Patent Publication Number: US-10310348-B2

Title: Liquid crystal display device and electronic apparatus having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/904,634, filed Oct. 14, 2010, now allowed, which claims the benefit of foreign priority applications filed in Japan as Serial No. 2009-238916 on Oct. 16, 2009, Serial No. 2009-273913 on Dec. 1, 2009, and Serial No. 2009-278999 on Dec. 8, 2009, all of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to liquid crystal display devices. The present invention relates to electronic apparatuses having the liquid crystal display devices. 
     BACKGROUND ART 
     Thin film transistors formed over a flat plate such as a glass substrate have been manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Thin film transistors manufactured using amorphous silicon have low field effect mobility but can be formed over a large glass substrate. On the other hand, thin film transistors manufactured using crystalline silicon have high field effect mobility, but due to a crystallization step such as laser annealing, such a transistor is not necessarily suitable for being formed over a large glass substrate. 
     In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 discloses a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn-0-based oxide semiconductor as an oxide semiconductor film, and such a transistor is used as, for example, a switching element of a liquid crystal display device. 
     REFERENCE 
     
         
         Patent Document 1: Japanese Published Patent Application No. 2006-165528 
       
    
     DISCLOSURE OF INVENTION 
     It is said that a thin film transistor in which an oxide semiconductor is used to form a channel region achieves higher field effect mobility than a thin film transistor in which amorphous silicon is used to form a channel region. A pixel including such a thin film transistor using an oxide semiconductor is expected to be applied to a display device such as a liquid crystal display device. 
     Each pixel included in a liquid crystal display device is provided with a storage capacitor in which a voltage for controlling the orientation of a liquid crystal element is held. Off-leakage current (hereinafter referred to as off-state current) of a thin film transistor is one factor by which the amount of the holding capacitance is determined. Reduction of the off-state current which leads to increase of the period for holding a voltage in the storage capacitor is important for reduction of power consumption when a still image or the like is displayed. 
     Further, it is important for the enhancement of the added value of a display device to manufacture the display device such that a moving image can be displayed in addition to low power consumption when a still image or the like is displayed. Therefore, it is important that whether an image is a still image or a moving image is determined and display is performed by switching between a still image and a moving image so that power consumption is further reduced by reducing the power consumption when a still image is displayed. 
     Note that in this specification, off-state current is current which flows between a source and a drain when a thin film transistor is in an off state (also called a non-conductive state). In the case of an n-channel thin film transistor (for example, with a threshold voltage of about 0 V to 2 V), the off-state current means a current which flows between a source and a drain when a negative voltage is applied between a gate and the source. 
     Further, in a liquid crystal display device with higher value added, such as a 3D display or a 4k2k display, the area per pixel is expected to be small and the aperture ratio needs to be improved. It is important to reduce the area of the storage capacitor in order to improve the aperture ratio. Accordingly, the off-state current of a thin film transistor needs to be decreased. 
     In view of the foregoing, it is an object of one embodiment of the present invention to provide a liquid crystal display device with power consumption reduced in which an off-state current of a thin film transistor using an oxide semiconductor is reduced in a pixel. 
     An embodiment of the present invention is a liquid crystal display device including: a display panel including a driver circuit portion and a pixel portion in which a transistor including a semiconductor layer using an oxide semiconductor is provided in each pixel; a signal generation circuit for generating a control signal for driving the driver circuit portion and an image signal which is supplied to the pixel portion; a memory circuit for storing the image signal for each frame period; a comparison circuit for detecting a difference of image signals for a series of frame periods among the image signals stored for respective frame periods in the memory circuit; a selection circuit which selects and outputs the image signals for the series of frame periods when the difference is detected in the comparison circuit; and a display control circuit which supplies the control signal and the image signals output from the selection circuit, to the driver circuit portion when the difference is detected in the comparison circuit, and stops supplying the control signal to the driver circuit portion when the difference is not detected in the comparison circuit. 
     The control signal in the liquid crystal display device may be any of a high power supply potential, a low power supply potential, a clock signal, a start pulse signal, and a reset signal. 
     The oxide semiconductor in the liquid crystal display device may have a hydrogen concentration of 1×10 16 /cm 3  or less which is detected by secondary ion mass spectrometry. 
     The oxide semiconductor in the liquid crystal display device may have a carrier density which is less than 1×10 14 /cm 3 . 
     In accordance with the present invention, in a pixel including a thin film transistor using an oxide semiconductor, the off-state current can be reduced. Therefore, the period for holding voltage in a storage capacitor can be extended, so that a liquid crystal display device in which the power consumption when a still image or the like is displayed can be decreased can be provided. Further, the aperture ratio can be improved, so that a liquid crystal display device including a high-definition display portion can be provided. 
     Further, a display device which displays not only a still image but also a moving image can be provided, so that the added value of the display device can be enhanced. Whether an image is a still image or a moving image is determined, and display is performed by switching between a still image and a moving image, so that the power consumption when a still image is displayed can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing one example of a block diagram of a liquid crystal display device; 
         FIGS. 2A to 2C  are diagrams showing one example of a driver circuit. 
         FIG. 3  is a timing chart of a driver circuit. 
         FIGS. 4A to 4C  are diagrams showing one example of a driver circuit. 
         FIGS. 5A and 5B  illustrate a thin film transistor. 
         FIGS. 6A to 6E  illustrate a method for manufacturing a thin film transistor. 
         FIGS. 7A and 7B  illustrate a thin film transistor. 
         FIGS. 8A to 8E  illustrate a method for manufacturing a thin film transistor. 
         FIGS. 9A and 9B  each illustrate a thin film transistor; 
         FIGS. 10A to 10E  illustrate a method for manufacturing a thin film transistor. 
         FIGS. 11A to 11E  illustrate a method for manufacturing a thin film transistor. 
         FIGS. 12A to 12D  illustrate a method for manufacturing a thin film transistor. 
         FIGS. 13A to 13D  illustrate a method for manufacturing a thin film transistor. 
         FIG. 14  illustrates a thin film transistor. 
         FIGS. 15A to 15C  illustrate a liquid crystal panel. 
         FIGS. 16A to 16C  each illustrate an electronic apparatus. 
         FIGS. 17A to 17C  each illustrate an electronic apparatus. 
         FIGS. 18A and 18B  illustrate a display panel and a thin film transistor. 
         FIG. 19  is a diagram for describing Embodiment 13. 
         FIGS. 20A and 20B  are diagrams for describing Embodiment 13. 
         FIGS. 21A and 21B  are diagrams for describing Embodiment 13. 
         FIG. 22  is a diagram for describing Embodiment 13. 
         FIG. 23  is a graph for describing Embodiment 14. 
         FIGS. 24A and 24B  are photographs for describing Embodiment 14. 
         FIGS. 25A and 25B  are graphs for describing Embodiment 14. 
         FIGS. 26A to 26D  are diagrams for describing Embodiment 1. 
         FIG. 27  is a photograph for describing Example 1. 
         FIG. 28  is a graph for describing Example 1. 
         FIG. 29  is a photograph for describing Example 2. 
         FIG. 30  is a graph for describing Example 2. 
         FIG. 31  is a photograph for describing Example 3. 
         FIG. 32  is a graph for describing Example 3. 
         FIG. 33  is a photograph for describing Example 4. 
         FIG. 34  is a diagram for describing Example 5. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments and examples of the present invention will be described with reference to the accompanying drawings. However, it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments and examples. Note that in the structures of the present invention described below, the same portions are denoted by the same reference numerals throughout the drawings. 
     Note that the size, the thickness of a layer, or a region of each structure illustrated in the drawings or the like in the embodiments is exaggerated for simplicity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. 
     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, a block diagram of a display device and a stop sequence and a start sequence of an operation in a driver circuit are described. First, a block diagram of a display device is described using  FIG. 1 . 
     A liquid crystal display device  1000  described in Embodiment 1 includes a display panel  1001 , a signal generation circuit  1002 , a memory circuit  1003 , a comparison circuit  1004 , a selection circuit  1005 , and a display control circuit  1006 . 
     The display panel  1001  includes, for example, a driver circuit portion  1007  and a pixel portion  1008 . A gate line driver circuit  1009 A and a signal line driver circuit  1009 B are included, which are driver circuits for driving the pixel portion  1008  including a plurality of pixels. The gate line driver circuit  1009 A, the signal line driver circuit  1009 B, and the pixel portion  1008  may be formed using transistors formed over one substrate. 
     The gate line driver circuit  1009 A, the signal line driver circuit  1009 B, and the pixel portion  1008  can be formed using n-channel transistors in each of which a semiconductor layer is formed using an oxide semiconductor. The gate line driver circuit  1009 A and/or the signal line driver circuit  1009 B may be formed over the same substrate as the pixel portion or a different substrate. 
     As a display method in the pixel portion  1008 , a progressive method, an interlace method or the like can be employed. Color components controlled in the pixel at the time of color display are not limited to three colors of R, G, and B (R, G, and B correspond to red, green, and blue, respectively); for example, R, G, B, and W (W corresponds to white), or R, G, B, and one or more of yellow, cyan, magenta, and the like can be employed. Further, the size of a display region may be different depending on respective dots of the color components. The present invention is not limited to the application to a display device for color display but can also be applied to a display device for monochrome display. 
     Next, an oxide semiconductor layer used as the semiconductor layer of the transistor included in any of the gate line driver circuit  1009 A, the signal line driver circuit  1009 B, and the pixel portion  1008  is described. 
     As for the oxide semiconductor used in this embodiment, hydrogen is contained at 1×10 16 /cm 3  or less in the oxide semiconductor, and hydrogen or an contained in the oxide semiconductor is removed. An oxide semiconductor film has a carrier density which is less than 1×10 14 /cm 3 , preferably equal to or less than 1×10 12 /cm 3 , and is used to form a channel region of a thin film transistor. In this specification, an oxide semiconductor having a carrier density which is less than 1×10 12 /cm 3  is called an intrinsic (I-type) oxide semiconductor, and an oxide semiconductor having a carrier density equal to or greater than 1×10 12 /cm 3  but equal to or less than 1×10 14 /cm 3  is called a substantially-intrinsic oxide semiconductor. In this specification, the concentration of hydrogen in the oxide semiconductor layer is measured by secondary ion mass spectrometry (SIMS). 
     The number of carriers caused by thermal excitation is negligible in the case where the bandgap of an oxide semiconductor is 2 eV or more, preferably 2.5 eV or more, far preferably 3 eV or more. Therefore, impurities such as hydrogen which may serve as a donor are reduced as much as possible so that the carrier density is less than 1×10 14 /cm 3 , preferably equal to or less than 1×10 12 /cm 3 . That is, the carrier density of the oxide semiconductor layer is reduced as much as possible to be extremely close to zero. 
     Such an oxide semiconductor which is highly purified by removing hydrogen from the oxide semiconductor as much as possible is used for the channel formation region of the thin film transistor, whereby the drain current is equal to or less than 1×10 −13  A at a drain voltage in the range of 1 V to 10 V and a gate voltage in the range of −5 V to −20 V even when the channel width is 10 mm. 
     In the case where a display device is manufactured using such a thin film transistor the off-state current of which is extremely small, the leakage current is reduced, so that a period for holding display data can be extended. 
     Specifically, in a transistor including the above-described oxide semiconductor layer with a channel width of 10 μm, the off-state current per micrometer of the channel width can be equal to or less than 10 aA/μm (1×10 −17  A/μm), and further can be equal to or less than 1 aA/μm (1×10 −18  A/μm). Such a transistor, whose off-state current is extremely small, is used as a transistor included in any of the gate line driver circuit  1009 A, the signal line driver circuit  1009 B, and the pixel portion  1008 , whereby a holding time of an electrical signal such as a video signal can be increased. Since the holding time can be increased, for example, the holding time after the writing of a video signal is set to 10 seconds or more, preferably 30 seconds or more, far preferably one minute or more and less than ten minutes. By increasing the holding time, the interval between writing timings can be increased, so that power consumption can be further suppressed. 
     The resistance to flow of off-state current in a transistor can be referred to as the off-state resistivity. The off-state resistivity is the resistivity of a channel formation region when the transistor is off, which can be calculated from the off-state current. 
     Specifically, the resistance when the transistor is off (off-state resistance R) can be calculated using Ohm&#39;s law from the off-state current and the drain voltage, which leads to the off-state resistivity p which can be calculated using Formula, ρ=RAIL (R is the off-state resistance), from the cross-sectional area A of the channel formation region and the length L of the channel formation region (which corresponds to the distance between a source electrode and a drain electrode). 
     The cross-sectional area A can be calculated from A=dW where the thickness of the channel formation region is d and the channel width is W The length L of the channel formation region is the channel length L. In this manner, the off-state resistivity can be calculated from the off-state current. 
     The off-state resistivity of the transistor including the oxide semiconductor layer in this embodiment is preferably 1×10 9  Ω·m or more, far preferably 1×10 10  Ω·m or more. 
     On the other hand, for example, in the case of a transistor using low-temperature polysilicon, design or the like is performed assuming that the off-state current is about 1×10 −12  A/μm. Therefore, in the transistor including the oxide semiconductor, the holding period of the voltage can be extended to a period about 10 5  times as long as that of the transistor using low-temperature poly silicon when the holding capacitances are equal to each other (about 0.1 pF). Further, in the case of a transistor using amorphous silicon, the off-state current per micrometer of the channel width is 1×10 −13  A/μm or more. Therefore, in the transistor including an oxide semiconductor with high purity, the holding period of the voltage can be extended to a period 10 4  times or more as long as that of the transistor using amorphous silicon when the holding capacitances are equal to each other (about 0.1 pF). 
     For example, in the case of a pixel using the transistor using low-temperature polysilicon, image display is generally performed at 60 frames per second (for 16 msec per frame). The same can be applied to the case of still-image display, and this is because if the rate is decreased (the interval between writing timings is increased), the voltage of the pixel is decreased, which adversely affects the image display. On the other hand, in the case of using the above-described transistor including the oxide semiconductor layer, the holding period per signal writing can be extended to 1600 seconds which is about 10 5  times as long as that of the transistor using low-temperature polysilicon since the off-state current is small. 
     In this manner, still image display can be performed on a display portion even by less frequent writing of image signals. Since the holding period can be extended, the frequency of performing writing of signals can be decreased particularly when a still image is displayed. For example, the number of times of signal writing in a display period of one still image can be one or n (n is greater than or equal to 2 and less than or equal to 10 3 ). Thus, low power consumption of a display device can be achieved. 
     Generally, each pixel is provided with a storage capacitor formed by a pair of electrodes and an insulating layer provided as a dielectric between the pair of electrodes. The size of the storage capacitor can be set considering the off-state current of a transistor provided in each pixel, or the like. In this embodiment, since the transistor including a high-purity oxide semiconductor layer is used as the transistor provided in each pixel, a storage capacitor having capacitance which is less than or equal to ⅓, preferably less than or equal to ⅕ with respect to the liquid crystal capacitance of each pixel is sufficient to be provided. 
     Since the holding period can be long in the above-described transistor including the high-purity oxide semiconductor layer, the frequency of signal writing can be extremely decreased particularly when a still image is displayed. Therefore, the number of times of signal writing to pixels can be reduced in displaying, for example, a still image which involves less frequent switching of display, so that low power consumption can be achieved. 
     In displaying a still image, refresh operation can be performed as appropriate considering the holding rate of the voltage applied to a liquid crystal element during the holding period. For example, the refresh operation can be performed at the time when the voltage in the storage capacitor reaches a predetermined level with respect to a value (initial value) of a voltage which is just after the signal writing into a pixel electrode of the liquid crystal element. It is preferable to set the predetermined level of the voltage such that flicker is not sensed with respect to the initial value. Specifically, it is preferable to perform the refresh operation (rewriting) every time the voltage reaches a voltage which is less than the initial value by 10%, far preferably 3%. 
     In the holding period in displaying a still image, a counter electrode (also called a common electrode) can be made in the floating state. Specifically, a switch may be provided between a power source which supplies a common potential to the counter electrode and the counter electrode, the switch is turned on to supply the common potential from the power source to the counter electrode in a writing period, and then, the switch is turned off to make the counter electrode in the floating state in the holding period. It is preferable to use the transistor including the above-described high-purity oxide semiconductor layer as the switch. 
     The signal generation circuit  1002  is a circuit for generating a signal for driving the gate line driver circuit  1009 A and a signal for driving the signal line driver circuit  1009 B. The signal generation circuit  1002  is also a circuit for outputting a signal for driving the driver circuit portion  1007  through a wiring, and is a circuit for outputting an image signal (also called a video voltage, a video signal, or video data) to the memory circuit  1003  through a wiring. In other words, the signal generation circuit  1002  is a circuit for generating and outputting a control signal for controlling the driver circuit portion  1007  and an image signal to be supplied to the pixel portion  1008 . 
     Specifically, the signal generation circuit  1002  supplies, as control signals, a high power supply potential VDD and a low power supply potential VSS to the gate line driver circuit  1009 A and the signal line driver circuit  1009 B, a start pulse SP and a clock pulse CK for the gate line driver circuit  1009 A, and a start pulse SP and a clock pulse CK for the signal line driver circuit  1009 B. Further, the signal generation circuit  1002  supplies an image signal Data for displaying a moving image or a still image to the memory circuit  1003 . 
     The moving image refers to an image which is recognized as a moving image with human eyes by rapid switch of a plurality of images which are time-divided into a plurality of frames. Specifically, the moving image refers to a series of image signals which are recognized as a moving image with less flicker with human eyes by switching images at least 60 times (60 frames) per second. The still image refers to image signals which do not change in a series of frame periods, for example, in the n-th frame and (n+1)-th frame, unlike the moving image, though a plurality of images which are time-divided into a plurality of frame periods are switched rapidly. 
     The signal generation circuit  1002  may further generates another signal such as an image signal or a latch signal. The signal generation circuit  1002  may output a reset signal Res for stopping the output of the pulse signal of each driver circuit, to the gate line driver circuit  1009 A and/or the signal line driver circuit  1009 B. Each signal may include a plurality of signals such as a first clock signal and a second clock signal. 
     The high power supply potential VDD refers to a potential which is higher than a reference potential, and the low power supply potential VSS refers to a potential which is lower than or equal to the reference potential. It is preferable that the high power supply potential and the low power supply potential are potentials as high as potentials high enough for the transistor to operate. 
     The voltage refers to a potential difference between a given potential and a reference potential (e.g., a ground potential) in many cases. Accordingly, the voltage, the potential, and the potential difference can also be referred to as a potential, a voltage, and a voltage difference, respectively. 
     In the case where the image signal which is output from the signal generation circuit  1002  to the memory circuit  1003  is an analog signal, the analog signal may be converted into a digital signal through an A/D converter or the like to be output to the memory circuit  1003 . 
     The memory circuit  1003  includes a plurality of frame memories  1010  for storing image signals for a plurality of frames. The frame memory may be formed using a memory element such as Dynamic Random Access Memory (DRAM) or Static Random Access Memory (SRAM). 
     The number of frame memories  1010  is not particularly limited as long as an image signal can be stored for each frame period. The image signals of the frame memories  1010  are selectively read out by the comparison circuit  1004  and the selection circuit  1005 . 
     The comparison circuit  1004  is a circuit which selectively reads out image signals in a series of frame periods stored in the memory circuit  1003 , compares the image signals, and detects a difference thereof. An image of the series of frame periods is determined as a moving image in the case where the difference is detected by the comparison of the image signals in the comparison circuit  1004 , and is determined as a still image in the case where the difference is not detected by the comparison of the image signals in the comparison circuit  1004 . That is, whether image signals in a series of frame periods are image signals for displaying a moving image or image signals for displaying a still image is determined by the detection of the difference in the comparison circuit  1004 . The difference obtained by the comparison may be set so as to be determined as a difference to be detected when it is over a predetermined level. 
     The selection circuit  1005  includes a plurality of switches such as thin film transistors, and is a circuit which selects, when image signals for displaying a moving image are determined by the difference detection in the comparison circuit  1004 , the image signals from the frame memories  1010  in which the image signals are stored, and outputs to the display control circuit  1006 . When the difference of image signals between a series of frames compared in the comparison circuit  1004  is not detected, an image displayed in the series of frames is a still image, and in that case, the selection circuit  1005  may output no signal of the image signal of the latter frame to the display control circuit  1006 . 
     The display control circuit  1006  is a circuit which switches supplying and stop of supplying of the image signal and the control signal such as the high power supply potential VDD, the low power supply potential VSS, the start pulse SP, the clock pulse CK, and the reset signal Res to the driver circuit portion  1007 . Specifically, when an image is determined to be a moving image by the comparison circuit  1004 , that is, a difference of image signals in a series of frames is detected, the image signals are supplied from the selection circuit  1005  to the driver circuit portion  1007  through the display control circuit  1006 , and the control signals are supplied to the driver circuit portion  1007  through the display control circuit  1006 . On the other hand, when an image is determined to be a still image by the comparison circuit  1004 , that is, a difference of image signals in a series of frames is not detected, the image signal of the latter frame is not supplied from the selection circuit  1005 , so that the image signal is not supplied to the driver circuit portion  1007  through the display control circuit  1006 , and the display control circuit  1006  stops supplying the control signals to the driver circuit portion  1007 . 
     Note that in the case where the still image is determined, when the period during which an image is assumed to be a still image is short, stop of supplying of the high power supply potential VDD and the low power supply potential VSS among the control signals is not necessarily performed. This is because an increase of the power consumption due to frequent stop and start of supplying of the high power supply potential VDD and the low power supply potential VSS can be reduced, which is preferable. 
     It is preferable that the stop of supplying of the image signals and the control signals is performed entirely in the period for holding an image signal in each pixel in the pixel portion  1008 , and the image signals and the control signals which the display control circuit  1006  supplies before are supplied again, such that the image signal is supplied again after the holding period of each pixel. 
     The supplying of any signal refers to supplying a predetermined potential to a wiring. The stop of supplying of any signal refers to stop of supplying of the predetermined potential to the wiring, and connection to a wiring to which a predetermined fixed potential is supplied, for example, a wiring to which the low power supply potential VSS is supplied. The stop of supplying of any signal also refers to cut of an electrical connection to a wiring to which a predetermined potential is supplied, to make a floating state. 
     As described above, in the thin film transistor including the oxide semiconductor layer, the off-state current can be reduced to less than or equal to 1×10 −12  A/μm, so that the holding period can be extended. Accordingly, a synergistic effect is expected to be generated in reduction of power consumption when a still image is displayed in this embodiment. 
     In this manner, image signals are compared to determine whether an image thereof is a moving image or a still image, and supplying or stop of supplying of control signals such as a clock signal or a start pulse is selectively performed, whereby power consumption can be reduced. 
     Next, an example of a structure of a shift register included in each of the gate line driver circuit  1009 A and the signal line driver circuit  1009 B of the driver circuit portion  1007  is described using  FIGS. 2A to 2C . 
     The shift register shown in  FIG. 2A  includes first to N-th pulse output circuits  10 _ 1  to  10 _N (N is a natural number of 3 or more). A first clock signal CK 1  from a first wiring  11 , a second clock signal CK 2  from a second wiring  12 , a third clock signal CK 3  from a third wiring  13 , and a fourth clock signal CK 4  from a fourth wiring  14  are supplied to the first to the N-th pulse output circuits  10 _ 1  to  10 _N of the shift register shown in  FIG. 2A . A start pulse SP 1  (a first start pulse) from a fifth wiring  15  is input to the first pulse output circuit  10 _ 1 . A signal from the pulse output circuit in the previous stage (the signal called a previous stage signal OUT(n−1)) (n is a natural number of more than or equal to 2 and lower than or equal to/V) is input to the N-th pulse output circuit  10 _N in the second or later stage. A signal from the third pulse output circuit  10 _ 3  in the stage two stages after the first pulse output circuit  10 _ 1  is input to the first pulse output circuit  10 _ 1 ; similarly, a signal from the (N+2)-th pulse output circuit  10 _(n+2) in the stage two stages after the N-th pulse output circuit  10 _N (the signal called a subsequent-stage signal OUT(n+2)) is input to the N-th pulse output circuit. In this manner, a first output signal (corresponding one of OUT(N)(SR) to OUT(N)(SR)) to be input to the pulse output circuit of the next stage and/or the two-stage-previous stage and a second output signal (corresponding one of OUT( 1 ) to OUT(N)) which is input to another circuit or the like are output from each of the pulse output circuits. Note that as shown in  FIG. 2A , the subsequent-stage signal OUT(n+2) is not input to the last two stages of the shift register; therefore, as an example, a second start pulse SP 2  may be input to one of the last two stages of the shift register and a third start pulse SP 3  may be input to the other of the same. Alternatively, signals may be generated inside to be input thereto. For example, a (N+1)-th pulse output circuit  10   (N+1)  and a (N+2)-th pulse output circuit  10   (N+2)  which do not contribute to output of pulses to the display portion (such circuits are also referred to as dummy stages) may be provided, and signals corresponding to the second start pulse (SP 2 ) and the third start pulse (SP 3 ) may be generated in the dummy stages. 
     Note that the first to the fourth clock signals (CK 1 ) to (CK 4 ) each are a signal which oscillates between an H-level signal and an L-level signal at a constant cycle. The first to the fourth clock signals (CK 1 ) to (CK 4 ) are delayed by ¼ period sequentially. In this embodiment, by using the first to fourth clock signals (CK 1 ) to (CK 4 ), control of driving of the pulse output circuit or the like is performed. Note that the clock signal is also called GCK or SCK depending on a driver circuit to which the clock signal is input; however, description is made in this embodiment by using CK as the clock signal. 
     Note that when it is explicitly described that “A and B are connected,” the case where A and B are electrically connected, the case where A and B are functionally connected, and the case where A and B are directly connected are included therein. Here, each of A and B corresponds to an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). Accordingly, other connection relations are included without being limited to a predetermined connection relation, for example, the connection relation shown in the drawings and the texts. 
     Each of the first to N-th pulse output circuits  10 _ 1  to  10 _N includes a first input terminal  21 , a second input terminal  22 , a third input terminal  23 , a fourth input terminal  24 , a fifth input terminal  25 , a first output terminal  26 , and a second output terminal  27  (see  FIG. 2B ). 
     The first input terminal  21 , the second input terminal  22 , and the third input terminal  23  are electrically connected to any of the first to fourth wirings  11  to  14 . For example, in  FIGS. 2A and 2B , the first input terminal  21  of the first pulse output circuit  10 _ 1  is electrically connected to the first wiring  11 , the second input terminal  22  of the first pulse output circuit  10 _ 1  is electrically connected to the second wiring  12 , and the third input terminal  23  of the first pulse output circuit  10 _ 1  is electrically connected to the third wiring  13 . In addition, the first input terminal  21  of the second pulse output circuit  10 _ 2  is electrically connected to the second wiring  12 , the second input terminal  22  of the second pulse output circuit  10 _ 2  is electrically connected to the third wiring  13 , and the third input terminal  23  of the second pulse output circuit  10 _ 2  is electrically connected to the fourth wiring  14 . 
     In  FIGS. 2A and 2B , in the first pulse output circuit  10 _ 1 , the first start pulse SP 1  is input to the fourth input terminal  24 , a subsequent-stage signal OUT( 3 ) is input to the fifth input terminal  25 , the first output signal OUT( 1 )(SR) is output from the first output terminal  26 , and the second output signal OUT( 1 ) is output from the second output terminal  27 . 
     Next, an example of a specific circuit structure of the pulse output circuit is described with reference to  FIG. 2C . 
     In  FIG. 2C , a first terminal of the first transistor  31  is electrically connected to the power supply line  51 , a second terminal of the first transistor  31  is electrically connected to a first terminal of the ninth transistor  39 , and a gate electrode of the first transistor  31  is electrically connected to the fourth input terminal  24 . A first terminal of the second transistor  32  is electrically connected to the power supply line  52 , a second terminal of the second transistor  32  is electrically connected to the first terminal of the ninth transistor  39 , and a gate electrode of the second transistor  32  is electrically connected to a gate electrode of the fourth transistor  34 . A first terminal of the third transistor  33  is electrically connected to the first input terminal  21 , and a second terminal of the third transistor  33  is electrically connected to the first output terminal  26 . A first terminal of the fourth transistor  34  is electrically connected to the power supply line  52 , and a second terminal of the fourth transistor  34  is electrically connected to the first output terminal  26 . A first terminal of the fifth transistor  35  is electrically connected to the power supply line  52 , a second terminal of the fifth transistor  35  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 , and a gate electrode of the fifth transistor  35  is electrically connected to the fourth input terminal  24 . A first terminal of the sixth transistor  36  is electrically connected to the power supply line  51 , a second terminal of the sixth transistor  36  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 , and a gate electrode of the sixth transistor  36  is electrically connected to the fifth input terminal  25 . A first terminal of the seventh transistor  37  is electrically connected to the power supply line  51 , a second terminal of the seventh transistor  37  is electrically connected to a second terminal of the eighth transistor  38 , and a gate electrode of the seventh transistor  37  is electrically connected to the third input terminal  23 . A first terminal of the eighth transistor  38  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 , and a gate electrode of the eighth transistor  38  is electrically connected to the second input terminal  22 . The first terminal of the ninth transistor  39  is electrically connected to the second terminal of the first transistor  31  and the second terminal of the second transistor  32 , a second terminal of the ninth transistor  39  is electrically connected to the gate electrode of the third transistor  33  and the gate electrode of the tenth transistor  40 , and a gate electrode of the ninth transistor  39  is electrically connected to the power supply line  51 . A first terminal of the tenth transistor  40  is electrically connected to the first input terminal  21 , a second terminal of the tenth transistor  40  is electrically connected to the second output terminal  27 , and the gate electrode of the tenth transistor  40  is electrically connected to the second terminal of the ninth transistor  39 . A first terminal of the eleventh transistor  41  is electrically connected to the power supply line  52 , a second terminal of the eleventh transistor  41  is electrically connected to the second output terminal  27 , and a gate electrode of the eleventh transistor  41  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 . 
     In  FIG. 2C , a connection point of the gate electrode of the third transistor  33 , the gate electrode of the tenth transistor  40 , and the second terminal of the ninth transistor  39  is referred to as a node NA. In addition, a connection point of the gate electrode of the second transistor  32 , the gate electrode of the fourth transistor  34 , the second terminal of the fifth transistor  35 , the second terminal of the sixth transistor  36 , the first terminal of the eighth transistor  38 , and the gate electrode of the eleventh transistor  41  is referred to as a node NB. 
     In the case where the pulse output circuit in  FIG. 2C  is the first pulse output circuit  10 _ 1 , the first clock signal CK 1  is input to the first input terminal  21 , the second clock signal CK 2  is input to the second input terminal  22 , the third clock signal CK 3  is input to the third input terminal  23 , the start pulse SP is input to the fourth input terminal  24 , a subsequent-stage signal OUT( 3 ) is input to the fifth input terminal  25 , the first output signal OUT( 1 )(SR) is output from the first output terminal  26 , and the second output signal OUT( 1 ) is output from the second output terminal  27 . 
       FIG. 3  shows a timing chart of a shift register including the plurality of pulse output circuits shown in  FIG. 2C . In the case where the shift register is a scan line driver circuit, a period  61  in  FIG. 3  is a vertical retrace period and a period  62  is a gate selection period. 
     The order of supplying or stop of supplying of potentials of wirings in the driver circuit including a plurality of n-channel transistors described as an example in  FIGS. 2A to 2C  and  FIG. 3  in the case where a still image and a moving image are displayed is described below. 
     First, in the case where operation of the driver circuit portion  1007  is stopped, supplying of the start pulse SP is stopped by the display control circuit  1006 . Next, after the supplying of the start pulse SP is stopped, supplying of each clock signal CK is stopped after pulse output reaches the last stage of the shift register. Then, supplying of the high power supply potential VDD and the low power supply potential VSS of the power supply voltage is stopped (see  FIG. 26A ). In the case where the operation of the driver circuit portion  1007  is started, first, the display control circuit  1006  supplies the high power supply potential VDD and the low power supply potential VSS of the power supply voltage to the driver circuit portion  1007 . Then, each clock signal CK is supplied, and then, supplying of the start pulse SP is started (see  FIG. 26B ). 
     In the description of  FIGS. 2A to 2C  and  FIG. 3 , the reset signal Res is not supplied to the driver circuit. A structure to which the reset signal Res is supplied is shown and described in  FIGS. 4A to 4C . 
     A shift register shown in  FIG. 4A  includes first to N-th pulse output circuits  10 _ 1  to  10 _N (N is a natural number of 3 or more). A first clock signal CK 1  from a first wiring  11 , a second clock signal CK 2  from a second wiring  12 , a third clock signal CK 3  from a third wiring  13 , and a fourth clock signal CK 4  from a fourth wiring  14  are supplied to the first to the N-th pulse output circuits  10 _ 1  to  10 _N of the shift register shown in  FIG. 4A . A start pulse SP 1  (a first start pulse) from a fifth wiring  15  is input to the first pulse output circuit  10 _ 1 . A signal from the pulse output circuit in the previous stage (the signal called a previous stage signal OUT(n−1)) (n is a natural number of more than or equal to 2 and less than or equal to/V) is input to the N-th pulse output circuit  10 _N in the second or later stage. A signal from the third pulse output circuit  10 _ 3  in the stage two stages after the first pulse output circuit  10 _ 1  is input to the first pulse output circuit  10 _ 1 ; similarly, a signal from the (N+2)-th pulse output circuit  10 _(N+2) in the stage two stages after the N-th pulse output circuit  10 _N (the signal called a subsequent-stage signal OUT(n+2)) is input to the N-th pulse output circuit. In this manner, a first output signal (corresponding one of OUT( 1 )(SR) to OUT(N)(SR)) to be input to the pulse output circuit of the next stage and/or the two-stage-previous stage and a second output signal (corresponding one of OUT( 1 ) to OUT(N)) which is input to another circuit or the like are output from each of the pulse output circuits. To the pulse output circuit in each stage, a reset signal Res is supplied from a sixth wiring  16 . 
     The pulse output circuit shown in  FIGS. 4A to 4C  is different from the pulse output circuit shown in  FIGS. 2A to 2C  in that the sixth wiring  16  for supplying the reset signal Res is provided; the other portions is as described in  FIGS. 2A to 2C . 
     Each of the first to N-th pulse output circuits  10 _ 1  to  10 _N includes a first input terminal  21 , a second input terminal  22 , a third input terminal  23 , a fourth input terminal  24 , a fifth input terminal  25 , a first output terminal  26 , a second output terminal  27 , and a sixth input terminal  28  (see  FIG. 4B ). 
     The first input terminal  21 , the second input terminal  22 , and the third input terminal  23  are electrically connected to any of the first to fourth wirings  11  to  14 . For example, in  FIGS. 4A and 4B , the first input terminal  21  of the first pulse output circuit  10 _ 1  is electrically connected to the first wiring  11 , the second input terminal  22  of the first pulse output circuit  10 _ 1  is electrically connected to the second wiring  12 , and the third input terminal  23  of the first pulse output circuit  10 _ 1  is electrically connected to the third wiring  13 . In addition, the first input terminal  21  of the second pulse output circuit  10 _ 2  is electrically connected to the second wiring  12 , the second input terminal  22  of the second pulse output circuit  10 _ 2  is electrically connected to the third wiring  13 , and the third input terminal  23  of the second pulse output circuit  10 _ 2  is electrically connected to the fourth wiring  14 . 
     In  FIGS. 4A and 4B , in the first pulse output circuit  10 _ 1 , the first start pulse SP 1  is input to the fifth input terminal  24 , the subsequent-stage signal OUT( 3 ) is input to the fourth input terminal  25 , the first output signal OUT( 1 )(SR) is output from the first output terminal  26 , the second output signal OUT( 1 ) is output from the second output terminal  27 , and the reset signal Res is input from the sixth input terminal  28 . 
     Next, an example of a specific circuit structure of the pulse output circuit is described with reference to  FIG. 4C . 
     In  FIG. 4C , a first terminal of the first transistor  31  is electrically connected to the power supply line  51 , a second terminal of the first transistor  31  is electrically connected to a first terminal of the ninth transistor  39 , and a gate electrode of the first transistor  31  is electrically connected to the fourth input terminal  24 . A first terminal of the second transistor  32  is electrically connected to the power supply line  52 , a second terminal of the second transistor  32  is electrically connected to the first terminal of the ninth transistor  39 , and a gate electrode of the second transistor  32  is electrically connected to a gate electrode of the fourth transistor  34 . A first terminal of the third transistor  33  is electrically connected to the first input terminal  21 , and a second terminal of the third transistor  33  is electrically connected to the first output terminal  26 . A first terminal of the fourth transistor  34  is electrically connected to the power supply line  52 , and a second terminal of the fourth transistor  34  is electrically connected to the first output terminal  26 . A first terminal of the fifth transistor  35  is electrically connected to the power supply line  52 , a second terminal of the fifth transistor  35  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 , and a gate electrode of the fifth transistor  35  is electrically connected to the fourth input terminal  24 . A first terminal of the sixth transistor  36  is electrically connected to the power supply line  51 , a second terminal of the sixth transistor  36  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 , and a gate electrode of the sixth transistor  36  is electrically connected to the fifth input terminal  25 . A first terminal of the seventh transistor  37  is electrically connected to the power supply line  51 , a second terminal of the seventh transistor  37  is electrically connected to a second terminal of the eighth transistor  38 , and a gate electrode of the seventh transistor  37  is electrically connected to the third input terminal  23 . A first terminal of the eighth transistor  38  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 , and a gate electrode of the eighth transistor  38  is electrically connected to the second input terminal  22 . The first terminal of the ninth transistor  39  is electrically connected to the second terminal of the first transistor  31  and the second terminal of the second transistor  32 , a second terminal of the ninth transistor  39  is electrically connected to the gate electrode of the third transistor  33  and the gate electrode of the tenth transistor  40 , and a gate electrode of the ninth transistor  39  is electrically connected to the power supply line  51 . A first terminal of the tenth transistor  40  is electrically connected to the first input terminal  21 , a second terminal of the tenth transistor  40  is electrically connected to the second output terminal  27 , and the gate electrode of the tenth transistor  40  is electrically connected to the second terminal of the ninth transistor  39 . A first terminal of the eleventh transistor  41  is electrically connected to the power supply line  52 , a second terminal of the eleventh transistor  41  is electrically connected to the second output terminal  27 , and a gate electrode of the eleventh transistor  41  is electrically connected to the gate electrode of the second transistor  32  and the gate electrode of the fourth transistor  34 . The gate electrode of the second transistor  32 , the gate electrode of the fourth transistor  34 , the second terminal of the fifth transistor  35 , the second terminal of the sixth transistor  36 , the first terminal of the eighth transistor  38 , and the gate electrode of the eleventh transistor  41  are electrically connected to a wiring  53  for supplying the reset signal Res. The reset signal Res is a signal by which a signal with a high power supply potential level is supplied to the gate electrode of the second transistor  32 , the gate electrode of the fourth transistor  34 , the second terminal of the fifth transistor  35 , the second terminal of the sixth transistor  36 , the first terminal of the eighth transistor  38 , and the gate electrode of the eleventh transistor  41  to reduce the output from the pulse output circuit to a signal with a low power supply potential level. 
     In  FIG. 4C , a connection point of the gate electrode of the third transistor  33 , the gate electrode of the tenth transistor  40 , and the second terminal of the ninth transistor  39  is referred to as a node NA. In addition, a connection point of the gate electrode of the second transistor  32 , the gate electrode of the fourth transistor  34 , the second terminal of the fifth transistor  35 , the second terminal of the sixth transistor  36 , the first terminal of the eighth transistor  38 , and the gate electrode of the eleventh transistor  41  is referred to as a node NB. 
     In the case where the pulse output circuit in  FIG. 4C  is the first pulse output circuit  10 _ 1 , the first clock signal CK 1  is input to the first input terminal  21 , the second clock signal CK 2  is input to the second input terminal  22 , the third clock signal CK 3  is input to the third input terminal  23 , the start pulse SP is input to the fourth input terminal  24 , a subsequent-stage signal OUT( 3 ) is input to the fifth input terminal  25 , the first output signal OUT( 1 )(SR) is output from the first output terminal  26 , the second output signal OUT( 1 ) is output from the second output terminal  27 , and the reset signal Res is input from the sixth input terminal  28 . 
     The timing chart of the shift register including a plurality of pulse output circuits shown in  FIG. 4C  is similar to that of  FIG. 2C , shown in  FIG. 3 . 
     The order of supplying or stop of supplying of potentials of wirings in the driver circuit including a plurality of n-channel transistors described as an example in  FIGS. 4A to 4C  in the case where a still image or a moving image is displayed are described below. 
     First, in the case where operation of the driver circuit portion  1007  is stopped, supplying of the start pulse SP is stopped by the display control circuit  1006 . Next, after the supplying of the start pulse SP is stopped, supplying of each clock signal CK is stopped after pulse output reaches the last stage of the shift register. Then, the reset signal Res is supplied. Next, supplying of the high power supply potential VDD and the low power supply potential VSS of the power supply voltage is stopped (see  FIG. 26C ). In the case where the operation of the driver circuit portion  1007  is started, first, the display control circuit  1006  supplies the high power supply potential VDD and the low power supply potential VSS of the power supply voltage to the driver circuit portion  1007 . Then, the reset signal Res is supplied. Next, each clock signal CK is supplied, and then, supplying of the start pulse SP is started (see  FIG. 26D ). 
     The structure as described in  FIGS. 4A to 4C  in which the reset signal is supplied in addition to the structure shown in  FIGS. 2A to 2C  and  FIG. 3  is preferable because malfunction due to signal delay at the time of switching between a still image and a moving image or the like can be reduced. 
     In the case where a still image is displayed, a common potential electrode provided over the thin film transistor included in the driver circuit portion may be cut off from a common potential line to be made in the floating state. Then, after the still-image mode, in the case where operation of the driver circuit is started again, the common potential electrode is connected to the common potential line. Accordingly, malfunction of the thin film transistor in the driver circuit portion can be prevented. 
       FIG. 18A  illustrates a display panel  1800  having such a structure, and  FIG. 18B  is a view for describing the cross-sectional structure thereof. The display panel  1800  includes driver circuits  1802  and  1804  and a pixel portion  1806 . A common potential electrode  1808  is provided so as to overlap the driver circuit  1802 . A switch  1810  for controlling connection/non-connection between the common potential electrode  1808  and a common potential terminal  1812  is provided therebetween. 
     The common potential electrode  1808  is provided over a TFT  1803  of the driver circuit as shown in  FIG. 18B , thereby shielding the TFT  1803  from static electricity, so that a change of the threshold voltage or generation of a parasitic channel is prevented. 
     The same structure as the TFT  1803  can be used as the switch  1810 . Such an element in which the leakage current in the off-state is extremely small contributes to stabilization of operation of the display panel. That is, in the case where a still image is displayed, even when the switch  1803  is turned off to make the common potential electrode in the floating state, the potential can be kept constant. 
     In this manner, by using the TFT formed using an oxide semiconductor having wide bandgap and providing the common potential electrode to shield the external electric field, a still image can be displayed even in the state where the operation of the driver circuit is stopped. Further, by controlling the potential of the common potential electrode as appropriate in accordance with the operation of the driver circuit, the operation of the display panel can be stabilized. 
     As described above, by using a feature of less off-state current of the thin film transistor using an oxide semiconductor, for a liquid crystal display device, a period for holding voltage in a storage capacitor can be extended, and power consumption when a still image or the like is displayed can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 1 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 2 
     A thin film transistor of this embodiment and an embodiment of a method for manufacturing the thin film transistor are described using  FIGS. 5A and 5B  and  FIGS. 6A to 6E . 
     In Embodiment 2, an example of a thin film transistor which can be applied to a liquid crystal display device described in this specification will be described. A thin film transistor  410  described in Embodiment 2 can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
       FIG. 5A  illustrates an example of a planar structure of the thin film transistor, and  FIG. 5B  illustrates an example of a cross-sectional structure thereof. The thin film transistor  410  shown in  FIGS. 5A and 5B  is a top-gate thin film transistor. 
       FIG. 5A  is a plane view of the top-gate thin film transistor  410  and  FIG. 5B  is a cross-sectional view along line C 1 -C 2  in  FIG. 5A . 
     The thin film transistor  410  includes over a substrate  400  having an insulating surface, an insulating layer  407 , an oxide semiconductor layer  412 , a source and drain electrode layers  415   a  and  415   b , a gate insulating layer  402 , and a gate electrode layer  411 . Wiring layers  414   a  and  414   b  are provided in contact with the source and drain electrode layers  415   a  and  415   b , respectively to be electrically connected thereto. 
     The thin film transistor  410  is described as a single-gate thin film transistor; a multi-gate thin film transistor including a plurality of channel formation regions can be formed when needed. 
     A process for manufacturing the thin film transistor  410  over the substrate  400  is described below with reference to  FIGS. 6A to 6E . 
     There is no particular limitation on a substrate which can be used as the substrate  400  having an insulating surface as long as the substrate has enough heat resistance to a heat treatment to be performed later. 
     As the substrate  400 , a glass substrate whose strain point is higher than or equal to 730° C. may be used when the temperature of the heat treatment performed later is high. As a material of the glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. Note that by containing a larger amount of barium oxide (BaO) than boron oxide, a heat-resistant glass substrate which is of more practical use can be formed. Therefore, it is preferable that a glass substrate containing a larger amount of BaO than B 2 O 3  be used. 
     Note that a substrate formed using an insulator such as a ceramic substrate, a quartz substrate, or a sapphire substrate may be used instead of the above-described glass substrate, as the substrate  400 . Alternatively, a crystallized glass substrate or the like may be used. Further alternatively, a plastic substrate or the like may be used. 
     First, the insulating layer  407  which functions as a base film is formed over the substrate  400  having an insulating surface. It is preferable that an oxide insulating layer such as a silicon oxide layer, a silicon oxynitride layer, an aluminum oxide layer, or an aluminum oxynitride layer be used as the insulating layer  407  which is in contact with the oxide semiconductor layer. The insulating layer  407  can be formed by a plasma CVD method, a sputtering method, or the like. It is preferable to form the insulating layer  407  by a sputtering method in order not to contain hydrogen in the insulating layer  407 . 
     In this embodiment, a silicon oxide layer is formed as the insulating layer  407  by a sputtering method. The substrate  400  is transferred into a chamber, a sputtering gas containing high-purity oxygen in which hydrogen and moisture are removed is introduced into the chamber, and a target is used, so that the silicon oxide layer is deposited on the substrate  400  as the insulating layer  407 . The substrate  400  may be at room temperature or may be heated. 
     For example, a silicon oxide film is formed as follows: quartz (preferably quart) is used as the target; the substrate temperature is 108° C.; the distance between the target and the substrate (T-S distance) is 60 mm; the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; the atmosphere is oxygen and argon (flow rate ratio of oxygen to argon is 25 sccm:25 sccm=1:1); and an RF sputtering method is used. The thickness of the silicon oxide film is 100 nm in this embodiment. A silicon target may be used instead of the quartz (preferably quart) to form the silicon oxide film. As a sputtering gas, oxygen or a mixed gas of oxygen and argon is used in this embodiment. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the insulating layer  407 . This is in order to prevent the insulating layer  407  from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cry opump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the insulating layer  407  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the insulating layer  407 . 
     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 in which a DC power source is used, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. The RF sputtering method is mainly used in the case where an insulating film is formed, and the DC sputtering method is mainly used in the case where a metal film is formed. 
     A multi-target sputtering apparatus in which a plurality of targets which are formed of different materials from each other can be set may be used. With the multi-target sputtering apparatus, films of different materials can be stacked to be formed in the same chamber, or plural kinds of materials can be deposited by electric discharge at the same time in the same chamber. 
     Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge may be used. 
     Further, as the deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, or a bias sputtering method in which a voltage is also applied to a substrate during deposition may be used. 
     The insulating layer  407  may have a stacked-layer structure; for example, a stacked-layer structure in which a nitride insulating layer such as a silicon nitride layer, a silicon nitride oxide layer, an aluminum nitride layer, or an aluminum nitride oxide layer and the above-described oxide insulating layer are stacked in this order over the substrate  400  may be used. 
     For example, a silicon nitride layer is formed between the silicon oxide layer and the substrate  400  by introducing a sputtering gas containing high-purity nitrogen in which hydrogen and moisture are removed and using a silicon target. In that case also, it is preferable to remove residual moisture in the chamber in the formation of the silicon nitride layer as is the case of the deposition of the silicon oxide layer. 
     The substrate may be heated at the time of the film deposition of the silicon nitride layer. 
     In the case where the silicon nitride layer and the silicon oxide layer are stacked to form the insulating layer  407 , the silicon nitride layer and the silicon oxide layer can be formed in the same chamber with the same silicon target. For example, first, a sputtering gas containing nitrogen is introduced and a silicon target placed inside the chamber is used to form the silicon nitride layer, and then, the sputtering gas is switched to a sputtering gas containing oxygen and the same silicon target is used to form the silicon oxide layer. Since the silicon nitride layer and the silicon oxide layer can be formed in succession without exposure to the air, an impurity such as hydrogen or moisture can be prevented from being adsorbed on a surface of the silicon nitride layer. 
     Next, an oxide semiconductor film is formed to a thickness of greater than or equal to 2 nm and less than or equal to 200 nm over the insulating layer  407 . 
     In order for the oxide semiconductor film not to contain impurities such as hydrogen, a hydroxyl group, and moisture as much as possible, it is preferable to preheat the substrate  400  provided with the insulating layer  407  in a preheating chamber of the sputtering apparatus before the film formation so that an impurity such as hydrogen or moisture adsorbed on the substrate  400  is eliminated, and perform exhaustion. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. This preheating step is not necessarily performed. 
     Note that before the oxide semiconductor film is formed by a sputtering method, it is preferable to perform reverse sputtering in which an argon gas is introduced and plasma is generated so that dust on a surface of the insulating layer  407  is removed. The reverse sputtering is a method by which voltage is applied to a substrate side with a high-frequency power source in an argon atmosphere to generate plasma on the substrate side without applying voltage to a target side, so that a surface is modified. Instead of the argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. 
     The oxide semiconductor film is formed by the sputtering method. The oxide semiconductor film is formed using 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, a Sn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, an In—Zn—O-based oxide semiconductor film, a Sn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxide semiconductor film, an In—O-based oxide semiconductor film, a Sn—O-based oxide semiconductor film, or a Zn—O-based oxide semiconductor film. In this embodiment, the oxide semiconductor film is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. Specifically, a target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [mol %] (that is, In:Ga:Zn=1:1:0.5 [atom %]) is used. Alternatively, a target having a composition ratio of In:Ga:Zn=1:1:1 [atom %] or In:Ga:Zn=1:1:2 [atom %] can be used. In this embodiment, the filling rate of the oxide semiconductor target is equal to or greater than 90% and equal to or less than 100%, preferably equal to or greater than 95% and equal to or less than 99.9%. With use of the oxide semiconductor target having high filling rate, the deposited oxide semiconductor film has high density. The atmosphere in the sputtering may be an atmosphere of a rare gas (typically argon), an atmosphere of oxygen, or a mixed atmosphere of a rare gas and oxygen. The target may contain SiO 2  at 2 wt % or more and 10 wt % or less. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide semiconductor film. 
     The oxide semiconductor film is formed over the substrate  400  as follows: the substrate is held in the chamber with pressure reduced, residual moisture in the chamber is removed, a sputtering gas from which hydrogen and moisture are removed is introduced, and the above-described target is used. In order to remove the residual moisture in the chamber, it is preferable to use an adsorption-type vacuum pump. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed using a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), a compound including a carbon atom, or the like is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film formed in the chamber can be reduced. The substrate may be heated at the time of the film deposition of the oxide semiconductor film. 
     As an example of the film deposition condition, the following condition is employed: the temperature of the substrate is room temperature; the distance between the substrate and the target is 110 mm; the pressure is 0.4 Pa; the direct current (DC) power is 0.5 kW; and the atmosphere is oxygen and argon (the flow rate ratio of oxygen to argon is 15 sccm:30 sccm). It is preferable that a pulsed direct current (DC) power supply be used because powder substances (also referred to as particles or dust) generated in the film deposition can be reduced and the film thickness can be made uniform. The oxide semiconductor film has a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm. Note that appropriate thickness of the oxide semiconductor film varies depending on a material thereof; therefore, the thickness may be determined as appropriate depending on the material. 
     Next, the oxide semiconductor film is processed into the island-shaped oxide semiconductor layer  412  by a first photolithography step (see  FIG. 6A ). A resist mask for forming the island-shaped oxide semiconductor layer  412  may be formed using an ink jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     For the etching of the oxide semiconductor film, either one or both of wet etching and dry etching may be employed. 
     As an etching gas for the dry etching, a gas containing chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon chloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. 
     Alternatively, a gas containing fluorine (fluorine-based gas such as carbon tetrafluoride (CF 4 ), sulfur fluoride (SF 6 ), nitrogen fluoride (NF 3 ), or trifluoromethane (CHF 3 )); hydrogen bromide (HBr); oxygen (O 2 ); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used. 
     As the dry etching method, a parallel-plate RIE (reactive ion etching) method or an ICP (inductively coupled plasma) etching method can be used. In order to etch the layer into a desired shape, the etching conditions (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) are adjusted as appropriate. 
     As an etchant used for wet etching, a mixed solution of phosphoric acid, acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), an ammonium hydroxide/hydrogen peroxide mixture (a 31 wt % hydrogen peroxide solution: 28 wt % ammonia water:water=5:2:2), or the like can be used. ITO07N (produced by KANTO CHEMICAL CO., INC.) may be used. 
     After the wet etching, the etchant is removed by cleaning together with the material which is etched off. Waste liquid of the etchant containing the removed material may be purified and the material contained in the waste liquid may be reused. The resources can be efficiently used and the cost can be reduced by collecting and reusing a material such as indium included in the oxide semiconductor from the waste liquid after the etching. 
     The etching conditions (such as an etchant, etching time, or temperature) are appropriately adjusted depending on a material so that the material can be etched into a desired shape. 
     In this embodiment, the oxide semiconductor film is processed into the island-shaped oxide semiconductor layer  412  by a wet etching method using a solution obtained by mixing phosphoric acid, acetic acid, and nitric acid. 
     In this embodiment, a first heat treatment is performed on the oxide semiconductor layer  412 . The temperature of the first heat treatment is higher than or equal to 400° C. and lower than or equal to 750° C., and higher than or equal to 400° C. and lower than the strain point of the substrate  400  when the strain point of the substrate  400  is lower than or equal to 750° C. In this embodiment, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layer at 450° C. for one hour in a nitrogen atmosphere, and then, the temperature is reduced to room temperature and water or hydrogen is prevented from entering the oxide semiconductor layer, without exposure to the air; thus, an oxide semiconductor layer is obtained. The oxide semiconductor layer  412  can be dehydrated or dehydrogenated by the first heat treatment. 
     The heat treatment apparatus is not limited to an electric furnace and may be provided with a device that heats an object to be processed by thermal conduction or thermal radiation from a heater such as a resistance heater or the like. For example, an RTA (rapid thermal annaling) apparatus such as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermal annealing) apparatus can be used. The 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. The GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with the object to be processed, by heat treatment, such as nitrogen or a rare gas such as argon is used. 
     For example, as the first heat treatment, GRTA may be performed as follows: the substrate is transferred into an inert gas heated to a high temperature of 650° C. to 700° C., heated for several minutes, and transferred and taken out of the inert gas heated to the high temperature. GRTA enables a high-temperature heat treatment for a short time. 
     In the first heat treatment, it is preferable that water, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. It is preferable that nitrogen or a rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus have a purity of 6N (99.9999%) or more, far preferably 7N (99.99999%) or more (that is, the concentration of impurities be 1 ppm or less, far preferably 0.1 ppm or less). 
     Further, depending on the condition of the first heat treatment or a material of the oxide semiconductor layer, the oxide semiconductor layer  412  might be crystallized to be a microcrystalline film or a polycrystalline film. For example, the oxide semiconductor layer may be crystallized to be a microcrystalline oxide semiconductor film in which the degree of crystallization is greater than or equal to 90% or greater than or equal to 80%. Further, depending on the condition of the first heat treatment or the material of the oxide semiconductor layer, the oxide semiconductor layer  412  may be an amorphous oxide semiconductor film which does not contain crystalline components. The oxide semiconductor layer may become an oxide semiconductor film in which a microcrystalline portion (with a grain diameter greater than or equal to 1 nm and less than or equal to 20 nm, typically greater than or equal to 2 nm and less than or equal to 4 nm) is mixed into an amorphous oxide semiconductor. 
     The first heat treatment of the oxide semiconductor layer can also be performed on the oxide semiconductor film before being processed into the island-like oxide semiconductor layer  412 . In that case, the substrate is taken out from the heat apparatus after the first heat treatment, and then a photolithography step is performed thereon. 
     The example in which the heat treatment for dehydration and/or dehydrogenation on the oxide semiconductor layer is performed just after the formation of the oxide semiconductor layer  412  is described above. However, the heat treatment for dehydration and/or dehydrogenation may be performed after a source electrode and a drain electrode are stacked on the oxide semiconductor layer or after a gate insulating layer is formed over a source electrode and a drain electrode as long as it is performed after the deposition of the oxide semiconductor layer. 
     A conductive film is formed over the insulating layer  407  and the oxide semiconductor layer  412 . The conductive film may be formed by a sputtering method or a vacuum evaporation method. As a material of the conductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy containing any of these elements as a component; an alloy film containing any of these elements in combination; and the like can be given. Further, one or more materials selected from manganese, magnesium, zirconium, beryllium, and yttrium may be used. Further, the conductive film may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum (Al) and one or a plurality of elements selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may be used. In this embodiment, a titanium film with a thickness of 150 nm is formed as the conductive film by a sputtering method. 
     Next, a resist mask is formed over the conductive film by a second photolithography step. The resist mask may be formed using an ink jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. After that, etching is selectively performed thereon, so that the source and drain electrode layers  415   a  and  415   b  are formed, and then, the resist mask is removed (see  FIG. 6B ). It is preferable that an end portion of the each of the source and drain electrode layers have a tapered shape because coverage with a gate insulating layer stacked thereover is improved. 
     Note that each material and etching conditions are adjusted as appropriate so that the oxide semiconductor layer  412  is not removed by the etching of the conductive film and the insulating layer  407  under the oxide semiconductor layer  412  is not exposed. 
     In this embodiment, since the Ti film is used as the conductive film and the In—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductor layer  412 , an ammonium hydroxide/hydrogen peroxide mixture (a 31 wt % hydrogen peroxide solution:28 wt % ammonia water:water=5:2:2) is used as an etchant. 
     In the second photolithography step, in some cases, part of the oxide semiconductor layer  412  is etched, whereby an oxide semiconductor layer having a groove (a depression portion) may be formed. 
     Light exposure at the time of the formation of the resist mask in the second photolithography step may be performed using ultraviolet light, KrF laser light, or ArF laser light. The channel length L of a thin film transistor to be formed is determined by a pitch between a lower end of the source electrode layer and a lower end of the drain electrode layer, which are adjacent to each other over the oxide semiconductor layer  412 . In the case where light exposure is performed for a channel length L of less than 25 nm, the light exposure at the time of the formation of the resist mask in the second photolithography step is performed using extreme ultraviolet light having an extremely short wavelength of several nanometers to several tens of nanometers. In the light exposure by extreme ultraviolet light, the resolution is high and the focus depth is large. Accordingly, the channel length L of the thin film transistor can be made to be greater than or equal to 10 nm and less than or equal to 1000 nm, the operation rate of a circuit can be increased, and low power consumption can be achieved by extremely small off-state current. 
     Next, the gate insulating layer  402  is formed over the insulating layer  407 , the oxide semiconductor layer  412 , and the source and drain electrode layers  415   a  and  415   b  (see  FIG. 6C ). 
     The gate insulating layer  402  can be formed with a single-layer structure or a stacked-layer structure using one or more of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, and an aluminum oxide layer by a plasma CVD method, a sputtering method, or the like. In order to prevent the gate insulating layer  402  from containing hydrogen as much as possible, it is preferable to form the gate insulating layer  402  by a sputtering method. In the case of forming a silicon oxide film by a sputtering method, a silicon target or a quartz target is used as a target, and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas. In this embodiment, a 100-nm-thick silicon oxide layer is formed as follows: the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; the atmosphere is oxygen and argon (flow rate ratio of oxygen to argon is 25 sccm:25 sccm=1:1); and an RF sputtering method is used. 
     The gate insulating layer  402  can have a structure in which a silicon oxide layer and a silicon nitride layer are stacked in this order. For example, a gate insulating layer with a thickness of greater than or equal to 70 nm and less than or equal to 400 nm, for example, a thickness of 100 nm is formed in such a manner that a silicon oxide layer (SiO x  (x&gt;0)) having a thickness of greater than or equal to 5 nm and less than or equal to 300 nm is formed by a sputtering method as a first gate insulating layer and then a silicon nitride layer (SiN y  (y&gt;0)) having a thickness of greater than or equal to 50 nm and less than or equal to 200 nm is stacked as a second gate insulating layer over the first gate insulating layer. 
     Next, a resist mask is formed by a third photolithography step, and etching is selectively performed to remove parts of the gate insulating layer  402 , so that openings  421   a  and  421   b  reaching the source and drain electrode layers  415   a  and  415   b  are formed (see  FIG. 6D ). 
     Next, a conductive film is formed over the gate insulating layer  402  and the openings  421   a  and  421   b . In this embodiment, a titanium film with a thickness of 150 nm is formed by a sputtering method. After that, a fourth photolithography step is performed thereon, so that the gate electrode layer  411  and the wiring layers  414   a  and  414   b  are formed. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     The gate electrode layer  411  and the wiring layers  414   a  and  414   b  can be formed each to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these materials as its main component. 
     For example, as a two-layer structure of each of the gate electrode layer  411  and the wiring layers  414   a  and  414   b , any of the following structures is preferable: a two-layer structure of an aluminum layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a titanium nitride layer or a tantalum nitride layer stacked thereover, and a two-layer structure of a titanium nitride layer and a molybdenum layer. As a three-layer structure, a stack of a tungsten layer or a tungsten nitride layer, a layer of an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer is preferable. The gate electrode layer can be formed using a light-transmitting conductive film. As an example of a material of the light-transmitting conductive film, a light-transmitting conductive oxide can be given. 
     Next, a second heat treatment (preferably at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C.) is performed in an inert gas atmosphere or an oxygen gas atmosphere. In this embodiment, the second heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. The second heat treatment may be performed after a protective insulating layer or a planarization insulating layer is formed over the thin film transistor  410 . 
     Furthermore, heat treatment may be performed at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. for one hour to 30 hours both inclusive in an air atmosphere. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. This heat treatment may be performed before the formation of the oxide insulating layer under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. 
     Through the above-described process, the thin film transistor  410  including the oxide semiconductor layer  412  in which the concentration of hydrogen, moisture, hydride, and hydroxide is reduced can be formed (see  FIG. 6E ). The thin film transistor  410  can be used as the thin film transistor described in Embodiment 1. 
     A protective insulating layer or a planarization insulating layer for planarization may be provided over the thin film transistor  410 . For example, the protective insulating layer can be formed with a single-layer structure or a stacked-layer structure using one or more of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, and an aluminum oxide layer. 
     The planarization insulating layer can be formed using a heat-resistant organic material such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy. Other than such organic materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. The planarization insulating layer may be formed by stacking a plurality of insulating films formed using these materials. 
     Note that the siloxane-based resin corresponds to a resin including a Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may include as a substituent an organic group (e.g., an alkyl group or an aryl group) or a fluoro group. The organic group may include a fluoro group. 
     There is no particular limitation on the method for forming the planarization insulating layer. The planarization insulating layer can be formed, depending on a material thereof, by a method such as a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, or a droplet discharge method (e.g., an inkjet method, screen printing, or offset printing), or with the use of a tool such as a doctor knife, a roll coater, a curtain coater, or a knife coater. 
     By removing residual moisture in the reaction atmosphere at the time of the film deposition of the oxide semiconductor film as described above, the concentration of hydrogen and hydride in the oxide semiconductor film can be reduced. Accordingly, the oxide semiconductor film can be stabilized. 
     By using the thin film transistor manufactured as described above in each of a plurality of pixels of a display portion of a liquid crystal display device, the leakage current from the pixel can be suppressed. Accordingly, a period for holding voltage in a storage capacitor can be increased, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 2 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 3 
     In Embodiment 3, another example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. Note that Embodiment 2 can be applied to the same portions and the portions and steps having similar functions as/to Embodiment 2, and description thereof is not repeated. Further, a specific description for the same portions is omitted. A thin film transistor  460  described in this embodiment can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
     A thin film transistor of this embodiment and an embodiment of a method for manufacturing the thin film transistor are described using  FIGS. 7A and 7B  and  FIGS. 8A to 8E . 
       FIG. 7A  illustrates an example of a planar structure of the thin film transistor, and  FIG. 7B  illustrates an example of a cross-sectional structure thereof. The thin film transistor  460  shown in  FIGS. 7A and 7B  is a top-gate thin film transistor. 
       FIG. 7A  is a plane view of the top-gate thin film transistor  460  and  FIG. 7B  is a cross-sectional view along line D 1 -D 2  in  FIG. 7A . 
     The thin film transistor  460  includes over a substrate  450  having an insulating surface, an insulating layer  457 , a source or drain electrode layer  465   a  ( 465   a   1  and  465   a   2 ), an oxide semiconductor layer  462 , a source or drain electrode layer  465   b , a wiring layer  468 , a gate insulating layer  452 , and a gate electrode layer  461  ( 461   a  and  461   b ). The source or drain electrode layer  465   a  ( 465   a   1  and  465   a   2 ) is electrically connected to a wiring layer  464  through the wiring layer  468 . Further, not shown in the drawing, the source or drain electrode layer  465   b  is also electrically connected to the wiring layer in an opening formed in the gate insulating layer  452 . 
     A process for manufacturing the thin film transistor  460  over the substrate  450  is described below with reference to  FIGS. 8A to 8E . 
     First, the insulating layer  457  which functions as a base film is formed over the substrate  450  having an insulating surface. 
     In this embodiment, a silicon oxide layer is formed as the insulating layer  457  by a sputtering method. The substrate  450  is transferred into a chamber, a sputtering gas containing high-purity oxygen in which hydrogen and moisture are removed is introduced into the chamber, and a silicon target or quartz (preferably quart) is used, so that the silicon oxide layer is deposited on the substrate  450  as the insulating layer  457 . In this embodiment, oxygen or a mixed gas of oxygen and argon is used as the sputtering gas. 
     For example, a silicon oxide film is formed in this embodiment as follows: quartz (preferably quart) which has a purity of 6N is use as the target; the substrate temperature is 108° C.; the distance between the target and the substrate (T-S distance) is 60 mm; the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; the atmosphere is oxygen and argon (flow rate ratio of oxygen to argon is 25 sccm:25 sccm=1:1); and an RF sputtering method is used. The thickness of the silicon oxide film is 100 nm in this embodiment. A silicon target may be used instead of the quartz (preferably quart) to form the silicon oxide film. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the insulating layer  457 . This is in order to prevent the insulating layer  457  from containing hydrogen, a hydroxyl group, and/or moisture. In the chamber in which exhaustion is performed with the use of a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the insulating layer  457  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the insulating layer  457 . 
     The insulating layer  457  may have a stacked-layer structure; for example, a stacked-layer structure in which a nitride insulating layer such as a silicon nitride layer, a silicon nitride oxide layer, an aluminum nitride layer, or an aluminum nitride oxide layer and the above-described oxide insulating layer are stacked in this order over the substrate  450  may be used. 
     For example, a silicon nitride layer is formed between the silicon oxide layer and the substrate  450  by introducing a sputtering gas containing high-purity nitrogen in which hydrogen and moisture are removed and using a silicon target. In that case also, it is preferable to remove residual moisture in the chamber in the formation of the silicon nitride layer as is the case of the deposition of the silicon oxide layer. 
     A conductive film is formed over the insulating layer  457 . As a material of the conductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy containing any of these elements as a component; an alloy film containing any of these elements in combination; and the like can be given. Further, one or more materials selected from manganese, magnesium, zirconium, beryllium, and yttrium may be used. Further, the conductive film may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum (Al) and one or a plurality of elements selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may be used. In this embodiment, a titanium film with a thickness of 150 nm is formed as the conductive film by a sputtering method. Next, a resist mask is formed over the conductive film by a first photolithography step, etching is selectively thereon, so that the source and drain electrode layers  465   a   1  and  465   a   2  are formed, and then, the resist mask is removed (see  FIG. 8A ). The source and drain electrode layers  465   a   1  and  465   a   2 , which are shown as being cut in the cross-sectional view, are one film having a torus-shaped portion as shown in  FIG. 7A . It is preferable that an end portion of the each of the source and drain electrode layers  465   a   1  and  465   a   2  have a tapered shape because coverage with a gate insulating layer stacked thereover is improved. 
     Next, an oxide semiconductor film with a thickness greater than or equal to 2 nm and less than or equal to 200 nm, for example, greater than or equal to 5 nm and less than or equal to 30 nm is formed. Note that appropriate thickness of the oxide semiconductor film varies depending on a material thereof; therefore, the thickness may be determined as appropriate depending on the material. In this embodiment, the oxide semiconductor film is formed by a sputtering method with the use of an In—Ga—Zn—O-based oxide semiconductor target. 
     The oxide semiconductor film is formed over the substrate  450  as follows: the substrate is held in the chamber with pressure reduced, residual moisture in the chamber is removed, a sputtering gas from which hydrogen and moisture are removed is introduced, and a target is used. In order to remove the residual moisture in the chamber, it is preferable to use an adsorption-type vacuum pump. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed using a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), a compound including a carbon atom, or the like is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film formed in the chamber can be reduced. The substrate may be heated at the time of the film deposition of the oxide semiconductor film. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide semiconductor film. 
     As an example of the film deposition condition, the following condition is employed: the temperature of the substrate is room temperature; the distance between the substrate and the target is 110 mm; the pressure is 0.4 Pa; the direct current (DC) power supply is 0.5 kW; and the atmosphere is oxygen and argon (the flow rate ratio of oxygen to argon is 15 sccm:30 sccm). 
     Next, the oxide semiconductor film is processed into an island-shaped oxide semiconductor layer  462  by a second photolithography step (see  FIG. 8B ). In this embodiment, the oxide semiconductor film is processed into the island-shaped oxide semiconductor layer  462  by a wet etching method using a solution obtained by mixing phosphoric acid, acetic acid, and nitric acid. 
     In this embodiment, a first heat treatment is performed on the oxide semiconductor layer  462 . The temperature of the first heat treatment is higher than or equal to 400° C. and lower than or equal to 750° C., and higher than or equal to 400° C. and lower than the strain point of the substrate  450  when the strain point of the substrate  450  is lower than or equal to 750° C. In this embodiment, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layer at 450° C. for one hour in a nitrogen atmosphere, and then, the temperature is reduced to room temperature without exposure to the air and water or hydrogen is prevented from entering the oxide semiconductor layer; thus, an oxide semiconductor layer is obtained. The oxide semiconductor layer  462  can be dehydrated or dehydrogenated by the first heat treatment. 
     The heat treatment apparatus is not limited to an electric furnace and may be provided with a device that heats an object to be processed by thermal conduction or thermal radiation from a heater such as a resistance heater or the like. For example, an RTA (rapid thermal annaling) apparatus such as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermal annealing) apparatus can be used. For example, as the first heat treatment, GRTA may be performed as follows: the substrate is transferred into an inert gas heated to a high temperature of 650° C. to 700° C., heated for several minutes, and transferred and taken out of the inert gas heated to the high temperature. GRTA enables a high-temperature heat treatment for a short time. 
     In the first heat treatment, it is preferable that water, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. It is preferable that nitrogen or a rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus have a purity of 6N (99.9999%) or more, far preferably 7N (99.99999%) or more (that is, the concentration of impurities be 1 ppm or less, far preferably 0.1 ppm or less). 
     Further, depending on the condition of the first heat treatment or a material of the oxide semiconductor layer, the oxide semiconductor layer  462  might be crystallized to be a microcrystalline film or a polycrystalline film. 
     The first heat treatment of the oxide semiconductor layer can also be performed on the oxide semiconductor film before being processed into the island-like oxide semiconductor layer  462 . In that case, the substrate is taken out from the heat apparatus after the first heat treatment, and then a photolithography step is performed thereon. 
     The example in which the heat treatment for dehydration and/or dehydrogenation on the oxide semiconductor layer is performed just after the formation of the oxide semiconductor layer  462  is described above. However, the heat treatment for dehydration and/or dehydrogenation may be performed after the source or drain electrode layer  465   b  is stacked on the oxide semiconductor layer or after the gate insulating layer  452  is formed over the source or drain electrode layer  465   b  as long as it is performed after the deposition of the oxide semiconductor layer. 
     Next, a conductive film is formed over the insulating layer  457  and the oxide semiconductor layer  462 . After that, a resist mask is formed over the conductive film by a third photolithography step, the conductive film is selectively etched to form the source or drain electrode layer  465   b  and the wiring layer  468 , and then, the resist mask is removed (see  FIG. 8C ). The source or drain electrode layer  465   b  and the wiring layer  468  may be each formed by a similar material and a similar step to the material and the step of each of the source or drain electrode layers  465   a   1  and  465   a   2 . 
     In this embodiment, a 150-nm-thick titanium film is formed as each of the source or drain electrode layer  465   b  and the wiring layer  468  by a sputtering method. In this embodiment, since the source or drain electrode layers  465   a   1  and  465   a   2  and the source or drain electrode layer  465   b  are the titanium films which are the same as each other, etching selectivity between the source or drain electrode layer  465   b  and each of the source or drain electrode layers  465   a   1  and  465   a   2  cannot be provided. Therefore, in order to prevent the source or drain electrode layers  465   a   1  and  465   a   2  from being etched when the source or drain electrode layer  465   b  is etched, the wiring layer  468  is provided over the source or drain electrode layer  465   a   2  which is not covered with the oxide semiconductor layer  462 . In the case where different materials which have high selectivity at the time of etching are used to form the source or drain electrode layers  465   a   1  and  465   a   2  and the source or drain electrode layer  465   b , the wiring layer  468  by which the source or drain electrode layer  465   a   2  is protected at the time of etching is not necessarily provided 
     The oxide semiconductor layer  462  may be partly etched off by the etching of the conductive film. Materials and the etching conditions are controlled as appropriate so as not to remove the oxide semiconductor layer  462  beyond necessity. 
     In this embodiment, since the Ti film is used as the conductive film and the In—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductor layer  462 , an ammonium hydroxide/hydrogen peroxide mixture (a 31 wt % hydrogen peroxide solution:28 wt % ammonia water:water=5:2:2) is used as an etchant. 
     In the third photolithography step, in some cases, part of the oxide semiconductor layer  462  is etched, whereby an oxide semiconductor layer having a groove (a depression portion) may be formed. The resist mask used for forming the source or drain electrode layer  465   b  and the wiring layer  468  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Next, the gate insulating layer  452  is formed over the insulating layer  457 , the oxide semiconductor layer  462 , the source or drain electrode layers  465   a   1  and  465   a   2 , and the source or drain electrode layer  465   b.    
     The gate insulating layer  452  can be formed with a single-layer structure or a stacked-layer structure using one or more of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, and an aluminum oxide layer by a plasma CVD method, a sputtering method, or the like. In order to prevent the gate insulating layer  452  from containing hydrogen as much as possible, it is preferable to form the gate insulating layer  452  by a sputtering method. In the case of forming a silicon oxide film by a sputtering method, a silicon target or a quartz target is used as a target, and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas. 
     The gate insulating layer  452  may have a structure in which a silicon oxide layer and a silicon nitride layer are stacked in this order over the source or drain electrode layers  465   a   1  and  465   a   2  and the source or drain electrode layer  465   b . In this embodiment, a 100-nm-thick silicon oxide film is formed as follows: the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; the atmosphere is oxygen and argon (flow rate ratio of oxygen to argon is 25 sccm:25 sccm=1:1); and an RF sputtering method is used. 
     Next, a resist mask is formed by a fourth photolithography step, and etching is selectively performed to remove part of the gate insulating layer  452 , so that an opening  423  reaching the wiring layer  438  is formed (see  FIG. 8D ). An opening reaching the source or drain electrode layer  465   b  may be formed when the opening  423  is formed, though not shown. In this embodiment, the opening reaching the source or drain electrode layer  465   b  is formed after stacking an interlayer insulating layer, and a wiring layer for electrical connection is formed in the opening. 
     Next, a conductive film is formed over the gate insulating layer  452  and the opening  423 . After that, a fifth photolithography step is performed thereon, so that the gate electrode layer  461  ( 461   a  and  461   b ) and the wiring layer  464  are formed. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     The gate electrode layer  461  ( 461   a  and  461   b ) and the wiring layer  464  can be formed each to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these materials as its main component. 
     In this embodiment, a 150-nm-thick titanium film is formed as each of the gate electrode layer  461  ( 461   a  and  461   b ) and the wiring layer  464  by a sputtering method. Although the gate electrode layer  461  ( 461   a  and  461   b ) is shown as being divided in  FIG. 8E , the gate electrode layer  461  ( 461   a  and  461   b ) is formed so as to overlap a torus-shaped void formed by the source or drain electrode layers  465   a   1  and  465   a   2  and the source or drain electrode layer  465   b  as shown in  FIG. 7A . 
     Next, a second heat treatment (preferably at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C.) is performed in an inert gas atmosphere or an oxygen gas atmosphere. In this embodiment, the second heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. The second heat treatment may be performed after a protective insulating layer or a planarization insulating layer is formed over the thin film transistor  460 . 
     Furthermore, heat treatment may be performed at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. for one hour to 30 hours both inclusive in an air atmosphere. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. This heat treatment may be performed before the formation of the oxide insulating layer under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. 
     Through the above-described process, the thin film transistor  460  including the oxide semiconductor layer  462  in which the concentration of hydrogen, moisture, hydride, and hydroxide is reduced can be formed (see  FIG. 8E ). The thin film transistor  460  can be used as the thin film transistor used in each pixel of the pixel portion  1008  described in Embodiment 1. 
     A protective insulating layer or a planarization insulating layer for planarization may be provided over the thin film transistor  460 . Although not shown, in this embodiment, an opening reaching the source or drain electrode layer  465   b  is formed in the gate insulating layer  452  and the protective insulating layer and/or the planarization insulating layer, and a wiring layer which is electrically connected to the source or drain electrode layer  465   b  is formed in the opening. 
     By removing residual moisture in the reaction atmosphere at the time of the film deposition of the oxide semiconductor film as described above, the concentration of hydrogen and hydride in the oxide semiconductor film can be reduced. Accordingly, the oxide semiconductor film can be stabilized. 
     In this manner, in a plurality of pixels included in a display portion of a liquid crystal display device including a thin film transistor using an oxide semiconductor layer, the off-state current can be suppressed. Accordingly, a period for holding voltage in a storage capacitor can be extended, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. In this embodiment, the shape of a channel is circular and the source electrode layer and the drain electrode layer are formed using different layers, whereby the channel length can be decreased and the channel width can be increased. In this manner, a thin film transistor having a large channel width can be formed even in a relatively small area, which enables switching for large current. In addition, although the channel width is large, the off-state current is extremely small since the oxide semiconductor is highly purified. 
     Embodiment 3 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 4 
     Thin film transistors in this embodiment are described using  FIGS. 9A and 9B . In Embodiment 4, other examples of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. Note that Embodiment 2 can be applied to the same portions and the portions and steps having similar functions as/to Embodiment 2, and description thereof is not repeated. Further, a specific description for the same portions is omitted. Thin film transistors  425  and  426  described in this embodiment each can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
       FIGS. 9A and 9B  illustrate examples of a cross-sectional structure of a thin film transistor. The thin film transistors  425  and  426  shown in  FIGS. 9A and 9B  each are a kind of thin film transistor having a structure in which an oxide semiconductor layer is interposed between a conductive layer and a gate electrode layer. 
     In  FIGS. 9A and 9B , a silicon substrate  420  is used, and the thin film transistors  425  and  426  are provided over an insulating layer  422  provided over the silicon substrate  420 , respectively. 
     In  FIG. 9A , a conductive layer  427  is provided between the insulating layer  422  provided over the silicon substrate  420  and an insulating layer  407  so as to overlap at least an oxide semiconductor layer  412  entirely. 
       FIG. 9B  is an example in which a conductive layer between the insulating layer  422  and the insulating layer  407  is processed by etching to be a conductive layer  424  and overlaps at least part including a channel region of the oxide semiconductor layer  412 . 
     The conductive layers  427  and  424  each are formed by a metal material which is resistant to the temperature of a heat treatment performed later. An element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy including any of the above elements as its component, an alloy film including a combination of any of these elements, a nitride including any of the above elements as its component, or the like can be used. A single layer structure or a stacked-layer structure may be used; for example, a single layer of a tungsten layer, a stacked-layer structure of a tungsten nitride layer and a tungsten layer, or the like can be used. 
     The potential of each of the conductive layers  427  and  424  may be equal to or different from the potential of the gate electrode layer  411  of each of the thin film transistors  425  and  426 , respectively, and each of the conductive layers  427  and  424  may function as a second gate electrode layer. The potentials of the conductive layers  427  and  424  each may be a fixed potential such as GND or 0 V. 
     Electric characteristics of the thin film transistors  425  and  426  can be controlled by the conductive layers  427  and  424 , respectively. 
     Embodiment 4 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 5 
     In Embodiment 5, an example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. 
     A thin film transistor of this embodiment and an embodiment of a method for manufacturing the thin film transistor are described using  FIGS. 10A to 10E . 
       FIGS. 10A to 10E  illustrate an example of a cross-sectional structure of a thin film transistor. A thin film transistor  390  shown in  FIGS. 10A to 10E  is a kind of bottom-gate structure which is also referred to as an inverted staggered thin film transistor. 
     Although the thin film transistor  390  is described using a single-gate thin film transistor, a multi-gate thin film transistor including a plurality of channel formation regions can be formed as necessary. 
     Hereinafter, a process for manufacturing the thin film transistor  390  over a substrate  394  is described using  FIGS. 10A to 10E . 
     First, a conductive film is formed over the substrate  394  having an insulating surface, and then, a first photolithography step is performed thereon, so that a gate electrode layer  391  is formed. It is preferable that an end portion of the gate electrode layer have a tapered shape because coverage with a gate insulating layer stacked thereover is improved. Note that a resist mask may be formed by an ink-jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Although there is no particular limitation on a substrate which can be used as the substrate  394  having an insulating surface, it is necessary that the substrate  394  has at least heat resistance high enough to withstand heat treatment to be performed later. 
     For example, in the case where a glass substrate is used as the substrate  394 , if the temperature of the heat treatment to be performed later is high, it is preferable to use a glass substrate whose strain point is 730° C. or higher. As the glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. Note that by containing a larger amount of barium oxide (BaO) than boron oxide, a glass substrate is heat-resistant and of more practical use. Therefore, it is preferable that a glass substrate containing more BaO than B 2 O 3  be used. 
     Note that a substrate formed of an insulator such as a ceramic substrate, a quartz glass substrate, or a sapphire substrate may be used instead of the glass substrate as the substrate  394 . Alternatively, a crystallized glass substrate or the like may be used. Further alternatively, a plastic substrate or the like may be used as appropriate. 
     An insulating film which functions as a base film may be provided between the substrate  394  and the gate electrode layer  391 . The base film has a function of preventing diffusion of an impurity element from the substrate  394 , and can be formed with a single-layer structure or a stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. 
     The gate electrode layer  391  can be formed with a single-layer structure or a stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as a main component. 
     For example, as a two-layer structure of the gate electrode layer  391 , any of the following structures is preferable: a two-layer structure of an aluminum layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a titanium nitride layer or a tantalum nitride layer stacked thereover, a two-layer structure of a titanium nitride layer and a molybdenum layer, and a two-layer structure of a tungsten nitride layer and a tungsten layer. As a three-layer structure, a stack of a tungsten layer or a tungsten nitride layer, a layer of an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer is preferable. The gate electrode layer may also be formed using a light-transmitting conductive film. As an example of a material of the light-transmitting conductive film, a light-transmitting conductive oxide or the like can be given. 
     Next, a gate insulating layer  397  is formed over the gate electrode layer  391 . 
     The gate insulating layer  397  can be formed with a single-layer structure or a stacked-layer structure using one or more of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, and an aluminum oxide layer by a plasma CVD method, a sputtering method, or the like. In order to prevent the gate insulating layer  397  from containing hydrogen as much as possible, it is preferable to form the gate insulating layer  397  by a sputtering method. In the case of forming a silicon oxide film by a sputtering method, a silicon target or a quartz target is used as a target, and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas. 
     The gate insulating layer  397  can have a structure in which a silicon nitride layer and a silicon oxide layer are stacked in this order over the gate electrode layer  391 . For example, a 100-nm-thick gate insulating layer is formed in such a manner that a silicon nitride layer (SiN y  (y&gt;0)) having a thickness of greater than or equal to 50 nm and less than or equal to 200 nm is formed by a sputtering method as a first gate insulating layer and then a silicon oxide layer (SiO x  (x&gt;0)) having a thickness of greater than or equal to 5 nm and less than or equal to 300 nm is stacked as a second gate insulating layer over the first gate insulating layer. 
     In order for the oxide semiconductor film not to contain hydrogen, a hydroxyl group, and moisture as much as possible in the gate insulating layer  397  and an oxide semiconductor film  393 , it is preferable to preheat the substrate  394  provided with the gate electrode layer  391  or the substrate  394  provided with the gate electrode layer  391  and the gate insulating layer  397  in a preheating chamber of a sputtering apparatus before the film formation so that an impurity such as hydrogen or moisture adsorbed on the substrate  394  is eliminated, and perform exhaustion. The temperature of the preheating be higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. This preheating step is not necessarily performed. This preheating step may be performed in a similar manner on the substrate  394  with which a source electrode layer  395   a  and a drain electrode layer  395   b  shown in  FIG. 10C  are provided before an oxide insulating layer  396  is formed. 
     Next, over the gate insulating layer  397 , the oxide semiconductor film  393  is formed to a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm by a sputtering method (see  FIG. 10A ). 
     Before the oxide semiconductor film  393  is formed by a sputtering method, it is preferable to perform reverse sputtering in which an argon gas is introduced and plasma is generated so that dust on a surface of the gate insulating layer  397  is removed. 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 to modify a surface. Instead of the argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. 
     The oxide semiconductor film  393  is formed using 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, a Sn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, an In—Zn—O-based oxide semiconductor film, a Sn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxide semiconductor film, an In—O-based oxide semiconductor film, a Sn—O-based oxide semiconductor film, or a Zn—O-based oxide semiconductor film. In this embodiment, the oxide semiconductor film  393  is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. Specifically, a target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [mol %] (that is, In:Ga:Zn=1:1:0.5 [atom %]) is used. Alternatively, a target having a composition ratio of In:Ga:Zn=1:1:1 [atom %] or In:Ga:Zn=1:1:2 [atom %] can be used. In this embodiment, the filling rate of the oxide semiconductor target is equal to or greater than 90% and equal to or less than 100%, preferably equal to or greater than 95% and equal to or less than 99.9%. With use of the oxide semiconductor target having high filling rate, the deposited oxide semiconductor film has high density. The atmosphere in the sputtering of the oxide semiconductor film  393  may be an atmosphere of a rare gas (typically argon), an oxygen atmosphere, or a mixed atmosphere of a rare gas (typically argon) and oxygen. The target may contain SiO 2  at 2 wt % or more and 10 wt % or less. 
     The oxide semiconductor film  393  is formed over the substrate  394  as follows: the substrate is held in the chamber with pressure reduced, and the substrate is heated to room temperature or a temperature lower than 400° C.; and residual moisture in the chamber is removed, a sputtering gas from which hydrogen and moisture are removed is introduced, and the above-described target is used. In order to remove the residual moisture in the chamber, it is preferable to use an adsorption-type vacuum pump. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed using a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), a compound including a carbon atom, or the like is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film formed in the chamber can be reduced. Moisture left in the chamber is removed by a cryopump in the sputtering film deposition, the substrate temperature at the time of film deposition of the oxide semiconductor film  393  can have a temperature equal to or higher than room temperature and lower than 400° C. 
     As an example of the film deposition condition, the following condition is employed: 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 flow rate ratio of oxygen is 100%). It is preferable that a pulsed direct current (DC) power supply be used because powder substances (also referred to as particles or dust) generated in the film deposition can be reduced and the film thickness can be made uniform. 
     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 in which a DC power source is used, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. The RF sputtering method is mainly used in the case where an insulating film is formed, and the DC sputtering method is mainly used in the case where a metal film is formed. 
     A multi-target sputtering apparatus in which a plurality of targets which are formed of different materials from each other can be set may be used. With the multi-target sputtering apparatus, films of different materials can be stacked to be formed in the same chamber, or plural kinds of materials can be deposited by electric discharge at the same time in the same chamber. 
     Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge may be used. 
     Further, as the deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, or a bias sputtering method in which a voltage is also applied to a substrate during deposition may be used. 
     Next, the oxide semiconductor film is processed into an island-shaped oxide semiconductor layer  399  by a second photolithography step (see  FIG. 10B ). A resist mask for forming the island-shaped oxide semiconductor layer  399  may be formed using an ink jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     In the case where a contact hole is formed in the gate insulating layer  397 , a step thereof can be performed at the time of the formation of the oxide semiconductor layer  399 . 
     For the etching of the oxide semiconductor film  393 , either one or both of wet etching and dry etching may be employed. 
     As an etching gas for the dry etching, a gas containing chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon chloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. 
     Alternatively, a gas containing fluorine (fluorine-based gas such as carbon tetrafluoride (CF 4 ), sulfur fluoride (SF 6 ), nitrogen fluoride (NF 3 ), or trifluoromethane (CHF 3 )); hydrogen bromide (HBr); oxygen (O 2 ); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used. 
     As the dry etching method, a parallel-plate RIE (reactive ion etching) method or an ICP (inductively coupled plasma) etching method can be used. In order to etch the layer into a desired shape, the etching conditions (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) are adjusted as appropriate. 
     As an etchant used for wet etching, a mixed solution of phosphoric acid, acetic acid, and nitric acid, an ammonium hydroxide/hydrogen peroxide mixture (a 31 wt % hydrogen peroxide solution:28 wt % ammonia water:water=5:2:2), or the like can be used. ITO07N (produced by KANTO CHEMICAL CO., INC.) may be used. 
     After the wet etching, the etchant is removed by cleaning together with the material which is etched off. Waste liquid of the etchant containing the removed material may be purified and the material contained in the waste liquid may be reused. The resources can be efficiently used and the cost can be reduced by collecting and reusing a material such as indium included in the oxide semiconductor from the waste liquid after the etching. 
     The etching conditions (such as an etchant, etching time, or temperature) are appropriately adjusted depending on a material so that the material can be etched into a desired shape. 
     Note that in that case, it is preferable to perform reverse sputtering before a conductive film is formed by the following step to remove a resist residue or the like from a surface of the oxide semiconductor layer  399  and the gate insulating layer  397 . 
     Next, a conductive film is formed over the gate insulating layer  397  and the oxide semiconductor layer  399 . The conductive film may be formed by a sputtering method or a vacuum evaporation method. As a material of the conductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy containing any of these elements as a component; an alloy film containing any of these elements in combination; and the like can be given. Further, one or more materials selected from manganese, magnesium, zirconium, beryllium, and yttrium may be used. Further, the conductive film may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum (Al) and one or a plurality of elements selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may be used. 
     Next, a resist mask is formed over the conductive film by a third photolithography step. After that, etching is selectively thereon, so that the source and drain electrode layers  395   a  and  395   b  are formed, and then, the resist mask is removed (see  FIG. 10C ). 
     Light exposure at the time of the formation of the resist mask in the third photolithography step may be performed using ultraviolet light, KrF laser light, or ArF laser light. The channel length L of a thin film transistor to be formed is determined by a pitch between a lower end of the source electrode layer and a lower end of the drain electrode layer, which are adjacent to each other over the oxide semiconductor layer  399 . In the case where light exposure is performed for a channel length L of less than 25 nm, the light exposure at the time of the formation of the resist mask in the third photolithography step is performed using extreme ultraviolet light having an extremely short wavelength of several nanometers to several tens of nanometers. In the light exposure by extreme ultraviolet light, the resolution is high and the focus depth is large. Accordingly, the channel length L of the thin film transistor can be made to be greater than or equal to 10 nm and less than or equal to 1000 nm, the operation rate of a circuit can be increased, and low power consumption can be achieved by extremely small off-state current. 
     The oxide semiconductor layer  399  may be partly etched off by the etching of the conductive film. Materials and the etching conditions are controlled as appropriate so as not to remove the oxide semiconductor layer  399  at the time of etching of the conductive film. 
     In this embodiment, since the Ti film is used as the conductive film and the In—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductor layer  399 , an ammonium hydroxide/hydrogen peroxide mixture (a mixture of ammonia, water, and a hydrogen peroxide solution) is used as an etchant. 
     In the third photolithography step, in some cases, part of the oxide semiconductor layer  399  is etched, whereby an oxide semiconductor layer having a groove (a depression portion) may be formed. The resist mask used for forming the source and drain electrode layers  395   a  and  395   b  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     In order to reduce the number of photomasks and steps in the photolithography step, etching may be performed with the use of a resist mask formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. Since a resist mask formed using a multi-tone mask has a plurality of thicknesses and can be further changed in shape by performing etching, the resist mask can be used in a plurality of etching steps to provide different patterns. Therefore, a resist mask corresponding to at least two kinds of different patterns can be formed by using one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of corresponding photolithography steps can also be reduced, whereby simplification of the manufacturing process can be realized. 
     After the removal of the resist mask, plasma treatment using a gas such as N 2 O, N 2 , or Ar may be performed to remove water or the like adsorbed on a surface of the oxide semiconductor layer  399  which is exposed. Plasma treatment may be performed using a mixed gas of oxygen and argon. 
     Next, the oxide insulating layer  396  is formed as an oxide insulating layer which functions as a protective insulating layer which is in contact with part of the oxide semiconductor layer (see  FIG. 10D ). In the case where the plasma treatment is performed, the oxide insulating layer  396  may be formed without exposure of the oxide semiconductor layer  399  to the air successively after the plasma treatment. In this embodiment, the oxide semiconductor layer  399  is in contact with the oxide insulating layer  396  in a region where the oxide semiconductor layer  399  overlaps neither the source electrode layer  395   a  nor the drain electrode layer  395   b.    
     In this embodiment, as the oxide insulating layer  396 , a silicon oxide layer including a defect is formed as follows: the substrate  394  over which the island-shaped oxide semiconductor layer  399 , the source electrode layer  395   a , and the drain electrode layer  395   b  are formed is heated at room temperature to a temperature lower than 100° C.; a sputtering gas containing high-purity oxygen from which hydrogen and moisture are removed is introduced; and a silicon semiconductor target is used. 
     For example, the silicon oxide film is formed in this embodiment as follows: a silicon target doped with boron (with a resistivity of 0.01 Ω·cm) and which has a purity of 6N is used; the distance between the target and the substrate (T-S distance) is 89 mm; the pressure is 0.4 Pa; the direct current (DC) power source is 6 kW; the atmosphere is oxygen (flow rate ratio of oxygen is 100%); and a pulsed DC sputtering method is used. The thickness of the silicon oxide film is 300 nm in this embodiment. Quartz (preferably quart) may be used instead of the silicon target to form the silicon oxide film. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide insulating layer  396 . This is in order to prevent the oxide semiconductor layer  399  and the oxide insulating layer  396  from containing hydrogen, a hydroxyl group, and/or moisture. 
     In order to remove the residual moisture in the chamber, it is preferable to use an adsorption-type vacuum pump. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed using a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like is exhausted. Accordingly, the concentration of impurities included in the oxide insulating layer  396  formed in the chamber can be reduced. 
     As the oxide insulating layer  396 , instead of the silicon oxide layer, a silicon oxynitride layer, an aluminum oxide layer, an aluminum oxynitride layer, or the like can be used. 
     Further, heat treatment at 100° C. to 400° C. may be performed on the state where the oxide insulating layer  396  is in contact with the oxide semiconductor layer  399  after the formation of the oxide insulating layer  396 . Since the oxide insulating layer  396  in this embodiment includes many defects, an impurity such as hydrogen, moisture, a hydroxyl group, or hydride included in the oxide semiconductor layer  399  is diffused into the oxide insulating layer  396  by this heat treatment, so that the impurity included in the oxide semiconductor layer  399  is further reduced. 
     Through the above-described process, the thin film transistor  390  including an oxide semiconductor layer  392  in which the concentration of hydrogen, moisture, a hydroxyl group, and/or hydride is reduced can be formed (see  FIG. 10E ). 
     By removing residual moisture in the reaction atmosphere at the time of the film deposition of the oxide semiconductor film as described above, the concentration of hydrogen and hydride in the oxide semiconductor film can be reduced. Accordingly, the oxide semiconductor film can be stabilized. 
     A protective insulating layer may be provided over the oxide insulating layer. In this embodiment, a protective insulating layer  398  is formed over the oxide insulating layer  396 . As the protective insulating layer  398 , a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film, or the like can be used. In this embodiment, the protective insulating layer  398  is formed using a silicon nitride film. 
     As the protective insulating layer  398 , a silicon nitride film is formed by heating the substrate  394  over which layers up to and including the oxide insulating layer  396  are formed, to a temperature of 100° C. to 400° C., introducing a sputtering gas containing high-purity nitrogen from which hydrogen and moisture are removed, and using a target of silicon semiconductor. In that case also, it is preferable that residual moisture be removed from the treatment chamber in the formation of the protective insulating layer  398  as is the case of the oxide insulating layer  396 . 
     In the case where the protective insulating layer  398  is formed, the substrate  394  is heated to a temperature of 100° C. to 400° C. at the time of the formation of the protective insulating layer  398 , whereby hydrogen and/or moisture included in the oxide semiconductor layer can be diffused into the oxide insulating layer. In such a case, heat treatment after the formation of the oxide insulating layer  396  is not necessarily performed. 
     In the case where the silicon oxide layer is formed as the oxide insulating layer  396  and the silicon nitride layer is stacked as the protective insulating layer  398 , the silicon oxide layer and the silicon nitride layer can be formed in the same chamber using a common silicon target. First, a sputtering gas containing oxygen is introduced and a silicon target placed inside the chamber is used, so that a silicon oxide layer is formed; and then, the sputtering gas is switched to a sputtering gas containing nitrogen and the same silicon target is used, so that a silicon nitride layer is formed. Since the silicon oxide layer and the silicon nitride layer can be formed in succession without exposure to the air, an impurity such as hydrogen or moisture can be prevented from being adsorbed on a surface of the silicon oxide layer. In that case, after silicon oxide layer is formed as the oxide insulating layer  396  and the silicon nitride layer is stacked as the protective insulating layer  398 , heat treatment (at a temperature of 100° C. to 400° C.) for diffusing hydrogen or moisture included in the oxide semiconductor layer into the oxide insulating layer may be performed. 
     After the formation of the protective insulating layer, heat treatment may be performed at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in the air for 1 hour to 30 hours both inclusive. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. Further, this heat treatment may be performed before the formation of the oxide insulating film under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. With this heat treatment, a normally-off thin film transistor (threshold voltage thereof is positive in the case of an re-channel transistor) can be obtained. Therefore, reliability of the liquid crystal display device can be improved. 
     Further, by removing residual moisture in the reaction atmosphere at the time of the formation of the oxide semiconductor layer, in which a channel formation region is to be formed, over the gate insulating layer, the concentration of hydrogen or hydride in the oxide semiconductor layer can be reduced. 
     The above-described process can be used for manufacturing a backplane (a substrate over which a thin film transistor is formed) of a liquid crystal display panel, an electroluminescent display panel, a display device using electronic ink, or the like. Since the above-described process is performed at a temperature lower than or equal to 400° C., the process can be applied to a manufacturing process using a glass substrate having a side longer than or equal to one meter and a thickness less than or equal to one millimeter. Further, since the whole process can be performed at a treatment temperature of 400° C. or lower, a display panel can be manufactured without consuming too much energy. 
     The off-state current can be reduced in the thin film transistor using an oxide semiconductor layer, manufactured as described above. Therefore, by using the thin film transistor in each of a plurality of pixels of a display portion of a liquid crystal display device, a period for holding voltage in a storage capacitor can be extended, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 5 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 6 
     A thin film transistor of this embodiment and an embodiment of a method for manufacturing the thin film transistor are described using  FIGS. 11A to 11E . 
     In Embodiment 6, another example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. A thin film transistor  310  described in this embodiment can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
       FIGS. 11A to 11E  illustrate an example of a cross-sectional structure of a thin film transistor. The thin film transistor  310  shown in  FIGS. 11A to 11E  is a kind of bottom-gate structure which is also referred to as an inverted staggered thin film transistor. 
     Although the thin film transistor  310  is described using a single-gate thin film transistor, a multi-gate thin film transistor including a plurality of channel formation regions can be formed as necessary. 
     Hereinafter, a process for manufacturing the thin film transistor  310  over a substrate  300  is described using  FIGS. 11A to 11E . 
     First, a conductive film is formed over the substrate  300  having an insulating surface, and then, a first photolithography step is performed thereon, so that a gate electrode layer  311  is formed. Note that a resist mask may be formed by an ink-jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Although there is no particular limitation on a substrate which can be used as the substrate  300  having an insulating surface, it is necessary that the substrate  300  has at least heat resistance high enough to withstand heat treatment to be performed later. 
     For example, in the case where a glass substrate is used as the substrate  300 , if the temperature of the heat treatment to be performed later is high, it is preferable to use a glass substrate whose strain point is 730° C. or higher. As the glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. Note that by containing a larger amount of barium oxide (BaO) than boron oxide, a glass substrate is heat-resistant and of more practical use. Therefore, it is preferable that a glass substrate containing more BaO than B 2 O 3  be used. 
     Note that a substrate formed of an insulator such as a ceramic substrate, a quartz glass substrate, or a sapphire substrate may be used instead of the glass substrate as the substrate  300 . Alternatively, a crystallized glass substrate or the like may be used. 
     An insulating film which functions as a base film may be provided between the substrate  300  and the gate electrode layer  311 . The base film has a function of preventing diffusion of an impurity element from the substrate  300 , and can be formed with a single-layer structure or a stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. 
     The gate electrode layer  311  can be formed with a single-layer structure or a stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as a main component. 
     For example, as a two-layer structure of the gate electrode layer  311 , any of the following structures is preferable: a two-layer structure of an aluminum layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a titanium nitride layer or a tantalum nitride layer stacked thereover, a two-layer structure of a titanium nitride layer and a molybdenum layer, and a two-layer structure of a tungsten nitride layer and a tungsten layer. As a three-layer structure, a stack of a tungsten layer or a tungsten nitride layer, a layer of an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer is preferable. 
     Next, a gate insulating layer  302  is formed over the gate electrode layer  311 . 
     The gate insulating layer  302  can be formed to have a single layer of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, or an aluminum 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 using SiH 4 , oxygen, and nitrogen as a deposition gas. In this embodiment, the thickness of the gate insulating layer  302  is greater than or equal to 100 nm and less than or equal to 500 nm. In the case of a stacked-layer structure, a first gate insulating layer with a thickness greater than or equal to 50 nm and less than or equal to 200 nm and a second gate insulating layer with a thickness greater than or equal to 5 nm and less than or equal to 300 nm are stacked on the first gate insulating layer. 
     In this embodiment, a silicon oxynitride layer having a thickness of 100 nm or less is formed by a plasma CVD method as the gate insulating layer  302 . 
     Next, over the gate insulating layer  302 , an oxide semiconductor film  330  is formed by a sputtering method to a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm. Note that an appropriate thickness differs depending on an oxide semiconductor material, and the thickness may be set as appropriate depending on the material. A cross-sectional view in this step is  FIG. 11A . 
     Before the oxide semiconductor film  330  is formed by a sputtering method, it is preferable to perform reverse sputtering in which an argon gas is introduced and plasma is generated so that dust on a surface of the gate insulating layer  302  is removed. Instead of the argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. 
     The oxide semiconductor film  330  is formed using 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, a Sn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxide semiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, an In—Zn—O-based oxide semiconductor film, a Sn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxide semiconductor film, an In—O-based oxide semiconductor film, a Sn—O-based oxide semiconductor film, or a Zn—O-based oxide semiconductor film. In this embodiment, the oxide semiconductor film  330  is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. Specifically, a target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [mol %] (that is, In:Ga:Zn=1:1:0.5 [atom %]) is used. Alternatively, a target having a composition ratio of In:Ga:Zn=1:1:1 [atom %] or In:Ga:Zn=1:1:2 [atom %] can be used. In this embodiment, the filling rate of the oxide semiconductor target is equal to or greater than 90% and equal to or less than 100%, preferably equal to or greater than 95% and equal to or less than 99.9%. With use of the oxide semiconductor target having high filling rate, the deposited oxide semiconductor film has high density. The target may contain SiO 2  at 2 wt % or more and 10 wt % or less. The atmosphere in the sputtering of the oxide semiconductor film  330  may be an atmosphere of a rare gas (typically argon), an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide semiconductor film  330 . 
     The sputtering is performed by holding the substrate in the chamber with pressure reduced at a substrate temperature higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. By heating the substrate in the film deposition, the concentration of impurities contained in the oxide semiconductor film can be decreased. Further, damage by the sputtering can be suppressed. Then, residual moisture in the chamber is removed, a sputtering gas from which hydrogen and moisture are removed is introduced, and the above-described target is used, so that the oxide semiconductor film  330  is formed over the substrate  300 . In order to remove the residual moisture in the chamber, it is preferable to use an adsorption-type vacuum pump. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed using a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), a compound including a carbon atom, or the like is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film formed in the chamber can be reduced. 
     As an example of the film deposition condition, the following condition is employed: 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 flow rate ratio of oxygen is 100%). It is preferable that a pulsed direct current (DC) power supply be used because powder substances (also referred to as particles or dust) generated in the film deposition can be reduced and the film thickness can be made uniform. 
     Next, the oxide semiconductor film  330  is processed into an island-shaped oxide semiconductor layer  331  by a second photolithography step. A resist mask for forming the island-shaped oxide semiconductor layer may be formed using an ink jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Next, a first heat treatment is performed on the oxide semiconductor layer  331 . The oxide semiconductor layer  331  can be dehydrated or dehydrogenated by the first heat treatment. The temperature of the first heat treatment is higher than or equal to 400° C. and lower than or equal to 750° C., preferably higher than or equal to 400° C. and lower than the strain point of the substrate. In this embodiment, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layer at 450° C. for one hour in a nitrogen atmosphere, and then, the temperature is reduced to room temperature without exposure to the air and water or hydrogen is prevented from entering the oxide semiconductor layer; thus, the oxide semiconductor layer  331  is obtained (see  FIG. 11B ). 
     The heat treatment apparatus is not limited to an electric furnace and may be provided with a device that heats an object to be processed by thermal conduction or thermal radiation from a heater such as a resistance heater or the like. For example, an RTA (rapid thermal annaling) apparatus such as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermal annealing) apparatus can be used. The 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. The GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with the object to be processed, by heat treatment, such as nitrogen or a rare gas such as argon is used. 
     For example, as the first heat treatment, GRTA may be performed as follows: the substrate is transferred into an inert gas heated to a high temperature of 650° C. to 700° C., heated for several minutes, and transferred and taken out of the inert gas heated to the high temperature. GRTA enables a high-temperature heat treatment for a short time. 
     In the first heat treatment, it is preferable that water, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. It is preferable that nitrogen or a rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus have a purity of 6N (99.9999%) or more, far preferably 7N (99.99999%) or more (that is, the concentration of impurities be 1 ppm or less, far preferably 0.1 ppm or less). 
     By the first heat treatment, hydrogen and/or the like contained in the oxide semiconductor layer  331  can be removed and oxygen loss is generated, so that the oxide semiconductor layer  331  becomes an n-type semiconductor (a semiconductor with resistance reduced). Further, depending on the condition of the first heat treatment or a material of the oxide semiconductor layer  331 , the oxide semiconductor layer  331  might be crystallized to be a microcrystalline film or a polycrystalline film. For example, the oxide semiconductor layer may be crystallized to be a microcrystalline oxide semiconductor film in which the degree of crystallization is greater than or equal to 90% or greater than or equal to 80%. Further, depending on the condition of the first heat treatment or the material of the oxide semiconductor layer  331 , the oxide semiconductor layer  331  may be an amorphous oxide semiconductor film which does not contain crystalline components. The oxide semiconductor layer  331  may become an oxide semiconductor film in which a microcrystalline portion (with a grain diameter greater than or equal to 1 nm and less than or equal to 20 nm, typically greater than or equal to 2 nm and less than or equal to 4 nm) is mixed into an amorphous oxide semiconductor. 
     The first heat treatment of the oxide semiconductor layer can also be performed on the oxide semiconductor film  330  before being processed into the island-like oxide semiconductor layer  331 . In that case, the substrate is taken out from the heat apparatus after the first heat treatment, and then a photolithography step is performed thereon. 
     The heat treatment for dehydration and/or dehydrogenation may be performed after a source electrode and a drain electrode are stacked on the oxide semiconductor layer or after a protective insulating film is formed over a source electrode and a drain electrode as long as it is performed after the deposition of the oxide semiconductor layer. 
     In the case where a contact hole is formed in the gate insulating layer  302 , a step thereof can be performed before or after the heat treatment for dehydration and/or dehydrogenation is performed on the oxide semiconductor film  330  or the oxide semiconductor layer  331 . 
     For the etching of the oxide semiconductor film, dry etching may be employed as well as wet etching. 
     The etching conditions (such as an etchant, etching time, or temperature) are appropriately adjusted depending on a material so that the material can be etched into a desired shape. 
     Next, a conductive film is formed over the gate insulating layer  302  and the oxide semiconductor layer  331 . The conductive film may be formed by a sputtering method or a vacuum evaporation method. As a material of the conductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy containing any of these elements as a component; an alloy film containing any of these elements in combination; and the like can be given. Further, one or more materials selected from manganese, magnesium, zirconium, beryllium, and yttrium may be used. Further, the conductive film may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum (Al) and one or a plurality of elements selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may be used. 
     In the case where heat treatment is performed after the deposition of the conductive film, it is preferable that the conductive film have heat resistance high enough to withstand the heat treatment. 
     Next, a resist mask is formed over the conductive film by a third photolithography step. After that, etching is selectively thereon, so that a source and drain electrode layers  315   a  and  315   b  are formed, and then, the resist mask is removed (see  FIG. 11C ). 
     Light exposure at the time of the formation of the resist mask in the third photolithography step may be performed using ultraviolet light, KrF laser light, or ArF laser light. The channel length L of a thin film transistor to be formed is determined by a pitch between a lower end of the source electrode layer and a lower end of the drain electrode layer, which are adjacent to each other over the oxide semiconductor layer  331 . In the case where light exposure is performed for a channel length L of less than 25 nm, the light exposure at the time of the formation of the resist mask in the third photolithography step is performed using extreme ultraviolet light having an extremely short wavelength of several nanometers to several tens of nanometers. In the light exposure by extreme ultraviolet light, the resolution is high and the focus depth is large. Accordingly, the channel length L of the thin film transistor can be made to be greater than or equal to 10 nm and less than or equal to 1000 nm, the operation rate of a circuit can be increased, and low power consumption can be achieved by extremely small off-state current. 
     Materials and the etching conditions are controlled as appropriate so as not to remove the oxide semiconductor layer  331  at the time of etching of the conductive film. 
     In this embodiment, since the Ti film is used as the conductive film and the In—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductor layer  331 , an ammonium hydroxide/hydrogen peroxide mixture (a mixture of ammonia, water, and a hydrogen peroxide solution) is used as an etchant. 
     In the third photolithography step, in some cases, part of the oxide semiconductor layer  331  is etched, whereby an oxide semiconductor layer having a groove (a depression portion) may be formed. The resist mask used for forming the source and drain electrode layers  315   a  and  315   b  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Further, an oxide conductive layer may be formed between the oxide semiconductor layer and the source and drain electrode layers. The oxide conductive layer and the metal layer for forming the source and drain electrode layers can be formed successively. The oxide conductive layer can function as a source and drain regions. 
     By providing the oxide conductive layer as the source region and the drain region between the oxide semiconductor layer and the source and drain electrode layers, the resistance of the source region and the drain region can be decreased and the transistor can be operated at high speed. 
     In order to reduce the number of photomasks and steps in the photolithography step, etching may be performed with the use of a resist mask formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. Since a resist mask formed using a multi-tone mask has a plurality of thicknesses and can be further changed in shape by performing etching, the resist mask can be used in a plurality of etching steps to provide different patterns. Therefore, a resist mask corresponding to at least two kinds of different patterns can be formed by using one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of corresponding photolithography steps can also be reduced, whereby simplification of the manufacturing process can be realized. 
     Next, plasma treatment using a gas such as N 2 O, N 2 , or Ar may be performed to remove water or the like adsorbed on a surface of the oxide semiconductor layer which is exposed. Plasma treatment may be performed using a mixed gas of oxygen and argon. 
     After the plasma treatment, an oxide insulating layer  316  which functions as a protective insulating film and is in contact with part of the oxide semiconductor layer is formed without exposure to the air. 
     The oxide insulating layer  316  can be formed to a thickness at least 1 nm by a method by which an impurity such as water or hydrogen does not enter the oxide insulating layer  316 , such as a sputtering method as appropriate. When hydrogen is contained in the oxide insulating layer  316 , entry of the hydrogen to the oxide semiconductor layer or extraction of oxygen in the oxide semiconductor layer by hydrogen may be caused, thereby making the backchannel of the oxide semiconductor layer n-type (making the resistance thereof low), so that a parasitic channel may be formed. Therefore, it is important that a formation method in which hydrogen is used as little as possible is employed such that the oxide insulating layer  316  contains hydrogen as little as possible. 
     In this embodiment, a 200-nm-thick silicon oxide film is deposited as the oxide insulating layer  316  by a sputtering method. The substrate temperature at the time of film deposition may be higher than or equal to room temperature and lower than or equal to 300° C. and in this embodiment, is 100° C. The silicon oxide film can be formed with a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas and oxygen. As a target, a silicon oxide target or a silicon target may be used. For example, with the use of a silicon target, silicon oxide can be deposited by a sputtering method under an atmosphere of oxygen and nitrogen. As the oxide insulating layer  316  which is formed in contact with the oxide semiconductor layer whose resistance is reduced, an inorganic insulating film which does not include impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of these from the outside is used. Typically, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or the like is used. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide insulating layer  316 . This is in order to prevent the oxide semiconductor layer  331  and the oxide insulating layer  316  from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the oxide insulating layer  316  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide insulating layer  316 . 
     Next, a second heat treatment (preferably at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C.) is performed in an inert gas atmosphere or an oxygen gas atmosphere. For example, the second heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. With the second heat treatment, heat is applied on the state where part of the oxide semiconductor layer (the channel formation region) is in contact with the oxide insulating layer  316 . 
     Through the above process, heat treatment for dehydration and/or dehydrogenation is performed on the deposited oxide semiconductor film to lower the resistance, and then, a part of the oxide semiconductor film is selectively made to include excessive oxygen. As a result, a channel formation region  313  overlapping the gate electrode layer  311  becomes i-type, a high-resistance source region  314   a  which overlaps the source electrode layer  315   a  and is formed using a low-resistance oxide semiconductor, and a high-resistance drain region  314   b  which overlaps the drain electrode layer  315   b  and is formed using a low-resistance oxide semiconductor are formed in a self-aligned manner. Through the above steps, the thin film transistor  310  is formed (see  FIG. 11D ). 
     Furthermore, heat treatment at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in the air for 1 hour to 30 hours both inclusive may be performed. In this embodiment, heat treatment is performed at 150° C. for 10 hours. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. Further, this heat treatment may be performed before the formation of the oxide insulating film under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. With this heat treatment, hydrogen is introduced from the oxide semiconductor layer to the oxide insulating layer; thus, a normally-off thin film transistor can be obtained. Therefore, reliability of the liquid crystal display device can be improved. Further, by using a silicon oxide layer containing many defects as the oxide insulating layer, impurities such as hydrogen, moisture, a hydroxyl group, or hydride contained in the oxide semiconductor layer are diffused into the oxide insulating layer by this heat treatment to further reduce the impurities contained in the oxide semiconductor layer. 
     The high-resistance drain region  314   b  (or the high-resistance source region  314   a ) is formed in a portion of the oxide semiconductor layer which overlaps the drain electrode layer  315   b  (or the source electrode layer  315   a ), so that the reliability of the thin film transistor can be increased. Specifically, with the formation of the high-resistance drain region  314   b , the conductivity can be gradually varied from the drain electrode layer  315   b  to the high-resistance drain region  314   b  and the channel formation region  313  in the transistor. Therefore, in the case where the thin film transistor operates using the drain electrode layer  315   b  connected to a wiring for supplying a high power supply potential VDD, the high-resistance drain region serves as a buffer and a high electric field is not applied locally even if a high electric field is applied between the gate electrode layer  311  and the drain electrode layer  315   b , so that the withstand voltage of the transistor can be improved. 
     The high-resistance source region and the high-resistance drain region may be formed at all the depths in the film thickness direction in the oxide semiconductor layer in the case where the oxide semiconductor layer is as thin as 15 nm or less; whereas in the case where the oxide semiconductor layer is as thick as a thickness greater than or equal to 30 nm and less than or equal to 50 nm, parts of the oxide semiconductor layer, that is, regions of the oxide semiconductor layer, which are in contact with the source and drain electrode layers and the vicinity thereof may be reduced in the resistance, so that the high-resistance source region and the high-resistance drain region are formed and a region of the oxide semiconductor layer, near the gate insulating layer can be made to be i-type. 
     A protective insulating layer may be formed over the oxide insulating layer  316 . For example, a silicon nitride film is formed by an RF sputtering method. An RF sputtering method is preferable as a method for forming a protective insulating layer because it has high productivity. As the protective insulating layer, an inorganic insulating film which does not contain impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of these from the outside is used; a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, an aluminum nitride oxide film, or the like is used. In this embodiment, the protective insulating layer  303  is formed using a silicon nitride film as the protective insulating layer (see  FIG. 11E ). 
     In this embodiment, as the protective insulating layer  303 , a silicon nitride film is formed by heating the substrate  300  over which layers up to and including the oxide insulating layer  316  are formed, to a temperature of 100° C. to 400° C., introducing a sputtering gas containing high-purity nitrogen from which hydrogen and moisture are removed, and using a target of silicon semiconductor. In that case also, it is preferable that residual moisture be removed from the treatment chamber in the formation of the protective insulating layer  303  as is the case of the oxide insulating layer  316 . 
     A planarization insulating layer for planarization may be provided over the protective insulating layer  303 . 
     The off-state current can be reduced in each of a plurality of pixels of a display portion of a liquid crystal display device using the thin film transistor using an oxide semiconductor layer as described above. Therefore, a period for holding voltage in a storage capacitor can be extended, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 6 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 7 
     A thin film transistor of this embodiment and an embodiment of a method for manufacturing the thin film transistor are described using  FIGS. 12A to 12D . 
     In Embodiment 7, another example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. A thin film transistor  360  described in this embodiment can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
       FIGS. 12A to 12D  illustrate an example of a cross-sectional structure of a thin film transistor. The thin film transistor  360  shown in  FIGS. 12A to 12D  is a kind of bottom-gate structure called a channel-protective structure (also referred to as a channel-stop structure) and is also referred to as an inverted staggered thin film transistor. 
     Although the thin film transistor  360  is described using a single-gate thin film transistor, a multi-gate thin film transistor including a plurality of channel formation regions can be formed as necessary. 
     Hereinafter, a process for manufacturing the thin film transistor  360  over a substrate  320  is described using  FIGS. 12A to 12D . 
     First, a conductive film is formed over the substrate  320  having an insulating surface, a first photolithography step is performed to form a resist mask, and the conductive film is selectively etched by using the resist mask, so that a gate electrode layer  361  is formed. After that, the resist mask is removed. Note that the resist mask may be formed by an ink-jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     The gate electrode layer  361  can be formed with a single-layer structure or a stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as a main component. 
     Next, a gate insulating layer  322  is formed over the gate electrode layer  361 . 
     In this embodiment, a silicon oxynitride layer having a thickness of 100 nm or less is formed by a plasma CVD method as the gate insulating layer  322 . 
     Next, an oxide semiconductor film having a thickness greater than or equal to 2 nm and less than or equal to 200 nm is formed over the gate insulating layer  322 , and is processed into an island-shaped oxide semiconductor layer  332  by a second photolithography step. In this embodiment, the oxide semiconductor film is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide semiconductor film. This is in order to prevent the oxide semiconductor film from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cry opump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide semiconductor film. 
     Next, dehydration and/or dehydrogenation of the oxide semiconductor layer are/is performed. The temperature of the first heat treatment for dehydration and/or dehydrogenation is higher than or equal to 400° C. and lower than or equal to 750° C., preferably higher than or equal to 400° C. and lower than the stain point of the substrate. In this embodiment, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layer at 450° C. for one hour in a nitrogen atmosphere, and then, water or hydrogen is prevented from entering the oxide semiconductor layer, without exposure to the air; thus, an oxide semiconductor layer  332  is obtained (see  FIG. 12A ). 
     Next, plasma treatment using a gas such as N 2 O, N 2 , or Ar is performed. This plasma treatment removes water or the like adsorbed on a surface of the oxide semiconductor layer which is exposed. In addition, plasma treatment may be performed using a mixed gas of oxygen and argon. 
     Next, an oxide insulating layer is formed over the gate insulating layer  322  and the oxide semiconductor layer  332 . After that, a resist mask is formed by a third photolithography step, and etching is performed selectively thereon, so that an oxide insulating layer  366  is formed. After that, the resist mask is removed. 
     In this embodiment, a 200-nm-thick silicon oxide film is deposited as the oxide insulating layer  366  by a sputtering method. The substrate temperature at the time of film deposition may be higher than or equal to room temperature and lower than or equal to 300° C. and in this embodiment, is 100° C. The silicon oxide film can be formed with a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas and oxygen. As a target, a silicon oxide target or a silicon target may be used. For example, with the use of a silicon target, silicon oxide can be deposited by a sputtering method under an atmosphere of oxygen and nitrogen. As the oxide insulating layer  366  which is formed in contact with the oxide semiconductor layer, an inorganic insulating film which does not include impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of these from the outside is used. Typically, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or the like is used. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide insulating layer  366 . This is in order to prevent the oxide semiconductor layer  332  and the oxide insulating layer  366  from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the oxide insulating layer  366  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide insulating layer  366 . 
     Next, a second heat treatment (preferably at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C.) is performed in an inert gas atmosphere or an oxygen gas atmosphere. For example, the second heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. With the second heat treatment, heat is applied on the state where part of the oxide semiconductor layer (the channel formation region) is in contact with the oxide insulating layer  366 . 
     In this embodiment, the oxide semiconductor layer  332  which is provided with the oxide insulating layer  366  and is partly exposed is further subjected to heat treatment in a nitrogen atmosphere or an inert gas atmosphere or under a reduced pressure. By the heat treatment in a nitrogen atmosphere or an inert gas atmosphere or under a reduced pressure, the resistance of the exposed region of the oxide semiconductor layer  332 , which is not covered by the oxide insulating layer  366  can be reduced. For example, heat treatment is performed at 250° C. in a nitrogen atmosphere for one hour. 
     With the heat treatment on the oxide semiconductor layer  332  provided with the oxide insulating layer  366  in a nitrogen atmosphere, the resistance of the exposed region of the oxide semiconductor layer  332  is reduced, so that an oxide semiconductor layer  362  including regions with different resistances (indicated as a shaded region and a white region in  FIG. 12B ) is formed. 
     Next, a conductive film is formed over the gate insulating layer  322 , the oxide semiconductor layer  362 , and the oxide insulating layer  366 . After that, a resist mask is formed by a fourth photolithography step, and selective etching is performed thereon, so that a source electrode layer  365   a  and a drain electrode layer  365   b  are formed. After that, the resist mask is removed (see  FIG. 12C ). 
     The source electrode layer  365   a  and the drain electrode layer  365   b  each are formed by an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, an alloy including any of the above elements as its component, an alloy film including a combination of any of these elements, or the like can be used. A single layer structure or a stacked-layer structure including two or more layer may be used as the conductive film. 
     Through the above process, a part of the oxide semiconductor film is selectively made to include excessive oxygen. As a result, a channel formation region  363  overlapping the gate electrode layer  361  becomes i-type, and a high-resistance source region  364   a  which overlaps the source electrode layer  365   a  and a high-resistance drain region  364   b  which overlaps the drain electrode layer  365   b  are formed in a self-aligned manner. Through the above steps, the thin film transistor  360  is formed. 
     Furthermore, heat treatment at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in the air for 1 hour to 30 hours both inclusive may be performed. In this embodiment, heat treatment is performed at 150° C. for 10 hours. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. Further, this heat treatment may be performed before the formation of the oxide insulating film under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. With this heat treatment, hydrogen is introduced from the oxide semiconductor layer to the oxide insulating layer; thus, a normally-off thin film transistor can be obtained. Therefore, reliability of the liquid crystal display device can be improved. 
     The high-resistance drain region  364   b  (or the high-resistance source region  364   a ) is formed in a portion of the oxide semiconductor layer which overlaps the drain electrode layer  365   b  (or the source electrode layer  365   a ), so that the reliability of the thin film transistor can be increased. Specifically, with the formation of the high-resistance drain region  364   b , the conductivity can be gradually varied from the drain electrode layer  365   b  to the high-resistance drain region  364   b  and the channel formation region  363  in the transistor. Therefore, in the case where the thin film transistor operates using the drain electrode layer  365   b  connected to a wiring for supplying a high power supply potential VDD, the high-resistance drain region serves as a buffer and a high electric field is not applied locally even if a high electric field is applied between the gate electrode layer  361  and the drain electrode layer  365   b , so that the withstand voltage of the transistor can be improved. 
     A protective insulating layer  323  is formed over the source electrode layer  365   a , the drain electrode layer  365   b , and the oxide insulating layer  366 . In this embodiment, the protective insulating layer  323  is formed using a silicon nitride film (see  FIG. 12D ). 
     An oxide insulating layer may be formed over the source electrode layer  365   a , the drain electrode layer  365   b , and the oxide insulating layer  366 , and the protective insulating layer  323  may be stacked over the oxide insulating layer. 
     The off-state current can be reduced in each of a plurality of pixels of a display portion of a liquid crystal display device using the thin film transistor using an oxide semiconductor layer as described above. Therefore, a period for holding voltage in a storage capacitor can be extended, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 7 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 8 
     In Embodiment 8, another example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. A thin film transistor  350  described in this embodiment can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
     A thin film transistor of this embodiment and an embodiment of a method for manufacturing the thin film transistor are described using  FIGS. 13A to 13D . 
     Although the thin film transistor  350  is described using a single-gate thin film transistor, a multi-gate thin film transistor including a plurality of channel formation regions can be formed as necessary. 
     Hereinafter, a process for manufacturing the thin film transistor  350  over a substrate  340  is described using  FIGS. 13A to 13D . 
     First, a conductive film is formed over the substrate  340  having an insulating surface, and a first photolithography step is performed, so that a gate electrode layer  351  is formed. In this embodiment, a 150-nm-thick tungsten film is formed by a sputtering method as the gate electrode layer  351 . 
     Next, a gate insulating layer  342  is formed over the gate electrode layer  351 . In this embodiment, a silicon oxynitride layer having a thickness of 100 nm or less is formed by a plasma CVD method as the gate insulating layer  342 . 
     Next, a conductive film is formed over the gate insulating layer  342 , and a resist mask is formed over the conductive film by a second photolithography step, and selective etching is performed thereon, so that a source electrode layer  355   a  and a drain electrode layer  355   b  are formed. After that, the resist mask is removed (see  FIG. 13A ). 
     Next, an oxide semiconductor film  345  is formed (see  FIG. 13B ). In this embodiment, the oxide semiconductor film  345  is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. The oxide semiconductor film  345  is processed into an island-shaped oxide semiconductor layer by a third photolithography step. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide semiconductor film  345 . This is in order to prevent the oxide semiconductor film  345  from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cry opump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film  345  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide semiconductor film  345 . 
     Next, dehydration and/or dehydrogenation of the oxide semiconductor layer are/is performed. The temperature of the first heat treatment for dehydration and/or dehydrogenation is higher than or equal to 400° C. and lower than or equal to 750° C., preferably higher than or equal to 400° C. and lower than the stain point of the substrate. In this embodiment, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layer at 450° C. for one hour in a nitrogen atmosphere, and then, water or hydrogen is prevented from entering the oxide semiconductor layer, without exposure to the air; thus, an oxide semiconductor layer  346  is obtained (see  FIG. 13C ). 
     For example, as the first heat treatment, GRTA may be performed as follows: the substrate is transferred into an inert gas heated to a high temperature of 650° C. to 700° C., heated for several minutes, and transferred and taken out of the inert gas heated to the high temperature. GRTA enables a high-temperature heat treatment for a short time. 
     Next, an oxide insulating layer  356  serving as a protective insulating film which is in contact with the oxide semiconductor layer  346  is formed. 
     The oxide insulating layer  356  can be formed to a thickness at least 1 nm by a method by which an impurity such as water or hydrogen does not enter the oxide insulating layer  356 , such as a sputtering method as appropriate. When hydrogen is contained in the oxide insulating layer  356 , entry of the hydrogen to the oxide semiconductor layer or extraction of oxygen in the oxide semiconductor layer by hydrogen may be caused, thereby making the backchannel of the oxide semiconductor layer n-type (making the resistance thereof low), so that a parasitic channel may be formed. Therefore, it is important that a formation method in which hydrogen is used as little as possible is employed such that the oxide insulating layer  356  contains hydrogen as little as possible. 
     In this embodiment, a 200-nm-thick silicon oxide film is deposited as the oxide insulating layer  356  by a sputtering method. The substrate temperature at the time of film deposition may be higher than or equal to room temperature and lower than or equal to 300° C. and in this embodiment, is 100° C. The silicon oxide film can be formed with a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas and oxygen. As a target, a silicon oxide target or a silicon target may be used. For example, with the use of a silicon target, silicon oxide can be deposited by a sputtering method under an atmosphere of oxygen and nitrogen. As the oxide insulating layer  356  which is formed in contact with the oxide semiconductor layer, an inorganic insulating film which does not include impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of these from the outside is used. Typically, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or the like is used. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide insulating layer  356 . This is in order to prevent the oxide semiconductor layer  346  and the oxide insulating layer  356  from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cry opump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the oxide insulating layer  356  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide insulating layer  356 . 
     Next, a second heat treatment (preferably at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C.) is performed in an inert gas atmosphere or an oxygen gas atmosphere. For example, the second heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. With the second heat treatment, heat is applied on the state where part of the oxide semiconductor layer (the channel formation region) is in contact with the oxide insulating layer  356 . 
     Through the above process, the oxide semiconductor film is selectively made to include excessive oxygen. As a result, an i-type oxide semiconductor layer  352  is formed. Through the above steps, the thin film transistor  350  is formed. 
     Furthermore, heat treatment at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in the air for 1 hour to 30 hours both inclusive may be performed. In this embodiment, heat treatment is performed at 150° C. for 10 hours. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. Further, this heat treatment may be performed before the formation of the oxide insulating film under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. With this heat treatment, hydrogen is introduced from the oxide semiconductor layer to the oxide insulating layer; thus, a normally-off thin film transistor can be obtained. Therefore, reliability of the liquid crystal display device can be improved. 
     A protective insulating layer may be formed over the oxide insulating layer  356 . For example, a silicon nitride film is formed by an RF sputtering method. In this embodiment, a protective insulating layer  343  is formed using a silicon nitride film as the protective insulating layer (see  FIG. 13D ). 
     A planarization insulating layer for planarization may be provided over the protective insulating layer  343 . 
     The off-state current can be reduced in the thin film transistor using an oxide semiconductor layer, manufactured as described above. Therefore, by using the thin film transistor in each of a plurality of pixels of a display portion of a liquid crystal display device, a period for holding voltage in a storage capacitor can be extended, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 8 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 9 
     In Embodiment 9, an example which is different from Embodiment 6 in the manufacturing process of a thin film transistor will be described using  FIG. 14 . Since  FIG. 14  is the same as  FIGS. 11A to 11E  except for part of the steps, the same reference numerals are used for the same portions, and detailed description of the same portions is not repeated. 
     In Embodiment 9, another example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. A thin film transistor  380  described in this embodiment can be used as a thin film transistor in each pixel of the pixel portion  1008  described in Embodiment 1. 
     In accordance with Embodiment 6, a gate electrode layer  381  is formed over a substrate  370 , and a first gate insulating layer  372   a  and a second gate insulating layer  372   b  are stacked. In this embodiment, a gate insulating layer has a two-layer structure, in which a nitride insulating layer is used as the first gate insulating layer  372   a  and an oxide insulating layer is used as the second gate insulating layer  372   b.    
     As the oxide insulating layer, a silicon oxide layer, a silicon oxynitride layer, an aluminum oxide layer, an aluminum oxynitride layer, or the like can be used. As the nitride insulating layer, a silicon nitride layer, a silicon nitride oxide layer, an aluminum nitride layer, an aluminum nitride oxide layer, or the like can be used. 
     In this embodiment, the gate insulating layer has a structure in which a silicon nitride layer and a silicon oxide layer are stacked over the gate electrode layer  381 . For example, a 150-nm-thick gate insulating layer is formed in such a manner that a silicon nitride layer (SiN y  (y&gt;0)) having a thickness of greater than or equal to 50 nm and less than or equal to 200 nm (50 nm in this embodiment) is formed by a sputtering method as the first gate insulating layer  372   a  and then a silicon oxide layer (SiO x  (x&gt;0)) having a thickness of greater than or equal to 5 nm and less than or equal to 300 nm (100 nm in this embodiment) is stacked as the second gate insulating layer  372   b  over the first gate insulating layer  372   a.    
     Next, an oxide semiconductor film is formed and is processed into an island-shaped oxide semiconductor layer by a photolithography step. In this embodiment, the oxide semiconductor film is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide semiconductor film. This is in order to prevent the oxide semiconductor film from containing hydrogen, a hydroxyl group, or moisture. 
     In order to remove residual moisture from the chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed with the use of a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like, for example, is exhausted. Accordingly, the concentration of impurities included in the oxide semiconductor film formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide semiconductor film. 
     Next, dehydration and/or dehydrogenation of the oxide semiconductor layer are/is performed. The temperature of the first heat treatment for dehydration and/or dehydrogenation is higher than or equal to 400° C. and lower than or equal to 750° C., preferably higher than or equal to 425° C. The heat treatment time is one hour or shorter at a temperature of higher than or equal to 425° C. and is longer than one hour at a temperature of lower than 425° C. In this embodiment, the substrate is put in an electric furnace which is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor layer in a nitrogen atmosphere, and then, water or hydrogen is prevented from entering the oxide semiconductor layer, without exposure to the air; thus, an oxide semiconductor layer is obtained. After that, cooling is performed by introduction of a high-purity oxygen gas, a high-purity N 2 O gas, or ultra-dry air (having a dew point of −40° C. or lower, preferably −60° C. or lower) into the same furnace. It is preferable that the oxygen gas or the N 2 O gas do not contain water, hydrogen, or the like. Alternatively, the purity of an oxygen gas or an N 2 O gas which is introduced into the heat treatment apparatus is preferably 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration of the oxygen gas or the N 2 O gas is 1 ppm or lower, preferably 0.1 ppm or lower). 
     The heat treatment apparatus is not limited to an electric furnace. For example, an RTA (rapid thermal annaling) apparatus such as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermal annealing) apparatus can be used. The 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. The LRTA apparatus may be provided with not only a lamp but also a device that heats an object to be processed by thermal conduction or thermal radiation from a heater such as a resistance heater or the like. The GRTA is a method for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with the object to be processed, by heat treatment, such as nitrogen or a rare gas such as argon is used. The heat treatment may be performed at 600° C. to 750° C. for several minutes by an RTA method. 
     After the first heat treatment for dehydration and/or dehydrogenation, heat treatment may be performed at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., preferably higher than or equal to 200° C. and lower than or equal to 300° C., in an oxygen gas atmosphere or a N 2 O gas atmosphere. 
     The first heat treatment of the oxide semiconductor layer can also be performed on the oxide semiconductor film before being processed into the island-like oxide semiconductor layer. In that case, the substrate is taken out from the heat apparatus after the first heat treatment, and then a photolithography step is performed thereon. 
     The entire oxide semiconductor film is made to contain an excess amount of oxygen through the above steps, whereby the oxide semiconductor film has higher resistance, that is, becomes i-type. Accordingly, an oxide semiconductor layer  382  whose entire region is i-type is formed. 
     Next, a resist mask is formed by a photolithography step over the oxide semiconductor layer  382 , and is selectively etched to form a source electrode layer  385   a  and a drain electrode layer  385   b , and then, an oxide insulating layer  386  is formed by a sputtering method. 
     In that case, it is preferable to remove residual moisture in the chamber in the deposition of the oxide insulating layer  386 . This is in order to prevent the oxide semiconductor layer  382  and the oxide insulating layer  386  from containing hydrogen, a hydroxyl group, and/or moisture. 
     In order to remove the residual moisture in the chamber, it is preferable to use an adsorption-type vacuum pump. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the chamber in which exhaustion is performed using a cryopump, a hydrogen molecule, a compound including a hydrogen atom such as water (H 2 O), or the like is exhausted. Accordingly, the concentration of impurities included in the oxide insulating layer  386  formed in the chamber can be reduced. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to 1 ppm or less, preferably 10 ppb or less be used as the sputtering gas for the deposition of the oxide insulating layer  386 . 
     Through the above steps, the thin film transistor  380  can be manufactured. 
     Next, heat treatment (preferably at a temperature higher than or equal to 150° C. and lower than 350° C.) may be performed in an inert gas atmosphere or a nitrogen gas atmosphere in order to suppress variation of electrical characteristics of the thin film transistor. For example, heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere. 
     Furthermore, heat treatment at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in the air for 1 hour to 30 hours both inclusive may be performed. In this embodiment, heat treatment is performed at 150° C. for 10 hours. This heat treatment may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. Further, this heat treatment may be performed before the formation of the oxide insulating film under a reduced pressure. Under the reduced pressure, the heat treatment time can be shortened. With this heat treatment, hydrogen is introduced from the oxide semiconductor layer to the oxide insulating layer; thus, a normally-off thin film transistor can be obtained. Therefore, reliability of the liquid crystal display device can be improved. 
     A protective insulating layer  373  is formed over the oxide insulating layer  386 . In this embodiment, a 100-nm-thick silicon nitride film is formed as the protective insulating layer  373  by a sputtering method. 
     The protective insulating layer  373  and the first gate insulating layer  372   a , which are nitride insulating layers, do not contain impurities such as moisture, hydrogen, hydride, or hydroxide and blocks them from entering from the outside. 
     Therefore, in the manufacturing process after the formation of the protective insulating layer  373 , entry of impurities such as moisture from the outside can be prevented. Further, even after a device is completed as a semiconductor device such as a liquid crystal display device, entry of impurities such as moisture from the outside can be prevented in the long term; therefore, long-term reliability of the device can be improved. 
     The insulating layers provided between the protective insulating layer  373  which is a nitride insulating layer and the first gate insulating layer  372   a  may be removed to make the protective insulating layer  373  in contact with the first gate insulating layer  372   a.    
     Accordingly, impurities such as moisture, hydrogen, hydride, or hydroxide in the oxide semiconductor layer can be reduced and entry thereof is prevented, so that the concentration of impurities in the oxide semiconductor layer can be kept to be low. 
     A planarization insulating layer for planarization may be provided over the protective insulating layer  373 . 
     The off-state current can be reduced in each of a plurality of pixels of a display portion of a liquid crystal display device using the thin film transistor using an oxide semiconductor layer as described above. Therefore, a period for holding voltage in a storage capacitor can be extended, and the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 9 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 10 
     In Embodiment 10, another example of a thin film transistor which can be applied to a liquid crystal display device disclosed in this specification will be described. A thin film transistor described in this embodiment can be used as the thin film transistor in any of Embodiments 2 to 8, which can be used as the thin film transistor in Embodiment 1. 
     In Embodiment 10, an example of using a conductive material having a light-transmitting property as any of a gate electrode layer, a source electrode layer, and a drain electrode layer will be described. Note that the above embodiment can be applied to the same portions and the portions and steps having similar functions as/to the above embodiment, and description thereof is not repeated. Further, a specific description for the same portions is omitted. 
     As a material of any of a gate electrode layer, a source electrode layer, and a drain electrode layer, a conductive material that transmits visible light can be used. For example, any of the following metal oxides can be used: an In—Sn—O-based metal oxide; an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide. The thickness thereof is set in the range of greater than or equal to 50 nm and less than or equal to 300 nm as appropriate. As a deposition method of the metal oxide used for any of the gate electrode layer, the source electrode layer, and the drain electrode layer, a sputtering method, a vacuum evaporation method (an electron beam evaporation method or the like), an arc discharge ion plating method, or a spray method is used. In the case where a sputtering method is employed, it is preferable that deposition be performed using a target containing SiO 2  at 2 wt % to 10 wt % both inclusive and SiO x  (x&gt;0) which inhibits crystallization be contained in the light-transmitting conductive film so as to prevent crystallization at the time of the heat treatment in a later step. 
     Note that the unit of the percentage of components in the light-transmitting conductive film is atomic percent, and the percentage of components is evaluated by analysis using an electron probe X-ray microanalyzer (EPMA). 
     In a pixel provided with a thin film transistor, when a pixel electrode layer, another electrode layer (such as a capacitor electrode layer), or a wiring layer (such as a capacitor wiring layer) is formed using a conductive film that transmits visible light, a display device having high aperture ratio can be realized. Needless to say, it is preferable that a gate insulating layer, an oxide insulating layer, a protective insulating layer, and a planarization insulating layer in the pixel be also each formed using a film that transmits visible light. 
     In this specification, a film that transmits visible light means a film having such a thickness as to have transmittance of visible light of 75% to 100%. In the case where the film has conductivity, the film is also referred to as a transparent conductive film. Further, a conductive film which is semi-transmissive with respect to visible light may be used as metal oxide applied to the gate electrode layer, the source electrode layer, the drain electrode layer, the pixel electrode layer, another electrode layer, or another wiring layer. The conductive film which is semi-transmissive with respect to visible light indicates a film having transmittance of visible light of 50% to 75%. 
     When a thin film transistor has a light-transmitting property, the aperture ratio can be increased. For small liquid crystal display panels of 10 inches or smaller in particular, a high aperture ratio can be achieved even when the size of pixels is decreased in order to realize higher resolution of display images by increasing the number of gate wirings, for example. Further, by using a film having a light-transmitting property for components of a thin film transistor, a high aperture ratio can be achieved even when one pixel is divided into a plurality of sub-pixels in order to realize a wide viewing angle. That is, a high aperture ratio can be maintained even when a group of high-density thin film transistors is provided, so that a sufficient area of a display region can be secured. For example, in the case where one pixel includes two to four sub-pixels, an aperture ratio can be improved because the thin film transistor has a light-transmitting property. Further, a storage capacitor may be formed using the same material by the same step as the component in the thin film transistor so that the storage capacitor can have a light-transmitting property, whereby the aperture ratio can be further improved. 
     Embodiment 10 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 11 
     The appearance and the cross section of a liquid crystal display panel, which is an embodiment of a liquid crystal display device, are described with reference to  FIGS. 15A to 15C .  FIGS. 15A and 15C  are each a top view of a panel in which thin film transistors  4010  and  4011  and a liquid crystal element  4013 , which are formed over a first substrate  4001 , are sealed between the first substrate  4001  and a second substrate  4006  with a sealant  4005 .  FIG. 15B  corresponds to a cross-sectional view of  FIG. 15A or 15C  along line M-N. 
     The sealant  4005  is provided so as to surround a pixel portion  4002  and a scan line driver circuit  4004  which are provided over the first substrate  4001 . The second substrate  4006  is provided over the pixel portion  4002  and the scan line driver circuit  4004 . Therefore, the pixel portion  4002  and the scan line driver circuit  4004  are sealed together with a liquid crystal layer  4008 , by the first substrate  4001 , the sealant  4005 , and the second substrate  4006 . A signal line driver circuit  4003  that is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant  4005  over the first substrate  4001 . 
     Note that a connection method of a driver circuit which is separately formed is not particularly limited; a COG method, a wire bonding method, a TAB method, or the like can be used.  FIG. 15A  illustrates an example of mounting the signal line driver circuit  4003  by a COG method, and  FIG. 15C  illustrates an example of mounting the signal line driver circuit  4003  by a TAB method. 
     Further, the pixel portion  4002  and the scan line driver circuit  4004  provided over the first substrate  4001  each include a plurality of thin film transistors.  FIG. 15B  illustrates the thin film transistor  4010  included in the pixel portion  4002  and the thin film transistor  4011  included in the scan line driver circuit  4004 . Over the thin film transistors  4010  and  4011 , insulating layers  4041 ,  4042 ,  4020 , and  4021  are provided. 
     Any one of the thin film transistors described in Embodiments 2 to 9 can be used as each of the thin film transistors  4010  and  4011  as appropriate, and can be formed using a similar process and a similar material. In the oxide semiconductor layer of each of the thin film transistors  4010  and  4011 , hydrogen or water is reduced. Thus, the thin film transistors  4010  and  4011  have high reliability. In this embodiment, the thin film transistors  4010  and  4011  are n-channel thin film transistors. 
     A conductive layer  4040  is provided over part of the insulating layer  4021 , which overlaps a channel formation region of the oxide semiconductor layer in the thin film transistor  4011  for the drive circuit. The conductive layer  4040  is provided at the position overlapping the channel formation region of the oxide semiconductor layer, whereby the amount of change in the threshold voltage of the thin film transistor  4011  by a BT test can be reduced. A potential of the conductive layer  4040  may be the same as or different from that of a gate electrode layer of the thin film transistor  4011 . The conductive layer  4040  can also function as a second gate electrode layer. In addition, the potential of the conductive layer  4040  may be GND or 0 V, or the conductive layer  4040  may be in a floating state. 
     A pixel electrode layer  4030  included in the liquid crystal element  4013  is electrically connected to a source electrode layer or a drain electrode layer of the thin film transistor  4010 . A counter electrode layer  4031  of the liquid crystal element  4013  is provided on the second substrate  4006 . A portion where the pixel electrode layer  4030 , the counter electrode layer  4031 , and the liquid crystal layer  4008  overlap each other corresponds to the liquid crystal element  4013 . Note that the pixel electrode layer  4030  and the counter electrode layer  4031  are provided with an insulating layer  4032  and an insulating layer  4033  respectively which each function as an alignment film, and the liquid crystal layer  4008  is sandwiched between the pixel electrode layer  4030  and the counter electrode layer  4031  with the insulating layers  4032  and  4033  interposed therebetween. 
     As each of the first substrate  4001  and the second substrate  4006 , a light-transmitting substrate can be used; glass, ceramic, or plastic can be used. As plastic, a fiberglass-reinforced plastic (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. 
     A spacer  4035  is a columnar spacer obtained by selective etching of an insulating film and is provided in order to control the distance (a cell gap) between the pixel electrode layer  4030  and the counter electrode layer  4031 . Alternatively, a spherical spacer may be used. In addition, the counter electrode layer  4031  is electrically connected to a common potential line formed over the same substrate as the thin film transistor  4010 . With the use of a common connection portion, the counter electrode layer  4031  and the common potential line can be electrically connected to each other by conductive particles arranged between a pair of substrates. The conductive particles are included in the sealant  4005 . 
     As the liquid crystal, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer-dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like is used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     Alternatively, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase is only generated within a narrow range of temperatures, a liquid crystal composition containing a chiral agent at greater than or equal to 5 wt % is used for the liquid crystal layer  4008  in order to widen the temperature range. The liquid crystal composition which includes a liquid crystal exhibiting a blue phase and a chiral agent has a short response time of less than or equal to 1 msec, has optical isotropy, which makes the alignment process unneeded, and has a small viewing angle dependence. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. Thus, productivity of the liquid crystal display device can be improved. A thin film transistor including an oxide semiconductor layer particularly has a possibility that electrical characteristics of the thin film transistor may significantly change and deviate from the designed range by the influence of static electricity. Therefore, it is more effective to use a blue phase liquid crystal material for a liquid crystal display device having a thin film transistor including an oxide semiconductor layer. 
     The specific resistance of the liquid crystal material in this embodiment is 1×10 12  Ω·cm or more, preferably 1×10 13  Ω·cm or more, far preferably 1×10 14  Ω·cm or more. The resistance in the case where a liquid crystal cell using the liquid crystal material is 1×10 11  Ω·cm or more, in which an impurity from the alignment film or the sealant may enter, and is preferably over 1×10 12  Ω·cm. The value of the specific resistance in this specification is measured at 20° C. 
     As the specific resistance of the liquid crystal material increases, the amount of charge which leaks through the liquid crystal material can be decreased, so that a decrease over time of a voltage for holding the operation state of the liquid crystal element can be suppressed. As a result, the holding period can be extended, the frequency of signal writing can be decreased, and low power consumption of the display device can be achieved. 
     This embodiment of the present invention can also be applied to either a semi-transmissive (transflective) or reflective liquid crystal display device as well as a transmissive liquid crystal display device. The display device of this embodiment is not limited to a liquid crystal display device, and may be an EL display device using a light-emitting element such as an electroluminescent element (also called an EL element) as a display element. 
     An example of the liquid crystal display device is illustrated in which a polarizing plate is provided on the outer surface of the substrate (on the viewer side) and a coloring layer and an electrode layer used for a display element are provided on the inner surface of the substrate in this order; however, the polarizing plate may be provided on the inner surface of the substrate. The stacked-layer structure of the polarizing plate and the coloring layer is not limited to that described in this embodiment and may be set as appropriate depending on materials of the polarizing plate and the coloring layer or conditions of the manufacturing process. Further, a light blocking film serving as a black matrix may be provided in a region other than a display portion. 
     Over the thin film transistors  4011  and  4010 , the insulating layer  4041  is formed in contact with the oxide semiconductor layers. The insulating layer  4041  may be formed using a similar material by a similar method to the oxide insulating layer  416  described in Embodiment 2. In this embodiment, as the insulating layer  4041 , a silicon oxide layer is formed by a sputtering method, using Embodiment 2. Further, the protective insulating layer  4042  is formed on and in contact with the insulating layer  4041 . The protective insulating layer  4042  may be formed in a similar manner to the protective insulating layer  403  described in Embodiment 2; for example, a silicon nitride film can be used. In addition, in order to reduce the surface roughness of the thin film transistors, the protective insulating layer  4042  is covered with the insulating layer  4021  functioning as a planarization insulating film. 
     The insulating layer  4021  is formed as the planarization insulating film. As the insulating layer  4021 , an organic material having heat resistance such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. The insulating layer  4021  may be formed by stacking a plurality of insulating films formed using these materials. 
     There is no particular limitation on the method for forming the insulating layer  4021 . The insulating layer  4021  can be formed, depending on the material, by a method such as a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, or a droplet discharge method (e.g., an ink-jet method, screen printing, or offset printing), or a tool (equipment) such as a doctor knife, a roll coater, a curtain coater, or a knife coater. The baking step of the insulating layer  4021  also serves as annealing of the semiconductor layer, whereby a liquid crystal display device can be manufactured efficiently. 
     The pixel electrode layer  4030  and the counter electrode layer  4031  can be formed using a light transmitting conductive material, such as indium tin oxide (ITO), indium zinc oxide (IZO) in which zinc oxide (ZnO) is mixed in indium oxide, a conductive material in which silicon oxide (SiO 2 ) is mixed in indium oxide, organoindium, organotin, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, or indium tin oxide containing titanium oxide. Alternatively, in the case where a light transmitting property is not needed or a reflective property is needed for the pixel electrode layer  4030  or the counter electrode layer  4031  in a reflective liquid crystal display device, the pixel electrode layer  4030  or the counter electrode layer  4031  can be formed using one kind or plural kinds selected from metal such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), or silver (Ag), an alloy thereof, and a nitride thereof. 
     A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) can be used for the pixel electrode layer  4030  and the counter electrode layer  4031 . The pixel electrode formed using the conductive composition preferably has a sheet resistance of 10000 ohms per square or less and a transmittance of 70% or more at a wavelength of 550 nm. Further, the resistivity of the conductive high molecule contained in the conductive composition is preferably 0.1 Ω·cm or less. 
     As the conductive high molecule, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more kinds of them, and the like can be given. 
     Furthermore, a variety of signals and potentials are supplied to the signal line driver circuit  4003  which is formed separately, the scan line driver circuit  4004 , or the pixel portion  4002  from an FPC  4018 . 
     A connection terminal electrode  4015  is formed from the same conductive film as the pixel electrode layer  4030  included in the liquid crystal element  4013 , and a terminal electrode  4016  is formed from the same conductive film as the source and drain electrode layers of the thin film transistors  4010  and  4011 . 
     The connection terminal electrode  4015  is electrically connected to a terminal included in the FPC  4018  via an anisotropic conductive film  4019 . 
       FIGS. 15A to 15C  illustrate an example in which the signal line driver circuit  4003  is formed separately and mounted on the first substrate  4001 ; however, this embodiment is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted. 
     A black matrix (a light blocking layer), an optical member (an optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, and the like are provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source. 
     In an active matrix liquid crystal display device, display patterns are formed on a screen by driving of pixel electrodes that are arranged in matrix. Specifically, voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the counter electrode is optically modulated. This optical modulation is recognized as a display pattern by a viewer. 
     Further, since the thin film transistor is easily damaged due to static electricity or the like, a protective circuit is preferably provided over the same substrate as the pixel portion or the driver circuit portion. The protective circuit is preferably formed with a non-linear element including an oxide semiconductor layer. For example, a protective circuit is provided between the pixel portion, and a scan line input terminal and a signal line input terminal. In this embodiment, a plurality of protective circuits is provided so that the pixel transistor and the like are not damaged when surge voltage due to static electricity or the like is applied to the scan line, the signal line, or a capacitor bus line. Accordingly, the protective circuit is configured to release charges to a common wiring when surge voltage is applied to the protective circuit. The protective circuit includes non-linear elements which are arranged in parallel between the common wiring and the scan line, the signal line, or the capacitor bus line. Each of the non-linear elements includes a two-terminal element such as a diode or a three-terminal element such as a transistor. For example, the non-linear element can be formed through the same steps as the thin film transistor of the pixel portion. For example, characteristics similar to those of a diode can be achieved by connecting a gate terminal to a drain terminal. 
     Further, for a liquid crystal display module, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     There is no particular limitation on the liquid crystal display device disclosed in this specification; a TN liquid crystal, an OCB liquid crystal, an STN liquid crystal, a VA liquid crystal, an ECB liquid crystal, a GH liquid crystal, a polymer dispersed liquid crystal, a discotic liquid crystal, or the like can be used. In particular, a normally black liquid crystal panel such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode is preferable. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, or the like can be employed. 
     Furthermore, this embodiment can be applied to a VA liquid crystal display device. The VA liquid crystal display device has a kind of form in which alignment of liquid crystal molecules of a liquid crystal display panel is controlled. In the VA liquid crystal display device, liquid crystal molecules are aligned in a vertical direction with respect to a panel surface when no voltage is applied. Furthermore, it is possible to use a method called domain multiplication or multi-domain design, in which a pixel is divided into some regions (subpixels) and molecules are aligned in different directions in their respective regions. 
     Embodiment 11 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 12 
     In Embodiment 12, examples of electronic apparatuses including any of the liquid crystal display devices of the embodiments described above will be described. 
       FIG. 16A  illustrates a portable game machine, which can include a housing  9630 , a display portion  9631 , a speaker  9633 , operation keys  9635 , a connection terminal  9636 , a recording medium reading portion  9672 , and the like. The portable game machine illustrated in  FIG. 16A  can have a function of reading a program or data stored in a recording medium to display it on the display portion, a function of sharing information with another portable game machine by wireless communication, and the like. The portable game machine illustrated in  FIG. 16A  can have various functions besides those given above. 
       FIG. 16B  illustrates a digital camera, which can include a housing  9630 , a display portion  9631 , a speaker  9633 , operation keys  9635 , a connection terminal  9636 , a shutter button  9676 , an image receiving portion  9677 , and the like. The digital camera having a television reception function in  FIG. 16B  can have a function of photographing a still image and/or a moving image, a function of automatically or manually correcting the photographed image, a function of obtaining various kinds of information from an antenna, a function of storing the photographed image or the information obtained from the antenna, and a function of displaying the photographed image or the information obtained from the antenna on the display portion. The digital camera having the television reception function in  FIG. 16B  can have various functions besides those given above. 
       FIG. 16C  illustrates a television set, which can include a housing  9630 , a display portion  9631 , a speaker  9633 , operation keys  9635 , a connection terminal  9636 , and the like. The television set in  FIG. 16C  can have a function of processing and converting an electric wave for television into an image signal, a function of processing and converting the image signal into a signal suitable for display, a function of converting a frame frequency of the image signal, and the like. The television set in  FIG. 16C  can have various functions besides those given above. 
       FIG. 17A  illustrates a computer, which can include a housing  9630 , a display portion  9631 , a speaker  9633 , operation keys  9635 , a connection terminal  9636 , a pointing device  9681 , an external connection port  9680 , and the like. The computer in  FIG. 17A  can have a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a function of controlling processing by a variety of software (programs), a communication function such as wireless communication or wired communication, a function of being connected to various computer networks with the communication function, a function of transmitting or receiving a variety of data with the communication function, and the like. The computer illustrated in  FIG. 17A  can have various functions besides those given above. 
       FIG. 17B  illustrates a mobile phone, which can include a housing  9630 , a display portion  9631 , a speaker  9633 , operation keys  9635 , a microphone  9638 , and the like. The mobile phone in  FIG. 17B  can have a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a function of displaying a calendar, a date, the time, or the like on the display portion, a function of operating or editing the information displayed on the display portion, a function of controlling processing by various kinds of software (programs), and the like. The mobile phone in  FIG. 17B  can have various functions besides those given above. 
       FIG. 17C  illustrates electronic paper (also referred to as an e-book), which can include a housing  9630 , a display portion  9631 , operation key  9635 , and the like. The electronic paper in  FIG. 17C  can have a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a function of displaying a calendar, a date, the time, and the like on the display portion, a function of operating or editing the information displayed on the display portion, a function of controlling processing by various kinds of software (programs), and the like. The electronic paper in  FIG. 17C  can have various functions besides those given above. 
     In each of the electronic apparatuses described in this embodiment, in each of a plurality of pixels of a display portion of a liquid crystal display device, the off-state current can be decreased. Accordingly, a period for holding voltage in a storage capacitor can be increased, and the electronic apparatuses in which the power consumption when a still image or the like is displayed in the liquid crystal display device can be decreased can be manufactured. Further, by stopping supplying a control signal in the case where a still image is displayed, power consumption can be further decreased. In addition, a still image and a moving image can be switched without malfunction. 
     Embodiment 12 can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 13 
     In Embodiment 13, a principle of operation of a bottom-gate transistor including an oxide semiconductor will be described. 
       FIG. 19  is a cross-sectional view of an inverted-staggered insulated-gate transistor including an oxide semiconductor. An oxide semiconductor layer (OS) is provided over a gate electrode (G 1 ) with a first gate insulating film (GI 1 ) interposed therebetween, and a source electrode (S) and a drain electrode (D) are provided thereover. Further, a second gate insulating film (GI 2 ) is provided over the source electrode (S) and the drain electrode (D), and a second gate electrode (G 2 ) is provided thereover. G 2  is maintained to ground potential. 
     Hereinafter, description is made using energy band diagrams. The energy band diagrams described here are so simplified as much as possible for understanding that they are not strict.  FIGS. 20A and 20B  are energy band diagrams (schematic diagrams) along an A-A′ section illustrated in  FIG. 19 .  FIG. 20A  illustrates the case where the potential of a voltage applied to the source is equal to the potential of a voltage applied to the drain (VD=0 V), and  FIG. 20B  illustrates the case where a positive potential with respect to the source is applied to the drain (VD&gt;0). 
       FIGS. 21A and 21B  are energy band diagrams (schematic diagrams) along a B-B′ section illustrated in  FIG. 19 .  FIG. 21A  illustrates an on state in which a positive potential (+VG) is applied to the gate (G 1 ) and carriers (electrons) flow between the source and the drain.  FIG. 21B  illustrates an off state in which a negative potential (−VG) is applied to the gate (G 1 ) and minority carriers do not flow. 
       FIG. 22  illustrates the relationships between the vacuum level and the work function of a metal (ϕM) and between the vacuum level and the electron affinity (χ) of an oxide semiconductor. 
     Since metal is degenerated, the conduction band and the Fermi level correspond to each other. On the other hand, a conventional oxide semiconductor is typically an n-type semiconductor, in which case the Fermi level (Ef) is away from the intrinsic Fermi level (Ei) located in the middle of a band gap and is located closer to the conduction band. Note that it is known that hydrogen is a donor in an oxide semiconductor and is one factor causing an oxide semiconductor to be an n-type semiconductor. 
     On the other hand, an oxide semiconductor of the present invention is an intrinsic (i-type) or a substantially intrinsic oxide semiconductor which is obtained by removing hydrogen that is an n-type impurity from an oxide semiconductor and purifying the oxide semiconductor such that an impurity other than a main component of the oxide semiconductor is prevented from being contained therein as much as possible. In other words, a feature is that a purified i-type (intrinsic) semiconductor, or a semiconductor close thereto, is obtained not by adding an impurity but by removing an impurity such as hydrogen or water as much as possible. This enables the Fermi level (Ef) to be at the same level as the intrinsic Fermi level (Ei). 
     In the case where the band gap (Eg) of an oxide semiconductor is 3.15 eV, the electron affinity (χ) is said to be 4.3 eV. The work function of titanium (Ti) included in the source electrode and the drain electrode is substantially equal to the electron affinity (χ) of the oxide semiconductor. In that case, a Schottky barrier for electrons is not formed at an interface between the metal and the oxide semiconductor. 
     In other words, in the case where the work function of metal (ϕM) and the electron affinity (χ) of the oxide semiconductor are equal to each other and the metal and the oxide semiconductor are in contact with each other, an energy band diagram (a schematic diagram) as illustrated in  FIG. 20A  is obtained. 
     In  FIG. 20B , a black circle (●) represents an electron, and when a positive potential is applied to the gate and the drain, the electron is injected into the oxide semiconductor over the barrier (h) and flows toward the drain. In that case, the height of the barrier (h) changes depending on the gate voltage and the drain voltage; in the case where a positive drain voltage is applied, the height of the barrier (h) is smaller than the height of the barrier, in  FIG. 20A  where no voltage is applied, i.e., ½ of the band gap (Eg). 
     The electron injected into the oxide semiconductor at this time flows in the oxide semiconductor as illustrated in  FIG. 21A . In addition, in  FIG. 21B , when a negative potential (reverse bias) is applied to the gate electrode (G 1 ), the value of current is extremely close to zero because holes that are minority carriers are substantially zero. 
     For example, even when an insulated-gate transistor as described above has a channel width W of 1×10 4  μm and a channel length of 3 μm, the off-state current is 10 −13  A or less and the subthreshold swing (S value) can be 0.1 V/dec (the thickness of the gate insulating film: 100 nm). 
     Note that the intrinsic carrier concentration of a silicon semiconductor is 1.45×10 10 /cm 3  (300 K) and carriers exist even at room temperature. This means that thermally excited carriers exist even at room temperature. A silicon wafer to which an impurity such as phosphorus or boron is added is practically used. In addition, even in a so-called intrinsic silicon wafer, impurities that cannot be controlled exist. Therefore, carriers exist in practice in a silicon semiconductor at 1×10 14 /cm 3  or more, which contributes to a conduction between the source and the drain. Furthermore, the band gap of a silicon semiconductor is 1.12 eV, and thus the off-state current of a transistor including a silicon semiconductor significantly changes depending on temperature. 
     Therefore, not by simply using an oxide semiconductor having a wide band gap for a transistor but by purifying the oxide semiconductor such that an impurity other than a main component can be prevented from being contained therein as much as possible so that the carrier concentration becomes less than 1×10 14 /cm 3 , preferably 1×10 12 /cm 3  or less, carriers to be thermally excited at a practical operation temperature can be eliminated, and the transistor can be operated only with carriers that are injected from the source side. This makes it possible to decrease the off-state current to 1×10 −13  A or less and to obtain a transistor whose off-state current hardly changes with a change in temperature and which is capable of extremely stable operation. 
     A technical idea of the present invention is that an impurity is not added to an oxide semiconductor and on the contrary the oxide semiconductor itself is purified by removing an impurity such as water or hydrogen which undesirably exists therein. In other words, a feature of an embodiment of the present invention is that an oxide semiconductor itself is purified by removing water or hydrogen which forms a donor level and further by sufficiently supplying oxygen to eliminate oxygen defects. 
     In an oxide semiconductor, even just after the deposition, hydrogen is observed on the order of 10 20 /cm 3  by secondary ion mass spectrometry (SIMS). One technical idea of the present invention is to purify an oxide semiconductor and obtain an electrically i-type (intrinsic) semiconductor by intentionally removing an impurity such as water or hydrogen which forms a donor level and further by adding oxygen (one of components of the oxide semiconductor), which decreases at the same time as removing water or hydrogen, to the oxide semiconductor. 
     As a result, it is preferable that the amount of hydrogen be as small as possible, and it is also preferable that the number of carriers in the oxide semiconductor be as small as possible. The oxide semiconductor is a purified i-type (intrinsic) semiconductor from which carriers have been eliminated and to which a meaning as a path of carriers as a semiconductor is given, rather than intentionally including carriers as a semiconductor, when used for an insulated-gate transistor. 
     As a result, by completely eliminating carriers from an oxide semiconductor or significantly reducing carries therein, the off-state current of an insulated-gate transistor can be decreased, which is a technical idea of an embodiment of the present invention. In other words, as a criterion, the hydrogen concentration is 1×10 16 /cm 3  or less and the carrier concentration is less than 1×10 14 /cm 3 , preferably 1×10 12 /cm 3  or less. According to a technical idea of the present invention, the ideal hydrogen concentration and carrier concentration are zero or close to zero. 
     In addition, as a result, the oxide semiconductor functions as a path, and the oxide semiconductor itself is an i-type (intrinsic) semiconductor which is purified so as to include no carriers or extremely few carriers, and carriers are supplied by an electrode on the source side. The degree of supply is determined by the barrier height that is obtained from the electron affinity χ of the oxide semiconductor, the Fermi level, which ideally corresponds to the intrinsic Fermi level, and the work function of the source or drain electrode. 
     Therefore, it is preferable that off-state current be as small as possible, and a feature of an embodiment of the present invention is that in characteristics of an insulated-gate transistor to which a drain voltage of 1 V to 10 V is applied, the off-state current per micrometer of channel width is 100 aA/μm or less, preferably 10 aA/μm or less, far preferably 1 aA/μm or less. 
     Embodiment 14 
     In Embodiment 14, measured values of the off-state current using a test element group (also referred to as a TEG) will be described below. 
       FIG. 23  shows initial characteristics of a thin film transistor with L/W=3 μm/10000 μm in which 200 thin film transistors each with L/W=3 μm/50 μm are connected in parallel. In addition, a top view thereof is  FIG. 24A  and a partially enlarged top view thereof is  FIG. 24B . The region enclosed by a dotted line in  FIG. 24B  is a thin film transistor of one stage with L/W=3 μm/50 μm and Lov=1.5 μm. In order to measure initial characteristics of the thin film transistor, the changing characteristics of the source-drain current (hereinafter referred to as a drain current or Id), i.e., Vg−Id characteristics, were measured, under the conditions where the substrate temperature was set to room temperature, the voltage between source and drain (hereinafter, a drain voltage or Vd) was set to 10 V, and the voltage between source and gate (hereinafter, a gate voltage or Vg) was changed from −20 V to +20 V. Note that  FIG. 23  shows Vg in the range of from −20 V to +5 V. 
     As shown in  FIG. 23 , the thin film transistor having a channel width W of 10000 μm has an off-state current of 1×10 −13  A or less at Vd of 1 V and 10 V, which is less than or equal to the resolution (100 fA) of a measurement device (a semiconductor parameter analyzer, Agilent 4156C manufactured by Agilent Technologies Inc.). 
     A method for manufacturing the thin film transistor used for the measurement is described. 
     First, a silicon nitride layer was formed as a base layer over a glass substrate by a CVD method, and a silicon oxynitride layer was formed over the silicon nitride layer. A tungsten layer was formed as a gate electrode layer over the silicon oxynitride layer by a sputtering method. In this embodiment, the tungsten layer was selectively etched into the gate electrode layer. 
     Then, a silicon oxynitride layer having a thickness of 100 nm was formed as a gate insulating layer over the gate electrode layer by a CVD method. 
     Then, an oxide semiconductor layer having a thickness of 50 nm was formed over the gate insulating layer by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target (at a molar ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2). Here, the oxide semiconductor layer was selectively etched into an island-shaped oxide semiconductor layer. 
     Then, first heat treatment was performed on the oxide semiconductor layer in a nitrogen atmosphere in a clean oven at 450° C. for 1 hour. 
     Then, a titanium layer (having a thickness of 150 nm) was formed as a source electrode layer and a drain electrode layer over the oxide semiconductor layer by a sputtering method. Here, the source electrode layer and the drain electrode layer were selectively etched such that 200 thin film transistors each having a channel length L of 3 μm and a channel width W of 50 μm were connected in parallel to obtain a thin film transistor with L/W=3 μm/10000 μm. 
     Next, a silicon oxide layer having a thickness of 300 nm was formed as a protective insulating layer in contact with the oxide semiconductor layer by a reactive sputtering method. Here, the silicon oxide layer which is a protective layer was selectively etched to form opening portions over the gate electrode layer, the source electrode layer, and the drain electrode layer. After that, second heat treatment was performed in a nitrogen atmosphere at 250° C. for 1 hour. 
     Then, heat treatment was performed at 150° C. for 10 hours before the measurement of Vg−Id characteristics. 
     Through the above process, a bottom-gate thin film transistor was manufactured. 
     The reason why the thin film transistor has an off-state current of about 1×10 −13  A as shown in  FIG. 23  is that the concentration of hydrogen in the oxide semiconductor layer could be sufficiently reduced in the above manufacturing process. The concentration of hydrogen in the oxide semiconductor layer is 1×10 16 /cm 3  or less. Note that the concentration of hydrogen in the oxide semiconductor layer was measured by secondary ion mass spectrometry (SIMS). 
     Although the example of using an In—Ga—Zn—O-based oxide semiconductor is described, this embodiment is not particularly limited thereto. Another oxide semiconductor material, such as an In—Sn—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, a Sn—Al—Zn—O-based oxide semiconductor, an In—Zn—O-based oxide semiconductor, an In—Sn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor, can also be used. As an oxide semiconductor material, an In—Al—Zn—O-based oxide semiconductor mixed with AlO x  of 2.5 wt % to 10 wt % or an In—Zn—O-based oxide semiconductor mixed with SiO x  of 2.5 wt % to 10 wt % can be used. 
     The carrier concentration of the oxide semiconductor layer which is measured by a carrier measurement device is less than 1×10 14 /cm 3 , preferably 1×10 12 /cm 3  or less. In other words, the carrier concentration of the oxide semiconductor layer can be made as close to zero as possible. 
     The thin film transistor can also have a channel length L of greater than or equal to 10 nm and less than or equal to 1000 nm, which enables an increase in circuit operation speed, and the off-state current is extremely small, which enables a further reduction in power consumption. 
     In addition, in circuit design, the oxide semiconductor layer can be regarded as an insulator when the thin film transistor is in an off state. 
     After that, the temperature characteristics of off-state current of the thin film transistor manufactured in this embodiment were evaluated. Temperature characteristics are important in considering the environmental resistance, maintenance of performance, or the like of an end product in which the thin film transistor is used. It is to be understood that a smaller amount of change is more preferable, which increases the degree of freedom for product designing. 
     For the temperature characteristics, the Vg−Id characteristics were obtained using a constant-temperature chamber under the conditions where substrates provided with thin film transistors were kept at respective constant temperatures of −30° C., 0° C., 25° C., 40° C., 60° C., 80° C., 100° C., and 120° C., the drain voltage was set to 6 V, and the gate voltage was changed from −20 V to +20V. 
       FIG. 25A  shows Vg−Id characteristics measured at the above temperatures and superimposed on one another, and  FIG. 25B  shows an enlarged view of a range of off-state current enclosed by a dotted line in  FIG. 25A . The rightmost curve indicated by an arrow in the diagram is a curve obtained at −30° C.; the leftmost curve is a curve obtained at 120° C.; and curves obtained at the other temperatures are located therebetween. The temperature dependence of on-state currents can hardly be observed. On the other hand, as clearly shown also in the enlarged view of  FIG. 25B , the off-state currents are less than or equal to 1×10 −12  A, which is near the resolution of the measurement device, at all temperatures except in the vicinity of a gate voltage of −20 V, and the temperature dependence thereof is not observed. In other words, even at a high temperature of 120° C., the off-state current is kept less than or equal to 1×10 −12  A, and given that the channel width W is 10000 μm, it can be seen that the off-state current is significantly small. 
     A thin film transistor including a purified oxide semiconductor (purified OS) as described above shows almost no dependence of off-state current on temperature. It can be said that an oxide semiconductor does not show temperature dependence when purified because the conductivity type becomes extremely close to an intrinsic type and the Fermi level is located in the middle of the forbidden band, as illustrated in the band diagram of  FIG. 19 . This also results from the fact that the oxide semiconductor has an energy gap of 3 eV or more and includes very few thermally excited carriers. In addition, the source region and the drain region are in a degenerated state, which is also a factor for showing no temperature dependence. The thin film transistor is mainly operated with carriers which are injected from the degenerated source region to the oxide semiconductor, and the above characteristics (independence of off-state current on temperature) can be explained by independence of carrier concentration on temperature. 
     In the case where a display device is manufactured using such a thin film transistor the off-state current of which is extremely small, the leakage current is reduced, so that a period for holding display data can be extended. 
     Example 1 
     In Example 1, the results of evaluation of image signal holding characteristics of the liquid crystal display device, which is described in the above embodiment, shown in  FIG. 1 , and actually manufactured, at the time of displaying a still image will be described. 
     First, regarding an upper-side layout diagram of a plurality of pixels included in a pixel portion, a photograph of elements such as thin film transistors formed over a substrate, which is taken from the rear side, is shown in  FIG. 27 . 
     From the photograph of the pixels shown in  FIG. 27 , it can be seen that rectangular pixels are provided and gate lines  2701  and signal lines  2702  are provided at right angles to each other. It can also be seen that capacitor lines  2703  are provided in a position parallel with the gate lines  2701 . In a region where the gate line  2701  and the capacitor line  2703 , and the signal line  2702  overlap each other, an insulating film is provided in order to reduce parasitic capacitance, and can be observed as a bump in  FIG. 27 . The liquid crystal display device described in this example is a reflective liquid crystal display device, and a red (R) color filter  2704 R, a green (G) color filter  2704 G, and a blue (B) color filter  2704 B are observed. In  FIG. 27 , in a region controlled by the gate line  2701 , an In—Ga—Zn—O-based non-single-crystal film which is an oxide semiconductor is provided as a light-transmitting semiconductor layer, and a thin film transistor is formed. 
       FIG. 28  shows a graph of changes in luminance over time, of each pixel shown in  FIG. 27  at the time of displaying a still image according to the above embodiment. 
     It can be seen from  FIG. 28  that in the case of the upper-side layout of the pixel of  FIG. 27 , the image signal holding period is about 1 minute long. Therefore, at the time of displaying a still image, a constant luminance may be maintained by performing the operation to regularly supply the same image signal (in the diagram, “refresh”). As a result, the length of time to apply a voltage to a transistor included in a driver circuit portion can be drastically shortened. Furthermore, deterioration of a driver circuit over time can be drastically slowed down, which produces advantageous effects such as an improvement in reliability of a liquid crystal display device. 
     Example 2 
     In Example 2, the results of evaluation of image signal holding characteristics of the liquid crystal display device, which is described in the above embodiment, shown in  FIG. 1 , and actually manufactured to have a structure which is different from Example 1, at the time of displaying a still image will be described. 
     First, regarding an upper-side layout diagram of a plurality of pixels included in a pixel portion, a photograph of elements such as thin film transistors formed over a substrate, which is taken from the rear side, is shown in  FIG. 29 . 
     From the photograph of the pixels shown in  FIG. 29 , it can be seen that rectangular pixels are provided and gate lines  2901  and signal lines  2902  are provided at right angles to each other. It can also be seen that capacitor lines  2903  are provided in a position parallel with the gate lines  2901 . In a region where the gate line  2901  and the capacitor line  2903 , and the signal line  2902  overlap each other, an insulating film is provided in order to reduce parasitic capacitance, and can be observed as a bump in  FIG. 29 . The liquid crystal display device described in this example is a reflective liquid crystal display device, and a reflective electrode  2904 R overlapping a red (R) color filter, a reflective electrode  2904 G overlapping a green (G) color filter, and a reflective electrode  2904 B overlapping a blue (B) color filter are observed. In  FIG. 29 , in a region controlled by the gate line  2901 , an In—Ga—Zn—O-based non-single-crystal film which is an oxide semiconductor is provided as a light-transmitting semiconductor layer, and a thin film transistor is formed. 
       FIG. 30  shows a graph of changes in luminance over time, of each pixel shown in  FIG. 29  at the time of displaying a still image according to the above embodiment. 
     It can be seen from  FIG. 30  that in the case of the upper-side layout of the pixel of  FIG. 29 , the image signal holding period is about 1 minute long. Therefore, at the time of displaying a still image, a constant luminance may be maintained by performing the operation to regularly supply the same image signal (in the diagram, “refresh”). As a result, the length of time to apply a voltage to a transistor included in a driver circuit portion can be drastically shortened. Furthermore, deterioration of a driver circuit over time can be drastically slowed down, which produces advantageous effects such as an improvement in reliability of a liquid crystal display device. 
     Example 3 
     In Example 3, the results of evaluation of image signal holding characteristics of the liquid crystal display device, which is described in the above embodiment, shown in  FIG. 1 , and actually manufactured to have a structure which is different from Examples 1 and 2, at the time of displaying a still image will be described. 
     First, regarding an upper-side layout diagram of a plurality of pixels included in a pixel portion, a photograph of elements such as thin film transistors formed over a substrate, which is taken from the rear side, is shown in  FIG. 31 . 
     From the photograph of the pixels shown in  FIG. 31 , it can be seen that rectangular pixels are provided and gate lines  3101  and signal lines  3102  are provided at right angles to each other. It can also be seen that capacitor lines  3103  are provided in a position parallel with the gate lines  3101 . In a region where the gate line  3101  and the capacitor line  3103 , and the signal line  3102  overlap each other, an insulating film is provided in order to reduce parasitic capacitance, and can be observed as a bump in  FIG. 31 . The liquid crystal display device described in this example is a liquid crystal display device using a polymer dispersed liquid crystal, and a reflective electrode  3104  is observed. In  FIG. 31 , in a region controlled by the gate line  3101 , an In—Ga—Zn—O-based non-single-crystal film which is an oxide semiconductor is provided as a light-transmitting semiconductor layer, and a thin film transistor is formed. 
       FIG. 32  shows a graph of changes in luminance over time, of each pixel shown in  FIG. 31  at the time of displaying a still image according to the above embodiment. 
     It can be seen from  FIG. 32  that in the case of the upper-side layout of the pixel of  FIG. 31 , the image signal holding period can be longer than that of any of Examples 1 and 2 since the polymer dispersed liquid crystal has a property of holding an image signal. Therefore, at the time of displaying a still image, the interval of operation of supplying the same image signal can be extended. As a result, the length of time to apply a voltage to a transistor included in a driver circuit portion can be drastically shortened. Furthermore, deterioration of a driver circuit over time can be drastically slowed down, which produces advantageous effects such as an improvement in reliability of a liquid crystal display device. 
     Example 4 
     In Example 4, the results of evaluation of image signal holding characteristics of the liquid crystal display device, which is described in the above embodiment, shown in  FIG. 1 , and actually manufactured to have a structure which is different from Examples 1 to 3, at the time of displaying a still image will be described. Particularly in this example, an example which is different from the upper-side layout diagram of a plurality of pixels described in any of Examples 1 to 3 will be described. A photograph of elements such as thin film transistors formed over a substrate, which is taken from the rear side, is shown in  FIG. 33 . 
     From the photograph of the pixels shown in  FIG. 33 , it can be seen that rectangular pixels are provided and gate lines  3301  and signal lines  3302  are provided at right angles to each other. Unlike the photograph of the pixels described any of Examples 1 to 3, an upper-side layout diagram in which a capacitor line is omitted I is described. The liquid crystal display device described in this example is a transmissive liquid crystal display device, and a pixel electrode  3304  is observed. In  FIG. 33 , in a region controlled by the gate line  3301 , an In—Ga—Zn—O-based non-single-crystal film which is an oxide semiconductor is provided as a light-transmitting semiconductor layer, and a thin film transistor is formed. 
     Example 5 
     In Example 5, an example of operation method of the liquid crystal display device shown in  FIG. 1  and described in the above embodiment will be described. The procedure of supplying, or stopping the supply of, a potential to each wiring of the driver circuit portion during the operations to display a still image and a moving image, or the operation to rewrite a voltage to be applied to a liquid crystal element (hereinafter also referred to as refresh operation), in the driver circuit manufactured using a plurality of n-channel transistors, which is given as an example in  FIGS. 2A to 2C  and  FIG. 3 , will be described with reference to  FIG. 34 . Note that  FIG. 34  illustrates changes in potentials, before and after a period T 1 , of a wiring for supplying a high power supply potential (VDD), a wiring for supplying a low power supply potential (VSS), a wiring for supplying a start pulse (SP), and wirings for supplying first to fourth clock signals (CK 1  to CK 4 ) to a shift register. 
     The liquid crystal display device of this embodiment can display a still image without constantly operating the driver circuit portion. Therefore, as illustrated in  FIG. 34 , there are a period in which control signals such as the high power supply potential (VDD), the first to fourth clock signals (CK 1  to CK 4 ), and the start pulse are supplied to a shift register and a period in which control signals are not supplied. Note that the period T 1  illustrated in  FIG. 34  corresponds to the period in which control signals are supplied, in other words, a period in which a moving image is displayed and a period in which refresh operation is performed. The period T 2  illustrated in  FIG. 34  corresponds to the period in which control signals are not supplied, in other words, a period in which a still image is displayed. 
     In  FIG. 34 , a period in which the high power supply potential (VDD) is supplied is provided not only in the period T 1  but also in part of the period T 2 . In addition, in  FIG. 34 , a period in which the first to fourth clock signals (CK 1  to CK 4 ) are supplied is provided between the start of the supply of the high power supply potential (VDD) and the stop of the supply of the high power potential (VDD). 
     Moreover, as illustrated in  FIG. 34 , the first to fourth clock signals (CK 1  to CK 4 ) may be set so as to start to oscillate at a constant frequency after being set to a high potential once before the period T 1  begins and stop oscillating after being set to a low potential after the period T 1  ends. 
     As described above, in the liquid crystal display device of this example, the supply of control signals such as the high power supply potential (VDD), the first to fourth clock signals (CK 1  to CK 4 ), and the start pulse to the shift register is stopped in the period T 2 . Then, in the period in which the supply of control signals is stopped, whether each transistor is turned on or turned off is controlled and the output of a pulse signal from the shift register is also stopped. Therefore, power consumption of the shift register and power consumption of the pixel portion which is driven by the shift register can be reduced. 
     The aforementioned refresh operation needs to be performed regularly because there is a possibility that the quality of a displayed still image may deteriorate. In the liquid crystal display device of this example, the aforementioned transistor including an oxide semiconductor is employed as a switching element for controlling a voltage to be applied to a liquid crystal element of each pixel. Accordingly, off-state current can be drastically decreased, and a change in voltage to be applied to the liquid crystal element of each pixel can be reduced. In other words, even when the period in which the operation of the shift register is stopped is long due to display of a still image, the deterioration of image quality can be suppressed. For example, even when the period is 3 minutes long, the quality of a displayed still image can be maintained. For example, if a liquid crystal display device in which rewrite is performed 60 times per second and a liquid crystal display device in which refresh operation is performed once in 3 minutes are compared with each other, power consumption can be reduced to about 1/10000. 
     Note that the stop of the supply of the high power supply potential (VDD) is to set a potential equal to the low power supply potential (VSS) as illustrated in  FIG. 34 . In addition, the stop of the supply of the high power supply potential (VDD) may be to set the potential of a wiring, to which the high power supply potential is supplied, in a floating state. 
     Note that when the potential of the wiring to which the high power supply potential (VDD) is supplied is increased, which means that the potential is increased from the low power supply potential (VSS) to the high power supply potential (VDD) before the period Ti, it is preferable that the potential of the wiring is controlled so as to change gradually. If the gradient of the change in potential of the wiring is steep, there is a possibility that the change in potential may become noise and an fault pulse may be output from the shift register. In the case where the shift register is included in a gate line driver circuit, the fault pulse serves as a signal for turning on a transistor. Thus, there is a possibility that a voltage to be applied to a liquid crystal element may be changed by the fault pulse and the quality of a still image may be changed. Therefore, it is preferable to control the change in potential of the wiring as described above. In view of the above content,  FIG. 34  illustrates an example in which a rise in signal to the high power supply potential (VDD) is more gradual than a fall. In particular, in the liquid crystal display device of this embodiment, when a still image is displayed in the pixel portion, the stop of the supply, and the resupply, of the high power supply potential (VDD) to the shift register are performed as appropriate. In other words, in the case where a change in potential of the wiring for supplying the high power supply potential (VDD) adversely affects the pixel portion as noise, the noise directly leads to deterioration of a display image. Therefore, it is important to control the liquid crystal display device of this embodiment so as to prevent a change in potential (particularly, an increase in potential) of the wiring from entering the pixel portion as noise. 
     This application is based on Japanese Patent Application serial no. 2009-238916, 2009-273913, and 2009-278999 filed with Japan Patent Office on Oct. 16, 2009, Dec. 1, 2009, and Dec. 8, 2009, respectively, the entire contents of which are hereby incorporated by reference. 
     EXPLANATION OF REFERENCE 
     
         
         
           
               1000 : liquid crystal display device;  1001 : display panel;  1002 : signal generation circuit;  1003 : memory circuit;  1004 : comparison portion;  1005 : selection circuit;  1006 : display control circuit;  1007 : driver circuit portion;  1008 : pixel portion;  1010 : frame memory;  1009 A: gate line driver circuit;  1009 B: signal line driver circuit