Patent Publication Number: US-2023162697-A1

Title: Semiconductor device, display module, and electronic device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a semiconductor device, a display device, a display module, and an electronic device. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them. 
     2. Description of the Related Art 
     A shift register is used for a driver circuit of a memory device, an image sensor, a display device, or the like. In particular, a shift register formed using transistors having the same polarity has been developed. A technique relating to such a shift register is disclosed in Patent Documents 1 and 2. 
     The shift direction of the shift registers disclosed in Patent Documents 1 and 2 is only one way. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2004-103226 
         [Patent Document 2] Japanese Published Patent Application No. 2005-050502 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a novel circuit or a method for driving the circuit. In particular, an object of one embodiment of the present invention is to provide a novel circuit which can be used for at least part of a shift register in which the shift direction can be changed or a method for driving the circuit. 
     Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a semiconductor device including a shift register. The shift register includes first to third flipflops. The first flipflop has a function of outputting a first signal to a first wiring. The second flipflop has a function of outputting a second signal to a second wiring. The third flipflop has a function of outputting a third signal to a third wiring. The first signal has a value synchronized with a first clock signal. The second signal has a value synchronized with a second clock signal. The third signal has a value synchronized with a third clock signal. The second flipflop includes first to third transistors. One of a source and a drain of the first transistor is electrically connected to a fourth wiring. The other of the source and the drain of the first transistor is electrically connected to the second wiring. One of a source and a drain of the second transistor is electrically connected to the first wiring. The other of the source and the drain of the second transistor is electrically connected to a gate of the first transistor. A gate of the second transistor is electrically connected to a fifth wiring. One of a source and a drain of the third transistor is electrically connected to the third wiring. The other of the source and the drain of the third transistor is electrically connected to the gate of the first transistor. A gate of the third transistor is electrically connected to a sixth wiring. The fourth wiring has a function of transmitting the second clock signal. The fifth wiring has a function of transmitting the first clock signal. The sixth wiring has a function of transmitting the third clock signal. 
     One embodiment of the present invention is a semiconductor device including a shift register. The shift register includes first to third flipflops. The first flipflop has a function of outputting a first signal to a first wiring. The second flipflop has a function of outputting a second signal to a second wiring. The third flipflop has a function of outputting a third signal to a third wiring. The first signal has a value synchronized with a first clock signal. The second signal has a value synchronized with a second clock signal. The third signal has a value synchronized with a third clock signal. The second flipflop includes first to third transistors. One of a source and a drain of the first transistor is electrically connected to a fourth wiring. The other of the source and the drain of the first transistor is electrically connected to the second wiring. One of a source and a drain of the second transistor is electrically connected to a fifth wiring. The other of the source and the drain of the second transistor is electrically connected to a gate of the first transistor. A gate of the second transistor is electrically connected to the first wiring. One of a source and a drain of the third transistor is electrically connected to a sixth wiring. The other of the source and the drain of the third transistor is electrically connected to the gate of the first transistor. A gate of the third transistor is electrically connected to the third wiring. The fourth wiring has a function of transmitting the second clock signal. The fifth wiring has a function of transmitting the first clock signal. The sixth wiring has a function of transmitting the third clock signal. 
     One embodiment of the present invention is a semiconductor device including a shift register. The shift register includes first to fifth flipflops. The first flipflop has a function of outputting a first signal to a first wiring. The second flipflop has a function of outputting a second signal to a second wiring. The third flipflop has a function of outputting a third signal to a third wiring. The fourth flipflop has a function of outputting a fourth signal to a fourth wiring. The fifth flipflop has a function of outputting a fifth signal to a fifth wiring. The first signal has a value synchronized with a first clock signal. The second signal has a value synchronized with a second clock signal. The third signal has a value synchronized with a third clock signal. The fourth signal has a value synchronized with a fourth clock signal. The fifth signal has a value synchronized with the first clock signal. The third flipflop includes first to fifth transistors. One of a source and a drain of the first transistor is electrically connected to a sixth wiring. The other of the source and the drain of the first transistor is electrically connected to the third wiring. One of a source and a drain of the second transistor is electrically connected to the second wiring. The other of the source and the drain of the second transistor is electrically connected to a gate of the first transistor. One of a source and a drain of the third transistor is electrically connected to the fourth wiring. The other of the source and the drain of the third transistor is electrically connected to the gate of the first transistor. At least one of a gate and one of a source and a drain of the fourth transistor is electrically connected to the first wiring. The other of the source and the drain of the fourth transistor is electrically connected to a gate of the second transistor. At least one of a gate and one of a source and a drain of the fifth transistor is electrically connected to the fifth wiring. The other of the source and the drain of the fifth transistor is electrically connected to a gate of the third transistor. The sixth wiring has a function of transmitting the third clock signal. 
     One embodiment of the present invention is a semiconductor device including a shift register. The shift register includes first to third flipflops. The first flipflop has a function of outputting a first signal to a first wiring. The second flipflop has a function of outputting a second signal to a second wiring. The third flipflop has a function of outputting a third signal to a third wiring. The first signal has a value synchronized with a first clock signal. The second signal has a value synchronized with a second clock signal. The third signal has a value synchronized with a third clock signal. The second flipflop includes first to fifth transistors. One of a source and a drain of the first transistor is electrically connected to a fourth wiring. The other of the source and the drain of the first transistor is electrically connected to the second wiring. One of a source and a drain of the second transistor is electrically connected to the first wiring. The other of the source and the drain of the second transistor is electrically connected to a gate of the first transistor. One of a source and a drain of the third transistor is electrically connected to the third wiring. The other of the source and the drain of the third transistor is electrically connected to the gate of the first transistor. At least one of a gate and one of a source and a drain of the fourth transistor is electrically connected to the first wiring. The other of the source and the drain of the fourth transistor is electrically connected to a gate of the second transistor. At least one of a gate and one of a source and a drain of the fifth transistor is electrically connected to the third wiring. The other of the source and the drain of the fifth transistor is electrically connected to a gate of the third transistor. The fourth wiring has a function of transmitting the second clock signal. 
     Note that W (W refers to a channel width)/L (L refers to a channel length) of the fourth transistor is preferably 0.8 to 1.2 times W/L of the fifth transistor. 
     Note that W (W refers to a channel width)/L (L refers to a channel length) of the second transistor is preferably 0.8 to 1.2 times W/L of the third transistor. 
     Note that the first transistor preferably includes an oxide semiconductor in a channel formation region. 
     One embodiment of the present invention is a display module including the above semiconductor device and an FPC. 
     One embodiment of the present invention is an electronic device including the above semiconductor device or the above display module, a speaker, an operation button, and/or an antenna. 
     According to one embodiment of the present invention, a novel circuit or a method for driving the circuit can be provided. In particular, a novel circuit which can be applied to at least part of a shift register in which the shift direction can be switched or a method for driving the circuit can be provided. 
     Note that the descriptions of these effects do not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all the effects. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG.  1    is a circuit diagram illustrating a device of one embodiment of the present invention; 
         FIG.  2    is a timing chart showing operation of a device of one embodiment of the present invention; 
         FIG.  3    is a timing chart showing operation of a device of one embodiment of the present invention; 
         FIG.  4    is a circuit diagram illustrating a device of one embodiment of the present invention; 
         FIG.  5    is a timing chart showing operation of a device of one embodiment of the present invention; 
         FIGS.  6 A and  6 B  are schematic diagrams illustrating a device of one embodiment of the present invention; 
         FIGS.  7 A and  7 B  are schematic diagrams illustrating a device of one embodiment of the present invention; 
         FIGS.  8 A and  8 B  are schematic diagrams illustrating a device of one embodiment of the present invention; 
         FIGS.  9 A and  9 B  are schematic diagrams illustrating a device of one embodiment of the present invention; 
         FIG.  10    is a timing chart showing operation of a device of one embodiment of the present invention; 
         FIGS.  11 A and  11 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  12 A and  12 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  13 A and  13 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  14 A and  14 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  15 A and  15 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  16 A and  16 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  17 A and  17 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  18 A and  18 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  19 A and  19 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  20 A and  20 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  21 A and  21 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  22 A and  22 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  23 A and  23 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  24 A and  24 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  25 A and  25 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  26 A and  26 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  27 A and  27 B  are circuit diagrams each illustrating a device of one embodiment of the present invention; 
         FIGS.  28 A to  28 C  are circuit diagrams illustrating a display device of one embodiment of the present invention; 
         FIGS.  29 A and  29 B  illustrate a structure example of a transistor of one embodiment of the present invention; 
         FIGS.  30 A to  30 D  illustrate an example of a method for manufacturing a transistor of one embodiment of the present invention; 
         FIGS.  31 A and  31 B  each illustrate a structure example of a transistor of one embodiment of the present invention; 
         FIGS.  32 A to  32 C  each illustrate a structure example of a transistor of one embodiment of the present invention; 
         FIG.  33    is a top view illustrating one embodiment of a display device; 
         FIG.  34    is a cross-sectional view illustrating one embodiment of a display device; 
         FIGS.  35 A to  35 C  are projection drawings illustrating a structure of an input/output device of one embodiment; 
         FIG.  36    is a cross-sectional view illustrating a structure of an input/output device of one embodiment; 
         FIGS.  37 A to  37 C  each illustrate an electronic device of one embodiment of the present invention; and 
         FIGS.  38 A and  38 B  are circuit diagrams each illustrating a device of one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the scope and spirit of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. 
     One embodiment of the present invention includes, in its category, an integrated circuit, a display device, an RF tag, and any other devices. The display device includes, in its category, a display device including an integrated circuit, such as a liquid crystal display device, a light-emitting device in which a light-emitting element typified by an organic light-emitting element is provided in each pixel, an electronic paper, a digital micromirror device (DMD), a plasma display panel (PDP), and a field emission display (FED). 
     In describing structures of the present invention with reference to the drawings, common reference numerals are used for the same portions in different drawings. 
     Note that in this specification and the like, part of a diagram or text described in one embodiment can be taken out to constitute one embodiment of the invention. Thus, in the case where a diagram or text related to a certain portion is described, the contents taken out from part of the diagram or the text are also disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the present invention is clear. Therefore, for example, in a diagram or text in which one or more active elements (e.g., transistors), wirings, passive elements (e.g., capacitors), conductive layers, insulating layers, semiconductor layers, components, devices, operating methods, manufacturing methods, or the like are described, part of the diagram or the text is taken out, and one embodiment of the invention can be constituted. For example, from a circuit diagram in which N circuit elements (e.g., transistors or capacitors; N is an integer) are provided, it is possible to take out M circuit elements (e.g., transistors or capacitors; M is an integer, where M&lt;N) and constitute one embodiment of the invention. For another example, it is possible to take out some given elements from a sentence “A includes B, C, D, E, or F” and constitute one embodiment of the invention, for example, “A includes B and E”, “A includes E and F”, “A includes C, E, and F”, or “A includes B, C, D, and E”. 
     Note that in the case where at least one specific example is described in a diagram or text described in one embodiment in this specification and the like, it will be readily appreciated by those skilled in the art that a broader concept of the specific example can be derived. Therefore, in the diagram or the text described in one embodiment, in the case where at least one specific example is described, a broader concept of the specific example is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the present invention is clear. 
     Note that in this specification and the like, what is illustrated in at least a diagram (which may be part of the diagram) is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. Therefore, when a certain content is described in a diagram, the content is disclosed as one embodiment of the invention even when the content is not described with a text, and one embodiment of the invention can be constituted. In a similar manner, part of a diagram, which is taken out from the diagram, is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the present invention is clear. 
     Embodiment 1 
     In this embodiment, a device of one embodiment of the present invention is described. 
     A device of one embodiment of the present invention illustrated in  FIG.  1    includes a circuit  100 . The circuit  100  is connected to a wiring CK 1 , a wiring CK 2 , a wiring CK 3 , a wiring CK 4 , a wiring SP 1 , a wiring SP 2 , and N (N is a natural number of 3 or more) wirings OUT (also referred to as wirings OUT[ 1 ] to [N]). 
     Each of the wiring CK 1 , the wiring CK 2 , the wiring CK 3 , the wiring CK 4 , the wiring SP 1 , the wiring SP 2 , and the wirings OUT[ 1 ] to [N] has a function of transmitting a signal, a potential, a current, or the like. In other words, each of the wiring CK 1 , the wiring CK 2 , the wiring CK 3 , the wiring CK 4 , the wiring SP 1 , the wiring SP 2 , and the wirings OUT[ 1 ] to [N] has a function of a signal line, a power supply line, or a current supply line. For example, a signal is input to each of the wiring CK 1 , the wiring CK 2 , the wiring CK 3 , and the wiring CK 4 . Examples of a signal input to the wiring CK 1  (also referred to as a signal V CK1 ), a signal input to the wiring CK 2  (also referred to as a signal V CK2 ), a signal input to the wiring CK 3  (also referred to as a signal V CK3 ), and a signal input to the wiring CK 4  (also referred to as a signal V CK4 ) are clock signals. Note that the signal V CK1  to the signal V CK4  preferably have different phases. For example, a signal is input to each of the wiring SP 1  and the wiring SP 2 . Examples of a signal input to the wiring SP 1  (also referred to as a signal V SP1 ) and a signal input to the wiring SP 2  (also referred to as a signal V SP2 ) are start pulses. As described later, the shift direction of data can be controlled by the signal V SP1  and the signal V SP2 . For example, signals are output from the circuit  100  to the corresponding wirings OUT[ 1 ] to [N]. Examples of signals output to the corresponding wirings OUT[ 1 ] to [N] (also referred to as signals V OUT [ 1 ] to [N]) are output signals of the circuit  100 . As described later, the signals V OUT [ 1 ] to [N]are signals delayed from the signal V SP1  or the signal V SP2 . 
     The circuit  100  has a function of controlling the potentials of the wirings OUT[ 1 ] to [N]. Specifically, the circuit  100  has a function of shifting the signal V SP1  and a function of shifting the signal V SP2 . Furthermore, the circuit  100  has a function of outputting the signals V OUT [ 1 ] to [N] delayed from the signal V SP1  to the corresponding wirings OUT[ 1 ] to [N] and a function of outputting the signals V OUT [ 1 ] to [N] delayed from the signal V SP2  to the corresponding wirings OUT[ 1 ] to [N]. As exemplified in  FIG.  2   , when the signal V SP1  is set at high level (in an active state), the signal V SP1  is shifted in a direction from the wiring OUT[ 1 ] to the wiring OUT[N]. The signal V OUT [ 1 ] is delayed from the signal V SP1 , the signal V OUT [i] (i is one of 2 to N−1) is delayed from the signal V OUT [i−1], and the signal V OUT [N] is delayed from the signal V OUT [N−1]. As exemplified in  FIG.  3   , when the signal V SP2  is set at high level (in an active state), the signal V SP2  is shifted in a direction from the wiring OUT[N] to the wiring OUT[ 1 ]. The signal V OUT [N] is delayed from the signal V SP2 , the signal V OUT [i] is delayed from the signal V OUT [i+1], and the signal V OUT [ 1 ] is delayed from the signal V OUT [ 2 ]. As described above, the circuit  100  has a function of a shift register, particularly a bidirectional shift register. The shift direction of data is determined depending on which of the signal V SP1  and the signal V SP2  is shifted. Which of the signal V SP1  and the signal V SP2  is shifted depends on which of the signal V SP1  and the signal V SP2  is set in the active state. Note that the function of the circuit  100  is not limited thereto. 
     Next, a structure example of the circuit  100  is described with reference to  FIG.  1   . The circuit  100  includes N circuits SR (also referred to as circuits SR[ 1 ] to [N]). In  FIG.  1   , the circuits SR[ 1 ] to [ 5 ] and the circuit SR[N] are shown. A terminal C 1  of the circuit SR[ 1 ] is connected to a corresponding wiring of the wirings CK 1  to CK 4 , a terminal C 2  thereof is connected to a corresponding wiring of the wirings CK 1  to CK 4 , a terminal C 3  thereof is connected to a corresponding wiring of the wirings CK 1  to CK 4 , a terminal S 1  thereof is connected to the wiring SP 1 , a terminal S 2  thereof is connected to the wiring OUT[ 2 ], and a terminal O thereof is connected to the wiring OUT[ 1 ]. The circuit SR[i] is different from the circuit SR[ 1 ] in that the terminal S 1  is connected to the wiring OUT[i−1]. The circuit SR[N] is different from the circuit SR[ 1 ] in that the terminal S 1  is connected to the wiring OUT[N−1] and the terminal S 2  is connected to the wiring SP 2 . Note that a terminal C 1  of the circuit SR[ 4   m +1] (m is 0 or a positive integer) is connected to the wiring CK 1 , a terminal C 2  thereof is connected to the wiring CK 4 , and a terminal C 3  thereof is connected to the wiring CK 2 . A terminal C 1  of the circuit SR[ 4   m+ 2] is connected to the wiring CK 2 , a terminal C 2  thereof is connected to the wiring CK 1 , and a terminal C 3  thereof is connected to the wiring CK 3 . A terminal C 1  of the circuit SR[ 4   m+ 3] is connected to the wiring CK 3 , a terminal C 2  thereof is connected to the wiring CK 2 , and a terminal C 3  thereof is connected to the wiring CK 4 . A terminal C 1  of the circuit SR[ 4   m+ 4] (4m+4≤N) is connected to the wiring CK 4 , a terminal C 2  thereof is connected to the wiring CK 3 , and a terminal C 3  thereof is connected to the wiring CK 1 . 
     The circuit SR has a function of controlling the potential of the terminal O. Specifically, the circuit SR has a function of shifting a signal of the terminal S 1  and a function of shifting a signal of the terminal S 2 . Furthermore, the circuit SR has a function of outputting a signal delayed from the signal of the terminal S 1  from the terminal O and a function of outputting a signal delayed from the signal of the terminal S 2  from the terminal O. As exemplified in  FIG.  2   , in the case where the signal VSP is shifted, a signal delayed from a signal input to the terminal S 1  is output from the terminal O. For example, in the circuit SR[i], a signal V OUT [i] delayed from a signal V OUT [i−1] input to the terminal S 1  is output from the terminal O. In contrast, as exemplified in  FIG.  3   , in the case where the signal V SP2  is shifted, a signal delayed from a signal input to the terminal S 2  is output from the terminal O. For example, in the circuit SR[i], a signal V OUT [i] delayed from a signal V OUT [i+1] input to the terminal S 2  is output from the terminal O. As described above, the circuit SR has a function of a stage of a sequential circuit, a flipflop, or a shift register. Note that the function of the circuit SR is not limited thereto. 
     Next, a specific example of the circuit SR is described with reference to  FIG.  4   . The circuit SR includes a transistor  101 , a transistor  102 , a transistor  103 , a transistor  104 , and a transistor  105 . A first terminal and a second terminal of the transistor  101  are connected to a terminal C 1  and a terminal O, respectively. A first terminal and a second terminal of the transistor  102  are connected to a wiring VSS 1  and the terminal O, respectively. A first terminal and a second terminal of the transistor  103  are connected to a wiring VSS 2  and a gate of the transistor  101 , respectively. A first terminal, a second terminal, and a gate of the transistor  104  are connected to a terminal S 1 , the gate of the transistor  101 , and a terminal C 2 , respectively. A first terminal, a second terminal, and a gate of the transistor  105  are connected to a terminal S 2 , the gate of the transistor  101 , and a terminal C 3 , respectively. Note that the gate of the transistor  101 , the second terminal of the transistor  103 , the second terminal of the transistor  104 , or the second terminal of the transistor  105  is referred to as a node ND 1 . 
     Each of the transistors  101  to  105  has a function of controlling electrical conduction and non-conduction between a portion to which the first terminal (also referred to as one of a source and a drain) is connected and a portion to which the second terminal (also referred to as the other of the source and the drain) is connected. That is, each of the transistors  101  to  105  has a function of a switch. Note that the functions of the transistors  101  to  105  are not limited thereto. 
     Each of the wiring VSS 1  and the wiring VSS 2  has a function of transmitting a signal, a potential, or a current In other words, each of the wiring VSS 1  and the wiring VSS 2  has a function of a signal line, a power supply line, or a current supply line. For example, a potential is input to each of the wiring VSS 1  and the wiring VSS 2 . The potential input to each of the wiring VSS 1  and the wiring VSS 2  is a potential corresponding to the low-level or high-level signal V CK1 , the low-level or high-level signal V CK2 , the low-level or high-level signal V CK3 , or the low-level or high-level signal V CK4 . In particular, when the transistor  101  is an n-channel transistor, a potential corresponding to the low-level signal V CK1 , the low-level signal V CK2 , the low-level signal V CK3 , or the low-level signal V CK4  is preferably input to each of the wiring VSS 1  and the wiring VSS 2 . In contrast, when the transistor  101  is a p-channel transistor, a potential corresponding to the high-level signal V CK1 , the high-level signal V CK2 , the high-level signal V CK3 , or the high-level signal V CK4  is preferably input to each of the wiring VSS 1  and the wiring VSS 2 . 
     A potential corresponding to a low-level signal is a potential that is equal or substantially equal to the low-level signal. Note that the potential corresponding to the low-level signal may be a potential lower than the high-level signal or a potential lower than an intermediate potential between the high-level and low-level signals. Similarly, a potential corresponding to a high-level signal is a potential that is equal or substantially equal to the high-level signal. Note that the potential corresponding to the high-level signal may be a potential higher than the low-level signal or a potential higher than an intermediate potential between the high-level and low-level signals. 
     In the case where the same potential is input to the wiring VSS 1  and the wiring VSS 2 , the wiring VSS 1  and the wiring VSS 2  may be one wiring. That is, the first terminal of the transistor  102  and the first terminal of the transistor  103  may be connected to the same wiring. 
     Next, an example of operation of the circuit SR exemplified in  FIG.  4    is described using the circuit SR[i] as an example. For convenience, the terminal C 1 , the terminal C 2 , and the terminal C 3  of the circuit SR[i] are connected to the wiring CK 2 , the wiring CK 1 , and the wiring CK 3 , respectively. Moreover, for convenience, at least the transistor  101 , the transistor  104 , and the transistor  105  are n-channel transistors. 
     The case where a signal delayed from a signal of the terminal S 1  is output from the terminal O and the case where a signal delayed from a signal of the terminal S 2  is output from the terminal O are separately described. 
     First, an example of operation in the case where a signal delayed from a signal of the terminal S 1  is output from the terminal O is described with reference to  FIG.  5   . 
     An example of operation of the circuit SR in a period A is described.  FIG.  6 A  is an example of a schematic diagram of the operation in the period A. 
     The transistor  104  is turned on because a signal (signal V CK1 ) of the terminal C 2  is set at high level. The transistor  105  is turned off because a signal (signal V CK3 ) of the terminal C 3  is set at low level. The transistor  103  is turned off. Thus, the potential of the node ND 1  is increased because a signal (high-level signal V OUT [i−1]) of the terminal S 1  is supplied to the node ND 1 . After that, when the potential of the node ND 1  becomes equal or substantially equal to a value obtained by subtracting the threshold voltage of the transistor  104  from the potential of the gate of the transistor  104  (signal (high-level signal V CK1 ) of the terminal C 2 ), the transistor  104  is turned off. Thus, the node ND 1  is set in the floating state. 
     The transistor  101  is turned on because the potential of the node ND 1  is set to a high value. The transistor  102  is turned on. Thus, the signal (signal V OUT [i]) of the terminal O is set at low level because the signal (low-level signal V CK2 ) of the terminal C 1  and the potential of the wiring VSS 1  are supplied to the terminal O. 
     Note that the high value of the potential of the node ND 1  refers to a value by which the transistor  101  is turned on. Specifically, it is a value higher than the sum of the potential of the first terminal or the second terminal of the transistor  101  and the threshold voltage of the transistor  101 . 
     Note that in the period A, the transistor  102  may be off. 
     An example of operation of the circuit SR in a period B is described.  FIG.  6 B  is an example of a schematic diagram of the operation in the period B. 
     The transistor  104  is turned off because the signal (signal V CK1 ) of the terminal C 2  is set at low level. Since the signal (signal V CK3 ) of the terminal C 3  is at low level, the transistor  105  is kept off. The transistor  103  is kept off. Thus, the potential of the node ND 1  is maintained at the high value because the node ND 1  is set in the floating state. 
     Since the potential of the node ND 1  has the high value, the transistor  101  is kept on. The transistor  102  is turned off. Thus, the potential of the terminal O is increased because the signal (high-level signal V CK2 ) of the terminal C 1  is supplied to the terminal O. The potential difference between the terminal O and the node ND 1  is maintained by parasitic capacitance between the gate and the second terminal of the transistor  101  and the node ND 1  is in the floating state. Thus, when the potential of the terminal O is increased, the potential of the node ND 1  is also increased. When the potential of the node ND 1  is higher than the sum of the potential of the first terminal of the transistor  101  (signal (high-level signal V CK2 ) of the terminal C 1 ) and the threshold voltage of the transistor  101 , the potential of the terminal O is increased to the same level as the signal (high-level signal V CK2 ) of the terminal C 1 . Consequently, the signal (signal V OUT [i]) of the terminal O is set at high level. 
     An example of operation of the circuit SR in a period C is described.  FIG.  7 A  is an example of a schematic diagram of the operation in the period C. 
     Since the signal (signal V CK1 ) of the terminal C 2  is at low level, the transistor  104  is kept off. The transistor  105  is turned on because the signal (signal V CK3 ) of the terminal C 3  is set at high level. The transistor  103  is kept off. Thus, the potential of the node ND 1  is increased because the signal (high-level signal V OUT [i+1]) of the terminal S 2  is supplied to the node ND 1 . After that, when the potential of the node ND 1  becomes equal or substantially equal to a value obtained by subtracting the threshold voltage of the transistor  105  from the potential of the gate of the transistor  105  (signal (high-level signal V CK3 ) of the terminal C 3 ), the transistor  105  is turned off. Thus, the node ND 1  is set in the floating state. 
     Since the potential of the node ND 1  has the high value, the transistor  101  is kept on. The transistor  102  is turned on. Thus, the signal (signal V OUT [i]) of the terminal O is set at low level because the signal (low-level signal V CK2 ) of the terminal C 1  and the potential of the wiring VSS 1  are supplied to the terminal O. 
     In the period C, the signal (the low-level signal V CK2 ) of the terminal C 1  is supplied to the terminal O through the transistor  101 . The current supply capability of the transistor  101  is large in many cases. Thus, the fall time of the signal (signal V OUT [i]) of the terminal O can be shortened. 
     Note that when the potential of the terminal O is decreased by supplying the signal (low-level signal V CK2 ) of the terminal C 1  and the potential of the wiring VSS 1  to the terminal O, the potential of the node ND 1  is also decreased. As described above, the transistor  105  is turned on in the case where the potential of the node ND 1  is lower than a value obtained by subtracting the threshold voltage of the transistor  105  from the potential of the gate of the transistor  105  (signal (high-level signal V CK3 ) of the terminal C 3 ). The transistor  105  is not turned on in the case where the potential of the node ND 1  is higher than the value obtained by subtracting the threshold voltage of the transistor  105  from the potential of the gate of the transistor  105  (signal (high-level signal V CK3 ) of the terminal C 3 ). 
     Note that in the period C, the transistor  102  may be off. 
     An example of operation of the circuit SR in a period D is described.  FIG.  7 B  is an example of a schematic diagram of the operation in the period D. 
     Since the signal (signal V CK1 ) of the terminal C 2  is at low level, the transistor  104  is kept off. The transistor  105  is turned off because the signal (signal V CK3 ) of the terminal C 3  is set at low level. The transistor  103  is turned on. Thus, the potential of the node ND 1  is decreased because the potential of the wiring VSS 2  is supplied to the node ND 1 . 
     The transistor  101  is turned off because the potential of the node ND 1  is set to a low value. The transistor  102  is kept on. Thus, the signal (signal V OUT [i]) of the terminal O is set at low level because the potential of the wiring VSS 1  is supplied to the terminal O. 
     Note that the low value of the potential of the node ND 1  refers to a value by which the transistor  101  is turned off. Specifically, it is a value lower than the sum of the potential of the first terminal or the second terminal of the transistor  101  and the threshold voltage of the transistor  101 . 
     Note that in the period D, the transistor  102  may be off. In that case, the signal (signal V OUT [i]) of the terminal O is maintained at low level because the terminal O is set in the floating state. 
     Note that in the period D, the transistor  103  may be off. In that case, the potential of the node ND 1  is maintained at the high value because the node ND 1  is set in the floating state. The transistor  101  is turned on because the potential of the node ND 1  has the high value. Thus, the low-level signal V CK2  is supplied to the terminal O. 
     An example of operation of the circuit SR in a period E is described.  FIG.  8 A  is an example of a schematic diagram of the operation in the period E. 
     The transistor  104  is turned on because the signal (signal V CK1 ) of the terminal C 2  is set at high level. Since the signal (signal V CK3 ) of the terminal C 3  is at low level, the transistor  105  is kept off. The transistor  103  is kept on. Thus, the potential of the node ND 1  is set to a low value because the signal (low-level signal V OUT [i−1]) of the terminal S 1  and the potential of the wiring VSS 2  are supplied to the node ND 1 . 
     Since the potential of the node ND 1  has the low value, the transistor  101  is kept off. The transistor  102  is kept on. Thus, the signal (signal V OUT [i]) of the terminal O is set at low level because the potential of the wiring VSS 1  is supplied to the terminal O. 
     In the period E, the wiring VSS 2  and the terminal S 1  are brought into electrical conduction through the transistor  103  and the transistor  104 . Thus, noise of the wiring OUT[i−1] can be reduced because the potential of the wiring VSS 2  is supplied to the wiring OUT[i−1]. 
     Note that in the period E, the transistor  102  may be off. In that case, the signal (signal V OUT [i]) of the terminal O is maintained at low level because the terminal O is set in the floating state. 
     Note that in the period E, the transistor  103  may be off. 
     An example of operation of the circuit SR in a period F is described.  FIG.  8 B  is an example of a schematic diagram of the operation in the period F. 
     The transistor  104  is turned off because the signal (signal V CK1 ) of the terminal C 2  is set at low level. Since the signal (signal V CK3 ) of the terminal C 3  is at low level, the transistor  105  is kept off. The transistor  103  is kept on. Thus, the potential of the node ND 1  is set to a low value because the potential of the wiring VSS 2  is supplied to the node ND 1 . 
     Since the potential of the node ND 1  has the low value, the transistor  101  is kept off. The transistor  102  is kept on. Thus, the signal (signal V OUT [i]) of the terminal O is at low level because the potential of the wiring VSS 1  is supplied to the terminal O. 
     Note that in the period F, the transistor  102  may be off. In that case, the signal (signal V OUT [i]) of the terminal O is maintained at low level because the terminal O is set in the floating state. 
     Note that in the period F, the transistor  103  may be off. In that case, the potential of the node ND 1  is maintained at the low value because the node ND 1  is set in the floating state. 
     An example of operation of the circuit SR in a period G is described.  FIG.  9 A  is an example of a schematic diagram of the operation in the period G. 
     Since the signal (signal V CK1 ) of the terminal C 2  is at low level, the transistor  104  is kept off. The transistor  105  is turned on because the signal (signal V CK3 ) of the terminal C 3  is set at high level. The transistor  103  is kept on. Thus, the potential of the node ND 1  is set to a low value because the signal (low-level signal V OUT [i+1]) of the terminal S 2  and the potential of the wiring VSS 2  are supplied to the node ND 1 . 
     Since the potential of the node ND 1  has the low value, the transistor  101  is kept off. The transistor  102  is kept on. Thus, the signal (signal V OUT [i]) of the terminal O is at low level because the potential of the wiring VSS 1  is supplied to the terminal O. 
     In the period G, the wiring VSS 2  and the terminal S 2  are brought into electrical conduction through the transistor  103  and the transistor  105 . Thus, noise of the wiring OUT[i+1] can be reduced because the potential of the wiring VSS 2  is supplied to the wiring OUT[i+1]. 
     Note that in the period G, the transistor  102  may be off. In that case, the signal (signal V OUT [i]) of the terminal O is maintained at low level because the terminal O is set in the floating state. 
     Note that in the period G, the transistor  103  may be off. 
     An example of operation of the circuit SR in a period H is described.  FIG.  9 B  is an example of a schematic diagram of the operation in the period H. 
     Since the signal (signal V CK1 ) of the terminal C 2  is at low level, the transistor  104  is kept off. The transistor  105  is turned off because the signal (signal V CK1 ) of the terminal C 3  is set at low level. The transistor  103  is kept on. Thus, the potential of the node ND 1  is set to a low value because the potential of the wiring VSS 2  is supplied to the node ND 1 . 
     Since the potential of the node ND 1  has the low value, the transistor  101  is kept off. The transistor  102  is kept on. Thus, the signal (signal V OUT [ 1 ]) of the terminal O is at low level because the potential of the wiring VSS 1  is supplied to the terminal O. 
     Note that in the period H, the transistor  102  may be off. In that case, the signal (signal V OUT [i]) of the terminal O is maintained at low level because the terminal O is set in the floating state. 
     Note that in the period H, the transistor  103  may be off. In that case, the potential of the node ND 1  is maintained at the low value because the node ND 1  is set in the floating state. 
     Next, an example of operation in the case where a signal delayed from a signal of the terminal S 2  is output from the terminal O is described with reference to  FIG.  10   . A timing chart in  FIG.  10    is different from the timing chart in  FIG.  5    in that the signal (signal V OUT [i−1]) of the terminal S 1  is set at high level in the period C and the signal (signal V OUT [i+1]) of the terminal S 2  is set at high level in the period A. Furthermore, the timing chart in  FIG.  10    is different from the timing chart in  FIG.  5    in that the signal (signal V CK1 ) of the terminal C 2  is set at high level in the period C and the period G and the signal (signal V CK3 ) of the terminal C 3  is set at high level in the period A and the period E. Note that the description of the operation that is the same as the operation in the case where a signal delayed from a signal of the terminal S 1  is output from the terminal O is not repeated. 
     An example of operation of the circuit SR in a period A is described. 
     The transistor  104  is turned off because the signal (signal V CK1 ) of the terminal C 2  is set at low level. The transistor  105  is turned on because the signal (signal V CK3 ) of the terminal C 3  is set at high level. The transistor  103  is turned off. Thus, the potential of the node ND 1  is increased because the signal (high-level signal V OUT [i+1]) of the terminal S 2  is supplied to the node ND 1 . After that, when the potential of the node ND 1  becomes equal or substantially equal to a value obtained by subtracting the threshold voltage of the transistor  105  from the potential of the gate of the transistor  105  (signal (high-level signal V CK3 ) of the terminal C 3 ), the transistor  105  is turned off. Thus, the node ND 1  is set in the floating state. 
     The transistor  101  is turned on because the potential of the node ND 1  is set to a high value. The transistor  102  is turned on. Thus, the signal (signal V OUT [i]) of the terminal O is set at low level because the signal (low-level signal V CK2 ) of the terminal C 1  and the potential of the wiring VSS 1  are supplied to the terminal O. 
     Note that in the period A, the transistor  102  may be off. 
     The operation of the circuit SR in the period B is similar to the operation in the period B in the case where the signal delayed from the signal of the terminal S 1  is output from the terminal O. 
     An example of operation of the circuit SR in a period C is described. 
     The transistor  104  is turned on because the signal (signal V CK1 ) of the terminal C 2  is set at high level. Since the signal (signal V CK3 ) of the terminal C 3  is at low level, the transistor  105  is kept off. The transistor  103  is kept off. Thus, the potential of the node ND 1  is increased because the signal (high-level signal V OUT [i−1]) of the terminal S 1  is supplied to the node ND 1 . After that, when the potential of the node ND 1  becomes equal or substantially equal to a value obtained by subtracting the threshold voltage of the transistor  104  from the potential of the gate of the transistor  104  (signal (high-level signal V CK1 ) of the terminal C 2 ), the transistor  104  is turned off. Thus, the node ND 1  is set in the floating state. 
     Since the potential of the node ND 1  has the high value, the transistor  101  is kept on. The transistor  102  is turned on. Thus, the signal (signal V OUT [i]) of the terminal O is set at low level because the signal (low-level signal V CK2 ) of the terminal C 1  and the potential of the wiring VSS 1  are supplied to the terminal O. 
     Note that when the potential of the terminal O is decreased by supplying the signal (low-level signal V CK2 ) of the terminal C 1  and the potential of the wiring VSS 1  to the terminal O, the potential of the node ND 1  is also decreased. As described above, the transistor  104  is turned on in the case where the potential of the node ND 1  is lower than a value obtained by subtracting the threshold voltage of the transistor  104  from the potential of the gate of the transistor  104  (signal (high-level signal V CK2 ) of the terminal C 2 ). The transistor  104  is not turned on in the case where the potential of the node ND 1  is higher than the value obtained by subtracting the threshold voltage of the transistor  104  from the potential of the gate of the transistor  104  (signal (high-level signal V CK1 ) of the terminal C 2 ). 
     Note that in the period C, the transistor  102  may be off. 
     The operation of the circuit SR in the period D is similar to the operation in the period D in the case where the signal delayed from the signal of the terminal S 1  is output from the terminal O. 
     The operation of the circuit SR in the period E is similar to the operation in the period G in the case where the signal delayed from the signal of the terminal S 1  is output from the terminal O. 
     The operation of the circuit SR in the period F is similar to the operation in the period F or H in the case where the signal delayed from the signal of the terminal S 1  is output from the terminal O. 
     The operation of the circuit SR in the period G is similar to the operation in the period E in the case where the signal delayed from the signal of the terminal S 1  is output from the terminal O. 
     The operation of the circuit SR in the period H is similar to the operation in the period F or H in the case where the signal delayed from the signal of the terminal S 1  is output from the terminal O. 
     Next, a preferred example of the device of one embodiment of the present invention is described. 
     The transistors  101  to  105  preferably have the same polarity. That is, the transistors  101  to  105  are preferably n-channel transistors. Alternatively, the transistors  101  to  105  are preferably p-channel transistors. Thus, manufacturing steps can be simplified, which enables improvement in yield and/or a reduction in costs. In particular, in the case where the transistors  101  to  105  are n-channel transistors, a transistor including an oxide semiconductor in a channel formation region (also referred to as an OS transistor) can be used as each of the transistors  101  to  105 . The OS transistor has higher mobility than a transistor including amorphous silicon in a channel formation region, and has an extremely low off-state current. Therefore, the transistors  101  to  105  can be downsized. 
     A first conductor (also referred to as a conductive film or a conductive layer) includes a region serving as a first terminal (one of a source electrode and a drain electrode) on the terminal C 1  side of the transistor  101 . A second conductor includes a region serving as a second terminal (the other of the source electrode and the drain electrode) on the terminal O side of the transistor  101 . A third conductor includes a region serving as a gate (gate electrode) of the transistor  101 . The third conductor includes a first region overlapping with the first conductor and a second region overlapping with the second conductor. In that case, the area of the second region is preferably larger than the area of the first region. Thus, the capacitance between the second terminal and the gate of the transistor  101  can be large, which can increase the amount of rise in the potential of the node ND 1  in the period B. 
     Note that the first region may be an area where the third conductor and the first conductor overlap with each other without a semiconductor layer interposed therebetween, and the second region may be an area where the third conductor and the second conductor overlap with each other without a semiconductor layer interposed therebetween. The semiconductor layer includes the channel formation region of the transistor  101 . 
     Note that W (channel width)/L (channel length) of the transistor  101  is preferably higher than W/L of the transistor  102 . Furthermore, W/L of the transistor  101  is preferably higher than W/L of the transistor  103 . Furthermore, W/L of the transistor  101  is preferably higher than W/L of the transistor  104 . Furthermore, W/L of the transistor is preferably higher than W/L of the transistor  105 . That is, the transistor  101  preferably has the highest W/L among the transistors  101  to  105 . Thus, the current supply capability of the transistor  101  can be increased, so that the rise time and fall time of the signal of the terminal O can be shortened. 
     In the case where the transistor is formed using a plurality of transistors, W/L of the transistor is a value of the sum of W/L of the plurality of transistors. For example, in the case where the plurality of transistors are connected in parallel, W is the sum of W of the plurality of transistors and L is the average value of L of the plurality of transistors. 
     Note that W/L of the transistor  104  is preferably equal or substantially equal to W/L of the transistor  105 . The phrase “W/L of the transistor  104  is substantially equal to W/L of the transistor  105 ” means that W/L of the transistor  104  is 0.8 to 1.2 times W/L of the transistor  105 . More preferably, W/L of the transistor  104  is 0.9 to 1.1 times W/L of the transistor  105 . Thus, the same operation can be performed in the circuit SR even in the case where the signal of the terminal S 1  is shifted or the case where the signal of the terminal S 2  is shifted. 
     Here, the configuration of the circuit SR is not limited to the configuration exemplified in  FIG.  4   . Modification examples of the circuit SR exemplified in  FIG.  4    are described. Note that components in common with those in  FIG.  4    are denoted by the same reference numerals, and description thereof is omitted. 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4    or the like, the first terminal of the transistor  103  may be connected to the wiring VSS 1 , the terminal S 1 , the terminal S 2 , the terminal C 1 , the terminal C 2 , the terminal C 3 , the terminal C 4 , the terminal S 3 , the terminal S 4 , or the terminal O. The terminal C 4 , the terminal S 3 , and the terminal S 4  are described later.  FIG.  11 A  illustrates a configuration example where the first terminal of the transistor  103  is connected to the wiring VSS 1  in  FIG.  4   .  FIG.  11 B  illustrates a configuration example where the first terminal of the transistor  103  is connected to the terminal S 2  in  FIG.  4   .  FIG.  12 A  illustrates a configuration example where the first terminal of the transistor  103  is connected to the terminal S 1  in  FIG.  4   . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A , or the like, the first terminal of the transistor  104  may be connected to the wiring VSS 2 , the terminal S 1 , the terminal S 2 , the terminal C 1 , the terminal C 2 , the terminal C 3 , the terminal C 4 , the terminal S 3 , or the terminal S 4 .  FIG.  12 B  illustrates a configuration example where the first terminal of the transistor  102  is connected to the terminal S 2  in  FIG.  4   .  FIG.  13 A  illustrates a configuration example where the first terminal of the transistor  102  is connected to the terminal S 1  in  FIG.  4   . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A , or the like, the gate of the transistor  102  may be connected to the gate of the transistor  103 .  FIG.  13 B  illustrates a configuration example where the gate of the transistor  102  is connected to the gate of the transistor  103  in  FIG.  4   . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B , or the like, one of the first terminal and the gate of the transistor  104  is connected to the terminal S 1 . One of the first terminal and the gate of the transistor  105  is connected to the terminal S 2 .  FIG.  14 A  illustrates a configuration example where the gate of the transistor  104  is connected to the terminal S 1  and the gate of the transistor  105  is connected to the terminal S 2  in  FIG.  4   .  FIG.  14 B  illustrates a configuration example where the first terminal of the transistor  104  is connected to the terminal C 2 , the gate of the transistor  104  is connected to the terminal S 1 , the first terminal of the transistor  105  is connected to the terminal C 3 , and the gate of the transistor  105  is connected to the terminal S 2  in  FIG.  4   . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B , or the like, a configuration for controlling the potential of the gate of the transistor  104  may be added. Furthermore, a configuration for controlling the potential of the gate of the transistor  105  may be added.  FIG.  15 A  illustrates a configuration example where a transistor  106 , a transistor  107 , a transistor  108 , and a transistor  109  are added in  FIG.  4   . A first terminal, a second terminal, and a gate of the transistor  106  are connected to the terminal C 4 , the gate of the transistor  104 , and the terminal S 3 , respectively. A first terminal, a second terminal, and a gate of the transistor  107  are connected to the wiring VSS 3 , the gate of the transistor  104 , and the terminal C 1 , respectively. A first terminal, a second terminal, and a gate of the transistor  108  are connected to the terminal C 4 , the gate of the transistor  105 , and the terminal S 4 , respectively. A first terminal, a second terminal, and a gate of the transistor  109  are connected to the wiring VSS 4 , the gate of the transistor  105 , and the terminal C 1 , respectively. 
     Each of the wiring VSS 3  and the wiring VSS 4  has a function of transmitting a signal, a potential, or a current. In other words, each of the wiring VSS 3  and the wiring VSS 4  has a function of a signal line, a power supply line, or a current supply line. For example, a potential is input to each of the wiring VSS 3  and the wiring VSS 4 . The potential input to each of the wiring VSS 3  and the wiring VSS 4  is a potential corresponding to the low-level or high-level signal V CK1 , the low-level or high-level signal V CK2 , the low-level or high-level signal V CK3 , or the low-level or high-level signal V CK4 . In particular, when the transistor  104  and the transistor  105  are n-channel transistors, a potential corresponding to the low-level signal V CK1 , the low-level signal V CK2 , the low-level signal V CK3 , or the low-level signal V CK4  is preferably input to each of the wiring VSS 3  and the wiring VSS 4 . Furthermore, when the transistor  104  and the transistor  105  are p-channel transistors, a potential corresponding to the high-level signal V CK1 , the high-level signal V CK2 , the high-level signal V CK3 , or the high-level signal V CK4  is preferably input to each of the wiring VSS 3  and the wiring VSS 4 . 
     The terminal S 3  is preferably connected to the wiring OUT[i−2]. The terminal S 4  is preferably connected to the wiring OUT[i+2]. The terminal C 4  is preferably connected to a corresponding wiring of the wirings CK 1  to CK 4 . Specifically, the terminal C 4  in the circuit SR[ 4   m+ 1] is connected to the wiring CK 3 , the terminal C 4  in the circuit SR[ 4   m+ 2] is connected to the wiring CK 4 , the terminal C 4  in the circuit SR[ 4   m+ 3] is connected to the wiring CK 1 , and the terminal C 4  in the circuit SR[ 4   m+ 4] is connected to the wiring CK 2 . For example, the terminal C 4  is connected to the wiring CK 4  in the case where the terminal C 1  is connected to the wiring CK 2 , the terminal C 2  is connected to the wiring CK 1 , and the terminal C 3  is connected to the wiring CK 3 . 
     An example of operation of the circuit SR exemplified in  FIG.  15 A  is described. Note that an example of the operation in the case where a signal delayed from the signal of the terminal S 1  is output from the terminal O is described. 
     An example of operation of the circuit SR in a period H immediately before a period A is described. 
     The transistor  106  is turned on because the signal (signal V OUT [i−2]) of the terminal S 3  is set at high level. The transistor  107  is turned off because the signal (signal V CK2 ) of the terminal C 1  is set at low level. Thus, the potential of the gate of the transistor  104  is increased because the signal (high-level signal V CK4 ) of the terminal C 4  is supplied to the gate of the transistor  104 . When the potential of the gate of the transistor becomes a value obtained by subtracting the threshold voltage of the transistor  106  from the potential of the gate of the transistor  106  (signal (high-level signal V OUT [i−2]) of the terminal S 3 ), the transistor  106  is turned off. Thus, the gate of the transistor  104  is set in the floating state. 
     The transistor  108  is turned off because the signal (signal V OUT [i+2]) of the terminal S 4  is set at low level. The transistor  109  is turned off because the signal (signal V CK2 ) of the terminal C 1  is set at low level. Thus, the gate of the transistor  105  is set in the floating state. In the case where the initial value of the potential of the gate of the transistor  105  is low, the potential of the gate of the transistor  105  is maintained at the low value. 
     The transistor  104  is turned on because the potential of the gate of the transistor becomes a high value. Furthermore, the transistor  105  is turned off because the potential of the gate of the transistor  105  becomes a low value. Thus, the signal (low-level signal V OUT [i−1]) of the terminal S 1  is supplied to the node ND 1 . 
     An example of operation of the circuit SR in the period A is described. 
     The transistor  106  is turned off because the signal (signal V OUT [i−2]) of the terminal S 3  is set at low level. Since the signal (signal V CK2 ) of the terminal C 1  is at low level, the transistor  107  is kept off. Thus, the potential of the gate of the transistor  104  is maintained at the high value because the gate of the transistor  104  is set in the floating state. 
     Since the signal (signal V OUT [i+2]) of the terminal S 4  is at low level, the transistor is kept off. Since the signal (signal V CK2 ) of the terminal C 1  is at low level, the transistor  109  is kept off. Thus, the potential of the gate of the transistor  105  is maintained at the low value because the gate of the transistor  105  is in the floating state. 
     Since the potential of the gate of the transistor  104  has the high value, the transistor  104  is kept on. Furthermore, since the potential of the gate of the transistor  105  has the low value, the transistor  105  is kept off. Thus, the signal (high-level signal V OUT [i−1]) of the terminal S 1  is supplied to the node ND 1 , so that the potential of the node ND 1  is increased. The potential difference between the gate and the second terminal of the transistor  104  is maintained by parasitic capacitance between the gate and the second terminal of the transistor  104 , and the gate of the transistor  104  is in the floating state. Thus, as the potential of the node ND 1  is increased, the potential of the gate of the transistor  104  is also increased. In the case where the potential of the gate of the transistor  104  is higher than the sum of the potential of the first terminal of the transistor (signal (high-level signal V OUT [i−1]) of the terminal S 1 ) and the threshold voltage of the transistor  104 , the potential of the node ND 1  is increased to the same level as the high-level signal V OUT [i−1]. Since the potential difference between the gate and the second terminal of the transistor  104  can be large as described above, the drain current of the transistor  104  can be increased. Thus, the rise time of the potential of the node ND 1  can be shortened. Alternatively, the transistor  104  can be downsized, which enables a reduction in layout. 
     An example of operation of the circuit SR in a period B is described. 
     Since the signal (signal V OUT [i−2]) of the terminal S 3  is at low level, the transistor is kept off. The transistor  107  is turned on because the signal (signal V CK2 ) of the terminal C 1  is set at high level. Thus, the potential of the gate of the transistor  104  is decreased because the potential of the wiring VSS 3  is supplied to the gate of the transistor  104 . 
     Since the signal (signal V OUT [i+2]) of the terminal S 4  is at low level, the transistor is kept off. The transistor  109  is turned on because the signal (signal V CK2 ) of the terminal C 1  is set at high level. Thus, the potential of the gate of the transistor  105  has the low value because the potential of the wiring VSS 4  is supplied to the gate of the transistor  105 . 
     The transistor  104  is turned off because the potential of the gate of the transistor becomes a low value. Since the potential of the gate of the transistor  105  has the low value, the transistor  105  is kept off. 
     An example of operation of the circuit SR in a period C is described. 
     Since the signal (signal V OUT [i−2]) of the terminal S 3  is at low level, the transistor  106  is kept off. The transistor  107  is turned off because the signal (signal V CK2 ) of the terminal C 1  is set at low level. Thus, the potential of the gate of the transistor  104  is maintained at the low value because the gate of the transistor  104  is set in the floating state. 
     Since the signal (signal V OUT [i+2]) of the terminal S 4  is at low level, the transistor  104  is kept off. The transistor  109  is turned off because the signal (signal V CK2 ) of the terminal C 1  is set at low level. Thus, the potential of the gate of the transistor  105  is maintained at the low value because the gate of the transistor  105  is set in the floating state. 
     Since the potential of the gate of the transistor  104  has the low value, the transistor  108  is kept off. Since the potential of the gate of the transistor  105  has the low value, the transistor  105  is kept off. 
     An example of operation of the circuit SR in a period D is described. 
     Since the signal (signal V OUT [i−2]) of the terminal S 3  is at low level, the transistor is kept off. Since the signal (signal V CK2 ) of the terminal C 1  is at low level, the transistor  107  is kept off. Thus, the potential of the gate of the transistor  104  is maintained at the low value because the gate of the transistor  104  is set in the floating state. 
     The transistor  108  is turned on because the signal (signal V OUT [i+2]) of the terminal S 4  is set at high level. Since the signal (signal V CK2 ) of the terminal C 1  is at low level, the transistor  109  is kept off. Thus, the potential of the gate of the transistor  105  is increased because the signal (high-level signal V CK4 ) of the terminal C 4  is supplied to the gate of the transistor  105 . 
     Since the potential of the gate of the transistor  104  has the low value, the transistor is kept off. The transistor  105  is turned on because the potential of the gate of the transistor  105  becomes a high value. Thus, the signal (low-level signal V OUT [i+1]) of the terminal S 2  is supplied to the node ND 1 . 
     An example of operation in a period E immediately after the period D is described. 
     Since the signal (signal V OUT [i−2]) of the terminal S 3  is at low level, the transistor  106  is kept off. Since the signal (signal V CK2 ) of the terminal C 1  is at low level, the transistor  107  is kept off. Thus, since the gate of the transistor  104  is in the floating state, the potential of the gate of the transistor  104  is maintained at the low value. 
     The transistor  108  is turned off because the signal (signal V OUT [i+2]) of the terminal S 4  is set at low level. Since the signal (signal V CK2 ) of the terminal C 1  is at low level, the transistor  109  is kept off. Thus, the potential of the gate of the transistor  105  is maintained at the high value because the gate of the transistor  105  is set in the floating state. 
     Since the potential of the gate of the transistor  104  has the low value, the transistor is kept off. Since the potential of the gate of the transistor  105  has the high value, the transistor  105  is kept on. Thus, the signal (low-level signal V OUT [i+1]) of the terminal S 2  is supplied to the node ND 1 . 
     The operation in the period E except immediately after the period D is similar to the operation in the period C. 
     The operation in the period F is similar to the operation in the period B. 
     The operation in the period G is similar to the operation in the period C. 
     The operation in the period H except immediately before the period A is similar to the operation in the period C. 
     Note that at least one of the first terminal and the gate of the transistor  106  is connected to the terminal S 3 . At least one of the first terminal and the gate of the transistor  108  is connected to the terminal S 4 .  FIG.  15 B  illustrates a configuration example where the first terminal of the transistor  106  is connected to the terminal S 3  and the first terminal of the transistor  108  is connected to the terminal S 4  in  FIG.  15 A .  FIG.  16 A  illustrates a configuration example where the first terminal of the transistor  106  is connected to the terminal S 3 , the gate of the transistor  106  is connected to the terminal C 4 , the first terminal of the transistor  108  is connected to the terminal S 4 , and the gate of the transistor  108  is connected to the terminal C 4  in  FIG.  4   . 
     Note that at least one of the first terminal and the gate of the transistor  106  may be connected to the terminal S 1 . At least one of the first terminal and the gate of the transistor  108  may be connected to the terminal S 2 .  FIG.  16 B  illustrates a configuration example where the first terminal of the transistor  106  is connected to the terminal C 2 , the gate of the transistor  106  is connected to the terminal S 1 , the first terminal of the transistor is connected to the terminal C 3 , and the gate of the transistor  108  is connected to the terminal S 2  in  FIG.  15 A .  FIG.  17 A  illustrates a configuration example where the first terminal of the transistor  106  is connected to the terminal S 1 , the gate of the transistor  106  is connected to the terminal S 1 , the first terminal of the transistor  108  is connected to the terminal S 2 , and the gate of the transistor  108  is connected to the terminal S 2  in  FIG.  15 A .  FIG.  17 B  illustrates a configuration example where the first terminal of the transistor  106  is connected to the terminal S 1 , the gate of the transistor  106  is connected to the terminal C 2 , the first terminal of the transistor  108  is connected to the terminal S 2 , and the gate of the transistor  108  is connected to the terminal C 3  in  FIG.  15 A . 
     Note that the first terminal of the transistor  107  may be connected to the wiring VSS 1 , the wiring VSS 2 , the wiring VSS 4 , the terminal S 1 , the terminal S 2 , the terminal S 3 , the terminal S 4 , the terminal C 2 , the terminal C 3 , or the terminal C 4 . Furthermore, the first terminal of the transistor  109  may be connected to the wiring VSS 1 , the wiring VSS 2 , the wiring VSS 3 , the terminal S 1 , the terminal S 2 , the terminal S 3 , the terminal S 4 , the terminal C 2 , the terminal C 3 , or the terminal C 4 .  FIG.  18 A  illustrates a configuration example where the first terminal of the transistor  107  is connected to the terminal S 3  and the first terminal of the transistor  109  is connected to the terminal S 4  in  FIG.  15 A .  FIG.  18 B  illustrates a configuration example where the first terminal of the transistor  107  is connected to the wiring VSS 1  and the first terminal of the transistor  109  is connected to the wiring VSS 1  in  FIG.  15 A . 
     Note that W/L of the transistor  106  is preferably equal or substantially equal to W/L of the transistor  108 . The phrase “W/L of the transistor  106  is substantially equal to W/L of the transistor  108 ” means that W/L of the transistor  106  is 0.8 to 1.2 times W/L of the transistor  108 . More preferably, W/L of the transistor  106  is 0.9 to 1.1 times W/L of the transistor  108 . 
     Note that W/L of the transistor  107  is preferably equal or substantially equal to W/L of the transistor  109 . The phrase “W/L of the transistor  107  is substantially equal to W/L of the transistor  109 ” means that W/L of the transistor  107  is 0.8 to 1.2 times W/L of the transistor  109 . More preferably, W/L of the transistor  107  is 0.9 to 1.1 times W/L of the transistor  109 . 
     Note that the transistors  106  to  109  preferably have the same polarity as the transistor  101 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B , or the like, a transistor may be added between the terminal O and the node ND 1 . FIG.  19 A illustrates a configuration example whom a transistor  110  is added in  FIG.  4   . A first terminal, a second terminal, and a gate of the transistor  110  are connected to the terminal O, the node ND 1 , and the terminal C 1 , respectively. 
     Note that the transistor  110  preferably has the same polarity as the transistor  101 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A , or the like, a transistor may be added between the terminal C 1  and the node ND 1 .  FIG.  19 B  illustrates a configuration example where a transistor  111  is added in  FIG.  4   . A first terminal and a second terminal of the transistor  111  are connected to the terminal C 1  and the node ND 1 , respectively. 
     Note that the transistor  111  preferably has the same polarity as the transistor  101 . 
     On/off of the transistor  111  may be controlled with a signal input to the gate of the transistor  111 . 
     Note that the gate of the transistor  111  may be connected to the gate of the transistor  102 . Alternatively, the gate of the transistor  111  may be connected to the gate of the transistor  103 . Alternatively, the gate of the transistor  111  may be connected to the gate of the transistor  102  and the gate of the transistor  103 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B , or the like, a transistor directly connected to the transistor  103  may be added.  FIG.  20 A  illustrates a configuration example where a transistor  112  is added in  FIG.  4   . A first terminal and a second terminal of the transistor  112  are connected to the second terminal of the transistor  103  and the node ND 1 , respectively. 
     Note that the transistor  112  preferably has the same polarity as the transistor  101 . 
     On/off of the transistor  112  may be controlled with a signal input to the gate of the transistor  112 . 
     Note that the transistor  112  may be connected between the first terminal of the transistor  101  and the wiring VSS 1 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A , or the like, a transistor directly connected to the transistor may be added.  FIG.  20 B  illustrates a configuration example where a transistor  113  is added in  FIG.  4   . A first terminal and a second terminal of the transistor  113  are connected to the second terminal of the transistor  102  and the terminal O, respectively. 
     Note that the transistor  113  preferably has the same polarity as the transistor  101 . 
     On/off of the transistor  113  may be controlled with a signal input to the gate of the transistor  113 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B , or the like, a transistor connected among the gate of the transistor  101 , the second terminal of the transistor  104 , and the second terminal of the transistor  105  may be added.  FIG.  21 A  illustrates a configuration example where a transistor  114  is added in  FIG.  4   . A first terminal of the transistor  114  is connected to the gate of the transistor  101  and a second terminal of the transistor  114  is connected to the second terminal of the transistor  104 , the second terminal of the transistor  105 , and the second terminal of the transistor  103 . 
     Note that the transistor  114  preferably has the same polarity as the transistor  101 . 
     As illustrated in  FIG.  21 B , the second terminal of the transistor  103  may be connected to the first terminal of the transistor  114 . 
     A gate of the transistor  114  may be connected to the terminal C 1 . Alternatively, the gate of the transistor  114  may be supplied with a potential corresponding to a high-level signal of the terminal C 1 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B ,  FIG.  21 A ,  FIG.  21 B , or the like, the gate of the transistor  103  may be connected to the terminal S 3 , the terminal S 4 , or the terminal C 4 .  FIG.  22 A  illustrates a configuration example where the gate of the transistor  103  is connected to the terminal C 4  in  FIG.  4   . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B ,  FIG.  21 A ,  FIG.  21 B ,  FIG.  22 A , or the like, a configuration for controlling the potential of the gate of the transistor  103  may be added.  FIG.  22 B  illustrates a configuration example where a transistor  115  and a transistor  116  are added in  FIG.  4   . A first terminal, a second terminal, and a gate of the transistor  115  are connected to the terminal C 4 , the gate of the transistor  103 , and the terminal C 4 , respectively. A first terminal, a second terminal, and a gate of the transistor  116  are connected to the wiring VSS 2 , the gate of the transistor  103 , and the node ND 1 , respectively. 
     In the period A, the period B, and the period C, the transistor  115  is off because the signal (signal V CK4 ) of the terminal C 4  is at low level. The transistor  116  is turned on because the potential of the node ND 1  is set to a high value. Thus, the potential of the gate of the transistor  103  is set to a low value because the potential of the wiring VSS 2  is supplied to the gate of the transistor  103 . Since the potential of the gate of the transistor is set to the low value, the transistor  103  is turned off. 
     In the period D, the transistor  115  is turned on because the signal (signal V CK4 ) of the terminal C 4  is set at high level. The transistor  116  is turned off because the potential of the node ND 1  is set to a low value. Thus, the potential of the gate of the transistor  103  is set to a high value because the signal (high-level signal V CK4 ) of the terminal C 4  is supplied to the gate of the transistor  103 . Since the potential of the gate of the transistor  103  is set to the high value, the transistor  103  is turned on. 
     In the period E, the period F, and the period G, the transistor  115  is turned off because the signal (signal V CK4 ) of the terminal C 4  is set at low level. Since the potential of the node ND 1  has the low value, the transistor  116  is kept off. Thus, the potential of the gate of the transistor  103  has the high value because the gate of the transistor  103  is set in the floating state. Since the potential of the gate of the transistor  103  has the high value, the transistor  103  is kept on. 
     In the period H, the transistor  115  is turned on because the signal (signal V CK4 ) of the terminal C 4  is set at high level. Since the potential of the node ND 1  has the low value, the transistor  116  is kept off. Thus, since the signal (high-level signal V CK4 ) of the terminal C 4  is supplied to the gate of the transistor  103 , the potential of the gate of the transistor  103  has the high value. Since the potential of the gate of the transistor  103  has the high value, the transistor  103  is kept on. 
     Note that at least one of the first terminal and the gate of the transistor  115  is connected to the terminal C 4 . For example, the first terminal of the transistor  115  may be connected to the terminal C 4  and the gate of the transistor  115  may be connected to a wiring (not illustrated) to which a potential corresponding to the high-level signal V CK1 , the high-level signal V CK2 , the high-level signal V CK3 , or the high-level signal V CK4  is input. 
     Note that the gate of the transistor  116  may be connected to the terminal C 1 , the terminal C 2 , the terminal C 3 , the terminal S 1 , the terminal S 2 , or the terminal O.  FIG.  23 A  illustrates a configuration example where the gate of the transistor  116  is connected to the terminal S 1  in  FIG.  22 B .  FIG.  23 B  illustrates a configuration example where the gate of the transistor  116  is connected to the terminal C 1  in  FIG.  22 B . 
     Note that the first terminal of the transistor  116  may be connected to the wiring VSS 1 , the wiring VSS 3 , the wiring VSS 4 , the terminal S 3 , the terminal S 4 , or the terminal C 4 . 
     Note that the transistor  115  and the transistor  116  preferably have the same polarity as the transistor  101 . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B ,  FIG.  21 A ,  FIG.  21 B ,  FIG.  22 A ,  FIG.  22 B ,  FIG.  23 A ,  FIG.  23 B , or the like, the gate of the transistor  102  may be connected to the terminal S 1 , the terminal S 2 , the terminal S 3 , the terminal S 4 , the terminal C 2 , the terminal C 3 , or the terminal C 4 .  FIG.  24 A  illustrates a configuration example where the gate of the transistor  102  is connected to the terminal C 4  in  FIG.  4   .  FIG.  24 B  illustrates a configuration example where the gate of the transistor  102  is connected to the terminal S 1  in  FIG.  4   . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B ,  FIG.  21 A ,  FIG.  21 B ,  FIG.  22 A ,  FIG.  22 B ,  FIG.  23 A ,  FIG.  23 B ,  FIG.  24 A ,  FIG.  24 B , or the like, in the case where a plurality of configurations can be applied to each transistor, a transistor having two or more of the plurality of configurations may be provided. In other words, in the case where a first configuration, a second configuration, and a third configuration can be applied to one transistor, two or more of transistors corresponding to the first configuration, the second configuration, and the third configuration may be provided. 
     For example, the first terminal of the transistor  103  is connected to the wiring VSS 2  (see  FIG.  4   ), the wiring VSS 1  (see  FIG.  11 A ), the terminal S 2  (see  FIG.  11 B ), the terminal S 1  (see  FIG.  12 A ), the terminal C 1 , the terminal C 2 , the terminal C 3 , the terminal C 4 , the terminal S 3 , the terminal S 4 , or the terminal O.  FIG.  25 A  illustrates a configuration example where a transistor  103 A corresponding to the transistor  103  whose first terminal is connected to the terminal S 2 , and a transistor  103 B corresponding to the transistor  103  whose first terminal is connected to the terminal S 1  are provided in the circuit SR illustrated in  FIG.  4   . 
     For example, the gate of the transistor  103  is connected to the terminal S 3 , the terminal S 4 , the terminal C 2 , the terminal C 3 , or the terminal C 4  (see  FIG.  22 A ).  FIG.  25 B  illustrates a configuration example where a transistor  103 C corresponding to the transistor  103  whose gate is connected to the terminal S 2  and a transistor  103 D corresponding to the transistor  103  whose gate is connected to the terminal S 1  are provided in the circuit SR illustrated in  FIG.  4   . 
     For example, the gate of the transistor  116  is connected to the node ND 1  (see  FIG.  22 B ), the terminal C 1  (see  FIG.  23 B ), the terminal C 2 , the terminal C 3 , the terminal S 1  (see  FIG.  23 A ), the terminal S 2 , or the terminal O.  FIG.  26 A  illustrates a configuration example where a transistor  116 A corresponding to the transistor  116  whose gate is connected to the terminal S 1  and a transistor  116 B corresponding to the transistor  116  whose gate is connected to the terminal S 2  are provided in the circuit SR illustrated in  FIG.  22 B . 
     All or some of the circuits SR in this embodiment that are described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B ,  FIG.  21 A ,  FIG.  21 B ,  FIG.  22 A ,  FIG.  22 B ,  FIG.  23 A ,  FIG.  23 B ,  FIG.  24 A ,  FIG.  24 B ,  FIG.  25 A ,  FIG.  25 B ,  FIG.  26 A , and the like can be combined. 
     For example,  FIG.  26 B  illustrates a configuration example where, in  FIG.  4   , the first terminal of the transistor  103  is connected to the wiring VSS 1  as illustrated in FIG. A, the transistors  106  to  109  are added as illustrated in  FIG.  15 A , the first terminal of the transistor  107  is connected to the wiring VSS 1  as illustrated in  FIG.  18 B , and the first terminal of the transistor  109  is connected to the wiring VSS 1  as illustrated in  FIG.  18 B . 
     For example,  FIG.  27 A  illustrates a configuration example where, in  FIG.  4   , the transistor  110  is added as illustrated in  FIG.  19 A  and the transistor  111  is added as illustrated in  FIG.  19 B . 
     For example,  FIG.  27 B  illustrates a configuration example where, in  FIG.  4   , the gate of the transistor  104  is connected to the terminal S 1  as illustrated in  FIG.  14 A , the gate of the transistor  105  is connected to the terminal S 2  as illustrated in  FIG.  14 A , the transistor  103 A is provided as illustrated in  FIG.  25 A , and the transistor  103 B is provided as illustrated in  FIG.  25 A . 
     In the circuit SR in this embodiment that is described with or without reference to  FIG.  4   ,  FIG.  11 A ,  FIG.  11 B ,  FIG.  12 A ,  FIG.  12 B ,  FIG.  13 A ,  FIG.  13 B ,  FIG.  14 A ,  FIG.  14 B ,  FIG.  15 A ,  FIG.  15 B ,  FIG.  16 A ,  FIG.  16 B ,  FIG.  17 A ,  FIG.  17 B ,  FIG.  18 A ,  FIG.  18 B ,  FIG.  19 A ,  FIG.  19 B ,  FIG.  20 A ,  FIG.  20 B ,  FIG.  21 A ,  FIG.  21 B ,  FIG.  22 A ,  FIG.  22 B ,  FIG.  23 A ,  FIG.  23 B ,  FIG.  24 A ,  FIG.  24 B ,  FIG.  25 A ,  FIG.  25 B ,  FIG.  26 A ,  FIG.  26 B ,  FIG.  27 A ,  FIG.  27 B , or the like, all or at least one of the plurality of transistors may have a double-gate structure. The transistor having a double-gate structure includes a first gate and a second gate. A channel formation region or a semiconductor layer including the channel formation region of the transistor having a double-gate structure includes a region sandwiched between the first gate and the second gate. The first gate may be provided below or above the second gate. The first gate corresponds to the above “gate” and a portion to which the first gate is connected is the same as a portion to which the above “gate” is connected. The second gate may be connected to the first gate or a dedicated wiring. In the case where the second gate is connected to the first gate, the mobility of the transistor becomes high and the on-state current of the transistor is increased. Thus, W/L of the transistor can be small. In the case where the second gate is connected to a dedicated wiring, electrical characteristics of the transistor can be controlled with a potential or a signal of the dedicated wiring. 
     For example,  FIG.  38 A  illustrates a configuration example where, in  FIG.  4   , the transistors  101  to  105  each have a double-gate structure in which the first gate and the second gate are connected to each other. 
     For example,  FIG.  38 B  illustrates a configuration example where, in  FIG.  4   , the transistors  101  to  105  each have a double-gate structure in which the second gate is connected to a wiring BG. 
     In this specification and the like, any of a variety of switches can be used as a switch. The switch has a function of determining whether current flows or not by being turning on or off (being brought into an on state or an off state). Alternatively, the switch has a function of selecting and changing a current path. For example, the switch has a function of determining whether current can flow through a path  1  or a path  2  and switching the paths. For example, an electrical switch or a mechanical switch can be used as a switch. That is, any element can be used as a switch as long as it can control current, without limitation to a certain element. For example, a transistor (e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky diode, a metal-insulator-metal (MIM) diode, a metal-insulator-semiconductor (MIS) diode, or a diode-connected transistor), or a logic circuit in which such elements are combined can be used as a switch. An example of a mechanical switch is a switch formed using a micro electro mechanical system (MEMS) technology, such as a digital micromirror device (DMD). Such a switch includes an electrode which can be moved mechanically, and operates by controlling electrical conduction and non-conduction in accordance with movement of the electrode. 
     In the case where a transistor is used as a switch, the polarity (conductivity type) of the transistor is not particularly limited to a certain type because it operates just as a switch. However, a transistor of polarity with a smaller off-state current is preferably used when off-state current is to be suppressed. Examples of a transistor with a smaller off-state current are a transistor provided with an LDD region and a transistor with a multi-gate structure. 
     Note that in the case where a transistor is used as a switch, an n-channel transistor is preferably used as the switch when the potential of a source of the transistor which operates as the switch is close to a potential of a low-potential-side power supply (e.g., Vss, GND, or 0 V). A p-channel transistor is preferably used as the switch when the potential of the source is close to a potential of a high-potential-side power supply (e.g., Vdd). This is because the absolute value of the gate-source voltage can be increased when the potential of a source of the n-channel transistor is close to the potential of a low-potential-side power supply or when the potential of a source of the p-channel transistor is close to the potential of a high-potential-side power supply, so that the transistor can more accurately operate as a switch. This is also because the transistor does not often perform source follower operation, so that the decrease in output voltage does not often occur. 
     Note that a CMOS switch including both n-channel and p-channel transistors may be employed as a switch. The use of a CMOS switch as a switch allows more accurate operation of the switch because a current can flow when either the p-channel transistor or the n-channel transistor is turned on. Thus, a voltage can be appropriately output regardless of whether the voltage of an input signal to the switch is high or low. Alternatively, the voltage amplitude of a signal for turning on or off the switch can be made small, so that power consumption can be reduced. 
     Note that when a transistor is used as a switch, the switch includes an input terminal (one of a source and a drain), an output terminal (the other of the source and the drain), and a terminal for controlling electrical conduction (a gate) in some cases. When a diode is used as a switch, the switch does not have a terminal for controlling electrical conduction in some cases. Therefore, when a diode is used as a switch, the number of wirings for controlling terminals can be small as compared with the case of using a transistor as a switch. 
     In this specification and the like, for example, transistors with a variety of structures can be used as a transistor, without limitation to a certain type. For example, a transistor including single crystal silicon or a non-single-crystal semiconductor film typified by amorphous silicon, polycrystalline silicon, microcrystalline (also referred to as microcrystal, nanocrystal, or semi-amorphous) silicon, or the like can be used as a transistor. Alternatively, a thin film transistor (TFT) whose semiconductor film is thinned can be used. In the case of using the TFT, there are various advantages. For example, since the TFT can be formed at temperature lower than that of the case of using single crystal silicon, manufacturing cost can be reduced or a manufacturing apparatus can be made larger. Since the manufacturing apparatus can be made larger, the TFT can be formed using a large substrate. Therefore, many display devices can be formed at the same time at low cost. In addition, a substrate having low heat resistance can be used because of low manufacturing temperature. Therefore, the transistor can be formed using a light-transmitting substrate. Alternatively, transmission of light in a display element can be controlled by using the transistor formed using the light-transmitting substrate. Alternatively, part of a film included in the transistor can transmit light because of a small thickness of the transistor. Therefore, the aperture ratio can be improved. 
     Note that when a catalyst (e.g., nickel) is used in the case of forming polycrystalline silicon, crystallinity can be further improved and a transistor having excellent electrical characteristics can be formed. Accordingly, a gate driver circuit (e.g., a scan line driver circuit), a source driver circuit (e.g., a signal line driver circuit), and a signal processing circuit (e.g., a signal generation circuit, a gamma correction circuit, or a DA converter circuit) can be formed using the same substrate. 
     Note that when a catalyst (e.g., nickel) is used in the case of forming microcrystalline silicon, crystallinity can be further improved and a transistor having excellent electrical characteristics can be formed. In that case, crystallinity can be improved by just performing heat treatment without performing laser irradiation. Accordingly, a gate driver circuit (e.g., a scan line driver circuit) and part of a source driver circuit (e.g., an analog switch) can be formed over the same substrate. Note that when laser irradiation for crystallization is not performed, unevenness in crystallinity of silicon can be suppressed. Therefore, high-quality images can be displayed. Note that it is possible to form polycrystalline silicon or microcrystalline silicon without a catalyst (e.g., nickel). 
     Note that although the crystallinity of silicon is preferably improved to polycrystal, microcrystal, or the like in the whole panel, the present invention is not limited to this. The crystallinity of silicon may be improved only in part of the panel. Selective increase in crystallinity can be achieved by selective laser irradiation or the like. For example, only a peripheral circuit region excluding pixels may be irradiated with laser light. Alternatively, only a region of a gate driver circuit, a source driver circuit, or the like may be irradiated with laser light. Alternatively, only part of a source driver circuit (e.g., an analog switch) may be irradiated with laser light. Accordingly, the crystallinity of silicon can be improved only in a region in which a circuit needs to be operated at high speed. Because a pixel region is not particularly needed to be operated at high speed, even if crystallinity is not improved, the pixel circuit can be operated without any problem. Thus, a region whose crystallinity is improved is small, so that manufacturing steps can be decreased. This can increase throughput and reduce manufacturing cost. Alternatively, since the number of necessary manufacturing apparatuses is small, manufacturing cost can be reduced. 
     Examples of the transistor include a transistor including a compound semiconductor (e.g., SiGe or GaAs) or an oxide semiconductor (e.g., Zn—O, In—Ga—Zn—O, In—Zn—O, In—Sn—O (ITO), Sn—O, Ti—O, Al—Zn—Sn—O (AZTO), or In—Sn—Zn—O) and a thin film transistor including a thin film of such a compound semiconductor or an oxide semiconductor. Because manufacturing temperature can be lowered, such a transistor can be formed at room temperature, for example. The transistor can thus be formed directly on a substrate having low heat resistance, such as a plastic substrate or a film substrate. Note that such a compound semiconductor or an oxide semiconductor can be used not only for a channel portion of the transistor but also for other applications. For example, such a compound semiconductor or an oxide semiconductor can be used for a wiring, a resistor, a pixel electrode, a light-transmitting electrode, or the like. Such an element can be formed at the same time as the transistor; thus, cost can be reduced. 
     Note that for example, a transistor formed by an ink-jet method or a printing method can be used as a transistor. Accordingly, such a transistor can be formed at room temperature, can be formed at a low vacuum, or can be formed using a large substrate. Thus, the transistor can be formed without using a mask (reticle), which enables the layout of the transistor to be easily changed. Alternatively, the transistor can be formed without using a resist, leading to reductions in material cost and the number of steps. Furthermore, a film can be formed only in a portion where the film is needed, a material is not wasted as compared with the case of employing a manufacturing method by which etching is performed after the film is formed over the entire surface, so that the cost can be reduced. 
     Note that for example, a transistor including an organic semiconductor or a carbon nanotube can be used as a transistor. Thus, such a transistor can be formed over a flexible substrate. A device including a transistor which includes an organic semiconductor or a carbon nanotube can resist a shock. 
     Note that transistors with a variety of different structures can be used for a transistor. For example, a MOS transistor, a junction transistor, a bipolar transistor, or the like can be used as a transistor. By using a MOS transistor as a transistor, the size of the transistor can be reduced. Thus, a number of transistors can be mounted. By using a bipolar transistor as a transistor, a large amount of current can flow. Thus, a circuit can be operated at high speed. Note that a MOS transistor and a bipolar transistor may be formed over one substrate, in which case reductions in power consumption and size, high-speed operation, and the like can be achieved. 
     Note that in this specification and the like, for example, a transistor with a multi-gate structure having two or more gate electrodes can be used as a transistor. With the multi-gate structure, a structure where a plurality of transistors are connected in series is provided because channel regions are connected in series. Thus, with the multi-gate structure, the amount of off-state current can be reduced and the withstand voltage of the transistor can be increased (reliability can be improved). Alternatively, with the multi-gate structure, the drain-source current does not change so much even if the drain-source voltage fluctuates when the transistor operates in a saturation region, so that a flat slope of the voltage-current characteristics can be obtained. By utilizing the flat slope of the voltage-current characteristics, an ideal current source circuit or an active load having extremely high resistance can be obtained. Accordingly, a differential circuit, a current mirror circuit, or the like having excellent properties can be obtained. 
     Note that a transistor with a structure where gate electrodes are formed above and below a channel can be used, for example. With the structure where gate electrodes are formed above and below a channel, a circuit structure where a plurality of transistors are connected in parallel is provided. Thus, a channel region is increased, so that the amount of current can be increased. Alternatively, by using the structure where gate electrodes are formed above and below a channel, a depletion layer can be easily formed, resulting in lower subthreshold swing. 
     Note that for example, a transistor with a structure where a gate electrode is formed above a channel region, a structure where a gate electrode is formed below a channel region, a staggered structure, an inverted staggered structure, a structure where a channel region is divided into a plurality of regions, a structure where channel regions are connected in parallel or in series, or the like can be used as a transistor. A transistor with any of a variety of structures such as a planar type, a FIN-type, a TRI-Gate type, a top-gate type, a bottom-gate type, and a double-gate type (with gates above and below a channel) can be used. 
     Note that for example, a transistor with a structure where a source electrode or a drain electrode overlaps with a channel region (or part of it) can be used as a transistor. By using the structure where a source electrode or a drain electrode overlaps with a channel region (or part of it), unstable operation due to accumulation of electric charge in part of the channel region can be prevented. 
     Note that for example, a transistor with a structure where an LDD region is provided can be used as a transistor. Provision of the LDD region enables a reduction in off-state current or an increase in the withstand voltage of the transistor (an improvement in reliability). Alternatively, by providing the LDD region, the drain current does not change so much even when the drain-source voltage fluctuates when the transistor operates in a saturation region, so that a flat slope of the voltage-current characteristics can be obtained. 
     Note that in this specification and the like, a transistor can be formed using any of a variety of substrates, for example. The type of a substrate is not limited to a certain type. Examples of the substrate are a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, and a base material film. Examples of the glass substrate are a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. Examples of the flexible substrate, the attachment film, and the base material film are plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES), a synthetic resin of acrylic or the like, polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, polyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film, and paper. Specifically, when a transistor is formed using a semiconductor substrate, a single crystal substrate, an SOI substrate, or the like, the transistor can have few variations in characteristics, size, shape, or the like, high current supply capability, and a small size. By forming a circuit with the use of such a transistor, power consumption of the circuit can be reduced or the circuit can be highly integrated. 
     A flexible substrate may be used as the substrate, and the transistor may be provided directly on the flexible substrate. Moreover, a separation layer may be provided between the substrate and the transistor. 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 and transferred onto another substrate. In such a case, the transistor can be transferred to a substrate having low heat resistance or a flexible substrate. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, or an organic resin film of polyimide or the like formed over a substrate can be used, for example. 
     That is, a transistor may be formed using a substrate, and then, the transistor may be transferred to another substrate. Examples of a substrate to which a transistor is transferred are, in addition to the above substrate over which the transistor can be formed, a paper substrate, a cellophane substrate, an aramid substrate, a polyimide film substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), and the like), a leather substrate, and a rubber substrate. The use of such a substrate enables formation of a transistor with excellent properties, a transistor with low power consumption, or a device with high durability, high heat resistance, or a reduction in weight or thickness. 
     Note that all the circuits which are necessary to realize a predetermined function can be formed using one substrate (e.g., a glass substrate, a plastic substrate, a single crystal substrate, or an SOI substrate). In this manner, the cost can be reduced by a reduction in the number of components or reliability can be improved by a reduction in the number of connection points to circuit components. 
     Note that not all the circuits which are necessary to realize the predetermined function are needed to be formed using one substrate. That is, part of the circuits which are necessary to realize the predetermined function may be formed using a substrate and another part of the circuits which are necessary to realize the predetermined function may be formed using another substrate. For example, part of the circuits which are necessary to realize the predetermined function can be formed using a glass substrate and another part of the circuits which are necessary to realize the predetermined function can be formed using a single crystal substrate (or an SOI substrate). The single crystal substrate over which the another part of the circuits which are necessary to realize the predetermined function (such a substrate is also referred to as an IC chip) can be connected to the glass substrate by chip on glass (COG), and the IC chip can be provided over the glass substrate. Alternatively, the IC chip can be connected to the glass substrate by tape automated bonding (TAB), chip on film (COF), surface mount technology (SMT), a printed circuit board, or the like. When part of the circuits is formed over the same substrate as a pixel portion in this manner, the cost can be reduced by a reduction in the number of components or reliability can be improved by a reduction in the number of connection points between circuit components. In particular, a circuit in a portion where a driving voltage is high, a circuit in a portion where a driving frequency is high, or the like consumes much power in many cases. In view of the above, such a circuit is formed over a substrate (e.g., a single crystal substrate) different from a substrate over which a pixel portion is formed, whereby an IC chip is formed. The use of this IC chip allows prevention of increase in power consumption. 
     For example, in this specification and the like, a transistor is an element having at least three terminals: a gate, a drain, and a source. The transistor includes a channel region between the drain (a drain terminal, a drain region, or a drain electrode) and the source (a source terminal, a source region, or a source electrode), and a current can flow through the drain, the channel region, and the source. Here, since the source and the drain of the transistor change depending on the structure, the operating condition, or the like of the transistor, it is difficult to define which is a source or a drain. Therefore, a portion functioning as a source or a drain is not called a source or a drain in some cases. In that case, for example, one of the source and the drain is referred to as a first terminal, a first electrode, or a first region, and the other of the source and the drain is referred to as a second terminal, a second electrode, or a second region in some cases. 
     Note that a transistor may be an element having at least three terminals of a base, an emitter, and a collector. In that case also, one of the emitter and the collector is referred to as a first terminal, a first electrode, or a first region, and the other of the emitter and the collector is referred to as a second terminal, a second electrode, or a second region in some cases. Note that in the case where a bipolar transistor is used as a transistor, a gate can be rephrased as a base. 
     For example, in this specification and the like, when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected, the case where X and Y are functionally connected, and the case where X and Y are directly connected are included therein. Accordingly, a connection relation other than connection relations shown in drawings and texts is also included, without being limited to a predetermined connection relation, for example, a connection relation shown in the drawings and the texts. 
     Here, each of X and Y denotes an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, a layer, or the like). 
     In the case where X and Y are electrically connected, one or more elements (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, and a load) that enable an electrical connection between X and Y can be connected between X and Y, for example. Note that the switch is controlled to be turned on or off. That is, the switch has a function of determining whether current flows or not by being turned on or off (becoming an on state or an off state). Alternatively, the switch has a function of selecting and changing a current path. 
     In the case where X and Y are functionally connected, one or more circuits (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit, a signal converter circuit such as a DA converter circuit, an AD converter circuit, or a gamma correction circuit; a potential level converter circuit such as a power supply circuit (e.g., a step-up circuit or a step-down circuit) or a level shifter circuit for changing the potential level of a signal; a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a storage circuit; and a control circuit) that enable a functional connection between X and Y can be connected between X and Y, for example. Note that for example, in the case where a signal output from X is transmitted to Y even when another circuit is interposed between X and Y, X and Y are functionally connected. 
     Note that when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), the case where X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another element or another circuit provided therebetween), and the case where X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween) are included therein. That is, when it is explicitly described that “X and Y are electrically connected”, the description is the same as the case where it is explicitly only described that “X and Yare connected”. 
     Note that, for example, the case where a source (or a first terminal or the like) of a transistor is electrically connected to X through (or not through) Z 1  and a drain (or a second terminal or the like) of the transistor is electrically connected to Y through (or not through) Z 2 , or the case where a source (or a first terminal or the like) of a transistor is directly connected to one part of Z 1  and another part of Z 1  is directly connected to X while a drain (or a second terminal or the like) of the transistor is directly connected to one part of Z 2  and another part of Z 2  is directly connected to Y, can be expressed by using any of the following expressions. 
     The expressions include, for example, “X, Y, a source (or a first terminal or the like) of a transistor, and a drain (or a second terminal or the like) of the transistor are electrically connected to each other, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in this order”, “a source (or a first terminal or the like) of a transistor is electrically connected to X, a drain (or a second terminal or the like) of the transistor is electrically connected to Y, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in this order”, and “X is electrically connected to Y through a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are provided to be connected in this order”. When the connection order in a circuit configuration is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope. Note that these expressions are examples and there is no limitation on the expressions. Here, each of X, Y, Z 1 , and Z 2  denotes an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, a layer, or the like). 
     Even when independent components are electrically connected to each other in a circuit diagram, one component has functions of a plurality of components in some cases. For example, when part of a wiring also functions as an electrode, one conductive film functions as the wiring and the electrode. Thus, “electrical connection” in this specification includes in its category such a case where one conductive film has functions of a plurality of components. 
     Note that this embodiment can be combined with any of the other embodiments as appropriate. Thus, a content (or part thereof) described in this embodiment can be applied to, combined with, or replaced with a different content in the embodiment and/or a content (or part thereof) described in one or a plurality of different embodiments. Note that in each embodiment, a content described in the embodiment is a content described with reference to a variety of diagrams or a content described with a text described in this specification. In addition, by combining a diagram (or part thereof) described in one embodiment with another part of the diagram, a different diagram (or part thereof) described in the same embodiment, and/or a diagram (or part thereof) described in one or a plurality of different embodiments, much more diagrams can be formed. This applies also to other embodiments. 
     Embodiment 2 
     In this embodiment, a display device including the device of one embodiment of the present invention is described. 
     A display device illustrated in  FIG.  28 A  includes the circuit  100  and a pixel portion  130 . In the pixel portion  130 , N (N is a natural number of 3 or more) wirings OUT and M (M is a natural number) wirings SL (also referred to as wirings SL[ 1 ] to SL[M]) are provided. Pixels  131  are provided for the N wirings OUT and M wirings SL. The circuit  100  has a function of a gate driver (also referred to as a gate line driver circuit, a gate signal line driver circuit, or a scan line driver circuit). The N wirings OUT have a function of gate lines (also referred to as gate signal lines or scan lines). The M wirings SL have a function of transmitting video signals. That is, the M wirings SL have a function of source lines (also referred to as source signal lines or signal lines). The M wirings SL are connected to a circuit having a function of a source driver (also referred to as a source line driver circuit, a source signal line driver circuit, or a signal line driver circuit). 
     The selection and non-selection of the pixel  131  are controlled based on the potential of the wiring OUT. That is, the selection and non-selection of the pixel  131  are controlled with the circuit  100 . When the pixel  131  is selected, a video signal is written from the wiring SL to the pixel  131 . While the video signal is held in the pixel  131 , the pixel  131  displays an image in accordance with the video signal. After that, when the pixel  131  is not selected, the pixel  131  keeps displaying the image in accordance with the held video signal. 
     Next, a specific structure example of the pixel  131  is described. 
     The pixel  131  illustrated in  FIG.  28 B  includes a transistor  132 , a liquid crystal element  133 , and a capacitor  134 . A first terminal of the transistor  132  is connected to the wiring SL, a second terminal of the transistor  132  is electrically connected to a first electrode of the liquid crystal element  133  and a first electrode (also referred to as a pixel electrode) of the capacitor  134 , and a gate of the transistor  132  is connected to the wiring OUT. A second electrode (also referred to as a common electrode) of the liquid crystal element  133  is shared by all or two or more of the plurality of pixels  131 . In other words, a conductor including a region serving as the second electrode of the liquid crystal element of a first pixel  131  includes a region serving as the second electrode of the liquid crystal element  133  of a second pixel  131 . A second electrode of the capacitor  134  is connected to a wiring having a function of a capacitor line. The second electrode of the capacitor  134  is connected to the same wiring in all or two or more of the plurality of pixels  131 . Note that the second electrode of the capacitor  134  may be connected to the second electrode of the liquid crystal element  133 . On/off of the transistor  132  is controlled with the potential of the wiring OUT. When the transistor  132  is turned on, a video signal of the wiring SL is input to the pixel  131 . The liquid crystal element  133  includes a liquid crystal material. The alignment of the liquid crystal material is controlled with the potential difference between the first electrode of the liquid crystal element  133  and the second electrode of the liquid crystal element  133 . The capacitor has a function of accumulating electric charge based on the video signal. In other words, the capacitor  134  has a function of maintaining the potential of the first electrode of the liquid crystal element  133  at a value corresponding to the video signal. 
     The pixel  131  illustrated in  FIG.  28 C  includes a transistor  135 , a transistor  136 , and an EL element  137 . A first terminal, a second terminal, and a gate of the transistor are connected to the wiring SL, a gate of the transistor  136 , and the wiring OUT, respectively. A first terminal and a second terminal of the transistor  136  are connected to a wiring having a function of supplying a current flowing through the EL element  137  and a first electrode (also referred to as a pixel electrode) of the EL element  137 , respectively. A second electrode (also referred to as a common electrode) of the EL element  137  is shared by all or two or more of the plurality of pixels  131 . In other words, a conductor including a region serving as the second electrode of the EL element  137  of the first pixel includes a region serving as the second electrode of the EL element  137  of the second pixel  131 . On/off of the transistor  135  is controlled with the potential of the wiring OUT. When the transistor  135  is turned on, the video signal of the wiring SL is input to the pixel  131 . The transistor  136  has a function of supplying a current to the EL element  137 . The current supplied to the EL element  137  by the transistor  136  has a value corresponding to the video signal. The EL element  137  has a function of emitting light in accordance with the current supplied from the transistor  136 . 
     The structure of the pixel  131  is not limited to those in  FIGS.  28 B and  28 C . The pixel  131  has any structure as long as it includes a transistor whose gate is connected to the wiring OUT and first terminal is connected to the wiring SL and a display element which displays an image on the basis of a video signal input through the transistor. Alternatively, the pixel  131  has any structure as long as it includes a transistor whose gate is connected to the wiring OUT and first terminal is connected to the wiring SL and a pixel electrode to which a potential or a current based on a video signal input through the transistor is supplied. Alternatively, the pixel  131  has any structure as long as it includes a transistor whose gate is connected to the wiring OUT and first terminal is connected to the wiring SL and a transistor which supplies a current based on a video signal input through the transistor to a display element or a pixel electrode. 
     This embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     Embodiment 3 
     In this embodiment, a structure example of a transistor is described with reference to drawings. A transistor described in this embodiment can be applied to the transistors  101  to  116  described in Embodiment 1 and the transistors  132 ,  135 , and  136  described in Embodiment 2. 
     &lt;Structure Example of Transistor&gt; 
       FIG.  29 A  is a schematic top view of a transistor  600  described below as an example.  FIG.  29 B  is a schematic cross-sectional view of the transistor  600  taken along line A-B in  FIG.  29 A . The transistor  600  described as an example in  FIGS.  29 A and  29 B  is a bottom-gate transistor. 
     The transistor  600  includes a gate electrode  602  over a substrate  601 , an insulating layer  603  over the substrate  601  and the gate electrode  602 , an oxide semiconductor layer provided over the insulating layer  603  to overlap with the gate electrode  602 , and a pair of electrodes  605   a  and  605   b  in contact with a top surface of the oxide semiconductor layer  604 . An insulating layer  606  covers the insulating layer  603 , the oxide semiconductor layer  604 , and the pair of electrodes  605   a  and  605   b . An insulating layer is provided over the insulating layer  606 . 
     There is no particular limitation on the properties of a material and the like of the substrate  601  as long as the material has heat resistance high enough to withstand at least heat treatment performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or an yttria-stabilized zirconia (YSZ) substrate may be used as the substrate  601 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium, an SOI substrate, or the like can be used as the substrate  601 . Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate  601 . 
     A flexible substrate such as a plastic substrate may be used as the substrate  601 , and the transistor  600  may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate  601  and the transistor  600 . The separation layer can be used when part or the whole of the transistor formed over the separation layer is formed, separated from the substrate  601 , and then transferred to another substrate. Thus, the transistor  600  can be transferred to a substrate having low heat resistance or a flexible substrate. 
     The gate electrode  602  can be formed using a metal selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metals as a component; an alloy containing any of these metals in combination; or the like. Furthermore, one or both of manganese and zirconium may be used. The gate electrode  602  may have a single-layer structure or a stacked structure of two or more layers. For example, the gate electrode  602  can have a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum 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 three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, or the like. Alternatively, 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 electrode  602  can also be formed using a light-transmitting conductive material such as indium tin oxide, 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. The gate electrode  602  can have a stacked structure using the above light-transmitting conductive material and the above metal. 
     Furthermore, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-based oxynitride semiconductor film, an In—Ga-based oxynitride semiconductor film, an In—Zn-based oxynitride semiconductor film, a Sn-based oxynitride semiconductor film, an In-based oxynitride semiconductor film, a film of metal nitride (such as InN or ZnN), or the like may be provided between the gate electrode  602  and the insulating layer  603 . These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which is higher than the electron affinity of an oxide semiconductor, thus, the threshold voltage of a transistor including the oxide semiconductor can be shifted in the positive direction, and what is called a normally-off switching element can be achieved. For example, in the case of using an In—Ga—Zn-based oxynitride semiconductor film, an In—Ga—Zn-based oxynitride semiconductor film having a higher nitrogen concentration than at least the oxide semiconductor layer  604 , specifically, an In—Ga—Zn-based oxynitride semiconductor film having a nitrogen concentration of 7 atomic % or higher is used. 
     The insulating layer  603  functions as a gate insulating film. The insulating layer in contact with a bottom surface of the oxide semiconductor layer  604  is preferably an oxide insulating film. 
     The insulating layer  603  has a single-layer structure or a stacked structure using silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, or Ga—Zn-based metal oxide, for example. 
     The insulating layer  603  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, in which case gate leakage current of the transistor can be reduced. 
     The pair of electrodes  605   a  and  605   b  function as a source electrode and a drain electrode of the transistor. 
     The pair of electrodes  605   a  and  605   b  can be formed to have a single-layer structure or a stacked structure using, as a conductive material, any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten or an alloy containing any of these metals as its main component. For example, the pair of electrodes  605   a  and  605   b  can have a single-layer structure of an aluminum film containing silicon; a two-layer structure in which a titanium film is stacked over an aluminum film; a two-layer structure in which a titanium 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 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; or 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. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used. 
     The insulating layer  606  is preferably an oxide insulating film which contains oxygen in excess of that in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film which contains oxygen in excess of that in the stoichiometric composition. The oxide insulating film which contains oxygen in excess of that in the stoichiometric composition is an oxide insulating film in which the amount of released oxygen converted into oxygen atoms is 1.0×10 18  atoms/cm 3  or more, preferably 3.0×10 20  atoms/cm 3  or more in thermal desorption spectroscopy (TDS) analysis. Note that the temperature of the film surface in the TDS analysis is preferably 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. 
     As the insulating layer  606 , a silicon oxide film, a silicon oxynitride film, or the like can be used. 
     Note that the insulating layer  606  also functions as a film that relieves damage to the oxide semiconductor layer  604  at the time of forming the insulating layer  607  later. 
     Moreover, an oxide film transmitting oxygen may be provided between the insulating layer  606  and the oxide semiconductor layer  604 . 
     As the oxide film transmitting oxygen, a silicon oxide film, a silicon oxynitride film, or the like can be used. Note that in this specification, a “silicon oxynitride film” refers to a film that contains more oxygen than nitrogen, and a “silicon nitride oxide film” refers to a film that contains more nitrogen than oxygen. 
     The insulating layer  607  can be an insulating film having a blocking effect against oxygen, hydrogen, water, and the like. Providing the insulating layer  607  over the insulating layer  606  can prevent outward diffusion of oxygen from the oxide semiconductor layer  604  and entry of hydrogen, water, or the like into the oxide semiconductor layer  604  from the outside. Examples of the insulating film having a blocking effect against oxygen, hydrogen, water, and the like are a silicon nitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, and a hafnium oxynitride film. 
     &lt;Example of Manufacturing Method of Transistor&gt; 
     Next, an example of a manufacturing method of the transistor  600  in  FIGS.  29 A and  29 B  is described. 
     First, as illustrated in  FIG.  30 A , the gate electrode  602  is formed over the substrate  601 , and the insulating layer  603  is formed over the gate electrode  602 . 
     Here, a glass substrate is used as the substrate  601 . 
     A method for forming the gate electrode  602  is described below. First, a conductive film is formed by a sputtering method, a CVD method, an evaporation method, or the like and then a resist mask is formed over the conductive film using a first photomask by a photolithography process. Next, part of the conductive film is etched using the resist mask to form the gate electrode  602 . After that, the resist mask is removed. 
     Note that the gate electrode  602  may be formed by an electrolytic plating method, a printing method, an inkjet method, or the like instead of the above formation method. 
     The insulating layer  603  is formed by a sputtering method, a PECVD method, an evaporation method, or the like. 
     When a silicon oxide film, a silicon oxynitride film, or a silicon nitride oxide film is formed as the insulating layer  603 , a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide. 
     When a silicon nitride film is formed as the insulating layer  603 , it is preferable to use a two-step formation method. First, a first silicon nitride film with few defects is formed by a plasma CVD method using a mixed gas of silane, nitrogen, and ammonia as a source gas. Then, a second silicon nitride film that has low hydrogen concentration and can block hydrogen is formed by switching the source gas to a mixed gas of silane and nitrogen. With such a formation method, a silicon nitride film having few defects and a blocking property against hydrogen can be formed as the insulating layer  603 . 
     When a gallium oxide film is formed as the insulating layer  603 , a metal organic chemical vapor deposition (MOCVD) method can be employed. 
     Next, as illustrated in  FIG.  30 B , the oxide semiconductor layer  604  is formed over the insulating layer  603 . 
     A method for forming the oxide semiconductor layer  604  is described below. First, an oxide semiconductor film is formed. Then, a resist mask is formed over the oxide semiconductor film using a second photomask by a photolithography process. Then, part of the oxide semiconductor film is etched using the resist mask to form the oxide semiconductor layer  604 . After that, the resist mask is removed. 
     After that, heat treatment may be performed. In such a case, the heat treatment is preferably performed under an atmosphere containing oxygen. The temperature of the heat treatment may be, for example, higher than or equal to 150° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. 
     Next, as illustrated in  FIG.  30 C , the pair of electrodes  605   a  and  605   b  are formed. 
     A method for forming the pair of electrodes  605   a  and  605   b  is described below. First, a conductive film is formed by a sputtering method, a PECVD method, an evaporation method, or the like. Next, a resist mask is formed over the conductive film using a third photomask by a photolithography process. Then, part of the conductive film is etched using the resist mask to form the pair of electrodes  605   a  and  605   b . After that, the resist mask is removed. 
     Note that as illustrated in  FIG.  30 C , the upper part of the oxide semiconductor layer  604  is partly etched and thinned by the etching of the conductive film in some cases. For this reason, the oxide semiconductor film is preferably formed thick. 
     Next, as illustrated in  FIG.  30 D , the insulating layer  606  is formed over the oxide semiconductor layer  604  and the pair of electrodes  605   a  and  605   b , and the insulating layer is successively formed over the insulating layer  606 . 
     When a silicon oxide film or a silicon oxynitride film is formed as the insulating layer  606 , a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide. 
     For example, a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in an vacuum-evacuated treatment chamber of a plasma CVD apparatus is held at a temperature ranging from 180° C. to 260° C., preferably from 200° C. to 240° C.; the pressure of the treatment chamber into which the source gas is introduced is set in the range from 100 Pa to 250 Pa, preferably from 100 Pa to 200 Pa; and an electrode provided in the treatment chamber is supplied with a high-frequency power ranging from 0.17 W/cm 2  to 0.5 W/cm 2 , preferably from 0.25 W/cm 2  to 0.35 W/cm 2 . 
     As the film formation conditions, the high-frequency power with the above power density is supplied to the treatment 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, therefore, the oxygen content in the oxide insulating film becomes higher than that in the stoichiometric composition. However, when the substrate temperature is within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen is released by heating. Thus, it is possible to form an oxide insulating film which contains oxygen in excess of that in the stoichiometric composition and from which part of oxygen is released by heating. 
     In the case where an oxide insulating film is provided between the oxide semiconductor layer  604  and the insulating layer  606 , the oxide insulating film serves as a protection film of the oxide semiconductor layer  604  in the step of forming the insulating layer  606 . Thus, the insulating layer  606  can be formed using the high-frequency power with high power density while damage to the oxide semiconductor layer  604  is reduced. 
     For example, a silicon oxide film or a silicon oxynitride film can be formed as the oxide insulating film under the following conditions: the substrate placed in an vacuum-evacuated treatment chamber of the PECVD apparatus is held at a temperature ranging from 180° C. to 400° C., preferably from 200° C. to 370° C.; the pressure of the treatment chamber into which the source gas is introduced is set in the range from 20 Pa to 250 Pa, preferably from 100 Pa to 250 Pa; and high-frequency power is supplied to the electrode provided in the treatment chamber. Setting the pressure in the treatment chamber in the range from 100 Pa to 250 Pa can reduce damage to the oxide semiconductor layer  604  at the time of forming the oxide insulating film. 
     A deposition gas containing silicon and an oxidizing gas are preferably used as a source gas of the oxide insulating film. Typical examples of the deposition gas containing silicon are silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas are oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide. 
     The insulating layer  607  can be formed by a sputtering method, a PECVD method, or the like. 
     When a silicon nitride film or a silicon nitride oxide film is formed as the insulating layer  607 , a deposition gas containing silicon, an oxidizing gas, and a gas containing nitrogen are preferably used as a source gas. Typical examples of the deposition gas containing silicon are silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas are oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide. Examples of the gas containing nitrogen are nitrogen and ammonia. 
     Through the above steps, the transistor  600  can be formed. 
     &lt;Modification Example of Transistor&gt; 
     Examples of the structure of a transistor that is partly different from the transistor are described below. 
       FIG.  31 A  is a schematic cross-sectional diagram of a transistor  610  described below as an example. The transistor  610  differs from the transistor  600  in the structure of the oxide semiconductor layer. 
     An oxide semiconductor layer  614  included in the transistor  610  is a stack of an oxide semiconductor layer  614   a  and an oxide semiconductor layer  614   b.    
     Note that a boundary between the oxide semiconductor layers  614   a  and  614   b  is shown by broken lines in  FIG.  31 A  and the like because the boundary is not clear in some cases. 
     Typical examples of a material of the oxide semiconductor layer  614   a  are an In—Ga oxide, an In—Zn oxide, and an In-M-Zn oxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). When the oxide semiconductor layer  614   a  is In-M-Zn oxide, without taking Zn and O into consideration, the proportion of In and the proportion of M are preferably set to less than 50 atomic % and greater than or equal to 50 atomic %, respectively, more preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. For example, a material that has an energy gap of 2 eV or more, preferably 2.5 eV or more, more preferably 3 eV or more is used for the oxide semiconductor layer  614   a.    
     The oxide semiconductor layer  614   b  contains In or Ga, and typically contains an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). The energy at the conduction band bottom of the oxide semiconductor layer  614   b  is closer to a vacuum level than that of the oxide semiconductor layer  614   a , and typically, the difference in the energy at the conduction band bottom between the oxide semiconductor layer  614   b  and the oxide semiconductor layer  614   a  is preferably 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     For example, when the oxide semiconductor layer  614   b  is an In-M-Zn oxide, the proportion of In and the proportion of M, not taking Zn and O into consideration, are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, more preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively. 
     For example, for the oxide semiconductor layer  614   a , an In—Ga—Zn oxide with an atomic ratio of In:Ga:Zn=1:1:1, 1:1:1.2, or 3:1:2 can be used. For the oxide semiconductor layer  614   b , an In—Ga—Zn oxide with an atomic ratio of In:Ga:Zn=1:3:2, 1:6:4, or 1:9:6 can be used. Note that the atomic ratio of each of the oxide semiconductor layers  614   a  and  614   b  may vary within a margin of ±20% of the corresponding atomic ratio. 
     The oxide with a high content of Ga serving as a stabilizer is used for the oxide semiconductor layer  614   b  provided over the oxide semiconductor layer  614   a , thereby preventing release of oxygen from the oxide semiconductor layers  614   a  and  614   b.    
     Note that without limitation to the materials given above, a material with an appropriate composition depending on intended semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor can be used. In order to obtain intended semiconductor characteristics of the transistor, 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 layers  614   a  and  614   b  be set to appropriate values. 
     Although the oxide semiconductor layer  614  is a stack of two oxide semiconductor layers in the above structure, it may be a stack of three or more oxide semiconductor layers. 
       FIG.  31 B  is a schematic cross-sectional diagram of a transistor  620  described below as an example. The transistor  620  differs from the transistor  600  and the transistor in the structure of the oxide semiconductor layer. 
     In an oxide semiconductor layer  624  included in the transistor  620 , an oxide semiconductor layer  624   a , an oxide semiconductor layer  624   b , and an oxide semiconductor layer  624   c  are stacked in this order. 
     The oxide semiconductor layers  624   a  and  624   b  are stacked over the insulating layer  603 . The oxide semiconductor layer  624   c  is provided in contact with a top surface of the oxide semiconductor layer  624   b  and top and side surfaces of the pair of electrodes  605   a  and  605   b.    
     The oxide semiconductor layer  624   b  can have a structure similar to that of the oxide semiconductor layer  614   a  exemplified in Modification Example 1, for example. Moreover, the oxide semiconductor layers  624   a  and  624   c  can have a structure similar to that of the oxide semiconductor layer  614   b  exemplified in Modification Example 1, for example. 
     For example, when an oxide with a high content of Ga serving as a stabilizer is used for the oxide semiconductor layer  624   a  provided under the oxide semiconductor layer  624   b  and the oxide semiconductor layer  624   c  provided over the oxide semiconductor layer  624   b , oxygen can be prevented from being released from the oxide semiconductor layers  624   a  to  624   c.    
     For example, in the case where a channel is formed mainly in the oxide semiconductor layer  624   b , the on-state current of the transistor  620  can be increased when an oxide with a high content of In is used for the oxide semiconductor layer  624   b  and the pair of electrodes  605   a  and  605   b  are provided in contact with the oxide semiconductor layer  624   b.    
     &lt;Another Structure Example of Transistor&gt; 
     A structural example of a top-gate transistor to which the oxide semiconductor film of one embodiment of the present invention can be applied is described below. 
     Note that components having structures or functions similar to the above are denoted by the same reference numerals, and the description thereof is omitted below. 
       FIG.  32 A  is a schematic cross-sectional diagram of a top-gate transistor  650  described below as an example. 
     The transistor  650  includes the oxide semiconductor layer  604  over the substrate provided with an insulating layer  651 , the pair of electrodes  605   a  and  605   b  in contact with the top surface of the oxide semiconductor layer  604 , the insulating layer  603  over the oxide semiconductor layer  604  and the pair of electrodes  605   a  and  605   b , and the gate electrode  602  provided over the insulating layer  603  to overlap with the oxide semiconductor layer  604 . An insulating layer  652  is provided to cover the insulating layer  603  and the gate electrode  602 . 
     The insulating layer  651  has a function of suppressing diffusion of impurities from the substrate  601  to the oxide semiconductor layer  604 . For example, a structure similar to that of the insulating layer  607  can be employed. Note that the insulating layer  651  is not necessarily provided. 
     Like the insulating layer  607 , the insulating layer  652  can be an insulating film having a blocking effect against oxygen, hydrogen, water, and the like. Note that the insulating layer  607  is not necessarily provided. 
     Examples of the structure of a transistor that is partly different from the transistor are described below. 
       FIG.  32 B  is a schematic cross-sectional diagram of a transistor  660 . The transistor  660  differs from the transistor  650  in the structure of the oxide semiconductor layer. 
     In an oxide semiconductor layer  664  included in the transistor  660 , an oxide semiconductor layer  664   a , an oxide semiconductor layer  664   b , and an oxide semiconductor layer  664   c  are stacked in this order. 
     The oxide semiconductor film described above can be used as one or more of the oxide semiconductor layers  664   a  to  664   c.    
     The oxide semiconductor layer  664   b  can have a structure similar to that of the oxide semiconductor layer  614   a  exemplified in Modification Example 1, for example. The oxide semiconductor layers  664   a  and  664   c  can have a structure similar to that of the oxide semiconductor layer  614   b  exemplified in Modification Example 1, for example. 
     For example, when an oxide with a high content of Ga serving as a stabilizer is used for the oxide semiconductor layer  664   a  provided under the oxide semiconductor layer  664   b  and the oxide semiconductor layer  664   c  provided over the oxide semiconductor layer  664   b , oxygen can be prevented from being released from the oxide semiconductor layers  664   a  to  664   c.    
     A structure example of a transistor, which is partly different from the transistor  650 , is described below. 
       FIG.  32 C  is a schematic cross-sectional diagram of a transistor  670  exemplified below. The transistor  670  differs from the transistor  650  in the shapes of the pair of electrodes  605   a  and  605   b  in contact with the oxide semiconductor layer  604 , the shape of the gate electrode  602 , and the like. 
     The transistor  670  includes the oxide semiconductor layer  604  over the substrate provided with the insulating layer  651 , the insulating layer  603  over the oxide semiconductor layer  604 , the gate electrode  602  over the insulating layer  603 , an insulating layer  654  over the insulating layer  651  and the oxide semiconductor layer  604 , an insulating layer  656  over the insulating layer  654 , the pair of electrodes  605   a  and  605   b  electrically connected to the oxide semiconductor layer  604  through openings provided in the insulating layers  654  and  656 , and the insulating layer  652  over the insulating layer  656  and the pair of electrodes  605   a  and  605   b.    
     The insulating layer  654  is formed using, for example, an insulating film containing hydrogen. As an example of the insulating film containing hydrogen, a silicon nitride film can be given. Hydrogen contained in the insulating layer  654  is bonded to oxygen vacancies in the oxide semiconductor layer  604  to be carriers in the oxide semiconductor layer  604 . Thus, in the structure illustrated in  FIG.  32 C , regions where the oxide semiconductor layer  604  and the insulating layer  654  are in contact with each other refer to an n-type region  604   b  and an n-type region  604   c . Note that a region between the n-type region  604   b  and the n-type region  604   c  serves as a channel region  604   a.    
     By providing the n-type regions  604   b  and  604   c  in the oxide semiconductor layer  604 , contact resistance with the pair of electrodes  605   a  and  605   b  can be reduced. The n-type regions  604   b  and  604   c  can be formed using the insulating layer  654  covering the gate electrode  602  at the time of forming the gate electrode  602  in a self-aligned manner. The transistor  670  illustrated in  FIG.  32 C  is a so-called top-gate transistor having a self-aligned structure. With a top-gate transistor having a self-aligned structure, an overlap of the gate electrode  602  and the pair of electrodes  605   a  and  605   b  serving as a source electrode and a drain electrode is not formed, which enables a reduction in parasitic capacitance between the electrodes. 
     The insulating layer  656  included in the transistor  670  can be formed using a silicon oxynitride film, for example. 
     This embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     Embodiment 4 
     In this embodiment, an oxide semiconductor layer that can be used as a semiconductor layer of the transistor with low off-state current described in the above embodiment is described. 
     An oxide semiconductor used for a channel formation region in the semiconductor layer of the transistor preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. A stabilizer for strongly bonding oxygen is preferably contained in addition to In and Zn. As a stabilizer, at least one of gallium (Ga), tin (Sn), zirconium (Zr), hafnium (Hf), and aluminum (Al) may be contained. 
     As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) may be contained. 
     As the oxide semiconductor used for the semiconductor layer of the transistor, for example, any of the following can be used: indium oxide, tin oxide, zinc oxide, an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, an In—Ga-based oxide, an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, an In—Sc—Zn-based oxide, an In—Y—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and an In—Hf—Al—Zn-based oxide. 
     For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1, 3:1:2, or 2:1:3, or an oxide with an atomic ratio close to the above atomic ratios can be used. 
     When the oxide semiconductor film forming the semiconductor layer contains a large amount of hydrogen, the hydrogen and the oxide semiconductor are bonded to each other, so that part of the hydrogen serves as a donor and causes generation of an electron that is a carrier. As a result, the threshold voltage of the transistor shifts in the negative direction. Therefore, it is preferable that, after formation of the oxide semiconductor film, dehydration treatment (dehydrogenation treatment) be performed to remove hydrogen or moisture from the oxide semiconductor film so that the oxide semiconductor film is highly purified to contain impurities as little as possible. 
     Note that oxygen in the oxide semiconductor film is also reduced by the dehydration treatment (dehydrogenation treatment) in some cases. Therefore, it is preferable that oxygen be added to the oxide semiconductor film to fill oxygen vacancies increased by the dehydration treatment (dehydrogenation treatment). In this specification and the like, supplying oxygen to an oxide semiconductor film may be expressed as oxygen adding treatment, and treatment for making the oxygen content of an oxide semiconductor film be in excess of that in the stoichiometric composition may be expressed as treatment for making an oxygen-excess state. 
     In this manner, hydrogen or moisture is removed from the oxide semiconductor film by the dehydration treatment (dehydrogenation treatment) and oxygen vacancies therein are filled by the oxygen adding treatment, whereby the oxide semiconductor film can be turned into an i-type (intrinsic) oxide semiconductor film or a substantially i-type (intrinsic) oxide semiconductor film which is extremely close to an i-type oxide semiconductor film. Note that “substantially intrinsic” means that the oxide semiconductor film contains extremely few (close to zero) carriers derived from a donor and has a carrier density which is 1×10 17 /cm 3  or lower, 1×10 16 /cm 3  or lower, 1×10 15 /cm 3  or lower, 1×10 14 /cm 3  or lower, or 1×10 13 /cm 3  or lower. 
     In this manner, the transistor including an i-type (intrinsic) or substantially i-type oxide semiconductor film can have extremely favorable off-state current characteristics. For example, the drain current at the time when the transistor including an oxide semiconductor film is in an off-state at room temperature (approximately 25° C.) can be less than or equal to 1×10 −18  A, preferably less than or equal to 1×10 −21  A, further preferably less than or equal to 1×10 −24  A; or at 85° C., less than or equal to 1×10 −15  A, preferably less than or equal to 1×10 −18  A, further preferably less than or equal to 1×10 −12  A. Note that the off state of an n-channel transistor refers to a state where a gate voltage is sufficiently lower than the threshold voltage. Specifically, the transistor is off when the gate voltage is lower than the threshold voltage by 1 V or more, 2 V or more, or 3 V or more. 
     A structure of the oxide semiconductor film is described below. 
     An oxide semiconductor film is classified roughly into a non-single-crystal oxide semiconductor film and a single-crystal oxide semiconductor film. The non-single-crystal oxide semiconductor film includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, a polycrystalline oxide semiconductor film, a microcrystalline oxide semiconductor film, an amorphous oxide semiconductor film, and the like. 
     First, a CAAC-OS film is described. 
     The CAAC-OS film is an oxide semiconductor film having a plurality of c-axis aligned crystal parts. 
     With a transmission electron microscope (TEM), a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of the CAAC-OS film is observed. Consequently, a plurality of crystal parts are observed clearly. However, in the high-resolution TEM image, a boundary between crystal parts, i.e., a grain boundary is not observed clearly. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the high-resolution cross-sectional TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface, metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a shape reflecting 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. 
     On the other hand, according to the high-resolution plan-view TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface, 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. 
     A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO 4  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. 
     Note that when the CAAC-OS film with an InGaZnO 4  crystal 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 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 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 decrease in crystallinity. Furthermore, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic order of the oxide semiconductor film and causes a decrease 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. 
     The CAAC-OS film is an oxide semiconductor film having a low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein. 
     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 a “highly purified intrinsic” or “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 little variation in electrical characteristics and high reliability. Charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released, and might behave like fixed 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. 
     In a transistor including the CAAC-OS film, changes in electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light are small. 
     Next, a microcrystalline oxide semiconductor film is described. 
     A microcrystalline oxide semiconductor film has a region in which a crystal part is observed and a region in which a crystal part is not observed clearly in a high-resolution TEM image. In most cases, 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. 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 nanocrystal (nc). An oxide semiconductor film including nanocrystal is referred to as a nanocrystalline oxide semiconductor (nc-OS) film. In a high resolution TEM image of the nc-OS film, a grain boundary cannot be found clearly in the nc-OS film in some cases. 
     In the nc-OS film, a microscopic region (for example, 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. Note that 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. Furthermore, a halo pattern is shown 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 probe diameter (e.g., larger than or equal to nm) larger than the diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter close to, or smaller than the diameter of a crystal part. Furthermore, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are shown in some cases. Also 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. 
     The nc-OS film is an oxide semiconductor film that has high regularity as compared to an amorphous oxide semiconductor film. Therefore, the nc-OS film has a lower density of defect states than an amorphous oxide semiconductor film. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film. Therefore, the nc-OS film has a higher density of defect states than the CAAC-OS film. 
     Next, an amorphous oxide semiconductor film is described. 
     The amorphous oxide semiconductor film has disordered atomic arrangement and no crystal part. For example, the amorphous oxide semiconductor film does not have a specific state as in quartz. 
     In a high-resolution TEM image of the amorphous oxide semiconductor film, crystal parts cannot be found. 
     When the amorphous oxide semiconductor film is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak which shows a crystal plane does not appear. A halo pattern is shown in an electron diffraction pattern of the amorphous oxide semiconductor film. Furthermore, a halo pattern is shown but a spot is not shown in a nanobeam electron diffraction pattern of the amorphous oxide semiconductor film. 
     Note that an oxide semiconductor film may have a structure having physical properties between the nc-OS film and the amorphous oxide semiconductor film. The oxide semiconductor film having such a structure is specifically referred to as an amorphous-like oxide semiconductor (amorphous-like OS) film. 
     In a high-resolution TEM image of the amorphous-like OS film, a void may be seen. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed. In the amorphous-like OS film, crystallization by a slight amount of electron beam used for TEM observation occurs and growth of the crystal part is found sometimes. In contrast, crystallization by a slight amount of electron beam used for TEM observation is less observed in the nc-OS film having good quality. 
     Note that the crystal part size in the amorphous-like OS film and the nc-OS film can be measured using high-resolution TEM images. For example, an InGaZnO 4  crystal has a layered structure in which two Ga—Zn—O layers are included between In—O layers. A unit cell of the InGaZnO 4  crystal has a structure in which nine layers of three In—O layers and six Ga—Zn—O layers are layered in the c-axis direction. Accordingly, the spacing between these adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to 0.29 nm from crystal structure analysis. Thus, each of the lattice fringes in which the spacing therebetween is from 0.28 nm to 0.30 nm corresponds to the a-b plane of the InGaZnO 4  crystal, focusing on the lattice fringes in the high-resolution TEM image. 
     Note that an oxide semiconductor film may be a stacked film including two or more films of an amorphous oxide semiconductor film, an amorphous-like OS film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example. 
     In this specification, the 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, the 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 also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system. 
     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, an example of a display module is described below with reference to  FIG.  33    and  FIG.  34   . 
       FIG.  33    is a top view of an example of a display module. A display module  700  illustrated in  FIG.  33    includes a pixel portion  702  provided over a first substrate  701 ; a source driver circuit portion  704  and a gate driver circuit portion  706  provided over the first substrate  701 ; a sealant  712  provided to surround the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706 ; and a second substrate provided to face the first substrate  701 . The first substrate  701  and the second substrate  705  are sealed with the sealant  712 . That is, the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  are sealed with the first substrate  701 , the sealant  712 , and the second substrate  705 . Although not illustrated in  FIG.  33   , a display element is provided between the first substrate  701  and the second substrate  705 . 
     In the display module  700 , a flexible printed circuit (FPC) terminal portion  708  electrically connected to the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  is provided in a region different from the region which is surrounded by the sealant  712  and positioned over the first substrate  701 . Furthermore, an FPC  716  is connected to the FPC terminal portion  708 , and a variety of signals and the like are supplied to the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  through the FPC  716 . Furthermore, a signal line  710  is connected to the pixel portion  702 , the source driver circuit portion  704 , the gate driver circuit portion  706 , and the FPC terminal portion  708 . The variety of signals and the like are applied to the pixel portion  702 , the source driver circuit portion  704 , the gate driver circuit portion  706 , and the FPC terminal portion  708  via the signal line  710  from the FPC  716 . 
     A plurality of gate driver circuit portions  706  may be provided in the display module  700 . An example of the display module  700  in which the source driver circuit portion  704  and the gate driver circuit portion  706  are formed over the first substrate  701  where the pixel portion  702  is also formed is described; however, the structure is not limited thereto. For example, only the gate driver circuit portion  706  may be formed over the first substrate  701  or only the source driver circuit portion  704  may be formed over the first substrate  701 . In this case, a substrate where a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver-circuit substrate formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be mounted on the first substrate  701 . Note that there is no particular limitation on the method of connecting a separately prepared driver circuit substrate, and a chip on glass (COG) method, a wire bonding method, or the like can be used. 
     The pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  included in the display module  700  include a plurality of transistors. As the plurality of transistors, any of the transistors that are described in the above embodiments can be used. 
     The display module  700  can include any of a variety of elements. The display module  700  includes, for example, at least one of a liquid crystal element, an electroluminescence (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), an interferometric modulator (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, and a display element including a carbon nanotube. Other than the above, display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by an electrical or magnetic effect may be included. Examples of display devices having EL elements include an EL display. Examples of display devices including electron emitters include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including electronic ink or electrophoretic elements is electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to include aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption. Note that in this embodiment, a structure including a liquid crystal element as a display element is described below. 
     As a display method in the display module  700 , a progressive method, an interlace method, or the like can be employed. Furthermore, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel may be included. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ among color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Furthermore, the size of a display region may be different depending on respective dots of the color components. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display. 
     A coloring layer (also referred to as a color filter) may be used in order to obtain a full-color display device in which white light (W) for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp) is used. As the coloring layer, red (R), green (G), blue (B), yellow (Y), or the like may be combined as appropriate, for example. With the use of the coloring layer, higher color reproducibility can be obtained than in the case without the coloring layer. In this case, by providing a region with the coloring layer and a region without the coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without the coloring layer, a decrease in luminance due to the coloring layer can be suppressed, and 20% to 30% of power consumption can be reduced in some cases when an image is displayed brightly. Note that in the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light of their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption can be further reduced as compared to the case of using the coloring layer in some cases. Note that in this embodiment, a structure in which a backlight and the like are not provided, that is, a so-called reflective liquid crystal display module is described below. 
       FIG.  34    is a cross-sectional view taken along dashed-dotted line Q-R in  FIG.  33   . The display module illustrated in  FIG.  34    is described in detail below. 
     &lt;Display Module&gt; 
     The display module  700  illustrated in  FIG.  34    includes a lead wiring portion  711 , the pixel portion  702 , the source driver circuit portion  704 , and the FPC terminal portion  708 . Note that the lead wiring portion  711  includes the signal line  710 . The pixel portion  702  includes a transistor  750  and a capacitor  790 . The source driver circuit portion  704  includes a transistor  752 . 
     Any of the transistors described above can be used as the transistors  750  and  752 . 
     The transistors used in this embodiment each include an oxide semiconductor film which is highly purified and in which formation of oxygen vacancies is suppressed. In the transistor, the current in an off state (off-state current) can be made small. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption. 
     In addition, the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high speed operation. For example, with such a transistor which can operate at high speed used for a display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced. In addition, the transistor which can operate at high speed can be used also in the pixel portion, whereby a high-quality image can be provided. 
     The capacitor  790  includes a dielectric between a pair of electrodes. Specifically, a conductive film which is formed using the same step as a conductive film functioning as a gate electrode of the transistor  750  is used as one electrode of the capacitor  790 , and a conductive film functioning as a source electrode or a drain electrode of the transistor  750  is used as the other electrode of the capacitor  790 . Furthermore, an insulating film functioning as a gate insulating film of the transistor  750  is used as the dielectric between the pair of electrodes. 
     In  FIG.  34   , insulating films  764  and  768  and a planarization insulating film  770  are formed over the transistor  750 , the transistor  752 , and the capacitor  790 . 
     As the insulating film  764 , a silicon oxide film, a silicon oxynitride film, or the like may be formed by a PECVD device, for example. As the insulating film  768 , a silicon nitride film or the like may be formed by a PECVD device, for example. The planarization insulating film  770  can be formed using a heat-resistant organic material, such as a polyimide resin, an acrylic resin, a polyimide amide resin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. Alternatively, a structure without the planarization insulating film  770  may be employed. 
     The signal line  710  is formed in the same process as conductive films functioning as a source electrode and a drain electrode of the transistor  750  or  752 . Note that the signal line  710  may be formed using a conductive film which is formed in a different process as a source electrode and a drain electrode of the transistor  750  or  752 , e.g., a conductive film formed in the same process as a conductive film functioning as a gate electrode may be used. In the case where the signal line  710  is formed using a material including a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen. 
     The FPC terminal portion  708  includes a connection electrode  760 , an anisotropic conductive film  780 , and the FPC  716 . Note that the connection electrode  760  is formed in the same process as conductive films functioning as a source electrode and a drain electrode of the transistor  750  or  752 . The connection electrode  760  is electrically connected to a terminal included in the FPC  716  through the anisotropic conductive film  780 . 
     For example, a glass substrate can be used as the first substrate  701  and the second substrate  705 . A flexible substrate may be used as the first substrate  701  and the second substrate  705 . Examples of the flexible substrate include a plastic substrate. 
     A structure body  778  is provided between the first substrate  701  and the second substrate  705 . The structure body  778  is a columnar spacer obtained by selective etching of an insulating film and provided to control the distance (cell gap) between the first substrate  701  and the second substrate  705 . Note that a spherical spacer may be used as the structure body  778 . Although the structure in which the structure body  778  is provided on the first substrate  701  side is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which the structure body  778  is provided on the second substrate  705  side, or a structure in which both of the first substrate  701  and the second substrate  705  are provided with the structure body  778  may be employed. 
     Furthermore, a light-blocking film  738  functioning as a black matrix, a coloring film  736  functioning as a color filter, and an insulating film  734  in contact with the light-blocking film  738  and the coloring film  736  are provided on the second substrate  705  side. 
     &lt;Structure Example of Liquid Crystal Element as Display Element&gt; 
     The display module  700  illustrated in  FIG.  34    includes a liquid crystal element  775 . The liquid crystal element  775  includes a conductive film  772 , a conductive film  774 , and a liquid crystal layer  776 . The liquid crystal layer  776  is formed using a liquid crystal material having a dielectric constant anisotropy of greater than or equal to 2 and less than or equal to 3.8. The conductive film  774  is provided on the second substrate side and functions as a counter electrode. The display module  700  in  FIG.  34    is capable of displaying an image in such a manner that transmission or non-transmission of light is controlled by change in the alignment state of the liquid crystal layer  776  depending on a voltage applied to the conductive film  772  and the conductive film  774 . 
     The conductive film  772  is connected to the conductive films functioning as a source electrode and a drain electrode included in the transistor  750 . The conductive film is formed over the planarization insulating film  770  to function as a pixel electrode, i.e., one electrode of the display element. The conductive film  772  has a function of a reflective electrode. The display module  700  in  FIG.  34    is what is called a reflective color liquid crystal display device in which external light is reflected by the conductive film  772  to display an image through the coloring film  736 . 
     A conductive film that transmits visible light or a conductive film that reflects visible light can be used for the conductive film  772 . For example, a material including one kind selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material including aluminum or silver may be used for the conductive film that reflects visible light. In this embodiment, the conductive film that reflects visible light is used for the conductive film  772 . 
     In the case where a conductive film which reflects visible light is used as the conductive film  772 , the conductive film may have a stacked-layer structure. For example, a 100-nm-thick aluminum film is formed as the bottom layer, and a 30-nm-thick silver alloy film (e.g., an alloy film including silver, palladium, and copper) is formed as the top layer. Such a structure makes it possible to obtain the following effects. 
     (1) Adhesion between the base film and the conductive film  772  can be improved. 
     (2) The aluminum film and the silver alloy film can be collectively etched depending on a chemical solution. 
     (3) The conductive film  772  can have a favorable cross-sectional shape (e.g., a tapered shape). 
     The reason for (3) is as follows: the etching rate of the aluminum film with the chemical solution is lower than that of the silver alloy film, or etching of the aluminum film that is the bottom layer is developed faster than that of the silver alloy film because when the aluminum film that is the bottom layer is exposed after the etching of the silver alloy film that is the top layer, electrons are extracted from metal that is less noble than the silver alloy film, i.e., aluminum that is metal having a high ionization tendency, and thus etching of the silver alloy film is suppressed. 
     Note that projections and depressions are provided in part of the planarization insulating film  770  of the pixel portion  702  in the display module  700  in  FIG.  34   . The projections and depressions can be formed in such a manner that the planarization insulating film  770  is formed using an organic resin film or the like, and projections and depressions are formed on the surface of the organic resin film. The conductive film  772  functioning as a reflective electrode is formed along the projections and depressions. Therefore, when external light is incident on the conductive film  772 , the light is reflected diffusely at the surface of the conductive film  772 , whereby visibility can be improved. As illustrated in  FIG.  34   , a reflective color liquid crystal display device can display an image without a backlight, which enables a reduction in power consumption. 
     Note that the display module  700  illustrated in  FIG.  34    is a reflective color liquid crystal display module given as an example, but a display type is not limited thereto. For example, a transmissive color liquid crystal display module in which the conductive film  772  is a conductive film that transmits visible light may be used. In the case of a transmissive color liquid crystal display module, projections and depressions are not necessarily provided on the planarization insulating film  770 . 
     Although not illustrated in  FIG.  34   , an alignment film may be provided on a side of the conductive film  772  in contact with the liquid crystal layer  776  and on a side of the conductive film  774  in contact with the liquid crystal layer  776 . Although not illustrated in  FIG.  34   , an optical member (an optical substrate) and the like such as a polarizing member, a retardation member, or an anti-reflection member may be provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In the case of the transmissive display module or the semi-transmissive display module, a backlight, a sidelight, or the like may be used as a light source. 
     As the liquid crystal element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has a short response time, and has optical isotropy, which makes the alignment process unneeded and viewing angle dependence small. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. 
     In the case where a liquid crystal element is used as the display element, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     Furthermore, a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, or the like can be employed. 
     The structure described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 6 
     In this embodiment, an input/output device (also referred to as a touch panel) obtained by providing a touch sensor (a contact sensor device) for the display module described in the above embodiment is described with reference to  FIGS.  35 A to  35 C  and  FIG.  36   . Hereinafter, the description of the same portions as the above embodiments is omitted in some cases. 
       FIGS.  35 A to  35 C  are projection drawings illustrating a structure of the input/output device. 
       FIG.  35 A  is a projection drawing of an input/output device  800 , and  FIG.  35 B  is a projection drawing illustrating a structure of a sensor unit  820 U included in the input/output device  800 . 
       FIG.  36    is a cross-sectional view taken along line Z 1 -Z 2  of the input/output device in  FIG.  35 A . 
     &lt;Structure Example 1 of Input/Output Device&gt; 
     The input/output device  800  described in this embodiment includes an input device  850  and a display module  801 . The input device  850  includes a plurality of sensor units  820 U arranged in matrix and each provided with window portions  834  transmitting visible light, a scan line G 1  electrically connected to a plurality of sensor units  820 U placed in the row direction (indicated by arrow Rx in the drawing), a signal line DL electrically connected to a plurality of sensor units  820 U placed in the column direction (indicated by arrow Ry in the drawing), and a first base material  836  supporting the sensor unit  820 U, the scan line G 1 , and the signal line DL. The display module  801  includes a plurality of pixels  802  overlapping with the window portions  834  and arranged in matrix and a second base material  810  supporting the pixels  802  (see  FIGS.  35 A to  35 C ). 
     The sensor unit  820 U includes a sensor element Ca overlapping with the window portion  834  and a sensor circuit  839  electrically connected to the sensor element Ca (see  FIG.  35 B ). 
     The sensor element Ca includes an insulating layer  823 , and a first electrode  821  and a second electrode  822  between which the insulating layer  823  (not illustrated in  FIG.  35 B ) is sandwiched (see  FIG.  35 B ). 
     A selection signal is supplied to the sensor circuit  839 , and the sensor circuit  839  supplies a sensor signal DATA based on the change in capacitance of the sensor element Ca. 
     The scan line G 1  can supply the selection signal, the signal line DL can supply the sensor signal DATA, and the sensor circuit  839  is placed to overlap with gaps between the plurality of window portions  834 . 
     In addition, the input/output device  800  described in this embodiment includes a coloring layer between the sensor unit  820 U and the pixel  802  overlapping with the window portion  834  of the sensor unit  820 U. 
     The input/output device  800  described in this embodiment includes the input device  850  including the plurality of sensor units  820 U, each of which is provided with the window portions  834  transmitting visible light, and the display module  801  including the plurality of pixels  802  overlapping with the window portions  834 . The coloring layer is included between the window portion  834  and the pixel  802 . 
     With such a structure, the input/output device can supply a sensor signal based on the change in the capacitance and positional information of the sensor unit supplying the sensor signal and can display image data relating to the positional information of the sensor unit. As a result, a novel input/output device with high convenience or high reliability can be provided. 
     The input/output device  800  may include a flexible substrate FPC  1  to which a signal from the input device  850  is supplied and/or a flexible substrate FPC  2  supplying a signal including image data to the display module  801 . 
     In addition, a protective base material  837  or a protective layer  837   p  that protects the input/output device  800  by preventing damage and/or an anti-reflective layer  867   p  that weakens the intensity of external light reflected by the input/output device  800  may be included. 
     Moreover, the input/output device  800  includes a scan line driver circuit  803   g  that supplies the selection signal to a scan line of the display module  801 , a wiring  811  supplying a signal, and a terminal  819  electrically connected to the flexible substrate FPC  2 . 
     Components of the input/output device  800  are described below. Note that these components cannot be clearly distinguished and one component also serves as another component or includes part of another component in some cases. For example, the input device  850  including the coloring layer overlapping with the plurality of window portions also serves as a color filter. 
     The input/output device  800  includes the input device  850  and the display module (see  FIG.  35 A ). 
     The input device  850  includes the plurality of sensor units  820 U and the first base material  836  supporting the sensor units  820 U. For example, the plurality of sensor units  820 U are arranged in matrix with 40 rows and 15 columns on the first base material  836 . 
     The window portion  834  transmits visible light. 
     A coloring layer transmitting light of a predetermined color is provided to overlap with the window portion  834 . For example, a coloring layer CFB transmitting blue light, a coloring layer CFG transmitting green light, and a coloring layer CFR transmitting red light are included (see  FIG.  35 B ). 
     Note that, in addition to the coloring layers transmitting blue light, green light, and/or red light, coloring layers transmitting light of various colors such as a coloring layer transmitting white light and a coloring layer transmitting yellow light can be included. 
     For a coloring layer, a metal material, a pigment, dye, or the like can be used. 
     A light-blocking layer BM is provided to surround the window portions  834 . The light-blocking layer BM does not easily transmit light as compared to the window portion  834 . 
     For the light-blocking layer BM, carbon black, a metal oxide, a composite oxide containing a solid solution of a plurality of metal oxides, or the like can be used. 
     The scan line G 1 , the signal line DL, a wiring VPI, a wiring RES, a wiring VRES, and the sensor circuit  839  are provided to overlap with the light-blocking layer BM. 
     Note that a light-transmitting overcoat layer covering the coloring layer and the light-blocking layer BM can be provided. 
     The sensor element Ca includes the first electrode  821 , the second electrode  822 , and the insulating layer  823  between the first electrode  821  and the second electrode  822  (see  FIG.  36   ). 
     The first electrode  821  is formed in, for example, an island shape so as to be apart from other regions. A layer that can be formed in the same process as that of the first electrode  821  is preferably placed close to the first electrode  821  so that the user of the input/output device  800  does not recognize the first electrode  821 . Further preferably, the number of the window portions  834  placed in the gap between the first electrode  821  and the layer placed close to the first electrode  821  is reduced as much as possible. In particular, the window portion  834  is preferably not placed in the gap. 
     When an object whose dielectric constant is different from that of the air gets closer to the first electrode  821  or the second electrode  822  of the sensor element Ca that is put in the air, the capacitance of the sensor element Ca is changed. Specifically, when a finger or the like gets closer to the sensor element Ca, the capacitance of the sensor element Ca is changed. Thus, the sensor element Ca can be used in a proximity sensor. 
     The first electrode  821  and the second electrode  822  include a conductive material. 
     For example, an inorganic conductive material, an organic conductive material, a metal material, a conductive ceramic material, or the like can be used for the first electrode and the second electrode  822 . 
     Specifically, a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten, nickel, silver, and manganese; an alloy including any of the above-described metal elements; an alloy including any of the above-described metal elements in combination; or the like can be used for the first electrode  821  and the second electrode  822 . 
     Alternatively, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used for the first electrode  821  and the second electrode  822 . 
     Alternatively, graphene or graphite can be used for the first electrode  821  and the second electrode  822 . The film including graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat, a method using a reducing agent, or the like can be employed. 
     Alternatively, a conductive polymer can be used for the first electrode  821  and the second electrode  822 . 
     The sensor circuit  839  includes transistors M 1  to M 3 , for example. In addition, the sensor circuit  839  includes wirings supplying a power supply potential and a signal. For example, the signal line DL, the wiring VPI, a wiring CS, the scan line G 1 , the wiring RES, and the wiring VRES are included. 
     Note that the sensor circuit  839  may be placed not to overlap with the window portion  834 . 
     A conductive material can be used for the wirings (e.g., the signal line DL, the wiring VPI, the wiring CS, the scan line G 1 , the wiring RES, and the wiring VRES). For example, an inorganic conductive material, an organic conductive material, a metal material, a conductive ceramic material, or the like can be used for the wirings. Alternatively, a material which is the same as those of the first electrode  821  and the second electrode  822  may be used. 
     For the scan line G 1 , the signal line DL, the wiring VPI, the wiring RES, and the wiring VRES, a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium, or an alloy material containing any of these metal materials can be used. 
     The sensor circuit  839  may be formed on the first base material  836 . Alternatively, the sensor circuit  839  formed on another base material may be transferred to the first base material  836 . 
     Examples of the materials of the first base material  836  and the second base material  810  are glass substrates or flexible materials (e.g., a resin, a resin film, and a plastic film). 
     More specifically, non-alkali glass, soda-lime glass, potash glass, crystal glass, or the like can be used for the first base material  836  and the second base material  810 . Alternatively, a resin film or resin plate of polyester, polyolefin, polyamide, polyimide, polycarbonate, an acrylic resin, or the like can be used for the first base material  836 . 
     For example, a glass substrate, a resin film or resin plate of polyester, polyolefin, polyamide, polyimide, polycarbonate, an acrylic resin, or the like, a stack thereof, or the like can be used as the protective base material  837  and/or the protective layer  837   p.    
     For example, a hard coat layer or a ceramic coat layer can be used as the protective layer  837   p . Specifically, a layer containing a UV curable resin or aluminum oxide may be formed to overlap with the second electrode  822 . 
     The display module  801  includes the plurality of pixels  802  arranged in matrix (see  FIG.  35 C ). 
     For example, the pixel  802  includes a sub-pixel  802 B, a sub-pixel  802 G, and a sub-pixel  802 R, and each sub-pixel includes a display element and a pixel circuit for driving the display element. 
     In the pixel  802 , the sub-pixel  802 B is placed to overlap with the coloring layer CFB, the sub-pixel  802 G is placed to overlap with the coloring layer CFG, and the sub-pixel  802 R is placed to overlap with the coloring layer CFR. 
     The coloring layer CFR is positioned in a region overlapping with the liquid crystal element  880 . Note that the liquid crystal element  880  includes a reflective electrode  872  as one electrode (see  FIG.  36   ). Thus, part of external light reflected by the reflective electrode  872  passes through the coloring layer CFR and is emitted in a direction indicated by an arrow in the drawing. The reflective electrode  872  can have a structure similar to that of the conductive film  772  serving as the reflective electrode in the above embodiment. The liquid crystal element  880  includes a liquid crystal layer having a dielectric constant anisotropy of greater than or equal to 2 and less than or equal to 3.8. 
     The light-blocking layer BM is provided to surround the coloring layer (e.g., the coloring layer CFR). 
     The scan line driver circuit  803   g  includes a transistor  803   t  and a capacitor  803   c  (see  FIG.  36   ). 
     Various circuits that can convert the sensor signal DATA supplied from the sensor unit  820 U and supply the converted signal to the flexible substrate FPC  1  can be used as a converter CONV (see  FIG.  35 A  and  FIG.  36   ). 
     For example, a transistor M 4  can be used in the converter CONV. 
     The display module  801  includes the anti-reflective layer  867   p  positioned in a region overlapping with pixels. As the anti-reflective layer  867   p , a circular polarizing plate can be used, for example. 
     As illustrated in  FIG.  35 A , the display module  801  includes the wirings  811  through which signals can be supplied. The wirings  811  are provided with the terminal  819 . Note that the flexible substrate FPC  2  through which a signal such as an image signal or a synchronization signal is supplied is electrically connected to the terminal  819 . 
     Note that a printed wiring board (PWB) may be attached to the flexible substrate FPC  2 . 
     The display module  801  includes wirings such as scan lines, signal lines, and power supply lines. Any of various conductive films can be used as the wirings. 
     For the wirings included in the display module  801 , for example, a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten, nickel, yttrium, zirconium, silver, and manganese; an alloy including any of the above-described metal elements; or an alloy including any of the above-described metal elements in combination can be used. In particular, one or more elements selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten are preferably included. In particular, an alloy of copper and manganese is suitably used in microfabrication with the use of a wet etching method. 
     As specific structures of the wirings included in the display module  801 , a two-layer structure in which a titanium film is stacked over an aluminum 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 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 stacked structure in which an alloy film or a nitride film which contains one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium is stacked over an aluminum film can be used. Alternatively, a light-transmitting conductive material including indium oxide, tin oxide, or zinc oxide may be used. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 7 
     In this embodiment, specific examples of electronic devices each of which is manufactured using the liquid crystal display device described in the above embodiment are described with reference to  FIGS.  37 A to  37 C . 
     Examples of electronic devices to which one embodiment of the present invention can be applied include a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, a music reproducing device, a game machine (e.g., a pachinko machine or a slot machine), and a game console.  FIGS.  37 A to  37 C  illustrate specific examples of these electronic devices. 
       FIG.  37 A  illustrates a portable information terminal  1400  including a display portion. The portable information terminal  1400  includes a display portion  1402  and an operation button  1403  which are incorporated in a housing  1401 . The liquid crystal display device of one embodiment of the present invention can be used for the display portion  1402 . 
       FIG.  37 B  illustrates a cellular phone  1410 . The cellular phone  1410  includes a display portion  1412 , an operation button  1413 , a speaker  1414 , and a microphone  1415  which are incorporated in a housing  1411 . The liquid crystal display device of one embodiment of the present invention can be used for the display portion  1412 . 
       FIG.  37 C  illustrates a music reproducing device  1420 . The music reproducing device  1420  includes a display portion  1422 , an operation button  1423 , and an antenna which are incorporated in a housing  1421 . In addition, the antenna  1424  transmits and receives data via a wireless signal. The liquid crystal display device of one embodiment of the present invention can be used for the display portion  1422 . 
     The display portions  1402 ,  1412 , and  1422  each have a touch-input function. When a user touches a displayed button (not illustrated) which is displayed on the display portion  1402 ,  1412 , or  1422  with his/her fingers or the like, the user can carry out operation on the screen and input of information. 
     When the liquid crystal display device described in the above embodiment is used for the display portions  1402 ,  1412 , and  1422 , the display quality of the display portions  1402 ,  1412 , and  1422  can be improved. 
     This embodiment can be implemented in an appropriate combination with any of the structures described in the other embodiments. 
     Note that contents that are not specified in any drawing or text in the specification can be excluded from one embodiment of the invention. Alternatively, when the range of a value that is defined by the maximum and minimum values is described, part of the range is appropriately narrowed or part of the range is removed, whereby one embodiment of the invention excluding part of the range can be constituted. In this manner, it is possible to specify the technical scope of one embodiment of the present invention so that a conventional technology is excluded, for example. 
     As a specific example, a diagram of a circuit including a first transistor to a fifth transistor is illustrated. In that case, it can be specified that the circuit does not include a sixth transistor in the invention. It can be specified that the circuit does not include a capacitor in the invention. It can be specified that the circuit does not include a sixth transistor with a particular connection structure in the invention. It can be specified that the circuit does not include a capacitor with a particular connection structure in the invention. For example, it can be specified that a sixth transistor whose gate is connected to a gate of the third transistor is not included in the invention. For example, it can be specified that a capacitor whose first electrode is connected to the gate of the third transistor is not included in the invention. 
     As another specific example, a description of a value, “a voltage is preferably higher than or equal to 3 V and lower than or equal to 10 V” is given. In that case, for example, it can be specified that the case where the voltage is higher than or equal to −2 V and lower than or equal to 1 V is excluded from one embodiment of the invention. For example, it can be specified that the case where the voltage is higher than or equal to 13 V is excluded from one embodiment of the invention. Note that, for example, it can be specified that the voltage is higher than or equal to 5 V and lower than or equal to 8 V in the invention. For example, it can be specified that the voltage is approximately 9 V in the invention. For example, it can be specified that the voltage is higher than or equal to 3 V and lower than or equal to 10 V but is not 9 V in the invention. Note that even when the description “a value is preferably in a certain range” or “a value preferably satisfies a certain condition” is given, the value is not limited to the description. In other words, a description of a value that includes a term “preferable”, “preferably”, or the like does not necessarily limit the value. 
     As another specific example, a description “a voltage is preferred to be 10 V” is given. In that case, for example, it can be specified that the case where the voltage is higher than or equal to −2 V and lower than or equal to 1 V is excluded from one embodiment of the invention. For example, it can be specified that the case where the voltage is higher than or equal to 13 V is excluded from one embodiment of the invention. 
     As another specific example, a description “a film is an insulating film” is given to describe properties of a material. In that case, for example, it can be specified that the case where the insulating film is an organic insulating film is excluded from one embodiment of the invention. For example, it can be specified that the case where the insulating film is an inorganic insulating film is excluded from one embodiment of the invention. For example, it can be specified that the case where the insulating film is a conductive film is excluded from one embodiment of the invention. For example, it can be specified that the case where the insulating film is a semiconductor film is excluded from one embodiment of the invention. 
     As another specific example, the description of a stacked structure, “a film is provided between an A film and a B film” is given. In that case, for example, it can be specified that the case where the film is a stacked film of four or more layers is excluded from the invention. For example, it can be specified that the case where a conductive film is provided between the A film and the film is excluded from the invention. 
     Note that various people can implement one embodiment of the invention described in this specification and the like. However, different people may be involved in the implementation of the invention. For example, in the case of a transmission/reception system, the following case is possible: Company A manufactures and sells transmitting devices, and Company B manufactures and sells receiving devices. As another example, in the case of a light-emitting device including a TFT and a light-emitting element, the following case is possible: Company A manufactures and sells semiconductor devices including TFTs, and Company B purchases the semiconductor devices, provides light-emitting elements for the semiconductor devices, and completes light-emitting devices. 
     In such a case, one embodiment of the invention can be constituted so that a patent infringement can be claimed against each of Company A and Company B. In other words, one embodiment of the invention can be constituted so that only Company A implements the embodiment, and another embodiment of the invention can be constituted so that only Company B implements the embodiment. One embodiment of the invention with which a patent infringement suit can be filed against Company A or Company B is clear and can be regarded as being disclosed in this specification or the like. For example, in the case of a transmission/reception system, even when this specification or the like does not include a description of the case where a transmitting device is used alone or the case where a receiving device is used alone, one embodiment of the invention can be constituted by only the transmitting device and another embodiment of the invention can be constituted by only the receiving device. Those embodiments of the invention are clear and can be regarded as being disclosed in this specification or the like. Another example is as follows: in the case of a light-emitting device including a TFT and a light-emitting element, even when this specification or the like does not include a description of the case where a semiconductor device including the TFT is used alone or the case where a light-emitting device including the light-emitting element is used alone, one embodiment of the invention can be constituted by only the semiconductor device including the TFT and another embodiment of the invention can be constituted by only the light-emitting device including the light-emitting element. Those embodiments of the invention are clear and can be regarded as being disclosed in this specification or the like. 
     Note that in this specification and the like, it might be possible for those skilled in the art to constitute one embodiment of the invention even when portions to which all the terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected are not specified. In other words, one embodiment of the invention can be clear even when connection portions are not specified. Furthermore, in the case where a connection portion is disclosed in this specification and the like, it can be determined that one embodiment of the invention in which a connection portion is not specified is disclosed in this specification and the like, in some cases. In particular, in the case where the number of portions to which the terminal is connected might be plural, it is not necessary to specify the portions to which the terminal is connected. Therefore, it might be possible to constitute one embodiment of the invention by specifying only portions to which some of terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected. 
     Note that in this specification and the like, it might be possible for those skilled in the art to specify the invention when at least the connection portion of a circuit is specified. Alternatively, it might be possible for those skilled in the art to specify the invention when at least a function of a circuit is specified. In other words, when a function of a circuit is specified, one embodiment of the present invention can be clear. Furthermore, it can be determined that one embodiment of the present invention whose function is specified is disclosed in this specification and the like. Therefore, when a connection portion of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a function is not specified, and one embodiment of the invention can be constituted. Alternatively, when a function of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a connection portion is not specified, and one embodiment of the invention can be constituted. 
     Note that in this specification and the like, in a diagram or a text described in one embodiment, it is possible to take out part of the diagram or the text and constitute an embodiment of the invention. Thus, in the case where a diagram or a text related to a certain portion is described, the content taken out from part of the diagram or the text is also disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the present invention is clear. Therefore, for example, in a diagram or text in which one or more active elements (e.g., transistors or diodes), wirings, passive elements (e.g., capacitors or resistors), conductive layers, insulating layers, semiconductor layers, organic materials, inorganic materials, components, devices, operating methods, manufacturing methods, or the like are described, part of the diagram or the text is taken out, and one embodiment of the invention can be constituted. For example, from a circuit diagram in which N circuit elements (e.g., transistors or capacitors; N is an integer) are provided, it is possible to constitute one embodiment of the invention by taking out M circuit elements (e.g., transistors or capacitors; M is an integer, where M&lt;N). As another example, it is possible to constitute one embodiment of the invention by taking out M layers (M is an integer, where M&lt;N) from a cross-sectional view in which N layers (N is an integer) are provided. As another example, it is possible to constitute one embodiment of the invention by taking out M elements (M is an integer, where M&lt;N) from a flow chart in which N elements (Nis an integer) are provided. For another example, it is possible to take out some given elements from a sentence “A includes B, C, D, E, or F” and constitute one embodiment of the invention, for example, “A includes B and E”, “A includes E and F”, “A includes C, E, and F”, or “A includes B, C, D, and E”. 
     Note that in the case where at least one specific example is described in a diagram or a text described in one embodiment in this specification and the like, it will be readily appreciated by those skilled in the art that a broader concept of the specific example can be derived. Therefore, in the diagram or the text described in one embodiment, in the case where at least one specific example is described, a broader concept of the specific example is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the present invention is clear. 
     Note that in this specification and the like, a content described in at least a diagram (which may be part of the diagram) is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. Therefore, when a certain content is described in a diagram, the content is disclosed as one embodiment of the invention even when the content is not described with a text, and one embodiment of the invention can be constituted. In a similar manner, part of a diagram, which is taken out from the diagram, is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the present invention is clear. 
     This application is based on Japanese Patent Application serial No. 2014-090314 filed with Japan Patent Office on Apr. 24, 2014, the entire contents of which are hereby incorporated by reference.