Patent Publication Number: US-2022215810-A1

Title: Semiconductor Device, Driver Circuit, and Display Device

Description:
This application is a continuation of copending U.S. application Ser. No. 16/850,401, filed on Apr. 16, 2020 which is a continuation of U.S. application Ser. No. 15/366,368, filed on Dec. 1, 2016 (now U.S. Pat. No. 10,629,149 issued Apr. 21, 2020) which is a continuation of U.S. application Ser. No. 14/959,777, filed on Dec. 4, 2015 (now U.S. Pat. No. 9,514,696 issued Dec. 6, 2016) which is a continuation of U.S. application Ser. No. 14/325,603, filed on Jul. 8, 2014 (now U.S. Pat. No. 9,208,742 issued Dec. 8, 2015, which are all incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, or manufacture. The present invention particularly relates to, for example, a semiconductor device, a display device, a light-emitting device, a power storage device, a driving method thereof, or a manufacturing method thereof. In particular, the present invention relates to, for example, a semiconductor device including a transistor and a method for manufacturing the semiconductor device. 
     2. Description of the Related Art 
     As a means of downsizing, weight saving, and obtaining a narrowed bezel of a flat panel display typified by a liquid crystal display device and a light-emitting display device, it is known that a gate driver and a pixel portion are formed on one substrate. To obtain a bezel which is further narrowed, downsizing of the gate driver is required. Main circuits of the gate driver include a shift register. 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2002-49333 
     SUMMARY OF THE INVENTION 
     A reduction in width of a shift register which is a main circuit of a gate driver is an effective means of obtaining a narrowed bezel because the reduction in width of the shift register leads to a reduction in width of the whole of a gate driver circuit. 
     One object of one embodiment of the present invention is to provide a gate driver circuit in which the width of a shift register unit part is reduced. Another object of one embodiment of the present invention is to provide a semiconductor device in which the width of a gate driver circuit is reduced without increasing the delay time of a signal of a signal line of the gate driver circuit. Another object of one embodiment of the present invention is to provide a semiconductor device in which a gate driver circuit is designed to achieve a narrowed bezel. 
     Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a driver circuit which includes a shift register unit, a demultiplexer circuit electrically connected to the shift register unit, and n signal lines (n is a natural number of four or more). The shift register unit is electrically connected to one or more of the n signal lines. The demultiplexer circuit is electrically connected to one to (n-3) of the n signal lines. 
     Another embodiment of the present invention is a driver circuit which includes m shift register units (m is a natural number of three or more), m demultiplexer circuits electrically connected to the m shift register units, and n signal lines (n is a natural number of four or more). Each of the m shift register units is electrically connected to one or more of the n signal lines. Each of the m demultiplexer circuits is electrically connected to one to (n-3) of the n signal lines. To one of the m shift register units, one of outputs of a demultiplexer circuit electrically connected to a shift register unit in the previous stage of one of the m shift register units is input. To one of the m shift register units, one of outputs of the demultiplexer circuit electrically connected to the shift register unit in the next stage of the one of the m shift register units is input. 
     Another embodiment of the present invention is a driver circuit including a shift register unit including a set signal line, a first transistor, a second transistor, a third transistor, a fourth transistor, a fifth transistor, and a sixth transistor; a demultiplexer circuit; and n signal lines (n is a natural number of four or more). In the driver circuit, one of a source and a drain of the first transistor is electrically connected to a high power supply potential line; the other of the source and the drain of the first transistor is electrically connected to one of a source and a drain of the second transistor and the demultiplexer circuit; a gate of the first transistor is electrically connected to the set signal line; the other of the source and the drain of the second transistor is electrically connected to a low power supply potential line; a gate of the second transistor is electrically connected to the demultiplexer circuit, one of a source and a drain of the fourth transistor, one of a source and a drain of the fifth transistor, and one of a source and a drain of the sixth transistor; one of a source and a drain of the third transistor is electrically connected to the high power supply potential line; the other of the source and the drain of the third transistor is electrically connected to the other of the source and the drain of the fourth transistor; a gate of the third transistor is electrically connected to one of the n signal lines; a gate of the fourth transistor is electrically connected to one of the n signal lines; the other of the source and the drain of the fifth transistor is electrically connected to the low power supply potential line; a gate of the fifth transistor is electrically connected to the set signal line; the other of the source and the drain of the sixth transistor is electrically connected to the high power supply potential line; a gate of the sixth transistor is electrically connected to a reset signal line; the demultiplexer circuit includes a buffers (a is a natural number of one or more and (n-3) or less); each of the a buffers is electrically connected to the other of the source and the drain of the first transistor and the gate of the second transistor; the a buffers are electrically connected to the respective n signal lines; and each of the a buffers includes an output terminal. 
     According to one embodiment of the present invention, a semiconductor device with a narrowed bezel can be formed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a block diagram and a circuit diagram illustrating one embodiment of a semiconductor device. 
         FIG. 2  is a cross-sectional view illustrating one embodiment of a semiconductor device. 
         FIGS. 3A to 3E  are cross-sectional views illustrating one embodiment of a method for manufacturing a semiconductor device. 
         FIGS. 4A to 4D  are cross-sectional views illustrating one embodiment of a method for manufacturing a semiconductor device. 
         FIG. 5  is a cross-sectional view illustrating one embodiment of a semiconductor device. 
         FIG. 6  is a cross-sectional view illustrating one embodiment of a semiconductor device. 
         FIGS. 7A to 7C  illustrate electronic devices each including a semiconductor device which is one embodiment of the present invention. 
         FIGS. 8A to 8C  illustrate an electronic device including a semiconductor device which is one embodiment of the present invention. 
         FIG. 9  illustrates an overall view of a gate driver circuit. 
         FIGS. 10A and 10B  illustrate a shift register unit. 
         FIGS. 11A and 11B  illustrate another shift register unit which is a dummy stage. 
         FIGS. 12A and 12B  illustrate a demultiplexer. 
         FIGS. 13A and 13B  illustrate a demultiplexer. 
         FIG. 14  illustrates a buffer. 
         FIGS. 15A and 15B  illustrate another shift register unit. 
         FIGS. 16A and 16B  illustrate another shift register unit which is a dummy stage. 
         FIGS. 17A and 17B  illustrate another buffer. 
         FIGS. 18A and 18B  illustrate a way to obtain a narrower bezel. 
         FIG. 19  is a timing diagram of a shift register unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the following description, and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. In addition, in the following embodiments, the same portions or portions having similar functions are denoted by the same reference numerals or the same hatching patterns in different drawings, and description thereof will not be repeated. 
     Note that in each drawing described in this specification, the size, the film thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. 
     In addition, terms such as “first”, “second”, and “third” in this specification are used in order to avoid confusion among components, and the terms do not limit the components numerically. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate. 
     Functions of a “source” and a “drain” are sometimes replaced with each other when the direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification. 
     Note that a voltage refers to a difference between potentials of two points, and a potential refers to electrostatic energy (electric potential energy) of a unit charge at a given point in an electrostatic field. Note that in general, a difference between a potential of one point and a reference potential (e.g., a ground potential) is merely called a potential or a voltage, and a potential and a voltage are used as synonymous words in many cases. Thus, in this specification, a potential may be rephrased as a voltage and a voltage may be rephrased as a potential unless otherwise specified. 
     In this specification, in the case where an etching step is performed after a photolithography step, removal of a mask formed in the photolithography step that is performed after the etching step is not described in some cases for simplicity. 
     Embodiment 1 
     In this embodiment, a structure of a semiconductor device that is one embodiment of the present invention and a method for manufacturing the semiconductor device are described with reference to drawings. 
       FIG. 1A  illustrates a liquid crystal display device as one example of a semiconductor device. The liquid crystal display device in  FIG. 1A  includes a pixel portion  101 , a gate driver  104 , a source driver  106 , m scan lines  107  which are arranged in parallel or substantially in parallel and whose potentials are controlled by the gate driver  104 , and n signal lines  109  which are arranged in parallel or substantially in parallel and whose potentials are controlled by the source driver  106 . Further, the pixel portion  101  includes a plurality of pixels  301  arranged in a matrix. Capacitor lines  115  which are arranged in parallel or substantially in parallel to the scan lines  107  are also provided. The capacitor lines  115  may be arranged in parallel or substantially in parallel to the signal lines  109 . The gate driver  104  and the source driver  106  are collectively referred to as a driver circuit portion in some cases. 
     Each scan line  107  is electrically connected to the n pixels  301  in the corresponding row among the pixels  301  arranged in m rows and n columns in the pixel portion  101 . Each signal line  109  is electrically connected to the m pixels  301  in the corresponding column among the pixels  301  arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each capacitor line  115  is electrically connected to the n pixels  301  in the corresponding row among the pixels  301  arranged in m rows and n columns. Note that in the case where the capacitor lines  115  are arranged in parallel or substantially in parallel to the signal lines  109 , each capacitor line  115  is electrically connected to the m pixels  301  in the corresponding column among the pixels  301  arranged in m rows and n columns. 
       FIG. 1B  illustrates a circuit structure that can be used for the pixel  301  in the liquid crystal display device illustrated in  FIG. 1A . 
     The pixel  301  illustrated in  FIG. 1B  includes a liquid crystal element  132 , a transistor  131 , and a capacitor  133 . 
     The potential of one of a pair of electrodes of the liquid crystal element  132  is set according to the specifications of the pixels  301  as appropriate. The alignment state of the liquid crystal element  132  depends on written data. A common potential may be applied to one of the pair of electrodes of the liquid crystal element  132  included in each of the plurality of pixels  301 . Further, the potential supplied to one of the pair of electrodes of the liquid crystal element  132  in the pixel  301  in one row may be different from the potential supplied to one of the pair of electrodes of the liquid crystal element  132  in the pixel  301  in another row. Alternatively, in the IPS mode or the FFS mode, one of the pair of electrodes of the liquid crystal element  132  can be connected to a capacitor line CL. 
     As examples of a driving method of the liquid crystal display device including the liquid crystal element  132 , any of the following modes can be given: a TN mode, an STN mode, a VA mode, an ASM (axially symmetric aligned micro-cell) mode, an OCB (optically compensated birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode, an MVA mode, a PVA (patterned vertical alignment) mode, an IPS mode, an FFS mode, a TBA (transverse bend alignment) mode, and the like. Other examples of the driving method of the liquid crystal display device include an ECB (electrically controlled birefringence) mode, a PDLC (polymer dispersed liquid crystal) mode, a PNLC (polymer network liquid crystal) mode, and a guest-host mode. Note that one embodiment of the present invention is not limited to this, and various liquid crystal elements and driving methods can be used as a liquid crystal element and a driving method thereof. 
     The liquid crystal element may be formed using a liquid crystal composition including liquid crystal exhibiting a blue phase and a chiral material. The liquid crystal exhibiting a blue phase has a short response time of 1 msec or less and is optically isotropic; therefore, alignment treatment is not necessary and viewing angle dependence is small. 
     In the pixel  301  in the mth row and the nth column, one of a source electrode and a drain electrode of the transistor  131  is electrically connected to a signal line DL_n, and the other is electrically connected to one of a pair of electrodes of the capacitor  133  and the other of the pair of electrodes of the liquid crystal element  132 . A gate electrode of the transistor  131  is electrically connected to a scan line GL_m. The transistor  131  has a function of controlling whether to write a data signal by being turned on or off. 
     The other of the pair of electrodes of the capacitor  133  is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as capacitor line CL). The potential of the capacitor line CL is set according to the specifications of the pixel  301  as appropriate. The capacitor  133  functions as a storage capacitor for retaining written data. Note that in the IPS mode or the FFS mode, the other of the pair of electrodes of the capacitor  133  can be electrically connected to one of the pair of electrodes of the liquid crystal element  132 . 
     For example, in the liquid crystal display device including the pixel  301  of  FIG. 1B , the pixels  301  are sequentially selected row by row by the gate driver  104 , whereby the transistors  131  are turned on and a data signal is written. 
     When the transistors  131  are turned off, the pixels  301  in which the data has been written are brought into a holding state. This operation is sequentially performed row by row; thus, an image is displayed. 
     Note that in this specification and the like, examples of liquid crystal display devices having liquid crystal elements are 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. An example of the liquid crystal element is an element that controls transmission or 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 crystal is controlled by an electric field applied to the liquid crystal (including a horizontal electric field, a vertical electric field and a diagonal electric field). Note that specifically, the following can be used for a liquid crystal element, for example: 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, a banana-shaped liquid crystal, and the like. 
     Instead of a liquid crystal display device, one example of a semiconductor device can be a display element, a display device, a light-emitting device, and the like. A display element, a display device which is a device including a display element, a light-emitting element, and a light-emitting device which is a device including a light-emitting element can employ various modes and can include various elements. Examples of a display element, a display device, a light-emitting element, or a light-emitting device include an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor which emits light depending on current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator display (IMOD), a piezoelectric ceramic display, or a carbon nanotube, which are display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by electromagnetic action. Examples of display devices having electron emitters include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Display devices having electronic ink or electrophoretic elements include electronic paper and the like. 
     Next, a specific example of a liquid crystal display device including a liquid crystal element in the pixel  301  is described.  FIG. 2  is a cross-sectional view for illustrating a cross-sectional structure of the liquid crystal display device.  FIG. 2  illustrates cross-sectional structures of a gate driver and a pixel circuit. In this embodiment, as a semiconductor device, a liquid crystal display device of a vertical electric field mode is described. 
     In the liquid crystal display device described in this embodiment, a liquid crystal element  209  is provided between a pair of substrates (a substrate  200  and a substrate  242 ). 
     The liquid crystal element  209  includes a conductive layer  206  over the substrate  200 , films controlling alignment (hereinafter referred to as alignment films  251  and  252 ), a liquid crystal layer  207 , and a conductive layer  208 . The conductive layer  206  functions as one electrode of the liquid crystal element  209 , and the conductive layer  208  functions as the other electrode of the liquid crystal element  209 . 
     Thus, “liquid crystal display device” refers to a device including a liquid crystal element. The liquid crystal display device includes a driver circuit for driving a plurality of pixels, for example. The liquid crystal display device may also be referred to as a liquid crystal module including a control circuit, a power supply circuit, a signal generation circuit, a backlight module, and the like provided over another substrate. 
     The liquid crystal display device shown in  FIG. 2  is provided with a transistor  211  included in a pixel portion  220  and a transistor  221  included in a driver circuit portion  230  over the substrate  200 . Furthermore, the liquid crystal element  209  including the conductive layer  206 , the liquid crystal layer  207 , and the conductive layer  208  is included in the pixel portion  220 . 
     In the liquid crystal display device shown in  FIG. 2 , the transistor  211  provided in the pixel portion  220  includes a semiconductor layer  212  in which a channel region is formed, and the transistor  221  provided in the driver circuit portion  230  includes a semiconductor layer  222  in which a channel region is formed. 
     Here, components of the display device shown in  FIG. 2  are described below. 
     An insulating film  201  and an insulating film  202  are formed over the substrate  200 . The semiconductor layer  212  and the semiconductor layer  222  in which channel regions of the transistors are formed are formed in island-like shapes over the insulating film  202 . 
     There is no particular limitation on the property of a material and the like of the substrate  200  as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like may be used as the substrate  200 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate  200 . Furthermore, any of these substrates further provided with a semiconductor element may be used as the substrate  200 . In the case where a glass substrate is used as the substrate  200 , a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized liquid crystal display device can be manufactured. 
     Alternatively, a flexible substrate may be used as the substrate  200 , and the transistor may be provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate  200  and the transistor. The separation layer can be used when part or the whole of an element portion formed over the separation layer is completed and separated from the substrate  200  and transferred to another substrate. In such a case, the transistor can be transferred to a substrate having low heat resistance or a flexible substrate as well. 
     Each of the insulating film  201  and the insulating film  202  can have a single layer structure or a stacked-layer structure including an insulating film formed of silicon oxide, silicon oxynitride, silicon nitride, or the like by a chemical vapor deposition (CVD) method, a sputtering method, a thermal oxidation method, or the like. As a combination of the insulating film  201  and the insulating film  202 , a combination of silicon oxynitride and silicon oxide can be used for example. 
     The semiconductor layer  212  and the semiconductor layer  222  are preferably formed using crystalline silicon, but may be formed using amorphous silicon. Crystalline silicon is formed in such a manner that, after an amorphous silicon film is formed, the amorphous silicon film is crystallized by laser irradiation. Alternatively, after a metal film such as a Ni film is formed over an amorphous silicon film, the amorphous silicon film may be thermally crystallized. Further alternatively, a crystalline silicon film may be formed by a CVD method. 
     An insulating film  231  is a gate insulating film. The insulating film  231  can be formed with a single layer structure or a stacked-layer structure using an insulating film of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, or the like. 
     Further, when a silicon oxide film is formed by a CVD method using an organosilane gas as the insulating film  231 , the crystallinity of a semiconductor film to be formed later can be increased, whereby the on-state current and the field-effect mobility of the transistor can be increased. As an organosilane gas, a silicon-containing compound such as tetraethoxysilane (TEOS) (chemical formula: Si(OC 2 H 5 ) 4 ), tetramethylsilane (TMS) (chemical formula: Si(CH 3 ) 4 ), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC 2 H 5 ) 3 ), or trisdimethylaminosilane (SiH(N(CH 3 ) 2 ) 3 ) can be used. 
     Alternatively, the insulating film  231  may be formed by performing plasma treatment on the semiconductor layers  212  and  222  to oxidize or nitride the surfaces of the semiconductor layers  212  and  222 . For example, the insulating film  231  is formed by plasma treatment with a mixed gas of a rare gas such as He, Ar, Kr or Xe, and oxygen, nitrogen oxide (NO 2 ), ammonia, nitrogen, hydrogen, or the like. In this case, when excitation of plasma is performed by introducing a microwave, plasma with a low electron temperature and high density can be generated. The surface of a semiconductor film can be oxidized or nitrided by oxygen radicals (OH radicals are included in some cases) or nitrogen radicals (NH radicals are included in some cases) generated by this high-density plasma. 
     By treatment using such high-density plasma, an insulating film with a thickness of 1 nm to 20 nm inclusive, typically 5 nm to 10 nm inclusive, is formed over the semiconductor film. Since reaction in this case is solid-phase reaction, interface state density between the insulating film and the semiconductor film can be made quite low. By such high density plasma treatment, since the semiconductor film is directly oxidized (or nitrided), variation in thickness of the formed insulating film can be made extremely small. By solid-phase oxidation of the surface of the semiconductor film through such high-density plasma treatment, an insulating film which has favorable uniformity and low interface state density can be formed. 
     As for the insulating film  231 , only the insulating film formed by high density plasma treatment may be used, or one or more of insulating films of silicon oxide, silicon oxynitride, or silicon nitride may be deposited and stacked by a CVD method, a sputtering method, or the like. In any case, when transistors include an insulating film formed by high-density plasma in a part or the whole of a gate insulating film, variations in characteristics can be reduced. 
     Next, a first conductive layer  272  and a second conductive layer  273  are formed over the insulating film  231 . The first conductive layer  272  and the second conductive layer  273  can be formed using an element selected from among tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and the like, or an alloy material or a compound material containing such an element as a main component (for example, tantalum nitride). Alternatively, they are each formed using a semiconductor material typified by polycrystalline silicon doped with an impurity element such as phosphorus. Note that the first conductive layer  272  and the second conductive layer  273  may be formed of the same conductive material or different conductive materials. 
     As a combination of the first conductive layer  272  and the second conductive layer  273 , a combination of tantalum nitride and tungsten, a combination of tungsten nitride and tungsten, a combination of molybdenum nitride and molybdenum, or the like can be used for example. Here, the first conductive layer is formed to a thickness greater than or equal to 20 nm and less than or equal to 100 nm by a CVD method, a sputtering method, or the like. The second conductive layer is formed to a thickness greater than or equal to 100 nm and less than or equal to 400 nm. Although a stacked-layer structure of two conductive films is used in this embodiment, one layer or a stacked-layer structure of three or more layers may be alternatively used. In the case of a three-layer structure, a stacked-layer structure of a molybdenum layer, an aluminum layer, and a molybdenum layer may be employed. 
     Impurity regions  216  and impurity regions  217  are formed in the semiconductor layer  212 , and impurity regions  226  and impurity regions  227  are formed in the semiconductor layer  222 . An impurity element can be introduced by an ion doping method, an ion implantation method, or the like with the use of an n-type impurity element or a p-type impurity element. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. 
     An insulating film  236  is an interlayer insulating film. Conductive layers  218  and conductive layers  228  are source electrodes and drain electrodes. 
     The insulating film  236  can be formed using an insulating film made of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, or the like. Alternatively, an organic resin such as an acrylic resin, a polyimide resin, a benzocyclobutene-based resin, a siloxane-based resin, a polyamide resin, or an epoxy resin can be used. 
     Each of the conductive layers  218  and the conductive layers  228  can have a single layer structure or a stacked-layer structure including one element selected from aluminum, tungsten, titanium, tantalum, molybdenum, nickel, and neodymium, or an alloy containing a plurality of any of these elements. For example, a conductive layer that is formed using an alloy that contains a plurality of any of the elements given above can be formed from an aluminum alloy that contains titanium, an aluminum alloy that contains neodymium, or the like can be used. In the case of forming a stacked-layer structure, for example, a structure where an aluminum layer or the above-described aluminum alloy layer is provided between titanium layers can be used. Note that the conductive layers  218  and  228  serve as source electrodes and drain electrodes of the transistors. 
     An insulating film  238  is an interlayer insulating film. The conductive layer  206  formed over the insulating film  238  is a pixel electrode. The conductive layer  206  serves as an anode or a cathode in a light-emitting device. 
     The insulating film  238  can be formed using an insulating film made of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, or the like. Alternatively, an organic resin such as an acrylic resin, a polyimide resin, a benzocyclobutene-based resin, a siloxane-based resin, a polyamide resin, or an epoxy resin can be used. 
     As the conductive layer  206 , a transparent conductive film composed of a light-transmitting conductive material may be used, and indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, and the like can be used. Needless to say, it is possible to use indium tin oxide, indium zinc oxide, indium tin oxide with silicon oxide added, or the like. As the conductive layer  206 , a material that has a high work function, for example, an element selected from nickel (Ni), tungsten (W), chromium (Cr), platinum (Pt), zinc (Zn), tin (Sn), indium (In), or molybdenum (Mo), or an alloy material including any of the metal elements as its main component, for example, titanium nitride, titanium silicon nitride, tungsten silicide, tungsten nitride, tungsten silicide nitride, or niobium nitride may be used to form a single layer film or a film of stacked layers. 
     Reference numeral  251  denotes an alignment film. The alignment film  251  can be formed using an organic resin such as polyimide. The thickness of the alignment film  251  is greater than or equal to 40 nm and less than or equal to 100 nm, preferably greater than or equal to 50 nm and less than or equal to 90 nm. With such a thickness, the pretilt angle of a liquid crystal material can be made large, which can reduce disclination. 
     A film having a coloring property (hereinafter referred to as a coloring film  246 ) is formed on the substrate  242 . The coloring film  246  functions as a color filter. Further, a light-blocking film  244  adjacent to the coloring film  246  is formed on the substrate  242 . The light-blocking film  244  functions as a black matrix. The coloring film  246  is not necessarily provided in the case where the liquid crystal display device is a monochrome display device, for example. 
     The coloring film  246  is a coloring film that transmits light in a specific wavelength range. For example, a red (R) color filter for transmitting light in a red wavelength range, a green (G) color filter for transmitting light in a green wavelength range, a blue (B) color filter for transmitting light in a blue wavelength range, or the like can be used. 
     The light-blocking film  244  preferably has a function of blocking light in a particular wavelength range, and can be a metal film, an organic insulating film including a black pigment, or the like. 
     An insulating film  248  is formed on the coloring film  246 . The insulating film  248  functions as a planarization film or suppresses diffusion of impurities in the coloring film  246  to the liquid crystal element side. 
     The conductive layer  208  is formed on the insulating film  248 . The conductive layer  208  functions as the other of the pair of electrodes of the liquid crystal element in the pixel portion. Note that the alignment film  251  is formed over the conductive layer  206 , and the alignment film  252  is formed on the conductive layer  208 . 
     The liquid crystal layer  207  is formed between the conductive layer  206  and the conductive layer  208 . The liquid crystal layer  207  is sealed between the substrate  200  and the substrate  242  with the use of a sealant (not illustrated). The sealant is preferably in contact with an inorganic material to prevent entry of moisture and the like from the outside. 
     A spacer may be provided between the conductive layer  206  and the conductive layer  208  to maintain the thickness of the liquid crystal layer  207  (also referred to as a cell gap). 
     Next, a method for manufacturing the transistors  211  and  221  of the liquid crystal display device illustrated in  FIG. 1A  is described with reference to  FIGS. 3A to 3E  and  FIGS. 4A to 4D . 
     First, the substrate  200  is prepared. Here, a glass substrate is used as the substrate  200 . 
     Next, the insulating film  201  and the insulating film  202  are sequentially stacked over the substrate  200 . Each of the insulating film  201  and the insulating film  202  can have a single layer structure or a stacked-layer structure using an insulating film formed of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, a thermal oxidation method, or the like. As a combination of the insulating film  201  and the insulating film  202 , a combination of silicon oxynitride and silicon oxide can be used for example. 
     Next, the semiconductor film is formed over the insulating film  202  and is selectively etched to form the semiconductor layer  212  and the semiconductor layer  222 . The semiconductor layer  212  and the semiconductor layer  222  are preferably formed using crystalline silicon. In this embodiment, after amorphous silicon is formed by a CVD method, the amorphous silicon is crystallized by laser irradiation. Note that heat treatment for removing hydrogen may be performed before the laser irradiation ( FIG. 3A ). 
     Next, the insulating film  231  is formed to cover the semiconductor layer  212  and the semiconductor layer  222 . The insulating film  231  can be formed with a single layer structure or a stacked-layer structure using an insulating film of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, a thermal oxidation method, or the like. Here, silicon oxide is used for the gate insulating film ( FIG. 3B ). 
     Next, a first conductive film  292  and a second conductive film  293  are sequentially stacked over the insulating film  231 . Each of the first conductive film  292  and the second conductive film  293  can be formed using an element selected from among tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and the like, or an alloy material or a compound material containing such an element as a main component (for example, tantalum nitride). Alternatively, they are each formed using a semiconductor material typified by polycrystalline silicon doped with an impurity element such as phosphorus. Note that the first conductive film  292  and the second conductive film  293  may be formed of the same conductive material or different conductive materials. Here, the first conductive film and the second conductive film are formed using a tantalum nitride film and a tungsten film, respectively ( FIG. 3C ). 
     Then, resist masks  234  are selectively formed over the second conductive film  293 , and first etching treatment and second etching treatment are performed using the resist masks  234 . By performing the first etching treatment, the first conductive film  292  and the second conductive film  293  that are formed over the insulating film  231  are selectively etched. Thus, a stacked-layer structure including a first conductive layer  232   a  and a second conductive layer  233   a  that can serve as a gate electrode is made to remain over the semiconductor layer  212 , and a stacked-layer structure including a first conductive layer  232   b  and a second conductive layer  233   b  that can serve as a gate electrode is made to remain over the semiconductor layer  222  ( FIG. 3D ). 
     By performing the second etching treatment after that, end portions of the second conductive layer  233   a  and the second conductive layer  233   b  are selectively etched, thereby obtaining a structure where the second conductive layer  233   a  and the second conductive layer  233   b  each have a smaller width than the first conductive layer  232   a  and the first conductive layer  232   b  ( FIG. 3E ). 
     An etching method for carrying out the first etching treatment and the second etching treatment may be selected as appropriate. In order to improve the etching rate, a dry etching apparatus using a high density plasma source of ECR (electron cyclotron resonance) or ICP (inductively coupled plasma) or the like may be used. With appropriate control of the etching conditions of the first etching treatment and the second etching treatment, the end portions of the first conductive layers  232   a  and  232   b  and the second conductive layers  233   a  and  233   b  can be formed into desired tapered shapes. 
     Next, an impurity element is introduced into the semiconductor layer  212  and the semiconductor layer  222  using the first conductive layer  232   a,  the first conductive layer  232   b,  the second conductive layer  233   a,  and the second conductive layer  233   b  as masks to form low-concentration impurity regions  215  in the semiconductor layer  212  and low-concentration impurity regions  225  in the semiconductor layer  222  ( FIG. 4A ). 
     An impurity element can be introduced by an ion doping method, an ion implantation method, or the like with the use of an n-type impurity element or a p-type impurity element. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. 
     Here, an example is described in which the impurity regions  215  are formed in regions which do not overlap with the first conductive layer  232   a  in the semiconductor layer  212 ; however, the impurity regions  215  can be formed in regions overlapping with the first conductive layer  232   a  depending on the condition of introducing the impurity element. Further, an example is described in which the impurity regions  225  are formed in regions which do not overlap with the first conductive layer  232   b  in the semiconductor layer  222 ; however, the impurity regions  225  can be formed in regions overlapping with the first conductive layer  232   b  depending on the condition of introducing the impurity element. 
     A resist mask  235  is selectively formed over the first conductive layer  232   a,  the second conductive layer  233   a,  and the semiconductor layer  212 , and an impurity element is introduced into the semiconductor layers  212  and  222  with the use of the resist mask  235 , the first conductive layer  232   b,  and the second conductive layer  233   b  as masks. As a result, the impurity regions  216  and the impurity regions  217  are formed in the semiconductor layer  212 , and the impurity regions  226  and the impurity regions  227  are formed in the semiconductor layer  222 . Note that the impurity element goes through the first conductive layer  232   b  to be introduced into the semiconductor layer  222  ( FIG. 4B ). 
     The impurity element can be introduced by an ion doping method, an ion implantation method, or the like. As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. Here, the impurity regions  216 ,  217 ,  226 , and  227  are subjected to ion doping using phosphorus (P). 
     In the semiconductor layer  212 , the high-concentration impurity regions  217  formed in regions which are not covered with the resist mask  235  serve as a source region and a drain region of the transistor, and the low-concentration impurity regions  216  formed in regions which are covered with the resist mask  235  and do not overlap with the first conductive layer  232   a  serve as LDD regions of the transistor. Further, in the semiconductor layer  222 , the high-concentration impurity regions  227  formed in regions which do not overlap with the first conductive layer  232   b  serve as a source region and a drain region of the transistor, and the low-concentration impurity regions  226  formed in regions which overlap with the first conductive layer  232   b  but do not overlap with the second conductive layer  233   b  serve as LDD regions of the transistor. 
     The LDD region is a region to which an impurity element is added at low concentration between a channel forming region and a source region or a drain region which is formed by being doped with an impurity element at high concentration. By providing the LDD region, the effects of reducing the electric field in the vicinity of the drain region and preventing deterioration due to hot carrier injection can be obtained. In order to prevent deterioration of on-state current due to hot carriers, a structure (also referred to as a GOLD (gate-drain overlapped LDD) structure) may be employed in which a LDD region overlaps with a gate electrode with a gate insulating film interposed therebetween. This embodiment shows an example in which a LDD region is used in the n-channel transistor  211  included in the pixel portion and a GOLD structure is used in the n-channel transistor  221  included in the driver circuit portion. However, one embodiment of the present invention is not limited thereto, and a GOLD structure may be used in the transistor included in the pixel portion  220 . 
     Next, an interlayer insulating film is formed. Here, the insulating film  236  is formed as the interlayer insulating film. Then, opening portions are selectively formed in the insulating film  231  and the insulating film  236 , and the conductive layers  218  and the conductive layers  228  that serve as source electrodes and drain electrodes are formed ( FIG. 4C ). 
     The insulating film  236  can be formed using an insulating film made of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, or the like. Alternatively, an organic resin such as an acrylic resin, a polyimide resin, a benzocyclobutene-based resin, a siloxane-based resin, a polyamide resin, or an epoxy resin can be used. Here, the insulating film  236  is formed using silicon oxide, silicon oxynitride, or silicon nitride by a CVD method. 
     The conductive layers  218  and the conductive layers  228  can be formed with a single layer structure or a stacked-layer structure using one element selected from aluminum, tungsten, titanium, tantalum, molybdenum, nickel, and neodymium, or an alloy containing a plurality of any of these elements. For example, a conductive layer that is formed using an alloy that contains a plurality of any of the elements given above can be formed from an aluminum alloy that contains titanium, an aluminum alloy that contains neodymium, or the like can be used. In the case of forming a stacked-layer structure, for example, a structure where an aluminum layer or the above-described aluminum alloy layer is provided between titanium layers can be used. Note that the conductive layers  218  and  228  serve as the source electrodes and the drain electrodes of the transistors. 
     Next, the insulating film  238  is formed. Then, an opening portion is provided in the insulating film  238 , and the conductive layer  206  serving as a pixel electrode is formed so as to be electrically connected to the conductive layer  218 . The conductive layer  206  serves as an anode or a cathode in a light-emitting device ( FIG. 4D ). 
     The insulating film  238  can be formed using an insulating film made of silicon oxide, silicon oxynitride, silicon nitride, or the like by a CVD method, a sputtering method, or the like. Alternatively, an organic resin such as an acrylic resin, a polyimide resin, a benzocyclobutene-based resin, a siloxane-based resin, a polyamide resin, or an epoxy resin can be used. 
     A transparent conductive film formed using a light-transmitting conductive material may be used for the conductive layer  206  which serves as a pixel electrode, and 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 can be used. Needless to say, it is possible to use indium tin oxide, indium zinc oxide, indium tin oxide with silicon oxide added, or the like. Furthermore, a material that has a high work function, for example, an element selected from nickel (Ni), tungsten (W), chromium (Cr), platinum (Pt), zinc (Zn), tin (Sn), indium (In), or molybdenum (Mo), or an alloy material including any of the metal elements as its main component, for example, titanium nitride, titanium silicon nitride, tungsten silicide, tungsten nitride, tungsten silicide nitride, or niobium nitride may be used to form a single layer film or a film of stacked layers. 
     Thus, the transistor  211  and the transistor  221  of the liquid crystal display device illustrated in  FIG. 1A  can be manufactured. 
     Note that the structure and the like described in this embodiment can be used as appropriate in combination with any of the structures and the like in the other embodiments. 
     Modification Example 
     A modification example of the gate electrode in Embodiment 1 is described with reference to  FIG. 5 . 
     In  FIG. 2 , the gate electrode has a two-layer structure including the conductive films. In  FIG. 5 , the gate electrode has a single-layer structure including a conductive film  261 . 
     Furthermore, impurity regions  266  and impurity regions  276  in  FIG. 5  are low-concentration impurity regions and serve as LDD regions of transistors. Impurity regions  267  and impurity regions  277  in  FIG. 5  are high-concentration impurity regions and serve as source regions and drain regions of the transistors. The low-concentration impurity regions  266  and  276  and the high-concentration impurity regions  267  and  277  are formed using a resist mask in a manner similar to that of the low-concentration impurity regions  216  and the high-concentration impurity regions  217 , which is illustrated in  FIGS. 4A and 4B . 
     The transistors each including the gate electrode having a single-layer structure can be manufactured by a simple process, whereby cost reduction can be achieved. 
     Embodiment 2 
     In this embodiment, a structure of a semiconductor device different from that of the semiconductor device of Embodiment 1 is described with reference to the drawings. 
     In  FIG. 6 , a transistor  811  is a transistor included in a pixel portion, and a transistor  821  is a transistor included in a driver circuit portion. 
     As illustrated in  FIG. 6 , a conductive layer  832  serving as a gate electrode is formed over a substrate  800 . An insulating film  831  serving as a gate insulating film is formed so as to cover the conductive layer  832 . A semiconductor layer  812  and a semiconductor layer  822  are formed over the insulating film  831 . A channel region  816 , a channel region  826 , impurity regions  817 , and impurity regions  827  are formed in the semiconductor layer  812  and the semiconductor layer  822 . The impurity regions  817  and the impurity regions  827  serve as source regions and drain regions. 
     As the substrate  800 , the substrate  200  described in Embodiment 1 can be used as appropriate. 
     The conductive layer  832  is formed with a single layer or stacked layers using an element selected from molybdenum (Mo), aluminum (Al), tantalum (Ta), tungsten (W), titanium (Ti), copper (Cu), chromium (Cr), niobium (Nb), neodymium, scandium, nickel, and the like, or an alloy material or a chemical compound material containing any of the above elements as its main component (for example, tantalum nitride). Alternatively, the conductive layer  832  is formed using a semiconductor material typified by polycrystalline silicon doped with an impurity element such as phosphorus. 
     The following are preferable examples of the two-layer structure of the conductive layer  832 : a two-layer structure in which a molybdenum film is provided over an aluminum film, a two-layer structure in which a molybdenum film is provided over a copper film, a two-layer structure in which a titanium nitride film or a tantalum nitride film is provided over a copper film, a two-layer structure in which a titanium nitride film and a molybdenum film are stacked, a two-layer structure in which a film of a copper-magnesium alloy containing oxygen and a copper film are stacked, a two-layer structure in which a film of a copper-manganese alloy containing oxygen and a copper film are stacked, a two-layer structure in which a copper-manganese alloy film and a copper film are stacked, or the like. As a three-layer structure, it is preferable to stack a tungsten film or a tungsten nitride film, an alloy film of aluminum and silicon or an alloy film of aluminum and titanium, and a titanium nitride film or a titanium film. By stacking a metal film functioning as a barrier film over a film having low electric resistance, electric resistance can be low and diffusion of a metal element from the metal film into a semiconductor film can be prevented. 
     Through the step of forming the conductive layer  832 , a gate wiring (a scan line) and a capacitor wiring can also be formed at the same time. The scan line means a wiring for selecting a pixel, while the capacitor wiring means a wiring which is connected to one of electrodes of a storage capacitor in a pixel. However, one embodiment of the present invention is not limited thereto, and either one or both of the gate wiring and the capacitor wiring may be formed by a separate step from that of the conductive layer  832 . 
     The insulating film  831  can be formed by a CVD method, a sputtering method, or the like. 
     Further, by the formation of a silicon oxide film by a CVD method using an organosilane gas as the insulating film  831 , the crystallinity of the semiconductor film to be formed later can be increased, whereby the on-state current and the field-effect mobility of the transistor can be increased. As an organosilane gas, a silicon-containing compound such as tetraethoxysilane (TEOS) (chemical formula: Si(OC 2 H 5 ) 4 ), tetramethylsilane (TMS) (chemical formula: Si(CH 3 ) 4 ), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC 2 H 5 ) 3 ), or trisdimethylaminosilane (SiH(N(CH 3 ) 2 ) 3 ) can be used. 
     The semiconductor layer  812  and the semiconductor layer  822  are preferably formed using a crystalline silicon layer, but may be formed using an amorphous silicon layer. The crystalline silicon layer is formed in such a manner that, after an amorphous silicon film is formed, the amorphous silicon film is crystallized by laser irradiation. Alternatively, after a metal film such as a Ni film is formed over an amorphous silicon film, the amorphous silicon film may be thermally crystallized. Further alternatively, a crystalline silicon film may be formed by a CVD method. An impurity element can be introduced by an ion doping method, an ion implantation method, or the like with the use of an n-type impurity element or a p-type impurity element to form the impurity regions  817  and the impurity regions  827 . As the n-type impurity element, phosphorus (P), arsenic (As), or the like can be used. As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga), or the like can be used. Here, ion doping is performed using phosphorus (P) to form the n-channel transistors  811  and  821 . 
     Conductive layers  818  and conductive layers  828  are source electrodes and drain electrodes. Each of the conductive layers  818  and the conductive layers  828  can be formed with a single layer structure or a stacked-layer structure using an element selected from aluminum, tungsten, titanium, tantalum, molybdenum, nickel, and neodymium, or an alloy containing a plurality of any of these elements. For example, a conductive layer that is formed using an alloy that contains a plurality of any of the elements given above can be formed from an aluminum alloy that contains titanium, an aluminum alloy that contains neodymium, or the like can be used. In the case of forming a stacked-layer structure, for example, a structure where an aluminum layer or the above-described aluminum alloy layer is sandwiched between titanium layers can be used. Alternatively, crystalline silicon to which an impurity element which serves as a donor is added may be used. Further, a stacked-layer structure in which a film on the side that is in contact with the crystalline silicon to which an impurity element serving as a donor is added is formed using titanium, tantalum, molybdenum, tungsten, or a nitride of any of these elements, and a layer of aluminum or an aluminum alloy is formed thereover may be formed. Alternatively, the conductive film may have a stacked-layer structure where aluminum or an aluminum alloy is sandwiched with titanium, tantalum, molybdenum, tungsten, or nitride of any of these elements, on an upper side and a lower side thereof. The conductive layers  818  and the conductive layers  828  are formed by a CVD method, a sputtering method, or a vacuum evaporation method. Note that one of the conductive layer  818  and the conductive layer  828  serves as a signal line as well as a source electrode or a drain electrode. However, one embodiment of the present invention is not limited thereto, and a signal line may be provided separately from the source and drain electrodes. 
     An insulating film  837  and an insulating film  838  are interlayer insulating films. A conductive layer  806  is a pixel electrode. The conductive layer  806  serves as an anode or a cathode in a light-emitting device. Here, an example is described in which the conductive layer  806  is formed over the insulating film  838  provided over the conductive layer  818 , but one embodiment of the present invention is not limited thereto. For example, the conductive layer  806  may be provided over the insulating film  837 . 
     As the insulating film  837  and the insulating film  838 , insulating films of silicon oxide, silicon oxynitride, silicon nitride, or the like which are formed by a CVD method, a sputtering method, or the like can be used. Alternatively, an organic resin such as an acrylic resin, a polyimide resin, a benzocyclobutene-based resin, a siloxane-based resin, a polyamide resin, or an epoxy resin can be used. 
     As the conductive layer  806 , a transparent conductive film composed of a light-transmitting conductive material may be used, and indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, and the like can be used. Needless to say, it is possible to use indium tin oxide, indium zinc oxide, indium tin oxide with silicon oxide added, or the like. Furthermore, a material that has a high work function, for example, an element selected from nickel (Ni), tungsten (W), chromium (Cr), platinum (Pt), zinc (Zn), tin (Sn), indium (In), or molybdenum (Mo), or an alloy material including any of the metal elements as its main component, for example, titanium nitride, titanium silicon nitride, tungsten silicide, tungsten nitride, tungsten silicide nitride, or niobium nitride may be used to form a single layer film or a film of stacked layers. 
     Reference numeral  851  denotes an alignment film. The alignment film  851  can be formed using an organic resin such as polyimide. The thickness of the alignment film  851  is greater than or equal to 40 nm and less than or equal to 100 nm, preferably greater than or equal to 50 nm and less than or equal to 90 nm. With such a thickness, the pretilt angle of a liquid crystal material can be made large, which can reduce disclination. 
     A film having a coloring property (hereinafter referred to as a coloring film  846 ) is formed on the substrate  842 . The coloring film  846  functions as a color filter. Further, a light-blocking film  844  adjacent to the coloring film  846  is formed on the substrate  842 . The light-blocking film  844  functions as a black matrix. The coloring film  846  is not necessarily provided in the case where the liquid crystal display device is a monochrome display device, for example. 
     The coloring film  846  is a coloring film that transmits light in a specific wavelength range. For example, a red (R) color filter for transmitting light in a red wavelength range, a green (G) color filter for transmitting light in a green wavelength range, a blue (B) color filter for transmitting light in a blue wavelength range, or the like can be used. 
     The light-blocking film  844  preferably has a function of blocking light in a particular wavelength range, and can be a metal film or an organic insulating film including a black pigment. 
     An insulating film  848  is formed on the coloring film  846 . The insulating film  848  functions as a planarization film or suppresses diffusion of impurities in the coloring film  846  to the liquid crystal element side. 
     A conductive layer  808  is formed on the insulating film  848 . The conductive layer  808  functions as the other of the pair of electrodes of the liquid crystal element in the pixel portion. Note that the alignment film  851  is formed over the conductive layer  806  and an alignment film  852  is formed on the conductive layer  808 . 
     A liquid crystal layer  807  is formed between the conductive layer  806  and the conductive layer  808 . The liquid crystal layer  807  is sealed between the substrate  800  and the substrate  842  with the use of a sealant (not illustrated). The sealant is preferably in contact with an inorganic material to prevent entry of moisture and the like from the outside. 
     A spacer may be provided between the conductive layer  806  and the conductive layer  808  to maintain the thickness of the liquid crystal layer  807  (also referred to as a cell gap). 
     Note that the structure and the like described in this embodiment can be used as appropriate in combination with any of the structures and the like in the other embodiments. 
     Embodiment 3 
     In this embodiment, the driver circuit portion of the display device described in the above embodiments will be described. 
       FIG. 9  is a diagram of an entire gate driver circuit as one example of a driver circuit of a display device. A gate driver circuit  600  includes a plurality of shift register units  601 , a shift register unit  602  which is a dummy stage, demultiplexer circuits  603  electrically connected to the shift register units  601 , a demultiplexer circuit  604  electrically connected to the shift register unit  602 , and signal lines transmitting a start pulse SP and clock signals (CLK 1  to CLK 8 ). 
     To the shift register unit  601  (here, description is made by using a shift register unit in the first stage), as shown in  FIG. 10A , a set signal LIN (here, the start pulse SP), a reset signal RIN, and clock signals (here, CLK 6  and CLK 7 ) are input.  FIG. 10B  illustrates a specific circuit structure example. The shift register unit  601  includes a first transistor  611  to a sixth transistor  616 . 
     One of a source and a drain of the first transistor  611  is connected to a high power supply potential line VDD. The other of the source and the drain of the first transistor  611  is connected to one of a source and a drain of the second transistor  612  and an input terminal FN 1  of the demultiplexer circuit  603 . The set signal LIN is input to a gate of the first transistor  611 . The other of the source and the drain of the second transistor  612  is connected to a low power supply potential line VSS. A gate of the second transistor  612  is connected to an input terminal FN 2  of the demultiplexer circuit  603 , one of a source and a drain of the fourth transistor  614 , one of a source and a drain of the fifth transistor  615 , and one of a source and a drain of the sixth transistor  616 . One of a source and a drain of the third transistor  613  is connected to the high power supply potential line VDD. The other of the source and the drain of the third transistor  613  is connected to the other of the source and the drain of the fourth transistor  614 . The clock signal CLK 7  is input to a gate of the third transistor  613 . The clock signal CLK 6  is input to a gate of the fourth transistor  614 . The other of the source and the drain of the fifth transistor  615  is connected to the low power supply potential line VSS. The set signal LIN is input to a gate of the fifth transistor  615 . The other of the source and the drain of the sixth transistor  616  is connected to the high power supply potential line VDD. The reset signal RIN is input to a gate of the sixth transistor  616 . Note that a portion in which the other of the source and the drain of the first transistor  611  and the one of the source and the drain of the second transistor  612  are electrically connected is referred to as a node FN 1 . A portion in which the gate of the second transistor  612 , the one of the source and the drain of the fourth transistor  614 , the one of the source and the drain of the fifth transistor  615 , and the one of the source and the drain of the sixth transistor  616  are electrically connected is referred to as a node FN 2 . 
     The clock signals CLK 6  and CLK 7  are input to the shift register unit  601  in the (8a+1)th stage (a is zero or a natural number). The clock signals CLK 3  and CLK 4  are input to the shift register unit  601  in the (8a+2)th stage (a is zero or a natural number). The clock signals CLK 1  and CLK 8  are input to the shift register unit  601  in the (8a+3)th stage (a is zero or a natural number). The clock signals CLK 5  and CLK 6  are input to the shift register unit  601  in the (8a+4)th stage (a is zero or a natural number). The clock signals CLK 2  and CLK 3  are input to the shift register unit  601  in the (8a+5)th stage (a is zero or a natural number). The clock signals CLK 7  and CLK 8  are input to the shift register unit  601  in the (8a+6)th stage (a is zero or a natural number). 
     The clock signals CLK 4  and CLK 5  are input to the shift register unit  601  in the (8a+7)th stage (a is zero or a natural number). The clock signals CLK 1  and CLK 2  are input to the shift register unit  601  in the 8(a+1)th stage (a is zero or a natural number). 
     The set signal LIN and clock signals (here, CLK 3  and CLK 4 ) are input to the shift register unit  602  which is a dummy stage, as illustrated in  FIG. 11A .  FIG. 11B  illustrates a specific circuit structure example. The shift register unit  602  includes the first transistor  611  to the fifth transistor  615 . 
     The one of the source and the drain of the first transistor  611  is connected to the high power supply potential line VDD. The other of the source and the drain of the first transistor  611  is connected to the one of the source and the drain of the second transistor  612  and the input terminal FN 1  of the demultiplexer circuit  604 . The set signal LIN is input to the gate of the first transistor  611 . The other of the source and the drain of the second transistor  612  is connected to the low power supply potential line VSS. The gate of the second transistor  612  is connected to the input terminal FN 2  of the demultiplexer circuit  604 , the one of the source and the drain of the fourth transistor  614 , and the one of the source and the drain of the fifth transistor  615 . The one of the source and the drain of the third transistor  613  is connected to the high power supply potential line VDD. The other of the source and the drain of the third transistor  613  is connected to the other of the source and the drain of the fourth transistor  614 . The clock signal CLK 4  is input to the gate of the third transistor  613 . The clock signal CLK 3  is input of the gate of the fourth transistor  614 . The other of the source and the drain of the fifth transistor  615  is connected to the low power supply potential line VSS. The set signal LIN is input to the gate of the fifth transistor  615 . Note that a portion in which the other of the source and the drain of the first transistor  611  and the one of the source and the drain of the second transistor  612  are electrically connected is referred to as the node FN 1 . A portion in which the gate of the second transistor  612 , the one of the source and the drain of the fourth transistor  614 , and the one of the source and the drain of the fifth transistor  615  are electrically connected is referred to as the node FN 2 . 
     As illustrated in  FIG. 12A  and  FIG. 13A , clock signals and output signals are input from the shift register unit  601  and the shift register unit  602  (signals input to the input terminal FN 1  and the input terminal FN 2 ) to the demultiplexer circuit  603  and the demultiplexer circuit  604 , and the demultiplexer circuit  603  and the demultiplexer circuit  604  output output signals.  FIG. 12B  and  FIG. 13B  each illustrate a specific circuit structure example. The demultiplexer circuit  603  and the demultiplexer circuit  604  each include a buffer  605 . 
       FIG. 14  illustrates one example of a specific circuit structure of the buffer  605 . A clock signal CLK (one of the clock signals CLK 1  to CLK 8 ) is input to one of a source and a drain of a seventh transistor  617 . The other of the source and the drain of the seventh transistor  617  is connected to one of a source and a drain of an eighth transistor  618  and an output terminal. A gate of the seventh transistor  617  is connected to the node FN 1 . The other of the source and the drain of the eighth transistor  618  is connected to the low power supply potential line VSS. A gate of the eighth transistor  618  is connected to the node FN 2 . 
     A shift register unit may be a shift register unit  601   a  which is illustrated in  FIG. 15A  and  FIG. 15B  and in which a transistor  621 , a transistor  622 , a transistor  623 , and a capacitor  624  are added to the shift register unit  601 . Note that a reset signal RES is input to a gate of the transistor  623 . 
     Similarly, a shift register unit which is a dummy stage may be a shift register unit  602   a  which is illustrated in  FIG. 16A  and  FIG. 16B  and in which the transistor  621 , the transistor  622 , the transistor  623 , and the capacitor  624  are added to the shift register unit  602 . Note that the reset signal RES is input to the gate of the transistor  623 . 
     To initialize the shift register unit, a pulse of the reset signal RES is input to turn on the transistor  623 , so that the potential of the node FN 2  becomes equal to the potential of the high power supply potential line VDD. The second transistor  612  and the transistor  621  are turned on with the potential of the node FN 2 , so that the potential of the node FN 1  becomes equal to the potential of the low power supply potential line VSS. Consequently, the shift register unit can be initialized. Note that the reset signal RES is input to all of the shift register units using a common signal line. 
     As illustrated in  FIG. 17A  and  FIG. 17B , the buffer  605  can be replaced with a buffer  605   a  further provided with a transistor  625  and a capacitor  619 . 
     The capacitor serves as storage capacitor for holding charge. 
     In the shift register unit  601  in the first stage, the clock signals CLK 1  to CLK 5  are input to the demultiplexer circuit  603  and the demultiplexer circuit  603  outputs output signals OUT 1  to OUT 5 . 
     The potential of the node FN 2  is fixed to a high potential in a period in which a gate selection output is not output, so that the second transistor  612  and the eighth transistor  618  are always on. In this way, the output is a low potential stably. However, in the case where the cutoff current (a drain current flowing at a gate voltage of 0 V) of the fifth transistor  615  is high, charge of the node FN 2  leaks through the fifth transistor  615 ; therefore, charge needs to be regularly compensated. Thus, the third transistor  613  and the fourth transistor  614  are turned on with the clock signals CLK 6  and CLK 7 , so that charge for the node FN 2  is supplied from the high power supply potential line VDD. Note that a gate selection output period (the period in which the node FN 1  is at high potential) of the shift register unit  601  in the first stage is a period from the rising (set) of the start pulse SP to the rising (reset) of the clock signal CLK 7 , which is described later. In the period, the gate selection output period and timing of regular compensation of charge are set not to overlap each other with two clock signals. 
     In the shift register unit  601  in the first stage, the clock signal CLK 8  is not input to anywhere. The clock signal is also provided to avoid overlapping timing of regular compensation of charge. 
     Similarly, in the shift register unit  601  in the second stage, the clock signals CLK 1 , CLK 2 , and CLK 6  to CLK 8  are input to the demultiplexer circuit  603 , and the demultiplexer circuit  603  outputs the output signals OUT 1  to OUT 5 . The clock signals CLK 3  and CLK 4  have a function of regularly compensating charge. In the shift register unit  601  in the second stage, the clock signal CLK 5  is not input to anywhere. 
     The same can be applied to the shift register units  601  in the third and the following stages. In other words, one stage of the shift register unit inputs five clock signals to the demultiplexer circuit  603 , and the demultiplexer circuit  603  outputs five output signals. Other two clock signals have a function of regularly compensating charge and are input to the shift register unit  601 . The other clock signal is not input to anywhere. 
     The same is also applied to the shift register unit  602  which is a dummy stage. The clock signals CLK 1  and CLK 2  are input to the demultiplexer circuit  604 , and the demultiplexer circuit  604  outputs output signals DUMOUT 1  and DUMOUT 2 . The clock signals CLK 3  and CLK 4  have a function of regularly compensating charge. 
     The number of clock signals are eight in this embodiment, but the present invention is not limited thereto. The number of clock signals may be any number as long as it is four or more. For example, when the number of clock signals is n, the number of output signals is (n-3) since three clock signals do not contribute to output signals. 
     In other words, by connecting n signal lines for transmitting clock signals to one stage of the shift register unit, (n-3) output signals can be output. The larger n becomes, the smaller the rate of signal lines for transmitting clock signals which do not contribute to output becomes; accordingly, the area of the shift register unit part is small compared to a conventional structure in which one stage of a shift register unit outputs one output signal. Therefore, the width of the gate driver circuit  600  can be reduced. 
     Here, to narrow the width of the gate driver circuit  600  is briefly described.  FIG. 18A  is a block diagram of a conventional gate driver circuit.  FIG. 18B  is a block diagram of a gate driver circuit in this embodiment. 
     In a conventional gate driver circuit illustrated in  FIG. 18A , one stage of a shift register unit SR is connected to four signal lines CLK_LINE for transmitting a clock signal and one buffer BUF outputs one signal. On the other hand, in the gate driver circuit in this embodiment illustrated in  FIG. 18B , one shift register unit SR is connected to eight signal lines CLK_LINE for transmitting clock signals and five buffers BUF output five signals. 
     The gate driver circuit in this embodiment can have a smaller horizontal layout width of one shift register unit than that of the conventional gate driver circuit. The vertical layout width increases because of increased buffers BUF (here, five times as much as the conventional one), but the increase does not contribute to the bezel of the gate driver circuit. The horizontal layout width of one shift register unit can be reduced, so that the bezel can be narrower. In comparison with the conventional one, the number of the signal lines CLK_LINE for transmitting a clock signal is increased, and accordingly, load capacitance for each signal line CLK_LINE can be reduced. Therefore, even when the signal line CLK_LINE is set to thin to increase load resistance, delay time is not changed (because time constant=load capacitance×load resistance). Accordingly, by making the width of the signal line thin to obtain the same time constant, increase in layout width can be prevented; therefore, even if the number of the signal lines CLK_LINE is increased, the width of the gate driver circuit can be reduced. 
     Next, the operation of the gate driver circuit  600  is described with reference to a timing diagram in  FIG. 19 . Here, high potentials of the set signal LIN, the reset signal MN, and the clock signals CLK 1  to CLK 8  correspond to the potential of the high power supply potential line VDD, while low potentials of the set signal LIN, the reset signal MN, and the clock signals CLK 1  to CLK 8  correspond to the potential of the low power supply potential line VSS. 
     In the driving method of the gate driver circuit  600  shown in  FIG. 19 , first, the start pulse SP is set to a high potential to turn on the first transistor  611  and the fifth transistor  615 . Since the reset signal RIN (the output signal OUT 7 ) is a low potential, the sixth transistor  616  is turned off. Since the clock signals CLK 1  to CLK 6  are low potentials and the clock signals CLK 7  and CLK 8  are high potentials, the fourth transistor  614  and the seventh transistor  617  are turned off and the third transistor  613  is turned on. 
     At that time, the potential of the node FN 1  has a value obtained by subtracting the threshold voltage of the first transistor  611  from the potential of the high power supply potential line VDD (VDD−Vth( 611 )), while the potential of the node FN 2  becomes equal to the potential of the low power supply potential line VSS. Accordingly, the seventh transistor  617  is turned on and the eighth transistor  618  is turned off, and thus, the output signals OUT 1  to OUT 5  are low potentials, as in the clock signals CLK 1  to CLK 5 . 
     Then, the clock signal CLK 7  is set to a low potential, so that the third transistor  613  is turned off. Note that a high potential is held at a node in which the other of the source and the drain of the third transistor  613  and the one of the source and the drain of the fourth transistor  614  are electrically connected. 
     Next, the clock signal CLK 1  changes from a low potential to a high potential, and the potential of the node FN 1  increases by a voltage corresponding to the amplitude of the clock signal CLK 1  by a bootstrap operation. As a result, the seventh transistor  617  is turned on, and a high potential (the potential of the clock signal CLK 1 ) is output as the output signal OUT 1 . Note that the bootstrap operation occurs similarly when the clock signals followed by the clock signal CLK 2  changes from a low potential to a high potential. Next, the clock signal CLK 8  becomes a low potential, but change does not occur because a signal of the clock signal CLK 8  is not used for the shift register unit  601  in the first stage. Then, the clock signal CLK 2  becomes a high potential, and a high potential is output as the output signal OUT 2 . After that, the clock signal CLK 1  becomes a low potential, and a low potential is output as the output signal OUT 1 . The same can be applied to the following operation associated with the output signals OUT 3  and OUT 4 . When the clock signal CLK 5  becomes a high potential and the output signal OUT 5  becomes a high potential, the set signal LIN of the shift register unit  601  in the second stage becomes a high potential. 
     In the shift register unit  601  in the first stage, when the clock signal CLK 6  becomes a high potential, the fourth transistor  614  is turned on. Then, the clock signal CLK 5  becomes a low potential, and a low potential is output as the output signal OUT 5 . 
     In the shift register unit  601  in the second stage, the set signal LIN (the output signal OUT 5 ) becomes a high potential, and the first transistor  611  and the fifth transistor  615  are turned on. Since the reset signal RIN (an output signal OUT 12 ) is a low potential, the sixth transistor  616  is turned off. Since the clock signals CLK 1 , CLK 2 , and CLK 6  to CLK 8  become a low potential and the clock signals CLK 4  and CLK 5  become high potential, the fourth transistor  614  and the seventh transistor  617  are turned off and the third transistor  613  is turned on. 
     At this time, the potential of the node FN 1  has a value obtained by subtracting the threshold voltage of the first transistor  611  from the potential of the high power supply potential line VDD (VDD−Vth( 611 )), while the potential of the node FN 2  becomes equal to the potential of the low power supply potential line VSS. Accordingly, the seventh transistor  617  is turned on and the eighth transistor  618  is turned off, and thus, output signals OUT 6  to OUT 10  become a low potential, as in the clock signals CLK 1 , CLK 2 , and CLK 6  to CLK 8 . 
     Next, the clock signal CLK 4  becomes a low potential, and the third transistor  613  is turned off. Note that a high potential is held at a node in which the other of the source and the drain of the third transistor  613  and the one of the source and the drain of the fourth transistor  614  are electrically connected. 
     Next, the clock signal CLK 6  changes from a low potential to a high potential, and the potential of the node FN 1  increases by a voltage corresponding to the amplitude of the clock signal CLK 6  by a bootstrap operation. As a result, the seventh transistor  617  is turned on, and a high potential (the potential of the clock signal CLK 6 ) is output as the output signal OUT 6 . Next, the clock signal CLK 5  becomes a low potential, but change does not occur because a signal of the clock signal CLK 5  is not used for the shift register unit  601  in the second stage. Then, the clock signal CLK 7  becomes a high potential, and a high potential is output as the output signal OUT 7 . 
     At that time, in the shift register unit  601  in the first stage, the reset signal RIN (the output signal OUT 7 ) becomes a high potential, and the sixth transistor  616  is turned on, so that the potential of the node FN 2  becomes equal to the potential of the high power supply potential line VDD. The second transistor  612  is turned on with the potential of the node FN 2 , so that the potential of the node FN 1  becomes the potential of the low power supply potential line VSS and then is reset. 
     The shift register unit  601  in the second stage is driven like the shift register unit  601  in the first stage. 
     That is, the output signal OUT 5 (m−1) of the shift register unit  601  in the (m−1)th stage is input as the set signal LIN of the shift register unit  601  in the m-th stage (m is a natural number). The output signal OUT 5 (m+2) of the shift register unit  601  in the (m+1)th stage is input as the reset signal RIN of the shift register unit  601  in the m-th stage. Note that the set signal LIN when m is 1 corresponds to the start pulse SP. 
     The shift register unit  602  which is a dummy stage is similar to the shift register unit  601 . The reset signal RIN can be input to the shift register unit  601  in the final stage with the shift register unit  602 . 
     Note that pulses of a clock signal and the next clock signal overlap by one third of the pulse width in this embodiment, but the present invention is not limited thereto. The overlap width may be any value as long as it is half of the pulse width or less. The falling of the pulse of the clock signal and the rising of the pulse of the next clock signal may be at the same timing. In that case, the gate selection output period of the shift register unit  601  in the first stage is a period from the rising (set) of the start pulse SP to the rising (reset) of the clock signal CLK 6 ; accordingly, the number of clock signals needed for regularly compensating charge is only one. 
     Note that the structure and the like described in this embodiment can be used as appropriate in combination with any of the structures and the like in the other embodiments. 
     Embodiment 4 
     The semiconductor device of one embodiment of the present invention can be used in a sensor that can detect proximity or touch of an object (e.g., a capacitive, a resistive, a surface acoustic wave, an infrared, and an optical touch sensor) and a radiographic image detection device that can obtain a medical radiographic image. The semiconductor device which is one embodiment of the present invention can be applied to a variety of electronic appliances (including game machines). Examples of electronic appliances include a television device (also referred to as television or television receiver), a monitor of a computer or the like, a digital camera, a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, an audio reproducing device, a game machine (e.g., a pachinko machine or a slot machine), and a game console, and the like. Examples of these electronic appliances are illustrated in  FIGS. 7A to 7C . 
       FIG. 7A  illustrates a table  9000  having a display portion. In the table  9000 , a display portion  9003  is incorporated in a housing  9001  and an image can be displayed on the display portion  9003 . The housing  9001  is supported by four leg portions  9002 . Further, a power cord  9005  for supplying power is provided for the housing  9001 . 
     The semiconductor device described in any of the above embodiments can be used for the display portion  9003 . Thus, the display portion  9003  can have high display quality. 
     The display portion  9003  has a touch-input function. When a user touches displayed buttons  9004  which are displayed on the display portion  9003  of the table  9000  with his/her fingers or the like, the user can carry out operation of the screen and input of information. Further, when the table may be made to communicate with home appliances or control the home appliances, the display portion  9003  may function as a control device which controls the home appliances by operation on the screen. For example, with use of a semiconductor device having an image sensor function, the display portion  9003  can have a touch-input function. 
     Further, the screen of the display portion  9003  can be placed perpendicular to a floor with a hinge provided for the housing  9001 ; thus, the table can also be used as a television device. When a television device having a large screen is set in a small room, an open space is reduced; however, when a display portion is incorporated in a table, a space in the room can be efficiently used. 
       FIG. 7B  illustrates a television device  9100 . In the television device  9100 , a display portion  9103  is incorporated in a housing  9101  and an image can be displayed on the display portion  9103 . Note that the housing  9101  is supported by a stand  9105  here. 
     The television device  9100  can be operated with an operation switch of the housing  9101  or a separate remote controller  9110 . Channels and volume can be controlled with an operation key  9109  of the remote controller  9110  so that an image displayed on the display portion  9103  can be controlled. Furthermore, the remote controller  9110  may be provided with a display portion  9107  for displaying data output from the remote controller  9110 . 
     The television device  9100  illustrated in  FIG. 7B  is provided with a receiver, a modem, and the like. With the receiver, general television broadcasts can be received in the television device  9100 . Further, when the television device  9100  is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers) data communication can be performed. 
     Any of the semiconductor devices described in the above embodiments can be used for the display portions  9103  and  9107 . Thus, the television device can have high display quality. 
       FIG. 7C  illustrates a computer  9200 , which includes a main body  9201 , a housing  9202 , a display portion  9203 , a keyboard  9204 , an external connection port  9205 , a pointing device  9206 , and the like. 
     Any of the semiconductor devices described in the above embodiments can be used for the display portion  9203 . Thus, the computer  9200  can have high display quality. 
       FIGS. 8A and 8B  illustrate a foldable tablet terminal. The tablet terminal is opened in  FIG. 8A . The tablet terminal includes a housing  9630 , a display portion  9631   a,  a display portion  9631   b,  a display mode switch  9034 , a power switch  9035 , a power saver switch  9036 , a clasp  9033 , and an operation switch  9038 . 
     The semiconductor device described in any of the above embodiments can be used for the display portion  9631   a  and the display portion  9631   b.  Thus, the display quality of the tablet terminal can be improved. 
     Part of the display portion  9631   a  can be a touch panel region  9632   a  and data can be input when a displayed operation key  9638  is touched. Although a structure in which a half region in the display portion  9631   a  has only a display function and the other half region also has a touch panel function is illustrated as an example, the structure of the display portion  9631   a  is not limited thereto. The whole area of the display portion  9631   a  may have a touch screen function. For example, the whole area of the display portion  9631   a  can display keyboard buttons and serve as a touch screen while the display portion  9631   b  can be used as a display screen. 
     Like the display portion  9631   a,  part of the display portion  9631   b  can be a touch screen region  9632   b.  When a keyboard display switching button  9639  displayed on the touch panel is touched with a finger, a stylus, or the like, a keyboard can be displayed on the display portion  9631   b.    
     Touch input can be performed concurrently on the touch screen regions  9632   a  and  9632   b.    
     The display-mode switching switch  9034  can switch display orientation (e.g., between landscape mode and portrait mode) and select a display mode (switch between monochrome display and color display), for example. The power-saving-mode switching switch  9036  can control display luminance in accordance with the amount of external light in use of the tablet terminal detected by an optical sensor incorporated in the tablet. The tablet terminal may include another detection device such as a sensor for detecting orientation (e.g., a gyroscope or an acceleration sensor) in addition to the optical sensor. 
     Although the display portion  9631   a  and the display portion  9631   b  have the same display area in  FIG. 8A , one embodiment of the present invention is not limited to this example. The display portion  9631   a  and the display portion  9631   b  may have different areas or different display quality. For example, one of them may be a display panel that can display higher-definition images than the other. 
     In  FIG. 8B , the tablet terminal is folded and includes the housing  9630 , a solar cell  9633 , and a charge and discharge control circuit  9634 . Note that in  FIG. 8B , an example in which the charge and discharge control circuit  9634  includes the battery  9635  and the DCDC converter  9636  is illustrated. 
     Since the tablet can be folded in two, the housing  9630  can be closed when not in use. Thus, the display portions  9631   a  and  9631   b  can be protected, thereby providing a tablet with high endurance and high reliability for long-term use. 
     In addition, the tablet terminal illustrated in  FIGS. 8A and 8B  can have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, a function of controlling processing by a variety of kinds of software (programs), and the like. 
     The solar cell  9633 , which is attached on the surface of the tablet terminal, supplies electric power to a touch screen, a display portion, an image signal processor, and the like. Note that the solar cell  9633  can be provided on one or both surfaces of the housing  9630 , so that the battery  9635  can be charged efficiently. The use of a lithium ion battery as the battery  9635  is advantageous in downsizing or the like. 
     The structure and operation of the charge and discharge control circuit  9634  illustrated in  FIG. 8B  are described with reference to a block diagram of  FIG. 8C . The solar cell  9633 , the battery  9635 , the DCDC converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and the display portion  9631  are shown in  FIG. 8C , and the battery  9635 , the DCDC converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond to the charge/discharge control circuit  9634  in  FIG. 8B . 
     First, an example of the operation in the case where power is generated by the solar cell  9633  using external light is described. The voltage of power generated by the solar battery is raised or lowered by the DCDC converter  9636  so that the power has a voltage for charging the battery  9635 . Then, when the power from the solar cell  9633  is used for the operation of the display portion  9631 , the switch SW 1  is turned on and the voltage of the power is raised or lowered by the converter  9637  so as to be a voltage needed for the display portion  9631 . In addition, when display on the display portion  9631  is not performed, the switch SW 1  is turned off and a switch SW 2  is turned on so that charge of the battery  9635  may be performed. 
     Note that the solar cell  9633  is described as an example of a power generation means; however, without limitation thereon, the battery  9635  may be charged using another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery  9635  may be charged with a non-contact power transmission module that transmits and receives power wirelessly (without contact) to charge the battery or with a combination of other charging means. 
     Note that the structures and the like described in this embodiment can be combined as appropriate with any of the structures and the like described in the other embodiments. 
     This application is based on Japanese Patent Application serial No. 2013-144276 filed with Japan Patent Office on Jul. 10, 2013, the entire contents of which are hereby incorporated by reference.