Patent Publication Number: US-9899420-B2

Title: Display device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention disclosed in this specification and the like relates to a display device and a method for manufacturing the display device. One embodiment of the present invention particularly relates to a display device having a transistor including an oxide semiconductor film and a method for manufacturing the display device. 
     2. Description of the Related Art 
     Since display devices using light-emitting elements such as organic electroluminescence (hereinafter, also referred to as EL) have high visibility, are suitable for reduction in thickness, and do not have limitations on viewing angles, they have attracted attention as display devices which can take the place of cathode ray tubes (CRTs) or liquid crystal display devices. In an active matrix display device using a light-emitting element, in general, at least a light-emitting element, a transistor (a switching (selection) transistor) which controls input of a video signal to a pixel, and a transistor (a driving transistor) which controls a value of current supplied to the light-emitting element are provided in each pixel. 
     Transistors used for display devices are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. 
     In recent years, instead of the silicon semiconductor, a technique in which a metal oxide having semiconductor characteristics (an oxide semiconductor) is used for transistors has attracted attention. For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn oxide as an oxide semiconductor and the transistor is used as a switching transistor or the like of a pixel of a display device (see Patent Document 1). 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2007-123861 
     SUMMARY OF THE INVENTION 
     In a display device including a light-emitting element, drain current of a driver transistor is supplied to the light-emitting element. Thus, when the threshold voltage fluctuates because of degradation of the driver transistor, the luminance of the light-emitting element also fluctuates. This means that in order to improve the image quality of the display device, it is an important object to suppress fluctuations in the threshold voltage of the driver transistor. 
     Further, in order to achieve high-speed driving of the display device, it is desirable to use a transistor with high field-effect mobility as the driver transistor. 
     In contrast, in order to reduce power consumption of the display device, it is desirable to use a transistor having positive threshold voltage (normally-off characteristics) as a selection transistor. Alternatively, as a selection transistor, it is desirable to use a transistor in which drain current is reduced at a gate voltage of 0 V (cutoff current: Icut) in the drain current−gate voltage (I d −V g ) characteristic curve. 
     In view of the above, an object of one embodiment of the present invention is to provide a display device that is less influenced by fluctuations in the threshold voltage of a transistor. Another object of one embodiment of the present invention is to provide a highly reliable display device. 
     Another object of one embodiment of the present invention is to provide a display device with favorable display characteristics. 
     Another object of one embodiment of the present invention is to provide a display device with low power consumption. 
     Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects are apparent from and can be derived from the description of the specification and the like. 
     In a pixel including a selection transistor, a driver transistor, and a light-emitting element, as the driver transistor, a transistor is used in which a channel is formed in an oxide semiconductor film and its channel length is greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Further, the driver transistor includes a first gate electrode over an oxide semiconductor film and a second gate electrode below the oxide semiconductor film. The first gate electrode and the second gate electrode are electrically connected to each other and overlap with the oxide semiconductor film. With such a structure, the field-effect mobility and the on-state current of the driver transistor can be improved, and the display device can display favorably even at a high drive frequency. Further, in the selection transistor of a pixel, which does not need to have field-effect mobility as high as that of the driver transistor, a channel length is made longer than at least the channel length of the driver transistor, whereby the power consumption of the pixel can be reduced while the aperture ratio is increased. 
     Specifically, the following structures can be employed for example. 
     One embodiment of the present invention is a display device including a pixel having a light-emitting element, a first transistor serving as a selection transistor for the light-emitting element, and a second transistor that serves as a driver transistor and is electrically connected to the first transistor. The first transistor includes a first gate electrode over an insulating surface, a first insulating film over the first gate electrode, a first oxide semiconductor film overlapping with the first gate electrode with the first insulating film positioned therebetween, a first pair of electrodes electrically connected to the first oxide semiconductor film, a second insulating film over the first oxide semiconductor film, and a second gate electrode over the first oxide semiconductor film with the second insulating film positioned therebetween. End portions of the first pair of electrodes are over the first oxide semiconductor film. The second gate electrode overlaps with the first gate electrode and includes, in a channel width direction, a region that faces a side surface of the first oxide semiconductor film with the second insulating film positioned therebetween. The second transistor includes a third gate electrode over the insulating surface, the first insulating film over the third gate electrode, a second oxide semiconductor film overlapping with the third gate electrode with the first insulating film positioned therebetween, and a second pair of electrodes electrically connected to the second oxide semiconductor film. End portions of the second pair of electrodes are over the second oxide semiconductor film. A distance between the first pair of electrodes of the first transistor is greater than or equal to 0.5 μm and less than or equal to 4.5 μm. A distance between the second pair of electrodes of the second transistor is longer than that between the first pair of electrodes of the first transistor. 
     Another embodiment of the present invention is a display device including a pixel having a light-emitting element, a first transistor serving as a selection transistor for the light-emitting element, and a second transistor that serves as a driver transistor and is electrically connected to the first transistor. The first transistor includes a first gate electrode over an insulating surface, a first insulating film over the first gate electrode, a first oxide semiconductor film overlapping with the first gate electrode with the first insulating film positioned therebetween, a first pair of electrodes electrically connected to the first oxide semiconductor film, a second insulating film over the first oxide semiconductor film, and a second gate electrode over the first oxide semiconductor film with the second insulating film positioned therebetween. End portions of the first pair of electrodes are over the first oxide semiconductor film. The second gate electrode overlaps with the first gate electrode and includes, in a channel width direction, a region that faces a side surface of the first oxide semiconductor film with the second insulating film positioned therebetween. The second transistor includes a third gate electrode over the insulating surface, the first insulating film over the third gate electrode, a second oxide semiconductor film overlapping with the third gate electrode with the first insulating film positioned therebetween, and a second pair of electrodes electrically connected to the second oxide semiconductor film. End portions of the second pair of electrodes are over the second oxide semiconductor film. A distance between the first pair of electrodes of the first transistor is greater than or equal to 0.5 μm and less than or equal to 4.5 μm. A distance between the second pair of electrodes of the second transistor is longer than that between the first pair of electrodes of the first transistor. In addition, cutoff current of the second transistor is lower than that of the first transistor. 
     In any of the above display devices, the second transistor may further include a fourth gate electrode over the second oxide semiconductor film with the second insulating film positioned therebetween. The fourth gate electrode overlaps with the third gate electrode. The third gate electrode and the fourth gate electrode include a region where the third gate electrode and the fourth gate electrode are in contact with each other. 
     In any of the above display devices, the first gate electrode and the second gate electrode preferably include a region where the first gate electrode and the second gate electrode are in contact with each other. 
     In any of the above display devices, one or both of the first insulating film and the second insulating film preferably includes an oxide insulating film containing oxygen at a higher proportion than oxygen in the stoichiometric composition. 
     One embodiment of the present invention can provide a display device that is less influenced by fluctuations in the threshold voltage of a transistor including an oxide semiconductor film. 
     One embodiment of the present invention can provide a highly reliable display device. 
     One embodiment of the present invention can provide a display device with favorable display characteristics. 
     One embodiment of the present invention can provide a display device with low power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a conceptual diagram illustrating one embodiment of a display device and a circuit diagram illustrating one embodiment of a pixel. 
       FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D are plan views and cross-sectional views illustrating one embodiment of a display device. 
         FIGS. 3A to 3D  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
         FIGS. 4A and 4B  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
         FIGS. 5A and 5B  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
         FIGS. 6A and 6B  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
       FIGS.  7 A 1 ,  7 A 2 ,  7 B,  7 C 1 , and  7 C 2  are plan views and cross-sectional views illustrating one embodiment of a display device. 
       FIGS.  8 A 1 ,  8 A 2 ,  8 B,  8 C 1 , and  8 C 2  are plan views and cross-sectional views illustrating one embodiment of a display device. 
         FIGS. 9A to 9D  are a circuit diagram, a plan view, and cross-sectional views illustrating a display device. 
         FIGS. 10A ,  10 B 1 ,  10 B 2 ,  10 C 1 , and  10 C 2  illustrate circuit symbols and structures of a transistor. 
       FIGS.  11 A 1 ,  11 A 2 ,  11 B,  11 C 1 , and  11 C 2  are plan views and cross-sectional views illustrating one embodiment of a display device. 
       FIGS.  12 A 1 ,  12 A 2 ,  12 B,  12 C 1 , and  12 C 2  are plan views and cross-sectional views illustrating one embodiment of a display device. 
         FIGS. 13A and 13B  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
         FIGS. 14A and 14B  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
         FIGS. 15A and 15B  are cross-sectional views illustrating one embodiment of a method for manufacturing a display device. 
         FIG. 16  is a plan view illustrating one embodiment of a pixel structure of a display device. 
         FIG. 17  is a cross-sectional view illustrating one embodiment of a pixel structure of a display device. 
         FIG. 18  is a cross-sectional view illustrating one embodiment of a pixel structure of a display device. 
         FIG. 19  is a cross-sectional view illustrating one embodiment of a display device. 
       FIGS.  20 A 1 ,  20 A 2 ,  20 B,  20 C 1 , and  20 C 2  are plan views and cross-sectional views illustrating one embodiment of a display device. 
         FIG. 21  is a cross-sectional view illustrating one embodiment of a display device. 
         FIGS. 22A to 22C  illustrate band structures of transistors. 
         FIG. 23  is a circuit diagram illustrating one embodiment of a pixel structure of a display device. 
         FIGS. 24A and 24B  are a top view and a cross-sectional view illustrating a display device. 
         FIGS. 25A to 25F  each illustrate an example of an electronic device. 
         FIGS. 26A to 26C  each show V g −I d  characteristics of a transistor of Example. 
         FIGS. 27A to 27C  each show V g −I d  characteristics of a transistor of Example. 
         FIGS. 28A to 28C  each show V g −I d  characteristics of a transistor of Example. 
         FIGS. 29A to 29C  show a structure of a transistor used for calculation, field-effect mobility obtained by the calculation, and on-state current obtained by the calculation. 
         FIG. 30  illustrates a transistor model used for calculation. 
         FIGS. 31A and 31B  each show channel length dependence of saturation mobility obtained by calculation. 
         FIG. 32  shows channel length dependence of on-state current obtained by calculation. 
         FIG. 33A  shows the V g −I d  characteristics of a transistor obtained by calculation and  FIGS. 33B and 33C  each show current distribution in an oxide semiconductor film obtained by calculation. 
         FIG. 34A  shows the V g −I d  characteristics of a transistor obtained by calculation and  FIGS. 34B and 34C  each show current distribution in a silicon film obtained by calculation. 
         FIG. 35A  shows an electron trap used for calculation and  FIGS. 35B and 35C  each show channel length dependence of saturation mobility obtained by calculation. 
         FIGS. 36A to 36C  show flow of carriers in a transistor in an off state and an on state. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments and examples. In addition, in the following embodiments and example, the same portions or portions having similar functions are denoted by the same reference numerals or the same hatching patterns in different drawings, and description thereof will not be repeated. 
     Note that in each drawing described in this specification, the size, the film thickness, or the region of each component may be exaggerated for clarity. Therefore, embodiments of the present invention are not limited to such scales. 
     In this specification, ordinal numbers such as “first”, “second”, and the like are used in order to avoid confusion among components, and the terms do not limit the components numerically. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. 
     Functions of a “source” and a “drain” are sometimes replaced with each other when the direction of flow of current is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification and the like. 
     Note that a voltage refers to a difference between potentials of two points, and a potential refers to electrostatic energy (electric potential energy) of a unit charge at a given point in an electrostatic field. Note that in general, a difference between a potential of one point and a reference potential (e.g., a ground potential) is merely called a potential or a voltage, and a potential and a voltage are used as synonymous words in many cases. Thus, in this specification, a potential may be rephrased as a voltage and a voltage may be rephrased as a potential unless otherwise specified. 
     In this specification, in the case where an etching step is performed after a photolithography process, a mask formed in the photolithography process is removed. 
     Embodiment 1 
     In this embodiment, a display device that is one embodiment of the present invention and a manufacturing method thereof will be described with reference to drawings. 
     Structural Example of Display Device 
       FIG. 1A  is a block diagram of an example of a display device. The display device illustrated in  FIG. 1A  includes a pixel portion  600 , a scan line driver circuit  604 , a signal line driver circuit  606 , m scan lines  607  arranged parallel or substantially parallel to each other, whose potentials are controlled by the scan line driver circuit  604 , and n signal lines  609  arranged parallel or substantially parallel to each other, whose potentials are controlled by the signal line driver circuit  606 . The pixel portion  600  includes a plurality of pixels  601  arranged in a matrix. The scan line driver circuit  604  and the signal line driver circuit  606  are collectively referred to as a driver circuit portion in some cases. 
     Each scan line  607  is electrically connected to the n pixels  601  in the corresponding row among the pixels  601  arranged in m rows and n columns in the pixel portion  600 . Each signal line  609  is electrically connected to the m pixels  601  in the corresponding column among the pixels  601  arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each capacitor line  615  is electrically connected to the n pixels  601  in the corresponding row among the pixels  601  arranged in m rows and n columns. Note that in the case where the capacitor lines  615  are arranged in parallel or almost in parallel along the signal lines  609 , each capacitor line  615  is electrically connected to the m pixels  601  in the corresponding column among the pixels  601  arranged in m rows and n columns. 
     Configuration Example of Pixel 
       FIG. 1B  illustrates an example of a circuit configuration that can be used for the pixel  601  in the display device illustrated in  FIG. 1A . 
     The pixel  601  illustrated in  FIG. 1B  includes a transistor  400   b  serving as a selection transistor, a transistor  400   a  serving as a driver transistor, a capacitor  370 , and a light-emitting element  350 . 
     One of a source electrode and a drain electrode of the transistor  400   a  is electrically connected to one electrode of the light-emitting element  350 . The other of the source electrode and the drain electrode of the transistor  400   a  is electrically connected to an anode line (not illustrated) which is supplied with a high power supply potential. The transistor  400   a  also includes a pair of gate electrodes overlapping with each other with a semiconductor film provided therebetween. One gate electrode of the transistor  400   a  is electrically connected to the other gate electrode of the transistor  400   a , one of a source electrode and a drain electrode of the transistor  400   b , and one electrode of the capacitor  370 . A gate electrode of the transistor  400   b  is electrically connected to the scan line  607 . The other of the source electrode and the drain electrode of the transistor  400   b  is electrically connected to the signal line  609 . The other electrode of the capacitor  370  is electrically connected to the capacitor line  615 . The other electrode of the light-emitting element  350  is electrically connected to a cathode line (not illustrated) which is supplied with a low power supply potential. 
     The transistor  400   a  has a function of controlling current flowing in the light-emitting element  350  by being turned on or off. 
     To obtain sufficient luminance for the light-emitting element  350 , a transistor having high on-state current needs to be used as the transistor  400   a  serving as a driver transistor. Further, to improve the drive frequency of the display device and achieve smoother image display, the transistor needs to have high field-effect mobility. 
     Thus, as the transistor  400   a  in the display device of this embodiment, a transistor is used whose channel length is greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Because the on-state current of a transistor increases as the ratio of the channel length to the channel width (L/W ratio) becomes smaller, the on-state current of the transistor  400   a  can be improved when the channel length is reduced to about the above ranges. Alternatively, when the channel length is reduced to about the above ranges and the channel width is also reduced, the transistor can be reduced in size while the on-state current is kept constant, leading to an improvement in the aperture ratio of the pixel. 
     The transistor  400   a  further includes an oxide semiconductor film in which a channel is formed, and a first gate electrode and a second gate electrode overlapping with each other with the oxide semiconductor film provided therebetween. The first gate electrode and the second gate electrode included in the transistor  400   a  are electrically connected to each other. As described above, the oxide semiconductor film is sandwiched between the pair of gate electrodes electrically connected to each other, whereby the pair of gate electrodes can be supplied with the same potential unlike in the case where a constant potential is supplied to only one of the pair of gate electrodes; thus, a channel formation region can be increased, resulting in an increase in the drain current of the transistor  400   a . Accordingly, the transistor  400   a  can be reduced in size while a reduction in the on-state current is suppressed. 
     In addition, to make a connection between the second gate electrode and the first gate electrode, the second gate electrode is positioned so as to overlap with at least one side surface of the oxide semiconductor film in the channel width direction. Accordingly, an electric field is also applied to the side surface of the oxide semiconductor film in the channel width direction, so that a region where current flows can be increased. Thus, the field-effect mobility of the transistor  400   a  can be improved. 
     When the pair of gate electrodes electrically connected to each other is provided, a depletion layer is easily formed in the oxide semiconductor film, which enables the subthreshold characteristics of the transistor  400   a  to be improved. 
     A reduction in the channel length shifts the threshold voltage of the transistor in the negative direction in some cases. However, the transistor  400   a  includes, in addition to the first gate electrode, the second gate electrode (back gate electrode) on a back channel region side; this can prevent negative charge from generating in the back channel region and can suppress a shift in the threshold voltage of the transistor in the negative direction. 
     The transistor  400   b  has a function of controlling writing of a data signal by being turned on or off. 
     A transistor having positive threshold voltage (normally-off characteristics) is preferably used as the transistor  400   b . The cutoff current of the transistor is preferably reduced. 
     As described above, the transistor with a short channel length can have high on-state current; at the same time, the threshold voltage of the transistor might shift in the negative direction (negative shift) in some cases. In the display device of this embodiment, the channel length of the transistor  400   a , which serves as a driver transistor that needs to have high on-state current and high field-effect mobility, is set to greater than or equal to 0.5 μm and less than or equal to 4.5 μm, and a pair of gate electrodes electrically connected to each other is provided; thus, the on-state current and the field-effect mobility are improved and a negative shift in the threshold voltage is suppressed. 
     In contrast, because the transistor  400   b  serving as a selection transistor does not require field-effect mobility as high as that of the transistor  400   a , the channel length of the transistor  400   b  is made longer than that of the transistor  400   a . As a result, a negative shift in the threshold voltage of the transistor  400   b  is suppressed. With such a structure, the display device can operate at high speed with low power consumption. 
     In the case where the channel length of the transistor  400   a  is set greater than or equal to 0.5 μm and less than or equal to 4.5 μm, for example, the channel length of the transistor  400   b  can be 6 μm. Note that the channel length of the transistor  400   b  can be set as appropriate depending on characteristics required for the display device, as long as it is at least longer than the channel length of the transistor  400   a.    
     Further, the cutoff current of the transistor  400   b  is preferably lower than that of the transistor  400   a . When the ratio of the channel length to the channel width (L/W ratio) of the transistor  400   b  is made larger than the L/W ratio of the transistor  400   a , the cutoff current of the transistor  400   b  can be lower than that of the transistor  400   a , for example. In the case where the channel widths of the transistor  400   a  and the transistor  400   b  are substantially the same, the channel length of the transistor  400   b  is made longer than that of the transistor  400   a , whereby the cutoff current of the transistor  400   b  can be reduced. 
     Similarly to the transistor  400   a , the transistor  400   b  may include a pair of gate electrodes electrically connected to each other. However, in the case where the transistor  400   b  has such a structure, a region where the pair of gate electrodes is connected to each other is necessary, which results in an increase in the area of the transistor  400   b  and a reduction in the aperture ratio of the pixel. For this reason, the transistor  400   b  preferably has a single-gate structure. In the case of a large display device, the parasitic capacitance of a gate wiring of the transistor  400   b  affects the operation speed of the display device. For this reason, the transistor  400   b  preferably has a single-gate structure in which the parasitic capacitance of the gate wiring is small. 
     As the light-emitting element  350 , an organic electroluminescent element (organic EL element) or an inorganic EL element can be used, for example. 
     &lt;Circuit Symbol of Transistor&gt; 
     Here,  FIG. 10A  illustrates a circuit symbol of the transistor in this specification, which includes the pair of gate electrodes overlapping with each other with the oxide semiconductor film provided therebetween. In the circuit symbol in  FIG. 10A , the pair of gate electrodes, the source electrode, and the drain electrode are represented by FG and BG, S, and D, respectively. 
     FIG.  10 B 2  is an example of a cross-sectional view of the transistor  400   a  that can be represented by a circuit symbol in FIG.  10 B 1 . In the transistor  400   a  illustrated in FIG.  10 B 2 , a pair of electrodes  20   a  and  20   b  serving as a source electrode and a drain electrode partly overlaps with a gate electrode  31  over an oxide semiconductor film  17   a . In the circuit symbol in FIG.  10 B 1 , similarly in the circuit symbol in  FIG. 10A , the pair of gate electrodes, the source electrode, and the drain electrode are represented by FG and BG, S, and D, respectively. 
     In the channel length direction of the transistor  400   a  illustrated in FIG.  10 B 2 , a distance Wsd between the pair of electrodes  20   a  and  20   b  is shorter than a distance Wbg between end portions of the gate electrode  31 . Further, in the cross-sectional view in the channel length direction, the end portions of the gate electrode  31  overlap with the pair of electrodes  20   a  and  20   b.    
     FIG.  10 C 2  is an example of a cross-sectional view of the transistor  400   a  that can be represented by a circuit symbol in FIG.  10 C 1 . In the transistor  400   a  illustrated in FIG.  10 C 2 , the pair of electrodes  20   a  and  20   b  serving as a source electrode and a drain electrode does not overlap with the gate electrode  31  over the oxide semiconductor film  17   a . In the circuit symbol in FIG.  10 C 1 , similarly in the circuit symbol in  FIG. 10A , the pair of gate electrodes, the source electrode, and the drain electrode are represented by FG and BG, S, and D, respectively. 
     In the channel length direction of the transistor  400   a  illustrated in FIG.  10 C 2 , the distance Wsd between the pair of electrodes  20   a  and  20   b  is longer than the distance Wbg between end portions of the gate electrode  31 . Further, in the cross-sectional view in the channel length direction, the end portions of the gate electrode  31  do not overlap with the pair of electrodes  20   a  and  20   b.    
     In the drawings attached to this specification, the circuit symbol in  FIG. 10A  can represent the transistor  400   a  having a structure represented by the circuit symbol in FIG.  10 B 1  and the transistor  400   a  having a structure represented by the circuit symbol in FIG.  10 C 1 . 
     Structural Example of Transistor Included in Pixel 
     Next, a specific structure of the transistor included in the pixel of the display device will be described. 
     FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 , and  2 C 2  are top views and cross-sectional views of the transistor  400   a  and the transistor  400   b  included in the pixel  601  of the display device. FIG.  2 A 1  is a top view of the transistor  400   a  serving as a driver transistor. FIG.  2 A 2  is a top view of the transistor  400   b  serving as a selection transistor.  FIG. 2B  shows cross-sectional views taken along dashed-dotted line A 1 -B 1  in FIG.  2 A 1  and dashed-dotted line A 2 -B 2  in FIG.  2 A 2 . FIG.  2 C 1  is a cross-sectional view taken along dashed-dotted line C 1 -D 1  in FIG.  2 A 1 . FIG.  2 C 2  is a cross-sectional view taken along dashed-dotted line C 2 -D 2  in FIG.  2 A 2 . Note that in FIGS.  2 A 1  and  2 A 2 , a substrate  11 , insulating films, and the like are omitted for simplicity. 
     The transistor  400   a  illustrated in FIGS.  2 A 1 ,  2 B, and  2 C 1  is a channel-etched transistor. The transistor  400   a  includes a gate electrode  13   a  provided over the substrate  11 , an insulating film  15  formed over the substrate  11  and the gate electrode  13   a , the oxide semiconductor film  17   a  overlapping with the gate electrode  13   a  with the insulating film  15  provided therebetween, and the pair of electrodes  20   a  and  20   b  in contact with the oxide semiconductor film  17   a . The transistor  400   a  also includes an insulating film  28  that is composed of an oxide insulating film  23 , an oxide insulating film  25 , and a nitride insulating film  27  and is over the insulating film  15 , the oxide semiconductor film  17   a , and the pair of electrodes  20   a  and  20   b ; and the gate electrode  31  formed over the insulating film  28 . The gate electrode  31  is connected to the gate electrode  13   a  through opening portions  42  and  43  provided in the insulating film  15  and the insulating film  28 . Furthermore, an electrode  32  connected to one of the pair of electrodes  20   a  and  20   b  (here, the electrode  20   b ) is formed over the nitride insulating film  27 . Note that the electrode  32  serves as a pixel electrode. 
     The transistor  400   b  illustrated in  FIGS. 2B  and  2 C 2  is a channel-etched transistor. The transistor  400   b  includes a gate electrode  13   b  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   b , an oxide semiconductor film  17   b  overlapping with the gate electrode  13   b  with the insulating film  15  provided therebetween, and a pair of electrodes  20   c  and  20   d  in contact with the oxide semiconductor film  17   b . The transistor  400   b  also includes an insulating film  28  that is composed of the oxide insulating film  23 , the oxide insulating film  25 , and the nitride insulating film  27  and is over the insulating film  15 , the oxide semiconductor film  17   b , and the pair of electrodes  20   c  and  20   d.    
     In each of the transistor  400   a  and the transistor  400   b , the insulating film  15  serves as a gate insulating film (in the transistor  400   a , a first gate insulating film). Further, the insulating film  28  serves as a second gate insulating film in the transistor  400   a  and as a protective insulating film in the transistor  400   b.    
     The transistor  400   a  of this embodiment has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Furthermore, in the channel width direction of the transistor  400   a , the oxide semiconductor film  17   a  is provided between the gate electrode  13   a  and the gate electrode  31  with the insulating film  15  provided between the gate electrode  13   a  and the oxide semiconductor film  17   a  and with the insulating film  28  provided between the gate electrode  31  and the oxide semiconductor film  17   a . In addition, as illustrated in FIG.  2 A 1 , the gate electrode  31  overlaps with end portions of the oxide semiconductor film  17   a  with the insulating film  28  provided therebetween, when seen from the above. 
     The oxide semiconductor film  17   a  included in the transistor  400   a  has a structure in which side surfaces in the channel length direction overlap with the pair of electrodes  20   a  and  20   b  and side surfaces in the channel width direction overlap with the gate electrode  31 . In the end portion of the oxide semiconductor film  17   a , metal elements constituting an oxide semiconductor are easily bonded to chlorine radicals, fluorine radicals, and the like generated from an etching gas when the oxide semiconductor film  17   a  is exposed to plasma at the time of etching treatment for processing into an island shape. Thus, in the end portions of the oxide semiconductor film  17   a , oxygen bonded to the metal element is easily eliminated, so that an oxygen vacancy is easily formed and the end portions easily become n-type in some cases. In particular, when regions surrounded by dashed lines  33  and  34  in the end portions of the oxide semiconductor film  17   a  have n-type conductivity, leakage current easily flows between the pair of electrodes  20   a  and  20   b  through the regions. In the transistor  400   a , the regions overlap with the gate electrode  31 , so that an electric field applied to the regions can be controlled by controlling the potential of the gate electrode  31  (including the gate electrode  13   a  at the same potential as the gate electrode  31 ). Accordingly, leakage current which might flow between the pair of electrodes  20   a  and  20   b  can be controlled by a potential applied to the pair of gate electrodes even when the end portions of the oxide semiconductor film  17   a  are n-type. 
     Specifically, in the case where a potential with which the transistor  400   a  is turned off is applied to the pair of gate electrodes, off-state current that flows between the pair of electrodes  20   a  and  20   b  through the end portions of the oxide semiconductor film  17   a  surrounded by the dashed lines  33  and  34  can be low. Thus, even when the channel length of the transistor  400   a  is made short to obtain high on-state current, which causes the distance between the pair of electrodes  20   a  and  20   b  in the end portions of the oxide semiconductor film  17   a  to be short, off-state current can be low. That is, high on-state current can be obtained in the case where the transistor  400   a  is on, and off-state current can be low in the case where the transistor  400   a  is off. 
     Further, the transistor  400   b  described in this embodiment has a longer channel length than the transistor  400   a . Thus, a negative shift in the threshold voltage of the transistor  400   b  having a single-gate structure can be suppressed, so that cutoff current can be low. 
     A plurality of opening portions are provided in the insulating film  15  and the insulating film  28 . As a typical example, as illustrated in  FIG. 2B , an opening portion  41  that reaches one of the pair of electrodes  20   a  and  20   b  is provided. Further, the opening portions  42  and  43  are provided with the oxide semiconductor film  17   a  provided therebetween in the channel width direction as illustrated in FIG.  2 C 1 . In other words, the opening portions  42  and  43  are provided on outer sides of the side surfaces of the oxide semiconductor film  17   a . In the opening portion  41 , one of the pair of electrodes  20   a  and  20   b  (here, the electrode  20   b ) is connected to the electrode  32 . Further, in the opening portions  42  and  43 , the gate electrode  13   a  is connected to the gate electrode  31 . This means that the gate electrode  13   a  and the gate electrode  31  surround the oxide semiconductor film  17   a  in the channel width direction with the insulating film  15  and the insulating film  28  provided between the oxide semiconductor film  17   a  and each of the gate electrode  13   a  and the gate electrode  31 . Furthermore, the gate electrode  31  faces the side surfaces of the oxide semiconductor film  17   a  at side surfaces of the opening portions  42  and  43 . 
     Note that as illustrated in FIG.  2 C 1 , a distance d between the side surface of the oxide semiconductor film  17   a  in the channel width direction and the gate electrode  31  in the opening portion  42  or  43  is 1 to 7.5 times as long as the sum of a thickness t1 of the insulating film  15  and a thickness t2 of the insulating film  28 . In the case where the distance d between the side surface of the oxide semiconductor film  17   a  and the gate electrode  31  in the opening portion  42  or  43  is equal to or longer than the sum of the thickness t1 of the insulating film  15  and the thickness t2 of the insulating film  28 , an electric field from the gate electrode  31  affects the side surface of the oxide semiconductor film  17   a  or the end portion including the side surface and its vicinity as indicated by lines  444  of electric force in  FIG. 2D . Thus, a parasitic channel can be prevented from being generated at the side surface of the oxide semiconductor film  17   a  or the end portion thereof. In contrast, in the case where the distance d between the side surface of the oxide semiconductor film  17   a  and the gate electrode  31  in the opening portion  42  or  43  is 7.5 or less times as long as the sum of the thickness t1 of the insulating film  15  and the thickness t2 of the insulating film  28 , the area of the transistor can be smaller. 
     The oxide semiconductor film  17   a  included in the transistor  400   a  and the oxide semiconductor film  17   b  included in the transistor  400   b  can be formed in the same manufacturing process. The oxide semiconductor film  17   a  and the oxide semiconductor film  17   b  are each formed using a metal oxide containing at least In or Zn; as a typical example, an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M is Al, Ga, Y, Zr, La, Ce, or Nd) can be given. 
     Note that in the case where the oxide semiconductor films  17   a  and  17   b  each contain an In-M-Zn oxide, the proportion of In and the proportion of M, not taking Zn and O into consideration, are greater than or equal to 25 atomic % and less than 75 atomic %, respectively, preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively. 
     The energy gap of each of the oxide semiconductor films  17   a  and  17   b  is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistors  400   a  and  400   b  can be reduced. 
     The thickness of each of the oxide semiconductor films  17   a  and  17   b  is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm. 
     In the case where the oxide semiconductor films  17   a  and  17   b  each contains an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd), it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In-M-Zn oxide satisfy In≧M and Zn≧M. As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that the atomic ratios of metal elements in the formed oxide semiconductor films  17   a  and  17   b  vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error. 
     An oxide semiconductor film with low carrier density is used for the oxide semiconductor films  17   a  and  17   b . For example, an oxide semiconductor film whose carrier density is 1×10 17 /cm 3  or lower, preferably 1×10 15 /cm 3  or lower, further preferably 1×10 13 /cm 3  or lower, still further preferably 1×10 11 /cm 3  or lower is used for the oxide semiconductor films  17   a  and  17   b.    
     Note that, without limitation to that described above, a material with an appropriate composition may be used for each of the oxide semiconductor films  17   a  and  17   b  depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of the transistors. Further, in order to obtain required semiconductor characteristics of the transistors, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of each of the oxide semiconductor films  17   a  and  17   b  be set to be appropriate. 
     Note that it is preferable to use, as the oxide semiconductor films  17   a  and  17   b , oxide semiconductor films in which the impurity concentration is low and density of defect states is low, in which case the transistors can have more excellent electrical characteristics. Note that examples of the impurities include hydrogen, nitrogen, an alkali metal, and an alkaline earth metal. In this specification, the state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus has a low carrier density. Thus, a transistor including the oxide semiconductor film in which a channel region is formed is likely to have positive threshold voltage (normally-off characteristics). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has few carrier traps in some cases. Furthermore, a transistor including the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10 −13  A, at voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. Thus, the transistor whose channel region is formed in the oxide semiconductor film has a small variation in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. 
     Hydrogen contained in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and oxygen reacted with hydrogen is released from a metal atom, whereby an oxygen vacancy is formed in a lattice (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Further, in some cases, bonding of part of hydrogen to oxygen bonded to a metal element causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor that contains hydrogen is likely to be normally on. 
     Accordingly, it is preferable that hydrogen be reduced as much as possible as well as the oxygen vacancies in the oxide semiconductor films  17   a  and  17   b . Specifically, the hydrogen concentration of the oxide semiconductor films  17   a  and  17   b , which is measured by secondary ion mass spectrometry (SIMS), is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , further preferably lower than or equal to 1×10 19  atoms/cm 3 , even further preferably lower than or equal to 5×10 18  atoms/cm 3 , still further preferably lower than or equal to 1×10 18  atoms/cm 3 , yet still further preferably lower than or equal to 5×10 17  atoms/cm 3 , even further more preferably lower than or equal to 1×10 16  atoms/cm 3 . 
     When silicon or carbon that is one of elements belonging to Group 14 is contained in the oxide semiconductor films  17   a  and  17   b , oxygen vacancies are increased in the oxide semiconductor films  17   a  and  17   b , and the oxide semiconductor films  17   a  and  17   b  become n-type films. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) of the oxide semiconductor films  17   a  and  17   b  is lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     Further, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor films  17   a  and  17   b , which is measured by SIMS, is lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Thus, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the oxide semiconductor films  17   a  and  17   b.    
     Further, when containing nitrogen, the oxide semiconductor films  17   a  and  17   b  easily become n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor that contains nitrogen is likely to be normally on. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen that is measured by SIMS is preferably set to, for example, lower than or equal to 5×10 18  atoms/cm 3 . 
     When the concentration of an impurity contained in the oxide semiconductor films  17   a  and  17   b  is reduced to the above value, the transistors  400   a  and  400   b  can each have positive threshold voltage (normally-off characteristics). 
     The oxide semiconductor films  17   a  and  17   b  may each have a non-single-crystal structure, for example. The non-single-crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) that is described later, a polycrystalline structure, a microcrystalline structure described later, or an amorphous structure, for example. Among the non-single-crystal structure, the amorphous structure has the highest density of defect levels, whereas CAAC-OS has the lowest density of defect levels. 
     It is preferable to form the oxide semiconductor film  17   a  using a CAAC-OS film because a channel-etched transistor can be manufactured even when its channel length is, typified by as short as greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. 
     Note that the oxide semiconductor films  17   a  and  17   b  may each be a mixed film including two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure. The mixed film includes, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Further, the mixed film has a layered structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. 
     A transistor including an oxide semiconductor film is an accumulation-type transistor. Here, carrier flow in the transistor including an oxide semiconductor film in an off state and in an on state is described with reference to schematic diagrams illustrated in  FIGS. 36A to 36C .  FIGS. 36A and 36B  are cross-sectional views in the channel length direction and  FIG. 36C  is a cross-sectional view in the channel width direction. 
     The transistor including an oxide semiconductor film illustrated in  FIGS. 36A to 36C  includes a gate electrode GE_ 1 , a gate insulating film GI_ 1  over the gate electrode GE_ 1 , an oxide semiconductor film OS over the gate insulating film GI_ 1 , electrodes S and D over the oxide semiconductor film OS, a gate insulating film GI_ 2  over the oxide semiconductor film OS and the electrodes S and D, and a gate electrode GE_ 2  over the gate insulating film GI_ 2 . The oxide semiconductor film OS includes a channel region i, and low-resistance regions n+ in contact with the electrodes S and D. The gate electrode GE_ 1  and the gate electrode GE_ 2  are connected to each other as illustrated in  FIG. 36C . 
     In the case where the transistor is in an off state, electrons are forced away from the channel region i of the oxide semiconductor film OS when negative voltage is applied to the gate electrodes GE_ 1  and GE_ 2 , so that the channel region i is completely depleted as illustrated in  FIG. 36A . As a result, the off-state current of the transistor becomes extremely low. 
     In contrast, in the case of an on state, electrons are accumulated from the low-resistance region n +  in contact with the electrode S to the low-resistance region n +  in contact with the electrode D, and a current path is formed as indicated by arrows in  FIG. 36B . As illustrated in  FIG. 36C , the gate electrodes GE_ 1  and GE_ 2  are set to the same potential, side surfaces of the oxide semiconductor film OS face the gate electrode GE_ 2 , and the gate electrodes GE_ 1  and GE_ 2  surround the oxide semiconductor film OS in the channel width direction with the gate insulating film GI_ 1  provided between the oxide semiconductor film OS and the gate electrode GE_ 1  and the gate insulating film GI_ 2  provided between the oxide semiconductor film OS and the gate electrode GE_ 2 . In that case, carriers flow not only at the interfaces between the oxide semiconductor film OS and the gate insulating films GI_ 1  and GI_ 2  but also in a wide region in the oxide semiconductor film OS as illustrated in  FIG. 36B , which results in an increase in the amount of transferred carriers in the transistor. As a result, the on-state current of the transistor is increased, and the field-effect mobility is increased to greater than or equal to 10 cm 2 /V·s or to greater than or equal to 20 cm 2 /V·s, for example. Note that here, the field-effect mobility is not an approximate value of the mobility as the physical property of the oxide semiconductor film but is the field-effect mobility of a saturation region of the transistor. Note that an increase in field-effect mobility becomes significant when the channel length (also referred to as L length) of the transistor is greater than or equal to 0.5 μm and less than or equal to 6.5 μm, preferably greater than 1 μm and less than 6 μm, further preferably greater than 1 μm and less than or equal to 4 μm, still further preferably greater than 1 μm and less than or equal to 3.5 μm, yet still further preferably greater than 1 μm and less than or equal to 2.5 μm. Further, with a short channel length greater than or equal to 0.5 μm and less than or equal to 6.5 μm, the channel width can also be short, in which case the area of the transistor can be reduced even when a region which is a connection portion of the gate electrode GE_ 1  and the gate electrode GE_ 2  is provided as illustrated in FIG.  36 C. 
     The transistor  400   a  illustrated in FIGS.  2 A 1 ,  2 B,  2 C 1 , and  2 D includes the gate electrode  13   a  and the gate electrode  31 , each of which has a function of blocking an external electric field; thus, fixed charges between the substrate  11  and the gate electrode  13   a  and over the gate electrode  31  do not affect the oxide semiconductor film  17   a . Thus, degradation due to a stress test (e.g., a negative gate bias temperature (−GBT) stress test in which a negative potential is applied to a gate electrode) can be reduced, and changes in the rising voltages of on-state current at different drain voltages can be suppressed. 
     The BT stress test is one kind of accelerated test and can evaluate, in a short time, change in characteristics (i.e., a change over time) of transistors, which is caused by long-term use. In particular, the amount of change in the threshold voltage of the transistor between before and after the BT stress test is an important indicator when examining the reliability of the transistor. As the amount of change in the threshold voltage between before and after the BT stress test is small, the transistor has higher reliability. 
     When the channel length of a transistor is reduced, threshold voltage might shift in the negative direction. However, in the display device of this embodiment, the transistor  400   b  serving as a selection transistor in the pixel has a longer channel length than the transistor  400   a  serving as a driver transistor, whereby the display device can operate at high speed with low power consumption. 
     Further, the insulating film  28  over the oxide semiconductor films  17   a  and  17   b  preferably includes an oxide insulating film containing oxygen at higher proportion than the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film containing oxygen at higher proportion than the stoichiometric composition. The oxide insulating film containing oxygen at higher proportion than the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 18  atoms/cm 3 , or greater than or equal to 3.0×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS) analysis performed at a surface temperature higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. 
     In the case where the insulating film  28  includes the oxide insulating film containing oxygen at higher proportion than the stoichiometric composition, part of oxygen contained in the insulating film  28  can be transferred to the oxide semiconductor films  17   a  and  17   b  to reduce oxygen vacancies that might be in the oxide semiconductor films. 
     In a transistor formed using an oxide semiconductor film including oxygen vacancies, the threshold voltage is likely to shift in the negative direction to have normally-on characteristics. This is because charges are generated because of oxygen vacancies in the oxide semiconductor film and the resistance of the oxide semiconductor film is thus reduced. The transistor having normally-on characteristics causes various problems in that malfunction is likely to be caused when in operation and that power consumption is increased when not in operation. Furthermore, there is a problem in that the amount of change in electrical characteristics, typified by threshold voltage, of the transistor is increased with passage of time or by a stress test. 
     However, in the transistor  400   a  and the transistor  400   b  described in this embodiment, an oxide insulating film containing oxygen at higher proportion than the stoichiometric composition is included in the insulating film  28  provided over the oxide semiconductor films  17   a  and  17   b ; thus, oxygen contained in the insulating film  28  can be transferred to the oxide semiconductor films  17   a  and  17   b  to reduce oxygen vacancies in the oxide semiconductor films. In addition, because the insulating film  28  is not exposed to an etching atmosphere, the insulating film  28  has few defects. As a result, the transistors have normally-off characteristics. Further, the amount of change in electrical characteristics, typified by threshold voltage with respect to operation time, of the transistors with passage of time or due to a stress test can be reduced. Furthermore, a change in the threshold voltage can be reduced even when a stress test is repeatedly performed. 
     It is also effective for an improvement in electrical characteristics that the transistor  400   a  and the transistor  400   b  are channel-etched transistors. Here, a channel-etched transistor and a channel protective transistor are compared. In a channel protective transistor including two gate electrodes with an oxide semiconductor film provided therebetween, for example, a first gate insulating film is provided over a first gate electrode, the oxide semiconductor film is provided over the first gate insulating film, a channel protective film is provided over the oxide semiconductor film, a pair of electrodes in contact with the oxide semiconductor film is provided over the channel protective film, a second gate insulating film is provided over the channel protective film and the pair of electrodes, and a second gate electrode is provided over the second gate insulating film. 
     The channel protective film is damaged by exposure to plasma in an etching process for forming the pair of electrodes. Thus, defects are easily formed in the channel protective film. As a result, carriers that flow in the oxide semiconductor film are trapped by the defects in the channel protective film and the electrical characteristics of the transistor fluctuate over operation time, resulting in a reduction in the reliability. However, in the transistor  400   a  and the transistor  400   b  described in this embodiment, which are channel-etched transistors, a region in the insulating film  28 , which overlaps with the oxide semiconductor film  17   a  or the oxide semiconductor film  17   b  is not exposed to an etching atmosphere. Thus, there are few defects in the insulating film  28  and the transistor has high reliability. 
     Further, in the channel protective transistor, the pair of electrodes blocks an electric field applied from the second gate electrode to regions in the oxide semiconductor film which overlap with the pair of electrodes, so that the electric field from the second gate electrode does not evenly affect the oxide semiconductor film. As a result, the amount of carriers that flow in the oxide semiconductor film when induced by the electric field from the second gate electrode is reduced. In contrast, in the transistor  400   a  described in this embodiment, which is a channel-etched transistor, the electric field from the gate electrode  31  evenly affects a back channel of the oxide semiconductor film  17   a . Further, the electric field from the gate electrode  31  also affects the side surface of the oxide semiconductor film  17   a . As a result, carriers flow in a wide region in the oxide semiconductor film  17   a , so that the on-state current of the transistor and the field-effect mobility are increased. 
     Furthermore, in the channel protective transistor, one end portion of each of the pair of electrodes is positioned over the channel protective film to make a connection between the oxide semiconductor film and each of the pair of electrodes. The one end portion of each of the pair of electrodes is positioned on an inner side of a connection region of the oxide semiconductor film and each of the pair of electrodes. For this reason, in consideration of misalignment of a photomask, the distance between the connection regions of the oxide semiconductor film and the pair of electrodes needs to be designed to be long. In contrast, in a channel-etched transistor, an oxide semiconductor film is directly in contact with one end portion of each of a pair of electrodes. Thus, the distance between the pair of electrodes in the channel-etched transistor can make short easily in comparison with the channel protective transistor. In particular, in the display device of one embodiment of the present invention, the transistor  400   a  serving as a driver transistor in the pixel has a short channel length, in which case the display device can be manufactured with a high yield when a channel-etched transistor is used as the transistor  400   a.    
     Here, both of the transistor  400   a  and the transistor  400   b  are channel-etched transistors. The structure of the transistor  400   a  is different from that of the transistor  400   b  in that the channel length is short and the gate electrode  31  serving as a back gate electrode is included. Thus, the transistor  400   a  and the transistor  400   b  can be formed in the same process, which can lead to simplification of the manufacturing process of the display device. 
     &lt;Details of Components of Transistor&gt; 
     Components of the transistor  400   a  and the transistor  400   b  will be described in detail below. 
     There is no particular limitation on the property of a material and the like of the substrate  11  as long as the material has heat resistance enough to withstand at least later heat treatment. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate  11 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate  11 . Furthermore, any of these substrates further provided with a semiconductor element may be used as the substrate  11 . Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate  11 . In the case where a glass substrate is used as the substrate  11 , a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured. 
     Alternatively, a flexible substrate may be used as the substrate  11 , and the transistors  400   a  and  400   b  may be provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate  11  and the transistors  400   a  and  400   b . The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate  11  and transferred onto another substrate. In that case, the transistors  400   a  and  400   b  can be transferred to a substrate having low heat resistance or a flexible substrate. 
     The gate electrodes  13   a  and  13   b  can be formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing these metal elements in combination; or the like. Further, one or more metal elements selected from manganese and zirconium may be used. The gate electrodes  13   a  and  13   b  may each have a single-layer structure or a layered structure of two or more layers. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a two-layer structure in which a copper film is stacked over a titanium film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film that contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used. 
     The gate electrodes  13   a  and  13   b  can also be formed using a light-transmitting conductive material such as indium tin oxide (ITO), indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a layered structure formed using the above light-transmitting conductive material and the above metal element. 
     The insulating film  15  can be formed to have a single-layer structure or a layered structure using, for example, one or more of a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, a gallium oxide film, a Ga—Zn-based metal oxide film, and a silicon nitride film. 
     The insulating film  15  may be formed using a high-k material such as hafnium silicate (HfSiO x ), hafnium silicate to which nitrogen is added (HfSi x O y N z ), hafnium aluminate to which nitrogen is added (HfAl x O y N z ), hafnium oxide, or yttrium oxide, so that gate leakage current of the transistor can be reduced. 
     The thickness of the insulating film  15  is greater than or equal to 5 nm and less than or equal to 400 nm, preferably greater than or equal to 10 nm and less than or equal to 300 nm, more preferably greater than or equal to 50 nm and less than or equal to 250 nm. 
     The pair of electrodes  20   a  and  20   b  and the pair of electrodes  20   c  and  20   d  are formed with a single-layer structure or a layered structure using any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten and an alloy containing any of these metals as a main component. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is formed over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is formed over a titanium film, a two-layer structure in which a copper film is formed over a tungsten film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order, and the like can be given. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used. 
     The insulating film  28  includes the oxide insulating film  23  in contact with the oxide semiconductor films  17   a  and  17   b , the oxide insulating film  25  in contact with the oxide insulating film  23 , and the nitride insulating film  27  in contact with the oxide insulating film  25 . The insulating film  28  preferably includes at least an oxide insulating film containing oxygen at higher proportion than the stoichiometric composition. Here, as the oxide insulating film  23 , an oxide insulating film through which oxygen passes is formed. As the oxide insulating film  25 , an oxide insulating film containing oxygen at higher proportion than the stoichiometric composition is formed. As the nitride insulating film  27 , a nitride insulating film that blocks hydrogen and oxygen is formed. Although the insulating film  28  has a three-layer structure here, the insulating film  28  can have a single layer structure, a two-layer structure, or a layered structure including four or more layers as appropriate. Note that in these cases, at least an oxide insulating film containing oxygen at higher proportion than the stoichiometric composition is preferably included. 
     The oxide insulating film  23  is an oxide insulating film through which oxygen passes. Thus, oxygen released from the oxide insulating film  25  provided over the oxide insulating film  23  can be moved to the oxide semiconductor films  17   a  and  17   b  through the oxide insulating film  23 . Moreover, the oxide insulating film  23  also serves as a film which relieves damage to the oxide semiconductor films  17   a  and  17   b  at the time of forming the oxide insulating film  25  later. 
     A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the oxide insulating film  23 . Note that in this specification, a “silicon oxynitride film” refers to a film that contains oxygen at a higher proportion than nitrogen, and a “silicon nitride oxide film” refers to a film that contains nitrogen at a higher proportion than oxygen. 
     Further, it is preferable that the amount of defects in the oxide insulating film  23  be small; as a typical example, the spin density of a signal that appears at g=2.001 due to a dangling bond of silicon is preferably lower than or equal to 3×10 17  spins/cm 3  by electron spin resonance (ESR) measurement. This is because if the density of defects in the oxide insulating film  23  is high, oxygen is bonded to the defects and the amount of oxygen that passes through the oxide insulating film  23  is reduced. 
     Further, it is preferable that the amount of defects at the interfaces between the oxide insulating film  23  and each of the oxide semiconductor films  17   a  and  17   b  be small; as a typical example, the spin density of a signal that appears at g=1.93 due to an oxygen vacancy in the oxide semiconductor films  17   a  and  17   b  is preferably lower than or equal to 1×10 17  spins/cm 3 , more preferably lower than or equal to the lower limit of detection by ESR measurement. 
     Note that in the oxide insulating film  23 , all oxygen that enters the oxide insulating film  23  from the outside is transferred to the outside of the oxide insulating film  23  in some cases. Alternatively, some oxygen that enters the oxide insulating film  23  from the outside remains in the oxide insulating film  23 . Furthermore, movement of oxygen occurs in the oxide insulating film  23  in some cases in such a manner that oxygen enters the oxide insulating film  23  from the outside and oxygen contained in the oxide insulating film  23  is transferred to the outside of the oxide insulating film  23 . 
     The oxide insulating film  25  is formed in contact with the oxide insulating film  23 . The oxide insulating film  25  is formed using an oxide insulating film that contains oxygen at higher proportion than the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film that contains oxygen at higher proportion than the stoichiometric composition. The oxide insulating film containing oxygen at higher proportion than the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 18  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3 , in TDS analysis performed at a surface temperature higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. 
     A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 50 nm and less than or equal to 400 nm can be used for the oxide insulating film  25 . 
     Further, it is preferable that the amount of defects in the oxide insulating film  25  be small; as a typical example, the spin density of a signal that appears at g=2.001 originating from a dangling bond of silicon is preferably lower than 1.5×10 18  spins/cm 3 , further preferably lower than or equal to 1×10 18  spins/cm 3  by ESR measurement. Note that the oxide insulating film  25  is provided more apart from the oxide semiconductor films  17   a  and  17   b  than the oxide insulating film  23  is; thus, the oxide insulating film  25  may have higher defect density than the oxide insulating film  23 . 
     The nitride insulating film  27  has an effect of blocking at least hydrogen and oxygen. Preferably, the nitride insulating film  27  has an effect of blocking oxygen, hydrogen, water, an alkali metal, an alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor films  17   a  and  17   b  and entry of hydrogen, water, or the like into the oxide semiconductor films  17   a  and  17   b  from the outside by providing the nitride insulating film  27  over the insulating film  28 . 
     The nitride insulating film  27  is formed using a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like having a thickness greater than or equal to 50 nm and less than or equal to 300 nm, preferably greater than or equal to 100 nm and less than or equal to 200 nm. 
     Note that instead of the nitride insulating film  27 , an oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like may be provided. As the oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, and hafnium oxynitride can be given. 
     In the transistor  400   a , a transparent conductive film is used for the gate electrode  31  and the electrode  32 . The transparent conductive film is formed using an indium tin oxide (hereinafter, also referred to as ITO), an indium zinc oxide, an indium oxide containing tungsten oxide, an indium zinc oxide containing tungsten oxide, an indium oxide containing titanium oxide, an indium tin oxide containing titanium oxide, an indium tin oxide containing silicon oxide, or the like. 
     &lt;Improvement in Current Drive Capability Due to Dual-Gate Driving&gt; 
     In the display device of this embodiment, a “dual-gate transistor” is used as the transistor  400   a  serving as a driver transistor in the pixel, which needs to have high on-state current and high field-effect mobility. In the dual-gate transistor, the gate electrode  13   a  and the gate electrode  31  overlapping with each other are provided over and below the oxide semiconductor film  17   a  where a channel is formed, respectively, and the gate electrodes are electrically connected to each other. The channel length of the dual-gate transistor is made as short as greater than or equal to 0.5 μm and less than or equal to 4.5 μm. Description is given below of an improvement in the current drive capability of a dual-gate transistor due to a short channel length L. In the transistor, gate electrodes facing each other with an oxide semiconductor film provided therebetween are connected to each other and have the same potential. 
     &lt;&lt;Saturation Mobility of Ideal Model&gt;&gt; 
     First, calculation was performed on an ideal model regardless of effects of an interface state, interface scattering, and the like.  FIG. 30  illustrates a model of a transistor used for the calculation. Note that device simulation software “Atlas” (manufactured by Silvaco Data Systems Inc.) was used for the calculation. 
     In the transistor illustrated in  FIG. 30 , a gate insulating film GI_ 1  is formed over a gate electrode GE_ 1 ; an oxide semiconductor film OS is formed over the gate insulating film GI_ 1 ; a source electrode S and a drain electrode D are formed over the gate insulating film GI_ 1  and the oxide semiconductor film OS; a gate insulating film GI_ 2  is formed over the oxide semiconductor film OS, the source electrode S, and the drain electrode D; and a gate electrode GE_ 2  is formed over the gate insulating film GI_ 2 . The gate electrode GE_ 1  is connected to the gate electrode GE_ 2  through an opening portion (not illustrated) formed in the gate insulating film GI_ 1  and the gate insulating film GI_ 2 . 
     Calculation conditions are shown in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Channel length 
                 2 μm, 3 μm, 6 μm, or 10 μm 
               
               
                 Channel width 
                  50 μm 
               
               
                 Thickness of GI_1 (T1) 
                 450 nm 
               
               
                 Thickness of GI_2 (T2) 
                 450 nm 
               
               
                 Dielectric constants of GI_1 and GI_2 
                 3.9 
               
               
                 (Er) 
               
               
                 Thickness of OS 
                  35 nm 
               
               
                 Mobility of OS 
                 10 cm 2 /Vsec 
               
               
                 Dielectric constant of OS (Er) 
                 15   
               
               
                 Donor density of OS under SD 
                 1 × 10 19 /cm 3   
               
               
                 Donor density of OS in channel region 
                 6.6 × 10 −9 /cm 3   
               
               
                 Drain voltage 
                 1 V or 10 V 
               
               
                   
               
            
           
         
       
     
     The gate electrode GE_ 1  and the gate electrode GE_ 2  are connected to each other and therefore have potentials equal to each other at all times. Furthermore, an effect in a channel width direction is not considered because a two-dimensional simulation was performed on the model. Saturation mobility μ FE  was obtained by substituting a value of V g −I d  characteristics at a drain voltage V d  of 10 V into Formula 1. Note that here, field-effect mobility in a saturation region is described as the saturation mobility. The maximum value of the saturation mobility obtained by calculation is an index of current drive capability in the saturation region (gate voltage V g &lt;drain voltage V d +threshold voltage V th ), and is not an approximate value of the mobility as the physical property of the oxide semiconductor film. 
     
       
         
           
             
               
                 
                   
                     μ 
                     FE 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             ∂ 
                             
                               
                                 I 
                                 d 
                               
                             
                           
                           
                             ∂ 
                             
                               V 
                               
                                 g 
                                 ⁢ 
                                 
                                     
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                     ⁢ 
                     
                       
                         2 
                         ⁢ 
                         L 
                       
                       
                         
                           C 
                           Bottom 
                         
                         ⁢ 
                         W 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Formula 1, W represents the channel width of a transistor and C Bottom  represents capacitance per unit area between the gate electrode GE_ 1  and the oxide semiconductor film OS. 
       FIG. 31A  shows calculation result of the dual-gate transistor, and  FIG. 31B  shows calculation result of the single-gate transistor that does not include a gate electrode GE_ 2 . 
       FIGS. 31A and 31B  show that the saturation mobility has a sharp peak in both of the dual-gate transistor and the single-gate transistor. The peak value of the saturation mobility is increased as the channel length L becomes shorter. 
     Description is given below of whether an increase in the saturation mobility with decreasing the channel length L corresponds to the improvement in the current drive capability of the transistor. 
       FIG. 32  shows on-state current versus channel length plots at gate voltages V g  of (V th +5) V and (V th +10) V, which are obtained from the calculation result of the ideal model. In  FIG. 32 , the top plots each show on-state current and the bottom plots each show a value obtained by multiplying the on-state current by the channel length. Note that in  FIG. 32 , the left plots show the calculation results at a drain voltage V d  of 1 V and the right plots show the calculation results at a drain voltage V d  of 10 V. 
     In  FIG. 32 , the on-state current is inversely proportional to the channel length L. This is because the on-state current is inversely proportional to the channel length L. 
     When the on-state current is completely inversely proportional to the channel length, a value obtained by multiplying the on-state current by the channel length is constant regardless of the channel length. In  FIG. 32 , a value obtained by multiplying the on-state current by the channel length at a drain voltage V d  of 1 V is substantially constant to the channel length L. In contrast, a value obtained by multiplying the on-state current by the channel length at a drain voltage V d  of 10 V is increased as the channel length L becomes shorter. This suggests that in the case of a drain voltage V d  of 10 V, an effective channel length which is described later is shorter than the channel length (a distance between the source electrode S and the drain electrode D) determined in  FIG. 30 . 
     &lt;&lt;Theory of Bulk Current&gt;&gt; 
     The reason why a peak of the saturation mobility is caused at low gate voltage in the transistor with the ideal model is described below. 
     It is assumed that in the transistor illustrated in  FIG. 30 , the density of electrons in the oxide semiconductor film OS is constant in the thickness direction of the oxide semiconductor film OS and represented by a value n 0 (y); y represents a given position in the oxide semiconductor film OS in the channel length direction. Potential φ in the thickness direction of the oxide semiconductor film OS is represented by Formula 2, which is constant. Note that it is assumed that a gate voltage V g   _   1  of the gate electrode GE_ 1  and a gate voltage V g   _   2  of the gate electrode GE_ 2  are equal to each other, and a flat band voltage on the gate electrode GE_ 1  side and a flat band voltage on the gate electrode GE_ 2  side are collectively referred to as a flat band voltage V FB .
 
φ− V ( y )= V   g   −V   FB   −V ( y )  [Formula 2]
 
     In this case, in an accumulation-type transistor including an oxide semiconductor film, a drain current I d  can be approximately represented by only a bulk current I bulk  as shown in Formula 3. 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       d 
                     
                     ≅ 
                     
                       I 
                       bulk 
                     
                   
                   = 
                   
                     
                       Wt 
                       
                         L 
                         eff 
                       
                     
                     ⁢ 
                     μ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       k 
                       B 
                     
                     ⁢ 
                     
                       T 
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               n 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               0 
                               ) 
                             
                           
                           - 
                           
                             
                               n 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               
                                 L 
                                 eff 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Note that in Formula 3, t represents the thickness of the oxide semiconductor film, μ represents the electron mobility of the oxide semiconductor film, k B  represents the Boltzmann&#39;s constant, T represents the absolute temperature, and L eff  represents an effective channel length. The effective channel length refers to the distance between an n region which extends under the source electrode and an n region which extends under the drain electrode in the oxide semiconductor film. The effective channel length becomes shorter than the channel length particularly in the case where the channel length is short or the drain voltage is high. 
     Note that n 0 (0) represents the electron density of a source-electrode-side end portion of a region determined by the effective channel length, which can be represented by Formula 4. Further, n 0 (L eff ) represents the electron density of a drain-electrode-side end portion of the region determined by the effective channel length, which can be represented by Formula 5. In Formulae 4 and 5, N D  represents the donor density of the channel region in the oxide semiconductor film and q represents elementary charge.
 
 n   0 (0)= N   D   e   qφ/k     B     T   =N   D   e   q(V     g     −V     FB     )/k     B     T   [Formula 4]
 
 n   0 ( L   eff )= N   D   e   q(φ−V     d     )/k     B     T   =N   D   e   q(V     g     −V     FB     −V     d     )/k     B     T   [Formula 5]
 
     In the case of a saturation region satisfying V d &gt;V g −V th  and V g &gt;V th , the drain voltage V d  can be replaced with V g −V th , so that Formula 3 is changed into Formula 6. 
     
       
         
           
             
               
                 
                   
                     I 
                     d 
                   
                   = 
                   
                     
                       Wt 
                       
                         L 
                         eff 
                       
                     
                     ⁢ 
                     μ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       k 
                       B 
                     
                     ⁢ 
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       N 
                       D 
                     
                     ⁢ 
                     
                       
                         e 
                         
                           
                             - 
                             q 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               V 
                               FB 
                             
                             / 
                             
                               k 
                               B 
                             
                           
                           ⁢ 
                           T 
                         
                       
                       ( 
                       
                         
                           e 
                           
                             q 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 V 
                                 g 
                               
                               / 
                               
                                 k 
                                 B 
                               
                             
                             ⁢ 
                             T 
                           
                         
                         - 
                         
                           e 
                           
                             q 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 V 
                                 th 
                               
                               / 
                               
                                 k 
                                 B 
                               
                             
                             ⁢ 
                             T 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     Saturation mobility μ FE   sat  to the drain current I d  obtained by Formula 6 is represented by Formula 7. 
     
       
         
           
             
               
                 
                   
                     
                       μ 
                       FE 
                       sat 
                     
                     ≡ 
                     
                       
                         
                           ( 
                           
                             
                               d 
                               ⁢ 
                               
                                 
                                   I 
                                   d 
                                 
                               
                             
                             
                               d 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 V 
                                 g 
                               
                             
                           
                           ) 
                         
                         2 
                       
                       ⁢ 
                       
                         
                           2 
                           ⁢ 
                           L 
                         
                         
                           
                             C 
                             GI 
                           
                           ⁢ 
                           W 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         Lt 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         μ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           q 
                           2 
                         
                         ⁢ 
                         
                           N 
                           D 
                         
                         ⁢ 
                         
                           e 
                           
                             
                               - 
                               q 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 V 
                                 FB 
                               
                               / 
                               
                                 k 
                                 B 
                               
                             
                             ⁢ 
                             T 
                           
                         
                       
                       
                         2 
                         ⁢ 
                         
                           L 
                           eff 
                         
                         ⁢ 
                         
                           C 
                           GI 
                         
                         ⁢ 
                         
                           k 
                           B 
                         
                         ⁢ 
                         T 
                       
                     
                     ⁢ 
                     
                       
                         e 
                         
                           q 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               V 
                               g 
                             
                             / 
                             
                               k 
                               B 
                             
                           
                           ⁢ 
                           T 
                         
                       
                       
                         1 
                         - 
                         
                           e 
                           
                             
                               
                                 q 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       V 
                                       g 
                                     
                                     - 
                                     
                                       V 
                                       th 
                                     
                                   
                                   ) 
                                 
                               
                               / 
                               
                                 k 
                                 B 
                               
                             
                             ⁢ 
                             T 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     In Formula 7, a denominator is 0 when V g  is V th , in which case the saturation mobility μ FE   sat  diverges to infinity. This is the cause of the peak of the saturation mobility at low gate voltage V g , which is shown in  FIGS. 31A and 31B . That is, the more the drain current is affected by the bulk current which flows inside the oxide semiconductor film OS, the clearer the peak is as shown in the saturation mobility observed in the case of a channel length of 2 μm in  FIGS. 31A and 31B . 
     In addition, the saturation mobility is probably increased when the effective channel length L eff  is shorter than the channel length L. In the oxide semiconductor film OS, when the n regions are formed in vicinities of regions of the oxide semiconductor film OS each of which is in contact with the source electrode S or the drain electrode D, the effective channel length L eff  becomes shorter than the channel length L, for example. This effect is apparent from the saturation mobility μ FE   sat  shown in Formula 7 that is proportional to L/L eff . 
     &lt;&lt;Current Density in Oxide Semiconductor Film&gt;&gt; 
     The effect of the bulk current on saturation mobility is a phenomenon peculiar to a transistor including an oxide semiconductor film, which is an accumulation-type device. The effect of the bulk current is small in a transistor including a silicon film as a semiconductor film, which is an inversion-type device. 
       FIGS. 33B and 33C  are plots of current density distributions obtained by device simulation.  FIG. 33A  illustrates V g −I d  characteristics obtained by calculation with a drain voltage of 10 V.  FIGS. 33B and 33C  each show a current density distribution in the oxide semiconductor film in a cross-sectional direction along A 1 -A 2  in  FIG. 30 .  FIG. 33B  shows the current density distribution in a saturation region (V g =0.5 V), and  FIG. 33C  shows the current density distribution in a linear region (V g =15 V). Note that in the calculation, a transistor had a channel length L of 2 μm and a channel width W of 50 μm, and the drain voltage V d  was 10 V. 
       FIG. 33B  shows that the current density distribution in the oxide semiconductor film OS is almost uniform in the saturation region (at low gate voltage V g ). In contrast,  FIG. 33C  shows that current flowing in the vicinity of a surface of the oxide semiconductor film OS is dominant in the linear region (at high gate voltage V g ). Since the current density distribution is almost uniform in the oxide semiconductor film OS in the saturation region as shown in  FIG. 33B , a cause of the peak in the saturation mobility is the bulk current. 
       FIGS. 34B and 34C  each show a current density distribution in a semiconductor film of an inversion-type device, which is obtained by device simulation.  FIGS. 34A to 34C  show the calculation results of a transistor obtained by replacing the oxide semiconductor film OS with a semiconductor (silicon) film including an n-p-n junction in the transistor illustrated in  FIG. 30 . A channel region of the semiconductor film is assumed to have acceptor impurities with a density of 1×17/cm 3 . 
       FIG. 34A  illustrates V g −I d  characteristics obtained by calculation with a drain voltage of 10 V.  FIGS. 34B and 34C  each show a current density distribution in the semiconductor film in a cross-sectional direction along A 1 -A 2  in  FIG. 30 .  FIG. 34B  shows the current density distribution in a saturation region (V g =0.5 V), and  FIG. 34C  shows the current density distribution in a linear region (V g =15 V). Note that in the calculation, a transistor had a channel length L of 2 μm and a channel width W of 50 μm, and the drain voltage V d  was 10 V. 
     Unlike in the transistor including the oxide semiconductor film that is an accumulation-type device, in the transistor including the semiconductor film that is an inversion-type device, current flowing through a surface of the semiconductor film is increased even at around the threshold voltage as shown in  FIG. 34B . The effect of the bulk current in the inversion-type device is smaller than that in the accumulation-type device. 
     The above results show that in a transistor including an oxide semiconductor film that is an accumulation-type device with the ideal model, the saturation mobility has a sharp peak due to the bulk current. Furthermore, because of the bulk current, the saturation mobility is improved as the channel length L becomes short. 
     As the channel length L becomes shorter, the peak value of the saturation mobility due to the bulk current is increased. This is probably because the effective channel length L eff  becomes shorter than the channel length L due to formation of the n regions in the vicinities of the regions of the oxide semiconductor film OS each of which is in contact with the source electrode S or the drain electrode D. Furthermore, it is probable that when the channel length L is short, the effective channel length L eff  becomes shorter than the channel length L by a phenomenon (conduction band lowering effect: CBL effect) in which the conduction band minimum (Ec) of the oxide semiconductor film OS is reduced and the conduction band minimum gets closer to the Fermi energy because of the effect of the source electrode S and the drain electrode D. As shown in Formula 7, as the effective channel length L eff  becomes short, the saturation mobility is increased in proportion to L/L eff . This effect becomes significant as the channel length L becomes short; therefore, it is probable that the saturation mobility is increased as the channel length L becomes short. 
     &lt;&lt;Model Assuming Shallow Electron Trap State&gt;&gt; 
     Next, to approximate the saturation mobility of the transistor with the ideal model to that of an actual transistor, calculation was performed on the assumption that there is an acceptor level that is negatively charged when trapping an electron, i.e., a shallow electron trap state, at the interface between the gate insulating film GI_ 1  and the oxide semiconductor film OS in the transistor with the ideal model. Calculation results are shown in  FIGS. 35A to 35C . 
       FIG. 35A  shows density of states (DOS) of the electron trap state which is assumed to be at the interface between the gate insulating film GI_ 1  and the oxide semiconductor film OS. 
     Next, the saturation mobilities of a dual-gate transistor and a single-gate transistor were calculated.  FIG. 35B  shows the calculation result of the dual-gate transistor, and  FIG. 35C  shows the calculation result of the single-gate transistor. 
     As shown in  FIGS. 35B and 35C , unlike in the saturation mobility of the transistor with the ideal model, a sharp peak is not observed in the saturation mobilities of the dual-gate transistor and the single-gate transistor. In  FIG. 35C , the peak values of the saturation mobilities are each around 5 cm 2 /V·sec, which shows that the saturation mobility of the transistor in the single-gate driving does not depend too much on the channel length L. In contrast, the peak values of the saturation mobilities of the dual-gate transistor are each greater than or equal to 15 cm 2 /V·sec and a little less than 20 cm 2 /V·sec, and are increased as the channel length L becomes short. These results are similar to those in Example described later. 
     Consequently, it is found that the saturation mobility is increased as the channel length L becomes short in a dual-gate transistor. 
     &lt;Comparison Between Channel-Etched Transistor and Channel-Protective Transistor Operated by Dual-Gate Driving&gt; 
     The field-effect mobility μ FE  and the on-state current I on  of a channel-etched transistor are compared with those of a channel protective transistor below using calculation results of the electrical characteristics of the transistors. Note that the transistors are operated by dual-gate driving. In each of the transistors, gate electrodes facing each other with an oxide semiconductor film provided therebetween are connected to each other and have the same potential. 
       FIG. 29A  illustrates a structure of the channel-protective transistor used for the calculation. Note that device simulation software “Atlas” (manufactured by Silvaco Data Systems Inc.) was used for the calculation. 
     In the channel-protective transistor, a gate insulating film GI_ 1  is formed over a gate electrode GE_ 1 , an oxide semiconductor film OS is formed over the gate insulating film GI_ 1 , and a source electrode S and a drain electrode D are formed over the gate insulating film GI_ 1  and the oxide semiconductor film OS. Note that a channel protective film CS is formed between the oxide semiconductor film OS and end portions of the source electrode S and the drain electrode D. In addition, a gate insulating film GI_ 2  is formed over the oxide semiconductor film OS, the source electrode S, the drain electrode D, and the channel protective film CS; and a gate electrode GE_ 2  is formed over the gate insulating film GI_ 2 . The gate electrode GE_ 1  is connected to the gate electrode GE_ 2  in an opening portion (not illustrated) formed in the gate insulating film GI_ 1  and the gate insulating film GI_ 2 . 
     In the channel-etched transistor, a channel protective film CS is not provided, and end portions of a source electrode S and a drain electrode D are in contact with an oxide semiconductor film OS. 
     Calculation conditions are shown in Table 2. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 Channel length (L) 
                  10 μm 
               
               
                 Channel width (W) 
                  50 μm 
               
               
                 Thickness of GI_1 (T1) 
                 450 nm 
               
               
                 Thickness of GI_2 (T2) 
                 450 nm 
               
               
                 Thickness of channel protective film CS (T3) 
                 100 nm 
               
               
                 Dielectric constants of GI_1, GI_2, and CS (Er) 
                 3.9 
               
               
                 Thickness of OS 
                  35 nm 
               
               
                 Mobility of OS 
                 10 cm 2 /Vsec 
               
               
                 Dielectric constant of OS (Er) 
                 15   
               
               
                 Donor density of OS under SD 
                 1 × 10 19 /cm 3   
               
               
                 Donor density of OS in channel region 
                 6.6 × 10 −9 /cm 3   
               
               
                   
               
            
           
         
       
     
     As a comparison example of the dual-gate transistor which is illustrated in  FIG. 29A , a transistor operated by a single-gate driving that does not include a gate electrode GE_ 2  was used and subjected to calculation in the manner similar to that of the dual-gate transistor. The single-gate transistor corresponds to the transistor  400   b  serving as a selection transistor illustrated in FIGS.  2 A 2 ,  2 B, and  2 C 2 . 
     In the channel-protective transistor, Sov denotes the length of a region where the source electrode S or the drain electrode D overlaps with the oxide semiconductor film OS with the channel protective film CS provided therebetween. A region which is in the source electrode S or the drain electrode D and overlaps with the oxide semiconductor film OS with the channel protective film CS provided therebetween is referred to as an Sov region.  FIG. 29B  shows the calculation result of the relationship between Sov and field-effect mobility.  FIG. 29C  shows the calculation result of the relationship between Sov and on-state current. 
     Furthermore, the field-effect mobility and the on-state current of the channel-etched transistor were calculated on the assumption that Sov was 0 μm. The calculation result of the field-effect mobility and that of the on-state current are shown in  FIGS. 29B and 29C , respectively. 
     Note that  FIG. 29B  shows the result at a drain voltage V d  of 1 V.  FIG. 29C  shows the result at a drain voltage V d  of 1 V and a gate voltage V g  of 10 V. 
     As shown in  FIG. 29B , the field-effect mobility of the channel-etched transistor (Sov=0 μm) operated by the dual-gate driving is approximately twice that of the channel-etched transistor operated by the single-gate driving. In contrast, the field-effect mobility of the channel-protective transistor operated by the dual-gate driving is decreased as the Sov becomes larger. 
     As shown in  FIG. 29C , the on-state current of the channel-etched transistor (Sov=0 μm) operated by the dual-gate driving is approximately twice that of the transistor operated by the single-gate driving. In contrast, the on-state current of the channel-protective transistor operated by the dual-gate driving is reduced as the Sov becomes larger. 
     In the channel-protective transistor, the Sov regions of the source electrode S and the drain electrode D block an electric field of the gate electrode GE_ 2 . Accordingly, a region whose carrier density cannot be controlled by the voltage of the gate electrode GE_ 2  is increased in the oxide semiconductor film OS. It is probable that the field-effect mobility and the on-state current are thus decreased as the Sov becomes larger. In view of the above, the channel-etched dual-gate transistor is much more effective in increasing the field-effect mobility and the on-state current than the channel-protective dual-gate transistor is. In other words, it is effective to use a channel-etched transistor as the transistor  400   a  serving as a driver transistor for the light-emitting element in the display device of this embodiment. 
     The distance between the pair of electrodes in the channel-etched transistor can be made small easily in comparison with the channel protective transistor. Thus, the transistor  400   a  can have a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. 
     &lt;Method for Manufacturing Display Device&gt; 
     Next, a method for manufacturing the display device including the transistor  400   a  and the transistor  400   b  illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D is described with reference to  FIGS. 3A to 3D ,  FIGS. 4A and 4B ,  FIGS. 5A and 5B , and  FIGS. 6A and 6B . In each of  FIGS. 3A to 3D ,  FIGS. 4A and 4B ,  FIGS. 5A and 5B , and  FIGS. 6A and 6B , a cross-sectional view in the channel length direction along line A 1 -B 1  and a cross-sectional view in the channel width direction along line C 1 -D 1  are used for describing a method for manufacturing the transistor  400   a , and a cross-sectional view in the channel length direction along line A 2 -B 2  is used for describing a method for manufacturing the transistor  400   b.    
     Note that the cross-sectional view of the transistor  400   b  in the channel width direction is similar to that of the transistor  400   a  except that the gate electrode  31  which is in contact with the gate electrode  13   a  in the opening portions  42  and  43  is not included. 
     As illustrated in  FIG. 3A , a conductive film  12  to be the gate electrodes  13   a  and  13   b  is formed over the substrate  11 . 
     In this case, a glass substrate is used as the substrate  11 . 
     The conductive film  12  is formed by a sputtering method, a CVD method, an evaporation method, or the like. 
     As the conductive film  12 , a 100 nm-thick tungsten film is formed by a sputtering method. 
     Then, a mask is formed over the conductive film  12  by a photolithography process using a first photomask. Next, the conductive film  12  is partly etched using the mask to form the gate electrodes  13   a  and  13   b . After that, the mask is removed (see  FIG. 3B ). 
     The conductive film  12  can be partly etched by one or both of wet etching and dry etching. 
     Here, a mask is formed by a photolithography process and the conductive film  12  is dry-etched using the mask to form the gate electrodes  13   a  and  13   b.    
     Note that the gate electrodes  13   a  and  13   b  may be formed by an electrolytic plating method, a printing method, an ink-jet method, or the like instead of the above formation method. 
     Next, as illustrated in  FIG. 3C , an insulating film  14  to be the insulating film  15  is formed over the substrate  11  and the gate electrodes  13   a  and  13   b , and an oxide semiconductor film  16  to be the oxide semiconductor films  17   a  and  17   b  is formed over the insulating film  14 . 
     The insulating film  14  is formed by a sputtering method, a CVD method, an evaporation method, or the like. 
     In the case where a silicon oxide film, a silicon oxynitride film, or a silicon nitride oxide film is formed as the insulating film  14 , a deposition gas containing silicon and an oxidizing gas are preferred to be used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be given as examples. 
     Moreover, in the case of forming a gallium oxide film as the insulating film  14 , a metal organic chemical vapor deposition (MOCVD) method can be employed. 
     The oxide semiconductor film  16  can be formed by a sputtering method, a coating method, a pulsed laser deposition method, a laser ablation method, or the like. 
     In the case where the oxide semiconductor film  16  is formed by a sputtering method, a power supply device for generating plasma can be an RF power supply device, an AC power supply device, a DC power supply device, or the like as appropriate. 
     As a sputtering gas, a rare gas (argon as a typical example) atmosphere, an oxidizing atmosphere, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased. 
     Furthermore, a target may be appropriately selected in accordance with the composition of the oxide semiconductor film  16  to be formed. 
     To make the oxide semiconductor film  16  intrinsic or substantially intrinsic, besides the high vacuum evacuation of the chamber, a highly purification of a sputtering gas is also needed. As an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film  16  can be prevented as much as possible. 
     Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxide semiconductor film  16  by a sputtering method using an In—Ga—Zn-oxide target (In:Ga:Zn=1:1:1). 
     Then, after a mask is formed over the oxide semiconductor film  16  by a photolithography process using a second photomask, the oxide semiconductor film  16  is partly etched using the mask. Thus, the oxide semiconductor films  17   a  and  17   b  subjected to element isolation are formed. After that, the mask is removed (see  FIG. 3D ). 
     The oxide semiconductor film  16  can be partly etched by one or both of wet etching and dry etching. 
     Here, a mask is formed by a photolithography process and the oxide semiconductor film  16  is wet-etched using the mask to form the oxide semiconductor films  17   a  and  17   b.    
     After that, heat treatment may be performed at higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 200° C. and lower than or equal to 450° C., further preferably higher than or equal to 300° C. and lower than or equal to 450° C. This heat treatment can reduce the amount of hydrogen, water, and the like contained in the oxide semiconductor films  17   a  and  17   b , which results in a reduction in impurities contained in the oxide semiconductor films  17   a  and  17   b.    
     Next, as illustrated in  FIG. 4A , a conductive film  18  to be the pair of electrodes  20   a  and  20   b  of the transistor  400   a  and the pair of electrodes  20   c  and  20   d  of the transistor  400   b  is formed. 
     The conductive film  18  is formed by a sputtering method, a CVD method, an evaporation method, or the like. 
     Here, a 50-nm-thick tungsten film and a 300-nm-thick copper film are sequentially stacked by a sputtering method to form the conductive film  18 . 
     Next, a mask is formed over the conductive film  18  by a photolithography process using a third photomask. Then, the conductive film  18  is etched using the mask, so that the pair of electrodes  20   a  and  20   b  and the pair of electrodes  20   c  and  20   d  are formed. After that, the mask is removed (see  FIG. 4B ). 
     Here, the tungsten film and the copper film are dry-etched using the mask to form the pair of electrodes  20   a  and  20   b  and the pair of electrodes  20   c  and  20   d . Note that in the case where the copper film is etched by a wet etching method first and then the tungsten film is etched by a dry etching method using SF 6 , fluoride is formed on the surface of the copper film. By the fluoride, diffusion of copper elements from the copper film is reduced and thus the copper concentration in the oxide semiconductor films  17   a  and  17   b  can be reduced. 
     Next, as illustrated in  FIG. 5A , an oxide insulating film  22  to be the oxide insulating film  23  and an oxide insulating film  24  to be the oxide insulating film  25  are formed over the oxide semiconductor film  17   a  and the pair of electrodes  20   a  and  20   b  and over the oxide semiconductor film  17   b  and the pair of electrodes  20   c  and  20   d.    
     Note that after the oxide insulating film  22  is formed, the oxide insulating film  24  is preferably formed in succession without exposure to the air. After the oxide insulating film  22  is formed, the oxide insulating film  24  is formed in succession by adjusting at least one of the flow rate of a source gas, pressure, a high-frequency power, and a substrate temperature without exposure to the air, whereby the concentration of impurities attributed to the atmospheric component at the interface between the oxide insulating film  22  and the oxide insulating film  24  can be reduced and oxygen in the oxide insulating film  24  can be moved to the oxide semiconductor films  17   a  and  17   b ; accordingly, the amount of oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced. 
     As the oxide insulating film  22 , a silicon oxide film or a silicon oxynitride film can be formed under the following conditions: the substrate placed in a treatment chamber of a plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 280° C. and lower than or equal to 400° C., the pressure is greater than or equal to 20 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 250 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power is supplied to an electrode provided in the treatment chamber. 
     A deposition gas containing silicon and an oxidizing gas are preferably used as the source gas of the oxide insulating film  22 . Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be given as examples. 
     With the use of the above conditions, an oxide insulating film which permeates oxygen can be formed as the oxide insulating film  22 . Further, by providing the oxide insulating film  22 , damage to the oxide semiconductor films  17   a  and  17   b  can be reduced in a step of forming the oxide insulating film  25  which is formed later. 
     Under the above film formation conditions, the bonding strength of silicon and oxygen becomes strong in the above substrate temperature range. Thus, as the oxide insulating film  22 , a dense and hard oxide insulating film which permeates oxygen, as a typical example, a silicon oxide film or a silicon oxynitride film having an etching rate lower than or equal to 10 nm/min, preferably lower than or equal to 8 nm/min when etching is performed at 25° C. with 0.5 weight % using hydrofluoric acid can be formed. 
     In the case where hydrogen, water, and the like are contained in the oxide semiconductor films  17   a  and  17   b , the hydrogen, water, and the like can be removed in this step because the oxide insulating film  22  is formed while heating is performed. Hydrogen contained in the oxide semiconductor films  17   a  and  17   b  is bonded to an oxygen radical formed in plasma to form water. Since the substrate is heated in the step of forming the oxide insulating film  22 , water formed by bonding of oxygen and hydrogen is released from the oxide semiconductor films  17   a  and  17   b . That is, when the oxide insulating film  22  is formed by a plasma CVD method, the amount of water and hydrogen contained in the oxide semiconductor films  17   a  and  17   b  can be reduced. 
     Further, time for heating in a state where the oxide semiconductor films are exposed can be shortened because heating is performed in a step of forming the oxide insulating film  22 . Thus, the amount of oxygen released from the oxide semiconductor film by heat treatment can be reduced. That is, the amount of oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced. 
     Note that by setting the pressure in the treatment chamber to be greater than or equal to 100 Pa and less than or equal to 250 Pa, the amount of water contained in the oxide insulating film  23  is reduced; thus, variation in electrical characteristics of the transistors  400   a  and  400   b  can be reduced and change in threshold voltage can be inhibited. 
     Further, by setting the pressure in the treatment chamber to be greater than or equal to 100 Pa and less than or equal to 250 Pa, damage to the oxide semiconductor films  17   a  and  17   b  can be reduced when the oxide insulating film  22  is formed, so that the amount of oxygen vacancies contained in the oxide semiconductor films  17   a  and  17   b  can be reduced. In particular, when the film formation temperature of the oxide insulating film  22  or the oxide insulating film  24  which is formed later is set to be high, typified by a temperature higher than 220° C., part of oxygen contained in the oxide semiconductor films  17   a  and  17   b  is released and oxygen vacancies are easily formed. Further, when the film formation conditions for reducing the amount of defects in the oxide insulating film  24  which is formed later are used to increase reliability of the transistor, the amount of released oxygen is easily reduced. Thus, it is difficult to reduce oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  in some cases. However, by setting the pressure in the treatment chamber to be greater than or equal to 100 Pa and less than or equal to 250 Pa to reduce damage to the oxide semiconductor films  17   a  and  17   b  at the time of forming the oxide insulating film  22 , oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced even when the amount of oxygen released from the oxide insulating film  24  is small. 
     Note that when the ratio of the amount of the oxidizing gas to the amount of the deposition gas containing silicon is 100 or higher, the hydrogen content in the oxide insulating film  22  can be reduced. Consequently, the amount of hydrogen entering the oxide semiconductor films  17   a  and  17   b  can be reduced; thus, the negative shift in the threshold voltage of the transistor can be inhibited. 
     Here, as the oxide insulating film  22 , a 50-nm-thick silicon oxynitride film is formed by a plasma CVD method in which silane at a flow rate of 30 sccm and dinitrogen monoxide at a flow rate of 4000 sccm are used as a source gas, the pressure in the treatment chamber is 200 Pa, the substrate temperature is 220° C., and a high-frequency power of 150 W is supplied to parallel-plate electrodes with the use of a 27.12 MHz high-frequency power source. Under the above conditions, a silicon oxynitride film which permeates oxygen can be formed. 
     As the oxide insulating film  24 , a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in a treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., preferably higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power greater than or equal to 0.17 W/cm 2  and less than or equal to 0.5 W/cm 2 , preferably greater than or equal to 0.25 W/cm 2  and less than or equal to 0.35 W/cm 2  is supplied to an electrode provided in the treatment chamber. 
     A deposition gas containing silicon and an oxidizing gas are preferably used as the source gas of the oxide insulating film  24 . Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be given as examples. 
     As the film formation conditions of the oxide insulating film  24 , the high-frequency power having the above power density is supplied to a reaction chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; thus, the oxygen content in the oxide insulating film  25  becomes higher than that in the stoichiometric composition. On the other hand, in the film formed at a substrate temperature within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the film is released by heat treatment in the later step. Thus, it is possible to form an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition and from which part of oxygen is released by heating. Further, the oxide insulating film  22  is provided over the oxide semiconductor film  17 . Accordingly, in the step of forming the oxide insulating film  24 , the oxide insulating film  22  serves as a protective film of the oxide semiconductor film  17 . Consequently, the oxide insulating film  24  can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film  17  is reduced. 
     Here, as the oxide insulating film  24 , a 400-nm-thick silicon oxynitride film is formed by a plasma CVD method in which silane at a flow rate of 200 sccm and dinitrogen monoxide at a flow rate of 4000 sccm are used as the source gas, the pressure in the reaction chamber is 200 Pa, the substrate temperature is 220° C., and the high-frequency power of 1500 W is supplied to the parallel-plate electrodes with the use of a 27.12 MHz high-frequency power source. Note that a plasma CVD apparatus used here is a parallel-plate plasma CVD apparatus in which the electrode area is 6000 cm 2 , and the power per unit area (power density) into which the supplied power is converted is 0.25 W/cm 2 . 
     Next, heat treatment is performed. The heat treatment is performed at, as a typical example, a temperature higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., further preferably higher than or equal to 320° C. and lower than or equal to 370° C. 
     An electric furnace, an RTA apparatus, or the like can be used for the heat treatment. With the use of an RTA apparatus, the heat treatment can be performed at a temperature higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened. 
     The heat treatment may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or less, preferably 1 ppm or less, further preferably 10 ppb or less), or a rare gas (argon, helium, or the like). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like. 
     By the heat treatment, part of oxygen contained in the oxide insulating film  24  can be moved to the oxide semiconductor films  17   a  and  17   b , so that the amount of oxygen vacancies contained in the oxide semiconductor films  17   a  and  17   b  can be further reduced. 
     In the case where the oxide insulating film  22  and the oxide insulating film  24  contain water, hydrogen, or the like, water, hydrogen, or the like contained in the oxide insulating film  22  and the oxide insulating film  24  is moved to the oxide semiconductor films  17   a  and  17   b  by heat treatment performed after a nitride insulating film  26  that blocks water, hydrogen, and the like is formed, so that defects are generated in the oxide semiconductor films  17   a  and  17   b . However, when the heat treatment is performed prior to formation of the nitride insulating film  26 , water, hydrogen, or the like contained in the oxide insulating film  22  and the oxide insulating film  24  can be released; thus, variation in electrical characteristics of the transistors  400   a  and  400   b  can be reduced, and change in threshold voltage can be inhibited. 
     Note that when the oxide insulating film  24  is formed over the oxide insulating film  22  while being heated, oxygen can be moved to the oxide semiconductor films  17   a  and  17   b  to reduce the oxygen vacancies in the oxide semiconductor films  17   a  and  17   b ; thus, the heat treatment needs not to be performed. 
     Here, heat treatment is performed at 350° C. for one hour in an atmosphere of nitrogen and oxygen. 
     Furthermore, when the pair of electrodes  20   a  and  20   b  and the pair of electrodes  20   c  and  20   d  are formed, the oxide semiconductor films  17   a  and  17   b  are damaged by the etching of the conductive film, so that oxygen vacancies are generated on the back channel side (the side of the oxide semiconductor films  17   a  and  17   b  which is opposite to the side facing to the gate electrodes  13   a  and  13   b ) of the oxide semiconductor films  17   a  and  17   b . However, with the use of the oxide insulating film containing oxygen at a higher proportion than the stoichiometric composition as the oxide insulating film  24 , the oxygen vacancies generated on the back channel side can be reduced by heat treatment. As a result, the reliability of the transistors  400   a  and  400   b  can be improved. 
     Next, the nitride insulating film  26  to be the nitride insulating film  27  is formed by a sputtering method, a CVD method, or the like. 
     Note that in the case where the nitride insulating film  26  is formed by a plasma CVD method, the substrate placed in the treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is preferably set to be higher than or equal to 300° C. and lower than or equal to 400° C., more preferably, higher than or equal to 320° C. and lower than or equal to 370° C., so that a dense nitride insulating film can be formed. 
     In the case where a silicon nitride film is formed by the plasma CVD method as the nitride insulating film  26 , a deposition gas containing silicon, nitrogen, and ammonia are preferably used as a source gas. As the source gas, ammonia whose amount is smaller than the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species cut a bond between silicon and hydrogen which are contained in a deposition gas containing silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen is few, can be formed. On the other hand, when the amount of ammonia is larger than the amount of nitrogen in a source gas, decomposition of a deposition gas containing silicon and decomposition of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Thus, in a source gas, the flow ratio of the nitrogen to the ammonia is set to be preferably greater than or equal to 5 and less than or equal to 50, more preferably greater than or equal to 10 and less than or equal to 50. 
     Here, in the reaction chamber of a plasma CVD apparatus, a 50-nm-thick silicon nitride film is formed as the nitride insulating film  26  by a plasma CVD method in which silane at a flow rate of 50 sccm, nitrogen at a flow rate of 5000 sccm, and ammonia at a flow rate of 100 sccm are used as the source gas, the pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and high-frequency power of 1000 W is supplied to parallel-plate electrodes with a high-frequency power supply of 27.12 MHz. Note that the plasma CVD apparatus is a parallel-plate plasma CVD apparatus in which the electrode area is 6000 cm 2 , and the power per unit area (power density) into which the supplied power is converted is 1.7×10 −1  W/cm 2 . 
     Through the above-described steps, the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  can be formed. 
     Next, heat treatment is performed. The heat treatment is performed at, as a typical example, a temperature higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., further preferably higher than or equal to 320° C. and lower than or equal to 370° C. 
     Next, a mask is formed over the nitride insulating film  26  by a photolithography process using a fourth photomask, and then each of the insulating film  14 , the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  is partly etched using the mask, so that the insulating film  15  and the insulating film  28  including the oxide insulating film  23 , the oxide insulating film  25 , and the nitride insulating film  27  are formed. As illustrated in the cross-sectional view along line A 1 -B 1  in  FIG. 5B , which is the cross-sectional view of the transistor  400   a  in the channel length direction, the opening portion  41  is provided in the insulating film  28 . Further, as illustrated in the cross-sectional view along line C 1 -D 1  in  FIG. 5B , which is the cross-sectional view of the transistor  400   a  in the channel width direction, the opening portions  42  and  43  are provided in the insulating film  15  and the insulating film  28 . 
     Then, as illustrated in  FIG. 6A , a conductive film  30  to be the gate electrode  31  and the electrode  32  is formed. 
     The conductive film  30  is formed by a sputtering method, a CVD method, an evaporation method, or the like. 
     Here, an ITO film with a thickness of 100 nm is formed as the conductive film  30  by a sputtering method. 
     Then, a mask is formed over the conductive film  30  by a photolithography process using a fifth photomask. Next, the conductive film  30  is partly etched using the mask to form the gate electrode  31  and the electrode  32 . After that, the mask is removed. 
     Note that as illustrated in  FIG. 6B , in the channel width direction of the transistor  400   a , the gate electrode  31  is formed so as to face the side surfaces of the oxide semiconductor film  17   a  at the side surfaces of the opening portions provided in the insulating film  15  and the insulating film  28 . 
     Through the above process, the transistor  400   a  and the transistor  400   b  can be manufactured. 
     In the transistor described in this embodiment, the oxide insulating film containing oxygen at higher proportion than the stoichiometric composition is formed to overlap with the oxide semiconductor film that serves as a channel region, and thus, oxygen in the oxide insulating film can be transferred to the oxide semiconductor film. Consequently, oxygen vacancies contained in the oxide semiconductor film can be reduced, so that the transistor can have high reliability. 
     Further, in the channel width direction of the transistor  400   a  serving as a driver transistor for the light-emitting element, the gate electrode  31  faces the side surfaces of the oxide semiconductor film  17   a  at the side surfaces of the opening portions  42  and  43  provided in the insulating film  15  and the insulating film  28 , whereby the electric field from the gate electrode  31  also affects the side surfaces of the oxide semiconductor film  17   a  and carriers flow in a wide region in the oxide semiconductor film  17   a , so that the field-effect mobility and the on-state current of the transistor are increased. 
     When the transistor  400   a  serving as a driver transistor for the light-emitting element has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm, the field-effect mobility of the transistor can be further increased. Accordingly, the display device can operate at high speed. 
     In addition, when the transistor  400   b  serving as a selection transistor in the pixel has a longer channel length than the transistor  400   a , cutoff current can be low. Accordingly, low power consumption of the display device can be achieved. 
     Through the above steps, a display device which includes a transistor having an oxide semiconductor film and has favorable electrical characteristics can be obtained. Further, the display device including the transistor having the oxide semiconductor film can have high reliability. 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Modification Example 1 of Display Device 
     A display device having a structure that is different from that illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D is described with reference to FIGS.  7 A 1 ,  7 A 2 ,  7 B,  7 C 1 , and  7 C 2 . The display device illustrated in FIGS.  7 A 1 ,  7 A 2 ,  7 B,  7 C 1 , and  7 C 2  includes a pixel having a transistor  410   a  serving as a driver transistor and the transistor  400   b  serving as a selection transistor. FIG.  7 A 1  is a top view of the transistor  410   a  serving as a driver transistor. FIG.  7 A 2  is a top view of the transistor  400   b  serving as a selection transistor.  FIG. 7B  shows cross-sectional views taken along dashed-dotted line A 3 -B 3  in FIG.  7 A 1  and dashed-dotted line A 2 -B 2  in FIG.  7 A 2 . FIG.  7 C 1  is a cross-sectional view taken along dashed-dotted line C 3 -D 3  in FIG.  7 A 1 . FIG.  7 C 2  is a cross-sectional view taken along dashed-dotted line C 2 -D 2  in FIG.  7 A 2 . Note that in FIGS.  7 A 1  and  7 A 2 , the substrate  11 , insulating films, and the like are omitted for simplicity. 
     The transistor  400   b  serving as a selection transistor illustrated in FIGS.  7 A 2 ,  7 B, and  7 C 2  has a structure similar to that in FIGS.  2 A 2 ,  2 B, and  2 C 2 . 
     Unlike the above-described transistor  400   a , the transistor  410   a  has a structure in which, in the channel width direction, the gate electrode  13   a  and a gate electrode  51  are connected to each other on an outer side of one side surface of the oxide semiconductor film  17   a , and the gate electrode  13   a  and the gate electrode  51  face each other with the insulating film  15  and the insulating film  28  provided therebetween on an outer side of the other side surface of the oxide semiconductor film  17   a.    
     The transistor  410   a  illustrated in FIGS.  7 A 1 ,  7 B, and  7 C 1  is a channel-etched transistor. The transistor  410   a  includes the gate electrode  13   a  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   a , the oxide semiconductor film  17   a  overlapping with the gate electrode  13   a  with the insulating film  15  provided therebetween, and the pair of electrodes  20   a  and  20   b  in contact with the oxide semiconductor film  17   a . The transistor  410   a  also includes an insulating film  28  that is composed of the oxide insulating film  23 , the oxide insulating film  25 , and the nitride insulating film  27  and is over the insulating film  15 , the oxide semiconductor film  17   a , and the pair of electrodes  20   a  and  20   b ; and the gate electrode  51  formed over the insulating film  28 . The gate electrode  51  is connected to the gate electrode  13   a  through the opening portion  42  provided in the insulating film  15  and the insulating film  28 . Furthermore, the electrode  32  connected to one of the pair of electrodes  20   a  and  20   b  (here, the electrode  20   b ) is formed over the insulating film  28 . Note that the electrode  32  serves as a pixel electrode. 
     The transistor  410   a  has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Furthermore, in the transistor  410   a , the oxide semiconductor film  17   a  is provided between the gate electrode  13   a  and the gate electrode  51 . In addition, as illustrated in FIG.  7 A 1 , the gate electrode  51  overlaps with the end portions of the oxide semiconductor film  17   a  with the insulating film  28  provided therebetween, when seen from the above. 
     Further, a plurality of opening portions are provided in the insulating film  15  and the insulating film  28 . As a typical example, as illustrated in  FIG. 7B , the opening portion  41  that reaches one of the pair of electrodes  20   a  and  20   b  is provided. Furthermore, as illustrated in FIG.  7 C 1 , the opening portion  42  is provided in the insulating film  15  and the insulating film  28  on an outer side of one side surface of the oxide semiconductor film  17   a  in the channel width direction, and the gate electrode  51  and the gate electrode  13   a  are connected to each other through the opening portion  42 . The gate electrode  51  faces the side surface of the oxide semiconductor film  17   a  in the channel width direction at the side surface of the opening portion  42 . The gate electrode  51  and the gate electrode  13   a  are not connected to each other on an outer side of the other side surface of the oxide semiconductor film  17   a  in the channel width direction. End portions of the gate electrode  51  are positioned on the outer sides of the side surfaces of the oxide semiconductor film  17   a.    
     Note that as illustrated in FIG.  7 C 1 , a distance d between the side surface of the oxide semiconductor film  17   a  in the channel width direction and a projected end portion of the gate electrode  51  on the interface between the insulating film  15  and the insulating film  28  is preferably 1 to 7.5 times as long as the sum of the thickness t1 of the insulating film  15  and the thickness t2 of the insulating film  28 . In the case where the distance d is equal to or longer than the sum of the thickness t1 of the insulating film  15  and the thickness t2 of the insulating film  28 , an electric field from the gate electrode  51  affects the side surface of the oxide semiconductor film  17   a  or the end portion including the side surface and its vicinity. Thus, a parasitic channel can be prevented from being generated at the side surface of the oxide semiconductor film  17   a  or the end portion thereof. In addition, in the case where the distance d is 7.5 or less times as long as the sum of the thickness t1 of the insulating film  15  and the thickness t2 of the insulating film  28 , the area of the transistor can be smaller. 
     Next, a manufacturing process of the transistor  410   a  will be described. 
     Through steps of  FIGS. 3A to 3D ,  FIGS. 4A and 4B , and  FIG. 5A , the gate electrode  13   a , the insulating film  14 , the oxide semiconductor film  17   a , the pair of electrodes  20   a  and  20   b , the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  are formed over the substrate  11 . In the steps, a photolithography process is performed using the first to third photomasks. 
     Next, after a mask is formed over the nitride insulating film  26  by a photolithography process using a fourth photomask, the insulating film  14 , the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  are partly etched to form the opening portions  41  and  42  illustrated in FIGS.  7 A 1 ,  7 B, and  7 C 1 . 
     Subsequently, the conductive film  30  is formed as in the step of  FIG. 6A . Then, after a mask is formed over the conductive film  30  by a photolithography process using a fifth photomask, the conductive film  30  is partly etched to form the gate electrode  51  and the electrode  32  illustrated in FIGS.  7 A 1 ,  7 B, and  7 C 1 . 
     Through the above process, the transistor  410   a  can be manufactured. 
     Modification Example 2 of Display Device 
     A display device having a structure that is different from the structures illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D and FIGS.  7 A 1 ,  7 A 2 ,  7 B,  7 C 1 , and  7 C 2  is described with reference to FIGS.  8 A 1 ,  8 A 2 ,  8 B,  8 C 1 , and  8 C 2 . The display device illustrated in FIGS.  8 A 1 ,  8 A 2 ,  8 B,  8 C 1 , and  8 C 2  includes a pixel having a transistor  420   a  serving as a driver transistor and the transistor  400   b  serving as a selection transistor. FIG.  8 A 1  is a top view of the transistor  420   a  serving as a driver transistor. FIG.  8 A 2  is a top view of the transistor  400   b  serving as a selection transistor.  FIG. 8B  shows cross-sectional views taken along dashed-dotted line A 4 -B 4  in FIG.  8 A 1  and dashed-dotted line A 2 -B 2  in FIG.  8 A 2 . FIG.  8 C 1  is a cross-sectional view taken along dashed-dotted line C 4 -D 4  in FIG.  8 A 1 . FIG.  8 C 2  is a cross-sectional view taken along dashed-dotted line C 2 -D 2  in FIG.  8 A 2 . Note that in FIGS.  8 A 1  and  8 A 2 , the substrate  11 , insulating films, and the like are omitted for simplicity. 
     The transistor  400   b  serving as a selection transistor illustrated in FIGS.  8 A 2 ,  8 B, and  8 C 2  has a structure similar to that in FIGS.  2 A 2 ,  2 B, and  2 C 2 . Unlike in the above-described driver transistor  410   a , the gate electrode  13   a  and a gate electrode  64  are connected to each other through a conductive film  62  in the transistor  420   a  serving as a driver transistor in FIGS.  8 A 1 ,  8 B, and  8 C 1 . 
     The transistor  420   a  illustrated in FIGS.  8 A 1 ,  8 B, and  8 C 1  is a channel-etched transistor. The transistor  420   a  includes the gate electrode  13   a  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   a , the oxide semiconductor film  17   a  overlapping with the gate electrode  13   a  with the insulating film  15  provided therebetween, and the pair of electrodes  20   a  and  20   b  in contact with the oxide semiconductor film  17   a . The transistor  420   a  also includes an insulating film  28  that is composed of the oxide insulating film  23 , the oxide insulating film  25 , and the nitride insulating film  27  and is over the insulating film  15 , the oxide semiconductor film  17   a , and the pair of electrodes  20   a  and  20   b ; and the gate electrode  64  formed over the insulating film  28 . The gate electrode  64  is connected to the gate electrode  13   a  through the conductive film  62 . Furthermore, the electrode  32  connected to one of the pair of electrodes  20   a  and  20   b  (here, the electrode  20   b ) is formed over the insulating film  28 . Note that the electrode  32  serves as a pixel electrode. 
     The transistor  420   a  has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Furthermore, in the transistor  420   a , the oxide semiconductor film  17   a  is provided between the gate electrode  13   a  and the gate electrode  64 . In addition, as illustrated in FIG.  8 A 1 , the gate electrode  64  overlaps with the end portions of the oxide semiconductor film  17   a  with the insulating film  28  provided therebetween, when seen from the above. 
     Further, a plurality of opening portions are provided in the insulating film  15  and the insulating film  28 . As a typical example, as illustrated in  FIG. 8B , the opening portion  41  that reaches one of the pair of electrodes  20   a  and  20   b  is provided in the transistor  420   a . Furthermore, as illustrated in FIG.  8 C 1 , the conductive film  62  is connected to the gate electrode  13   a  through an opening portion  61  provided in the insulating film  15 . Note that the conductive film  62  is formed at the same time as the pair of electrodes  20   a  and  20   b . The gate electrode  64  is connected to the conductive film  62  through an opening portion  63  provided in the insulating film  28 . That is, the gate electrode  13   a  and the gate electrode  64  are electrically connected to each other through the conductive film  62 . In addition, the conductive film  62  having the same potential as the gate electrode  13   a  and the gate electrode  64  faces the side surface of the oxide semiconductor film  17   a.    
     Note that although the transistor  420   a  has a structure in which the gate electrode  13   a  and the gate electrode  64  are connected to each other through the conductive film  62  on an outer side of only one side surface of the oxide semiconductor film  17   a  in the channel width direction as illustrated in FIG.  8 C 1 , the gate electrode  13   a  and the gate electrode  64  may be connected to each other through the conductive film  62  on the outer sides of both side surfaces of the oxide semiconductor film  17   a  in the channel width direction. 
     Next, a manufacturing process of the transistor  420   a  will be described. 
     Through steps of  FIGS. 3A to 3D , the gate electrode  13   a , the insulating film  14 , and the oxide semiconductor film  17   a  are formed over the substrate  11 . In the steps, a photolithography process is performed using the first and second photomasks. 
     Then, after a mask is formed over the insulating film  14  by a photolithography process using a third photomask, the insulating film  14  is partly etched to form the opening portion  61  illustrated in FIGS.  8 A 1  and  8 C 1 . 
     Next, as in the steps of  FIGS. 4A and 4B , a mask is formed over the conductive film  18  by a photolithography process using a fourth photomask and then, the conductive film  18  is partly etched to form the pair of electrodes  20   a  and  20   b  and the conductive film  62 . 
     Subsequently, the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  are formed as in the step of  FIG. 5A . Next, after a mask is formed over the nitride insulating film  26  by a photolithography process using a fifth photomask, the nitride insulating film  26  is partly etched to form the opening portion  63  illustrated in FIGS.  8 A 1  and  8 C 1 . 
     After that, the conductive film  30  is formed as in the step of  FIG. 6A . Then, after a mask is formed over the conductive film  30  by a photolithography process using a sixth photomask, the conductive film  30  is partly etched to form the gate electrode  64  and the electrode  32  illustrated in FIGS.  8 A 1 ,  8 B, and  8 C 1 . 
     Through the above process, the transistor  420   a  can be manufactured. 
     Modification Example 3 of Display Device 
     A display device having a structure that is different from the structures illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D, FIGS.  7 A 1 ,  7 A 2 ,  7 B,  7 C 1 , and  7 C 2 , and FIGS.  8 A 1 ,  8 A 2 ,  8 B,  8 C 1 , and  8 C 2  is described with reference to FIGS.  11 A 1 ,  11 A 2 ,  11 B,  11 C 1 , and  11 C 2 . The display device illustrated in FIGS.  11 A 1 ,  11 A 2 ,  11 B,  11 C 1 , and  11 C 2  includes a pixel having a transistor  430   a  serving as a driver transistor and a transistor  430   b  serving as a selection transistor. FIG.  11 A 1  is a top view of the transistor  430   a  serving as a driver transistor. FIG.  11 A 2  is a top view of the transistor  430   b  serving as a selection transistor.  FIG. 11B  shows cross-sectional views taken along dashed-dotted line A 5 -B 5  in FIG.  11 A 1  and dashed-dotted line A 6 -B 6  in FIG.  11 A 2 . FIG.  11 C 1  is a cross-sectional view taken along dashed-dotted line C 5 -D 5  in FIG.  11 A 1 . FIG.  11 C 2  is a cross-sectional view taken along dashed-dotted line C 6 -D 6  in FIG.  11 A 2 . Note that in FIGS.  11 A 1  and  11 A 2 , the substrate, insulating films, and the like are omitted for simplicity. 
     In the transistor  430   a  illustrated in FIGS.  11 A 1 ,  11 B, and  11 C 1 , an electrode  77  connected to one of the pair of electrodes  20   a  and  20   b  is provided over the insulating film  15 . Unlike the other driver transistors described above, the transistor  430   a  includes an insulating film  74   a , which is isolated from an insulating film  74   b , over the oxide semiconductor film  17   a  and the pair of electrodes  20   a  and  20   b.    
     In addition, unlike the transistor  400   b  described above, the transistor  430   b  illustrated in FIGS.  11 A 2 ,  11 B, and  11 C 2  includes the insulating film  74   b , which is isolated from the insulating film  74   a , over the oxide semiconductor film  17   b  and the pair of electrodes  20   c  and  20   d.    
     The transistor  430   a  illustrated in FIGS.  11 A 1 ,  11 B, and  11 C 1  is a channel-etched transistor. The transistor  430   a  includes the gate electrode  13   a  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   a , the oxide semiconductor film  17   a  overlapping with the gate electrode  13   a  with the insulating film  15  provided therebetween, and the pair of electrodes  20   a  and  20   b  in contact with the oxide semiconductor film  17   a . The transistor  430   a  also includes an insulating film  74   a  serving as a gate insulating film, which is composed of an oxide insulating film  71   a , an oxide insulating film  72   a , and a nitride insulating film  73   a  and is over the insulating film  15 , the oxide semiconductor film  17   a , and the pair of electrodes  20   a  and  20   b ; and a gate electrode  76  formed over the insulating film  74   a  and a nitride insulating film  15   a . The gate electrode  76  is connected to the gate electrode  13   a  through an opening portion  75  provided in the nitride insulating film  15   a . Furthermore, the electrode  77  connected to one of the pair of electrodes  20   a  and  20   b  (here, the electrode  20   b ) is formed over the insulating film  15 . Note that the electrode  77  serves as a pixel electrode. 
     The transistor  430   b  illustrated in FIGS.  11 A 2 ,  11 B, and  11 C 2  is a channel-etched transistor. The transistor  430   b  includes the gate electrode  13   b  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   b , the oxide semiconductor film  17   b  overlapping with the gate electrode  13   b  with the insulating film  15  provided therebetween, and the pair of electrodes  20   c  and  20   d  in contact with the oxide semiconductor film  17   b . The transistor  430   b  also includes an insulating film  74   b  that is composed of an oxide insulating film  71   b , an oxide insulating film  72   b , and a nitride insulating film  73   b  and is over the insulating film  15 , the oxide semiconductor film  17   b , and the pair of electrodes  20   c  and  20   d.    
     The insulating film  15  included in the transistors  430   a  and  430   b  is formed using the nitride insulating film  15   a  and an oxide insulating film  15   b . The oxide insulating film  15   b  is formed in a region overlapping with any of the oxide semiconductor film  17   a , the pair of electrodes  20   a  and  20   b , the oxide insulating film  71   a , the oxide semiconductor film  17   b , the pair of electrodes  20   c  and  20   d , and the oxide insulating film  71   b.    
     The insulating films  74   a  and  74   b  are isolated from each other and overlap with the oxide semiconductor film  17   a  and the oxide semiconductor film  17   b , respectively. Specifically, in the channel length direction of the transistor  430   a  illustrated in  FIG. 11B , end portions of the insulating film  74   a  are positioned over the pair of electrodes  20   a  and  20   b , whereas in the channel width direction of the transistor  430   a  illustrated in FIG.  11 C 1 , end portions of the insulating film  74   a  are positioned on the outer sides of the oxide semiconductor film  17   a . Further, in the channel length direction of the transistor  430   b  illustrated in  FIG. 11B , end portions of the insulating film  74   b  are positioned over the pair of electrodes  20   c  and  20   d , whereas in the channel width direction of the transistor  430   b  illustrated in FIG.  11 C 2 , end portions of the insulating film  74   b  are positioned on outer sides of the oxide semiconductor film  17   b.    
     Furthermore, in the channel width direction in FIG.  11 C 1 , the gate electrode  76  faces the side surfaces of the oxide semiconductor film  17   a  at side surfaces of the insulating film  74   a  serving as a gate insulating film. Note that in the channel length direction, the end portions of the insulating film  74   a  may be positioned over the insulating film  15  instead of over the pair of electrodes  20   a  and  20   b . In such a case, the electrode  77  is formed over the insulating film  74   a  and is connected to one of the pair of electrodes  20   a  and  20   b  through an opening portion in the insulating film  74   a.    
     The transistor  430   a  has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Furthermore, in the channel width direction of the transistor  430   a , the oxide semiconductor film  17   a  is provided between the gate electrode  13   a  and the gate electrode  76  with the insulating film  15  provided between the gate electrode  13   a  and the oxide semiconductor film  17   a  and with the insulating film  74   a  provided between the gate electrode  76  and the oxide semiconductor film  17   a . In addition, as illustrated in FIG.  11 A 1 , the gate electrode  76  overlaps with the end portions of the oxide semiconductor film  17   a  with the insulating film  74   a  provided therebetween, when seen from the above. 
     Furthermore, the transistor  430   b  has a longer channel length than the transistor  430   a . Thus, the cutoff current of the transistor  430   b  serving as a selection transistor in the pixel can be reduced. 
     Note that although FIG.  11 C 1  illustrates an example where the gate electrode  13   a  and the gate electrode  76  are connected to each other on an outer side of only one side surface of the oxide semiconductor film  17   a  in the channel width direction, the gate electrode  13   a  and the gate electrode  76  may be connected to each other on the outer sides of both side surfaces of the oxide semiconductor film  17   a  in the channel width direction. 
     Next, a manufacturing process of the transistors  430   a  and  430   b  will be described. 
     In the manufacturing process of the transistors  430   a  and  430   b , through steps of  FIGS. 3A to 3D ,  FIGS. 4A and 4B , and  FIG. 5A , the gate electrodes  13   a  and  13   b , the insulating film  14 , the oxide semiconductor films  17   a  and  17   b , the pair of electrodes  20   a  and  20   b , the pair of electrodes  20   c  and  20   d , the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  are formed over the substrate  11 . In the steps, a photolithography process is performed using the first to third photomasks. 
     Next, after a mask is formed over the nitride insulating film  26  by a photolithography process using a fourth photomask as in the step of  FIG. 5B , the oxide insulating film  22 , the oxide insulating film  24 , and the nitride insulating film  26  are partly etched to form the insulating films  74   a  and  74   b  isolated from each other. Note that in the case where the insulating film  14  is a stack including a nitride insulating film and an oxide insulating film, the insulating film  14  is partly etched in the etching of the oxide insulating film  23 . As a result, the insulating film  15  including a step formed by the nitride insulating film  15   a  and the oxide insulating film  15   b  is formed as illustrated in  FIG. 11B . 
     After that, the gate electrode  76  and the electrode  77  are formed through the process illustrated in  FIGS. 6A and 6B . 
     Through the above process, the transistors  430   a  and  430   b  can be manufactured. 
     Modification Example 4 of Display Device 
     A display device having a structure that is different from the structures illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D, FIGS.  7 A 1 ,  7 A 2 ,  7 B,  7 C 1 , and  7 C 2 , FIGS.  8 A 1 ,  8 A 2 ,  8 B,  8 C 1 , and  8 C 2 , and FIGS.  11 A 1 ,  11 A 2 ,  11 B,  11 C 1 , and  11 C 2  is described with reference to  FIGS. 9A to 9D .  FIG. 9A  is an equivalent circuit diagram of a pixel  602  included in the display device. 
     As illustrated in  FIG. 9A , the pixel  602  of this modification example includes the light-emitting element  350 , the transistor  400   a  serving as a driver transistor for the light-emitting element  350 , a transistor  450   b  serving as a selection transistor, and the capacitor  370 . Each of the transistor  400   a  and the transistor  450   b  is what is called a dual-gate transistor including gate electrodes positioned over and below an oxide semiconductor film. 
     The transistor  400   a  included in the pixel  602  can have a structure similar to that illustrated in FIGS.  2 A 1 ,  2 B,  2 C 1 , and  2 D. 
       FIG. 9B  is a top view of the transistor  450   b  serving as a selection transistor.  FIG. 9C  is a cross-sectional view taken along dashed-dotted line C 9 -D 9  in  FIG. 9B .  FIG. 9D  shows cross-sectional views taken along dashed-dotted line A 1 -B 1  of the transistor  400   a  illustrated in FIG.  2 A 1  and dashed-dotted line A 9 -B 9  in  FIG. 9B . Note that in  FIG. 9B , the substrate  11 , insulating films, and the like are omitted for simplicity. 
     The transistor  450   b  illustrated in  FIGS. 9A to 9D  includes a gate electrode  109  overlapping with the oxide semiconductor film  17   b  and the gate electrode  13   b  over the insulating film  28 , unlike the other selection transistors described above. 
     The transistor  450   b  illustrated in  FIGS. 9A to 9D  is a channel-etched transistor. The transistor  450   b  includes the gate electrode  13   b  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   b , the oxide semiconductor film  17   b  overlapping with the gate electrode  13   b  with the insulating film  15  provided therebetween, and the pair of electrodes  20   c  and  20   d  in contact with the oxide semiconductor film  17   b . The transistor  450   b  also includes the insulating film  28  serving as a gate insulating film, which is composed of the oxide insulating film  23 , the oxide insulating film  25 , and the nitride insulating film  27  and is over the insulating film  15 , the oxide semiconductor film  17   b , and the pair of electrodes  20   c  and  20   d ; and the gate electrode  109  formed over the insulating film  28  and the insulating film  15 . The gate electrode  109  is connected to the gate electrode  13   b  through opening portions  102  and  103  provided in the insulating film  15  and the insulating film  28 . 
     The gate electrode  109  included in the transistor  450   b  is formed using the same layer as the gate electrode  31  included in the transistor  400   a.    
     Further, the channel length of the transistor  450   b  is longer than at least the channel length of the transistor  400   a . Further, in the channel width direction, the oxide semiconductor film  17   b  is provided between the gate electrode  13   b  and the gate electrode  109  with the insulating film  15  provided between the gate electrode  13   b  and the oxide semiconductor film  17   b  and with the insulating film  28  provided between the gate electrode  109  and the oxide semiconductor film  17   b . Furthermore, as illustrated in  FIG. 9B , the gate electrode  109  overlaps with the end portions of the oxide semiconductor film  17   b  with the insulating film  28  provided therebetween, when seen from the above. 
     In each of the transistors  400   a  and  450   b , defects might be formed at the end portion of the oxide semiconductor film processed into an island shape by etching or the like because of damage due to the processing, and the end portion of the oxide semiconductor film is polluted by attachment of impurities or the like. For this reason, in the case where one gate electrode is formed either over or below an oxide semiconductor film in a transistor, even when the oxide semiconductor film is intrinsic or substantially intrinsic, end portions of the oxide semiconductor film are easily activated to be n-type (low-resistance regions) by application of stress such as an electric field. In the case where the n-type end portions overlap with regions between the pair of electrodes  20   c  and  20   d , which are surrounded by the dashed lines  33  and  34  in  FIG. 9B , for example, the n-type regions serve as carrier paths, resulting in formation of a parasitic channel. As a result, drain current with respect to the threshold voltage is gradually increased, so that the threshold voltage of the transistor shifts in the negative direction. 
     However, the transistor  450   b  illustrated in  FIGS. 9A to 9D  includes the gate electrode  13   b  and the gate electrode  109  having the same potentials and the gate electrode  109  faces the side surfaces of the oxide semiconductor film  17   b  in the channel width direction at the side surfaces of the insulating film  28 , whereby an electric field from the gate electrode  109  affects the oxide semiconductor film  17   b  also from the side surfaces of the oxide semiconductor film  17   b . As a result, a parasitic channel is prevented from being generated at the side surface of the oxide semiconductor film  17   b  or the end portion including the side surface and its vicinity. Thus, the transistor  450   b  can have favorable electrical characteristics of a sharp increase in drain current with respect to the threshold voltage. Note that the above descriptions can be also used for the gate electrode  13   a , the gate electrode  31 , and the oxide semiconductor film  17   a  included in the transistor  400   a.    
     Although the above-described structures of the display device of this embodiment and the display devices of the modification examples are partly different from each other, the structures can be freely combined with each other. 
     The structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in other embodiments. 
     Embodiment 2 
     In this embodiment, a display device and a manufacturing method thereof, which are different from those in Embodiment 1, will be described with reference to drawings. In this embodiment, a transistor in which oxygen vacancies contained in an oxide semiconductor film are further reduced will be described with reference to FIGS.  12 A 1 ,  12 A 2 ,  12 B,  12 C 1 , and  12 C 2 ,  FIGS. 13A and 13B ,  FIGS. 14A and 14B , and  FIGS. 15A and 15B . 
     FIGS.  12 A 1 ,  12 A 2 ,  12 B,  12 C 1 , and  12 C 2  are top views and cross-sectional views of transistors included in a pixel of the display device of this embodiment. 
     The display device of this embodiment includes a pixel having a transistor  440   a  serving as a driver transistor and a transistor  440   b  serving as a selection transistor. FIG.  12 A 1  is a top view of the transistor  440   a  serving as a driver transistor. FIG.  12 A 2  is a top view of the transistor  440   b  serving as a selection transistor.  FIG. 12B  shows cross-sectional views taken along dashed-dotted line A 7 -B 7  in FIG.  12 A 1  and dashed-dotted line A 8 -B 8  in FIG.  12 A 2 . FIG.  12 C 1  is a cross-sectional view taken along dashed-dotted line C 7 -D 7  in FIG.  12 A 1 . FIG.  12 C 2  is a cross-sectional view taken along dashed-dotted line C 8 -D 8  in FIG.  12 A 2 . Note that in FIGS.  12 A 1  and  12 A 2 , the substrate  11 , insulating films, and the like are omitted for simplicity. 
     The transistor  440   a  illustrated in FIGS.  12 A 1 ,  12 B, and  12 C 1  is a channel-etched transistor. The transistor  440   a  includes the gate electrode  13   a  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   a , the oxide semiconductor film  17   a  overlapping with the gate electrode  13   a  with the insulating film  15  provided therebetween, and the pair of electrodes  20   a  and  20   b  in contact with the oxide semiconductor film  17   a . The transistor  440   a  includes, over the insulating film  15 , the oxide semiconductor film  17   a , and the pair of electrodes  20   a  and  20   b , an insulating film  88   a  that is composed of an oxide insulating film  83   a , an oxide insulating film  85   a , and a nitride insulating film  87 , and a gate electrode  91  formed over the insulating film  88   a . The gate electrode  91  is connected to the gate electrode  13   a  through an opening portion  96  provided in the insulating film  15  and the nitride insulating film  87 . Furthermore, an electrode  92  connected to one of the pair of electrodes  20   a  and  20   b  (here, the electrode  20   b ) is formed over the nitride insulating film  87 . The electrode  92  is connected to the electrode  20   b  through an opening portion  95  provided in the nitride insulating film  87 . Note that the electrode  92  serves as a pixel electrode. 
     The transistor  440   b  illustrated in FIGS.  12 A 2 ,  12 B, and  12 C 2  is a channel-etched transistor. The transistor  440   b  includes a gate electrode  13   b  provided over the substrate  11 , the insulating film  15  formed over the substrate  11  and the gate electrode  13   b , an oxide semiconductor film  17   b  overlapping with the gate electrode  13   b  with the insulating film  15  provided therebetween, and a pair of electrodes  20   c  and  20   d  in contact with the oxide semiconductor film  17   b . The transistor  440   b  also includes an insulating film  88   b  that is composed of an oxide insulating film  83   b , an oxide insulating film  85   b , and the nitride insulating film  87  and is over the insulating film  15 , the oxide semiconductor film  17   b , and the pair of electrodes  20   c  and  20   d.    
     In the transistor  440   a , the insulating film  15  and the insulating film  88   a  each serve as a gate insulating film. Further, in the transistor  440   b , the insulating film  15  serves as a gate insulating film. The insulating film  15  includes the nitride insulating film  15   a  and the oxide insulating film  15   b . The oxide insulating film  15   b  is formed in a region overlapping with any of the oxide semiconductor films  17   a  and  17   b , the pair of electrodes  20   a  and  20   b , the pair of electrodes  20   c  and  20   d , and the oxide insulating film  83   a.    
     Note that in the transistor  440   b , a second gate electrode may be provided in a region which is over the insulating film  88   b  and which overlaps with the gate electrode  13   b  and the oxide semiconductor film  17   b . In that case, the second gate electrode is preferably connected to the gate electrode  13   b  through an opening portion provided in the insulating film  15  and the nitride insulating film  87 . 
     In this embodiment, the nitride insulating film  15   a  is formed using a silicon nitride film. For the oxide insulating film  15   b , any of the oxides listed for the insulating film  15  in Embodiment 1 can be used as appropriate. The nitride insulating film  15   a  and the oxide insulating film  15   b  each can be formed by any of the listed methods for forming the insulating film  14  as appropriate. The oxide insulating films  83   a  and  83   b  can be formed using a material and a formation method similar to those of the oxide insulating film  23  described in Embodiment 1, as appropriate. The oxide insulating films  85   a  and  85   b  can be formed using a material and a formation method similar to those of the oxide insulating film  25  described in Embodiment 1, as appropriate. The nitride insulating film  87  can be formed using a material and a formation method similar to those of the nitride insulating film  27  described in Embodiment 1, as appropriate. The gate electrode  91  and the electrode  92  can be formed using a material and a formation method similar to those of the gate electrode  31  and the electrode  32  described in Embodiment 1, as appropriate. 
     The oxide insulating films  83   a  and  85   a  are isolated from the oxide insulating films  83   b  and  85   b . Further, the oxide insulating films  83   a  and  85   a  overlap with the oxide semiconductor film  17   a  and the oxide insulating films  83   b  and  85   b  overlap with the oxide semiconductor film  17   b . Specifically, in the cross-sectional view in the channel length direction of the transistor  440   a  illustrated in  FIG. 12B , end portions of the oxide insulating films  83   a  and  85   a  are positioned over the pair of electrodes  20   a  and  20   b , whereas in the cross-sectional view in the channel width direction of the transistor  440   a  illustrated in FIG.  12 C 1 , end portions of the oxide insulating films  83   a  and  85   a  are positioned on the outer sides of the side surfaces of the oxide semiconductor film  17   a . Similarly, in the cross-sectional view in the channel length direction of the transistor  440   b  illustrated in  FIG. 12B , end portions of the oxide insulating films  83   b  and  85   b  are positioned over the pair of electrodes  20   c  and  20   d , whereas in the cross-sectional view in the channel width direction of the transistor  440   b  illustrated in FIG.  12 C 2 , end portions of the oxide insulating films  83   b  and  85   b  are positioned on the outer sides of the side surfaces of the oxide semiconductor film  17   b.    
     The nitride insulating film  87  is formed so as to cover the top surfaces and side surfaces of the oxide insulating films  83   a ,  83   b ,  85   a , and  85   b , and is in contact with the nitride insulating film  15   a.    
     Note that in the channel length direction of the transistor  440   a , the end portions of the oxide insulating films  83   a  and  85   a  may be positioned over the nitride insulating film  15   a  instead of over the pair of electrodes  20   a  and  20   b . Further, in the channel length direction of the transistor  440   b , the end portions of the oxide insulating films  83   b  and  85   b  may be positioned over the nitride insulating film  15   a  instead of over the pair of electrodes  20   c  and  20   d.    
     In the cross-sectional view of the transistor  440   a  in the channel width direction illustrated in FIG.  12 C 1 , the gate electrode  91  faces the side surfaces of the oxide semiconductor film  17   a  with side surfaces of the oxide insulating films  83   a  and  85   a  positioned therebetween. 
     The transistor  440   a  of this embodiment has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, preferably greater than 1 μm and less than or equal to 4 μm, further preferably greater than 1 μm and less than or equal to 3.5 μm, still further preferably greater than 1 μm and less than or equal to 2.5 μm. Further, in the channel width direction of the transistor  440   a , the oxide semiconductor film  17   a  is provided between the gate electrode  13   a  and the gate electrode  91  with the insulating film  15  provided between the gate electrode  13   a  and the oxide semiconductor film  17   a  and with the insulating film  88   a  provided between the gate electrode  91  and the oxide semiconductor film  17   a . Furthermore, as illustrated in FIG.  12 A 1 , the gate electrode  91  overlaps with end portions of the oxide semiconductor film  17   a  with the insulating film  88   a  provided therebetween, when seen from the above. 
     Further, the transistor  440   b  has a longer channel length than the transistor  440   a . Thus, the cutoff current of the transistor  440   b  serving as a selection transistor in the pixel can be reduced. 
     In the channel width direction of the transistor  440   a , as illustrated in FIG.  12 C 1 , the opening portion  96  is provided in the insulating film  15  and the nitride insulating film  87  on an outer side of one side surface of the oxide semiconductor film  17   a . In the opening portion  96 , the gate electrode  91  is connected to the gate electrode  13   a . The gate electrode  91  faces the side surface of the oxide semiconductor film  17   a  in the channel width direction at the side surfaces of the oxide insulating films  83   a  and  85   a . The gate electrode  91  and the gate electrode  13   a  are not connected to each other on an outer side of the other side surface of the oxide semiconductor film  17   a  in the channel width direction. End portions of the gate electrode  91  are positioned on outer sides of the side surfaces of the oxide semiconductor film  17   a.    
     Note that although the transistor  440   a  has a structure in which the gate electrode  13   a  and the gate electrode  91  are connected to each other on an outer side of only one side surface of the oxide semiconductor film  17   a  in the channel width direction as illustrated in FIG.  12 C 1 , the gate electrode  13   a  and the gate electrode  91  may be connected to each other on the outer sides of both side surfaces of the oxide semiconductor film  17   a  in the channel width direction. 
     In the transistor  440   a  or the transistor  440   b  described in this embodiment, the oxide semiconductor film  17   a  and the oxide insulating film  85   a , or the oxide semiconductor film  17   b  and the oxide insulating film  85   b  are surrounded by the nitride insulating film  15   a  and the nitride insulating film  87 . The nitride insulating film  15   a  and the nitride insulating film  87  each have a small oxygen diffusion coefficient and have a barrier property against oxygen. Thus, part of oxygen contained in the oxide insulating films  85   a  and  85   b  can be moved to the oxide semiconductor films  17   a  and  17   b , so that the amount of oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced. In addition, the nitride insulating film  15   a  and the nitride insulating film  87  each have a small diffusion coefficient of water, hydrogen, and the like and have a barrier property against water, hydrogen, and the like, which can prevent diffusion of water, hydrogen, and the like from the outside into the oxide semiconductor films  17   a  and  17   b . For these reason, the transistor  440   a  and the transistor  440   b  have high reliability. 
     Next, a manufacturing process of the display device of this embodiment including the transistors  440   a  and  440   b  will be described with reference to  FIGS. 13A and 13B ,  FIGS. 14A and 14B , and  FIGS. 15A and 15B . 
     In each of  FIGS. 13A and 13B ,  FIGS. 14A and 14B , and  FIGS. 15A and 15B , a cross-sectional view in the channel length direction along line A 7 -B 7  and a cross-sectional view in the channel width direction along line C 7 -D 7  are used for describing a method for manufacturing the transistor  440   a , and a cross-sectional view in the channel length direction along line A 8 -B 8  is used for describing a method for manufacturing the transistor  440   b.    
     Note that the cross-sectional view of the transistor  440   b  in the channel width direction is similar to that of the transistor  440   a  except that the gate electrode  91  which is in contact with the gate electrode  13   a  in the opening portion  96  is not included. 
     In the manufacturing process of the transistors  440   a  and  440   b , the gate electrodes  13   a  and  13   b , the nitride insulating film  15   a , the oxide insulating film  14   b , the oxide semiconductor films  17   a  and  17   b , the pair of electrodes  20   a  and  20   b , and the pair of electrodes  20   c  and  20   d  are formed over the substrate  11  through steps similar to those in  FIGS. 3A to 3D  and  FIGS. 4A and 4B . In the steps, a photolithography process is performed using the first to third photomasks. 
     Then, the oxide insulating film  22  and the oxide insulating film  24  are formed as illustrated in  FIG. 13A . Subsequently, oxygen contained in the oxide insulating film  24  is partly transferred to the oxide semiconductor film  17   a  and the oxide semiconductor film  17   b  by heat treatment. This heat treatment can reduce the amount of oxygen vacancies contained in the oxide semiconductor films  17   a  and  17   b.    
     Next, a mask is formed over the oxide insulating film  24  by a photolithography process using a fourth photomask and then, the oxide insulating film  22  and the oxide insulating film  24  are partly etched to form the oxide insulating films  83   a  and  85   a , and the oxide insulating films  83   b  and  85   b  which are isolated from each other. Note that the oxide insulating film  14   b  is partly etched in the etching of the oxide insulating film  24 , whereby the oxide insulating film  15   b  is formed. As a result, the nitride insulating film  15   a  is exposed as illustrated in  FIG. 13B . In other words, the insulating film  15  including a step is formed. 
     After that, a nitride insulating film  86  illustrated in  FIG. 14A  is formed. In this step, in the cross-sectional view of the transistor  440   a  along line C 7 -D 7  in the channel width direction, the nitride insulating film  15   a  and the nitride insulating film  86  are in contact with each other. That is, the oxide semiconductor film  17   a  and the oxide insulating film  85   a  are surrounded by the nitride insulating film  15   a  and the nitride insulating film  86 . 
     Although not illustrated in the drawings, when the nitride insulating film  86  is formed, also in the cross-sectional view of the transistor  440   b  in the channel width direction, the oxide semiconductor film  17   b  and the oxide insulating film  85   b  are surrounded by the nitride insulating film  15   a  and the nitride insulating film  86 . 
     Next, after a mask is formed over the nitride insulating film  86  by a photolithography process using a fifth photomask, the nitride insulating film  86  is partly etched to form the opening portion  95 . Further, the nitride insulating film  15   a  and the nitride insulating film  86  are partly etched to form the opening portion  96  (see  FIG. 14B ). 
     After that, a conductive film  90  to be the gate electrode  91  and the electrode  92  of the transistor  440   a  is formed as illustrated in  FIG. 15A . The conductive film  90  can be formed in a manner similar to that of the conductive film  30  described in Embodiment 1. 
     Then, a mask is formed over the conductive film  90  by a photolithography process using a sixth photomask. Next, the conductive film  90  is partly etched using the mask to form the gate electrode  91  and the electrode  92 . After that, the mask is removed (see  FIG. 15B ). 
     Note that as illustrated in  FIG. 15B , in the cross-sectional view of the transistor  440   a  in the channel width direction, the gate electrode  91  is formed so as to face the side surfaces of the oxide semiconductor film  17   a  at the side surfaces of the oxide insulating film  83   a  and the oxide insulating film  85   a.    
     After that, heat treatment may be performed. The oxide insulating films  85   a  and  85   b  are each formed using an oxide insulating film containing oxygen at higher proportion than the stoichiometric composition. Further, the nitride insulating film  15   a  and the nitride insulating film  87  each have a high barrier property against oxygen. Accordingly, the heat treatment can reduce diffusion of oxygen contained in the oxide insulating films  85   a  and  85   b  to the outside. In addition, diffusion of oxygen contained in the oxide semiconductor films  17   a  and  17   b  to the outside can be also reduced. As a result, oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced. Further, the nitride insulating film  15   a  and the nitride insulating film  87  each have a high bather property against hydrogen, water, and the like, which can prevent diffusion of hydrogen, water, and the like from the outside into the oxide semiconductor films  17   a  and  17   b . Thus, hydrogen, water, and the like in the oxide semiconductor films  17   a  and  17   b  can be reduced. As a result, the highly reliable transistor  440   a  and the highly reliable transistor  440   b  can be manufactured. 
     Through the above process, the display device including the transistor  440   a  and the transistor  440   b  can be manufactured. 
     The structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in other embodiments. 
     Embodiment 3 
     In each of the selection transistors and driver transistors described in Embodiments 1 and 2, a base insulating film can be provided between the substrate  11  and the gate electrode  13   a  and between the substrate  11  and the gate electrode  13   b  as necessary. For the base insulating film, silicon oxide, silicon oxynitride, silicon nitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, aluminum oxynitride, and the like can be used, for example. Note that when silicon nitride, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, or the like is used for the base insulating film, it is possible to suppress diffusion of impurities such as alkali metal, water, and hydrogen from the substrate  11  into the oxide semiconductor films  17   a  and  17   b.    
     The base insulating film can be formed by a sputtering method, a CVD method, or the like. 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 4 
     In this embodiment, a display device in which the oxide semiconductor film  17   a , the pair of electrodes  20   a  and  20   b , the oxide semiconductor film  17   b , and the pair of electrodes  20   c  and  20   d  are different from those in the transistor  400   a  and the transistor  400   b  illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D will be described with reference to  FIG. 19 . Note that this embodiment can be applied to other transistors as appropriate. 
     As for the pair of electrodes provided in the transistor, it is possible to use a conductive material which is easily bonded to oxygen, such as tungsten, titanium, aluminum, copper, molybdenum, chromium, or tantalum, or an alloy thereof. Thus, oxygen contained in the oxide semiconductor films  17   a  and  17   b  and the conductive material contained in the electrodes  20   a  to  20   d  are bonded to each other, so that an oxygen deficient region is formed in the oxide semiconductor films  17   a  and  17   b . Furthermore, in some cases, part of constituent elements of the conductive material that forms the electrodes  20   a  to  20   d  is mixed into the oxide semiconductor films  17   a  and  17   b . Consequently, as shown in  FIG. 19 , low-resistance regions  21   a  to  21   d  are formed in the vicinity of regions of the oxide semiconductor films  17   a  and  17   b  which are in contact with the electrodes  20   a  to  20   d . Specifically, the low-resistance region  21   a  is in contact with the electrode  20   a  and is formed between the insulating film  15  and the electrode  20   a , whereas the low-resistance region  21   b  is in contact with the electrode  20   b  and is formed between the insulating film  15  and the electrode  20   b . In addition, the low-resistance region  21   c  is in contact with the electrode  20   c  and is formed between the insulating film  15  and the electrode  20   c , whereas the low-resistance region  21   d  is in contact with the electrode  20   d  and is formed between the insulating film  15  and the electrode  20   d.    
     Since the low-resistance regions  21   a  to  21   d  have high conductivity, contact resistance between the oxide semiconductor films  17   a  and  17   b  and the electrodes  20   a  to  20   d  can be reduced, and thus, the on-state current of the transistor can be increased. 
     Note that end portions of the low-resistance regions  21   a  and  21   b  may be substantially aligned with the end portions of the pair of electrodes  20   a  and  20   b . Alternatively, as illustrated in  FIG. 19 , the end portions of the low-resistance regions  21   a  and  21   b  may overlap with a region between the end portions of the pair of electrodes  20   a  and  20   b . Similarly, end portions of the low-resistance regions  21   c  and  21   d  may be substantially aligned with the end portions of the pair of electrodes  20   c  and  20   d . Alternatively, as illustrated in  FIG. 19 , the end portions of the low-resistance regions  21   c  and  21   d  may overlap with a region between the end portions of the pair of electrodes  20   c  and  20   d . In the case where the low-resistance regions  21   a  to  21   d  are formed in the oxide semiconductor films  17   a  and  17   b , a channel length is the distance between the low-resistance regions at the interface between the insulating film  28  and each of the oxide semiconductor films. 
     Furthermore, the electrodes  20   a  to  20   d  may each have a stacked-layer structure including the conductive material which is easily bonded to oxygen and a conductive material which is not easily bonded to oxygen, such as titanium nitride, tantalum nitride, or ruthenium. With such a stacked-layer structure, oxidization of the electrodes  20   a  to  20   d  can be prevented at the interface between the oxide insulating film  23  and the electrodes  20   a  to  20   d , so that an increase in the resistance of the electrodes  20   a  to  20   d  can be inhibited. 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 5 
     In this embodiment, a display device including a transistor in which the number of defects in an oxide semiconductor film can be further reduced as compared to Embodiments 1 to 4 is described with reference to drawings. The transistor described in this embodiment is different from any of the transistors in Embodiments 1 to 4 in that a multilayer film including a plurality of oxide semiconductor films is provided. 
     FIGS.  20 A 1 ,  20 A 2 ,  20 B,  20 C 1 , and  20 C 2  are top views and cross-sectional views of a transistor  105   a  and a transistor  105   b  included in the display device. The transistor  105   a  serves as a driver transistor for a light-emitting element included in a pixel. Further, the transistor  105   b  serves as a selection transistor of the pixel. 
     FIG.  20 A 1  is a top view of the transistor  105   a  and FIG.  20 A 2  is a top view of the transistor  105   b .  FIG. 20B  shows cross-sectional views taken along dashed-dotted line A 10 -B 10  in FIG.  20 A 1  and dashed-dotted line A 11 -B 11  in FIG.  20 A 2 . FIG.  20 C 1  is a cross-sectional view taken along dashed-dotted line C 10 -D 10  in FIG.  20 A 1 . FIG.  20 C 2  is a cross-sectional view taken along dashed-dotted line C 11 -D 11  in FIG.  20 A 2 . Note that in FIGS.  20 A 1  and  20 A 2 , the substrate  11 , insulating films, and the like are omitted for simplicity. 
     Unlike the transistor  400   a  and the transistor  400   b  included in the display device illustrated in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D, the transistor  105   a  and the transistor  105   b  included in the display device illustrated in FIGS.  20 A 1 ,  20 A 2 ,  20 B,  20 C 1 , and  20 C 2  include a multilayer film  98   a  and a multilayer film  98   b , respectively, between the insulating film  15  and the insulating film  28 . The other components are similar to those in FIGS.  2 A 1 ,  2 A 2 ,  2 B,  2 C 1 ,  2 C 2 , and  2 D; thus, the above description can be referred to. 
     In the transistor  105   a  described in this embodiment, the multilayer film  98   a  includes the oxide semiconductor film  17   a  and an oxide semiconductor film  97   a . Furthermore, in the transistor  105   b , the multilayer film  98   b  includes the oxide semiconductor film  17   b  and an oxide semiconductor film  97   b . That is, each of the multilayer film  98   a  and the multilayer film  98   b  has a two-layer structure. 
     In the transistor  105   a , part of the oxide semiconductor film  17   a  serves as a channel region, and in the transistor  105   b , part of the oxide semiconductor film  17   b  serves as a channel region. Further, the oxide insulating film  23  is formed in contact with the multilayer film  98   a  and the multilayer film  98   b , and the oxide insulating film  25  is formed in contact with the oxide insulating film  23 . In other words, the oxide semiconductor film  97   a  is provided between the oxide semiconductor film  17   a  and the oxide insulating film  23 , and the oxide semiconductor film  97   b  is provided between the oxide semiconductor film  17   b  and the oxide insulating film  23 . 
     The oxide semiconductor films  97   a  and  97   b  contain one or more elements that form the oxide semiconductor films  17   a  and  17   b . Thus, interface scattering is unlikely to occur at the interface between the oxide semiconductor films  17   a  and  97   a  and the interface between the oxide semiconductor films  17   b  and  97   b . Thus, the transistor can have high field-effect mobility because the movement of carriers is not hindered at the interfaces. 
     An oxide semiconductor film (hereinafter, an oxide semiconductor film  97 ) used for the oxide semiconductor films  97   a  and  97   b  is formed using a metal oxide containing at least In or Zn. Typical examples of the metal oxide include an In—Ga oxide, an In—Zn oxide, and an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd). The conduction band minimum of the oxide semiconductor film  97  is closer to a vacuum level than that of an oxide semiconductor film (hereinafter, the oxide semiconductor film  17 ) that is used for the oxide semiconductor films  17   a  and  17   b  is; as a typical example, the difference between the conduction band minimum of the oxide semiconductor film  97  and the conduction band minimum of the oxide semiconductor film  17  is any one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. That is, the difference between the electron affinity of the oxide semiconductor film  97  and the electron affinity of the oxide semiconductor film  17  is any one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     The oxide semiconductor film  97  preferably contains In because carrier mobility (electron mobility) can be increased. 
     When the oxide semiconductor film  97  contains a larger amount of Al, Ga, Y, Zr, La, Ce, or Nd than the amount of In in an atomic ratio, any of the following effects may be obtained: (1) the energy gap of the oxide semiconductor film  97  is widened; (2) the electron affinity of the oxide semiconductor film  97  decreases; (3) impurity diffusion from the outside is suppressed; and (4) an insulating property of the oxide semiconductor film  97  increases as compared to that of the oxide semiconductor film  17 . 
     Furthermore, an oxygen vacancy is less likely to be generated in the oxide semiconductor film  97  containing a larger amount of, Ga, Y, Zr, La, Ce, or Nd in an atomic ratio than the amount of In in an atomic ratio because, Ga, Y, Zr, La, Ce, or Nd is a metal element that is strongly bonded to oxygen. 
     In the case where the oxide semiconductor film  97  is an In-M-Zn oxide film, the proportion of In and the proportion of M, not taking Zn and O into consideration, are less than 50 atomic % and greater than or equal to 50 atomic %, respectively, and preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. 
     Furthermore, in the case where each of the oxide semiconductor films  17  and  97  contains an In-M-Zn oxide (M represents Ga, Y, Zr, La, Ce, or Nd), the proportion of M atoms (M represents Ga, Y, Zr, La, Ce, or Nd) in the oxide semiconductor film  97  is higher than that in the oxide semiconductor film  17 . As a typical example, the proportion of M in the oxide semiconductor film  17  is 1.5 or more times, preferably twice or more, further preferably three or more times as high as that in the oxide semiconductor film  17 . 
     Furthermore, in the case where each of the oxide semiconductor films  17  and  97  contains an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd), when In:M:Zn=x 1 :y 1 :z 1  [atomic ratio] is satisfied in the oxide semiconductor film  97  and In:M:Zn=x 2 :y 2 :z 2  [atomic ratio] is satisfied in the oxide semiconductor film  17 , y 1 /x 1  is higher than y 2 /x 2 , and preferably, y 1 /x 1  be 1.5 or more times as high as y 2 /x 2 . Alternatively, y 1 /x 1  is preferably twice or more as high as y 2 /x 2 . Further alternatively, y 1 /x 1  is preferably three or more times as high as y 2 /x 2 . In this case, it is preferable that in the oxide semiconductor film, y 2  be higher than or equal to x 2  because a transistor including the oxide semiconductor film can have stable electrical characteristics. However, when y 2  is 3 times or more as large as x 2 , the field-effect mobility of the transistor including the oxide semiconductor film is reduced; accordingly, y z  is preferably less than 3 times x 2 . 
     In the case where the oxide semiconductor film  17  contains an In-M-Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x 1 :y 1 :z 1  is used for forming the oxide semiconductor film  17 , x 1 /y 1  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6, and z 1 /y 1  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z 1 /y 1  is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film to be described later as the oxide semiconductor film  17  is easily formed. Typical examples of the atomic ratio of the metal elements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2. 
     In the case where the oxide semiconductor film  97  contains an In-M-Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x 2 :y 2 :z 2  is used for forming the oxide semiconductor film  97 , x 2 /y 2  is preferably less than x 1 /y 1 , and z 2 /y 2  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z 2 /y 2  is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film to be described later as the oxide semiconductor film  97  is easily formed. Typical examples of the atomic ratio of the metal elements of the target are In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, and the like. 
     Note that the proportion of each metal element in the atomic ratio of each of the oxide semiconductor films  17  and the oxide semiconductor film  97  varies within a range of ±40% of that in the above atomic ratio as an error. 
     The oxide semiconductor films  97   a  and  97   b  can relieve damage to the oxide semiconductor films  17   a  and  17   b  at the time of forming the oxide insulating film  25  later. Thus, the oxide insulating film  25  may be formed over the oxide semiconductor films  97   a  and  97   b  without the oxide insulating film  23 . 
     The thickness of each of the oxide semiconductor films  97   a  and  97   b  is greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm. 
     The oxide semiconductor films  97   a  and  97   b  may each have a non-single-crystal structure, for example, like the oxide semiconductor films  17   a  and  17   b . The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) that is described later, a polycrystalline structure, a microcrystalline structure described later, or an amorphous structure, for example. 
     The oxide semiconductor films  97   a  and  97   b  may each have an amorphous structure, for example. An amorphous oxide semiconductor film, for example, has disordered atomic arrangement and no crystalline component. Alternatively, an amorphous oxide film is, for example, absolutely amorphous and has no crystal part. 
     Note that the oxide semiconductor films  17  and  97  may each be a mixed film including two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure. The mixed film includes, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Furthermore, in some cases, the mixed film has a stacked-layer structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure. 
     In this case, the oxide semiconductor film  97   a  is provided between the oxide semiconductor film  17   a  and the oxide insulating film  23 , and the oxide semiconductor film  97   b  is provided between the oxide semiconductor film  17   b  and the oxide insulating film  23 . Thus, if trap states are generated between the oxide insulating film  23  and each of the oxide semiconductor films  97   a  and  97   b  by impurities and defects, electrons flowing in the oxide semiconductor films  17   a  and  17   b  are less likely to be captured by the trap states because there is a distance between the region where the trap states are generated and the oxide semiconductor film  17   a  or the oxide semiconductor film  17   b . Accordingly, the amount of on-state current of the transistor can be increased, and the field-effect mobility can be increased. When the electrons are captured by the trap states, the electrons become negative fixed charges. As a result, the threshold voltage of the transistor varies. However, by the distance between the region where the trap states are generated and the oxide semiconductor film  17   a  or the oxide semiconductor film  17   b , capture of the electrons by the trap states can be reduced, and accordingly fluctuations of the threshold voltage of each of the transistors  105   a  and  105   b  can be reduced. 
     The oxide semiconductor films  97   a  and  97   b  can block impurities from the outside, and accordingly, the amount of impurities that are transferred from the outside to the oxide semiconductor films  17   a  and  17   b  can be reduced. Furthermore, an oxygen vacancy is less likely to be formed in the oxide semiconductor films  97   a  and  97   b . Consequently, the impurity concentration and the number of oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced. 
     Note that the oxide semiconductor films  17  and  97  are not formed by simply stacking each film, but are formed to form a continuous junction (here, in particular, a structure in which the conduction band minimum is changed continuously between each film). In other words, a stacked-layer structure in which there exists no impurity that forms a defect level such as a trap center or a recombination center at each interface is provided. If an impurity exists between the oxide semiconductor films  17  and  97  that are stacked, a continuity of the energy band is damaged, and the carrier is captured or recombined at the interface and then disappears. 
     In order to form such a continuous energy band, it is necessary to form films continuously without being exposed to the air, with use of a multi-chamber deposition apparatus (sputtering apparatus) including a load lock chamber. Each chamber in the sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10 −7  Pa to 1×10 −4  Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity against the oxide semiconductor film, as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas containing carbon or hydrogen from an exhaust system to the inside of the chamber. 
     Note that as in a transistor  106   a  and a transistor  106   b  illustrated in  FIG. 21 , multilayer films  94   a  and  94   b  may be included instead of the multilayer films  98   a  and  98   b . The transistor  106   a  serves as a driver transistor in a pixel, and the transistor  106   b  serves as a selection transistor of the pixel. 
     An oxide semiconductor film  99   a , the oxide semiconductor film  17   a , and the oxide semiconductor film  97   a  are stacked in this order in the multilayer film  94   a . Further, an oxide semiconductor film  99   b , the oxide semiconductor film  17   b , and the oxide semiconductor film  97   b  are stacked in this order in the multilayer film  94   b . That is, the multilayer film  94   a  and the multilayer film  94   b  each have a three-layer structure. Note that the multilayer film  94   a  and the multilayer film  94   b  are formed in the same process. In the transistor  106   a , the oxide semiconductor film  17   a  serves as a channel region, and in the transistor  106   b , the oxide semiconductor film  17   b  serves as a channel region. 
     In the transistor  106   a  and the transistor  106   b , the insulating film  15  is in contact with the oxide semiconductor film  99   a  and the oxide semiconductor film  99   b . In other words, the oxide semiconductor film  99   a  or the oxide semiconductor film  99   b  is provided between the insulating film  15  and the oxide semiconductor film  17   a  or the oxide semiconductor film  17   b.    
     Furthermore, the oxide semiconductor films  97   a  and  97   b  are in contact with the oxide insulating film  23 . That is, the oxide semiconductor film  97   a  is provided between the oxide semiconductor film  17   a  and the oxide insulating film  23 , and the oxide semiconductor film  97   b  is provided between the oxide semiconductor film  17   b  and the oxide insulating film  23 . 
     An oxide semiconductor film (hereinafter, an oxide semiconductor film  99 ) used for the oxide semiconductor films  99   a  and  99   b  can be formed using a material and a formation method similar to those of the oxide semiconductor film  97 . 
     It is preferable that the thicknesses of the oxide semiconductor films  99   a  and  99   b  be smaller than those of the oxide semiconductor films  17   a  and  17   b . When the thickness of each of the oxide semiconductor films  99   a  and  99   b  is greater than or equal to 1 nm and less than or equal to 5 nm, preferably greater than or equal to 1 nm and less than or equal to 3 nm, the amount of change in the threshold voltage of the transistor can be reduced. 
     Note that as in the transistors  105   a  and  105   b , the oxide semiconductor films  97   a  and  97   b  included in the transistors  106   a  and  106   b  also serve as films that relieve damage to the oxide semiconductor films  17   a  and  17   b  at the time of forming the oxide insulating film  25  later. Thus, the oxide insulating film  25  may be formed over the oxide semiconductor films  97   a  and  97   b  without the oxide insulating film  23 . 
     In the transistors described in this embodiment, the oxide semiconductor film  97   a  and the oxide semiconductor film  97   b  are provided between the oxide insulating film  23  and the oxide semiconductor film  17   a  and between the oxide insulating film  23  and the oxide semiconductor film  17   b . A channel is formed in each of the oxide semiconductor films  17   a  and  17   b . Thus, if trap states are generated in the region between the oxide insulating film  23  and each of the oxide semiconductor films  97   a  and  97   b  by impurities and defects, electrons flowing in the oxide semiconductor films  17   a  and  17   b  are less likely to be captured by the trap states because there is a distance between the region where the trap states are generated and the oxide semiconductor film  17   a  or the oxide semiconductor film  17   b . Accordingly, the amount of on-state current of the transistor can be increased, and the field-effect mobility can be increased. When the electrons are captured by the trap states, the electrons become negative fixed charges. As a result, the threshold voltage of the transistor varies. However, by the distance between the region where the trap states are generated and the oxide semiconductor film  17   a  or the oxide semiconductor film  17   b , the number of electrons captured by the trap states can be reduced, and accordingly, fluctuations of the threshold voltage can be reduced. 
     The oxide semiconductor films  97   a  and  97   b  block entry of impurities from the outside, and accordingly, the amount of impurities that enter the oxide semiconductor films  17   a  and  17   b  from the outside can be reduced. Furthermore, an oxygen vacancy is less likely to be formed in the oxide semiconductor films  97   a  and  97   b . Consequently, the impurity concentration and the number of oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  in the transistor described in this embodiment can be reduced. 
     The oxide semiconductor film  99   a  and the oxide semiconductor film  99   b  are provided between the insulating film  15  and the oxide semiconductor film  17   a  and between the insulating film  15  and the oxide semiconductor film  17   b , respectively. Further, the oxide semiconductor film  97   a  and the oxide semiconductor film  97   b  are provided between the oxide semiconductor film  17   a  and the oxide insulating film  23  and between the oxide semiconductor film  17   b  and the oxide insulating film  23 , respectively. Thus, it is possible to reduce the concentration of silicon or carbon in the vicinity of the interface between the oxide semiconductor film  99   a  and the oxide semiconductor film  17   a  and in the vicinity of the interface between the oxide semiconductor film  99   b  and the oxide semiconductor film  17   b , in the oxide semiconductor films  17   a  and  17   b , or in the vicinity of the interface between the oxide semiconductor film  97   a  and the oxide semiconductor film  17   a  and in the vicinity of the interface between the oxide semiconductor film  97   b  and the oxide semiconductor film  17   b.    
     The transistor of this embodiment having such a structure includes very few defects in the multilayer film including the oxide semiconductor film where a channel is formed; thus, the electrical characteristics, typified by the on-state current and the field-effect mobility, of these transistors can be improved. Further, in a BT stress test and a BT photostress test that are examples of a stress test, the amount of change in threshold voltage is small, and thus, reliability is high. 
     &lt;Band Structure of Transistor&gt; 
     Next, band structures of the multilayer film  98   a  included in the transistor  105   a  illustrated in FIGS.  20 A 1 ,  20 B, and  20 C 1  and the multilayer film  94   a  included in the transistor  106   a  illustrated in  FIG. 21  are described with reference to  FIGS. 22A to 22C . Note that the multilayer film  98   b  included in the transistor  105   b  has a structure similar to that of the multilayer film  98   a . Further, the multilayer film  94   b  included in the transistor  106   b  has a structure similar to that of the multilayer film  94   a . Thus, the multilayer film  98   a  and the multilayer film  94   a  can be alternately referred to as the multilayer film  98   b  and the multilayer film  94   b , respectively, in the description below. 
     Here, for example, an In—Ga—Zn oxide having an energy gap of 3.15 eV is used for the oxide semiconductor film  17   a , and an In—Ga—Zn oxide having an energy gap of 3.5 eV is used for the oxide semiconductor film  97   a . The energy gaps were measured using a spectroscopic ellipsometer (UT-300 manufactured by HORIBA JOBIN YVON SAS.). 
     The energy difference between the vacuum level and the valence band maximum (also called ionization potential) of the oxide semiconductor film  17   a  and the energy difference between the vacuum level and the valence band maximum of the oxide semiconductor film  97   a  were 8 eV and 8.2 eV, respectively. Note that the energy difference between the vacuum level and the valence band maximum was measured using an ultraviolet photoelectron spectroscopy (UPS) device (VersaProbe manufactured by ULVAC-PHI, Inc.). 
     Thus, the energy difference between the vacuum level and the conduction band minimum (also called electron affinity) of the oxide semiconductor film  17   a  and the energy gap therebetween of the oxide semiconductor film  97   a  were 4.85 eV and 4.7 eV, respectively. 
       FIG. 22A  schematically illustrates a part of the band structure of the multilayer film  98   a . Here, the case where silicon oxide films are used for the insulating film  15  and the oxide insulating film  23  and the silicon oxide films are provided in contact with the multilayer film  98   a  is described. In  FIG. 22A , EcI1 denotes the energy of the bottom of the conduction band in the silicon oxide film; EcS1 denotes the energy of the bottom of the conduction band in the oxide semiconductor film  17   a ; EcS2 denotes the energy of the bottom of the conduction band in the oxide semiconductor film  97   a ; and EcI2 denotes the energy of the bottom of the conduction band in the silicon oxide film. Furthermore, EcI1 and EcI2 correspond to the insulating film  15  and the oxide insulating film  23  in  FIG. 20B , respectively. 
     As illustrated in  FIG. 22A , there is no energy barrier between the oxide semiconductor films  17   a  and  97   a , and the energy level of the bottom of the conduction band gradually changes therebetween. In other words, the energy level of the conduction band minimum is continuously changed. This is because the multilayer film  98   a  contains an element contained in the oxide semiconductor film  17   a  and oxygen is transferred between the oxide semiconductor films  17   a  and  97   a , so that a mixed layer is formed. 
     As shown in  FIG. 22A , the oxide semiconductor film  17   a  in the multilayer film  98   a  serves as a well and a channel region of the transistor including the multilayer film  98   a  is formed in the oxide semiconductor film  17   a . Note that since the conduction band minimum of the multilayer film  98   a  is continuously changed, it can be said that the oxide semiconductor films  17   a  and  97   a  are continuous. 
     Although trap states due to impurities or defects might be generated in the vicinity of the interface between the oxide semiconductor film  97   a  and the oxide insulating film  23  as shown in  FIG. 22A , the oxide semiconductor film  17   a  can be distanced from the region where the trap states are generated owing to the existence of the oxide semiconductor film  97   a . However, when the energy difference between EcS1 and EcS2 is small, an electron in the oxide semiconductor film  17   a  might reach the trap state across the energy difference. When the electron is captured by the trap state, a negative fixed charge is generated at the interface with the insulating film, whereby the threshold voltage of the transistor shifts in the positive direction. Thus, it is preferable that the energy difference between EcS1 and EcS2 be 0.1 eV or more, further preferably 0.15 eV or more, because change in the threshold voltage of the transistor is reduced and stable electrical characteristics are obtained. 
       FIG. 22B  schematically illustrates a part of the band structure of the multilayer film  98   a , which is a variation of the band structure shown in  FIG. 22A . Here, a structure where silicon oxide films are used for the insulating film  15  and the oxide insulating film  23  and the silicon oxide films are in contact with the multilayer film  98   a  is described. In  FIG. 22B , EcI1 denotes the conduction band minimum in the silicon oxide film; EcS1 denotes the conduction band minimum in the oxide semiconductor film  17   a ; and EcI2 denotes the conduction band minimum in the silicon oxide film. Further, EcI1 and EcI2 correspond to the insulating film  15  and the oxide insulating film  23  in  FIG. 20B , respectively. 
     In the transistor illustrated in  FIG. 20B , an upper portion of the multilayer film  98   a , that is, the oxide semiconductor film  97   a  might be etched in formation of the pair of electrodes  20   a  and  20   b . Furthermore, a mixed layer of the oxide semiconductor films  17   a  and  97   a  is likely to be formed on the top surface of the oxide semiconductor film  17   a  in formation of the oxide semiconductor film  97   a.    
     For example, when the oxide semiconductor film  17   a  is an oxide semiconductor film formed with use of, as a sputtering target, In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 or In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 3:1:2, and the oxide semiconductor film  97   a  is an oxide film formed with use of, as a sputtering target, In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:4, or In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, the Ga content in the oxide semiconductor film  97   a  is higher than that in the oxide semiconductor film  17   a . Thus, a GaO x  layer or a mixed layer whose Ga content is higher than that in the oxide semiconductor film  17   a  can be formed on the top surface of the oxide semiconductor film  17   a.    
     For that reason, even in the case where the oxide semiconductor film  97   a  is etched, the conduction band minimum of EcS1 on the EcI2 side is increased and the band structure shown in  FIG. 22B  can be obtained in some cases. 
     As in the band structure shown in  FIG. 22B , in observation of a cross section of a channel region, only the oxide semiconductor film  17   a  in the multilayer film  98   a  is apparently observed in some cases. However, a mixed layer that contains Ga more than the oxide semiconductor film  17   a  does is formed over the oxide semiconductor film  17   a  in fact, and thus the mixed layer can be regarded as a 1.5-th layer. Note that the mixed layer can be confirmed by analyzing a composition in the upper portion of the oxide semiconductor film  17   a , when the elements contained in the multilayer film  98   a  are measured by an EDX analysis, for example. The mixed layer can be confirmed, for example, in such a manner that the Ga content in the composition in the upper portion of the oxide semiconductor film  17   a  is larger than the Ga content in the oxide semiconductor film  17   a.    
       FIG. 22C  schematically illustrates a part of the band structure of the multilayer film  94   a  illustrated in  FIG. 21 . Here, the case where silicon oxide films are used for the insulating film  15  and the oxide insulating film  23  and the silicon oxide films are in contact with the multilayer film  94   a  is described. In  FIG. 22C , EcI1 denotes the energy of the conduction band minimum of the silicon oxide film; EcS1 denotes the energy of the conduction band minimum of the oxide semiconductor film  17   a ; EcS2 denotes the energy of the conduction band minimum of the oxide semiconductor film  97   a ; EcS3 denotes the energy of the conduction band minimum of the oxide semiconductor film  99   a ; and EcI2 denotes the energy of the conduction band minimum of the silicon oxide film. Further, EcI1 and EcI2 correspond to the insulating film  15  and the oxide insulating film  23  in  FIG. 21 , respectively. 
     As illustrated in  FIG. 22C , there is no energy barrier between the oxide semiconductor films  99   a ,  17   a , and  97   a , and the conduction band minimums thereof smoothly vary. In other words, the conduction band minimums are continuous. This is because the multilayer film  94   a  contains an element contained in the oxide semiconductor film  17   a  and oxygen is transferred between the oxide semiconductor films  17   a  and  97 , so that a mixed layer is formed. 
     As shown in  FIG. 22C , the oxide semiconductor film  17   a  in the multilayer film  94   a  serves as a well and a channel region of the transistor including the multilayer film  94   a  is formed in the oxide semiconductor film  17   a . Note that since the energy of the bottom of the conduction band of the multilayer film  94   a  is continuously changed, it can be said that the oxide semiconductor films  99   a ,  17   a , and  97   a  are continuous. 
     Although trap states due to impurities or defects might be generated in the vicinity of the interface between the multilayer film  94   a  and the oxide insulating film  23  and in the vicinity of the interface between multilayer film  94   a  and the insulating film  15 , as illustrated in  FIG. 22C , the oxide semiconductor film  17   a  can be distanced from the region where the trap states are generated owing to the existence of the oxide semiconductor films  97   a  and  99   a . However, when the energy difference between EcS1 and EcS2 and the energy difference between EcS1 and EcS3 are small, electrons in the oxide semiconductor film  17   a  might reach the trap state across the energy difference. When the electrons are captured by the trap state, a negative fixed charge is generated at the interface with the insulating film, whereby the threshold voltage of the transistor shifts in the positive direction. Thus, it is preferable that the energy difference between EcS1 and EcS2 and the energy difference between EcS1 and EcS3 be 0.1 eV or more, further preferably 0.15 eV or more, because change in the threshold voltage of the transistor is reduced and stable electrical characteristics are obtained. 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 6 
     In this embodiment, one embodiment that can be applied to an oxide semiconductor film in the transistor included in the semiconductor device described in the above embodiment is described. 
     It is preferable that the oxide semiconductor film include a CAAC-OS film. The CAAC-OS film includes crystals whose c-axes are aligned; however, no grain boundary between the crystals is clearly observed. The crystals whose c-axes are aligned are less likely to be etched, so that in a channel-etched transistor, a small amount of an oxide semiconductor film is overetched when a pair of electrodes is formed. For this reason, with the use of a CAAC-OS film for the oxide semiconductor film, a channel-etched transistor can be manufactured. In particular, a channel-etched transistor used as a driver transistor can have a structure in which the distance between a pair of electrodes, that is, a channel length, is as short as greater than or equal to 0.5 μm and less than or equal to 4.5 μm. 
     The oxide semiconductor film may include one or more of an oxide semiconductor having a single-crystal structure (hereinafter referred to as a single-crystal oxide semiconductor), an oxide semiconductor having a polycrystalline structure (hereinafter referred to as a polycrystalline oxide semiconductor), an oxide semiconductor having a microcrystalline structure (hereinafter referred to as a microcrystalline oxide semiconductor), and an oxide semiconductor having an amorphous structure (hereinafter referred to as an amorphous oxide semiconductor). 
     Described below are the CAAC-OS, the single-crystal oxide semiconductor, the polycrystalline oxide semiconductor, the microcrystalline oxide semiconductor, and the amorphous oxide semiconductor. 
     &lt;CAAC-OS&gt; 
     The CAAC-OS film is one of oxide semiconductor films having a plurality of crystal parts. The crystal parts included in the CAAC-OS film each have c-axis alignment. In a plan TEM image, the area of the crystal parts included in the CAAC oxide film is greater than or equal to 2500 nm 2 , preferably greater than or equal to 5 μm 2 , further preferably greater than or equal to 1000 μm 2 . Furthermore, in a cross-sectional TEM image, when the proportion of the crystal parts is greater than or equal to 50%, preferably greater than or equal to 80%, further preferably greater than or equal to 95% of the CAAC-OS film, the CAAC-OS film is a thin film having physical properties similar to those of a single crystal. 
     In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film. Note that in this specification, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     Note that in an electron diffraction pattern of the CAAC-OS film, spots (luminescent spots) having alignment are shown. 
     From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film. 
     A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. When the CAAC-OS film is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is assigned to the (00x) plane (x is an integer) of an InGaZn oxide crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is assigned to the (110) plane of the InGaZn oxide crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZn oxide, six peaks appear. The six peaks are assigned to crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when φ scan is performed with 2φ fixed at around 56°. 
     According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axes of the crystal part are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axes of the crystal part might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. 
     Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies depending on regions. 
     Note that when the CAAC-OS film is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal part having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ do not appear at around 36°. 
     The CAAC-OS film is an oxide semiconductor film having low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor film, such as silicon, disturbs the atomic arrangement of the oxide semiconductor film by depriving the oxide semiconductor film of oxygen and causes a reduction in crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor film and causes a reduction in crystallinity when it is contained in the oxide semiconductor film. Note that the impurity contained in the oxide semiconductor film might serve as a carrier trap or a carrier generation source. 
     Further, the CAAC-OS film is an oxide semiconductor film having a low density of defect states. For example, an oxygen vacancy in the oxide semiconductor film serves as a carrier trap or a carrier generation source in some cases when hydrogen is captured therein. 
     The state in which the impurity concentration is low and the density of defect states is low (the number of oxygen vacancies is small) is referred to as a highly purified intrinsic state or a substantially highly purified intrinsic state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor including the oxide semiconductor film rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has small variations in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released, and might behave like fixed electric charge. Thus, the transistor that includes the oxide semiconductor film having high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases. 
     With the use of the CAAC-OS film in a transistor, variations in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light are small. 
     &lt;Single Crystal Oxide Semiconductor&gt; 
     The single-crystal oxide semiconductor film has a lower impurity concentration and a lower density of defect states (few oxygen vacancies). Thus, the carrier density can be decreased. Accordingly, a transistor including the single-crystal oxide semiconductor film is unlikely to be normally on. Moreover, since the single-crystal oxide semiconductor film has a lower impurity concentration and a lower density of defect states, carrier traps might be reduced. Thus, the transistor including the single-crystal oxide semiconductor film has small variation in electrical characteristics and accordingly has high reliability. 
     Note that when the oxide semiconductor film has few defects, the density thereof is increased. When the oxide semiconductor film has high crystallinity, the density thereof is increased. When the oxide semiconductor film has a lower concentration of impurities such as hydrogen, the density thereof is increased. The single-crystal oxide semiconductor film has a higher density than the CAAC-OS film. The CAAC-OS film has a higher density than the microcrystalline oxide semiconductor film. The polycrystalline oxide semiconductor film has a higher density than the microcrystalline oxide semiconductor film. The microcrystalline oxide semiconductor film has a higher density than the amorphous oxide semiconductor film. 
     &lt;Polycrystalline Oxide Semiconductor&gt; 
     In an image obtained with a TEM, for example, crystal grains can be found in the polycrystalline oxide semiconductor film. In most cases, the size of a crystal grain in the polycrystalline oxide semiconductor film is greater than or equal to 2 nm and less than or equal to 300 nm, greater than or equal to 3 nm and less than or equal to 100 nm, or greater than or equal to 5 nm and less than or equal to 50 nm in an image obtained with the TEM, for example. Moreover, in an image obtained with the TEM, a boundary between crystals can be found in the polycrystalline oxide semiconductor film in some cases. 
     The polycrystalline oxide semiconductor film may include a plurality of crystal grains, and alignment of crystals may be different in the plurality of crystal grains. The polycrystalline oxide semiconductor film is subjected to structural analysis with an XRD apparatus. For example, when the polycrystalline oxide semiconductor film including an InGaZn oxide crystal is analyzed by an out-of-plane method, peaks of 2θ appear at around 31°, 36°, and the like in some cases. 
     The polycrystalline oxide semiconductor film has high crystallinity and thus has high electron mobility in some cases. Accordingly, a transistor including the polycrystalline oxide semiconductor film has high field-effect mobility. Note that there are cases in which an impurity is segregated at the boundary in the polycrystalline oxide semiconductor film. Moreover, the boundary of the polycrystalline oxide semiconductor film serves as a defect state. Since the boundary of the polycrystalline oxide semiconductor film may serve as a carrier trap or a carrier generation source, a transistor including the polycrystalline oxide semiconductor film has larger variation in electrical characteristics and lower reliability than a transistor including a CAAC-OS film in some cases. 
     &lt;Microcrystalline Oxide Semiconductor&gt; 
     In an image obtained with a TEM, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor in some cases. In most cases, the size of a crystal part in the microcrystalline oxide semiconductor film is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. An oxide semiconductor film including nanocrystal (nc), which is a microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm, is specifically referred to as a nanocrystalline oxide semiconductor (nc-OS) film. In an image of the nc-OS film obtained with a TEM, for example, a crystal grain cannot be observed clearly in some cases. 
     In the nc-OS film, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Further, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor film depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Further, a halo pattern is observed in an electron diffraction pattern (also referred to as a selected-area electron diffraction pattern) of the nc-OS film obtained by using an electron beam having a diameter (e.g., larger than or equal to 50 nm) larger than the diameter of a crystal part. Meanwhile, spots are observed in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a diameter (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm) close to, or smaller than or equal to the diameter of a crystal part. In some cases, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are observed. Further, in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots are shown in a ring-like region in some cases. 
     Since an nc-OS film is an oxide semiconductor film having more regularity than an amorphous oxide semiconductor film, the nc-OS film has a lower density of defect states than the amorphous oxide semiconductor film. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; hence, the nc-OS film has a higher density of defect states than a CAAC-OS film. 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 7 
     In the method for manufacturing any of the display devices described in Embodiments 1 to 6, after the electrodes  20   a  to  20   d  are formed over the oxide semiconductor films  17   a  and  17   b , the oxide semiconductor films  17   a  and  17   b  may be exposed to plasma generated in an oxidizing atmosphere to be supplied with oxygen. Atmospheres of oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be given as examples of oxidizing atmospheres. Further, in the plasma treatment, the oxide semiconductor films  17   a  and  17   b  are preferably exposed to plasma generated with no bias applied to the substrate  11  side. Consequently, the oxide semiconductor films  17   a  and  17   b  can be supplied with oxygen without being damaged; accordingly, the number of oxygen vacancies in the oxide semiconductor films  17   a  and  17   b  can be reduced. Moreover, impurities, e.g., halogen such as fluorine or chlorine remaining on a surface of the oxide semiconductor films  17   a  and  17   b  due to the etching treatment can be removed. The plasma treatment is preferably performed while heating is performed at a temperature higher than or equal to 300° C. Oxygen in the plasma is bonded to hydrogen contained in the oxide semiconductor films  17   a  and  17   b  to form water. Since the substrate is heated, the water is released from the oxide semiconductor films  17   a  and  17   b . Consequently, the amount of hydrogen and water in the oxide semiconductor films  17   a  and  17   b  can be reduced. 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 8 
     In this embodiment, the structure of a pixel included in the display device of one embodiment of the present invention will be described with reference to drawings. Note that in the drawings, portions similar to those denoted by the reference numerals in the above embodiments and portions having functions similar to those denoted by the reference numerals in above embodiments are given by the same reference numerals, and detailed description thereof is omitted. 
       FIG. 23  illustrates a circuit configuration that can be used for the pixel in the display device. 
     A pixel  603  illustrated in  FIG. 23  includes a transistor  300   b  serving as a selection transistor which controls writing of a data signal, a transistor  300   a  serving as a driver transistor, a transistor  300   c , the capacitor  370 , and the light-emitting element  350 . 
     One of a source electrode and a drain electrode of the transistor  300   a  is electrically connected to a wiring ANO1 serving as an anode line. The other of the source electrode and the drain electrode of the transistor  300   a  is electrically connected to one electrode of the light-emitting element  350 . Further, gate electrodes of the transistor  300   a  are electrically connected to one of a source electrode and a drain electrode of the transistor  300   b  and one electrode of the capacitor  370 . 
     The transistor  300   a  has a function of controlling current flowing in the light-emitting element  350  by being turned on or off. In this embodiment, the transistor  300   a  has a channel length greater than or equal to 0.5 μm and less than or equal to 4.5 μm, and includes a first gate electrode over an oxide semiconductor film and a second gate electrode below the oxide semiconductor film. Note that the first gate electrode and the second gate electrode are electrically connected to each other. Thus, in the transistor  300   a , on-state current and field-effect mobility are improved and a negative shift in the threshold voltage is suppressed. 
     The other of a source electrode and a drain electrode of the transistor  300   b  is electrically connected to a signal line SL to which a data signal is supplied. A gate electrode of the transistor  300   b  is electrically connected to a scan line GL to which a gate signal is supplied. 
     The transistor  300   b  has a function of controlling writing of a data signal by being turned on or off. This means that the transistor  300   b  serves as a selection transistor. 
     In this embodiment, the transistor  300   b  has a longer channel length than the transistor  300   a . A negative shift in the threshold voltage of the transistor  300   b  is suppressed, so that cutoff current is low. 
     One of a source electrode and a drain electrode of the transistor  300   c  is connected to a wiring ML to which a reference potential of data is supplied, and the other is electrically connected to the one electrode of the light-emitting element  350  and the other electrode of the capacitor  370 . Moreover, a gate electrode of the transistor  300   c  is electrically connected to the scan line GL to which the gate signal is supplied. 
     The transistor  300   c  has a function of adjusting the current flowing in the light-emitting element  350 . For example, in the case where the threshold voltage or the field-effect mobility of the transistor  300   a  varies or the transistor  300   a  deteriorates, current that flows in each light-emitting element  350  can be corrected through monitoring of current flowing in the wiring ML. The wiring ML can be supplied with, for example, voltage lower than or equal to the threshold voltage of the light-emitting element  350 . 
     In this embodiment, the channel length of the transistor  300   c  is preferably longer than the channel length of the transistor  300   a , for example. Note that the transistor  300   c  can have a single-gate structure, or have a dual-gate structure like the transistor  300   a . It is preferable that the transistor  300   c  have a single-gate structure because a region where a first gate electrode and a second gate electrode are in contact with each other is not necessary, so that the area of the transistor can be reduced, which can increase the aperture ratio of the pixel. 
     One of the pair of electrodes of the capacitor  370  is electrically connected to one of the source electrode and the drain electrode of the transistor  300   b  and the gate electrode of the transistor  300   a , and the other of the pair of electrodes of the capacitor  370  is electrically connected to the other of the source electrode and the drain electrode of the transistor  300   c  and one electrode of the light-emitting element  350 . 
     In the structure of the pixel  603  in  FIG. 23 , the capacitor  370  serves as a storage capacitor for storing written data. 
     The one of the pair of electrodes of the light-emitting element  350  is electrically connected to the other of the source and drain electrodes of the transistor  300   a , the other electrode of the capacitor  370 , and the other of the source and drain electrodes of the transistor  300   c . Furthermore, the other of the pair of electrodes of the light-emitting element  350  is electrically connected to a wiring CAT serving as a cathode. 
     As the light-emitting element  350 , an organic electroluminescent element (also referred to as an organic EL element) or an inorganic EL element can be used, for example. 
     A wiring ANO2 extends in a direction parallel to the wiring ML. The wiring ANO2 is connected to the wiring ANO1 serving as an anode line, so that the wiring resistance of the wiring ANO1 can be reduced. Thus, voltage drop of the wiring in the display device using a large substrate can be reduced; accordingly, unevenness in luminance of the display device can be reduced. 
     A high power supply potential VDD is supplied to either of the wirings ANO1 and ANO2 and the wiring CAT, and a low power supply potential VSS is supplied to the other. In the structure of  FIG. 23 , the high power supply potential VDD is supplied to the wirings ANO1 and ANO2, and the low power supply potential VSS is supplied to the wiring CAT. 
     In the display device including the pixel  603  in  FIG. 23 , the pixels  603  are sequentially selected row by row by the scan line driver circuit, whereby the transistor  300   b  is turned on and a data signal is written. 
     When the transistor  300   b  is turned off, the pixels  603  in which the data has been written are brought into a holding state. Moreover, the transistor  300   b  is connected to the capacitor  370 ; thus, the written data can be held for a long time. Further, the amount of current flowing between the source electrode and the drain electrode of the transistor  300   a  is controlled. The light-emitting element  350  emits light with a luminance corresponding to the amount of flowing current. 
     Next, a structure which can be used for the pixel  603  illustrated in  FIG. 23  will be described below with reference to  FIG. 16 ,  FIG. 17 , and  FIG. 18 . 
       FIG. 16  shows a part of a top view of a pixel circuit that can be used for the pixel  603 .  FIG. 17  shows a cross section taken along dashed-dotted line X 1 -X 2  in  FIG. 16 .  FIG. 18  shows a cross section taken along dashed-dotted lines X 3 -X 4  and X 5 -X 6  in  FIG. 16 . 
     In  FIG. 16 , a wiring GL serving as a scan line extends substantially perpendicularly to a signal line (in the horizontal direction in the drawing). A wiring SL serving as the signal line extends substantially perpendicularly to the scan line (in the vertical direction in the drawing). A wiring ML which is supplied with a reference potential of data extends in a direction parallel to the wiring SL. The wiring ANO2 serving as an anode line extends in a direction parallel to the wiring SL and the wiring ML. 
     The transistors  300   a ,  300   b , and  300   c  are provided in the pixel  603 . Note that each of the transistors  300   a ,  300   b , and  300   c  includes a conductive film serving as a gate electrode, a gate insulating film, an oxide semiconductor film which is over the gate insulating film and in which a channel region is formed, and conductive films serving as a pair of electrodes. For example, the transistor  300   a  includes the gate electrode  13   a , a gate insulating film (not illustrated), the oxide semiconductor film  17   a , and the electrodes  20   a  and  20   b.    
     The transistor  300   b  includes the gate electrode  13   b , a gate insulating film (not illustrated), the oxide semiconductor film  17   b , and the electrode  20   c  and  20   d.    
     Note that although the structure of the transistor  300   c  is not particularly mentioned, the transistor  300   c  can have a structure similar to the structure of the transistor  300   b.    
     An electrode  13   c  is electrically connected to the electrode  20   a  through an opening portion  352   a . The electrode  20   b  is electrically connected to a pixel electrode  322  through opening portions  354  and  356   b . An electrode  13   d  is electrically connected to an electrode  20   e  through opening portions  352   b  and  352   c.    
     The gate electrode  13   a  is formed under the electrode  20   b . The electrode  20   b , a dielectric film formed over the gate electrode  13   a , and the gate electrode  13   a  form a capacitor. The capacitor corresponds to the capacitor  370  illustrated in  FIG. 23 . 
     Next, cross sections taken along dashed-dotted lines X 1 -X 2 , X 3 -X 4 , and X 5 -X 6  in  FIG. 16  will be described with reference to  FIG. 17  and  FIG. 18 . 
     The pixel illustrated in  FIG. 17  and  FIG. 18  includes the substrate  11 ; the gate electrodes  13   a  and  13   b  and the electrodes  13   c  and  13   d  over the substrate  11 ; insulating films  306   a  and  306   b  which are formed over the substrate  11 , the gate electrodes  13   a  and  13   b , and the electrodes  13   c  and  13   d ; the oxide semiconductor films  17   a  and  17   b  over the insulating film  306   b ; the pair of electrodes  20   a  and  20   b  and the pair of electrodes  20   c  and  20   d  over the oxide semiconductor film  17   a  and the oxide semiconductor film  17   b , respectively; the electrode  20   e  which is formed in the same step as the electrodes  20   a  to  20   d ; an oxide insulating film  314  formed over the oxide semiconductor films  17   a  and  17   b  and the electrodes  20   a  to  20   e ; a nitride insulating film  316  formed over the oxide insulating film  314 ; a gate electrode  320  formed over the nitride insulating film  316  to overlap with the oxide semiconductor film  17   a ; an insulating film  318  which is over the nitride insulating film  316  and has an opening portion  356   a  in a position overlapping with the oxide semiconductor film  17   a ; the pixel electrode  322  which is formed in the same step as the gate electrode  320  and formed over the insulating film  318 ; and an insulating film  324  which is formed to cover the transistor and an end portion of the pixel electrode  322 . 
     The insulating films  306   a  and  306   b  serve as gate insulating films of the transistors  300   a  and  300   b  (a first gate insulating film in the transistor  300   a ), and the oxide insulating film  314  and the nitride insulating film  316  serve as a second gate insulating film of the transistor  300   a.    
     In a region between the electrode  20   b  and the gate electrode  13   a , the insulating films  306   a  and  306   b  serve as a dielectric. That is, the electrode  20   b , the insulating films  306   a  and  306   b , and the gate electrode  13   a  form the capacitor. 
     An EL layer  326  is formed over the pixel electrode  322  and the insulating film  324 . An electrode  328  is formed over the EL layer  326 . The pixel electrode  322 , the EL layer  326 , and the electrode  328  form the light-emitting element  350 . The EL layer  326  includes at least a light-emitting layer containing a light-emitting substance. In addition to the light-emitting layer, a functional layer such as a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, or a charge generation layer may be formed. In the EL layer  326 , electrons and holes are injected from a pair of electrodes (here, the pixel electrode  322  and the electrode  328 ), and current flows. The electrons and holes are recombined, and thus, the light-emitting substance is excited. The light-emitting substance returns to a ground state from the excited state, thereby emitting light. 
     The insulating film  318  may have a function of planarizing unevenness formed under the pixel electrode  322 . The insulating film  318  can be formed using an organic insulating film, for example. 
     The insulating film  324  has a function of separating EL layers  326  in adjacent pixels; that is, the insulating film  324  has a function as a partition wall. The insulating film  324  has an insulating property and can be formed using an organic insulating film or an inorganic insulating film, for example. As the organic insulating film, for example, a polyimide-based resin, a polyamide-based resin, an acrylic-based resin, a siloxane-based resin, an epoxy-based resin, a phenol-based resin, or the like can be used. As the inorganic insulating material, silicon oxide, silicon oxynitride, or the like can be used. In particular, a photosensitive organic resin material is preferably used, in which case the insulating film  324  is easily formed. 
     The opening portion  352   a  is formed in the insulating films  306   a  and  306   b  over the electrode  13   c . The electrode  13   c  is connected to the electrode  20   a  through the opening portion  352   a . The opening portions  352   b  and  352   c  are formed in the insulating films  306   a  and  306   b  over the electrode  13   d . The electrode  13   d  is electrically connected to the electrode  20   e  through the opening portions  352   b  and  352   c . With a plurality of opening portions such as the opening portions  352   b  and  352   c  formed over the electrode  13   d , contact resistance between the electrode  13   d  and the electrode  20   e  can be reduced. Note that  FIG. 18  illustrates the case of providing two opening portions, i.e., the opening portions  352   b  and  352   c . However, the number of opening portions is not limited thereto. One opening portion, or three or more opening portions may be formed. 
     The opening portion  356   a  is formed in the insulating film  318  over the transistor  300   a . The opening portion  356   a  can shorten the distance between the gate electrode  320  and the oxide semiconductor film  17   a . Therefore, an electric field from the gate electrode  320  can be preferably applied to the oxide semiconductor film  17   a.    
     The opening portion  354  is formed in the oxide insulating film  314  and the nitride insulating film  316  over the electrode  20   b . The opening portion  356   b  is formed in the insulating film  318  over the opening portion  354 . The electrode  20   b  is electrically connected to the pixel electrode  322  through the opening portions  354  and  356   b.    
     Note that the description in Embodiment 1 or Embodiment 2 can be referred to for materials which can be used for the pixel illustrated in  FIGS. 17 and 18 . 
     Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 9 
     In this embodiment, an example of an active matrix display device that is one embodiment of the present invention will be described with reference to  FIGS. 24A and 24B . 
       FIG. 24A  is a top view of the display device of one embodiment of the present invention.  FIG. 24B  is a cross-sectional view taken along the dashed-dotted lines M 1 -M 2  and N 1 -N 2 . 
     An active matrix display device illustrated in  FIGS. 24A and 24B  includes, over a support substrate  801 , a light-emitting portion  802 , a driver circuit portion  803  (e.g., gate line driver circuit), a driver circuit portion  804  (e.g., signal line driver circuit), and a sealant  805 . The light-emitting portion  802  and the driver circuit portions  803  and  804  are sealed in a space  810  surrounded by the support substrate  801 , the sealing substrate  806 , and the sealant  805 . 
     The driver circuit portions  803  and  804  can be formed using the transistor described in any of the above embodiment. Note that the driver circuit portions or the like may each be divided so that parts of the driver circuit portions or the like face each other with pixels provided therebetween. 
     The light-emitting portion  802  shown in  FIG. 24B  includes a plurality of pixels each having a first transistor (not shown) serving as a selection transistor which controls writing of a data signal, a second transistor  811  serving as a driver transistor which adjusts current flowing in a light-emitting element, and a first electrode  831  which is electrically connected to a wiring (a source electrode or a drain electrode) of the second transistor  811 . 
     The light-emitting element  840  has a top-emission structure and includes the first electrode  831 , an EL layer  833 , and a second electrode  835 . An insulating film  839  covering an end portion of the first electrode  831  and serving as a partition wall is formed. 
     The transistor  811  has a dual-gate structure and includes, over an insulating film  844 , a gate electrode  832  which is formed concurrently with the first electrode  831 . When the transistor  811  is provided under the insulating film  839 , reflection of external light on the gate electrode  832  can be reduced. 
     Over the support substrate  801 , a lead wiring  809  for connecting an external input terminal through which a signal (e.g., a video signal, a clock signal, a start signal, or a reset signal) or a potential from the outside is transmitted to the driver circuit portions  803  and  804  is provided. Here, an example in which a flexible printed circuit (FPC)  808  is provided as an external input wiring is described. 
     The driver circuit portions  803  and  804  have a plurality of transistors.  FIG. 24B  illustrates an example in which the driver circuit portion  803  includes an NMOS circuit having n-channel transistors  852  and  853 . A circuit included in the driver circuit portion can be formed with various types of circuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although a driver-integrated type in which a driver circuit is formed over the substrate over which the light-emitting portion is formed is described in this embodiment, the structure of the present invention is not limited to the driver-integrated type. The driver circuit can be formed over a substrate that is different from the substrate over which the light-emitting portion is formed. 
     To prevent increase in the number of manufacturing steps, the lead wiring  809  is preferably formed using the same material and the same step(s) as those of the electrode or the wiring in the light-emitting portion or the driver circuit portion. For example, the lead wiring  809  can be formed using the same material in the same step(s) as those of the gate electrode of the transistor included in the light-emitting portion  802  and the driver circuit portion  803 . 
     As the support substrate  801 , a substrate having heat resistance high enough to resist the manufacturing process of the display device can be used. The substrate is not particularly limited in thickness and size as long as it can be used in a manufacturing apparatus. 
     The support substrate  801  preferably has a gas barrier property. Alternatively, a film having a gas barrier property may be stacked over the support substrate  801 . 
     Specifically, when a material having a gas barrier property such that the vapor permeability is lower than or equal to 10 −5  g/m 2 ·day, preferably lower than or equal to 10 −6  g/m 2 ·day, is used, the reliability of the display device can be improved. 
     Further, the support substrate  801  may be a flexible substrate. As a flexible substrate, a plastic substrate can be given as a typical example. In addition, a thin glass substrate with a thickness greater than or equal to 50 μm and less than or equal to 500 μm, a metal foil, or the like can be used. 
     For example, as a substrate which can be used as the support substrate  801 , a non-alkali glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, a metal substrate, a stainless-steel substrate, a plastic substrate, a polyethylene terephthalate substrate, a polyimide substrate, or the like can be used. 
     The structure of the transistors in the driver circuit portion  803  is not particularly limited. Although transistors having a channel-etched bottom gate structure are illustrated as an example in  FIG. 24B , the transistors may have a channel-protective bottom gate structure, a self-aligned top gate structure, or a non-self-aligned top gate structure. 
     A transistor including an oxide semiconductor in a channel formation region has characteristics of very low off-state current in some cases. When such a transistor is used, the capability of holding a signal input to a pixel (capacitor) can be high, so that the frame frequency can be reduced for example in the case of displaying a still image. Low frame frequency enables a reduction in power consumption of the display device. 
     The insulating film  839  is formed to cover an end portion of the first electrode  831 . The insulating film  839  is preferably formed to have a curved end portion in order to improve the coverage with the EL layer  833  or the second electrode  835  which is formed over the partition wall. 
     The insulating film  839  is preferably formed using a material having a low refractive index than the EL layer  833 . When the insulating film  839  is formed using such a material, total reflection can be caused at an interface between the EL layer  833  and the insulating film  839  and the amount of light entering the insulating film  839  can be reduced, whereby light extraction efficiency can be increased. 
     The light-emitting element included in the display device includes a pair of electrodes (the first electrode  831  and the second electrode  835 ), and the EL layer  833  between the pair of electrodes. One of the pair of electrodes functions as an anode and the other functions as a cathode. 
     In the case where the light-emitting element has a top-emission structure, a conductive film that transmits visible light is used for an upper electrode, and a conductive film that reflects visible light is preferably used for a lower electrode. In the case where the light-emitting element has a bottom-emission structure, a conductive film that transmits visible light is used for a lower electrode, and a conductive film that reflects visible light is preferably used for an upper electrode. In the case where the light-emitting element has a dual-emission structure, a conductive film that transmits visible light is used for upper and lower electrodes. 
     When a voltage higher than the threshold voltage of the light-emitting element is applied between the first electrode  831  and the second electrode  835 , holes are injected from the first electrode  831  side to the EL layer  833  and electrons are injected from the second electrode  835  side to the EL layer  833 . The injected electrons and holes recombine in the EL layer  833  and a light-emitting substance contained in the EL layer  833  emits light. 
     The EL layer  833  includes a light-emitting layer. In addition to the light-emitting layer, the EL layer  833  may further include one or more layers containing any of the following substances: a substance with a high hole-injection property, a substance with a high hole-transport property, a hole-blocking material, a substance with a high electron-transport property, a substance with a high electron-injection property, a substance with a bipolar property (a substance with a high electron- and hole-transport property), and the like. 
     For the EL layer  833 , either a low molecular compound or a high molecular compound can be used, and an inorganic compound may also be used. The above-described layers included in the EL layer  833  can be formed separately by any of the following methods: an evaporation method (including a vacuum evaporation method), a transfer method, a printing method, an inkjet method, a coating method, and the like. 
     With the first electrode  831  and the second electrode  835  of the light-emitting element  840 , a micro resonator (also referred to as microcavity) can be formed. For example, the first electrode  831  is formed using a conductive film which reflects light emitted from the EL layer  833 , and the second electrode  835  is formed using a semi-transmissive and semi-reflective, conductive film which reflects part of the light and transmits part of the light. 
     An optical adjustment layer can be provided between the first electrode  831  and the second electrode  835 . The optical adjustment layer is a layer which adjusts the optical path length between the reflective first electrode  831  and the semi-transmissive and semi-reflective second electrode  835 . By adjusting the optical adjustment layer, the wavelength of light preferentially extracted from the second electrode  835  can be controlled. 
     The EL layer can be employed for a material that can be used for the optical adjustment layer. For example, the thickness of the optical adjustment layer may be adjusted using a charge generation region. A region containing a substance having a high hole-transport property and an acceptor substance is especially preferably used for the optical adjustment layer, in which case an increase in drive voltage can be inhibited even when the optical adjustment layer is thick. 
     Alternatively, for a material that can be used for the optical adjustment layer, a conductive light-transmitting film that transmits light emitted from the EL layer  833  can also be employed. For example, the light-transmitting conductive film is stacked over a surface of the reflective conductive film to form the first electrode  831 . Such a structure is preferable because the thickness of an optical adjustment layer of an adjacent first electrode  831  is easily changed. 
     The oxide insulating film and the nitride insulating film described in Embodiment 1 can be used for the insulating film  844 . 
     As the insulating film  846 , an insulating film that functions as a planarization film is preferably selected in order to reduce surface unevenness due to the transistor. 
     The sealant  805  and the sealing substrate  806  are desirably formed using a material which does not pass impurities in the air (such as water and/or oxygen) as much as possible. An epoxy-based resin, glass frit, or the like can be used for the sealant  805 . 
     For a material that can be used for the sealing substrate  806 , any of the substrate that can be used as the support substrate  801 , a plastic substrate formed of polyvinyl fluoride (PVF), polyester, acrylic, or the like, fiberglass-reinforced plastics (FRP), or the like can be used. 
     A light-transmitting material may be contained in the space  810  where the structure formed on the support substrate  801  side and the structure formed on the sealing substrate  806  side are not in contact with each other. 
     The light-transmitting material may be a material that reacts with or adsorbs impurities (such as water and/or hydrogen) which lower the reliability of the light-emitting element. Thus, impurities preferentially react with or are preferentially adsorbed by the material contained in the space  810  and become inactive before lowering the reliability of the light-emitting element. Accordingly, reliability of the display device can be increased. 
     As the light-transmitting material, a substance having a high hole-transport property, a light-emitting substance, a host material, a substance having a high electron-transport property, a substance having a high electron-injection property, an accepter substance, and/or the like can be used. 
     Specifically, a conductive high molecule, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS), a desiccating agent, a material which can be used in the EL layer  833 , 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), or the like can be used. 
     The light-transmitting material can optically connect the second electrode  835  to the sealing substrate  806  (including the structure formed on the sealing substrate  806 ). Accordingly, a drastic change in the refractive index (also referred to as a step portion in the refractive index) of light emitted from the light-emitting element  840  can be suppressed in the optical path from the second electrode  835  to the sealing substrate  806 , whereby light of the light-emitting element  840  can be efficiently extracted to the sealing substrate  806  through the second electrode  835 . Thus, luminous efficiency of the display device can be improved. 
     The light-transmitting material is preferably a material having a higher refractive index than the second electrode  835 . With such a material, total reflection at an interface between the second electrode  835  and the material can be inhibited, whereby light extraction efficiency can be increased. 
     Note that as the material that can optically connect the second electrode  835  to the sealing substrate  806 , as well as the above-described materials, a liquid crystal material, a fluorine-based inactive liquid (such as perfluorocarbon), or a light-transmitting resin can be used. These materials may be used after impurities which lower the reliability of the light-emitting element are removed therefrom, as appropriate. In addition, a material that reacts with or adsorbs the impurities may be dispersed in the above materials. 
     As examples of a liquid crystal material, any of the following can be used: a nematic liquid crystal, a cholesteric liquid crystal, a smectic liquid crystal, a discotic liquid crystal, a thermotropic liquid crystal, a lyotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal (PDLC), a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, a main-chain liquid crystal, a side-chain high-molecular liquid crystal, a banana-shaped liquid crystal, and a mixed material such as a mixture of any of the above-described liquid crystal materials and a chiral material. 
     The color filter  866  is provided in order to adjust the color of light emitted from a light source to increase the color purity. For example, in a full-color display device using white light-emitting elements, a plurality of pixels provided with color filters of different colors are used. In that case, the color filters may be those of three colors of red (R), green (G), and blue (B) or four colors (yellow (Y) in addition to these three colors). Further, a white (W) pixel may be added to R, G, and B pixels (and a Y pixel). 
     A black matrix  864  is provided between the adjacent color filters  866 . The black matrix  864  blocks light emitted from an adjacent pixel, thereby preventing color mixture between the adjacent pixels. The black matrix  864  may be provided only between adjacent pixels of different emission colors and not between pixels of the same emission color. Here, the color filter  866  is provided so that its end portions overlap with the black matrix  864 , whereby light leakage can be reduced. 
     The black matrix  864  can be formed using a material that blocks light, for example, a metal material or a resin material including a pigment. Note that when the black matrix  864  is provided in a region other than the light-emitting portion  802 , such as a driver circuit portion, undesired leakage of guided light or the like can be prevented. 
     As illustrated in  FIG. 24B , by providing an overcoat  868  covering the color filter  866  and the black matrix  864 , an impurity such as a pigment included in the color filter  866  or the black matrix  864  can be prevented from diffusing into the light-emitting element or the like. For the overcoat  868 , a light-transmitting inorganic or organic insulating material can be used. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 10 
     In this embodiment, electronic devices in which a display device of one embodiment of the present invention can be incorporated will be described. 
     Examples of the electronic device to which the display device is applied include television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phones (also referred to as portable telephone devices), portable game machines, portable information terminals, audio reproducing devices, large game machines such as pin-ball machines, and the like. Specific examples of these electronic devices are illustrated in  FIGS. 25A to 25F . 
       FIG. 25A  illustrates an example of a television device. In the television device  7100 , a display portion  7103  is incorporated in a housing  7101 . Images can be displayed by the display portion  7103 , and the display device can be used for the display portion  7103 . In addition, here, the housing  7101  is supported by a stand  7105 . 
     The television device  7100  can be operated by an operation switch of the housing  7101  or a separate remote controller  7110 . With operation keys  7109  of the remote controller  7110 , channels and volume can be controlled and images displayed on the display portion  7103  can be controlled. Furthermore, the remote controller  7110  may be provided with a display portion  7107  for displaying data output from the remote controller  7110 . 
     Note that the television device  7100  is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, when the television set is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed. 
       FIG. 25B  illustrates a computer, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connecting port  7205 , a pointing device  7206 , and the like. Note that this computer is manufactured using the display device for the display portion  7203 . 
       FIG. 25C  illustrates a portable game machine, which includes two housings, i.e., a housing  7301  and a housing  7302 , connected to each other via a joint portion  7303  so that the portable game machine can be opened or closed. A display portion  7304  is incorporated in the housing  7301  and a display portion  7305  is incorporated in the housing  7302 . In addition, the portable game machine illustrated in  FIG. 25C  includes a speaker portion  7306 , a recording medium insertion portion  7307 , an LED lamp  7308 , input means (an operation key  7309 , a connection terminal  7310 , a sensor  7311  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, electric current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), and a microphone  7312 ), and the like. Needless to say, the structure of the portable game machine is not limited to the above as long as a display device can be used for at least either the display portion  7304  or the display portion  7305 , or both, and may include other accessories as appropriate. The portable game machine illustrated in  FIG. 25C  has a function of reading out a program or data stored in a storage medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. The portable amusement machine illustrated in  FIG. 25C  can have various functions without limitation to the above. 
       FIG. 25D  illustrates an example of a mobile phone. A mobile phone  7400  is provided with a display portion  7402  incorporated in a housing  7401 , operation buttons  7403 , an external connection port  7404 , a speaker  7405 , a microphone  7406 , and the like. Note that the mobile phone  7400  is manufactured using a display device for the display portion  7402 . 
     When the display portion  7402  of the mobile phone  7400  illustrated in  FIG. 25D  is touched with a finger or the like, data can be input into the mobile phone  7400 . Further, operations such as making a call and creating an e-mail can be performed by touching the display portion  7402  with a finger or the like. 
     There are mainly three screen modes of the display portion  7402 . 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 characters. 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 the case of making a call or creating e-mail, a character input mode mainly for inputting characters is selected for the display portion  7402  so that characters displayed on the screen can be input. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion  7402 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the mobile phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the mobile phone  7400  (whether the mobile phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are switched by touching the display portion  7402  or operating the operation buttons  7403  of the housing  7401 . The screen modes can be switched depending on the kind of images displayed on the display portion  7402 . 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. 
     Moreover, in the input mode, if a signal detected by an optical sensor in the display portion  7402  is detected and the input by touch on the display portion  7402  is not performed for a certain period, the screen mode may be controlled so as to be changed from the input mode to the display mode. 
     The display portion  7402  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion  7402  with the palm or the finger, whereby personal authentication 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. 25E  illustrates an example of a folding computer. A folding computer  7450  includes a housing  7451 L and a housing  7451 R connected by hinges  7454 . The computer  7450  further includes an operation button  7453 , a left speaker  7455 L, and a right speaker  7455 R. In addition, a side surface of the computer  7450  is provided with an external connection port  7456 , which is not illustrated. Note that when the computer  7450  is folded on the hinges  7454  so that a display portion  7452 L provided in the housing  7451 L and a display portion  7452 R provided in the housing  7451 R can face each other, the display portions can be protected by the housings. 
     Each of the display portions  7452 L and  7452 R is a component which can display images and to which data can be input by touch with a finger or the like. For example, the icon for the installed program is selected by touch with a finger, so that the program can be started. Further, changing the distance between fingers touching two positions of the displayed image enables zooming in or out on the image. Drag of a finger touching one position of the displayed image enables drag and drop of the image. Selection of the displayed character or symbol on the displayed image of a keyboard by touch with a finger enables information input. 
     Further, the computer  7450  can also include a gyroscope, an acceleration sensor, a global positioning system (GPS) receiver, fingerprint sensor, or a video camera. For example, when a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided, the orientation of the display screen can be automatically changed by determining the orientation of the computer  7450  (whether the computer  7450  is placed horizontally or vertically). 
     Furthermore, the computer  7450  can be connected to a network. The computer  7450  not only can display data on the Internet but also can be used as a terminal which controls another electronic device connected to the network from a distant place. 
       FIG. 25F  illustrates an example in which the display device of this embodiment is used in a lighting device. A lighting device  7500  includes, in a housing  7501 , a light-emitting portion  7503   a , a light-emitting portion  7503   b , a light-emitting portion  7503   c , and a light-emitting portion  7503   d  in which the display devices of one embodiment of the present invention are incorporated as light sources. The lighting device  7500  can be attached to a ceiling, a wall, or the like. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Example 
     In this example, examination results of the V g −I d  characteristics and the reliabilities of fabricated transistors will be described. 
     &lt;Fabrication of Samples&gt; 
     In this example, samples 1 and 2 each of which can be used as a driver transistor in the pixel in the display device of one embodiment of the present invention, and a sample 3 that can be used as a selection transistor were fabricated. Specifically, a transistor having a structure corresponding to that of the transistor  410   a  illustrated in FIGS.  7 A 1 ,  7 B, and  7 C 1  was fabricated as the sample 1, which is one embodiment of the present invention. Further, a transistor having a structure corresponding to that of the transistor  440   a  illustrated in FIGS.  12 A 1 ,  12 B, and  12 C 1  was fabricated as the sample 2, which is one embodiment of the present invention. In addition, a transistor having a structure corresponding to that of the transistor  400   b  illustrated in FIGS.  2 A 2 ,  2 B, and  2 C 2  was fabricated as the sample 3, which is one embodiment of the present invention. 
     &lt;Sample 1&gt; 
     First, a glass substrate was used as a substrate, and a gate electrode was formed over the substrate. 
     The gate electrode was formed in the following manner: a 200-nm-thick tungsten film was formed by a sputtering method, a mask was formed over the tungsten film by a photolithography process, and the tungsten film was partly etched using the mask. 
     Next, an insulating film serving as a gate insulating film was formed over the gate electrode. 
     As the gate insulating film, a stack including a 400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitride film was used. 
     Note that the silicon nitride film was formed to have a three-layer structure of a first silicon nitride film, a second silicon nitride film, and a third silicon nitride film. 
     The first silicon nitride film was formed to have a thickness of 50 nm under the following conditions: silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm were supplied to a reaction chamber of a plasma CVD apparatus as a source gas; the pressure in the reaction chamber was controlled to 100 Pa, and power of 2000 W was supplied with the use of a 27.12 MHz high-frequency power source. The second silicon nitride film was formed to have a thickness of 300 nm under the following conditions: silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm were supplied to the reaction chamber of the plasma CVD apparatus as a source gas; the pressure in the reaction chamber was controlled to 100 Pa, and power of 2000 W was supplied with the use of a 27.12 MHz high-frequency power source. The third silicon nitride film was formed to have a thickness of 50 nm under the following conditions: silane at a flow rate of 200 sccm and nitrogen at a flow rate of 5000 sccm were supplied to the reaction chamber of the plasma CVD apparatus as a source gas; the pressure in the reaction chamber was controlled to 100 Pa, and power of 2000 W was supplied with the use of a 27.12 MHz high-frequency power source. Note that the first silicon nitride film, the second silicon nitride film, and the third silicon nitride film were each formed at a substrate temperature of 350° C. 
     The silicon oxynitride film was formed under the following conditions: silane at a flow rate of 20 sccm and dinitrogen monoxide at a flow rate of 3000 sccm were supplied to the reaction chamber of the plasma CVD apparatus as a source gas; the pressure in the reaction chamber was controlled to 40 Pa, and power of 100 W was supplied with the use of a 27.12 MHz high-frequency power source. Note that the silicon oxynitride film was formed at a substrate temperature of 350° C. 
     Next, an oxide semiconductor film was formed to overlap with the gate electrode with the gate insulating film positioned therebetween. 
     Here, a 35-nm-thick oxide semiconductor film was formed over the gate insulating film by a sputtering method. 
     The oxide semiconductor film was formed under the following conditions: a sputtering target containing In, Ga, and Zn at an atomic ratio of 1:1:1 was used; oxygen at a flow rate of 100 sccm was supplied as a sputtering gas into a reaction chamber of a sputtering apparatus; the pressure in the reaction chamber was controlled to 0.6 Pa; and direct-current power of 5 kW was supplied. Note that the oxide semiconductor film was formed at a substrate temperature of 170° C. 
     Next, a pair of electrodes in contact with the oxide semiconductor film was formed. 
     First, a conductive film was formed over the gate insulating film and the oxide semiconductor film. As the conductive film, a 400-nm-thick aluminum film was formed over a 50-nm-thick tungsten film, and a 200-nm-thick titanium film was formed over the aluminum film. Then, a mask was formed over the conductive film by a photolithography process, and the conductive film was partly etched using the mask. Through the above steps, the pair of electrodes was formed. 
     Next, the substrate was transferred to a reaction chamber in a reduced pressure and heated at 350° C. Then, the oxide semiconductor film was exposed to oxygen plasma that was generated in a dinitrogen monoxide atmosphere by supply of a high-frequency power of 150 W to an upper electrode in the reaction chamber with the use of a 27.12 MHz high-frequency power source. 
     After that, a second gate insulating film was formed over the oxide semiconductor film and the pair of electrodes. In this case, the second gate insulating film was formed to have a three-layer structure of a first oxide insulating film, a second oxide insulating film, and a nitride insulating film. 
     The first oxide insulating film was formed by a plasma CVD method under the following conditions: silane at a flow rate of 20 sccm and dinitrogen monoxide at a flow rate of 3000 sccm were used as a source gas; the pressure in the reaction chamber was 200 Pa; the substrate temperature was 350° C.; and high-frequency power of 100 W was supplied to parallel-plate electrodes. 
     The second oxide insulating film was formed by a plasma CVD method under the following conditions: silane at a flow rate of 160 sccm and dinitrogen monoxide at a flow rate of 4000 sccm were used as a source gas, the pressure in the reaction chamber was 200 Pa, the substrate temperature was 220° C., and high-frequency power of 1500 W was supplied to the parallel-plate electrodes. Under the above conditions, it is possible to form a silicon oxynitride film which contains oxygen at a higher proportion than the stoichiometric composition so that part of oxygen is released by heating. 
     Next, heat treatment was performed to release water, nitrogen, hydrogen, and the like from the first oxide insulating film and the second oxide insulating film and to supply part of oxygen contained in the second oxide insulating film into the oxide semiconductor film. Here, the heat treatment was performed at 350° C. in a mixed atmosphere of nitrogen and oxygen for one hour. 
     Then, a 100-nm-thick nitride insulating film was formed over the second oxide insulating film. The nitride insulating film was formed by a plasma CVD method under the following conditions: silane at a flow rate of 50 sccm, nitrogen at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm were used as a source gas, the pressure in the reaction chamber was 100 Pa, the substrate temperature was 350° C., and high-frequency power of 1000 W was supplied to the parallel-plate electrodes. 
     Subsequently, in a region where the oxide semiconductor film and the pair of electrodes were not provided, an opening portion that reaches the gate electrode was formed in the gate insulating film and the second gate insulating film. The opening portion was formed in the following manner: a mask was formed over the second gate insulating film by a photolithography process, and the gate insulating film and the second gate insulating film were partly etched using the mask. 
     After that, a second gate electrode serving as a back gate electrode was formed over the second gate insulating film. The back gate electrode was electrically connected to the gate electrode through the opening portion provided in the gate insulating film and the second gate insulating film. 
     Here, as the back gate electrode, a 100-nm-thick conductive film of an indium oxide-tin oxide compound (ITO—SiO 2 ) containing silicon oxide was formed by a sputtering method. Note that the composition of a target used for forming the conductive film was In 2 O 3 :SnO 2 :SiO 2 =85:10:5 [wt %]. After that, heat treatment was performed at 250° C. in a nitrogen atmosphere for one hour. 
     Through the above process, the sample 1 of this example was fabricated. 
     &lt;Sample 2&gt; 
     The sample 2 includes a second gate insulating film and a back gate electrode having structures different from those in the sample 1. Specifically, as illustrated in FIG.  12 C 1 , the sample 2 has a structure in which the back gate electrode covers side surfaces of a first oxide insulating film and a second oxide insulating film in the channel width direction of the transistor. 
     In the fabrication process of the sample 2, the first oxide insulating film and the second oxide insulating film were formed, heat treatment was performed, and then a mask was formed over the second oxide insulating film by a photolithography process. Subsequently, the first oxide insulating film and the second oxide insulating film were partly etched using the mask. Other steps of fabricating the sample 2 are similar to those for fabricating the sample 1; thus, the descriptions for the sample 1 can be referred to. 
     &lt;Sample 3&gt; 
     The sample 3 is different from the sample 1 in that a back gate electrode is not included. 
     The sample 3 was fabricated using the above-described fabrication process of the sample 1, excluding the step of forming the back gate electrode. Other steps of fabricating the sample 3 are similar to those for fabricating the sample 1; thus, the descriptions for the sample 1 can be referred to. 
     Note that as each of the samples 1 to 3, three kinds of transistors were fabricated in which channel widths W were 50 μm and channel lengths L were 2 μm, 3 μm, and 6 μm. 
     &lt;V g −I d  Characteristics&gt; 
     Next, the initial V g −I d  characteristics of the samples 1 to 3 used as transistors were measured. Here, changes in characteristics of current flowing between a source electrode and a drain electrode (hereinafter referred to as drain current: I d ), that is, V g −I d  characteristics were measured under the following conditions: the substrate temperature was 25° C., the potential difference between the source and drain electrodes (hereinafter referred to as drain voltage: V d ) was 1 V or 10 V, and the potential difference between the source and the gate electrodes (hereinafter referred to as gate voltage: V g ) was changed from −15 V to 15 V. 
     Here, the sample 1 and the sample 2 were driven by a method in which gate voltage is applied in a state where the gate electrode and the back gate electrode are electrically short-circuited. Such a driving method is called dual-gate driving. In the dual-gate driving, the gate electrode always has the same gate voltage as the back gate electrode. 
       FIGS. 26A to 26C  show the V g −I d  characteristics of the sample 3.  FIGS. 26A to 26C  show results of the transistors with channel lengths L of 2 μm, 3 μm, and 6 μm, respectively. Similarly,  FIGS. 27A to 27C  show the V g −I d  characteristics of the sample 1 and  FIGS. 28A to 28C  show the V g −I d  characteristics of the sample 2. 
     In each of  FIGS. 26A to 26C ,  FIGS. 27A to 27C , and  FIGS. 28A to 28C , the lateral axis, the first longitudinal axis, and the second longitudinal axis represent gate voltage V g , drain current I d , and field-effect mobility Mobility, respectively. Here, to show field-effect mobility in a saturation region, field-effect mobility calculated when V d =10V is shown. 
     The results of the sample 3 (a transistor that can be suitably used as a selection transistor of the pixel in the display device of one embodiment of the present invention) in  FIGS. 26A to 26C  show that a shift of the threshold voltage in the negative direction is further suppressed as the channel length L becomes longer. An effect of suppressing the shift of the threshold voltage in the negative direction is particularly significant when the drain voltage V d  is high. Note that the field-effect mobility hardly changes regardless of the channel length L. 
     Further, as shown in  FIGS. 27A to 27C , the field-effect mobility of the sample 1 (a transistor that can be suitably used as a driver transistor in the pixel in the display device of one embodiment of the present invention) is improved in comparison with the sample 3 regardless of the channel length L. In addition, the field-effect mobility is further improved as the channel length L becomes shorter. Furthermore, the dual-gate driving leads to an extremely small change in the threshold voltage with respect to the drain voltage V d  even when the channel length L is short (i.e., L=2 μm). 
     Further, as shown in  FIGS. 28A to 28C , the field-effect mobility of the sample 2 (a transistor that can be suitably used as a driver transistor in the pixel in the display device of one embodiment of the present invention) is improved in comparison with the sample 3 regardless of the channel length L. In addition, the field-effect mobility is further improved as the channel length L becomes shorter. Furthermore, the dual-gate driving leads to an extremely small change in the threshold voltage with respect to the drain voltage V d  when the channel length L is short (i.e., L=2 μm). 
     The above results show that a shift of the threshold voltage in the negative direction is further suppressed as the channel length L becomes longer and that the field-effect mobility is further improved as the channel length L becomes shorter. Furthermore, the dual-gate driving allows an electric field to be efficiently applied to an oxide semiconductor in which a channel is formed; as a result, a change in the threshold voltage with respect to the drain voltage V d  can be small even when the channel length L is short. Thus, when the channel length L is made short (specifically, L=2 μm) and the dual-gate driving is employed in a driver transistor in the pixel that requires high field-effect mobility, and the channel length L of a selection transistor of the pixel that requires normally-off characteristics is made longer than that of the driver transistor, a display device that can operate at high speed with low power consumption can be provided. 
     This application is based on Japanese Patent Application serial no. 2013-119149 filed with Japan Patent Office on Jun. 5, 2013, the entire contents of which are hereby incorporated by reference.