Patent Publication Number: US-2022216242-A1

Title: Shift register and display device and driving method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/806,418, filed Nov. 8, 2017, now allowed, which is a continuation of U.S. application Ser. No. 14/010,843, filed Aug. 27, 2013, now abandoned, which is a continuation of U.S. application Ser. No. 12/897,360, filed Oct. 4, 2010, now U.S. Pat. No. 8,526,567, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2009-234845 on Oct. 9, 2009, all of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a shift register and a display device including the shift register. 
     BACKGROUND ART 
     A thin film transistor (TFT) formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. Although a TFT which is formed using amorphous silicon has low field-effect mobility, such a TFT can be formed over a larger glass substrate. On the other hand, a TFT formed using polycrystalline silicon has high field-effect mobility, but such a TFT needs a crystallization step such as laser annealing and is not always suitable for being formed over a larger glass substrate. 
     In view of the foregoing, a TFT formed using an oxide semiconductor as a semiconductor material has attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used for a switching element in an image display device. 
     A TFT in which a channel formation region is formed in an oxide semiconductor can have higher electric-field mobility than a TFT formed using amorphous silicon. Further, an oxide semiconductor film can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and a manufacturing process of the TFT formed using an oxide semiconductor is simpler than that of the TFT formed using polycrystalline silicon. 
     TFTs which are formed using such an oxide semiconductor are expected to be applied to switching elements included in a pixel portion and a driver circuit of a display device such as a liquid crystal display, an electroluminescent display, and electronic paper. For example, Non-Patent Document 1 discloses a technique by which a pixel portion and a driver circuit of a display device include TFTs formed using the above oxide semiconductor. 
     Note that the TFTs which are formed using an oxide semiconductor are all n-channel transistors. Therefore, in the case where a driver circuit is formed using a TFT formed using an oxide semiconductor, the driver circuit includes only n-channel TFTs (hereinafter also referred to as unipolar TFTs). 
     [Patent Document 1] Japanese Published Patent Application No. 2007-123861 
     [Patent Document 2] Japanese Published Patent Application No. 2007-096055 
     [Non-Patent Document 1] T. Osada, et al., SID 09 DIGEST, pp. 184-187 (2009) 
     DISCLOSURE OF INVENTION 
     A driver circuit includes a shift register, a buffer, and the like. For example, in the case where the shift register includes only unipolar TFTs, there is a problem in that a signal decreases or increases by the same amount as the threshold voltage of the TFT. Therefore, bootstrap operation is often performed in a portion where such a problem occurs. Specifically, the bootstrap operation is often performed in an analog switch by which a signal line or a scan line in a display device is driven. 
     Further, in the case where the load of a driver circuit utilizing bootstrap is heavy, a gate width of a TFT included in the driver circuit needs to be made larger. In accordance with that, parasitic capacitance caused in the TFT becomes larger. In particular, parasitic capacitance becomes larger in a TFT in which a conductive layer serving as a gate terminal and a conductive layer serving as a source or drain terminal need to be overlapped with each other with a gate insulating layer provided therebetween (e.g., a so-called inverted staggered TFT). As a result, there is a problem in that the power consumption of a clock signal inputted to the driver circuit is made large by the parasitic capacitance. 
     In view of the above problems, an object of one embodiment of the present invention is to reduce the power consumption of a shift register or a display device including the shift register. 
     The above problems can be solved by dividing a clock signal line included in a shift register into a plurality of pulse signal lines. In other words, a plurality of flip-flops included in the shift register is not electrically connected to one clock signal line; and a plurality of pulse signal lines is provided and part of the plurality of flip-flops is electrically connected to one of the plurality of pulse signal lines. The pulse signal line supplies a clock signal in part of an operation period of the shift register, not during the whole operation period of the shift register. Therefore, the capacity load caused with the supply of clock signals to the shift register can be reduced, leading to reduction in power consumption of the shift register. 
     One embodiment of the present invention is a shift register including a first pulse signal line serving as a wiring for supplying a clock signal that periodically alternates between a high power supply potential and a low power supply potential during a first period; a second pulse signal line serving as a wiring for supplying a clock signal during a second period; a third pulse signal line serving as a wiring for supplying an inverted clock signal that is an inversion of the clock signal during a third period; a fourth pulse signal line serving as a wiring for supplying an inverted clock signal during a fourth period; a first flip-flop which is electrically connected to the first pulse signal line and outputs the high power supply potential in the first period; a second flip-flop which is electrically connected to the second pulse signal line and outputs the high power supply potential in the second period; a third flip-flop which is electrically connected to the first flip-flop and the third pulse signal line and outputs the high power supply potential in the third period; and a fourth flip-flop which is electrically connected to the second flip-flop and the fourth pulse signal line and outputs the high power supply potential in the fourth period. The third period overlaps with the first period, and the fourth period overlaps with the second period. 
     Another embodiment of the present invention is a shift register with the above-mentioned structure, in which the first pulse signal line serves as a wiring for supplying the low power supply potential in a period except the first period; the second pulse signal line serves as a wiring for supplying the low power supply potential in a period except the second period; the third pulse signal line serves as a wiring for supplying the low power supply potential in a period except the third period; and the fourth pulse signal line serves as a wiring for supplying the low power supply potential in a period except the fourth period. 
     Another embodiment of the present invention is a shift register with any one of the above-mentioned structures, in which a flip-flop includes a transistor whose channel formation region is formed using an oxide semiconductor. 
     Another embodiment of the present invention is a shift register with any one of the above-mentioned structures, in which a pulse signal line is electrically connected to a reference clock signal line or an inverted reference signal line via a transistor which is kept on in a period when the pulse signal line supplies a clock signal or an inverted clock signal. 
     Another embodiment of the present invention is a shift register with any one of the above-mentioned structures, in which a pulse signal line is electrically connected to a wiring for supplying a low power supply potential via a transistor which is kept on in the period when the pulse signal line does not supply a clock signal or an inverted clock signal. 
     Another embodiment of the present invention is a display device which includes a shift register with any one of the above-mentioned structures. 
     In a shift register which is one embodiment of the present invention, a clock signal is supplied by a plurality of wirings, not by one wiring. Any one of the plurality of wirings supplies a clock signal in only part of the operation period of the shift register, not during the whole operation period of the shift register. Therefore, the capacity load caused with the supply of clock signals can be reduced, leading to reduction in power consumption of the shift register. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are a diagram illustrating a configuration example and a timing chart, respectively, of a shift register described in Embodiment 1; 
         FIGS. 2A and 2B  are a diagram illustrating a configuration example and a timing chart, respectively, of a flip-flop described in Embodiment 1; 
         FIGS. 3A and 3B  are a diagram illustrating a configuration example and a timing chart, respectively, of a pulse signal line described in Embodiment 1; 
         FIGS. 4A and 4B  are a diagram illustrating a configuration example and a timing chart, respectively, of a pulse signal line described in Embodiment 1; 
         FIGS. 5A to 5C  are diagrams illustrating modification examples of the shift register described in Embodiment 1; 
         FIGS. 6A and 6B  are diagrams and  FIG. 6C  is a timing chart illustrating modification examples of the shift register described in Embodiment 1; 
         FIGS. 7A and 7B  are a diagram and a timing chart, respectively, illustrating a modification example of the flip-flop described in Embodiment 1; 
         FIGS. 8A to 8C  are a top diagram and cross-sectional diagrams, respectively, illustrating a transistor described in Embodiment 2; 
         FIGS. 9A and 9B  are a top diagram and a cross-sectional diagram, respectively, of a transistor described in Embodiment 2; 
         FIGS. 10A and 10B  are a top diagram and a cross-sectional diagram, respectively, of a plurality of transistors described in Embodiment 2; 
         FIGS. 11A to 11D  are cross-sectional diagrams illustrating a process for manufacturing a transistor described in Embodiment 2; 
         FIGS. 12A and 12B  are a top diagram and a cross-sectional diagram, respectively, of a plurality of transistors described in Embodiment 3; 
         FIGS. 13A and 13B  are a top diagram and a cross-sectional diagram, respectively, of a plurality of transistors described in Embodiment 4; 
         FIGS. 14A to 14C  are a block diagram, a block diagram of a scan line driver circuit, and a block diagram of a signal line driver circuit, respectively, of a display device described in Embodiment 5; 
         FIGS. 15A to 15C  are a circuit diagram, a top diagram, and a cross-sectional diagram, respectively, of a pixel of a liquid crystal display device described in Embodiment 6; 
         FIG. 16A  is a circuit diagram and  FIGS. 16B to 16D  are cross-sectional diagrams, of a pixel of a light-emitting display device described in Embodiment 7; 
         FIGS. 17A and 17B  are a top diagram and a cross-sectional diagram, respectively, of a light-emitting display device described in Embodiment 7; 
         FIGS. 18A to 18C  are a circuit diagram, a top diagram, and a cross-sectional diagram, respectively, of a pixel of electronic paper described in Embodiment 7; 
         FIG. 19  is a diagram illustrating an example of an electronic book described in Embodiment 8; 
         FIGS. 20A and 20B  are diagrams each illustrating an example of an electronic appliance described in Embodiment 9; 
         FIGS. 21A and 21B  are diagrams each illustrating an example of an electronic appliance described in Embodiment 9; and 
         FIGS. 22A and 22B  are diagrams each illustrating an example of an electronic appliance described in Embodiment 9. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that a variety of changes and modifications in modes and details thereof can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of the embodiments below. 
     Note that it is difficult to determine which one of the terminals of a transistor is a source terminal or a drain terminal because it changes depending on the structure, operating conditions, and the like of the transistor. Therefore, in this document, one of a source terminal and a drain terminal is referred to as a first terminal and the other thereof is referred to as a second terminal for distinction. 
     Note that the size, the thickness of a layer, or a region of each structure illustrated in drawings or the like in embodiments is exaggerated for simplicity in some cases. Therefore, embodiments of the present invention are not limited to such scales. Further, in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically. 
     Embodiment 1 
     In this embodiment, examples of a structure and operation of a shift register are described with reference to  FIGS. 1A and 1B ,  FIGS. 2A and 2B ,  FIGS. 3A and 3B ,  FIGS. 4A and 4B ,  FIGS. 5A to 5C ,  FIGS. 6A to 6C , and  FIGS. 7A and 7B . Specifically, a shift register which includes a pulse signal line and a flip-flop is described. The pulse signal line serves as a wiring for supplying a clock signal in part of the operation period of the shift register and serves as a wiring for supplying a low power supply potential in the other periods. The flip-flop is electrically connected to the pulse signal line. 
     Example of Structure of Shift Register 
     A shift register in this embodiment includes first to sixth pulse signal lines and first to tenth flip-flops. 
     The first pulse signal line (PS 1 ) is electrically connected to the first flip-flop (FF 1 ) and the third flip-flop (FF 3 ). The second pulse signal line (PS 2 ) is electrically connected to the fifth flip-flop (FF 5 ) and the seventh flip-flop (FF 7 ). The third pulse signal line (PS 3 ) is electrically connected to the ninth flip-flop (FF 9 ). The fourth pulse signal line (PS 4 ) is electrically connected to the second flip-flop (FF 2 ) and the fourth flip-flop (FF 4 ). The fifth pulse signal line (PS 5 ) is electrically connected to the sixth flip-flop (FF 6 ) and the eighth flip-flop (FF 8 ). The sixth pulse signal line (PS 6 ) is electrically connected to the tenth flip-flop (FF 10 ) (see  FIG. 1A ). 
     An output terminal of each flip-flop is electrically connected to an input terminal of the subsequent flip-flop. Note that an input terminal of the first flip-flop (FF 1 ) is electrically connected to a wiring for supplying a start pulse (SP). 
     In a first period (t 1 ), the first pulse signal line (PS 1 ) serves as a wiring for supplying a clock signal that periodically alternates between a high power supply potential and a low power supply potential. In a second period (t 2 ), the second pulse signal line (PS 2 ) serves as a wiring for supplying a clock signal. In a third period (t 3 ), the third pulse signal line (PS 3 ) serves as a wiring for supplying a clock signal. In a fourth period (t 4 ), the fourth pulse signal line (PS 4 ) serves as a wiring for supplying an inverted clock signal that is the inversion of a clock signal. In a fifth period (t 5 ), the fifth pulse signal line (PS 5 ) serves as a wiring for supplying an inverted clock signal. In a sixth period (t 6 ), the sixth pulse signal line (PS 6 ) serves as a wiring for supplying an inverted clock signal (see  FIG. 1B ). 
     Example of Operation of Shift Register 
     The operation of the shift register in this embodiment is described below. 
     A high power supply potential signal is inputted as a start pulse (SP) to the input terminal of the first flip-flop (FF 1 ). The first flip-flop (FF 1 ) operates on the basis of the inputted signal, and outputs a high power supply potential signal as an output signal (FF 1 out) of the first flip-flop with a half clock cycle delay. 
     The output signal (FF 1 out) is inputted to an input terminal of the second flip-flop (FF 2 ). Like the first flip-flop (FF 1 ), the second flip-flop (FF 2 ) operates on the basis of the inputted signal, and outputs a high power supply potential signal as an output signal (FF 2 out) of the second flip-flop with a half clock cycle delay. 
     Similarly, as for the other flip-flops, a high power supply potential signal is inputted to an input terminal of the subsequent flip-flop, and the flip-flop outputs a high power supply potential signal with a half clock cycle delay. 
     Specific Eexample of Flip-Flop 
     A specific example of a circuit configuration of the flip-flop in this embodiment is illustrated in  FIG. 2A . Note that only configurations of the first flip-flop (FF 1 ) and the second flip-flop (FF 2 ) are illustrated in  FIG. 2A  for convenience. 
     The first flip-flop (FF 1 ) includes transistors  101  to  106 . Note that in this case, the transistors  101  to  106  are n-channel transistors. 
     A gate terminal of the transistor  101  is electrically connected to the output terminal of the second flip-flop (FF 2 ). A first terminal of the transistor  101  is electrically connected to a wiring for supplying a high power supply potential (VDD) (hereinafter also referred to as a high power supply potential line). 
     A gate terminal of the transistor  102  is electrically connected to a wiring for supplying a start pulse (SP) (hereinafter also referred to as a start pulse line). A first terminal of the transistor  102  is electrically connected to a second terminal of the transistor  101 . A second terminal of the transistor  102  is electrically connected to a wiring for supplying a low power supply potential (VSS) (hereinafter also referred to as a low power supply potential line). 
     A gate terminal of the transistor  103  is electrically connected to the start pulse line. A first terminal of the transistor  103  is electrically connected to the high power supply potential line. 
     A gate terminal of the transistor  104  is electrically connected to the second terminal of the transistor  101  and the first terminal of the transistor  102 . A first terminal of the transistor  104  is electrically connected to a second terminal of the transistor  103 . A second terminal of the transistor  104  is electrically connected to the low power supply potential line. 
     A gate terminal of the transistor  105  is electrically connected to the second terminal of the transistor  103  and the first terminal of the transistor  104 . A first terminal of the transistor  105  is electrically connected to the first pulse signal line (PS 1 ). 
     A gate terminal of the transistor  106  is electrically connected to the second terminal of the transistor  101 , the first terminal of the transistor  102 , and the gate terminal of the transistor  104 . A first terminal of the transistor  106  is electrically connected to a second terminal of the transistor  105 . A second terminal of the transistor  106  is electrically connected to the low power supply potential line. 
     Note that hereinafter, for convenience, a node in which the second terminal of the transistor  101 , the first terminal of the transistor  102 , the gate terminal of the transistor  104 , and the gate terminal of the transistor  106  are electrically connected is referred to as a node A; and a node in which the second terminal of the transistor  103 , the first terminal of the transistor  104 , and the gate terminal of the transistor  105  are electrically connected to each other is referred to as a node B. 
     In addition to the above configuration, a capacitor may be provided between the gate terminal and the source terminal of the transistor  105 . The capacitor is provided, so that a bootstrap operation, which is described below, can be surely performed. 
     Example of Operation of Flip-Flop 
     As an example of operation of the above-mentioned flip-flops, operation of the first flip-flop (FF 1 ) is described below with reference to  FIG. 2B . 
     First, the potential of the start pulse line electrically connected to the first flip-flop (FF 1 ) is increased to a high level (hereinafter referred to as an H level). Accordingly, H-level signals are inputted to the gate terminals of the transistors  102  and  103 . Thus, the transistors  102  and  103  are turned on. Then, the potential of the node A is decreased to a low level (hereinafter referred to as an L level) and the potential of the node B is increased to an H level. In accordance with this, the transistor  105  is also turned on. As a result, an L-level potential, which is the potential of the first pulse signal line (PS 1 ) in this period, is outputted as the output signal (FF 1 out) of the first flip-flop. 
     In the subsequent period, the potential of the start pulse line is decreased to an L level. Thus, the transistors  102  and  103  are turned off. As a result, the potential of the node A and the potential of the node B go into a floating state. At this time, there is a potential difference from the L level to the H level between the source terminal and the gate terminal of the transistor  105 , and the potential difference is maintained because the node B goes into a floating state. That is, the transistor  105  is kept on regardless of the state of the source terminal. Moreover, the potential of the first pulse signal line (PS 1 ) is increased to an H level. Thus, the potential of the node B which is in a floating state and electrically connected to the gate terminal of the transistor  105  is further increased by an H-level potential of the first pulse signal line (PS 1 ) in this period. The operation in which the potential of the node B is increased by capacitive coupling of the source terminal and the gate terminal that is electrically connected to the node B in the floating state, of the transistor  105  as described above, is called bootstrap. Accordingly, an H-level potential, which is the potential of the first pulse signal line (PS 1 ), is outputted as the output signal (FF 1 out) of the first flip-flop. 
     Note that in this case, the transistor  105  is an n-channel transistor. Thus, in this period when the potential of the first pulse signal line (PS 1 ) is set to an H level, a terminal of the transistor  105  which is electrically connected to the output terminal of the first flip-flop (FF 1 ) functions as a source terminal, and a terminal of the transistor  105  which is electrically connected to the first pulse signal line (PS 1 ) functions as a drain terminal. A transistor is turned on or turned off depending on the potential difference between a source terminal and a gate terminal. Therefore, in the case where an H-level potential of the first pulse signal line (PS 1 ) is outputted as the output signal (FF 1 out) of the first flip-flop via an n-channel transistor in which bootstrap does not occur, the outputted potential is reduced from the H-level potential by the threshold voltage (Vth) of the n-channel transistor. However, since the bootstrap occurs in the transistor  105 , the output signal (FF 1 out) of the first flip-flop can be outputted without a reduction in the potential of the first pulse signal line (PS 1 ). 
     An H-level signal which is the output signal of the first flip-flop (FF 1 ) is inputted to the second flip-flop (FF 2 ). In this case, the second flip-flop (FF 2 ) has the same structure as that of the first flip-flop (FF 1 ) except that the fourth pulse signal line (PS 4 ), instead of the first pulse signal line (PS 1 ), is electrically connected to the second flip-flop (FF 2 ). Thus, the above-mentioned description of the first flip-flop (FF 1 ) applies to detailed description of the circuit operation of the second flip-flop (FF 2 ). In this period, the second flip-flop (FF 2 ) outputs an L-level potential which is the potential of the fourth pulse signal line (PS 4 ) in this period. 
     In the subsequent period, the potential of the first pulse signal (PS 1 ) is decreased to an L level and the potential of the fourth pulse signal line (PS 4 ) is increased to an H level. As a result, the output signal (FFlout) of the first flip-flop is decreased to an L level. An H-level potential which is the potential of the fourth pulse signal line (PS 4 ) is outputted as the output signal (FF 2 out) of the second flip-flop. 
     The output signal (FF 2 out) of the second flip-flop is inputted to the third flip-flop (not illustrated) and to the gate terminal of the transistor  101  included in the first flip-flop (FF 1 ). Thus, the transistor  101  included in the first flip-flop (FF 1 ) is turned on, and then the potential of the node A is set to an H level. Accordingly, the transistors  104  and  106  are also turned on. Since the transistor  104  is turned on, the potential of the node B is decreased to an L level. That is, the potential of the gate terminal of the transistor  105  is decreased to an L level. Therefore, the transistor  105  is turned off. In addition, since the transistor  106  is turned on, the output signal (FF 1 out) of the first flip-flop is changed from the L level of the first pulse signal line (PS 1 ) via the transistor  105  in this period to an L level of the low power supply potential (VSS) via the transistor  106 . In other words, the output signal (FF 1 out) of the first flip-flop is outputted via a different transistor without being substantially changed. 
     In the subsequent period, the potential of the fourth pulse signal line (PS 4 ) is decreased to an L level. That is, the output signal (FF 2 out) of the second flip-flop is decreased to an L level. Thus, the transistor  101  included in the first flip-flop (FF 1 ) is turned off. As a result, a node electrically connected to the gate terminal of the transistor  104  and a node electrically connected to the gate terminal of the transistor  106  go into a floating state while such nodes keep the H-level signals. Thus, the transistors  104  and  106  are kept on and the output signal (FF 1 out) of the first flip-flop is maintained at an L level. Such a state is continued until an H-level potential is inputted to the input terminal of the first flip-flop (FF 1 ) again. 
     In the above-mentioned manner, the first flip-flop (FF 1 ) illustrated in  FIG. 2A  can output a signal which is inputted with a half clock cycle delay. 
     Example of Pulse Signal Line 
     The first pulse signal line (PS 1 ) to the sixth pulse signal line (PS 6 ) each serve as a wiring for supplying a clock signal in part of the operation period, and the first to sixth pulse signal lines each serve as a wiring for supplying a low power supply potential in the other periods. An example of a wiring having such functions is described with reference to  FIGS. 3A and 3B  and  FIGS. 4A and 4B . 
     Each of the first pulse signal line (PS 1 ) to the sixth pulse signal line (PS 6 ) illustrated in  FIG. 3A  is electrically connected to a reference clock signal line (CK) or an inverted reference clock signal line (CKB) via a source terminal and a drain terminal of any one of clock signal selection transistors  111 ,  112 , and  113  and inverted clock signal selection transistors  114 ,  115 , and  116 . Note that in this case, the clock signal selection transistors  111 ,  112 , and  113  and the inverted clock signal selection transistors  114 ,  115 , and  116  are n-channel transistors. 
     Specifically, a gate terminal of the clock signal selection transistor  111  is electrically connected to a control terminal a. A first terminal of the clock signal selection transistor  111  is electrically connected to the first pulse signal line (PS 1 ). A second terminal of the clock signal selection transistor  111  is electrically connected to the reference clock signal line (CK). A gate terminal of the clock signal selection transistor  112  is electrically connected to a control terminal b. A first terminal of the clock signal selection transistor  112  is electrically connected to the second pulse signal line (PS 2 ). A second terminal of the clock signal selection transistor  112  is electrically connected to the reference clock signal line (CK). A gate terminal of the clock signal selection transistor  113  is electrically connected to a control terminal c. A first terminal of the clock signal selection transistor  113  is electrically connected to the third pulse signal line (PS 3 ). A second terminal of the clock signal selection transistor  113  is electrically connected to the reference clock signal line (CK). 
     A gate terminal of the inverted clock signal selection transistor  114  is electrically connected to a control terminal d. A first terminal of the inverted clock signal selection transistor  114  is electrically connected to the fourth pulse signal line (PS 4 ). A second terminal of the inverted clock signal selection transistor  114  is electrically connected to the inverted reference clock signal line (CKB). A gate terminal of the inverted clock signal selection transistor  115  is electrically connected to a control terminal e. A first terminal of the inverted clock signal selection transistor  115  is electrically connected to the fifth pulse signal line (PS 5 ). A second terminal of the inverted clock signal selection transistor  115  is electrically connected to the inverted reference clock signal line (CKB). A gate terminal of the inverted clock signal selection transistor  116  is electrically connected to a control terminal f. A first terminal of the inverted clock signal selection transistor  116  is electrically connected to the sixth pulse signal line (PS 6 ). A second terminal of the inverted clock signal selection transistor  116  is electrically connected to the inverted reference clock signal line (CKB). 
     As illustrated in  FIG. 3B , the reference clock signal line is a wiring for supplying a clock signal that periodically alternates between a high power supply potential and a low power supply potential regardless of a period, and the inverted reference clock signal line is a wiring for supplying an inverted clock signal that is the inversion of a clock signal, regardless of a period. 
     The potential of the control terminal a is set to an H level in the first period (t 1 ), and is set to an L level in the other periods. Thus, in the first period (t 1 ), the first pulse signal line (PS 1 ) can serve as a wiring for supplying a clock signal. In other words, the first period (t 1 ) is a period in which the potential of the control terminal a is set to an H level. 
     Similarly, the potentials of the control terminals b to f are set to an H level in the second period (t 2 ) to the sixth period (t 6 ), respectively, and are set to an L level in the other periods. Thus, the second pulse signal line in the second period and the third pulse signal line in the third period can serve as a wiring for supplying a clock signal; the fourth pulse signal line in the fourth period, the fifth pulse signal line in the fifth period, and the sixth pulse signal line in the sixth period can serve as a wiring for supplying an inverted clock signal. In other words, the second period (t 2 ) to the sixth period (t 6 ) are periods in which the potentials of the respective control terminals b to fare set to an H level. 
     Each of the first pulse signal line (PS 1 ) to the sixth pulse signal line (PS 6 ) illustrated in  FIG. 4A  is electrically connected to a wiring for supplying a low power supply potential (VSS) via a source terminal and a drain terminal of one of low power supply potential selection transistors  121  to  126 . Note that in this case, the low power supply potential selection transistors  121  to  126  are n-channel transistors. 
     A gate terminal of the low power supply potential selection transistor  121  is electrically connected to a control terminal g. A first terminal of the low power supply potential selection transistor  121  is electrically connected to the first pulse signal line (PS 1 ). A second terminal of the low power supply potential selection transistor  121  is electrically connected to the wiring for supplying a low power supply potential (VSS). A gate terminal of the low power supply potential selection transistor  122  is electrically connected to a control terminal h. A first terminal of the low power supply potential selection transistor  122  is electrically connected to the second pulse signal line (PS 2 ). A second terminal of the low power supply potential selection transistor  122  is electrically connected to the wiring for supplying a low power supply potential (VSS). A gate terminal of the low power supply potential selection transistor  123  is electrically connected to a control terminal i. A first terminal of the low power supply potential selection transistor  123  is electrically connected to the third pulse signal line (PS 3 ). A second terminal of the low power supply potential selection transistor  123  is electrically connected to the wiring for supplying a low power supply potential (VSS). A gate terminal of the low power supply potential selection transistor  124  is electrically connected to a control terminal j. A first terminal of the low power supply potential selection transistor  124  is electrically connected to the fourth pulse signal line (PS 4 ). A second terminal of the low power supply potential selection transistor  124  is electrically connected to the wiring for supplying a low power supply potential (VSS). A gate terminal of the low power supply potential selection transistor  125  is electrically connected to a control terminal k. A first terminal of the low power supply potential selection transistor  125  is electrically connected to the fifth pulse signal line (PS 5 ). A second terminal of the low power supply potential selection transistor  125  is electrically connected to the wiring for supplying a low power supply potential (VSS). A gate terminal of the low power supply potential selection transistor  126  is electrically connected to a control terminal l. A first terminal of the low power supply potential selection transistor  126  is electrically connected to the sixth pulse signal line (PS 6 ). A second terminal of the low power supply potential selection transistor  126  is electrically connected to the wiring for supplying a low power supply potential (VSS). 
     The potential of the control terminal g is set to an L level in the first period (t 1 ) and is set to an H level in the other periods. Thus, in the periods except the first period (t 1 ), the first pulse signal line (PS 1 ) can serve as a wiring for supplying a low power supply potential (VSS). 
     Similarly, the potentials of the control terminals h to l are set to an L level in the second period (t 2 ) to the sixth period (t 6 ), respectively, and are set to an H level in the other periods. Thus, the second pulse signal line in the periods except the second period, the third pulse signal line in the periods except the third period, the fourth pulse signal line in the periods except the fourth period, the fifth pulse signal line in the periods except the fifth period, and the sixth pulse signal line in the periods except the sixth period can serve as a wiring for supplying a low power supply potential (VSS). 
     In the shift register in this embodiment, clock signals are supplied by a plurality of wirings, not by one wiring. Further, any one of the plurality of wirings supplies clock signals in a part of period, not in the entire operation period of the shift register. Therefore, the capacity load caused with the supply of clock signals can be reduced, leading to a reduction in power consumption of the shift register. 
     Modification Example 
     The above-described shift register is an example of this embodiment, and a shift register which has a different point from the above description is included in this embodiment. 
     For example, in the above shift register, two flip-flops are electrically connected to each of the pulse signal lines (see  FIG. 1A ); however, a larger number of flip-flops may be electrically connected to each of the pulse signal lines. Specifically, as illustrated in  FIG. 5A , x (x is a natural number of 3 or more) flip-flops can be electrically connected to each of the pulse signal lines, for example. 
     Further, six pulse signal lines are included in the above shift register (see  FIG. 1A ), but a larger number of pulse signal lines may be included. Specifically, a structure illustrated in  FIG. 5B  can be employed, which includes the first pulse signal line (PS 1 ) to the y-th (y is a natural number of 4 or more) pulse signal line (PSy) which supply clock signals in part of the operation period; and the (y+1)th pulse signal line (PSy+1) to the 2y-th pulse signal line (PS2y) which supply inverted clock signals in part of the operation period. Two flip-flops are electrically connected to each of the pulse signal lines in the structure illustrated in  FIG. 5B . 
     Furthermore, two flip-flops are electrically connected to each of the pulse signal lines and six pulse signal lines are included in the above shift register (see  FIG. 1A ); however, a larger number of flip-flops may be electrically connected to each of the pulse signal lines and a larger number of pulse signal lines may be included. Specifically, a structure illustrated in  FIG. 5C  can be employed: the first pulse signal line (PS 1 ) to the y-th (y is a natural number of 4 or more) pulse signal line (PSy) which supply clock signals in part of the operation period are included; the (y+1)th pulse signal line (PSy+1) to the 2y-th pulse signal line (PS2y) which supply inverted clock signals in part of the operation period are included; and x flip-flops are electrically connected to each of the pulse signal lines, for example. 
     In the above shift register, the numbers of flip-flops which are electrically connected to each of pulse signal lines are the same (see  FIG. 1A  and  FIGS. 5A to 5C ), but the number of flip-flops which are electrically connected to each of pulse signal lines may be different depending on a pulse signal line. Specifically, as illustrated in  FIG. 6A , x flip-flops can be electrically connected to the first pulse signal line (PS 1 ) and the fourth pulse signal line (PS 4 ), and z flip-flops (z is a natural number of 2 or more and different from x) can be electrically connected to the second pulse signal line (PS 2 ) and the fifth pulse signal line (PS 5 ). 
     In the above shift register, the number of flip-flops which are electrically connected to the first pulse signal line (PS 1 ) and the number of flip-flops which are electrically connected to the fourth pulse signal line (PS 4 ) are the same (see  FIG. 1A ,  FIG. 5A , and  FIG. 6A ), but the number of flip-flops which are electrically connected to the first pulse signal line (PS 1 ) and the number of flip-flops which are electrically connected to the fourth pulse signal line (PS 4 ) may be different from each other. Specifically, as illustrated in  FIG. 6B , x flip-flops can be electrically connected to the first pulse signal line (PS 1 ) and (x+z) flip-flops can be electrically connected to the fourth pulse signal line (PS 4 ). 
     In the above shift register, the first period (t 1 ) and the second period (t 2 ) do not overlap with each other (see  FIG. 1B ), but the first period (t 1 ) and the second period (t 2 ) may be overlap with each other. Specifically, as illustrated in  FIG. 6C , a period (T) in which the first period (t 1 ) and the second period (t 2 ) overlap with each other can be provided, for example. As illustrated in  FIG. 1B  and  FIG. 6C , periods need to be provided so that at least one of plural pulse signal lines serves as a wiring for supplying a clock signal and at least one of a plurality of pulse signal lines serves as a wiring for supplying an inverted clock signal. 
     The circuit configuration of a flip-flop illustrated in  FIG. 2A  is an example, and any circuit configuration may be employed as long as an inputted signal is outputted with a delay. Specifically, the circuit illustrated in  FIG. 7A  can be applied to the flip-flop in this embodiment. 
     The first flip-flop (FF 1 ) illustrated in  FIG. 7A  includes transistors  131  to  134 . Note that in this case, the transistors  131  to  134  are n-channel transistors. 
     A gate terminal and a first terminal of the transistor  131  are electrically connected to the start pulse line. 
     A gate terminal of the transistor  132  is electrically connected to the output terminal of the second flip-flop (FF 2 ), a first terminal of the transistor  132  is electrically connected to a second terminal of the transistor  131 , and a second terminal of the transistor  132  is electrically connected to the low power supply potential line. 
     A gate terminal of the transistor  133  is electrically connected to the second terminal of the transistor  131  and the first terminal of the transistor  132 , and a first terminal of the transistor  133  is electrically connected to the first pulse signal line (PS 1 ). 
     A gate terminal of the transistor  134  is electrically connected to the output terminal of the second flip-flop (FF 2 ), a first terminal of the transistor  134  is electrically connected to a second terminal of the transistor  133 , and a second terminal of the transistor  134  is electrically connected to the low power supply potential line. 
     Note that hereinafter, for convenience, a node in which the second terminal of the transistor  131 , the first terminal of the transistor  132 , and the gate terminal of the transistor  133  are electrically connected to each other is referred to as a node C. 
     The operation of the first flip-flop (FF 1 ) illustrated in  FIG. 7A  is described below with reference to  FIG. 7B . 
     First, the potential of the start pulse line electrically connected to the first flip-flop (FF 1 ) is increased to an H level. Accordingly, H-level signals are inputted to the gate terminal of the transistor  131  and the first terminal of the transistor  131 , and the transistor  131  which is diode-connected is turned on. Thus, the potential of the node C is increased to an H level. Accordingly, the transistor  133  is also turned on. As a result, an L-level potential, which is the potential of the first pulse signal line (PS 1 ) in this period, is outputted as the output signal (FF 1 out) of the first flip-flop. 
     In the subsequent period, the potential of the start pulse line is decreased to an L level. Thus, the transistor  131  is turned off. As a result, the potential of the node C goes into a floating state. At this time, there is a potential difference from the L level to the H level between the source terminal and the gate terminal of the transistor  131 , and the potential difference is maintained because the node C goes into a floating state. That is, the transistor  131  is kept on regardless of the state of the source terminal. Moreover, the potential of the first pulse signal line (PS 1 ) is increased to an H level. Thus, the potential of the node C which is in a floating state and electrically connected to the gate terminal of the transistor  133  is further increased by an H-level potential of the first pulse signal line (PS 1 ) in this period. Accordingly, an H-level potential, which is the potential of the first pulse signal line (PS 1 ), is outputted as the output signal (FF 1 out) of the first flip-flop. 
     An H-level signal which is the output signal of the first flip-flop (FF 1 ) is inputted to the second flip-flop (FF 2 ). In this case, the second flip-flop (FF 2 ) has the same structure as that of the first flip-flop (FF 1 ) except that the fourth pulse signal line (PS 4 ), instead of the first pulse signal line (PS 1 ), is electrically connected to the second flip-flop (FF 2 ). Thus, the above-mentioned description of the first flip-flop (FF 1 ) applies to detailed description of the circuit operation of the second flip-flop (FF 2 ). In this period, the second flip-flop (FF 2 ) outputs an L-level potential which is the potential of the fourth pulse signal line (PS 4 ) in this period. 
     In the subsequent period, the potential of the first pulse signal (PS 1 ) is decreased to an L level and the potential of the fourth pulse signal line (PS 4 ) is increased to an H level. As a result, the output signal (FF 1 out) of the first flip-flop is decreased to an L level. An H-level potential which is the potential of the fourth pulse signal line (PS 4 ) is outputted as the output signal (FF 2 out) of the second flip-flop. 
     The output signal (FF 2 out) of the second flip-flop is inputted to the third flip-flop (not illustrated) and to the gate terminals of the transistors  132  and  134  included in the first flip-flop (FF 1 ). Thus, the transistors  132  and  134  included in the first flip-flop (FF 1 ) are turned on. Accordingly, the potential of the gate terminal of the transistor  132  (the node C) is set to an L level, and the output signal (FF 1 out) of the first flip-flop is changed from the L level of the first pulse signal (PS 1 ) via the transistor  133  in this period to an L level of the low power supply potential (VSS) via the transistor  134 . 
     In the subsequent period, the potential of the fourth pulse signal line (PS 4 ) is decreased to an L level. That is, the output signal (FF 2 out) of the second flip-flop is decreased to an L level. Thus, the transistors  132  and  134  included in the first flip-flop (FF 1 ) are turned off. Such a state is continued until an H-level potential is inputted to the input terminal of the first flip-flop (FF 1 ) again. 
     In the above-mentioned manner, the first flip-flop (FF 1 ) illustrated in  FIG. 7A  can output a signal which is inputted with a half clock cycle delay. Therefore, the first flip-flop (FF 1 ) can be applied to the flip-flop in this embodiment. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 2 
     In this embodiment, an example of a transistor which is applicable to the shift register described in Embodiment 1 will be described. 
     Examples of structures of the transistors of this embodiment will be described with reference to  FIGS. 8A to 8C .  FIGS. 8A to 8C  illustrate examples of the structures of the transistors of this embodiment.  FIG. 8A  is a top diagram and  FIG. 8B  is a cross-sectional diagram taken along line Z 1 -Z 2  in  FIG. 8A . 
     The transistor in  FIGS. 8A and 8B  includes a conductive layer  211  over a substrate  201 , an insulating layer  202  over the conductive layer  211 , an oxide semiconductor layer  213  over the insulating layer  202 , and a conductive layer  215   a  and a conductive layer  215   b  over the oxide semiconductor layer  213 . 
     Note that in the transistor, the conductive layer  211  serves as a gate terminal, the insulating layer  202  serves as a gate insulating layer, one of the conductive layers  215   a  and  215   b  serves as a source terminal, and the other of the conductive layers  215   a  and  215   b  serves as a drain terminal. The oxide semiconductor layer  213  includes a channel formation region. The oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment when formed. 
     Further, as for the transistor in  FIGS. 8A and 8B , the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment, and moreover, an oxide insulating layer  207  is formed in contact with part of the oxide semiconductor layer  213 . In the transistor including the oxide semiconductor layer  213  as a channel formation region, in which the oxide insulating layer  207  is formed after the dehydration or dehydrogenation treatment is performed, the threshold voltage (Vth) shift due to long-term use and large load hardly occurs and thus reliability is high. 
     Note that a nitride insulating layer may be provided over the oxide insulating layer  207 . It is preferable that the nitride insulating layer be in contact with the insulating layer  202  provided below the oxide insulating layer  207  or an insulating layer serving as a base, so that impurities such as moisture, hydrogen ions, and OH −  from the vicinity of side surfaces of the substrate are prevented from entering. In particular, it is effective to use a silicon nitride layer for the insulating layer  202  in contact with the oxide insulating layer  207  or the insulating layer serving as a base. That is, when the silicon nitride layers are provided so as to surround a lower surface, an upper surface, and side surfaces of the oxide semiconductor layer  213 , reliability of the transistor is improved. 
     Further, a planarization insulating layer may be provided over the oxide insulating layer  207  (over the nitride insulating layer in the case where the nitride insulating layer is provided). 
     Alternatively, as illustrated in  FIG. 8C , the transistor may have a structure in which an oxide conductive layer  214   a  and an oxide conductive layer  214   b  are provided over parts of the oxide semiconductor layer  213 , the conductive layer  215   a  is provided so as to be in contact with the oxide conductive layer  214   a,  and the conductive layer  215   b  is provided so as to be in contact with the oxide conductive layer  214   b.    
     The oxide conductive layer  214   a  and the oxide conductive layer  214   b  have conductivity higher than that of the oxide semiconductor layer  213  and serves as a source region (also referred to as a low-resistance source region) and a drain region (also referred to as a low-resistance drain region) of the transistor  251 . 
     As a material of the oxide conductive film used for forming the oxide conductive layer  214   a  and the oxide conductive layer  214   b,  a conductive material having a light-transmitting property with respect to visible light, such as an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, an Sn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, an Sn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, an Sn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, an In—Sn—O-based metal oxide, an In—O-based metal oxide, an Sn—O-based metal oxide, or a Zn—O-based metal oxide, can be employed. The thickness of the oxide conductive film is selected as appropriate in the range of greater than or equal to 1 nm and less than or equal to 300 nm. In the case of employing a sputtering method, film deposition is performed with the use of a target containing SiO 2  at greater than or equal to 2 wt % and less than or equal to 10 wt %, so that SiO x  (x&gt;0) which hinders crystallization is contained in the light-transmitting conductive film. Thus, the oxide semiconductor layer  213  can be prevented from being crystallized in heat treatment for dehydration or dehydrogenation performed later. 
     For example, in the case where an In—Ga—Zn—O-based film is used for the oxide semiconductor layer, the oxide semiconductor layer  213  serving as a channel formation region and the oxide conductive layer  214   a  and the oxide conductive layer  214   b  can be separately formed under different deposition conditions. 
     For example, in the case where the deposition is performed by a sputtering method, the oxide conductive layer  214   a  and the oxide conductive layer  214   b  which are formed using an oxide semiconductor film formed in an argon gas each have n-type conductivity and an activation energy (ΔE) of greater than or equal to 0.01 eV and less than or equal to 0.1 eV. 
     Note that in this embodiment, the oxide conductive layers  214   a  and  214   b  are In—Ga—Zn—O-based films and include at least amorphous components. Moreover, the oxide conductive layers  214   a  and  214   b  may include crystal grains (nanocrystals). The crystal grains in the oxide conductive layers  214   a  and  214   b  have a diameter of approximately 1 nm to 10 nm, typically approximately 2 nm to 4 nm. 
     The oxide conductive layers  214   a  and  214   b  are not necessarily provided, but when the oxide conductive layers  214   a  and  214   b  are provided between the oxide semiconductor layer  213  serving as a channel formation region and the conductive layers  215   a  to  215   d  serving as a source terminal and a drain terminal, good electrical junctions can be obtained and the transistor  251  can operate stably. Further, excellent mobility can be maintained even when the drain voltage is high. 
     Alternatively, the transistor in  FIGS. 8A and 8B  may have a structure in  FIGS. 9A and 9B , in which a conductive layer  217  is provided over the oxide semiconductor layer  213  with the oxide insulating layer  207  therebetween (with the oxide insulating layer  207  and the nitride insulating layer therebetween in the case where the nitride insulating layer is provided).  FIGS. 9A and 9B  illustrate an example of the structure of the transistor of this embodiment.  FIG. 9A  is a top diagram of the transistor and  FIG. 9B  is a cross-sectional diagram taken along line Z 1 -Z 2  in  FIG. 9A . The conductive layer  217  functions as a second gate terminal. A second gate voltage is applied to the conductive layer  217 , whereby the threshold voltage of the transistor  251  can be controlled. In the case where a planarization insulating layer is provided, the conductive layer  217  can be provided over the planarization insulating layer. 
     For example, when a voltage of the second gate terminal is higher than that of the source terminal, the threshold voltage of the transistor is shifted to a negative side; when a voltage is lower than that of the source terminal, the threshold voltage of the transistor is shifted to a positive side. 
     As illustrated in  FIGS. 8A to 8C  and  FIGS. 9A and 9B  as examples, the transistors of this embodiment are transistors each including an oxide semiconductor layer serving as a channel formation region. Accordingly, the transistors have mobility higher than that of the conventional transistor including amorphous silicon in a channel formation region, and thus high-speed operation is possible. 
     One embodiment in the case where a plurality of transistors in  FIGS. 8A and 8C  is used will be described with reference to  FIGS. 10A and 10B . As an example,  FIGS. 10A and 10B  illustrate a structure of the plurality of transistors which are applicable to the shift register of one embodiment of the present invention.  FIG. 10A  is a top diagram of two transistors and  FIG. 10B  is a cross-sectional diagram taken along line X 1 -X 2  in  FIG. 10A . 
     The transistor  251  and a transistor  252  are illustrated in  FIG. 10A . Note that as an example, a structure is illustrated in which an oxide conductive layer is provided between an oxide semiconductor layer and the conductive layer serving as a source terminal or a drain terminal. 
     The transistor  251  is the transistor illustrated in  FIGS. 8A and 8C . Therefore, the above description is to be referred to. 
     The transistor  252  includes a conductive layer  211  over the substrate  201 ; an insulating layer  202  over the conductive layer  211 ; an oxide semiconductor layer  213  over the insulating layer  202 ; oxide conductive layers  214   a  and  214   b  over the oxide semiconductor layer  213 ; and conductive layers  215   a  and  215   b.    
     In the transistor  252 , the conductive layer  211  serves as a gate terminal, the insulating layer  202  serves as a gate insulating layer, the oxide conductive layers  214   a  and  214   b  the conductivity of which is higher than that of the oxide semiconductor layer  213  each serve as a source region (also referred to as a low-resistance source region) or a drain region (also referred to as a low-resistance drain region), and the conductive layers  215   a  and  215   b  each serve as a source terminal or a drain terminal. The oxide semiconductor layer  213  includes a channel formation region. Note that the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment when the oxide semiconductor layer  213  is formed. 
     The transistors  251  and  252  in  FIGS. 10A and 10B  are provided with an oxide insulating layer  207  to be in contact with part of the oxide semiconductor layer  213  and part of the oxide semiconductor layer  2132 , respectively, in addition to performing dehydration or dehydrogenation treatment on the oxide semiconductor layers. 
     Further, the conductive layer  211  of the transistor  251  is in contact with the conductive layer  215   b  in an opening portion formed in the gate insulating layer  202 . Accordingly, favorable contact can be obtained, which leads to reduction in contact resistance. Thus, the number of openings can be reduced, which results in reducing the area occupied by openings. Therefore, a logic circuit (e.g., an inverter) which has this structure with the use of two transistors can be formed, for example. 
     As illustrated in  FIGS. 10A and 10B , in the shift register described in Embodiment 1, a conductive layer serving as a gate terminal of a transistor may be electrically connected to a source terminal or a drain terminal of another transistor in an opening portion formed in an insulating layer serving as a gate insulating layer. 
     Next, an example of a method for manufacturing the transistor illustrated in  FIGS. 8A and 8B  is described with reference to  FIGS. 11A to 11D .  FIGS. 11A to 11D  are cross-sectional diagrams illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 8A and 8B . 
     In the below description, the word “film” means something formed on the entire surface of a substrate to be processed into a desired shape in a subsequent photolithography step or the like, and something before the processing. The word “layer” means something obtained by processing and shaping a “film” into a desired shape by a photolithography step or the like, or something that is to be formed on the entire surface of a substrate. 
     First, the substrate  201  is prepared. A conductive film is formed over the substrate  201 , and then the conductive layer  211  is formed by a first photolithography step (see  FIG. 11A ). Note that the conductive layer  211  is preferably tapered. When the conductive layer  211  is tapered, adhesion between the conductive layer  211  and a film formed on the conductive layer  211  can be increased. 
     The substrate  201  needs to have an insulating surface and heat resistance high enough to withstand at least heat treatment to be performed later. As the substrate  201 , a glass substrate or the like can be used, for example. 
     As the glass substrate, in the case where the temperature of the heat treatment to be performed later is high, the one whose strain point is 730° C. or higher is preferably used. As the glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used. In general, by containing a larger amount of barium oxide (BaO) than boric acid (B 2 O 3 ), more practical heat-resistant glass can be obtained. Therefore, a glass substrate containing BaO and B 2 O 3  so that the amount of BaO is larger than that of B 2 O 3  is preferably used. 
     Note that a substrate formed of an insulator, such as a ceramic substrate, a quartz substrate, or a sapphire substrate may be used for the substrate  201 , instead of the glass substrate. Alternatively, a crystallized glass substrate or the like may be used. 
     An insulating layer serving as a base layer may be provided between the substrate  201  and the conductive layer  211 . The base layer has a function of preventing diffusion of an impurity element from the substrate  201  and can be formed to have a single-layer structure or a stacked structure of one or more of a silicon nitride layer, a silicon oxide layer, a silicon nitride oxide layer, and a silicon oxynitride layer. 
     As an example of a material of the conductive film for forming the conductive layer  211 , a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium or an alloy material containing any of these materials as a main component can be used. The conductive film for forming the conductive layer  211  can be formed with a single film containing one or more of these materials or a stacked film thereof. 
     As the conductive film for forming the conductive layer  211 , a stacked film of three layers in which a titanium layer, an aluminum layer, and a titanium layer are stacked in this order, or a stacked film of three layers in which a molybdenum layer, an aluminum layer, and a molybdenum layer are stacked in this order is preferably used. A single-layer film, a stacked film of two layers, or a stacked film of four or more layers may also be used as the conductive film. When a stacked conductive film of a titanium film, an aluminum film, and a titanium film is used as the conductive film, etching can be performed by a dry etching method with the use of a chlorine gas. 
     Next, the insulating layer  202  is formed over the conductive layer  211 . 
     The insulating layer  202  can be formed to have a single layer of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a silicon nitride oxide layer or a stacked layer thereof by a plasma CVD method, a sputtering method, or the like. For example, a silicon oxynitride layer may be formed by a plasma CVD method with the use of SiH 4 , oxygen, and nitrogen as a deposition gas. The thickness of the insulating layer  202  is greater than or equal to 100 nm and less than or equal to 500 nm; in the case where the insulating layer  202  is formed using a stacked layer, for example, a first insulating layer with a thickness of greater than or equal to 50 nm and less than or equal to 200 nm and a second insulating layer with a thickness of greater than or equal to 5 nm and less than or equal to 300 nm are stacked. When a silicon oxide film which is formed using a silicon target doped with phosphorus or boron is used for the insulating layer  202 , entry of impurities (such as moisture, hydrogen ions, and OH − ) can be suppressed. 
     In this embodiment, the insulating layer  202  is formed using a silicon nitride film with a thickness of 200 nm by a plasma CVD method as an example. 
     Next, an oxide semiconductor film is formed over the insulating layer  202 . The thickness of the oxide semiconductor film is preferably greater than or equal to 2 nm and less than or equal to 200 nm. For example, when the thickness of the oxide semiconductor film is as small as 50 nm or less, the oxide semiconductor film can be in an amorphous state even when heat treatment for dehydration or dehydrogenation is performed after formation of the oxide semiconductor film. By making the thickness of the oxide semiconductor film small, crystallization of the oxide semiconductor film can be suppressed when heat treatment is performed after the formation of the oxide semiconductor film. 
     Note that before the oxide semiconductor film is formed by a sputtering method, particles which are attached on a surface of the insulating layer  202  may be removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering refers to a method in which, without application of a voltage to a target side, an RF power source is used for application of a voltage to a substrate side in an argon atmosphere so that plasma is generated in the vicinity of the substrate to modify a surface. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. 
     As the oxide semiconductor film, any of the following can be used: an In—Ga—Zn—O-based oxide semiconductor film, an In—Sn—Zn—O-based oxide semiconductor film, an In—Al—Zn—O-based oxide semiconductor film, an Sn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film, an Sn—Al—Zn—O-based oxide semiconductor film, an In—Zn—O-based oxide semiconductor film, an Sn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxide semiconductor film, an In—Sn—O-based oxide semiconductor film, an In—O-based oxide semiconductor film, an Sn—O-based oxide semiconductor film, and a Zn—O-based oxide semiconductor film. In this embodiment, the oxide semiconductor film is formed using an In—Ga—Zn—O-based metal oxide semiconductor target by a sputtering method. Alternatively, the oxide semiconductor film can be formed by a sputtering method in a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas (typically, argon) and oxygen. In the case of employing a sputtering method, film deposition may be performed using a target containing SiO 2  at greater than or equal to 2 wt % and less than or equal to 10 wt % and SiOx (x&gt;0) which hinders crystallization may be contained in the oxide semiconductor film. Accordingly, crystallization of the oxide semiconductor layer can be suppressed in heat treatment for dehydration or dehydrogenation which is to be performed later. 
     Here, the oxide semiconductor film is formed using a metal oxide semiconductor target for film deposition including In, Ga, and Zn (at a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [mol], In:Ga:Zn=1:1:0.5 [at]) under the following condition: the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power supply is 0.5 kW, and the atmosphere is oxygen (the proportion of the oxygen flow is 100%). Note that a pulse direct-current (DC) power supply is preferable because powder substances (also referred to as a particle generated at the time of film deposition can be reduced and the film thickness can be made uniform. In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-based film is formed by a sputtering method with the use of an In—Ga—Zn—O-based metal oxide semiconductor target for film deposition. 
     As the composition ratio of the metal oxide semiconductor target, In 2 O 3 :Ga 2 O 3 :ZnO=1:1:0.5 [mol], In 2 O 3 :Ga 2 O 3 :ZnO=1:1:0.25 [at], In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2 [mol], In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [at], or the like can be employed instead of the above-mentioned composition ratio. 
     Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used as a sputtering power source, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case of forming an insulating film, and a DC sputtering method is mainly used in the case of forming a metal conductive film. 
     Further, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber. 
     Moreover, there are a sputtering apparatus provided with a magnet system inside a chamber and used for a magnetron sputtering method, and a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge. 
     As a film deposition method employing a sputtering method, there are a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during film formation to form a thin film of a compound thereof, and a bias sputtering method in which voltage is also applied to a substrate during film deposition. 
     As an evacuation means of the deposition chamber where sputtering is performed, a cryopump is preferably used. When the cryopump is used for evacuation, impurities such as moisture in the deposition chamber can be removed. 
     Next, the oxide semiconductor film is processed into an island shape by a second photolithography step to form the oxide semiconductor layer  213  (see  FIG. 11B ). Note that after the second photolithography step, the oxide semiconductor layer  213  may be subjected to heat treatment (at higher than or equal to 400° C. and lower than 750° C.) in an inert gas atmosphere (e.g., nitrogen, helium, neon, or argon) so that impurities such as hydrogen and water contained in the layer are removed. 
     Next, the oxide semiconductor layer  213  is dehydrated or dehydrogenated. First heat treatment for dehydration or dehydrogenation is performed at a temperature higher than or equal to 400° C. and lower than 750° C., preferably higher than or equal to 425° C. and lower than 750° C. Note that in the case of the temperature that is 425° C. or higher, the heat treatment time may be one hour or shorter, whereas in the case of the temperature lower than 425° C., the heat treatment time is longer than one hour. In this embodiment, the substrate is introduced into an electric furnace, which is one of heat treatment apparatuses, and heat treatment is performed on the oxide semiconductor layer  213  in a nitrogen atmosphere. Then, the oxide semiconductor layer  213  is not exposed to air, which prevents entry of water and hydrogen into the oxide semiconductor layer  213 . In this embodiment, slow cooling is performed from the heating temperature at which the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation to a temperature low enough to prevent water from entering again, specifically to a temperature that is lower than the heating temperature by 100° C. or more, in one furnace. The atmosphere is not limited to a nitrogen atmosphere, and any atmosphere may be employed as long as it is an inert gas atmosphere such as helium, neon, or argon. 
     Note that the heat treatment apparatus is not limited to an electric furnace, and may have a device for heating an object by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, a rapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment with a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment is used. For example, nitrogen or a rare gas such as argon is used. 
     When the oxide semiconductor layer  213  is subjected to heat treatment at a temperature of higher than or equal to 400° C. and lower than 750° C., the dehydration or dehydrogenation of the oxide semiconductor layer can be achieved; thus, water (H 2 O) can be prevented from being contained again in the oxide semiconductor layer later. 
     In the first heat treatment, it is preferable that water, hydrogen, and the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. Further, the purity of nitrogen or a rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably greater than or equal to 6N (99.9999%), more preferably greater than or equal to 7N (99.99999%) (i.e., the impurity concentration is preferably less than or equal to 1 ppm, more preferably less than or equal to 0.1 ppm). 
     The oxide semiconductor layer  213  includes microcrystalline grains and an amorphous region, or includes only microcrystalline grains, depending on conditions of the first heat treatment or a material of the oxide semiconductor layer  213 . For example, the oxide semiconductor layer  213  may become a microcrystalline semiconductor layer having a crystallinity of 90% or more, or 80% or more. Further, depending on conditions of the first heat treatment or a material of the oxide semiconductor layer  213 , the oxide semiconductor layer  213  may become an amorphous oxide semiconductor layer containing no crystalline grains. 
     The oxide semiconductor layer  213  is changed to an oxygen-deficient type after the first heat treatment so as to be low-resistance. The oxide semiconductor film after the first heat treatment has a higher carrier concentration than the oxide semiconductor layer shortly after the formation and preferably has a carrier concentration of 1×10 18 /cm 3  or more. 
     Note that the conductive layer  211  becomes a microcrystalline layer or a polycrystalline layer depending on the conditions of the first heat treatment or the material of the conductive layer  211  in some cases. For example, in the case of using a film of an alloy of indium oxide and tin oxide as the conductive layer  211 , it is crystallized by the first heat treatment at 450° C. for one hour, whereas in the case of using a film of an alloy of indium oxide and tin oxide containing silicon oxide as the conductive layer  211 , it is not crystallized. 
     The first heat treatment can be performed on the oxide semiconductor layer  213  which has not been processed into the island-shaped oxide semiconductor layer. In that case, the substrate is taken out of the heat treatment apparatus after the first heat treatment, and then a photolithography step is performed. 
     Next, a conductive film is formed over the insulating layer  202  and the oxide semiconductor layer  213 . 
     For the conductive film, an element selected from titanium (Ti), molybdenum (Mo), tungsten (W), aluminum (Al), chromium (Cr), copper (Cu), and tantalum (Ta), an alloy containing any of these elements as a component, a compound containing any of these elements in combination, or the like is used. The conductive film is not limited to a single layer containing the above element and can be formed in a stacked layer of two or more layers. In this embodiment, a three-layer conductive film in which a titanium film (with a thickness of 100 nm), an aluminum film (with a thickness of 200 nm), and a titanium film (with a thickness of 100 nm) are stacked is formed. Instead of a titanium film, a titanium nitride film may be used. 
     In the case where heat treatment at 200° C. to 600° C. is performed later, it is preferable that the conductive film have heat resistance high enough to withstand the heat treatment. For example, it is preferable to use an aluminum alloy to which an element for preventing hillocks is added or a conductive film stacked with a heat-resistance conductive film. As the formation method of the conductive film, a sputtering method, a vacuum evaporation method (e.g., an electron beam evaporation method), an arc discharge ion plating method, or a spray method is used. Alternatively, the conductive film may be formed by discharging a conductive nanopaste of silver, gold, copper, or the like by a screen printing method, an ink-jet method, or the like and baking the nanopaste. 
     Next, a third photolithography step is performed so that a resist mask  233   a  and a resist mask  233   b  are formed. Then the conductive film is selectively etched, so that the conductive layer  215   a  and the conductive layer  215   b  are formed (see  FIG. 11C ). 
     In the third photolithography step, only part of the conductive film which is on the oxide semiconductor layer  213  is selectively removed. For example, when an ammonia peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2 in a weight ratio) or the like is used as an alkaline etchant in order to selectively remove only part of the metal conductive film, which is on the In—Ga—Zn—O-based oxide semiconductor layer, the metal conductive film can be selectively removed, and the oxide semiconductor layer formed of an oxide semiconductor can be left. 
     In the third photolithography step, an exposed region of the oxide semiconductor layer  213  is etched in some cases depending on an etching condition. In that case, the oxide semiconductor layer in a region which is sandwiched between the conductive layer  215   a  and the conductive layer  215   b  is thinner than the oxide semiconductor layer in a region which overlaps with the conductive layer  215   a  and the conductive layer  215   b  over the conductive layer  211 . 
     Next, the oxide insulating layer  207  is formed over the insulating layer  202  and the oxide semiconductor layer  213 . At this stage, part of the oxide semiconductor layer  213  is in contact with the oxide insulating layer  207 . Note that a region of the oxide semiconductor layer, which overlaps with the conductive layer  211  with the insulating layer  202  therebetween, is a channel formation region. 
     The oxide insulating layer  207  can be formed to have a thickness of at least 1 nm by a method with which impurities such as water and hydrogen are not mixed into the oxide insulating layer, such as a sputtering method, as appropriate. In this embodiment, a silicon oxide film is formed as the oxide insulating layer by a sputtering method. The substrate temperature in deposition may be higher than or equal to room temperature and lower than or equal to 300° C. The substrate temperature is 10 ° C. in this embodiment. The silicon oxide film can be formed by a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere containing a rare gas (typically argon) and oxygen. Moreover, a silicon oxide target or a silicon target can be used as a target. For example, with the use of a silicon target, a silicon oxide film can be formed by a sputtering method in a mixed atmosphere containing oxygen and a rare gas. The oxide insulating layer which is formed to be in contact with the oxide semiconductor layer whose resistance is reduced is formed using an inorganic insulating film which does not contain impurities such as moisture, hydrogen ions, and OH −  and blocks entry of such impurities from the outside; a silicon oxide film, a silicon nitride oxide film, an aluminum oxide film, an aluminum oxynitride film, or the like is typically used. Note that an oxide insulating layer formed by a sputtering method is particularly dense, and even a single layer can be used as a protective film for suppressing a phenomenon in which impurities are diffused into a layer in contact therewith. Also, a target doped with phosphorus (P) or boron (B) can be used so that phosphorus (P) or boron (B) is added to the oxide insulating layer. 
     In this embodiment, the film deposition is performed by a pulsed DC sputtering method using a columnar polycrystalline, boron-doped silicon target which has a purity of 6N (the resistivity is 0.01 Ω·cm), in which the distance between the substrate and the target (T-S distance) is 89 mm, the pressure is 0.4 Pa, the direct-current (DC) power source is 6 kW, and the atmosphere is oxygen (the proportion of the oxygen flow is 100%). The film thickness is 300 nm. 
     The oxide insulating layer  207  is provided on the channel formation region of the oxide semiconductor layer  213  and also functions as a channel protective layer. 
     Next, second heat treatment (for example, at higher than or equal to 200° C. and lower than or equal to 400° C., preferably, at higher than or equal to 250° C. and lower than or equal to 350° C.) may be performed in an inert gas atmosphere or a nitrogen atmosphere. For example, the second heat treatment is performed in a nitrogen atmosphere at 250° C. for one hour. When the second heat treatment is performed, the oxide semiconductor layer  213  is heated while part thereof is in contact with the oxide insulating layer  207  and other parts thereof are in contact with the conductive layer  215   a  and the conductive layer  215   b.    
     When the second heat treatment is performed while the oxide semiconductor layer  213  whose resistance is reduced in the first heat treatment is in contact with the oxide insulating layer  207 , the region in contact with the oxide insulating layer  207  becomes in an oxygen-excess state. Accordingly, the region of the oxide semiconductor layer  213 , which is in contact with the oxide insulating layer  207 , increases resistance (becomes an i-type) in a depth direction of the oxide semiconductor layer  213  (see  FIG. 11D ). 
     The timing of performing the second heat treatment is not limited to the timing shortly after the third photolithography step as long as it is after the third photolithography step. 
     Thus, the transistor illustrated in  FIGS. 8A and 8B . 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 3 
     In this embodiment, another example of a transistor which is applicable to the shift register described in Embodiment 1 and different from the transistor described in Embodiment 2 will be described. 
     An example of a structure of the transistor of this embodiment will be described with reference to  FIGS. 12A and 12B .  FIGS. 12A and 12B  illustrate an example of a structure of the transistor of this embodiment.  FIG. 12A  is a top diagram of the transistor and  FIG. 12B  is a cross-sectional diagram taken along line Z 1 -Z 2  in  FIG. 12A . 
     The transistor illustrated in  FIGS. 12A and 12B  includes the conductive layer  211  over the substrate  201 ; the insulating layer  202  over the conductive layer  211 ; the conductive layers  215   a  and  215   b  over the insulating layer  202 ; the oxide semiconductor layer  213  over the insulating layer  202  and the conductive layers  215   a  and  215   b.    
     In the transistor, the conductive layer  211  serves as a gate terminal, the insulating layer  202  serves as a gate insulating layer, one of the conductive layers  215   a  and  215   b  serves as a source terminal, and the other of the conductive layers  215   a  and  215   b  serves as a drain terminal. The oxide semiconductor layer  213  includes a channel formation region. Note that the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment when the oxide semiconductor layer  213  is formed. 
     Further, as for the transistor in  FIGS. 12A and 12B , the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment, and moreover, the oxide insulating layer  207  is formed to be in contact with part of the oxide semiconductor layer  213 . A transistor including the oxide semiconductor layer  213 , which undergoes dehydration or dehydrogenation treatment and then brought into contact with the oxide insulating layer  207  formed thereon, as a channel formation region has high reliability because a threshold voltage (V-th) shift due to a long-term use or high load hardly occurs. 
     Note that a nitride insulating layer may be provided over the oxide insulating layer  207 . It is preferable that the nitride insulating layer be in contact with the insulating layer  202  provided below the oxide insulating layer  207  or an insulating layer serving as a base, so that impurities such as moisture, hydrogen ions, and OH −  from the vicinity of side surfaces of the substrate are prevented from entering. In particular, it is effective to use a silicon nitride layer for the insulating layer  202  in contact with the oxide insulating layer  207  or the insulating layer serving as a base. That is, when the silicon nitride layers are provided so as to surround a lower surface, an upper surface, and side surfaces of the oxide semiconductor layer  213 , reliability of the transistor is improved. 
     Further, a planarization insulating layer may be provided over the oxide insulating layer  207  (over the nitride insulating layer in the case where the nitride insulating layer is provided). 
     As in  FIGS. 9A and 9B , the transistor  251  in  FIGS. 12A and 12B  may have a structure in which a conductive layer is provided in a region which overlaps with the oxide semiconductor layer  213  over the oxide insulating layer  207  (over the planarization insulating layer in the case where the planarization insulating layer is provided). The conductive layer serves as a second gate terminal. Second gate voltage is applied to the conductive layer, whereby the threshold voltage of the transistor can be controlled. 
     Note that the planarization insulating layer is not necessarily provided. When the planarization insulating layer is not provided, a conductive layer serving as the second gate terminal can be formed over the oxide insulating layer  207  (over the nitride insulating layer in the case where the nitride insulating layer is formed). 
     For example, when the potential of the second gate terminal is higher than the potential of the source terminal, the threshold voltage of the transistor shifts in a negative direction. When the potential of the second gate terminal is lower than the potential of the source terminal, the threshold voltage of the transistor shifts in a positive direction. 
     As illustrated in  FIGS. 12A and 12B , the transistor of this embodiment is a so-called bottom-contact transistor in which an oxide semiconductor layer is provided over a conductive layer serving as a source terminal or a drain terminal. Therefore, high-speed operation can be performed since the transistor of this embodiment has higher mobility than that of the conventional transistor including amorphous silicon in a channel formation region. Further, the bottom-contact transistor is applied, so that an area where the oxide semiconductor layer is in contact with the conductive layer serving as the source terminal or the drain terminal can be increased, leading to prevention of peeling or the like. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 4 
     In this embodiment, another example of a transistor which is applicable to the shift register described in Embodiment 1 and different from the transistors described in Embodiments 2 and 3 will be described. 
     The example of the structure of the transistor of this embodiment will be described with reference to  FIGS. 13A and 13B .  FIGS. 13A and 13B  illustrate an example of the structure of the transistor of this embodiment.  FIG. 13A  is a top diagram and  FIG. 13B  is a cross-sectional diagram taken along line Z 1 -Z 2  in  FIG. 13A . 
     As in the transistors illustrated in  FIGS. 8A to 8C , the transistor illustrated in  FIGS. 13A and 13B  includes the conductive layer  211  over the substrate  201 , the insulating layer  202  over the conductive layer  211 , the oxide semiconductor layer  213  over the insulating layer  202 , the conductive layer  215   a  and the conductive layer  215   b  over the oxide semiconductor layer  213 . 
     In the transistor, the conductive layer  211  serves as a gate terminal, the insulating layer  202  serves as a gate insulating layer, one of the conductive layers  215   a  and  215   b  serves as a source terminal, and the other of the conductive layers  215   a  and  215   b  serves as a drain terminal. The oxide semiconductor layer  213  includes a channel formation region. Note that the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment when the oxide semiconductor layer  213  is formed. 
     Further, as for the transistor illustrated in  FIGS. 13A and 13B , the oxide semiconductor layer  213  is subjected to dehydration or dehydrogenation treatment, and moreover, the oxide insulating layer  207  is provided below the conductive layers  215   a  and  215   b  so that the oxide insulating layer  207  is in contact with part of the oxide semiconductor layer  213 . The oxide insulating layer  207  in  FIGS. 13A and 13B  has a function of a channel protective layer. 
     A nitride insulating layer may be provided over the oxide insulating layer  207  and the conductive layers  215   a  and  215   b.  It is preferable that the nitride insulating layer  207  be in contact with the insulating layer  202  provided below the oxide insulating layer  207  or an insulating layer serving as a base, so that impurities such as moisture, hydrogen ions, and OH −  from the vicinity of side surfaces of the substrate are prevented from entering. In particular, it is effective to use a silicon nitride layer for the insulating layer  202  in contact with the oxide insulating layer  207  or the insulating layer serving as a base. That is, when the silicon nitride layers are provided so as to surround a lower surface, an upper surface, and side surfaces of the oxide semiconductor layer  213 , reliability of the transistor is improved. 
     Further, a planarization insulating layer may be provided over the oxide insulating layer  207  and the conductive layers  215   a  and  215   b  (over the nitride insulating layer in the case where the nitride insulating layer is provided). 
     Further, a conductive layer may be provided over the oxide insulating layer  207  (over the planarization insulating layer in the case where the planarization insulating layer is provided) so that the oxide insulating layer  207  is sandwiched between the conductive layer and the oxide semiconductor layer  213 . The conductive layer serves as a second gate terminal. Second gate voltage is applied to the conductive layer, whereby the threshold voltage of the transistor  251  can be controlled. 
     Note that the planarization insulating layer is not necessarily provided. When the planarization insulating layer is not provided, a conductive layer can be formed over the oxide insulating layer  207  (over the nitride insulating layer in the case where the nitride insulating layer is formed). 
     For example, when the potential of the second gate terminal is higher than the potential of the source terminal, the threshold voltage of the transistor shifts in a negative direction. When the potential of the second gate terminal is lower than the potential of the source terminal, the threshold voltage of the transistor shifts in a positive direction. 
     Further, as in the transistor in  FIG. 8C , the transistor of this embodiment may have a structure in which a pair of oxide conductive layers serving as buffer layers is provided over parts of the oxide semiconductor layer  213 , and the conductive layers  215   a  and  215   b  which serve as a pair of electrodes are provided so as to be in contact with the pair of the oxide conductive layers. 
     As described above, the transistors of this embodiment are so-called channel protective transistors each including an insulating layer serving as a channel formation layer over part of the oxide semiconductor layer. Accordingly, the transistors have mobility higher than that of the conventional transistor including amorphous silicon in a channel formation region, and thus high-speed operation is possible. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 5 
     In this embodiment, an example of a display device including the shift register described in Embodiment 1 is described with reference to  FIGS. 14A to 14C . 
     A variety of display devices, such as a liquid crystal display device or an electroluminescence (hereinafter also referred to as EL) display device, is given as the display device including the shift register described in Embodiment 1. A structure of a display device in this embodiment is described with reference to  FIG. 14A .  FIG. 14A  is a block diagram illustrating a structure of the display device in this embodiment. 
     The display device illustrated in  FIG. 14A  includes a pixel portion  701 , a scan line driver circuit  702 , and a signal line driver circuit  703 . 
     The pixel portion  701  has a dot matrix structure in which a plurality of pixels  704  is provided. Specifically, the plurality of pixels  704  is arranged in the row and column directions. Each of the pixels  704  is electrically connected to the scan line driver circuit  702  through a scan line  705  and electrically connected to the signal line driver circuit  703  through a signal line  706 . 
     The scan line driver circuit  702  is a circuit for selecting the pixel  704  to which a data signal is inputted, and outputs a selection signal to the pixel  704  through the scan line  705 . p The signal line driver circuit  703  is a circuit for outputting data written to the pixel  704  as a signal, and outputs pixel data as a signal through the signal line  706  to the pixel  704  selected by the scan line driver circuit  702 . 
     The pixel  704  includes at least a display element and a switching element. A liquid crystal element or a light-emitting element such as an EL element can be applied to the display element, for example. A transistor can be applied to the switching element, for example. 
     Next, an example of structures of the scan line driver circuit  702  and the signal line driver circuit  703  is described with reference to  FIGS. 14B and 14C .  FIGS. 14B and 14C  are block diagrams illustrating a structure of the driver circuit.  FIG. 14B  is a block diagram illustrating a structure of the scan line driver circuit  702 .  FIG. 14C  is a block diagram illustrating a structure of the signal line driver circuit  703 . 
     As illustrated in  FIG. 14B , the scan line driver circuit  702  includes a shift register  900 , a level shifter  901 , and a buffer  902 . 
     Signals such as a scan line driver circuit start pulse signal (GSP) and a scan line driver circuit reference clock signal (GCK) are inputted to the shift register  900 , and selection signals are sequentially output from sequential logic circuits. As illustrated in Embodiment 1, the shift register  900  in this embodiment includes a plurality of wirings for supplying scan line driver circuit reference clock signals (GCK) in part of the operation period. 
     As illustrated in  FIG. 14C , the signal line driver circuit  703  includes a shift register  903 , a first latch circuit  904 , a second latch circuit  905 , a level shifter  906 , and a buffer  907 . 
     Signals such as a signal line driver circuit start pulse signal (SSP) and a signal line driver circuit reference clock signal (SCK) are inputted to the shift register  903 , and selection signals are sequentially output from sequential logic circuits. As in Embodiment 1, the shift register  903  in this embodiment includes a plurality of wirings by which signal line driver circuit reference signals (SCK) are supplied in part of the operation period. 
     Note that the shift register described in Embodiment 1 may be employed for only one of the shift register  900  and the shift register  903 . 
     A data signal (DATA) is inputted to the first latch circuit  904 . The first latch circuit  904  can be constituted using a logic circuit. 
     The buffer  907  has a function of amplifying a signal and includes an operational amplifier or the like. The buffer  907  can be constituted using a logic circuit. 
     The second latch circuit  905  can hold a latch (LAT) signal temporally and outputs the held latch signals all at once to the pixel portion  701  in  FIG. 14A . This is referred to as line sequential driving. Therefore, in the case of using a pixel in which not line sequential driving but dot sequential driving is performed, the second latch circuit  905  is not necessary. The second latch circuit  905  can be constituted using a logic circuit. 
     Next, operation of the display device in this embodiment is described. 
     First, the scan line  705  is selected by the scan line driver circuit  702 . A data signal is inputted from the signal line driver circuit  703  to the pixel  704  electrically connected to the selected scan line  705 , via the signal line  706 . Accordingly, data is written to the pixel  704 , and the pixel  704  enters into a display state. The scan lines  705  are sequentially selected by the scan line driver circuit  702 ; thus, data is written to all the pixels  704 . The above is the operation of the display device in this embodiment. 
     The circuits of the display device illustrated in  FIGS. 14A to 14C  can be provided over one substrate. Further, the circuits of the display device illustrated in  FIGS. 14A to 14C  can be configured using transistors having the same conductivity type. Providing the circuits over one substrate leads to the reduction in the size of the display device, and using transistors having the same conductivity type can simplify the process. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 6 
     In this embodiment, as an example of the display device described in Embodiment 5, a liquid crystal display device is described with reference to  FIGS. 15A to 15C . 
       FIG. 15A  is a circuit diagram of a pixel included in the liquid crystal display device in this embodiment. The pixel illustrated in  FIG. 15A  includes a transistor  821 , a liquid crystal element  822 , and a capacitor  823 . 
     A gate terminal of the transistor  821  is electrically connected to a scan line  804 , and a first terminal of the transistor  821  is electrically connected to a signal line  805 . Note that the transistor  821  serves as a selection transistor which controls application of voltage to a liquid crystal element  822  included in the pixel. 
     One terminal of the liquid crystal element  822  is electrically connected to a second terminal of the transistor  821  and the other terminal of the liquid crystal element  822  is electrically connected to a wiring for supplying a common potential (Vcom) (hereinafter also referred to as a common potential line). The liquid crystal element  822  includes a first electrode which serves as part or the whole of one terminal, a second electrode which serves as part or the whole of the other terminal, and a layer including liquid crystal molecules whose alignment is changed by applying voltage between the first electrode and the second electrode (such a layer is referred to as a liquid crystal layer). 
     One terminal of the capacitor  823  is electrically connected to the second terminal of the transistor  821  and the other terminal of the capacitor  823  is electrically connected to the common potential line. The capacitor  823  includes a first electrode which serves as part or the whole of one terminal, a second electrode which serves as part or the whole of the other terminal, and a dielectric layer provided between the first electrode and the second electrode. The capacitor  823  has a function of a storage capacitor in a pixel. Note that although the capacitor  823  is not necessarily provided, the provision of the capacitor  823  can reduce adverse effects due to leakage current of the transistor  821 . 
     As a method for driving liquid crystals of the liquid crystal display device in this embodiment, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, and the like are given. 
     Alternatively, blue-phase liquid crystal for which an alignment film is not necessary may be used. The blue phase is a kind of liquid crystal phase and appears just before phase transition from a cholesteric phase to an isotropic phase when temperature of cholesteric liquid crystal rises. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which 5 wt. % 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 contains blue-phase liquid crystal and the chiral material has the following characteristics: the response speed is 10 μs to 100 μs, which is short; alignment treatment is not necessary due to optical isotropy; and viewing angle dependence is low. 
     When a signal is inputted to a pixel, first, a pixel to which data is written is selected, and in the selected pixel, the transistor  821  is turned on by a signal inputted from the scan line  804 . 
     At this time, a data signal from the signal line  805  is inputted to the pixel via the transistor  821 , and the potential of one terminal of the liquid crystal element  822  corresponds to the potential of the data signal. Therefore, the alignment-state of the liquid crystal element  822  is set depending on the voltage applied between one terminal and the other terminal. After data writing, the transistor  821  is turned off by a signal inputted from the scan line  804 , the alignment-state of the liquid crystal element  822  is maintained during a display period, and the pixel enters into a display state. The above operation is sequentially performed per scan line  804 , and in all the pixels included in the liquid crystal display device. 
     In displaying moving images in a liquid crystal display device, there is a problem in that an afterimage or motion blur occurs because of slow response of liquid crystal molecules themselves. In order to improve moving image characteristics of the liquid crystal display device, there is a driving technique called black insertion, in which the entire screen is displayed as black every other frame. 
     Moreover, there is a driving technique called double-frame rate driving, in which a vertical synchronizing frequency is 1.5 times or more, preferably 2 times or more as high as a normal vertical synchronizing frequency. 
     Further, in order to improve the moving image characteristics of the liquid crystal display device, there is a driving technique in which a plurality of LED (light-emitting diode) light sources, a plurality of EL light sources, or the like are used as backlights to form an area light source, and the light sources forming the area light source are independently lit intermittently in one frame period. For the area light source, LEDs of three kinds or more or an LED which emits white light may be used. Since a plurality of LEDs can be independently controlled, the timing when the LED emits light can be synchronized with the timing when optical modulation of the liquid crystal layer is changed. Part of the LEDs can be turned off in this driving technique, so that power consumption can be reduced particularly in the case of displaying an image in which a black display region occupies a large area in one screen. 
     By combining these driving techniques, display characteristics such as moving image characteristics of the liquid crystal display device can be improved as compared to those of a conventional liquid crystal display device. 
     Next, a structure of the display device in this embodiment, which includes the above pixel, is described with reference to  FIGS. 15B and 15C .  FIGS. 15B and 15C  illustrate a structure of the pixel in the display device in this embodiment.  FIG. 15B  is a top diagram of the pixel, and  FIG. 15C  is a cross-sectional diagram taken along line A 1 -A 2  and line B 1 -B 2  of  FIG. 15B . 
     The liquid crystal display device illustrated in  FIGS. 15B and 15C  includes, in the cross section A 1 -A 2 , a conductive layer  2001  provided over a substrate  2000 ; an insulating layer  2002  provided over the conductive layer  2001 ; an oxide semiconductor layer  2003  provided over the insulating layer  2002 ; a pair of conductive layers  2005   a  and  2005   b  provided over the oxide semiconductor layer  2003 ; an oxide insulating layer  2007  provided over the conductive layers  2005   a  and  2005   b  and the oxide semiconductor layer  2003 ; and a transparent conductive layer  2020  which is in contact with the conductive layer  2005   b  through an opening provided in the oxide insulating layer  2007 . 
     Note that the conductive layer  2001  serves as a gate terminal, the insulating layer  2002  serves as a gate insulating layer, one of the conductive layers  2005   a  and  2005   b  serves as a first terminal, and the other of the conductive layers  2005   a  and  2005   b  serves as a second terminal. Although the transistor described in Embodiment 2 (see  FIG. 8B ) is employed here, the transistor described in Embodiment 3 or 4 can be employed as the transistor. 
     The liquid crystal display device illustrated in  FIGS. 15B and 15C  includes, in the cross section B 1 -B 2 , a conductive layer  2008  provided over the substrate  2000 ; the insulating layer  2002  provided over the conductive layer  2008 ; the oxide insulating layer  2007  provided over the insulating layer  2002 ; and the transparent conductive layer  2020  provided over the oxide insulating layer  2007 . 
     Further, the liquid crystal display device in this embodiment includes a conductive layer  2022  and a transparent conductive layer  2029 , a conductive layer  2023 , a conductive layer  2024 , and a transparent conductive layer  2028  which serve as an electrode or a wiring for connecting flexible printed circuits (FPC). 
     The transparent conductive layers  2020 ,  2029 , and  2028  are formed using indium oxide (In 2 O 3 ), an alloy of indium oxide and tin oxide (In 2 O 3 —SnO 2 , referred to as ITO), or the like by a sputtering method, a vacuum evaporation method, or the like. Such a material is etched with a hydrochloric acid-based solution. Note that since etching of ITO particularly tends to leave residue, an alloy of indium oxide and zinc oxide (In 2 O 3 —ZnO) may be used in order to improve the etching processability. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 7 
     In this embodiment, as an example of the display device described in Embodiment 5, a light-emitting display device including a light-emitting element utilizing electroluminescence is described with reference to  FIGS. 16A and 16B  and  FIGS. 17A and 17B . 
     Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter as an inorganic EL element. 
     In an organic EL element, by application of voltage to the light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and there flows a current. Then, the carriers (electrons and holes) are recombined, thereby emitting light. Based on such a mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element. 
     Inorganic EL elements are classified, according to the element structures, into a dispersion inorganic EL elements and thin-film inorganic EL elements. A dispersion inorganic EL element includes a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination light emission utilizing a donor level and an acceptor level. A thin-film inorganic EL element has a structure in which a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized light emission utilizing inner-shell electron transition of metal ions. Note that, an organic EL element is described as a light-emitting element here. 
       FIG. 16A  is a circuit diagram illustrating a circuit configuration of a pixel in the light-emitting display device in this embodiment. 
     As illustrated in  FIG. 16A , the pixel of the display device in this embodiment includes a transistor  851 , a capacitor  852  serving as a storage capacitor in the pixel, a transistor  853 , and a light-emitting element  854 . 
     A gate terminal of the transistor  851  is electrically connected to a scan line  855 . A first terminal of the transistor  851  is electrically connected to a signal line  856 . 
     One terminal of the capacitor  852  is electrically connected to a second terminal of the transistor  851 . The other terminal of the capacitor  852  is electrically connected to the low power supply potential line. 
     A gate terminal of the transistor  853  is electrically connected to the second terminal of the transistor  851  and the one terminal of the capacitor  852 . A first terminal of the transistor  853  is electrically connected to the low power supply potential line. 
     A first terminal of the light-emitting element  854  is electrically connected to a second terminal of the transistor  853 . A second terminal of the light-emitting element  854  is electrically connected to the high power supply potential line. 
     When a signal is inputted to a pixel, first, a pixel to which data is written is selected. In the selected pixel, the transistor  851  is turned on by a scan signal inputted from the scan line  855 , and a video signal (also referred to as a data signal) which is a fixed voltage is inputted from the signal line  856  to the gate terminal of the transistor  853 . 
     The transistor  853  is turned on or off by a potential in response to the data signal inputted to the gate terminal. At this time, current flows depending on the voltage applied between one terminal and the other terminal of the light-emitting element  854 , and the light-emitting element  854  emits light with luminance in response to the amount of current flowing therethrough. Further, the gate voltage of the transistor  853  is held for a certain period of time by the capacitor  852 ; thus, the light-emitting element  854  maintains a light-emitting state for a certain period of time. 
     When the data signal inputted from the signal line  856  to the pixel is digital, the light-emitting state of the pixel is controlled by switching on and off of the transistor. Accordingly, gradation can be expressed by an area ratio grayscale method or a time ratio grayscale method. An area ratio grayscale method refers to a driving method by which one pixel is divided into a plurality of subpixels and each of the subpixels with the structure illustrated in  FIG. 16A  is independently driven based on a data signal so that gradation is expressed. Further, a time ratio grayscale method refers to a driving method by which a period during which a pixel is in a light-emitting state is controlled so that gradation is expressed. 
     Since the response speed of light-emitting elements is higher than that of liquid crystal elements or the like, the light-emitting elements are suitable for a time ratio grayscale method as compared to the liquid crystal elements. Specifically, when display is performed by a time gray scale method, one frame period is divided into a plurality of subframe periods. Then, in accordance with video signals, the light-emitting state of the light-emitting element is controlled in each subframe period. 
     By dividing one frame period into a plurality of subframe periods, the total length of a period in which pixels actually emit light in one frame period can be controlled with video signals, and gradation can be expressed. 
     Next, a structure of a light-emitting element is described with reference to  FIGS. 16B to 16D . Here, a cross-sectional structure of a pixel in the case where the transistor  853  is an n-channel transistor is described as an example. Note that the transistor  853  used in a light-emitting display device illustrated in  FIGS. 16B to 16D  is a driving transistor. 
     In order to extract light emitted from the light-emitting element  854 , at least one of an anode and a cathode needs to be transparent. A transistor and a light-emitting element are formed over a substrate. There are light-emitting elements having a top emission structure in which light is extracted through the surface opposite to the substrate, having a bottom emission structure in which light is extracted through the surface on the substrate side, and having a dual emission structure in which light is extracted through the surface on the substrate side and the surface opposite to the substrate. The pixel structure of the present invention can be applied to a light-emitting element having any of these emission structures. 
     A light-emitting element having a top emission structure is described with reference to  FIG. 16B . 
       FIG. 16B  is a cross-sectional diagram of a pixel in the case where the transistor  853 , which is a driving transistor, is an n-channel transistor and light emitted from the light-emitting element  854  passes through an anode  7005 . In  FIG. 16B , a cathode  7003  of the light-emitting element  854  and the transistor  853 , which is the driving transistor, are electrically connected to each other, and a light-emitting layer  7004  and the anode  7005  are sequentially stacked over the cathode  7003 . As the cathode  7003 , any conductive film can be used as long as it has a low work function and reflects light. For example, Ca, Al, CaF, MgAg, AlLi, or the like is preferably used. The light-emitting layer  7004  may be formed using a single layer or by stacking a plurality of layers. When the light-emitting layer  7004  is formed using a plurality of layers, the light-emitting layer  7004  is formed by stacking an electron-injection layer, an electron-transport layer, a light-emitting layer, a hole-transport layer, and a hole-injection layer sequentially over the cathode  7003 . Note that it is not necessary to form all of these layers. The anode  7005  is formed using a light-transmitting conductive material such as a material of indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter also referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added. 
     The light-emitting element  854  corresponds to a region where the light-emitting layer  7004  is sandwiched between the cathode  7003  and the anode  7005 . In the pixel illustrated in  FIG. 16B , light is emitted from the light-emitting element  854  to the anode  7005  side as shown by an arrow. 
     Next, a light-emitting element having a bottom emission structure is described with reference to  FIG. 16C .  FIG. 16C  is a cross-sectional diagram of a pixel in the case where the transistor  853  is an n-channel transistor and light is emitted from the light-emitting element  854  to a cathode  7013  side. In  FIG. 16C , the cathode  7013  of the light-emitting element  854  is formed over a light-transmitting conductive layer  7017  which is electrically connected to the transistor  853 , and a light-emitting layer  7014  and an anode  7015  are sequentially stacked over the cathode  7013 . Note that when the anode  7015  has a light-transmitting property, a light-blocking layer  7016  for reflecting or blocking light may be formed so as to cover the anode  7015 . As in the case of  FIG. 16B , a variety of materials can be used for the cathode  7013  as long as a material is a conductive material having a low work function. Note that the cathode  7013  has a thickness that can transmit light (preferably has approximately 5 nm to 30 nm). For example, a 20-nm-thick aluminum layer can be used as the cathode  7013 . The light-emitting layer  7014  may be formed using a single layer or by stacking a plurality of layers as in  FIG. 16B . The anode  7015  is not necessary to transmit light, but can be formed using a light-transmitting conductive material as in  FIG. 16B . The light-blocking layer  7016  can be formed using, for example, metal or the like which reflects light; however, one embodiment of the present invention is not limited to metal. For example, a resin to which a black pigment is added can be used. 
     The light-emitting element  854  corresponds to a region where the light-emitting layer  7014  is sandwiched between the cathode  7013  and the anode  7015 . In the pixel illustrated in  FIG. 16C , light is emitted from the light-emitting element  854  to the cathode  7013  side as shown by an arrow. 
     Next, a light-emitting element having a dual emission structure is described with reference to  FIG. 16D . In  FIG. 16D , a cathode  7023  of the light-emitting element  854  is formed over a light-transmitting conductive layer  7027  which is electrically connected to the transistor  853 , and a light-emitting layer  7024  and an anode  7025  are sequentially stacked over the cathode  7023 . As in the case of  FIG. 16B , a variety of materials can be used for the cathode  7023  as long as a material is a conductive material having a low work function. Note that the cathode  7023  has a thickness that can transmit light. For example, a 20-nm-thick aluminum layer can be used as the cathode  7023 . The light-emitting layer  7024  may be formed using a single layer or by stacking a plurality of layers as in  FIG. 16B . The anode  7025  can be formed using a light-transmitting conductive material as in  FIG. 16B . 
     The light-emitting element  854  corresponds to a region where the cathode  7023 , the light-emitting layer  7024 , and the anode  7025  overlap with each other. In the pixel illustrated in  FIG. 16D , light is emitted from the light-emitting element  854  to both the anode  7025  side and the cathode  7023  side as shown by arrows. 
     Note that, although the organic EL elements are described here as the light-emitting elements, an inorganic EL element can also be provided as a light-emitting element. 
     Note that in this embodiment, the example is described in which a transistor (also referred to as a driving transistor) which controls driving of a light-emitting element is electrically connected to the light-emitting element; alternatively, a structure may be employed in which a transistor for current control is connected between the driving transistor and the light-emitting element. 
     Next, the appearance and cross section of the light-emitting display device (also referred to as a light-emitting panel) in this embodiment are described with reference to  FIGS. 17A and 17B .  FIG. 17A  is a top diagram of the light-emitting display device, in which a transistor and a light-emitting element formed over a first substrate are sealed between the first substrate and a second substrate by a sealing material.  FIG. 17B  is a cross-sectional diagram taken along line H-I of  FIG. 17A . 
     A sealing material  4505  is provided so as to surround a pixel portion  4502 , a signal line driver circuits  4503   a  and  4503   b,  and scan line driver circuits  4504   a  and  4504   b  which are provided over a first substrate  4501 . Moreover, a second substrate  4506  is provided over the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b,  and the scan line driver circuits  4504   a  and  4504   b.  Accordingly, the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b,  and the scan line driver circuits  4504   a  and  4504   b  are sealed, together with a filler  4507 , by the first substrate  4501 , the sealing material  4505 , and the second substrate  4506 . In such a manner, it is preferable to pack (seal) the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b,  and the scan line driver circuits  4504   a  and  4504   b  with a protective film (such as an attachment film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b,  and the scan line driver circuits  4504   a  and  4504   b  are not exposed to the air. 
     The pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b,  and the scan line driver circuits  4504   a  and  4504   b,  which are formed over the first substrate  4501 , each include a plurality of transistors. In  FIG. 17B , a transistor  4510  included in the pixel portion  4502  and transistors  4509  and  4555  included in the signal line driver circuit  4503   a  are illustrated as an example. 
     As the transistors  4509 ,  4510 , and  4555 , any of the highly reliable transistors described in Embodiments 2 to 4 which include an oxide semiconductor layer as a semiconductor layer, can be used. In this embodiment, the transistors  4509 ,  4510 , and  4555  are n-channel transistors. An insulating layer  4542  is formed over the transistors  4509 ,  4510 , and  4555 . An insulating layer  4544  is formed over the insulating layer  4542 . A conductive layer  4540  is formed over the transistor  4509  with the insulating layers  4542  and  4544  provided therebetween. The conductive layer  4540  has a function of a second gate terminal. 
     Note that in the pixel portion  4502 , a planarization insulating layer  4545  is provided over the insulating layer  4542 , and an insulating layer  4543  is provided over the planarization insulating layer  4545 . 
     Moreover, reference numeral  4511  denotes a light-emitting element. A first electrode  4517  which is a pixel electrode included in the light-emitting element  4511  is electrically connected to a second terminal of the transistor  4510 . Note that the light-emitting element  4511  has a stacked structure of the first electrode  4517 , a light-emitting layer  4512 , and a second electrode  4513 ; however, the structure of the light-emitting element is not limited to that shown in this embodiment. The structure of the light-emitting element  4511  can be changed as appropriate depending on the direction in which light is extracted from the light-emitting element  4511 , or the like. 
     A bank  4520  is formed using an organic resin film, an inorganic insulating film, or organic polysiloxane. In particular, it is preferable that the bank  4520  be formed using a photosensitive material to have an opening portion over the first electrode  4517 , and a sidewall of the opening portion be formed as an inclined surface with a continuous curvature. 
     The light-emitting layer  4512  may be formed using a single layer or a stack of a plurality of layers. 
     In order to prevent oxygen, hydrogen, moisture, carbon dioxide, or the like from entering the light-emitting element  4511 , a protective layer may be formed over the second electrode  4513  and the bank  4520 . As the protective layer, a silicon nitride layer, a silicon nitride oxide layer, a DLC (diamond like carbon) layer, or the like can be formed. 
     Further, a variety of signals and voltages are supplied to the signal line driver circuits  4503   a  and  4503   b,  the scan line driver circuits  4504   a  and  4504   b,  or the pixel portion  4502  from FPCs  4518   a  and  4518   b.    
     In the light-emitting display device illustrated in  FIGS. 17A and 17B , a connection terminal electrode  4515  is formed using the same conductive film as the conductive film for forming the first electrode  4517  included in the light-emitting element  4511 . A terminal electrode  4516  is formed using the same conductive film as the conductive film for forming the source electrodes and the drain electrodes of the transistors  4509 ,  4510 , and  4555 . 
     The connection terminal electrode  4515  is electrically connected to a terminal of the FPC  4518   a  through an anisotropic conductive layer  4519 . 
     The substrate located in the direction in which light is extracted from the light-emitting element  4511  needs to have a light-transmitting property. In that case, a light-transmitting material such as a glass, a plastic, a polyester film, or an acrylic film is used as the substrate. 
     As the filler  4507 , an ultraviolet curable resin or a thermosetting resin can be used other than an inert gas such as nitrogen or argon. For example, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. In this embodiment, nitrogen is used for the filler  4507 . 
     If necessary, an optical film such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, a polarizing plate or a circularly polarizing plate may be provided with an anti-reflection film. Further, a polarizing plate or a circularly polarizing plate may be provided with an anti-reflection film. 
     As the signal line driver circuits  4503   a  and  4503   b  and the scan line driver circuits  4504   a  and  4504   b,  a driver circuit formed using a single crystal semiconductor layer or a polycrystalline semiconductor layer may be mounted on a substrate separately prepared. Alternatively, only the signal line driver circuits  4503   a  and  4503   b  or part thereof, or the scan line driver circuits  4504   a  and  4504   b,  or part thereof may be separately formed to be mounted. This embodiment is not limited to the structure in  FIGS. 17A and 17B . 
     Through the above steps, a light-emitting display device (display panel) can be manufactured. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 8 
     In this embodiment, as an example of the display device described in Embodiment 5, electronic paper which can display images without an external connection line such as an FPC is described with reference to  FIGS. 18A to 18C  and  FIG. 19 . 
     Note that the electronic paper in this embodiment has a period for holding an image (an image holding period) and a period for rewriting an image (an image rewriting period). In the image holding period, electronic power for holding the image is not needed. Accordingly, the electronic paper is a display device with less power consumption. 
     The electronic paper includes a display element; display can be controlled by application of voltage to the display element and can be maintained in a state where voltage is not applied to the display element. Examples of the display element include an element using electrophoresis (an electrophoretic element), a particle rotation type element using a twisting ball, a particle transfer type element using a charged toner or Electronic Liquid Powder (registered trademark), a magnetophoretic element in which gradation is expressed by magnetism, a liquid transfer type element, a light-scattering element, and a phase-change element. In this embodiment, electronic paper including an electrophoretic element is described as an example of electronic paper. 
     An example of the electrophoretic elements is an element having a microcapsule containing positively charged first particles, negatively charged second particles which exhibit color different from that of the first particles, and liquid serving as a solvent. By application of voltage to the electrophoretic element, the first particles or the second particles are collected on one side of the microcapsule; thus, display can be performed. Note that in a state where voltage is not applied to the electrophoretic element, the first particles and the second particles are not transferred. That is, the display of the electrophoretic element is maintained. Alternatively, as the electrophoretic element, an element having a microcapsule containing positively or negatively charged particles and liquid which exhibits color different from that of the particles and serves as a solvent can be used, for example. 
     Note that for the positively or negatively charged particles in the microcapsule, one or a composite material of a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, or a magnetophoretic material may be used. 
     Next, an example of a structure of the electronic paper in this embodiment is described with reference to  FIGS. 18A to 18C . Note that  FIG. 18A  is a circuit diagram of a pixel of the electronic paper,  FIG. 18B  is a top diagram of the pixel, and  FIG. 18C  is a cross-sectional diagram taken along line A-B of  FIG. 18B . 
     The pixel of the electronic paper in this embodiment includes a transistor  601 , a capacitor  602 , and an electrophoretic element  603  (see  FIG. 18A ). A gate terminal of the transistor  601  is electrically connected to a scan line  630 . A first terminal of the transistor  601  is electrically connected to a signal line  631 . One terminal of the capacitor  602  is electrically connected to a second terminal of the transistor  601 . The other terminal of the capacitor  602  is electrically connected to a common potential line. One terminal of the electrophoretic element  603  is electrically connected to the second terminal of the transistor  601  and one terminal of the capacitor  602 . The other terminal of the electrophoretic element  603  is electrically connected to the common potential line. Note that in this embodiment, a ground potential, 0V, or the like is given as a common potential (V com ). 
     The pixel includes a substrate  600 , the transistor  601  and the capacitor  602  which are provided over the substrate  600 , the electrophoretic element  603  provided over the transistor  601  and the capacitor  602 , and a substrate  604  provided over the electrophoretic element  603  (see  FIGS. 18B and 18C ). Note that the electrophoretic element  603  is not illustrated in  FIG. 18B . 
     The transistor  601  includes a conductive layer  610  which is electrically connected to the scan line  630 , an insulating layer  611  over the conductive layer  610 , a semiconductor layer  612  over the insulating layer  611 , and conductive layers  613  and  614  which are electrically connected to the signal line  631  over the semiconductor layer  612 . Note that the conductive layer  610  serves as a gate terminal, the insulating layer  611  serves as a gate insulating layer, the conductive layer  613  serves as a first terminal, and the conductive layer  614  serves as a second terminal. Further, the conductive layer  610  and the conductive layer  613  can be expressed as part of the scan line  630  and part of the signal line  631 , respectively. 
     The capacitor  602  includes the conductive layer  614 , the insulating layer  611 , and a conductive layer  615  which is electrically connected to a common potential line  632 . Note that the conductive layer  614  serves as one terminal, the insulating layer  611  serves as a dielectric, and the conductive layer  615  serves as the other terminal. The conductive layer  615  can be expressed as part of the common potential line  632 . 
     The electrophoretic element  603  includes a pixel electrode  616  which is electrically connected to the conductive layer  614  in an opening portion provided in an insulating layer  620 , a counter electrode  617  to which the same potential as that of the conductive layer  615  is applied, and a layer  618  which includes charged particles and is provided between the pixel electrode  616  and the counter electrode  617 . Note that the pixel electrode  616  serves as one terminal and the counter electrode  617  serves as the other terminal. 
     In the electronic paper in this embodiment, voltage applied to the layer  618  including the charged particles is controlled; thus, transfer of the charged particles which are dispersed in the layer  618  can be controlled. The counter electrode  617  and the substrate  604  of the electronic paper in this embodiment have a light-transmitting property. That is, the display device in this embodiment is a reflective display device in which the substrate  604  side corresponds to a display surface. 
     Materials which can be used for components of the electronic paper in this embodiment are given below. 
     Examples of the substrate  600  include a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a conductive substrate whose top surface is provided with an insulating layer, flexible substrates such as a plastic substrate, a bonding film, paper containing a fibrous material, and a base film. As examples of the glass substrate, there are a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. As examples of the flexible substrate, there are flexible synthetic resins such as plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and acrylic. 
     As the conductive layers  610  and  615 , the scan line  630 , and the common potential line  632 , an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements, or a nitride containing any of these elements can be used. A stacked structure of these materials can also be used. 
     As the insulating layer  611 , an insulator such as silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, or tantalum oxide can be employed. A stacked structure of these materials can also be used. Note that silicon oxynitride refers to a substance which contains more oxygen than nitrogen and contains oxygen, nitrogen, silicon, and hydrogen at given concentrations ranging from 55 atomic % to 65 atomic %, 1 atomic % to 20 atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %, respectively, where the total percentage of atoms is 100 atomic %. Further, silicon nitride oxide refers to a substance which contains more nitrogen than oxygen and contains oxygen, nitrogen, silicon, and hydrogen at given concentrations ranging from 15 atomic % to 30 atomic %, 20 atomic % to 35 atomic %, 25 atomic % to 35 atomic %, and 15 atomic % to 25 atomic %, respectively, where the total percentage of atoms is 100 atomic %. 
     The semiconductor layer  612  can be formed using any of the following semiconductor material, for example: a material containing an element belonging to Group  14  of the periodic table, such as silicon (Si) or germanium (Ge), as its main component; a compound such as silicon germanium (SiGe) or gallium arsenide (GaAs); oxide such as zinc oxide (ZnO) or zinc oxide containing indium (In) and gallium (Ga); or an organic compound exhibiting semiconductor characteristics can be used. A stacked structure of layers formed using these semiconductor materials can also be used. 
     As the conductive layers  613  and  614 , and the signal line  631 , an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements, or a nitride containing any of these elements can be used. A stacked structure of these materials can also be used. 
     As the insulating layer  620 , an insulator such as silicon oxide, silicon oxynitride, silicon nitride, silicon nitride oxide, aluminum oxide, or tantalum oxide can be used. Further, the insulating layer  620  can also be formed using an organic material such as polyimide, polyamide, polyvinylphenol, benzocyclobutene, acrylic, or epoxy; a siloxane material such as a siloxane resin; an oxazole resin; or the like. Note that a siloxane material corresponds to a material having a Si—O—Si bond. Siloxane has a skeleton structure with a bond of silicon (Si) and oxygen (O). As a substituent, an organic group (e.g., an alkyl group or aromatic hydrocarbon) or a fluoro group may be used. The organic group may include a fluoro group. 
     As the pixel electrode  616 , an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements, or a nitride containing any of these elements can be used. A stacked structure of these materials can also be used. Further, a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added can be used. 
     As the charged particles contained in the layer  618  including charged particles, titanium oxide or the like can be used as positively charged particles, and carbon black or the like can be used as negatively charged particles. In addition, a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, or a magnetophoretic material, or a composite material formed using any of these materials can also be used. 
     As the counter electrode  617 , a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added can be used. 
     As the substrate  604 , a light-transmitting substrate can be used. The light-transmitting substrate is typified by a glass substrate formed using barium borosilicate glass, aluminoborosilicate glass, soda-lime glass, or the like; or a flexible substrate formed using polyethylene terephthalate (PET) or the like. 
     The electronic paper in this embodiment can be used in electronic appliances of various fields, which display information. For example, the electronic paper can be applied to e-book readers (electronic books), posters, advertisements on vehicles such as trains, or displays on a variety of cards such as credit cards. An example of such an electronic appliance is illustrated in  FIG. 19 .  FIG. 19  illustrates an example of an e-book reader. 
     As illustrated in  FIG. 19 , an e-book reader  2700  has two housings  2701  and  2703 . The housings  2701  and  2703  are united with an axis portion  2711 , and the e-book reader  2700  can be opened and closed with the axis portion  2711  as an axis. With such a structure, the e-book reader  2700  can be operated like a paper book. 
     A display portion  2705  is incorporated into the housing  2701 . A display portion  2707  is incorporated into the housing  2703 . The display portions  2705  and  2707  may display one image or different images. When the display portions display different images, text can be displayed on the right display portion (the display portion  2705  in  FIG. 19 ) and an image can be displayed on the left display portion (the display portion  2707  in  FIG. 19 ), for example. 
     Further,  FIG. 19  illustrates an example where the housing  2701  is provided with an operation portion and the like. For example, the housing  2701  is provided with a power supply switch  2721 , operation keys  2723 , a speaker  2725 , and the like. Pages can be turned by the operation key  2723 . Note that a keyboard, a pointing device, or the like may be provided on the same side as the display portion in the housing. Moreover, a terminal for external connection (e.g., an earphone terminal, a USB terminal, and a terminal capable of connecting a variety of cables such as an AC adapter and a USB cable), a portion for inserting recording media, or the like may be provided on a rear surface or a side surface of the housing. Furthermore, the e-book reader  2700  may functions as an electronic dictionary. 
     In addition, the e-book reader  2700  may wirelessly transmit and receive information. The e-book reader  2700  can have a structure where desired book data or the like is wirelessly purchased and downloaded from an e-book server. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     Embodiment 9 
     The display devices described in Embodiments 5 to 8 can be applied to a variety of electronic appliances (including amusement machines). Examples of electronic appliances are television devices (also referred to as televisions or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phone devices (also referred to as mobile phones or cellular phones), portable game machines, portable information terminals, sound reproducing devices, and large game machines such as pachinko machines. 
       FIG. 20A  illustrates an example of a television device. In a television device  9600 , a display portion  9603  is incorporated into a housing  9601 . The display portion  9603  can display an image. Further, the housing  9601  is supported by a stand  9605  here. 
     The television device  9600  can be operated with an operation switch of the housing  9601  or a separate remote controller  9610 . Channels and volume can be controlled with an operation key  9609  of the remote controller  9610  so that an image displayed on the display portion  9603  can be controlled. Further, the remote controller  9610  may be provided with a display portion  9607  for displaying data output from the remote controller  9610 . 
     Note that the television device  9600  is provided with a receiver, a modem, and the like. With the receiver, general television broadcast can be received. Further, when the television device  9600  is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers) data communication can be performed. 
       FIG. 20B  illustrates an example of a digital photo frame. For example, in a digital photo frame  9700 , a display portion  9703  is incorporated into a housing  9701 . The display portion  9703  can display a variety of images. For example, the display portion  9703  can display data of an image taken with a digital camera or the like and function as a normal photo frame. 
     Note that the digital photo frame  9700  is provided with an operation portion, an external connection portion (e.g., a USB terminal, or a terminal which can be connected to various cables such as a USB cable), a recording medium insertion portion, and the like. Although these components may be provided on the surface on which the display portion is provided, it is preferable to provide them on the side surface or the rear surface for the design of the digital photo frame  9700 . For example, a memory storing data of an image taken with a digital camera is inserted in the recording medium insertion portion of the digital photo frame, and the image data can be transferred and then displayed on the display portion  9703 . 
     Further, the digital photo frame  9700  may be configured to transmit and receive data wirelessly. The structure may be employed in which desired image data is transferred wirelessly to be displayed. 
       FIG. 21A  is a portable game machine and includes two housings of a housing  9881  and a housing  9891 , which are connected with a joint portion  9893  so that the portable game machine can be opened and folded. A display portion  9882  is incorporated into the housing  9881 , and a display portion  9883  is incorporated into the housing  9891 . Moreover, the portable game machine illustrated in  FIG. 21A  is provided with a speaker  9884 , a recording medium insertion portion  9886 , an LED lamp  9890 , input means (operation keys  9885 , a connection terminal  9887 , a sensor  9888  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, odor, or infrared ray), and a microphone  9889 ), and the like. It is needless to say that the structure of the portable game machine is not limited to that described above. The portable game machine may have a structure in which additional accessory equipment is provided as appropriate as long as at least a display device is provided. The portable game machine in  FIG. 21A  has a function of reading a program or data stored in a recording medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. Note that a function of the portable game machine in  FIG. 21A  is not limited to those described above, and the portable game machine can have a variety of functions. 
       FIG. 21B  illustrates an example of a slot machine, which is a large amusement machine. In a slot machine  9900 , a display portion  9903  is incorporated into a housing  9901 . Moreover, the slot machine  9900  is provided with operation means such as a start lever and a stop switch, a coin slot, a speaker, and the like. Needless to say, the structure of the slot machine  9900  is not limited to the above structure. The slot machine may have a structure in which additional accessory equipment is provided as appropriate as long as at least the display device described in any of the above embodiments is provided. 
       FIG. 22A  illustrates an example of a mobile phone. A mobile phone  9000  is provided with a display portion  9002  incorporated into a housing  9001 , an operation button  9003 , an external connection port  9004 , a speaker  9005 , a microphone  9006 , and the like. 
     When the display portion  9002  of the mobile phone  9000  illustrated in  FIG. 22A  is touched with a finger or the like, data can be inputted into the mobile phone  9000 . Further, operation such as making calls and texting can be performed by touching the display portion  9002  with a finger or the like. 
     There are mainly three screen modes of the display portion  9002 . The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting data such as text. The third mode is a display-and-input mode, which is a combination of the two modes, that is, a combination of the display mode and the input mode. 
     For example, in the case of making a call or texting, a text input mode mainly for inputting text is selected for the display portion  9002  so that characters displayed on a screen can be inputted. In that case, it is preferable to display a keyboard or number buttons on almost all area of the screen of the display portion  9002 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the mobile phone  9000 , display on the screen of the display portion  9002  can be automatically changed by determining the orientation of the mobile phone  9000  (whether the mobile phone  9000  stands upright or is laid down on its side). 
     The screen modes are changed by touching the display portion  9002  or using the operation button  9003  of the housing  9001 . Alternatively, the screen modes may be changed depending on the kind of the image displayed on the display portion  9002 . For example, when a signal of an image displayed on the display portion  9002  is data of moving images, the screen mode is changed to the display mode. When the signal is text data, the screen mode is changed to the input mode. 
     Further, in the input mode, when input by touching the display portion  9002  is not performed for a certain period while a signal detected by an optical sensor in the display portion  9002  is detected, the screen mode may be controlled so as to be changed from the input mode to the display mode. 
     The display portion  9002  can also function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken when the display portion  9002  is touched with a palm or a finger, whereby personal identification can be performed. Further, when a backlight or a sensing light source which emits near-infrared light is provided in the display portion, an image of a finger vein, a palm vein, or the like can be taken. 
       FIG. 22B  illustrates another example of a mobile phone. The mobile phone in  FIG. 22B  includes a display device  9410  in a housing  9411 , which includes a display portion  9412  and operation buttons  9413 ; and a communication device  9400  in a housing  9401 , which includes scan buttons  9402 , an external input terminal  9403 , a microphone  9404 , a speaker  9405 , and a light-emitting portion  9406  that emits light when receiving a call. The display device  9410  having a display function can be detached from and attached to the communication device  9400  having a telephone function in two directions shown by arrows. Accordingly, short axes of the display device  9410  and the communication device  9400  can be attached to each other, or long axes of the display device  9410  and the communication device  9400  can be attached to each other. Further, when only a display function is necessary, the display device  9410  may be detached from the communication device  9400  so that the semiconductor device  9410  can be used by itself. The communication device  9400  and the display device  9410  can transmit and receive images or input information to/from each other by wireless communication or wired communication, and each of the communication device  9400  and the display device  9410  has a rechargeable battery. 
     Note that this embodiment or part of this embodiment can be freely combined with the other embodiments or part of the other embodiments. 
     This application is based on Japanese Patent Application serial no. 2009-234845 filed with Japan Patent Office on Oct. 9, 2009, the entire contents of which are hereby incorporated by reference. 
     EXPLANATION OF REFERENCE 
       101 : transistor,  102 : transistor,  103 : transistor,  104 : transistor,  105 : transistor,  106 : transistor,  111 : clock signal selection transistor, transistor,  112 : clock signal selection transistor, transistor,  113 : clock signal selection transistor,  114 : clock signal selection transistor,  115 : clock signal selection transistor,  116 : clock signal selection transistor,  121 : low power supply potential selection transistor,  122 : low power supply potential selection transistor,  123 : low power supply potential selection transistor,  124 : low power supply potential selection transistor,  125 : low power supply potential selection transistor,  126 : low power supply potential selection transistor,  131 : transistor,  132 : transistor,  133 : transistor,  134 : transistor,  201 : substrate,  202 : insulating layer,  207 : oxide semiconductor layer,  211 : conductive layer,  213 : oxide semiconductor layer,  214   a : oxide conductive layer,  214   b : oxide conductive layer,  215   a : conductive layer,  215   b : conductive layer,  215   c : conductive layer,  217 : conductive layer,  233   a : resist mask,  233   b : resist mask,  251 : transistor,  252 : transistor,  600 : substrate,  601 : transistor,  602 : capacitor,  603 : electrophoretic element,  604 : substrate,  610 : conductive layer,  611 : insulating layer,  612 : semiconductor layer,  613 : conductive layer,  614 : conductive layer,  615 : conductive layer,  616 : pixel electrode,  617 : counter electrode,  618 : layer including charged particles,  620 : insulating layer,  630 : scan line,  631 : signal line,  632 : common potential line,  701 : pixel portion,  702 : scan line driver circuit,  703 : signal line driver circuit,  704 : pixel,  705 : scan line,  706 : signal line,  804 : scan line,  805 : signal line,  821 : transistor,  822 : liquid crystal element,  823 : capacitor,  851 : transistor,  852 : capacitor,  853 : transistor,  854 : light-emitting element,  855 : scan line,  856 : signal line,  900 : shift register,  901 : level shifter,  902 : buffer,  903 : shift register,  904 : latch circuit,  905 : latch circuit,  906 : level shifter,  907 : buffer,  2000 : substrate,  2001 : conductive layer,  2002 : insulating layer,  2003 : oxide semiconductor layer,  2005   a : conductive layer,  2005   b : conductive layer,  2007 : oxide semiconductor layer,  2008 : conductive layer,  2020 : transparent conductive layer,  2022 : conductive layer,  2023 : conductive layer,  2024 : conductive layer,  2028 : transparent conductive layer,  2029 : transparent conductive layer,  2112 : conductive layer,  2132 : oxide semiconductor layer,  2142   a : oxide conductive layer,  2142   b : oxide conductive layer,  2700 : e-book reader,  2701 : housing,  2703 : housing,  2705 : display portion,  2707 : display portion,  2711 : axis portion,  2721 : power supply switch,  2723 : operation key,  2725 : speaker,  4501 : substrate,  4502 : pixel portion,  4503   a : signal line driver circuit,  4503   b : signal line driver circuit,  4504   a : scan line driver circuit,  4504   b : scan line driver circuit,  4505 : sealing material,  4506 : substrate,  4507 : filler,  4509 : transistor,  4510 : transistor,  4511 : light-emitting element,  4512 : light-emitting layer,  4513 : electrode,  4515 : connection terminal electrode,  4516 : terminal electrode,  4517 : electrode,  4518   a : FPC,  4518   b : FPC,  4519 : anisotropic conductive layer,  4520 : bank,  4540 : conductive layer,  4542 : insulating layer,  4543 : insulating layer,  4544 : insulating layer,  4545 : planarization insulating layer,  4555 : transistor,  7003 : cathode,  7004 : light-emitting layer,  7005 : anode,  7013 : cathode,  7014 : light-emitting layer,  7015 : anode,  7016 : light-blocking layer,  7017 : conductive layer,  7023 : cathode,  7024 : light-emitting layer,  7025 : anode,  7027 : conductive layer,  9000 : mobile phone device,  9001 : housing,  9002 : display portion,  9003 : operation button,  9004 : external connection port,  9005 : speaker,  9006 : microphone,  9400 : communication device,  9401 : housing,  9402 : operation button,  9403 : external input terminal,  9404 : microphone,  9405 : speaker,  9406 : light-emitting portion,  9410 : display device,  9411 : housing,  9412 : display portion,  9413 : operation button,  9600 : television device,  9601 : housing,  9603 : display portion,  9605 : stand,  9607 : display portion,  9609 : operation key,  9610 : remote controller,  9700 : digital photo frame,  9701 : housing,  9703 : display portion,  9881 : housing,  9882 : display portion,  9883 : display portion,  9884 : speaker,  9885 : operation key,  9886 : recording medium insertion portion,  9887 : connection terminal,  9888 : sensor,  9889 : microphone,  9890 : LED lamp,  9891 : housing,  9893 : joint portion,  9900 : slot machine,  9901 : housing,  9903 : display portion