Abstract:
Objects are to provide a semiconductor device with a novel structure, to provide a semiconductor device with low power consumption, and to provide a semiconductor device with a small chip area. A digital-analog converter and a frame memory are included. The frame memory includes a sample-and-hold circuit, a correction circuit, and a source follower circuit. The sample-and-hold circuit retains the analog voltage output from the digital-analog converter. The correction circuit corrects the analog voltage retained in the sample-and-hold circuit. The source-follower circuit outputs the corrected analog voltage. The sample-and-hold-circuit, the correction circuit, and the source follower circuit each comprise a first transistor. The first transistor comprises an oxide semiconductor layer in a semiconductor layer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    One embodiment of the present invention relates to a semiconductor device, a display panel, and an electronic device. 
         [0003]    Note that one embodiment of the present invention is not limited to the above technical field. The technical field of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. Furthermore, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them. 
         [0004]    In this specification and the like, a semiconductor device refers to an element, a circuit, a device, or the like that can function by utilizing semiconductor characteristics. An example of the semiconductor device is a semiconductor element such as a transistor or a diode. Another example of the semiconductor device is a circuit including a semiconductor element. Another example of the semiconductor device is a device provided with a circuit including a semiconductor element. 
         [0005]    2. Description of the Related Art 
         [0006]    A source driver integrated circuit (IC) in which a frame memory and a source driver are included (for example, see Patent Document 1) has been known. Static random access memory (SRAM) is generally used for the frame memory. 
       REFERENCE 
     Patent Document 
     [Patent Document 1] United States Published Patent Application No. 2008/0186266 
     SUMMARY OF THE INVENTION 
       [0007]    By including the frame memory in the source driver IC, transmitting/receiving data to/from a host can be reduced and thus the source driver IC can reduce power consumption. However, data stored in SRAM is digital data. Therefore, the source driver IC cannot reduce the power consumed by converting digital data to analog data. 
         [0008]    Furthermore, as the number of pixels increases, the amount of data retained in SRAM also increases. To deal with the increase in the amount of data, miniaturization of transistors that constitute SRAM has progressed to reduce the cell areas. However, transistor miniaturization causes the increase in leakage current. As a result, power consumption is increased in a source driver IC embedded with a frame memory using SRAM. 
         [0009]    Furthermore, SRAM has a large number of transistors and a large cell area. Therefore, the source driver IC including the frame memory with the use of SRAM causes problems such as an increase in a chip area. 
         [0010]    In view of the above, an object of one embodiment of the present invention is to provide a novel semiconductor device that has a structure different from that of an existing semiconductor device functioning as a source driver IC, a novel display panel, and a novel electronic device. Another object of one embodiment of the present invention is to provide a semiconductor device or the like with a novel structure, in which power consumption is reduced. Alternatively, an object of one embodiment of the present invention is to provide a semiconductor device or the like with a novel structure in which a chip area is reduced. 
         [0011]    Note that the objects of one embodiment of the present invention are not limited to the above objects. The objects described above do not disturb the existence of other objects. The other objects are objects that are not described above and will be described below. The other objects will be apparent from and can be derived from the description of the specification, the drawings, and the like by those skilled in the art. One embodiment of the present invention is to achieve at least one of the aforementioned objects and the other objects. 
         [0012]    One embodiment of the present invention is a digital-analog converter and a frame memory. The frame memory includes a sample-and-hold circuit, a correction circuit, and a source follower circuit. The correction circuit is configured to correct the analog voltage retained in the sample-and-hold circuit. The source follower is configured to output the corrected analog voltage. The sample-and-hold-circuit, the correction circuit, and the source follower circuit each include a first transistor. The first transistor includes an oxide semiconductor layer in the semiconductor layer. 
         [0013]    One embodiment of the present invention is a digital-analog converter, a frame memory, and a buffer circuit. The frame memory includes a sample-and-hold circuit, a correction circuit, and a source follower circuit. The correction circuit is configured to correct the analog voltage retained in the sample-and-hold circuit. The source follower is configured to output the corrected analog voltage to the buffer circuit. The sample-and-hold-circuit, the correction circuit, and the source follower circuit each include a first transistor. The first transistor includes an oxide semiconductor layer in the semiconductor layer. 
         [0014]    One embodiment of the present invention is a digital-analog converter, a frame memory, and a buffer circuit. The frame memory includes a sample-and-hold circuit, a correction circuit, and a source follower circuit. The correction circuit is configured to correct the analog voltage retained in the sample-and-hold circuit. The source follower is configured to output the corrected analog voltage to the buffer circuit. The sample-and-hold-circuit, the correction circuit, and the source follower circuit each include a first transistor. The digital analog converter and the buffer circuit includes a second transistor. The first transistor includes an oxide semiconductor layer in the semiconductor layer. The second transistor includes silicon in the semiconductor layer. 
         [0015]    In the semiconductor device of one embodiment of the present invention, a layer including the first transistor is preferably placed above a layer including the second transistor. 
         [0016]    Note that other embodiments of the present invention will be described in the following embodiments with reference to the drawings. 
         [0017]    One embodiment of the present invention can provide a novel semiconductor device that has a structure different from that of an existing semiconductor device functioning as a source driver IC, a novel display panel, and a novel electronic device. Another embodiment of the present invention can provide a semiconductor device or the like with a novel structure, in which power consumption is reduced. Another embodiment of the present invention can provide a semiconductor device or the like with a novel structure in which a chip area is reduced. 
         [0018]    Note that the effects of one embodiment of the present invention are not limited to the above effects. The effects described above do not disturb the existence of other effects. The other effects are effects that are not described above and will be described below. The other effects will be apparent from and can be derived from the description of the specification, the drawings, and the like by those skilled in the art. One embodiment of the present invention is to have at least one of the aforementioned effects and the other effects. Accordingly, one embodiment of the present invention does not have the aforementioned effects in some cases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In the accompanying drawings: 
           [0020]      FIG. 1  illustrates an example of a semiconductor device; 
           [0021]      FIG. 2  illustrates an example of a semiconductor device; 
           [0022]      FIG. 3  illustrates an example of a semiconductor device; 
           [0023]      FIG. 4  illustrate an example of a semiconductor device; 
           [0024]      FIGS. 5A and 5B  illustrate an example of a semiconductor device and an operation of the semiconductor device; 
           [0025]      FIGS. 6A and 6B  illustrate an example of an operation of a semiconductor device; 
           [0026]      FIG. 7  illustrates an example of an operation of a semiconductor device; 
           [0027]      FIGS. 8A and 8B  illustrate an example of a semiconductor device; 
           [0028]      FIG. 9  illustrates an example of a semiconductor device; 
           [0029]      FIG. 10  illustrates an example of a semiconductor device; 
           [0030]      FIGS. 11A and 11B  illustrate an example of a semiconductor device and an operation of the semiconductor device; 
           [0031]      FIG. 12  illustrates an example of a semiconductor device; 
           [0032]      FIG. 13  illustrates an example of an operation of a semiconductor device; 
           [0033]      FIGS. 14A and 14B  illustrate an example of a semiconductor device and an operation of the semiconductor device; 
           [0034]      FIG. 15  illustrates an example of a semiconductor device; 
           [0035]      FIG. 16  illustrates an example of a semiconductor device; 
           [0036]      FIG. 17  illustrates an example of a semiconductor device; 
           [0037]      FIG. 18  illustrates an example of a display panel; 
           [0038]      FIG. 19  illustrates an example of a display panel; 
           [0039]      FIG. 20  illustrates an example of a display panel; 
           [0040]      FIGS. 21A to 21C  illustrate an example of a display panel; 
           [0041]      FIGS. 22A and 22B  each illustrate an example of a display panel; 
           [0042]      FIG. 23  illustrates an example of a schematic cross-sectional diagram; 
           [0043]      FIGS. 24A and 24B  illustrate examples of schematic cross-sectional diagrams; 
           [0044]      FIGS. 25A and 25B  each illustrate an example of a schematic cross-sectional diagram; 
           [0045]      FIGS. 26A to 26C  each illustrate an atomic ratio range of an oxide semiconductor; 
           [0046]      FIG. 27  illustrates an InMZnO 4  crystal; 
           [0047]      FIGS. 28A to 28C  are each a band diagram of a layered structure including an oxide semiconductor; 
           [0048]      FIGS. 29A to 29D  illustrate an example of a method for manufacturing a semiconductor device; 
           [0049]      FIGS. 30A to 30C  illustrate an example of a method for manufacturing a semiconductor device; 
           [0050]      FIGS. 31A to 31C  illustrate an example of a method for manufacturing a semiconductor device; 
           [0051]      FIGS. 32A and 32B  illustrate an example of a method for manufacturing a semiconductor device; 
           [0052]      FIGS. 33A and 33B  illustrate an example of a method for manufacturing a semiconductor device; 
           [0053]      FIGS. 34A and 34B  illustrate an example of a method for manufacturing a semiconductor device; 
           [0054]      FIG. 35  illustrates an example of a method for manufacturing a semiconductor device; 
           [0055]      FIGS. 36A and 36B  each illustrate an example of a display panel; 
           [0056]      FIG. 37  illustrates an example of a display module; and 
           [0057]      FIGS. 38A to 38E  each illustrate an example of an electronic device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0058]    Hereinafter, embodiments will be described with reference to drawings. However, the embodiments can be implemented with various modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments. 
         [0059]    In this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components. Thus, the terms do not limit the number or order of components. In the present specification and the like, a “first” component in one embodiment can be referred to as a “second” component in other embodiments or claims. Furthermore, in this specification and the like, a “first” component in one embodiment can be omitted in other embodiments or claims. 
         [0060]    The same elements or elements having similar functions, elements formed using the same material, elements formed at the same time, or the like in the drawings are denoted by the same reference numerals in some cases, and the description thereof is not repeated in some cases. 
       Embodiment 1 
       [0061]    In this embodiment, an example of a semiconductor device functioning as a source driver IC will be described. 
         [0062]      FIG. 1  is an example of a block diagram that schematically illustrates a configuration of the semiconductor device. 
         [0063]    In  FIG. 1 , a semiconductor device  100  (shown as SDIC) includes an interface  101  (shown as I/F), a logic circuit  102  (shown as LOGIC), a latch circuit  103  (shown as LAT), a digital-analog converter  104  (shown as D/A), a frame memory  105  (shown as RAM), and a buffer circuit  106  (shown as AMP). 
         [0064]    In  FIG. 1 , a digital signal output from a host processor  110  (shown as Host) is input to the semiconductor device  100 . A data signal, which is an analog signal, is output to a display device  120  (shown as Display) from the semiconductor device  100 . 
         [0065]    The interface  101  has a function of decoding the digital signal input from the host processor  110 . 
         [0066]    The logic circuit  102  has a function of arithmetically processing the digital signal, a function of distributing the digital signal to the latch circuit  103  by a shift register, or the like. 
         [0067]    The latch circuit  103  has a function of retaining the digital signal, which is an image signal to be output to pixels of the display device. 
         [0068]    The digital-analog converter  104  has a function of converting the digital signal into an analog signal and outputting the analog signal. The analog signal is an input signal D IN  of the frame memory  105 . 
         [0069]    The buffer circuit  106  has a function of increasing the current supply capability of the input analog signal and outputting the resulting signal. The analog signal input to the buffer circuit  106  is an output signal D OUT  of the frame memory  105 . The analog signal whose current supply capability is improved in the buffer circuit  106  is output to the display device  120 . 
         [0070]    The frame memory  105  has a function of retaining the input signal D IN , which is an analog signal. The frame memory  105  has a function of outputting the retained analog signal as the output signal D OUT  to the buffer circuit  106 . 
         [0071]    A memory cell included in the frame memory  105  includes a transistor including an oxide semiconductor in a channel formation region (hereinafter, such a transistor is referred to as an OS transistor). The OS transistor has a low off-state current which flows in an off state. Therefore, the frame memory  105  including the OS transistor can retain charge corresponding to the analog signal. Furthermore, the analog signal corresponding to the charge can be output. 
         [0072]      FIG. 2  is a block diagram of the digital-analog converter  104 , the frame memory  105 , and the buffer circuit  106  of the semiconductor device  100 , and the display device  120  in  FIG. 1 . 
         [0073]    In  FIG. 2 , the display device  120  includes a plurality of pixels  121 .  FIG. 2  shows an example of the pixels  121  arranged in m rows and n columns (m and n are natural numbers). 
         [0074]    In  FIG. 2 , the frame memory  105  includes a plurality of memory cells  140 .  FIG. 2  shows an example of the memory cells  140  arranged in m rows and n columns (m and n are natural numbers). Note that the number of the memory cells  140  is the same as that of the pixels  121 . 
         [0075]    Note that it is effective to arrange a greater number of the memory cells  140  than the number of the pixels  121 . With this structure, an analog signal corresponding to a data signal supplied to each pixel can be retained in the frame memory  105 . 
         [0076]    In the frame memory  105  in  FIG. 2 , input signals D IN   _   1  to D IN   _   n , which are output signals of the digital-analog converter  104 , are input to the respective lines. In the frame memory  105  in  FIG. 2 , output signals D OUT   _   1  to D OUT   _   n , which are input signals of the buffer circuit  106 , are output to the respective lines of the display device  120 . 
         [0077]    The digital-analog converter  104  and the buffer circuit  106  in  FIG. 2  are required to operate at high speed. Thus, a transistor including silicon in a channel formation region (hereinafter, a Si transistor) is preferably included in a channel formation region. In contrast, the frame memory  105  illustrated in  FIG. 2 , as described above, includes the OS transistor to retain charge corresponding to the analog signal. 
         [0078]    Therefore, it is possible that the digital-analog converter  104  and the buffer circuit  106  are provided in a layer and the frame memory  105  is provided in another layer.  FIG. 3  is a schematic block diagram of the digital-analog converter  104  and the buffer circuit  106 , and the frame memory  105  provided in different layers. 
         [0079]    As shown in  FIG. 3 , the digital-analog converter  104  and the buffer circuit  106  are provided in a first layer  141 . Furthermore, the frame memory  105  is provided in a second layer  142 , which is the upper layer of the first layer  141 . With this structure, it is possible that in the semiconductor device  100 , which functions as a source driver IC, the OS transistor included in the frame memory  105  is provided over the Si transistor included in a circuit other than the frame memory  105 , for example, the digital-analog converter  104  and the buffer circuit  106 . 
         [0080]    It is necessary that the number of the memory cells  140  of the frame memory  105  is determined in accordance with the number of pixels  121  of the display device  120 . Thus, the circuit area occupied by the frame memory  105  increases. As described above, with the structure in which the frame memory  105  including the OS transistor is provided over a circuit other than the frame memory  105 , the circuit area occupied by the frame memory  105  is not increased; thus, the increase in the circuit area can be suppressed. 
         [0081]      FIG. 4  is a block diagram of the memory cell  140  of the frame memory  105  in  FIG. 2 . The memory cell  140  illustrated in  FIG. 4  includes a sample-and-hold circuit  131  (shown as S/H), a correction circuit  132  (shown as COR), and a source follower circuit  133  (shown as S/F). The input signal D IN  is supplied to the sample-and-hold circuit  131  and the correction circuit  132 . The output signal D OUT  is output from the source follower circuit  133 . 
         [0082]      FIG. 5A  shows an example of a specific circuit configuration of the memory cell  140  in  FIG. 4 . 
         [0083]    A memory cell  140 A illustrated in  FIG. 5A  includes transistors M 1  to M 5  and capacitors C 1  and C 2 . All the transistors M 1  to M 5  are n-channel transistors in the following description. Gates of the transistors M 1  and M 3  are connected to a wiring for supplying a control signal EN 1 . A gate of the transistor M 2  is connected to a wiring for supplying a control signal EN 2 . One electrode of the capacitor C 1  is referred to as a node ND 1  in  FIG. 5A . One electrode of the capacitor C 2  is referred to as a node ND 2  in  FIG. 5A . A gate of a transistor M 5  is supplied with a reference voltage V REF . A current flowing through the transistor M 5  is made constant by the reference voltage V REF . A voltage V DD  is applied to either one of a source and a drain of the transistor M 4 . A voltage V SS  is applied to either one of a source and a drain of the transistor M 5 . Note that the voltage V DD  is higher than the voltage V SS . The other electrode of the capacitor C 2  is supplied with a voltage V 0 . The voltage V 0  is preferably a fixed voltage, for example, a ground voltage (GND). 
         [0084]      FIG. 5B  is a timing chart showing an operation of the circuit of  FIG. 5A .  FIG. 5B  illustrates signal waveforms of the control signals EN 1  and EN 2 . Furthermore,  FIGS. 6A and 6B , and  FIG. 7  illustrate voltage of each of the transistors M 1  to M 5  and the nodes ND 1  and ND 2  in periods P 1  to P 3  in the timing chart in  FIG. 5B . 
         [0085]    In the first period P 1 , the control signal EN 1  is set at a high level and the control signal EN 2  is set at a low level. Here, the states of each of the transistors are illustrated in  FIG. 6A . The transistors M 1  and M 3  are brought into an on state. The transistor M 2  is brought into an off state. Transistors in an off state are represented by a cross in  FIG. 6A . 
         [0086]    The transistor M 1  is turned on, so that the voltage of the node ND 1  becomes a voltage V DATA  which is the input signal D IN . 
         [0087]    The current flowing in the transistor M 5  flows through the transistors M 4  and M 5 . As the voltage between a gate and a source of the transistor M 4  (also referred to as a gate-source voltage), a voltage for making the above-described current flow is applied.  FIG. 6A  illustrates the gate-source voltage of the transistor M 4  as V GS . Here, a voltage of the output signal D OUT  becomes (V DATA −V GS ). The transistor M 3  is in an on state and thus, a voltage of the node ND 2  becomes (V DATA −V GS ). 
         [0088]    In the second period P 2 , the control signal EN 1  is set at a low level and the control signal EN 2  is set at a high level. Here, the states of each transistors at that time are illustrated in  FIG. 6B . The transistor M 2  is brought into an on state. The transistors M 1  and M 3  are brought into an off state. Transistors in an off state are represented by a cross in  FIG. 6B . 
         [0089]    The transistor M 2  is turned on, so that the voltage of the node ND 2  changes from the voltage (V DATA −V GS ) to the voltage V DATA . Here, the transistor M 1  is in an off state and thus, the node ND 1  is in an electrically floating state. Therefore, the voltage of the node ND 1  increases in accordance with the change of the voltage of the node ND 2  from the voltage (V DATA −V GS ) to the voltage V DATA . The voltage of the ND 1  is increased to a voltage (V DATA +V GS ) when the capacitance component of the capacitor C 1  is sufficiently larger than that of the node ND 1 . The voltage of the output signal D OUT  becomes a voltage V DATA  and thus, the voltage can be corrected to a voltage V DATA  of the input signal D IN  because the V GS  of the transistor M 4  does not change. 
         [0090]    In the third period P 3 , the control signal EN 1  is kept at a low level and the control signal EN 2  is set at a low level. Here, the states of each transistors at that time are illustrated in  FIG. 7 . The transistors M 1  to M 3  are brought into an off state. Transistors in an off state are represented by a cross in  FIG. 7 . 
         [0091]    The transistors M 1  to M 3  are in an off state, so that voltages of the nodes ND 1  and ND 2  are retained at the voltage (V DATA +V GS ) and the voltage V DATA , respectively. The voltage of the output signal D OUT  becomes the voltage V DATA  and thus, the voltage corrected to the voltage V DATA  of the input signal D IN  can keep being output because the V GS  of the transistor M 4  does not change. 
         [0092]    As described above, the memory cell included in the frame memory  105  includes the OS transistor. In other words, the transistors M 1  to M 5  are OS transistors. The OS transistor has a low off-state current which flows in an off state. Therefore, the transistors M 1  to M 3  are brought into an off state and thus, voltages of the node ND 1  and the node ND 2  can be kept at the voltage (V DATA +V GS ) and the voltage V DATA , respectively. Furthermore, the voltage V DATA , which is an analog signal corresponding to the voltages, can be output. 
         [0093]    Instead of the memory cell  140 A illustrated in  FIG. 5A , a memory cell  140 B illustrated in  FIG. 8A  can be used.  FIG. 8A  illustrates a configuration in which a backgate electrode for controlling a threshold voltage is included in each of the transistors M 1  to M 3 , which retain the voltages of the nodes ND 1  and ND 2 . The threshold voltage of each of the transistors M 1  to M 3  can be controlled by supplying a fixed voltage, for example, a voltage V 0  to the backgate electrode of each of the transistors M 1  to M 3 . By controlling the threshold voltage, for example, by applying a voltage for shifting the threshold voltage in a positive direction to the backgate electrode, the off-current can more surely be reduced. 
         [0094]    As another example, a memory cell  140 C illustrated in  FIG. 8B  has a configuration including a backgate electrode in each of the transistors M 4  and M 5  supplying a constant current. By supplying the same voltages as those of the gate electrodes to the backgate electrodes of the transistors M 4  and M 5 , electric fields are applied from both above and below the channel formation regions and thus, the amount of current flowing through the transistors M 4  and M 5  can be increased without increasing the size of the transistors M 4  and M 5 . 
         [0095]      FIG. 9  is a diagram formed by adding a driver circuit  143  for controlling the operation of the frame memory  105  to a block diagram of the semiconductor device  100  in  FIG. 2 . Note that the display device  120  is omitted in  FIG. 9 . 
         [0096]    The operation of the memory cells  140  is controlled row by row from the row [ 1 ] to [m] by the driving circuit  143 . The driving circuit  143  includes, for example, a shift register. Writing, retaining, and reading of the data signal can be controlled row by row by the driving circuit  143  in a manner similar to a gate driver controlling the pixels of the display device  120 . 
         [0097]    The data signal output from the digital-analog converter  104  is directly output to the display device  120  in the case where the data signal retained in the frame memory  105  is different from the image displayed in the successive frames. Therefore, as illustrated in  FIG. 10 , switching circuits  144  are preferably provided between the frame memory  105  and the buffer circuit  106 . 
         [0098]    The switching circuits  144  perform switching such that the data signal output from the digital-analog converter  104  is output to the buffer circuit  106  in the case where images displayed are different in successive frames and the data signal output from the frame memory  105  is output to the buffer circuit  106  in the case where images displayed are the same in successive frames. Therefore, by providing the switching circuits  144 , power necessary for an operation of the interface  101 , the logic circuit  102 , the latch circuit  103 , and the digital-analog converter  104  can be reduced, which leads to a reduction in power consumption in the semiconductor device  100 . 
         [0099]    A circuit configuration of a memory cell in the case where the data signal output from the frame memory  105  is stopped is illustrated in  FIG. 11A . A memory cell  140 D illustrated in  FIG. 11A  corresponds to a configuration where transistors M 6  and M 7  are added to the circuit configuration of the memory cell  140 A illustrated in  FIG. 5A . The transistors M 6  and M 7  are n-channel transistors here, like the transistors M 1  to M 5 . 
         [0100]    Gates of the transistors M 6  and M 7  are connected to a wiring for supplying a control signal EN 3 . The transistors M 6  and M 7  are arranged on a path where a current of the source follower circuit  133  flows. 
         [0101]      FIG. 11B  is a timing chart showing an operation of the circuit of  FIG. 11A .  FIG. 11B  illustrates signal waveforms of the control signal EN 1 , the control signal EN 2 , and the control signal EN 3 . 
         [0102]    The operation in periods P 1  to P 3  in  FIG. 11B  is basically similar to the operation in  FIG. 5B . Specifically, in the periods P 1  and P 2 , the control signal EN 3  is set at a high level and the same operation as the operation in  FIG. 5B  is performed. In the period P 3  other than the periods P 1  and P 2 , the control signal EN 3  is set at a low level and the transistors M 6  and M 7  on the path of the source follower circuit  133  where current flows are controlled to be in an off state. 
         [0103]    As described above, the memory cell included in the frame memory  105  includes the OS transistor. That is, the transistors M 1  to M 7  are OS transistors. The OS transistor has a low off-state current which flows in an off state. Therefore, the transistors M 1  to M 3  are brought into an off state and thus, the voltages of the node ND 1  and the node ND 2  may be kept at the voltage (V DATA +V GS ) and the voltage V DATA , respectively. Furthermore, the voltage V DATA , which is an analog signal corresponding to the voltages of the node ND 1  and the node ND 2  can be output. 
         [0104]    Note that the transistor M 5  can be shared by the memory cells  140 D in the same column, which have the configuration in  FIG. 11A .  FIG. 12  illustrates a circuit configuration in which the transistor M 5  is shared by memory cells  140 D_ 1  and  140 _ 2  in the same column. 
         [0105]    Note that the memory cell  140 D_ 1  receives an input signal D IN   _   1 [ 1 ] and performs retention and performs output of an output signal D OUT   _   1 [ 1 ] corresponding to the first column of the memory cell. The memory cell  140 D_ 2  performs input and retention of an input signal D IN   _   1 [ 2 ] and output of an output signal D OUT   _   1 [ 2 ] corresponding to the second column of the memory cell. Control signals EN 1 [ 1 ], EN 2 [ 1 ], and EN 3 [ 1 ] are signals that control the operation of the memory cell  140 D_ 1 . Control signals EN 1 [ 2 ], EN 2 [ 2 ], and EN 3 [ 2 ] are signals that control the operation of the memory cell  140 D_ 2 . 
         [0106]      FIG. 13  is a timing chart showing an operation of the circuit configuration of  FIG. 11A .  FIG. 13  illustrates signal waveforms of the control signals EN 1 [ 1 ], EN  2 [ 1 ], and EN 3 [ 1 ] and the control signals EN 1 [ 2 ], EN  2 [ 2 ], and EN 3 [ 2 ]. 
         [0107]    Instead of the memory cell  140 A illustrated in  FIG. 5A , a memory cell  140 E illustrated in  FIG. 14A  can be used.  FIG. 14A  illustrates a structure in which one electrode of the capacitor C 3  is connected to the node ND 2 . The other electrode of the capacitor C 3  is connected to a wiring for supplying a control signal EN 2 _B. The control signal EN 2 _B is an inverted signal of the control signal EN 2 . 
         [0108]      FIG. 14B  is a timing chart that illustrates an operation of the circuit configuration of  FIG. 14A .  FIG. 14B  illustrates signal waves of the control signals EN 1 , EN  2 , and EN 2 _B. 
         [0109]    The configurations illustrated in  FIGS. 14A and 14B  can prevent a decrease in the voltage due to parasitic capacitance of the node ND 2  and the transistor M 2  when the control signal EN 2  is set at a low level from a high level in a second period P 2 . Specifically, when the control signal EN 2 _B is set at a high level from a low level in the second period P 2 , the voltage of the node ND 2  is increased by the voltage decrease. Thus, the voltage of the node ND 1 , which is in an electrically floating state, can easily increase to the voltage (V DATA +V GS ). 
         [0110]    As another configuration, a memory cell  140 F of  FIG. 15  has a gate capacitance of a transistor M 8  instead of the capacitor C 3  in the circuit configuration of  FIG. 14A . 
         [0111]    Furthermore, as another configuration, a memory cell  140 G of  FIG. 16  has a circuit configuration in which the structure added to the structure of  FIG. 14A  is applied to the circuit configuration of  FIG. 11A . 
         [0112]    Furthermore, as another configuration, a memory cell  140 H of  FIG. 17  has a circuit configuration in which the structure added to the structure of  FIG. 15  is applied to the circuit configuration of  FIG. 11A . 
         [0113]    As described above, according to one embodiment of the present invention, a semiconductor device with low power consumption can be provided. Furthermore, the semiconductor device can have a reduced chip area. 
       Embodiment 2 
       [0114]    This embodiment will describe the semiconductor device that is explained in Embodiment 1 and functions as a source driver IC, a display device operated by the semiconductor device, and their variation examples. 
         [0115]    A block diagram in  FIG. 18  illustrates the semiconductor device  100 , the host processor  110 , a game driver  150  (shown as GD), and the display device  120 .  FIG. 18  also illustrates a plurality of scan lines XL, a plurality of signal lines YL, and pixels  121  in the display device  120 . The semiconductor device  100  has a structure similar to that shown in  FIG. 1  of Embodiment 1. 
         [0116]    The gate driver  150  has a function of supplying scan signals to the scan lines XL. The semiconductor device  100  serving as a source driver IC has a function of supplying data signals, which are analog signals, to the signal lines YL. 
         [0117]    In the display device  120 , the scan lines XL and the signal lines YL are provided to be substantially orthogonal. The pixels  121  are provided at the intersections of the scan lines XL and the signal lines YL. For color display, the pixels  121  corresponding to the respective colors of red, green, and blue (RGB) are arranged in sequence. Note that the pixels of RGB can be arranged in a stripe pattern, a mosaic pattern, a delta pattern, or the like as appropriate. Without limitation to RGB, a pixel corresponding to white, yellow, or the like can be added for color display. 
         [0118]    In the case of adding a touch sensor function to the display device  120 , a touch sensor  160  is added as in a semiconductor device  100 A illustrated in  FIG. 19 . Note that it is possible to obtain an in-cell touch panel by combining the touch sensor  160  and the display device  120 . A signal obtained by the touch sensor  160  can be processed by a semiconductor device  100 A that includes a touch sensor driver circuit  181  in addition to the components of the semiconductor device  100 . In the structure of  FIG. 19 , controlling driving of the touch sensor and driving of the display device at different timings enables the reduction in malfunction of the touch sensor due to noise. 
         [0119]    A semiconductor device  100 B in a block diagram of  FIG. 20  includes an arithmetic device  182 . The arithmetic device  182  has a function of performing arithmetic processing on data. As an example of arithmetic processing, the arithmetic device  182  can execute image rotation processing, control for turning on or off a backlight, super-resolution processing, or the like. The semiconductor device  100  to which the arithmetic device  182  is added achieves higher performance. 
         [0120]    A semiconductor device  100 C in a block diagram of  FIG. 21A  includes an FPGA  183 . The FPGA  183  has a function of performing arithmetic processing on data. As an example of arithmetic processing, like the arithmetic device  182 , the FPGA  183  can execute image rotation processing, control for turning on or off a backlight, super-resolution processing, or the like. 
         [0121]      FIG. 21B  is a block diagram illustrating a configuration memory that stores configuration data. For example, the on/off state of a switch  184 , which controls a connection of logic elements  185 , is controlled by a configuration memory  186 . In  FIG. 21C , an example of a circuit configuration which can be used for the configuration memory  186  is illustrated. The configuration memory  186  includes transistors  187  and  188  and charge corresponding to the configuration data at a floating node FN is retained. The function of the switch  184  is achieved by switching the on/off state of the transistor  188  in accordance with the voltage of the floating node FN. The circuit configuration of  FIG. 21C  can be similar to that of the memory cell  140  described in Embodiment 1, in which case it is useful to use a transistor containing an oxide semiconductor as the transistor  187 . With this structure, the configuration memory  186  of the FPGA  183  can be fabricated through the same process as the memory cell  140 . 
         [0122]      FIGS. 22A and 22B  illustrate configuration examples of the pixel  121 . 
         [0123]    A pixel  162 A in  FIG. 22A  is an example of a pixel included in a liquid crystal display device. The pixel  162 A includes a transistor  191 , a capacitor  192 , and a liquid crystal element  193 . 
         [0124]    The transistor  191  serves as a switching element for controlling the connection between the liquid crystal element  193  and the signal line YL. The on/off state of the transistor  191  is controlled by a scan voltage input to its gate via the scan line XL. 
         [0125]    The capacitor  192  is, for example, an element formed by stacking conductive layers. 
         [0126]    The liquid crystal element  193  includes a common electrode, a pixel electrode, and a liquid crystal layer, for example. Alignment of a liquid crystal material of the liquid crystal layer is changed by the action of an electric field generated between the common electrode and the pixel electrode. 
         [0127]    A pixel  162 B in  FIG. 22B  is an example of a pixel included in an EL display device and includes a transistor  194 , a transistor  195 , and an EL element  196 . Note that in  FIG. 22B , a current supply line ZL in addition to the scan line XL and the signal line YL is illustrated. The current supply line ZL is a wiring for supplying current to the EL element  196 . 
         [0128]    The transistor  194  serves as a switching element for controlling the connection between a gate of the transistor  195  and the signal line YL. The on state of the transistor  194  is controlled by a scan voltage input to its gate through the scan line XL. 
         [0129]    The transistor  195  has a function of controlling current flowing between the current supply line ZL and the EL element  196 , in accordance with voltage applied to the gate of the transistor  195 . 
         [0130]    The EL element  196  is, for example, an element including a light-emitting layer provided between electrodes. The luminance of the EL element  196  can be controlled by the amount of current that flows in the light-emitting layer. 
       Embodiment 3 
       [0131]    In this embodiment, an example of a cross-sectional structure of a semiconductor device in one embodiment of the present invention will be described with reference to  FIGS. 23 to 35 . 
         [0132]    The semiconductor device described in the above embodiments can be fabricated by stacking a layer including a transistor using silicon and the like (Si transistor), a layer including a transistor using oxide semiconductor (OS transistor), and a wiring layer. 
       &lt;Layer Structure of Semiconductor Device&gt; 
       [0133]    A schematic view of a layer structure of the semiconductor device is illustrated in  FIG. 23 . A transistor layer  10 , a wiring layer  20 , a transistor layer  30 , and a wiring layer  40  are stacked in this order. The wiring layer  20  shown as an example includes wiring layers  20 A and  20 B. Furthermore, the wiring layer  40  includes a plurality of wiring layers  40 A and  40 B. In the wiring layer  20  and/or the wiring layer  40 , a capacitor can be formed such that an insulator is sandwiched between conductors. 
         [0134]    The transistor layer  10  includes a plurality of transistors  12 . The transistor  12  includes a semiconductor layer  14  and a gate electrode  16 . Although a layer processed into an island shape is shown as the semiconductor layer  14 , the semiconductor layer  14  may be a semiconductor layer obtained by element isolation from a semiconductor substrate. Although a gate electrode for a top-gate transistor is shown as the gate electrode  16 , the gate electrode  16  may be a gate electrode for a bottom-gate, a double-gate, or a dual-gate transistor, for example. 
         [0135]    Each of the wiring layers  20 A and  20 B includes a wiring  22  that is embedded in an opening provided in an insulating layer  24 . The wiring  22  functions as a wiring for connecting elements such as transistors. 
         [0136]    The transistor layer  30  includes a plurality of transistors  32 . The transistor  32  includes a semiconductor layer  34  and a gate electrode  36 . Although a layer processed into an island shape is shown as the semiconductor layer  34 , the semiconductor layer  34  may be a semiconductor layer obtained by element isolation from a semiconductor substrate. Although a gate electrode for a top-gate transistor is shown as the gate electrode  36 , the gate electrode  36  may be a gate electrode for a bottom-gate, a double-gate, or a dual-gate transistor, for example. 
         [0137]    Each of the wiring layers  40 A and  40 B includes a wiring  42  that is embedded in an opening provided in an insulating layer  44 . The wiring  42  functions as a wiring for connecting elements such as transistors. 
         [0138]    The semiconductor layer  14  is formed using a semiconductor material different from that for the semiconductor layer  34 . For example, given that the transistor  12  is a Si transistor and the transistor  32  is an OS transistor, the semiconductor material for the semiconductor layer  14  is silicon and that for the semiconductor layer  34  is an oxide semiconductor. 
       [Structure Example] 
       [0139]      FIG. 24A  illustrates an example of a cross-sectional view of the semiconductor device.  FIG. 24B  is an enlarged view of part of the structure in  FIG. 24A . 
         [0140]    The semiconductor device illustrated in  FIG. 24A  includes a capacitor  300 , a transistor  400 , and a transistor  500 . 
         [0141]    The capacitor  300  is provided over an insulator  602  and includes a conductor  604 , an insulator  612 , and a conductor  616 . 
         [0142]    The conductor  604  can be formed using a conductive material such as a metal material, an alloy material, or a metal oxide material. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. When the conductor  604  is formed concurrently with another component such as a plug or a wiring, a low-resistance metal material such as copper (Cu) or aluminum (Al) can be used. 
         [0143]    The insulator  612  is provided to cover a side surface and a top surface of the conductor  604 . The insulator  612  has a single-layer structure or a stacked-layer structure formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitride oxide, or hafnium nitride. 
         [0144]    The conductor  616  is provided to cover the side surface and the top surface of the conductor  604  with the insulator  612  positioned therebetween. 
         [0145]    Note that the conductor  616  can be formed using a conductive material such as a metal material, an alloy material, or a metal oxide material. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. When the conductor  616  is formed concurrently with another component such as a conductor, a low-resistance metal material such as copper (Cu) or aluminum (Al) can be used. 
         [0146]    With the structure where the conductor  616  included in the capacitor  300  covers the side surfaces and the top surface of the conductor  604  with the insulator  612  positioned therebetween, the capacitance per projected area of the capacitor  300  can be increased. Thus, the semiconductor device can be reduced in area, highly integrated, and miniaturized. 
         [0147]    The transistor  500  is provided over a substrate  301  and includes a conductor  306 , an insulator  304 , a semiconductor region  302  that is part of the substrate  301 , and low-resistance regions  308   a  and  308   b  functioning as a source region and a drain region. 
         [0148]    The transistor  500  is either a p-channel transistor or an n-channel transistor. 
         [0149]    A channel formation region of the semiconductor region  302 , a region around the channel formation region, the low-resistance regions  308   a  and  308   b  serving as the source region and the drain region, and the like contain preferably a semiconductor such as a silicon-based semiconductor, more preferably single crystal silicon. Alternatively, they may contain a material containing germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs), gallium aluminum arsenide (GaAlAs), or the like. They may contain silicon whose effective mass is controlled by applying stress to the crystal lattice and thereby changing the lattice spacing. Alternatively, the transistor  500  may be a high-electron-mobility transistor (HEMT) using GaAs and GaAlAs, or the like. 
         [0150]    The low-resistance regions  308   a  and  308   b  contain an element that imparts n-type conductivity (e.g., arsenic or phosphorus) or an element that imparts p-type conductivity (e.g., as boron) in addition to a semiconductor material used for the semiconductor region  302 . 
         [0151]    The conductor  306  functioning as a gate electrode can be formed using a semiconductor material such as silicon containing an element that imparts n-type conductivity (e.g., arsenic or phosphorus) or an element that imparts p-type conductivity (e.g., boron), or a conductive material such as a metal material, an alloy material, or a metal oxide material. 
         [0152]    Note that the threshold voltage can be adjusted by setting the work function with a material of the conductor. Specifically, it is preferable to use titanium nitride, tantalum nitride, or the like as the conductor. Furthermore, in order to ensure the conductivity and embeddability of the conductor, it is preferable to use a laminated layer of metal materials such as tungsten and aluminum as the conductor. In particular, tungsten is preferable in terms of heat resistance. 
         [0153]    In the transistor  500  illustrated in  FIG. 24A , the semiconductor region  302  (part of the substrate  301 ) in which a channel is formed includes a protruding portion. Furthermore, the conductor  306  is provided to cover a side surface and a top surface of the semiconductor region  302  with the insulator  304  therebetween. Note that the conductor  306  may be formed using a material for adjusting a work function. The transistor  500  with such a structure is also referred to as a FIN transistor because it utilizes the protruding portion of the semiconductor substrate. An insulator serving as a mask for forming the protruding portion may be provided in contact with a top surface of the protruding portion. Although the case where the protruding portion is formed by processing part of the semiconductor substrate is described here, a semiconductor film having a protruding shape may be formed by processing an SOI substrate. 
         [0154]    Note that the transistor  500  illustrated in  FIG. 24A  is just an example; without limitation to the structure shown in  FIG. 24A , an appropriate transistor can be used in accordance with a circuit configuration or a driving method. For example, a planar transistor  500 A illustrated in  FIG. 25A  may be used. 
         [0155]    An insulator  320 , an insulator  322 , an insulator  324 , and an insulator  326  are sequentially stacked and cover the transistor  500 . 
         [0156]    The insulator  322  functions as a planarization film for eliminating a level difference caused by the transistor  500  or the like underlying the insulator  322 . A top surface of the insulator  322  may be planarized by planarization treatment using a chemical mechanical polishing (CMP) method or the like to increase the level of planarity. 
         [0157]    The insulator  324  functions as a barrier film that prevents hydrogen or impurities from diffusing from the substrate  301 , the transistor  500 , or the like into a region where the transistor  400  is formed. For example, the insulator  324  can be formed using nitride such as silicon nitride. 
         [0158]    A conductor  328 , a conductor  330 , and the like that are electrically connected to the capacitor  300  or the transistor  400  are embedded in the insulator  320 , the insulator  322 , the insulator  324 , and the insulator  326 . The conductor  328  and the conductor  330  each function as a plug or a wiring. Note that a plurality of conductors functioning as plugs or wirings are collectively denoted by the same reference numeral in some cases, as described later. Furthermore, in this specification and the like, a wiring and a plug electrically connected to the wiring may be a single component. That is, there are cases where part of a conductor functions as a wiring and where part of a conductor functions as a plug. 
         [0159]    For each of the plugs and wirings (e.g., the conductor  328  and the conductor  330 ), a single-layer structure or a stacked-layer structure using a conductive material such as a metal material, an alloy material, or a metal oxide material can be used. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. It is particularly preferable to use a low-resistance conductive material such as aluminum or copper. The use of the above material can reduce the wiring resistance. 
         [0160]    A wiring layer may be provided over the insulator  326  and the conductor  330 . For example, an insulator  350 , an insulator  352 , and an insulator  354  are sequentially stacked in  FIG. 24A . A conductor  356  and a conductor  358  are embedded in the insulator  350 , the insulator  352 , and the insulator  354 . The conductor  356  and the conductor  358  each function as a plug or a wiring. 
         [0161]    Note that for example, the insulator  350  is preferably formed using an insulator with a barrier property with respect to hydrogen, like the insulator  324 . The conductor  356  and the conductor  358  are preferably formed using a conductor with a barrier property with respect to hydrogen. The conductor with a barrier property with respect to hydrogen is formed in an opening in the insulator  350  with a barrier property with respect to hydrogen. This structure can separate the transistor  500  and the transistor  400  by the barrier layer, and thus can prevent diffusion of hydrogen from the transistor  500  to the transistor  400 . 
         [0162]    As the conductor with a barrier property with respect to hydrogen, tantalum nitride can be used, for example. Stacking tantalum nitride and tungsten, which has high conductivity, can prevent diffusion of hydrogen from the transistor  500  while the conductivity of a wiring is ensured. 
         [0163]    The transistor  400  is provided over the insulator  354 .  FIG. 24B  is an enlarged view of the transistor  400 . Note that the transistor  400  illustrated in  FIG. 24B  is just an example; without limitation to the structure shown in  FIG. 24B , an appropriate transistor can be used in accordance with a circuit configuration or a driving method. 
         [0164]    The transistor  400  is a transistor in which a channel is formed in a semiconductor layer containing an oxide semiconductor. The off-state current of the transistor  400  is low; thus, using the transistor  400  in a frame memory of a semiconductor device enables stored data to be retained for a long time. 
         [0165]    An insulator  210 , an insulator  212 , an insulator  214 , and an insulator  216  are sequentially stacked over the insulator  354 . A conductor  218 , a conductor  205 , and the like are embedded in the insulator  210 , the insulator  212 , the insulator  214 , and the insulator  216 . The conductor  218  functions as a plug or a wiring that is electrically connected to the capacitor  300  or the transistor  500 . The conductor  205  functions as a gate electrode of the transistor  400 . 
         [0166]    A material with a barrier property with respect to oxygen or hydrogen is preferably used for any of the insulators  210 ,  212 ,  214 , and  216 . In particular, in the case of using an oxide semiconductor in the transistor  400 , the reliability of the transistor  400  can be increased when an insulator including an oxygen excess region is provided as an interlayer film or the like around the transistor  400 . Accordingly, in order to diffuse oxygen from the interlayer film around the transistor  400  to the transistor  400  efficiently, layers with barrier properties with respect to hydrogen and oxygen are preferably provided such that the transistor  400  and the interlayer film are sandwiched therebetween. 
         [0167]    For example, aluminum oxide, hafnium oxide, or tantalum oxide is preferably used for the barrier layers. Stacking the barrier layers achieves the function of diffusing oxygen more reliably. 
         [0168]    An insulator  220 , an insulator  222 , and an insulator  224  are sequentially stacked over the insulator  216 . Part of a conductor  244  is embedded in the insulator  220 , the insulator  222 , and the insulator  224 . Note that the conductor  218  functions as a plug or a wiring that is electrically connected to the capacitor  300  or the transistor  500 . 
         [0169]    Each of the insulators  220  and  224  is preferably an insulator containing oxygen, such as a silicon oxide film or a silicon oxynitride film. In particular, the insulator  224  is preferably an insulator containing excess oxygen (containing oxygen in excess of that in the stoichiometric composition). When such an insulator containing excess oxygen is provided in contact with an oxide  230  in which a channel region of the transistor  400  is formed, oxygen vacancies in the oxide can be filled. Note that the insulators  220  and  224  are not necessarily formed of the same material. 
         [0170]    The insulator  222  preferably has a single-layer structure or a stacked-layer structure using an insulator containing silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO 3 ), (Ba,Sr)TiO 3  (BST), or the like. Aluminum oxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxide, for example, may be added to the insulator. The insulator may be subjected to nitriding treatment. A layer of silicon oxide, silicon oxynitride, or silicon nitride may be stacked over the insulator. 
         [0171]    Note that the insulator  222  may have a stacked-layer structure of two or more layers. In this case, the stacked layers are not necessarily formed of the same material and may be formed of different materials. 
         [0172]    When the insulator  222  containing a high-k material is provided between the insulator  220  and the insulator  224 , electrons can be trapped in the insulator  222  under specific conditions, resulting in higher threshold voltage. In other words, the insulator  222  is negatively charged in some cases. 
         [0173]    For example, when the insulator  220  and the insulator  224  are formed using silicon oxide and the insulator  222  is formed using a material having a lot of electron trap states (e.g., hafnium oxide, aluminum oxide, or tantalum oxide), electrons move from the oxide  230  toward the conductor  205  under the following conditions: the potential of the conductor  205  is kept higher than the potential of a source electrode and a drain electrode for 10 milliseconds or longer, typically 1 minute or longer at temperatures higher than the operating temperature or the storage temperature of the semiconductor device (e.g., at temperatures ranging from 125° C. to 450° C., typically from 150° C. to 300° C.). At this time, some of the moving electrons are trapped by the electron trap states of the insulator  222 . 
         [0174]    In the transistor in which a necessary amount of electrons is trapped by the electron trap states of the insulator  222 , the threshold voltage is shifted in the positive direction. By controlling the voltage of the conductor  205 , the amount of electrons to be trapped can be controlled, and the threshold voltage can be controlled accordingly. The transistor  400  having this structure is a normally-off transistor, which is in a non-conduction state (also referred to as off state) even when the gate voltage is 0 V. 
         [0175]    The treatment for trapping the electrons can be performed in the manufacturing process of the transistor. For example, the treatment can be performed at any step before factory shipment, such as after the formation of a conductor connected to a source conductor or a drain conductor of the transistor, after a wafer process, after a wafer-dicing step, or after packaging. 
         [0176]    The insulator  222  is preferably formed using a material with a barrier property with respect to oxygen or hydrogen. The use of such a material can prevent release of oxygen from the oxide  230  and entry of impurities such as hydrogen from the outside. 
         [0177]    An oxide  230   a , an oxide  230   b , and an oxide  230   c  are formed using a metal oxide such as an In-M-Zn oxide (M is Al, Ga, Y, or Sn). An In—Ga oxide or In—Zn oxide may be used as the oxide  230 . Hereinafter the oxide  230   a , the oxide  230   b , and the oxide  230   c  may be collectively referred to as the oxide  230 . 
         [0178]    The oxide  230  according to the present invention is described below. 
         [0179]    An oxide used as the oxide  230  preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like may be contained. 
         [0180]    Here, the case where an oxide contains indium, an element M, and zinc is considered. The element M is aluminum, gallium, yttrium, tin, or the like. Other elements that can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like. Note that two or more of the above elements may be used in combination as the element M. 
         [0181]    First, preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide according to the present invention are described with reference to  FIGS. 26A to 26C . Note that the proportion of oxygen atoms is not shown in  FIGS. 26A to 26C . The terms of the atomic ratio of indium, the element M, and zinc contained in the oxide are denoted by [In], [M], and [Zn], respectively. 
         [0182]    In  FIGS. 26A to 26C , broken lines indicate a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):1 (where −1≦α≦1), a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio [In]:[M]:[Zn] is (1+a):(1−α):4, and a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):5. 
         [0183]    Dashed-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn] is 1:1:β (where β≧0), a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:3:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:4:β, a line where the atomic ratio [In]:[M]:[Zn] is 2:1:β, and a line where the atomic ratio [In]:[M]:[Zn] is 5:1:β. 
         [0184]    A dashed double-dotted line indicates a line where the atomic ratio [In]:[M]:[Zn] is (1+γ):2:(1−γ), where −1≦γ≦1. An oxide with the atomic ratio [In]:[M]:[Zn] of 0:2:1 or around 0:2:1 in  FIGS. 26A to 26C  tends to have a spinel crystal structure. 
         [0185]      FIGS. 26A and 26B  illustrate examples of the preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide of one embodiment of the present invention. 
         [0186]      FIG. 27  illustrates an example of the crystal structure of InMZnO 4  with an atomic ratio [In]:[M]:[Zn] of 1:1:1. The crystal structure illustrated in  FIG. 27  is InMZnO 4  observed from a direction parallel to the b-axis. Note that a metal element in a layer that contains the element M, Zn, and oxygen (hereinafter this layer is referred to as “(M,Zn) layer”) in  FIG. 27  represents the element M or zinc. In that case, the proportion of the element M is the same as the proportion of zinc. The element M and zinc can be replaced with each other, and their arrangement is random. 
         [0187]    Note that InMZnO 4  has a layered crystal structure (also referred to as layered structure) and includes two (M,Zn) layers that contain the element M, zinc, and oxygen with respect to one layer that contains indium and oxygen (hereinafter referred to as In layer), as illustrated in  FIG. 27 . 
         [0188]    Indium and the element M can be replaced with each other. Accordingly, when the element M in the (M,Zn) layer is replaced by indium, the layer can also be referred to as (In,M,Zn) layer. In that case, a layered structure that includes two (In,M,Zn) layers with respect to one In layer is obtained. 
         [0189]    An oxide with an atomic ratio [In]:[M]:[Zn] of 1:1:2 has a layered structure that includes three (M,Zn) layers with respect to one In layer. In other words, if [Zn] is larger than [In] and [M], the proportion of the (M,Zn) layer to the In layer becomes higher when the oxide is crystallized. 
         [0190]    Note that in the case where the number of (M,Zn) layers with respect to one In layer is not an integer in the oxide, the oxide might have plural kinds of layered structures where the number of (M,Zn) layers with respect to one In layer is an integer. For example, in the case of [In]:[M]:[Zn]=1:1:1.5, the oxide may have a mix of a layered structure including one In layer for every two (M,Zn) layers and a layered structure including one In layer for every three (M,Zn) layers. 
         [0191]    For example, when the oxide is deposited with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of a target is formed. In particular, [Zn] in the film might be smaller than [Zn] in the target depending on the substrate temperature in deposition. 
         [0192]    A plurality of phases (e.g., two phases or three phases) exist in the oxide in some cases. For example, with an atomic ratio [In]:[M]:[Zn] around 0:2:1, two phases of a spinel crystal structure and a layered crystal structure are likely to exist. In addition, with an atomic ratio [In]:[M]:[Zn] around 1:0:0, two phases of a bixbyite crystal structure and a layered crystal structure are likely to exist. In the case where a plurality of phases exist in the oxide, a grain boundary might be formed between different crystal structures. 
         [0193]    In addition, the oxide with a higher content of indium can have high carrier mobility (electron mobility). This is because in an oxide containing indium, the element M, and zinc, the s orbital of heavy metal mainly contributes to carrier transfer, and a higher indium content in the oxide enlarges a region where the s orbitals of indium atoms overlap; therefore, an oxide with a high indium content has higher carrier mobility than an oxide with a low indium content. 
         [0194]    In contrast, when the indium content and the zinc content in an oxide become lower, the carrier mobility becomes lower. Thus, with an atomic ratio [In]:[M]:[Zn] of 0:1:0 or around 0:1:0 (e.g., a region C in  FIG. 26C ), insulation performance becomes better. 
         [0195]    Accordingly, an oxide in one embodiment of the present invention preferably has an atomic ratio represented by a region A in  FIG. 26A . With this atomic ratio, a layered structure with high carrier mobility and a few grain boundaries is easily obtained. 
         [0196]    A region B in  FIG. 26B  represents an atomic ratio [In]:[M]:[Zn] of 4:2:3 to 4:2:4.1 and the vicinity thereof. The vicinity includes an atomic ratio [In]:[M]:[Zn] of 5:3:4, for example. An oxide with an atomic ratio represented by the region B is an excellent oxide that has particularly high crystallinity and high carrier mobility. 
         [0197]    Note that a condition where an oxide has a layered structure is not uniquely determined by an atomic ratio. The atomic ratio affects difficulty in forming a layered structure. Even with the same atomic ratio, whether a layered structure is formed or not depends on a formation condition. Therefore, the illustrated regions each represent an atomic ratio with which an oxide has a layered structure, and boundaries of the regions A to C are not clear. 
         [0198]    Next, the case where the oxide is used for a transistor is described. 
         [0199]    When the oxide is used for a transistor, carrier scattering or the like at a grain boundary can be reduced; thus, the transistor can have high field-effect mobility. Moreover, the transistor can have high reliability. 
         [0200]    An oxide with a low carrier density is preferably used for a transistor. For example, an oxide whose carrier density is lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , further preferably lower than 1×10 10 /cm 3 , and greater than or equal to 1×10 −9 /cm 3  is used. 
         [0201]    A highly purified intrinsic or substantially highly purified intrinsic oxide has few carrier generation sources and thus can have a low carrier density. A highly purified intrinsic or substantially highly purified intrinsic oxide has a low density of defect states and accordingly has a low density of trap states in some cases. 
         [0202]    Charge trapped by the trap states in the oxide takes a long time to be released and may behave like fixed charge. Thus, a transistor whose channel region is formed in an oxide with a high density of trap states has unstable electrical characteristics in some cases. 
         [0203]    In view of the above, to obtain stable electrical characteristics of a transistor, it is effective to reduce the concentration of impurities in the oxide. To reduce the concentration of impurities in the oxide, the concentration of impurities in a film that is adjacent to the oxide is preferably reduced. Examples of impurities include hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, and silicon. 
         [0204]    Here, the influence of impurities in the oxide is described. 
         [0205]    When silicon or carbon, which is a Group 14 element, is contained in the oxide, defect states are formed in the oxide. Thus, the concentration of silicon or carbon in the oxide and around an interface with the oxide (the concentration obtained by secondary ion mass spectrometry (SIMS)) is set lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
         [0206]    When the oxide contains alkali metal or alkaline earth metal, defect states are formed and carriers are generated in some cases. Thus, a transistor using an oxide that contains alkali metal or alkaline earth metal is likely to have normally-on characteristics. Accordingly, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide. Specifically, the concentration of alkali metal or alkaline earth metal in the oxide measured by SIMS is set lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . 
         [0207]    When the oxide contains nitrogen, the oxide easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor in which an oxide containing nitrogen is used as a semiconductor is likely to have normally-on characteristics. For this reason, nitrogen in the oxide is preferably reduced as much as possible. For example, the nitrogen concentration in the oxide measured by SIMS is set lower than 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , further preferably lower than or equal to 1×10 18  atoms/cm 3 , still further preferably lower than or equal to 5×10 17  atoms/cm 3 . 
         [0208]    Hydrogen contained in an oxide reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy in some cases. Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is sometimes generated. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor using an oxide that contains hydrogen is likely to have normally-on characteristics. Accordingly, it is preferred that hydrogen in the oxide be reduced as much as possible. Specifically, the hydrogen concentration in the oxide measured by SIMS is set lower than 1×10 20  atoms/cm 3 , preferably lower than 1×10 19  atoms/cm 3 , further preferably lower than 5×10 18  atoms/cm 3 , still further preferably lower than 1×10 18  atoms/cm 3 . 
         [0209]    When an oxide with sufficiently reduced impurity concentration is used for a channel region in a transistor, the transistor can have stable electrical characteristics. 
         [0210]    Next, the case where the oxide has a two-layer structure or a three-layer structure will be described. With reference to  FIGS. 28A to 28C , the description is made on a band diagram of a layered structure of an oxide S 1 , an oxide S 2 , and an oxide S 3  and insulators that are in contact with the layered structure of an oxide S 1 , an oxide S 2 , and an oxide S 3 ; a layered structure of the oxide S 1  and the oxide S 2  and insulators that are in contact with the layered structure of the oxide S 1  and the oxide S 2 ; and a band diagram of a layered structure of the oxide S 2  and the oxide S 3  and insulators that are in contact with a layered structure of the oxide S 2  and the oxide S 3 . 
         [0211]      FIG. 28A  is an example of a band diagram of a layered structure including an insulator I 1 , the oxide S 1 , the oxide S 2 , the oxide S 3 , and an insulator I 2  in the thickness direction.  FIG. 28B  is an example of a band diagram of a layered structure including the insulator I 1 , the oxide S 2 , the oxide S 3 , and the insulator I 2  in the thickness direction.  FIG. 28C  is an example of a band diagram of a layered structure including the insulator I 1 , the oxide S 1 , the oxide S 2 , and the insulator I 2  in the thickness direction. Note that for easy understanding, the band diagrams show the energy level of the conduction band minimum (Ec) of each of the insulator I 1 , the oxide S 1 , the oxide S 2 , the oxide S 3 , and the insulator I 2 . 
         [0212]    The energy level of the conduction band minimum of each of the oxides S 1  and S 3  is closer to the vacuum level than that of the oxide S 2 . Typically, a difference in the energy level of the conduction band minimum between the oxide S 2  and each of the oxides S 1  and S 3  is preferably greater than or equal to 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2 eV or less than or equal to 1 eV. That is, the difference in the electron affinity between the oxide S 2  and each of the oxides S 1  and S 3  is preferably greater than or equal to 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2 eV or less than or equal to 1 eV. 
         [0213]    As illustrated in  FIGS. 28A to 28C , the energy level of the conduction band minimum of each of the oxides S 1  to S 3  is gradually varied. In other words, the energy level of the conduction band minimum is continuously varied or continuous junction is formed. To obtain such a band diagram, the density of defect states in a mixed layer formed at an interface between the oxides S 1  and S 2  or an interface between the oxides S 2  and S 3  is preferably made low. 
         [0214]    Specifically, when the oxides S 1  and S 2  or the oxides S 2  and S 3  contain the same element (as a main component) in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, when the oxide S 2  is an In—Ga—Zn oxide, it is preferable to use an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like as the oxides S 1  and S 3 . 
         [0215]    At this time, the oxide S 2  serves as a main carrier path. Since the density of defect states at the interface between the oxides S 1  and S 2  and the interface between the oxides S 2  and S 3  can be made low, the influence of interface scattering on carrier conduction is small, and a high on-state current can be obtained. 
         [0216]    When an electron is trapped in a trap state, the trapped electron behaves like fixed charge; thus, the threshold voltage of a transistor is shifted in the positive direction. The oxides S 1  and S 3  can make the trap state apart from the oxide S 2 . This structure can prevent the positive shift of the threshold voltage of the transistor. 
         [0217]    A material whose conductivity is sufficiently lower than that of the oxide S 2  is used for the oxides S 1  and S 3 . Accordingly, the oxide S 2 , the interface between the oxides S 1  and S 2 , and the interface between the oxides S 2  and S 3  mainly function as a channel region. For example, an oxide with high insulation performance and the atomic ratio represented by the region C in  FIG. 26C  can be used as the oxides S 1  and S 3 . Note that the region C in  FIG. 26C  represents the atomic ratio [In]:[M]:[Zn] of 0:1:0 or around 0:1:0. 
         [0218]    In the case where an oxide with the atomic ratio represented by the region A is used as the oxide S 2 , it is particularly preferable to use an oxide with an atomic ratio where [M]/[In] is greater than or equal to 1, preferably greater than or equal to 2 as each of the oxides S 1  and S 3 . In addition, it is suitable to use an oxide with sufficiently high insulation performance and an atomic ratio where [M]/([Zn]+[In]) is greater than or equal to 1 as the oxide S 3 . 
         [0219]    One of a conductor  240   a  and a conductor  240   b  functions as a source electrode, and the other functions as a drain electrode. 
         [0220]    The conductor  240   a  and the conductor  240   b  are formed to have a single-layer structure or a stacked-layer structure using any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten, or an alloy containing any of these metals as a main component. For example, the conductor  240   a  and the conductor  240   b  can have any of the following structures: a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a tantalum film or a tantalum nitride film is stacked, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, and a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used. 
         [0221]    An insulator  250  can have a single-layer structure or a stacked-layer structure using an insulator containing silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO 3 ), (Ba,Sr)TiO 3  (BST), or the like. Aluminum oxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxide, for example, may be added to the insulator. The insulator may be subjected to nitriding treatment. A layer of silicon oxide, silicon oxynitride, or silicon nitride may be stacked over the insulator. 
         [0222]    Like the insulator  224 , the insulator  250  is preferably an oxide insulator that contains oxygen in excess of that in the stoichiometric composition. 
         [0223]    Note that the insulator  250  may have a stacked-layer structure similar to that of the insulator  220 , the insulator  222 , and the insulator  224 . When the insulator  250  contains an insulator in which a necessary amount of electrons is trapped by electron trap states, the threshold voltage of the transistor  400  can be shifted in the positive direction. The transistor  400  having this structure is a normally-off transistor, which is in a non-conduction state (also referred to as off state) even when the gate voltage is 0 V. 
         [0224]    A conductor  260  functioning as a gate electrode can be formed using a metal selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloy containing any of these metals as its component, or an alloy containing any of these metals in combination, for example. Furthermore, one or both of manganese and zirconium may be used. A semiconductor typified by polycrystalline silicon doped with an impurity element such as phosphorus, or a silicide such as nickel silicide may be used. For example, the conductor  260  can have any of the following structures: a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, and a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order. Alternatively, an alloy film or a nitride film that contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used. 
         [0225]    The conductor  260  can also be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. The conductor  260  can have a stacked-layer structure using the above light-transmitting conductive material and the above metal. 
         [0226]    An insulator  280  is preferably formed using an oxide material from which oxygen is partly released due to heating. 
         [0227]    As the oxide material from which oxygen is released due to heating, an oxide containing oxygen in excess of that in the stoichiometric composition is preferably used. Part of oxygen is released by heating from an oxide film containing oxygen more than that in the stoichiometric composition. The oxide film containing oxygen in excess of that in the stoichiometric composition is an oxide film in which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 18  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS) analysis. Note that the temperature of the film surface in the TDS analysis preferably ranges from 100° C. to 700° C. or from 100° C. to 500° C. 
         [0228]    As such a material, a material containing silicon oxide or silicon oxynitride is preferably used, for example. Alternatively, a metal oxide can be used. Note that in this specification, silicon oxynitride refers to a material that has a higher oxygen content than a nitrogen content, and silicon nitride oxide refers to a material that has a higher nitrogen content than an oxygen content. 
         [0229]    The insulator  280  covering the transistor  400  may function as a planarization film that covers roughness thereunder. 
         [0230]    An insulator  270  may be provided to cover the conductor  260 . When the insulator  280  is formed using an oxide material from which oxygen is released, the insulator  270  is formed using a material with a barrier property with respect to oxygen to prevent the conductor  260  from being oxidized by the released oxygen. With this structure, oxidation of the conductor  260  can be prevented, and oxygen released from the insulator  280  can be efficiently supplied to the oxide  230 . 
         [0231]    An insulator  282  and an insulator  284  are sequentially stacked over the insulator  280 . The conductor  244 , a conductor  246   a , a conductor  246   b , and the like are embedded in the insulator  280 , the insulator  282 , and the insulator  284 . The conductor  244  functions as a plug or a wiring that is electrically connected to the capacitor  300  or the transistor  500 . Each of the conductors  246   a  and  246   b  functions as a plug or a wiring that is electrically connected to the capacitor  300  or the transistor  400 . 
         [0232]    A material with a barrier property with respect to oxygen or hydrogen is preferably used for one or both of the insulator  282  and the insulator  284 . Accordingly, oxygen released from the interlayer film around the transistor  400  can be efficiently diffused into the transistor  400 . 
         [0233]    The capacitor  300  is provided above the insulator  284 . 
         [0234]    The conductor  604  and a conductor  624  are provided over the insulator  602 . The conductor  624  functions as a plug or a wiring that is electrically connected to the transistor  400  or the transistor  500 . 
         [0235]    The insulator  612  is provided over the conductor  604 , and the conductor  616  is provided over the insulator  612 . The conductor  616  covers a side surface of the conductor  604  with the insulator  612  placed therebetween. That is, a capacitance is formed also on the side surface of the conductor  604 , so that the capacitance per projected area of the capacitor can be increased. Thus, the semiconductor device can be reduced in area, highly integrated, and miniaturized. 
         [0236]    Note that the insulator  602  is provided at least in a region overlapped by the conductor  604 . For example, as in a capacitor  300 A illustrated in  FIG. 25B , the insulator  602  may be provided only in regions overlapped by the conductor  604  or the conductor  624  so that the insulator  602  is in contact with the insulator  612 . 
         [0237]    An insulator  620  and an insulator  622  are sequentially stacked over the conductor  616 . A conductor  626  and a conductor  628  are embedded in the insulator  620 , the insulator  622 , and the insulator  602 . Each of the conductors  626  and the conductor  628  functions as a plug or a wiring that is electrically connected to the transistor  400  or the transistor  500 . 
         [0238]    The insulator  620  covering the capacitor  300  may function as a planarization film that covers roughness thereunder. 
         [0239]    The above is the description of the structure example. 
       [Example of Manufacturing Method] 
       [0240]    An example of a method for manufacturing the semiconductor device shown in the above structure example will be described below with reference to  FIG. 29A  to  FIG. 35 . 
         [0241]    First, the substrate  301  is prepared. A semiconductor substrate is used as the substrate  301 . For example, a single crystal silicon substrate (including a p-type semiconductor substrate or an n-type semiconductor substrate) or a compound semiconductor substrate containing silicon carbide or gallium nitride can be used. An SOI substrate may alternatively be used as the substrate  301 . The case where a single crystal silicon is used as the substrate  301  is described below. 
         [0242]    Next, an element isolation layer is formed in the substrate  301 . The element isolation layer can be formed by a local oxidation of silicon (LOCOS) method, a shallow trench isolation (STI) method, or the like. 
         [0243]    When a p-channel transistor and an n-channel transistor are formed on one substrate, an n-well or a p-well may be formed in part of the substrate  301 . For example, a p-well may be formed by adding an impurity element that imparts p-type conductivity (e.g., boron) to an n-type substrate  301 , and an n-channel transistor and a p-channel transistor may be formed on the same substrate. 
         [0244]    Then, an insulator to be the insulator  304  is formed on the substrate  301 . For example, after surface nitriding treatment, oxidizing treatment may be performed to oxidize the interface between silicon and silicon nitride, whereby a silicon oxynitride film may be formed. For example, a silicon oxynitride film is obtained by performing oxygen radical oxidation after a thermal silicon nitride film is formed on the surface of the substrate  301  at 700° C. in an NH 3  atmosphere. 
         [0245]    The insulator may be formed by a sputtering method, a chemical vapor deposition (CVD) method (including a thermal CVD method, a metal organic CVD (MOCVD) method, and a plasma-enhanced CVD (PECVD) method), a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, a pulsed laser deposition (PLD) method, or the like. 
         [0246]    Then, a conductive film to be the conductor  306  is formed. It is preferred that the conductive film be formed using a metal selected from tantalum, tungsten, titanium, molybdenum, chromium, niobium, and the like, or an alloy material or a compound material containing any of the metals as its main component. Alternatively, polycrystalline silicon to which an impurity such as phosphorus is added can be used. Further alternatively, a stacked-layer structure of a film of a metal nitride and a film of any of the above metals may be used. As a metal nitride, tungsten nitride, molybdenum nitride, or titanium nitride can be used. When the metal nitride film is provided, adhesiveness of the metal film can be increased; thus, separation can be prevented. Note that the threshold voltage of the transistor  500  can be adjusted by determining a work function of the conductor  306 , and therefore, a material of the conductive film is selected as appropriate in accordance with the characteristics that the transistor  500  needs to have. 
         [0247]    The conductive film can be formed by a sputtering method, an evaporation method, a CVD method (including a thermal CVD method, an MOCVD method, and a PECVD method), or the like. A thermal CVD method, an MOCVD method, or an ALD method is preferably used to reduce plasma damage. 
         [0248]    Next, a resist mask is formed over the conductive film by a photolithography process or the like, and an unnecessary portion of the conductive film is removed. After that, the resist mask is removed, whereby the conductor  306  is formed. 
         [0249]    Here, a method for processing a film is described. To process a film finely, a variety of fine processing techniques can be used. For example, it is possible to use a method in which a resist mask formed by a photolithography process or the like is subjected to slimming treatment. Alternatively, it is possible that a dummy pattern is formed by a photolithography process or the like, a sidewall is formed on the dummy pattern, the dummy pattern is then removed, and a film is etched using the remaining sidewall as a resist mask. In order to achieve a high aspect ratio, anisotropic dry etching is preferably used for film etching. Alternatively, a hard mask formed of an inorganic film or a metal film may be used. 
         [0250]    As light used to form the resist mask, it is possible to use light with the i-line (wavelength: 365 nm), the g-line (wavelength: 436 nm), or the h-line (wavelength: 405 nm) or light in which the i-line, the g-line, and the h-line are mixed. Alternatively, ultraviolet light, KrF laser light, ArF laser light, or the like can be used. Light exposure may be performed by liquid immersion exposure technique. As the light for the exposure, extreme ultraviolet (EUV) light or X-rays may be used. Moreover, an electron beam can be used instead of the light for the exposure. It is preferable to use EUV light, X-rays, or an electron beam because extremely minute processing can be performed. Note that a photomask is not needed in the case of performing exposure by scanning of a beam such as an electron beam. 
         [0251]    An organic resin film having a function of improving the adhesion between a film to be processed and a resist film may be formed before the resist film serving as a resist mask is formed. The organic resin film can be formed by a spin coating method or the like to planarize a surface by covering a step under the film, and thus can reduce variation in thickness of the resist mask over the organic resin film. For fine processing in particular, a material serving as a film preventing reflection of light for the exposure is preferably used for the organic resin film. An example of the organic resin film having such a function includes a bottom anti-reflective coating (BARC) film. The organic resin film can be removed at the same time as the removal of the resist mask or after the removal of the resist mask. 
         [0252]    After the conductor  306  is formed, a sidewall covering a side surface of the conductor  306  may be formed. The sidewall can be formed in such a manner that an insulator thicker than the conductor  306  is formed and subjected to anisotropic etching so that the insulator remains only on the side surface of the conductor  306 . 
         [0253]    The insulator to be the insulator  304  is etched concurrently with the formation of the sidewall, whereby the insulator  304  is formed under the conductor  306  and the sidewall. Alternatively, the insulator  304  may be formed by etching the insulator with the conductor  306  or the resist mask for processing the conductor  306  used as an etching mask after the conductor  306  is formed. In this case, the insulator  304  is formed under the conductor  306 . Further alternatively, the insulator can be used as the insulator  304  without being processed by etching. 
         [0254]    Then, an element that imparts n-type conductivity (e.g., phosphorus) or an element that imparts p-type conductivity (e.g., boron) is added to a region of the substrate  301  where the conductor  306  (and the sidewall) is not provided. 
         [0255]    Subsequently, the insulator  320  is formed, and then heat treatment is performed to activate the aforementioned element that imparts conductivity. 
         [0256]    The insulator  320  is formed with a single-layer structure or a stacked-layer structure using silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, or aluminum nitride, for example. The insulator  320  is preferably formed using silicon nitride containing oxygen and hydrogen (SiNOH) because the amount of hydrogen released by heating can be increased. The insulator  320  can also be formed using silicon oxide with high step coverage that is formed by reacting tetraethyl orthosilicate (TEOS), silane, or the like with oxygen, nitrous oxide, or the like. 
         [0257]    The insulator  320  can be formed by a sputtering method, a CVD method (including a thermal CVD method, an MOCVD method, and a PECVD method), an MBE method, an ALD method, or a PLD method, for example. In particular, the insulator is formed preferably by a CVD method, more preferably a PECVD method because coverage can be further improved. A thermal CVD method, an MOCVD method, or an ALD method is preferably used to reduce plasma damage. 
         [0258]    The heat treatment can be performed at a temperature higher than or equal to 400° C. and lower than the strain point of the substrate in an inert gas atmosphere such as a rare gas atmosphere or a nitrogen gas atmosphere or in a reduced-pressure atmosphere. 
         [0259]    At this stage, the transistor  500  is completed. 
         [0260]    Subsequently, the insulator  322  is formed over the insulator  320 . The insulator  322  can be formed using a material and a method similar to those used for forming the insulator  320 . Moreover, the top surface of the insulator  322  is planarized by a CMP method or the like ( FIG. 29A ). 
         [0261]    Then, openings that reach the low-resistance region  308   a , the low-resistance region  308   b , the conductor  306 , and the like are formed in the insulator  320  and the insulator  322  ( FIG. 29B ). After that, a conductive film is formed to fill the openings (see  FIG. 29C ). The conductive film can be formed by a sputtering method, a CVD method (including a thermal CVD method, an MOCVD method, and a PECVD method), an MBE method, an ALD method, or a PLD method, for example. 
         [0262]    Next, planarization treatment is performed on the conductive film to expose the top surface of the insulator  322 , whereby a conductor  328   a , a conductor  328   b , a conductor  328   c , and the like are formed ( FIG. 29D ). Note that arrows in  FIG. 29D  represent CMP treatment. In the specification and the drawings, the conductor  328   a , the conductor  328   b , and the conductor  328   c  each function as a plug or a wiring and are collectively referred to as “conductor  328 ” in some cases. Note that in this specification, conductors functioning as a plug or a wiring are treated in a similar manner. 
         [0263]    After the insulator  322  and the insulator  324  are formed over the insulator  320 , a conductor  330   a , a conductor  330   b , and a conductor  330   c  are formed by a damascene process or the like ( FIG. 30A ). The insulator  322  and the insulator  324  can be formed using a material and a method similar to those used for forming the insulator  320 . A conductive film to be the conductor  330  can be formed using a material and a method similar to those used for forming the conductor  328 . 
         [0264]    Then, the insulator  352  and the insulator  354  are formed, and after that, a conductor  358   a , a conductor  358   b , and a conductor  358   c  are formed in the insulator  352  and the insulator  354  by a dual damascene process or the like ( FIG. 30B ). The insulator  352  and the insulator  354  can be formed using a material and a method similar to those used for forming the insulator  320 . A conductive film to be the conductor  358  can be formed using a material and a method similar to those used for forming the conductor  328 . 
         [0265]    Next, the transistor  400  is formed in the following manner. After the insulator  210  is formed, the insulator  212  and the insulator  214  that have a barrier property against hydrogen or oxygen are formed. The insulator  210  can be formed using a material and a method similar to those used for forming the insulator  320 . 
         [0266]    The insulator  212  and the insulator  214  can be formed by a sputtering method, a CVD method (including a thermal CVD method, an MOCVD method, and a PECVD method), an MBE method, an ALD method, or a PLD method, for example. In particular, when the insulator  212  or the insulator  214  is formed by an ALD method, it is possible to form a dense insulator that includes a small number of defects such as cracks or pinholes or has a uniform thickness. 
         [0267]    Then, the insulator  216  is formed over the insulator  214 . The insulator  216  can be formed using a material and a method similar to those used for forming the insulator  210  ( FIG. 30C ). 
         [0268]    Next, openings that reach the conductor  358   a , the conductor  358   b , the conductor  358   c , and the like are formed in the insulator  210 , the insulator  212 , the insulator  214 , and the insulator  216  ( FIG. 31A ). 
         [0269]    Subsequently, an opening is formed in a region of the insulator  216  where the gate electrode of the transistor  400  is to be formed. At this time, the openings that have been formed in the insulator  216  may be widened ( FIG. 31B ). By widening the openings formed in the insulator  216 , an adequate design margin for plugs or wirings to be formed in a later step can be provided. 
         [0270]    After that, a conductive film is formed to fill the openings (see  FIG. 31C ). The conductive film can be formed using a material and a method similar to those used for forming the conductor  328 . Then, planarization treatment is performed on the conductive film to expose a top surface of the insulator  216 , whereby a conductor  218   a , a conductor  218   b , a conductor  218   c , and the conductor  205  are formed ( FIG. 32A ). Note that arrows in  FIG. 32A  represent CMP treatment. 
         [0271]    Then, the insulator  220 , the insulator  222 , and the insulator  224  are formed. The insulator  220 , the insulator  222 , and the insulator  224  can be formed using a material and a method similar to those used for forming the insulator  210 . It is particularly preferable to use a high-k material as the insulator  222 . 
         [0272]    Next, an oxide to be the oxide  230   a  and an oxide to be the oxide  230   b  are sequentially formed. The oxides are preferably formed successively without exposure to the air. 
         [0273]    After the oxide to be the oxide  230   b  is formed, heat treatment is preferably performed. The heat treatment is performed at a temperature ranging from 250° C. to 650° C., preferably from 300° C. to 500° C. in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, or a reduced-pressure state. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidization gas at 10 ppm or more, in order to compensate released oxygen. The heat treatment may be performed directly after the formation of the oxide to be the oxide  230   b  or may be performed after the oxide to be the oxide  230   b  is processed into an island shape. By the heat treatment, oxygen can be supplied from the insulator formed under the oxide  230   a  to the oxide  230   a  and the oxide  230   b , so that oxygen vacancies in the oxides can be reduced. 
         [0274]    Then, a conductive film to be the conductor  240   a  and the conductor  240   b  is formed over the oxide to be the oxide  230   b . Subsequently, a resist mask is formed by a method similar to that described above, and an unnecessary portion of the conductive film is removed by etching. After that, unnecessary portions of the oxides are removed by etching using the conductive film as a mask. Then, the resist mask is removed. In this manner, a stack including the island-shaped oxide  230   a , the island-shaped oxide  230   b , and the island-shaped conductive film can be formed. 
         [0275]    Subsequently, a resist mask is formed over the island-shaped conductive film by a method similar to that described above, and an unnecessary portion of the conductive film is removed by etching. Next, the resist mask is removed; thus, the conductor  240   a  and the conductor  240   b  are formed. 
         [0276]    Then, an oxide to be the oxide  230   c , an insulator to be the insulator  250 , and a conductive film to be the conductor  260  are sequentially formed. Next, a resist mask is formed over the conductive film by a method similar to that described above, and an unnecessary portion of the conductive film is removed by etching, whereby the conductor  260  is formed. 
         [0277]    Then, an insulator to be the insulator  270  is formed over the insulator to be the insulator  250  and the conductor  260 . The insulator to be the insulator  270  is preferably formed using a material with barrier properties against hydrogen and oxygen. Then, a resist mask is formed over the insulator by a method similar to that described above, and unnecessary portions of the insulator to be the insulator  270 , the insulator to be the insulator  250 , and the oxide to be the oxide  230   c  are removed by etching. After that, the resist mask is removed. Thus, the transistor  400  is completed. 
         [0278]    Next, the insulator  280  is formed. The insulator  280  is preferably formed using an oxide containing oxygen in excess of that in the stoichiometric composition. After an insulator to be the insulator  280  is formed, planarization treatment using a CMP method or the like may be performed to improve the planarity of a top surface of the insulator. 
         [0279]    To make the insulator  280  contain excess oxygen, the insulator  280  can be formed in an oxygen atmosphere, for example. Alternatively, a region containing excess oxygen may be formed by introducing oxygen into the insulator  280  that has been formed. Both the methods may be used in combination. 
         [0280]    For example, oxygen (at least including any of oxygen radicals, oxygen atoms, and oxygen ions) is introduced into the insulator  280  that has been formed, whereby a region containing excess oxygen is formed. Oxygen can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. 
         [0281]    A gas containing oxygen can be used for the oxygen introduction treatment. Examples of a gas containing oxygen include oxygen, dinitrogen monoxide, nitrogen dioxide, carbon dioxide, and carbon monoxide. In the oxygen introduction treatment, a rare gas may be contained in the gas containing oxygen. For example, a mixed gas of carbon dioxide, hydrogen, and argon can be used. 
         [0282]    An example of the oxygen introduction treatment is a method of stacking an oxide over the insulator  280  using a sputtering apparatus. For example, when the insulator  282  is formed in an oxygen gas atmosphere with a sputtering apparatus, oxygen can be introduced into the insulator  280  while the insulator  282  is formed. 
         [0283]    Next, the insulator  284  is formed. The insulator  284  can be formed using a material and a method similar to those used for forming the insulator  210 . The insulator  284  is preferably formed using aluminum oxide with a barrier property against oxygen or hydrogen, for example. In particular, when the insulator  284  is formed by an ALD method, it is possible to form a dense insulator that includes a small number of defects such as cracks or pinholes or has a uniform thickness. 
         [0284]    By stacking the insulator  284  having dense film quality over the insulator  282 , excess oxygen introduced into the insulator  280  can be effectively sealed on the transistor  400  side ( FIG. 32B ). 
         [0285]    Next, the capacitor  300  is formed in the following manner. First, the insulator  602  is formed over the insulator  284 . The insulator  602  can be formed using a material and a method similar to those used for forming the insulator  210 . 
         [0286]    Then, openings that reach the conductor  218   a , the conductor  218   b , the conductor  218   c , the conductor  240   a , the conductor  240   b , and the like are formed in the insulator  220 , the insulator  222 , the insulator  224 , the insulator  280 , the insulator  282 , and the insulator  284 . 
         [0287]    After that, a conductive film is formed to fill the openings, and planarization treatment is performed on the conductive film to expose a top surface of the insulator  216 . Thus, a conductor  244   a , a conductor  244   b , a conductor  244   c , the conductor  246   a , and the conductor  246   b  are formed. The conductive film can be formed using a material and a method similar to those used for forming the conductor  328 . 
         [0288]    Next, a conductive film  604 A is formed over the insulator  602 . The conductive film  604 A can be formed using a material and a method similar to those used for forming the conductor  328 . Then, a resist mask  690  is formed over the conductive film  604 A ( FIG. 33A ). 
         [0289]    A conductor  624   a , a conductor  624   b , a conductor  624   c , and the conductor  604  are formed by etching the conductive film  604 A. Over-etching is performed as this etching treatment, whereby part of the insulator  602  can be removed at the same time ( FIG. 33B ). The depth of the removed portion of the insulator  602  needs to be larger than the thickness of the insulator  612  that is formed later. Formation of the conductor  604  with over-etching enables etching without an etching residue. 
         [0290]    By switching the types of etching gases during the etching treatment, part of the insulator  602  can be removed efficiently. 
         [0291]    As an alternative example, after the conductor  604  is formed, the resist mask  690  may be removed and part of the insulator  602  may be removed using the conductor  604  as a hard mask. 
         [0292]    After the conductor  604  is formed, a surface of the conductor  604  may be subjected to cleaning treatment. By the cleaning treatment, an etching residue or the like can be removed. 
         [0293]    When the insulator  602  and the insulator  284  are films of different types, the insulator  284  may serve as an etching stopper film. In this case, the insulator  602  is formed in regions overlapped by the conductor  624  or the conductor  604  as illustrated in  FIG. 25B . 
         [0294]    Then, the insulator  612  that covers the side surface and the top surface of the conductor  604  is formed ( FIG. 34A ). The insulator  612  has a single-layer structure or a stacked-layer structure formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitride oxide, or hafnium nitride. 
         [0295]    For example, the insulator  612  preferably has a stacked-layer structure of a high-k material (e.g., aluminum oxide) and a material with high dielectric strength (e.g., silicon oxynitride). Such a structure enables the capacitor  300  to have sufficient capacitance due to the high-k material and increased dielectric strength due to the material with high dielectric strength. Thus, the capacitor  300  can be prevented from being damaged by electrostatic discharge, which leads to improvement in the reliability of the capacitor  300 . 
         [0296]    Then, a conductive film  616 A is formed over the insulator  612  ( FIG. 34A ). The conductive film  616 A can be formed using a material and a method similar to those used for forming the conductor  604 . Subsequently, a resist mask is formed over the conductive film  616 A, and an unnecessary portion of the conductive film  616 A is removed by etching. After that, the resist mask is removed, whereby the conductor  616  is formed. 
         [0297]    Then, the insulator  620  covering the capacitor  300  is formed ( FIG. 34B ). An insulator to be the insulator  620  can be formed using a material and a method similar to those used for forming the insulator  602  and the like. 
         [0298]    Next, openings that reach the conductor  624   a , the conductor  624   b , the conductor  624   c , the conductor  604 , and the like are formed in the insulator  620 . 
         [0299]    Then, a conductive film is formed to fill the openings, and planarization treatment is performed on the conductive film to expose a top surface of the insulator  620 . Thus, a conductor  626   a , a conductor  626   b , a conductor  626   c , and a conductor  626   d  are formed. Note that the conductive film can be formed using a material and a method similar to those used for forming the conductor  244 . 
         [0300]    Subsequently, a conductive film to be the conductor  626  is formed. The conductive film can be formed by a sputtering method, a CVD method (including a thermal CVD method, an MOCVD method, and a PECVD method), an MBE method, an ALD method, a PLD method, or others. In particular, the conductive film is formed preferably by a CVD method, more preferably a PECVD method because coverage can be further improved. A thermal CVD method, an MOCVD method, or an ALD method is preferably used to reduce plasma damage. 
         [0301]    The conductive film to be the conductor  626  can be formed using, for example, a metal selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metals as a component; or an alloy containing any of these metals in combination. Moreover, one or both of manganese and zirconium may be used. A semiconductor typified by polycrystalline silicon doped with an impurity element (e.g., phosphorus) or a silicide such as nickel silicide may be used. For example, the conductive film can have any of the following structures: a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, and a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order. Alternatively, an alloy film or a nitride film that contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used. 
         [0302]    Next, a resist mask is formed over the conductive film to be the conductor  626  by a method similar to that described above, and an unnecessary portion of the conductive film is removed by etching. Then, the resist mask is removed, whereby the conductor  626   a , the conductor  626   b , the conductor  626   c , and the conductor  626   d  are formed. 
         [0303]    Then, the insulator  622  is formed over the insulator  620  ( FIG. 35 ). The insulator  622  can be formed using a material and a method similar to those used for forming the insulator  602  and the like. 
         [0304]    Next, openings that reach the conductor  626   a , the conductor  626   b , the conductor  626   c , and the conductor  626   d  are formed in the insulator  622 . 
         [0305]    Then, a conductive film is formed to fill the openings, and planarization treatment is performed on the conductive film to expose a top surface of the insulator  622 ; thus, a conductor  628   a , a conductor  628   b , a conductor  628   c , and a conductor  628   d  are formed. Note that the conductive film can be formed using a material and a method similar to those used for forming the conductor  244 . 
         [0306]    Through the above steps, the semiconductor device in one embodiment of the present invention can be manufactured. 
         [0307]    At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
       Embodiment 4 
       [0308]    In this embodiment, application examples of the semiconductor device described in the foregoing embodiment to a display panel, an application example of a display module, and application examples of the display module to an electronic device will be described with reference to  FIGS. 36A and 36B ,  FIG. 37 , and  FIGS. 38A to 38E . 
       &lt;Examples of Mounting Semiconductor Device on Display Panel&gt; 
       [0309]    Application examples of a semiconductor device functioning as a source driver IC to a display panel will be described with reference to  FIGS. 36A and 36B . 
         [0310]    In the example of  FIG. 36A , a source driver  712  and gate drivers  712 A and  712 B are provided around a display portion  711  of a display panel, and a source driver IC  714  including a semiconductor device is mounted on a substrate  713  as the source driver  712 . 
         [0311]    The source driver IC  714  is mounted on the substrate  713  using an anisotropic conductive adhesive and an anisotropic conductive film. 
         [0312]    The source driver IC  714  is connected to an external circuit board  716  via an FPC  715 . 
         [0313]    In the example of  FIG. 36B , the source driver  712  and the gate drivers  712 A and  712 B are provided around the display portion  711 , and the source driver IC  714  is mounted on the FPC  715  as the source driver  712 . 
         [0314]    Mounting the source driver IC  714  on the FPC  715  allows a larger display portion  711  to be provided over the substrate  713 , resulting in a narrower frame. 
       &lt;Application Example of Display Module&gt; 
       [0315]    Next, an application example of a display module using the display panel illustrated in  FIG. 36A  or  FIG. 36B  will be described with reference to  FIG. 37 . 
         [0316]    In a display module  8000  in  FIG. 37 , a touch panel  8004  connected to an FPC  8003 , a display panel  8006  connected to an FPC  8005 , a frame  8009 , a printed circuit board  8010 , and a battery  8011  are provided between an upper cover  8001  and a lower cover  8002 . Note that the battery  8011 , the touch panel  8004 , and the like are not provided in some cases. 
         [0317]    The display panel illustrated in  FIG. 36A  or  FIG. 36B  can be used as the display panel  8006  in  FIG. 37 . 
         [0318]    The shape and/or size of the upper cover  8001  and the lower cover  8002  can be changed as appropriate in accordance with the size of the touch panel  8004  and the display panel  8006 . 
         [0319]    The touch panel  8004  can be a resistive touch panel or a capacitive touch panel and can overlap the display panel  8006 . A counter substrate (sealing substrate) of the display panel  8006  can have a touch panel function. Alternatively, a photosensor may be provided in each pixel of the display panel  8006  so that an optical touch panel is obtained. Further alternatively, an electrode for a touch sensor may be provided in each pixel of the display panel  8006  so that a capacitive touch panel is obtained. In such cases, the touch panel  8004  can be omitted. 
         [0320]    The frame  8009  protects the display panel  8006  and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed circuit board  8010 . The frame  8009  may also function as a radiator plate. 
         [0321]    The printed circuit board  8010  is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a separate power source using the battery  8011  may be used. The battery  8011  can be omitted in the case of using a commercial power source. 
         [0322]    The display module  8000  may be additionally provided with a polarizing plate, a retardation plate, a prism sheet, or the like. 
       &lt;Application Examples of Display Module to Electronic Device&gt; 
       [0323]    Next, an electronic device using the above display module for a display panel will be described. Examples of the electronic device include a computer, a portable information appliance (including a mobile phone, a portable game machine, and an audio reproducing device), electronic paper, a television device (also referred to as television or television receiver), and a digital video camera. 
         [0324]      FIG. 38A  illustrates a portable information appliance that includes a housing  901 , a housing  902 , a first display portion  903   a , a second display portion  903   b , and the like. At least one of the housings  901  and  902  is provided with the display module including the semiconductor device of the foregoing embodiment. It is thus possible to obtain a portable information appliance with a smaller circuit area. 
         [0325]    The first display portion  903   a  is a panel having a touch input function, and for example, as illustrated in the left of  FIG. 38A , which of “touch input” and “keyboard input” is performed can be selected by a selection button  904  displayed on the first display portion  903   a . Since selection buttons with a variety of sizes can be displayed, the information appliance can be easily used by people of any generation. For example, when “keyboard input” is selected, a keyboard  905  is displayed on the first display portion  903   a  as illustrated in the right of  FIG. 38A . Thus, letters can be input quickly by keyboard input as in a conventional information appliance, for example. 
         [0326]    One of the first display portion  903   a  and the second display portion  903   b  can be detached from the portable information appliance as shown in the right of  FIG. 38A . Providing the second display portion  903   b  with a touch input function makes the information appliance convenient because a weight to carry around can be further reduced and the information appliance can operate with one hand while the other hand supports the housing  902 . 
         [0327]    The portable information appliance in  FIG. 38A  can be equipped with a function of displaying a variety of information (e.g., a still image, a moving image, and a text image); a function of displaying a calendar, a date, the time, or the like on the display portion; a function of operating or editing information displayed on the display portion; a function of controlling processing by various kinds of software (programs); and the like. An external connection terminal (e.g., an earphone terminal or a USB terminal), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. 
         [0328]    The portable information appliance illustrated in  FIG. 38A  may transmit and receive data wirelessly. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server. 
         [0329]    Furthermore, the housing  902  in  FIG. 38A  may be equipped with an antenna, a microphone function, and/or a wireless communication function so that the information appliance can be used as a mobile phone. 
         [0330]      FIG. 38B  illustrates an e-book reader  910  including electronic paper. The e-book reader  910  includes two housings  911  and  912 . The housing  911  and the housing  912  are provided with a display portion  913  and a display portion  914 , respectively. The housings  911  and  912  are connected by a hinge  915  and can be opened and closed with the hinge  915  as an axis. The housing  911  is provided with a power switch  916 , an operation key  917 , a speaker  918 , and the like. The display module including the semiconductor device of the foregoing embodiment is provided in at least one of the housings  911  and  912 . It is thus possible to obtain an e-book reader with a smaller circuit area. 
         [0331]      FIG. 38C  illustrates a television device including a housing  921 , a display portion  922 , a stand  923 , and the like. The television device can be controlled by a switch of the housing  921  and/or a remote controller  924 . The display module including the semiconductor device of the foregoing embodiment is mounted on the housing  921  and the remote controller  924 . Consequently, it is possible to obtain a television device with a smaller circuit area. 
         [0332]      FIG. 38D  illustrates a smartphone in which a main body  930  is provided with a display portion  931 , a speaker  932 , a microphone  933 , an operation button  934 , and the like. The display module including the semiconductor device of the foregoing embodiment is provided in the main body  930 . It is thus possible to obtain a smartphone with a smaller circuit area. 
         [0333]      FIG. 38E  illustrates a digital camera including a main body  941 , a display portion  942 , an operation switch  943 , and the like. The display module including the semiconductor device of the foregoing embodiment is provided in the main body  941 . Thus, it is possible to obtain a digital camera with a smaller circuit area. 
         [0334]    As described above, the display module including the semiconductor device of the foregoing embodiment is provided in the electronic device shown in this embodiment. It is thus possible to obtain an electronic device with a smaller circuit area. 
         [0000]    (Supplementary Notes on Description in this Specification and the Like) 
         [0335]    The following are notes on the description of Embodiments 1 to 4 and the structures in Embodiments 1 to 4. 
       &lt;Notes on One Embodiment of the Present Invention Described in Embodiments&gt; 
       [0336]    One embodiment of the present invention can be constituted by appropriately combining the structure described in an embodiment with any of the structures described in the other embodiments. In the case where a plurality of structure examples are described in one embodiment, any of the structure examples can be combined as appropriate. 
         [0337]    Note that a content (or part thereof) described in one embodiment can be applied to, combined with, or replaced with another content (or part thereof) described in the same embodiment and/or a content (or part thereof) described in another embodiment or other embodiments. 
         [0338]    Note that in each embodiment, a content described in the embodiment is a content described with reference to a variety of diagrams or a content described with a text in the specification. 
         [0339]    By combining a diagram (or part thereof) described in one embodiment with another part of the diagram, a different diagram (or part thereof) described in the embodiment, and/or a diagram (or part thereof) described in another embodiment or other embodiments, much more diagrams can be created. 
       &lt;Notes on Description for Drawings&gt; 
       [0340]    In this specification and the like, terms for explaining arrangement, such as “over” and “under,” are used for convenience to indicate a positional relation between components with reference to drawings. The positional relation between components is changed as appropriate in accordance with a direction in which the components are described. Therefore, the terms for explaining arrangement are not limited to those used in the specification and can be changed to other terms as appropriate depending on the situation. 
         [0341]    The term “over” or “below” does not necessarily mean that a component is placed directly on or directly below and directly in contact with another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is on and in direct contact with the insulating layer A and can also mean the case where another component is provided between the insulating layer A and the electrode B. 
         [0342]    In a block diagram in this specification and the like, components are functionally classified and shown by blocks that are independent of each other. However, in an actual circuit and the like, such components are sometimes hard to classify functionally, and there is a case where one circuit is associated with a plurality of functions or a case where a plurality of circuits are associated with one function. Therefore, the segmentation of blocks in a block diagram is not limited by any of the components described in the specification and can be differently determined as appropriate depending on the situation. 
         [0343]    In the drawings, the size, the layer thickness, or the region is determined arbitrarily for description convenience; therefore, embodiments of the present invention are not limited to the illustrated scale. Note that the drawings are schematically shown for clarity, and embodiments of the present invention are not limited to shapes or values shown in the drawings. For example, the following can be included: variation in signal, voltage, or current due to noise or difference in timing. 
         [0000]    &lt;Notes on Expressions that can be Rephrased&gt; 
         [0344]    In this specification and the like, the terms “one of a source and a drain” (or first electrode or first terminal) and “the other of the source and the drain” (or second electrode or second terminal) are used to describe the connection relation of a transistor. This is because the source and the drain of a transistor are interchangeable depending on the structure, operation conditions, or the like of the transistor. Note that the source or the drain of the transistor can also be referred to as a source (or drain) terminal, a source (or drain) electrode, or the like as appropriate depending on the situation. 
         [0345]    In this specification and the like, the term “electrode” or “wiring” does not limit a function of a component. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa. Moreover, the term “electrode” or “wiring” can also mean a combination of a plurality of electrodes or wirings formed in an integrated manner. 
         [0346]    In this specification and the like, “voltage” and “potential” can be replaced with each other. The term “voltage” refers to a potential difference from a reference potential. When the reference potential is a ground voltage, for example, “voltage” can be replaced with “potential.” A ground potential does not necessarily mean 0 V. Potentials are relative values, and a potential supplied to a wiring or the like is sometimes changed depending on the reference potential. 
         [0347]    In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, in some cases, the term “conductive film” can be used instead of “conductive layer,” and the term “insulating film” can be used instead of “insulating layer.” 
         [0348]    This specification and the like show a 1T-1C circuit configuration where one pixel has one transistor and one capacitor and a 2T-1C circuit configuration where one pixel has two transistors and one capacitor; however, one embodiment of the present invention is not limited to these. It is possible to employ a circuit configuration where one pixel has three or more transistors and two or more capacitors. Moreover, a variety of circuit configurations can be obtained by formation of an additional wiring. 
       &lt;Notes on Term Definitions&gt; 
       [0349]    The following are definitions of the terms mentioned in the above embodiments. 
       &lt;&lt;Switch&gt;&gt; 
       [0350]    In this specification and the like, a switch is conducting or not conducting (is turned on or off) to determine whether current flows therethrough or not. Alternatively, a switch has a function of selecting and changing a current path. 
         [0351]    For example, an electrical switch or a mechanical switch can be used. That is, a switch is not limited to a certain element and can be any element capable of controlling current. 
         [0352]    Examples of an electrical switch include a transistor (e.g., a bipolar transistor and a MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky diode, a metal-insulator-metal (MIM) diode, a metal-insulator-semiconductor (MIS) diode, and a diode-connected transistor), and a logic circuit in which such elements are combined. 
         [0353]    In the case of using a transistor as a switch, the “on state” of the transistor refers to a state in which a source and a drain of the transistor are regarded as being electrically short-circuited. The “off state” of the transistor refers to a state in which the source and the drain of the transistor are regarded as being electrically disconnected. In the case where a transistor operates just as a switch, there is no particular limitation on the polarity (conductivity type) of the transistor. 
         [0354]    An example of a mechanical switch is a switch formed using a microelectromechanical system (MEMS) technology, such as a digital micromirror device (DMD). Such a switch includes an electrode that can be moved mechanically, and its conduction and non-conduction is controlled with movement of the electrode. 
       &lt;&lt;Pixel&gt;&gt; 
       [0355]    In this specification and the like, one pixel refers to one element whose brightness can be controlled, for example. Therefore, for example, one pixel corresponds to one color element by which brightness is expressed. Accordingly, in a color display device using color elements of red (R), green (G), and blue (B), the smallest unit of an image is formed of three pixels of an R pixel, a G pixel, and a B pixel. 
         [0356]    Note that the number of colors for color elements is not limited to three, and more colors may be used. For example, RGBW (W: white) can be employed, or yellow, cyan, or magenta can be added to RGB. 
       &lt;&lt;Display Element&gt;&gt; 
       [0357]    In this specification and the like, a display element includes a display medium whose contrast, luminance, reflectivity, transmittance, or the like is changed by electrical or magnetic effect. Examples of a display element include an electroluminescent (EL) element, an LED chip (e.g., a white LED chip, a red LED chip, a green LED chip, and a blue LED chip), a transistor (a transistor that emits light depending on current), an electron emitter, a display element using a carbon nanotube, a liquid crystal element, electronic ink, an electrowetting element, an electrophoretic element, a plasma display panel (PDP), a display element using microelectromechanical systems (MEMS) (e.g., a grating light valve (GLV), a digital micromirror device (DMD), a digital micro shutter (DMS), Mirasol (registered trademark), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, and a piezoelectric ceramic display), and a display element using a quantum dot. An example of a display device including EL elements is an EL display. Examples of a display device including electron emitters are a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of a display device including liquid crystal elements include liquid crystal displays (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, and a projection liquid crystal display). An example of a display device including electronic ink, electronic liquid powder (registered trademark), or electrophoretic elements is electronic paper. An example of a display device containing quantum dots in each pixel is a quantum dot display. Note that quantum dots may be provided not as display elements but as part of a backlight. The use of quantum dots enables display with high color purity. In a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as SRAM can be provided under the reflective electrodes; thus, power consumption can be further reduced. In the case of using an LED chip, graphene or graphite may be provided under an electrode or a nitride semiconductor of the LED chip. Graphene or graphite may be a multilayer film in which a plurality of layers are stacked. When graphene or graphite is provided in this manner, a nitride semiconductor, for example, an n-type GaN semiconductor layer including crystals can be easily formed thereover. Furthermore, a p-type GaN semiconductor layer including crystals or the like can be provided thereover, and thus the LED chip can be formed. Note that an AlN layer may be provided between graphene or graphite and the n-type GaN semiconductor layer including crystals. The GaN semiconductor layers included in the LED chip may be formed by MOCVD. Note that when graphene is provided, the GaN semiconductor layers included in the LED chip can also be formed by a sputtering method. In a display element using MEMS, a drying agent may be provided in a space where the display element is sealed (e.g., a space between an element substrate where the display element is placed and a counter substrate opposite to the element substrate). Providing a drying agent can prevent MEMS and the like from becoming difficult to move and/or deteriorating easily because of moisture or the like. 
       &lt;&lt;Connection&gt;&gt; 
       [0358]    In this specification and the like, when it is described that “A and B are connected to each other,” the case where A and B are electrically connected to each other is included in addition to the case where A and B are directly connected to each other. Here, the expression “A and B are electrically connected” means that electric signals can be transmitted and received between A and B when an object having any electric action exists between A and B. 
         [0359]    This application is based on Japanese Patent Application serial No. 2016-014992 filed with Japan Patent Office on Jan. 29, 2016, the entire contents of which are hereby incorporated by reference.