor  40201  (or the transistor  40211 ) is an N-channel transistor, an H level of the scan signal is a potential which can turn on the transistor  40201  (or the transistor  40211 ) and an L level of the scan signal is a potential which can turn off the transistor  40201  (or the transistor  40211 ). Alternatively, in the case where the transistor  40201  (or the transistor  40211 ) is a P-channel transistor, the H level of the scan signal is a potential which can turn off the transistor  40201  (or the transistor  40211 ) and the L level of the scan signal is a potential which can turn on the transistor  40201  (or the transistor  40211 ). Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current. In addition, current of the video signal may be either analog or digital. The video signal has a potential which is lower than the H level of the scan signal and higher than the L level of the scan signal. 
     Operations of the pixel  40200  are described by dividing the whole operations into the case where the transistor  40201  is on and the case where the transistor  40201  is off. 
     In the case where the transistor  40201  is on, the wiring  40204  is electrically connected to the first electrode (a pixel electrode) of the liquid crystal element  40202  and the first electrode of the capacitor  40203 . Therefore, the video signal is input to the first electrode (the pixel electrode) of the liquid crystal element  40202  and the first electrode of the capacitor  40203  from the wiring  40204  through the transistor  40201 . In addition, the capacitor  40203  holds a potential difference between a potential of the video signal and a potential supplied to the wiring which is the same as the wiring connected to the gate of the transistor of the previous row. 
     In the case where the transistor  40201  is off, the wiring  40204  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40202  and the first electrode of the capacitor  40203 . Therefore, each of the first electrode of the liquid crystal element  40202  and the first electrode of the capacitor  40203  is set in a floating state. Since the capacitor  40203  holds the potential difference between the potential of the video signal and the potential of the wiring which is the same as the wiring connected to the gate of the transistor of the previous row, each of the first electrode of the liquid crystal element  40202  and the first electrode of the capacitor  40203  holds a potential which is the same as (corresponds to) the video signal. Note that the liquid crystal element  40202  has transmittivity in accordance with the video signal. 
     Operations of the pixel  40210  are described by dividing the whole operations into the case where the transistor  40211  is on and the case where the transistor  40211  is off. 
     In the case where the transistor  40211  is on, the wiring  40214  is electrically connected to the first electrode (a pixel electrode) of the liquid crystal element  40212  and the first electrode of the capacitor  40213 . Therefore, the video signal is input to the first electrode (the pixel electrode) of the liquid crystal element  40212  and the first electrode of the capacitor  40213  from the wiring  40214  through the transistor  40211 . In addition, the capacitor  40213  holds a potential difference between a potential of the video signal and a potential supplied to a wiring which is the same as the wiring connected to the gate of the transistor of the previous row (the wiring  40205 ). 
     In the case where the transistor  40211  is off, the wiring  40214  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40212  and the first electrode of the capacitor  40213 . Therefore, each of the first electrode of the liquid crystal element  40212  and the first electrode of the capacitor  40213  is set in a floating state. Since the capacitor  40103  holds the potential difference between the potential of the video signal and the potential of the wiring which is the same as the wiring connected to the gate of the transistor of the previous row (the wiring  40215 ), each of the first electrode (the pixel electrode) of the liquid crystal element  40212  and the first electrode of the capacitor  40213  holds a potential which is the same as (corresponds to) the video signal. Note that the liquid crystal element  40212  has transmittivity in accordance with the video signal. 
       FIG. 133  shows an example of a pixel structure which can be applied to the liquid crystal display device. In particular,  FIG. 133  shows an example of a pixel structure in which the viewing angle can be improved by using a subpixel. 
     A pixel  40320  includes a subpixel  40300  and a subpixel  40310 . Although the case in which the pixel  40320  includes two subpixels is described, the pixel  40320  may include three or more subpixels. 
     The subpixel  40300  includes a transistor  40301 , a liquid crystal element  40302 , and a capacitor  40303 . A gate of the transistor  40301  is connected to a wiring  40305 . A first terminal of the transistor  40301  is connected to a wiring  40304 . A second terminal of the transistor  40301  is connected to a first electrode of the liquid crystal element  40302  and a first electrode of the capacitor  40303 . A second electrode of the liquid crystal element  40302  corresponds to a counter electrode  40307 . A second electrode of the capacitor  40303  is connected to a wiring  40306 . 
     The subpixel  40310  includes a transistor  40311 , a liquid crystal element  40312 , and a capacitor  40313 . A gate of the transistor  40311  is connected to a wiring  40315 . A first terminal of the transistor  40311  is connected to the wiring  40304 . A second terminal of the transistor  40311  is connected to a first electrode of the liquid crystal element  40312  and a first electrode of the capacitor  40313 . A second electrode of the liquid crystal element  40312  corresponds to a counter electrode  40317 . A second electrode of the capacitor  40313  is connected to a wiring  40306 . 
     The wiring  40304  functions as a signal line. The wiring  40305  functions as a scan line. The wiring  40315  functions as a signal line. The wiring  40306  functions as a capacitor line. Each of the transistor  40301  and the transistor  40311  functions as a switch. Each of the capacitor  40303  and the capacitor  40313  functions as a storage capacitor. 
     It is acceptable as long as each of the transistor  40301  and the transistor  40311  functions as a switch, and each of the transistor  40301  and the transistor  40311  may be either a P-channel transistor or an N-channel transistor. 
     A video signal is input to the wiring  40304 . A scan signal is input to the wiring  40305 . A scan signal is input to the wiring  40315 . A constant potential is supplied to the wiring  40306 . 
     The scan signal is an H-level or L-level digital voltage signal. In the case where the transistor  40301  (or the transistor  40311 ) is an N-channel transistor, an H level of the scan signal is a potential which can turn on the transistor  40301  (or the transistor  40311 ) and an L level of the scan signal is a potential which can turn off the transistor  40301  (or the transistor  40311 ). Alternatively, in the case where the transistor  40301  (or the transistor  40311 ) is a P-channel transistor, the H level of the scan signal is a potential which can turn off the transistor  40301  (or the transistor  40311 ) and the L level of the scan signal is a potential which can turn on the transistor  40301  (or the transistor  40311 ). Note that the video signal has analog voltage. Note that the present invention is not limited to this, the video signal may have digital voltage. Alternatively, the video signal may be current. In addition, current of the video signal may be either analog or digital. The video signal has a potential which is lower than the H level of the scan signal and higher than the L level of the scan signal. Note that the constant potential supplied to the wiring  40306  is preferably equal to a potential of the counter electrode  40307 . 
     Operations of the pixel  40320  are described by dividing the whole operations into the case where the transistor  40301  is on and the transistor  40311  is off, the case where the transistor  40301  is off and the transistor  40311  is on, and the case where the transistor  40301  and the transistor  40311  are off. 
     In the case where the transistor  40301  is on and the transistor  40311  is off, the wiring  40304  is electrically connected to the first electrode (a pixel electrode) of the liquid crystal element  40302  and the first electrode of the capacitor  40303  in the subpixel  40300 . Therefore, the video signal is input to the first electrode (the pixel electrode) of the liquid crystal element  40302  and the first electrode of the capacitor  40303  from the wiring  40304  through the transistor  40301 . In addition, the capacitor  40303  holds a potential difference between a potential of the video signal and a potential supplied to the wiring  40306 . At this time, the wiring  40304  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40312  and the first electrode of the capacitor  40313  in the subpixel  40310 . Therefore, the video signal is not input to the subpixel  40310 . 
     In the case where the transistor  40301  is off and the transistor  40311  is on, the wiring  40304  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40302  and the first electrode of the capacitor  40303  in the subpixel  40300 . Therefore, each of the first electrode of the liquid crystal element  40302  and the first electrode of the capacitor  40303  is set in a floating state. Since the capacitor  40303  holds the potential difference between the potential of the video signal and the potential supplied to the wiring  40306 , each of the first electrode of the liquid crystal element  40302  and the first electrode of the capacitor  40303  holds a potential which is the same as (corresponds to) the video signal. At this time, the wiring  40304  is electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40312  and the first electrode of the capacitor  40313  in the subpixel  40310 . Therefore, the video signal is input to the first electrode (the pixel electrode) of the liquid crystal element  40312  and the first electrode of the capacitor  40313  from the wiring  40304  through the transistor  40311 . In addition, the capacitor  40313  holds a potential difference between a potential of the video signal and a potential supplied to the wiring  40306 . 
     In the case where the transistor  40301  and the transistor  40311  are off, the wiring  40304  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40302  and the first electrode of the capacitor  40303  in the subpixel  40300 . Therefore, each of the first electrode of the liquid crystal element  40302  and the first electrode of the capacitor  40303  is set in a floating state. Since the capacitor  40303  holds the potential difference between the potential of the video signal and the potential supplied to the wiring  40306 , each of the first electrode of the liquid crystal element  40302  and the first electrode of the capacitor  40303  holds a potential which is the same as (corresponds to) the video signal. Note that the liquid crystal element  40302  has transmittivity in accordance with the video signal. At this time, the wiring  40304  is not electrically connected to the first electrode (the pixel electrode) of the liquid crystal element  40312  and the first electrode of the capacitor  40313  similarly in the subpixel  40310 . Therefore, each of the first electrode of the liquid crystal element  40312  and the first electrode of the capacitor  40313  is set in a floating state. Since the capacitor  40313  holds the potential difference between the potential of the video signal and the potential of the wiring  40316 , each of the first electrode of the liquid crystal element  40312  and the first electrode of the capacitor  40313  holds a potential which is the same as (corresponds to) the video signal. Note that the liquid crystal element  40312  has transmittivity in accordance with the video signal. 
     A video signal input to the subpixel  40300  may be a value which is different from that of a video signal input to the subpixel  40310 . In this case, the viewing angle can be widened because alignment of liquid crystal molecules of the liquid crystal element  40302  and alignment of liquid crystal molecules of the liquid crystal element  40312  can be varied from each other. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 8 
     In this embodiment mode, a driving method of a display device is described. In particular, a driving method of a liquid crystal display device is described. 
     A liquid crystal display panel which can be used for the liquid crystal display device described in this embodiment mode has a structure in which a liquid crystal material is sandwiched between two substrates. An electrode for controlling an electric field applied to the liquid crystal material is provided in each of the two substrates. A liquid crystal material corresponds to a material, optical and electrical properties of which, is changed by an electric field applied from outside. Therefore, a liquid crystal panel corresponds to a device in which desired optical and electrical properties can be obtained by controlling voltage applied to the liquid crystal material using the electrode included in each of the two substrates. In addition, a large number of electrodes are arranged in a planar manner, each of the electrodes corresponds to a pixel, and voltages applied to the pixels are individually controlled. Therefore, a liquid crystal display panel which can display a clear image can be obtained. 
     Here, response time of the liquid crystal material with respect to change in an electric field depends on a gap between the two substrates (a cell gap) and a type or the like of the liquid crystal material, and is generally several milli-seconds to several ten milli-seconds. Further, in the case where the amount of change in the electric field is small, the response time of the liquid crystal material is further lengthened. This characteristic causes a defect in image display such as an after image, a phenomenon in which traces can be seen, or decrease in contrast when the liquid crystal panel displays a moving image. In particular, when a half tone is changed into another half tone (change in the electric field is small), the degree of the above-described defect becomes noticeable. 
     Meanwhile, as a particular problem of a liquid crystal panel using an active matrix method, fluctuation in writing voltage due to constant electric charge driving is given. Constant electric charge driving in this embodiment mode is described below. 
     A pixel circuit using an active matrix method includes a switch which controls writing and a capacitor which holds an electric charge. A method for driving the pixel circuit using the active matrix method corresponds to a method in which predetermined voltage is written to a pixel circuit with a switch in an on state, and immediately after that, an electric charge in the pixel circuit is held (a hold state) with the switch in an off state. At the time of hold state, exchange of the electric charge between inside and outside of the pixel circuit is not performed (a constant electric charge). Usually, a period in which the switch is in an off state is approximately several hundreds of times (the number of scan lines) longer than a period in which the switch is in an on state. Therefore, it may be considered that the switch of the pixel circuit be almost always in an off state. As described above, constant electric charge driving in this embodiment mode corresponds to a driving method in which a pixel circuit is in a hold state in almost all periods in driving a liquid crystal panel. 
     Next, electrical properties of the liquid crystal material are described. A dielectric constant as well as optical properties of the liquid crystal material are changed when an electric field applied from outside is changed. That is, when it is considered that each pixel of the liquid crystal panel be a capacitor (a liquid crystal element) sandwiched between two electrodes, the capacitor corresponds to a capacitor, capacitance of which is changed in accordance with applied voltage. This phenomenon is called dynamic capacitance. 
     When a capacitor, capacitance of which is changed in accordance with applied voltage in this manner, is driven by constant electric charge driving, the following problem occurs. When capacitance of a liquid crystal element is changed in a hold state in which an electric charge is not moved, applied voltage is also changed. This is not difficult to understand from the fact that the amount of electric charges is constant in a relational expression of (the amount of electric charges)=(capacitance) x (applied voltage). 
     Because of the above-described reasons, voltage at the time of a hold state is changed from voltage at the time of writing because constant electric charge driving is performed in a liquid crystal panel using an active matrix method. Accordingly, change in transmittivity of the liquid crystal element is different from change in transmittivity of a liquid crystal element in a driving method which does not take a hold state.  FIGS. 83A to 83C  show this state.  FIG. 83A  shows an example of controlling voltage written to a pixel circuit in the case where time is represented by the horizontal axis and the absolute value of the voltage is represented by the vertical axis.  FIG. 83B  shows an example of controlling voltage written to the pixel circuit in the case where time is represented by the horizontal axis and the voltage is represented by the vertical axis.  FIG. 83C  shows time change in transmittivity of the liquid crystal element in the case where the voltage shown in  FIG. 83A or 83B  is written to the pixel circuit when time is represented by the horizontal axis and transmittivity of the liquid crystal element is represented by the vertical axis. In each of  FIGS. 83A to 83C , a period F shows a period for rewriting the voltage and time for rewriting the voltage is described as t 1 , t 2 , t 3 , and t 4 . 
     Here, writing voltage corresponding to image data input to the liquid crystal display device corresponds to |V 1 | in rewriting at the time of 0 and corresponds to |V 2 | in rewriting at the time of t 1 , t 2 , t 3 , and t 4  (see  FIG. 83A ). 
     Note that polarity of the writing voltage corresponding to image data input to the liquid crystal display device may be switched periodically (inversion driving: see  FIG. 83B ). Since direct voltage can be prevented from being applied to a liquid crystal as much as possible by using this method, burn-in or the like caused by deterioration of the liquid crystal element can be prevented. Note that a period of switching the polarity (an inversion period) may be the same as a period of rewriting voltage. In this case, generation of flickers caused by inversion driving can be reduced because the inversion period is short. Further, the inversion period may be a period which is integral times of the period of rewriting voltage. In this case, power consumption can be reduced because the inversion period is long and frequency of writing voltage can be decreased by changing the polarity. 
       FIG. 83C  shows time change in transmittivity of the liquid crystal element in the case where voltage as shown in  FIG. 83A or 83B  is applied to the liquid crystal element. Here, the voltage |V 1 | is applied to the liquid crystal element and transmittivity of the liquid crystal element after time passes sufficiently corresponds to TR 1 . Similarly, the voltage |V 2 | is applied to the liquid crystal element and transmittivity of the liquid crystal element after time passes sufficiently corresponds to TR 2 . When the voltage applied to the liquid crystal element is changed from |V 1 | to |V 2 | at the time of t 1 , transmittivity of the liquid crystal element does not immediately become TR 2  as shown by a dashed line  30401  but slowly changes. For example, when the period of rewriting voltage is the same as a frame period of an image signal of 60 Hz (16.7 milli-seconds), time for several frames is necessary until transmittivity is changed to TR 2 . 
     Note that smooth time change in transmittivity as shown in the dashed line  30401  corresponds to time change in transmittivity when the voltage |V 2 | is accurately applied to the liquid crystal element. In an actual liquid crystal panel, for example, a liquid crystal panel using an active matrix method, transmittivity of the liquid crystal does not have time change as shown by the dashed line  30401  but has gradual time change as shown by a solid line  30402  because voltage at the time of a hold state is changed from voltage at the time of writing due to constant electric charge driving. This is because the voltage is changed due to constant electric charge driving, so that it is impossible to reach intended voltage only by one writing. Accordingly, the response time of transmittivity of the liquid crystal element becomes further longer than original response time (the dashed line  30401 ) in appearance, so that a defect in image display such as an after image, a phenomenon in which traces can be seen, or decrease in contrast occurs. 
     When overdriving is used, it is possible to solve a phenomenon in which the response time in appearance becomes further longer because of shortage of writing by dynamic capacitance and constant electric charge driving as well as length of the original response time of the liquid crystal element.  FIGS. 84A to 84C  show this state.  FIG. 84A  shows an example of controlling voltage written to a pixel circuit in the case where time is represented by the horizontal axis and the absolute value of the voltage is represented by the vertical axis.  FIG. 84B  shows an example of controlling voltage written to the pixel circuit in the case where time is represented by the horizontal axis and the voltage is represented by the vertical axis.  FIG. 84C  shows time change in transmittivity of the liquid crystal element in the case where the voltage shown in  FIG. 84A or 84B  is written to the pixel circuit when time is represented by the horizontal axis and transmittivity of the liquid crystal element is represented by the vertical axis. In each of  FIGS. 84A to 84C , a period F shows a period for rewriting the voltage and time for rewriting the voltage is described as t 1 , t 2 , t 3 , and t 4 . 
     Here, writing voltage corresponding to image data input to the liquid crystal display device corresponds to |V 1 | in rewriting at the time of 0, corresponds to |V 3 | in rewriting at the time of t 1 , and corresponds to |V 3 | in writing at the time of t 2 , t 3 , and t 4  (see  FIG. 84A ). 
     Note that polarity of the writing voltage corresponding to image data input to the liquid crystal display device may be switched periodically (inversion driving: see  FIG. 84B ). Since direct voltage can be prevented from being applied to a liquid crystal as much as possible by using this method, burn-in or the like caused by deterioration of the liquid crystal element can be prevented. Note that a period of switching the polarity (an inversion period) may be the same as a period of rewriting voltage. In this case, generation of flickers caused by inversion driving can be reduced because the inversion period is short. Further, the inversion period may be a period which is integral times of the period of rewriting voltage. In this case, power consumption can be reduced because the inversion period is long and frequency of writing voltage can be decreased by changing the polarity. 
       FIG. 84C  shows time change in transmittivity of the liquid crystal element in the case where voltage as shown in  FIG. 84A or 84B  is applied to the liquid crystal element. Here, the voltage |V 1 | is applied to the liquid crystal element and transmittivity of the liquid crystal element after time passes sufficiently corresponds to TR 1 . Similarly, the voltage |V 2 | is applied to the liquid crystal element and transmittivity of the liquid crystal element after time passes sufficiently corresponds to TR 2 . Similarly, the voltage |V 3 | is applied to the liquid crystal element and transmittivity of the liquid crystal element after time passes sufficiently corresponds to TR 3 . When the voltage applied to the liquid crystal element is changed from |V 1 | to |V 3 | at the time of t 1 , transmittivity of the liquid crystal element is tried to be changed to TR 3  for several frames as shown by a dashed line  30501 . However, application of the voltage |V 3 | is terminated at the time t 2  and the voltage |V 2 | is applied after the time t 2 . Therefore, transmittivity of the liquid crystal element does not become as shown by the dashed line  30501  but becomes as shown by a solid line  30502 . Here, it is preferable that a value of the voltage |V 3 | be set so that transmittivity is approximately TR 2  at the time of t 2 . Here, the voltage |V 3 | is also referred to as overdriving voltage. 
     That is, the response time of the liquid crystal element can be controlled to some extent by changing |V 3 | which is the overdriving voltage. This is because the response time of the liquid crystal element is changed by strength of an electric field. Specifically, the response time of the liquid crystal element becomes shorter as the electric field is strong, and the response time of the liquid crystal element becomes longer as the electric field is weak. 
     Note that it is preferable that |V 3 | which is the overdriving voltage be changed in accordance with the amount of change in the voltage, i.e., the voltage |V 1 | and the voltage |V 2 | which supply intended transmittivity TR 1  and TR 2 . This is because appropriate response time can be always obtained by changing |V 3 | which is the overdriving voltage in accordance with change in the response time of the liquid crystal element even when the response time of the liquid crystal element is changed by the amount of change in the voltage. 
     Note that it is preferable that |V 3 | which is the overdriving voltage be changed by a mode of the liquid crystal element such as a TN mode, a VA mode, an IPS mode, or an OCB mode. This is because appropriate response time can be always obtained by changing |V 3 | which is the overdriving voltage in accordance with change in the response time of the liquid crystal element even when the response time of the liquid crystal element is changed by the mode of the liquid crystal element. 
     Note that the voltage rewriting period F may be the same as a frame period of an input signal. In this case, a liquid crystal display device with low manufacturing cost can be obtained because a peripheral driver circuit of the liquid crystal display device can be simplified. 
     Note also that the voltage rewriting period F may be shorter than the frame period of the input signal. For example, the voltage rewriting period F may be one half the frame period of the input signal, one third the frame period of the input signal, or one third or less the frame period of the input signal. It is effective to combine this method with a countermeasure against deterioration in quality of moving images caused by hold driving of the liquid crystal display device such as black data insertion driving, backlight blinking, backlight scanning, or intermediate image insertion driving by motion compensation. That is, since required response time of the liquid crystal element is short in the countermeasure against deterioration in quality of moving images caused by hold driving of the liquid crystal display device, the response time of the liquid crystal element can be relatively shortened easily by using overdriving described in this embodiment mode. Although the response time of the liquid crystal element can be essentially shortened by a cell gap, a liquid crystal material, a mode of the liquid crystal element, or the like, it is technically difficult to shorten the response time of the liquid crystal element. Therefore, it is very important to use a method for shortening the response time of the liquid crystal element by a driving method such as overdriving. 
     Note that the voltage rewriting period F may be longer than the frame period of the input signal. For example, the voltage rewriting period F may be twice the frame period of the input signal, three times the frame period of the input signal, or three times or more the frame period of the input signal. It is effective to combine this method with a unit (a circuit) which determines whether voltage is not rewritten for a long period or not. That is, when the voltage is not rewritten for a long period, an operation of the circuit can be stopped during a period where no voltage is rewritten without performing a rewriting operation itself of the voltage. Therefore, a liquid crystal display device with low power consumption can be obtained. 
     Next, a specific method for changing |V 3 | which is the overdriving voltage in accordance with the voltage |V 1 | and the voltage |V 2 | which supply intended transmittivity TR 1  and TR 2  is described. 
     Since an overdriving circuit corresponds to a circuit for appropriately controlling |V 3 | which is the overdriving voltage in accordance with the voltage |V 1 | and the voltage |V 2 | which supply intended transmittivity TR 1  and TR 2 , signals input to the overdriving circuit are a signal which is related to the voltage |V 1 | which supplies intended transmittivity TR 1  and a signal which is related to the voltage |V 2 | which supplies intended transmittivity TR 2 , and a signal output from the overdriving circuit is a signal which is related to |V 3 | which is the overdriving voltage. Here, each of these signals may have an analog voltage value such as the voltage applied to the liquid crystal element (e.g., |V 1 |, |V 2 |, or |V 3 |) or may be a digital signal for supplying the voltage applied to the liquid crystal element. Here, the signal which is related to the overdriving circuit is described as a digital signal. 
     First, a general structure of the overdriving circuit is described with reference to  FIG. 80A . Here, input image signals  30101   a  and  30101   b  are used as signals for controlling the overdriving voltage. As a result of processing these signals, an output image signal  30104  is to be output as a signal which supplies the overdriving voltage. 
     Here, since the voltage |V 1 | and the voltage |V 2 | which supply intended transmittivity TR 1  and TR 2  are image signals in adjacent frames, it is preferable that the input image signals  30101   a  and  30101   b  be similarly image signals in adjacent frames. In order to obtain such signals, the input image signal  30101   a  is input to a delay circuit  30102  in  FIG. 80A  and a signal which is consequently output can be used as the input image signal  30101   b . For example, a memory can be given as the delay circuit  30102 . That is, the input image signal  30101   a  is stored in the memory in order to delay the input image signal  30101   a  for one frame; a signal stored in the previous frame is taken out from the memory as the input image signal  30101   b  at the same time; and the input image signal  30101   a  and the input image signal  30101   b  are simultaneously input to a correction circuit  30103 . Therefore, the image signals in adjacent frames can be handled. When the image signals in adjacent frames are input to the correction circuit  30103 , the output image signal  30104  can be obtained. Note that when a memory is used as the delay circuit  30102 , a memory having capacity for storing an image signal for one frame in order to delay the input image signal  30101   a  for one frame (i.e., a frame memory) can be obtained. Thus, the memory can have a function as a delay circuit without causing excess and deficiency of memory capacity. 
     Next, the delay circuit  30102  formed mainly for reducing memory capacity is described. Since memory capacity can be reduced by using such a circuit as the delay circuit  30102 , manufacturing cost can be reduced. 
     Specifically, a delay circuit as shown in  FIG. 80B  can be used as the delay circuit  30102  having such characteristics. The delay circuit shown in  FIG. 80B  includes an encoder  30105 , a memory  30106 , and a decoder  30107 . 
     Operations of the delay circuit  30102  shown in  FIG. 80B  are as follows. First, compression treatment is performed by the encoder  30105  before the input image signal  30101   a  is stored in the memory  30106 . Thus, the size of data to be stored in the memory  30106  can be reduced. Accordingly, since memory capacity can be reduced, manufacturing cost can also be reduced. Then, a compressed image signal is transferred to the decoder  30107  and extension treatment is performed here. Thus, the previous signal which is compressed by the encoder  30105  can be restored. Here, compression and extension treatment which is performed by the encoder  30105  and the decoder  30107  may be reversible treatment. Thus, since the image signal does not deteriorate even after compression and extension treatment is performed, memory capacity can be reduced without causing deterioration of quality of an image, which is finally displayed on a device. Further, compression and extension treatment which is performed by the encoder  30105  and the decoder  30107  may be non-reversible treatment. Thus, since the size of data of the compressed image signal can be extremely made small, memory capacity can be significantly reduced. 
     Note that as a method for reducing memory capacity, various methods as well as the above-described method can be used. A method in which color information included in an image signal is reduced (e.g., tone reduction from 2.6 hundred thousand colors to 65 thousand colors is performed) or the amount of data is reduced (e.g., resolution is made smaller) without performing image compression by an encoder, or the like can be used. 
     Next, specific examples of the correction circuit  30103  are described with reference to  FIGS. 80C to 80E . The correction circuit  30103  corresponds to a circuit for outputting an output image signal having a certain value from two input image signals. Here, when relation between the two input image signals and the output image signal is non-linear and it is difficult to calculate the relation by simple operation, a look up table (a LUT) may be used as the correction circuit  30103 . Since the relation between the two input image signals and the output image signal is calculated in advance by measurement in a LUT, the output image signal corresponding to the two input image signals can be calculated only by seeing the LUT (see  FIG. 80C ). When a LUT  30108  is used as the correction circuit  30103 , the correction circuit  30103  can be realized without performing complicated circuit design or the like. 
     Here, since the LUT is one of memories, it is preferable to reduce memory capacity as much as possible in order to reduce manufacturing cost. As an example of the correction circuit  30103  for realizing reduction in memory capacity, a circuit shown in  FIG. 80D  can be given. The correction circuit  30103  shown in  FIG. 80D  includes a LUT  30109  and an adder  30110 . Data of difference between the input image signal  30101   a  and the output image signal  30104  to be output is stored in the LUT  30109 . That is, corresponding difference data from the input image signal  30101   a  and the input image signal  30101   b  is taken out from the LUT  30109  and taken out difference data and the input image signal  30101   a  are added by the adder  30110 , so that the output image signal  30104  can be obtained. Note that when data stored in the LUT  30109  is difference data, memory capacity of the LUT can be reduced. This is because data size of difference data is smaller than data size of the output image signal  30104  itself, so that memory capacity necessary for the LUT  30109  can be made smaller. 
     In addition, when the output image signal can be calculated by simple operation such as four arithmetic operations of the two input image signals, the correction circuit  30103  can be realized by combination of simple circuits such as an adder, a subtracter, and a multiplier. Accordingly, it is not necessary to use the LUT, so that manufacturing cost can be significantly reduced. As such a circuit, a circuit shown in  FIG. 80E  can be given. The correction circuit  30103  shown in  FIG. 80E  includes a subtracter  30111 , a multiplier  30112 , and an adder  30113 . First, difference between the input image signal  30101   a  and the input image signal  30101   b  is calculated by the subtracter  30111 . After that, a differential value is multiplied by an appropriate coefficient by using the multiplier  30112 . Then, when the differential value multiplied by an appropriate coefficient is added to the input image signal  30101   a  by the adder  30113 , the output image signal  30104  can be obtained. When such a circuit is used, it is not necessary to use the LUT. Therefore, manufacturing cost can be significantly reduced. 
     Note that when the correction circuit  30103  shown in  FIG. 88E  is used under a certain condition, output of the inappropriate output image signal  30104  can be prevented. The condition is as follows. The output image signal  30104  which supplies the overdriving voltage and a differential value between the input image signals  30101   a  and  30101   b  have linearity. In addition, the differential value corresponds to a coefficient multiplied by inclination of this linearity by using the multiplier  30112 . That is, it is preferable that the correction circuit  30103  shown in  FIG. 80E  be used for a liquid crystal element having such properties. As a liquid crystal element having such properties, an IPS-mode liquid crystal element in which response time has low dependency on a gray scale can be given. For example, when the correction circuit  30103  shown in  FIG. 80E  is used for an IPS-mode liquid crystal element in this manner, manufacturing cost can be significantly reduced and an overdriving circuit which can prevent output of the inappropriate output image signal  30104  can be obtained. 
     Note that operations which are similar to those of the circuit shown in  FIGS. 80A to 80E  may be realized by software processing. As for the memory used for the delay circuit, another memory included in the liquid crystal display device, a memory included in a device which transfers an image displayed on the liquid crystal display device (e.g., a video card or the like included in a personal computer or a device similar to the personal computer) can be used. Thus, intensity of overdriving, availability, or the like can be selected in accordance with user&#39;s preference in addition to reduction in manufacturing cost. 
     Driving which controls a potential of a common line is described with reference to  FIGS. 81A and 81B .  FIG. 81A  is a diagram showing a plurality of pixel circuits in which one common line is provided with respect to one scan line in a display device using a display element which has capacitive properties like a liquid crystal element. Each of the pixel circuits shown in  FIG. 81A  includes a transistor  30201 , an auxiliary capacitor  30202 , a display element  30203 , a video signal line  30204 , a scan line  30205 , and a common line  30206 . 
     A gate electrode of the transistor  30201  is electrically connected to the scan line  30205 ; one of a source electrode and a drain electrode of the transistor  30201  is electrically connected to the video signal line  30204 ; and the other of the source electrode and the drain electrode of the transistor  30201  is electrically connected to one of electrodes of the auxiliary capacitor  30202  and one of electrodes of the display element  30203 . In addition, the other of the electrodes of the auxiliary capacitor  30202  is electrically connected to the common line  30206 . 
     First, in each of pixels selected by the scan line  30205 , voltage corresponding to an image signal is applied to the display element  30203  and the auxiliary capacitor  30202  through the video signal line  30204  because the transistor  30201  is turned on. At this time, when the image signal is a signal which makes all pixels connected to the common line  30206  display a minimum gray scale or when the image signal is a signal which makes all the pixels connected to the common line  30206  display a maximum gray scale, it is not necessary that the image signal be written to each of the pixels through the video signal line  30204 . Voltage applied to the display element  30203  can be changed by changing a potential of the common line  30206  instead of writing the image signal through the video signal line  30204 . 
     Next,  FIG. 81B  is a diagram showing a plurality of pixel circuits in which two common lines are provided with respect to one scan line in a display device using a display element which has capacitive properties like a liquid crystal element. Each of the pixel circuits shown in  FIG. 81B  includes a transistor  30211 , an auxiliary capacitor  30212 , a display element  30213 , a video signal line  30214 , a scan line  30215 , a first common line  30216 , and a second common line  30217 . 
     Agate electrode of the transistor  30211  is electrically connected to the scan line  30215 ; one of a source electrode and a drain electrode of the transistor  30211  is electrically connected to the video signal line  30214 ; and the other of the source electrode and the drain electrode of the transistor  30211  is electrically connected to one of electrodes of the auxiliary capacitor  30212  and one of electrodes of the display element  30213 . In addition, the other of the electrodes of the auxiliary capacitor  30212  is electrically connected to the first common line  30216 . Further, in a pixel which is adjacent to the pixel, the other of the electrodes of the auxiliary capacitor  30212  is electrically connected to the second common line  30217 . 
     In the pixel circuits shown in  FIG. 81B , the number of pixels which are electrically connected to one common line is small. Therefore, when a potential of the first common line  30216  or the second common line  30217  is changed instead of writing an image signal through the video signal line  30214 , frequency of changing voltage applied to the display element  30213  is significantly increased. In addition, source inversion driving or dot inversion driving can be performed. When source inversion driving or dot inversion driving is performed, reliability of the element can be improved and a flicker can be suppressed. 
     A scanning backlight is described with reference to  FIGS. 82A to 82C .  FIG. 82A  shows a scanning backlight in which cold cathode fluorescent lamps are arranged. The scanning backlight shown in  FIG. 82A  includes a diffusion plate  30301  and N pieces of cold cathode fluorescent lamps  30302 - 1  to  30302 -N. The N pieces of the cold cathode fluorescent lamps  30302 - 1  to  30302 -N are arranged on the back side of the diffusion plate  30301 , so that the N pieces of the cold cathode fluorescent lamps  30302 - 1  to  30302 -N can be scanned while luminance thereof is changed. 
     Change in luminance of each of the cold cathode fluorescent lamps in scanning is described with reference to  FIG. 82C . First, luminance of the cold cathode fluorescent lamp  30302 - 1  is changed for a certain period. After that, luminance of the cold cathode fluorescent lamp  30302 - 2  which is provided adjacent to the cold cathode fluorescent lamp  30302 - 1  is changed for the same period. In this manner, luminance is changed sequentially from the cold cathode fluorescent lamp  30302 - 1  to the cold cathode fluorescent lamp  30302 -N. Although luminance which is changed for a certain period is set to be lower than original luminance in  FIG. 82C , it may also be higher than original luminance. In addition, although scanning is performed from the cold cathode fluorescent lamps  30302 - 1  to  30302 -N, scanning may also be performed from the cold cathode fluorescent lamps  30302 -N to  30302 - 1 , which is in a reversed order. 
     When driving is performed as in  FIGS. 82A to 82C , average luminance of the backlight can be decreased. Therefore, power consumption of the backlight, which mainly takes up power consumption of the liquid crystal display device, can be reduced. 
     Note that an LED may be used as a light source of the scanning backlight. The scanning backlight in that case is as shown in  FIG. 82B . The scanning backlight shown in  FIG. 82B  includes a diffusion plate  30311  and light sources  30312 - 1  to  30312 -N, in each of which LEDs are arranged. When the LED is used as the light source of the scanning backlight, there is an advantage in that the backlight can be thin and lightweight. In addition, there is also an advantage that a color reproduction area can be widened. Further, since the LEDs which are arranged in each of the light sources  30312 - 1  to  30312 -N can be similarly scanned, a dot scanning backlight can also be obtained. When the dot scanning backlight is used, image quality of moving images can be further improved. 
     Note that when the LED is used as the light source of the backlight, driving can be performed by changing luminance, as shown in  FIG. 82C . 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 9 
     In this embodiment mode, various liquid crystal modes are described. 
     First, various liquid crystal modes are described with reference to cross-sectional views. 
       FIGS. 134A and 134B  are schematic views of cross sections of a TN mode. 
     A liquid crystal layer  50100  is held between a first substrate  50101  and a second substrate  50102  which are provided so as to be opposite to each other. A first electrode  50105  is formed on a top surface of the first substrate  50101 . A second electrode  50106  is formed on a top surface of the second substrate  50102 . A first polarizing plate  50103  is provided on a surface of the first substrate  50101 , which does not face the liquid crystal layer. A second polarizing plate  50104  is provided on a surface of the second substrate  50102 , which does not face the liquid crystal layer. Note that the first polarizing plate  50103  and the second polarizing plate  50104  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50103  may be provided on the top surface of the first substrate  50101 . The second polarizing plate  50104  may be provided on the top surface of the second substrate  50102 . 
     It is acceptable as long as at least one of or both the first electrode  50105  and the second electrode  50106  have light-transmitting properties (a transmissive or reflective liquid crystal display device). Alternatively, both the first electrode  50105  and the second electrode  50106  may have light-transmitting properties, and part of one of the electrodes may have reflectivity (a semi-transmissive liquid crystal display device). 
       FIG. 134A  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50105  and the second electrode  50106  (referred to as a vertical electric field mode). Since liquid crystal molecules are aligned longitudinally, light emitted from a backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50103  and the second polarizing plate  50104  are provided so as to be in a cross nicol state, light emitted from the backlight cannot pass through the substrate. Therefore, black display is performed. 
     Note that when voltage applied to the first electrode  50105  and the second electrode  50106  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 134B  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50105  and the second electrode  50106 . Since the liquid crystal molecules are aligned laterally and rotated in a plane, light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50103  and the second polarizing plate  50104  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. This is a so-called normally white mode. 
     A liquid crystal display device having the structure shown in  FIG. 134A  or  FIG. 134B  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50101  side or a second substrate  50102  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for a TN mode. 
       FIGS. 135A and 135B  are schematic views of cross sections of a VA mode. In the VA mode, liquid crystal molecules are aligned such that they are vertical to a substrate when there is no electric field. 
     A liquid crystal layer  50200  is held between a first substrate  50201  and a second substrate  50202  which are provided so as to be opposite to each other. A first electrode  50205  is formed on a top surface of the first substrate  50201 . A second electrode  50206  is formed on a top surface of the second substrate  50202 . A first polarizing plate  50203  is provided on a surface of the first substrate  50201 , which does not face the liquid crystal layer. A second polarizing plate  50204  is provided on a surface of the second substrate  50202 , which does not face the liquid crystal layer. Note that the first polarizing plate  50203  and the second polarizing plate  50204  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50203  may be provided on the top surface of the first substrate  50201 . The second polarizing plate  50204  may be provided on the top surface of the second substrate  50202 . 
     It is acceptable as long as at least one of or both the first electrode  50205  and the second electrode  50206  have light-transmitting properties (a transmissive or reflective liquid crystal display device). Alternatively, both the first electrode  50205  and the second electrode  50206  may have light-transmitting properties, and part of one of the electrodes may have reflectivity (a semi-transmissive liquid crystal display device). 
       FIG. 135A  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50205  and the second electrode  50206  (referred to as a vertical electric field mode). Since liquid crystal molecules are aligned laterally, light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50203  and the second polarizing plate  50204  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. 
     Note that when voltage applied to the first electrode  50205  and the second electrode  50206  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 135B  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50205  and the second electrode  50206 . Since liquid crystal molecules are aligned longitudinally, light emitted from a backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50203  and the second polarizing plate  50204  are provided so as to be in a cross nicol state, light emitted from the backlight does not pass through the substrate. Therefore, black display is performed. This is a so-called normally black mode. 
     A liquid crystal display device having the structure shown in  FIG. 135A  or  FIG. 135B  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50201  side or a second substrate  50202  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for a VA mode. 
       FIGS. 135C and 135D  are schematic views of cross sections of an MVA mode. In the MVA mode, viewing angle dependency of each portion is compensated by each other. 
     A liquid crystal layer  50210  is held between a first substrate  50211  and a second substrate  50212  which are provided so as to be opposite to each other. A first electrode  50215  is formed on a top surface of the first substrate  50211 . A second electrode  50216  is formed on a top surface of the second substrate  50212 . A first protrusion  50217  for controlling alignment is formed on the first electrode  50215 . A second protrusion  50218  for controlling alignment is formed over the second electrode  50216 . A first polarizing plate  50213  is provided on a surface of the first substrate  50211 , which does not face the liquid crystal layer. A second polarizing plate  50214  is provided on a surface of the second substrate  50212 , which does not face the liquid crystal layer. Note that the first polarizing plate  50213  and the second polarizing plate  50214  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50213  may be provided on the top surface of the first substrate  50211 . The second polarizing plate  50214  may be provided on the top surface of the second substrate  50212 . 
     It is acceptable as long as at least one of or both the first electrode  50215  and the second electrode  50216  have light-transmitting properties (a transmissive or reflective liquid crystal display device). Alternatively, both the first electrode  50215  and the second electrode  50216  may have light-transmitting properties, and part of one of the electrodes may have reflectivity (a semi-transmissive liquid crystal display device). 
       FIG. 135C  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50215  and the second electrode  50216  (referred to as a vertical electric field mode). Since liquid crystal molecules are aligned so as to tilt toward the first protrusion  50217  and the second protrusion  50218 , light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50213  and the second polarizing plate  50214  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. 
     Note that when voltage applied to the first electrode  50215  and the second electrode  50216  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 135D  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50215  and the second electrode  50216 . Since liquid crystal molecules are aligned longitudinally, light emitted from a backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50213  and the second polarizing plate  50214  are provided so as to be in a cross nicol state, light emitted from the backlight does not pass through the substrate. Therefore, black display is performed. This is a so-called normally black mode. 
     A liquid crystal display device having the structure shown in  FIG. 135C  or  FIG. 135D  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50211  side or a second substrate  50212  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an MVA mode. 
       FIGS. 136A and 136B  are schematic views of cross sections of an OCB mode. In the OCB mode, viewing angle dependency is low because alignment of liquid crystal molecules in a liquid crystal layer can be optically compensated. This state of the liquid crystal molecules is referred to as bend alignment. 
     A liquid crystal layer  50300  is held between a first substrate  50301  and a second substrate  50302  which are provided so as to be opposite to each other. A first electrode  50305  is formed on a top surface of the first substrate  50301 . A second electrode  50306  is formed on a top surface of the second substrate  50302 . A first polarizing plate  50303  is provided on a surface of the first substrate  50301 , which does not face the liquid crystal layer. A second polarizing plate  50304  is provided on a surface of the second substrate  50302 , which does not face the liquid crystal layer. Note that the first polarizing plate  50303  and the second polarizing plate  50304  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50303  may be provided on the top surface of the first substrate  50301 . The second polarizing plate  50304  may be provided on the top surface of the second substrate  50302 . 
     It is acceptable as long as at least one of or both the first electrode  50305  and the second electrode  50306  have light-transmitting properties (a transmissive or reflective liquid crystal display device). Alternatively, both the first electrode  50305  and the second electrode  50306  may have light-transmitting properties, and part of one of the electrodes may have reflectivity (a semi-transmissive liquid crystal display device). 
       FIG. 136A  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50305  and the second electrode  50306  (referred to as a vertical electric field mode). Since liquid crystal molecules are aligned longitudinally, light emitted from a backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50303  and the second polarizing plate  50304  are provided so as to be in a cross nicol state, light emitted from the backlight does not pass through the substrate. Therefore, black display is performed. 
     Note that when voltage applied to the first electrode  50305  and the second electrode  50306  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 136B  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50305  and the second electrode  50306 . Since liquid crystal molecules are in a bend alignment state, light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50303  and the second polarizing plate  50304  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. This is a so-called normally white mode. 
     A liquid crystal display device having the structure shown in  FIG. 136A  or  FIG. 136B  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50301  side or a second substrate  50302  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an OCB mode. 
       FIGS. 136C and 136D  are schematic views of cross sections of an FLC mode or an AFLC mode. 
     A liquid crystal layer  50310  is held between a first substrate  50311  and a second substrate  50312  which are provided so as to be opposite to each other. A first electrode  50315  is formed on a top surface of the first substrate  50311 . A second electrode  50316  is formed on a top surface of the second substrate  50312 . A first polarizing plate  50313  is provided on a surface of the first substrate  50311 , which does not face the liquid crystal layer. A second polarizing plate  50314  is provided on a surface of the second substrate  50312 , which does not face the liquid crystal layer. Note that the first polarizing plate  50313  and the second polarizing plate  50314  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50313  may be provided on the top surface of the first substrate  50311 . The second polarizing plate  50314  may be provided on the top surface of the second substrate  50312 . 
     It is acceptable as long as at least one of or both the first electrode  50315  and the second electrode  50316  have light-transmitting properties (a transmissive or reflective liquid crystal display device). Alternatively, both the first electrode  50315  and the second electrode  50316  may have light-transmitting properties, and part of one of the electrodes may have reflectivity (a semi-transmissive liquid crystal display device). 
       FIG. 136C  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50315  and the second electrode  50316  (referred to as a vertical electric field mode). Since liquid crystal molecules are aligned laterally in a direction which is deviated from a rubbing direction, light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50313  and the second polarizing plate  50314  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. 
     Note that when voltage applied to the first electrode  50315  and the second electrode  50316  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 136D  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50315  and the second electrode  50316 . Since liquid crystal molecules are aligned laterally in a rubbing direction, light emitted from a backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50313  and the second polarizing plate  50314  are provided so as to be in a cross nicol state, light emitted from the backlight does not pass through the substrate. Therefore, black display is performed. This is a so-called normally black mode. 
     A liquid crystal display device having the structure shown in  FIG. 136C  or  FIG. 136D  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50311  side or a second substrate  50312  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an FLC mode or an AFLC mode. 
       FIGS. 137A and 137B  are schematic views of cross sections of an IPS mode. In the IPS mode, alignment of liquid crystal molecules in a liquid crystal layer can be optically compensated, the liquid crystal molecules are constantly rotated in a plane parallel to a substrate, and a horizontal electric field method in which electrodes are provided only on one substrate side is used. 
     A liquid crystal layer  50400  is held between a first substrate  50401  and a second substrate  50402  which are provided so as to be opposite to each other. A first electrode  50405  and a second electrode  50406  are formed on a top surface of the second substrate  50402 . A first polarizing plate  50403  is provided on a surface of the first substrate  50401 , which does not face the liquid crystal layer. A second polarizing plate  50404  is provided on a surface of the second substrate  50402 , which does not face the liquid crystal layer. Note that the first polarizing plate  50403  and the second polarizing plate  50404  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50403  may be provided on the top surface of the first substrate  50401 . The second polarizing plate  50404  may be provided on the top surface of the second substrate  50402 . 
     It is acceptable as long as both the first electrode  50405  and the second electrode  50406  have light-transmitting properties. Alternatively, part of one of the first electrode  50405  and the second electrode  50406  may have reflectivity. 
       FIG. 137A  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50405  and the second electrode  50406  (referred to as a horizontal electric field mode). Since liquid crystal molecules are aligned along a line of electric force which is deviated from a rubbing direction, light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50403  and the second polarizing plate  50404  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. 
     Note that when voltage applied to the first electrode  50405  and the second electrode  50406  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 137B  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50405  and the second electrode  50406 . Since liquid crystal molecules are aligned laterally in a rubbing direction, light emitted from a backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50403  and the second polarizing plate  50404  are provided so as to be in a cross nicol state, light emitted from the backlight does not pass through the substrate. Therefore, black display is performed. This is a so-called normally black mode. 
     A liquid crystal display device having the structure shown in  FIG. 137A  or  FIG. 137B  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50401  side or a second substrate  50402  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an IPS mode. 
       FIGS. 137C and 137D  are schematic views of cross sections of an FFS mode. In the FFS mode, alignment of liquid crystal molecules in a liquid crystal layer can be optically compensated, the liquid crystal molecules are constantly rotated in a plane parallel to a substrate, and a horizontal electric field method in which electrodes are provided only on one substrate side is used. 
     A liquid crystal layer  50410  is held between a first substrate  50411  and a second substrate  50412  which are provided so as to be opposite to each other. A second electrode  50416  is formed on a top surface of the second substrate  50412 . An insulating film  50417  is formed on a top surface of the second electrode  50416 . A first electrode  50415  is formed over the insulating film  50417 . A first polarizing plate  50413  is provided on a surface of the first substrate  50411 , which does not face the liquid crystal layer. A second polarizing plate  50414  is provided on a surface of the second substrate  50412 , which does not face the liquid crystal layer. Note that the first polarizing plate  50413  and the second polarizing plate  50414  are provided so as to be in a cross nicol state. 
     The first polarizing plate  50413  may be provided on the top surface of the first substrate  50411 . The second polarizing plate  50414  may be provided on the top surface of the second substrate  50412 . 
     It is acceptable as long as both the first electrode  50415  and the second electrode  50416  have light-transmitting properties. Alternatively, part of one of the electrodes may have reflectivity. 
       FIG. 137C  is a schematic view of a cross section in the case where voltage is applied to the first electrode  50415  and the second electrode  50416  (referred to as a horizontal electric field mode). Since liquid crystal molecules are aligned along a line of electric force which is deviated from a rubbing direction, light emitted from a backlight is affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50413  and the second polarizing plate  50414  are provided so as to be in a cross nicol state, light emitted from the backlight passes through the substrate. Therefore, white display is performed. 
     Note that when voltage applied to the first electrode  50415  and the second electrode  50416  is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
       FIG. 137D  is a schematic view of a cross section in the case where voltage is not applied to the first electrode  50415  and the second electrode  50416 . Since liquid crystal molecules are aligned laterally in a rubbing direction, light emitted from the backlight is not affected by birefringence of the liquid crystal molecules. In addition, since the first polarizing plate  50413  and the second polarizing plate  50414  are provided so as to be in a cross nicol state, light emitted from the backlight does not pass through the substrate. Therefore, black display is performed. This is a so-called normally black mode. 
     A liquid crystal display device having the structure shown in  FIG. 137C  or  FIG. 137D  can perform full-color display by being provided with a color filter. The color filter can be provided on a first substrate  50411  side or a second substrate  50412  side. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an FFS mode. 
     Next, various liquid crystal modes are described with reference to top views. 
       FIG. 138  is a top view of a pixel portion to which an MVA mode is applied. In the MVA mode, viewing angle dependency of each portion is compensated by each other. 
       FIG. 138  shows a first pixel electrode  50501 , second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ), and a protrusion  50503 . The first pixel electrode  50501  is formed over the entire surface of a counter substrate. The protrusion  50503  is formed so as to be a dogleg shape. In addition, the second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ) are formed over the first pixel electrode  50501  so as to have shapes corresponding to the protrusion  50503 . 
     Opening portions of the second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ) function like protrusions. 
     In the case where voltage is applied to the first pixel electrode  50501  and the second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ) (referred to as a vertical electric field mode), liquid crystal molecules are aligned so as to tilt toward the opening portions of the second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ) and the protrusion  50503 . Since light emitted from a backlight passes through a substrate when a pair of polarizing plates is provided so as to be in a cross nicol state, white display is performed. 
     Note that when voltage applied to the first pixel electrode  50501  and the second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ) is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
     In the case where voltage is not applied to the first pixel electrode  50501  and the second pixel electrodes ( 50502   a ,  50502   b , and  50502   c ), the liquid crystal molecules are aligned longitudinally. Since light emitted from the backlight does not pass through a panel when the pair of polarizing plates is provided so as to be in the cross nicol state, black display is performed. This is a so-called normally black mode. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an MVA mode. 
       FIGS. 139A to 139D  are top views of a pixel portion to which an IPS mode is applied. In the IPS mode, alignment of liquid crystal molecules in a liquid crystal layer can be optically compensated, the liquid crystal molecules are constantly rotated in a plane parallel to a substrate, and a horizontal electric field method in which electrodes are provided only on one substrate side is used. 
     In the IPS mode, a pair of electrodes is formed so as to have different shapes. 
       FIG. 139A  shows a first pixel electrode  50601  and a second pixel electrode  50602 . The first pixel electrode  50601  and the second pixel electrode  50602  are wavy shapes. 
       FIG. 139B  shows a first pixel electrode  50611  and a second pixel electrode  50612 . The first pixel electrode  50611  and the second pixel electrode  50612  have shapes having concentric openings. 
       FIG. 139C  shows a first pixel electrode  50631  and a second pixel electrode  50632 . The first pixel electrode  50631  and the second pixel electrode  50632  are comb shapes and partially overlap with each other. 
       FIG. 139D  shows a first pixel electrode  50641  and a second pixel electrode  50642 . The first pixel electrode  50641  and the second pixel electrode  50642  are comb shapes in which electrodes engage with each other. 
     In the case where voltage is applied to the first pixel electrodes ( 50601 ,  50611 ,  50621 , and  50631 ) and the second pixel electrodes ( 50602 ,  50612 ,  50622 , and  50623 ) (referred to as a horizontal electric field mode), liquid crystal molecules are aligned along a line of electric force which is deviated from a rubbing direction. Since light emitted from a backlight passes through a substrate when a pair of polarizing plates is provided so as to be in a cross nicol state, white display is performed. 
     Note that when voltage applied to the first pixel electrodes ( 50601 ,  50611 ,  50621 , and  50631 ) and the second pixel electrodes ( 50602 ,  50612 ,  50622 , and  50623 ) is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
     In the case where voltage is not applied to the first pixel electrodes ( 50601 ,  50611 ,  50621 , and  50631 ) and the second pixel electrodes ( 50602 ,  50612 ,  50622 , and  50623 ), the liquid crystal molecules are aligned laterally in the rubbing direction. Since light emitted from the backlight does not pass through the substrate when the pair of polarizing plates is provided so as to be in the cross nicol state, black display is performed. This is a so-called normally black mode. 
     It is acceptable as long as a known material be used for a liquid crystal material used for an IPS mode. 
       FIGS. 140A to 140D  are top views of a pixel portion to which an FFS mode is applied. In the FFS mode, alignment of liquid crystal molecules in a liquid crystal layer can be optically compensated, the liquid crystal molecules are constantly rotated in a plane parallel to a substrate, and a horizontal electric field method in which electrodes are provided only on one substrate side is used. 
     In the FFS mode, a first electrode is formed over a top surface of a second electrode so as to be various shapes. 
       FIG. 140A  shows a first pixel electrode  50701  and a second pixel electrode  50702 . The first pixel electrode  50701  is a bent dogleg shape. The second pixel electrode  50702  is not necessarily patterned. 
       FIG. 140B  shows a first pixel electrode  50711  and a second pixel electrode  50712 . The first pixel electrode  50711  is a concentric shape. The second pixel electrode  50712  is not necessarily patterned. 
       FIG. 140C  shows a first pixel electrode  50731  and a second pixel electrode  50732 . The first pixel electrode  50731  is a comb shape in which electrodes engage with each other. The second pixel electrode  50732  is not necessarily patterned. 
       FIG. 140D  shows a first pixel electrode  50741  and a second pixel electrode  50742 . The first pixel electrode  50741  is a comb shape. The second pixel electrode  50742  is not necessarily patterned. 
     In the case where voltage is applied to the first pixel electrodes ( 50701 ,  50711 ,  50721 , and  50731 ) and the second pixel electrodes ( 50702 ,  50712 ,  50722 , and  50723 ) (referred to as a horizontal electric field mode), liquid crystal molecules are aligned along a line of electric force which is deviated from a rubbing direction. Since light emitted from a backlight passes through a substrate when a pair of polarizing plates is provided so as to be in a cross nicol state, white display is performed. 
     Note that when voltage applied to the first pixel electrodes ( 50701 ,  50711 ,  50721 , and  50731 ) and the second pixel electrodes ( 50702 ,  50712 ,  50722 , and  50723 ) is controlled, conditions of the liquid crystal molecules can be controlled. Therefore, since the amount of light emitted from the backlight passing through the substrate can be controlled, predetermined image display can be performed. 
     In the case where voltage is not applied to the first pixel electrodes ( 50701 ,  50711 ,  50721 , and  50731 ) and the second pixel electrodes ( 50702 ,  50712 ,  50722 , and  50723 ), the liquid crystal molecules are aligned laterally in the rubbing direction. Since light emitted from the backlight does not pass through the substrate when the pair of polarizing plates is provided so as to be in the cross nicol state, black display is performed. This is a so-called normally black mode. 
     It is acceptable as long as a known material is used for a liquid crystal material used for an FFS mode. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 10 
     In this embodiment mode, a pixel structure of a display device is described. In particular, a pixel structure of a liquid crystal display device is described. 
     A pixel structure in the case where each liquid crystal mode and a transistor are combined is described with reference to cross-sectional views of a pixel. 
     Note that as the transistor, a thin film transistor (TFT) or the like including a non-single-crystal semiconductor layer typified by amorphous silicon, polycrystalline silicon, micro crystalline (also referred to as semi-amorphous) silicon, or the like can be used. 
     As the structure of the transistor, a top-gate structure, a bottom-gate structure, or the like can be used. Note that a channel-etched transistor, a channel-protective transistor, or the like can be used as a bottom-gate transistor. 
       FIG. 85  is an example of a cross-sectional view of a pixel in the case where a TN mode and a transistor are combined. When the pixel structure shown in  FIG. 85  is applied to a liquid crystal display device, a liquid crystal display device can be formed at low cost. 
     Features of the pixel structure shown in  FIG. 85  are described. Liquid crystal molecules  10118  shown in  FIG. 85  are long and narrow molecules each having a major axis and a minor axis. In  FIG. 85 , a direction of each of the liquid crystal molecules  10118  is expressed by the length thereof. That is, the direction of the major axis of the liquid crystal molecule  10118 , which is expressed as long, is parallel to the page, and as the liquid crystal molecule  10118  is expressed to be shorter, the direction of the major axis becomes closer to a normal direction of the page. That is, among the liquid crystal molecules  10118  shown in  FIG. 85 , the direction of the major axis of the liquid crystal molecule  10118  which is close to the first substrate  10101  and the direction of the major axis of the liquid crystal molecule  10118  which is close to the second substrate  10116  are different from each other by 90 degrees, and the directions of the major axes of the liquid crystal molecules  10118  located therebetween are arranged so as to link the above two directions smoothly. That is, the liquid crystal molecules  10118  shown in  FIG. 85  are aligned to be twisted by 90 degrees between the first substrate  10101  and the second substrate  10116 . 
     Note that the case is described in which a bottom-gate transistor using an amorphous semiconductor is used as the transistor. In the case where a transistor using an amorphous semiconductor is used, a liquid crystal display device can be formed at low cost by using a large substrate. 
     A liquid crystal display device includes a basic portion displaying images, which is called a liquid crystal panel. The liquid crystal panel is manufactured as follows: two processed substrates are attached to each other with a gap of several m therebetween, and a liquid crystal material is injected into a space between the two substrates. In  FIG. 85 , the two substrates correspond to the first substrate  10101  and the second substrate  10116 . A transistor and a pixel electrode are formed over the first substrate. A light-shielding film  10114 , a color filter  10115 , a fourth conductive layer  10113 , a spacer  10117 , and a second alignment film  10112  are formed on the second substrate. 
     The light-shielding film  10114  is not necessarily formed on the second substrate  10116 . When the light-shielding film  10114  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the light-shielding film  10114  is formed, a display device with little light leakage at the time of black display can be obtained. 
     The color filter  10115  is not necessarily formed on the second substrate  10116 . When the color filter  10115  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Note that even when the color filter  10115  is not formed, a display device which can perform color display can be obtained by field sequential driving. Alternatively, when the color filter  10115  is formed, a display device which can perform color display can be obtained. 
     Spherical spacers may be dispersed on the second substrate  10116  instead of forming the spacer  10117 . When the spherical spacers are dispersed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the spacer  10117  is formed, a distance between the two substrates can be uniform because a position of the spacer is not varied, so that a display device with little display unevenness can be obtained. 
     A process to be performed on the first substrate  10101  is described. 
     First, a first insulating film  10102  is formed over the first substrate  10101  by sputtering, a printing method, a coating method, or the like. Note that the first insulating film  10102  is not necessarily formed. The first insulating film  10102  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects a semiconductor layer. 
     Next, a first conductive layer  10103  is formed over the first insulating film  10102  by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a second insulating film  10104  is formed over the entire surface by sputtering, a printing method, a coating method, or the like. The second insulating film  10104  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects the semiconductor layer. 
     Next, a first semiconductor layer  10105  and a second semiconductor layer  10106  are formed. Note that the first semiconductor layer  10105  and the second semiconductor layer  10106  are formed sequentially and shapes thereof are processed at the same time. 
     Next, a second conductive layer  10107  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Note that as a method for etching which is performed at the time of processing the shape of the second conductive layer  10107 , dry etching is preferable. Note that either a light-transmitting material or a reflective material may be used for the second conductive layer  10107 . 
     Next, a channel region of the transistor is formed. Here, an example of a step thereof is described. The second semiconductor layer  10106  is etched by using the second conductive layer  10107  as a mask. Alternatively, the second semiconductor layer  10106  is etched by using a mask for processing the shape of the second conductive layer  10107 . Then, the first conductive layer  10103  at a position where the second semiconductor layer  10106  is removed serves as the channel region of the transistor. Thus, the number of masks can be reduced, so that manufacturing cost can be reduced. 
     Next, a third insulating film  10108  is formed and a contact hole is selectively formed in the third insulating film  10108 . Note that a contact hole may be formed also in the second insulating film  10104  at the same time as forming the contact hole in the third insulating film  10108 . Note that the surface of the third insulating film  10108  is preferably as even as possible. This is because alignment of the liquid crystal molecules are affected by unevenness of a surface with which the liquid crystal is in contact. 
     Next, a third conductive layer  10109  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a first alignment film  10110  is formed. Note that after the first alignment film  10110  is formed, rubbing may be performed so as to control the alignment of the liquid crystal molecules. Rubbing is a step of forming stripes on an alignment film by rubbing the alignment film with a cloth. When rubbing is performed, the alignment film can have alignment properties. 
     The first substrate  10101  which is manufactured as described above and the second substrate  10116  on which the light-shielding film  10114 , the color filter  10115 , the fourth conductive layer  10113 , the spacer  10117 , and the second alignment film  10112  are formed are attached to each other by a sealant with a gap of several m therebetween. Then, a liquid crystal material is injected into a space between the two substrates. Note that in the TN mode, the fourth conductive layer  10113  is formed over the entire surface of the second substrate  10116 . 
       FIG. 86A  is an example of a cross-sectional view of a pixel in the case where an MVA (multi-domain vertical alignment) mode and a transistor are combined. When the pixel structure shown in  FIG. 86A  is applied to a liquid crystal display device, a liquid crystal display device having a wide viewing angle, high response speed, and high contrast can be obtained. 
     Features of the pixel structure shown in  FIG. 86A  are described. Liquid crystal molecules  10218  shown in  FIG. 86A  are long and narrow molecules each having a major axis and a minor axis. In  FIG. 86A , a direction of each of the liquid crystal molecules  10218  is expressed by the length thereof. That is, the direction of the major axis of the liquid crystal molecule  10218 , which is expressed as long, is parallel to the page, and as the liquid crystal molecule  10218  is expressed to be shorter, the direction of the major axis becomes closer to a normal direction of the page. That is, each of the liquid crystal molecules  10218  shown in  FIG. 86A  is aligned such that the direction of the major axis is normal to the alignment film. Thus, the liquid crystal molecules  10218  at a position where an alignment control protrusion  10219  is formed are aligned radially with the alignment control protrusion  10219  as a center. With this state, a liquid crystal display device having a wide viewing angle can be obtained. 
     Note that the case is described in which a bottom-gate transistor using an amorphous semiconductor is used as the transistor. In the case where a transistor using an amorphous semiconductor is used, a liquid crystal display device can be formed at low cost by using a large substrate. 
     A liquid crystal display device includes a basic portion displaying images, which is called a liquid crystal panel. The liquid crystal panel is manufactured as follows: two processed substrates are attached to each other with a gap of several m therebetween, and a liquid crystal material is injected into a space between the two substrates. In  FIG. 86A , the two substrates correspond to the first substrate  10201  and the second substrate  10216 . A transistor and a pixel electrode are formed over the first substrate. A light-shielding film  10214 , a color filter  10215 , a fourth conductive layer  10213 , a spacer  10217 , a second alignment film  10212 , and an alignment control protrusion  10219  are formed on the second substrate. 
     The light-shielding film  10214  is not necessarily formed on the second substrate  10216 . When the light-shielding film  10214  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the light-shielding film  10214  is formed, a display device with little light leakage at the time of black display can be obtained. 
     The color filter  10215  is not necessarily formed on the second substrate  10216 . When the color filter  10215  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Note that even when the color filter  10215  is not formed, a display device which can perform color display can be obtained by field sequential driving. Alternatively, when the color filter  10215  is formed, a display device which can perform color display can be obtained. 
     Spherical spacers may be dispersed on the second substrate  10216  instead of forming the spacer  10217 . When the spherical spacers are dispersed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the spacer  10217  is formed, a distance between the two substrates can be uniform because a position of the spacer is not varied, so that a display device with little display unevenness can be obtained. 
     A process to be performed on the first substrate  10201  is described. 
     First, a first insulating film  10202  is formed over the first substrate  10201  by sputtering, a printing method, a coating method, or the like. Note that the first insulating film  10202  is not necessarily formed. The first insulating film  10202  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects a semiconductor layer. 
     Next, a first conductive layer  10203  is formed over the first insulating film  10202  by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a second insulating film  10204  is formed over the entire surface by sputtering, a printing method, a coating method, or the like. The second insulating film  10204  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects the semiconductor layer. 
     Next, a first semiconductor layer  10205  and a second semiconductor layer  10206  are formed. Note that the first semiconductor layer  10205  and the second semiconductor layer  10206  are formed sequentially and shapes thereof are processed at the same time. 
     Next, a second conductive layer  10207  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Note that as a method for etching which is performed at the time of processing the shape of the second conductive layer  10207 , dry etching is preferable. Note that as the second conductive layer  10207 , either a light-transmitting material or a reflective material may be used. 
     Next, a channel region of the transistor is formed. Here, an example of a step thereof is described. The second semiconductor layer  10206  is etched by using the second conductive layer  10207  as a mask. Alternatively, the second semiconductor layer  10206  is etched by using a mask for processing the shape of the second conductive layer  10207 . Then, the first conductive layer  10203  at a position where the second semiconductor layer  10206  is removed serves as the channel region of the transistor. Thus, the number of masks can be reduced, so that manufacturing cost can be reduced. 
     Next, a third insulating film  10208  is formed and a contact hole is selectively formed in the third insulating film  10208 . Note that a contact hole may be formed also in the second insulating film  10204  at the same time as forming the contact hole in the third insulating film  10208 . 
     Next, a third conductive layer  10209  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a first alignment film  10210  is formed. Note that after the first alignment film  10210  is formed, rubbing may be performed so as to control the alignment of the liquid crystal molecules. Rubbing is a step of forming stripes on an alignment film by rubbing the alignment film with a cloth. When rubbing is performed, the alignment film can have alignment properties. 
     The first substrate  10201  which is manufactured as described above and the second substrate  10216  on which the light-shielding film  10214 , the color filter  10215 , the fourth conductive layer  10213 , the spacer  10217 , and the second alignment film  10212  are manufactured are attached to each other by a sealant with a gap of several m therebetween. Then, a liquid crystal material is injected into a space between the two substrates. Note that in the MVA mode, the fourth conductive layer  10213  is formed over the entire surface of the second substrate  10216 . Note that the alignment control protrusion  10219  is formed so as to be in contact with the fourth conductive layer  10213 . The alignment control protrusion  10219  preferably has a shape with a smooth curved surface. Thus, alignment of the adjacent liquid crystal molecules  10218  is extremely similar, so that an alignment defect can be reduced. Further, a defect of the alignment film caused by breaking of the alignment film can be reduced. 
       FIG. 86B  is an example of a cross-sectional view of a pixel in the case where a PVA (patterned vertical alignment) mode and a transistor are combined. When the pixel structure shown in  FIG. 86B  is applied to a liquid crystal display device, a liquid crystal display device having a wide viewing angle, high response speed, and high contrast can be obtained. 
     Features of the pixel structure shown in  FIG. 86B  are described. Liquid crystal molecules  10248  shown in  FIG. 86B  are long and narrow molecules each having a major axis and a minor axis. In  FIG. 86B , direction of each of the liquid crystal molecules  10248  is expressed by the length thereof. That is, the direction of the major axis of the liquid crystal molecule  10248 , which is expressed as long, is parallel to the page, and as the liquid crystal molecule  10248  is expressed to be shorter, the direction of the major axis becomes closer to a normal direction of the page. That is, each of the liquid crystal molecules  10248  shown in  FIG. 86B  is aligned such that the direction of the major axis is normal to the alignment film. Thus, the liquid crystal molecules  10248  at a position where an electrode notch portion  10249  is formed are aligned radially with a boundary of the electrode notch portion  10249  and the fourth conductive layer  10243  as a center. With this state, a liquid crystal display device having a wide viewing angle can be obtained. 
     Note that the case is described in which a bottom-gate transistor using an amorphous semiconductor is used as the transistor. In the case where a transistor using an amorphous semiconductor is used, a liquid crystal display device can be formed at low cost by using a large substrate. 
     A liquid crystal display device includes a basic portion displaying images, which is called a liquid crystal panel. The liquid crystal panel is manufactured as follows: two processed substrates are attached to each other with a gap of several m therebetween, and a liquid crystal material is injected into a space between the two substrates. In  FIG. 23B , the two substrates correspond to the first substrate  10231  and the second substrate  10246 . A transistor and a pixel electrode are formed over the first substrate. A light-shielding film  10244 , a color filter  10245 , a fourth conductive layer  10243 , a spacer  10247 , and a second alignment film  10242  are formed on the second substrate. 
     The light-shielding film  10244  is not necessarily formed on the second substrate  10246 . When the light-shielding film  10244  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the light-shielding film  10244  is formed, a display device with little light leakage at the time of black display can be obtained. 
     The color filter  10245  is not necessarily formed on the second substrate  10246 . When the color filter  10245  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Note that even when the color filter  10245  is not formed, a display device which can perform color display can be obtained by field sequential driving. Alternatively, when the color filter  10245  is formed, a display device which can perform color display can be obtained. 
     Spherical spacers may be dispersed on the second substrate  10246  instead of forming the spacer  10247 . When the spherical spacers are dispersed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the spacer  10247  is formed, a distance between the two substrates can be uniform because a position of the spacer is not varied, so that a display device with little display unevenness can be obtained. 
     A process to be performed on the first substrate  10231  is described. 
     First, a first insulating film  10232  is formed over the first substrate  10231  by sputtering, a printing method, a coating method, or the like. Note that the first insulating film  10232  is not necessarily formed. The first insulating film  10232  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects a semiconductor layer. 
     Next, a first conductive layer  10233  is formed over the first insulating film  10232  by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a second insulating film  10234  is formed over the entire surface by sputtering, a printing method, a coating method, or the like. The second insulating film  10234  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects the semiconductor layer. 
     Next, a first semiconductor layer  10235  and a second semiconductor layer  10236  are formed. Note that the first semiconductor layer  10235  and the second semiconductor layer  10236  are formed sequentially and shapes thereof are processed at the same time. 
     Next, a second conductive layer  10237  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Note that as a method for etching which is performed at the time of processing a shape of the second conductive layer  10237 , dry etching is preferable. Note that as the second conductive layer  10237 , either a light-transmitting material or a reflective material may be used. 
     Next, a channel region of the transistor is formed. Here, an example of a step thereof is described. The second semiconductor layer  10236  is etched by using the second conductive layer  10237  as a mask. Alternatively, the second semiconductor layer  10236  is etched by using a mask for processing the shape of the second conductive layer  10237 . Then, the first conductive layer  10233  at a position where the second semiconductor layer  10236  is removed serves as the channel region of the transistor. Thus, the number of masks can be reduced, so that manufacturing cost can be reduced. 
     Next, a third insulating film  10238  is formed and a contact hole is selectively formed in the third insulating film  10238 . Note that a contact hole may be formed also in the second insulating film  10234  at the same time as forming the contact hole in the third insulating film  10238 . Note that the surface of the third insulating film  10238  is preferably as even as possible. This is because alignment of the liquid crystal molecules are affected by unevenness of a surface with which the liquid crystal is in contact. 
     Next, a third conductive layer  10239  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a first alignment film  10240  is formed. Note that after the first alignment film  10240  is formed, rubbing may be performed so as to control the alignment of the liquid crystal molecules. Rubbing is a step of forming stripes on an alignment film by rubbing the alignment film with a cloth. When rubbing is performed, the alignment film can have alignment properties. 
     The first substrate  10231  which is manufactured as described above and the second substrate  10246  on which the light-shielding film  10244 , the color filter  10245 , the fourth conductive layer  10243 , the spacer  10247 , and the second alignment film  10242  are manufactured are attached to each other by a sealant with a gap of several m therebetween. Then, a liquid crystal material is injected into a space between the two substrates. Note that in the PVA mode, the fourth conductive layer  10243  is patterned and is provided with the electrode notch portion  10249 . Although the shape of the electrode notch portion  10249  is not particularly limited to a certain shape, the electrode notch portion  10249  preferably has a shape in which a plurality of rectangles having different directions are combined. Thus, a plurality of regions having different alignment can be formed, so that a liquid crystal display device having a wide viewing angle can be obtained. Note that the fourth conductive layer  10243  at the boundary between the electrode notch portion  10249  and the fourth conductive layer  10243  preferably has a shape with a smooth curved surface. Thus, alignment of the adjacent liquid crystal molecules  10248  is extremely similar, so that an alignment defect is reduced. Further, a defect of the alignment film caused by breaking of the second alignment film  10242  by the electrode notch portion  10249  can be prevented. 
       FIG. 87A  is an example of a cross-sectional view of a pixel in the case where an IPS (in-plane-switching) mode and a transistor are combined. When the pixel structure shown in  FIG. 87A  is applied to a liquid crystal display device, a liquid crystal display device theoretically having a wide viewing angle and response speed which has low dependency on a gray scale can be obtained. 
     Features of the pixel structure shown in  FIG. 87A  are described. Liquid crystal molecules  10318  shown in  FIG. 87A  are long and narrow molecules each having a major axis and a minor axis. In  FIG. 87A , a direction of each of the liquid crystal molecules  10318  is expressed by the length thereof. That is, the direction of the major axis of the liquid crystal molecule  10318 , which is expressed as long, is parallel to the page, and as the liquid crystal molecule  10318  is expressed to be shorter, the direction of the major axis becomes closer to a normal direction of the page. That is, each of the liquid crystal molecules  10318  shown in  FIG. 87A  is aligned so that the direction of the major axis thereof is always horizontal to the substrate. Although  FIG. 87A  shows alignment with no electric field, when an electric field is applied to each of the liquid crystal molecules  10318 , each of the liquid crystal molecules  10318  rotates in a horizontal plane as the direction of the major axis thereof is always horizontal to the substrate. With this state, a liquid crystal display device having a wide viewing angle can be obtained. 
     Note that the case is described in which a bottom-gate transistor using an amorphous semiconductor is used as the transistor. In the case where a transistor using an amorphous semiconductor is used, a liquid crystal display device can be formed at low cost by using a large substrate. 
     A liquid crystal display device includes a basic portion displaying images, which is called a liquid crystal panel. The liquid crystal panel is manufactured as follows: two processed substrates are attached to each other with a gap of several m therebetween, and a liquid crystal material is injected into a space between the two substrates. In  FIG. 87A , the two substrates correspond to the first substrate  10301  and the second substrate  10316 . A transistor and a pixel electrode are formed over the first substrate. A light-shielding film  10314 , a color filter  10315 , a fourth conductive layer  10313 , a spacer  10317 , and a second alignment film  10312  are formed on the second substrate. 
     The light-shielding film  10314  is not necessarily formed on the second substrate  10316 . When the light-shielding film  10314  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the light-shielding film  10314  is formed, a display device with little light leakage at the time of black display can be obtained. 
     The color filter  10315  is not necessarily formed on the second substrate  10316 . When the color filter  10315  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Note that even when the color filter  10315  is not formed, a display device which can perform color display can be obtained by field sequential driving. Alternatively, when the color filter  10315  is formed, a display device which can perform color display can be obtained. 
     Spherical spacers may be dispersed on the second substrate  10316  instead of forming the spacer  10317 . When the spherical spacers are dispersed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the spacer  10317  is formed, a distance between the two substrates can be uniform because a position of the spacer is not varied, so that a display device with little display unevenness can be obtained. 
     A process to be performed on the first substrate  10301  is described. 
     First, a first insulating film  10302  is formed over the first substrate  10301  by sputtering, a printing method, a coating method, or the like. Note that the first insulating film  10302  is not necessarily formed. The first insulating film  10302  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects a semiconductor layer. 
     Next, a first conductive layer  10303  is formed over the first insulating film  10302  by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a second insulating film  10304  is formed over the entire surface by sputtering, a printing method, a coating method, or the like. The second insulating film  10304  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects the semiconductor layer. 
     Next, a first semiconductor layer  10305  and a second semiconductor layer  10306  are formed. Note that the first semiconductor layer  10305  and the second semiconductor layer  10306  are formed sequentially and shapes thereof are processed at the same time. 
     Next, a second conductive layer  10307  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Note that as a method for etching which is performed at the time of processing the shape of the second conductive layer  10307 , dry etching is preferable. Note that as the second conductive layer  10307 , either a light-transmitting material or a reflective material may be used. 
     Next, a channel region of the transistor is formed. Here, an example of a step thereof is described. The second semiconductor layer  10106  is etched by using the second conductive layer  10307  as a mask. Alternatively, the second semiconductor layer  10306  is etched by using a mask for processing the shape of the second conductive layer  10307 . Then, the first conductive layer  10303  at a position where the second semiconductor layer  10306  is removed serves as the channel region of the transistor. Thus, the number of masks can be reduced, so that manufacturing cost can be reduced. 
     Next, a third insulating film  10308  is formed and a contact hole is selectively formed in the third insulating film  10308 . Note that a contact hole may be formed also in the second insulating film  10304  at the same time as forming the contact hole in the third insulating film  10308 . 
     Next, a third conductive layer  10309  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Here, the third conductive layer  10309  has a shape in which two comb-shaped electrodes engage with each other. One of the comb-shaped electrodes is electrically connected to one of a source electrode and a drain electrode of the transistor, and the other of the comb-shaped electrodes is electrically connected to a common electrode. Thus, a horizontal electric field can be effectively applied to the liquid crystal molecules  10318 . 
     Next, a first alignment film  10310  is formed. Note that after the first alignment film  10310  is formed, rubbing may be performed so as to control the alignment of the liquid crystal molecules. Rubbing is a step of forming stripes on an alignment film by rubbing the alignment film with a cloth. When rubbing is performed, the alignment film can have alignment properties. 
     The first substrate  10301  which is manufactured as described above and the second substrate  10316  on which the light-shielding film  10314 , the color filter  10315 , the spacer  10317 , and the second alignment film  10312  are formed are attached to each other by a sealant with a gap of several pm therebetween. Then, a liquid crystal material is injected into a space between the two substrates. 
       FIG. 87B  is an example of a cross-sectional view of a pixel in the case where an FFS (fringe field switching) mode and a transistor are combined. When the pixel structure shown in  FIG. 87B  is applied to a liquid crystal display device, a liquid crystal display device theoretically having a wide viewing angle and response speed which has low dependency on a gray scale can be obtained. 
     Features of the pixel structure shown in  FIG. 87B  are described. Liquid crystal molecules  10348  shown in  FIG. 87B  are long and narrow molecules each having a major axis and a minor axis. In  FIG. 87B , direction of each of the liquid crystal molecules  10348  is expressed by the length thereof. That is, the direction of the major axis of the liquid crystal molecule  10348 , which is expressed as long, is parallel to the page, and as the liquid crystal molecule  10348  is expressed to be shorter, the direction of the major axis becomes closer to a normal direction of the page. That is, each of the liquid crystal molecules  10348  shown in  FIG. 87B  is aligned so that the direction of the major axis thereof is always horizontal to the substrate. Although  FIG. 87B  shows alignment with no electric field, when an electric field is applied to each of the liquid crystal molecules  10348 , each of the liquid crystal molecules  10348  rotates in a horizontal plane as the direction of the major axis thereof is always horizontal to the substrate. With this state, a liquid crystal display device having a wide viewing angle can be obtained. 
     Note that the case is described in which a bottom-gate transistor using an amorphous semiconductor is used as the transistor. In the case where a transistor using an amorphous semiconductor is used, a liquid crystal display device can be formed at low cost by using a large substrate. 
     A liquid crystal display device includes a basic portion displaying images, which is called a liquid crystal panel. The liquid crystal panel is manufactured as follows: two processed substrates are attached to each other with a gap of several μm therebetween, and a liquid crystal material is injected into a space between the two substrates. In  FIG. 87B , the two substrates correspond to the first substrate  10331  and the second substrate  10346 . A transistor and a pixel electrode are formed over the first substrate. A light-shielding film  10344 , a color filter  10345 , a fourth conductive layer  10343 , a spacer  10347 , and a second alignment film  10342  are formed on the second substrate. 
     The light-shielding film  10344  is not necessarily formed on the second substrate  10346 . When the light-shielding film  10344  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the light-shielding film  10344  is formed, a display device with little light leakage at the time of black display can be obtained. 
     The color filter  10345  is not necessarily formed on the second substrate  10346 . When the color filter  10345  is not formed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Note that even when the color filter  10345  is not formed, a display device which can perform color display can be obtained by field sequential driving. Alternatively, when the color filter  10345  is formed, a display device which can perform color display can be obtained. 
     Spherical spacers may be dispersed on the second substrate  10346  instead of forming the spacer  10347 . When the spherical spacers are dispersed, the number of steps is reduced, so that manufacturing cost can be reduced. In addition, since the structure is simple, yield can be improved. Alternatively, when the spacer  10347  is formed, a distance between the two substrates can be uniform because a position of the spacer is not varied, so that a display device with little display unevenness can be obtained. 
     A process to be performed on the first substrate  10331  is described. 
     First, a first insulating film  10332  is formed over the first substrate  10331  by sputtering, a printing method, a coating method, or the like. Note that the first insulating film  10332  is not necessarily formed. The first insulating film  10332  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects a semiconductor layer. 
     Next, a first conductive layer  10333  is formed over the first insulating film  10332  by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a second insulating film  10334  is formed over the entire surface by sputtering, a printing method, a coating method, or the like. The second insulating film  10334  has a function of preventing change in characteristics of the transistor due to an impurity from the substrate, which affects the semiconductor layer. 
     Next, a first semiconductor layer  10335  and a second semiconductor layer  10336  are formed. Note that the first semiconductor layer  10335  and the second semiconductor layer  10336  are formed sequentially and shapes thereof are processed at the same time. 
     Next, a second conductive layer  10337  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Note that as a method for etching which is performed at the time of processing the shape of the second conductive layer  10337 , dry etching is preferable. Note that as the second conductive layer  10337 , either a light-transmitting material or a reflective material may be used. 
     Next, a channel region of the transistor is formed. Here, an example of a step thereof is described. The second semiconductor layer  10106  is etched by using the second conductive layer  10337  as a mask. Alternatively, the second semiconductor layer  10336  is etched by using a mask for processing the shape of the second conductive layer  10337 . Then, the first conductive layer  10333  at a position where the second semiconductor layer  10336  is removed serves as the channel region of the transistor. Thus, the number of masks can be reduced, so that manufacturing cost can be reduced. 
     Next, a third insulating film  10338  is formed and a contact hole is selectively formed in the third insulating film  10338 . 
     Next, a fourth conductive layer  10343  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. 
     Next, a fourth insulating film  10349  is formed and a contact hole is selectively formed in the fourth insulating film  10349 . Note that the surface of the fourth insulating film  10349  is preferably as even as possible. This is because alignment of the liquid crystal molecules are affected by unevenness of a surface with which the liquid crystal is in contact. 
     Next, a third conductive layer  10339  is formed by photolithography, a laser direct writing method, an inkjet method, or the like. Here, the third conductive layer  10339  is comb-shaped. 
     Next, a first alignment film  10340  is formed. Note that after the first alignment film  10340  is formed, rubbing may be performed so as to control the alignment of the liquid crystal molecules. Rubbing is a step of forming stripes on an alignment film by rubbing the alignment film with a cloth. When rubbing is performed, the alignment film can have alignment properties. 
     The first substrate  10331  which is manufactured as described above and the second substrate  10346  on which the light-shielding film  10344 , the color filter  10345 , the spacer  10347 , and the second alignment film  10342  are formed are attached to each other by a sealant with a gap of several m therebetween. Then, a liquid crystal material is injected into a space between the two substrates. Therefore, a liquid crystal panel can be manufactured. 
     Here, materials which can be used for conductive layers or insulating films are described. 
     As the first insulating film  10102  in  FIG. 85 , the first insulating film  10202  in  FIG. 86A , the first insulating film  10232  in  FIG. 86B , the first insulating film  10302  in  FIG. 87A , or the first insulating film  10332  in  FIG. 87B , an insulating film such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride (SiO x N y ) film can be used. Alternatively, an insulating film having a stacked-layer structure in which two or more of a silicon oxide film, a silicon nitride film, a silicon oxynitride (SiO x N y ) film, and the like are combined can be used as. 
     As the first conductive layer  10103  in  FIG. 85 , the first conductive layer  10203  in  FIG. 86A , the first conductive layer  10233  in  FIG. 86B , the first conductive layer  10303  in  FIG. 87A , or the first conductive layer  10333  in  FIG. 87B , Mo, Ti, Al, Nd, Cr, or the like can be used. Alternatively, a stacked-layer structure in which two or more of Mo, Ti, Al, Nd, Cr, and the like are combined can be used. 
     As the second insulating film  10104  in  FIG. 85 , the second insulating film  10204  in  FIG. 86A , the second insulating film  10234  in  FIG. 86B , the second insulating film  10304  in  FIG. 87A , or the second insulating film  10334  in  FIG. 87B , a thermal oxide film, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or the like can be used. Alternatively, a stacked-layer structure in which two or more of a thermal oxide film, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and the like are combined can be used. Note that a silicon oxide film is preferable in a portion which is in contact with a semiconductor layer. This is because a trap level at an interface with the semiconductor layer decreases when a silicon oxide film is used. Note that a silicon nitride film is preferable in a portion which is in contact with Mo. This is because a silicon nitride film does not oxidize Mo. 
     As the first semiconductor layer  10105  in  FIG. 85 , the first semiconductor layer  10205  in  FIG. 86A , the first semiconductor layer  10235  in  FIG. 86B , the first semiconductor layer  10305  in  FIG. 87A , or the first semiconductor layer  10335  in  FIG. 87B , silicon, silicon germanium (SiGe), or the like can be used. 
     As the second semiconductor layer  10106  in  FIG. 85 , the second semiconductor layer  10206  in  FIG. 86A , the second semiconductor layer  10236  in  FIG. 86B , the second semiconductor layer  10306  in  FIG. 87A , or the second semiconductor layer  10336  in  FIG. 87B , silicon or the like including phosphorus can be used, for example. 
     As a light-transmitting material of the second conductive layer  10107  and the third conductive layer  10109  in  FIG. 85 ; the second conductive layer  10207  and the third conductive layer  10209  in  FIG. 86A ; the second conductive layer  10237  and the third conductive layer  10239  in  FIG. 86B ; the second conductive layer  10307  and the third conductive layer  10309  in  FIG. 87A ; or the second conductive layer  10337 , the third conductive layer  10339 , and a fourth conductive layer  10343  in  FIG. 87B , an indium tin oxide (ITO) film formed by mixing tin oxide into indium oxide, an indium tin silicon oxide (ITSO) film formed by mixing silicon oxide into indium tin oxide (ITO), an indium zinc oxide (IZO) film formed by mixing zinc oxide into indium oxide, a zinc oxide film, a tin oxide film, or the like can be used. Note that IZO is a light-transmitting conductive material formed by sputtering using a target in which zinc oxide (ZnO) is mixed into ITO at 2 to 20 wt %. 
     As a reflective material of the second conductive layer  10107  and the third conductive layer  10109  in  FIG. 85 ; the second conductive layer  10207  and the third conductive layer  10209  in  FIG. 86A ; the second conductive layer  10237  and the third conductive layer  10239  in  FIG. 86B ; the second conductive layer  10307  and the third conductive layer  10309  in  FIG. 87A ; or the second conductive layer  10337 , the third conductive layer  10339 , and the fourth conductive layer  10343  in  FIG. 87B , Ti, Mo, Ta, Cr, W, Al, or the like can be used. Alternatively, a two-layer structure in which Al and Ti, Mo, Ta, Cr, or W are stacked, or a three-layer structure in which Al is interposed between metals such as Ti, Mo, Ta, Cr, and W may be used. 
     As the third insulating film  10108  in  FIG. 85 , the third insulating film  10208  in  FIG. 86A , the third insulating film  10238  in  FIG. 23B , the third conductive layer  10239  in  FIG. 86B , the third insulating film  10308  in  FIG. 87A , or the third insulating film  10338  and the fourth insulating film  10349  in  FIG. 87B , an inorganic material (e.g., silicon oxide, silicon nitride, or silicon oxynitride), an organic compound material having a low dielectric constant (e.g., a photosensitive or nonphotosensitive organic resin material), or the like can be used. Alternatively, a material including siloxane can be used. Note that siloxane is a material in which a skeleton structure is formed by a bond of silicon (Si) and oxygen (O). As a substituent, an organic group including at least hydrogen (e.g., an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group may be used as the substituent. Further alternatively, the organic group including at least hydrogen and the fluoro group may be used as the substituent. 
     As the first alignment film  10110  in  FIG. 85 , the first alignment film  10210  in  FIG. 86A , the first alignment film  10240  in  FIG. 86B , the first alignment film  10310  in  FIG. 87A , or the first alignment film  10340  in  FIG. 87B , a film of a high molecular compound such as polyimide can be used. 
     Next, the pixel structure in the case where each liquid crystal mode and the transistor are combined is described with reference to a top view (a layout diagram) of the pixel. 
     Note that as a liquid crystal mode, a TN (twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA (multi-domain vertical alignment) mode, a PVA (patterned vertical alignment) mode, an ASM (axially symmetric aligned micro-cell) mode, an OCB (optical compensated birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode, or the like can be used. 
     As the transistor, a thin film transistor (TFT) including a non-single-crystal semiconductor layer typified by amorphous silicon, polycrystalline silicon, microcrystalline (also referred to as semi-amorphous) silicon, or the like can be used. 
     Note that as the structure of the transistor, a top-gate structure, a bottom-gate structure, or the like can be used. A channel-etched transistor, a channel-protective transistor, or the like can be used as a bottom-gate transistor. 
       FIG. 88  is an example of a top view of a pixel in the case where a TN mode and a transistor are combined. When the pixel structure shown in  FIG. 88  is applied to a liquid crystal display device, a liquid crystal display device can be formed at low cost. 
     The pixel shown in  FIG. 88  includes a scan line  10401 , an image signal line  10402 , a capacitor line  10403 , a transistor  10404 , a pixel electrode  10405 , and a pixel capacitor  10406 . 
     The scan line  10401  has a function of transmitting a signal (a scan signal) to the pixel. The image signal line  10402  has a function for transmitting a signal (an image signal) to the pixel. Note that since the scan line  10401  and the image signal line  10402  are arranged in matrix, they are formed using conductive layers in different layers. Note that a semiconductor layer may be provided at an intersection of the scan line  10401  and the image signal line  10402 . Thus, intersection capacitance formed between the scan line  10401  and the image signal line  10402  can be reduced. 
     The capacitor line  10403  is provided in parallel to the pixel electrode  10405 . A portion where the capacitor line  10403  and the pixel electrode  10405  overlap with each other corresponds to the pixel capacitor  10406 . Note that part of the capacitor line  10403  is extended along the image signal line  10402  so as to surround the image signal line  10402 . Thus, crosstalk can be reduced. Crosstalk is a phenomenon in which a potential of an electrode, which should hold the potential, is changed in accordance with change in potential of the image signal line  10402 . Note that intersection capacitance can be reduced by providing a semiconductor layer between the capacitor line  10403  and the image signal line  10402 . Note that the capacitor line  10403  is formed using a material which is similar to that of the scan line  10401 . 
     The transistor  10404  has a function as a switch which turns on the image signal line  10402  and the pixel electrode  10405 . Note that one of a source region and a drain region of the transistor  10404  is provided so as to be surrounded by the other of the source region and the drain region of the transistor  10404 . Thus, the channel width of the transistor  10404  increases, so that switching capability can be improved. Note that a gate electrode of the transistor  10404  is provided so as to surround the semiconductor layer. 
     The pixel electrode  10405  is electrically connected to one of a source electrode and a drain electrode of the transistor  10404 . The pixel electrode  10405  is an electrode for applying signal voltage which is transmitted by the image signal line  10402  to a liquid crystal element. Note that the pixel electrode  10405  is rectangular. Thus, the aperture ratio can be improved. Note that as the pixel electrode  10405 , a light-transmitting material or a reflective material may be used. Alternatively, the pixel electrode  10405  may be formed by combining a light-transmitting material and a reflective material. 
       FIG. 89A  is an example of a top view of a pixel in the case where an MVA mode and a transistor are combined. When the pixel structure shown in  FIG. 89A  is applied to a liquid crystal display device, a liquid crystal display device having a wide viewing angle, high response speed, and high contrast can be obtained. 
     The pixel shown in  FIG. 89A  includes a scan line  10501 , a video signal line  10502 , a capacitor line  10503 , a transistor  10504 , a pixel electrode  10505 , a pixel capacitor  10506 , and an alignment control protrusion  10507 . 
     The scan line  10501  has a function of transmitting a signal (a scan signal) to the pixel. The image signal line  10502  has a function for transmitting a signal (an image signal) to the pixel. Note that since the scan line  10501  and the image signal line  10502  are arranged in matrix, they are formed using conductive layers in different layers. Note that a semiconductor layer may be provided at an intersection of the scan line  10501  and the image signal line  10502 . Thus, intersection capacitance formed between the scan line  10501  and the image signal line  10502  can be reduced. 
     The capacitor line  10503  is provided in parallel to the pixel electrode  10505 . A portion where the capacitor line  10503  and the pixel electrode  10505  overlap with each other corresponds to the pixel capacitor  10506 . Note that part of the capacitor line  10503  is extended along the image signal line  10502  so as to surround the image signal line  10502 . Thus, crosstalk can be reduced. Crosstalk is a phenomenon in which a potential of an electrode, which should hold the potential, is changed in accordance with change in potential of the image signal line  10502 . Note that intersection capacitance can be reduced by providing a semiconductor layer between the capacitor line  10503  and the image signal line  10502 . Note that the capacitor line  10503  is formed using a material which is similar to that of the scan line  10501 . 
     The transistor  10504  has a function as a switch which turns on the image signal line  10502  and the pixel electrode  10505 . Note that one of a source region and a drain region of the transistor  10504  is provided so as to be surrounded by the other of the source region and the drain region of the transistor  10504 . Thus, the channel width of the transistor  10504  increases, so that switching capability can be improved. Note that a gate electrode of the transistor  10504  is provided so as to surround the semiconductor layer. 
     The pixel electrode  10505  is electrically connected to one of a source electrode and a drain electrode of the transistor  10504 . The pixel electrode  10505  is an electrode for applying signal voltage which is transmitted by the image signal line  10502  to a liquid crystal element. Note that the pixel electrode  10505  is rectangular. Thus, the aperture ratio can be improved. Note that as the pixel electrode  10505 , a light-transmitting material or a reflective material may be used. Alternatively, the pixel electrode  10505  may be formed by combining a light-transmitting material and a reflective material. 
     The alignment control protrusion  10507  is formed on a counter substrate. The alignment control protrusion  10507  has a function of aligning liquid crystal molecules radially. Note that a shape of the alignment control protrusion  10507  is not particularly limited. For example, the alignment control protrusion  10507  may be a dogleg shape. Thus, a plurality of regions having different alignment of the liquid crystal molecules can be formed, so that the viewing angle can be improved. 
       FIG. 89B  is an example of a top view of a pixel in the case where a PVA mode and a transistor are combined. When the pixel structure shown in  FIG. 89B  is applied to a liquid crystal display device, a liquid crystal display device having a wide viewing angle, high response speed, and high contrast can be obtained. 
     The pixel shown in  FIG. 89B  includes a scan line  10511 , a video signal line  10512 , a capacitor line  10513 , a transistor  10514 , a pixel electrode  10515 , a pixel capacitor  10516 , and an electrode notch portion  10517 . 
     The scan line  10511  has a function of transmitting a signal (a scan signal) to the pixel. The image signal line  10512  has a function for transmitting a signal (an image signal) to the pixel. Note that since the scan line  10511  and the image signal line  10512  are arranged in matrix, they are formed using conductive layers in different layers. Note that a semiconductor layer may be provided at an intersection of the scan line  10511  and the image signal line  10512 . Thus, intersection capacitance formed between the scan line  10511  and the image signal line  10512  can be reduced. 
     The capacitor line  10513  is provided in parallel to the pixel electrode  10515 . A portion where the capacitor line  10513  and the pixel electrode overlap with each other corresponds to the pixel capacitor  10516 . Note that part of the capacitor line  10513  is extended along the image signal line  10512  so as to surround the image signal line  10512 . Thus, crosstalk can be reduced. Crosstalk is a phenomenon in which a potential of an electrode, which should hold the potential, is changed in accordance with change in potential of the image signal line  10512 . Note that intersection capacitance can be reduced by providing a semiconductor layer between the capacitor line  10513  and the image signal line  10512 . Note that the capacitor line  10513  is formed using a material which is similar to that of the scan line  10511 . 
     The transistor  10514  has a function as a switch which turns on the image signal line  10512  and the pixel electrode  10515 . Note that one of a source region and a drain region of the transistor  10514  is provided so as to be surrounded by the other of the source region and the drain region of the transistor  10514 . Thus, the channel width of the transistor  10514  increases, so that switching capability can be improved. Note that a gate electrode of the transistor  10514  is provided so as to surround the semiconductor layer. 
     The pixel electrode  10515  is electrically connected to one of a source electrode and a drain electrode of the transistor  10514 . The pixel electrode  10515  is an electrode for applying signal voltage which is transmitted by the image signal line  10512  to a liquid crystal element. Note that the pixel electrode  10515  has a shape which is formed in accordance with a shape of the electrode notch portion  10517 . Specifically, the pixel electrode  10515  has a shape in which a portion where the pixel electrode  10515  is notched is formed in a portion where the electrode notch portion  10517  is not formed. Thus, a plurality of regions having different alignment of the liquid crystal molecules can be formed, so that the viewing angle can be improved. Note that as the pixel electrode  10515 , a light-transmitting material or a reflective material may be used. Alternatively, the pixel electrode  10515  may be formed by combining a light-transmitting material and a reflective material. 
       FIG. 90A  is an example of a top view of a pixel in the case where an IPS mode and a transistor are combined. When the pixel structure shown in  FIG. 90A  is applied to a liquid crystal display device, a liquid crystal display device theoretically having a wide viewing angle and response speed which has low dependency on a gray scale can be obtained. 
     The pixel shown in  FIG. 90A  includes a scan line  10601 , a video signal line  10602 , a common electrode  10603 , a transistor  10604 , and a pixel electrode  10605 . 
     The scan line  10601  has a function of transmitting a signal (a scan signal) to the pixel. The image signal line  10602  has a function of transmitting a signal (an image signal) to the pixel. Note that since the scan line  10601  and the image signal line  10602  are arranged in matrix, they are formed using conductive layers in different layers. Note that a semiconductor layer may be provided at an intersection of the scan line  10601  and the image signal line  10602 . Thus, intersection capacitance formed between the scan line  10601  and the image signal line  10602  can be reduced. Note that the image signal line  10602  is formed in accordance with a shape of the pixel electrode  10605 . 
     The common electrode  10603  is provided in parallel to the pixel electrode  10605 . The common electrode  10603  is an electrode for generating a horizontal electric field. Note that the common electrode  10603  is bent comb-shaped. Note that part of the common electrode  10603  is extended along the image signal line  10602  so as to surround the image signal line  10602 . Thus, crosstalk can be reduced. Crosstalk is a phenomenon in which a potential of an electrode, which should hold the potential, is changed in accordance with change in potential of the image signal line  10602 . Note that intersection capacitance can be reduced by providing a semiconductor layer between the common electrode  10603  and the image signal line  10602 . Par of the common electrode  10603 , which is provided in parallel to the scan line  10601 , is formed using a material which is similar to that of the scan line  10601 . Part of the common electrode  10603 , which is provided in parallel to the pixel electrode  10605 , is formed using a material which is similar to that of the pixel electrode  10605 . 
     The transistor  10604  has a function as a switch which turns on the image signal line  10602  and the pixel electrode  10605 . Note that one of a source region and a drain region of the transistor  10604  is provided so as to be surrounded by the other of the source region and the drain region of the transistor  10604 . Thus, the channel width of the transistor  10604  increases, so that switching capability can be improved. Note that a gate electrode of the transistor  10604  is provided so as to surround the semiconductor layer. 
     The pixel electrode  10605  is electrically connected to one of a source electrode and a drain electrode of the transistor  10604 . The pixel electrode  10605  is an electrode for applying signal voltage which is transmitted by the image signal line  10602  to a liquid crystal element. Note that the pixel electrode  10605  is bent comb-shaped. Thus, a horizontal electric field can be applied to liquid crystal molecules. In addition, since a plurality of regions having different alignment of the liquid crystal molecules can be formed, the viewing angle can be improved. Note that as the pixel electrode  10605 , a light-transmitting material or a reflective material may be used. Alternatively, the pixel electrode  10605  may be formed by combining a light-transmitting material and a reflective material. 
     Note that a comb-shaped portion in the common electrode  10603  and the pixel electrode  10605  may be formed using different conductive layers. For example, the comb-shaped portion in the common electrode  10603  may be formed using a conductive layer which is the same as that of the scan line  10601  or the image signal line  10602 . Similarly, the pixel electrode  10605  may be formed using a conductive layer which is the same as that of the scan line  10601  or the image signal line  10602 . 
       FIG. 90B  is a top view of a pixel in the case where an FFS mode and a transistor are combined. When the pixel structure shown in  FIG. 90B  is applied to a liquid crystal display device, a liquid crystal display device theoretically having a wide viewing angle and response speed which has low dependency on a gray scale can be obtained. 
     The pixel shown in  FIG. 90B  may include a scan line  10611 , a video signal line  10612 , a common electrode  10613 , a transistor  10614 , and a pixel electrode  10615 . 
     The scan line  10611  has a function of transmitting a signal (a scan signal) to the pixel. The image signal line  10612  has a function of transmitting a signal (an image signal) to the pixel. Note that since the scan line  10611  and the image signal line  10612  are arranged in matrix, they are formed using conductive layers in different layers. Note that a semiconductor layer may be provided at an intersection of the scan line  10611  and the image signal line  10612 . Thus, intersection capacitance formed between the scan line  10611  and the image signal line  10612  can be reduced. Note that the image signal line  10612  is formed in accordance with a shape of the pixel electrode  10615 . 
     The common electrode  10613  is formed uniformly below the pixel electrode  10615  and below and between the pixel electrodes  10615 . Note that as the common electrode  10613 , either a light-transmitting material or a reflective material may be used. Alternatively, the common electrode  10613  may be formed by combining a material in which a light-transmitting material and a reflective material. 
     The transistor  10614  has a function as a switch which turns on the image signal line  10612  and the pixel electrode  10615 . Note that one of a source region and a drain region of the transistor  10614  is provided so as to be surrounded by the other of the source region and the drain region of the transistor  10614 . Thus, the channel width of the transistor  10614  increases, so that switching capability can be improved. Note that a gate electrode of the transistor  10614  is provided so as to surround the semiconductor layer. 
     The pixel electrode  10615  is electrically connected to one of a source electrode and a drain electrode of the transistor  10614 . The pixel electrode  10615  is an electrode for applying signal voltage which is transmitted by the image signal line  10612  to a liquid crystal element. Note that the pixel electrode  10615  is bent comb-shaped. The comb-shaped pixel electrode  10615  is provided to be closer to a liquid crystal layer than a uniform portion of the common electrode  10613 . Thus, a horizontal electric field can be applied to liquid crystal molecules. In addition, a plurality of regions having different alignment of the liquid crystal molecules can be formed, so that the viewing angle can be improved. Note that as the pixel electrode  10615 , a light-transmitting material or a reflective material may be used. Alternatively, the pixel electrode  10615  may be formed by combining a light-transmitting material and a reflective material. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 11 
     In this embodiment mode, steps of manufacturing a liquid crystal cell (also referred to as a liquid crystal panel) are described. 
     Steps of manufacturing a liquid crystal cell in the case where a vacuum injection method is used as a method for filling with liquid crystals are described with reference to  FIGS. 91A to 91E and 92A to 92C . 
       FIG. 92C  is a cross-sectional view of a liquid crystal cell. A first substrate  70101  and a second substrate  70107  are attached with spacers  70106  and a sealant  70105  interposed therebetween. Liquid crystals  70109  are arranged between the first substrate  70101  and the second substrate  70107 . Note that an alignment film  70102  is formed over the first substrate  70101 , and an alignment film  70108  is formed on the second substrate  70107 . 
     The first substrate  70101  is provided with a plurality of pixels arranged in matrix. Each of the plurality of pixels may include a transistor. Note that the first substrate  70101  may be referred to as a TFT substrate, an array substrate, or a TFT array substrate. As the first substrate  70101 , a single-crystal substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), and a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester)), a leather substrate, a rubber substrate, a stainless steel substrate, and a substrate including stainless steel foil can be used. Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissue of an animal such as a human may be used as the substrate. Note that the present invention is not limited to this, and various substrates can be used. 
     A common electrode, a color filter, a black matrix, and the like are provided on the second substrate  70107 . Note that the second substrate  70107  may be referred to as a counter substrate or a color filter substrate. 
     The alignment film  70102  has a function of aligning liquid crystal molecules in a certain direction. For the alignment film  70102 , a polyimide resin or the like can be used. Note that the present invention is not limited to this, and various materials can be used. Note that the alignment film  70108  is similar to the alignment film  70102 . 
     The sealant  70105  has a function of bonding the first substrate  70101  and the second substrate  70107  so that the liquid crystals  70109  do not leak. That is, the sealant  70105  functions as a sealant. 
     The spacer  70106  has a function of maintaining a fixed space between the first substrate  70101  and the second substrate  70107  (a cell gap of the liquid crystal). As the spacer  70106 , a granular spacer or a columnar spacer can be used. Examples of the granular spacer are a fiber-shaped spacer and a spherical spacer. Examples of a material for the granular spacer are plastic and glass. Note that a spherical spacer formed by using plastic is referred to as a plastic bead and is widely used. A fiber-shaped spacer formed by using glass is referred to as a glass fiber and mixed in a sealant when used. 
       FIG. 91A  is a cross-sectional view of a step of forming the alignment film  70102  over the first substrate  70101 . The alignment film  70102  is formed over the first substrate  70101  by a roller coating method using a roller  70103 . Note that other than a roller coating method, an offset printing method, a dip coating method, an air-knife method, a curtain coating method, a wire-bar coating method, a gravure coating method, an extrusion coating method, or the like can be used. After that, pre-baking and main-baking are sequentially performed on the alignment film  70102 . 
       FIG. 91B  is a cross-sectional view of a step of performing rubbing treatment on the alignment film  70102 . The rubbing treatment is performed by rotating a roller  70104  for rubbing, in which a cloth is wrapped around a drum, to rub the alignment film  70102 . When the rubbing treatment is performed on the alignment film  70102 , a groove for aligning liquid crystal molecules in a certain direction is formed in the alignment film  70102 . Note that the present invention is not limited to this, and a groove may be formed in the alignment film by using an ion beam. After that, water washing treatment is performed on the first substrate  70101 . Accordingly, contaminant, dirt, or the like on a surface of the first substrate  70101  can be removed. 
     Note that although not shown, in a similar manner that in the first substrate  70101 , the alignment film  70108  is formed on the second substrate  70107 , and rubbing treatment is performed on the alignment film  70108 . Note that the present invention is not limited to this, and a groove may be formed in the alignment film by using an ion beam. 
       FIG. 91C  is a cross-sectional view of a step of forming the sealant  70105  over the alignment film  70102 . The sealant  70105  is applied by a lithography device, screen printing, or the like, and a seal pattern is formed. The seal pattern is formed along the periphery of the first substrate  70101 , and a liquid crystal inlet is provided in part of the seal pattern. A UV resin for temporal fixing is spot-applied to a region other than a display region of the first substrate  70101  by a dispenser or the like. 
     Note that the sealant  70105  may be provided for the second substrate  70107 . 
       FIG. 91D  is a cross-sectional view of a step of dispersing the spacers  70106  over the first substrate  70101 . The spacers  70106  are ejected by a nozzle together with a compressed gas and dispersed (dry dispersion). Alternatively, the spacers  70106  are mixed in a volatile liquid, and the liquid is sprayed so as to be dispersed (wet dispersion). By such dry dispersion or wet dispersion, the spacers  70106  can be uniformly dispersed over the first substrate  70101 . 
     In this embodiment mode, the case where the spherical spacer of the granular spacer is used as the spacer  70106  is described. However, the present invention is not limited to this, and a columnar spacer may be used. The columnar spacer may be provided for either the first substrate  70101  or the second substrate  70107 . Alternatively, part of the spacers may be provided for the first substrate  70101  and the other part thereof may be provided for the second substrate  70107 . 
     Note that a spacer may be mixed in the sealant. Accordingly, the cell gap of the liquid crystal can be maintained constant more easily. 
       FIG. 91E  is a cross-sectional view of a step of attaching the first substrate  70101  and the second substrate  70107 . The first substrate  70101  and the second substrate  70107  are attached in the atmosphere. Then, the first substrate  70101  and the second substrate  70107  are pressurized so that a gap between the first substrate  70101  and the second substrate  70107  is constant. After that, ultraviolet ray irradiation or heat treatment is performed on the sealant  70105 , so that the sealant  70105  is hardened. 
       FIGS. 92A and 92B  are top views of steps of filling a cell with liquid crystals. A cell in which the first substrate  70101  and the second substrate  70107  are attached (also referred to as an empty cell) is placed in a vacuum chamber. After that, the pressure in the vacuum chamber is reduced, and then, a liquid crystal inlet  70113  of the empty cell is immersed in liquid crystals. Then, when the vacuum chamber is opened to the atmosphere, the empty cell is filled with the liquid crystals  70109  due to pressure difference and capillary action. 
     When the empty cell is filled with the needed amount of liquid crystals  70109 , the liquid crystal inlet is sealed by a resin  70110 . Then, extra liquid crystals attached to the empty cell are washed out. After that, realignment treatment is performed on the liquid crystals  70109  by annealing treatment. Accordingly, the liquid crystal cell is completed. 
     Next, steps of manufacturing a liquid crystal cell in the case where a dropping method is used as a method for filling with liquid crystals are described with reference to  FIGS. 93A to 93D and 94A to 94C . 
       FIG. 94C  is a cross-sectional view of a liquid crystal cell. A first substrate  70301  and a second substrate  70307  are attached with spacers  70306  and a sealant  70305  interposed therebetween. Liquid crystals  70309  are arranged between the first substrate  70301  and the second substrate  70307 . Note that an alignment film  70302  is formed over the first substrate  70301 , and an alignment film  70308  is formed on the second substrate  70307 . 
     The first substrate  70301  is provided with a plurality of pixels arranged in matrix. Each of the plurality of pixels may include a transistor. Note that the first substrate  70301  may be referred to as a TFT substrate, an array substrate, or a TFT array substrate. As the first substrate  70301 , a single-crystal substrate, an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), and a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester)), a leather substrate, a rubber substrate, a stainless steel substrate, and a substrate including stainless steel foil can be used. Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissue of an animal such as a human may be used as the substrate. Note that the present invention is not limited to this, and various substrates can be used. 
     A common electrode, a color filter, a black matrix, and the like are provided on the second substrate  70307 . Note that the second substrate  70307  may be referred to as a counter substrate or a color filter substrate. 
     The alignment film  70302  has a function of aligning liquid crystal molecules in a certain direction. As the alignment film  70302 , a polyimide resin or the like can be used. Note that the present invention is not limited to this, and various materials can be used. Note that the alignment film  70308  is similar to the alignment film  70302 . 
     The sealant  70305  has a function of bonding the first substrate  70301  and the second substrate  70307  so that the liquid crystals  70309  do not leak. That is, the sealant  70305  functions as a sealant. 
     The spacer  70306  has a function of maintaining a fixed space between the first substrate  70301  and the second substrate  70307  (a cell gap of the liquid crystal). As the spacer  70306 , a granular spacer or a columnar spacer can be used. Examples of the granular spacer are a fiber-shaped spacer and a spherical spacer. Examples of a material for the granular spacer are plastic and glass. A spherical spacer formed by using plastic is referred to as a plastic bead and has been widely used. A fiber-shaped spacer formed by using glass is referred to as a glass fiber and mixed in a sealant when used. 
       FIG. 93A  is a cross-sectional view of a step of forming the alignment film  70302  over the first substrate  70301 . The alignment film  70302  is formed over the first substrate  70301  by a roller coating method using a roller  70303 . Note that other than a roller coating method, an offset printing method, a dip coating method, an air-knife method, a curtain coating method, a wire-bar coating method, a gravure coating method, an extrusion coating method, or the like can be used. After that, pre-baking and main-baking are sequentially performed on the alignment film  70302 . 
       FIG. 93B  is a cross-sectional view of a step of performing rubbing treatment on the alignment film  70302 . The rubbing treatment is performed by rotating a roller  70304  for rubbing, in which a cloth is wrapped around a drum, to rub the alignment film  70302 . When the rubbing treatment is performed on the alignment film  70302 , a groove for aligning liquid crystal molecules in a certain direction is formed in the alignment film  70302 . Note that the present invention is not limited to this, and a groove may be formed in the alignment film by using an ion beam. After that, water washing treatment is performed on the first substrate  70301 . Accordingly, contaminant, dirt, or the like on a surface of the first substrate  70301  can be removed. 
     Note that although not shown, in a similar manner that in the first substrate  70301 , the alignment film  70308  is formed on the second substrate  70307 , and rubbing treatment is performed on the alignment film  70308 . Note that the present invention is not limited to this, and a groove may be formed in the alignment film by using an ion beam. 
       FIG. 93C  is a cross-sectional view of a step of forming the sealant  70305  over the alignment film  70302 . The sealant  70305  is applied by a lithography device, screen printing, or the like, and a seal pattern is formed. The seal pattern is formed along the periphery of the first substrate  70301 . In this embodiment mode, a radical UV resin or a cationic UV resin is used for the sealant  70305 . Then, a conductive resin is spot-applied by a dispenser. 
     Note that the sealant  70305  may be provided for the second substrate  70307 . 
       FIG. 93D  is a cross-sectional view of a step of dispersing the spacers  70306  over the first substrate  70301 . The spacers  70306  are ejected by a nozzle together with a compressed gas and dispersed (dry dispersion). Alternatively, the spacers  70306  are mixed in a volatile liquid, and the liquid is sprayed so as to be dispersed (wet dispersion). By such dry dispersion or wet dispersion, the spacer  70306  can be uniformly dispersed over the first substrate  70301 . 
     In this embodiment mode, the case where the spherical spacer of the granular spacer is used as the spacer  70306  is described. However, the present invention is not limited to this, and a columnar spacer may be used. The columnar spacer may be provided for the first substrate  70301  or the second substrate  70307 . Alternatively, a part of the spacers may be provided for the first substrate  70301  and the other part thereof may be provided for the second substrate  70307 . 
     Note that a spacer may be mixed in the sealant. Accordingly, the cell gap of the liquid crystal can be maintained constant more easily. 
       FIG. 94A  is a cross-sectional view of a step of dropping the liquid crystals  70309 . Defoaming treatment is performed on the liquid crystals  70309 , and then, the liquid crystals  70309  are dropped inside the sealant  70305 . 
       FIG. 94B  is a top view after the liquid crystals  70309  are dropped. Since the sealant  70305  is formed along the periphery of the first substrate  70301 , the liquid crystals  70309  do not leak. 
       FIG. 94C  is a cross-sectional view of a step of attaching the first substrate  70301  and the second substrate  70307 . The first substrate  70301  and the second substrate  70307  are attached in a vacuum chamber. Then, the first substrate  70301  and the second substrate  70307  are pressurized so that a gap between the first substrate  70301  and the second substrate  70307  is constant. After that, ultraviolet ray irradiation is performed on the sealant  70305 , so that the sealant  70305  is hardened. It is preferable to perform ultraviolet ray irradiation on the sealant  70305  while a display portion is covered with a mask because deterioration of the liquid crystals  70309  due to ultraviolet rays can be prevented. After that, realignment treatment is performed on the liquid crystals  70309  by annealing treatment. Accordingly, the liquid crystal cell is completed. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 12 
     In this embodiment mode, a structure and an operation of a pixel in a display device are described. 
       FIGS. 95A and 95B  are timing charts showing an example of digital time gray scale driving. The timing chart of  FIG. 95A  shows a driving method in the case where a signal writing period (an address period) to a pixel and a light-emitting period (a sustain period) are separated. 
     One frame period refers to a period for fully displaying an image for one display region. One frame period includes a plurality of subframe periods, and one subframe period includes an address period and a sustain period. Address periods T a   1  to T a   4  indicate time for writing signals to pixels in all rows, and periods T b   1  to T b   4  indicate time for writing signals to pixels in one row (or one pixel). Sustain periods T s   1  to T s   4  indicate time for maintaining a lighting state or a non-lighting state in accordance with a video signal written to the pixel, and a ratio of the length of the sustain periods is set to satisfy T s   1 :T s   2 :T s   3 :T s   4 =2 3 :2 2 :2 1 :2 0 =8:4:2:1. A gray scale is expressed depending on in which sustain period light emission is performed. 
     Operations are described. First, in the address period T a   1 , pixel selection signals are sequentially input to scan lines from a first row, and a pixel is selected. Then, while the pixel is selected, a video signal is input to the pixel from a signal line. Then, when the video signal is written to the pixel, the pixel maintains the signal until a signal is input again. Lighting and non-lighting of each pixel in the sustain period T s   1  are controlled by the written video signal. Similarly, in the address periods T a   2 , T a   3 , and T a   4 , a video signal is input to pixels, and lighting and non-lighting of each pixel in the sustain periods T s   2 , T s   3 , and T s   4  are controlled by the video signal. Then, in each subframe period, a pixel to which a signal for not lighting in the address period and for lighting when the sustain period starts after the address period ends is written is lit. 
     Here, the i-th pixel row is described with reference to  FIG. 95B . First, in the address period Ta 1 , pixel selection signals are input to scan lines from a first row, and in a period T b   1 ( i ) in the address period T a   1 , a pixel in the i-th row is selected. Then, while the pixel in the i-th row is selected, a video signal is input to the pixel in the i-th row from a signal line. Then, when the video signal is written to the pixel in the i-th row, the pixel in the i-th row maintains the signal until a signal is input again. Lighting and non-lighting of the pixel in the i-th row in the sustain period Ts 1  are controlled by the written video signal. Similarly, in the address periods T a   2 , T a   3 , and T a   4 , a video signal is input to the pixel in the i-th row, and lighting and non-lighting of the pixel in the i-th row in the sustain periods T s   2 , T s   3 , and T s   4  are controlled by the video signal. Then, in each subframe period, a pixel to which a signal for not lighting in the address period and for lighting when the sustain period starts after the address period ends is written is lit. 
     Here, the case where a 4-bit gray scale is expressed is described here; however, the number of bits and the number of gray scales are not limited thereto. Note that lighting is not needed to be performed in order of T s   1 , T s   2 , T s   3 , and T s   4 , and the order may be random or light may be emitted by dividing the whole period into a plurality of periods. The ratio of lighting time of T s   1 , T s   2 , T s   3 , and T s   4  is not needed to be a power of two, and may be the same length or slightly different from a power of two. 
     Next, a driving method in the case where a period for writing a signal to a pixel (an address period) and a light-emitting period (a sustain period) are not separated is described. That is, a pixel in a row in which a writing operation of a video signal is completed maintains the signal until another signal is written to the pixel (or the signal is erased). A period between the writing operation and writing of another signal to the pixel is referred to as data holding time. In the data holding time, the pixel is lit or not lit in accordance with the video signal written to the pixel. The same operations are performed until the last row, and the address period ends. Then, an operation proceeds to a signal writing operation of the next subframe period sequentially from a row in which the data holding time ends. 
     As described above, in the case of a driving method in which a pixel is lit or not lit in accordance with a video signal written to the pixel immediately after the signal writing operation is completed and the data holding time starts, signals cannot be input to two rows at the same time. Accordingly, address periods need to be prevented from overlapping. Therefore, the data holding time cannot be made shorter than the address period. As a result, it becomes difficult to perform high-level gray scale display. 
     Thus, the data holding time is set to be shorter than the address period by providing an erasing period. A driving method in the case where the data holding time shorter than the address period is set by providing an erasing period is described with reference to  FIG. 96A . 
     First, in the address period Ta 1 , pixel scan signals are input to scan lines from a first row, and a pixel is selected. Then, while the pixel is selected, a video signal is input to the pixel from a signal line. Then, when the video signal is written to the pixel, the pixel maintains the signal until a signal is input again. Lighting and non-lighting of the pixel in the sustain period T s   1  are controlled by the written video signal. In a row in which a writing operation of a video signal is completed, a pixel is immediately lit or not lit in accordance with the written video signal. The same operations are performed until the last row, and the address period T a   1  ends. Then, an operation proceeds to a signal writing operation of the next subframe period sequentially from a row in which the data holding time ends. Similarly, in the address periods T a   2 , T a   3 , and T a   4 , a video signal is input to the pixel, and lighting and non-lighting of the pixel in the sustain periods T s   2 , T s   3 , and T s   4  are controlled by the video signal. The end of the sustain period T s   4  is set by the start of an erasing operation. This is because when a signal written to a pixel is erased in an erasing time Te of each row, the pixel is forced to be not lit regardless of the video signal written to the pixel in the address period until another signal is written to the pixel. That is, the data holding time ends from a pixel in which the erasing time T e  starts. 
     Here, the i-th pixel row is described with reference to  FIG. 96B . In the address period T a   1 , pixel scan signals are input to scan lines from a first row, and a pixel is selected. Then, in the period T b   1 ( i ), while the pixel in the i-th row is selected, a video signal is input to the pixel in the i-th row. Then, when the video signal is written to the pixel in the i-th row, the pixel in the i-th row maintains the signal until a signal is input again. Lighting and non-lighting of the pixel in the i-th row in a sustain period T s   1 ( i ) are controlled by the written video signal. That is, the pixel in the i-th row is immediately lit or not lit in accordance with the video signal written to the pixel after the writing operation of the video signal to the i-th row is completed. Similarly, in the address periods T a   2 , T a   3 , and T a   4 , a video signal is input to the pixel in the i-th row, and lighting and non-lighting of the pixel in the i-th row in the sustain periods T s   2 , T s   3 , and T s   4  are controlled by the video signal. The end of a sustain period T s   4 ( i ) is set by the start of an erasing operation. This is because the pixel is forced to be not lit regardless of the video signal written to the pixel in the i-th row in an erasing time T e (i) in the i-th row. That is, the data holding time of the pixel in the i-th row ends when the erasing time T e (i) starts. 
     Thus, a display device with a high-level gray scale and a high duty ratio (a ratio of a lighting period in one frame period) can be provided, in which data holding time is shorter than an address period without separating the address period and a sustain period. Since instantaneous luminance can be lowered, reliability of a display element can be improved. 
     Here, the case where a 4-bit gray scale is expressed is described here; however, the number of bits and the number of gray scales are not limited thereto. Note that lighting is not needed to be performed in order of T s   1 , T s   2 , T s   3 , and T s   4 , and the order may be random or light may be emitted by dividing the whole period into a plurality of periods. The ratio of lighting time of T s   1 , T s   2 , T s   3 , and T s   4 , is not needed to be a power of two, and may be the same length or slightly different from a power of two. 
     Next, a structure and an operation of a pixel to which digital time gray scale driving can be applied are described. 
       FIG. 97  shows an example of a pixel structure to which digital time gray scale driving can be applied. 
     A pixel  80300  includes a switching transistor  80301 , a driving transistor  80302 , a light-emitting element  80304 , and a capacitor  80303 . A gate of the switching transistor  80301  is connected to a scan line  80306 ; a first electrode (one of a source electrode and a drain electrode) of the switching transistor  80301  is connected to a signal line  80305 ; and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor  80301  is connected to a gate of the driving transistor  80302 . The gate of the driving transistor  80302  is connected to a power supply line  80307  through the capacitor  80303 ; a first electrode of the driving transistor  80302  is connected to the power supply line  80307 ; and a second electrode of the driving transistor  80302  is connected to a first electrode (a pixel electrode) of the light-emitting element  80304 . A second electrode of the light-emitting element  80304  corresponds to a common electrode  80308 . 
     Note that the second electrode (the common electrode  80308 ) of the light-emitting element  80304  is set to have a low power supply potential. A low power supply potential refers to a potential satisfying (the low power supply potential)&lt;(a high power supply potential) based on the high power supply potential set to the power supply line  80307 . As the low power supply potential, GND, 0 V, or the like may be set, for example. In order to make the light-emitting element  80304  emit light by applying a potential difference between the high power supply potential and the low power supply potential to the light-emitting element  80304  so that current is supplied to the light-emitting element  80304 , each of the potentials is set so that the potential difference between the high power supply potential and the low power supply potential is equal to or higher than forward threshold voltage. 
     Note that gate capacitance of the driving transistor  80302  may be used as a substitute for the capacitor  80303 , so that the capacitor  80303  can be omitted. The gate capacitance of the driving transistor  80302  may be formed in a region where a source region, a drain region, an LDD region, or the like overlaps with the gate electrode. Alternatively, capacitance may be formed between a channel region and the gate electrode. 
     When a pixel is selected by the scan line  80306 , that is, when the switching transistor  80301  is on, a video signal is input to the pixel from the signal line  80305 . Then, charge for voltage corresponding to the video signal is stored in the capacitor  80303 , and the capacitor  80303  maintains the voltage. The voltage is voltage between the gate and the first electrode of the driving transistor  80302  and corresponds to gate-source voltage V gs  of the driving transistor  80302 . 
     In general, an operation region of a transistor can be divided into a linear region and a saturation region. When drain-source voltage is denoted by V ds , gate-source voltage is denoted by V gs , and threshold voltage is denoted by Vth, a boundary between the linear region and the saturation region sets so as to satisfy (V gs −Vth)=V ds . When (V gs −Vth)&gt;V ds , the transistor operates in a linear region, and a current value is determined in accordance with the level of Vds and Vgs. On the other hand, when (V gs −Vth)&lt;V ds , the transistor operates in a saturation region and ideally, a current value hardly changes even when Vds changes. That is, the current value is determined only by the level of Vgs. 
     Here, in the case of a voltage-input voltage driving method, a video signal is input to the gate of the driving transistor  80302  so that the driving transistor  80302  is in either of two states of being sufficiently turned on and turned off. That is, the driving transistor  80302  operates in a linear region. 
     Thus, when a video signal which makes the driving transistor  80302  turned on is input, a power supply potential V DD  set to the power supply line  80307  without change is ideally set to the first electrode of the light-emitting element  80304 . 
     That is, ideally, constant voltage is applied to the light-emitting element  80304  to obtain constant luminance from the light-emitting element  80304 . Then, a plurality of subframe periods are provided in one frame period. A video signal is written to a pixel in each subframe period, lighting and non-lighting of the pixel are controlled in each subframe period, and a gray scale is expressed by the sum of lighting subframe periods. 
     Note that when the video signal by which the driving transistor  80302  operates in a saturation region is input, current can be supplied to the light-emitting element  80304 . When the light-emitting element  80304  is an element luminance of which is determined in accordance with current, luminance decay due to deterioration of the light-emitting element  80304  can be suppressed. Further, when the video signal is an analog signal, current in accordance with the video signal can be supplied to the light-emitting element  80304 . In this case, analog gray scale driving can be performed. 
       FIG. 98  shows another example of a pixel structure to which digital time gray scale driving can be applied. 
     A pixel  80400  includes a switching transistor  80401 , a driving transistor  80402 , a capacitor  80403 , a light-emitting element  80404 , and a rectifier element  80409 . A gate of the switching transistor  80401  is connected to a first scan line  80406 ; a first electrode (one of a source electrode and a drain electrode) of the switching transistor  80401  is connected to a signal line  80405 ; and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor  80401  is connected to a gate of the driving transistor  80402 . The gate of the driving transistor  80402  is connected to a power supply line  80407  through the capacitor  80403 , and is also connected to a second scan line  80410  through the rectifier element  80409 . A first electrode of the driving transistor  80402  is connected to the power supply line  80407 , and a second electrode of the driving transistor  80402  is connected to a first electrode (a pixel electrode) of the light-emitting element  80404 . A second electrode of the light-emitting element  80404  corresponds to a common electrode  80408 . 
     The second electrode (the common electrode  80408 ) of the light-emitting element  80404  is set to have a low power supply potential. Note that a low power supply potential refers to a potential satisfying (the low power supply potential)&lt;(a high power supply potential) based on the high power supply potential set to the power supply line  80407 . As the low power supply potential, GND, 0 V, or the like may be set, for example. In order to make the light-emitting element  80404  emit light by applying a potential difference between the high power supply potential and the low power supply potential to the light-emitting element  80404  so that current is supplied to the light-emitting element  80404 , each of the potentials is set so that the potential difference between the high power supply potential and the low power supply potential is equal to or higher than forward threshold voltage. 
     Note that gate capacitance of the driving transistor  80402  may be used as a substitute for the capacitor  80403 , so that the capacitor  80403  can be omitted. The gate capacitance of the driving transistor  80402  may be formed in a region where a source region, a drain region, an LDD region, or the like overlaps with the gate electrode. Alternatively, capacitance may be formed between a channel region and the gate electrode. 
     As the rectifier element  80409 , a diode-connected transistor can be used. A PN junction diode, a PIN junction diode, a Schottky diode, a diode formed using a carbon nanotube, or the like may be used other than a diode-connected transistor. A diode-connected transistor may be either an n-channel transistor or a p-channel transistor. 
     The pixel  80400  is such that the rectifier element  80409  and the second scan line  80410  are added to the pixel shown in  FIG. 97 . Accordingly, the switching transistor  80401 , the driving transistor  80402 , the capacitor  80403 , the light-emitting element  80404 , the signal line  80405 , the first scan line  80406 , the power supply line  80407 , and the common electrode  80408  shown in  FIG. 98  correspond to the switching transistor  80301 , the driving transistor  80302 , the capacitor  80303 , the light-emitting element  80304 , the signal line  80305 , the scan line  80306 , the power supply line  80307 , and the common electrode  80308  shown in  FIG. 97 . Accordingly, a writing operation and a light-emitting operation in  FIG. 98  are similar to those described in  FIG. 97 , so that description thereof is omitted. 
     An erasing operation is described. In the erasing operation, an H-level signal is input to the second scan line  80410 . Thus, current is supplied to the rectifier element  80409 , and a gate potential of the driving transistor  80402  held by the capacitor  80403  can be set to a certain potential. That is, the potential of the gate of the driving transistor  80402  is set to a certain value, and the driving transistor  80402  can be forcibly turned off regardless of a video signal written to the pixel. 
     Note that an L-level signal input to the second scan line  80410  has a potential such that current is not supplied to the rectifier element  80409  when a video signal for non-lighting is written to a pixel. An H-level signal input to the second scan line  80410  has a potential such that a potential to turn off the driving transistor  80302  can be set to the gate regardless of a video signal written to a pixel. 
     As the rectifier element  80409 , a diode-connected transistor can be used. A PN junction diode, a PIN junction diode, a Schottky diode, a diode formed using a carbon nanotube, or the like may be used other than a diode-connected transistor. A diode-connected transistor may be either an n-channel transistor or a p-channel transistor. 
       FIG. 99  shows another example of a pixel structure to which digital time gray scale driving can be applied. 
     A pixel  80500  includes a switching transistor  80501 , a driving transistor  80502 , a capacitor  80503 , a light-emitting element  80504 , and an erasing transistor  80509 . A gate of the switching transistor  80501  is connected to a first scan line  80506 , a first electrode (one of a source electrode and a drain electrode) of the switching transistor  80501  is connected to a signal line  80505 , and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor  80501  is connected to a gate of the driving transistor  80502 . The gate of the driving transistor  80502  is connected to a power supply line  80507  through the capacitor  80503 , and is also connected to a first electrode of the erasing transistor  80509 . A first electrode of the driving transistor  80502  is connected to the power supply line  80507 , and a second electrode of the driving transistor  80502  is connected to a first electrode (a pixel electrode) of the light-emitting element  80504 . A gate of the erasing transistor  80509  is connected to a second scan line  80510 , and a second electrode of the erasing transistor  80509  is connected to the power supply line  80507 . A second electrode of the light-emitting element  80504  corresponds to a common electrode  80508 . 
     The second electrode (the common electrode  80508 ) of the light-emitting element  80504  is set to have a low power supply potential. Note that a low power supply potential refers to a potential satisfying (the low power supply potential)&lt;(a high power supply potential) based on the high power supply potential set to the power supply line  80507 . As the low power supply potential, GND, 0 V, or the like may be set, for example. In order to make the light-emitting element  80504  emit light by applying a potential difference between the high power supply potential and the low power supply potential to the light-emitting element  80504  so that current is supplied to the light-emitting element  80504 , each of the potentials is set so that the potential difference between the high power supply potential and the low power supply potential is equal to or higher than forward threshold voltage. 
     Note that gate capacitance of the driving transistor  80502  may be used as a substitute for the capacitor  80503 , so that the capacitor  80503  can be omitted. The gate capacitance of the driving transistor  80502  may be formed in a region where a source region, a drain region, an LDD region, or the like overlaps with the gate electrode. Alternatively, capacitance may be formed between a channel region and the gate electrode. 
     The pixel  80500  is such that the erasing transistor  80509  and the second scan line  80510  are added to the pixel shown in  FIG. 97 . Accordingly, the switching transistor  80501 , the driving transistor  80502 , the capacitor  80503 , the light-emitting element  80504 , the signal line  80505 , the first scan line  80506 , the power supply line  80507 , and the common electrode  80508  shown in  FIG. 99  correspond to the switching transistor  80301 , the driving transistor  80302 , the capacitor  80303 , the light-emitting element  80304 , the signal line  80305 , the scan line  80306 , the power supply line  80307 , and the common electrode  80308  shown in  FIG. 97 . Accordingly, a writing operation and a light-emitting operation in  FIG. 99  are similar to those described in  FIG. 97 , so that description thereof is omitted. 
     An erasing operation is described. In the erasing operation, an H-level signal is input to the second scan line  80510 . Thus, the erasing transistor  80509  is turned on, and the gate and the first electrode of the driving transistor  80502  can be made to have the same potential. That is, Vgs of the driving transistor  80502  can be 0 V. Accordingly, the driving transistor  80502  can be forcibly turned off. 
     Next, a structure and an operation of a pixel called a threshold voltage compensation pixel are described. A threshold voltage compensation pixel can be applied to digital time gray scale driving and analog gray scale driving. 
       FIG. 100  shows an example of a structure of a pixel called a threshold voltage compensation pixel. 
     The pixel shown in  FIG. 100  includes a driving transistor  80600 , a first switch  80601 , a second switch  80602 , a third switch  80603 , a first capacitor  80604 , a second capacitor  80605 , and a light-emitting element  80620 . A gate of the driving transistor  80600  is connected to a signal line  80611  through the first capacitor  80604  and the first switch  80601  in that order. Further, the gate of the driving transistor  80600  is connected to a power supply line  80612  through the second capacitor  80605 . A first electrode of the driving transistor  80600  is connected to the power supply line  80612 . A second electrode of the driving transistor  80600  is connected to a first electrode of the light-emitting element  80620  through the third switch  80603 . Further, the second electrode of the driving transistor  80600  is connected to the gate of the driving transistor  80600  through the second switch  80602 . A second electrode of the light-emitting element  80620  corresponds to a common electrode  80621 . 
     The second electrode of the light-emitting element  80620  is set to a low power supply potential. Note that a low power supply potential refers to a potential satisfying (the low power supply potential)&lt;(a high power supply potential) based on the high power supply potential set to the power supply line  80612 . As the low power supply potential, GND, 0 V, or the like may be set, for example. In order to make the light-emitting element  80620  emit light by applying a potential difference between the high power supply potential and the low power supply potential to the light-emitting element  80620  so that current is supplied to the light-emitting element  80620 , each of the potentials is set so that the potential difference between the high power supply potential and the low power supply potential is equal to or higher than forward threshold voltage. Note that gate capacitance of the driving transistor  80600  may be used as a substitute for the second capacitor  80605 , so that the second capacitor  80605  can be omitted. The gate capacitance of the driving transistor  80600  may be formed in a region where a source region, a drain region, an LDD region, or the like overlaps with the gate electrode. Alternatively, capacitance may be formed between a channel formation region and the gate electrode. Note that on/off of the first switch  80601 , the second switch  80602 , and the third switch  80603  is controlled by a first scan line  80613 , a second scan line  80614 , and a third scan line  80615 , respectively. 
     A method for driving the pixel shown in  FIG. 100  is described by dividing an operation period into an initialization period, a data writing period, a threshold acquiring period, and a light-emitting period. 
     In the initialization period, the second switch  80602  and the third switch  80603  are turned on. Then, a potential of the gate of the driving transistor  80600  is lower than at least a potential of the power supply line  80612 . At this time, the first switch  80601  may be either on or off. Note that the initialization period is not necessarily required. 
     In the threshold acquiring period, a pixel is selected by the first scan line  80613 . That is, the first switch  80601  is turned on, and constant voltage is input from the signal line  80611 . At this time, the second switch  80602  is turned on and the third switch  80603  is turned off. Accordingly, the driving transistor  80600  is diode-connected, and the second electrode and the gate of the driving transistor  80600  are set in a floating state. Then, a potential of the gate of the driving transistor  80600  is a value obtained by subtracting threshold voltage of the driving transistor  80600  from the potential of the power supply line  80612 . Thus, the threshold voltage of the driving transistor  80600  is held in the first capacitor  80604 . A potential difference between the potential of the gate of the driving transistor  80600  and the constant voltage input from the signal line  80611  is held in the second capacitor  80605 . 
     In the data writing period, a video signal (voltage) is input from the signal line  80611 . At this time, the first switch  80601  is kept on, the second switch  80602  is turned off, and the third switch  80603  is kept off. Since the gate of the driving transistor  80600  is in a floating state, the potential of the gate of the driving transistor  80600  changes depending on a potential difference between the constant voltage input from the signal line  80611  in the threshold acquiring period and the video signal input from the signal line  80611  in the data writing period. For example, when (a capacitance value of the first capacitor  80604 )&lt;&lt;(a capacitance value of the second capacitor  80605 ) is satisfied, the potential of the gate of the driving transistor  80600  in the data writing period is approximately equal to the sum of a potential difference (the amount of change) between the potential of the signal line  80611  in the threshold acquiring period and the potential of the signal line  80611  in the data writing period; and a value obtained by subtracting the threshold voltage of the driving transistor  80600  from the potential of the power supply line  80612 . That is, the potential of the gate of the driving transistor  80600  becomes a potential obtained by correcting the threshold voltage of the driving transistor  80600 . 
     In the light-emitting period, current in accordance with a potential difference (V gs ) between the gate of the driving transistor  80600  and the power supply line  80612  is supplied to the light-emitting element  80620 . At this time, the first switch  80601  is turned off, the second switch  80602  is kept off, and the third switch  80603  is turned on. Note that current flowing to the light-emitting element  80620  is constant regardless of the threshold voltage of the driving transistor  80600 . 
     Note that a pixel structure of the present invention is not limited to the pixel structure shown in  FIG. 100 . For example, a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like may be added to the pixel shown in  FIG. 100 . For example, the second switch  80602  may include a p-channel transistor or an n-channel transistor, the third switch  80603  may include a transistor with polarity different from that of the second switch  80602 , and the second switch  80602  and the third switch  80603  may be controlled by the same scan line. 
     A structure and an operation of a pixel called a current input pixel are described. A current input pixel can be applied to digital gray scale driving and analog gray scale driving. 
       FIG. 101  shows an example of a structure of a current input pixel. 
     The pixel shown in  FIG. 101  includes a driving transistor  80700 , a first switch  80701 , a second switch  80702 , a third switch  80703 , a capacitor  80704 , and a light-emitting element  80730 . A gate of the driving transistor  80700  is connected to a signal line  80711  through the second switch  80702  and the first switch  80701  in this order. Further, the gate of the driving transistor  80700  is connected to a power supply line  80712  through the capacitor  80704 . A first electrode of the driving transistor  80700  is connected to the power supply line  80712 . A second electrode of the driving transistor  80700  is connected to the signal line  80711  through the first switch  80701 . Further, the second electrode of the driving transistor  80700  is connected to a first electrode of the light-emitting element  80730  through the third switch  80703 . A second electrode of the light-emitting element  80730  corresponds to a common electrode  80731 . 
     The second electrode of the light-emitting element  80730  is set to a low power supply potential. Note that a low power supply potential refers to a potential satisfying (the low power supply potential)&lt;(a high power supply potential) based on the high power supply potential set to the power supply line  80712 . As the low power supply potential, GND, 0 V, or the like may be set, for example. In order to make the light-emitting element  80730  emit light by applying a potential difference between the high power supply potential and the low power supply potential to the light-emitting element  80730  so that current is supplied to the light-emitting element  80730 , each of the potentials is set so that the potential difference between the high power supply potential and the low power supply potential is equal to or higher than forward threshold voltage. Note that gate capacitance of the driving transistor  80700  may be used as a substitute for the capacitor  80704 , so that the capacitor  80704  can be omitted. The gate capacitance of the driving transistor  80700  may be formed in a region where a source region, a drain region, an LDD region, or the like overlaps with the gate electrode. Alternatively, capacitance may be formed between a channel region and the gate electrode. Note that on/off of the first switch  80701 , the second switch  80702 , and the third switch  80703  is controlled by a first scan line  80713 , a second scan line  80714 , and a third scan line  80715 , respectively. 
     A method for driving the pixel shown in  FIG. 101  is described by dividing an operation period into a data writing period and a light-emitting period. 
     In the data writing period, a pixel is selected by the first scan line  80713 . That is, the first switch  80701  is turned on, and current is input as a video signal from the signal line  80711 . At this time, the second switch  80702  is turned on and the third switch  80703  is turned off. Accordingly, a potential of the gate of the driving transistor  80700  becomes a potential in accordance with the video signal. That is, voltage between the gate electrode and the source electrode of the driving transistor  80700 , which is such that the driving transistor  80700  supplies the same current as the video signal, is held in the capacitor  80704 . 
     Next, in the light-emitting period, the first switch  80701  and the second switch  80702  are turned off, and the third switch  80703  is turned on. Thus, current with the same value as the video signal is supplied to the light-emitting element  80730 . 
     Note that the present invention is not limited to the pixel structure shown in  FIG. 101 . For example, a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like may be added to the pixel shown in  FIG. 101 . For example, the first switch  80701  may include a p-channel transistor or an n-channel transistor, the second switch  80702  may include a transistor with the same polarity as that of the first switch  80701 , and the first switch  80701  and the second switch  80702  may be controlled by the same scan line. The second switch  80702  may be provided between the gate of the driving transistor  80700  and the signal line  80711 . 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 13 
     In this embodiment mode, a pixel structure of a display device is described. In particular, a pixel structure of a display device using an organic EL element is described. 
       FIG. 102A  shows an example of a top plan view (a layout diagram) of a pixel including two transistors.  FIG. 102B  shows an example of a cross-sectional view along X-X′ in  FIG. 102A . 
       FIGS. 102A and 102B  show a first transistor  60105 , a first wiring  60106 , a second wiring  60107 , a second transistor  60108 , a third wiring  60111 , a counter electrode  60112 , a capacitor  60113 , a pixel electrode  60115 , a partition wall  60116 , an organic conductive film  60117 , an organic thin film  60118 , and a substrate  60119 . Note that it is preferable that the first transistor  60105  be used as a switching transistor, the first wiring  60106  as a gate signal line, the second wiring  60107  as a source signal line, the second transistor  60108  as a driving transistor, and the third wiring  60111  as a current supply line. 
     A gate electrode of the first transistor  60105  is electrically connected to the first wiring  60106 . One of a source electrode and a drain electrode of the first transistor  60105  is electrically connected to the second wiring  60107 . The other of the source electrode and the drain electrode of the first transistor  60105  is electrically connected to a gate electrode of the second transistor  60108  and one electrode of the capacitor  60113 . Note that the gate electrode of the first transistor  60105  includes a plurality of gate electrodes. Accordingly, leakage current in the off state of the first transistor  60105  can be reduced. 
     One of a source electrode and a drain electrode of the second transistor  60108  is electrically connected to the third wiring  60111 , and the other of the source electrode and the drain electrode of the second transistor  60108  is electrically connected to the pixel electrode  60115 . Accordingly, current flowing through the pixel electrode  60115  can be controlled by the second transistor  60108 . 
     The organic conductive film  60117  is provided over the pixel electrode  60115 , and the organic thin film  60118  (an organic compound layer) is provided thereover. The counter electrode  60112  is provided over the organic thin film  60118  (the organic compound layer). Note that the counter electrode  60112  may be formed without patterning so as to be connected to all the pixels in common, or may be patterned using a shadow mask or the like. 
     Light emitted from the organic thin film  60118  (the organic compound layer) is transmitted through either the pixel electrode  60115  or the counter electrode  60112 . 
     In  FIG. 102B , the case where light is emitted to the pixel electrode side, that is, a side on which the transistor and the like are formed is referred to as bottom emission; and the case where light is emitted to the counter electrode side is referred to as top emission. 
     In the case of bottom emission, it is preferable that the pixel electrode  60115  be formed of a light-transmitting conductive film. On the other hand, in the case of top emission, it is preferable that the counter electrode  60112  be formed of a light-transmitting conductive film. 
     In a light-emitting device for color display, EL elements having respective light emission colors of R, G, and B may be separately formed, or an EL element with a single color may be formed without patterning and light emission of R, G, and B can be obtained by using a color filter. 
     Note that the structures shown in  FIGS. 102A and 102B  are examples, and various structures can be employed for a pixel layout, a cross-sectional structure, a stacking order of electrodes of an EL element, and the like, other than the structures shown in  FIGS. 102A and 102B . Further, for a light-emitting layer, various elements such as a crystalline element such as an LED, and an element formed using an inorganic thin film as well as the element formed using the organic thin film shown in the drawing can be used. 
       FIG. 103A  shows an example of a top plan view (a layout diagram) of a pixel including three transistors.  FIG. 103B  shows an example of a cross-sectional view along X-X′ in  FIG. 103A . 
       FIGS. 103A and 103B  show a substrate  60200 , a first wiring  60201 , a second wiring  60202 , a third wiring  60203 , a fourth wiring  60204 , a first transistor  60205 , a second transistor  60206 , a third transistor  60207 , a pixel electrode  60208 , a partition wall  60211 , an organic conductive film  60212 , an organic thin film  60213 , and a counter electrode  60214 . Note that it is preferable that the first wiring  60201  be used as a source signal line, the second wiring  60202  as a gate signal line for writing, the third wiring  60203  as a gate signal line for erasing, the fourth wiring  60204  as a current supply line, the first transistor  60205  as a switching transistor, the second transistor  60206  as an erasing transistor, and the third transistor  60207  as a driving transistor. 
     A gate electrode of the first transistor  60205  is electrically connected to the second wiring  60202 . One of a source electrode and a drain electrode of the first transistor  60205  is electrically connected to the first wiring  60201 . The other of the source electrode and the drain electrode of the first transistor  60205  is electrically connected to a gate electrode of the third transistor  60207 . Note that the gate electrode of the first transistor  60205  includes a plurality of gate electrodes. Accordingly, leakage current in the off state of the first transistor  60205  can be reduced. 
     A gate electrode of the second transistor  60206  is electrically connected to the third wiring  60203 . One of a source electrode and a drain electrode of the second transistor  60206  is electrically connected to the fourth wiring  60204 . The other of the source electrode and the drain electrode of the second transistor  60206  is electrically connected to the gate electrode of the third transistor  60207 . Note that the gate electrode of the second transistor  60206  includes a plurality of gate electrodes. Accordingly, leakage current in the off state of the second transistor  60206  can be reduced. 
     One of a source electrode and a drain electrode of the third transistor  60207  is electrically connected to the fourth wiring  60204 , and the other of the source electrode and the drain electrode of the third transistor  60207  is electrically connected to the pixel electrode  60208 . Accordingly, current flowing through the pixel electrode  60208  can be controlled by the third transistor  60207 . 
     The organic conductive film  60212  is provided over the pixel electrode  60208 , and the organic thin film  60213  (an organic compound layer) is provided thereover. The counter electrode  60214  is provided over the organic thin film  60213  (the organic compound layer). Note that the counter electrode  60214  may be formed without patterning so as to be connected to all the pixels in common, or may be patterned using a shadow mask or the like. 
     Light emitted from the organic thin film  60213  (the organic compound layer) is transmitted through either the pixel electrode  60208  or the counter electrode  60214 . 
     In  FIG. 103B , the case where light is emitted to the pixel electrode side, that is, a side on which the transistor and the like are formed is referred to as bottom emission; and the case where light is emitted to the counter electrode side is referred to as top emission. 
     In the case of bottom emission, it is preferable that the pixel electrode  60208  be formed of a light-transmitting conductive film. On the other hand, in the case of top emission, it is preferable that the counter electrode  60214  be formed of a light-transmitting conductive film. 
     In a light-emitting device for color display, EL elements having respective light emission colors of R, G, and B may be separately formed, or an EL element with a single color may be formed without patterning and light emission of R, G, and B can be obtained by using a color filter. 
     Note that the structures shown in  FIGS. 103A and 103B  are examples, and various structures can be employed for a pixel layout, a cross-sectional structure, a stacking order of electrodes of an EL element, and the like, other than the structures shown in  FIGS. 103A and 103B . Further, as a light-emitting layer, various elements such as a crystalline element such as an LED, and an element formed using an inorganic thin film as well as the element formed using the organic thin film shown in the drawing can be used. 
       FIG. 104A  shows an example of a top plan view (a layout diagram) of a pixel including four transistors.  FIG. 104B  shows an example of a cross-sectional view along X-X′ in  FIG. 104A . 
       FIGS. 104A and 104B  show a substrate  60300 , a first wiring  60301 , a second wiring  60302 , a third wiring  60303 , a fourth wiring  60304 , a first transistor  60305 , a second transistor  60306 , a third transistor  60307 , a fourth transistor  60308 , a pixel electrode  60309 , a fifth wiring  60311 , a sixth wiring  60312 , a partition wall  60321 , an organic conductive film  60322 , an organic thin film  60323 , and a counter electrode  60324 . Note that it is preferable that the first wiring  60301  be used as a source signal line, the second wiring  60302  as a gate signal line for writing, the third wiring  60303  as a gate signal line for erasing, the fourth wiring  60304  as a signal line for reverse bias, the first transistor  60305  as a switching transistor, the second transistor  60306  as an erasing transistor, the third transistor  60307  as a driving transistor, the fourth transistor  60308  as a transistor for reverse bias, the fifth wiring  60311  as a current supply line, and the sixth wiring  60312  as a power supply line for reverse bias. 
     A gate electrode of the first transistor  60305  is electrically connected to the second wiring  60302 . One of a source electrode and a drain electrode of the first transistor  60305  is electrically connected to the first wiring  60301 . The other of the source electrode and the drain electrode of the first transistor  60305  is electrically connected to a gate electrode of the third transistor  60307 . Note that the gate electrode of the first transistor  60305  includes a plurality of gate electrodes. Accordingly, leakage current in the off state of the first transistor  60305  can be reduced. 
     A gate electrode of the second transistor  60306  is electrically connected to the third wiring  60303 . One of a source electrode and a drain electrode of the second transistor  60306  is electrically connected to the fifth wiring  60311 . The other of the source electrode and the drain electrode of the second transistor  60306  is electrically connected to the gate electrode of the third transistor  60307 . Note that the gate electrode of the second transistor  60306  includes a plurality of gate electrodes. Accordingly, leakage current in the off state of the second transistor  60306  can be reduced. 
     One of a source electrode and a drain electrode of the third transistor  60307  is electrically connected to the fifth wiring  60311 , and the other of the source electrode and the drain electrode of the third transistor  60307  is electrically connected to the pixel electrode  60309 . Accordingly, current flowing through the pixel electrode  60309  can be controlled by the third transistor  60307 . 
     A gate electrode of the fourth transistor  60308  is electrically connected to the fourth wiring  60304 . One of a source electrode and a drain electrode of the fourth transistor  60308  is electrically connected to the sixth wiring  60312 . The other of the source electrode and the drain electrode of the fourth transistor  60308  is electrically connected to the pixel electrode  60309 . Accordingly, a potential of the pixel electrode  60309  can be controlled by the fourth transistor  60308 , so that reverse bias can be applied to the organic conductive film  60322  and the organic thin film  60323 . When reverse bias is applied to a light-emitting element including the organic conductive film  60322 , the organic thin film  60323 , and the like, reliability of the light-emitting element can be significantly improved. 
     The organic conductive film  60322  is provided over the pixel electrode  60309 , and the organic thin film  60323  (an organic compound layer) is provided thereover. The counter electrode  60324  is provided over the organic thin film  60213  (the organic compound layer). Note that the counter electrode  60324  may be formed without patterning so as to be connected to all the pixels in common, or may be patterned using a shadow mask or the like. 
     Light emitted from the organic thin film  60323  (the organic compound layer) is transmitted through either the pixel electrode  60309  or the counter electrode  60324 . 
     In  FIG. 104B , the case where light is emitted to the pixel electrode side, that is, a side on which the transistor and the like are formed is referred to as bottom emission; and the case where light is emitted to the counter electrode side is referred to as top emission. 
     In the case of bottom emission, it is preferable that the pixel electrode  60309  be formed of a light-transmitting conductive film. On the other hand, in the case of top emission, it is preferable that the counter electrode  60324  be formed of a light-transmitting conductive film. 
     In a light-emitting device for color display, EL elements having respective light emission colors of R, G, and B may be separately formed, or an EL element with a single color may be formed without patterning and light emission of R, G, and B can be obtained by using a color filter. 
     Note that the structures shown in  FIGS. 104A and 104B  are examples, and various structures can be employed for a pixel layout, a cross-sectional structure, a stacking order of electrodes of an EL element, and the like, other than the structures shown in  FIGS. 104A and 104B . Further, as a light-emitting layer, various elements such as a crystalline element such as an LED, and an element formed using an inorganic thin film as well as the element formed using the organic thin film shown in the drawing can be used. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 14 
     In this embodiment mode, a structure of an EL element is described. In particular, a structure of an organic EL element is described. 
     A structure of a mixed junction EL element is described. As an example, a structure is described, which includes a layer (a mixed layer) in which a plurality of materials among a hole injecting material, a hole transporting material, a light-emitting material, an electron transporting material, an electron injecting material, and the like are mixed (hereinafter referred to as a mixed junction type EL element), which is different from a stacked-layer structure where a hole injecting layer formed of a hole injecting material, a hole transporting layer formed of a hole transporting material, a light-emitting layer formed of a light-emitting material, an electron transporting layer formed of an electron transporting material, an electron injecting layer formed of an electron injecting material, and the like are clearly distinguished. 
       FIGS. 105A to 105E  are schematic views each showing a structure of a mixed junction type EL element. Note that a layer interposed between an anode  190101  and a cathode  190102  corresponds to an EL layer. 
       FIG. 105A  shows a structure in which an EL layer includes a hole transporting region  190103  formed of a hole transporting material and an electron transporting region  190104  formed of an electron transporting material. The hole transporting region  190103  is closer to the anode than the electron transporting region  190104 . A mixed region  190105  including both the hole transporting material and the electron transporting material is provided between the hole transporting region  190103  and the electron transporting region  190104 . 
     In a direction from the anode  190101  to the cathode  190102 , a concentration of the hole transporting material in the mixed region  190105  is decreased and a concentration of the electron transporting material in the mixed region  190105  is increased. 
     Note that a concentration gradient can be freely set. For example, a ratio of concentrations of each functional material may be changed (a concentration gradient may be formed) in the mixed region  190105  including both the hole transporting material and the electron transporting material, without including the hole transporting region  190103  formed of only the hole transporting material. Alternatively, a ratio of concentrations of each functional material may be changed (a concentration gradient may be formed) in the mixed region  190105  including both the hole transporting material and the electron transporting material, without including the hole transporting region  190103  formed of only the hole transporting material and the electron transporting region  190104  formed of only the electron transporting material. Further alternatively, a ratio of concentrations may be changed depending on a distance from the anode or the cathode. Note that the ratio of concentrations may be changed continuously. 
     A region  190106  to which a light-emitting material is added is included in the mixed region  190105 . A light emission color of the EL element can be controlled by the light-emitting material. Further, carriers can be trapped by the light-emitting material. As the light-emitting material, various fluorescent dyes as well as a metal complex having a quinoline skeleton, a benzoxazole skeleton, or a benzothiazole skeleton can be used. The light emission color of the EL element can be controlled by adding the light-emitting material. 
     As the anode  190101 , an electrode material having a high work function is preferably used in order to inject holes efficiently. For example, a transparent electrode formed using indium tin oxide (ITO), indium zinc oxide (IZO), ZnO, SnO 2 , In 2 O 3 , or the like can be used. When a light-transmitting property is not needed, the anode  190101  may be formed using an opaque metal material. 
     As the hole transporting material, an aromatic amine compound or the like can be used. 
     As the electron transporting material, a metal complex having a quinoline derivative, 8-quinolinol, or a derivative thereof as a ligand (especially tris(8-quinolinolato)aluminum (Alq 3 )), or the like can be used. 
     As the cathode  190102 , an electrode material having a low work function is preferably used in order to inject electrons efficiently. A metal such as aluminum, indium, magnesium, silver, calcium, barium, or lithium can be used by itself. Alternatively, an alloy of the aforementioned metal or an alloy of the aforementioned metal and another metal may be used. 
       FIG. 105B  is the schematic view of the structure of the EL element, which is different from that of  FIG. 105A . Note that the same portions as those in  FIG. 105A  are denoted by the same reference numerals, and description thereof is omitted. 
     In  FIG. 105B , a region to which a light-emitting material is added is not included. However, when a material (electron-transporting and light-emitting material) having both an electron transporting property and a light-emitting property, for example, tris(8-quinolinolato)aluminum (Alq 3 ) is used as a material added to the electron transporting region  190104 , light emission can be performed. 
     Alternatively, as a material added to the hole transporting region  190103 , a material (a hole-transporting and light-emitting material) having both a hole transporting property and a light-emitting property may be used. 
       FIG. 105C  is the schematic view of the structure of the EL element, which is different from those of  FIGS. 105A and 105B . Note that the same portions as those in  FIGS. 105A and 105B  are denoted by the same reference numerals, and description thereof is omitted. 
     In  FIG. 105C , a region  190107  included in the mixed region  190105  is provided, to which a hole blocking material having a larger energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital than the hole transporting material is added. The region  190107  to which the hole blocking material is added is provided closer to the cathode  190102  than the region  190106  in the mixed region  190105 , to which the light-emitting material is added; thus, a recombination rate of carriers can be increased, and light emission efficiency can be increased. The structure provided with the region  190107  to which the hole blocking material is added is especially effective in an EL element which utilizes light emission (phosphorescence) by a triplet exciton. 
       FIG. 105D  is the schematic view of the structure of the EL element, which is different from those of  FIGS. 105A to 105C . Note that the same portions as those in  FIGS. 105A to 105C  are denoted by the same reference numerals, and description thereof is omitted. 
     In  FIG. 105D , a region  190108  included in the mixed region  190105  is provided, to which an electron blocking material having a larger energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital than the electron transporting material is added. The region  190108  to which the electron blocking material is added is provided closer to the anode  190101  than the region  190106  in the mixed region  190105 , to which the light-emitting material is added; thus, a recombination rate of carriers can be increased, and light emission efficiency can be increased. The structure provided with the region  190108  to which the electron blocking material is added is especially effective in an EL element which utilizes light emission (phosphorescence) by a triplet exciton. 
       FIG. 105E  is the schematic view of the structure of the mixed junction type EL element, which is different from those of  FIGS. 105A to 105D .  FIG. 105E  shows an example of a structure where a region  190109  to which a metal material is added is included in part of an EL layer in contact with an electrode of the EL element. In  FIG. 105E , the same portions as those in  FIGS. 105A to 105D  are denoted by the same reference numerals, and description thereof is omitted. In the structure shown in  FIG. 105E , MgAg (an Mg—Ag alloy) may be used as the cathode  190102 , and the region  190109  to which an Al (aluminum) alloy is added may be included in a region of the electron transporting region  190104  to which the electron transporting material is added, which is in contact with the cathode  190102 , for example. With the aforementioned structure, oxidation of the cathode can be prevented, and electron injection efficiency from the cathode can be increased. Accordingly, the lifetime of the mixed junction type EL element can be extended. Further, driving voltage can be lowered. 
     As a method for forming the mixed junction type EL element, a co-evaporation method or the like can be used. 
     In the mixed junction type EL elements as shown in  FIGS. 105A to 105E , a clear interface between the layers does not exist, and charge accumulation can be reduced. Accordingly, the lifetime of the EL element can be extended. Further, driving voltage can be lowered. 
     Note that the structures shown in  FIGS. 105A to 105E  can be implemented in free combination with each other. 
     In addition, a structure of the mixed junction type EL element is not limited to those described above. A known structure can be freely used. 
     An organic material which forms an EL layer of an EL element may be a low molecular material or a high molecular material. Alternatively, both the materials may be used. When a low molecular material is used for an organic compound material, a film can be formed by an evaporation method. When a high molecular material is used for the EL layer, the high molecular material is dissolved in a solvent and a film can be formed by a spin coating method or an inkjet method. 
     The EL layer may be formed using a middle molecular material. In this specification, a middle molecule organic light-emitting material refers to an organic light-emitting material without a sublimation property and with a polymerization degree of approximately 20 or less. When a middle molecular material is used for the EL layer, a film can be formed by an inkjet method or the like. 
     Note that a low molecular material, a high molecular material, and a middle molecular material may be used in combination. 
     An EL element may utilize either light emission (fluorescence) by a singlet exciton or light emission (phosphorescence) by a triplet exciton. 
     Next, an evaporation device for manufacturing a display device is described with reference to the drawings. 
     A display device may be manufactured to include an EL layer. The EL layer is formed including at least partially a material which exhibits electroluminescence. The EL layer may be formed of a plurality of layers having different functions. In this case, the EL layer may be formed of a combination of layers having different functions, which are also referred to as a hole injecting and transporting layer, a light-emitting layer, an electron injecting and transporting layer, and the like. 
       FIG. 106  shows a structure of an evaporation device for forming an EL layer over an element substrate provided with a transistor. In the evaporation device, a plurality of treatment chambers are connected to transfer chambers  190260  and  190261 . Each treatment chamber includes a loading chamber  190262  for supplying a substrate, an unloading chamber  190263  for collecting the substrate, a heat treatment chamber  190268 , a plasma treatment chamber  190272 , deposition treatment chambers  190269  to  190275  for depositing an EL material, and a deposition treatment chamber  190276  for forming a conductive film which is formed of aluminum or contains aluminum as its main component as one electrode of an EL element. Gate valves  190277   a  to  190277   m  are provided between the transfer chambers and the treatment chambers, so that the pressure in each treatment chamber can be controlled independently, and cross contamination between the treatment chambers is prevented. 
     A substrate introduced into the transfer chamber  190260  from the loading chamber  190262  is transferred to a predetermined treatment chamber by an arm type transfer means  190266  capable of rotating. The substrate is transferred from a certain treatment chamber to another treatment chamber by the transfer means  190266 . The transfer chambers  190260  and  190261  are connected by the deposition treatment chamber  190270  at which the substrate is transported by the transfer means  190266  and a transfer means  190267 . 
     Each treatment chamber connected to the transfer chambers  190260  and  190261  is maintained in a reduced pressure state. Accordingly, in the evaporation device, deposition treatment of an EL layer is continuously performed without exposing the substrate to the room air. A display panel in which formation of the EL layer is finished is deteriorated due to moisture or the like in some cases. Accordingly, in the evaporation device, a sealing treatment chamber  190265  for performing sealing treatment before exposure to the room air in order to maintain the quality is connected to the transfer chamber  190261 . Since the sealing treatment chamber  190265  is under atmospheric pressure or reduced pressure near atmospheric pressure, an intermediate treatment chamber  190264  is also provided between the transfer chamber  190261  and the sealing treatment chamber  190265 . The intermediate treatment chamber  190264  is provided for transporting the substrate and buffering the pressure between the chambers. 
     An exhaust means is provided in the loading chamber, the unloading chamber, the transfer chamber, and the deposition treatment chamber in order to maintain reduced pressure in the chamber. As the exhaust means, various vacuum pumps such as a dry pump, a turbo-molecular pump, and a diffusion pump can be used. 
     In the evaporation device in  FIG. 106 , the number of treatment chambers connected to the transfer chambers  190260  and  190261  and structures thereof can be combined as appropriate in accordance with a stacked-layer structure of the EL element. An example of a combination is described below. 
     In the heat treatment chamber  190268 , degasification treatment is performed by heating a substrate over which a lower electrode, an insulating partition wall, or the like is formed. In the plasma treatment chamber  190272 , a surface of the lower electrode is treated with a rare gas or oxygen plasma. This plasma treatment is performed for cleaning the surface, stabilizing a surface state, or stabilizing a physical or chemical state (e.g., a work function) of the surface. 
     The deposition treatment chamber  190269  is for forming an electrode buffer layer which is in contact with one electrode of the EL element. The electrode buffer layer has a carrier injection property (hole injection or electron injection) and suppresses generation of a short-circuit or a black spot defect of the EL element. Typically, the electrode buffer layer is formed of an organic-inorganic hybrid material, has a resistivity of 5×10 4  to 1×10 6 Ωcm, and is formed having a thickness of 30 to 300 nm. Note that the deposition treatment chamber  190271  is for forming a hole transporting layer. 
     A light-emitting layer in an EL element has a different structure between the case of emitting single color light and the case of emitting white light. Deposition treatment chambers in the evaporation device are preferably arranged depending on the structure. For example, when three kinds of EL elements each having a different light emission color are formed in a display panel, it is necessary to form light-emitting layers corresponding to respective light emission colors. In this case, the deposition treatment chamber  190270  can be used for forming a first light-emitting layer, the deposition treatment chamber  190273  can be used for forming a second light-emitting layer, and the deposition treatment chamber  190274  can be used for forming a third light-emitting layer. When different deposition treatment chambers are used for respective light-emitting layers, cross contamination due to different light-emitting materials can be prevented, and throughput of the deposition treatment can be improved. 
     Note that three kinds of EL elements each having a different light emission color may be sequentially deposited in each of the deposition treatment chambers  190270 ,  190273 , and  190274 . In this case, evaporation is performed by moving a shadow mask depending on a region to be deposited. 
     When an EL element which emits white light is formed, the EL element is formed by vertically stacking light-emitting layers of different light emission colors. In this case also, the element substrate can be sequentially transferred through the deposition treatment chambers so that each light-emitting layer is formed. Alternatively, different light-emitting layers can be formed continuously in the same deposition treatment chamber. 
     In the deposition treatment chamber  190276 , an electrode is formed over the EL layer. The electrode can be formed by an electron beam evaporation method or sputtering, and preferably by a resistance heating evaporation method. 
     The element substrate in which formation of the electrode is finished is transferred to the sealing treatment chamber  190265  through the intermediate treatment chamber  190264 . The sealing treatment chamber  190265  is filled with an inert gas such as helium, argon, neon, or nitrogen, and a sealing substrate is attached to a side of the element substrate where the EL layer is formed under the atmosphere so that the EL layer is sealed. In a sealed state, a space between the element substrate and the sealing substrate may be filled with an inert gas or a resin material. The sealing treatment chamber  190265  is provided with a dispenser which draws a sealing material, a mechanical element such as an arm or a fixing stage which fixes the sealing substrate to face the element substrate, a dispenser or a spin coater which fills the chamber with a resin material, or the like. 
       FIG. 107  shows an internal structure of a deposition treatment chamber. The deposition treatment chamber is maintained in a reduced pressure state. In  FIG. 107 , a space interposed between a top plate  190391  and a bottom plate  190392  corresponds to an internal space of the chamber, which is maintained in a reduced pressure state. 
     One or a plurality of evaporation sources are provided in the treatment chamber. This is because a plurality of evaporation sources are preferably provided when a plurality of layers having different compositions are formed or when different materials are co-evaporated. In  FIG. 107 , evaporation sources  190381   a ,  190381   b , and  190381   c  are attached to an evaporation source holder  190380 . The evaporation source holder  190380  is held by a multi-joint arm  190383 . The multi-joint arm  190383  allows the evaporation source holder  190380  to move within its movable range by stretching the joint. Alternatively, the evaporation source holder  190380  may be provided with a distance sensor  190382  to monitor a distance between the evaporation sources  190381   a  to  190381   c  and a substrate  190389  so that an optimal distance for evaporation is controlled. In this case, the multi-joint arm may be capable of moving toward upper and lower directions (Z direction) as well. 
     The substrate  190389  is fixed by using a substrate stage  190386  and a substrate chuck  190387  together. The substrate stage  190386  may have a structure where a heater is incorporated so that the substrate  190389  can be heated. The substrate  190389  is fixed to the substrate stage  190386  with the support of the substrate chuck  190387  and is transferred. At the time of evaporation, a shadow mask  190390  provided with an opening corresponding to an evaporation pattern can be used when needed. In this case, the shadow mask  190390  is arranged between the substrate  190389  and the evaporation sources  190381   a  to  190381   c . The shadow mask  190390  adheres to the substrate  190389  or is fixed to the substrate  190389  with a certain interval therebetween by a mask chuck  190388 . When alignment of the shadow mask  190390  is needed, the alignment is performed by arranging a camera in the treatment chamber and providing the mask chuck  190388  with a positioning means which slightly moves in X-Y-θ directions. 
     Each of the evaporation sources  190381   a  to  190381   c  is provided with an evaporation material supply means which continuously supplies an evaporation material to the evaporation source. The evaporation material supply means includes material supply sources  190385   a ,  190385   b , and  190385   c , which are provided apart from the evaporation sources  190381   a ,  190381   b , and  190381   c , and a material supply pipe  190384  which connects the evaporation source and the material supply source. Typically, the material supply sources  190385   a  to  190385   c  are provided corresponding to the evaporation sources  190381   a  to  190381   c . In  FIG. 74 , the material supply source  190385   a  corresponds to the evaporation source  190381   a , the material supply source  190385   b  corresponds to the evaporation source  190381   b , and the material supply source  190385   c  corresponds to the evaporation source  190381   c.    
     As a method for supplying an evaporation material, an airflow transfer method, an aerosol method, or the like can be employed. In an airflow transfer method, impalpable powder of an evaporation material is transferred in airflow to the evaporation sources  190381   a  to  190381   c  by using an inert gas or the like. In an aerosol method, evaporation is performed while material liquid in which an evaporation material is dissolved or dispersed in a solvent is transferred and aerosolized by an atomizer and the solvent in the aerosol is vaporized. In each case, the evaporation sources  190381   a  to  190381   c  are provided with a heating means, and a film is formed over the substrate  190389  by vaporizing the transferred evaporation material. In  FIG. 107 , the material supply pipe  190384  can be bent flexibly and is formed of a thin pipe which has enough rigidity not to be transformed even under reduced pressure. 
     When an airflow transfer method or an aerosol method is employed, film formation may be performed in the deposition treatment chamber under atmospheric pressure or lower, and preferably under a reduced pressure of 133 to 13300 Pa. The pressure can be adjusted while an inert gas such as helium, argon, neon, krypton, xenon, or nitrogen fills the deposition treatment chamber or is supplied (and exhausted at the same time) to the deposition treatment chamber. Note that an oxidizing atmosphere may be employed by introducing a gas such as oxygen or nitrous oxide in the deposition treatment chamber where an oxide film is formed. Alternately, a reducing atmosphere may be employed by introducing a gas such as hydrogen in the deposition treatment chamber where an organic material is deposited. 
     As another method for supplying an evaporation material, a screw may be provided in the material supply pipe  190384  to continuously push the evaporation material toward the evaporation source. 
     With this evaporation device, a film can be formed continuously with high uniformity even in the case of a large display panel. Since it is not necessary to supply an evaporation material to the evaporation source every time the evaporation material is run out, throughput can be improved. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 15 
     In this embodiment mode, a structure of an EL element is described. In particular, a structure of an inorganic EL element is described. 
     An inorganic EL element is classified as either a dispersion type inorganic EL element or a thin-film type inorganic EL element, depending on its element structure. These elements differ in that the former includes an electroluminescent layer in which particles of a light-emitting material are dispersed in a binder, whereas the latter includes an electroluminescent layer formed of a thin film of a light-emitting material. However, the former and the latter have in common in that they need electrons accelerated by a high electric field. Note that mechanisms for obtaining light emission are donor-acceptor recombination light emission which utilizes a donor level and an acceptor level; and localized light emission which utilizes inner-shell electron transition of a metal ion. In general, donor-acceptor recombination light emission is employed in dispersion type inorganic EL elements and localized light emission is employed in thin-film type inorganic EL elements in many cases. 
     A light-emitting material includes a base material and an impurity element to be a luminescence center. Light emission of various colors can be obtained by changing the impurity element to be included. The light-emitting material can be formed using various methods, such as a solid phase method or a liquid phase method (a coprecipitation method). Further, a liquid phase method such as a spray pyrolysis method, a double decomposition method, a method employing precursor pyrolysis, a reverse micelle method, a method in which one or more of these methods are combined with high-temperature baking, or a freeze-drying method, or the like can be used. 
     A solid phase method is a method in which a base material and an impurity element or a compound containing an impurity element are weighed, mixed in a mortar, and heated and baked in an electric furnace so as to be reacted; thus, the impurity element is included in the base material. The baking temperature is preferably 700 to 1500° C. This is because a solid-phase reaction does not proceed when the temperature is too low, and the base material decomposes when the temperature is too high. Note that although the materials may be baked in powder form, they are preferably baked in pellet form. Although a solid phase method needs a comparatively high temperature, it is a simple method, and thus has high productivity and is suitable for mass production. 
     A liquid phase method (a coprecipitation method) is a method in which a base material or a compound containing a base material, and an impurity element or a compound containing an impurity element are reacted in a solution, dried, and then baked. Particles of a light-emitting material are uniformly distributed, and the reaction can progress even when the particles are small and the baking temperature is low. 
     As a base material to be used for a light-emitting material, sulfide, oxide, or nitride can be used. As sulfide, zinc sulfide (ZnS), cadmium sulfide (CdS), calcium sulfide (CaS), yttrium sulfide (Y 2 S 3 ), gallium sulfide (Ga 2 S 3 ), strontium sulfide (SrS), barium sulfide (BaS), or the like can be used, for example. As oxide, zinc oxide (ZnO), yttrium oxide (Y 2 O 3 ), or the like can be used, for example. As nitride, aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), or the like can be used, for example. Further, zinc selenide (ZnSe), zinc telluride (ZnTe), or the like; or a ternary mixed crystal such as calcium gallium sulfide (CaGa 2 S 4 ), strontium gallium sulfide (SrGa 2 S 4 ), or barium gallium sulfide (BaGa 2 S 4 ) may be used. 
     As a luminescence center for localized light emission, manganese (Mn), copper (Cu), samarium (Sm), terbium (Tb), erbium (Er), thulium (Tm), europium (Eu), cerium (Ce), praseodymium (Pr), or the like can be used. Note that a halogen element such as fluorine (F) or chlorine (Cl) may be added for charge compensation. 
     On the other hand, as a luminescence center for donor-acceptor recombination light emission, a light-emitting material including a first impurity element forming a donor level and a second impurity element forming an acceptor level can be used. As the first impurity element, fluorine (F), chlorine (Cl), aluminum (Al), or the like can be used, for example. As the second impurity element, copper (Cu), silver (Ag), or the like can be used, for example. 
     When the light-emitting material for donor-acceptor recombination light emission is synthesized by a solid phase method, a base material, the first impurity element or a compound containing the first impurity element, and the second impurity element or a compound containing the second impurity element are weighed, mixed in a mortar, and heated and baked in an electric furnace. As the base material, the aforementioned base material can be used. As the first impurity element or the compound containing the first impurity element, fluorine (F), chlorine (Cl), aluminum sulfide (Al 2 S 3 ), or the like can be used, for example. As the second impurity element or the compound containing the second impurity element, copper (Cu), silver (Ag), copper sulfide (Cu 2 S), silver sulfide (Ag 2 S), or the like can be used, for example. The baking temperature is preferably 700 to 1500° C. This is because a solid-phase reaction does not proceed when the temperature is too low, and the base material decomposes when the temperature is too high. Note that although the materials may be baked in powder form, they are preferably baked in pellet form. 
     As the impurity element in the case of using a solid phase reaction, compounds including the first impurity element and the second impurity element may be used in combination. In this case, the impurity elements are easily diffused, and the solid phase reaction proceeds readily, so that a uniform light-emitting material can be obtained. Further, since an unnecessary impurity element is not included, a light-emitting material with high purity can be obtained. As the compound including the first impurity element and the second impurity element, copper chloride (CuCl), silver chloride (AgCl), or the like can be used, for example. 
     Note that the concentration of these impurity elements is in the range of 0.01 to 10 at. %, and is preferably in the range of 0.05 to 5 at. % with respect to the base material. 
     In the case of a thin-film type inorganic EL element, an electroluminescent layer includes the aforementioned light-emitting material, and can be formed using a physical vapor deposition (PVD) method such as sputtering or a vacuum evaporation method, for example, a resistance heating evaporation method or an electron beam evaporation (EB evaporation) method, a chemical vapor deposition (CVD) method such as a metal organic CVD method or a low-pressure hydride transport CVD method, an atomic layer epitaxy (ALE) method, or the like. 
       FIGS. 108A to 108C  each show an example of a thin-film type inorganic EL element which can be used as the light-emitting element. In  FIGS. 108A to 108C , a light-emitting element includes a first electrode layer  120100 , an electroluminescent layer  120102 , and a second electrode layer  120103 . 
     The light-emitting elements shown in  FIGS. 108B and 108C  each have a structure where an insulating film is provided between the electrode layer and the electroluminescent layer in the light-emitting element in  FIG. 108A . The light-emitting element shown in  FIG. 108B  includes an insulating film  120104  between the first electrode layer  120100  and the electroluminescent layer  120102 . The light-emitting element shown in  FIG. 108C  includes an insulating film  120105  between the first electrode layer  120100  and the electroluminescent layer  120102 , and an insulating film  120106  between the second electrode layer  120103  and the electroluminescent layer  120102 . 
     Note that the insulating film  120104  is provided so as to be in contact with the first electrode layer  120100  in  FIG. 61B ; however, the insulating film  120104  may be provided in contact with the second electrode layer  120103  by reversing the order of the insulating film and the electroluminescent layer. 
     In the case of a dispersion type inorganic EL, a film-shaped electroluminescent layer is formed by dispersing particulate light-emitting materials in a binder. When particles with a desired size cannot be sufficiently obtained by a method of forming the light-emitting material, the light-emitting materials may be processed into particles by being crushed in a mortar or the like. The binder is a substance for fixing the particulate light-emitting material in a dispersed state and maintaining the shape as the electroluminescent layer. The light-emitting material is uniformly dispersed in the electroluminescent layer and fixed by the binder. 
     In the case of a dispersion type inorganic EL, as a method of forming the electroluminescent layer, a droplet discharging method by which the electroluminescent layer can be selectively formed, a printing method (such as screen printing or offset printing), a coating method such as a spin coating method, a dipping method, a dispenser method, or the like can be used. The thickness of the electroluminescent layer is not particularly limited, but preferably in the range of 10 to 1000 nm. In the electroluminescent layer including the light-emitting material and the binder, a ratio of the light-emitting material is preferably equal to or more than 50 wt % and equal to or less than 80 wt %. 
       FIGS. 109A to 109C  each show an example of a dispersion type inorganic EL element which can be used as the light-emitting element. A light-emitting element in  FIG. 109A  has a stacked-layer structure of a first electrode layer  120200 , an electroluminescent layer  120202 , and a second electrode layer  120203 . The electroluminescent layer  120202  includes a light-emitting material  120201  held by a binder. 
     An insulating material can be used for the binder. As the insulating material, an organic material or an inorganic material can be used. Alternatively, a mixed material containing an organic material and an inorganic material may be used. As the organic insulating material, a polymer having a comparatively high dielectric constant, such as a cyanoethyl cellulose based resin, or a resin such as polyethylene, polypropylene, a polystyrene based resin, a silicone resin, an epoxy resin, or vinylidene fluoride can be used. Alternatively, a heat-resistant polymer such as aromatic polyamide or polybenzimidazole, or a siloxane resin may be used. Note that a siloxane resin corresponds to a resin having Si—O—Si bonds. Siloxane includes a skeleton structure of a bond of silicon (Si) and oxygen (O). As a substituent, an organic group containing at least hydrogen (such as an alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluoro group, or a fluoro group and an organic group containing at least hydrogen may be used as a substituent. Further alternately, a resin material, for example, a vinyl resin such as polyvinyl alcohol or polyvinylbutyral, a phenol resin, a novolac resin, an acrylic resin, a melamine resin, an urethane resin, or an oxazole resin (polybenzoxazole) may be used. A dielectric constant can be adjusted by appropriately mixing these resins with fine particles having a high dielectric constant, such as barium titanate (BaTiO 3 ) or strontium titanate (SrTiO 3 ). 
     The inorganic insulating material included in the binder can be formed using silicon oxide (SiO x ), silicon nitride (SiN x ), silicon containing oxygen and nitrogen, aluminum nitride (AlN), aluminum containing oxygen and nitrogen, aluminum oxide (Al 2 O 3 ) containing oxygen and nitrogen, titanium oxide (TiO 2 ), BaTiO 3 , SrTiO 3 , lead titanate (PbTiO 3 ), potassium niobate (KNbO 3 ), lead niobate (PbNbO 3 ), tantalum oxide (Ta 2 O 5 ), barium tantalite (BaTa 2 O 6 ), lithium tantalite (LiTaO 3 ), yttrium oxide (Y 2 O 3 ), zirconium oxide (ZrO 2 ), ZnS, or a substance containing another inorganic insulating material. When an inorganic material having a high dielectric constant is included in the organic material (by addition or the like), the dielectric constant of the electroluminescent layer formed of the light-emitting material and the binder can be more effectively controlled, and the dielectric constant can be further increased. 
     In a manufacturing step, the light-emitting material is dispersed in a solution containing the binder. As a solvent for the solution containing the binder, it is acceptable as long as a solvent dissolves a binder material and can make a solution having a viscosity suitable for a method of forming the electroluminescent layer (various wet processes) and for desired film thickness. For example, an organic solvent or the like can be used as the solvent. When a siloxane resin is used as the binder, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate (also referred to as PGMEA), 3-methoxy-3-methyl-1-butanol (also referred to as MMB), or the like can be used as the solvent. 
     The light-emitting elements shown in  FIGS. 109B and 109C  each have a structure where an insulating film is provided between the electrode layer and the electroluminescent layer in the light-emitting element in  FIG. 109A . The light-emitting element shown in  FIG. 109B  includes an insulating film  120204  between the first electrode layer  120200  and the electroluminescent layer  120202 . The light-emitting element shown in  FIG. 109C  includes an insulating film  120205  between the first electrode layer  120200  and the electroluminescent layer  120202 , and an insulating film  120206  between the second electrode layer  120203  and the electroluminescent layer  120202 . In such a manner, the insulating film may be provided between the electroluminescent layer and one of the electrode layers interposing the electroluminescent layer, or may be provided between the electroluminescent layer and each of the electrode layers interposing the electroluminescent layer. The insulating film may be a single layer or stacked layers including a plurality of layers. 
     Although the insulating film  120204  is provided in contact with the first electrode layer  120200  in  FIG. 109B , the insulating film  120204  may be provided in contact with the second electrode layer  120203  by reversing the order of the insulating film and the electroluminescent layer. 
     A material used for an insulating film such as the insulating film  120104  in  FIG. 108B  and the insulating film  120204  in  FIG. 109B  preferably has high withstand voltage and dense film quality. Further, the material preferably has a high dielectric constant. For example, silicon oxide (SiO 2 ), yttrium oxide (Y 2 O 3 ), titanium oxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), hafnium oxide (HfO 2 ), tantalum oxide (Ta 2 O 5 ), barium titanate (BaTiO 3 ), strontium titanate (SrTiO 3 ), lead titanate (PbTiO 3 ), silicon nitride (Si 3 N 4 ), zirconium oxide (ZrO 2 ), or the like; or a mixed film of these materials or a stacked-layer film including two or more of those materials can be used. The insulating film can be formed by sputtering, evaporation, CVD, or the like. The insulating film may be formed by dispersing particles of the insulating material in a binder. A binder material may be formed using a material and a method similar to those of the binder contained in the electroluminescent layer. The thickness of the insulating film is not particularly limited, but preferably in the range of 10 to 1000 nm. 
     Note that the light-emitting element can emit light when voltage is applied between the pair of electrode layers interposing the electroluminescent layer. The light-emitting element can operate with DC drive or AC drive. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 16 
     In this embodiment mode, an example of a display device is described. In particular, the case where optical treatment is performed is described. 
     A rear projection display device  130100  in  FIGS. 110A and 110B  is provided with a projector unit  130111 , a mirror  130112 , and a screen panel  130101 . The rear projection display device  130100  may also be provided with a speaker  130102  and operation switches  130104 . The projector unit  130111  is provided at a lower portion of a housing  130110  of the rear projection display device  130100 , and projects incident light which projects an image based on a video signal to the mirror  130112 . The rear projection display device  130100  displays an image projected from a rear surface of the screen panel  130101 . 
       FIG. 111  shows a front projection display device  130200 . The front projection display device  130200  is provided with the projector unit  130111  and a projection optical system  130201 . The projection optical system  130201  projects an image to a screen or the like provided at the front. 
     The structure of the projector unit  130111  which is applied to the rear projection display device  130100  in  FIGS. 110A and 110B  and the front projection display device  130200  in  FIG. 111  is described below. 
       FIG. 112  shows a structure example of the projector unit  130111 . The projector unit  130111  is provided with a light source unit  130301  and a modulation unit  130304 . The light source unit  130301  is provided with a light source optical system  130303  including lenses and a light source lamp  130302 . The light source lamp  130302  is stored in a housing so that stray light is not scattered. As the light source lamp  130302 , a high-pressure mercury lamp or a xenon lamp, for example, which can emit a large amount of light, is used. The light source optical system  130303  is provided with an optical lens, a film having a function of polarizing light, a film for adjusting phase difference, an IR film, or the like as appropriate. The light source unit  130301  is provided so that emitted light is incident on the modulation unit  130304 . The modulation unit  130304  is provided with a plurality of display panels  130308 , a color filter, a dichroic mirror  130305 , a total reflection mirror  130306 , a prism  130309 , and a projection optical system  130310 . Light emitted from the light source unit  130301  is split into a plurality of optical paths by the dichroic mirror  130305 . 
     The display panel  130308  and a color filter which transmits light with a predetermined wavelength or wavelength range are provided in each optical path. The transmissive display panel  130308  modulates transmitted light based on a video signal. Light of each color transmitted through the display panel  130308  is incident on the prism  130309 , and an image is displayed on a screen through the projection optical system  130310 . Note that a Fresnel lens may be provided between the mirror and the screen. Then, projected light which is projected by the projector unit  130111  and reflected by the mirror is converted into generally parallel light by the Fresnel lens and projected on the screen. 
       FIG. 113  shows the projector unit  130111  provided with reflective display panels  130407 ,  130408 , and  130409 . 
     The projector unit  130111  shown in  FIG. 113  includes the light source unit  130301  and a modulation unit  130400 . The light source unit  130301  may have a structure similar to the structure of  FIG. 112 . Light from the light source unit  130301  is split into a plurality of optical paths by dichroic mirrors  130401  and  130402  and a total reflection mirror  130403  to be incident on polarization beam splitters  130404 ,  130405 , and  130406 . The polarization beam splitters  130404 ,  130405 , and  130406  are provided corresponding to the reflective display panels  130407 ,  130408 , and  130409  which correspond to respective colors. The reflective display panels  130407 ,  130408 , and  130409  modulate reflected light based on a video signal. Light of respective colors which is reflected by the reflective display panels  130407 ,  130408 , and  130409  is incident on the prism  130109  to be synthesized, and projected through a projection optical system  130411 . 
     Among light emitted from the light source unit  130301 , only light in a wavelength region of red is transmitted through the dichroic mirror  130401  and light in wavelength regions of green and blue is reflected by the dichroic mirror  130401 . Further, only the light in the wavelength region of green is reflected by the dichroic mirror  130402 . The light in the wavelength region of red, which is transmitted through the dichroic mirror  130401 , is reflected by the total reflection mirror  130403  and incident on the polarization beam splitter  130404 . The light in the wavelength region of blue is incident on the polarization beam splitter  130405 . The light in the wavelength region of green is incident on the polarization beam splitter  130406 . The polarization beam splitters  130404 ,  130405 , and  130406  have a function of splitting incident light into p-polarized light and s-polarized light and a function of transmitting only p-polarized light. The reflective display panels  130407 ,  130408 , and  130409  polarize incident light based on a video signal. 
     Only s-polarized light corresponding to respective colors is incident on the reflective display panels  130407 ,  130408 , and  130409  corresponding to respective colors. Note that the reflective display panels  130407 ,  130408 , and  130409  may be liquid crystal panels. In this case, the liquid crystal panel operates in an electrically controlled birefringence (ECB) mode. Liquid crystal molecules are vertically aligned with respect to a substrate at a certain angle. Accordingly, in the reflective display panels  130407 ,  130408 , and  130409 , when a pixel is in an off state, display molecules are aligned so as to reflect incident light without changing a polarization state of the incident light. When the pixel is in an on state, alignment of the display molecules is changed, and the polarization state of the incident light is changed. 
     The projector unit  130111  in  FIG. 113  can be applied to the rear projection display device  130100  in  FIGS. 110A and 110B  and the front projection display device  130200  in  FIG. 111 . 
       FIGS. 114A to 114C  show single-panel type projector units. The projector unit  130111  shown in  FIG. 114A  includes the light source unit  130301 , a display panel  130507 , a projection optical system  130511 , and a retardation plate  130504 . The projection optical system  130511  includes one or a plurality of lenses. The display panel  130507  may include a color filter. 
       FIG. 114B  shows a structure of the projector unit  130111  operating in a field sequential mode. A field sequential mode refers to a mode in which color display is performed by light of respective colors such as red, green, and blue sequentially incident on a display panel with a time lag, without a color filter. High-definition image can be displayed particularly by combination with a display panel with high-speed response to change in input signal. In  FIG. 114B , a rotating color filter plate  130505  including a plurality of color filters with red, green, blue, or the like is provided between the light source unit  130301  and a display panel  130508 . 
       FIG. 114C  shows a structure of the projector unit  130111  with a color separation method using a micro lens, as a color display method. This method refers to a method in which color display is realized by providing a micro lens array  130506  on a light incident side of a display panel  130509  and emitting light of each color from each direction. The projector unit  130111  employing this method has little loss of light due to a color filter, so that light from the light source unit  130301  can be efficiently utilized. The projector unit  130111  shown in  FIG. 114C  includes dichroic mirrors  130501 ,  130502 , and  130503  so that light of each color is lit to the display panel  130509  from each direction. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     Embodiment Mode 17 
     In this embodiment mode, examples of electronic devices are described. 
       FIG. 115  shows a display panel module in which a display panel  900101  and a circuit board  900111  are combined. The display panel  900101  includes a pixel portion  900102 , a scan line driver circuit  900103 , and a signal line driver circuit  900104 . The circuit board  900111  is provided with a control circuit  900112 , a signal dividing circuit  900113 , and the like, for example. The display panel  900101  and the circuit board  900111  are connected by a connection wiring  900114 . As the connection wiring  900114 , an FPC or the like can be used. 
     In the display panel  900101 , the pixel portion  900102  and part of peripheral driver circuits (a driver circuit having low operation frequency among a plurality of driver circuits) may be formed over the same substrate by using transistors, and another part of the peripheral driver circuits (a driver circuit having high operation frequency among the plurality of driver circuits) may be formed over an IC chip. The IC chip may be mounted on the display panel  900101  by COG (chip on glass) or the like. Thus, the area of the circuit board  900111  can be reduced, so that a small display device can be obtained. Alternatively, the IC chip may be mounted on the display panel  900101  by using TAB (tape automated bonding) or a printed circuit board. Thus, the area of the circuit board  900111  can be reduced, so that a display device with a narrower frame can be obtained. 
     For example, in order to reduce power consumption, a pixel portion may be formed over a glass substrate by using transistors, and all peripheral circuits may be formed over an IC chip. The IC chip may be mounted on a display panel by COG or TAB. 
     A television receiver can be completed with the display panel module shown in  FIG. 115 .  FIG. 116  is a block diagram showing a main structure of a television receiver. A tuner  900201  receives a video signal and an audio signal. The video signal is processed by a video signal amplifier circuit  900202 , a video signal processing circuit  900203  for converting a signal output from the video signal amplifier circuit  900202  into a color signal corresponding to each color of red, green, and blue, and a control circuit  900212  for converting the video signal into a signal which meets input specifications of a driver circuit. The control circuit  900212  outputs signals to a scan line side and a signal line side. In the case of digital driving, a structure may be used in which a signal dividing circuit  900213  is provided on the signal line side and an input digital signal is divided into m (m is a positive integer) pieces to be supplied. 
     Among the signals received by the tuner  900201 , the audio signal is transmitted to an audio signal amplifier circuit  900205 , and output thereof is supplied to a speaker  900207  through an audio signal processing circuit  900206 . A control circuit  900208  receives control information on a receiving station (reception frequency) and sound volume from an input portion  900209 , and transmits a signal to the tuner  900201  or the audio signal processing circuit  900206 . 
       FIG. 117A  shows a television receiver incorporated with a display panel module which is different from that of  FIG. 116 . In  FIG. 117A , a display screen  900302  stored in a housing  900301  is formed using the display panel module. Note that speakers  900303 , operation switches  900304 , an input means  900305 , a sensor  900306  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  900307 , or the like may be provided as appropriate. 
       FIG. 117B  shows a television receiver, only a display of which can be carried wirelessly. A battery and a signal receiver are incorporated in a housing  900312 . The battery drives a display portion  900313 , speaker portions  900317 , a sensor  900319  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), and a microphone  900320 . Electricity can be repeatedly stored in the battery by a charger  900310 . The charger  900310  can transmit and receive a video signal and can transmit the video signal to the signal receiver of the display. The device shown in  FIG. 117B  is controlled by operation keys  900316 . Alternatively, the device shown in  FIG. 117B  can transmit a signal to the charger  900310  by operating the operation keys  900316 . That is, the device may be an image audio two-way communication device. Further alternatively, the device shown in  FIG. 117B  can transmit a signal to the charger  900310  by operating the operation keys  900316 , and can control communication of another electronic device when the electronic device is made to receive a signal which can be transmitted from the charger  900310 . That is, the device may be a general-purpose remote control device. Note that an input means  900318  or the like may be provided as appropriate. Note that the contents (or may be part of the contents) described in each drawing of this embodiment mode can be applied to the display portion  900313 . 
       FIG. 118A  shows a module in which a display panel  900401  and a printed wiring board  900402  are combined. The display panel  900401  may be provided with a pixel portion  900403  including a plurality of pixels, a first scan line driver circuit  900404 , a second scan line driver circuit  900405 , and a signal line driver circuit  900406  which supplies a video signal to a selected pixel. 
     The printed wiring board  900402  is provided with a controller  900407 , a central processing unit (CPU)  900408 , a memory  900409 , a power supply circuit  900410 , an audio processing circuit  900411 , a transmitting/receiving circuit  900412 , and the like. The printed wiring board  900402  and the display panel  900401  are connected by a flexible printed circuit (FPC)  900413 . The flexible printed circuit (FPC)  900413  may be provided with a storage capacitor, a buffer circuit, or the like so as to prevent noise on power supply voltage or a signal, and increase in rise time of a signal. Note that the controller  900407 , the audio processing circuit  900411 , the memory  900409 , the central processing unit (CPU)  900408 , the power supply circuit  900410 , and the like can be mounted on the display panel  900401  by using a COG (chip on glass) method. When a COG method is used, the size of the printed wiring board  900402  can be reduced. 
     Various control signals are input and output through an interface (I/F) portion  900414  provided for the printed wiring board  900402 . In addition, an antenna port  900415  for transmitting and receiving a signal to/from an antenna is provided for the printed wiring board  900402 . 
       FIG. 118B  is a block diagram of the module shown in  FIG. 118A . The module includes a VRAM  900416 , a DRAM  900417 , a flash memory  900418 , and the like as the memory  900409 . The VRAM  900416  stores data on an image displayed on the panel. The DRAM  900417  stores video data or audio data. The flash memory  900418  stores various programs. 
     The power supply circuit  900410  supplies electric power for operating the display panel  900401 , the controller  900407 , the central processing unit (CPU)  900408 , the audio processing circuit  900411 , the memory  900409 , and the transmitting/receiving circuit  900412 . Note that depending on panel specifications, the power supply circuit  900410  is provided with a current source in some cases. 
     The central processing unit (CPU)  900408  includes a control signal generation circuit  900420 , a decoder  900421 , a register  900422 , an arithmetic circuit  900423 , a RAM  900424 , an interface (I/F) portion  900419  for the central processing unit (CPU)  900408 , and the like. Various signals which are input to the central processing unit (CPU)  900408  through the interface (I/F) portion  900414  are once stored in the register  900422 , and then input to the arithmetic circuit  900423 , the decoder  900421 , and the like. The arithmetic circuit  900423  performs operation based on the input signal so as to designate a location to which various instructions are sent. On the other hand, the signal input to the decoder  900421  is decoded and input to the control signal generation circuit  900420 . The control signal generation circuit  900420  generates a signal including various instructions based on the input signal, and transmits the signal to locations designated by the arithmetic circuit  900423 , specifically the memory  900409 , the transmitting/receiving circuit  900412 , the audio processing circuit  900411 , the controller  900407 , and the like. 
     The memory  900409 , the transmitting/receiving circuit  900412 , the audio processing circuit  900411 , and the controller  900407  operate in accordance with respective instructions. Operations thereof are briefly described below. 
     A signal input from an input means  900425  is transmitted to the central processing unit (CPU)  900408  mounted on the printed wiring board  900402  through the interface (I/F) portion  900414 . The control signal generation circuit  900420  converts image data stored in the VRAM  900416  into a predetermined format based on the signal transmitted from the input means  900425  such as a pointing device or a keyboard, and transmits the converted data to the controller  900407 . 
     The controller  900407  performs data processing of the signal including the image data transmitted from the central processing unit (CPU)  900408  in accordance with the panel specifications, and supplies the signal to the display panel  900401 . The controller  900407  generates an Hsync signal, a V sync  signal, a clock signal (CLK), alternating voltage (AC Cont), and a switching signal L/R based on power supply voltage input from the power supply circuit  900410  or various signals input from the central processing unit (CPU)  900408 , and supplies the signals to the display panel  900401 . 
     The transmitting/receiving circuit  900412  processes a signal which is transmitted and received as a radio wave by an antenna  900428 . Specifically, the transmitting/receiving circuit  900412  may include a high-frequency circuit such as an isolator, a band pass filter, a VCO (voltage controlled oscillator), an LPF (low pass filter), a coupler, or a balun. Among signals transmitted and received by the transmitting/receiving circuit  900412 , a signal including audio information is transmitted to the audio processing circuit  900411  in accordance with an instruction from the central processing unit (CPU)  900408 . 
     The signal including the audio information, which is transmitted in accordance with the instruction from the central processing unit (CPU)  900408 , is demodulated into an audio signal by the audio processing circuit  900411  and is transmitted to a speaker  900427 . An audio signal transmitted from a microphone  900426  is modulated by the audio processing circuit  900411  and is transmitted to the transmitting/receiving circuit  900412  in accordance with an instruction from the central processing unit (CPU)  900408 . 
     The controller  900407 , the central processing unit (CPU)  900408 , the power supply circuit  900410 , the audio processing circuit  900411 , and the memory  900409  can be mounted as a package of this embodiment mode. 
     Needless to say, the present invention is not limited to the television receiver, and can be applied to various uses particularly as a large display medium such as an information display board at a train station, an airport, or the like, or an advertisement display board on the street, as well as a monitor of a personal computer. 
     Next, a structural example of a mobile phone is described with reference to  FIG. 119 . 
     A display panel  900501  is incorporated in a housing  900530  so as to be detachable. The shape and the size of the housing  900530  can be changed as appropriate in accordance with the size of the display panel  900501 . The housing  900530  to which the display panel  900501  is fixed is fitted into a printed circuit board  900531  and is assembled as a module. 
     The display panel  900501  is connected to the printed wiring board  900531  through an FPC  900513 . The printed wiring board  900531  is provided with a speaker  900532 , a microphone  900533 , a transmitting/receiving circuit  900534 , a signal processing circuit  900535  including a CPU, a controller, and the like, and a sensor  900541  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray). Such a module, an input means  900536 , and a battery  900537  are combined and stored in a housing  900539 . A pixel portion of the display panel  900501  is provided so as to be seen from an opening window formed in the housing  900539 . 
     In the display panel  900501 , the pixel portion and part of peripheral driver circuits (a driver circuit having low operation frequency among a plurality of driver circuits) may be formed over the same substrate by using transistors, and another part of the peripheral driver circuits (a driver circuit having high operation frequency among the plurality of driver circuits) may be formed over an IC chip. The IC chip may be mounted on the display panel  900501  by COG (chip on glass). Alternatively, the IC chip may be connected to a glass substrate by using TAB (tape automated bonding) or a printed circuit board. With such a structure, power consumption of the mobile phone can be reduced, so that operation time of the mobile phone per charge can be extended. In addition, cost of the mobile phone can be reduced. 
     The mobile phone shown in  FIG. 119  has various functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image); a function of displaying a calendar, a date, time, or the like on a display portion; a function of operating or editing the information displayed on the display portion; a function of controlling processing by a variety of software (programs); a wireless communication function; a function of communicating with another mobile phone, a fixed phone, or an audio communication device by using the wireless communication function; a function of connecting with a variety of computer networks by using the wireless communication function; a function of transmitting or receiving a variety of data by using the wireless communication function; a function of operating a vibrator in accordance with incoming call, reception of data, or an alarm; and a function of generating a sound in accordance with incoming call, reception of data, or an alarm. Note that functions of the mobile phone shown in  FIG. 119  are not limited to them, and the mobile phone can have various functions. 
     In a mobile phone shown in  FIG. 120 , a main body (A)  900601  which is provided with operation switches  900604 , a microphone  900605 , and the like is connected to a main body (B)  900602  which is provided with a display panel (A)  900608 , a display panel (B)  900609 , a speaker  900606 , a sensor  900611  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), an input means  900612 , and the like by using a hinge  900610  so that the mobile phone can be opened and closed. The display panel (A)  900608  and the display panel (B)  900609  are stored in a housing  900603  of the main body (B)  900602  together with a circuit board  900607 . Each of pixel portions of the display panel (A)  900608  and the display panel (B)  900609  is provided so as to be seen from an opening window formed in the housing  900603 . 
     Specifications of the display panel (A)  900608  and the display panel (B)  900609 , such as the number of pixels, can be set as appropriate in accordance with functions of a mobile phone  900600 . For example, the display panel (A)  900608  can be used as a main screen and the display panel (B)  900609  can be used as a sub-screen. 
     Each of the mobile phones of this embodiment mode can be changed in various modes depending on functions or applications thereof. For example, it may be a camera-equipped mobile phone by incorporating an imaging element in a portion of the hinge  900610 . When the operation switches  900604 , the display panel (A)  900608 , and the display panel (B)  900609  are stored in one housing, the above-described advantageous effects can be obtained. Further, similar advantageous effects can be obtained when the structure of this embodiment mode is applied to an information display terminal provided with a plurality of display portions. 
     The mobile phone shown in  FIG. 120  has various functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image); a function of displaying a calendar, a date, time, or the like on a display portion; a function of operating or editing the information displayed on the display portion; a function of controlling processing by a variety of software (programs); a wireless communication function; a function of communicating with another mobile phone, a fixed phone, or an audio communication device by using the wireless communication function; a function of connecting with a variety of computer networks by using the wireless communication function; a function of transmitting or receiving a variety of data by using the wireless communication function; a function of operating a vibrator in accordance with incoming call, reception of data, or an alarm; and a function of generating a sound in accordance with incoming call, reception of data, or an alarm. Note that functions of the mobile phone shown in  FIG. 120  are not limited to them, and the mobile phone can have various functions. 
     The contents (or may be part of the contents) described in each drawing of this embodiment mode can be applied to various electronic devices. Specifically, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be applied to display portions of electronic devices. Examples of such electronic devices are a video camera, a digital camera, a goggle-type display, a navigation system, an audio reproducing device (e.g., a car audio component or an audio component), a computer, a game machine, a portable information terminal (e.g., a mobile computer, a mobile phone, a mobile game machine, or an electronic book), an image reproducing device provided with a recording medium (specifically, a device which reproduces a recording medium such as a digital versatile disc (DVD) and has a display for displaying a reproduced image), and the like. 
       FIG. 121A  shows a display, which includes a housing  900711 , a support base  900712 , a display portion  900713 , an input means  900714 , a sensor  900715  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  900716 , a speaker  900717 , operation keys  900718 , an LED lamp  900719 , and the like. The display shown in  FIG. 121A  has a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion. Note that the display shown in  FIG. 121A  is not limited to having this function, and can have various functions. 
       FIG. 121B  shows a camera, which includes a main body  900731 , a display portion  900732 , an image receiving portion  900733 , operation keys  900734 , an external connection port  900735 , a shutter button  900736 , an input means  900737 , a sensor  900738  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  900739 , a speaker  900740 , an LED lamp  900741 , and the like. The camera shown in  FIG. 121B  has a function of photographing a still image and a moving image; a function of automatically correcting the photographed image (the still image or the moving image); a function of storing the photographed image in a recording medium (provided outside or incorporated in the camera); and a function of displaying the photographed image on the display portion. Note that the camera shown in  FIG. 121B  is not limited to having these functions, and can have various functions. 
       FIG. 121C  shows a computer, which includes a main body  900751 , a housing  900752 , a display portion  900753 , a keyboard  900754 , an external connection port  900755 , a pointing device  900756 , an input means  900757 , a sensor  900758  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  900759 , a speaker  900760 , an LED lamp  900761 , a reader/writer  900762 , and the like. The computer shown in  FIG. 121C  has a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion; a function of controlling processing by a variety of software (programs); a communication function such as wireless communication or wire communication; a function of connecting to various computer networks by using the communication function; and a function of transmitting or receiving a variety of data by using the communication function. Note that the computer shown in  FIG. 121C  is not limited to having these functions, and can have various functions. 
       FIG. 128A  shows a mobile computer, which includes a main body  901411 , a display portion  901412 , a switch  901413 , operation keys,  901414 , an infrared port  901415 , an input means  901416 , a sensor  901417  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  901418 , a speaker  901419 , an LED lamp  901420 , and the like. The mobile computer shown in  FIG. 128A  has a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion; a touch panel function on the display portion; a function of displaying a calendar, a date, the time, and the like on the display portion; a function of controlling processing by a variety of software (programs); a wireless communication function; a function of connecting to various computer networks by using the wireless communication function; and a function of transmitting or receiving a variety of data by using the wireless communication function. Note that the mobile computer shown in  FIG. 128A  is not limited to having these functions, and can have various functions. 
       FIG. 128B  shows a portable image reproducing device provided with a recording medium (e.g., a DVD reproducing device), which includes a main body  901431 , a housing  901432 , a display portion A  901433 , a display portion B  901434 , a recording medium (e.g., DVD) reading portion  901435 , operation keys  901436 , a speaker portion  901437 , an input means  901438 , a sensor  901439  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  901440 , an LED lamp  901441 , and the like. The display portion A  901433  can mainly display image information, and the display portion B  901434  can mainly display text information. 
       FIG. 128C  shows a goggle-type display, which includes a main body  901451 , a display portion  901452 , an earphone  901453 , a support portion  901454 , an input means  901455 , a sensor  901456  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  901457 , a speaker  901458 , and the like. The goggle-type display shown in  FIG. 128C  has a function of displaying an image (e.g., a still image, a moving image, or a text image) which is externally obtained on the display portion. Note that the goggle-type display shown in  FIG. 128C  is not limited to having these functions, and can have various functions. 
       FIG. 129A  shows a portable game machine, which includes a housing  901511 , a display portion  901512 , speaker portions  901513 , operation keys  901514 , a recording medium insert portion  901515 , an input means  901516 , a sensor  901517  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  901518 , an LED lamp  901519 , and the like. The portable game machine shown in  FIG. 129A  has a function of reading a program or data stored in the recording medium to display on the display portion, and a function of sharing information with another portable game machine by wireless communication. Note that the portable game machine shown in  FIG. 129A  is not limited to having these functions, and can have various functions. 
       FIG. 129B  shows a digital camera having a television reception function, which includes a main body  901531 , a display portion  901532 , operation keys  901533 , a speaker  901534 , a shutter button  901535 , an image receiving portion  901536 , an antenna  901537 , an input means  901538 , a sensor  901539  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  901540 , an LED lamp  901541 , and the like. The digital camera having the television reception function shown in  FIG. 129B  has a function of photographing a still image and a moving image; a function of automatically correcting the photographed image; a function of obtaining a variety of information from the antenna; a function of storing the photographed image or the information obtained from the antenna; and a function of displaying the photographed image or the information obtained from the antenna on the display portion. Note that the digital camera having the television reception function shown in  FIG. 129B  is not limited to having these functions, and can have various functions. 
       FIG. 130  shows a portable game machine, which includes a housing  901611 , a first display portion  901612 , a second display portion  901613 , speaker portions  901614 , operation keys  901615 , a recording medium insert portion  901616 , an input means  901617 , a sensor  901618  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical reaction, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, smell, or infrared ray), a microphone  901619 , an LED lamp  901620 , and the like. The portable game machine shown in  FIG. 130  has a function of reading a program or data stored in the recording medium to display on the display portion, and a function of sharing information with another portable game machine by wireless communication. Note that the portable game machine shown in  FIG. 130  is not limited to having these functions, and can have various functions. 
     As shown in  FIGS. 121A to 121C ,  FIGS. 128A to 128C ,  FIGS. 129A to 129C , and  FIG. 130 , an electronic device includes a display portion for displaying some information. The electronic device can include a display portion having a wide viewing angle. 
     Next, an application of a semiconductor device is described. 
       FIG. 122  shows an example in which the semiconductor device is incorporated in a structure.  FIG. 122  shows a housing  900810 , a display panel  900811 , a remote controller  900812  which is an operation portion, a speaker portion  900813 , and the like. The semiconductor device is incorporated in the structure as a wall-hanging type, so that the semiconductor device can be provided without requiring a wide space. 
       FIG. 123  shows another example in which the semiconductor device is incorporated in a structure. A display panel  900901  is incorporated in a prefabricated bath unit  900902 , so that a bather can view the display panel  900901 . The display panel  900901  has a function of displaying information by an operation of the bather. The display panel  900901  can be utilized for advertisement or an amusement means. 
     Note that the semiconductor device can be provided in various places as well as on a sidewall of the prefabricated bath unit  900902  shown in  FIG. 123 . For example, the semiconductor device may be incorporated in part of a mirror or the bathtub itself. At this time, the shape of the display panel  900901  may be a shape in accordance with the mirror or the bathtub. 
       FIG. 124  shows another example in which the semiconductor device is incorporated in a structure. Display panels  901002  are curved in accordance with curved surfaces of columnar objects  901001 . Note that here, the columnar objects  901001  are described as telephone poles. 
     The display panels  901002  shown in  FIG. 124  are provided in positions higher than a human eye level. When the display panels  901002  are provided for structures standing outside to each other in large numbers, such as telephone poles, advertisement can be performed to an unspecified number of viewers. Here, since the display panels  901002  can easily display the same images by control from outside and can easily switch images instantly, extremely effective information display and advertising effects can be expected. When self-luminous display elements are provided in the display panels  901002 , the display panels  901002  are effectively used as highly visible display media even at night. When the display panels  901002  are provided for the telephone poles, power supply means of the display panels  901002  can be easily secured. In an emergency such as a disaster, the display panels  901002  can be means for quickly transmitting precise information to victims. 
     Note that as each of the display panels  901002 , a display panel in which a display element is driven by providing a switching element such as an organic transistor over a film-shaped substrate so that an image is displayed can be used. 
     Note that although this embodiment describes the wall, the prefabricated bath unit, and the columnar object as examples of the structure, this embodiment mode is not limited to this, and the semiconductor device can be provided for various structures. 
     Next, an example is described in which the semiconductor device is incorporated in a moving object. 
       FIG. 125  shows an example in which the semiconductor device is incorporated in a car. A display panel  901102  is incorporated in a car body  901101  of the car and can display information on an operation of the car or information input from inside or outside of the car on an on-demand basis. Note that the display panel  901102  may have a navigation function. 
     Note that the semiconductor device can be provided in various positions as well as the car body  901101  shown in  FIG. 125 . For example, the semiconductor device may be incorporated in a glass window, a door, a steering wheel, a shift lever, a seat, a room mirror, or the like. At this time, the shape of the display panel  901102  may be a shape in accordance with a shape of an object in which the display panel  901102  is provided. 
       FIGS. 126A and 126B  each show an example in which the semiconductor device is incorporated in a train car. 
       FIG. 126A  shows an example in which display panels  901202  are provided for glasses of a door  901201  of the train car. The display panels  901202  have an advantage over conventional paper-based advertisement that labor cost which is necessary for switching advertisement is not needed. Since the display panels  901202  can instantly switch images displayed on display portions by external signals, images on the display panels can be switched as the type of train passenger changes in accordance with different time periods, for example, so that a more effective advertising effect can be expected. 
       FIG. 126B  shows an example in which display panels  901202  are provided for glass windows  901203  and a ceiling  901204  as well as the glasses of the doors  901201  of the train car. Since the semiconductor device can be easily provided in a position in which the semiconductor device is conventionally difficult to be provided in this manner, an effective advertisement effect can be obtained. Since the semiconductor device can instantly switch images displayed on the display portion by external signals, cost and time generated in advertisement switching can be reduced, so that more flexible advertisement operation and information transmission can be performed. 
     Note that the semiconductor device can be provided in various positions as well as the doors  901201 , the glass windows  901203 , and the ceiling  901204  which are shown in  FIGS. 126A and 126B . For example, the semiconductor device may be incorporated in a hand strap, a seat, a handrail, a floor, or the like. At this time, the shape of the display panel  901202  may be a shape in accordance with a shape of an object in which the display panel  901202  is provided. 
       FIGS. 127A and 127B  each show an example in which the semiconductor device is incorporated in a passenger airplane. 
       FIG. 127A  shows a shape in use when a display panel  901302  is provided for a ceiling  901301  above a seat of the passenger airplane. The display panel  901302  is incorporated in the ceiling  901301  through a hinge portion  901303 , and a passenger can view the display panel  901302  by a telescopic motion of the hinge portion  901303 . The display panel  901302  has a function of displaying information by an operation of the passenger. The display panel  901302  can be utilized for advertisement or an amusement means. When the display panel  901302  is stored on the ceiling  901301  by folding the hinge portion  901303  as shown in  FIG. 127B , safety during takeoff and landing can be secured. Note that the display panel  901302  can also be utilized as a medium and a guide light by lighting display elements of the display panel  901302  in an emergency. 
     Note that the semiconductor device can be incorporated in various positions as well as the ceiling  901301  shown in  FIGS. 127A and 127B . For example, the semiconductor device may be incorporated in a seat, a table, an armrest, a window, or the like. A large display panel which can be viewed simultaneously by a plurality of people may be provided on a wall of an airframe. At this time, the shape of the display panel  901302  may be a shape in accordance with a shape of an object in which the display panel  901302  is provided. 
     Note that although this embodiment mode describes the train car body, the car body, and the airplane body as examples of moving objects, the present invention is not limited to them, and the semiconductor device can be provided in various objects such as a motorbike, a four-wheeled vehicle (including a car, a bus, and the like), a train (including a monorail, a railroad, and the like), and a vessel. Since display on display panels in a moving object can be switched instantly by external signals, the semiconductor device can be used for an advertisement display board for an unspecified number of customers, an information display board in an emergency, or the like by providing the semiconductor device in the moving object. 
     Note that although this embodiment mode is described with reference to various drawings, the contents (or may be part of the contents) described in each drawing can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in another drawing. Further, even more drawings can be formed when each part is combined with another part in the above-described drawings. 
     Similarly, the contents (or may be part of the contents) described in each drawing of this embodiment mode can be freely applied to, combined with, or replaced with the contents (or may be part of the contents) described in a drawing in another embodiment mode. Further, even more drawings can be formed when each part is combined with part of another embodiment mode in the drawings of this embodiment mode. 
     Note that this embodiment mode shows an example of an embodied case of the contents (or may be part of the contents) described in other embodiment modes, an example of slight transformation thereof, an example of partial modification thereof, an example of improvement thereof, an example of detailed description thereof, an application example thereof, an example of related part thereof, or the like. Therefore, the contents described in other embodiment modes can be freely applied to, combined with, or replaced with this embodiment mode. 
     This application is based on Japanese Patent Application serial no. 2007-133533 filed with Japan Patent Office on May 18, 2007, the entire contents of which are hereby incorporated by reference.