Patent Publication Number: US-10332462-B2

Title: Semiconductor device, display module, and electronic device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a semiconductor device, a display module, and an electronic device. 
     Note that one embodiment of the present invention is not limited to the above technical field. Examples of the technical field of one embodiment of the present invention disclosed in this specification and the like include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a display module, a display system, an examination system, an electronic device, a lighting device, an input device, an input/output device, a driving method thereof, and a manufacturing method thereof. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor, a semiconductor circuit, an arithmetic device, a driver circuit, a memory device, and the like are each one embodiment of the semiconductor device. In addition, an imaging device, an electro-optical device, a power generation device (e.g., a thin film solar cell and an organic thin film solar cell), and an electronic device may each include a semiconductor device. 
     2. Description of the Related Art 
     Flat panel displays typified by liquid crystal display devices and light-emitting display devices are widely used for displaying images. Although transistors used in these display devices are mainly manufactured using silicon semiconductors, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used for transistors in recent years. For example, in Patent Documents 1 and 2, a technique is disclosed in which a transistor manufactured using zinc oxide or an In—Ga—Zn-based oxide for a semiconductor layer is used in a pixel of a display device. 
     In a display device including a light-emitting element, a driver transistor that controls current supplied to the light-emitting element in accordance with a video signal is provided. If the characteristics of the driver transistor vary among pixels, luminance of a light-emitting element varies among the pixels. In Patent Document 3, as a method for preventing such variation in luminance of light-emitting elements, a method for correcting variation in the threshold voltages of driver transistors in pixels (hereinafter also referred to as internal correction) is disclosed. 
     REFERENCES 
     Patent Documents 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2007-096055 
         [Patent Document 2] Japanese Published Patent Application No. 2007-123861 
         [Patent Document 3] Japanese Published Patent Application No. 2008-233933 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a novel semiconductor device, display module, or electronic device. Another object of one embodiment of the present invention is to provide a semiconductor device, a display module, or an electronic device that is capable of examining variations in element characteristics easily. Another object of one embodiment of the present invention is to provide a versatile semiconductor device, display module, or electronic device. Another object of one embodiment of the present invention is to provide a semiconductor device, a display module, or an electronic device that is capable of performing external correction with a high degree of freedom. 
     One embodiment of the present invention does not necessarily achieve all the objects listed above and only needs to achieve at least one of the objects. The description of the above objects does not preclude the existence of other objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     A semiconductor device of one embodiment of the present invention includes a controller, an image processing portion, a driver circuit, and an examination circuit. The controller has a function of controlling operations of the image processing portion and the examination circuit. The image processing portion has a function of generating a video signal using image data. The driver circuit has a function of outputting the video signal to a display portion. The examination circuit has a function of examining the degree of variations in characteristics of an element provided in the display portion. The examination results are output to the outside. 
     In the semiconductor device of one embodiment of the present invention, the examination may be performed on the basis of a signal including information on the characteristics of the element provided in the display portion. The signal may be input from the display portion to the examination circuit. 
     In the semiconductor device of one embodiment of the present invention, the examination circuit may include a converter circuit, an evaluation circuit, and a memory device. The converter circuit may have a function of converting the signal into a digital signal. The evaluation circuit may have a function of calculating a difference between first element characteristics corresponding to the digital signal and second element characteristics used as a reference. The memory device may have a function of storing the first element characteristics, the second element characteristics, and data calculated by the evaluation circuit. 
     In the semiconductor device of one embodiment of the present invention, the controller may have a function of outputting the signal to a transmitting portion. The controller may have a function of outputting image data corrected by the transmitting portion on the basis of the signal to the image processing portion. 
     A display module of one embodiment of the present invention includes a control portion including the semiconductor device in any of the above embodiments and a display portion. The display portion includes a light-emitting element and a transistor electrically connected to the light-emitting element. The examination circuit has a function of examining the degree of variations in the threshold voltage of the transistor, the field-effect mobility of the transistor, or the threshold voltage of the light-emitting element. 
     In the display module of one embodiment of the present invention, the display portion may include a first pixel group including a plurality of first pixels and a second pixel group including a plurality of second pixels. The first pixel may include a reflective liquid crystal element and the second pixel may include the light-emitting element. 
     An electronic device of one embodiment of the present invention includes the display module and a processor. The processor has a function of correcting image data on the basis of the variations in the characteristics of the element provided in the display portion. 
     According to one embodiment of the present invention, a novel semiconductor device, display module, or electronic device can be provided. According to one embodiment of the present invention, a semiconductor device, a display module, or an electronic device that is capable of examining variations in element characteristics easily can be provided. According to one embodiment of the present invention, a versatile semiconductor device, display module, or electronic device can be provided. According to one embodiment of the present invention, a semiconductor device, a display module, or an electronic device that is capable of performing external correction with a high degree of freedom can be provided. 
     Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily have all of the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  illustrate a configuration example of a system; 
         FIGS. 2A to 2C  illustrate a configuration example of an examination circuit; 
         FIGS. 3A to 3C  each illustrate a configuration example of a read circuit; 
         FIG. 4  illustrates an operation example of a system: 
         FIG. 5  illustrates an operation example of a system: 
         FIG. 6  illustrates a configuration example of a display portion; 
         FIGS. 7A and 7B  illustrate a configuration example and an operation example of a pixel; 
         FIG. 8  illustrates a structure example of a display module: 
         FIGS. 9A and 9B  each illustrate a structure example of an electronic device; 
         FIGS. 10A and 10B  each illustrate a configuration example of a pixel; 
         FIGS. 11A and 11B  each illustrate a configuration example of a pixel: 
         FIG. 12  illustrates a configuration example of a display portion; 
         FIG. 13  illustrates a configuration example of a display portion; 
         FIG. 14  illustrates a configuration example of a pixel unit: 
         FIGS. 15A to 15D  each illustrate a configuration example of a pixel unit; 
         FIG. 16  illustrates a configuration example of a pixel unit; 
         FIGS. 17A and 17B  each illustrate a configuration example of a pixel unit; 
         FIG. 18  illustrates a structure example of a display device: 
         FIG. 19  illustrates a structure example of a display device; 
         FIG. 20  illustrates a structure example of a display device; 
         FIG. 21  illustrates a structure example of a display device; 
         FIG. 22  illustrates a configuration example of a control portion; 
         FIGS. 23A to 23D  illustrate a structure example of a transistor; 
         FIGS. 24A to 24C  illustrate a structure example of a transistor, and 
         FIGS. 25A to 25D  each illustrate a structure example of an electronic device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the scope and spirit of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of the embodiments below. 
     One embodiment of the present invention includes, in its category, devices such as a semiconductor device, a memory device, a display device, an imaging device, and a radio frequency (RF) tag. Furthermore, the display device includes, in its category, a liquid crystal display device, a light-emitting device having pixels each provided with a light-emitting element typified by an organic light-emitting element, electronic paper, a digital micromirror device (DMD), a plasma display panel (PDP), a field emission display (FED), and the like. 
     In this specification and the like, a metal oxide means an oxide of metal in a broad sense. Metal oxides are classified into an oxide insulator, an oxide conductor (including a transparent oxide conductor), an oxide semiconductor (also simply referred to as an OS), and the like. For example, a metal oxide used in a channel formation region of a transistor is called an oxide semiconductor in some cases. That is, a metal oxide that has at least one of an amplifying function, a rectifying function, and a switching function can be called a metal oxide semiconductor, or OS for short. In the following description, a transistor including a metal oxide in a channel formation region is also called an OS transistor. 
     In this specification and the like, a metal oxide including nitrogen is also called a metal oxide in some cases. Moreover, a metal oxide including nitrogen may be called a metal oxynitride. The details of a metal oxide are described later. 
     Furthermore, in this specification and the like, an explicit description “X and Y are connected” means that X and Y are electrically connected, X and Y are functionally connected, and X and Y are directly connected. Accordingly, without being limited to a predetermined connection relation, for example, a connection relation shown in drawings or text, another connection relation is included in the drawings or the text. Here, X and Y each denote an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). 
     Examples of the case where X and Y are directly connected include the case where an element that allows an electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, and a load) is not connected between X and Y, and the case where X and Y are connected without the element that allows the electrical connection between X and Y provided therebetween. 
     For example, in the case where X and Y are electrically connected, one or more elements that enable an electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, or a load) can be connected between X and Y. Note that the switch is controlled to be turned on or off. That is, a switch is conducting or not conducting (is turned on or off) to determine whether current flows therethrough or not. Alternatively, the switch has a function of selecting and changing a current path. Note that the case where X and Y are electrically connected includes the case where X and Y are directly connected. 
     For example, in the case where X and Y are functionally connected, one or more circuits that enable functional connection between X and Y (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit; a signal converter circuit such as a DA converter circuit, an AD converter circuit, or a gamma correction circuit; a potential level converter circuit such as a power source circuit (e.g., a step-up circuit or a step-down circuit) or a level shifter circuit for changing the potential level of a signal, a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a memory circuit; and/or a control circuit) can be connected between X and Y. For example, even when another circuit is interposed between X and Y, X and Y are functionally connected if a signal output from X is transmitted to Y. Note that the case where X and Y are functionally connected includes the case where X and Y are directly connected and the case where X and Y are electrically connected. 
     Note that in this specification and the like, an explicit description “X and Y are electrically connected” means that X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween). That is, in this specification and the like, the explicit description “X and Y are electrically connected” is the same as the description “X and Y are connected”. 
     Even when independent components are electrically connected to each other in the drawing, one component has functions of a plurality of components in some cases. For example, when part of a wiring also functions as an electrode, one conductive film functions as the wiring and the electrode. Thus, “electrical connection” in this specification includes in its category such a case where one conductive film has functions of a plurality of components. 
     Embodiment 1 
     In this embodiment, a semiconductor device and a system, each of which is one embodiment of the present invention, are described. 
     &lt;Configuration Example of System&gt; 
       FIG. 1A  illustrates a configuration example of a system  10 . The system  10  includes a transmitting portion  11 , a control portion  12 , and a display portion  13 . The system  10  has a function of generating a signal for displaying an image on the basis of data transmitted from the transmitting portion  11  (hereinafter this signal is also referred to as a video signal) and displaying an image on the basis of the video signal. The system  10  also has a function of examining characteristics of an element used for displaying an image. That is, the system  10  functions as both a display system and an examination system. 
     The transmitting portion  11  has a function of transmitting data Di (hereinafter also referred to as image data) corresponding to an image displayed on the display portion  13  and a control signal (a signal CSd) for controlling display of an image to the control portion  12 . In addition, the transmitting portion  11  has a function of transmitting a control signal (a signal CSt) for controlling examination of element characteristics to the control portion  12 . 
     The transmitting portion  11  corresponds to a host that instructs the control portion  12  to execute examination of display of an image or element characteristics. The transmitting portion  11  can be formed using a processor or the like. 
     The control portion  12  has a function of generating a video signal on the basis of the data Di input from the transmitting portion  11  and outputting the video signal to the display portion  13 . The control portion  12  also has a function of examining characteristics of an element used for displaying an image and outputting examination results to the transmitting portion  11 . The control portion  12  includes an interface  20 , an interface  21 , a controller  22 , an image processing portion  23 , a driver circuit  24 , and an examination circuit  25 . 
     Note that the control portion  12  can be formed using a semiconductor device. Thus, the control portion  12  can also be referred to as a semiconductor device. The circuits included in the control portion  12  can be integrated into one integrated circuit. 
     The interface  20  and the interface  21  each have a function of transmitting and receiving a signal to and from the transmitting portion  11 . The data Di input from the transmitting portion  11  is output to the image processing portion  23  via the interface  20 , and the signal CSd input from the transmitting portion  11  is output to the controller  22  via the interface  20 . The signal CSt input from the transmitting portion  11  is output to the controller  22  via the interface  21 . Signals are transmitted from the control portion  12  to the transmitting portion  11  via the interface  20  or the interface  21 . 
     The controller  22  has a function of controlling operations of the circuits included in the control portion  12  on the basis of the signals input from the transmitting portion  11 . Specifically, the controller  22  has a function of generating a signal Cip for controlling the operation of the image processing portion  23  on the basis of the signal CSd input from the transmitting portion  11  via the interface  20  and outputting the signal Cip to the image processing portion  23 . In addition, the controller  22  has a function of generating a signal Ctc for controlling the operation of the examination circuit  25  on the basis of the signal CSt input from the transmitting portion  11  via the interface  21  and outputting the signal Ctc to the examination circuit  25 . 
     The image processing portion  23  has a function of generating a video signal using the image data. Specifically, the image processing portion  23  has a function of generating a signal Sv by performing various kinds of processing on the data Di input from the transmitting portion  11  and transmitting the signal Sv to the driver circuit  24 . Examples of the processing performed in the image processing portion  23  include gamma correction, dimming, and toning. 
     The driver circuit  24  has a function of performing signal processing on the video signal as appropriate and outputting the processed signal to the display portion  13 . Specifically, the driver circuit  24  has a function of performing processing such as a level shift and digital-to-analog (DA) conversion on the signal Sv input from the image processing portion  23  and transmitting the processed signal Sv to the display portion  13 . Note that the driver circuit  24  may be provided in the display portion  13 . 
     The display portion  13  includes a pixel portion  30  including a plurality of pixels  31 . When the signal Sv is input to the pixel portion  30 , an image corresponding to the signal Sv is displayed. 
     The pixel  31  includes a light-emitting element and a transistor having a function of controlling luminance of the light-emitting element.  FIG. 1B  illustrates a configuration example of the pixel  31  including a light-emitting element E 1  and a transistor Tr 1  connected to the light-emitting element E 1 . The transistor Tr 1  has a function of controlling the amount of current flowing through the light-emitting element E 1 . The luminance of the light-emitting element E 1  can be controlled by controlling the amount of current flowing through the light-emitting element E 1 ; thus, the pixel  31  can display a predetermined gray level. 
     As the light-emitting element E 1 , for example, a self-luminous light-emitting element such as an organic light-emitting diode (OLED), a light-emitting diode (LED), a quantum-dot light-emitting diode (QLED), and a semiconductor laser can be used. 
     Note that an image displayed on the display portion  13  is affected by variations in the characteristics of the elements (e.g., the transistor Tr 1  and the light-emitting element E 1 ) included in the pixel  31 . Thus, in order to control the quality of the image displayed on the display portion  13 , the degree of the variations in the characteristics of the elements included in the pixel  31  needs to be understood. 
     Note that the control portion  12  of one embodiment of the present invention has a function of examining the degree of the variations in the characteristics of the elements included in the pixel  31  on the basis of a signal Sch including information on the characteristics of the elements (e.g., the transistor Tr 1  and the light-emitting element E 1 ) included in the pixel  31 . Then, a signal Str corresponding to the results of the examination with the control portion  12  is output from the control portion  12  to the transmitting portion  11 . Accordingly, the transmitting portion  11  can understand the degree of the variations in the characteristics of the elements included in the pixel  31 . 
     In addition, the control portion  12  of one embodiment of the present invention has a function of outputting the signal Sch to the transmitting portion  11 . The transmitting portion  11  has a function of correcting the data Di transmitted to the control portion  12  on the basis of the variations in the element characteristics indicated by the signal Sch. Accordingly, even when the characteristics of the elements included in the pixel  31  vary, an image can be correctly displayed on the display portion  13 . 
     The element characteristics are examined when the signal Sch is input from the pixel  31  to the examination circuit  25  included in the control portion  12 . Note that current flowing through the pixel  31 , voltage output from the pixel  31 , or the like can be used as the signal Sch when the predetermined signal Sv is supplied to the pixel  31 . 
     The examination circuit  25  has a function of examining the degree of the variations in the element characteristics on the basis of the signal Sch. Specifically, the examination circuit  25  has a function of calculating the degree of the variations in the element characteristics on the basis of the signal Sch output from the pixel  31  when the predetermined signal Sv is supplied to the pixel  31  and outputting the calculation results as the signal Str to the controller  22 . The signal Str input to the controller  22  is output to the transmitting portion  11  via the interface  21 . Accordingly, the transmitting portion  11  can understand the degree of the variations in the element characteristics and determine whether the data Di needs to be corrected or not. 
     The signal Str can include information such as ranks of elements classified on the basis of deviation from ideal characteristics of the transistor Tr 1  and the number of elements belonging to the ranks. From the above-described information, the degree of the variations in the element characteristics can be understood. 
     The examination circuit  25  has a function of outputting the signal Sch to the controller  22 . The signal Sch input to the controller  22  is output to the transmitting portion  11  via the interface  21 . In the case where the correction of the data Di is determined to be necessary by the examination, the transmitting portion  11  corrects the data Di on the basis of the signal Sch. The corrected data Di is transmitted to the control portion  12  and a video signal is generated using the data Di. Accordingly, a video signal taking the variations in the element characteristics into account can be generated and the quality of the image displayed on the display portion  13  can be improved, so that a highly reliable display system can be achieved. 
     Note that the operation of the examination circuit  25  is controlled by the signal Ctc generated by the controller  22  on the basis of the signal CSt. Thus, when the predetermined control signal is input to the control portion  12 , the transmitting portion  11  can receive the examination results from the control portion  12 . 
     As described above, the control portion  12  of one embodiment of the present invention includes the examination circuit  25  as well as the image processing portion  23 , the driver circuit  24 , and the like used for displaying an image. Thus, when the predetermined control signal is input to the control portion  12 , information on the element characteristics of the display portion  13  can be easily obtained. 
     In the case where internal correction is performed in the pixel  31 , the number of elements included in the pixel  31  is increased; thus, the area of the pixel  31  is also increased. The internal correction is a method in which correction is performed inside the pixel  31 ; thus, it is difficult to control the content of the correction from the outside and the content of the correction may be limited. In contrast, in one embodiment of the present invention, the transmitting portion  11  receives the signal Sch output from the control portion  12 , so that the correction based on the variations in the element characteristics can be freely performed outside the pixel  31 . That is, external correction with a high degree of freedom can be performed. Accordingly, a wide range of content can be corrected while an increase in the area of the pixel  31  is suppressed. 
     &lt;Configuration Example of Examination Circuit&gt; 
       FIG. 2A  illustrates a configuration example of the examination circuit  25 . The examination circuit  25  includes a converter circuit  100 , an evaluation circuit  110 , and a memory device  120 . Operations of the converter circuit  100 , the evaluation circuit  110 , and the memory device  120  are controlled by the signal Ctc input from the controller  22 . 
     The converter circuit  100  has a function of converting the signal Sch into a predetermined signal and outputting the converted signal to the controller  22  or the evaluation circuit  110 . The converter circuit  100  has a function of, for example, performing analog-to-digital (AD) conversion on the signal Sch. 
       FIG. 2B  illustrates a specific configuration example of the converter circuit  100 . The converter circuit  100  includes a read circuit  101  and an AD converter circuit  102 . The read circuit  101  has a function of converting or amplifying the signal Sch, for example. The read circuit  101  can be omitted.  FIGS. 3A to 3C  each illustrate a configuration example of the read circuit. 
     When current is supplied as the signal Sch from the pixel  31 , a read circuit  101   a  illustrated in  FIG. 3A  has a function of outputting an integral value of the current. The read circuit  101   a  includes an operational amplifier OPa, a capacitor C 1 , and a switch SW 1 . 
     A reference potential is input to a non-inverting input terminal of the operational amplifier OPa, and the signal Sch is input to an inverting input terminal of the operational amplifier OPa. The inverting input terminal of the operational amplifier OPa is connected to one terminal of the switch SW 1  and one electrode of the capacitor C 1 , and an output terminal of the operational amplifier OPa is connected to the other terminal of the switch SW 1  and the other electrode of the capacitor C 1 . Thus, an integrator circuit is formed, and the read circuit  101   a  can output a potential corresponding to the integral value of the current input as the signal Sch to the AD converter circuit  102 . 
     When current is supplied as the signal Sch from the pixel  31 , a read circuit  101   b  illustrated in  FIG. 3B  has a function of converting the current into voltage and outputting the voltage. The read circuit  101   b  includes an operational amplifier OPb and a resistor R 1 . 
     A reference potential is input to a non-inverting input terminal of the operational amplifier OPb, and the signal Sch is input to an inverting input terminal of the operational amplifier OPb. An output terminal of the operational amplifier OPb is connected to the inverting input terminal through the resistor R 1 . Thus, the read circuit  101   b  can output a potential corresponding to the value of the current input as the signal Sch to the AD converter circuit  102 . 
     When a potential is supplied as the signal Sch from the pixel  31 , a read circuit  101   c  illustrated in  FIG. 3C  has a function of amplifying and outputting the potential. The read circuit  101   c  includes an operational amplifier OPc. 
     The signal Sch is input to a non-inverting input terminal of the operational amplifier OPc. An output terminal of the operational amplifier OPc is connected to an inverting input terminal. Thus, the read circuit  101   c  can amplify and output the potential input as the signal Sch to the AD converter circuit  102 . 
     The AD converter circuit  102  has a function of converting the signal Sch input as an analog signal into a digital signal and outputting the digital signal to the controller  22  or the evaluation circuit  110 . The signal Sch input as the analog signal may be either current or voltage. 
     The evaluation circuit  110  has a function of calculating the degree of the variations in the element characteristics. Specifically, the evaluation circuit  110  has a function of comparing element characteristics corresponding to the signal Sch input from the converter circuit  100  with reference element characteristics and calculating a difference therebetween.  FIG. 2C  illustrates a specific configuration example of the evaluation circuit  110 . The evaluation circuit  110  illustrated in  FIG. 2C  includes an arithmetic circuit  111  and a register  112 . 
     The arithmetic circuit  111  has a function of performing arithmetic operation for evaluating the element characteristics. Specifically, the arithmetic circuit  111  has a function of reading out the reference element characteristics and the element characteristics corresponding to the signal Sch by accessing the memory device  120  and comparing these element characteristics with each other to calculate the difference therebetween. In addition, the arithmetic circuit  111  has a function of ranking the elements on the basis of the calculated difference between the element characteristics and storing the calculation results in the memory device  120 . Note that as the reference element characteristics used for the arithmetic operation, ideal characteristics for the elements included in the pixel  31  can be used, for example. 
     The register  112  is connected to the arithmetic circuit  111  and has a function of temporarily holding data used for the arithmetic operation in the arithmetic circuit  111 . 
     The memory device  120  has a function of storing data used for the evaluation of the element characteristics. Specifically, the memory device  120  has a function of storing the reference element characteristics, a table showing a relationship between the signal Sch and the element characteristics, the evaluation results of the element characteristics calculated by the arithmetic circuit  111 , and the like. The evaluation results of the element characteristics stored in the memory device  120  are output as the signal Str to the controller  22 . 
     Examples of the element characteristics stored in the memory device  120  include the field-effect mobility and the threshold voltage of the transistor Tr 1  illustrated in  FIG. 1B  and the threshold voltage of the light-emitting element E 1  illustrated in  FIG. 1B . Note that the element characteristics stored in the memory device  120  can be rewritten using the controller  22 . Examples of the evaluation results of the element characteristics stored in the memory device  120  include the ranks of the elements calculated by the arithmetic circuit  111  and the number of elements belonging to the ranks. 
     The signal Sch output from the converter circuit  100  to the controller  22  and the signal Str output from the memory device  120  to the controller  22  are output to the transmitting portion  11  via the interface  21 . Accordingly, the transmitting portion  11  can obtain the examination results of the element characteristics and the information on the element characteristics. 
     &lt;Operation Example of System&gt; 
     Next, an operation example of the system  10  is described. The system  10  functions as an examination system  10   a  examining element characteristics, and also functions as a display system  10   b  displaying an image using image data corrected on the basis of variations in the element characteristics. An operation example of each of the systems is described below. 
     [Examination System] 
       FIG. 4  illustrates an operation example of the examination system  10   a . Here, the case where current Ich is read out as the signal Sch from the pixel  31  and variations in the threshold voltage and field-effect mobility of the transistor Tr 1  illustrated in  FIG. 1B  are examined is described as an example. 
     First, the signal Sv is supplied from the driver circuit  24  to the pixel  31 , and the current Ich flowing through the transistor Tr 1  at this time is input to the converter circuit  100 . Then, the current Ich is converted into a digital signal and input to the evaluation circuit  110 . 
     Next, the evaluation circuit  110  accesses the memory device  120  to read out data and calculates the variations in the element characteristics. Note that the memory device  120  includes a region  121 , a region  122 , and a region  123 . In the region  121 , reference threshold voltage V th  and reference field-effect mobility μ are stored. The reference threshold voltage V th  and the reference field-effect mobility μ are ideal threshold voltage and ideal field-effect mobility for the transistor Tr 1 , respectively. In the region  122 . N threshold voltages V th ′ (V th ′ 1  to V th ′ N ) and N field-effect mobility μ′ (μ′ 1  to μ′ N ) of N transistors Tr 1  that correspond to N currents Ich (Ich 1  to Ich N ) are stored. Note that N is a natural number. 
     First, the evaluation circuit  110  accesses the region  121  to read out the reference field-effect mobility μ and the reference threshold voltage V th  from the memory device  120 . In addition, the evaluation circuit  110  outputs the currents Ich to the memory device  120  and reads out the field-effect mobility μ′ and the threshold voltages V th ′ that correspond to the currents Ich from the region  122 . Then, ΔV th  that is an error between V th  and V th ′ and Δμ that is an error between μ and μ′ are calculated and ranks of the elements are determined on the basis of the errors. After that, data Drank corresponding to the results of the ranking is stored in the region  123 . 
     A method for ranking the elements can be freely determined without any particular limitation. For example, as shown in Table 1, the elements can be classified into Rank A to Rank F on the basis of ranges of ΔV th  and Δμ. Here, the elements are classified into six ranks; Rank A is the highest rank and Rank F is the lowest rank. Furthermore, the transistor Tr 1  classified into Rank F cannot be corrected. Note that criterion values of ΔV th  and Δμ for judging impossibility of correction are determined by a dynamic range of the driver circuit  24  generating the signal Sv, for example. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Rank 
                 ΔVth 
                 Δ μ 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 | ΔVth | ≤ 0.10 V 
                 | Δ μ | ≤ 10%  
               
               
                   
                 B 
                 0.10 V &lt; | ΔVth | ≤ 0.25 V 
                 10% &lt; | Δ μ | ≤ 25% 
               
               
                   
                 C 
                 0.25 V &lt; | ΔVth | ≤ 0.50 V 
                 25% &lt; | Δ μ | ≤ 50% 
               
               
                   
                 D 
                 0.50 V &lt; | ΔVth | ≤ 1.00 V 
                  50% &lt; | Δ μ | ≤ 100% 
               
               
                   
                 E 
                 1.00 V &lt; | ΔVth | ≤ 2.00 V 
                 100% &lt; | Δ μ | ≤ 200% 
               
               
                   
                 F 
                 | ΔVth | &gt; 2.00 V  
                 | Δ μ | &gt; 200% 
               
               
                   
                   
               
            
           
         
       
     
     In the region  123 , the data Drank and data corresponding to the number of elements classified into different ranks are stored. These pieces of data are output as the signal Str to the controller  22  and then output to the transmitting portion  11  via the interface  21  (see  FIG. 2A ). Accordingly, the transmitting portion  11  can determine whether the correction is needed or not or whether the correction can be performed or not on the basis of the rank of the transistor Tr 1 . 
     Note that the examination is performed in such a manner that the signal CSt is input from the transmitting portion  11  to the control portion  12  (see  FIG. 2A ) and the signal Ctc is input from the controller  22  to the examination circuit  25 . That is, when a predetermined command is input to the control portion  12 , the characteristics of the elements included in the pixel  31  can be examined and the examination results can be output to the outside of the control portion  12 . Note that the signal CSt transmitted from the transmitting portion  11  to the control portion  12  may be encrypted. 
     Through the above operation, the characteristics of the elements included in the pixel  31  can be examined. 
     [Display System] 
       FIG. 5  illustrates an operation example of the display system  10   b . When the correction of the image data is determined to be necessary by the examination, the display system  10   b  has a function of correcting the image data and displaying an image on the basis of the corrected image data. 
     First, the signal Sv is supplied from the driver circuit  24  to the pixel  31 , and the current Ich flowing through the transistor Tr 1  (see  FIG. 1B ) at this time is input to the converter circuit  100 . Then, the current Ich converted into a digital signal is input to the controller  22 . After that, the current Ich is output to the transmitting portion  11  via the interface  21 . 
     The transmitting portion  11  corrects the data Di transmitted to the control portion  12  on the basis of the current Ich. Specifically, the image data is corrected so that the current Ich flowing through the transistor Tr 1  is corrected to be ideal current that should flow when the signal Sv is supplied to the pixel  31 . Then, corrected image data Di′ is input to the image processing portion  23  via the interface  20 . After that, the image processing portion  23  generates a signal Sv′ on the basis of the data Di′ and outputs the signal Sv′ to the driver circuit  24 . 
     Through the above operation, the image data can be corrected on the basis of the examination results of the element characteristics. Note that the transmitting portion  11  can determine the content of the correction independently. Thus, the external correction with a high degree of freedom can be performed. 
     &lt;Configuration Example of Display Portion&gt; 
     Next, a specific configuration example of the display portion  13  is described.  FIG. 6  illustrates a configuration example of the display portion  13 . The display portion  13  includes the pixel portion  30  and a driver circuit  40 . 
     The driver circuit  40  has a function of supplying a signal for selecting the pixels  31  (hereinafter, this signal is also referred to as a selection signal) to the pixel portion  30 . Specifically, the driver circuit  40  has a function of supplying the selection signal to a wiring GL connected to the pixels  31  to which video signals are written and supplying the selection signal to a wiring RL connected to the pixels  31  from which the element characteristics are read out. The wiring GL and the wiring RL each have a function of transmitting the selection signal output from the driver circuit  40 . 
     The driver circuit  24  has a function of supplying the video signal to each wiring SL. The video signal supplied to each wiring SL is written to the pixels  31  selected by the driver circuit  40 . 
     The pixels  31  are connected to a wiring OL. The signal Sch including the information on the characteristics of the elements included in the pixels  31  is output to the wiring OL. The signal Sch output to the wiring OL is input to the examination circuit  25 . 
     Next, a configuration example of the pixel  31  connected to the wiring OL is described.  FIG. 7A  illustrates the configuration example of the pixel  31 . 
     The pixel  31  includes a transistor Tr 2 , a transistor Tr 3 , a transistor Tr 4 , a capacitor C 2 , and a light-emitting element E 2 . A gate of the transistor Tr 2  is connected to the wiring GL. One of a source and a drain of the transistor Tr 2  is connected to a gate of the transistor Tr 3  and one electrode of the capacitor C 2 . The other of the source and the drain of the transistor Tr 2  is connected to the wiring SL. One of a source and a drain of the transistor Tr 3  is connected to one electrode of the light-emitting element E 2 , the other electrode of the capacitor C 2 , and one of a source and a drain of the transistor Tr 4 . The other of the source and the drain of the transistor Tr 3  is connected to a wiring to which a potential Va is supplied. A gate of the transistor Tr 4  is connected to the wiring RL and the other of the source and the drain of the transistor Tr 4  is connected to the wiring OL. The other electrode of the light-emitting element E 2  is connected to a wiring to which a potential Vc (&lt;Va) is supplied. Here, a fixed potential is supplied to the wiring OL. 
       FIG. 7B  illustrates an operation example of the pixel  31 . The potentials of the wiring GL and the wiring RL are controlled to turn on the transistor Tr 2  and the transistor Tr 4 , whereby the potential (the signal Sv) of the wiring SL is supplied to the gate of the transistor Tr 3 . In addition, the potential of the wiring OL is supplied to one of the source and the drain of the transistor Tr 3 . At this time, the potential of the wiring OL is close to the potential Vc and current does not flow through the light-emitting element E 2 . Then, the potentials of the wiring GL and the wiring RL are controlled to turn off the transistor Tr 2  and the transistor Tr 4 . Accordingly, a gate potential of the transistor Tr 3  is increased while a potential between the gate and the source of the transistor Tr 3  is held. 
     Note that an OS transistor is preferably used as the transistor Tr 2 . A metal oxide has a larger energy gap and a lower minority carrier density than a semiconductor such as silicon; thus, the off-state current of an OS transistor is extremely low. Accordingly, when an OS transistor is used as the transistor Tr 2 , a video signal can be held in the pixel  31  for a long time as compared to the case where a transistor containing silicon in its channel formation region (such a transistor is also referred to as a Si transistor) is used. Consequently, the frequency of writing the video signal to the pixel  31  can be greatly reduced, whereby the power consumption can be reduced. The frequency of writing the video signal is, for example, less than once per second, preferably less than 0.1 times per second, further preferably less than 0.01 times per second. 
     In the case where the frequency of writing the video signal is reduced, power supply to the driver circuit  24  is preferably stopped in a period during which the driver circuit  24  does not generate the video signal. Accordingly, the power consumption of the control portion  12  can be reduced. The power supply to the driver circuit  24  is controlled by the controller  22 . 
     The transistor Tr 3  has a function of supplying current corresponding to a potential between the gate and the source, i.e., the video signal to the light-emitting element E 2 . The light-emitting element E 2  emits light with luminance corresponding to the current flowing through the light-emitting element E 2 . Accordingly, the pixel  31  can display a gray level corresponding to the video signal. The transistor Tr 3  and the light-emitting element E 2  correspond to the transistor Tr 1  and the light-emitting element E 1  in  FIG. 1B , respectively. 
     Note that the amount of current supplied to the light-emitting element E 2  is affected by the characteristics of the transistor Tr 3 . Thus, when the pixel  31  displays a gray level, the characteristics of the transistor Tr 3  are preferably examined by outputting a signal including the information on the characteristics of the transistor Tr 3 . Here, the case where the current Ich flowing through the transistor Tr 3  is output as the signal Sch (see  FIGS. 1A and 1B ) to the examination circuit  25  is described as an example. 
     When the current Ich is output, the potential of the wiring RL is controlled to turn on the transistor Tr 4  as illustrated in  FIG. 7B . Accordingly, the current flowing through the transistor Tr 3  is output to the wiring OL and then output as the current Ich to the examination circuit  25 . After that, the examination circuit  25  calculates the variations in the characteristics of the transistor Tr 3  (e.g., the threshold voltage and the field-effect mobility) on the basis of the current Ich. 
     Here, the current flowing through the transistor Tr 3  is used as the signal Sch; however, the other signals may be used. For example, the current flowing through the light-emitting element E 2  can also be used as the signal Sch. In this case, the characteristics of the light-emitting element E 2 , such as the threshold voltage, can be examined. 
     As described above, the element characteristics can be examined by outputting the signal Sch to the wiring OL. 
     Note that the transistor Tr 2  is not necessarily the OS transistor. For example, a transistor whose channel formation region is formed in part of a substrate containing a single-crystal semiconductor other than a metal oxide may be used. Examples of such a substrate include a single-crystal silicon substrate and a single-crystal germanium substrate. In addition, a transistor whose channel formation region is formed in a film containing a material other than a metal oxide can be used as the transistor Tr 2 . Examples of a material other than a metal oxide include silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, and an organic semiconductor. Each of the above materials may be a single-crystal semiconductor or a non-single-crystal semiconductor such as an amorphous semiconductor, a microcrystalline semiconductor, or a polycrystalline semiconductor. 
     A material that can be used for the transistors Tr 3  and Tr 4  is similar to that of the transistor Tr 2 . 
     &lt;Structure Example of Display Module&gt; 
     Next, a structure example of a display module including the control portion  12  and the display portion  13  illustrated in  FIG. 1A  is described.  FIG. 8  illustrates the structure example of the display module. 
     A display module  150  includes a touch panel  154  connected to an FPC  153  and a display device  156  connected to an FPC  155 . 
     The touch panel  154  can be a resistive touch panel or a capacitive touch panel and may be formed to overlap with the display device  156 . Instead of providing the touch panel  154 , the display device  156  can have a touch panel function. In addition, the display device  156  has a function of displaying an image using a light-emitting element. 
     The display module  150  may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet. 
     The control portion  12  and the display portion  13  illustrated in  FIG. 1A  can be provided in the display device  156 . That is, the display module  150  includes a display portion including a light-emitting element and a control portion including an examination circuit. Here, an integrated circuit  160  functioning as the control portion  12  in  FIG. 1A  is provided in the display device  156 . Note that the integrated circuit  160  can be mounted on the display device  156  by a chip on glass (COG) method, a chip on film (COF) method, or the like. 
     A user of the display module  150  can examine the characteristics of the elements included in the display device  156  by inputting the signal CSt to the integrated circuit  160  and can receive the examination results as the signal Str. In addition, the user of the display module  150  can receive the characteristics of the elements included in the display device  156  as the signal Sch by inputting the signal CSt to the integrated circuit  160 , and the data Di corrected on the basis of the signal Sch can be output to the integrated circuit  160 . Thus, after purchasing the display module  150 , the user can easily examine the element characteristics and can determine the content of the correction on the basis of the user&#39;s evaluation standard. 
     As described above, the display module  150  including the control portion  12  can achieve a versatile display module. 
     &lt;Structure Example of Electronic Device&gt; 
     Next, a structure example of an electronic device including the display module in  FIG. 8  is described.  FIGS. 9A and 9B  illustrate a structure example of a tablet information terminal as an example of an electronic device. 
       FIG. 9A  illustrates a structure example of a tablet information terminal. An information terminal  170  includes a housing  171 , a display portion  172 , operation keys  173 , and a speaker  174 . Note that a display device having a position-input function can be used as the display portion  172 . The position-input function can be added by providing a touch panel in a display device or by providing a pixel portion including a photoelectric conversion element in a display device, for example. The operation keys  173  can be used as any one of a power switch for starting the information terminal  170 , a button for operating an application of the information terminal  170 , a volume control button, and a switch for turning on or off the display portion  172 . 
     Although the number of operation keys  173  illustrated in  FIG. 9A  is four, the number and position of operation keys included in the information terminal  170  are not limited to this example. The information terminal  170  may also include a microphone. Thus, the information terminal  170  can have a telephone function like a cellular phone, for example. The information terminal  170  may also include a camera. The information terminal  170  may also include a light-emitting device for use as a flashlight or a lighting device. 
     The information terminal  170  may also include a sensor (which measures force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, a chemical substance, a sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, smell, infrared rays, or the like) inside the housing  171 . In particular, when a measuring device including a sensor such as a gyroscope sensor or an acceleration sensor for measuring inclination is provided, display on the screen of the display portion  172  can be automatically changed in accordance with the orientation of the information terminal  170  by determining the orientation of the information terminal  170  (the orientation of the information terminal with respect to the vertical direction). 
     The information terminal  170  can be provided with the display module  150  illustrated in  FIG. 8 . In this case, the display device  156  provided with the integrated circuit  160  is used as the display portion  172 . In addition, the information terminal  170  includes a processor  161  transmitting and receiving a signal to and from the integrated circuit  160 . In this manner, the information terminal  170  is provided with a system of one embodiment of the present invention. 
       FIG. 9B  illustrates a configuration example of a system  180  provided for the information terminal  170 . The system  180  includes the processor  161 , the integrated circuit  160 , and the display portion  172 . The processor  161 , the integrated circuit  160 , and the display portion  172  correspond to the transmitting portion  11 , the control portion  12 , and the display portion  13  in  FIG. 1A , respectively. 
     The processor  161  transmits the data Di to the integrated circuit  160 , and the integrated circuit  160  generates the signal Sv using the data Di and transmits the signal Sv to the display portion  172 . Then, the display portion  172  inputs the signal Sch including the information on the element characteristics to the integrated circuit  160 , and the element characteristics are examined in the integrated circuit  160 . 
     After that, the integrated circuit  160  outputs the signal Str or the signal Sch to the processor  161 . The processor  161  evaluates the display portion  172  using the signal Str or corrects the data Di using the signal Sch. The corrected data Di is transmitted to the integrated circuit  160 , and the signal Sv generated using the data Di is output from the integrated circuit  160  to the display portion  172 . 
     As described above, an electronic device including the system  180  can correct image data using the processor  161 . 
     A manufacturer of an electronic device can assemble an electronic device which includes the display module  150  in  FIG. 8  purchased by the manufacturer and the processor  161  manufactured by the manufacturer. Note that the processor  161  can execute correction set by the manufacturer of the electronic device. Accordingly, an electronic device having a high added value can be provided. 
     As described above, when a semiconductor device functioning as a control portion is provided with an examination circuit, one embodiment of the present invention can easily perform evaluation of element characteristics and correction of image data. In addition, when a control portion of one embodiment of the present invention is included in a display module, a versatile display module can be provided. In addition, when an electronic device is provided with a system of one embodiment of the present invention, an electronic device having a high added value can be provided. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 2 
     In this embodiment, modification examples of the pixel described in the above embodiment are described. 
     Modification examples of the pixel  31  illustrated in  FIGS. 7A and 7B  are illustrated in  FIGS. 10A and 10B  and  FIGS. 11A and 11B . 
     An element included in the pixel  31  can share a predetermined wiring with another element. The pixel  31  illustrated in  FIG. 10A  is different from that illustrated in  FIGS. 7A and 7B  in that the gate of the transistor Tr 4  is connected to the wiring GL. That is, the gate of the transistor Tr 2  and the gate of the transistor Tr 4  are connected to the same wiring. In this case, the on/off states of the transistor Tr 2  and the transistor Tr 4  are controlled at the same time by the potential of the wiring GL. 
     The polarity of the transistor, the orientation of the light-emitting element, the potential of the wiring, and the like in the pixel  31  can be changed as appropriate. The pixel  31  illustrated in  FIG. 10B  is different from that illustrated in  FIGS. 7A and 7B  in the polarity of the transistors Tr 2 , Tr 3 , and Tr 4 , that is, the transistors Tr 2 , Tr 3 , and Tr 4  are p-channel transistors. In addition, one electrode of the capacitor C 2  is connected to the gate of the transistor Tr 3  and the other electrode is connected to the wiring to which the potential Va is supplied. 
     An element other than the elements illustrated in  FIGS. 7A and 7B  can be provided in the pixel  31  as appropriate. For example, as illustrated in  FIG. 1A , a switch SW 2  can be provided between the transistor Tr 3  and the light-emitting element E 2 . The switch SW 2  is off in a period during which the element characteristics are read out, whereby the amount of current flowing through the transistor Tr 3  can be accurately transmitted to the wiring OL regardless of the potential of the wiring OL. 
     Transistors having different polarities may be provided in the pixel  31 . For example, as illustrated in  FIG. 11B , the transistors Tr 2  and Tr 4  can be n-channel transistors and the transistor Tr 3  can be a p-channel transistor. Note that a connection relationship between the capacitor C 2  and other components in  FIG. 11B  is the same as that in  FIG. 10B . 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, modification examples of the display portion described in the above embodiment are described. In particular, a configuration in which the display portion includes a plurality of pixel groups is described. 
     &lt;Configuration Example of Display Portion&gt; 
       FIG. 12  illustrates a configuration example of the display portion  13 . The display portion  13  illustrated in  FIG. 12  includes a plurality of driver circuits  40 . The pixel portion  30  includes a plurality of pixel groups  32 . A configuration is described below as an example in which the display portion  13  includes two pixel groups  32  (pixel groups  32   a  and  32   b ) and two driver circuits  40  (driver circuits  40   a  and  40   b ). Note that the number of these circuits may be three or more. 
     The pixel group  32   a  includes a plurality of pixels  31   a  and the pixel group  32   b  includes a plurality of pixels  31   b . The pixel group  32   a  is connected to the driver circuit  24   a  and the pixel group  32   b  is connected to the driver circuit  24   b . The pixels  31   a  and  31   b  each include a display element and have a function of displaying a predetermined gray level. The kind and characteristics of the display elements included in the pixels  31   a  may be the same as or different from those of the display elements included in the pixels  31   b . The circuit configuration of the pixels  31   a  may be the same as or different from that of the pixels  31   b . The plurality of pixels  31   a  or the plurality of pixels  31   b  each display a predetermined gray level, whereby the pixel portion  30  displays a predetermined image. 
     Examples of the display element include a liquid crystal element and a light-emitting element. As the liquid crystal element, a transmissive liquid crystal element, a reflective liquid crystal element, a transflective liquid crystal element, or the like can be used. As the display element, a micro electro mechanical systems (MEMS) shutter element, an optical interference type MEMS element, a display element using a microcapsule method, an electrophoretic method, an electrowetting method, an Electronic Liquid Powder (registered trademark) method, or the like can be used. 
     Examples of the light-emitting element include a self-luminous light-emitting element such as an OLED, an LED, a QLED, and a semiconductor laser. 
     An image may be displayed using either one or both of the pixel groups  32   a  and  32   b . In the case where both of the pixel groups  32   a  and  32   b  are used, the pixel groups  32   a  and  32   b  may display one image, or the pixel groups  32   a  and  32   b  may display different images from each other. 
     In the case where either one of the pixel groups  32   a  and  32   b  is used for displaying an image, the pixel group  32  which displays an image can be selected automatically or manually. Note that by providing different display elements in the pixels  31   a  and  31   b , the characteristics, the quality, and the like of images displayed by the pixel group  32   a  and the pixel group  32   b  can be made different from each other. In this case, the pixel group  32  which displays an image can be selected in accordance with the surroundings, the content of a displayed image, and the like. A configuration is described below as an example in which a reflective liquid crystal element is provided in the pixel  31   a  and a light-emitting element is provided in the pixel  31   b.    
     A driver circuit  40   a  has a function of supplying a selection signal to a wiring GLa connected to the pixels  31   a , and the wiring GLa has a function of transmitting the selection signal output from the driver circuit  40   a . A driver circuit  40   b  has a function of supplying a selection signal to a wiring GLb and the wiring RL that are connected to the pixels  31   b , and the wiring GLb and the wiring RL each have a function of transmitting the selection signal output from the driver circuit  40   b.    
     The driver circuit  24   a  has a function of supplying a video signal to a wiring SLa connected to the pixels  31   a , and the driver circuit  24   b  has a function of supplying a video signal to a wiring SLb connected to the pixels  31   b . The video signals supplied to the wirings SLa and SLb are written to the pixels  31   a  and  31   b  selected by the driver circuits  40   a  and  40   b.    
     Note that the pixel  31   b , the driver circuit  40   b , and the driver circuit  24   b  correspond to the pixel  31 , the driver circuit  40 , and the driver circuit  24  in  FIG. 6 , respectively. 
       FIG. 13  illustrates a more specific configuration example of the display portion  13 . The pixel portion  30  includes the pixels  31   a  and the pixels  31   b  arranged in m columns and n rows (m and n are each an integer of 2 or more). The pixel  31   a  in the i-th column and the j-th row (i is an integer greater than or equal to 1 and less than or equal to m, and j is an integer greater than or equal to 1 and less than or equal to n) is connected to a wiring SLa[i] and a wiring GLa[j]. The pixel  31   b  in the i-th column and the j-th row is connected to a wiring SLb[i], a wiring GLb[j], a wiring OL[i], and a wiring RL[j]. Wirings GLa[1] to GLa[n] are connected to the driver circuit  40   a , and wirings GLb[1] to GLb[n] and wirings RL[1] to RL[n] are connected to the driver circuit  40   b . Wirings SLa[1] to SLa[m] are connected to the driver circuit  24   a  and wirings SLb[1] to SLb[m] are connected to the driver circuit  24   b . Here, the pixels  31   a  and  31   b  are alternately provided in the column direction (the direction in which the wirings SLa and SLb extend, i.e., the vertical direction), and a pixel unit  33  includes the pixels  31   a  and  31   b . As described above, the pixels  31   a  and  31   b  can be provided in the same region of the pixel portion  30 . 
     The pixel unit  33  can display a gray level using one or both of the reflective liquid crystal element and the light-emitting element.  FIG. 14  is a schematic view of a configuration of the pixel unit  33  which performs display using a reflective liquid crystal element  60  and a light-emitting element  70 . The liquid crystal element  60  includes a reflective electrode  61 , a liquid crystal layer  62 , and a transparent electrode  63 . 
     A gray level of the liquid crystal element  60  is controlled by controlling transmittance of the liquid crystal layer  62  with respect to light  64  reflected by the reflective electrode  61 . Note that the transmittance is controlled with alignment of liquid crystals. The light  64  reflected by the reflective electrode  61  passes through the liquid crystal layer  62  and the transparent electrode  63  and is extracted to the outside. The reflective electrode  61  includes an opening  65 , and the light-emitting element  70  is provided to overlap with the opening  65 . A gray level of the light-emitting element  70  is controlled by controlling the intensity of light  71  emitted from the light-emitting element  70 . Note that the intensity of the light  71  is controlled by controlling current flowing through the light-emitting element  70 . The light  71  emitted from the light-emitting element  70  passes through the opening  65 , the liquid crystal layer  62 , and the transparent electrode  63  and is extracted to the outside. The light  64  and the light  71  are emitted toward a display surface of the display portion  13 . 
     With such a structure, the pixel portion  30  can display an image using the reflective liquid crystal element  60  and the light-emitting element  70 . 
     The display portion  13  has a first mode in which an image is displayed using a reflective liquid crystal element, a second mode in which an image is displayed using a light-emitting element, and a third mode in which an image is displayed using a reflective liquid crystal element and a light-emitting element. The display portion  13  can be switched between these modes automatically or manually. 
     In the first mode, an image is displayed using the reflective liquid crystal element and external light. Because a light source is unnecessary in the first mode, power consumed in this mode is extremely low. When sufficient external light enters a display device (e.g., in a bright environment), for example, an image can be displayed by using light reflected by the reflective liquid crystal element. The first mode is effective in the case where external light is white light or light near white light and is sufficiently strong, for example. The first mode is suitable for displaying text. Furthermore, the first mode enables eye-friendly display owing to the use of reflected external light, which leads to an effect of easing eyestrain. 
     In the second mode, an image is displayed using light emitted from the light-emitting element. Thus, an extremely vivid image (with high contrast and excellent color reproducibility) can be displayed regardless of the illuminance and the chromaticity of external light. The second mode is effective in the case of extremely low illuminance, such as in a night environment or in a dark room, for example. When a bright image is displayed in a dark environment, a user may feel that the image is too bright. To prevent this, an image with reduced luminance is preferably displayed in the second mode. In that case, glare can be reduced, and power consumption can also be reduced. The second mode is suitable for displaying a vivid (still and moving) image or the like. 
     In the third mode, an image is displayed using both light reflected by the reflective liquid crystal element and light emitted from the light-emitting element. An image displayed in the third mode can be more vivid than an image displayed in the first mode while power consumption can be lower than that in the second mode. The third mode is effective in the case where the illuminance is relatively low or in the case where the chromaticity of external light is not white, for example, in an environment under indoor illumination or in the morning or evening. With the use of the combination of reflected light and emitted light, an image that makes a viewer feel like looking at a painting can be displayed. 
     With such a structure, an all-weather display device or a highly convenient display device with high visibility regardless of the ambient brightness can be fabricated. 
     Each of the pixels  31   a  and the pixels  31   b  can include one or more sub-pixels. For example, each pixel can include one sub-pixel (e.g., a white (W) sub-pixel), three sub-pixels (e.g., red (R), green (G), and blue (B) sub-pixels, or yellow (Y), cyan (C), and magenta (M) sub-pixels), or four sub-pixels (e.g., red (R), green (G), blue (B), and white (W) sub-pixels, or red (R), green (G), blue (B), and yellow (Y) sub-pixels). 
     The display portion  13  can display a full-color image using either the pixels  31   a  or the pixels  31   b . Alternatively, the display portion  13  can display a black-and-white image or a grayscale image using the pixels  31   a  and can display a fill-color image using the pixels  31   b . The pixels  31   a  that can be used for displaying a black-and-white image or a grayscale image are suitable for displaying information that need not be displayed in color such as text information. 
     In the third mode, the color tone can be corrected by using light emission from the light-emitting element at the time of display of an image by the reflective liquid crystal element. For example, in the case where an image is displayed in a reddish environment at evening, a blue (B) component is not sufficient only with the display by the reflective liquid crystal element in some cases; thus, the color tone can be corrected by making the light-emitting element emit light. 
     In addition, in the third mode, a still image that is a background, text, and the like are displayed by the reflective liquid crystal element, whereas a moving image and the like are displayed by the light-emitting element, for example. Accordingly, a high-quality image display and a reduction in the power consumption both can be achieved. Such a structure is suitable for the case where a display device is used as a teaching material such as a textbook, a notebook, or the like. 
     The display portion  13  can be switched between the first mode or the second mode and the third mode depending on the definition of a displayed image. For example, an image or a picture with high resolution can be displayed in the third mode, whereas a background, text, and the like can be displayed in the first mode or the second mode. Accordingly, the definition can be changed with a displayed image; as a result, a versatile display device can be achieved. 
     Although an example in which the reflective liquid crystal element is provided in the pixel  31   a  and the light-emitting element is provided in the pixel  31   b  is described with reference to  FIG. 12  and  FIG. 13 , there is no particular limitation on the display elements provided in the pixels  31   a  and  31   b , and the kind of display element can be freely selected. For example, different kinds of light-emitting elements can be provided in the pixels  31   a  and  31   b . In this case, examination of element characteristics and correction of image data can be performed on the pixel groups  32   a  and  32   b.    
     &lt;Configuration Example of Pixel Unit&gt; 
     Next, configuration examples of the pixel unit  33  including a reflective liquid crystal element and a light-emitting element are described with reference to  FIGS. 15A to 15D ,  FIG. 16 , and  FIGS. 17A and 17B . 
       FIGS. 15A to 15D  illustrate configuration examples of an electrode  611  included in the pixel unit  33 . The electrode  611  serves as a reflective electrode of the liquid crystal element. The opening  601  is provided in the electrode  611  in  FIGS. 15A and 15B . 
     In  FIGS. 15A and 15B , a light-emitting element  660  positioned in a region overlapping with the electrode  611  is indicated by a broken line. The light-emitting element  660  overlaps with the opening  601  included in the electrode  611 . Thus, light from the light-emitting element  660  is emitted to the display surface side through the opening  601 . 
     In  FIG. 15A , the pixel units  33  adjacent in the direction indicated by an arrow R correspond to different emission colors. As illustrated in  FIG. 15A , the openings  601  are preferably provided in different positions in the electrodes  611  so as not to be aligned in the two pixel units  33  adjacent to each other in the direction indicated by the arrow R. This allows the two light-emitting elements  660  to be apart from each other, thereby preventing light emitted from the light-emitting element  660  from entering a coloring layer in the adjacent pixel unit  33  (such a phenomenon is also referred to as crosstalk). Furthermore, since the two adjacent light-emitting elements  660  can be arranged apart from each other, a high-resolution display device can be achieved even when EL layers of the light-emitting elements  660  are separately formed with a shadow mask or the like. 
     In  FIG. 15B , the pixel units  33  adjacent in a direction indicated by an arrow C correspond to different emission colors. Also in  FIG. 15B , the openings  601  are preferably provided in different positions in the electrodes  611  so as not to be aligned in the two pixel units  33  adjacent to each other in the direction indicated by the arrow C. 
     The smaller the ratio of the total area of the opening  601  to the total area except for the opening is, the brighter an image displayed using the liquid crystal element can be. Furthermore, the larger the ratio of the total area of the opening  601  to the total area except for the opening is, the brighter an image displayed using the light-emitting element  660  can be. 
     The opening  601  may have a polygonal shape, a quadrangular shape, an elliptical shape, a circular shape, a cross-like shape, a stripe shape, a slit-like shape, or a checkered pattern, for example. The opening  601  may be provided close to the adjacent pixel unit  33 . Preferably, the opening  601  is provided close to another pixel unit  33  emitting light of the same color, in which case crosstalk can be suppressed. 
     As illustrated in  FIGS. 15C and 15D , a light-emitting region of the light-emitting element  660  may be positioned in a region where the electrode  611  is not provided, in which case light emitted from the light-emitting element  660  is emitted to the display surface side. 
     In  FIG. 15C , the light-emitting elements  660  are not aligned in the two pixel units  33  adjacent in the direction indicated by the arrow R. In  FIG. 15D , the light-emitting elements  660  are aligned in the two pixel units  33  adjacent to each other in the direction indicated by the arrow R. 
     The structure illustrated in  FIG. 15C  can, as mentioned above, prevent crosstalk and increase the resolution because the light-emitting elements  660  included in the two adjacent pixel units  33  can be apart from each other. The structure illustrated in  FIG. 15D  can prevent light emitted from the light-emitting element  660  from being blocked by the electrode  611  because the electrode  611  is not positioned along a side of the light-emitting element  660  which is parallel to the direction indicated by the arrow C. Thus, high viewing angle characteristics can be achieved. 
     Next, a circuit configuration of the pixel unit  33  is described.  FIG. 16  is an example of a circuit diagram of the pixel units  33 .  FIG. 16  illustrates two adjacent pixel units  33 . 
     The pixel unit  33  includes the pixel  31   a  including a switch SW 11 , a capacitor C 11 , and a liquid crystal element  640  and the pixel  31   b  including a switch SW 12 , a switch SW 13 , a transistor M, a capacitor C 12 , and the light-emitting element  660 . The wiring GLa, the wiring GLb, a wiring ANO, a wiring CSCOM, the wiring SLa, the wiring SLb, the wiring RL, and the wiring OL are connected to the pixel unit  33 . FIG.  16  illustrates a wiring VCOM 1  connected to the liquid crystal element  640  and a wiring VCOM 2  connected to the light-emitting element  660 . 
       FIG. 16  illustrates an example in which a transistor is used as each of the switches SW 11 , SW 12 , and SW 13 . Note that the circuit configuration of the pixel  31   b  in  FIG. 16  corresponds to that in  FIG. 7A . The potential Va is supplied to the wiring ANO and the potential Vc is supplied to the wiring VCOM 2 . 
     A gate of the switch SW 11  is connected to the wiring GLa. One of a source and a drain of the switch SW 11  is connected to the wiring SLa, and the other of the source and the drain is connected to one electrode of the capacitor C 11  and one electrode of the liquid crystal element  640 . The other electrode of the capacitor C 11  is connected to the wiring CSCOM. The other electrode of the liquid crystal element  640  is connected to the wiring VCOM 1 . 
     A gate of the switch SW 12  is connected to the wiring GLb. One of a source and a drain of the switch SW 12  is connected to the wiring SLb, and the other of the source and the drain is connected to one electrode of the capacitor C 12  and a gate of the transistor M. The other electrode of the capacitor C 12  is connected to one of a source and a drain of the transistor M and the wiring ANO. The other of the source and the drain of the transistor M is connected to one electrode of the light-emitting element  660 . The other electrode of the light-emitting element  660  is connected to the wiring VCOM 2 . 
     A gate of the switch SW 13  is connected to the wiring RL. One of a source and a drain of the switch SW 13  is connected to the wiring OL, and the other of the source and the drain is connected to the other of the source and the drain of the transistor M. 
       FIG. 16  illustrates an example in which the transistor M includes two gates between which a semiconductor is provided and which are connected to each other. This structure can increase the amount of current flowing through the transistor M. 
     A predetermined potential can be supplied to each of the wirings VCOM 1  and CSCOM. 
     The wiring VCOM 2  and the wiring ANO can be supplied with potentials having a difference large enough to make the light-emitting element  660  emit light. 
     In the pixel unit  33  of  FIG. 16 , for example, an image can be displayed in a reflective mode by driving the pixel unit with the signals supplied to the wiring GLa and the wiring SLa and utilizing the optical modulation of the liquid crystal element  640 . In the case where an image is displayed in a transmissive mode, the pixel unit is driven with the signals supplied to the wiring GLb and the wiring SLb and the light-emitting element  660  emits light. In the case where both modes are performed at the same time, the pixel unit can be driven with the signals supplied to the wirings GLa, GLb, SLa, and SLb. 
     As the switches SW 11  and SW 12 , OS transistors are preferably used. With the use of the OS transistors, video signals can be held in the pixels  31   a  and  31   b  for an extremely long time; thus, gray levels displayed by the pixels  31   a  and  31   b  can be maintained for a long time. Accordingly, the frequency of writing a video signal can be reduced. The frequency of writing a video signal is, for example, less than once per second, preferably less than 0.1 times per second, further preferably less than 0.01 times per second. 
     In the case where the frequency of writing a video signal is reduced, the power supply to the driver circuits  24   a  and  24   b  (see  FIG. 12 ) is preferably stopped in a period during which the driver circuits  24   a  and  24   b  do not generate video signals. Thus, the power consumption can be reduced. 
     Although  FIG. 16  illustrates an example in which one liquid crystal element  640  and one light-emitting element  660  are provided in one pixel unit  33 , one embodiment of the present invention is not limited thereto.  FIG. 17A  illustrates an example in which one liquid crystal element  640  and four light-emitting elements  660  (light-emitting elements  660   r ,  660   g ,  660   b , and  660   w ) are provided in one pixel unit  33 . The pixel  31   b  illustrated in  FIG. 17A  differs from that in  FIG. 16  in being capable of displaying a full-color image with the use of the light-emitting elements by one pixel. 
     In  FIG. 17A , a wiring GLba, a wiring GLbb, a wiring SLba, a wiring SLbb, a wiring RLa, a wiring RLb, a wiring OLa, and a wiring OLb are connected to the pixel unit  33 . 
     In the example in  FIG. 17A , light-emitting elements emitting red light (R), green light (G), blue light (B), and white light (W) can be used as the four light-emitting elements  660 , for example. Furthermore, as the liquid crystal element  640 , a reflective liquid crystal element emitting white light can be used. Thus, in the case of performing display in the reflective mode, white display with high reflectivity can be performed. In the case of performing display in the transmissive mode, an image can be displayed with a higher color rendering property at low power consumption. 
       FIG. 17B  illustrates a configuration example of the pixel unit  33  corresponding to  FIG. 17A . The pixel unit  33  includes the light-emitting element  660   w  overlapping with the opening included in the electrode  611  as well as the light-emitting element  660   r , the light-emitting element  660   g , and the light-emitting element  660   b  which are provided around the electrode  611 . It is preferable that the light-emitting elements  660   r ,  660   g , and  660   b  have almost the same light-emitting area. 
     &lt;Structure Example of Display Device&gt; 
     Next, structure examples of a display device that can be used for the display portion  13  are described. 
     Structure Example 1 
       FIG. 18  is a schematic perspective view of a display device  600 . In the display device  600 , a substrate  651  and a substrate  661  are bonded to each other. In  FIG. 18 , the substrate  661  is denoted by a dashed line. 
     The display device  600  includes a display portion  662 , a circuit  664 , a wiring  665 , and the like.  FIG. 18  illustrates an example in which the display device  600  is provided with an integrated circuit (IC)  673  and an FPC  672 . Thus, the structure illustrated in  FIG. 18  can be regarded as a display module including the display device  600 , the IC  673 , and the FPC  672 . 
     As the circuit  664 , for example, a scan line driver circuit can be used. 
     The wiring  665  has a function of supplying a signal and power to the display portion  662  and the circuit  664 . The signal and power are input to the wiring  665  from the outside through the FPC  672  or from the IC  673 . 
       FIG. 18  illustrates an example in which the IC  673  is provided over the substrate  651  by a COG method, a COF method, or the like. An IC including a scan line driver circuit, a signal line driver circuit, or the like can be used as the IC  673 , for example. Note that the display device  600  and the display module are not necessarily provided with an IC. The IC may be provided over the FPC by a COF method or the like. 
       FIG. 18  also illustrates an enlarged view of part of the display portion  662 . Electrodes  611   b  included in a plurality of display elements are arranged in a matrix in the display portion  662 . The electrode  611   b  has a function of reflecting visible light, and serves as a reflective electrode of a liquid crystal element. 
     As illustrated in  FIG. 18 , the electrode  611   b  includes the opening  601 . In addition, the display portion  662  includes a light-emitting element that is positioned closer to the substrate  651  than the electrode  611   b  is. Light from the light-emitting element is emitted to the substrate  661  side through the opening  601  in the electrode  611   b . The area of the light-emitting region of the light-emitting element may be equal to the area of the opening  601 . One of the area of the light-emitting region of the light-emitting element and the area of the opening  601  is preferably larger than the other because a margin for misalignment can be increased. It is particularly preferable that the area of the opening  601  be larger than the area of the light-emitting region of the light-emitting element. When the area of the opening  601  is small, part of light from the light-emitting element is blocked by the electrode  611   b  and cannot be extracted to the outside, in some cases. The opening  601  with a sufficiently large area can reduce waste of light emitted from the light-emitting element. 
       FIG. 19  illustrates an example of cross sections of part of a region including the FPC  672 , part of a region including the circuit  664 , and part of a region including the display portion  662  of the display device  600  illustrated in  FIG. 18 . 
     The display device  600  illustrated in  FIG. 19  includes a transistor  501 , a transistor  503 , a transistor  505 , a transistor  506 , a liquid crystal element  480 , a light-emitting element  470 , an insulating layer  520 , a coloring layer  431 , a coloring layer  434 , and the like between the substrate  651  and the substrate  661 . The substrate  661  is bonded to the insulating layer  520  with an adhesive layer  441 . The substrate  651  is bonded to the insulating layer  520  with an adhesive layer  442 . 
     The substrate  661  is provided with the coloring layer  431 , a light-blocking layer  432 , an insulating layer  421 , an electrode  413  functioning as a common electrode of the liquid crystal element  480 , an alignment film  433   b , an insulating layer  417 , and the like. A polarizing plate  435  is provided on an outer surface of the substrate  661 . The insulating layer  421  may function as a planarization layer. The insulating layer  421  enables the electrode  413  to have a substantially flat surface, resulting in a uniform alignment state of a liquid crystal layer  412 . The insulating layer  417  serves as a spacer for holding a cell gap of the liquid crystal element  480 . In the case where the insulating layer  417  transmits visible light, the insulating layer  417  may be positioned to overlap with a display region of the liquid crystal element  480 . 
     The liquid crystal element  480  is a reflective liquid crystal element. The liquid crystal element  480  has a stacked-layer structure of an electrode  611   a  functioning as a pixel electrode, the liquid crystal layer  412 , and the electrode  413 . The electrode  611   b  that reflects visible light is provided in contact with a surface of the electrode  611   a  on the substrate  651  side. The electrode  611   b  includes the opening  601 . The electrode  611   a  and the electrode  413  transmit visible light. An alignment film  433   a  is provided between the liquid crystal layer  412  and the electrode  611   a . The alignment film  433   b  is provided between the liquid crystal layer  412  and the electrode  413 . 
     In the liquid crystal element  480 , the electrode  611   b  has a function of reflecting visible light, and the electrode  413  has a function of transmitting visible light. Light entering from the substrate  661  side is polarized by the polarizing plate  435 , transmitted through the electrode  413  and the liquid crystal layer  412 , and reflected by the electrode  611   b . Then, the light is transmitted through the liquid crystal layer  412  and the electrode  413  again to reach the polarizing plate  435 . In this case, alignment of liquid crystals can be controlled with a voltage that is applied between the electrode  611   b  and the electrode  413 , and thus optical modulation of light can be controlled. In other words, the intensity of light emitted through the polarizing plate  435  can be controlled. Light excluding light in a particular wavelength region is absorbed by the coloring layer  431 , and thus, emitted light is red light, for example. 
     As illustrated in  FIG. 19 , the electrode  611   a  that transmits visible light is preferably provided across the opening  601 . Accordingly, liquid crystals are aligned in a region overlapping with the opening  601  as in the other regions, in which case an alignment defect of the liquid crystals is prevented from being generated in a boundary portion of these regions and undesired light leakage can be suppressed. 
     At a connection portion  507 , the electrode  611   b  is connected to a conductive layer  522   a  included in the transistor  506  via a conductive layer  521   b . The transistor  506  has a function of controlling the driving of the liquid crystal element  480 . 
     A connection portion  552  is provided in part of a region where the adhesive layer  441  is provided. In the connection portion  552 , a conductive layer obtained by processing the same conductive film as the electrode  611   a  is connected to part of the electrode  413  with a connector  543 . Accordingly, a signal or a potential input from the FPC  672  connected to the substrate  651  side can be supplied to the electrode  413  formed on the substrate  661  side through the connection portion  552 . 
     As the connector  543 , a conductive particle can be used, for example. As the conductive particle, a particle of an organic resin, silica, or the like coated with a metal material can be used. It is preferable to use nickel or gold as the metal material because contact resistance can be decreased. It is also preferable to use a particle coated with layers of two or more kinds of metal materials, such as a particle coated with nickel and further with gold. As the connector  543 , a material capable of elastic deformation or plastic deformation is preferably used. As illustrated in  FIG. 19 , the connector  543 , which is the conductive particle, has a shape that is vertically crushed in some cases. With the crushed shape, the contact area between the connector  543  and a conductive layer electrically connected to the connector  543  can be increased, thereby reducing contact resistance and suppressing the generation of problems such as disconnection. 
     The connector  543  is preferably provided so as to be covered with the adhesive layer  441 . For example, the connectors  543  are dispersed in the adhesive layer  441  before curing of the adhesive layer  441 . 
     The light-emitting element  470  is a bottom-emission light-emitting element. The light-emitting element  470  has a stacked-layer structure in which an electrode  491  serving as a pixel electrode, an EL layer  492 , and an electrode  493  serving as a common electrode are stacked in this order from the insulating layer  520  side. The electrode  491  is connected to a conductive layer  522   b  included in the transistor  505  through an opening provided in an insulating layer  514 . The transistor  505  has a function of controlling the driving of the light-emitting element  470 . An insulating layer  516  covers an end portion of the electrode  491 . The electrode  493  includes a material that reflects visible light, and the electrode  491  includes a material that transmits visible light. An insulating layer  494  is provided to cover the electrode  493 . Light is emitted from the light-emitting element  470  to the substrate  661  side through the coloring layer  434 , the insulating layer  520 , the opening  601 , the electrode  611   a , and the like. 
     The liquid crystal element  480  and the light-emitting element  470  can exhibit various colors when the color of the coloring layer varies among pixels. The display device  600  can display a color image using the liquid crystal element  480 . The display device  600  can display a color image using the light-emitting element  470 . 
     The transistor  501 , the transistor  503 , the transistor  505 , and the transistor  506  are formed on a plane of the insulating layer  520  on the substrate  651  side. These transistors can be fabricated using the same process. 
     A circuit connected to the liquid crystal element  480  and a circuit connected to the light-emitting element  470  are preferably formed on the same plane. In that case, the thickness of the display device can be smaller than that in the case where the two circuits are formed on different planes. Furthermore, since two transistors can be formed in the same process, a manufacturing process can be simplified as compared to the case where two transistors are formed on different planes. 
     The pixel electrode of the liquid crystal element  480  is positioned on the opposite side of a gate insulating layer included in the transistor from the pixel electrode of the light-emitting element  470 . 
     In the case where an OS transistor is used as the transistor  506  or a memory element connected to the transistor  506  is used, for example, a gray level can be maintained even when writing operation to the pixel is stopped while a still image is displayed using the liquid crystal element  480 . That is, display can be maintained even when the frame rate is set to an extremely small value. In one embodiment of the present invention, the frame rate can be extremely low and driving with low power consumption can be performed. 
     The transistor  503  is used for controlling whether the pixel is selected or not (such a transistor is also referred to as a switching transistor or a selection transistor). The transistor  50  is used for controlling current flowing to the light-emitting element  470  (such a transistor is also referred to as a driving transistor). 
     Insulating layers such as an insulating layer  511 , an insulating layer  512 , an insulating layer  513 , and the insulating layer  514  are provided on the substrate  651  side of the insulating layer  520 . Part of the insulating layer  511  functions as a gate insulating layer of each transistor. The insulating layer  512  is provided to cover the transistor  506  and the like. The insulating layer  513  is provided to cover the transistor  505  and the like. The insulating layer  514  functions as a planarization layer. Note that the number of insulating layers covering the transistor is not limited and may be one or two or more. 
     A material through which impurities such as water and hydrogen do not easily diffuse is preferably used for at least one of the insulating layers that cover the transistors. This is because such an insulating layer can serve as a barrier film. Such a structure can effectively suppress diffusion of the impurities into the transistors from the outside, and a highly reliable display device can be achieved. 
     Each of the transistors  501 ,  503 ,  505 , and  506  includes a conductive layer  521   a  functioning as a gate, the insulating layer  511  functioning as a gate insulating layer, the conductive layer  522   a  and the conductive layer  522   b  functioning as a source and a drain, and a semiconductor layer  531 . Here, a plurality of layers obtained by processing the same conductive film are shown with the same hatching pattern. 
     The transistor  501  and the transistor  505  each include a conductive layer  523  functioning as a gate, in addition to the components of the transistor  503  or the transistor  506 . 
     The structure in which the semiconductor layer including a channel formation region is provided between two gates is used as an example of the transistors  501  and  505 . Such a structure enables the control of the threshold voltages of the transistors. The two gates may be connected to each other and supplied with the same signal to operate the transistors. Such transistors can have higher field-effect mobility and thus have higher on-state current than other transistors. Consequently, a circuit capable of high-speed operation can be obtained. Furthermore, the area occupied by a circuit portion can be reduced. The use of the transistor having high on-state current can reduce signal delay in wirings and can reduce display unevenness even in a display device in which the number of wirings is increased because of increase in size or definition. 
     Alternatively, by supplying a potential for controlling the threshold voltage to one of the two gates and a potential for driving to the other, the threshold voltage of the transistors can be controlled. 
     The structure of the transistors included in the display device is not limited. The transistor included in the circuit  664  and the transistor included in the display portion  662  may have the same structure or different structures. A plurality of transistors included in the circuit  664  may have the same structure or a combination of two or more kinds of structures. Similarly, a plurality of transistors included in the display portion  662  may have the same structure or a combination of two or more kinds of structures. 
     It is preferable to use a conductive material containing an oxide for the conductive layer  523 . A conductive film used for the conductive layer  523  is formed in an oxygen-containing atmosphere, whereby oxygen can be supplied to the insulating layer  512 . The proportion of an oxygen gas in a deposition gas is preferably higher than or equal to 90% and lower than or equal to 100%. Oxygen supplied to the insulating layer  512  is supplied to the semiconductor layer  531  by later heat treatment, so that oxygen vacancies in the semiconductor layer  531  can be reduced. 
     It is particularly preferable to use a low-resistance metal oxide for the conductive layer  523 . In that case, an insulating film that releases hydrogen, such as a silicon nitride film is preferably used for the insulating layer  513 , for example, because hydrogen can be supplied to the conductive layer  523  during the formation of the insulating layer  513  or by heat treatment performed after the formation of the insulating layer  513 , which leads to an effective reduction in the electric resistance of the conductive layer  523 . 
     The coloring layer  434  is provided in contact with the insulating layer  513 . The coloring layer  434  is covered with the insulating layer  514 . 
     A connection portion  504  is provided in a region where the substrates  651  and  661  do not overlap with each other. In the connection portion  504 , the wiring  665  is connected to the FPC  672  via a connection layer  542 . The connection portion  504  has a structure similar to that of the connection portion  507 . On the top surface of the connection portion  504 , a conductive layer obtained by processing the same conductive film as the electrode  611   a  is exposed. Thus, the connection portion  504  and the FPC  672  can be connected to each other via the connection layer  542 . 
     As the polarizing plate  435  provided on the outer surface of the substrate  661 , a linear polarizing plate or a circularly polarizing plate can be used. An example of a circularly polarizing plate is a stack including a linear polarizing plate and a quarter-wave retardation plate. Such a structure can reduce reflection of external light. The cell gap, alignment, drive voltage, and the like of the liquid crystal element used as the liquid crystal element  480  are controlled depending on the kind of the polarizing plate so that desirable contrast is obtained. 
     Note that a variety of optical members can be arranged on the outer surface of the substrate  661 . Examples of the optical members include a polarizing plate, a retardation plate, a light diffusion layer (e.g., a diffusion film), an anti-reflective layer, and a light-condensing film. Furthermore, an antistatic film preventing the attachment of dust, a water repellent film suppressing the attachment of stain, a hard coat film suppressing a scratch in use, or the like may be arranged on the outer surface of the substrate  661 . 
     For each of the substrates  651  and  661 , glass, quartz, ceramic, sapphire, an organic resin, or the like can be used. When the substrates  651  and  661  are formed using a flexible material, the flexibility of the display device can be increased. 
     In the case where the reflective liquid crystal element is used, the polarizing plate  435  is provided on the display surface side. In addition, a light diffusion plate is preferably provided on the display surface side to improve visibility. 
     A front light may be provided on the outer side of the polarizing plate  435 . As the front light, an edge-light front light is preferably used. A front light including a light-emitting diode (LED) is preferably used to reduce power consumption. 
     Structure Example 2 
     A display device  600 A illustrated in  FIG. 20  is different from the display device  600  mainly in that a transistor  581 , a transistor  584 , a transistor  585 , and a transistor  586  are included instead of the transistor  501 , the transistor  503 , the transistor  505 , and the transistor  506 . 
     Note that the positions of the insulating layer  417 , the connection portion  507 , and the like in  FIG. 20  are different from those in  FIG. 19 .  FIG. 20  illustrates an end portion of a pixel. The insulating layer  417  is provided so as to overlap with an end portion of the coloring layer  431  and an end portion of the light-blocking layer  432 . As in this structure, the insulating layer  417  may be provided in a region not overlapping with a display region (or in a region overlapping with the light-blocking layer  432 ). 
     Two transistors included in the display device may partly overlap with each other like the transistor  584  and the transistor  585 . In that case, the area occupied by a pixel circuit can be reduced, leading to an increase in resolution. Furthermore, the light-emitting area of the light-emitting element  470  can be increased, leading to an improvement in aperture ratio. The light-emitting element  470  with a high aperture ratio requires low current density to obtain necessary luminance; thus, the reliability is improved. 
     Each of the transistors  581 ,  584 , and  586  includes the conductive layer  521   a , the insulating layer  511 , the semiconductor layer  531 , the conductive layer  522   a , and the conductive layer  522   b . The conductive layer  521   a  overlaps with the semiconductor layer  531  with the insulating layer  511  positioned therebetween. The conductive layer  522   a  and the conductive layer  522   b  are electrically connected to the semiconductor layer  531 . The transistor  581  includes the conductive layer  523 . 
     The transistor  585  includes the conductive layer  522   b , an insulating layer  517 , a semiconductor layer  561 , the conductive layer  523 , the insulating layer  512 , the insulating layer  513 , a conductive layer  563   a , and a conductive layer  563   b . The conductive layer  522   b  overlaps with the semiconductor layer  561  with the insulating layer  517  positioned therebetween. The conductive layer  523  overlaps with the semiconductor layer  561  with the insulating layers  512  and  513  positioned therebetween. The conductive layer  563   a  and the conductive layer  563   b  are electrically connected to the semiconductor layer  561 . 
     The conductive layer  521   a  functions as a gate. The insulating layer  511  functions as a gate insulating layer. The conductive layer  522   a  functions as one of a source and a drain. The conductive layer  522   b  functions as the other of the source and the drain. 
     The conductive layer  522   b  shared by the transistor  584  and the transistor  585  has a portion functioning as the other of a source and a drain of the transistor  584  and a portion functioning as a gate of the transistor  585 . The insulating layer  517 , the insulating layer  512 , and the insulating layer  513  function as gate insulating layers. One of the conductive layers  563   a  and  563   b  functions as a source, and the other functions as a drain. The conductive layer  523  functions as a gate. 
     Structure Example 3 
       FIG. 21  is a cross-sectional view illustrating a display portion of a display device  600 B. 
     The display device  600 B illustrated in  FIG. 21  includes a transistor  540 , a transistor  580 , the liquid crystal element  480 , the light-emitting element  470 , the insulating layer  520 , the coloring layer  431 , the coloring layer  434 , and the like between the substrate  651  and the substrate  661 . 
     In the liquid crystal element  480 , the electrode  611   b  reflects external light to the substrate  661  side. The light-emitting element  470  emits light to the substrate  661  side. 
     The substrate  661  is provided with the coloring layer  431 , the insulating layer  421 , the electrode  413  functioning as a common electrode of the liquid crystal element  480 , and the alignment film  433   b.    
     The liquid crystal layer  412  is provided between the electrode  611   a  and the electrode  413  with the alignment film  433   a  and the alignment film  433   b  positioned therebetween. 
     The transistor  540  is covered with the insulating layer  512  and the insulating layer  513 . The insulating layer  513  and the coloring layer  434  are bonded to the insulating layer  494  with the adhesive layer  442 . 
     In the display device  600 B, the transistor  540  for driving the liquid crystal element  480  and the transistor  580  for driving the light-emitting element  470  are formed over different planes; thus, each of the transistors can be easily formed using a structure and a material suitable for driving the corresponding display element. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 4 
     In this embodiment, a specific configuration example of the control portion is described. Note that in this example, the display portion  13  includes a plurality of pixel groups  32 . 
       FIG. 22  illustrates a configuration example of the control portion  12 . The control portion  12  includes an interface  821 , a frame memory  822 , a decoder  823 , a sensor controller  824 , a controller  825 , a clock generation circuit  826 , an image processing portion  830 , a memory device  841 , a timing controller  842 , a register  843 , a driver circuit  850 , a touch sensor controller  861 , and an examination circuit  862 . The interface  821 , the controller  825 , and the examination circuit  862  correspond to the interfaces  20  and  21 , the controller  22 , and the examination circuit  25  in  FIG. 1A , respectively. 
     The display portion  13  includes the pixel group  32   a  and the pixel group  32   b .  FIG. 22  illustrates, as an example, a configuration in which the display portion  13  includes the pixel group  32   a  that performs display using a reflective liquid crystal element and the pixel group  32   b  that performs display using a light-emitting element. In addition, the display portion  13  may include a touch sensor unit  812  having a function of obtaining information on whether touch operation is performed or not, touch position, or the like. In the case where the display portion  13  does not include the touch sensor unit  812 , the touch sensor controller  861  can be omitted. 
     The driver circuit  850  includes a source driver  851 . The source driver  851  is a circuit having a function of supplying a video signal to the pixel group  32 . Since the display portion  13  includes the pixel groups  32   a  and  32   b  in  FIG. 22 , the driver circuit  850  includes source drivers  851   a  and  851   b . The source drivers  851   a  and  851   b  correspond to the driver circuits  24   a  and  24   b  in  FIG. 12 , respectively. 
     Information on whether touch operation is performed or not, touch position, or the like obtained by the touch sensor controller  861  is transmitted from the control portion  12  to the transmitting portion  11 . Note that the circuits included in the control portion  12  can be selected as appropriate in accordance with the standard of the transmitting portion  11 , the specifications of the display portion  13 , and the like. 
     The frame memory  822  is a memory circuit having a function of storing image data input to the control portion  12 . In the case where compressed image data is transmitted from the transmitting portion  11  to the control portion  12 , the frame memory  822  can store the compressed image data. The decoder  823  is a circuit for decompressing the compressed image data. When decompression of the image data is not needed, processing is not performed in the decoder  823 . Note that the decoder  823  can be provided between the frame memory  822  and the interface  821 . 
     The image processing portion  830  has a function of performing various kinds of image processing on image data input from the frame memory  822  or the decoder  823  and generating a video signal. For example, the image processing portion  830  includes a gamma correction circuit  831 , a dimming circuit  832 , and a toning circuit  833 . 
     A video signal generated in the image processing portion  830  is output to the driver circuit  850  through the memory device  841 . The memory device  841  has a function of temporarily storing image data. The source drivers  851   a  and  851   b  have a function of performing various kinds of processing on video signals input from the memory device  841  and outputting the signals to the pixel groups  32   a  and  32   b.    
     The timing controller  842  has a function of generating timing signals and the like used in the driver circuit  850 , the touch sensor controller  861 , and the driver circuit included in the pixel group  32 . 
     The touch sensor controller  861  has a function of controlling the operation of the touch sensor unit  812 . A signal including touch information sensed by the touch sensor unit  812  is processed in the touch sensor controller  861  and transmitted to the transmitting portion  11  via the interface  821 . The transmitting portion  11  generates image data reflecting the touch information and transmits the image data to the control portion  12 . The control portion  12  may reflect the touch information in the image data. The touch sensor controller  861  may be provided in the touch sensor unit  812 . 
     The clock generation circuit  826  has a function of generating a clock signal used in the control portion  12 . The controller  825  has a function of processing a variety of control signals transmitted from the transmitting portion  11  through the interface  821  and controlling a variety of circuits in the control portion  12 . The controller  825  also has a function of controlling power supply to the variety of circuits in the control portion  12 . For example, the controller  825  can temporarily interrupt the power supply to a circuit that is not driven. 
     The register  843  has a function of storing data used for the operation of the control portion  12 . Examples of the data stored in the register  843  include a parameter used to perform correction processing in the image processing portion  830  and parameters used to generate waveforms of a variety of timing signals in the timing controller  842 . The register  843  includes a scan chain register including a plurality of registers. 
     The sensor controller  824  connected to a photosensor  880  can be provided in the control portion  12 . The photosensor  880  has a function of sensing external light  881  and generating a sensing signal. The sensor controller  824  has a function of generating a control signal on the basis of the sensing signal. The control signal generated in the sensor controller  824  is output to the controller  825 , for example. 
     The image processing portion  830  has a function of separately generating a video signal of the pixel group  32   a  and a video signal of the pixel group  32   b . In that case, the reflection intensity of the reflective liquid crystal element included in the pixel group  32   a  and the emission intensity of the light-emitting element included in the pixel group  32   b  can be adjusted in response to the brightness of the external light  881  measured using the photosensor  880  and the sensor controller  824 . Here, the adjustment can be referred to as dimming or dimming treatment. In addition, a circuit that performs the dimming treatment is referred to as a dimming circuit. 
     The image processing portion  830  may include another processing circuit such as an RGB-RGBW conversion circuit depending on the specifications of the display portion  13 . The RGB-RGBW conversion circuit has a function of converting image data of red, green, and blue (RGB) into image signals of red, green, blue, and white (RGBW). That is, in the case where the display portion  13  includes pixels of four colors of RGBW, power consumption can be reduced by displaying a white (W) component in the image data using the white (W) pixel. Note that in the case where the display portion  13  includes pixels of four colors of RGBY an RGB-RGBY (red, green, blue, and yellow) conversion circuit can be used, for example. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 5 
     In this embodiment, a structure example of an OS transistor that can be used in the above embodiment is described. 
     Structure Example of Transistor 
     Structure Example 1 
       FIG. 23A  is a top view of a transistor  900 .  FIG. 23C  is a cross-sectional view taken along line X 1 -X 2  in  FIG. 23A .  FIG. 23D  is a cross-sectional view taken along line Y 1 -Y 2  in  FIG. 23A . Note that in  FIG. 23A , some components of the transistor  900  (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. In some cases, the direction of line X 1 -X 2  is referred to as a channel length direction and the direction of line Y 1 -Y 2  is referred to as a channel width direction. As in  FIG. 23A , some components are not illustrated in some cases in top views of transistors described below. 
     The transistor  900  includes a conductive film  904  functioning as a gate electrode over a substrate  902 , an insulating film  906  over the substrate  902  and the conductive film  904 , an insulating film  907  over the insulating film  906 , a metal oxide film  908  over the insulating film  907 , a conductive film  912   a  functioning as a source electrode connected to the metal oxide film  908 , and a conductive film  912   b  functioning as a drain electrode connected to the metal oxide film  908 . Over the transistor  900 , specifically, over the conductive films  912   a  and  912   b  and the metal oxide film  908 , an insulating film  914 , an insulating film  916 , and an insulating film  918  are provided. The insulating films  914 ,  916 , and  918  function as a protective insulating film for the transistor  900 . 
     The metal oxide film  908  includes a first metal oxide film  908   a  on the conductive film  904  side and a second metal oxide film  908   b  over the first metal oxide film  908   a . The insulating films  906  and  907  function as a gate insulating film of the transistor  900 . 
     An In-M oxide (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf) or an In-M-Zn oxide can be used for the metal oxide film  908 . It is particularly preferable to use an In-M-Zn oxide for the metal oxide film  908 . 
     The first metal oxide film  908   a  includes a first region in which the atomic proportion of In is larger than the atomic proportion of M. The second metal oxide film  908   b  includes a second region in which the atomic proportion of In is smaller than that in the first metal oxide film  908   a . The second region includes a portion thinner than the first region. 
     The first metal oxide film  908   a  including the first region in which the atomic proportion of In is larger than that of M can increase the field-effect mobility (also simply referred to as mobility or μFE) of the transistor  900 . Specifically, the field-effect mobility of the transistor  900  can exceed 10 cm 2 /Vs. 
     For example, the use of the transistor with high field-effect mobility for a driver circuit that generates a selection signal (specifically, a demultiplexer connected to an output terminal of a shift register included in the driver circuit) allows a semiconductor device or a display device to have a narrow frame. 
     On the other hand, the first metal oxide film  908   a  including the first region in which the atomic proportion of In is larger than that of M makes it easier to change electrical characteristics of the transistor  900  in light irradiation in some cases. However, in the semiconductor device of one embodiment of the present invention, the second metal oxide film  908   b  is formed over the first metal oxide film  908   a . In addition, the thickness of a channel formation region in the second metal oxide film  908   b  is smaller than the thickness of the first metal oxide film  908   a.    
     Furthermore, the second metal oxide film  908   b  includes the second region in which the atomic proportion of In is smaller than that in the first metal oxide film  908   a  and thus has larger Eg than the first metal oxide film  908   a . For this reason, the metal oxide film  908  that is a layered structure of the first metal oxide film  908   a  and the second metal oxide film  908   b  has high resistance to a negative bias stress test with light irradiation. 
     The amount of light absorbed by the metal oxide film  908  can be reduced during light irradiation. As a result, the change in electrical characteristics of the transistor  900  due to light irradiation can be reduced. In the semiconductor device of one embodiment of the present invention, the insulating film  914  or the insulating film  916  includes excess oxygen. This structure can further reduce the change in electrical characteristics of the transistor  900  due to light irradiation. 
     Here, the metal oxide film  908  is described in detail with reference to  FIG. 23B . 
       FIG. 23B  is an enlarged cross-sectional view of the metal oxide film  908  and the vicinity thereof in the transistor  900  illustrated in  FIG. 23C . 
     In  FIG. 23B , t 1 , t 2 - 1 , and t 2 - 2  denote a thickness of the first metal oxide film  908   a , one thickness of the second metal oxide film  908   b , and the other thickness of the second metal oxide film  908   b , respectively. The second metal oxide film  908   b  over the first metal oxide film  908   a  prevents the first metal oxide film  908   a  from being exposed to an etching gas, an etchant, or the like when the conductive films  912   a  and  912   b  are formed. This is why the first metal oxide film  908   a  is not or is hardly reduced in thickness. In contrast, in the second metal oxide film  908   b , a portion not overlapping with the conductive films  912   a  and  912   b  is etched by formation of the conductive films  912   a  and  912   b , so that a depression is formed in the etched region. In other words, a thickness of the second metal oxide film  908   b  in a region overlapping with the conductive films  912   a  and  912   b  is t 2 - 1 , and a thickness of the second metal oxide film  908   b  in a region not overlapping with the conductive films  912   a  and  912   b  is t 2 - 2 . 
     As for the relationships between the thicknesses of the first metal oxide film  908   a  and the second metal oxide film  908   b , t 2 - 1 &gt;t 1 &gt;t 2 - 2  is preferable. A transistor with the thickness relationships can have high field-effect mobility and less variation in threshold voltage in light irradiation. 
     When oxygen vacancies are formed in the metal oxide film  908  included in the transistor  900 , electrons serving as carriers are generated; as a result, the transistor  900  tends to be normally-on. Therefore, for stable transistor characteristics, it is important to reduce oxygen vacancies in the metal oxide film  908 , particularly oxygen vacancies in the first metal oxide film  908   a . In the structure of the transistor of one embodiment of the present invention, excess oxygen is introduced into an insulating film over the metal oxide film  908 , here, the insulating film  914  and/or the insulating film  916  over the metal oxide film  908 , whereby oxygen is moved from the insulating film  914  and/or the insulating film  916  to the metal oxide film  908  to fill oxygen vacancies in the metal oxide film  908 , particularly in the first metal oxide film  908   a.    
     Note that it is preferable that the insulating films  914  and  916  each include a region (oxygen excess region) including oxygen in excess of that in the stoichiometric composition. In other words, the insulating films  914  and  916  are insulating films capable of releasing oxygen. Note that the oxygen excess region is formed in the insulating films  914  and  916  in such a manner that oxygen is introduced into the insulating films  914  and  916  after the deposition, for example. Oxygen can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. 
     In order to fill oxygen vacancies in the first metal oxide film  908   a , the thickness of the portion including the channel formation region and the vicinity of the channel formation region in the second metal oxide film  908   b  is preferably small, and t 2 - 2 &lt;t 1  is preferably satisfied. For example, the thickness of the portion including the channel formation region and the vicinity of the channel formation region in the second metal oxide film  908   b  is preferably greater than or equal to 1 nm and less than or equal to 20 nm, further preferably greater than or equal to 3 nm and less than or equal to 10 nm. 
     Structure Example 2 
       FIGS. 24A to 24C  illustrate another structure example of the transistor  900 .  FIG. 24A  is a top view of the transistor  900 .  FIG. 24B  is a cross-sectional view taken along line X 1 -X 2  in  FIG. 24A , and  FIG. 24C  is a cross-sectional view taken along line Y I-Y 2  in  FIG. 24A . 
     The transistor  900  includes the conductive film  904  functioning as a first gate electrode over the substrate  902 , the insulating film  906  over the substrate  902  and the conductive film  904 , the insulating film  907  over the insulating film  906 , the metal oxide film  908  over the insulating film  907 , the conductive film  912   a  functioning as the source electrode electrically connected to the metal oxide film  908 , the conductive film  912   b  functioning as the drain electrode electrically connected to the metal oxide film  908 , the insulating films  914  and  916  over the metal oxide film  908  and the conductive films  912   a  and  912   b , a conductive film  920   a  that is over the insulating film  916  and electrically connected to the conductive film  912   b , a conductive film  920   b  over the insulating film  916 , and the insulating film  918  over the insulating film  916  and the conductive films  920   a  and  920   b.    
     The conductive film  920   b  can be used as a second gate electrode of the transistor  900 . In the case where the transistor  900  is used in a display portion of an input/output device, the conductive film  920   a  can be used as an electrode of a display element, or the like. 
     The conductive film  920   a  functioning as a conductive film and the conductive film  920   b  functioning as the second gate electrode each include a metal element that is the same as that included in the metal oxide film  908 . For example, the conductive film  920   b  functioning as the second gate electrode and the metal oxide film  908  include the same metal element; thus, the manufacturing cost can be reduced. 
     For example, in the case where the conductive film  920   a  functioning as a conductive film and the conductive film  920   b  functioning as the second gate electrode each include In-M-Zn oxide, the atomic ratio of metal elements in a sputtering target used for forming the In-M-Zn oxide preferably satisfies In In≥M. The atomic ratio of metal elements in such a sputtering target is, for example, In:M:Zn=2:1:3, In:M:Zn=3:1:2, or In:M:Zn=4:2:4.1. 
     The conductive film  920   a  functioning as a conductive film and the conductive film  920   b  functioning as the second gate electrode can each have a single-layer structure or a stacked-layer structure of two or more layers. Note that in the case where the conductive film  920   a  and the conductive film  920   b  each have a stacked-layer structure, the composition of the sputtering target is not limited to that described above. 
     In a step of forming the conductive films  920   a  and  920   b , the conductive films  920   a  and  920   b  serve as a protective film for suppressing release of oxygen from the insulating films  914  and  916 . The conductive films  920   a  and  920   b  serve as semiconductors before a step of forming the insulating film  918  and serve as conductors after the step of forming the insulating film  918 . 
     Oxygen vacancies are formed in the conductive films  920   a  and  920   b , and hydrogen is added from the insulating film  918  to the oxygen vacancies, whereby a donor level is formed in the vicinity of the conduction band. As a result, the conductivity of each of the conductive films  920   a  and  920   b  is increased, so that the conductive films  920   a  and  920   b  become conductors. The conductive films  920   a  and  920   b  having become conductors can each be referred to as an oxide conductor. Oxide semiconductors generally have a visible light transmitting property because of their large energy gap. An oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band. Therefore, the influence of absorption due to the donor level is small in an oxide conductor, and an oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor. 
     &lt;Metal Oxide&gt; 
     Next, a metal oxide that can be used in the OS transistor is described. In particular, the details of a metal oxide and a cloud-aligned composite (CAC)-OS are described below. 
     A CAC-OS or a CAC metal oxide has a conducting function in part of the material and has an insulating function in another part of the material; as a whole, the CAC-OS or the CAC metal oxide has a function of a semiconductor. In the case where the CAC-OS or the CAC metal oxide is used in a channel formation region of a transistor, the conducting function is to allow electrons (or holes) serving as carriers to flow, and the insulating function is to not allow electrons serving as carriers to flow. By the complementary action of the conducting function and the insulating function, the CAC-OS or the CAC metal oxide can have a switching function (on/off function). In the CAC-OS or CAC metal oxide, separation of the functions can maximize each function. 
     The CAC-OS or the CAC metal oxide includes conductive regions and insulating regions. The conductive regions have the above-described conducting function, and the insulating regions have the above-described insulating function. In some cases, the conductive regions and the insulating regions in the material are separated at the nanoparticle level. In some cases, the conductive regions and the insulating regions are unevenly distributed in the material. The conductive regions are observed to be coupled in a cloud-like manner with their boundaries blurred, in some cases. 
     Furthermore, in the CAC-OS or the CAC metal oxide, the conductive regions and the insulating regions each have a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 0.5 nm and less than or equal to 3 n and are dispersed in the material, in some cases. 
     The CAC-OS or the CAC metal oxide includes components having different bandgaps. For example, the CAC-OS or the CAC metal oxide includes a component having a wide gap due to the insulating region and a component having a narrow gap due to the conductive region. In the case of such a composition, carriers mainly flow in the component having a narrow gap. The component having a narrow gap complements the component having a wide gap, and carriers also flow in the component having a wide gap in conjunction with the component having a narrow gap. Therefore, in the case where the above-described CAC-OS or the CAC metal oxide is used in a channel formation region of a transistor, high current drive capability in the on state of the transistor, that is, a high on-state current and high field-effect mobility, can be obtained. 
     In other words, the CAC-OS or the CAC metal oxide can be called a matrix composite or a metal matrix composite. 
     The CAC-OS has, for example, a composition in which elements included in a metal oxide are unevenly distributed. Materials including unevenly distributed elements each have a size of greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size. Note that in the following description of a metal oxide, a state in which one or more metal elements are unevenly distributed and regions including the metal element(s) are mixed is referred to as a mosaic pattern or a patch-like pattern. The regions each have a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size. 
     Note that a metal oxide preferably contains at least indium. In particular, indium and zinc are preferably contained. In addition, one or more of aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like may be contained. 
     For example, of the CAC-OS, an In—Ga—Zn oxide with the CAC composition (such an In—Ga—Zn oxide may be particularly referred to as CAC-IGZO) has a composition in which materials are separated into indium oxide (InOn, where X 1  is a real number greater than 0) or indium zinc oxide (In X2 Zn Y2 O Z2 , where X 2 , Y 2 , and Z 2  are real numbers greater than 0), and gallium oxide (GaO X3 , where X 3  is a real number greater than 0) or gallium zinc oxide (Ga X4 Zn Y4 O Z4 , where X 4 , Y 4 , and Z 4  are real numbers greater than 0), and a mosaic pattern is formed. Then, InO X1  or In X2 Zn Y2 O Z2  forming the mosaic pattern is evenly distributed in the film. This composition is also referred to as a cloud-like composition. 
     That is, the CAC-OS is a composite metal oxide with a composition in which a region including GaO X3  as a main component and a region including In X2 Zn Y2 O Z2  or InO X1  as a main component are mixed. Note that in this specification, for example, when the atomic ratio of In to an element M in a first region is greater than the atomic ratio of In to an element M in a second region, the first region has higher In concentration than the second region. 
     Note that a compound including In, Ga, Zn, and O is also known as IGZO. Typical examples of IGZO include a crystalline compound represented by InGaO 3 (ZnO) m1  (m1 is a natural number) and a crystalline compound represented by In (1+x0) Ga (1-x0) O 3 (ZnO) m0  (−1≤x0≤1; m0 is a given number). 
     The above crystalline compounds have a single crystal structure, a polycrystalline structure, or a c-axis-aligned crystalline (CAAC) structure. Note that the CAAC structure is a crystal structure in which a plurality of IGZO nanocrystals have c-axis alignment and are connected in the a-b plane direction without alignment. 
     On the other hand, the CAC-OS relates to the material composition of a metal oxide. In a material composition of a CAC-OS including In, Ga, Zn, and O, nanoparticle regions including Ga as a main component are observed in part of the CAC-OS and nanoparticle regions including In as a main component are observed in part thereof. These nanoparticle regions are randomly dispersed to form a mosaic pattern. Therefore, the crystal structure is a secondary element for the CAC-OS. 
     Note that in the CAC-OS, a stacked-layer structure including two or more films with different atomic ratios is not included. For example, a two-layer structure of a film including In as a main component and a film including Ga as a main component is not included. 
     A boundary between the region including GaO X3  as a main component and the region including In X2 Zn Y2 O Z2  or InO X1  as a main component is not clearly observed in some cases. 
     In the case where one or more of aluminum, yttrium, copper, vanadium, berylliuim boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like are contained instead of gallium in a CAC-OS, nanoparticle regions including the selected metal element(s) as a main component(s) are observed in part of the CAC-OS and nanoparticle regions including In as a main component are observed in part thereof, and these nanoparticle regions are randomly dispersed to form a mosaic pattern in the CAC-OS. 
     The CAC-OS can be formed by a sputtering method under conditions where a substrate is not heated intentionally, for example. In the case of forming the CAC-OS by a sputtering method, one or more selected from an inert gas (typically, argon), an oxygen gas, and a nitrogen gas may be used as a deposition gas. The ratio of the flow rate of an oxygen gas to the total flow rate of the deposition gas at the time of deposition is preferably as low as possible, and for example, the flow ratio of an oxygen gas is preferably higher than or equal to 0% and lower than 30%, further preferably higher than or equal to 0% and lower than or equal to 10%. 
     The CAC-OS is characterized in that no clear peak is observed in measurement using θ/2θ scan by an out-of-plane method, which is an X-ray diffraction (XRD) measurement method. That is, X-ray diffraction shows no alignment in the a-b plane direction and the c-axis direction in a measured region. 
     In an electron diffraction pattern of the CAC-OS which is obtained by irradiation with an electron beam with a probe diameter of 1 nm (also referred to as a nanometer-sized electron beam), a ring-like region with high luminance and a plurality of bright spots in the ring-like region are observed. Therefore, the electron diffraction pattern indicates that the crystal structure of the CAC-OS includes a nanocrystal (nc) structure with no alignment in plan-view and cross-sectional directions. 
     For example, an energy dispersive X-ray spectroscopy (EDX) mapping image confirms that an In—Ga—Zn oxide with the CAC composition has a structure in which a region including GaO X3  as a main component and a region including In X2 Zn Y2 O Z2  or InO X1  as a main component are unevenly distributed and mixed. 
     The CAC-OS has a structure different from that of an IGZO compound in which metal elements are evenly distributed, and has characteristics different from those of the IGZO compound. That is, in the CAC-OS, regions including GaO X3  or the like as a main component and regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are separated to form a mosaic pattern. 
     The conductivity of a region including In X2 Zn Y2 O Z2  or InO X1  as a main component is higher than that of a region including GaO X3  or the like as a main component. In other words, when carriers flow through regions including In X2 Zn Y2 O Z2  or InO X1  as a main component, the conductivity of an oxide semiconductor is exhibited. Accordingly, when regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are distributed in an oxide semiconductor like a cloud, high field-effect mobility (μ) can be achieved. 
     In contrast, the insulating property of a region including GaO X3  or the like as a main component is higher than that of a region including In X2 Zn Y2 O Z2  or InOn as a main component. In other words, when regions including GaO X3  or the like as a main component are distributed in an oxide semiconductor, leakage current can be suppressed and favorable switching operation can be achieved. 
     Accordingly, when a CAC-OS is used for a semiconductor element, the insulating property derived from GaO X3  or the like and the conductivity derived from In X2 Zn Y2 O Z2  or InO X1  complement each other, whereby high on-state current (I on ) and high field-effect mobility (μ) can be achieved. 
     A semiconductor element including a CAC-OS has high reliability. Thus, the CAC-OS is suitably used in a variety of semiconductor devices. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 6 
     In this embodiment, other examples of the electronic devices described in the above embodiments are described. 
     The semiconductor device and the system of one embodiment of the present invention can be used in portable electronic devices, wearable electronic devices (wearable devices), e-book readers, and the like.  FIGS. 25A to 25D  illustrate examples of electronic devices including the semiconductor device or the system of one embodiment of the present invention. 
       FIGS. 25A and 25B  illustrate an example of a portable information terminal  2000 . The portable information terminal  2000  includes a housing  2001 , a housing  2002 , a display portion  2003 , a display portion  2004 , a hinge portion  2005 , and the like. 
     The housing  2001  and the housing  2002  are connected with the hinge portion  2005 . The portable information terminal  2000  folded as in  FIG. 25A  can be changed into the state illustrated in  FIG. 25B , in which the housing  2001  and the housing  2002  are opened. 
     For example, the portable information terminal  2000  can also be used as an e-book reader, in which the display portion  2003  and the display portion  2004  can each display text data. In addition, the display portion  2003  and the display portion  2004  can each display a still image or a moving image. Furthermore, the display portion  2003  may be provided with a touch panel. 
     In this manner, the portable information terminal  2000  has high versatility because it can be folded when carried. 
     Note that the housing  2001  and the housing  2002  may include a power switch, an operation button, an external connection port, a speaker, a microphone, and/or the like. 
     Note that the portable information terminal  2000  may have a function of identifying a character, a figure, or an image using a touch sensor provided in the display portion  2003 . In this case, learning in the following mode becomes possible, for example: an answer is written with a finger, a stylus pen, or the like on an information terminal that displays a workbook or the like for studying mathematics or for learning language, and then the portable information terminal  2000  determines whether the answer is correct or not. The portable information terminal  2000  may have a function of performing speech interpretation. In this case, for example, the portable information terminal  2000  can be used in learning a foreign language. Such a portable information terminal is suitable for use as a teaching material such as a textbook, a notebook, or the like. 
     Note that the touch information obtained by the touch sensor provided in the display portion  2003  can be used for prediction of the necessity of power supply by the semiconductor device of one embodiment of the present invention. 
       FIG. 25C  illustrates an example of a portable information terminal. A portable information terminal  2010  illustrated in  FIG. 25C  includes a housing  2011 , a display portion  2012 , an operation button  2013 , an external connection port  2014 , a speaker  2015 , a microphone  2016 , a camera  2017 , and the like. 
     The portable information terminal  2010  includes a touch sensor in the display portion  2012 . Operations such as making a call and inputting a letter can be performed by touch on the display portion  2012  with a finger, a stylus, or the like. 
     With the operation buttons  2013 , power on or off can be switched. In addition, types of images displayed on the display portion  2012  can be switched; for example, switching images from a mail creation screen to a main menu screen is performed. 
     When a sensing device such as a gyroscope sensor or an acceleration sensor is provided inside the portable information terminal  2010 , the direction of display on the screen of the display portion  2012  can be automatically changed by determining the orientation of the portable information terminal  2010  (whether the portable information terminal  2010  is placed horizontally or vertically). Furthermore, the direction of display on the screen can be changed by touch on the display portion  2012 , operation with the operation button  2013 , sound input using the microphone  2016 , or the like. 
     The portable information terminal  2010  functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. For example, the portable information terminal  2010  can be used as a smartphone. The portable information terminal  2010  is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, reproducing a moving image, Internet communication, and computer games, for example. 
       FIG. 25D  illustrates an example of a camera. A camera  2020  includes a housing  2021 , a display portion  2022 , operation buttons  2023 , a shutter button  2024 , and the like. Furthermore, a detachable lens  2026  is attached to the camera  2020 . 
     Although the lens  2026  of the camera  2020  here is detachable from the housing  2021  for replacement, the lens  2026  may be included in the housing. 
     Still and moving images can be taken with the camera  2020  at the press of the shutter button  2024 . In addition, images can be taken at the touch of the display portion  2022  which serves as a touch panel. 
     Note that a stroboscope, a viewfinder, and the like can be additionally attached to the camera  2020 . Alternatively, these components may be included in the housing  2021 . 
     The system described in the above embodiment can be provided in any of the electronic devices illustrated in  FIGS. 25A to 25D . 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     This application is based on Japanese Patent Application Serial No. 2016-159948 filed with Japan Patent Office on Aug. 17, 2016, the entire contents of which are hereby incorporated by reference.