Patent Publication Number: US-11652174-B2

Title: Display device and electronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/373,879, filed Jul. 13, 2021, now allowed, which is a continuation of U.S. application Ser. No. 16/666,584, filed Oct. 29, 2019, now U.S. Pat. No. 11,069,817, which is a continuation of U.S. application Ser. No. 16/191,609, filed Nov. 15, 2018, now U.S. Pat. No. 10,700,215, which is a continuation of U.S. application Ser. No. 15/017,704, filed Feb. 8, 2016, now U.S. Pat. No. 10,134,912, which is a continuation of U.S. application Ser. No. 12/872,861, filed Aug. 31, 2010, now U.S. Pat. No. 9,257,082, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2009-205136 on Sep. 4, 2009, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device, a display device, and a light-emitting device, or a method for manufacturing these devices. In particular, the present invention relates to a semiconductor device, a display device, and a light-emitting device each of which includes a circuit having a thin film transistor in which a light-transmitting semiconductor film is used for a channel formation region, or a method for manufacturing these devices. In particular, the present invention relates to a semiconductor device, a display device, and a light-emitting device each of which includes a circuit having a thin film transistor in which an oxide semiconductor film is used for a channel formation region, or a method for manufacturing these devices. 
     2. Description of the Related Art 
     Thin film transistors (TFTs) in which a silicon layer of amorphous silicon or the like is used as a channel layer have been widely used as switching elements in display devices typified by liquid crystal display devices. Although thin film transistors using amorphous silicon have low field-effect mobility, they have an advantage that larger glass substrates can be used. 
     Moreover, attention has been recently drawn to a technique by which a thin film transistor is manufactured using a metal oxide with semiconductor properties and such a transistor is applied to an electronic device or an optical device. For example, it is known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide have semiconductor properties. A thin film transistor in which a transparent semiconductor layer formed using such a metal oxide is used as a channel formation region is disclosed (e.g., see Reference 1). 
     Furthermore, a technique has been considered to increase the aperture ratio in such a manner that a channel layer of a transistor is formed using a light-transmitting oxide semiconductor layer and a gate electrode, a source electrode, and a drain electrode are formed using a transparent conductive film with light-transmitting properties (e.g., see Reference 2). 
     The increase in aperture ratio increases the light use efficiency, the reduction in power and size of display devices can be achieved. On the other hand, in terms of the increase in size of display devices and application of display devices to portable devices, a further reduction in power consumption as well as the increase in aperture ratio is required. 
     As a metal auxiliary wiring for a transparent electrode of an electro-optic element, there is known a wiring including a metal auxiliary wiring and a transparent electrode that overlap with each other to be brought into conduction on the upper side or the lower side of the transparent electrode (e.g., see Reference 3). 
     A structure is known in which an additional capacitor electrode provided on an active matrix substrate is formed using a transparent conductive film of ITO, SnO 2 , or the like and an auxiliary wiring formed using a metal film is provided in contact with the additional capacitor electrode in order to reduce the electrical resistance of the additional capacitor electrode (e.g., see Reference 4). 
     In an electric-field transistor including an amorphous oxide semiconductor film, a transparent electrode formed from indium tin oxide (ITO), indium zinc oxide, ZnO, SnO 2 , or the like; a metal electrode formed from Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like; a metal electrode formed from an alloy containing any of the above elements; or the like can be used for a gate electrode, a source electrode, and a drain electrode. It is known that a stack of two or more layers of such materials can reduce the contact resistance and increase the interface intensity (e.g., see Reference 5). 
     It is known that a metal such as indium (In), aluminum (Al), gold (Au), or silver (Ag); or an oxide material such as indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmium oxide (CdIn 2 O 4 ), cadmium tin oxide (Cd 2 SnO 4 ), or zinc tin oxide (Zn 2 SnO 4 ) can be used for a source electrode, a drain electrode, and a gate electrode of a transistor including an amorphous oxide semiconductor and an auxiliary capacitor electrode. The materials for the gate electrode, the source electrode, and the drain electrode may be the same or different from each other (e.g., see References 6 and 7). 
     A display device in which a memory is placed in a pixel is considered in order to reduce power consumption (e.g., see References 8 and 9). Moreover, in the display devices in References 8 and 9, pixel electrodes that reflect light are used. 
     REFERENCE 
     
         
         Reference 1: Japanese Published Patent Application No. 2004-103957 
         Reference 2: Japanese Published Patent Application No. 2007-081362 
         Reference 3: Japanese Published Patent Application No. H2-082221 
         Reference 4: Japanese Published Patent Application No. H2-310536 
         Reference 5: Japanese Published Patent Application No. 2008-243928 
         Reference 6: Japanese Published Patent Application No. 2007-109918 
         Reference 7: Japanese Published Patent Application No. 2007-115807 
         Reference 8: Japanese Published Patent Application No. 2001-264814 
         Reference 9: Japanese Published Patent Application No. 2003-076343 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a technique related to a pixel including a memory. Another object of one embodiment of the present invention is to increase the aperture ratio of a pixel. 
     Note that the description of a plurality of objects does not preclude the existence of another object. Moreover, one embodiment of the present invention is not necessary to achieve all the objects listed above. 
     For example, one embodiment of the present invention is a display device that includes a display element having a pixel electrode, a first circuit having a function of controlling input of an image signal, a second circuit having a function of holding the image signal, and a third circuit having a function of controlling the polarity of a voltage supplied to the display element. A pixel including a memory can be provided according to this embodiment. In this embodiment, the first to third circuits may be formed using a light-transmitting material, and the pixel electrode may be provided above the first to third circuits. 
     One embodiment of the present invention is a display device that includes a first circuit including a first switch; a second circuit including a first capacitor and a second capacitor to which a signal is input through the first switch, and an inverter having an input terminal electrically connected to the first capacitor and an output terminal electrically connected to the second capacitor; a third circuit including a second switch having a control terminal electrically connected to the first capacitor, and a third switch having a control terminal electrically connected to the second capacitor; and a display element having a pixel electrode electrically connected to the second switch and the third switch. A pixel including a memory can be provided according to this embodiment. 
     In the above embodiment, the display device may include first to third wirings. In this structure example, the first wiring is electrically connected to the first capacitor through the first switch; the input terminal of the inverter is electrically connected to the first capacitor; the output terminal of the inverter is electrically connected to the second capacitor; the first capacitor is electrically connected to the control terminal of the second switch; the second capacitor is electrically connected to the control terminal of the third switch; and the second wiring is connected to the third wiring through the second switch and the third switch. 
     In the above embodiment, the first to third switches, the first and second capacitors, and the inverter can be formed using a light-transmitting material. Moreover, the pixel electrode can be provided above the first to third switches, the first and second capacitors, and the inverter. 
     In each of the above-described embodiments of the present invention, a variety of switches can be used as a switch. For example, an electrical switch, a mechanical switch, or the like can be used as a switch. That is, there is no particular limitation on the kind of switch as long as the switch can control the flow of current. Examples of switches are a transistor (e.g., a bipolar transistor and a MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky diode, a metal-insulator-metal (MIM) diode, a metal-insulator-semiconductor (MIS) diode, and a diode-connected transistor), and a logic circuit combining such elements. An example of a mechanical switch is a switch formed using a MEMS (micro electro mechanical system) technology, such as a digital micromirror device (DMD). Such a switch includes an electrode that can be moved mechanically, and operates to control electrical connection or non-electrical-connection with the movement of the electrode. 
     When a transistor is used as a switch in the each of above-described embodiments, the polarity (conductivity type) of the transistor is not particularly limited to a certain type because the transistor operates just as a switch. Note that a transistor of polarity with smaller off-state current is preferably used when the off-state current should be small. Examples of a transistor with smaller off-state current are a transistor provided with a high-resistance region and a transistor with a multi-gate structure. 
     In each of the above-described embodiments of the present invention, an n-channel transistor is preferably used as a switch when a potential of a source of the transistor used as the switch is close to a potential of a low potential side power supply (e.g., Vss, GND, or 0 V). On the other hand, a p-channel transistor is preferably used as the switch when the potential of the source of the transistor is close to a potential of a high potential side power supply (e.g., Vdd). This is because the absolute value of gate-source voltage can be increased when the potential of the source of the n-channel transistor is close to a potential of a low potential side power supply and when the potential of the source of the p-channel transistor is close to a potential of a high potential side power supply; thus, the transistor can more accurately operate as a switch. Alternatively, this is because decrease in output voltage does not often occur since the transistor does not often perform source follower operation. 
     In each of the above-described embodiments of the present invention, a CMOS switch may be employed as a switch by using both n-channel and p-channel transistors. By using a CMOS switch, the switch can more accurately operate as a switch because current can flow when either the p-channel transistor or the n-channel transistor is turned on. Thus, appropriate voltage can be output regardless of whether voltage of an input signal to the switch is high or low. Alternatively, the voltage amplitude value of a signal for turning on or off the switch can be made small, so that 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 and a drain), an output terminal (the other of the source and the drain), and a terminal for controlling conduction (a gate) in some cases. On the other hand, when a diode is used as a switch, the switch does not have a terminal for controlling electrical conduction in some cases. Therefore, when a diode is used as a switch, the number of wirings for controlling terminals can be reduced as compared to the case of using a transistor. 
     In the invention disclosed in this specification, transistors with a variety of structures can be used. That is, there is no limitation on the structure of transistors to be used. 
     In this specification, a semiconductor device corresponds to a device having a circuit including a semiconductor element (e.g., a transistor, a diode, or a thyristor). Note that a semiconductor device may correspond to all devices that can function by utilizing semiconductor properties or a device including a semiconductor material. In this specification, a display device corresponds to a device including a display element. 
     In this specification, a driving device corresponds to a device including a semiconductor element, an electric circuit, or an electronic circuit. Examples of the driving device are a transistor that 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 that supplies voltage or current to a pixel electrode, and a transistor that supplies voltage or current to a light-emitting element. Moreover, examples of the driving device are a circuit that supplies a signal to a gate signal line (also referred to as a gate driver, a gate line driver circuit, or the like) and a circuit that supplies a signal to a source signal line (also referred to as a source driver, a source line driver circuit, or the like). 
     It is possible to combine any of a display device, a semiconductor device, a lighting device, a cooling device, a light-emitting device, a reflecting device, a driving device, and the like. Such a device is also included in an embodiment of the present invention. For example, a display device sometimes includes a semiconductor device and a light-emitting device. In some cases, a semiconductor device includes a display device and a driving device. 
     In each of the above-described embodiments of the present invention, all the circuits that are necessary to realize a predetermined function can be formed using one substrate (e.g., a glass substrate, a plastic substrate, a single crystal substrate, or an SOI substrate). In this manner, costs can be reduced by reduction in the number of components or the reliability can be improved by reduction in the number of connections to circuit components. 
     Furthermore, it is possible not to form all the circuits that are necessary to realize the predetermined function over one substrate. That is, 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. For example, some of the circuits which are necessary to realize the predetermined function can be formed over a glass substrate and some of the circuits which are necessary to realize the predetermined function can be formed using a single crystal substrate (or an SOI substrate). Then, the single crystal substrate where some of the circuits which are necessary to realize the predetermined function (such a substrate is also referred to as an IC chip) can be connected to the glass substrate by COG (chip on glass), and the IC chip can be provided over the glass substrate. Alternatively, the IC chip can be connected to the glass substrate with TAB (tape automated bonding), COF (chip on film), SMT (surface mount technology), a printed circuit board, or the like. 
     In this specification, when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected, the case where X and Y are functionally connected, and the case where X and Y are directly connected are included therein. Here, each of X and Y is an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). Accordingly, another connection relation shown in drawings and texts is included without being limited to a predetermined connection relation, for example, the connection relation shown in the drawings and the texts. 
     For example, in the case where X and Y are electrically connected, one or more elements that enable electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, and/or a diode) can be connected between X and Y. 
     For example, in the case where X and Y are functionally connected, one or more circuits that enable functional connection between X and Y (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 dc-dc converter, a step-up dc-dc converter, or a step-down dc-dc converter) or a level shifter circuit for changing a potential level of a signal; a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a memory circuit; and/or a control circuit) can be connected between X and Y. Note that for example, when a signal output from X is transmitted to Y, it can be said that X and Y are functionally connected even if another circuit is provided between X and Y. 
     Note that when it is explicitly described that X and Y are electrically connected, the case where X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), the case where X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and the case where X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween) are included therein. That is, when it is explicitly described that X and Y are electrically connected, the description is the same as the case where it is explicitly only described that X and Y are connected. 
     In this specification, explicit singular forms preferably mean singular forms. Note that in that case, the singular form can also include the plural. Similarly, explicit plural forms preferably mean plural forms. However, also in that case, the plural form can include the singular. 
     Note that the size, the thickness of layers, or regions in the drawings of this application are exaggerated for simplicity in some cases. Therefore, embodiments of the present invention are not limited to such scales. Note that a drawing schematically illustrates an ideal example, and embodiments of the present invention are not limited to the shape, value, or the like illustrated in the drawing. For example, it is possible to include variations in shape due to a manufacturing technique or an error, variations in signal, voltage, or current due to noise or difference in timing. 
     Note that technical terms are used in order to describe a specific embodiment or the like in many cases. Note that one embodiment of the present invention is not construed as being limited by the technical terms. 
     Note that terms that are not defined (including terms used for science and technology such as technical term or academic parlance) can be used as the terms having meaning equal to general meaning that an ordinary person skilled in the art understands. It is preferable that terms defined by dictionaries or the like be construed as consistent meaning with the background of related art. 
     The terms such as “first”, “second”, and “third” are used for distinguishing a variety of elements, members, regions, layers, areas, and the like from each other. Therefore, the terms such as “first”, “second”, and “third” do not limit the order and number of the elements, members, regions, layers, areas, and the like. Further, the term “first” can be replaced with the term “second”, “third”, or the like, for example. 
     Note that terms for describing spatial arrangement, such as “over”, “above”, “under”, “below”, “laterally”, “right”, “left”, “obliquely”, “behind”, “front”, “inside”, “outside”, and “in”, are often used for briefly showing a relation between an element and another element or between a feature and another feature with reference to a diagram. Note that embodiments of the present invention are not limited to this use of terms, and such terms for describing spatial arrangement can indicate not only the direction illustrated in a diagram but also another direction in some cases. For example, when it is explicitly described that “Y is over X”, it does not necessarily mean that Y is placed over X. Since a structure in a diagram can be inverted or rotated by 180°, the case where Y is placed under X can be included. Thus, “over” can refer to the direction described by “under” in addition to the direction described by “over”. Note that the embodiments of the present invention are not limited to this, and “over” can refer to any of the other directions described by “laterally”, “right”, “left”, “obliquely”, “behind”, “front”, “inside”, “outside”, “in”, and the like in addition to the directions described by “over” and “under” because a device in a diagram can be rotated in a variety of directions. That is, the terms for describing spatial arrangement can be construed adequately depending on the situation. 
     When it is explicitly described that Y is formed on or over X, it does not necessarily mean that Y is formed on and in direct contact with X. The description includes the case where X and Y are not in direct contact with each other, that is, the case where another object is placed between X and Y. Here, each of X and Y 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 Y is formed on (or over) a layer X”, it includes both the case where the layer Y is formed on and in direct contact with the layer X, and the case where another layer (e.g., a layer Z) is formed on and in direct contact with the layer X and the layer Y is formed on and in direct contact with the layer Z. Note that another layer (e.g., the layer Z) may be a single layer or a plurality of layers. 
     Similarly, when it is explicitly described that Y is formed above X, it does not necessarily mean that Y is formed on and in direct contact with X, and another object may be placed between X and Y. Therefore, for example, when it is described that “a layer Y is formed above a layer X”, it includes both the case where the layer Y is formed on and in direct contact with the layer X, and the case where another layer (e.g., a layer Z) is formed on and in direct contact with the layer X and the layer Y is formed on and in direct contact with the layer Z. Note that another layer (e.g., the layer Z) may be a single layer or a plurality of layers. 
     Note that when it is explicitly described that Y is formed over, on, or above X, it includes the case where Y is formed obliquely over/above X. 
     Note that the same can be said when it is explicitly described that Y is formed below or under X. 
     By forming a light-transmitting transistor or a light-transmitting capacitor, light can be transmitted also in a portion where the transistor or the capacitor is formed even when the transistor or the capacitor is provided in a pixel. Thus, the aperture ratio of the pixel can be increased. In addition, a memory is provided in a pixel by using such a transistor and a light-transmitting wiring, whereby it is possible to provide a transmissive display including a pixel having a memory. 
     Further, when a wiring for connecting a transistor and an element (e.g., another transistor) or a wiring for connecting a capacitor and an element (e.g., another capacitor) can be formed using a material with low resistivity and high conductivity, the distortion of the waveform of a signal and a voltage drop due to wiring resistance can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG.  1    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  2    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  3    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  4    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  5    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  6    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  7    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  8    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  9    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  10    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  11    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  12    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  13    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  14    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  15    is a block diagram illustrating an example of a structure of a display device; 
         FIG.  16    is a circuit diagram illustrating an example of a structure of a display device; 
         FIGS.  17 A to  17 C  are circuit diagrams illustrating an example of operation of the display device in  FIG.  16   ; 
         FIGS.  18 A to  18 C  are circuit diagrams illustrating an example of operation of the display device in  FIG.  16   ; 
         FIGS.  19 A to  19 C  are circuit diagrams illustrating an example of operation of the display device in  FIG.  16   ; 
         FIGS.  20 A to  20 C  are circuit diagrams illustrating an example of operation of the display device in  FIG.  16   ; 
         FIGS.  21 A to  21 E  are circuit diagrams illustrating an example of a structure of a display device; 
         FIGS.  22 A to  22 D  are circuit diagrams each illustrating an example of a structure of a display device; 
         FIG.  23    is a circuit diagram illustrating an example of operation of a display device; 
         FIG.  24    is a circuit diagram illustrating an example of operation of a display device; 
         FIG.  25    is a circuit diagram illustrating an example of operation of a display device; 
         FIG.  26    is a circuit diagram illustrating an example of operation of a display device; 
         FIG.  27    is a circuit diagram illustrating an example of operation of a display device; 
         FIGS.  28 A to  28 D  are circuit diagrams each illustrating an example of a structure of a display device; 
         FIG.  29    is a circuit diagram illustrating an example of a structure of a display device; 
         FIG.  30    is a circuit diagram illustrating an example of a structure of a display device; 
         FIG.  31    is a circuit diagram illustrating an example of a structure of a display device; 
         FIG.  32    is a plan view illustrating an example of a structure of a display device; 
         FIG.  33    is a plan view illustrating an example of a structure of a display device; 
         FIG.  34 A  is a plan view and  FIGS.  34 B and  34 C  are cross-sectional views illustrating an example of a structure of a display device; 
         FIG.  35    is a cross-sectional view illustrating an example of a structure of a display device; 
         FIG.  36    is a cross-sectional view illustrating an example of a structure of a display device; 
       FIGS.  37 A 1  and  37 A 2  are plan views and  FIGS.  37 B and  37 C  are cross-sectional views illustrating an example of a structure of a display device; 
         FIGS.  38 A to  38 F  are cross-sectional views illustrating an example of a method for manufacturing a display device; 
         FIGS.  39 A to  39 E  are cross-sectional views illustrating an example of a method for manufacturing a display device; 
         FIGS.  40 A to  40 H  each illustrate an example of a structure of an electronic device; 
         FIGS.  41 A to  41 D  each illustrate an example of a structure of an electronic device, and  FIGS.  41 E to  41 H  each illustrate an application example of a display device; 
         FIG.  42    is a circuit diagram illustrating an example of a structure of a display device; and 
         FIG.  43    is a circuit diagram illustrating an example of a structure of a display device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below. An embodiment of the invention disclosed in this specification can achieve any of the following objects, for example. Note that the description of a plurality of objects does not preclude the existence of another object. In addition, each embodiment of the present invention is not necessary to achieve all the following objects. 
     Objects are, for example, to provide a technique related to a pixel including a memory, to increase the aperture ratio of a pixel, to lower wiring resistance, to reduce contact resistance, to reduce a voltage drop, to reduce power consumption, to improve display quality, and to reduce the off-state current of a transistor. 
     The embodiments of the present invention can be carried out in many different modes, and it is easily understood by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention is not interpreted as being limited to the description of the embodiments below. Note that in structures described below, the same portions or portions having similar functions are denoted by the same reference numerals, and description thereof is not repeated. 
     What is described in one embodiment (or part of the content) can be combined or replaced with another content in the same embodiment and/or what is described (or part thereof) in another embodiment or other embodiments. Note that in each embodiment, what is described in the embodiment is the content described with reference to one or a plurality of diagrams and the content described in text form. 
     A combination of a diagram (or part of the diagram) used in one embodiment with another part of the diagram, a different diagram (or part thereof) used in the same embodiment, and/or a diagram (or part thereof) used in one or a plurality of different embodiments can form a diagram in which another structure example is illustrated. On the basis of part of a diagram or a text used in one embodiment, another embodiment can be constituted. Therefore, in the case where a diagram or a text related to some portion is described, the present invention also discloses another embodiment represented by the diagram or the text for that portion. 
     Therefore, for example, it is possible to constitute one embodiment of the invention by taking out part of a diagram (e.g., a cross-sectional view, a plan view, a circuit diagram, a block diagram, a flow chart, a process diagram, a perspective view, a cubic diagram, a layout diagram, a timing chart, a structure diagram, a schematic view, a graph, a list, a ray diagram, a vector diagram, a phase diagram, a waveform chart, a photograph, or a chemical formula) or a text in which one or more of an active element (e.g., a transistor and a diode), a wiring, a passive element (e.g., a capacitor and a resistor), a conductive layer, an insulating layer, a semiconductor layer, an organic material, an inorganic material, a component, a substrate, a module, a device, a solid, a liquid, a gas, an operating method, a manufacturing method, and the like are described. 
     For example, from a circuit diagram in which N circuit elements (e.g., transistors or capacitors; N is an integer) are provided, it is possible to constitute one embodiment of the invention by taking out M circuit elements (e.g., transistors or capacitors; M is an integer, where M&lt;N). As another example, it is possible to constitute one embodiment of the invention by taking out M layers (M is an integer, where M&lt;N) from a cross-sectional view in which N layers (N is an integer) are provided. As another example, it is possible to constitute one embodiment of the invention by taking out M elements (M is an integer, where M&lt;N) from a flow chart in which N elements (N is an integer) are provided. 
     Further, in the case where at least one specific example is described in a diagram or a text described in one embodiment, it is readily appreciated by those skilled in the art that a broader concept of the specific example can be derived. Therefore, in the case where at least one specific example is described in the diagram or the text described in one embodiment, a broader concept of the specific example can constitute one embodiment of the invention disclosed in this specification. 
     The content described in at least a diagram (or part of the diagram) is disclosed as one embodiment of the invention, and can constitute one embodiment of the invention. Therefore, when certain content is described in a diagram, one embodiment of the invention disclosed in this specification can be constituted by the content even when the content is not described with a text. Similarly, one embodiment of the invention disclosed in this specification can be constituted by a diagram obtained by taking out part of a diagram. 
     It might be possible for those skilled in the art to constitute one embodiment of the invention even when portions to which all terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected are not specified. In particular, in the case where the number of portions to which the terminal is connected is plural, it is not necessary to specify the portions to which the terminal is connected. Therefore, in some cases, it is possible to constitute an embodiment of the invention by only specifying portions to which only some of terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected. 
     It is sometimes possible for those skilled in the art to specify an embodiment of the invention when at least a connection portion of a circuit is specified, and such a case is included in the embodiment of the invention disclosed in this specification. Moreover, it is sometimes possible for those skilled in the art to specify an embodiment of the invention disclosed in this specification when at least a function of a circuit is specified. Such a case is included in the embodiment of the invention disclosed in this specification. 
     Embodiment 1 
     In this embodiment, a display device will be described. 
     An example of a structure of a display device (also referred to as a semiconductor device) in this embodiment is described with reference to  FIG.  1   . The display device includes a plurality of pixels.  FIG.  1    illustrates a cross-sectional structure of one pixel. 
     A circuit  102 , a circuit  103 , and a circuit  104  are provided above a substrate  101 . An insulating layer  105  is provided above the circuits  102  to  104 . A conductive layer  106  is provided above the insulating layer  105 . A conductive layer  109  is provided above a substrate  108  (provided below the substrate  108  in  FIG.  1   ). A medium  107  is provided between the conductive layer  106  and the conductive layer  109 . A display element can be formed using the medium  107 , the conductive layer  106 , and the conductive layer  109 . The conductive layer  106  can be connected to the circuit  102 , the circuit  103 , and/or the circuit  104 . The conductive layer  106  can be placed so as to cover all or most parts of the circuit  102 , the circuit  103 , and/or the circuit  104 . However, this embodiment is not limited to such a structure. 
     In the example of the structure in  FIG.  1   , it is possible not to provide the substrate  108  or the conductive layer  109 , and it is possible not to provide the circuit  102 , the circuit  103 , or the circuit  104 . 
     Here, for example, the circuit  102  has a function of controlling whether a signal (e.g., an image signal) is input to a pixel or not. Thus, the circuit  102  can include a selection transistor or a switching transistor. 
     For example, the circuit  103  has a function of holding a signal. That is, the circuit  103  has a memory function. The circuit  103  includes a DRAM, an SRAM, a nonvolatile memory, or the like as a memory, for example. Moreover, the circuit  103  can include a refresh circuit. Data in a DRAM can be refreshed by the refresh circuit. For that reason, the circuit  103  can include an inverter, a clocked inverter, a capacitor, an analog switch, or the like. 
     For example, the circuit  104  has a function of controlling the polarity of voltage supplied to the medium  107 . Consequently, the circuit  104  is not provided in some cases depending on the kind of the medium  107 . For that reason, the circuit  104  can include an inverter, a source follower, an analog switch, or the like. When a memory is placed in a pixel in such a manner, the frequency of signal writing can be lowered, so that power consumption can be reduced. However, this embodiment is not limited to such circuits. 
     When the circuits  102  to  104  have the above-described functions, a transistor or a wiring included in the circuit  102 , the circuit  103 , and/or the circuit  104  can be formed using a light-transmitting material. For example, it is possible to use a light-transmitting material for part or all of the following: a gate electrode, a semiconductor layer, a source electrode, and a drain electrode of a transistor. Thus, light can pass through a portion where the transistor or the wiring is provided. Similarly, wirings such as a source signal line, a gate signal line, a capacitor wiring, and a power supply line can be formed using a light-transmitting material. Thus, light can pass through most of a pixel region in which a plurality of pixels are arranged. 
     Note that although part or all of the wirings such as a source signal line, a gate signal line, a capacitor wiring, and a power supply line can be formed using a light-transmitting material, this embodiment is not limited to this structure and the wiring can be formed using a material with high conductivity. That is, the wiring can be formed using a material without light-transmitting properties. For example, the wiring can be a stack of a layer with light-transmitting properties and a layer without light-transmitting properties. In such cases, the aperture ratio is decreased since a region through which light passes is narrowed, whereas distortion of signals and a voltage drop can be reduced because of high conductivity. 
     In particular, in a circuit for driving a pixel, for example, a gate driver, a source driver, or a circuit for driving a common electrode (a counter electrode), a wiring and/or a transistor can be formed using a layer without light-transmitting properties. Light does not need to pass through a gate driver, a source driver, a circuit for driving a common electrode (a counter electrode), or the like. For that reason, a wiring and a transistor are formed using a wiring and an electrode that have high conductivity, whereby distortion of signals and a voltage drop can be reduced. 
     Note that the conductive layer  106  or the conductive layer  109  can be formed using a light-transmitting material. As illustrated in  FIG.  1   , the circuit  102 , the circuit  103 , or the circuit  104  can be placed under the conductive layer  106 . In this case, the circuit  102 , the circuit  103 , or the circuit  104  can be formed using a light-transmitting material, so that the aperture ratio can be increased, or a transmissive display device can be provided. In other words, it is possible to obtain a transmissive display device in which a memory is provided in a pixel. 
     Note that part of the conductive layer  106  and/or part of the conductive layer  109  can be formed using a material without light-transmitting properties, that is, a material with high conductivity. When part of the conductive layer  106  is formed using a material with high conductivity, the part can reflect light. Thus, a transflective display device can be provided. 
     Note that at least one of the circuit  102 , the circuit  103 , and the circuit  104  may be placed under the conductive layer  106 . Alternatively, part of the circuit  102 , the circuit  103 , and the circuit  104  may be placed under the conductive layer  106 . 
     The conductive layer  106  can have a function of a pixel electrode. The conductive layer  109  can have a function of a common electrode. 
     Note that the conductive layer  109  is not limited to being formed on the substrate  108  and can be formed over the substrate  101 . 
     The medium  107  includes a liquid crystal, an organic EL material, an inorganic EL material, an electrophoretic material, an electro liquid powder, or a toner, for example Optical properties of the medium  107  are controlled with voltage or current that is supplied from the conductive layer  106  and the conductive layer  109 . 
       FIG.  1    illustrates the example where the circuits  102 ,  103 , and  104  are provided in one pixel; however, this embodiment is not limited to this structure. It is possible to provide a larger number of circuits or a smaller number of circuits. 
     Note that the substrate  101  or the substrate  108  is preferably an insulating substrate. Examples of the insulating substrate are a glass substrate, a plastic substrate, a flexible substrate, a polyethylene terephthalate (PET) substrate, a stainless steel foil substrate, an SOI substrate, a silicon substrate, a ceramic substrate, a quartz substrate, and a sapphire substrate. A conductive substrate that is formed of a conductor such as metal or stainless steel and has a surface covered with an insulating material can also be used. When glass or plastics are used for the substrate, light can pass through the substrate. When a plastic substrate or a flexible substrate is used as the substrate  101  or the substrate  108 , the substrate can be bent and is not easily broken. 
     Note that one or a plurality of insulating layers may be formed on a surface of the substrate  101  or the substrate  108 , in which case diffusion of impurities included in the substrate can be suppressed. 
     Embodiment 2 
     In this embodiment, a display device will be described. 
     Examples of structures of a display device (also referred to as a semiconductor device) shown in this embodiment will be described with reference to  FIGS.  2  to  14   . The display device includes a plurality of pixels.  FIGS.  2  to  14    each illustrate a cross-sectional structure of one pixel. 
     As illustrated in  FIG.  2   , conductive layers  201   a  and  201   b  are placed above the substrate  101 . An insulating layer  202  is placed above the conductive layers  201   a  and  201   b . A semiconductor layer  203  is placed above the insulating layer  202 . Conductive layers  204   a ,  204   b , and  204   c  are placed above the semiconductor layer  203  or the insulating layer  202 . An insulating layer  205  is placed above the conductive layers  204   a ,  204   b , and  204   c  or the semiconductor layer  203 . A conductive layer  206  is placed above the insulating layer  205 . The conductive layer  204   b  is connected to the conductive layer  206  through a contact hole formed in the insulating layer  205 . An upper portion of the semiconductor layer  203  and lower portions of the conductive layers  204   a  and  204   b  are in contact with each other so that the semiconductor layer  203  and the conductive layers  204   a  and  204   b  are connected to each other. 
     Note that the conductive layers  201   a  and  201   b  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the conductive layers  201   a  and  201   b  contain approximately the same material. Similarly, the conductive layers  204   a ,  204   b , and  204   c  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the conductive layers  204   a  to  204   c  contain approximately the same material. 
     The conductive layer  201   a  can have a function of a gate electrode of a transistor  207  or a function of a gate signal line. The conductive layer  201   b  can have a function of capacitor electrodes of capacitors  208  and  209  or a function of a storage capacitor line. 
     The insulating layer  202  can have a function of a gate insulating layer of the transistor  207  or a function of insulating layers of the capacitors  208  and  209 . 
     The conductive layers  204   a  and  204   b  can have a function of a source electrode and a drain electrode of the transistor  207  or a function of a source signal line or a video signal line. 
     The conductive layer  204   c  can have a function of a capacitor electrode of the capacitor  208  or a function of a storage capacitor line. 
     The semiconductor layer  203  can have a function of an active layer of the transistor  207 , a function of a channel layer of the transistor  207 , a function of a high-resistance region of the transistor  207 , or a function of an impurity region of the transistor  207 . 
     The conductive layer  206  can have a function of a pixel electrode or a function of a capacitor electrode of the capacitor  209 . 
     The conductive layer  206  can correspond to the conductive layer  106  in  FIG.  1   . The insulating layer  205  can correspond to the insulating layer  105  in  FIG.  1   . 
     The transistor  207 , the capacitor  208 , and the like can be placed below the conductive layer  206  as described above. Since the transistor  207 , the capacitor  208 , and the like have light-transmitting properties, the aperture ratio can be increased. Alternatively, a transmissive display device can be obtained. Moreover, a selection transistor, a memory (e.g., a DRAM or an SRAM), an analog switch, an inverter, a clocked inverter, or the like can be formed using the transistor  207  and the capacitors  208  and  209 . 
     Since the conductive layer  201   a  is placed below the semiconductor layer  203 , the transistor  207  can be referred to as a bottom-gate transistor or an inverted staggered transistor. Since a channel protective film is not provided over the semiconductor layer  203  in the transistor  207 , the transistor  207  can be referred to as a channel-etched transistor. Moreover, the transistor  207  can also be called a thin film transistor. 
     Note that the structures of the transistor and the capacitor are not limited to those in  FIG.  2   . A variety of other structures can be employed. 
     For example, it is possible to form a transistor in which an electrode is provided on the opposite side to a gate electrode, with respect to a channel portion.  FIG.  3    illustrates an example of a structure of a pixel in the case where a conductive layer  206   a  is provided above the semiconductor layer  203  and the insulating layer  205 . The conductive layer  206   a  can function as a back gate of the transistor  207 . A potential different from that for the conductive layer  201   a  is supplied to the conductive layer  206   a , whereby stable operation of the transistor  207  can be achieved. Alternatively, the same potential as that for the conductive layer  201   a  is supplied to the conductive layer  206   a , whereby the channel of the transistor  207  substantially doubles; thus, the mobility can be substantially increased. 
     Note that the conductive layers  206   a  and  206  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the conductive layers  206   a  and  206  contain approximately the same material. 
     In the capacitor  208 , a semiconductor layer  203   a  can be provided between the conductive layer  204   c  and a conductive layer  201   c  as illustrated in  FIG.  3   . Here, the semiconductor layer  203  and  203   a  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the semiconductor layer  203  and  203   a  contain approximately the same material. 
     Note that a transistor in a peripheral circuit portion (e.g., a circuit portion for driving a pixel) and a transistor in a pixel portion can have different structures. For example, it is possible to employ a structure in which the transistor  207  in a pixel portion is not provided with the conductive layer  206   a  as illustrated in  FIG.  2   , whereas the transistor  207  in a circuit for driving a pixel portion includes the conductive layer  206   a  as illustrated in  FIG.  3   . In the circuit for driving a pixel portion, it is extremely important to control the threshold voltage of the transistor  207 . In the pixel portion, however, the transistor  207  can be operated in some cases even if the transistor  207  is in a normally-on state. Moreover, in the pixel portion, reduction in aperture ratio can be prevented by the absence of the conductive layer  206   a . Thus, when the transistor  207  in the pixel portion is not provided with the conductive layer  206   a  and the transistor  207  in the circuit for driving a pixel portion includes the conductive layer  206   a , a display device can be operated in an appropriate manner and the aperture ratio can be increased. 
     However, this embodiment is not limited to the structure in  FIG.  3   . A pixel can be formed using a layer that is different from the conductive layer  206   a .  FIG.  4    illustrates one example. 
     A conductive layer  406   a  and an insulating layer  405  are provided between the insulating layer  205  and the conductive layer  206 . The conductive layer  406   a  can function as the back gate of the transistor  207 . With the use of the layer which is different from the conductive layer  206  in such a manner, the transistor  207  and the conductive layer  406   a  can be placed under the conductive layer  206 . Thus, when the transistor  207  with this structure is used in a pixel, the aperture ratio can be increased. 
     Note that a capacitor  408  can be formed using a conductive film that is in the same layer as the conductive layer  406   a . The capacitor  408  can be formed using a conductive layer  406   b  and the conductive layer  201   c . A capacitor  409  can be formed using the conductive layer  406   b  and the conductive layer  206 . A capacitor  408   a  can be formed using the conductive layer  406   b  and the conductive layer  204   d.    
     Note that the conductive layers  201   a  and  201   c  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the conductive layers  201   a  and  201   c  contain approximately the same material. In addition, the conductive layers  204   a ,  204   b , and  204   d  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the conductive layers  204   a ,  204   b , and  204   d  contain approximately the same material. The conductive layers  406   a  and  406   b  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In that case, the conductive layers  406   a  and  406   b  contain approximately the same material. 
     Note that  FIGS.  2  to  4    each illustrate the example of the transistor  207  in which a channel protective film is not provided over the semiconductor layer  203 . However, this embodiment is not limited to these examples. It is possible to provide a channel protective film. As an example,  FIG.  5    illustrates the case where a channel protective film  503  is provided for the transistor  207  in  FIG.  2   . Similarly, the transistor in each of  FIGS.  3  and  4    can include a channel protective film. In the transistor  207  in  FIG.  5   , the thickness of the semiconductor layer  203  can be reduced by the placement of the channel protective film  503 . Thus, the off-state current can be reduced or the subthreshold swing value (S value) can be reduced. Further, because it is not necessary to consider the etching selectivity of the semiconductor layer  203  to the conductive layers  204   a  and  204   b , materials can be freely selected. 
     In the examples of the structures in  FIGS.  2  to  5   , a region where the semiconductor layer  203  is not provided is placed under the conductive layers  204   a  and  204   b . This embodiment is not limited to these structures, and the semiconductor layer  203  may be provided under the whole area of the conductive layers  204   a  and  204   b  as illustrated in  FIG.  6   . In this case, it is possible not to provide the channel protective film  503 . When the channel protective film  503  is not provided, the number of masks (reticles) can be reduced by using a multi-tone mask (also referred to as a half-tone mask or a gray-tone mask). The following steps may be performed, for example Films to be the semiconductor layer  203  and the conductive layers  204   a  and  204   b  are successively formed. A resist mask is formed, and these films are etched at the same time. The resist mask is subjected to ashing or the like so that a mask for only etching the conductive layers  204   a  and  204   b  is formed. With one light exposure mask, a channel portion in the semiconductor layer  203  and a resist mask for etching the conductive layers  204   a  and  204   b  can be formed. 
     As an example of the pixel structure,  FIGS.  2  to  6    each illustrate the example of the structure in which an upper portion of the semiconductor layer  203  and lower portions of the conductive layers  204   a  and  204   b  are in contact with each other so that the semiconductor layer  203  and the conductive layers  204   a  and  204   b  are electrically connected to each other. Needless to say, this embodiment is not limited to these examples. It is possible to provide a conductive layer that is in contact with a lower portion of the semiconductor layer  203  and is electrically connected to the semiconductor layer  203 . Such structure examples will be described below with reference to  FIGS.  7  to  10   . 
       FIG.  7    is a cross-sectional view of the transistor  207  in the case where an upper portion of the semiconductor layer  203  and lower portions of the conductive layers  204   a  and  204   b  are in contact with each other in the structure in  FIG.  2   . Similarly,  FIG.  8    is a cross-sectional view of the transistor  207  in the case where an upper portion of the semiconductor layer  203  and lower portions of the conductive layers  204   a  and  204   b  are in contact with each other in the structure in  FIG.  3   . Similarly,  FIG.  9    is a cross-sectional view of the transistor  207  in the case where an upper portion of the semiconductor layer  203  and lower portions of the conductive layers  204   a  and  204   b  are in contact with each other in the structure in  FIG.  4   . Similarly,  FIG.  10    is a cross-sectional view of the transistor  207  in the case where the channel protective film  503  is not provided and an upper portion of the semiconductor layer  203  and lower portions of the conductive layers  204   a  and  204   b  are in contact with each other in the structure in  FIG.  6   . Note that in  FIG.  10   , it is preferable that a portion of the semiconductor layer  203  which is in contact with the conductive layer  206  become a portion which has a sufficient characteristic of an n-type or a p-type. In other words, the portion where the conductive layer  206  is in contact with the semiconductor layer  203  is preferably ohmic contact. 
     Note that  FIGS.  2  to  10    each illustrate the example of the structure in which an insulating layer is not provided between the conductive layers  204   a  and  204   b  and the semiconductor layer  203 . Needless to say, this embodiment is not limited to these examples. An insulating layer can be provided between the conductive layers  204   a  and  204   b  and the semiconductor layer  203 . As an example,  FIG.  11    illustrates the case where an insulating layer  1105  is provided in the structure in  FIG.  2   . The conductive layers  204   a  and  204   b  are connected to the semiconductor layer  203  through a contact hole provided in the insulating layer  1105 . 
     In this case, an electrode can be provided on the opposite side to a gate electrode, with respect to a channel portion, by using a conductive layer that is in the same layer as the conductive layers  204   a  and  204   b .  FIG.  12    illustrates an example of this case. A conductive layer  204   e  is provided on the opposite side to a gate electrode, with respect to a channel portion. Since the conductive layer  204   e  is provided in the same layer as the conductive layer  204   a  in such a manner, reduction in aperture ratio can be prevented even when a transistor in a pixel portion has such a structure. 
     Note that the conductive layers  204   a ,  204   b , and  204   e  can be formed with an etching treatment for films (with a single-layer structure or a layered structure) formed through the same deposition step. In this case, the conductive layers  204   a ,  204   b , and  204   e  contain approximately the same material. 
     Note that the transistor  207  as in  FIG.  11    or  FIG.  12    can be applied to the pixel in  FIGS.  3  to  10   . 
     In order to connect conductive layers that are placed in different layers with an insulating layer therebetween, it is necessary to form a contact hole in the insulating layer. An example of a contact structure in that case is illustrated in  FIG.  13   . In a contact structure  1301 , contact holes are formed in the insulating layers  205  and  202  in order to electrically connect the conductive layer  201   b , the conductive layer  204   b , and the conductive layer  206 . These contact holes are formed at the same time. In this case, the number of masks (the number of reticles) and the number of process steps can be reduced. In the case where the conductive layers  204   b  and  201   b  are to be connected to each other, they need to be connected through the conductive layer  206 ; thus, it is possible that the contact resistance is increased or the layout area is increased. On the other hand, as in a contact structure  1302 , it is possible to form a contact hole in the insulating layer  202  so that the conductive layers  201   a  and  204   a  are directly connected to each other. In this case, it is possible to decrease the possibility that the contact resistance is increased or the layout area is increased. 
       FIG.  14    illustrates an example of a contact structure in the case where the conductive layers  406   a  and  406   b  are provided. In a contact structure  1401 , contact holes are formed in the insulating layers  405 ,  205 , and  202  at the same time, and the conductive layers  201   a ,  204   b ,  406   a , and  206  are connected to each other. In this case, the number of masks (the number of reticles) and the number of process steps can be reduced. In the case where the conductive layer  406   a  is to be connected to the conductive layers  204   b  and  201   a , they need to be connected through the conductive layer  206 ; thus, it is possible that the contact resistance is increased or the layout area is increased. On the other hand, as in a contact structure  1402 , it is possible to form contact holes in the insulating layers  202  and  205  so that the conductive layers  406   b ,  204   d  and  201   c  are directly connected to each other. In this case, it is possible to decrease the possibility that the contact resistance is increased or the layout area is increased. 
     Note that  FIGS.  2  to  12    each illustrate the example of a bottom-gate transistor; however, this embodiment is not limited to a bottom-gate transistor. A display device can include a top-gate transistor. Similarly, without limitation to an inverted staggered transistor, a display device can include a planar transistor. 
     The semiconductor layer  203  illustrated in  FIGS.  2  to  12    can be formed using a semiconductor film with a single-layer structure or a layered structure. The film for the semiconductor layer can be formed using a light-transmitting material such as indium tin oxide (ITO), indium tin oxide containing silicon oxide (ITSO), organoindium, organotin, or zinc oxide (ZnO). Further, the film may be formed using indium zinc oxide (IZO) containing zinc oxide, a material in which zinc oxide is doped with gallium (Ga), tin oxide (SnO 2 ), indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, or the like. The film formed using such a material can be formed by a sputtering method. 
     The conductive layers illustrated in  FIGS.  1  to  14   , for example, the conductive layers  201   a ,  201   b ,  204   a  to  204   e ,  206 ,  206   a ,  406   a , and  406   b  can be formed using a conductive film with a single-layer structure or a layered structure. The film for forming these conductive layers can be formed using a light-transmitting material such as indium tin oxide (ITO), indium tin oxide containing silicon oxide (ITSO), organoindium, organotin, or zinc oxide (ZnO). Moreover, the film may be formed using indium zinc oxide (IZO) containing zinc oxide, a material in which zinc oxide is doped with gallium (Ga), tin oxide (SnO 2 ), indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, or the like. The film formed using such a material can be formed by a sputtering method. Note that when the conductive layer illustrated in  FIGS.  1  to  14    is formed using a conductive film with a layered structure, it is preferable that the light transmittance of the layered structure be sufficiently high. 
     Note that part or all of the wirings such as a source signal line, a gate signal line, a capacitor wiring, and a power supply line can be formed using a material with high conductivity. That is, the wiring can be formed using a material without light-transmitting properties. For example, the wiring can be a stack of a layer with light-transmitting properties and a layer without light-transmitting properties. The wiring in such a case can be formed, for example, with a single-layer structure or a layered structure using a metal material such as aluminum (Al), tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), neodymium (Nd), niobium (Nb), cerium, (Ce), or chromium (Cr); an alloy material containing any of the above metal materials as its main component; or nitride containing any of the above metal materials as its component. 
     In the case where ITO is used for one of conductive layers and aluminum is used for another conductive layer, a chemical reaction might occur when the conductive layers are connected to each other. For that reason, a high melting point material is preferably used between the conductive layers in order to prevent a chemical reaction. Examples of the high melting point material are molybdenum, titanium, tungsten, tantalum, and chromium. Moreover, the conductive layer preferably has a multi-layer structure in which a material with high conductivity is used over a film formed using the high melting point material. Examples of the material with high conductivity are aluminum, copper, and silver. For example, in the case where the conductive layer is formed with a layered structure, the conductive layer can be formed using a stack in which the first layer is molybdenum, the second layer is aluminum, and the third layer is molybdenum; or a stack in which the first layer is molybdenum, the second layer is aluminum containing a small amount of neodymium, and the third layer is molybdenum. With such a structure, the formation of hillocks can be prevented. 
     The insulating layers illustrated in  FIGS.  1  to  14   , for example, the insulating layers  105 ,  202 ,  205 ,  405 , and  1105  can be formed with a single-layer structure or a layered structure of a silicon oxide film, a silicon oxynitride film, a silicon nitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, or a tantalum oxide film. Each of the insulating layers can be formed to a thickness of 50 nm to 250 nm by a sputtering method or the like. For example, as the insulating layer, a 100-nm-thick silicon oxide film can be formed by a sputtering method or a CVD method, or a 100-nm-thick aluminum oxide film can be formed by a sputtering method. Moreover, the insulating layer can be formed with a single-layer structure or a layered structure of an insulating film containing oxygen and/or nitrogen, such as silicon oxide, silicon nitride, silicon oxynitride, or silicon nitride oxide; a film containing carbon such as DLC (diamond-like carbon); and a film formed using an organic material such as epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic or a siloxane material such as a siloxane resin. 
     Further, each of the insulating layers illustrated in  FIGS.  1  to  14    can have a function of a color filter and/or a black matrix. When a color filter is provided on the substrate  101  side, it is not necessary to provide a color filter on the counter substrate side. Therefore, a margin for adjusting the positions of two substrates is not necessary, which can facilitate manufacture of a panel. 
     An oxide semiconductor containing In, M, or Zn, for example, can be used for the semiconductor layer illustrated in any of  FIGS.  2  to  14   , for example, the semiconductor layer  203 . Here, M represents one or a plurality of metal elements selected from Ga, Fe, Ni, Mn, Co, and the like. In addition, when Ga is employed as M, a semiconductor film formed using this material is referred to as an In—Ga—Zn—O-based non-single-crystal film. Further, the above oxide semiconductor may contain Fe or Ni, another transitional metal element, or an oxide of the transitional metal as an impurity element in addition to the metal element contained as M. The semiconductor layer  203  may contain an insulating impurity. As the impurity, insulating oxide typified by silicon oxide, germanium oxide, aluminum oxide, or the like; insulating nitride typified by silicon nitride, aluminum nitride, or the like; or insulating oxynitride such as silicon oxynitride or aluminum oxynitride is used. Such an insulating oxide or insulating nitride is added to the oxide semiconductor at a concentration at which electrical conductivity of the oxide semiconductor does not deteriorate. When the insulating impurity is contained in the oxide semiconductor, crystallization of the oxide semiconductor can be suppressed. By suppressing the crystallization of the oxide semiconductor, characteristics of a thin film transistor can be stabilized. 
     When an In—Ga—Zn—O-based oxide semiconductor is made to contain an impurity such as silicon oxide, crystallization of the oxide semiconductor or generation of microcrystal grains can be prevented even by heat treatment at 300° C. to 600° C. In a manufacturing process of a thin film transistor in which an In—Ga—Zn—O-based oxide semiconductor layer serves as a channel formation region, the S value (a subthreshold swing value) or field effect mobility can be improved by heat treatment. Even in such a case, the thin film transistor can be prevented from being normally-on. Further, even when heat stress or bias stress is added to the thin film transistor, variations in threshold voltage can be prevented. 
     As the oxide semiconductor applied to the channel formation region of the thin film transistor, any of the following oxide semiconductors can be used in addition to the above: an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, an Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, an Sn—Al—Zn—O-based oxide semiconductor, an In—Zn—O-based oxide semiconductor, an Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, an In—O-based oxide semiconductor, an Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductor. In other words, by addition of an impurity that suppresses crystallization to keep an amorphous state to such an oxide semiconductor, characteristics of the thin film transistor can be stabilized. Examples of the impurity are insulating oxide such as silicon oxide, germanium oxide, or aluminum oxide; insulating nitride such as silicon nitride or aluminum nitride; and insulating oxynitride such as silicon oxynitride or aluminum oxynitride. 
     For example, a semiconductor film can be formed by a sputtering method using an oxide semiconductor target including In, Ga, and Zn (In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1). The following conditions may be employed for the sputtering, for example: the distance between the substrate  101  and the target is 30 mm to 500 mm; the pressure is 0.1 Pa to 2.0 Pa; the direct current (DC) power supply output is 0.25 kW to 5.0 kW (when the target of 8 inches in diameter is used); and the atmosphere is an argon atmosphere, an oxygen atmosphere, or a mixed atmosphere of argon and oxygen. The semiconductor film may have a thickness of approximately 5 nm to 200 nm. 
     As the sputtering method, an RF sputtering method using a high frequency power supply as a power supply for sputtering, a DC sputtering method, a pulsed DC sputtering method in which a DC bias is applied in pulses, or the like can be employed. An RF sputtering method is mainly used for forming an insulating film, and a DC sputtering method is mainly used for forming a metal film. 
     A multi-target sputtering apparatus in which a plurality of targets that are formed of different materials from each other may be used. In a multi-target sputtering apparatus, a stack of different films can be formed in one chamber, or one film can be formed by sputtering using plural kinds of materials at the same time in one chamber. Moreover, a method using a magnetron sputtering apparatus in which a magnetic field generating system is provided inside the chamber (a magnetron sputtering method), an ECR sputtering method in which plasma generated using a micro wave is used, or the like may be employed. Alternatively, a reactive sputtering method in which a target substance and a sputtering gas component chemically react with each other to form a compound thereof at the time of film formation, a bias sputtering method in which voltage is applied also to the substrate at the time of film formation, or the like may be employed. 
     Note that a semiconductor material used for a channel layer of the transistor  207  is not limited to an oxide semiconductor. For example, a silicon layer (an amorphous silicon layer, a microcrystalline silicon layer, a polycrystalline silicon layer, or a single crystal silicon layer) may be used as the channel layer of the transistor  207 . Other than the above, a light-transmitting organic semiconductor material, a carbon nanotube, or a compound semiconductor such as gallium arsenide or indium phosphide may be used for the channel layer of the transistor  207 . 
     Note that after formation of the semiconductor layer  203 , it is preferable to perform heat treatment at 100° C. to 600° C., typically 200° C. to 400° C. in a nitrogen atmosphere or an air atmosphere. For example, heat treatment can be performed at 350° C. for one hour in a nitrogen atmosphere. Through the heat treatment, rearrangement at the atomic level is performed in the island-shaped semiconductor layer  203 . This heat treatment (including light annealing and the like) is important in terms of releasing distortion that interrupts carrier movement in the island-shape semiconductor layer  203 . Note that there is no particular limitation on the timing of the heat treatment as long as it is performed after the semiconductor layer  203  is formed. 
     A semiconductor device or a display device can be manufactured using the above-described materials, for example. 
     Embodiment 3 
     In this embodiment, a display device will be described. The display device according to this embodiment includes a first circuit that has a function of controlling input of an image signal, a second circuit that has a function of holding an image signal, a third circuit that has a function of controlling the polarity of voltage supplied to a display element such as a liquid crystal element, and a display element. The display device according to this embodiment has a memory function for storing data in a pixel. 
       FIG.  15    is a circuit diagram (a block diagram) of the entire display device (also referred to as semiconductor device). A plurality of pixels are arranged in a matrix in a pixel portion  1501 . A circuit  1502  and a circuit  1503  that are used for driving or controlling the pixel portion  1501  are provided near the pixel portion  1501 . Moreover, the display device includes a circuit  1504  that supplies signals to the circuits  1502  and  1503 . 
     The circuit  1502  can have a function of controlling a potential of a gate of a transistor placed in the pixel portion  1501 . For that reason, the circuit  1502  can have a function of a circuit called a gate line driver circuit, a gate driver, or a scan driver. The circuit  1503  can have a function of controlling a potential of a source or a drain of the transistor placed in the pixel portion  1501  or a function of supplying an image signal to the pixel portion  1501 . For that reason, the circuit  1503  can have a function of a circuit called a source line driver circuit, a source driver, or a data driver. The circuit  1503  can also be formed using only analog switches. A variety of signals such as a clock signal, a start pulse signal, a latch signal, an image signal, and an inversion signal of counter voltage are input to the circuits  1502  and  1503 . For that reason, the circuit  1504  can have a function of a so-called controller, pulse generator, or the like. 
     Next,  FIG.  16    illustrates an example of a pixel placed in the pixel portion  1501 .  FIG.  16    is a circuit diagram of one pixel. The pixel includes the circuit  102 , the circuit  103 , the circuit  104 , a capacitor  1612 , and a display element  1613  having a pixel electrode. Note that one example of this embodiment of the present invention is not limited to this structure. 
     The circuit  102  includes a switch  1602 . The circuit  103  includes an inverter  1603  and capacitors  1604  and  1605 . The circuit  104  includes switches  1606  and  1607 . The inverter  1603  may have a function of inverting a signal or a function of setting the output in a high impedance state (a floating state). 
     The switch  1602  is connected to a wiring  1601 . The capacitor  1605  is connected between a wiring  1611  and the switch  1602 . The capacitor  1604  is connected between a wiring  1610  and an output terminal of the inverter  1603 . An input terminal of the inverter  1603  is connected to the switch  1602 . The output terminal of the inverter  1603  is connected to the capacitor  1604 . A wiring  1608  and a wiring  1609  are connected to each other through the switches  1606  and  1607 . On and off (conduction and non-conduction) of the switch  1606  are controlled with an output signal of the inverter  1603  or a signal held in the capacitor  1604 . On and off (conduction and non-conduction) of the switch  1607  are controlled with an input signal of the inverter  1603  or a signal held in the capacitor  1605 . The display element  1613  is connected between a wiring  1615  and a node of the switch  1606  and the switch  1607 . The capacitor  1612  is connected between a wiring  1614  and the node of the switch  1607  and the switch  1606  or between the wiring  1614  and the pixel electrode of the display element  1613 . 
     In the example of the structure in  FIG.  16   , the capacitor  1612  can be omitted. Alternatively, the capacitor  1604  can be omitted. 
     When the circuit diagram in  FIG.  16    is associated with the cross-sectional view in  FIG.  1   , the conductive layer  109  can correspond to a counter electrode or a common electrode, for example. Moreover, the conductive layer  109  can correspond to the wiring  1615 . It can be said that the display element  1613  includes the medium  107 . The conductive layer  106  has a function of a pixel electrode and can correspond to the pixel electrode of the display element  1613 . 
     Note that the wiring  1610  and the wiring  1611  can be connected to each other to be formed as one wiring. Moreover, when the wiring  1610  and/or the wiring  1611  is/are connected to the wirings  1608 ,  1609 , and  1614 , they can be formed as one wiring. When the wiring  1614  is connected to the wiring  1608 , the wiring  1609 , the wiring  1610 , or the wiring  1611 , they can be formed as one wiring. 
     The switch  1602  can have a function of controlling whether a signal supplied to the wiring  1601  is input to a pixel (or the capacitors  1604  and  1605  and the inverter  1603 ). For that reason, the switch  1602  can have a switching function or a selection function. 
     The wiring  1601  is electrically connected to the circuit  1503  illustrated in  FIG.  15   . Thus, an image signal can be supplied from the circuit  1503  to the wiring  1601 . For that reason, the wiring  1601  can be referred to as a source line, a source signal line, a data line, a data signal line, or the like. 
     When the wiring  1601  is formed using a light-transmitting material, the aperture ratio can be increased. However, this embodiment is not limited to using a light-transmitting material. For example, when the wiring  1601  is formed using a material that does not have light-transmitting properties and has high conductivity, signal delay can be reduced. Moreover, the wiring  1601  can be formed using a stack including a layer of a material with high conductivity and a layer of a light-transmitting material. 
     A signal input to a pixel through the switch  1602  is held in the capacitor  1605 . The capacitor  1605  has a function of holding the signal. For that reason, the capacitor  1605  can be referred to as a memory. Furthermore, it can be said that the capacitor  1605  is a DRAM because the signal held in the capacitor  1605  might attenuate over time. 
     The inverter  1603  has a function of inverting a signal held in the capacitor  1605  or a signal supplied from the wiring  1601  through the switch  1602  and outputting the resulting signal. Then, the signal output from the inverter  1603  is held in the capacitor  1604 . It can be said that the capacitor  1604  is a DRAM because the signal held in the capacitor  1604  might attenuate over time. 
     Since the inverter  1603  is provided, the signal held in the capacitor  1605  and the signal held in the capacitor  1604  are usually inverse to each other. Thus, when one of the signals is an H signal (a high-level signal), the other of the signals is often an L signal (a low-level signal), except in the case where the inverter  1603  does not output a signal, for example, the case where the output of the inverter  1603  is in a high impedance state. 
     The switch  1606  has a function of controlling whether a potential of the wiring  1608  is supplied to the capacitor  1612  or the display element  1613 . Similarly, the switch  1607  has a function of controlling whether a potential of the wiring  1609  is supplied to the capacitor  1612  or the display element  1613 . 
     Since the signal held in the capacitor  1605  and the signal held in the capacitor  1604  are usually inverse to each other as described above, one of the switches  1606  and  1607  is on (in a conduction state) and the other is off (in a non-conduction state) in many cases. Therefore, in that case, either the potential of the wiring  1609  or the potential of the wiring  1608  is supplied to the display element  1613 . At this time, when the potential of the wiring  1609  and the potential of the wiring  1608  are different from each other, potentials supplied to the display element  1613  vary; thus, the display element  1613  can be controlled to be in different states (e.g., states where the display element  1613  transmits light and does not transmits light, states where the display element  1613  emits light and does not emit light, states where the display element  1613  is dark and bright, or states where the display element  1613  scatters light and transmits light). For that reason, the display state can be changed, so that gradation can be expressed to display images. 
     Next, an example of operation of the circuit illustrated in  FIG.  16    will be described. First, as illustrated in  FIG.  17 A , an H signal is supplied from the wiring  1601 . When the switch  1602  is on, an H signal is input to the capacitor  1605 . An L signal is input to the capacitor  1604  through the inverter  1603 . 
     Next, as illustrated in  FIG.  17 B , the switch  1602  is turned off. Then, the signals stored in the capacitors  1604  and  1605  are maintained. The L signal is stored in the capacitor  1604 , and the H signal is stored in the capacitor  1605 . Therefore, assuming that a switch is turned on when an H signal is supplied to a control terminal of the switch and the switch is turned off when an L signal is supplied to the control terminal, the switch  1606  is turned off and the switch  1607  is turned on. Thus, a potential V 1  of the wiring  1609  is supplied to the pixel electrode of the display element  1613 . Assuming that a potential Vcom is applied to the wiring  1615 , a voltage of the difference between V 1  and Vcom is applied to the display element  1613 . At this time, when the potential V 1  is larger than the potential Vcom, a positive voltage is applied to the display element  1613 . If the display element  1613  is normally black (i.e., if the display element  1613  is brought into a black state when voltage is not applied), the display element  1613  expresses white. In contrast, if the display element  1613  is normally white (i.e., if the display element  1613  is brought into a white state when voltage is not applied), the display element  1613  expresses black. 
     When the display element  1613  needs to be driven by alternating current, for example, when the display element  1613  is a liquid crystal element, it is necessary to apply negative voltage to the display element  1613 . In that case, the potential of the wiring  1609  is changed from V 1  to V 2  as illustrated in  FIG.  17 C . At this time, the potential V 2  is lower than the potential Vcom. As an example, (V 1 −Vcom) and (Vcom−V 2 ) are approximately the same. Thus, negative voltage is applied to the display element  1613 . After that, the state in  FIG.  17 B  and the state in  FIG.  17 C  are alternately repeated every predetermined cycle, whereby the display element  1613  can be driven by alternating current. 
     At this time, since the signals are held in the capacitors  1604  and  1605 , a signal does not need to be input from the wiring  1601  again and the display element  1613  can be driven by alternating current by alternately repeating the state in  FIG.  17 B  and the state in  FIG.  17 C . Thus, power consumption can be reduced. Then, when the signals in the capacitors  1604  and  1605  need to be refreshed, the operation returns to  FIG.  17 A  and a signal is input again from the wiring  1601 . 
       FIGS.  17 A to  17 C  illustrate the operation in the case where an H signal is input from the wiring  1601 ; the circuit similarly operates in the case where an L signal is input.  FIGS.  18 A to  18 C  illustrate an example in this case. 
     First, as illustrated in  FIG.  18 A , an L signal is supplied from the wiring  1601 . When the switch  1602  is on, an L signal is input to the capacitor  1605 . An H signal is input to the capacitor  1604  through the inverter  1603 . 
     Next, as illustrated in  FIG.  18 B , the switch  1602  is turned off. Then, the signals stored in the capacitors  1604  and  1605  are maintained. The H signal is stored in the capacitor  1604 , and the L signal is stored in the capacitor  1605 . Therefore, the switch  1606  is turned on and the switch  1607  is turned off. Thus, a potential V 3  of the wiring  1608  is supplied to the pixel electrode of the display element  1613 . Assuming that the potential Vcom is applied to the wiring  1615 , a voltage of the difference between V 3  and Vcom is applied to the display element  1613 . At this time, when the potential V 3  and the potential Vcom are approximately the same, almost no voltage is applied to the display element  1613 . If the display element  1613  is normally black (i.e., if the display element  1613  is brought into a black state when voltage is not applied), the display element  1613  expresses black. In contrast, if the display element  1613  is normally white (i.e., if the display element  1613  is brought into a white state when voltage is not applied), the display element  1613  expresses white. 
     When the display element  1613  needs to be driven by alternating current, for example, when the display element  1613  is a liquid crystal element, the potential of the wiring  1609  is changed from V 1  to V 2  as illustrated in  FIG.  18 C . However, the potential of the wiring  1608  is not changed. Thus, even when the potential of the wiring  1609  is changed, almost no voltage is applied to the display element  1613 . After that, the state in  FIG.  18 B  and the state in  FIG.  18 C  are alternately repeated every predetermined cycle. Then, when the signals in the capacitors  1604  and  1605  need to be refreshed, the operation returns to  FIG.  18 A  and a signal is input again from the wiring  1601 . 
     As described above, in both the case where an H signal is input from the wiring  1601  and the case where an L signal is input from the wiring  1601 , display can be performed with alternating driving or inversion driving. 
     Note that in the driving methods illustrated in  FIGS.  17 A to  17 C  and  FIGS.  18 A to  18 C , the potential of the wiring  1615  is not changed when the polarity of the display element  1613  is inverted, that is, when alternating-current driving is performed. In contrast, by changing the potential of the wiring  1615 , the amplitude of the potential of the wiring  1609  (the difference between V 1  and V 2 ) can be made smaller. This means that the potential of the wiring  1615 , that is, a potential of the counter electrode or the common electrode is changed, and such driving is called common inversion driving. 
     Then, an operation method with common inversion driving is illustrated in  FIGS.  19 A to  19 C  and  FIGS.  20 A to  20 C . 
     First, as illustrated in  FIG.  19 A , an H signal is supplied from the wiring  1601 . When the switch  1602  is on, an H signal is input to the capacitor  1605 . An L signal is input to the capacitor  1604  through the inverter  1603 . 
     Next, as illustrated in  FIG.  19 B , the switch  1602  is turned off. Then, the signals stored in the capacitors  1604  and  1605  are maintained. The L signal is stored in the capacitor  1604 , and the H signal is stored in the capacitor  1605 . Therefore, the switch  1606  is turned off and the switch  1607  is turned on. Thus, a potential V 5  of the wiring  1609  is supplied to the pixel electrode of the display element  1613 . Assuming that a potential V 6  is applied to the wiring  1615 , a voltage of the difference between V 5  and V 6  is applied to the display element  1613 . At this time, when the potential V 5  is larger than the potential V 6 , a positive voltage is applied to the display element  1613 . If the display element  1613  is normally black (i.e., if the display element  1613  is brought into a black state when voltage is not applied), the display element  1613  expresses white. In contrast, if the display element  1613  is normally white (i.e., if the display element  1613  is brought into a white state when voltage is not applied), the display element  1613  expresses black. 
     Note that at this time, a potential supplied to the wiring  1614  is not limited to a specific value. The potential of the wiring  1614  is preferably changed with the same amplitude as the potential supplied to the wiring  1615 . For that reason, the potential supplied to the wiring  1614  is preferably the same as that supplied to the wiring  1615 , for example. However, this embodiment is not limited thereto. 
     When the display element  1613  needs to be driven by alternating current, for example, when the display element  1613  is a liquid crystal element, it is necessary to apply negative voltage to the display element  1613 . In that case, the potential of the wiring  1609  is changed from V 5  to V 6  as illustrated in  FIG.  19 C . Moreover, the potentials of the wirings  1608 ,  1614  and  1615  are changed from V 6  to V 5 . At this time, the potential V 6  is lower than the potential V 5 . Thus, negative voltage is applied to the display element  1613 . After that, the state in  FIG.  19 B  and the state in  FIG.  19 C  are alternately repeated every predetermined cycle, whereby the display element  1613  can be driven by alternating current. Then, when the signals in the capacitors  1604  and  1605  need to be refreshed or the signals need to be rewritten, the operation returns to  FIG.  19 A  and a signal is input again from the wiring  1601 . 
     At this time, the potential of the wiring  1615  is also changed, so that the amount of change (amplitude) of the potential of the wiring  1609  can be reduced. Thus, power consumption can be reduced. 
       FIGS.  19 A to  19 C  illustrate the operation in the case where an H signal is input from the wiring  1601 ; the circuit similarly operates in the case where an L signal is input.  FIGS.  20 A to  20 C  illustrate an example in this case. 
     First, as illustrated in  FIG.  20 A , an L signal is supplied from the wiring  1601 . When the switch  1602  is on, an L signal is input to the capacitor  1605 . An H signal is input to the capacitor  1604  through the inverter  1603 . 
     Next, as illustrated in  FIG.  20 B , the switch  1602  is turned off. Then, the signals stored in the capacitors  1604  and  1605  are maintained. The H signal is stored in the capacitor  1604 , and the L signal is stored in the capacitor  1605 . Therefore, the switch  1606  is turned on and the switch  1607  is turned off. Thus, the potential V 6  of the wiring  1608  is supplied to the pixel electrode of the display element  1613 . Assuming that the potential V 6  is applied to the wiring  1615 , almost no voltage is applied to the display element  1613 . If the display element  1613  is normally black (i.e., if the display element  1613  is brought into a black state when voltage is not applied), the display element  1613  expresses black. In contrast, if the display element  1613  is normally white (i.e., if the display element  1613  is brought into a white state when voltage is not applied), the display element  1613  expresses white. 
     When the display element  1613  needs to be driven by alternating current, for example, when the display element  1613  is a liquid crystal element, the potential of the wiring  1609  is changed from V 5  to V 6  as illustrated in  FIG.  20 C . Moreover, the potentials of the wirings  1608 ,  1614  and  1615  are changed from V 6  to V 5 . Thus, even when the potential of the wiring  1614  is changed, almost no voltage is applied to the display element  1613 . After that, the state in  FIG.  20 B  and the state in  FIG.  20 C  are alternately repeated every predetermined cycle. Then, when the signals in the capacitors  1604  and  1605  need to be refreshed or the signals need to be rewritten, the operation returns to  FIG.  20 A  and a signal is input again from the wiring  1601 . 
     As described above, in both the case where an H signal is input from the wiring  1601  and the case where an L signal is input from the wiring  1601 , display can be performed with alternating driving or inversion driving. Moreover, the amplitude of an image signal can be decreased. Furthermore, inversion driving can be performed without inputting an image signal again, resulting in reduction in power consumption. 
     Embodiment 4 
     In this embodiment, a circuit included in a display device (a semiconductor device) will be described with reference to drawings. 
       FIGS.  21 A to  21 E  illustrate specific examples of the inverter  1603  illustrated in  FIG.  16    and the like.  FIG.  21 A  illustrates an example of the case where the inverter  1603  has a CMOS structure. A p-channel transistor  2101  and an n-channel transistor  2102  are connected in series between a wiring  2104  and a wiring  2103 . This CMOS structure has functions such that a low voltage is supplied to the wiring  2103  and a high voltage is supplied to the wiring  2104 . With such a CMOS structure, the direct tunneling current can be reduced, resulting in reduction in power consumption. 
     Note that a voltage that does not change over time can be supplied to the wiring  2103  or the wiring  2104 ; however, this embodiment is not limited thereto and a pulsed signal can be supplied. 
     Note that it is possible to form a p-channel transistor by using not only polycrystalline silicon but also an oxide semiconductor for a semiconductor layer. A p-type zinc oxide film can be realized with various kinds of p-type dopant and doping methods, for example, by using substitutional doping with dopant serving as an acceptor (e.g., N, B, Cu, Li, Na, K, Rb, P, or As or a mixture of such elements). However, this embodiment is not limited thereto. 
       FIG.  21 B  illustrates an example of the structure of the inverter  1603  including a resistor  2101   a  instead of the p-channel transistor  2101 . A semiconductor layer that is in the same layer as a semiconductor layer included in the n-channel transistor  2102  can be used for the resistor  2101   a . Thus, the resistor  2101   a  can be formed using an oxide semiconductor, for example. In this case, an oxide semiconductor layer that is used as a channel layer of the re-channel transistor  2102  and the oxide semiconductor layer used for the resistor  2101   a  can be formed using the layers in the same layer. 
       FIG.  21 C  illustrates an example of the structure of the inverter  1603  including a transistor  2101   b  instead of the p-channel transistor  2101 . The transistor  2101   b  is diode-connected. Note that the transistor  2101   b  can be a depletion (normally-on) transistor so that the transistor is on and current can flow therethrough even when the gate-source voltage is 0 V or lower. Since the n-channel transistor  2102  and the transistor  2101   b  have the same polarity, the number of process steps can be reduced. 
       FIG.  21 D  illustrates an example of the inverter  1603  having a bootstrap function. The inverter  1603  includes a transistor  2101   c , a transistor  2101   d , a transistor  2102   a , and a transistor  2102   b . A potential of a gate of the transistor  2101   d  is made sufficiently high by bootstrap operation. Thus, a potential of a wiring  2104   b  can be output without change. The inverter  1603  in  FIG.  21 D  has functions such that a low voltage is supplied to wirings  2103   a  and  2103   b  and a high voltage is supplied to a wiring  2104   a  and the wiring  2104   b . Note that the wiring  2103   a  and the wiring  2103   b  may be connected to be unified as one wiring. Similarly, the wiring  2104   a  and the wiring  2104   b  may be connected to be unified as one wiring. Note that since the transistors  2101   c ,  2101   d ,  2102   a , and  2102   b  have the same polarity, the number of process steps can be reduced. 
     Note that  FIGS.  21 B to  21 D  each illustrate the case where the transistor is an re-channel transistor; however, this embodiment is not limited thereto. The circuit can be similarly formed when a p-channel transistor is used. As an example,  FIG.  21 E  illustrates an example of the case where the transistors  2101   b  and  2102  in  FIG.  21 C  are p-channel transistors. The inverter is constituted by a p-channel transistor  2101   p  and a p-channel transistor  2102   p.    
     Next, specific examples of the switches  1602 ,  1606 ,  1607 , and the like illustrated in  FIG.  16    and the like are shown. The switches  1602 ,  1606 , and  1607  can be referred to as analog switches or transfer gates.  FIGS.  22 A to  22 D  illustrate examples of the structure of the switch  1602 . Note that the other switches such as the switches  1606  and  1607  can formed in a manner similar to that of the switch  1602 . 
       FIG.  22 A  illustrates a structure example of the switch  1602  with a CMOS structure. The switch  1602  is formed by connecting a p-channel transistor  2202  and an n-channel transistor  2201  in parallel with each other. Note that signals that are inverse to each other are preferably supplied to a gate of the p-channel transistor  2202  and a gate of the n-channel transistor  2201 . Thus, the p-channel transistor  2202  and the n-channel transistor  2201  can be turned on and off at the same time. With such a CMOS structure, the amplitude of voltage applied to the gates of the p-channel transistor  2202  and the n-channel transistor  2201  can be reduced. Consequently, power consumption can be reduced. 
       FIG.  22 B  illustrates an example of the structure of the switch  1602  that includes the re-channel transistor  2201  without using a p-channel transistor.  FIG.  22 C  illustrates an example of the structure of the switch  1602  constituted by the n-channel transistor  2201  with a multi-gate structure. 
     Note that the transistor  2201  is an n-channel transistor in the structure examples in  FIGS.  22 B and  22 C ; however, the structure of the switch  1602  is not limited to these examples. For example, the switch can be similarly formed by using a p-channel transistor. As an example,  FIG.  22 D  illustrates an example of the structure of the switch  1602  in the case where the n-channel transistor  2201  in  FIG.  22 B  is replaced with a p-channel transistor. The switch  1602  is constituted by a p-channel transistor  2201   p.    
     Next,  FIG.  23    illustrates an example of the case where the circuit illustrated in  FIG.  16   ,  FIGS.  17 A to  17 C ,  FIGS.  18 A to  18 C ,  FIGS.  19 A to  19 C , and  FIGS.  20 A to  20 C  is constituted by the circuits illustrated in  FIGS.  21 A to  21 E  and  FIGS.  22 A to  22 D . In  FIG.  23   , the switches  1602 ,  1606 , and  1607  are constituted by the circuit in  FIG.  22 B  and the inverter  1603  is constituted by the circuit in  FIG.  21 C . Therefore, the circuit in  FIG.  23    is constituted by the transistors of the same polarity. Thus, the number of process steps can be reduced. Needless to say, this embodiment is not limited to this structure and other structures can be employed. 
     In the circuit in  FIG.  23   , a gate of a transistor  1602   a  is connected to a wiring  2301 . The wiring  2301  can be connected to the circuit  1502  in  FIG.  15   . Thus, a selection signal can be supplied from the circuit  1502  to the wiring  2301 . For that reason, the wiring  2301  can be referred to as a gate line, a gate signal line, a scan line, a scan signal line, or the like. 
     When the wiring  2301  is formed using a light-transmitting material, the aperture ratio can be increased. However, this embodiment is not limited to using a light-transmitting material. For example, when the wiring  2301  is formed using a material that does not have light-transmitting properties and has high conductivity, signal delay can be reduced. When the wiring  2301  is formed using a material with high conductivity, the wiring can be formed with a multi-layer structure including a layer of a light-transmitting material. 
     In  FIG.  23   , a liquid crystal element  1613   a  is used as the display element  1613 . Transistors  1602   a ,  1606   a , and  1607   a  are provided as the switches  1602 ,  1606 , and  1607 , respectively. 
     Note that both the capacitor  1604  and the capacitor  1605  are connected to the wiring  1610  in  FIG.  23   . That is, it can be said that the wiring  1611  is omitted so that the wirings  1610  and  1611  are unified as the wiring  1610 . 
     Note that although the capacitor  1612  is omitted in  FIG.  23   , the capacitor  1612  may be provided. 
     Note that the wiring  2103  may be connected to the wiring  1610  so that they are unified as one wiring. 
     In  FIG.  23   , the inverter  1603  has the configuration in  FIG.  21 C . In this case, current might continue to flow through the transistors  2101   b  and  2102  depending on a signal input to the inverter  1603 . That is, it is possible that the direct tunneling current flows through the inverter  1603 . In this case, extra power might be consumed by the inverter  1603 . 
     The direct tunneling current can be reduced by devising a driving method.  FIGS.  24  to  27    illustrate a driving method for reducing the direct tunneling current of the inverter  1603 . The idea is that the potential of the wiring  2104  is lowered when it is not necessary to remain high, instead of being kept high. As a result, the transistor  2101   b  is turned off, and the flow of direct tunneling current can be reduced. In this case, the output of the inverter  1603  is sometimes in a high impedance state. 
     First, as illustrated in  FIG.  24   , an H signal is input from the wiring  1601  through the transistor  1602   a . At that time, a potential V 7  of the wiring  2104  is higher than a potential of the wiring  2103 . Then, the n-channel transistor  2102  is turned on and outputs an L signal to the capacitor  1604 . At this time, the transistor  2101   b  is also turned on; however, an L signal is output because of the difference of on-resistances between the transistor  2102  and the transistor  2101   b . At this time, the direct tunneling current continues to flow. In order for the inverter  1603  to output an L signal, WIL (the ratio of channel width W to channel length L) of the n-channel transistor  2102  is preferably larger than that of the transistor  2101   b.    
     Next, as illustrated in  FIG.  25   , the potential of the wiring  2104  is lowered to a potential V 8 . Here, V 7  is higher than V 8 , and the potential V 8  is substantially the same as the potential of the wiring  2103 . As a result, the transistor  2101   b  is turned off, so that the flow of direct tunneling current can be reduced. Moreover, the signal held by the capacitor  1604  remains an L signal. 
     Note that the transistor  1602   a  is off at this time; the transistor  1602   a  can be turned off before, after, or at the same time as changing the potential of the wiring  2104 . 
     Note that the potential V 8  of the wiring  2104  at this time is approximately the same as the potential of the wiring  2103 . The potential V 8  is preferably lower than the potential that is higher than the potential of the wiring  2103  by the threshold voltage of the transistor  2101   b . More preferably, the potential V 8  is equal to the potential of the wiring  2103 . Thus, the number of potentials needed can be reduced, so that the size of the device can be reduced. 
       FIGS.  24  and  25    illustrate the case where an H signal is input from the wiring  1601 ; the case where an L signal is input is similar to that case and illustrated in  FIGS.  26  and  27   . First, as illustrated in  FIG.  26   , an L signal is input from the wiring  1601  through the transistor  1602   a . At this time, the potential V 7  of the wiring  2104  is higher than the potential of the wiring  2103 . Then, since the n-channel transistor  2102  is off, the transistor  2101   b  is turned on and outputs an H signal to the capacitor  1604 . Since the n-channel transistor  2102  is off at this time, direct tunneling current does not flow. Note that the potential of the H signal at this time is lower than the potential V 7  of the wiring  2104  by the threshold voltage of the transistor  2101   b ; operation is not affected as long as the H signal is a voltage that can make the transistor  1606   a  turn on. 
     Next, as illustrated in  FIG.  27   , the potential of the wiring  2104  is lowered to the potential V 8 . Here, V 7  is higher than V 8 . As a result, the transistor  2101   b  is turned off. Moreover, the signal held by the capacitor  1604  remains an H signal. In this case, the output of the inverter  1603  can be said to be in a high impedance state. 
     In such a manner, the potential of the wiring  2104  is kept high only when the inverter  1603  needs to output a signal, that is, when a signal in the capacitor  1604  needs to be rewritten, and the potential of the wiring  2104  is lowered when the inverter  1603  does not need to output a signal; thus, the direct tunneling current in the inverter  1603  can be reduced. Consequently, power consumption can be reduced. 
     Note that the direct tunneling current in the inverter  1603  is reduced by devising the driving method in  FIGS.  24  to  27   ; however, this embodiment is not limited to such a way. It is possible to reduce the direct tunneling current by changing part of the circuit configuration of the inverter  1603 .  FIGS.  28 A to  28 D  illustrate examples in that case. 
       FIG.  28 A  illustrates a configuration example of the inverter  1603  in which a switch  2802   a  connected in series with the transistor  2101   b  is provided between the wiring  2104  and the output terminal of the inverter  1603 . Note that the switch  2802   a  may be connected between the transistor  2101   b  and the output terminal of the inverter  1603  or between the wiring  2104  and the transistor  2101   b . A circuit diagram of the inverter  1603  in the case where a transistor  2802  is used as the switch  2802   a  in  FIG.  28 A  is illustrated in  FIG.  28 B . An example of the structure in the case where the connection relation of the transistor  2802  in  FIG.  28 B  is changed is illustrated in  FIG.  28 C . The potential of a gate of the transistor  2802  is controlled with such a circuit configuration to control on and off of the transistor  2802 , whereby the direct tunneling current can be reduced. In this case, the potential of the gate of the transistor  2802  can be controlled in a manner similar to that for changing the potential of the wiring  2104 . 
     In such a case, the potential of the wiring  2104  may be changed as in  FIGS.  24  to  27   ; a problem does not occur even when the potential of the wiring  2104  is kept fixed because the transistor  2802  can reduce the direct tunneling current. 
     The transistor is placed between the wiring  2104  and the output terminal of the inverter  1603  in  FIGS.  28 A to  28 C ; however, this embodiment is not limited to this structure and the transistor can be placed between the output terminal of the inverter  1603  and the wiring  2103 .  FIG.  28 D  illustrates an example of the structure of the inverter  1603  in the case where a transistor  2803  is connected in series with the n-channel transistor  2102  in the circuit in  FIG.  28 B . Note that the transistor  2803  can be provided at a different position so as to be placed between the p-channel transistor  2101  and the wiring  2103  as in  FIG.  28 C . 
     Note that a wiring  2801  connected to gates of the transistors  2802  and  2803  can be connected to the wiring  2301  in  FIG.  23   . Thus, the transistors  2802  and  2803  can be turned on only when the transistor  1602   a  is on. Therefore, the direct tunneling current can be reduced when the inverter  1603  does not need to operate. Furthermore, by connecting the wiring  2801  to the wiring  2301 , the number of wirings can be reduced as compared to the case where an additional wiring is provided. However, this embodiment is not limited to such a structure. 
     The control of the transistors  2802  and  2803  can control whether the inverter  1603  in  FIG.  28 D  outputs a signal or is brought into a high impedance state. Therefore, the inverter  1603  can be referred to as a clocked inverter. 
     Embodiment 5 
     In this embodiment, a circuit included in a display device (a semiconductor device) will be described with reference to drawings. 
       FIG.  29    illustrates an example of a configuration of a circuit obtained by changing part of the circuit in  FIG.  16   . The circuit in  FIG.  29    corresponds to a circuit obtained by adding a switch  2901  and a switch  2902  to the circuit in  FIG.  16   . 
     The switch  2902  has a function of controlling conduction and non-conduction between the display element  1613  and the wiring  1601 . Consequently, by turning on the switch  2902 , a signal supplied to the wiring  1601  can be supplied directly to the display element  1613 . Therefore, in general, a signal supplied to the wiring  1601  is often a digital signal; when the signal is an analog signal, the analog signal can be input directly to the display element  1613  and thus display can be performed with analog gray scale. 
     The switch  2901  has a control function so that the potentials of the wirings  1608  and  1609  are not supplied to the display element  1613 . If the potential of the wiring  1608  or the wiring  1609  is supplied to the display element  1613  when the switch  2902  is turned on and a signal is supplied from the wiring  1601  to the display element  1613  or when the switch  2902  is turned off and an analog signal is held in the display element  1613  after the switch  2902  is turned on and the signal is supplied from the wiring  1601  to the display element  1613 , the signal value held in the display element  1613  is changed. The switch  2901  is made on and off in order to prevent such a situation. When a signal is input through the switch  1602 , the switch  2901  is turned on so that the potential of the wiring  1608  or the wiring  1609  is supplied to the display element  1613 . Such a structure can realize both improvement in display quality and reduction in power consumption. However, this embodiment is not limited to such a structure. 
     As other configuration examples,  FIGS.  30  and  31    each illustrate an example of the case where the number of gray scales expressed by one pixel, that is, the number of bits is increased. The number of bits of an image signal held in a pixel is 1 bit in  FIG.  16   ; therefore, two gray scales are expressed. In order to realize multiple gray scales, a plurality of subpixels are provided in one pixel. When a plurality of subpixels are provided so that the display area of a display element is controlled, multiple gray scales can be achieved by an area gray scale method. 
       FIG.  30    is a circuit diagram in the case where image signals are simultaneously input to two subpixels by using the wiring  2301 . A wiring  1601   a  is provided in addition to the wiring  1601  in order to input signals simultaneously, and a signal supplied to either the wiring  1601  or the wiring  1601   a  is input to each of the subpixels. 
       FIG.  31    is a circuit diagram in the case where image signals are sequentially input to two subpixels by using the wiring  2301  and a wiring  2301   a . An image signal is input to one subpixel from the wiring  2301  and input to the other subpixel from the wiring  2301   a . Since image signals are sequentially input, signals can be sequentially input to the subpixels from the wiring  1601 . 
     Note that  FIGS.  30  and  31    illustrate the examples of the case where one pixel has two subpixels; however, the number of bits is not limited to two and can be increased. In particular, when a transistor or a capacitor is formed using a light-transmitting material, the increase in the number of bits does not decrease the aperture ratio, so that the number of bits can be easily increased. 
     Note that the wirings  2103 ,  1610 ,  1611 ,  1614 ,  1609 ,  1608 ,  2104 , and the like can be shared with a plurality of subpixels so that a plurality of wirings are unified as one wiring. Thus, the number of wirings can be reduced. 
     As another configuration example,  FIG.  42    illustrates the case where part of the circuit  104  is changed. A switch  4206  is provided in series with the switch  1606 . Alternatively, a switch  4207  is provided in series with the switch  1607 .  FIG.  42    illustrates the case where both the switch  4206  and the switch  4207  are provided. By controlling on and off of the switch  4206  and/or the switch  4207 , the potential of the wiring  1608  or the wiring  1609  can be prevented from being supplied to the display element  1613  regardless of on and off of the switch  1606  or the switch  1607 . 
     As another modification example,  FIG.  43    illustrates the case where part of the circuit  104  is changed.  FIG.  43    corresponds to a diagram in which two switches corresponding to the switch  1607  and two wirings corresponding to the wiring  1609  are provided in  FIG.  16   . A voltage for an anode is supplied to a wiring  1609   a , and a voltage for a cathode is supplied to a wiring  1609   b . A voltage with which voltage is not supplied to the display element  1613 , for example, a voltage that is substantially the same as the wiring  1615  is supplied to the wiring  1608 . The switch  1607  and a switch  4307   a  are connected in series between the display element  1613  and the wiring  1609   a . Similarly, a switch  1607   b  and a switch  4307   b  are connected in series between the display element  1613  and the wiring  1609   b . Note that these switches can be connected with a different structure as long as they are connected in series with each other. As an example of the operation, the switch  4307   a  and the switch  4307   b  are alternately turned on and off. That is, the switch  4307   b  is off when the switch  4307   a  is on, whereas the switch  4307   b  is on when the switch  4307   a  is off. Thus, inversion driving can be performed. 
     Note that  FIG.  43    illustrates the configuration in which the capacitor  1612  is not provided; alternatively, the capacitor  1612  can be provided. 
     Embodiment 6 
     In this embodiment, a circuit included in a display device (a semiconductor device) will be described with reference to drawings. 
       FIG.  32    is a plan view illustrating a layout example of the circuit illustrated in  FIG.  23   . The transistor  207  and the capacitor  208  that are illustrated in  FIG.  2    are used as a transistor and a capacitor in  FIG.  32   . A contact structure is such that a contact hole is formed in the insulating layer  202  so that the conductive layers  204   a  and  201   a  are directly connected to each other as in the contact structure  1302  in  FIG.  13   . With such a contact structure, the aperture ratio of a pixel can be increased. Alternatively, the contact resistance can be reduced, and a voltage drop can be reduced. Further alternatively, since the layout area can be reduced, a larger number of circuits can be arranged. However, this embodiment is not limited to such a structure. It is possible to employ a variety of transistor structures, contact structures, capacitor structures, and the like. 
     As illustrated in  FIG.  32   , a contact hole  3201   b  connects a drain electrode (a source electrode) of the transistor  1602   a  and a gate electrode of the n-channel transistor  2102 . Similarly, a contact hole  3201   a  connects a drain electrode (a source electrode) and a gate electrode of the transistor  2101   b.    
     Further, a contact hole  3202  connects a drain electrode (a source electrode) of the transistor  1607   a  (or a drain electrode (a source electrode) of the transistor  1606   a ) and a pixel electrode  3203 . 
     As illustrated in  FIG.  32   , the transistor, the capacitor, the wiring, and the like can be formed using light-transmitting materials. Thus, the aperture ratio can be increased. Note that one example of this embodiment of the present invention is not limited to this. For example, the wiring can be formed using a material without light-transmitting properties. An example of this case is illustrated in  FIG.  33   . 
     In  FIG.  33   , the wirings  1601 ,  2301 , and  2104  are formed using a material with high conductivity, and thus do not have light-transmitting properties. A signal with high frequency is supplied to these wirings. As a result, the formation of the wiring using a material with high conductivity can reduce the distortion of the waveform of a signal. 
     A structure example of part of the transistor  1602   a , the wiring  1601 , and the wiring  2103  in  FIG.  33    is described with reference to  FIGS.  34 A to  34 C .  FIG.  34 A  is a plan view of the structure example  FIG.  34 B  illustrates one example of a cross-sectional structure along line A-B in  FIG.  34 A .  FIG.  34 C  illustrates another example of a cross-sectional structure along line A-B. 
     In  FIG.  34 B , the wiring  1601  is a stack of a conductive layer  204   ab  and a conductive layer  204   aa . Here, the conductive layer  204   ab  is formed using a material that does not have light-transmitting properties and has high conductivity. The conductive layer  204   aa  is formed using a light-transmitting material. Similarly, the wiring  2103  is a stack of a conductive layer  201   ab  and a conductive layer  201   aa . The conductive layer  201   ab  is formed using a material that does not have light-transmitting properties and has high conductivity. The conductive layer  201   aa  is formed using a light-transmitting material. In such a manner, a light-transmitting layer can be placed under a layer without light-transmitting properties. In that case, the number of masks (reticles) can be reduced by using a multi-tone mask (also referred to as a half-tone mask or a gray-tone mask). For example, a pattern including a light-transmitting region and a region without light-transmitting properties can be formed using one mask in such a manner that the conductive layers  201   aa  and  201   ab  are successively formed and are etched at the same time, and then, a resist is subjected to ashing or the like and only the conductive layer  201   ab  is etched. 
     However, this embodiment is not limited to such a structure. As illustrated in  FIG.  34 C , the conductive layers can be arranged so that a region where a conductive layer  201   ba  is not provided is placed under a conductive layer  201   bb  and a region where a conductive layer  204   ba  is not provided is placed under a conductive layer  204   bb.    
     Note that the layers without light-transmitting properties (e.g., the conductive layers  201   ab ,  204   ab ,  201   bb , and  204   bb ) are provided over the layers with light-transmitting properties (e.g., the conductive layers  201   aa ,  204   aa ,  201   ba , and  204   ba ) in  FIGS.  34 B and  34 C ; however, this embodiment is not limited to this structure. For example, the layers can be formed in the reverse order. Moreover, it is possible to employ a layer structure in which the light-transmitting layer is sandwiched between the layers without light-transmitting properties. 
     A transistor and a capacitor can be formed using conductive layers that are formed by providing a light-transmitting layer under a layer without light-transmitting properties as illustrated in  FIG.  34 B .  FIG.  35    illustrates an example of a cross-sectional view of the case where such a layer structure is employed for forming a transistor and a capacitor that have the structure in  FIG.  2   . 
     The gate electrode is formed using conductive layers  201   ca  and  201   cb . The conductive layer  201   ca  has light-transmitting properties. The conductive layer  201   cb  does not have light-transmitting properties and has high conductivity. The source electrode (drain electrode) is formed using conductive layers  204   ca  and  204   cb . The conductive layer  204   ca  has light-transmitting properties. The conductive layer  204   cb  does not have light-transmitting properties and has high conductivity. The drain electrode (source electrode) is formed using conductive layers  204   da  and  204   db . The conductive layer  204   da  has light-transmitting properties. The conductive layer  204   db  does not have light-transmitting properties and has high conductivity. One electrode of the capacitor is formed using conductive layers  201   da  and  201   db . The conductive layer  201   da  has light-transmitting properties. The conductive layer  201   db  does not have light-transmitting properties and has high conductivity. The other electrode of the capacitor is formed using conductive layers  204   ea  and  204   eb . The conductive layer  204   ea  has light-transmitting properties. The conductive layer  204   eb  does not have light-transmitting properties and has high conductivity. 
     Similarly, a transistor and a capacitor can be formed using conductive layers that are formed so that a region where a light-transmitting layer is not provided is placed under a layer without light-transmitting properties as illustrated in  FIG.  34 C .  FIG.  36    illustrates an example of a cross-sectional view of the case where such a layer structure is employed for forming a transistor and a capacitor that have the structure in  FIG.  2   . 
     The gate electrode is formed using a conductive layer  201   eb . The conductive layer  201   eb  does not have light-transmitting properties and has high conductivity. The source electrode (drain electrode) is formed using a conductive layer  204   fb . The conductive layer  204   fb  does not have light-transmitting properties and has high conductivity. The drain electrode (source electrode) is formed using a conductive layer  204   gb . The conductive layer  204   gb  does not have light-transmitting properties and has high conductivity. One electrode of the capacitor is formed using a conductive layer  201   fb . The conductive layer  201   fb  does not have light-transmitting properties and has high conductivity. The other electrode of the capacitor is formed using a conductive layer  204   hb . The conductive layer  204   hb  does not have light-transmitting properties and has high conductivity. 
     Note that an element including such layers can be formed in a similar manner even in the case of employing another transistor structure or capacitor structure, for example, any of the structures illustrated in  FIGS.  3  to  14   . 
     Note that the transistor and the capacitor illustrated in  FIGS.  34  and  35    are preferably used in a circuit for driving a pixel. This is because light-transmitting properties are not necessary in such a circuit and a wiring is preferably formed using a layer with low conductivity. However, this embodiment is not limited thereto. 
     Embodiment 7 
     One example of a method for manufacturing a display device (a semiconductor device) will be described with reference to FIGS.  37 A 1 ,  37 A 2 ,  37 B, and  37 C,  FIGS.  38 A to  38 F , and  FIGS.  39 A to  39 E . This embodiment shows an example of a method for manufacturing two thin film transistors with different structures over one substrate. 
     FIG.  37 A 1  is a plan view of one thin film transistor  410 . FIG.  37 A 2  is a plan view of the other thin film transistor  420 .  FIG.  37 B  illustrates a cross-sectional view along line C 1 -C 2  in FIG.  37 A 1  and a cross-sectional view along line D 1 -D 2  in FIG.  37 A 2 .  FIG.  37 C  illustrates a cross-sectional view along line C 3 -C 4  in FIG.  37 A 1  and a cross-sectional view along line D 3 -D 4  in FIG.  37 A 2 . 
     The thin film transistor  410  has a kind of bottom-gate structure called a channel-etched type, and the thin film transistor  420  has a kind of bottom-gate structure called a channel protection type (also referred to as a channel stop type). The thin film transistors  410  and  420  are also referred to as inverted staggered thin film transistors. The thin film transistor  410  is placed in a driver circuit in the semiconductor device. On the other hand, the thin film transistor  420  is placed in a pixel. First, a structure of the thin film transistor  410  placed in the driver circuit in the semiconductor device will be described. 
     The thin film transistor  410  includes a gate electrode layer  411 ; a first gate insulating layer  402   a ; a second gate insulating layer  402   b ; an oxide semiconductor layer  412  including at least a channel formation region  413 , a high-resistance source region  414   a , and a high-resistance drain region  414   b ; a source electrode layer  415   a ; and a drain electrode layer  415   b  over a substrate  400  having an insulating surface. Moreover, an oxide insulating layer  416  that covers the thin film transistor  410  and is in contact with the channel formation region  413  is provided. 
     The high-resistance source region  414   a  is formed in contact with a bottom surface of the source electrode layer  415   a  in a self-aligned manner. The high-resistance drain region  414   b  is formed in contact with a bottom surface of the drain electrode layer  415   b  in a self-aligned manner. The channel formation region  413  is in contact with the oxide insulating layer  416 , has a small thickness, and is a region with higher resistance than that of the high-resistance source region  414   a  and the high-resistance drain region  414   b  (an i-type region). 
     In order to make the resistance of a wiring lower in the thin film transistor  410 , a metal material is preferably used for the source electrode layer  415   a  and the drain electrode layer  415   b.    
     In addition, when a pixel portion and a driver circuit are formed over the same substrate in a liquid crystal display device, only one of positive voltage or negative voltage is applied between a source electrode and a drain electrode of a thin film transistor included in a logic gate and a thin film transistor included in an analog circuit in the driver circuit. Examples of the logic circuit are an inverter circuit, a NAND circuit, a NOR circuit, and a latch circuit. Examples of the analog circuit are a sense amplifier, a constant voltage generation circuit, and a VCO. Consequently, the width of the high-resistance drain region  414   b  which needs high withstand voltage may be designed to be larger than the width of the high-resistance source region  414   a . Moreover, the width of a region of each of the high-resistance source region  414   a  and the high-resistance drain region  414   b  which overlaps with the gate electrode layer  411  may be increased. 
     The thin film transistor  410  placed in the driver circuit is described using a single-gate thin film transistor; a multi-gate thin film transistor including a plurality of channel formation regions can be formed when needed. 
     Furthermore, a conductive layer  417  is provided above the channel formation region  413  so as to overlap with the channel formation region  413 . The conductive layer  417  is electrically connected to the gate electrode layer  411  so that the conductive layer  417  and the gate electrode layer  411  have the same electric potential, whereby a gate voltage can be applied from the upper side and lower side of the oxide semiconductor layer  412  placed between the gate electrode layer  411  and the conductive layer  417 . When the gate electrode layer  411  and the conductive layer  417  are made to have different potentials, for example, one of them has a fixed potential, a GND potential, or 0 V, electrical characteristics of the TFT, such as the threshold voltage, can be controlled. 
     Further, a protective insulating layer  403  and a planarization insulating layer  404  are stacked between the conductive layer  417  and the oxide insulating layer  416 . 
     The protective insulating layer  403  is preferably in contact with the first gate insulating layer  402   a  provided below the protective insulating layer  403  or an insulating film serving as a base, and blocks entry of impurities such as moisture, a hydrogen ion, and Oft from a side surface of the substrate. It is particularly effective to use a silicon nitride film as the first gate insulating layer  402   a  or the insulating film serving as a base, which is in contact with the protective insulating layer  403 . 
     Next, a structure of the channel protective thin film transistor  420  placed in the pixel will be described. 
     The thin film transistor  420  includes a gate electrode layer  421 , the first gate insulating layer  402   a , the second gate insulating layer  402   b , an oxide semiconductor layer  422  including a channel formation region, an oxide insulating layer  426  functioning as a channel protection layer, a source electrode layer  425   a , and a drain electrode layer  425   b  over the substrate  400  having an insulating surface. Moreover, the protective insulating layer  403  is provided so as to cover the thin film transistor  420  and to be in contact with the oxide insulating layer  426 , the source electrode layer  425   a , and the drain electrode layer  425   b , and the planarization insulating layer  404  is stacked over the protective insulating layer  403 . A pixel electrode layer  427  is provided over the planarization insulating layer  404  to be in contact with the drain electrode layer  425   b , and thus is electrically connected to the thin film transistor  420 . 
     In order to form the oxide semiconductor layer  422 , heat treatment (heat treatment for dehydration or dehydrogenation) for reducing impurities such as moisture is performed at least after a semiconductor film is deposited to form the oxide semiconductor layer  422 . Reduction of the carrier concentration of the oxide semiconductor layer  422 , for example, by formation of the oxide insulating layer  426  in contact with the oxide semiconductor layer  422  after the heat treatment for dehydration or dehydrogenation and slow cooling leads to improvement in the electrical characteristics and reliability of the thin film transistor  420 . 
     The channel formation region of the thin film transistor  420  placed in the pixel is a region of the oxide semiconductor layer  422 , which is in contact with the oxide insulating layer  426  which is a channel protection layer and overlaps with the gate electrode layer  421 . Since the thin film transistor  420  is protected by the oxide insulating layer  426 , the oxide semiconductor layer  422  can be prevented from being etched in an etching step for forming the source electrode layer  425   a  and the drain electrode layer  425   b.    
     In order to realize a display device with a high aperture ratio, a light-transmitting conductive film is used for the source electrode layer  425   a  and the drain electrode layer  425   b  so that the thin film transistor  420  can serve as a light-transmitting thin film transistor. 
     Moreover, a light-transmitting conductive film is also used for the gate electrode layer  421  in the thin film transistor  420 . 
     In the pixel in which the thin film transistor  420  is placed, a conductive film that transmits visible light is used for the pixel electrode layer  427  or another electrode layer (e.g., a capacitor electrode layer) or another wiring layer (e.g., a capacitor wiring layer), which realizes a display device with a high aperture ratio. Needless to say, it is preferable to use a conductive film that transmits visible light for the first gate insulating layer  402   a , the second gate insulating layer  402   b , and the oxide insulating layer  426 . 
     In this specification, a film that transmits visible light refers to a film whose transmittance of visible light is 75% to 100%. In the case where such a film has conductivity, it is also referred to as a transparent conductive film. A conductive film that is semi-transparent to visible light may be used as metal oxide for the gate electrode layer, the source electrode layer, the drain electrode layer, the pixel electrode layer, another electrode layer, or another wiring layer. Semi-transparency to visible light means that the visible light transmittance is 50% to 75%. 
     Steps for manufacturing the thin film transistor  410  and the thin film transistor  420  over one substrate will be described below with reference to  FIGS.  38 A to  38 F  and  FIGS.  39 A to  39 E . Cross-sectional structures illustrated in these drawings correspond to the cross-sectional structure in  FIG.  37 B . 
     First, as illustrated in  FIG.  38 A , a light-transmitting conductive film is formed over the substrate  400  having an insulating surface, and then the gate electrode layers  411  and  421  are formed in a first photolithography step. Moreover, a capacitor wiring layer is formed in a pixel portion in the first photolithography step for the light-transmitting conductive film. Furthermore, when a capacitor is necessary in a driver circuit in addition to in the pixel portion, the capacitor wiring layer is also formed in the driver circuit. Note that a resist mask may be formed by an ink-jet method. A photomask is not used when the resist mask is formed by an ink jet method, which results in reducing manufacturing costs. 
     Although there is no particular limitation on a substrate that can be used as the substrate  400  having an insulating surface, the substrate needs to have heat resistance high enough to withstand at least heat treatment to be performed later. For the substrate  400  having an insulating surface, a substrate formed using an insulator, such as a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate, can be used. 
     An insulating film serving as a base film may be provided between the substrate  400  and the gate electrode layers  411  and  421 . The base film has a function of preventing diffusion of an impurity element from the substrate  400 , and can be formed with a single-layer structure or a layered structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. 
     A conductive material that transmits visible light can be used as a material for the gate electrode layers  411  and  421  and the capacitor wiring in the pixel portion and the like. For example, an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, an Sn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, an Sn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, an Sn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, an In—O-based metal oxide, an Sn—O-based metal oxide, or a Zn—O-based metal oxide can be used. The thickness of the gate electrode layers  411  and  421  and the capacitor wiring in the pixel portion and the like is determined as appropriate within the range of 50 nm to 300 nm. As a deposition method of the metal oxide used for the gate electrode layers  411  and  421 , a sputtering method, a vacuum evaporation method (e.g., an electron beam evaporation method), an arc discharge ion plating method, or a spray method is used. In the case where a sputtering method is used, it is preferable that deposition be performed using a target containing SiO 2  at 2 to 10 percent by weight, and SiO x  (x&gt;0), which inhibits crystallization, be contained in the light-transmitting conductive film so that crystallization is suppressed when the heat treatment for dehydration or dehydrogenation is performed in a later step. 
     Next, a gate insulating layer is formed over the gate electrode layers  411  and  421 . 
     The gate insulating layer can be formed with a single-layer structure or a layered structure of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, and/or a silicon nitride oxide layer by a plasma CVD method, a sputtering method, or the like. For example, a silicon oxynitride layer may be formed using SiH 4 , oxygen, and nitrogen as a deposition gas by a plasma CVD method. 
     In this embodiment, a two-layer gate insulating layer including the first gate insulating layer  402   a  with a thickness of 50 nm to 200 nm and the second gate insulating layer  402   b  with a thickness of 50 nm to 300 nm is formed as illustrated in  FIG.  38 A . For the first gate insulating layer  402   a , a 100-nm-thick silicon nitride film or a 100-nm-thick silicon nitride oxide film is used. For the second gate insulating layer  402   b , a 100-nm-thick silicon oxide film is used. 
     An oxide semiconductor film  430  with a thickness of 2 nm to 200 nm is formed over the second gate insulating layer  402   b . The crystalline structure of the oxide semiconductor film  430  is an amorphous structure. 
     In this embodiment, heat treatment for dehydration or dehydrogenation is performed after the oxide semiconductor film  430  is formed. In order to keep the amorphous structure of the oxide semiconductor film  430  after the heat treatment, the oxide semiconductor film  430  preferably has a small thickness of less than or equal to 50 nm. The small thickness of the oxide semiconductor film  430  can prevent crystallization due to heat treatment after the formation of the oxide semiconductor film  430 . 
     Note that before the oxide semiconductor film  430  is formed by a sputtering method, dust attached to a surface of the second gate insulating layer  402   b  is preferably removed by reverse sputtering in which plasma is generated by introduction of an argon gas. The reverse sputtering refers to a method in which, without application of a voltage to a target side, an RF power source is used for application of a voltage to a substrate side in an argon atmosphere so that plasma is generated around the substrate to modify a surface. Note that sputtering may be performed in an atmosphere of nitrogen, helium, oxygen, or the like instead of argon. 
     As the oxide semiconductor film  430 , an In—Ga—Zn—O-based non-single-crystal film, an In—Sn—Zn—O-based oxide semiconductor film, an In—Al—Zn—O-based oxide semiconductor film, a Sn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, an In—Zn—O-based oxide semiconductor film, a Sn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxide semiconductor film, an In—O-based oxide semiconductor film, a Sn—O-based oxide semiconductor film, or a Zn—O-based oxide semiconductor film is used. In this embodiment, the oxide semiconductor film  430  is formed by a sputtering method with the use of an In—Ga—Zn—O-based oxide semiconductor target. Alternatively, the oxide semiconductor film  430  can be formed by a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere containing a rare gas (typically argon) and oxygen. When a sputtering method is employed, it is preferable that deposition be performed using a target containing SiO 2  of 2 to 10 percent by weight and SiOx (x&gt;0) which inhibits crystallization be contained in the oxide semiconductor film  430  so as to prevent crystallization at the time of the heat treatment for dehydration or dehydrogenation in a later step. 
     Then, the oxide semiconductor film  430  is processed into island-shaped oxide semiconductor layers in a second photolithography step. In addition, the resist mask for forming the island-like oxide semiconductor layers may be formed by an ink-jet method. A photomask is not used when the resist mask is formed by an ink jet method, which results in reducing manufacturing costs. 
     Next, the oxide semiconductor layer is subjected to dehydration or dehydrogenation. A temperature at which first heat treatment for dehydration or dehydrogenation is performed is higher than or equal to 350° C. and less than the strain point of the substrate, preferably higher than or equal to 400° C. Here, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layers in a nitrogen atmosphere, the oxide semiconductor layer is not exposed to the air until the oxide semiconductor layer is cooled to a predetermined temperature or lower so that water and hydrogen are prevented from being mixed into the oxide semiconductor layers again; thus, oxide semiconductor layers  431  and  432  are obtained (see  FIG.  38 B ). In this embodiment, after the heat treatment is performed at a temperature T in an electric furnace in a nitrogen atmosphere for performing dehydration or dehydrogenation on the oxide semiconductor layer, the substrate is cooled slowly to a temperature low enough to prevent water from coming in (specifically to a temperature more than 100° C. lower than the temperature T). Without limitation to a nitrogen atmosphere, the heat treatment can be performed in an atmosphere such as helium, neon, or argon. 
     Note that in the first heat treatment, it is preferable that water, hydrogen, and the like be not contained in the atmosphere of nitrogen or a rare gas such as helium, neon, or argon. In addition, nitrogen or a rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus preferably has a purity of 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (i.e., the concentration of impurities is 1 ppm or lower, preferably 0.1 ppm or lower). 
     In accordance with conditions of the first heat treatment or a material of the oxide semiconductor layer, the oxide semiconductor layer is crystallized and changed to a microcrystalline film or a polycrystalline film in some cases. 
     The first heat treatment of the oxide semiconductor layer may be performed on the oxide semiconductor film  430  before being processed into the island-shaped oxide semiconductor layers. In this case, after the first heat treatment, the substrate is taken out from the heat treatment apparatus and a photolithography step is performed. 
     Before the oxide semiconductor film  430  is deposited, the gate insulating layer may be subjected to heat treatment (400° C. or higher and lower than the strain point of the substrate) in an inert gas atmosphere (e.g., nitrogen, helium, neon, or argon) or an oxygen atmosphere to remove impurities such as hydrogen and water included in the layer. 
     Next, a metal conductive film is formed over the second gate insulating layer  402   b  and the oxide semiconductor layers  431  and  432 ; after that, in a third photolithography step, resist masks  433   a  and  433   b  are formed and the metal conductive film is selectively etched, so that metal electrode layers  434  and  435  are formed (see  FIG.  38 C ). 
     Examples of the material for the metal conductive film are an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy containing any of these elements as a component; and an alloy containing any of these elements in combination. The metal conductive film preferably has a three-layer structure in which an aluminum layer is stacked over a titanium layer and a titanium layer is stacked over the aluminum layer, or a three layer structure in which an aluminum layer is stacked over a molybdenum layer and a molybdenum layer is stacked over the aluminum layer. It is needless to say that the metal conductive film can be a single-layer structure, a two-layer structure, or a layered structure including four or more layers. 
     The resist masks  433   a  and  433   b  for forming the metal electrode layers  434  and  435  may be formed by an ink-jet method. A photomask is not used when the resist masks  433   a  and  433   b  are formed by an ink-jet method, which results in reducing manufacturing costs. 
     Then, the resist masks  433   a  and  433   b  are removed, and in a fourth photolithography step, resist masks  436   a  and  436   b  are formed and selective etching is performed to form a source electrode layer  415   a  and a drain electrode layer  415   b  (see  FIG.  38 D ). Note that in the fourth photolithography step, only part of the oxide semiconductor layer  431  is etched, whereby an oxide semiconductor layer  437  having a groove (a recessed portion) is formed. The resist masks  436   a  and  436   b  for forming the groove (the recessed portion) in the oxide semiconductor layer  431  may be formed by an ink jet method. A photomask is not used when the resist mask is formed by an ink-jet method, which results in reducing manufacturing costs. 
     Next, the resist masks  436   a  and  436   b  are removed, and in a fifth photolithography step, a resist mask  438  for covering the oxide semiconductor layer  437  is formed and the metal electrode layer  435  over the oxide semiconductor layer  432  is removed (see  FIG.  38 E ). 
     Note that in order to remove the metal electrode layer  435  overlapping with the oxide semiconductor layer  432  in the fifth photolithography step, the materials of the oxide semiconductor layer  432  and the metal electrode layer  435  and the etching conditions are adjusted as appropriate so that the oxide semiconductor layer  432  is not removed in etching of the metal electrode layer  435 . 
     After the resist mask  438  is removed, an oxide insulating film  439  is formed in contact with an upper surface and side surfaces of the oxide semiconductor layer  432  and the groove (the recessed portion) of the oxide semiconductor layer  437  as illustrated in  FIG.  38 F . The oxide insulating film  439  serves as a protective insulating film. 
     The oxide insulating film  439  has a thickness of at least 1 nm and can be formed by a method by which impurities such as water or hydrogen are not mixed into the oxide insulating film  439 , such as a sputtering method, as appropriate. In this embodiment, a 300-nm-thick silicon oxide film is formed as the oxide insulating film  439  by a sputtering method. The substrate temperature in the film formation may be higher than or equal to room temperature and lower than or equal to 300° C., and is set at 100° C. in this embodiment. The formation of the silicon oxide film by a sputtering method can be performed in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas (typically argon) and oxygen. As a target, a silicon oxide target or a silicon target can be used. For example, with the use of a silicon target, a silicon oxide film can be formed by a sputtering method in an atmosphere of oxygen and nitrogen. The oxide insulating film  439  which is formed in contact with the oxide semiconductor layers  432  and  437  whose resistance is reduced is formed using an inorganic insulating film that does not contain impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of such impurities from the outside, typically a silicon oxide film, a silicon nitride oxide film, an aluminum oxide film, or an aluminum oxynitride film. 
     Next, second heat treatment (preferably at 200° C. to 400° C., for example, 250° C. to 350° C.) is performed in an inert gas atmosphere or an oxygen gas atmosphere. For example, the second heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. With the second heat treatment, heating is performed with the groove in the oxide semiconductor layer  437  and the upper surface and side surfaces of the oxide semiconductor layer  432  in contact with the oxide insulating film  439 . 
       FIG.  39 A  illustrates a state after the second heat treatment. In  FIG.  39 A , an oxide semiconductor layer  412  is the oxide semiconductor layer  437  subjected to the second heat treatment, and the oxide semiconductor layer  422  is the oxide semiconductor layer  432  subjected to the second heat treatment. 
     Through the above steps, the oxide semiconductor film  430  after deposition is subjected to the first heat treatment for dehydration or dehydrogenation, and the second heat treatment in an inert gas atmosphere or an oxygen gas atmosphere. 
     Thus, the high-resistance source region  414   a  is formed in a self-aligned manner in a region of the oxide semiconductor layer  412  overlapping with the source electrode layer  415   a . The high-resistance drain region  414   b  is formed in a self-aligned manner in a region of the oxide semiconductor layer  412  overlapping with the drain electrode layer  415   b . The entire region of the oxide semiconductor layer  412  overlapping with the gate electrode layer  411  is an i-type region and serves as the channel formation region  413 . Moreover, the entire oxide semiconductor layer  432  is made to be in an oxygen-excess state with the second heat treatment, so that the oxide semiconductor layer  422  that is highly resistive as a whole (i.e., the i-type oxide semiconductor layer  422 ) is formed. 
     After the second heat treatment, if heat treatment is performed in a nitrogen or inert gas atmosphere or under reduced pressure with the oxide semiconductor layer  422  exposed, the resistance of the high-resistance (i-type) oxide semiconductor layer  422  is reduced. For that reason, in the steps after the second heat treatment, heat treatment performed with the oxide semiconductor layer  422  exposed is performed in an oxygen gas or N 2 O gas atmosphere or an ultra-dry air (with a dew point of −40° C. or lower, preferably −60° C. or lower). 
     Note that the high-resistance drain region  414   b  (or the high-resistance source region  414   a ) is formed in the oxide semiconductor layer  412  overlapping with the drain electrode layer  415   b  (and the source electrode layer  415   a ), so that the reliability of the driver circuit including the thin film transistor  410  can be increased. Specifically, with the formation of the high-resistance drain region  414   b , the conductivity can vary from the drain electrode layer  415   b  to the high-resistance drain region  414   b  and the channel formation region  413 . Thus, when the thin film transistor  410  is operated while the drain electrode layer  415   b  is connected to a wiring that supplies a high power supply potential VDD, even when a high electric field is applied between the gate electrode layer  411  and the drain electrode layer  415   b , the high-resistance drain region  414   b  serves as a buffer and the high electric field is not applied locally, so that the thin film transistor  410  can have increased withstand voltage. 
     The high-resistance drain region  414   b  (or the high-resistance source region  414   a ) is formed in the oxide semiconductor layer  412  overlapping with the drain electrode layer  415   b  (or the source electrode layer  415   a ), whereby leakage current in the channel formation region  413  can be reduced even when the thin film transistor  410  is provided in the driver circuit. 
     Then, in a sixth photolithography step, resist masks  440   a  and  440   b  are formed and the oxide insulating film  439  is selectively etched to form the oxide insulating layers  416  and  426  as illustrated in  FIG.  39 B . The oxide insulating layer  426  covers a region where the channel formation region is formed in the oxide semiconductor layer  422 , and functions as a channel protective layer. Note that when an oxide insulating layer is used as the second gate insulating layer  402   b  as in this embodiment, part of the second gate insulating layer  402   b  is also etched in the etching step of the oxide insulating film  439 , whereby the thickness of the second gate insulating layer  402   b  is reduced in some cases. When a nitride insulating film that has high selectivity to the oxide insulating film  439  is used as the second gate insulating layer  402   b , the second gate insulating layer  402   b  can be prevented from being thinned by etching. 
     Next, the resist masks  440   a  and  440   b  are removed, and after that, a light-transmitting conductive film is formed over the oxide semiconductor layer  422  and the oxide insulating layer  426 . Then, in a seventh photolithography step, a resist mask is formed and the light-transmitting conductive film is etched using the resist mask, so that the source electrode layer  425   a  and the drain electrode layer  425   b  are formed as illustrated in  FIG.  39 C . The resist mask is removed after the etching step. 
     As a deposition method of the light-transmitting conductive film, a sputtering method, a vacuum evaporation method (e.g., an electron beam evaporation method), an arc ion plating method, or a spray method can be used. As a material of the conductive film, a conductive material that transmits visible light, for example, an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, an Sn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, an Sn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, an Sn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, an In—O-based metal oxide, an Sn—O-based metal oxide, or a Zn—O-based metal oxide can be employed. The thickness of the light-transmitting conductive film is set in the range of 50 nm to 300 nm as appropriate. In addition, in the case where a sputtering method is used, it is preferable that deposition be performed using a target containing SiO 2  at 2 to 10 percent by weight, and SiO x  (x&gt;0), which inhibits crystallization, be contained in the light-transmitting conductive film so that crystallization is suppressed when the heat treatment for dehydration or dehydrogenation is performed in a later step. 
     Note that the resist mask for forming the source electrode layer  425   a  and the drain electrode layer  425   b  may be formed by an ink jet method instead of the photolithography step. A photomask is not used when the resist mask is formed by an ink-jet method, which results in reducing manufacturing costs. 
     Then, as illustrated in  FIG.  39 D , the protective insulating layer  403  is formed over the oxide insulating layers  416  and  426 , the source electrode layer  425   a , and the drain electrode layer  425   b . In this embodiment, a silicon nitride film is formed as the protective insulating layer  403  by an RF sputtering method. Since an RF sputtering method allows high productivity, it is preferably used for depositing the protective insulating layer  403 . The protective insulating layer  403  is formed using an inorganic insulating film that does not contain impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of such impurities, for example, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum oxynitride film. Needless to say, the protective insulating layer  403  is a light-transmitting insulating film. 
     The protective insulating layer  403  is preferably in contact with the first gate insulating layer  402   a  provided below the protective insulating layer  403  or the insulating film serving as a base, and blocks entry of impurities such as moisture, a hydrogen ion, and OH −  from the vicinity of a side surface of the substrate  400 . It is particularly effective to use a silicon nitride film as the first gate insulating layer  402   a  or the insulating film serving as a base, which is in contact with the protective insulating layer  403 . In other words, when a silicon nitride film is provided so as to surround a lower surface, an upper surface, and a side surface of the oxide semiconductor layer, the reliability of the display device is improved. 
     Next, the planarization insulating layer  404  is formed over the protective insulating layer  403 . The planarization insulating layer  404  can be formed using an organic material having heat resistance, such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. Note that the planarization insulating layer  404  may be formed by stacking a plurality of insulating films formed from these materials. 
     Note that the siloxane-based resin corresponds to a resin including a Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may include an organic group (e.g., an alkyl group or an aryl group) or a fluoro group as a substituent. In addition, the organic group may include a fluoro group. 
     There is no particular limitation on the method for forming the planarization insulating layer  404 , and the following method or means can be employed depending on the material: a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, or a droplet discharge method (e.g., an ink-jet method, screen printing, or offset printing); a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. 
     Then, an eighth photolithography step is performed so that a resist mask is formed and a contact hole  441  that reaches the drain electrode layer  425   b  is formed by etching of the planarization insulating layer  404  and the protective insulating layer  403 . Moreover, contact holes that reach the gate electrode layers  411  and  421  are also formed with that etching. Alternatively, a resist mask for forming the contact hole that reaches the drain electrode layer  425   b  may be formed by an ink-jet method. A photomask is not used when the resist mask is formed by an ink jet method, which results in reducing manufacturing costs. 
     Next, the resist mask is removed, and then a light-transmitting conductive film is formed. The light-transmitting conductive film is formed using indium oxide (In 2 O 3 ), an alloy of indium oxide and tin oxide (In 2 O 3 —SnO 2 , referred to as ITO), or the like by a sputtering method, a vacuum evaporation method, or the like. Alternatively, the light-transmitting conductive film can be formed using an Al—Zn—O-based non-single-crystal film containing nitrogen (i.e., an Al—Zn—O—N-based non-single-crystal film), a Zn—O-based non-single-crystal film containing nitrogen, or an Sn—Zn—O-based non-single-crystal film containing nitrogen. Note that the percentage (at. %) of zinc in the Al—Zn—O—N-based non-single-crystal film is less than or equal to 47 at. % and is higher than that of aluminum in the non-single-crystal film; the percentage (at. %) of aluminum in the non-single-crystal film is higher than that of nitrogen in the non-single-crystal film. Such a material is etched with a hydrochloric acid-based solution. However, since a residue is easily generated particularly in etching ITO, indium oxide-zinc oxide alloy (In 2 O 3 —ZnO) may be used to improve the etching processability. 
     Note that the unit of the relative proportion in the light-transmitting conductive film is atomic percent (at. %), and the relative proportion is evaluated by analysis using an electron probe X-ray microanalyzer (EPMA). 
     Next, a ninth photolithography step is performed so that a resist mask is formed and unnecessary portions are removed by etching, whereby the pixel electrode layer  427  and the conductive layer  417  are formed as illustrated in  FIG.  39 E . The pixel electrode layer  427  is electrically connected to the drain electrode layer  425   b  through the contact hole  441  formed in the planarization insulating layer  404  and the protective insulating layer  403 . 
     Through the above-described steps, the driver circuit including the thin film transistor  410  and the pixel portion including the thin film transistor  420  can be manufactured over the same substrate  400  with the use of nine masks for light exposure. The thin film transistor  410  for the driver circuit is a channel-etched thin film transistor including the oxide semiconductor layer  412  including the high-resistance source region  414   a , the high-resistance drain region  414   b , and the channel formation region  413 . The thin film transistor  420  for the pixel is a channel protection thin film transistor including the oxide semiconductor layer  422  which is entirely intrinsic. 
     In addition, a capacitor that is constituted by a capacitor wiring layer and a capacitor electrode with the first gate insulating layer  402   a  and the second gate insulating layer  402   b  used as dielectrics can be formed over the same substrate  400 . The thin film transistors  420  and the capacitors are arranged in a matrix so as to correspond to individual pixels so that the pixel portion is formed, and the driver circuit including the thin film transistor  410  is placed around the pixel portion, whereby one of the substrates for manufacturing an active matrix display device can be obtained. In this specification, such a substrate is referred to as an active matrix substrate for convenience. 
     The pixel electrode layer  427  is electrically connected to a capacitor electrode layer. A contact hole for electrically connecting these two electrode layers is formed at the same time as the contact hole  441 . Note that the capacitor electrode layer can be formed using the same light-transmitting material in the same step as the source electrode layer  425   a  and the drain electrode layer  425   b.    
     The conductive layer  417  is provided so as to overlap with the channel formation region  413  in the oxide semiconductor layer  412 , whereby the amount of change in threshold voltage of the thin film transistor  410  before and after a bias-temperature stress test (referred to as a BT test) for examining the reliability of a thin film transistor can be reduced. The potential of the conductive layer  417  may be the same or different from that of the gate electrode layer  411 . The conductive layer  417  can also function as a second gate electrode layer. The potential of the conductive layer  417  may be GND or 0 V, or the conductive layer  417  may be in a floating state. 
     The resist mask for forming the conductive layer  417  and the pixel electrode layer  427  may be formed by an ink-jet method. A photomask is not used when the resist mask is formed by an ink jet method, which results in reducing manufacturing costs. 
     Embodiment 8 
     In this embodiment, examples of an electronic device provided with a display device will be described. 
       FIGS.  40 A to  40 H  and  FIGS.  41 A to  41 D  each illustrate an electronic device. These electronic devices can include a housing  5000 , a display portion  5001 , a speaker  5003 , an LED lamp  5004 , operation keys  5005  (including a power switch or an operation switch), a connection terminal  5006 , a sensor  5007  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone  5008 , and the like. 
       FIG.  40 A  illustrates a mobile computer which can include a switch  5009 , an infrared port  5010 , and the like in addition to the above objects.  FIG.  40 B  illustrates a portable image reproducing device (e.g., a DVD reproducing device) provided with a memory medium. The portable image reproducing device can include a second display portion  5002 , a recording medium reading portion  5011 , and the like in addition to the above objects.  FIG.  40 C  illustrates a goggle-type display which can include the second display portion  5002 , a supporting portion  5012 , an earphone  5013 , and the like in addition to the above objects.  FIG.  40 D  illustrates a portable game machine which can include the recording medium reading portion  5011  and the like in addition to the above objects.  FIG.  40 E  illustrates a digital camera having a television reception function, which can include an antenna  5014 , a shutter button  5015 , an image receiver portion  5016 , and the like in addition to the above objects.  FIG.  40 F  illustrates a portable game machine which can include the second display portion  5002 , the recording medium reading portion  5011 , and the like in addition to the above objects.  FIG.  40 G  illustrates a television receiver which can include a tuner, an image processing unit, and the like in addition to the above objects.  FIG.  40 H  illustrates a portable television receiver which can include a charger  5017  that can transmit and receive signals and the like in addition to the above objects. 
       FIG.  41 A  illustrates a display which can include a support base  5018  and the like in addition to the above objects.  FIG.  41 B  illustrates a camera which can include an external connection port  5019 , the shutter button  5015 , the image receiver portion  5016 , and the like in addition to the above objects.  FIG.  41 C  illustrates a computer which can include a pointing device  5020 , the external connecting port  5019 , a reader/writer  5021 , and the like in addition to the above objects.  FIG.  41 D  illustrates a mobile phone which can include a transmitting portion, a receiving portion, a tuner of one-segment partial reception service for mobile phones and mobile terminals (“1seg”), and the like in addition to the above objects. 
     The electronic devices illustrated in  FIGS.  40 A to  40 H  and  FIGS.  41 A to  41 D  can have a variety of functions. The electronic devices illustrated in  FIGS.  40 A to  40 H  and  FIGS.  41 A to  41 D  can have a variety of functions, for example, a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on a display portion; a touch panel function; a function of displaying a calendar, date, time, and the like; a function of controlling processing with a variety of software (programs); a wireless communication function; a function of being connected to a variety of computer networks with a wireless communication function; a function of transmitting and receiving a variety of data with a wireless communication function; and a function of reading a program or data stored in a memory medium and displaying the program or data on a display portion. Further, the electronic device including a plurality of display portions can have a function of displaying image information mainly on one display portion while displaying text information on another display portion, a function of displaying a three-dimensional image by displaying images where parallax is considered on a plurality of display portions, or the like. Furthermore, the electronic device including an image receiver portion can have a function of shooting a still image, a function of shooting a moving image, a function of automatically or manually correcting a shot image, a function of storing a shot image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like. Note that functions that can be provided for the electronic devices illustrated in  FIGS.  40 A to  40 H  and  FIGS.  41 A to  41 D  are not limited to those described above, and the electronic devices can have a variety of functions. 
     One of features of the electronic device described in this embodiment is that the electronic deice includes a display portion for displaying some sort of information. 
     Next, application examples of a display device (a semiconductor device) will be described. First, an example where the display device (the semiconductor device) is applied to an object that does not move, for example, a building is described. 
       FIG.  41 E  illustrates an embodiment of a building into which the display device (the semiconductor device) is incorporated. The semiconductor device includes a housing  5022 , a display portion  5023 , a remote controller device  5024  which is an operation portion, a speaker  5025 , and the like. The semiconductor device is incorporated into a wall in a living room as a wall-hung display device. Since such an embodiment does not need a large installation space, it is preferably used as a method for incorporating a semiconductor device including the large-screen display portion  5023  (of 40 inches or more) into a building. 
       FIG.  41 F  illustrates an embodiment of a building into which the display device (the semiconductor device) is incorporated. The semiconductor device includes a display panel  5026 . The display panel  5026  is integrated with into a prefabricated bath  5027 , and a person who takes a bath can enjoy images on the display panel  5026 . 
     Note that  FIGS.  41 E and  41 F  illustrate the examples in which the display device (the semiconductor device) is incorporated into a wall or a prefabricated bath; however, buildings in this embodiment are not limited to the above and the semiconductor device can be incorporated into a variety of places in buildings. 
     Next, examples of a structure of a moving object into which the display device (the semiconductor device) is incorporated will be described. 
       FIG.  41 G  illustrates an embodiment of a vehicle provided with the display device (the semiconductor device). The semiconductor device includes a display panel  5028 . The display panel  5028  is provided in a body  5029  of the vehicle and can display information input from the operation of the body or the outside of the body on demand. Moreover, the semiconductor device may have a navigation function. 
       FIG.  41 H  illustrates an embodiment of a passenger plane provided with the display device (the semiconductor device). The semiconductor device includes a display panel  5031 . The display panel  5031  is attached to a ceiling  5030  above a seat of the passenger plane through a hinge portion  5032 . The display panel  5031  has a function of displaying information when operated by the passenger.  FIG.  41 H  illustrates the display panel  5031  in use. The passenger can watch images on the display panel  5031  by extending and contracting the hinge portion  5032 . 
     Note that  FIGS.  41 G and  41 H  illustrate examples of a vehicle and a passenger plane as moving objects; however, this embodiment is not limited thereto and can be provided to a variety of moving objects such as a two-wheeled motor vehicle, a four-wheeled vehicle (including a car, a bus, a truck, and the like), a train, a ship, and a plane. 
     Embodiment 9 
     In this embodiment, the semiconductor device, the display device, and the like according to the invention disclosed in this specification will be described. 
     In this specification, a display device corresponds to a device including a display element. The display device to which the invention disclosed in this specification is applied includes a display medium whose contrast, luminance, reflectivity, transmittance, or the like changes by electromagnetic action. Examples of a display element included in the display device disclosed in this specification are an EL (electroluminescence) element (e.g., an EL element including organic and inorganic materials, an organic EL element, and an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, and a blue LED), a transistor (a transistor that emits light depending on the amount of current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma tube, a digital micromirror device (DMD), a piezoelectric ceramic display, and a carbon nanotube. An example of a display devices including EL elements is an EL display. Examples of display devices including electron emitters are a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices having liquid crystal elements are a liquid crystal display device (e.g., a transmissive liquid crystal display device, a transflective liquid crystal display device, a reflective liquid crystal display device, a direct-view liquid crystal display device, and a projection liquid crystal display). Examples of display devices having electronic ink or electrophoretic elements are electronic paper. Note that a device including a light-emitting element that emits light, such as an EL element or an LED, is sometimes referred to as a display device or a light-emitting device. A light-emitting device including a light-emitting element as a display element is a specific example of a display device. 
     Examples of an EL element are an element including an anode, a cathode, and an EL layer sandwiched between the anode and the cathode. Examples of an EL layer are 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, and a layer including a high-molecular material and a low-molecular material. Note that various types of EL elements can be used without limitation to the above. 
     Examples of an electron emitter are an element in which electrons are extracted by high electric field concentration on a cathode, and the like. Specifically, examples of an electron emitter are 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 thin film type in which a metal, an insulator, a semiconductor, and a metal are stacked, a HEED type, an EL type, a porous silicon type, and a surface-conduction (SCE) type. Note that various elements can be used as an electron emitter without limitation to the above. 
     In this specification, a liquid crystal display device means a display device having a liquid crystal element. Liquid crystal display devices are classified into a direct-view liquid crystal display, a projection type liquid crystal display, and the like according to a method for displaying images. Moreover, liquid crystal display devices can be classified into a transmissive liquid crystal display device, a reflective liquid crystal display device, and a transflective liquid crystal display device according to whether a pixel transmits or reflects illumination light. An example of a liquid crystal element is an element that controls transmission and non-transmission of light by optical modulation action of liquid crystals. The element can include a pair of electrodes and a liquid crystal layer. The optical modulation action of liquid crystals is controlled by an electric field applied to the liquid crystal (including a lateral electric field, a vertical electric field, and a diagonal electric field). Examples of a liquid used for a liquid crystal element are 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 polymer dispersed liquid crystal (PDLC), a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, a main-chain liquid crystal, a side-chain high-molecular liquid crystal, and a banana-shaped liquid crystal. 
     Examples of a method for displaying a liquid crystal display device are 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 (optically 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 PNLC (polymer Network liquid crystal) mode, a guest-host mode, and a blue phase mode. 
     It is needless to say that, without limitation to the above-described examples of the structure, liquid crystal display devices with a variety of structures can be applied to the invention disclosed in this specification. 
     For example, a device for displaying images by molecules (a device that utilizes optical anisotropy, dye molecular orientation, or the like), a device for displaying images by particles (a device that utilizes electrophoresis, particle movement, particle rotation, phase change, or the like), a device for displaying images by movement of one end of a film, a device for displaying images by using coloring properties or phase change of molecules, a device for displaying images with the use of optical absorption by molecules, a device for displaying images by using self-light emission by combination of electrons and holes, or the like can be used as electronic paper. Specifically, examples of electronic paper are 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, electro wetting, light-scattering (transparent-opaque change), a cholesteric liquid crystal and a photoconductive layer, a cholesteric liquid crystal, 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, photochromism, electrochromism, electrodeposition, and flexible organic EL. Note that various types of electronic papers can be used without limitation to those described above. By using microcapsule electrophoresis, problems of electrophoresis, that is, aggregation or 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. 
     Examples of a plasma display panel are a plasma display panel that has a structure where a substrate having a surface provided with an electrode faces a substrate having a surface provided with an electrode and a minute groove in which a phosphor layer is formed at a narrow interval and a rare gas is sealed therein, and a plasma display panel that has a structure where a plasma tube is sandwiched between film-form electrodes from the top and the bottom. The plasma tube is formed by sealing a discharge gas, RGB fluorescent materials, and the like inside a glass tube. Note that display can be performed by applying voltage between the electrodes to generate an ultraviolet ray so that a phosphor emits light. Without limitation to the above-described examples of the structure, plasma displays with a variety of structures can be applied to the invention disclosed in this specification. 
     Some display devices need a lighting device. Examples of such display devices are a liquid crystal display device, a display device using a grating light valve (GLV), and a display device using a digital micromirror device (DMD). A lighting device including an EL Element, 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 lighting device. 
     Further, an example of a display device is a display device including a plurality of pixels each having a display element. In this case, the display device may include a peripheral driver circuit for driving the plurality of pixels. A peripheral driver circuit in the display device may be a circuit formed over a substrate where the plurality of pixels are formed or a circuit formed over a different substrate. Both of these circuits can be provided as peripheral driver circuits. An example of the circuit formed over a different substrate from pixels is a circuit placed over a substrate where the pixels are formed by wire bonding, bump bonding, or the like, that is, an IC chip connected by chip on glass (COG), TAB, or the like. 
     When some of the circuits are formed over a substrate where a pixel portion is formed, costs can be reduced by reduction in the number of component parts or the reliability can be improved by reduction in the number of connections between circuit components. Specifically, a circuit in a portion where a driving voltage is high, a circuit in a portion where a driving frequency is high, or the like might consume much power. In order to deal with it, such a circuit is formed over a substrate (e.g., a single crystal substrate) which is different from a substrate where the pixel portion is formed, so that an IC chip is formed. By the use of this IC chip, an increase in power consumption can be prevented. 
     In addition, the display device may include a flexible printed circuit (FPC) to which an IC chip, a resistor, a capacitor, an inductor, a transistor, or the like is attached. The display device may include a printed wiring board (PWB) that is connected through a flexible printed circuit (FPC) and provided with an IC chip, a resistor, a capacitor, an inductor, a transistor, or the like. The display device may 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. 
     In this specification, one pixel means one element capable of controlling brightness. For example, one pixel corresponds to one color element and brightness is expressed with the one color element. Therefore, in the case of a color display device including color elements of R (red), G (green), and B (blue), a minimum unit of an image is composed of three pixels of an R pixel, a G pixel, and a B pixel. Note that the color elements are not limited to having three colors, and color elements of more than three colors may be used or a color other than RGB may be added. For example, white may be added so that R, G, B, and W (W means white) can be used. One or more colors of yellow, cyan, magenta, emerald green, and vermilion, for example, and the like can be added to RGB. Moreover, a color similar to at least one of R, G, and B can 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 wavelengths. Similarly, R1, R2, G, and B can be used. By using such color elements, display that is closer to the real object can be performed or power consumption can be reduced. 
     In the case where one color element is controlled in brightness by using a plurality of regions, one region can correspond to one pixel. For example, when area ratio gray scale display is performed or subpixels are included, a plurality of regions that control brightness are provided in one color element and gray scales are expressed with all the regions in some cases. In that case, one region for controlling brightness can correspond to one pixel. That is, one color element is composed of a plurality of pixels. Note that even when a plurality of regions that control brightness are placed in one color element, they may be collectively referred to as one pixel. In this case, one color element is composed of one pixel. When brightness of one color element is controlled with a plurality of regions, regions that contribute to display may differ in size depending on pixels. Moreover, in the plurality of regions for controlling the brightness of one color element, the viewing angle may be expanded by supplying each pixel with a slightly different signal. In other words, potentials of pixel electrodes in a plurality of regions may be different from each other in one color element. Thus, voltages applied to display elements including liquid crystal molecules or the like vary between the pixel electrodes. As a result, the viewing angle of the liquid crystal display device can be widened. Furthermore, the size of regions that contribute to display may vary between color elements. Thus, power consumption can be reduced or the life of a display element can be prolonged. 
     In the case where the explicit description “one pixel (for three colors)” is used, three pixels of R, G, and B can be considered as one pixel. In the case where the explicit description “one pixel (for one color)” is used, a plurality of regions provided in one color element can be collectively considered as one pixel. 
     A plurality of pixels can be arranged (aligned) in a matrix, for example. Here, “pixels are arranged (aligned) in a matrix” is not limited to the case where pixels are arranged over a straight line in a vertical direction or a horizontal direction. In a display device that performs full color display with three color elements (e.g., RGB), pixels are arranged in the following manner, for example: a stripe pattern, or a delta pattern or Bayer arrangement in which dots of the three color elements are arranged. 
     Embodiment 10 
     In the invention disclosed in this specification, a transistor with a variety of structures can be used. That is, there is no limitation on the structure of transistors. For example, a display device including pixels with high aperture ratio can be manufactured by application of Embodiment 7. In this embodiment, structure examples of transistors will be described. 
     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 microcrystal, nanocrystal, or 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 a temperature lower than that of the case of using single crystalline silicon, manufacturing costs can be reduced or a manufacturing apparatus can be made larger. Since the manufacturing apparatus can be made larger, the TFT can be formed using a large substrate. Therefore, a large number of 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 over a light-transmitting substrate. Further, transmission of light in a display element can be controlled by using the transistors formed over the light-transmitting substrate. Alternatively, part of a film included in the transistor can transmit light because the thickness of the transistor is small. Thus, the aperture ratio can be increased. 
     Note that when a catalyst (e.g., nickel) is used for 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 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. 
     Note that when a catalyst (e.g., nickel) is used for 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 light irradiation. Thus, 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 using the same substrate. Note that when laser irradiation for crystallization is not performed, unevenness in crystallinity of silicon can be suppressed. Accordingly, an image with improved image quality can be displayed. Note that polycrystalline silicon or microcrystalline silicon can be formed without use of a catalyst (e.g., nickel). 
     The crystallinity of silicon is preferably enhanced to polycrystallinity or microcrystallinity in the entire panel, but not limited thereto. The 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, laser light may be emitted only to a peripheral driver circuit region which is a region excluding pixel, a region such as a gate driver circuit and a source driver circuit, or part of a source driver circuit (e.g., an analog switch). As a result, the crystallinity of silicon only in a region in which a circuit needs to operate at high speed can be improved. Since a pixel region is not particularly needed to operate at high speed, even if crystallinity is not improved, the pixel circuit can operate without problems. Thus, a region whose crystallinity is improved is small, so that manufacturing steps can be decreased. As a result, the throughput can be increased and manufacturing costs can be reduced. Since the number of manufacturing apparatuses needed is small, manufacturing costs can be reduced. 
     For example, a transistor including a compound semiconductor or an oxide semiconductor, such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO, SnO, TiO, or AlZnSnO (AZTO), or a thin film transistor obtained by thinning such a compound semiconductor or oxide semiconductor can be used as a transistor. Since manufacturing temperature can be lowered, such a transistor can be formed at room temperature, for example. Thus, 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 oxide semiconductor can be used for not only a channel portion of a transistor but also for other applications. For example, such a compound semiconductor or oxide semiconductor can be used for a wiring, a resistor, a pixel electrode, a light-transmitting electrode, or the like. Such an element can be formed in the process for manufacturing the transistor, so that costs can be reduced. 
     An example of a transistor is a transistor formed by an inkjet method or a printing method. Such a transistor can be formed at room temperature, can be formed at a low vacuum, or can be formed using a large substrate. Since the transistor can be formed without using a mask (reticle), a layout of the transistor can be easily changed. Alternatively, since the transistor can be formed without use of a resist, material cost is reduced and the number of steps can be reduced. Further, since a film can be formed where needed, a material is not wasted as compared to a manufacturing method by which etching is performed after the film is formed over the entire surface; thus, costs can be reduced. 
     An example of a transistor is a transistor including an organic semiconductor or a carbon nanotube. Such a transistor can be formed over a flexible substrate. A semiconductor device using such a substrate can resist a shock. 
     Transistors with a variety of different structures can be used as a transistor. 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 as the transistor, the size of the transistor can be reduced. Thus, the integration degree of transistors can be increased. By using a bipolar transistor as the transistor, large current can flow. Thus, a circuit can operate at high speed. Note that a MOS transistor and a bipolar transistor may be formed over one substrate. Consequently, reduction in power consumption, reduction in size, high speed operation, and the like can be realized. 
     For example, a transistor with a multi-gate structure having two or more gate electrodes can be used as a transistor. With the multi-gate structure, a structure where a plurality of transistors are connected in series is provided because channel regions are connected in series. Thus, with the multi-gate structure, the amount of off-state current can be reduced and the withstand voltage of the transistor can be increased (the reliability can be improved). Alternatively, with the multi-gate structure, drain-source current does not fluctuate much even when 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, a current mirror circuit, or the like having excellent properties can be realized. 
     For example, a transistor with a structure where gate electrodes are formed above and below a channel can be used as a transistor. With the structure where the gate electrodes are formed above and below the channel, a circuit structure where a plurality of transistors are connected in parallel is provided. Thus, a channel region is increased, so that the amount of current can be increased. Alternatively, with the structure where gate electrodes are formed above and below the channel, a depletion layer can be easily formed, so that subthreshold swing value (S value) can be improved. 
     A transistor with 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 inverted 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 as a transistor, for example. 
     Note that for example, a transistor with a structure where a source electrode or a drain electrode overlaps with a channel region (or part of it) can be used as a transistor. The structure where the source electrode or the drain electrode may overlap with the channel region (or part of it) can prevent unstable operation due to electric charge accumulated in part of the channel region. 
     For example, a transistor including a high-resistance region can be used. When the high-resistance region is provided, the off-state current can be reduced or the withstand voltage of the transistor can be increased (the reliability can be improved). Alternatively, by providing the high-resistance region, drain-source current is not changed much even when drain-source voltage is changed when the transistor operates in the saturation region, so that a flat slope of voltage-current characteristics can be obtained. 
     A transistor can be formed using a variety of substrates. The type of a substrate is not limited to a certain type. Examples of the substrate are a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, and a base material film. As the glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, or a soda-lime glass substrate can be used, for example. For the flexible substrate, a flexible synthetic resin such as plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or polyethersulfone (PES), or acrylic can be used, for example Examples of an attachment film are attachment films formed using polypropylene, polyester, vinyl, polyvinyl fluoride, polyvinyl chloride, or the like. For a base material film, polyester, polyamide, polyimide, an inorganic vapor deposition film, paper, or the like can be used, for example. In particular, by forming transistors with the use of a semiconductor substrate, a single crystal substrate, an SOI substrate, or the like, transistors that have fewer variations in characteristics, sizes, shapes, or the like, high current supply capability, and small sizes can be formed. By forming a circuit using such transistors, power consumption of the circuit can be reduced or the circuit can be highly integrated. 
     A transistor may be formed using one substrate, and then, the transistor may be transferred to another substrate so as to be provided over another substrate. Example of a substrate to which a transistor is transferred are 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), or a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester)), a leather substrate, and a rubber substrate in addition to the above-described substrates over which the transistor can be formed. By using such a substrate, transistors with excellent properties or transistors with low power consumption can be formed, a device with high durability or high heat resistance can be formed, or reduction in weight or thickness can be achieved. 
     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, a region functioning as the source and a region functioning as the drain are not called the source or the drain in some cases. In that case, for example, one of the source and the drain is referred to as a first terminal, a first electrode, or a first region and the other of the source and the drain is referred to as a second terminal, a second electrode, or a second region in some cases. 
     In addition, a transistor may be an element having at least three terminals of a base, an emitter, and a collector. In that case also, one of the emitter and the collector is referred to as a first terminal, a first electrode, or a first region, and the other of the emitter and the collector is referred to as a second terminal, a second electrode, or a second region in some cases. Note that in the case where a bipolar transistor is used as a transistor, a gate can be rephrased as a base. 
     A gate corresponds to the all or some of a gate electrode and a gate wiring (also called a gate line, a gate signal line, a scan line, a scan signal line, or the like). A gate electrode corresponds to part of a conductive film that overlaps with a semiconductor which forms a channel region with a gate insulating film therebetween. Note that part of a gate electrode can overlap with a high-resistance region or a source region (or a drain region) with a gate insulating film therebetween. A gate wiring corresponds to a wiring for connecting gate electrodes of transistors, a wiring for connecting gate electrodes in pixels, or a wiring for connecting a gate electrode to another wiring. 
     There is a portion (a region, a conductive film, a wiring, or the like) that 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 overlapping 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. 
     A portion (a region, a conductive film, a wiring, or the like) that 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) that 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 sense, 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) that 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. 
     For example, in a multi-gate transistor, a gate electrode is often connected to another gate electrode by using a conductive film that is formed using the same material as the gate electrode. In such a case, a portion (a region, a conductive film, a wiring, or the like) for connecting the gate electrode to another gate electrode may be referred to as a gate wiring. The portion may 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) that 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. As another example, part of a conductive film that 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 be referred to as either a gate electrode or a gate wiring. 
     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) that is electrically connected to the gate electrode. 
     When a wiring is called a gate wiring, a gate line, a gate signal line, a scan line, a scan signal line, or the like, a gate of a transistor is not connected to the wiring in some cases. 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 of 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. Examples are a wiring for a storage capacitor, a power supply line, and a reference potential supply line. 
     A source corresponds to all or some of a source region, a source electrode, and a source wiring (also called 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, in the case where a low-concentration impurity region with a low concentration of p-type impurities or n-type impurities is a high-resistance region whose resistance is high, it is often considered not to be included in the source region. A source electrode corresponds to a conductive layer that is formed of a material different from that of a source region and electrically connected to the source region. Note that a source electrode and a source region are collectively called a source electrode in some cases. A source wiring is a wiring for connecting source electrodes of transistors, a wiring for connecting source electrodes in pixels, or a wiring for connecting a source electrode to another wiring. 
     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 overlapping portion (region, conductive film, wiring, or the like) functions as both a source wiring and a source electrode. Therefore, 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) that 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; a portion (a region, a conductive film, a wiring, or the like) that connects a source electrode and another source electrode; and a portion (a region, a conductive film, a wiring, or the like) that overlaps with a source region may also be referred to as a source electrode. Similarly, a region that is formed of 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 called 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) that 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 that connects a source electrode and a source wiring and is formed of a material different from that of the source electrode or the source wiring may be called 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) that is electrically connected to the source electrode. 
     When a wiring is called a source wiring, a source line, a source signal line, a data line, a data signal line, or the like, a source (a drain) of a transistor is not connected to the wiring in some cases. 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. Examples are a wiring for a storage capacitor, a power supply line, and a reference potential supply line. 
     Since the description of the drain is similar to that of the source, the description of the source can be applied. 
     This application is based on Japanese Patent Application serial no. 2009-205136 filed with Japan Patent Office on Sep. 4, 2009, the entire contents of which are hereby incorporated by reference.