Patent Publication Number: US-2023155033-A1

Title: Semiconductor Device and Method for Manufacturing the Same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/002,971, filed Aug. 26, 2020, now allowed, which is a continuation of U.S. application Ser. No. 16/180,565, filed Nov. 5, 2018, now U.S. Pat. No. 10,763,372, which is a continuation of U.S. application Ser. No. 15/814,919, filed Nov. 16, 2017, now U.S. Pat. No. 10,153,380, which is a continuation of U.S. application Ser. No. 15/443,096, filed Feb. 27, 2017, now U.S. Pat. No. 10,170,632, which is a continuation of U.S. application Ser. No. 15/071,674, filed Mar. 16, 2016, now U.S. Pat. No. 9,601,603, which is a continuation of U.S. application Ser. No. 14/694,212, filed Apr. 23, 2015, now U.S. Pat. No. 9,318,512, which is a continuation of U.S. application Ser. No. 13/302,222, filed Nov. 22, 2011, now U.S. Pat. No. 9,029,851, which is a continuation of U.S. application Ser. No. 12/581,918, filed Oct. 20, 2009, now U.S. Pat. No. 8,067,775, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2008-274540 on Oct. 24, 2008, all of which are incorporated by reference. 
    
    
     Technical Field 
     The present invention relates to a semiconductor device using an oxide semiconductor and a method for manufacturing the semiconductor device. 
     Background Art 
     A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor formed using amorphous silicon has low electric field effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor formed using crystalline silicon has high electric field effect mobility, but a crystallization process such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate. 
     In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is formed using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device. 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2007-123861 
         [Patent Document 2] Japanese Published Patent Application No. 2007-096055 
       
    
     DISCLOSURE OF INVENTION 
     The electron field effect mobility of a thin film transistor in which a channel formation region is provided in an oxide semiconductor is higher than that of a thin film transistor using amorphous silicon. The oxide semiconductor film can be formed by a sputtering method or the like at a temperature of 300° C. or lower. Its manufacturing process is easier than that of a thin film transistor using polycrystalline silicon. 
     Such an oxide semiconductor is expected to be used for forming a thin film transistor over a glass substrate, a plastic substrate, or the like, and to be applied to a display device such as a liquid crystal display device, an electroluminescent display device, or electronic paper. 
     When the size of a display region of a display device is increased, the number of pixels is increased and thus the number of gate lines and signal lines is increased. In addition, as a display device has a higher definition, the number of pixels is increased and thus the number of gate lines and signal lines is increased. When the number of the gate lines and the signal lines is increased, it is difficult to mount IC chips including driver circuits for driving of the gate lines and the signal lines by bonding or the like, whereby manufacturing cost is increased. 
     Therefore, it is an object to reduce manufacturing cost by employing a thin film transistor using an oxide semiconductor in at least part of a driver circuit for driving a pixel portion. 
     In the case of employing a thin film transistor using an oxide semiconductor in at least part of a driver circuit for driving a pixel portion, high dynamic characteristics (on characteristics or frequency characteristics (referred to as f characteristics)) are required for the thin film transistor. It is another object to provide a thin film transistor having high dynamic characteristics (on characteristics) and to provide a driver circuit which enables high speed operation. 
     In addition, it is an object of an embodiment of the present invention to provide a semiconductor device provided with a highly reliable thin film transistor in which an oxide semiconductor layer is used for a channel. 
     Gate electrodes are provided above and below an oxide semiconductor layer to realize improvement of on characteristics and reliability of a thin film transistor. 
     Further, by controlling gate voltage applied to the upper and lower gate electrodes, threshold voltage can be controlled. The upper and lower gate electrodes may be electrically connected to each other so as to have the same potential, or the upper and lower gate electrodes may be connected to different wirings so as to have different potentials. For example, when the threshold voltage is set at 0 or close to 0 to reduce driving voltage, reduction of power consumption can be achieved. Alternatively, when the threshold voltage is set positive, the thin film transistor can function as an enhancement type transistor. Further alternatively, when the threshold voltage is set negative, the thin film transistor can function as a depletion type transistor. 
     For example, an inverter circuit including a combination of the enhancement type transistor and the depletion type transistor (hereinafter, such a circuit is referred to as an EDMOS circuit) can be used for a driver circuit. The driver circuit includes at least a logic circuit portion, and a switch portion or a buffer portion. The logic circuit portion has a circuit structure including the above EDMOS circuit. Further, a thin film transistor by which large on current can flow is preferably used for the switch portion or the buffer portion. A depletion type transistor or a thin film transistor including gate electrodes above and below an oxide semiconductor layer is used. 
     Thin film transistors having different structures can be formed over the same substrate without greatly increasing the number of steps. For example, an EDMOS circuit using a thin film transistor including gate electrodes above and below the oxide semiconductor layer may be formed for the driver circuit for high-speed driving, and a thin film transistor including a gate electrode only below the oxide semiconductor layer may be used for a pixel portion. 
     Note that an n-channel TFT whose threshold voltage is positive is referred to as an enhancement type transistor, and an n-channel TFT whose threshold voltage is negative is referred to as a depletion type transistor, throughout this specification. 
     Examples of a material for the gate electrode provided above the oxide semiconductor layer include an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), and an alloy containing any of the above elements as its component, and any conductive film can be used without particular limitation. Further, the gate electrode is not limited to a single layer structure containing any of the above elements, and can have a stacked structure of two or more layers. 
     As a material for the gate electrode provided above the oxide semiconductor layer, the same material as a pixel electrode can be used (a transparent conductive film or the like can be used in a case of a transmissive display device). For example, the gate electrode provided above the oxide semiconductor layer can be formed in the same step as a step for forming the pixel electrode which is electrically connected to the thin film transistor in the pixel portion. Consequently, the thin film transistor provided with the gate electrodes above and below the oxide semiconductor layer can be formed without greatly increasing the number of steps. In addition, by providing the gate electrode above the oxide semiconductor layer, in a bias-temperature stress test (hereinafter, referred to as a BT test) for examining reliability of a thin film transistor, the amount of change in threshold voltage of the thin film transistor between before and after the BT test can be reduced. That is, provision of the gate electrode above the oxide semiconductor layer can improve the reliability. 
     One embodiment of the present invention disclosed in this specification is a semiconductor device including a first gate electrode over an insulating surface; a first insulating layer over the first gate electrode; an oxide semiconductor layer over the first insulating layer; a source electrode and a drain electrode over the oxide semiconductor layer; a second insulating layer covering the source electrode and the drain electrode; a second gate electrode over the second insulating layer, wherein the oxide semiconductor layer has a region whose thickness is smaller than that of a region overlapping with the source electrode or the drain electrode, and the second insulating layer is in contact with the region whose thickness is smaller in the oxide semiconductor layer. 
     The above-described structure can achieve at least one of the above-described objects. 
     In the above-described structure, a width of the second gate electrode is made larger than a width of the first gate electrode, whereby a gate voltage can be applied to the whole oxide semiconductor layer from the second gate electrode. 
     Alternatively, in the above-described structure, when a width of the first gate electrode is made smaller than a width of the second gate electrode, an area of the first gate electrode, which overlaps with the source electrode and the drain electrode is reduced, so that parasitic capacitance can be reduced. Further alternatively, a width of the first gate electrode is made larger than the region whose thickness is smaller in the oxide semiconductor layer while a width of the second gate electrode is made smaller than the region whose thickness is smaller in the oxide semiconductor, so that the second gate electrode is not overlapped with the source electrode or the drain electrode to reduce the parasitic capacitance more. 
     Another embodiment of the present invention is a semiconductor device including a pixel portion and a driver circuit, the pixel portion includes at least a first thin film transistor having a first oxide semiconductor layer, the driver circuit includes an EDMOS circuit in which at least a second thin film transistor having a second oxide semiconductor layer and a third thin film transistor having a third oxide semiconductor layer, and the third thin film transistor includes a first gate electrode below the third oxide semiconductor layer and a second gate electrode above the third oxide semiconductor layer. 
     In the above-described structure, when the first thin film transistor in the pixel portion is electrically connected to a pixel electrode and the pixel electrode is of the same material as the second gate electrode in the driver circuit, the semiconductor device can be manufactured without increasing the number of steps. 
     In the above-described structure, when the first thin film transistor in the pixel portion is electrically connected to a pixel electrode and the pixel electrode is formed of a different material from the second gate electrode in the driver circuit, for example, when the pixel electrode is formed of a transparent conductive film and the second gate electrode is formed of an aluminum film, resistance of the second gate electrode in the driver circuit can be reduced. 
     Further, a so-called dual-gate structure is provided, in which the third oxide semiconductor layer of the driver circuit overlaps with the first gate electrode with the first insulating layer therebetween and also overlaps with the second gate electrode with the second insulating layer therebetween. 
     As a semiconductor device having a driver circuit, besides a liquid crystal display device, a light-emitting display device using a light-emitting element and a display device using an electrophoretic display element, which is also referred to electronic paper, can be given. 
     Note that the term “display device” in this specification means an image display device, a light-emitting device, or a light source (including a lighting device). Further, the “display device” includes the following modules in its category: a module including a connector such as an flexible printed circuit (FPC), a tape automated bonding (TAB) tape, or a tape carrier package (TCP) attached; a module having a TAB tape or a TCP which is provided with a printed wiring board at the end thereof; and a module having an integrated circuit (IC) which is directly mounted on a display element by a chip on glass (COG) method. 
     In a light-emitting display device using a light-emitting element, a plurality of thin film transistors are included in a pixel portion, and a portion in which a gate electrode of a thin film transistor is electrically connected to a source wiring or a drain wiring of another transistor is included in the pixel portion. 
     Since a thin film transistor is easily broken due to static electricity or the like, a protective circuit for protecting the driver circuit is preferably provided over the same substrate for a gate line or a source line. The protective circuit is preferably formed with a non-linear element including an oxide semiconductor. 
     The oxide semiconductor used in this specification is a thin film expressed by InMO 3 (ZnO) m (m&gt;0), and a thin film transistor using the thin film as a semiconductor layer is formed. Note that M denotes one metal element or a plurality of metal elements selected from Ga, Fe, Ni, Mn, and Co. For example, M denotes Ga in some cases; meanwhile, M denotes the above metal element such as Ni or Fe in addition to Ga (Ga and Ni or Ga and Fe) in other cases. Further, the above oxide semiconductor may contain Fe or Ni, another transitional metal element, or an oxide of the transitional metal as an impurity element in addition to the metal element contained as M. In this specification, this thin film is also referred to as an In—Ga—Zn—O-based non-single-crystal film. 
     The In—Ga—Zn—O-based non-single-crystal film is formed by a sputtering method, and is heated at 200° C. to 500° C., typically 300° C. to 400° C., for 10 to 100 minutes. Note that an amorphous structure is observed by XRD analysis as the crystal structure of the In—Ga—Zn—O-based non-single-crystal film analyzed. 
     An oxide semiconductor typified by the In—Ga—Zn—O-based non-single-crystal film is a material having a wide energy gap (Eg); therefore, even if two gate electrodes are provided above and below an oxide semiconductor layer, increase of off current can be suppressed. 
     Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps and the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention. 
     By forming the thin film transistor using the oxide semiconductor interposed between the two gate electrodes provided above and below the oxide semiconductor in a peripheral circuit such as a gate line driver circuit or a source line driver circuit, or a pixel portion, manufacturing cost is reduced. 
     With the thin film transistor using the oxide semiconductor interposed between the two gate electrodes provided above and below the oxide semiconductor, in a BT test, the amount of change in threshold voltage of the thin film transistor between before and after the BT test can be reduced. That is, the thin film transistor includes the oxide semiconductor interposed between the two gate electrodes provided above and below the oxide semiconductor, whereby reliability of the thin film transistor can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIGS.  1 A,  1 B, and  1 C  are cross-sectional views illustrating an example of a display device of Embodiment 1, another example of a display device of Embodiment 1, and another example of a display device of Embodiment 1, respectively; 
         FIGS.  2 A,  2 B, and  2 C  are a cross-sectional view, an equivalent circuit diagram, and a top view of a semiconductor device of Embodiment 2, respectively; 
         FIGS.  3 A and  3 B  are block diagrams entirely illustrating display devices of Embodiment 3; 
         FIG.  4    is a diagram illustrating arrangement of a wiring, an input terminal, and the like in a display device of Embodiment 3; 
         FIG.  5    is a block diagram illustrating a structure of a shift register circuit; 
         FIG.  6    is a diagram illustrating an example of a flip-flop circuit; 
         FIG.  7    is a view illustrating a layout view (a top view) of the flip-flop circuit; 
         FIG.  8    is a diagram illustrating a timing chart for showing operation of a shift register circuit; 
         FIGS.  9 A to  9 C  are views illustrating a method for manufacturing a semiconductor device of Embodiment 4; 
         FIGS.  10 A to  10 C  are views illustrating a method for manufacturing the semiconductor device of Embodiment 4; 
         FIG.  11    is a view illustrating a method for manufacturing the semiconductor device of Embodiment 4; 
         FIG.  12    is a view illustrating a method for manufacturing the semiconductor device of Embodiment 4; 
         FIG.  13    is a view illustrating a method for manufacturing the semiconductor device of Embodiment 4. 
         FIG.  14    is a view illustrating the semiconductor device of Embodiment 4; 
         FIGS.  15 A,  15 B,  15 C, and  15 D  are views illustrating semiconductor devices of Embodiment 4; 
         FIG.  16    is a view illustrating the semiconductor device of Embodiment 4; 
         FIG.  17    is a cross-sectional view illustrating a semiconductor device of Embodiment 5; 
         FIG.  18    is a diagram illustrating a pixel equivalent circuit of a semiconductor device of Embodiment 6; 
         FIGS.  19 A to  19 C  are cross-sectional views illustrating semiconductor devices of Embodiment 6; 
         FIGS.  20 A and  20 B  are a top view and a cross-sectional view illustrating a semiconductor device of Embodiment 6, respectively; 
         FIGS.  21 A and  21 B  are top views and  FIG.  21 C  is a cross-sectional view illustrating a semiconductor device of Embodiment 7; 
         FIG.  22    is a cross-sectional view illustrating a semiconductor device of Embodiment 7; 
         FIGS.  23 A to  23 D  are external views illustrating examples of electronic devices; 
         FIGS.  24 A and  24 B  are external views illustrating examples of a television device and a digital photo frame, respectively; 
         FIGS.  25 A and  25 B  are external views illustrating examples of mobile phones; and 
         FIG.  26    is a cross-sectional view illustrating a semiconductor device of Embodiment 9. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments will be described below. The present invention is not limited to the description below and it is easily understood by those skilled in the art that the mode and details can be changed variously without departing from the scope and spirit of the present invention. Therefore, the present invention should not be interpreted as being limited to description in the embodiments below. 
     Embodiment 1 
       FIG.  1 A  illustrates an example in which a first thin film transistor  430  used for a driver circuit and a second thin film transistor  170  used for a pixel portion are provided over the same substrate. Note that  FIG.  1 A  is also an example of a cross-sectional view of a display device. 
     The pixel portion and the driver circuit are formed over the same substrate. In the pixel portion, the second thin film transistors  170 , which are enhancement type transistors, arranged in a matrix form are each used for switching on/off of voltage application to a pixel electrode  110 . The second thin film transistor  170  arranged in the pixel portion is formed using an oxide semiconductor layer  103 . As for electric characteristics of the second thin film transistor, on/off ratio is 10 9  or more at a gate voltage ±20 V; therefore, display contrast can be improved, and further, leakage current is small, whereby low power consumption driving can be realized. The on/off ratio is a ratio of on current to off current (I on /I off ), and the higher the value of the I on /I off  is, the better switching characteristics is. Thus, high on/off ratio contributes to improvement of display contrast. Note that on current is current which flows between a source electrode and a drain electrode when a transistor is in an on state. Meanwhile, off current is current which flows between the source electrode and the drain electrode when the transistor is in an off state. For example, in an n-channel transistor, the off current is current which flows between a source electrode and a drain electrode when gate voltage is lower than threshold voltage of the transistor. Therefore, an enhancement type transistor is preferably used for the pixel portion to achieve high contrast and low-power-consumption driving. 
     In the driver circuit, at least one thin film transistor  430  including a first gate electrode  401  below an oxide semiconductor layer  405  and a second gate electrode  470  above the oxide semiconductor layer  405  is used. The second gate electrode  470  can also be called a back-gate electrode. When the back-gate electrode is formed, in a bias-temperature stress test (hereinafter, referred to as a BT test) for examining reliability of a thin film transistor, the amount of change in threshold voltage of the thin film transistor between before and after the BT test can be reduced. 
     A structure of this thin film transistor  430  is described with reference to  FIG.  1 A . The first gate electrode  401  provided over a substrate  400  having an insulating surface is covered with a first gate insulating layer  403 , and the oxide semiconductor layer  405  is provided over the first gate insulating layer  403  overlapping with the first gate electrode  401 . Over the oxide semiconductor layer  405 , a first wiring  409  and a second wiring  410  are provided. The oxide semiconductor layer  405  includes a region whose thickness is smaller than the thickness of a region which serves as a source electrode or a drain electrode and overlaps with the first wiring  409  and the second wiring  410 . A second gate insulating layer  412  is provided so as to be over and in contact with the region whose thickness is smaller in the oxide semiconductor layer  405 . Further, the second gate electrode  470  is provided over the second gate insulating layer  412 . 
     The oxide semiconductor layer  405  is formed, e.g., at argon gas flow rate of 10 sccm and oxygen flow rate of 5 sccm by a sputtering method using a target wherein In 2 O 3 : Ga 2 O 3 : ZnO=1:1:1 (In: Ga: Zn=1:1:0.5). In addition, an n +  layer  406   a  is provided between the oxide semiconductor layer  405  and the first wiring  409 , and an n +  layer  406   b  is provided between the oxide semiconductor layer  405  and the second wiring  410 . 
     In this embodiment, the n +  layers  406   a  and  406   b  serving as source and drain regions are In—Ga—Zn—O-based non-single-crystal films, which are formed under deposition conditions different from the deposition conditions of the oxide semiconductor layer  405 , and are oxide semiconductor layers having lower resistance. For example, the n +  layers  406   a  and  406   b  formed of oxide semiconductor layers obtained at argon gas flow rate of 40 sccm have n-type conductivity and activation energy (ΔE) of from 0.01 eV to 0.1 eV. Note that in this embodiment, the n +  layers  406   a  and  406   b  are In—Ga—Zn—O-based non-single-crystal films, which include at least amorphous component. The n +  layers  406   a  and  406   b  include crystal grains (nano crystals) in the amorphous component in some cases. The diameter of the crystal grains (nano crystals) included in the n +  layers  406   a  and  406   b  is about 1 nm to 10 nm, typically about 2 nm to 4 nm. 
     Further, the first gate electrode  401  and the second gate electrode  470  may be electrically connected to each other so as to have the same potential. When the first gate electrode  401  and the second gate electrode  470  have the same potential, gate voltage can be applied from upper and lower sides of the oxide semiconductor layer, so that the amount of current which flows in an on state can be increased. 
     Further, by electrically connecting a control signal line for shifting the threshold voltage to a negative value to either the first gate electrode  401  or the second gate electrode  470 , a depletion type TFT can be formed. 
     Alternatively, by electrically connecting a control signal line for shifting the threshold voltage to a positive value to either the first gate electrode  401  or the second gate electrode  470 , an enhancement type TFT can be formed. 
     Further, there is no particular limitation on a combination of two thin film transistors used for the driver circuit, and a combination of a thin film transistor including one gate electrode as the depletion type TFT and a thin film transistor including two gate electrodes as the enhancement type TFT may be employed. In that case, a thin film transistor in the pixel portion has a structure in which gate electrodes are provided above and below the oxide semiconductor layer. 
     Alternatively, the thin film transistor in the pixel portion may have a structure in which gate electrodes are provided above and below the oxide semiconductor layer, and the enhancement type TFT and the depletion type TFT in the driver circuit may each have a structure in which gate electrodes are provided above and below the oxide semiconductor layer. In that case, a structure in which a control signal line for controlling the threshold voltage is electrically connected to either of the upper and lower gate electrodes and the connected gate electrode controls the threshold voltage is employed. 
     Note that in  FIG.  1 A , the second gate electrode  470  is formed of the same material as the pixel electrode  110  in the pixel portion, for example, using a transparent conductive film in a transmissive liquid crystal display device, in order to reduce the number of steps. However, there is no particular limitation on the second gate electrode  470 . In addition, the example in which a width of the second gate electrode  470  is larger than a width of the first gate electrode  401  and also larger than a width of the oxide semiconductor layer is illustrated; however, there is no particular limitation on the width of the second gate electrode  470 . Note that the width of the first gate electrode  401  is larger than that of the region whose thickness is smaller in the oxide semiconductor layer. 
       FIG.  1 B  illustrates an example different from  FIG.  1 A  in the material and the width of the second gate electrode. Further,  FIG.  1 B  is also an example of a display device in which the second thin film transistor  170  connected to an organic light-emitting element or an inorganic light-emitting element is included in the pixel portion. 
     In  FIG.  1 B , as a material for an electrode  471  which functions as a second gate electrode of a thin film transistor  432 , a metal material (an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), or an alloy containing any of the above-described elements as its component) is used. A width of the electrode  471  in the cross section is smaller than that of the second gate electrode  470  in  FIG.  1 A . Further, the width of the electrode  471  is smaller than a width of the oxide semiconductor layer. By reducing the width of the electrode  471 , the overlapping area of the electrode  471  with the first wiring  409  and the second wiring  410  with the second gate insulating layer  412  therebetween can be reduced, so that parasitic capacitance can be reduced. Note that in  FIG.  1 B , the width of the electrode  471  is larger than that of the region whose thickness is smaller in the oxide semiconductor layer. 
     The light-emitting element includes at least a first electrode  472 , a light-emitting layer  475 , and a second electrode  474 . In  FIG.  1 B , the electrode  471  is formed of the same material as the first electrode  472  in the pixel portion, for example, using aluminum or the like, in order to reduce the number of steps; however, there is no particular limitation on the electrode  471 . Further, in  FIG.  1 B , an insulating layer  473  functions as a partition for insulating the first electrodes of the adjacent pixels from each other. 
     Further,  FIG.  1 C  illustrates an example different from  FIG.  1 A  in the material and the width of the second gate electrode. In  FIG.  1 C , as a material for an electrode  476  which functions as a second gate electrode of a thin film transistor  433 , a metal material (an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), or an alloy containing any of the above-described elements as its component) is used. A width of the second gate electrode in a cross section is smaller than that in  FIG.  1 B . When the width is still smaller than that in  FIG.  1 B , it is possible to form the second gate electrode so as not to overlap with the first wiring  409  and the second wiring  410  with the second gate insulating layer  412  therebetween, and thus the parasitic capacitance can further be reduced. The width of the electrode  476  illustrated in  FIG.  1 C  is smaller than that of the region whose thickness is smaller in the oxide semiconductor. In forming the electrode  476  having such a small width, a process using wet etching or the like is preferably performed so that both ends of the electrode  476  are positioned on an inner portion than end portions of a resist mask. However, in  FIG.  1 C , since a metal material different from that of the pixel electrode  110  is used, one more photolithography process is added to form the electrode  476 , and one more mask is needed. 
     By using the thin film transistor including the oxide semiconductor interposed between the two gate electrodes above and below the oxide semiconductor for a peripheral circuit such as a gate line driver circuit or a source line driver circuit, or a pixel portion, which is used for a liquid crystal display device, a light-emitting display device, or electronic paper, high speed driving or low power consumption can be achieved. Further, both the pixel portion and the driver circuit can be provided over the same substrate without greatly increasing the number of steps. By providing various circuits in addition to the pixel portion over the same substrate, manufacturing cost of a display device can be reduced. 
     Embodiment 2 
     Although one thin film transistor has been described as the thin film transistor in the driver circuit in Embodiment 1, an example of forming an inverter circuit of a driver circuit with use of two n-channel thin film transistors will be described below in Embodiment 2. A thin film transistor illustrated in  FIG.  2 A  is the same as the thin film transistor  430  illustrated in  FIG.  1 A  of Embodiment 1; therefore, the same parts are denoted by the same reference numerals. 
     The driver circuit for driving a pixel portion is formed using an inverter circuit, a capacitor, a resistor, and the like. When the inverter circuit is formed using two n-channel TFTs in combination, there are an inverter circuit having a combination of an enhancement type transistor and a depletion type transistor (hereinafter, referred to as an EDMOS circuit) and an inverter circuit having a combination of two enhancement type TFTs (hereinafter, referred to as an EEMOS circuit). 
     A cross-sectional structure of the inverter circuit of the driver circuit is illustrated in  FIG.  2 A . Note that the thin film transistor  430  and a second thin film transistor  431  illustrated in  FIG.  2 A to  2 C  are bottom-gate thin film transistors, and are examples of thin film transistors in which wirings are provided over a semiconductor layer with a source region and a drain region therebetween. 
     In  FIG.  2 A , the first gate electrode  401  and a gate electrode  402  are provided over the substrate  400 . The first gate electrode  401  and the gate electrode  402  can be formed to have a single-layer structure or a stacked structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium or an alloy material containing any of these as its main component. 
     As a two-layer stacked structure of the first gate electrode  401  and the gate electrode  402 , for example, a two-layer stacked structure in which a molybdenum layer is stacked over an aluminum layer, a two-layer structure in which a molybdenum layer is stacked over a copper layer, a two-layer structure in which a titanium nitride layer or a tantalum nitride layer is stacked over a copper layer, or a two-layer structure in which a titanium nitride layer and a molybdenum layer are stacked is preferable. As a three-layer stacked structure, stacked layers of a tungsten layer or a tungsten nitride layer, an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer is preferable. 
     In addition, over the first gate insulating layer  403  covering the first gate electrode  401  and the gate electrode  402 , an oxide semiconductor layer  405  and a second oxide semiconductor layer  407  are provided. 
     A first wiring  409  and a second wiring  410  are provided over the oxide semiconductor layer  405 , and the second wiring  410  is directly connected to the gate electrode  402  through a contact hole  404  formed in the first gate insulating layer  403 . Further, a third wiring  411  is provided over the second oxide semiconductor layer  407 . 
     The thin film transistor  430  includes the first gate electrode  401  and the oxide semiconductor layer  405  overlapping with the first gate electrode  401  with the first gate insulating layer  403  therebetween. The first wiring  409  is a power supply line to which negative voltage VDL is applied (a negative power supply line). This power supply line may be a power supply line with a ground potential (a ground potential power supply line). 
     Further, the second thin film transistor  431  includes the gate electrode  402  and the second oxide semiconductor layer  407  overlapping with the gate electrode  402  with the first gate insulating layer  403  interposed therebetween. The third wiring  411  is a power supply line to which positive voltage VDH is applied (a positive power supply line). 
     In addition, an n +  layer  408   a  is provided between the second oxide semiconductor layer  407  and the second wiring  410 , and an n +  layer  408   b  is provided between the second oxide semiconductor layer  407  and the third wiring  411 . 
     Further, a top view of the inverter circuit of the driver circuit is illustrated in  FIG.  2 C . In  FIG.  2 C , a cross-section taken along a chain line Z 1 -Z 2  corresponds to  FIG.  2 A . 
     Further, an equivalent circuit of the EDMOS circuit is illustrated in  FIG.  2 B . A circuit connection illustrated in  FIG.  2 A  corresponds to that in  FIG.  2 B , and is an example in which the thin film transistor  430  is an enhancement type n-channel transistor while the second thin film transistor  431  is a depletion type n-channel transistor. 
     In this embodiment, in order that the thin film transistor  430  can serve as an enhancement type n-channel transistor, the second gate insulating layer  412  is provided over the oxide semiconductor layer  405  and the second gate electrode  470  is provided over the second gate insulating layer  412  so that threshold value of the thin film transistor  430  is controlled by voltage applied to the second gate electrode  470 . 
     Further, the second gate insulating layer  412  also functions as a protective layer covering the second oxide semiconductor layer  407 . 
     Note that the example in which the second wiring  410  is directly connected to the gate electrode  402  through the contact hole  404  formed in the first gate insulating layer  403  is illustrated in  FIGS.  2 A and  2 C , but, without particular limitations, a connection electrode may be separately provided, thereby electrically connecting the second wiring  410  and the gate electrode  402 . 
     Further, this embodiment mode can be freely combined with Embodiment 1. 
     Embodiment 3 
     In Embodiment 3, a display device will be described with reference to block diagrams and the like. 
       FIG.  3 A  illustrates an example of a block diagram of an active matrix liquid crystal display device. The liquid crystal display device illustrated in  FIG.  3 A  includes, over a substrate  300 , a pixel portion  301  having a plurality of pixels each provided with a display element; a scan line driver circuit  302  which controls a scan line connected to a gate electrode of each pixel; and a signal line driver circuit  303  which controls video signal input to a selected pixel. 
       FIG.  3 B  illustrates an example of a block diagram of an active matrix light-emitting display device. The light-emitting display device illustrated in  FIG.  3 B  includes, over a substrate  310 , a pixel portion  311  having a plurality of pixels each provided with a display element; a first scan line driver circuit  312  and a second scan line driver circuit  313 , each of which controls a scan line connected to a gate electrode of a pixel; and a signal line driver circuit  314  which controls video signal input to a selected pixel. In a case where two TFTs (thin film transistor) of a switching TFT and a current controlling TFT are arranged in one pixel, in the light-emitting display device illustrated in  FIG.  3 B , a signal which is input to a first scan line connected to a gate electrode of the switching TFT is generated in the first scan line driver circuit  312 , and a signal which is input to a second scan line connected to a gate electrode of the current controlling TFT is generated in the second scan line driver circuit  313 . Note that a structure in which the signal input to the first scan line and the signal input to the second scan line are generated in one scan line driver circuit may also be employed. Alternatively, for example, a plurality of the first scan lines used for controlling operation of a switching element may be provided in each pixel, depending on the number of TFTs included in the switching element. In this case, all signals which are input to the plurality of the first scan lines may be generated in one scan line driver circuit, or may be generated separately by a plurality of scan line driver circuits. 
     Note that modes in which the scan line driver circuit  302 , the first scan line driver circuit  312 , the second scan line driver circuit  313 , and the signal line driver circuits  303  and  314  are formed in the display device are described here; however, part of the scan line driver circuit  302 , the first scan line driver circuit  312 , or the second scan line driver circuit  313  may be mounted with use of a semiconductor device such as an IC. Alternatively, part of the signal line driver circuits  303  or  314  may be mounted with a semiconductor device such as an IC. 
       FIG.  4    is a diagram illustrating a positional relation between a pixel portion and a protective circuit including a signal input terminal  321 , a scan line  323 , a signal line  324 , and a non-linear element, which constitute the display device. A pixel portion  327  includes scan lines  323  and signal lines  324  which are arranged over a substrate  320  having an insulating surface so as to intersect with each other. Note that the pixel portion  327  corresponds to the pixel portion  301  and the pixel portion  311  illustrated in  FIGS.  3 A and  3 B . 
     The pixel portion  301  is connected to the signal line driver circuit  303  by a plurality of signal lines S1 to Sm (not illustrated) which are arranged in columns and extended from the signal line driver circuit  303 , and connected to the scan line driver circuit  302  by a plurality of scan lines G1 to Gn (not illustrated) which are arranged in rows and extended from the scan line driver circuit  302 . The pixel portion  301  includes a plurality of pixels (not illustrated) arranged in a matrix form by the signal lines S1 to Sm and the scan lines G1 to Gn. Then, each pixel is connected to a signal line Sj (any one of the signal lines S1 to Sm) and a scan line Gi (any one of the scan lines G1 to Gn). 
     The pixel portion  327  includes a plurality of pixels  328  arranged in a matrix form. The pixel  328  includes a pixel TFT  329  connected to the scan line  323  and the signal line  324 , a storage capacitor  330 , and a pixel electrode  331 . 
     The pixel structure here illustrates a case where one electrode of the storage capacitor  330  is connected to the pixel TFT  329  and the other electrode thereof is connected to a capacitor line  332 . Further, the pixel electrode  331  serves as one electrode which drives a display element (a liquid crystal element, a light-emitting element, a contrast medium (electronic ink), or the like). The other electrode of such a display element is connected to a common terminal  333 . 
     Some protective circuits are provided between the pixel portion  327  and signal line input terminals  322 . In addition, other protective circuits are provided between the scan line driver circuit and the pixel portion  327 . In this embodiment, a plurality of protective circuits are provided so that the pixel TFT  329  and the like are not broken when surge voltage due to static electricity or the like is applied to the scan line  323 , the signal line  324 , and a capacitor bus line  337 . Therefore, the protective circuits are formed so that charge is released into a common wiring when the surge voltage is applied. 
     In this embodiment, an example in which a protective circuit  334 , a protective circuit  335 , and a protective circuit  336  are arranged on a scan line  323  side, on a signal line  324  side, and on a capacitor bus line  337  side, respectively is illustrated. Note that an arrangement position of the protective circuits is not limited thereto. In addition, in a case where the scan line driver circuit is not mounted with use of a semiconductor device such as an IC, the protective circuit  334  is not necessarily provided on the scan line  323  side. 
     By use of the TFT described in Embodiment 1 or Embodiment 2 for these circuits, the following advantages can be obtained. 
     The driver circuit is roughly divided into a logic circuit portion, and a switch portion or a buffer portion. A TFT provided in the logic circuit portion preferably has a structure in which control threshold voltage can be controlled. On the other hand, a TFT provided in the switch portion or the buffer portion preferably has large on current. By provision of a driver circuit including the TFTs described in Embodiment 1 or Embodiment 2, the threshold voltage of the TFT provided in the logic circuit portion can be controlled, and the on current of the TFT provided in the switch portion or the buffer portion can be increased. Furthermore, the TFTs described in Embodiment 1 or Embodiment 2 contribute to reducing an area occupied by the driver circuit and narrowing a frame. 
     A shift register circuit included in the scan line driver circuit is described below. A shift register circuit illustrated in  FIG.  5    includes a plurality of flip-flop circuits  351 , a control signal line  352 , a control signal line  353 , a control signal line  354 , a control signal line  355 , a control signal line  356 , and a reset line  357 . 
     As illustrated in the shift register circuit of  FIG.  5   , in the flip-flop circuits  351 , a start pulse SSP is input to an input terminal IN of the first stage through the control signal line  352 , and an output signal terminal S out  of the flip-flop circuit  351  of the preceding stage is connected to an input terminal IN of the next stage. Further, a reset terminal RES of the N-th stage (N is a natural number) is connected to an output signal terminal S out  of the flip-flop circuit of the (N+3)th stage through the reset line  357 . When it is assumed that a first clock signal CLK1 is input to a clock terminal CLK of the flip-flop circuit  351  of the N-th stage through the control signal line  353 , a second clock signal CLK2 is input to the clock terminal CLK of the flip-flop circuit  351  of the (N+1)th stage through the control signal line  354 . A third clock signal CLK3 is input to the clock terminal CLK of the flip-flop circuit  351  of the (N+2)th stage through the control signal line  355 . A fourth clock signal CLK4 is input to the clock terminal CLK of the flip-flop circuit  351  of the (N+3)th stage through the control signal line  356 . Then the first clock signal CLK1 is input to the clock terminal CLK of the flip-flop circuit  351  of the (N+4)th stage through the control signal line  353 . In addition, the flip-flop circuit  351  of the N-th stage outputs an output SR out N of the flip flop circuit of the N-th stage from a gate output terminal G out . 
     Note that connection between the flip-flop circuits  351 , and a power source and a power supply line is not illustrated; however, each flip-flop circuit  351  is supplied with a power supply potential Vdd and a power supply potential GND through the power supply line. 
     Note that the power supply potential described in this specification corresponds to a potential difference when a reference potential is 0 V. Therefore, the power supply potential is also referred to as power supply voltage, or the power supply voltage is referred to as the power supply potential in some cases. 
     Note that in this specification, description that “A and B are connected to each other” includes a case where A and B are electrically connected to each other in addition to a case where A and B are directly connected to each other. Here, the description that “A and B are electrically connected to each other” includes the following cases: when an object having any electrical function exists between A and B, A and B have substantially the same potential via the object. Specifically, the description that “A and B are electrically connected to each other” includes cases where A and B are considered to have substantially the same potential in light of circuit operation, e.g., a case where A and B are connected through a switching element such as a TFT and electricity transmission through the switching element causes A and B to have substantially the same potential, a case where A and B are connected via a resistor and a potential difference between potentials generated at both ends of the resistor does not affect operation of a circuit including A and B, and the like. 
     Next,  FIG.  6    illustrates one mode of the flip-flop circuit  351  included in the shift register circuit illustrated in  FIG.  5   . The flip-flop circuit  351  illustrated in  FIG.  6    includes a logic circuit portion  361  and a switch portion  362 . The logic circuit portion  361  includes TFTs  363  to  368 . Further, the switch portion  362  includes TFTs  369  to  372 . Note that the logic circuit portion is a circuit for switching a signal that is output to a switch portion, which is a circuit in the next stage, in response to a signal that is input from an external portion. In addition, the switch portion is a circuit for switching on/off of a TFT which functions as a switch in response to a signal input from an external portion and a control circuit portion, and for outputting current depending on the size and the structure of the TFT. 
     In the flip-flop circuit  351 , an input terminal IN is connected to a gate terminal of the TFT  364  and a gate terminal of the TFT  367 . A reset terminal RES is connected to a gate terminal of the TFT  363 . A clock terminal CLK is connected to a first terminal of the TFT  369  and a first terminal of the TFT  371 . A power supply line through which the power supply potential Vdd is supplied is connected to a first terminal of the TFT  364 , and a gate terminal and a second terminal of the TFT  366 . A power supply line through which the power supply potential GND is supplied is connected to a second terminal of the TFT  363 , a second terminal of the TFT  365 , a second terminal of the TFT  367 , a second terminal of the TFT  368 , a second terminal of the TFT  370 , and a second terminal of the TFT  372 . Further, a first terminal of the TFT  363 , a second terminal of the TFT  364 , a first terminal of the TFT  365 , a gate terminal of the TFT  368 , a gate terminal of the TFT  369 , and a gate terminal of the TFT  371  are connected to each other. A first terminal of the TFT  366  is connected to a gate terminal of the TFT  365 , a first terminal of the TFT  367 , a first terminal of the TFT  368 , a gate terminal of the TFT  370 , and a gate terminal of the TFT  372 . In addition, a gate output terminal G out  is connected to a second terminal of the TFT  369  and a first terminal of the TFT  370 . An output signal terminal S out  is connected to a second terminal of the TFT  371  and a first terminal of the TFT  372 . 
     Note that a case where the TFTs  363  to  372  are all n-channel TFTs is described here. 
     Note that a TFT is an element having at least three terminals of a gate, a drain, and a source, and has a channel formation region between a drain region and a source region. Current can flow through the drain region, the channel formation region, and the source region. Here, the source and the drain may be exchanged with each other in some cases depending on a structure, operation conditions of the TFT, or the like; therefore, it is difficult to determine which the source is or which the drain is. Therefore, regions functioning as the source and the drain are not referred to as a source and a drain but referred to, for example, as a first terminal and a second terminal, respectively, in some cases. In such a case, a terminal functioning as a gate is referred to as a gate terminal. 
     Next,  FIG.  7    illustrates an example of a layout view of the flip-flop circuit  351  illustrated in  FIG.  6   . 
     The flip-flop circuit of  FIG.  7    includes a power supply line  381  through which the power supply potential Vdd is supplied, a reset line  382 , the control signal line  353 , the control signal line  354 , the control signal line  355 , the control signal line  356 , a control signal line  383 , a power supply line  384  through which the power supply potential GND is supplied, the logic circuit portion  361 , and the switch portion  362 . The logic circuit portion  361  includes the TFTs  363  to  368 . The switch portion  362  includes the TFTs  369  to  372 . In  FIG.  7   , a wiring connected to the gate output terminal G out  and a wiring connected to the output signal terminal S out  are also illustrated. 
       FIG.  7    illustrates a semiconductor layer  385 , a first wiring layer  386 , a second wiring layer  387 , a third wiring layer  388 , and a contact hole  389 . Note that the first wiring layer  386  may be formed with a layer of a gate electrode, the second wiring layer  387  may be formed with a layer of source and drain electrodes of a TFT, and the third wiring layer  388  may be formed with a layer of a pixel electrode in the pixel portion. However, without being limited to this example, the third wiring layer  388  may be formed as a layer different from the layer of the pixel electrode for example. 
     Note that connections between circuit elements in  FIG.  7    are as illustrated in  FIG.  6   . Note that  FIG.  7    illustrates the flip-flop circuit to which the first clock signal is input; therefore, connections to the control signal lines  354  to  356  are not illustrated. 
     In the layout view of the flip-flop circuit of  FIG.  7   , by controlling threshold voltage of the TFT  366  or the TFT  367  included in the logic circuit portion  361 , an EDMOS circuit  373  can be formed. Typically, the EDMOS circuit  373  in which the TFT  366  is a depletion type and the TFT  367  is an enhancement type is formed, and the TFTs  369  to  372  included in the switch portion  362  are dual-gate TFTs or depletion type TFTs. Note that in  FIG.  6   , the TFT  366  and the TFT  367  in the EDMOS circuit  373  are different from the TFTs in the EDMOS circuit illustrated in  FIGS.  2 A to  2 C  in a connection position of the gate electrode of the depletion type TFT. 
     The TFT  366  or the TFT  367  is formed so as to be a dual-gate TFT and a potential of a back-gate electrode is controlled, so that a depletion type TFT or an enhancement type TFT can be formed. 
     In  FIG.  7   , a control signal line  390  which has the same potential as a back-gate electrode for controlling the threshold voltage of the TFT  366  is separately provided to form a depletion type. The TFT  366  is a dual-gate TFT, and a potential of the back-gate electrode is different from a potential of the power supply line  381  through which the power supply potential Vdd that is applied to the gate electrode is supplied. 
       FIG.  7    illustrates an example in which the TFTs  369  to  372  are dual-gate TFTs and the back-gate electrodes and the gate electrodes have the same potentials, and a potential of each of the back-gate electrodes is the same potential as that of the power supply line through which the power supply potential Vdd that is applied to the gate electrode is supplied. 
     In this manner, TFTs arranged in the pixel portion and the driver circuit of the display device can be formed using only n-channel TFTs in which an oxide semiconductor layer is used. 
     Further, the TFT  366  in the logic circuit portion  361  is a TFT for supplying current in response to the power supply potential Vdd. The TFT  366  is formed to be a dual-gate TFT or a depletion type TFT to increase the flowing current, whereby miniaturization of the TFT can be achieved without reducing performance. 
     Further, in the TFTs included in the switch portion  362 , the amount of current flowing in the TFTs can be increased and switching of on/off can be performed at high speed; therefore, an area occupied by the TFTs can be reduced without reducing performance. Accordingly, an area occupied by the circuit including the TFTs can also be reduced. Note that the TFTs  369  to  372  in the switch portion  362  may be formed to be dual-gate TFTs such that the semiconductor layer  385  is interposed between the first wiring layer  386  and the third wiring layer  388  as illustrated in the drawing. 
     The example that the dual-gate TFTs each have a structure in which the semiconductor layer  385  is interposed between the first wiring layer  386  and the third wiring layer  388  which have the same potential by being connected to each other through the contact hole  389  is illustrated in  FIG.  7   . However, there is no particular limitation; for example, a structure in which a control signal line is separately provided for the third wiring layer  388  to control a potential of the third wiring layer  388  independently from the first wiring layer  386  may be employed. 
     Note that in the layout view of the flip-flop circuit illustrated in  FIG.  7   , the shapes of the channel formation regions of the TFTs  363  to  372  may be U shapes (reversed C shapes or horseshoe shapes). In addition, although all the TFTs have the same size in  FIG.  7   , the size of each TFT which is connected to the output signal terminal S out  or the gate output terminal G out  may be changed as appropriate in accordance with the amount of a load of a subsequent stage. 
     Next, operation of the shift register circuit illustrated in  FIG.  5    is described with reference to a timing chart illustrated in  FIG.  8   .  FIG.  8    illustrates the start pulse SSP and the first to fourth clock signals CLK1 to CLK4, which are supplied to the control signal lines  352  to  356  illustrated in  FIG.  5   , respectively, and the Sout 1 to Sout 5 output from the output signal terminals S out  of the flip-flop circuits of the first to fifth stages. Note that in description of  FIG.  8   , the reference numerals denoting the respective elements in  FIG.  6    and  FIG.  7    are used. 
     Note that  FIG.  8    is a timing chart in the case where each TFT included in the flip-flop circuits is an n-channel TFT. Further, the first clock signal CLK1 is shifted from the fourth clock signal CLK4 by ¼ wavelength (a section divided by dotted lines) as illustrated. 
     First, in a period T1, the start pulse SSP is input to the flip-flop circuit of the first stage at an H level, and the logic circuit portion  361  turns the TFTs  369  and  371  on and the TFTs  370  and  372  off in the switch portion. At this time, since the first clock signal CLK1 is at an L level, the Sout 1 is at an L level. 
     Note that in the period T1, signals are not input to the IN terminals of the flip-flop circuits of the second and subsequent stages, so that the flip-flop circuits output L levels without operation. Note that description is made assuming that each flip-flop circuit of the shift register circuit outputs an L level in an initial state. 
     Next, in a period T2, the logic circuit portion  361  controls the switch portion  362  in the flip-flop circuit of the first stage in a manner similar to the period T1. In the period T2, the first clock signal CLK1 is at an H level, and thus the Sout 1 is at an H level. Further, in the period T2, the Sout 1 is input to the IN terminal of the flip-flop circuit of the second stage at an H level, and the logic circuit portion  361  turns the TFTs  369  and  371  on and the TFTs  370  and  372  off in the switch portion. At this time, since the second clock signal CLK2 is at an L level, the Sout 2 is at an L level. 
     Note that in the period T2, signals are not input to the IN terminals of the flip-flop circuits of the third and subsequent stages, so that the flip-flop circuits output L levels without operation. 
     Next, in a period T3, the logic circuit portion  361  controls the switch portion  362  so that a state of the period T2 is held in the flip-flop circuit of the first stage. Therefore, in the period T3, the first clock signal CLK1 is at an H level and the Sout 1 is at an H level. Further, in the period T3, the logic circuit portion  361  controls the switch portion  362  in the flip-flop circuit of the second stage in a manner similar to the period T2. In the period T3, since the second clock signal CLK2 is at an H level, the Sout 2 is at an H level. In addition, the Sout 2 is input to the IN terminal of the flip-flop circuit of the third stage at an H level in the period T3, and the logic circuit portion  361  turns the TFTs  369  and  371  on and the TFTs  370  and  372  off in the switch portion. At this time, the third clock signal CLK3 is at an L level, and thus the Sout 3 is at an L level. 
     Note that in the period T3, signals are not input to the IN terminals of the flip-flop circuits of the fourth and subsequent stages, so that the flip-flop circuits output L levels without operation. 
     Next, in the period T4, the logic circuit portion  361  controls the switch portion  362  so that a state of the period T3 is held in the flip-flop circuit of the first stage. Therefore, in the period T4, the first clock signal CLK1 is at an L level and the Sout 1 is at an L level. Further, in the period T4, the logic circuit portion  361  controls the switch portion  362  so that a state of the period T3 is held in the flip-flop circuit of the second stage. Therefore, in the period T4, the second clock signal CLK2 is at an H level and Sout 2 is at an H level. In addition, in the period T4, the logic circuit portion  361  controls the switch portion  362  in the flip-flop circuit of the third stage in a manner similar to the period T3. In the period T4, since the third clock signal CLK3 is at an H level, the Sout 3 is at an H level. The Sout 3 is input to the IN terminal of the flip-flop circuit of the fourth stage at an H level in the period T4, and the logic circuit portion  361  turns the TFTs  369  and  371  on and the TFTs  370  and  372  off in the switch portion  362 . At this time, since the fourth clock signal CLK4 is at an L level, the Sout 4 is at an L level. 
     Note that in the period T4, signals are not input to the IN terminals of the flip-flop circuits of the fifth and subsequent stages, so that the flip-flop circuits output L levels without operation. 
     Next, in a period T5, the logic circuit portion  361  controls the switch portion  362  so that a state of the period T3 is held in the flip-flop circuit of the second stage. Therefore, in the period T5, the second clock signal CLK2 is at an L level and the Sout 2 is at an L level. Further, in the period T5, the logic circuit portion  361  controls the switch portion  362  so that a state of the period T4 is held in the flip-flop circuit of the third stage. Therefore, in the period T5, the third clock signal CLK3 is at an H level and the Sout 3 is at an H level. In addition, in the period T5, the logic circuit portion  361  controls the switch portion  362  in the flip-flop circuit of the fourth stage in a manner similar to the period T4. In the period T5, since the fourth clock signal CLK4 is at an H level, the Sout 4 is at an H level. The flip-flop circuits of the fifth and subsequent stages have a wiring connection and a timing of signals to be input similar to those of the flip-flop circuits of the first to fourth stages; therefore, description thereof is omitted. 
     As illustrated in the shift register circuit of  FIG.  5   , the Sout 4 also functions as a reset signal of the flip-flop circuit of the first stage. In the period T5, the Sout 4 is at an H level and this signal is input to the reset terminal RES of the flip-flop circuit of the first stage. When the reset signal is input, the TFTs  369  and  371  are turned off and the TFTs  370  and  372  are turned on in the switch portion  362 . Then, the Sout 1 of the flip-flop circuit of the first stage outputs an L level until input of the next start pulse SSP. 
     By the above-described operation, in the flip-flop circuits of the second and subsequent stages, the logic circuit portions are also reset based on the reset signals which are output from the flip-flop circuits of subsequent stages. As shown by the Souts 1 to 5, a shift register circuit in which signals having waveforms shifted by ¼ wavelength of the clock signals are output can be formed. 
     When the flip-flop circuit has a structure in which an EDMOS circuit that is a combination of an enhancement type TFT and a depletion type TFT is provided in the logic circuit portion and a dual-gate TFT is provided in the switch portion, the amount of current flowing in the TFTs included in the logic circuit portion  361  can be increased and an area occupied by the TFTs and furthermore, an area occupied by the circuit including the TFTs can be reduced without reduction in performance. Further, in the TFT included in the switch portion  362 , the amount of current flowing in the TFTs can be increased and switching of on/off can be performed at high speed; therefore, an area occupied by the TFTs and furthermore, an area occupied by the circuit including the TFTs can be reduced without reduction in performance. Accordingly, a narrower frame, downsizing, high performance of a display device can be achieved. 
     Further, a latch circuit, a level shifter circuit, or the like can be provided in the signal line driver circuit illustrated in  FIGS.  3 A and  3 B . A buffer portion is provided in the last stage through which a signal is transmitted from the signal line driver circuit to the pixel portion, and an amplified signal is transmitted from the signal line driver circuit to the pixel portion. Thus, when a TFT having large on current, typically a dual-gate TFT or a depletion type TFT is provided in the buffer portion, an area of the TFT can be reduced and an area occupied by the signal line driver circuit can be reduced. Accordingly, a narrow frame, downsizing, and high performance of a display device can be achieved. Note that since high-speed operation is required for the shift register which is part of the signal line driver circuit, the shift register is preferably mounted on a display device by use of an IC or the like. 
     In addition, this embodiment can be freely combined with Embodiment 1 or Embodiment 2. 
     Embodiment 4 
     In Embodiment 4, a method for manufacturing a display device including the second thin film transistor  170  described in Embodiment 1 will be described with reference to  FIGS.  9 A to  9 C ,  FIGS.  10 A to  10 C ,  FIG.  11   ,  FIG.  12   ,  FIG.  13   ,  FIG.  14   ,  FIGS.  15 A,  15 B,  15 C, and  15 D , and  FIG.  16   . 
     In  FIG.  9 A , a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like can be used as a substrate  100  having a light-transmitting property. 
     Next, after a conductive layer is formed over an entire surface of the substrate  100 , a resist mask is formed by a first photolithography step. Then, unnecessary portions are removed by etching, thereby forming wirings and electrodes (a gate wiring including the gate electrode  101 , a capacitor wiring  108 , and a first terminal  121 ). At this time, etching is performed so that at least an end portion of the gate electrode  101  is tapered. A cross-sectional view at this stage is illustrated in  FIG.  9 A . Note that  FIG.  11    is a top view at this stage. 
     The gate wiring including the gate electrode  101 , the capacitor wiring  108 , and the first terminal  121  in the terminal portion are desirably formed of a low-resistant conductive material such as aluminum (Al) or copper (Cu). However, aluminum itself has the disadvantages of low heat resistance, being easily corroded, and the like; thus, it is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance, it is possible to use an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements as its component, an alloy film containing a combination of any of these elements, or a nitride containing any of these elements as its component. 
     Then, the gate insulating layer  102  is entirely formed over the gate electrode  101 . The gate insulating layer  102  is formed to a thickness of 50 nm to 400 nm by a sputtering method or the like. When yield of the thin film transistor is prioritized, the thickness of the gate insulating layer  102  is preferably large. 
     For example, as the gate insulating layer  102 , a silicon oxide film is formed to a thickness of 100 nm by a sputtering method. It is needless to say that the gate insulating layer  102  is not limited to such a silicon oxide film, and another insulating film such as a silicon oxynitride film, a silicon nitride film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, or a tantalum oxide film may be used to form a single-layer structure or a stacked structure. When a silicon oxynitride film, a silicon nitride film, or the like is used as the gate insulating layer  102 , an impurity from the glass substrate, sodium for example, can be preventing from diffusing into and entering an oxide semiconductor to be formed later. 
     Note that dust adhering to the surface of the gate insulating layer is preferably removed by reverse sputtering where plasma is generated by introduction of an argon gas, before the oxide semiconductor film is formed. In addition, nitrogen, helium or the like may be used instead of an argon atmosphere. Alternatively, the reserve sputtering may be conducted in an argon atmosphere to which oxygen, hydrogen, N 2 O and/or the like are/is added. Still alternatively, it may be conducted in an argon atmosphere to which Cl 2 , CF 4 , and/or the like are/is added. 
     Next, the first oxide semiconductor film (first In—Ga—Zn—O-based non-single-crystal film in this embodiment) is formed over the gate insulating layer  102 . The first In—Ga—Zn—O-based non-single-crystal film formed without being exposed to air after the plasma treatment, can avoid the trouble that dust or moisture adheres to the interface between the gate insulating layer and the semiconductor film. Here, the In—Ga—Zn—O-based non-single-crystal film is formed in an argon atmosphere or an oxygen atmosphere under the condition where the target is an oxide semiconductor target including In (indium), Ga (gallium), and Zn (zinc) (In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1) with a diameter of 8 inches, the distance between the substrate and the target is set at 170 mm, the pressure is set at 0.4 Pa, and the direct current (DC) power supply is set at 0.5 kW. Note that a pulse direct current (DC) power supply is preferable because dust can be reduced and the film thickness can be uniform. The thickness of the first In—Ga—Zn—O-based non-single-crystal film is set to from 5 nm to 200 nm. The thickness of the first In—Ga—Zn—O-based non-single-crystal film in this embodiment is 100 nm. 
     Next, a second oxide semiconductor film (second In—Ga—Zn—O-based non-single-crystal film in this embodiment) is formed by a sputtering method without being exposed to air. Here, sputtering deposition is performed under the condition where a target includes indium oxide (In 2 O 3 ), gallium oxide (Ga 2 O 3 ), and zinc oxide (ZnO) at a ratio of 1:1:1 (═In 2 O 3 :Ga 2 O 3 :ZnO), the pressure in a deposition chamber is set at 0.4 Pa, the electric power is set at 500 W, the deposition temperature is set to room temperature, and the argon gas flow rate is set at 40 sccm. Although the target of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 is used intentionally, an In—Ga—Zn—O-based non-single-crystal film including a crystal grain which has a size of 1 nm to 10 nm just after the film formation is obtained in some cases. It can be said that control of the presence or absence of crystal grains and the density of crystal grains and adjustment of the diameter of the crystal grain within 1 nm to 10 nm can be done by adjusting as appropriate, the deposition condition of reactive sputtering, such as the target composition ratio, the deposition pressure (0.1 Pa to 2.0 Pa), the electric power (250 W to 3000 W: 8 inchesφ) the temperature (room temperature to 100° C.), or the like. The thickness of the second In—Ga—Zn—O-based non-single-crystal film is set to 5 nm to 20 nm. Needless to say, the size of a grain included in the film does not exceed the film thickness. In this embodiment, the second In—Ga—Zn—O-based non-single-crystal film has a thickness of 5 nm. 
     The first In—Ga—Zn—O-based non-single-crystal film is formed under the different conditions from the second In—Ga—Zn—O-based non-single-crystal film. For example, as compared with the oxygen gas flow rate and the argon gas flow rate in the deposition condition of the second In—Ga—Zn—O-based non-single-crystal film, the oxygen gas flow rate in the deposition condition of the first In—Ga—Zn—O-based non-single-crystal film is increased. Specifically, the second In—Ga—Zn—O-based non-single-crystal film is formed in a rare gas (such as argon or helium) atmosphere (or a gas including oxygen at 10% or less and argon at 90% or more), while the first In—Ga—Zn—O-based non-single-crystal film is formed in an oxygen atmosphere (or the oxygen gas flow rate is equal to or larger than the argon gas flow rate). 
     The second In—Ga—Zn—O-based non-single-crystal film may be formed in the same chamber as the chamber where the reverse sputtering is performed previously, or may be formed in a different chamber from the chamber where the reverse sputtering is performed previously. 
     Examples of sputtering include an RF sputtering in which a high-frequency power source is used for a sputtering power source, a DC sputtering, and a pulsed DC sputtering in which a bias is applied in a pulsed manner. 
     In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be deposited to be stacked in the same chamber, or a plurality of kinds of materials can be deposited by electric discharge at the same time in the same chamber. 
     In addition, there are a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering, and a sputtering apparatus used for an ECR sputtering in which plasma generated with use of microwaves is used without using glow discharge. 
     Furthermore, as a deposition method by sputtering, there are also a reactive sputtering in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, and a bias sputtering in which voltage is also applied to a substrate during deposition. 
     Next, a second photolithography step is performed to form a resist mask, and the first In—Ga—Zn—O-based non-single-crystal film and the second In—Ga—Zn—O-based non-single-crystal film are etched. Here, unnecessary portions are removed by wet etching using ITO07N (manufactured by KANTO CHEMICAL CO., INC.), thereby forming the oxide semiconductor film  109  that is the first In—Ga—Zn—O-based non-single-crystal film and an oxide semiconductor film  111  that is the second In—Ga—Zn—O-based non-single-crystal film. Note that this etching step may be dry etching without being limited to wet etching. A cross-sectional view at this stage is illustrated in  FIG.  9 B . Note that  FIG.  12    is a top view at this stage. 
     Next, a third photolithography step is conducted to form a resist mask, and unnecessary portions are removed by etching to form a contact hole which reaches an electrode layer or a wiring made of the same materials as the gate electrode layer. The contact hole is provided for direct connection with a conductive film to be formed later. For example, in the driving circuit portion, a contact hole is formed when a thin film transistor whose gate electrode layer is direct contact with the source or drain electrode layer or a terminal that is electrically connected to a gate wiring of a terminal portion is formed. In this embodiment, the example of forming the contact hole for direct connection with the conductive film to be formed later by the third photolithography step is described, but there is no particular limitation and the contact hole which reaches the gate electrode layer may be formed later at the same process as a contact hole for connection with the pixel electrode, and electrical connection may be performed with use of the same material as the pixel electrode. In the case where electrical connection is performed with use of the same material as the pixel electrode, one mask can be reduced. 
     Then, a conductive film  132  formed from a metal material is formed over the oxide semiconductor film  109  and the oxide semiconductor film  111  by a sputtering method or a vacuum evaporation method. A cross-sectional view at this stage is illustrated in  FIG.  9 C . 
     As the material of the conductive film  132 , there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, and the like. If heat treatment at 200° C. to 600° C. is performed, the conductive film preferably has heat resistance enough to withstand the heat treatment. Since aluminum itself has the disadvantages of low heat resistance, being easily corroded, and the like, it is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance, which is combined with aluminum, it is possible to use an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, or a nitride containing any of these elements as its component. 
     Here, the conductive film  132  has a single-layer structure of a titanium film. The conductive film  132  may also have a two-layer structure in which a titanium film is stacked on an aluminum film. Alternatively, the conductive film  132  may have a three-layer structure in which a titanium (Ti) film, an aluminum film containing neodymium (Nd) (an Al—Nd film), and a titanium (Ti) film are stacked in order. Further alternatively, the conductive film  132  may have a single-layer structure of an aluminum film containing silicon. 
     Next, a fourth photolithography step is performed to form a resist mask  131 , and unnecessary portions are removed by etching to form the source and drain electrode layers  105   a  and  105   b , the n +  layers  104   a  and  104   b  serving as the source and drain regions, and a connection electrode  120 . This etching step is performed by wet etching or dry etching. For example, when an aluminum film or an aluminum alloy film is used as the conductive film  132 , wet etching can be performed using a solution in which phosphoric acid, acetic acid, and nitric acid are mixed. Here, with use of an ammonia hydrogen peroxide mixture (hydrogen peroxide: ammonia: water=5:2:2), the conductive film  132  made of titanium (Ti) is wet-etched to form the source and drain electrode layers  105   a  and  105   b , and the oxide semiconductor film  111  is wet-etched to form the n +  layers  104   a  and  104   b . In this etching step, an exposed region of the oxide semiconductor film  109  is partially etched to be an oxide semiconductor layer  103 . In this manner, a channel region of the oxide semiconductor layer  103  between the n +  layers  104   a  and  104   b  is a region whose thickness is smaller. In  FIG.  10 A , formation of the source and drain electrode layers  105   a  and  105   b  and the n +  layers  104   a  and  104   b  by etching are simultaneously conducted using an ammonia hydrogen peroxide mixture; therefore, the end portions of the source and drain electrode layers  105   a  and  105   b  are aligned with the end portions of the n +  layers  104   a  and  104   b , so that the end portions are continuous. In addition, wet etching allows the layers to be etched isotropically, so that the end portions of the source and drain electrode layers  105   a  and  105   b  are recessed from the resist mask  131 . Through the above steps, the second thin film transistor  170  including the oxide semiconductor layer  103  as its channel formation region can be formed. A cross-sectional view at this stage is illustrated in  FIG.  10 A . Note that  FIG.  13    is a top view at this stage. 
     Then, heat treatment is preferably performed at 200° C. to 600° C., and typically, 300° C. to 500° C. Here, heat treatment at 350° C. for one hour is performed in a furnace in a nitrogen atmosphere. This heat treatment involves the rearrangement at the atomic level in the In—Ga—Zn—O-based non-single-crystal film. The heat treatment (including light annealing) in this step is important because the strain that inhibits the movement of carriers can be released. Note that there is no particular limitation on the timing of the heat treatment, and the heat treatment may be performed at any time after the deposition of the second In—Ga—Zn—O-based non-single-crystal film, for example, after the formation of a pixel electrode. 
     Furthermore, the exposed channel formation region of the oxide semiconductor layer  103  may be subjected to oxygen radical treatment, so that a normally-off thin film transistor can be obtained. In addition, the radical treatment can repair damage due to the etching of the oxide semiconductor layer  103 . The radical treatment is preferably performed in an atmosphere of O 2  or N 2 O, and preferably an atmosphere of N 2 , He, or Ar each containing oxygen. The radical treatment may also be performed in an atmosphere in which Cl 2  and/or CF 4  is added to the above atmosphere. Note that the radical treatment is preferably performed with no bias applied. 
     In the fourth photolithography step, a second terminal  122  that is made of the same material as the source and drain electrode layers  105   a  and  105   b  is left in the terminal portion. Note that the second terminal  122  is electrically connected to a source wiring (a source wiring including the source and drain electrode layers  105   a  and  105   b ). 
     In addition, in the terminal portion, the connection electrode  120  is directly connected to the first terminal  121  of the terminal portion through a contact hole formed in the gate insulating film. Note that although not illustrated here, a source or drain wiring of the thin film transistor of the driver circuit is directly connected to the gate electrode through the same steps as the above-described steps. 
     Further, by use of a resist mask having regions with plural thicknesses (typically, two different thicknesses) which is formed using a multi-tone mask, the number of resist masks can be reduced, resulting in simplified process and lower costs. 
     Next, the resist mask  131  is removed, and a protective insulating layer  107  is formed to cover the second thin film transistor  170 . For the protective insulating layer  107 , a single layer or a stacked layer of a silicon nitride film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, a tantalum oxide film, and/or the like, which are/is obtained by a sputtering method or the like can be used. In the thin film transistor in part of the driver circuit, the protective insulating layer  107  functions as a second gate insulating layer and a second gate electrode is formed thereover. The protective insulating layer  107  has a thickness of 50 nm to 400 nm. When yield of the thin film transistor is prioritized, the thickness of the protective insulating layer  107  is preferably large. Further, when a silicon oxynitride film, a silicon nitride film, or the like is used as the protective insulating layer  107 , impurities attached for some reason after the formation of the protective insulating layer  107 , e.g., sodium, can be prevented from diffusing into and entering the oxide semiconductor. 
     Then, a fifth photolithography step is performed to form a resist mask, and the protective insulating layer  107  is etched to form a contact hole  125  reaching the drain electrode layer  105   b . In addition, a contact hole  126  reaching the connection electrode  120  and a contact hole  127  reaching the second terminal  122  are also formed by this etching. A cross-sectional view at this stage is illustrated in  FIG.  10 B . 
     Next, the resist mask is removed, and then a transparent conductive film is formed. The transparent conductive film is formed of indium oxide In 2 O 3 ), indium oxide-tin oxide alloy (In 2 O 3 —SnO 2 , abbreviated to ITO), or the like by a sputtering method, a vacuum evaporation method, or the like. Such a material is etched with a hydrochloric acid-based solution. However, since a residue is easily generated particularly in etching ITO, indium oxide-zinc oxide alloy (In 2 O 3 —ZnO) may be used to improve etching processability. 
     Next, a sixth photolithography step is performed to form a resist mask, and unnecessary portions are removed by etching, thereby forming the pixel electrode  110  in the pixel portion. In the sixth photolithography step, in the driver circuit, the same material as that of the pixel electrode  110  is used for part of the circuit to form an electrode layer (a back-gate electrode) for controlling the threshold value over the oxide semiconductor layer. Note that the thin film transistor having the back-gate electrode is described in Embodiment 1 with reference to  FIG.  1 A ; therefore, detailed description thereof is omitted here. 
     In the sixth photolithography step, a storage capacitor is formed from the capacitor wiring  108  and the pixel electrode  110  by using the gate insulating layer  102  and the protective insulating layer  107  in the capacitor portion as dielectrics. Note that an example in which the storage capacitor is formed from the capacitor wiring  108  and the pixel electrode  110  by using the gate insulating layer  102  and the protective insulating layer  107  as the dielectrics is described here. However, there is no particular limitation and a structure may also be employed in which an electrode formed of the same material as the source electrode or the drain electrode is provided above the capacitor wiring and a storage capacitor is formed from the electrode and the capacitor wiring by using the gate insulating layer  102  therebetween as a dielectric, thereby electrically connecting the electrode and the pixel electrode. 
     Furthermore, in the sixth photolithography step, the first terminal and the second terminal are covered with the resist mask so that transparent conductive films  128  and  129  are left in the terminal portion. The transparent conductive films  128  and  129  function as electrodes or wirings connected to an FPC. The transparent conductive film  128  formed over the connection electrode  120  which is directly connected to the first terminal  121  is a connection terminal electrode which functions as an input terminal of the gate wiring. The transparent conductive film  129  formed over the second terminal  122  is a connection terminal electrode which functions as an input terminal of the source wiring. 
     Then, the resist mask is removed. A cross-sectional view at this stage is illustrated in  FIG.  10 C . Note that  FIG.  14    is a top view at this stage. 
       FIGS.  15 A and  15 B  respectively illustrate a cross-sectional view and a top view of a gate wiring terminal portion at this stage.  FIG.  15 A  is a cross-sectional view taken along line C 1 -C 2  of  FIG.  15 B . In  FIG.  15 A , a transparent conductive film  155  formed over a protective insulating film  154  is a connection terminal electrode functioning as an input terminal. Further, in the terminal portion of  FIG.  15 A , a first terminal  151  made of the same material as the gate wiring and a connection electrode  153  made of the same material as the source wiring overlap with each other with a gate insulating layer  152  therebetween, and are in direct contact with each other so as to be electrically connected. In addition, the connection electrode  153  and the transparent conductive film  155  are in direct contact with each other through a contact hole provided in the protective insulating film  154  so as to be electrically connected. 
       FIGS.  15 C and  15 D  respectively illustrate a cross-sectional view and a top view of a source wiring terminal portion.  FIG.  15 C  is a cross-sectional view taken along line D 1 -D 2  of  FIG.  15 D . In  FIG.  15 C , the transparent conductive film  155  formed over the protective insulating film  154  is a connection terminal electrode functioning as an input terminal. Further, in the terminal portion of  FIG.  15 C , an electrode  156  made of the same material as the gate wiring is formed below a second terminal  150  which is electrically connected to the source wiring and overlaps with the second terminal  150  with the gate insulating layer  152  interposed therebetween. The electrode  156  is not electrically connected to the second terminal  150 , and a capacitor to prevent noise or static electricity can be formed if the potential of the electrode  156  is set to a potential different from that of the second terminal  150 , such as floating, GND, or 0 V. The second terminal  150  is electrically connected to the transparent conductive film  155  with the protective insulating film  154  therebetween. 
     A plurality of gate wirings, source wirings, and capacitor wirings are provided depending on the pixel density. Also in the terminal portion, the first terminal at the same potential as the gate wiring, the second terminal at the same potential as the source wiring, the third terminal at the same potential as the capacitor wiring, and the like are each arranged in plurality. The number of each of the terminals may be any number, and the number of the terminals may be determined by a practitioner as appropriate. 
     Through these six photolithography steps, the second thin film transistor  170  which is a bottom-gate n-channel thin film transistor and the storage capacitor can be completed using the six photomasks. By disposing the thin film transistor and the storage capacitor in each pixel of a pixel portion in which pixels are arranged in a matrix form, one of substrates for manufacturing an active matrix display device can be obtained. In this specification, such a substrate is referred to as an active matrix substrate for convenience. 
     When electrical connection to the gate wiring by using the same material as the pixel electrode is conducted, the third photolithography step can be omitted. Therefore, through the five photolithography steps, the second thin film transistor which is a bottom-gate n-channel thin film transistor and the storage capacitor can be completed using the five photomasks. 
     Further, when a material of the second gate electrode is different from a material of the pixel electrode as illustrated in  FIG.  1 C , one photolithography step is added, so that one photomask is added. 
     In the case of manufacturing an active matrix liquid crystal display device, an active matrix substrate and a counter substrate provided with a counter electrode are bonded to each other with a liquid crystal layer interposed therebetween. Note that a common electrode electrically connected to the counter electrode on the counter substrate is provided over the active matrix substrate, and a fourth terminal electrically connected to the common electrode is provided in the terminal portion. The fourth terminal is provided so that the common electrode is set to a fixed potential such as GND or 0 V. 
     Further, the pixel structure is not limited to that of  FIG.  14   , and an example of a top view which is different from  FIG.  14    is illustrated in  FIG.  16   .  FIG.  16    illustrates an example in which a capacitor wiring is not provided but a pixel electrode overlaps with a gate wiring of an adjacent pixel, with a protective insulating film and a gate insulating layer therebetween to form a storage capacitor. In that case, the capacitor wiring and the third terminal connected to the capacitor wiring can be omitted. Note that in  FIG.  16   , the same parts as those in  FIG.  14    are denoted by the same reference numerals. 
     In an active matrix liquid crystal display device, pixel electrodes arranged in a matrix form are driven to form a display pattern on a screen. Specifically, voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, so that a liquid crystal layer provided between the pixel electrode and the counter electrode is optically modulated and this optical modulation is recognized as a display pattern by an observer. 
     In displaying moving images, a liquid crystal display device has a problem that a long response time of liquid crystal molecules themselves causes afterimages or blurring of moving images. In order to improve the moving-image characteristics of a liquid crystal display device, a driving method called black insertion is employed in which black is displayed on the whole screen every other frame period. 
     Alternatively, a driving method called double-frame rate driving may be employed in which the vertical cycle is 1.5 or 2 times as long as usual to improve the moving-image characteristics. 
     Further alternatively, in order to improve the moving-image characteristics of a liquid crystal display device, a driving method may be employed, in which a plurality of LEDs (light-emitting diodes) or a plurality of EL light sources are used to form a surface light source as a backlight, and each light source of the surface light source is independently driven in a pulsed manner in one frame period. As the surface light source, three or more kinds of LEDs may be used and an LED emitting white light may be used. Since a plurality of LEDs can be controlled independently, the light emission timing of LEDs can be synchronized with the timing at which a liquid crystal layer is optically modulated. According to this driving method, LEDs can be partly turned off; therefore, an effect of reducing power consumption can be obtained particularly in the case of displaying an image having a large part on which black is displayed. 
     By combining these driving methods, the display characteristics of a liquid crystal display device, such as moving-image characteristics, can be improved as compared to those of conventional liquid crystal display devices. 
     The n-channel transistor obtained in this embodiment uses an In—Ga—Zn—O-based non-single-crystal film for its channel formation region and has favorable dynamic characteristics. Accordingly, these driving methods can be applied in combination with the n-channel transistor of this embodiment. 
     In manufacturing a light-emitting display device, one electrode (also referred to as a cathode) of an organic light-emitting element is set to a low power supply potential such as GND or 0 V; thus, a terminal portion is provided with a fourth terminal for setting the cathode to a low power supply potential such as GND or 0 V. Also in manufacturing a light-emitting display device, a power supply line is provided in addition to a source wiring and a gate wiring. Accordingly, the terminal portion is provided with a fifth terminal electrically connected to the power supply line. 
     With use of the thin film transistor using the oxide semiconductor in a gate line driver circuit or a source line driver circuit, manufacturing cost is reduced. Then, by directly connecting a gate electrode of the thin film transistor used in the driver circuit with a source wiring or a drain wiring, the number of contact holes can be reduced, so that a display device in which an area occupied by the driver circuit is reduced can be provided. 
     Accordingly, by applying this embodiment, a display device with excellent electric characteristics can be provided at lower cost. 
     This embodiment can be freely combined with any of Embodiment 1, Embodiment 2, and Embodiment 3. 
     Embodiment 5 
     In Embodiment 5, an example of electronic paper as a semiconductor device will be described. 
       FIG.  17    illustrates active matrix electronic paper as an example, which is different from a liquid crystal display device. A thin film transistor  581  used in a pixel portion of the semiconductor device can be formed in a manner similar to the thin film transistor of the pixel portion described in Embodiment 4 and is a thin film transistor including an In—Ga—Zn—O-based non-single-crystal film as a semiconductor layer. In addition, as described in Embodiment 1, the pixel portion and a driver circuit can be formed over the same substrate, and thereby electronic paper with low manufacturing cost can be realized. 
     The electronic paper in  FIG.  17    is an example of a display device using a twisting ball display system. The twisting ball display system refers to a method in which spherical particles each colored in black or white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed. 
     The thin film transistor  581  is a thin film transistor with a bottom gate structure provided, and a source or drain electrode layer thereof is in contact with a first electrode layer  587  in an opening formed in insulating layers  583 ,  584 ,  585 , whereby the thin film transistor  581  is electrically connected to the first electrode layer  587 . Between the first electrode layer  587  and a second electrode layer  588 , spherical particles  589  each having a black region  590   a , a white region  590   b , and a cavity  594  filled with liquid, around the regions, are provided between a pair of substrates  580  and  596 . A space around the spherical particles  589  is filled with a filler  595  such as a resin (see  FIG.  17   ). 
     Further, instead of the twisting ball, an electrophoretic element can also be used. A microcapsule having a diameter of about 10 μm to 200 μm in which transparent liquid, positively charged white microparticles, and negatively charged black microparticles are encapsulated, is used. In the microcapsule which is provided between the first electrode layer and the second electrode layer, when an electric field is applied by the first electrode layer and the second electrode layer, the white microparticles and the black microparticles move to opposite sides, so that white or black can be displayed. A display element using this principle is an electrophoretic display element and is called electronic paper. The electrophoretic display element has higher reflectance than a liquid crystal display element, and thus, an auxiliary light is unnecessary, power consumption is low, and a display portion can be recognized in a dim place. In addition, even when power is not supplied to the display portion, an image which has been displayed once can be maintained. Accordingly, a displayed image can be stored even if a semiconductor device having a display function (which may be referred to simply as a display device or a semiconductor device provided with a display device) is distanced from an electric wave source. 
     Through this process, electronic paper, which can be manufactured at lower cost, as a semiconductor device can be manufactured. 
     This embodiment can be combined with any content of Embodiment 1 or Embodiment 2 as appropriate. 
     Embodiment 6 
     In Embodiment 6, an example of a light-emitting display device as a semiconductor device will be described. As a display element included in a display device, a light-emitting element utilizing electroluminescence is described here. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element. 
     In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element. 
     The inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an example of an organic EL element as a light-emitting element is described here. 
       FIG.  18    illustrates an example of a pixel structure to which digital time grayscale driving can be applied, as an example of a semiconductor device. 
     A structure and operation of a pixel to which digital time grayscale driving can be applied are described. In this embodiment, one pixel includes two n-channel transistors each of which includes an oxide semiconductor layer (an In—Ga—Zn—O-based non-single-crystal film) as its channel formation region. 
     A pixel  6400  includes a switching transistor  6401 , a driver transistor  6402 , a light-emitting element  6404 , and a capacitor  6403 . A gate of the switching transistor  6401  is connected to a scan line  6406 , a first electrode (one of a source electrode and a drain electrode) of the switching transistor  6401  is connected to a signal line  6405 , and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor  6401  is connected to a gate of the driver transistor  6402 . The gate of the driver transistor  6402  is connected to a power supply line  6407  via the capacitor  6403 , a first electrode of the driver transistor  6402  is connected to the power supply line  6407 , and a second electrode of the driver transistor  6402  is connected to a first electrode (pixel electrode) of the light-emitting element  6404 . A second electrode of the light-emitting element  6404  corresponds to a common electrode  6408 . 
     The second electrode (common electrode  6408 ) of the light-emitting element  6404  is set to a low power supply potential. Note that the low power supply potential is a potential satisfying the low power supply potential &lt;a high power supply potential with reference to the high power supply potential that is set to the power supply line  6407 . As the low power supply potential, GND, 0 V, or the like may be employed, for example. A potential difference between the high power supply potential and the low power supply potential is applied to the light-emitting element  6404  and current is supplied to the light-emitting element  6404 , so that the light-emitting element  6404  emits light. Here, in order to make the light-emitting element  6404  emit light, each potential is set so that the potential difference between the high power supply potential and the low power supply potential is a forward threshold voltage or higher of the light-emitting element  6404 . 
     Note that gate capacitor of the driver transistor  6402  may be used as a substitute for the capacitor  6403 , so that the capacitor  6403  can be omitted. The gate capacitor of the driver transistor  6402  may be formed between the channel region and the gate electrode. 
     In the case of a voltage-input voltage driving method, a video signal is input to the gate of the driver transistor  6402  so that the driver transistor  6402  is in either of two states of being sufficiently turned on and turned off. That is, the driver transistor  6402  operates in a linear region. Since the driver transistor  6402  operates in the linear region, a voltage higher than the voltage of the power supply line  6407  is applied to the gate of the driver transistor  6402 . Note that a voltage higher than or equal to (voltage of the power supply line+Vth of the driver transistor  6402 ) is applied to the signal line  6405 . 
     In a case of performing analog grayscale driving instead of digital time grayscale driving, the same pixel structure as that in  FIG.  18    can be used by changing signal input. 
     In the case of performing analog grayscale driving, a voltage higher than or equal to (forward voltage of the light-emitting element  6404 +Vth of the driver transistor  6402 ) is applied to the gate of the driver transistor  6402 . The forward voltage of the light-emitting element  6404  indicates a voltage at which a desired luminance is obtained, and includes at least forward threshold voltage. The video signal by which the driver transistor  6402  operates in a saturation region is input, so that current can be supplied to the light-emitting element  6404 . In order for the driver transistor  6402  to operate in the saturation region, the potential of the power supply line  6407  is set higher than the gate potential of the driver transistor  6402 . When an analog video signal is used, it is possible to feed current to the light-emitting element  6404  in accordance with the video signal and perform analog grayscale driving. 
     Note that the pixel structure illustrated in  FIG.  18    is not limited thereto. For example, a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like may be added to the pixel illustrated in  FIG.  18   . 
     Next, structures of the light-emitting element will be described with reference to  FIGS.  19 A to  19 C . A cross-sectional structure of a pixel will be described by taking a case where the driving TFT is the thin film transistor  170  illustrated in  FIG.  1 B  as an example. Driving TFTs  7001 ,  7011 , and  7021  used for semiconductor devices illustrated in  FIGS.  19 A to  19 C  can be formed in a manner similar to the thin film transistor  170  described in Embodiment 1 and are thin film transistors which include In—Ga—Zn—O-based non-single-crystal films as their semiconductor layers and which have excellent electric characteristics. 
     In order to extract light emitted from the light-emitting element, at least one of an anode and a cathode is required to transmit light. A thin film transistor and a light-emitting element are formed over a substrate. A light-emitting element can have a top emission structure, in which light emission is extracted through the surface opposite to the substrate; a bottom emission structure, in which light emission is extracted through the surface on the substrate side; or a dual emission structure, in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side. The pixel structure illustrated in  FIG.  18    can be applied to a light-emitting element having any of these emission structures. 
     A light-emitting element having a top emission structure will be described with reference to  FIG.  19 A . 
       FIG.  19 A  is a cross-sectional view of a pixel in the case where the driving TFT  7001  is the thin film transistor  170  illustrated in  FIG.  1 B  and light is emitted from a light-emitting element  7002  to an anode  7005  side. In  FIG.  19 A , a cathode  7003  of the light-emitting element  7002  is electrically connected to the driving TFT  7001 , and a light-emitting layer  7004  and the anode  7005  are stacked in this order over the cathode  7003 . The cathode  7003  can be formed using a variety of conductive materials as long as they have a low work function and reflect light. For example, Ca, Al, MgAg, AlLi, or the like is preferably used. The light-emitting layer  7004  may be formed using a single layer or a plurality of layers stacked. When the light-emitting layer  7004  is formed using a plurality of layers, the light-emitting layer  7004  is formed by stacking an electron-injecting layer, an electron-transporting layer, a light-emitting layer, a hole-transporting layer, and a hole-injecting layer in this order over the cathode  7003 . It is not necessary to form all of these layers. The anode  7005  is formed using a light-transmitting conductive film such as a film of indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added. 
     The light-emitting element  7002  corresponds to a region where the light-emitting layer  7004  is sandwiched between the cathode  7003  and the anode  7005 . In the case of the pixel illustrated in  FIG.  19 A , light is emitted from the light-emitting element  7002  to the anode  7005  side as indicated by an arrow. 
     A second gate electrode provided over the oxide semiconductor layer in the driver circuit is preferably formed from the same material as the cathode  7003 , which leads to simplification of the process. 
     Next, a light-emitting element having a bottom emission structure will be described with reference to  FIG.  19 B .  FIG.  19 B  is a cross-sectional view of a pixel in the case where the driving TFT  7011  is the thin film transistor  170  illustrated in  FIG.  1 A  and light is emitted from a light-emitting element  7012  to a cathode  7013  side. In  FIG.  19 B , the cathode  7013  of the light-emitting element  7012  is formed over a light-transmitting conductive film  7017  that is electrically connected to the driving TFT  7011 , and a light-emitting layer  7014  and an anode  7015  are stacked in this order over the cathode  7013 . A light-blocking film  7016  for reflecting or blocking light may be formed to cover the anode  7015  when the anode  7015  has a light-transmitting property. For the cathode  7013 , a variety of materials can be used as in the case of  FIG.  19 A  as long as they are conductive materials having a low work function. The cathode  7013  is formed to have a thickness that can transmit light (preferably, approximately 5 nm to 30 nm). For example, an aluminum film with a thickness of 20 nm can be used as the cathode  7013 . Similar to the case of  FIG.  19 A , the light-emitting layer  7014  may be formed using either a single layer or a plurality of layers stacked. The anode  7015  is not required to transmit light, but can be formed using a light-transmitting conductive material as in the case of  FIG.  19 A . As the light-blocking film  7016 , a metal or the like that reflects light can be used for example; however, it is not limited to a metal film. For example, a resin or the like to which black pigments are added can also be used. 
     The light-emitting element  7012  corresponds to a region where the light-emitting layer  7014  is sandwiched between the cathode  7013  and the anode  7015 . In the case of the pixel illustrated in  FIG.  19 B , light is emitted from the light-emitting element  7012  to the cathode  7013  side as indicated by an arrow. 
     A second gate electrode provided over the oxide semiconductor layer in the driver circuit is preferably formed from the same material as the cathode  7013 , which leads to simplification of the process. 
     Next, a light-emitting element having a dual emission structure will be described with reference to  FIG.  19 C . In  FIG.  19 C , a cathode  7023  of a light-emitting element  7022  is formed over a light-transmitting conductive film  7027  which is electrically connected to the driving TFT  7021 , and a light-emitting layer  7024  and an anode  7025  are stacked in this order over the cathode  7023 . As in the case of  FIG.  19 A , the cathode  7023  can be formed using a variety of conductive materials as long as they have a low work function. The cathode  7023  is formed to have a thickness that can transmit light. For example, a film of Al having a thickness of 20 nm can be used as the cathode  7023 . As in  FIG.  19 A , the light-emitting layer  7024  may be formed using either a single layer or a plurality of layers stacked. The anode  7025  can be formed using a light-transmitting conductive material as in the case of  FIG.  19 A . 
     The light-emitting element  7022  corresponds to a region where the cathode  7023 , the light-emitting layer  7024 , and the anode  7025  overlap with one another. In the case of the pixel illustrated in  FIG.  19 C , light is emitted from the light-emitting element  7022  to both the anode  7025  side and the cathode  7023  side as indicated by arrows. 
     A second gate electrode provided over the oxide semiconductor layer in the driver circuit is preferably formed from the same material as the conductive film  7027 , which leads to simplification of the process. Further, the second gate electrode provided over the oxide semiconductor layer in the driver circuit is preferably formed with a stack of the same materials as the conductive film  7027  and the cathode  7023 , thereby lowering wiring resistance as well as simplification of the process. 
     Note that, although the organic EL elements are described here as the light-emitting elements, an inorganic EL element can also be provided as a light-emitting element. 
     In this embodiment, the example is described in which a thin film transistor (a driving TFT) which controls the driving of a light-emitting element is electrically connected to the light-emitting element; however, a structure may be employed in which a TFT for current control is connected between the driving TFT and the light-emitting element. 
     A semiconductor device described in this embodiment is not limited to the structures illustrated in  FIGS.  19 A to  19 C  and can be modified in various ways based on the spirit of techniques disclosed. 
     Next, a top view and a cross section of a light-emitting display panel (also referred to as a light-emitting panel), which is one embodiment of a semiconductor device, will be described with reference to  FIGS.  20 A and  20 B .  FIG.  20 A  is the top view of a panel in which a thin film transistor and a light-emitting element formed over a first substrate are sealed between the first substrate and a second substrate with a sealant.  FIG.  20 B  is a cross-sectional view taken along a line H-I of  FIG.  20 A . 
     A sealant  4505  is provided so as to surround a pixel portion  4502 , signal line driver circuits  4503   a  and  4503   b , and scan line driver circuits  4504   a  and  4504   b  which are provided over a first substrate  4501 . In addition, a second substrate  4506  is provided over the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b , and the scan line driver circuits  4504   a  and  4504   b . Accordingly, the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b , and the scan line driver circuits  4504   a  and  4504   b  are sealed together with a filler  4507 , by the first substrate  4501 , the sealant  4505 , and the second substrate  4506 . It is preferable that a panel be packaged (sealed) with a protective film (such as a laminate film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the panel is not exposed to the outside air, in this manner. 
     The pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b , and the scan line driver circuits  4504   a  and  4504   b  formed over the first substrate  4501  each include a plurality of thin film transistors. In  FIG.  20 B , a thin film transistor  4510  included in the pixel portion  4502  and a thin film transistor  4509  included in the signal line driver circuit  4503   a  are illustrated as an example. 
     For each of the thin film transistors  4509  and  4510 , a highly reliable thin film transistor including an In—Ga—Zn—O-based non-single-crystal film as its semiconductor layer as described in Embodiment 1 can be applied. In addition, the thin film transistor  4509  includes gate electrodes above and below the semiconductor layer as described in Embodiment 1 with reference to  FIG.  1 B . 
     Moreover, reference numeral  4511  denotes a light-emitting element. A first electrode layer  4517  which is a pixel electrode included in the light-emitting element  4511  is electrically connected to a source electrode layer or a drain electrode layer of the thin film transistor  4510 . Note that a structure of the light-emitting element  4511  is a stacked-layer structure of the first electrode layer  4517 , the electroluminescent layer  4512 , and the second electrode layer  4513 , but there is no particular limitation on the structure. The structure of the light-emitting element  4511  can be changed as appropriate depending on the direction in which light is extracted from the light-emitting element  4511 , or the like. 
     A partition  4520  is formed using an organic resin film, an inorganic insulating film, or organic polysiloxane. It is particularly preferable that the partition  4520  be formed using a photosensitive material and an opening be formed over the first electrode layer  4517  so that a sidewall of the opening is formed as an inclined surface with continuous curvature. 
     The electroluminescent layer  4512  may be formed with a single layer or a plurality of layers stacked. 
     A protective film may be formed over the second electrode layer  4513  and the partition  4520  in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element  4511 . As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed. 
     In addition, a variety of signals and potentials are supplied to the signal line driver circuits  4503   a  and  4503   b , the scan line driver circuits  4504   a  and  4504   b , or the pixel portion  4502  from FPCs  4518   a  and  4518   b.    
     In this embodiment, a connection terminal electrode  4515  is formed from the same conductive film as the first electrode layer  4517  included in the light-emitting element  4511 , and a terminal electrode  4516  is formed from the same conductive film as the source and drain electrode layers included in the thin film transistors  4509  and  4510 . 
     The connection terminal electrode  4515  is electrically connected to a terminal included in the FPC  4518   a  via an anisotropic conductive film  4519 . 
     The second substrate  4506  located in the direction in which light is extracted from the light-emitting element  4511  should have a light-transmitting property. In that case, a light-transmitting material such as a glass plate, a plastic plate, a polyester film, or an acrylic film is used for the second substrate  4506 . 
     As the filler  4507 , an ultraviolet curable resin or a thermosetting resin can be used, in addition to an inert gas such as nitrogen or argon. For example, PVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate) can be used. 
     In addition, if needed, an optical film, such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter, may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed. 
     The signal line driver circuits  4503   a  and  4503   b  and the scan line driver circuits  4504   a  and  4504   b  may be provided by mounting driver circuits formed using a single crystal semiconductor film or polycrystalline semiconductor film over a single-crystal substrate or an insulating substrate separately prepared. In addition, only the signal line driver circuits or part thereof, or the scan line driver circuits or part thereof may be separately formed and mounted. This embodiment is not limited to the structure illustrated in  FIGS.  20 A and  20 B . 
     Through this process, a display device (display panel) can be manufactured at lower manufacturing cost. 
     This embodiment can be combined with any content of Embodiment 1 or Embodiment 2 as appropriate. 
     Embodiment 7 
     In this embodiment, a top view and a cross section of a liquid crystal display panel, which is one embodiment of a semiconductor device, will be described with reference to  FIGS.  21 A,  21 B and  21 C .  FIGS.  21 A and  21 B  are top views of a panel in which highly reliable thin film transistors  4010  and  4011  each including the In—Ga—Zn—O-based non-single-crystal film as its semiconductor layer as described in Embodiment 1 and a liquid crystal element  4013 , which are formed over a first substrate  4001  are sealed between the first substrate  4001  and a second substrate  4006  with a sealant  4005 .  FIG.  21 C  is a cross-sectional view taken along a line M-N of  FIGS.  21 A and  21 B . 
     The sealant  4005  is provided so as to surround a pixel portion  4002  and a scan line driver circuit  4004  which are provided over the first substrate  4001 . The second substrate  4006  is provided over the pixel portion  4002  and the scan line driver circuit  4004 . Therefore, the pixel portion  4002  and the scan line driver circuit  4004  are sealed together with a liquid crystal layer  4008 , by the first substrate  4001 , the sealant  4005 , and the second substrate  4006 . A signal line driver circuit  4003  that is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant  4005  over the first substrate  4001 . 
     Note that the connection method of a driver circuit which is separately formed is not particularly limited, and a COG method, a wire bonding method, a TAB method, or the like can be used.  FIG.  21 A  illustrates an example of mounting the signal line driver circuit  4003  by a COG method, and  FIG.  21 B  illustrates an example of mounting the signal line driver circuit  4003  by a TAB method. 
     The pixel portion  4002  and the scan line driver circuit  4004  provided over the first substrate  4001  include a plurality of thin film transistors.  FIG.  21 C  illustrates the thin film transistor  4010  included in the pixel portion  4002  and the thin film transistor  4011  included in the scan line driver circuit  4004 . Over the thin film transistors  4010  and  4011 , insulating layers  4020  and  4021  are provided. 
     Each of the thin film transistors  4010  and  4011  can be the thin film transistor including an In—Ga—Zn—O-based non-single-crystal film as its semiconductor layer described in Embodiment 1. The thin film transistor  4011  corresponds to the thin film transistor having a back gate electrode described in Embodiment 2 with reference to  FIG.  2 A . 
     A pixel electrode layer  4030  included in the liquid crystal element  4013  is electrically connected to the thin film transistor  4010 . A counter electrode layer  4031  of the liquid crystal element  4013  is provided for the second substrate  4006 . A portion where the pixel electrode layer  4030 , the counter electrode layer  4031 , and the liquid crystal layer  4008  overlap with one another corresponds to the liquid crystal element  4013 . Note that the pixel electrode layer  4030  and the counter electrode layer  4031  are provided with an insulating layer  4032  and an insulating layer  4033  respectively which each function as an alignment film, and the liquid crystal layer  4008  is sandwiched between the pixel electrode layer  4030  and the counter electrode layer  4031  with the insulating layers  4032  and  4033  therebetween. 
     Note that the first substrate  4001  and the second substrate  4006  can be formed of glass, metal (typically, stainless steel), ceramic, or plastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. In addition, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used. 
     Reference numeral  4035  denotes a columnar spacer obtained by selectively etching an insulating film and is provided to control the distance between the pixel electrode layer  4030  and the counter electrode layer  4031  (a cell gap). Further, a spherical spacer may also be used. In addition, the counter electrode layer  4031  is electrically connected to a common potential line formed over the same substrate as the thin film transistor  4010 . With use of the common connection portion, the counter electrode layer  4031  and the common potential line can be electrically connected to each other by conductive particles arranged between a pair of substrates. Note that the conductive particles are included in the sealant  4005 . 
     Alternatively, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase is generated within an only narrow range of temperature, liquid crystal composition containing a chiral agent at 5 wt % or more so as to improve the temperature range is used for the liquid crystal layer  4008 . The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral agent have such characteristics that the response time is 10 μs to 100 μs, which is short, the alignment process is unnecessary because the liquid crystal composition has optical isotropy, and viewing angle dependency is small. 
     Although the example of a transmissive liquid crystal display device is described in this embodiment, one embodiment of the present invention can also be applied to a reflective liquid crystal display device and a transflective liquid crystal display device. 
     While an example of the liquid crystal display device in which the polarizing plate is provided on the outer side of the substrate (on the viewer side) and the coloring layer and the electrode layer used for a display element are provided on the inner side of the substrate in that order is described in this embodiment, the polarizing plate may be provided on the inner side of the substrate. The stacked structure of the polarizing plate and the coloring layer is not limited to this embodiment and may be set as appropriate depending on materials of the polarizing plate and the coloring layer or conditions of manufacturing process. Further, a light-blocking film serving as a black matrix may be provided. 
     In this embodiment, in order to reduce surface unevenness of the thin film transistor and to improve reliability of the thin film transistor, the thin film transistor obtained in Embodiment 1 is covered with the insulating layers (the insulating layer  4020  and the insulating layer  4021 ) functioning as a protective film or a planarizing insulating film. Note that the protective film is provided to prevent entry of contaminant impurities such as organic substance, metal, or moisture existing in air and is preferably a dense film. The protective film may be formed with a single layer or a stacked layer of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, aluminum oxynitride film, and/or an aluminum nitride oxide film by a sputtering method. 
     Although an example in which the protective film is formed by a sputtering method is described in this embodiment, there is no particular limitation and the protective film may be formed by a variety of methods such as PCVD. In part of the driver circuit, the protective film serves as a second gate insulating layer, and a thin film transistor having a back gate over the second gate insulating layer is provided. 
     In this embodiment, the insulating layer  4020  having a stacked-layer structure is formed as a protective film. Here, as a first layer of the insulating layer  4020 , a silicon oxide film is formed by a sputtering method. The use of a silicon oxide film as a protective film has an effect of preventing hillock of an aluminum film. 
     As a second layer of the protective film, an insulating layer is formed. In this embodiment, as the second layer of the insulating layer  4020 , a silicon nitride film is formed by a sputtering method. The use of the silicon nitride film as the protective film can prevent mobile ions of sodium or the like from entering a semiconductor region and changing electrical characteristics of the TFT. 
     After the protective film is formed, the semiconductor layer may be subjected to annealing (300° C. to 400° C.). In addition, the back gate is formed after the protective film is formed. 
     The insulating layer  4021  is formed as the planarizing insulating film. As the insulating layer  4021 , an organic material having heat resistance such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. Note that the insulating layer  4021  may be formed by stacking a plurality of insulating films formed of these materials. 
     Note that a siloxane-based resin is a resin formed from a siloxane material as a starting material and having the bond of Si—O—Si. The siloxane-based resin may include as a substituent an organic group (e.g., an alkyl group or an aryl group) or a fluoro group. In addition, the organic group may include a fluoro group. 
     A formation method of the insulating layer  4021  is not particularly limited, and the following method can be employed depending on the material: a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, a droplet discharge method (e.g., an ink-jet method, screen printing, offset printing, or the like), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. In a case of forming the insulating layer  4021  using a material solution, annealing (300° C. to 400° C.) of the semiconductor layer may be performed at the same time as a baking step. The baking step of the insulating layer  4021  also serves as annealing of the semiconductor layer, whereby a semiconductor device can be manufactured efficiently. 
     The pixel electrode layer  4030  and the counter electrode layer  4031  can be formed using a light-transmitting conductive material such as indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, indium tin oxide to which silicon oxide is added, or the like. 
     Conductive compositions including a conductive high molecule (also referred to as a conductive polymer) can be used for the pixel electrode layer  4030  and the counter electrode layer  4031 . The pixel electrode formed using the conductive composition preferably has a sheet resistance of less than or equal to 10000 ohms per square and a transmittance of greater than or equal to 70% at a wavelength of 550 nm. Further, the resistivity of the conductive high molecule included in the conductive composition is preferably less than or equal to 0.1 Ω·cm. 
     As the conductive high molecule, a so-called  7   c -electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more kinds of them, and the like can be given. 
     Further, a variety of signals and potentials are supplied to the signal line driver circuit  4003  which is formed separately, the scan line driver circuit  4004 , or the pixel portion  4002  from an FPC  4018 . 
     In this embodiment, a connection terminal electrode  4015  is formed from the same conductive film as that of the pixel electrode layer  4030  included in the liquid crystal element  4013 , and a terminal electrode  4016  is formed from the same conductive film as that of the source and drain electrode layers of the thin film transistors  4010  and  4011 . 
     The connection terminal electrode  4015  is electrically connected to a terminal included in the FPC  4018  via an anisotropic conductive film  4019 . 
       FIGS.  21 A and  21 B  illustrate an example in which the signal line driver circuit  4003  is formed separately and mounted on the first substrate  4001 ; however, this embodiment is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted. 
       FIG.  22    illustrates an example in which a liquid crystal display module is formed as a semiconductor device by using a TFT substrate  2600 . 
       FIG.  22    illustrates an example of a liquid crystal display module, in which the TFT substrate  2600  and a counter substrate  2601  are fixed to each other with a sealant  2602 , and a pixel portion  2603  including a TFT or the like, a display element  2604  including a liquid crystal layer, and a coloring layer  2605  are provided between the substrates to form a display region. The coloring layer  2605  is necessary to perform color display. In the RGB system, respective coloring layers corresponding to colors of red, green, and blue are provided for respective pixels. Polarizing plates  2606  and  2607  and a diffusion plate  2613  are provided outside the TFT substrate  2600  and the counter substrate  2601 . A light source includes a cold cathode tube  2610  and a reflective plate  2611 , and a circuit substrate  2612  is connected to a wiring circuit portion  2608  of the TFT substrate  2600  by a flexible wiring board  2609  and includes an external circuit such as a control circuit or a power source circuit. The polarizing plate and the liquid crystal layer may be stacked with a retardation plate therebetween. 
     For the liquid crystal display module, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     Through this process, a liquid crystal display device as a semiconductor device can be manufactured at lower manufacturing cost. 
     This embodiment can be combined with any content of Embodiment 1, 2 or 3 as appropriate. 
     Embodiment 8 
     A semiconductor device according to one embodiment of the present invention can be applied to a variety of electronic devices (including an amusement machine). Examples of electronic devices are a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game console, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like. 
       FIG.  23 A  illustrates an example of a portable information terminal device  9200 . The portable information terminal device  9200  incorporates a computer and thus can process various types of data. An example of the portable information terminal device  9200  is a personal digital assistant. 
     The portable information terminal device  9200  has two housings, a housing  9201  and a housing  9203 . The housing  9201  and the housing  9203  are joined with a joining portion  9207  such that the portable information terminal device  9200  can be foldable. A display portion  9202  is incorporated in the housing  9201 , and the housing  9203  includes a keyboard  9205 . Needless to say, the structure of the portable information terminal device  9200  is not limited to the above structure, and the structure may include at least a thin film transistor having a back gate electrode, and additional accessory may be provided as appropriate. A driver circuit and a pixel portion are formed over the same substrate, which leads to reduction of the manufacturing cost. Thus, a portable information terminal device having a thin film transistor having high electric characteristics can be realized. 
       FIG.  23 B  illustrates an example of a digital video camera  9500 . The digital video camera  9500  includes a display portion  9503  incorporated in a housing  9501  and various operation portions. Needless to say, the structure of the digital video camera  9500  is not limited to the above structure, and the structure may include at least a thin film transistor having a back gate electrode, and additional accessory may be provided as appropriate. A driver circuit and a pixel portion are formed over the same substrate, which leads to reduction of the manufacturing cost. Thus, a digital video camera having a thin film transistor having high electric characteristics can be realized. 
       FIG.  23 C  illustrates an example of a mobile phone  9100 . The mobile phone  9100  has two housings, a housing  9102  and a housing  9101 . The housing  9102  and the housing  9101  are joined with a joining portion such that the mobile phone is foldable. A display portion  9104  is incorporated in the housing  9102 , and the housing  9101  includes operation keys  9106 . Needless to say, the structure of the mobile phone  9100  is not limited to the above structure, and the structure may include at least a thin film transistor having a back gate electrode, and additional accessory may be provided as appropriate. A driver circuit and a pixel portion are formed over the same substrate, which leads to reduction of the manufacturing cost. Thus, a mobile phone having a thin film transistor having high electric characteristics can be realized. 
       FIG.  23 D  illustrates an example of a portable computer  9400 . The computer  9400  has two housings, a housing  9401  and a housing  9404 . The housing  9401  and the housing  9404  are joined such that the computer can be open and closed. A display portion  9402  is incorporated in the housing  9401 , and the housing  9404  includes a key board  9403  or the like. Needless to say, the structure of the computer  9400  is not particularly limited to the above structure, and the structure may include at least a thin film transistor having a back gate electrode, and additional accessory may be provided as appropriate. A driver circuit and a pixel portion are formed over the same substrate, which leads to reduction of the manufacturing cost. Thus, a computer having a thin film transistor having high electric characteristics can be realized. 
       FIG.  24 A  illustrates an example of a television set  9600 . In the television set  9600 , a display portion  9603  is incorporated in a housing  9601 . The display portion  9603  can display images. Further, the housing  9601  is supported by a stand  9605  in  FIG.  24 A . 
     The television set  9600  can be operated with an operation switch of the housing  9601  or a separate remote controller  9610 . Channels and volume can be controlled with an operation key  9609  of the remote controller  9610  so that an image displayed on the display portion  9603  can be controlled. Further, the remote controller  9610  may be provided with a display portion  9607  for displaying data output from the remote controller  9610 . 
     Note that the television set  9600  is provided with a receiver, a modem, and the like. With the receiver, a general television broadcast can be received. Further, when the television set  9600  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. 
       FIG.  24 B  illustrates an example of a digital photo frame  9700 . For example, in the digital photo frame  9700 , a display portion  9703  is incorporated in a housing  9701 . The display portion  9703  can display a variety of images. For example, the display portion  9703  can display data of an image taken with a digital camera or the like and function as a normal photo frame. 
     Note that the digital photo frame  9700  is provided with an operation portion, an external connection portion (a USB terminal, a terminal that can be connected to various cables such as a USB cable, or the like), a recording medium insertion portion, and the like. Although these components may be provided on the surface on which the display portion is provided, it is preferable to provide them on the side surface or the back surface for the design of the digital photo frame  9700 . For example, a memory storing data of an image taken with a digital camera is inserted in the recording medium insertion portion of the digital photo frame, whereby the image data can be transferred and then displayed on the display portion  9703 . 
     The digital photo frame  9700  may be configured to transmit and receive data wirelessly. The structure may be employed in which desired image data is transferred wirelessly to be displayed. 
       FIG.  25 A  illustrates an example of a mobile phone  1000  which is different from that illustrated in  FIG.  23 C . The mobile phone  1000  includes a display portion  1002  incorporated in a housing  1001 , an operation button  1003 , an external connection port  1004 , a speaker  1005 , a microphone  1006  and the like. 
     In the mobile phone  1000  illustrated in  FIG.  25 A , data can be input when a person touches the display portion  1002  with his/her finger or the like. In addition, operations such as phone call or mailing can be conducted when a person touches the display portion  1002  with his/her finger or the like. 
     There are mainly three screen modes of the display portion  1002 : the first mode is a display mode mainly for displaying an image; the second mode is an input mode mainly for inputting data such as text; and the third mode is a display-and-input mode in which two modes of the display mode and the input mode are combined. 
     For example, in a case of calling or mailing, a text input mode mainly for inputting text is selected for the display portion  1002  so that text displayed on a screen can be input. In that case, it is preferable to display a keyboard or number buttons on almost all area of the screen of the display portion  1002 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the mobile phone  1000 , display in the screen of the display portion  1002  can be automatically switched by determining the installation direction of the mobile phone  1000  (whether the mobile phone  1000  is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are switched by touching the display portion  1002  or operating the operation button  1003  of the housing  1001 . Alternatively, the screen modes may be switched depending on the kind of the image displayed on the display portion  1002 . For example, when a signal of an image displayed on the display portion is a signal of moving image data, the screen mode is switched to the display mode. When the signal is a signal of text data, the screen mode is switched to the input mode. 
     Further, in the input mode, when input by touching the display portion  1002  is not performed for a certain period while a signal detected by the optical sensor in the display portion  1002  is detected, the screen mode may be controlled so as to be switched from the input mode to the display mode. 
     The display portion  1002  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken when the display portion  1002  is touched with a palm or a finger, whereby personal identification can be performed. Further, by providing a backlight or a sensing light source which emits a near-infrared light in the display portion, an image of a finger vein, a palm vein, or the like can be taken. 
       FIG.  25 B  illustrates another example of a mobile phone. The mobile phone in  FIG.  25 B  has a display device  9410  in a housing  9411 , which includes a display portion  9412  and operation buttons  9413 , and a communication device  9400  in a housing  9401 , which includes operation buttons  9402 , an external input terminal  9403 , a microphone  9404 , a speaker  9405 , and a light-emitting portion  9406  that emits light when a phone call is received. The display device  9410  which has a display function can be detached from or attached to the communication device  9400  which has a phone function, in two directions represented by the allows. Thus, the display device  9410  and the communication device  9400  can be attached to each other along their short sides or long sides. In addition, when only the display function is needed, the display device  9410  can be detached from the communication device  9400  and used alone. Images or input information can be transmitted or received by wireless or wire communication between the communication device  9400  and the display device  9410 , each of which has a rechargeable battery. 
     Embodiment 9 
     In this embodiment, an example of a display device including a thin film transistor in which a wiring and an oxide semiconductor layer are in contact with each other is described with reference to  FIG.  26   . It is to be noted that the same portions in  FIG.  26    as those in  FIG.  1 A  will be described with use of the same reference numerals. 
     A first thin film transistor  480  illustrated in  FIG.  26    is a thin film transistor used in a driver circuit, in which a first wiring  409  and a second wiring  410  are provided in contact with an oxide semiconductor layer  405 . The first thin film transistor  480  includes a first gate electrode  401  below the oxide semiconductor layer  405  and a second gate electrode  470  above the oxide semiconductor layer  405 . 
     In addition, a second thin film transistor  481  is a thin film transistor used in a pixel portion, in which source or drain electrode layers  105   a  and  105   b  are provided in contact with the oxide semiconductor layer  103 . 
     The semiconductor device of this embodiment has a structure in which the wiring and the oxide semiconductor layer are in contact with each other, and thus the number of steps can be reduced as compared with that of Embodiment 1. 
     This embodiment can be combined with any structure of the other embodiments. 
     This application is based on Japanese Patent Application serial no. 2008-274540 filed with Japanese Patent Office on Oct. 24, 2008, the entire contents of which are hereby incorporated by reference.