Patent Publication Number: US-11024692-B2

Title: Display panel and method for driving the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a display panel. One embodiment of the present invention relates to a method for driving the display panel. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. Another embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them. 
     2. Description of the Related Art 
     A liquid crystal display device in which a light-condensing means and a pixel electrode are provided on one side of a substrate and a region transmitting visible light in the pixel electrode is provided to overlap with an optical axis of the light-condensing means is known. In addition, a liquid crystal display device which uses an anisotropic light-condensing means having a light-condensing direction X and a non-light-condensing direction Y, where the non-light-condensing direction Y corresponds to a longitudinal direction of a region transmitting visible light in the pixel electrode is known (Patent Document 1). 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2011-191750 
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a novel display panel that is highly convenient or reliable. Another object is to provide a display panel with a high pixel aperture ratio. 
     The description of these objects does not disturb the existence of other objects. Note that one embodiment of the present invention does not necessarily achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     (1) One embodiment of the present invention is a display panel including a signal line and a pixel. 
     The pixel is electrically connected to a signal line, and includes a first display element, a first conductive film, a second conductive film, a second insulating film, a pixel circuit, and a second display element. 
     The first conductive film is electrically connected to the first display element. The second conductive film has a region overlapping with the first conductive film. The second insulating film has a region sandwiched between the second conductive film and the first conductive film. The pixel circuit is electrically connected to the second conductive film. The second display element is electrically connected to the pixel circuit. 
     The second insulating film includes an opening, and is electrically connected to the first conductive film in the opening. The pixel circuit is electrically connected to the signal line. 
     (2) Another embodiment of the present invention is the above-described display panel in which the pixel circuit includes a first switch and a second switch. The first switch is electrically connected to the first display element, the second switch is electrically connected to the second display element, and the first switch and the second switch are electrically connected to the signal line. The switch is a transistor and the transistor includes an oxide semiconductor. 
     (3) Another embodiment of the present invention is the above-described display panel further including a first signal line driver circuit, a second signal line driver circuit, a first transistor, and a second transistor. The first signal line driver circuit is electrically connected to the signal line through the first transistor. The second signal line driver circuit is electrically connected to the signal line through the second transistor. 
     (4) Another embodiment of the present invention is the above-described display panel in which the second display element is configured to perform display in a direction the same as a direction in which the first display element performs display. 
     (5) Another embodiment of the present invention is the above-described display panel in which a display region of the second display element is surrounded by a display region of the first display element. 
     (6) Another embodiment of the present invention is the above-described display panel in which the first display element includes a reflective film and is configured to control the intensity of reflected light. 
     The reflective film has a function of reflecting incident light and includes an opening. The second display element is configured to emit light toward the opening. 
     (7) Another embodiment of the present invention is the above-described display panel further including a first group of pixels, a second group of pixels, and a scan line. 
     The first group of pixels include the above-described pixel, and are arranged in a row direction. The second group of pixels include the above-described pixel, and are arranged in a column direction that intersects with the row direction. 
     The scan line is electrically connected to the first group of pixels arranged in the row direction. 
     The second group of pixels arranged in the column direction are electrically connected to the signal line. 
     The positions of openings of pixels adjacent to each other in the row direction or the column direction are different. 
     (8) Another embodiment of the present invention is a method for driving any of the above-described display panels, in which one frame period has at least a first period and a second period, the first switch is selected in the first period, and the second switch is selected in the second period. 
     Although the block diagram attached to this specification shows components classified by their functions in independent blocks, it is difficult to classify actual components according to their functions completely and it is possible for one component to have a plurality of functions. 
     In this specification, the terms “source” and “drain” of a transistor interchange with each other depending on the polarity of the transistor or the levels of potentials applied to the terminals. In general, in an n-channel transistor, a terminal to which a lower potential is applied is called a source, and a terminal to which a higher potential is applied is called a drain. Furthermore, in a p-channel transistor, a terminal to which a lower potential is applied is called a drain, and a terminal to which a higher potential is applied is called a source. In this specification, although connection relation of the transistor is described assuming that the source and the drain are fixed in some cases for convenience, actually, the names of the source and the drain interchange with each other depending on the relation of the potentials. 
     In this specification, a “source” of a transistor means a source region that is part of a semiconductor film functioning as an active layer or a source electrode connected to the semiconductor film. Similarly, a “drain” of the transistor means a drain region that is part of the semiconductor film or a drain electrode connected to the semiconductor film. A “gate” means a gate electrode. 
     In this specification, a state in which transistors are connected to each other in series means, for example, a state in which only one of a source and a drain of a first transistor is connected to only one of a source and a drain of a second transistor. In addition, a state in which transistors are connected parallel to each other means a state in which one of a source and a drain of a first transistor is connected to one of a source and a drain of a second transistor and the other of the source and the drain of the first transistor is connected to the other of the source and the drain of the second transistor. 
     In this specification, the term “connection” means electrical connection and corresponds to a state where current, voltage, or a potential can be supplied or transmitted. Accordingly, a connection state means not only a state of direct connection but also a state of indirect connection through a circuit element such as a wiring, a resistor, a diode, or a transistor that allows current, voltage, or a potential to be supplied or transmitted. 
     In this specification, even when different components are connected to each other in a circuit diagram, there is actually a case where one conductive film has functions of a plurality of components such as a case where part of a wiring serves as an electrode. The term “connection” also means such a case where one conductive film has functions of a plurality of components. 
     In addition, in this specification, one of a first electrode and a second electrode of a transistor refers to a source electrode and the other refers to a drain electrode. 
     One embodiment of the present invention can provide a novel display panel that is highly convenient or reliable or a display panel having a high pixel aperture ratio. 
     Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all 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 
         FIGS. 1A, 1B-1, and 1B-2  illustrate a structure of a display panel of Embodiment. 
         FIGS. 2A to 2C  illustrate a structure of a display panel of Embodiment. 
         FIG. 3  is a block diagram illustrating a pixel circuit of Embodiment. 
         FIGS. 4A, 4B-1, and 4B-2  illustrate structures of display panels of Embodiment. 
         FIG. 5  is a circuit diagram illustrating a pixel circuit of Embodiment. 
         FIG. 6  is a timing chart showing a method for driving a display panel of Embodiment. 
         FIGS. 7A to 7D  are circuit diagrams each illustrating a pixel circuit of Embodiment. 
         FIG. 8  illustrates a structure of a display panel of Embodiment. 
         FIGS. 9A and 9B  illustrate a structure of a display panel of Embodiment. 
         FIG. 10  is a flow chart showing a method for manufacturing a display panel of Embodiment. 
         FIG. 11  illustrates a method for manufacturing a display panel of Embodiment. 
         FIG. 12  illustrates a method for manufacturing a display panel of Embodiment. 
         FIG. 13  illustrates a method for manufacturing a display panel of Embodiment. 
         FIG. 14  illustrates a method for manufacturing a display panel of Embodiment. 
         FIG. 15  illustrates a method for manufacturing a display panel of Embodiment. 
         FIG. 16  illustrates a method for manufacturing a display panel of Embodiment. 
         FIG. 17  illustrates a method for manufacturing a display panel of Embodiment. 
         FIGS. 18A to 18D  illustrate a structure of a transistor of Embodiment. 
         FIGS. 19A to 19C  illustrate a structure of a transistor of Embodiment. 
         FIG. 20  illustrates a structure of an input/output device of Embodiment. 
         FIGS. 21A and 21B  are a block diagram and a perspective view, respectively, illustrating a structure of an information processing device of Embodiment. 
         FIGS. 22A to 22C  are block diagrams and a circuit diagram, respectively, illustrating a display portion of Embodiment. 
         FIGS. 23A and 23B  are flow charts each showing a program of Embodiment. 
         FIG. 24  is a schematic diagram illustrating image information of Embodiment. 
         FIGS. 25A to 25C  are a cross-sectional view and circuit diagrams each illustrating a structure of a semiconductor device of Embodiment. 
         FIG. 26  is a block diagram illustrating a structure of a CPU of Embodiment. 
         FIG. 27  is a circuit diagram illustrating a structure of a memory element of Embodiment. 
         FIGS. 28A to 28H  each illustrate a structure of an electronic device of Embodiment. 
         FIG. 29  shows measured XRD spectra of samples. 
         FIGS. 30A and 30B  are TEM images of samples and  FIGS. 30C to 30L  are electron diffraction patterns thereof. 
         FIGS. 31A to 31C  show EDX mapping images of a sample. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below. Note that in the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description of such portions is not repeated. 
     Embodiment 1 
     In this embodiment, a structure of a display panel of one embodiment of the present invention will be described with reference to  FIGS. 1A to 1B-2 ,  FIGS. 2A to 2C ,  FIG. 3 ,  FIGS. 4A to 4B-2 ,  FIG. 5 , and  FIG. 6 . 
       FIGS. 1A, 1B-1, and 1B-2  illustrate the structure of a display panel  700  of one embodiment of the present invention.  FIG. 1A  is a bottom view illustrating a display panel  700  of one embodiment of the present invention.  FIG. 1B-1  is a bottom view illustrating part of  FIG. 1A , and  FIG. 1B-2  is a bottom view in which some components in  FIG. 1B-1  are not illustrated. 
       FIGS. 2A to 2C  illustrate the structures of the display panel  700  of one embodiment of the present invention.  FIG. 2A  is a cross-sectional view taken along lines X 1 -X 2 , X 3 -X 4 , X 5 -X 6 , X 7 -X 8 , X 9 -X 10 , and X 11 -X 12  in  FIG. 2A . The X 5 -X 6  cross section and the X 7 -X 8  cross section in  FIG. 2A  correspond to the lines X 5 -X 6  and X 7 -X 8  in  FIG. 1B-1  and  FIG. 1B-2 .  FIG. 2B  is a cross-sectional view illustrating part of the display panel.  FIG. 2C  is a cross-sectional view illustrating other part of the display panel. 
       FIG. 3  illustrates the structure of the display panel  700  of one embodiment of the present invention.  FIG. 3  is a block diagram illustrating a peripheral circuit and a pixel circuit  530 ( i, j ) that can be used in a pixel circuit included in the display panel  700  of one embodiment of the present invention. 
       FIGS. 4A, 4B-1, and 4B-2  illustrate the structure of the display panel  700  of one embodiment of the present invention.  FIG. 4A  is a block diagram illustrating arrangement of pixels, wirings, and the like in the display panel  700  of one embodiment of the present invention.  FIGS. 4B-1 and 4B-2  are schematic views each illustrating the positions of openings  751 H in the display panel  700  of one embodiment of the present invention. 
     &lt;Structure Example 1 of Display Panel&gt; 
     The display panel  700  described in this embodiment includes a signal line S(j) and a pixel  702 ( i, j ) (see  FIGS. 1B-1 and 1B-2 ). 
     The pixel  702 ( i, j ) is electrically connected to the signal line S(j). 
     The pixel  702 ( i, j ) includes a first display element  750 ( i, j ), the first conductive film, the second conductive film, a second insulating film  501 C, the pixel circuit  530 ( i, j ), and the second display element  550 ( i, j ) (see  FIG. 2A  and  FIG. 3 ). 
     The first conductive film is electrically connected to the first display element  750 ( i, j ) (see  FIG. 2A ). For example, the first conductive film can be used for a first electrode  751 ( i, j ) of the first display element  750 ( i, j ). 
     The second conductive film has a region overlapping with the first conductive film. For example, the second conductive film can be used as a conductive film  512 B that serves as a source electrode or a drain electrode of a transistor that can be used as a switch SW 1 . 
     The second insulating film  501 C has a region sandwiched between the second conductive film and the first conductive film. 
     The pixel circuit  530 ( i, j ) is electrically connected to the second conductive film. For example, a transistor in which the second conductive film is used as the conductive film  512 B that serves as a source electrode or a drain electrode can be used as the switch SW 1  of the pixel circuit  530 ( i, j ) (see  FIG. 2A  and  FIG. 3 ). 
     The second display element  550 ( i, j ) is electrically connected to the pixel circuit  530 ( i, j ). 
     A first connection portion  591 A is provided in the second insulating film  501 C (see  FIG. 2A ). 
     The second conductive film is electrically connected to the first conductive film in the first connection portion  591 A. For example, the conductive film  512 B is electrically connected to the first electrode  751 ( i, j ) that also serves as the first conductive film. 
     The pixel circuit  530 ( i, j ) is electrically connected to the signal line S(j) (see  FIG. 3 ). Note that a conductive film  512 A is electrically connected to the signal line S(j) (see  FIG. 2A  and  FIG. 3 ). 
     The first electrode  751 ( i, j ) is embedded in the second insulating film  501 C. 
     The pixel circuit  530 ( i, j ) of the display panel described in this embodiment includes the switch SW 1 . A transistor including an oxide semiconductor is used as the switch SW 1 . 
     The second display element  550 ( i, j ) of the display panel described in this embodiment has a function of performing display in the same direction as a direction in which the first display element  750 ( i, j ) performs display. For example, a dashed line arrow denotes the direction in which the first display element  750 ( i, j ) performs display by controlling the intensity of external light reflection, and a solid line arrow denotes the direction in which the second display element  550 ( i, j ) performs display (see  FIG. 2A ). 
     A display region of the second display element  550 ( i, j ) in the display panel described in this embodiment is surrounded by a display region of the first display element  750 ( i, j ) (see  FIG. 4B-1  or  FIG. 4B-2 ). The first display element  750 ( i, j ) performs display in a region overlapping with the first electrode  751 ( i, j ), and the second display element  550 ( i, j ) performs display in a region overlapping with the opening  751 H. 
     The first display element  750 ( i, j ) of the display panel described in this embodiment includes a reflective film having a function of reflecting incident light and has a function of controlling the intensity of reflected light. The reflective film includes the opening  751 H. Note that for the reflective film of the first display element  750 ( i, j ), the first conductive film or first electrode  751 ( i, j ) can be used, for example. 
     The second display element  550 ( i, j ) has a function of emitting light toward the opening  751 H. 
     The display panel described in this embodiment includes the pixel  702 ( i, j ), a first group of pixels  702 ( i ,  1 ) to  702 ( i, n ), a second group of pixels  702 ( 1 ,  j ) to  702 ( m, j ), and a scan line G 1 ( i ) (see  FIG. 4A ). Note that i is an integer greater than or equal to 1 and less than or equal to m, j is an integer greater than or equal to 1 and less than or equal to n, and each of m and n is an integer greater than or equal to 1. 
     The display panel described in this embodiment includes a scan line G 2 ( i ), a wiring CSCOM, and a wiring ANO. 
     The first group of pixels  702 ( i ,  1 ) to  702 ( i, n ) include a pixel  702 ( i, j ) and are arranged in the row direction (in the direction denoted by an arrow R). 
     The second group of pixels  702 ( 1 ,  j ) to  702 ( m, j ) include the pixel  702 ( i, j ) and are arranged in the column direction (in the direction denoted by an arrow C) that intersects with the row direction. 
     The scan line G 1 ( i ) is electrically connected to the first group of pixels  702 ( i ,  1 ) to  702 ( i, n ) arranged in the row direction. 
     The second group of pixels  702 ( 1 ,  j ) to  702 ( m, j ) arranged in the column direction are electrically connected to the signal line SU). 
     For example, the pixel  702 ( i, j+ 1) adjacent to the pixel  702 ( i, j ) in the row direction preferably includes an opening in a position different from that of the opening  751 H in the pixel  702 ( i, j ) (see  FIG. 4B-1 ). 
     For example, the pixel  702 ( i +1, j) adjacent to the pixel  702 ( i, j ) in the column direction preferably includes an opening in a position different from that of the opening  751 H in the pixel  702 ( i, j ) (see  FIG. 4B-2 ). Note that the first electrode  751 ( i, j ) can be used as the reflective film, for example. 
     The above-described display panel of one embodiment of the present invention includes the first display element, the first conductive film electrically connected to the first display element, the second conductive film having a region overlapping with the first conductive film, the second insulating film having a region sandwiched between the second conductive film and the first conductive film, a pixel circuit electrically connected to the second conductive film, and the second display element electrically connected to the pixel circuit. The second insulating film includes an opening, and the second conductive film is electrically connected to the first conductive film in the opening. 
     Accordingly, for example, with use of the pixel circuits formed through the same process, the first display element and the second display element that performs display by a different method from the first display element can be driven. Accordingly, a novel display panel that is highly convenient or reliable can be provided. 
     The display panel described in this embodiment includes a terminal  519 B and a conductive film  511 B (see  FIG. 2A ). 
     The second insulating film  501 C has a region sandwiched between the terminal  519 B and the conductive film  511 B. A third connection portion  591 B is provided in the second insulating film  501 C. 
     The terminal  519 B is electrically connected to the conductive film  511 B in the third connection portion  591 B. The conductive film  511 B is electrically connected to the pixel circuit  530 ( i, j ). In the case where, for example, the first electrode  751 ( i, j ) or the first conductive film is used as the reflective film, a plane of the terminal  519 B in contact with an anisotropic conductive film ACF 1  and a plane of the first electrode  751 ( i, j ) on the side where light enters the first display element  750 ( i, j ) face in the same direction. 
     Thus, electric power or a signal can be supplied to the pixel circuit through the terminal. Accordingly, a novel display panel that is highly convenient or reliable can be provided. Note that a flexible printed circuit FPC 1  is electrically connected to the terminal  519 B through the anisotropic conductive film ACF 1 . 
     The first display element  750 ( i, j ) of the display panel described in this embodiment includes the first electrode  751 ( i, j ), a second electrode  752 , and a layer  753  containing a liquid crystal material. The second electrode  752  is positioned such that an electric field which controls the alignment of the liquid crystal material is generated between the first electrode  751 ( i, j ) and the second electrode  752 . 
     The display panel described in this embodiment includes an alignment film AF 1  and an alignment film AF 2 . The alignment film AF 2  is disposed such that the layer  753  containing a liquid crystal material is sandwiched between the alignment film AF 1  and the alignment film AF 2 . 
     The second display element  550 ( i, j ) of the display panel described in this embodiment includes a third electrode  551 ( i, j ), a fourth electrode  552 , and a layer  553 ( j ) containing a light-emitting organic compound. 
     The fourth electrode  552  has a region overlapping with the third electrode  551 ( i, j ). The layer  553 ( j ) containing a light-emitting organic compound is provided between the third electrode  551 ( i, j ) and the fourth electrode  552 . The third electrode  551 ( i, j ) is electrically connected to the pixel circuit  530 ( i, j ) in a second connection portion  592 . 
     The pixel  702 ( i, j ) of the display panel described in this embodiment includes a color film CF 1 , a light-blocking film BM, an insulating film  771 , and a functional film  770 P. 
     The color film CF 1  has a region overlapping with the first display element  750 ( i, j ). The light-blocking film BM has an opening in the region overlapping with the first display element  750 ( i, j ). 
     The insulating film  771  is provided between the color film CF 1  and the layer  753  containing a liquid crystal material or between the light-blocking film BM and the layer  753  containing a liquid crystal material. Owing to the insulating film  771 , unevenness due to the thickness of the color film CF 1  can be avoided. Alternatively, impurities can be prevented from being diffused from the light blocking film BM, the color film CF 1 , or the like to the layer  753  containing a liquid crystal material. 
     The functional film  770 P has a region overlapping with the first display element  750 ( i, j ). The functional film  770 P is provided such that a substrate  770  is sandwiched between the functional film  770 P and the first display element  750 ( i, j ). 
     The display panel described in this embodiment includes a substrate  570 , the substrate  770 , and a functional layer  520 . 
     The substrate  770  has a region overlapping with the substrate  570 . The functional layer  520  is provided between the substrate  570  and the substrate  770 . 
     The functional layer  520  includes the pixel circuit  530 ( i, j ) including the switch SW 1 , the second display element  550 ( i, j ), an insulating film  521 , and an insulating film  528 . 
     The insulating film  521  is provided between the pixel circuit  530 ( i, j ) and the second display element  550 ( i, j ). 
     The insulating film  528  is provided between the insulating film  521  and the substrate  570  and includes an opening in a region overlapping with the second display element  550 ( i, j ). The insulating film  528  formed along the edges of the third electrode  551 ( i, j ) can prevent a short circuit between the third electrode  551 ( i, j ) and the fourth electrode  552 . 
     The display panel described in this embodiment includes a bonding layer  505 , a sealant  705 , and a structure KB 1 . 
     The bonding layer  505  is provided between the functional layer  520  and the substrate  570 , and has a function of bonding the functional layer  520  and the substrate  570 . 
     The sealant  705  is provided between the functional layer  520  and the substrate  770 , and has a function of bonding the functional layer  520  and the substrate  770 . 
     The structure KB 1  has a function of providing a certain space between the functional layer  520  and the substrate  770 . 
     The display panel described in this embodiment includes a terminal  519 C, a conductive film  511 C, and a conductor CP. 
     The second insulating film  501 C has a region sandwiched between the terminal  519 C and the conductive film  511 C. A third connection portion  591 C is provided in the second insulating film  501 C. 
     The terminal  519 C is electrically connected to the conductive film  511 C in the third connection portion  591 C. The conductive film  511 C is electrically connected to the pixel circuit  530 ( i, j ). 
     The conductor CP is sandwiched between the terminal  519 C and the second electrode  752 , and electrically connects the terminal  519 C and the second electrode  752 . For example, a conductive particle can be used as the conductor CP. 
     The display panel described in this embodiment includes a driver circuit GD, a driver circuit SD_LC, and a driver circuit SD_EL (see  FIG. 1A  and  FIG. 4A ). 
     The driver circuit GD is electrically connected to the scan line G 1 ( i ). The driver circuit GD includes, for example, a transistor MD. Specifically, a transistor including a semiconductor film which can be formed by the same step as a transistor included in the pixel circuit  530 ( i, j ) can be used as the transistor MD (see  FIGS. 2A and 2C ). 
     The driver circuit SD_LC is electrically connected to the signal line S(j) through a transistor M 1  (see  FIG. 3 ). The driver circuit SD_LC has a function of outputting a signal for driving the first display element  750 . The driver circuit SD_EL is electrically connected to the signal line S(j) through a transistor M 2 . The driver circuit SD_EL has a function of outputting a signal for driving the second display element  550 . Note that a driver circuit SD that has a function of selectively outputting signals for driving the first display element  750  and the second display element  550  may be used instead of the driver circuit SD_LC and the driver circuit SD_EL (see  FIG. 5 ). In  FIG. 3 , a wiring that connects the driver circuit SD_LC and the transistor M 1  is denoted as a wiring DATA_LC, and a wiring that connects the driver circuit SD_EL and the transistor M 2  is denoted as a wiring DATA_EL. The driver circuit SD_LC and the driver circuit SD_EL are collectively referred to as a signal line driver circuit. 
     The driver circuit SD_LC and the driver circuit SD_EL share the same signal line S(j), in which case the number of wirings included in the pixel circuit  530 ( i, j ) can be reduced. In a layout of the pixel circuit, in order to prevent influence of a potential of the signal line on an electrode of a display element, the signal line and the electrode are preferably provided not to overlap each other. Consequently, the display panel of one embodiment of the present invention can have a high pixel aperture ratio due to a reduced number of wirings included in the pixel circuit  530 ( i, j ). 
     A sampling control circuit SAMP_LC is electrically connected to a gate electrode of the transistor M 1 . The sampling control circuit SAMP_LC has a function of outputting a control signal to the driver circuit SD_LC. The sampling control circuit SAMP_EL is electrically connected to a gate electrode of the transistor M 2 . The sampling control circuit SAMP_EL has a function of outputting a control signal to the driver circuit SD_EL. 
     The driver circuit SD_LC and the driver circuit SD_EL are electrically connected to, for example, a terminal that can be formed by the same step as the terminal  519 B or the terminal  519 C. 
     Individual components included in the display panel will be described below. Note that these units cannot be clearly distinguished and one unit also serves as another unit or includes part of another unit in some cases. 
     For example, the first conductive film can be used for the first electrode  751 ( i, j ). The first conductive film can be used for the reflective film. 
     The second conductive film can be used as the conductive film  512 B that functions as a source electrode or a drain electrode of a transistor. 
     &lt;&lt;Substrate  570 &gt;&gt; 
     A material having heat resistance high enough to withstand heat treatment in the manufacturing process can be used for the substrate  570 . Specifically, non-alkali glass with a thickness of 0.7 mm can be used. 
     For example, a large-sized glass substrate having any of the following sizes can be used as the substrate  570 : the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured. 
     For the substrate  570 , an organic material, an inorganic material, a composite material of an organic material and an inorganic material, or the like can be used. For example, an inorganic material such as glass, ceramic, or a metal can be used for the substrate  570 . 
     Specifically, non-alkali glass, soda-lime glass, potash glass, crystal glass, quartz, sapphire, or the like can be used for the substrate  570 . Specifically, an inorganic oxide film, an inorganic nitride film, an inorganic oxynitride film, or the like can be used for the substrate  570 . For example, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxide film, or the like can be used for the substrate  570 . Stainless steel, aluminum, or the like can be used for the substrate  570 . 
     For example, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon or silicon carbide, a compound semiconductor substrate of silicon germanium, or an SOI substrate can be used as the substrate  570 . Thus, a semiconductor element can be formed over the substrate  570 . 
     An organic material such as a resin, a resin film, or plastic can be used for the substrate  570 . Specifically, a resin film or resin plate of polyester, polyolefin, polyamide, polyimide, polycarbonate, an acrylic resin, or the like can be used for the substrate  570 . 
     For example, a composite material, such as a resin film to which a metal plate, a thin glass plate, or an inorganic film is bonded can be used for the substrate  570 . For example, a composite material formed by dispersing a fibrous or particulate metal, glass, inorganic material, or the like into a resin film can be used for the substrate  570 . For example, a composite material formed by dispersing a fibrous or particulate resin, an organic material, or the like into an inorganic material can be used for the substrate  570 . 
     A single-layer material or a stacked-layer material in which a plurality of layers are stacked can be used for the substrate  570 . For example, a stacked-layer material in which a base, an insulating film that prevents diffusion of impurities contained in the base, and the like are stacked can be used for the substrate  570 . Specifically, a stacked-layer material in which glass and one or a plurality of films that prevent diffusion of impurities contained in the glass and that are selected from a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, and the like are stacked can be used for the substrate  570 . Alternatively, a stacked-layer material in which a resin and a film for preventing diffusion of impurities that penetrate the resin, such as a silicon oxide film, a silicon nitride film, and a silicon oxynitride film are stacked can be used for the substrate  570 . 
     Specifically, a resin film, a resin plate, a stack, or the like of polyester, polyolefin, polyamide, polyimide, polycarbonate, an acrylic resin, or the like can be used for the substrate  570 . 
     Specifically, a material including polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxy resin, a resin having a siloxane bond, such as a silicone resin, or the like can be used for the substrate  570 . 
     Specifically, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), acrylic, or the like can be used for the substrate  570 . 
     Alternatively, paper, wood, or the like can be used for the substrate  570 . 
     For example, a flexible substrate can be used as the substrate  570 . 
     Note that as a method for forming the second display element  550 ( i, j ) or the like over the substrate  570 , a method in which a transistor, a capacitor, or the like is directly formed on a substrate may be employed. Alternatively, a method in which a transistor, a capacitor, or the like is formed over a substrate for use in manufacturing processes which can withstand heat applied in the manufacturing process and is transferred to the substrate  570  can be employed. Thus, a transistor, a capacitor, or the like can be formed over a flexible substrate, for example. 
     &lt;&lt;Substrate  770 &gt;&gt; 
     A light-transmitting material can be used for the substrate  770 , for example. Specifically, a material selected from the materials that can be used for the substrate  570  can be used for the substrate  770 . More specifically, non-alkali glass polished to a thickness of approximately 0.7 mm or 0.1 mm can be used. 
     &lt;&lt;Structure KB 1 &gt;&gt; 
     An organic material, an inorganic material, or a composite material of an organic material and an inorganic material can be used for the structure KB 1 . Thus, a certain gap can be provided between components (e.g., the second electrode  752  and the alignment film AF 1 ) with the structure KB 1  sandwiched therebetween. 
     Specifically, for the structure KB 1 , polyester, polyolefin, polyamide, polyimide, polycarbonate, polysiloxane, an acrylic resin, or the like, or a composite material of a plurality of resins selected from these can be used. Alternatively, a photosensitive material may be used. 
     &lt;&lt;Sealant  705 &gt;&gt; 
     An inorganic material, an organic material, a composite material of an inorganic material and an organic material, or the like can be used for the sealant  705  or the like. 
     For example, an organic material, such as a resin having thermal fusibility or a curable resin, can be used for the sealant  705  or the like. 
     For example, an organic material, such as a reactive curable adhesive, a light curable adhesive, a thermosetting adhesive, and/or an anaerobic adhesive, can be used for the sealant  705  or the like. 
     Specifically, an adhesive containing an epoxy resin, an acrylic resin, a silicone resin, a phenol resin, a polyimide resin, an imide resin, a polyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB) resin, or an ethylene vinyl acetate (EVA) resin, or the like can be used for the sealant  705  or the like. 
     &lt;&lt;Bonding Layer  505 &gt;&gt; 
     For example, a material that can be used for the sealant  705  can be used for the bonding layer  505 . 
     &lt;&lt;Insulating Film  521 &gt;&gt; 
     For example, an insulating inorganic material, an insulating organic material, or an insulating composite material containing an inorganic material and an organic material can be used for the insulating film  521 . 
     Specifically, an inorganic oxide film, an inorganic nitride film, an inorganic oxynitride film, or a material obtained by stacking any of these films can be used for the insulating film  521 . For example, a film including any of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and an aluminum oxide film, or a film including a material stacking any of these films can be used for the insulating film  521 . 
     Specifically, for the insulating film  521 , polyester, polyolefin, polyamide, polyimide, polycarbonate, polysiloxane, an acrylic resin, or the like, or a stacked or composite material of a plurality of resins selected from these can be used. Alternatively, a photosensitive material may be used. 
     Thus, steps due to components overlapping with the insulating film  521 , for example, can be covered so that a flat surface can be formed. 
     &lt;&lt;Insulating Film  528 &gt;&gt; 
     For example, a material that can be used for the insulating film  521  can be used for the insulating film  528 . Specifically, a 1-μm-thick film containing polyimide can be used as the insulating film  528 . 
     &lt;&lt;Second Insulating Film  501 C&gt;&gt; 
     For example, a material that can be used for the insulating film  521  can be used for the second insulating film  501 C. Specifically, a material containing silicon and oxygen can be used for the second insulating film  501 C. Thus, impurity can be prevented from being diffused to the pixel circuit, the second display element, or the like. 
     For example, a 200-nm-thick film containing silicon, oxygen, and nitrogen can be used as the second insulating film  501 C. 
     Note that the first connection portion  591 A, the third connection portion  591 B, and the third connection portion  591 C are provided in the second insulating film  501 C. 
     &lt;&lt;Wiring, Terminal, Conductive Film&gt;&gt; 
     A conductive material can be used for wirings and the like. Specifically, a conductive material can be used for the signal line S(j), the scan line G 1 ( i ), the scan line G 2 ( i ), the wiring CSCOM, the wiring ANO, the terminal  519 B, the terminal  519 C, the conductive film  511 B, the conductive film  511 C, or the like. 
     For example, an inorganic conductive material, an organic conductive material, a metal material, or a conductive ceramic material can be used for the wirings and the like. 
     Specifically, a metal element selected from aluminum, gold, platinum, silver, copper, chromium, tantalum, titanium, molybdenum, tungsten, nickel, iron, cobalt, palladium, and manganese can be used for the wirings and the like. Alternatively, an alloy including any of the above-described metal elements, or the like can be used for the wirings and the like. In particular, an alloy of copper and manganese is preferably used in microfabrication using wet etching. 
     Specifically, the following structures can be used for the wirings and the like: a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, or the like. 
     Specifically, a conductive oxide, such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added, can be used for the wirings and the like. 
     Specifically, a film containing graphene or graphite can be used for the wirings and the like. 
     A film containing graphene can be formed, for example, by reducing a film containing graphene oxide. Examples of the reducing method include a method with application of heat and a method using a reducing agent. 
     Specifically, a conductive high molecule can be used for the wirings and the like. 
     &lt;&lt;First Conductive Film and Second Conductive Film&gt;&gt; 
     For example, any of the materials used for the wirings and the like can be used for the first conductive film or the second conductive film. 
     The first electrode  571 ( i, j ), the wiring, or the like can be used for the first conductive film. 
     The conductive film  512 B of a transistor that can be used as the switch SW 1 , the wiring, or the like can be used as the second conductive film. 
     &lt;&lt;Pixel Circuit  530 ( i, j )&gt;&gt; 
     The pixel circuit  530 ( i, j ) is electrically connected to the signal line S(j), the scan line G 1 ( i ), the scan line G 2 ( i ), the wiring CSCOM, and the wiring ANO (see  FIG. 3 ). 
     The pixel circuit  530 ( i, j ) includes the switch SW 1 , the capacitor C 1 , the switch SW 2 , a transistor M, and the capacitor C 2 . 
     The switch SW 1  is electrically connected to the first display element  750 . The switch SW 2  is electrically connected to the second display element  550  through the transistor M. The switch SW 1  and the switch SW 2  are electrically connected to the signal line S(j). 
     For example, a transistor including a gate electrode electrically connected to the scan line G 1 ( i ) and a first electrode electrically connected to the signal line S(j) can be used as the switch SW 1 . 
     The capacitor C 1  includes a first electrode electrically connected to a second electrode of a transistor used as the switch SW 1  and a second electrode electrically connected to the wiring CSCOM. 
     For example, a transistor including a gate electrode electrically connected to the scan line G 2 ( i ) and a first electrode electrically connected to the signal line S(j) can be used as the switch SW 2 . 
     The transistor M includes a gate electrode electrically connected to a second electrode of a transistor used as the switch SW 2  and a first electrode electrically connected to the wiring ANO. 
     A transistor in which a semiconductor film is sandwiched between a conductive film and a gate electrode can be used as the transistor M. For example, a conductive film electrically connected to a wiring that can supply a potential the same as that of the gate electrode of the transistor M can be used. The conductive film can be called a second gate electrode of the transistor M. 
     The capacitor C 2  includes a first electrode electrically connected to a second electrode of a transistor used as the switch SW 2  and a second electrode electrically connected to a first electrode of the transistor M. 
     Note that a first electrode of the first display element  750  is electrically connected to the second electrode of the transistor used as the switch SW 1 , and a second electrode of the first display element  750  is electrically connected to a wiring VCOM 1 . Thus, the first display element  750  can be driven. 
     Note that a first electrode of the second display element  550  is electrically connected to a second electrode of the transistor M, and a second electrode of the second display element  550  is electrically connected to a wiring VCOM 2 . Thus, the second display element  550  can be driven. 
     &lt;&lt;Switch SW 1 , Switch SW 2 , Transistor M, and Transistor MD&gt;&gt; 
     For example, a bottom gate transistor or a top gate transistor can be used as the switch SW 1 , the switch SW 2 , the transistor M, or the transistor MD. 
     For example, a transistor in which a semiconductor containing a Group 14 element is used for a semiconductor film can be used. Specifically, a semiconductor containing silicon can be used for the semiconductor film. For example, single crystal silicon, polysilicon, microcrystalline silicon, or amorphous silicon can be used for the semiconductor film of the transistor. 
     For example, a transistor in which an oxide semiconductor is used for a semiconductor film can be used. Specifically, an oxide semiconductor containing indium or an oxide semiconductor containing indium, gallium, and zinc can be used for the semiconductor film. 
     For example, a transistor having a lower leakage current in an off state than a transistor that uses amorphous silicon for a semiconductor film can be used as the switch SW 1 , the switch SW 2 , the transistor M, or the transistor MD. Specifically, a transistor in which an oxide semiconductor is used for a semiconductor film  508  can be used as the switch SW 1 , the switch SW 2 , the transistor M, or the transistor MD. 
     Thus, a pixel circuit including a transistor that uses an oxide semiconductor for a semiconductor film can hold an image signal for a longer time than a pixel circuit including a transistor that uses amorphous silicon for a semiconductor film. Specifically, the selection signal can be supplied at a frequency of lower than 30 Hz, preferably lower than 1 Hz, more preferably less than once per minute while flickering is suppressed. Consequently, eyestrain on a user of the information processing device can be reduced, and power consumption for driving can be reduced. 
     The transistor that can be used as the switch SW 1  includes the semiconductor film  508  and a conductive film  504  having a region overlapping with the semiconductor film  508  (see  FIG. 2B ). The transistor that can be used as the switch SW 1  includes the conductive film  512 A and the conductive film  512 B. 
     Note that the conductive film  504  and the insulating film  506  serve as a gate electrode and a gate insulating film, respectively. The conductive film  512 A serves as one of a source electrode and a drain electrode, and the conductive film  512 B serves as the other. 
     A transistor in which the semiconductor film  508  is sandwiched between a conductive film  524  and the conductive film  504  can be used as the transistor M (see  FIG. 2C ). 
     A conductive film in which a 10-nm-thick film containing tantalum and nitrogen and a 300-nm-thick film containing copper are stacked in this order can be used as the conductive film  504 . 
     A film in which a 400-nm-thick film containing silicon and nitrogen and a 200-nm-thick film containing silicon, oxygen, and nitrogen are stacked can be used as the insulating film  506 . 
     A 25-nm-thick film containing indium, gallium, and zinc can be used as the semiconductor film  508 . 
     A conductive film in which a 50-nm-thick film containing tungsten, a 400-nm-thick film containing aluminum, and a 100-nm-thick film containing titanium are stacked in this order can be used as the conductive film  512 A or the conductive film  512 B. 
     &lt;&lt;First Display Element  750 ( i, j )&gt;&gt; 
     For example, a display element having a function of controlling transmission or reflection of light can be used as the first display element  750 ( i, j ). For example, a combined structure of a polarizing plate and a liquid crystal element or a MEMS shutter display element can be used. The use of a reflective display element can reduce power consumption of a display panel. Specifically, a reflective liquid crystal display element can be used as the first display element  750 ( i, j ). 
     A liquid crystal element that can be driven by any of the following driving methods can be used: an in-plane switching (IPS) mode, a twisted nematic (TN) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, and the like. 
     In addition, a liquid crystal element that can be driven by any of the following driving methods can be used: a vertical alignment (VA) mode such as a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an electrically controlled birefringence (ECB) mode, a continuous pinwheel alignment (CPA) mode, and an advanced super view (ASV) mode. 
     For example, thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, polymer dispersed liquid crystal, ferroelectric liquid crystal, or anti-ferroelectric liquid crystal can be used. Alternatively, a liquid crystal material that exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like can be used. Further alternatively, a liquid crystal material that exhibits a blue phase can be used. 
     &lt;&lt;First Electrode  751 ( i, j )&gt;&gt; 
     For example, any of the materials used for the wirings and the like can be used for the first electrode  751 ( i, j ). Specifically, a reflective film can be used for the first electrode  751 ( i, j ). 
     &lt;&lt;Reflective Film&gt;&gt; 
     For example, a material that reflects visible light can be used for the reflective film. Specifically, a material containing silver can be used for the reflective film. For example, a material containing silver and palladium or a material containing silver and copper can be used for the reflective film. 
     The reflective film reflects, for example, light that passes through the layer  753  containing a liquid crystal material. This allows the first display element  750  to serve as a reflective liquid crystal element. A film with an uneven surface can be used for the reflective film, for example. In that case, incident light can be reflected in various directions, and a white image can be displayed. 
     Note that a structure other than the structure in which the first electrode  751 ( i, j ) is used for the reflective film can be employed. For example, a structure in which the reflective film is provided between the layer  753  containing a liquid crystal material and the first electrode  751 ( i, j ) may be employed. Alternatively, a structure in which a light-transmissive first electrode  751 ( i, j ) is provided between the reflective film and the layer  753  containing a liquid crystal material may be employed. 
     &lt;&lt;Opening  751 H&gt;&gt; 
     Considering the reflective film for each first display element  750 ( i, j ), the ratio of the total area of the opening  751 H to the total area of the non-opening portion is preferably larger than or equal to 0.052 and smaller than or equal to 0.6. If the ratio of the total area of the opening  751 H to the total area of the non-opening portion is too large, display performed using the first display element  750 ( i, j ) is dark. If the ratio of the total area of the opening  751 H to the total area of the non-opening portion is too small, display performed using the second display element  550 ( i, j ) is dark. 
     In the case where the first electrode  751 ( i, j ) is used as the reflective film, the area of one opening  751 H is larger than or equal to 3 μm 2  and smaller than or equal to 25 μm 2 . If the area of the opening  751 H is too large, an electric field is not uniformly applied to the layer  753  containing a liquid crystal material, which lowers the display performance of the first display element  750 . If the area of the opening  751 H in the first conductive film is too small, the light extraction efficiency of light emitted from the second display element  550  is reduced. 
     The opening  751 H may have a polygonal shape, a quadrangular shape, an elliptical shape, a circular shape, a cross shape, a stripe shape, a slit-like shape, or a checkered pattern, for example. The opening  751 H may be close to the adjacent pixel. The opening  751 H is preferably disposed close to another pixel that displays the same color. Thus, a phenomenon in which light emitted from the second display element  550  enters a color film provided in an adjacent pixel (also referred to as crosstalk) can be inhibited. 
     &lt;&lt;Second Electrode  752 &gt;&gt; 
     For example, a conductive material having a visible-light transmitting property can be used for the second electrode  752 . 
     For example, a conductive oxide, a metal film thin enough to transmit light, or a metal nanowire can be used for the second electrode  752 . 
     Specifically, a conductive oxide containing indium can be used for the second electrode  752 . Alternatively, a metal thin film with a thickness greater than or equal to 1 nm and less than or equal to 10 nm can be used for the second electrode  752 . Alternatively, a metal nanowire containing silver can be used for the second electrode  752 . 
     Specifically, indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, zinc oxide to which gallium is added, zinc oxide to which aluminum is added, or the like can be used for the second electrode  752 . 
     &lt;&lt;Alignment Films AF 1  and AF 2 &gt;&gt; 
     For example, a material containing polyimide or the like can be used for the alignment films AF 1  and AF 2 . Specifically, a material that is formed by rubbing treatment or optical alignment technology to be oriented in a certain direction can be used. 
     For example, a film containing soluble polyimide can be used as the alignment films AF 1  and AF 2 . 
     &lt;&lt;Color Film CF 1 &gt;&gt; 
     A material that transmits light of a certain color can be used for the color film CF 1 . In that case, the color film CF 1  can be used as a color filter, for example. 
     For example, a material transmitting light of blue, green, red, yellow, or white can be used for the color film CF 1 . 
     &lt;&lt;Light-Blocking Film BM&gt;&gt; 
     A material that interrupts light transmission can be used for the light-blocking film BM, in which case, the light-blocking film BM serves as a black matrix, for example. 
     &lt;&lt;Insulating Film  771 &gt;&gt; 
     The insulating film  771  can be formed with polyimide, an epoxy resin, an acrylic resin, or the like. 
     &lt;&lt;Functional Film  770 P&gt;&gt; 
     For example, a polarizing plate, a retardation plate, a diffusing film, an anti-reflection film, a condensing film, or the like can be used as the functional film  770 P. Alternatively, a polarizing plate containing a dichromatic pigment can be used for the functional film  770 P. 
     Alternatively, an antistatic film preventing the attachment of a foreign substance, a water repellent film suppressing the attachment of stain, a hard coat film suppressing generation of a scratch in use, or the like can be used for the functional film  770 P. 
     &lt;&lt;Second Display Element  550 ( i, j )&gt;&gt; 
     A light-emitting element, for example, can be used as the second display element  550 ( i, j ). Specifically, an organic electroluminescent element, an inorganic electroluminescent element, a light-emitting diode, or the like can be used for the second display element  550 ( i, j ). 
     For example, a stack formed to emit blue light, a stack formed to emit green light, or a stack formed to emit red light can be used as the layer  553 ( j ) containing a light-emitting organic compound. 
     For example, a belt-shaped stack extended in the column direction along the signal line S(j) can be used as the layer  553 ( j ) containing a light-emitting organic compound. A belt-shaped stack which is extended in the column direction along the signal line S(j+1) and emits light of a color different from a color of the light emitted from the layer  553 ( j ) containing a light-emitting organic compound can be used as a layer  553 ( j+ 1) containing a light-emitting organic compound. 
     For example, a stack formed to emit white light can be used as the layer  553 ( j ) containing a light-emitting organic compound and the layer  553 ( j+ 1) containing a light-emitting organic compound. Specifically, a stack of a layer containing a light-emitting organic compound containing a blue-light-emitting fluorescent material, a layer containing a green-light-emitting material that is other than a fluorescent material, and a layer containing a red-light emitting material that is other than a fluorescent material, or a stack of a layer containing a light-emitting organic material containing a blue-light-emitting fluorescent material and a layer containing a yellow-light-emitting material that is other than a fluorescent material can be used as the layer  553 ( j ) containing a light-emitting organic material and the layer  553 ( j+ 1) containing a light-emitting organic compound. 
     For example, any of the materials that can be used for the wirings and the like can be used for the third electrode  551 ( i, j ) or the fourth electrode  552 . 
     For example, a material having a visible-light transmitting property selected from the materials that can be used for the wirings and the like can be used for the third electrode  551 ( i, j ). 
     Specifically, conductive oxide, indium-containing conductive oxide, indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, zinc oxide to which gallium is added, or the like can be used for the third electrode  551 ( i, j ). Alternatively, a metal film thin enough to transmit light can be used as the third electrode  551 ( i, j ). 
     For example, a material that reflects visible light selected from the materials that can be used for the wirings and the like can be used for the fourth electrode  552 . 
     &lt;&lt;Driver Circuit GD&gt;&gt; 
     Any of a variety of sequential circuits, such as a shift register, can be used as the driver circuit GD. For example, the transistor MD, a capacitor, and the like can be used in the driver circuit GD. Specifically, a transistor including a semiconductor film that can be formed by the same step as the transistor M can be used. 
     Alternatively, the transistor MD may have a structure different from the structure of the transistor that can be used as the switch SW 1 . Specifically, a transistor including the conductive film  524  can be used as the transistor MD (see  FIG. 2C ). 
     The conductive film  524  is provided such that the semiconductor film  508  is sandwiched between the conductive films  504  and  524 . The insulating film  516  is provided between the conductive film  524  and the semiconductor film  508 . The insulating film  506  is provided between the semiconductor film  508  and the conductive film  504 . For example, the conductive film  524  is electrically connected to a wiring supplying the same potential as that supplied to the conductive film  504 . 
     Note that the transistor MD can have the same structure as the transistor M. 
     &lt;&lt;Driver Circuits SD_LC and SD_EL&gt;&gt; 
     For example, an integrated circuit can be used as the driver circuits SD_LC and SD_EL. Specifically, an integrated circuit formed over a silicon substrate can be used as the driver circuits SD_LC and SD_EL. 
     For example, a chip on glass (COG) method can be used to mount the driver circuits SD_LC and SD_EL on a pad electrically connected to the pixel circuit  530 ( i, j ). Specifically, an anisotropic conductive film can be used to mount the integrated circuit on the pad. 
     Note that the pad can be formed through the same step as the terminal  519 B or the terminal  519 C. 
     &lt;Driving Method of Display Panel&gt; 
     Here, a method for driving the display panel of one embodiment of the present invention is described.  FIG. 6  is a timing chart showing voltage applied to a wiring or a circuit illustrated in the circuit configuration of  FIG. 3 . Note that wirings GSP_LC and GSP_EL shown in  FIG. 6  have a function of supplying a start pulse signal to the driver circuit GD. Although a scan line G 1 (dum) and a scan line G 2 (dum) are dummy wirings, the display panel of one embodiment of the present invention does not necessarily include a dummy wiring. 
     One frame period has at least the period PT 1  and the period PT 2 . In the period PT 1 , the driver circuit SD_LC outputs a signal for driving the first display element  750 . In addition, in the period PT 1 , the transistor M 1  is brought into a selected state (an on state) by a sampling signal output by the sampling control circuit SAMP_LC. In the period PT 2 , the driver circuit SD_EL outputs a signal for driving the second display element  550 . In addition, in the period PT 2 , the transistor M 2  is brought into a selected state by the sampling signal output by the sampling control circuit SAMP_EL. 
     The switch SW 1  electrically connected to the first display element  750  of the pixel circuit  530 ( i, j ) is in the selected state in an i-th gate selection period in the period PT 1 , and is in a non-selected state (an off state) in a period other than the i-gate selection period. Then, a (i+1)th gate selection period is provided after the i-th gate selection period in the period PT 1 . The switch SW 2  electrically connected to the second display element  550  of the pixel circuit  530 ( i, j ) is in the selected state in an i-th gate selection period in the period PT 2 , and is in the non-selected state (the off state) in a period other than the i-gate selection period. Then, a (i+1)th gate selection period is provided after the i-th gate selection period in the period PT 2 . By performing sequential scanning in this manner, all the pixels are sequentially selected in one frame period. Note that in  FIG. 6 , a first gate selection period, a second gate selection period, and an m-th gate selection period are shown in each of the period PT 1  and the period PT 2 . 
     The driver circuit SD_LC outputs a data signal for performing display of the first display element  750  in each pixel circuit to the first display element  750  via the wiring DATA_LC while the transistor M 1  is in the selected state and the switches SW 1  in pixel circuits  530 ( i ,  1 ) to  530 ( i, n ) of the i-stage are in the selected state. During the non-selected state of the transistor M 1 , the potential of the wiring DATA_LC is not particularly limited. The driver circuit SD_EL outputs a data signal for performing display of the second display element  550  in each pixel circuit via the wiring DATA_EL while the transistor M 2  is in the selected state and switches SW 2  in pixel circuits  530 ( i ,  1 ) to  530 ( i, n ) of the i-stage are in the selected state. During the non-selected state of the transistor M 2 , the potential of the wiring DATA_EL is not particularly limited. 
     Since the driver circuit SD_LC and the driver circuit SD_EL share the same signal line S(j) and one frame period has the period PT 1  in which the first display element  750  is driven and the period PT 2  in which the second display element  550  is driven, the display panel  700  including the pixel circuits  530 ( i, j ) can perform display. 
     &lt;Method for Controlling Resistivity of Oxide Semiconductor Film&gt; 
     The method for controlling the resistivity of an oxide semiconductor film will be described. 
     An oxide semiconductor film with a certain resistivity can be used as the semiconductor film  508  or the conductive film  524 . 
     For example, a method for controlling the concentration of impurities such as hydrogen and water contained in an oxide semiconductor film and/or the oxygen vacancies in the film can be used as the method for controlling the resistivity of an oxide semiconductor film. 
     Specifically, plasma treatment can be used as a method for increasing or decreasing the concentration of impurities such as hydrogen and water and/or the oxygen vacancies in the film. 
     Specifically, plasma treatment using a gas containing one or more kinds selected from a rare gas (He, Ne, Ar, Kr, Xe), hydrogen, boron, phosphorus, and nitrogen can be employed. For example, plasma treatment in an Ar atmosphere, plasma treatment in a mixed gas atmosphere of Ar and hydrogen, plasma treatment in an ammonia atmosphere, plasma treatment in a mixed gas atmosphere of Ar and ammonia, or plasma treatment in a nitrogen atmosphere can be employed. Thus, the oxide semiconductor film can have a high carrier density and a low resistivity. 
     Alternatively, hydrogen, boron, phosphorus, or nitrogen is added to the oxide semiconductor film by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like, so that the oxide semiconductor film can have a low resistivity. 
     Alternatively, an insulating film containing hydrogen is formed in contact with the oxide semiconductor film, and the hydrogen is diffused from the insulating film to the oxide semiconductor film, so that the oxide semiconductor film can have a high carrier density and a low resistivity. 
     For example, an insulating film with a hydrogen concentration of greater than or equal to 1×10 22  atoms/cm 3  is formed in contact with the oxide semiconductor film, in which case hydrogen can be effectively supplied to the oxide semiconductor film. Specifically, a silicon nitride film can be used as the insulating film formed in contact with the oxide semiconductor film. 
     Hydrogen contained in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and an oxygen vacancy is formed in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier in some cases. Thus, the oxide semiconductor film can have a high carrier density and a low resistivity. 
     Specifically, an oxide semiconductor with a hydrogen concentration measured by secondary ion mass spectrometry (SIMS) of greater than or equal to 8×10 19  atoms/cm 3 , preferably greater than or equal to 1×10 20  atoms/cm 3 , more preferably greater than or equal to 5×10 20  atoms/cm 3  can be suitably used for the conductive film  524 . 
     On the other hand, an oxide semiconductor with a high resistivity can be used for a semiconductor film where a channel of a transistor is formed. Specifically, such an oxide semiconductor can be suitably used for the semiconductor film  508 . 
     For example, an insulating film containing oxygen, in other words, an insulating film capable of releasing oxygen, is formed in contact with an oxide semiconductor film, and the oxygen is supplied from the insulating film to the oxide semiconductor film, so that oxygen vacancies in the film or at the interface can be filled. Thus, the oxide semiconductor film can have a high resistivity. 
     For example, a silicon oxide film or a silicon oxynitride film can be used as the insulating film capable of releasing oxygen. 
     The oxide semiconductor film in which oxygen vacancies are filled and the hydrogen concentration is reduced can be referred to as a highly purified intrinsic oxide semiconductor film or a substantially highly purified intrinsic oxide semiconductor film. The term “substantially intrinsic” refers to the state in which an oxide semiconductor film has a carrier density lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , further preferably lower than 1×10 10 /cm 3 . A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources and thus can have a low carrier density. The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly can have a low density of trap states. 
     Furthermore, a transistor including the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10 6  μm and a channel length of 10 μm, the off-state current can be lower than or equal to the measurement limit of a semiconductor parameter analyzer, that is, lower than or equal to 1×10 −13  A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. 
     The transistor in which a channel region is formed in a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small change in electrical characteristics and high reliability. 
     Specifically, an oxide semiconductor has a hydrogen concentration which is measured by secondary ion mass spectrometry (SIMS) of lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , more preferably lower than or equal to 1×10 19  atoms/cm 3 , more preferably lower than 5×10 18  atoms/cm 3 , more preferably lower than or equal to 1×10 18  atoms/cm 3 , more preferably lower than or equal to 5×10 17  atoms/cm 3 , more preferably lower than or equal to 1×10 16  atoms/cm 3  can be favorably used for a semiconductor film where a channel of a transistor is formed. 
     An oxide semiconductor film that has a higher hydrogen concentration and/or a larger number of oxygen vacancies and that has a lower resistivity than the semiconductor film  508  is used as the conductive film  524 . 
     A film having a hydrogen concentration twice or more, preferably ten times or more that in the semiconductor film  508  can be used as the conductive film  524 . 
     A film having a resistivity greater than or equal to 1×10 −8  times and less than 1×10 −1  times that of the semiconductor film  508  can be used as the conductive film  524 . 
     Specifically, a film having a resistivity higher than or equal to 1×10 −3  Ωcm and lower than 1×10 4  Ωcm, preferably higher than or equal to 1×10 −3  Ωcm and lower than 1×10 −1  Ωcm can be used as the conductive film  524 . 
     &lt;Structure Example 2 of Display Panel&gt; 
     Another structure of the display panel of one embodiment of the present invention is described below. 
       FIGS. 7A to 7D  illustrate structures of a pixel circuit which can be used for the display panel of one embodiment of the present invention. The pixel circuit illustrated in  FIGS. 7A to 7D  can be used instead of the pixel circuit  730 ( i, j ) in  FIG. 3 . 
     Note that the pixel circuit  730 ( i, j ) illustrated in  FIG. 7A  differs from the pixel circuit  730 ( i, j ) illustrated in  FIG. 3  in that a second electrode of the capacitor C 2  is electrically connected to a second electrode of the transistor M, and that the transistor M does not include a second gate electrode. 
     The pixel circuit  730 ( i, j ) illustrated in  FIG. 7B  differs from the pixel circuit  730 ( i, j ) illustrated in  FIG. 7A  in that the pixel circuit  730 ( i, j ) is electrically connected to a wiring ML 1 ( j ) and includes a switch SW 3 . A transistor with the same structure as the switch SW 2  can be used as the switch SW 3 . A gate electrode of a transistor used as the switch SW 3  is electrically connected to the scan line G 2 ( i ), a first electrode thereof is electrically connected to the second electrode of the capacitor C 2 , and a second electrode thereof is electrically connected to the wiring ML 1 ( j ). Note that the wiring ML 1 ( j ) serves as a monitor line, for example. 
     The pixel circuit  730 ( i, j ) illustrated in  FIG. 7C  differs from the pixel circuit  730 ( i, j ) illustrated in  FIG. 7B  in that the pixel circuit  730 ( i, j ) is electrically connected to a scan line G 3 ( i ) and that the gate electrode of the transistor used as the switch SW 3  is electrically connected to the scan line G 3 ( i ). 
     The pixel circuit  730 ( i, j ) illustrated in  FIG. 7D  differs from the pixel circuit  730 ( i, j ) illustrated in  FIG. 7C  in that the pixel circuit  730 ( i, j ) is electrically connected to a wiring ML 2 ( j ), and that a first electrode of the transistor used as the switch SW 1  is electrically connected to the wiring ML 2 ( j ). Note that the wiring ML 2 ( j ) serves as a signal line and a monitor line, for example. Specifically, when a display panel including the pixel circuit  730 ( i, j ) illustrated in  FIG. 7D  is driven, the wiring ML 2 ( j ) serves as the signal line that transmits a signal for driving the first display element  750  in the period PT 1 , and serves as the monitor line that transmits a potential held in the capacitor C 2  in the period PT 2 . 
     &lt;Structure Example 3 of Display Panel&gt; 
       FIG. 8  illustrates the structure of the display panel of one embodiment of the present invention.  FIG. 8  is a cross-sectional view of a display panel  700 A of one embodiment of the present invention taken along lines X 1 -X 2 , X 3 -X 4 , X 5 -X 6 , X 7 -X 8 , X 9 -X 10 , and X 11 -X 12  in  FIG. 1A . 
     The display panel  700 A illustrated in  FIG. 8  differs from the display panel  700  illustrated in  FIG. 2A  in it includes the first electrode  751 ( i, j ) and the second electrode  752 ( i, j ) embedded in the second insulating film  501 C. 
     Specifically, the display panel  700 A includes a liquid crystal display element driven in an IPS mode or the like as the first display element  750 . 
     &lt;Structure Example 4 of Display Panel&gt; 
       FIGS. 9A and 9B  illustrate the structure of a display panel  700 B of one embodiment of the present invention.  FIG. 9A  is a cross-sectional view taken along lines X 1 -X 2 , X 3 -X 4 , X 5 -X 6 , X 7 -X 8 , X 9 -X 10 , and X 11 -X 12  in  FIG. 1A .  FIG. 9B  is a cross-sectional view illustrating part of the structure of the display panel  700 B. 
     The display panel  700 B differs from the display panel  700  in  FIGS. 2A to 2C  in that it includes a top-gate transistor instead of a bottom-gate transistor. Different structures will be described in detail below, and the above description is referred to for the other similar structures. 
     &lt;&lt;Switch SW 1 B and Transistors MB and MDB&gt;&gt; 
     Each of a transistor MB and a transistor MDB, which can be used as a switch SW 1 B, includes the conductive film  504  having a region overlapping with the second insulating film  501 C and the semiconductor film  508  having a region sandwiched between the second insulating film  501 C and the conductive film  504 . Note that the conductive film  504  serves as a gate electrode (see  FIG. 9B ). 
     The semiconductor film  508  has a first region  508 A, a second region  508 B, and a third region  508 C. The first region  508 A and the second region  508 B do not overlap with the conductive film  504 . The third region  508 C is positioned between the first region  508 A and the second region  508 B and overlaps with the conductive film  504 . 
     The transistor MDB includes an insulating film  506  between the third region  508 C and the conductive film  504 . Note that the insulating film  506  serves as a gate insulating film. 
     The first region  508 A and the second region  508 B have a lower resistivity than the third region  508 C, and function as a source region and a drain region. 
     Note that, for example, a method for controlling the resistivity of the oxide semiconductor, which is described in detail in this embodiment, can be used for forming the first region  508 A and the second region  508 B in the semiconductor film  508 . Specifically, plasma treatment using a gas containing a rare gas can be employed. 
     For example, the conductive film  504  can be a used as a mask. In that case, the third region  508 C can be formed in a self-aligned manner using the conductive film  504  as a mask. 
     The transistor MDB includes the conductive films  512 A and  512 B which are in contact with the first region  508 A and the second region  508 B, respectively. The conductive film  512 A and the conductive film  512 B function as a source electrode and a drain electrode. 
     A transistor that can be formed by the same step as the transistor MDB can be used as the transistor MB. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 2 
     In this embodiment, a method for manufacturing a display panel of one embodiment of the present invention will be described with reference to  FIG. 10 ,  FIG. 11 ,  FIG. 12 ,  FIG. 13 ,  FIG. 14 ,  FIG. 15 ,  FIG. 16 , and  FIG. 17 . 
       FIG. 10  is a flow chart showing a method for manufacturing the display panel  700  of one embodiment of the present invention.  FIG. 11  to  FIG. 17  are cross-sectional views of the display panel  700  in the manufacturing process taken along lines X 1 -X 2 , X 3 -X 4 , X 5 -X 6 , X 7 -X 8 , X 9 -X 10 , and X 11 -X 12  in  FIG. 2A . 
     &lt;Method for Manufacturing Display Panel&gt; 
     The method for manufacturing the display panel  700  described in this embodiment includes the following 11 steps. 
     &lt;&lt;Step 1&gt;&gt; 
     In a step 1, the first insulating film  501 A is formed over a substrate for use in manufacturing processes (see U 1  in  FIG. 10 ). For example, the first insulating film  501 A is formed such that a separation film  510 W is provided between the insulating film  501 A and a substrate  510 . 
     The substrate for use in manufacturing processes can include, for example, the substrate  510  and the separation film  510 W having a region overlapping with the substrate  510 . 
     The substrate  510  can be formed using a material having heat resistance high enough to withstand heat treatment in the manufacturing process. 
     For example, a large-sized glass substrate having any of the following sizes can be used as the substrate  510 : the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured. 
     For the substrate  510 , an organic material, an inorganic material, a composite material of an organic material and an inorganic material, or the like can be used. For example, an inorganic material such as glass, ceramic, or metal can be used for the substrate  510 . 
     Specifically, non-alkali glass, soda-lime glass, potash glass, crystal glass, quartz, sapphire, or the like can be used for the substrate  510 . Specifically, an inorganic oxide, an inorganic nitride, an inorganic oxynitride, or the like can be used for the substrate  510 . For example, a silicon oxide, a silicon nitride, a silicon oxynitride, or an aluminum oxide can be used for the substrate  510 . Stainless steel, aluminum, or the like can be used for the substrate  510 . 
     For example, an organic material such as a resin, a resin film, or plastic can be used for the substrate  510 . Specifically, a resin film or resin plate of polyester, polyolefin, polyamide, polyimide, polycarbonate, an acrylic resin, or the like can be used for the substrate  510 . 
     For example, a composite material such as a resin film to which a metal plate, a thin glass plate, or a film of an inorganic material or the like is attached can be used for the substrate  510 . For example, a composite material formed by dispersing a fibrous or particulate metal, glass, inorganic material, or the like into a resin film can be used for the substrate  510 . For example, a composite material formed by dispersing a fibrous or particulate resin, organic material, or the like into an inorganic material can be used for the substrate  510 . 
     A single-layer material or a stacked-layer material in which a plurality of layers are stacked can be used for the substrate  510 . For example, a stacked-layer material in which a base, an insulating film that prevents diffusion of impurities contained in the base, and the like are stacked can be used for the substrate  510 . 
     For example, the separation film  510 W can be formed using a material that allows the first insulating film  501 A to be separated from the substrate  510  in the step 9. 
     Note that the first insulating film  501 A can be separated from the substrate  510  while the separation film  510 W remains on the substrate  510  side. Alternatively, the separation film  510 W can be separated together with the first insulating film  501 A from the substrate  510 . 
     Specifically, the first insulating film  501 A can be separated from the substrate  510  while the separation film  510 W remains on the substrate  510  side in the case where the substrate  510 , the separation film  510 W, and the first insulating film  501 A are formed using a non-alkali glass substrate, a film containing tungsten or the like, and a film containing inorganic oxide or inorganic oxynitride, respectively. 
     The separation film  510 W can be separated together with the first insulating film  501 A from the substrate  510  when the substrate  510 , the separation film  510 W, and the first insulating film  501 A are formed using a non-alkali glass substrate, a film containing polyimide, and a film containing various materials, respectively. 
     For example, the first insulating film  501 A is formed in contact with the separation film  510 W. Specifically, a chemical vapor deposition method, a sputtering method, a coating method, or the like can be employed. Then, unnecessary portions are removed by a photolithography process or the like, so that the first insulating film  501 A is completed. 
     Note that it is preferable that the first insulating film  501 A be larger than the separation film  510 W so that the peripheral portion of the first insulating film  501 A is in contact with the substrate  510 , in which case occurrence of unintended separation of the first insulating film  501 A from the substrate for use in manufacturing processes can be reduced. 
     Specifically, a 0.7-mm-thick glass plate is used as the substrate  510 , and a stacked-layer material in which a 200-nm-thick silicon oxynitride film and a 30-nm-thick tungsten film are stacked in this order from the substrate  510  side is used for the separation film  510 W. In addition, a film including a stacked-layer material in which a 600-nm-thick silicon oxynitride film and a 200-nm-thick silicon nitride film are stacked in this order from the separation film  510 W side can be used for the first insulating film  501 A. Note that a silicon oxynitride film refers to a film that includes more oxygen than nitrogen, and a silicon nitride oxide film refers to a film that includes more nitrogen than oxygen. 
     Specifically, instead of the first insulating film  501 A, a film including a stacked-layer material in which a 600-nm-thick silicon oxynitride film, a 200-nm-thick silicon nitride film, a 200-nm-thick silicon oxynitride film, a 140-nm-thick silicon nitride oxide film, and a 100-nm-thick silicon oxynitride film are stacked in this order from the separation film  510 W side can be used. 
     &lt;&lt;Step 2&gt;&gt; 
     In a step 2, a conductive film and a terminal are formed (see U 2  in  FIG. 10 ). Note that in this embodiment, an example in which the conductive film has a stacked structure of a transparent conductive film and a reflective film, the first electrode  751 ( i, j ) is used as the transparent conductive film, and the electrode  751 A(i, j) is used as the reflective film is described. 
     The reflective film includes the opening  751 H. The terminals include the terminals  519 B and  519 C. 
     For example, a film containing a conductive material is formed in contact with the first insulating film  501 A. Specifically, a chemical vapor deposition method, a sputtering method, a coating method, or the like can be employed. Then, unnecessary portions are removed by a photolithography process or the like, so that the first electrode  751  used as the reflective film and the terminals  519 B and  519 C are completed. 
     &lt;&lt;Step 3&gt;&gt; 
     In a step 3, the second insulating film  501 C covering the conductive film and the terminal is formed (see U 3  in  FIG. 10 ). Note that a third insulating film having a region overlapping with the second insulating film  501 C may be formed successively after the second insulating film  501 C is formed. 
     The second insulating film  501 C includes an opening. 
     For example, a film having a function of preventing impurity diffusion is formed to cover the reflective film and the terminal. Specifically, a chemical vapor deposition method, a sputtering method, a coating method, or the like can be employed. 
     Then, an opening reaching the electrode  751 A and an opening reaching the terminals  519 B and  519 C are formed by a photolithography process or the like, so that the second insulating film  501 C is completed. 
     &lt;&lt;Step 4&gt;&gt; 
     In a step 4, the first connection portion  591 A and the third connection portions  591 B and  591 C are formed (see U 4  in  FIG. 10  and  FIG. 11 ). The reflective film is electrically connected to the first connection portion  591 A. The terminals  519 B and  519 C are electrically connected to the third connection portions  591 B and  591 C. Note that the conductive film  504  serving as a gate electrode of the transistor M, the transistor MD, or the transistor that can be used as the switch SW 1  may be formed together with the first connection portion  591 A and the terminals  519 B and  519 C. 
     A film containing a conductive material is formed to be in contact with the second insulating film  501 C, the opening reaching the electrode  751 A, and the opening reaching the terminals  519 B and  519 C. Specifically, a chemical vapor deposition method, a sputtering method, a coating method, or the like can be employed. 
     Then, unnecessary portions are removed by a photolithography process or the like, so that the first connection portion  591 A, third connection portions  593 B and  593 C, and the conductive film  504  are completed. 
     &lt;&lt;Step 5&gt;&gt; 
     In a step 5, a pixel circuit electrically connected to the first connection  591  and the third connection portion  593  is formed (see U 5  in  FIG. 10 ). 
     A film containing a conductive material, a film containing an insulating material, a film containing a semiconductor material, and the like are formed by a chemical vapor deposition method, a sputtering method, or the like. Then, unnecessary portions of the films are removed by a photolithography process or the like. With combination of a deposition method and a photolithography process or the like, the pixel circuit including the transistor M, the transistor MD, the transistor serving as the switch SW 1 , and the like is completed. 
     Next, the insulating films  516  and  518  protecting elements, such as transistors, of the pixel circuit are formed. Furthermore, the conductive film  524  serving as a second gate electrode is formed between the insulating films  516  and  518 . 
     Then, the insulating film  521  is formed. An opening reaching the pixel circuit is formed in the insulating films  516 ,  518 , and  521 . 
     &lt;&lt;Step 6&gt;&gt; 
     In a step 6, the second connection portion  592  electrically connected to the pixel circuit is formed (see U 6  in  FIG. 10  and  FIG. 12 ). Note that a wiring may be formed together with the second connection portion  592 . 
     For example, a film containing a conductive material is formed. Specifically, a chemical vapor deposition method, a sputtering method, a coating method, or the like can be employed. 
     Then, unnecessary portions of the films are removed by a photolithography process or the like to form the second connection portion  592 . 
     &lt;&lt;Step 7&gt;&gt; 
     In a step 7, the second display element  550  electrically connected to the second connection portion  592  is formed (see U 7  in  FIG. 10  and  FIG. 13 ). 
     The third electrode  551 ( i, j ) is formed to be electrically connected to the second connection portion  592 . For example, a film containing a conductive material is formed. Specifically, a chemical vapor deposition method, a sputtering method, or the like can be employed. Then, unnecessary portions are removed by a photolithography process or the like, so that the third electrode  551 ( i, j ) is completed. 
     Next, the insulating film  528  having an opening in a region overlapping with the third electrode  551 ( i, j ) is formed. Note that the ends of the third electrode  551 ( i, j ) are covered with the insulating film  528 . For example, a photosensitive polymer film is formed. Specifically, a coating method or the like can be employed. Then, unnecessary portions are removed by a photolithography process or the like, so that the insulating film  528  is completed. 
     Then, a layer  553 ( j ) containing a light-emitting organic compound is formed to cover the third electrode  551 ( i, j ) exposed in the opening of the insulating film  528 . Specifically, an evaporation method, a printing method, an ink-jet method, or the like using a shadow mask can be used. 
     Then, the fourth electrode  552  is formed such that the layer  553 ( j ) containing a light-emitting organic compound is sandwiched between the third electrode  551 ( i, j ) and the fourth electrode  552 . Specifically, an evaporation method, a sputtering method, or the like using a shadow mask can be used. 
     &lt;&lt;Step 8&gt;&gt; 
     In a step 8, the substrate  570  is stacked (see U 8  in  FIG. 10  and  FIG. 14 ). 
     A fluid resin or the like is applied to form the bonding layer  505 . Specifically, a coating method, a printing method, an ink-jet method, or the like can be used. Alternatively, a sheet-like fluid resin or the like is bonded to form the bonding layer  505 . 
     Then, the functional layer  520  and the substrate  570  are bonded using the bonding layer  505 . 
     &lt;&lt;Step 9&gt;&gt; 
     In a step 9, the substrate  510  for use in manufacturing processes is separated (see U 9  in  FIG. 10  and  FIG. 15 ). 
     For example, part of the separation film  510 W is separated from the first insulating film  501 A by sticking a sharp tip into the separation film  510 W from the substrate  510  for use in manufacturing processes, by a method using a laser (e.g., a laser ablation method), or the like, whereby a separation starting point can be formed. 
     Then, the substrate  510  for use in manufacturing processes is gradually separated from the separation starting point. 
     Note that the separation may be performed while the vicinity of the interface between the separation film  510 W and the first insulating film  501 A is irradiated with ions to remove static electricity. Specifically, the ions may be generated by an ionizer. Alternatively, a liquid may be ejected and sprayed from a nozzle to the interface between the separation film  510 W and the first insulating film  501 A. For example, as the liquid to be injected or the liquid to be sprayed, water, a polar solvent, or a liquid that dissolves the separation film  510 W can be used. By injecting such a liquid, influence of static electricity and the like accompanying the separation can be reduced. 
     Particularly in the case where a film containing tungsten oxide is used for the separation film  510 W, the substrate  510  is separated while a water-containing liquid is injected or sprayed, which leads to a reduction in stress caused by separation. 
     &lt;&lt;Step 10&gt;&gt; 
     In a step 10, the first insulating film  501 A is removed to expose the reflective film and the terminal (see U 10  in  FIG. 10  and  FIG. 16 ). 
     For example, the first insulating film  501 A can be removed by etching or chemical mechanical polishing. Specifically, a wet etching method, a dry etching method, or the like can be used. 
     &lt;&lt;Step 11&gt;&gt; 
     In a step 11, the first display element is formed (see U 11  in  FIG. 10  and  FIG. 17 ). 
     A counter substrate is prepared. Specifically, the substrate  770  including the light blocking film BM, the color film CF 1 , the insulating film  771 , the second electrode  752 , the structure KB 1 , and the alignment film AF 2  is prepared as the counter substrate. 
     Next, the alignment film AF 1  having a region overlapping with the second insulating film  501 C and the first electrode  751 ( i, j ) is formed. Specifically, the alignment film AF 1  is formed by a printing method, a rubbing method, or the like. 
     The sealant  705  is formed. Specifically, a fluid resin is applied to form a frame-like shape using a dispensing method, a printing method, or the like. Note that a material containing the conductive member CP is applied to a region of the sealant  705  overlapping with the terminal  519 C. 
     Next, a liquid crystal material is dropped into a region surrounded by the frame-shaped sealant  705 . Specifically, a dispenser method or the like is used. 
     Then, the substrate  770  is bonded to the second insulating film  501 C using the sealant  705 . Note that the structure KB 1  is provided between the second insulating film  501 C and the substrate  770  to electrically connect the terminal  519 C and the second electrode  752  using the conductive member CP. 
     The method for manufacturing the display panel  700  in this embodiment includes the step of separating the substrate  510  for use in manufacturing processes and the step of removing the first insulating film  501 A to expose the reflective film and the terminal. Accordingly, a step at the edge of the reflective film can be minimized to reduce the possibility of alignment defects due to the step. In addition, the surface of the terminal at which contact with other components is made can be exposed. Thus, a method for manufacturing a novel display panel that is highly convenient or reliable can be provided. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 3 
     In this embodiment, the structure of a transistor which can be used for the display panel of one embodiment of the present invention will be described with reference to  FIGS. 18A to 18D . 
     &lt;Structural Example of Semiconductor Device&gt; 
       FIG. 18A  is a top view of the transistor  100 .  FIG. 18C  is a cross-sectional view taken along the section line X 1 -X 2  in  FIG. 18A , and  FIG. 18D  is a cross-sectional view taken along the section line Y 1 -Y 2  in  FIG. 18A . Note that in  FIG. 18A , some components of the transistor  100  (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. In some cases, the direction of the section line Y 1 -Y 2  is referred to as a channel length direction and the direction of the section line X 1 -X 2  is referred to as a channel width direction. As in  FIG. 18A , some components might not be illustrated in some top views of transistors described below. 
     Note that the transistor  100  can be used in the display panel described in Embodiment 1. 
     For example, when the transistor  100  is used as the transistor M, a substrate  102 , a conductive film  104 , a stacked film of an insulating film  106  and an insulating film  107 , an oxide semiconductor film  108 , a conductive film  112   a , a conductive film  112   b , a stacked film of an insulating film  114  and an insulating film  116 , and an insulating film  118  can be referred to as the second insulating film  501 C, the conductive film  504 , the insulating film  506 , the semiconductor film  508 , the conductive film  512 A, the conductive film  512 B, an insulating film  516 , and the insulating film  518 , respectively. 
     The transistor  100  includes a conductive film  104  functioning as a gate electrode over a substrate  102 , an insulating film  106  over the substrate  102  and the conductive film  104 , an insulating film  107  over the insulating film  106 , an oxide semiconductor film  108  over the insulating film  107 , and conductive films  112   a  and  112   b  functioning as source and drain electrodes electrically connected to the oxide semiconductor film  108 . Over the transistor  100 , specifically, over the conductive films  112   a  and  112   b  and the oxide semiconductor film  108 , insulating films  114 ,  116 , and  118  are provided. The insulating films  114 ,  116 , and  118  function as protective insulating films for the transistor  100 . 
     The oxide semiconductor film  108  includes a first oxide semiconductor film  108   a  on the conductive film  104  side and a second oxide semiconductor film  108   b  over the first oxide semiconductor film  108   a . Furthermore, the insulating films  106  and  107  function as gate insulating films of the transistor  100 . 
     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 oxide semiconductor film  108 . It is particularly preferable to use an In-M-Zn oxide for the semiconductor film  108 . 
     The first oxide semiconductor film  108   a  includes a first region in which the atomic proportion of In is larger than the atomic proportion of M The second oxide semiconductor film  108   b  includes a second region in which the atomic proportion of In is smaller than that in the first oxide semiconductor film  108   a . The second region include a portion thinner than the first region. 
     The first oxide semiconductor film  108   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  100 . Specifically, the field-effect mobility of the transistor  100  can exceed 10 cm 2 /Vs. 
     For example, the use of the transistor with high field-effect mobility for a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame. 
     On the other hand, the first oxide semiconductor film  108   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  100  in light irradiation. However, in the semiconductor device of one embodiment of the present invention, the second oxide semiconductor film  108   b  is formed over the first oxide semiconductor film  108   a . In addition, the thickness of a portion including a channel region and the vicinity of the channel region in the second oxide semiconductor film  108   b  is smaller than the thickness of the first oxide semiconductor film  108   a.    
     Furthermore, the second oxide semiconductor film  108   b  includes the second region in which the atomic proportion of In is smaller than the first oxide semiconductor film  108   a  and thus has larger Eg than that of the first oxide semiconductor film  108   a . For this reason, the oxide semiconductor film  108  which is a layered structure of the first oxide semiconductor film  108   a  and the second oxide semiconductor film  108   b  has high resistance to a negative bias stress test with light irradiation. 
     The amount of light absorbed by the oxide semiconductor film  108  can be reduced during light irradiation. As a result, the change in electrical characteristics of the transistor  100  due to light irradiation can be reduced. In the semiconductor device of one embodiment of the present invention, the insulating film  114  or the insulating film  116  includes excess oxygen. This structure can further reduce the change in electrical characteristics of the transistor  100  due to light irradiation. 
     Here, the oxide semiconductor film  108  is described in detail with reference to  FIG. 18B . 
       FIG. 18B  is a cross-sectional enlarged view of the oxide semiconductor film  108  and the vicinity thereof in the transistor  100  illustrated in  FIG. 18C . 
     In  FIG. 18B , t1, t2-1, and t2-2 denote a thickness of the first oxide semiconductor film  108   a , one thickness of the second oxide semiconductor film  108   b , and the other thickness of the second oxide semiconductor film  108   b , respectively. The second oxide semiconductor film  108   b  over the first oxide semiconductor film  108   a  prevents the first oxide semiconductor film  108   a  from being exposed to an etching gas, an etchant, or the like when the conductive films  112   a  and  112   b  are formed. This is why the first oxide semiconductor film  108   a  is not or is hardly reduced in thickness. In contrast, in the second oxide semiconductor film  108   b , a portion not overlapping with the conductive films  112   a  and  112   b  is etched by formation of the conductive films  112   a  and  112   b , so that a depression is formed in the etched region. In other words, a thickness of the second oxide semiconductor film  108   b  in a region overlapping with the conductive films  112   a  and  112   b  is t2-1, and a thickness of the second oxide semiconductor film  108   b  in a region not overlapping with the conductive films  112   a  and  112   b  is t2-2. 
     As for the relationships between the thicknesses of the first oxide semiconductor film  108   a  and the second oxide semiconductor film  108   b , t2-1&gt;t1&gt;t2-2 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 vacancy is formed in the oxide semiconductor film  108  included in the transistor  100 , electrons serving as carriers are generated; as a result, the transistor  100  tends to be normally-on. Therefore, for stable transistor characteristics, it is important to reduce oxygen vacancy in the oxide semiconductor film  108  particularly oxygen vacancy in the first oxide semiconductor film  108   a . In the structure of the transistor of one embodiment of the present invention, excess oxygen is introduced into an insulating film over the oxide semiconductor film  108 , here, the insulating film  114  and/or the insulating film  116  over the oxide semiconductor film  108 , whereby oxygen is moved from the insulating film  114  and/or the insulating film  116  to the oxide semiconductor film  108  to fill oxygen vacancy in the oxide semiconductor film  108  particularly in the first oxide semiconductor film  108   a.    
     It is preferable that the insulating films  114  and  116  each include a region (oxygen excess region) including oxygen in excess of that in the stoichiometric composition. In other words, the insulating films  114  and  116  are insulating films capable of releasing oxygen. Note that the oxygen excess region is formed in the insulating films  114  and  116  in such a manner that oxygen is introduced into the insulating films  114  and  116  after the deposition, for example. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed. 
     In order to fill oxygen vacancy in the first oxide semiconductor film  108   a , the thickness of the portion including the channel region and the vicinity of the channel region in the second oxide semiconductor film  108   b  is preferably small, and t2-2&lt;t1 is preferably satisfied. For example, the thickness of the portion including the channel region and the vicinity of the channel region in the second oxide semiconductor film  108   b  is preferably more than or equal to 1 nm and less than or equal to 20 nm, more preferably more than or equal to 3 nm and less than or equal to 10 nm. 
     Other constituent elements of the semiconductor device of this embodiment are described below in detail. 
     &lt;&lt;Substrate&gt;&gt; 
     There is no particular limitation on the property of a material and the like of the substrate  102  as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate  102 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon or silicon carbide, a compound semiconductor substrate of silicon germanium, an SOI substrate, or the like can be used as the substrate  102 . Alternatively, any of these substrates provided with a semiconductor element, an insulating film, or the like may be used as the substrate  102 . In the case where a glass substrate is used as the substrate  102 , a large substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large display device can be manufactured. 
     Alternatively, a flexible substrate may be used as the substrate  102 , and the transistor  100  may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate  102  and the transistor  100 . The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate  102  and transferred onto another substrate. In such a case, the transistor  100  can be transferred to a substrate having low heat resistance or a flexible substrate as well. 
     &lt;&lt;Conductive Film Functioning as Gate Electrode and Source and Drain Electrodes&gt;&gt; 
     The conductive film  104  functioning as a gate electrode and the conductive films  112   a  and  112   b  functioning as a source electrode and a drain electrode, respectively, can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal element as its component; an alloy including a combination of any of these metal elements; or the like. 
     Furthermore, the conductive films  104 ,  112   a , and  112   b  may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, and a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order can be given. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used. 
     The conductive films  104 ,  112   a , and  112   b  can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. 
     A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films  104 ,  112   a , and  112   b . Use of a Cu—X alloy film enables the manufacturing cost to be reduced because wet etching process can be used in the processing. 
     &lt;&lt;Insulating Film Functioning as Gate Insulating Film&gt;&gt; 
     As each of the insulating films  106  and  107  functioning as gate insulating films of the transistor  100 , an insulating film including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that instead of a stacked-layer structure of the insulating films  106  and  107 , an insulating film of a single layer formed using a material selected from the above or an insulating film of three or more layers may be used. 
     The insulating film  106  has a function as a blocking film which inhibits penetration of oxygen. For example, in the case where excess oxygen is supplied to the insulating film  107 , the insulating film  114 , the insulating film  116 , and/or the oxide semiconductor film  108 , the insulating film  106  can inhibit penetration of oxygen. 
     Note that the insulating film  107  that is in contact with the oxide semiconductor film  108  functioning as a channel region of the transistor  100  is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film  107  is an insulating film capable of releasing oxygen. In order to provide the oxygen excess region in the insulating film  107 , the insulating film  107  is formed in an oxygen atmosphere, for example. Alternatively, the oxygen excess region may be formed by introduction of oxygen into the insulating film  107  after the deposition. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed. 
     In the case where hafnium oxide is used for the insulating film  107 , the following effect is attained. Hafnium oxide has a higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, by using hafnium oxide, the thickness of the insulating film  107  can be made large as compared with the case where silicon oxide is used; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide with a crystalline structure has higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with a low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited thereto. 
     In this embodiment, a silicon nitride film is formed as the insulating film  106 , and a silicon oxide film is formed as the insulating film  107 . The silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the silicon nitride film is included in the gate insulating film of the transistor  100 , the physical thickness of the insulating film can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor  100  and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor  100 . 
     &lt;&lt;Oxide Semiconductor Film&gt;&gt; 
     The oxide semiconductor film  108  can be formed using the materials described above. 
     In the case where the oxide semiconductor film  108  includes In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In≥M and Zn≥M. As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, and In:M:Zn=4:2:4.1 are preferable. 
     In the case where the oxide semiconductor film  108  is formed of In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor film  108  having crystallinity. Note that the atomic ratios of metal elements in the formed oxide semiconductor film  108  vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error. For example, when a sputtering target with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the oxide semiconductor film  108  may be 4:2:3 or in the vicinity of 4:2:3. 
     The oxide semiconductor film  108   a  can be formed using the sputtering target having an atomic ratio of In:M:Zn=2:1:3, In:M:Zn=3:1:2, or In:M:Zn=4:2:4.1. The second oxide semiconductor film  108   b  can be formed using the sputtering target having an atomic ratio of In:M:Zn=1:1:1 or In:M:Zn=1:1:1.2. Note that the atomic ratio of metal elements in a sputtering target used for forming the second oxide semiconductor film  108   b  does not necessarily satisfy In≥M and Zn≥M, and may satisfy In≥M and Zn&lt;M, such as In:M:Zn=1:3:2. 
     The energy gap of the oxide semiconductor film  108  is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. The use of an oxide semiconductor having a wide energy gap can reduce off-state current of the transistor  100 . In particular, an oxide semiconductor film having an energy gap more than or equal to 2 eV, preferably more than or equal to 2 eV and less than or equal to 3.0 eV is preferably used as the first oxide semiconductor film  108   a , and an oxide semiconductor film having an energy gap more than or equal to 2.5 eV and less than or equal to 3.5 eV is preferably used as the second oxide semiconductor film  108   b . Furthermore, the second oxide semiconductor film  108   b  preferably has a higher energy gap than that of the first oxide semiconductor film  108   a.    
     Each thickness of the oxide semiconductor film  108   a  and the second oxide semiconductor film  108   b  is more than or equal to 3 nm and less than or equal to 200 nm, preferably more than or equal to 3 nm and less than or equal to 100 nm, more preferably more than or equal to 3 nm and less than or equal to 50 nm. Note that the above-described thickness relationships between them are preferably satisfied. 
     An oxide semiconductor film with low carrier density is used as the second oxide semiconductor film  108   b . For example, the carrier density of the second oxide semiconductor film  108   b  is lower than or equal to 1×10 17 /cm 3 , preferably lower than or equal to 1×10 15 /cm 3 , further preferably lower than or equal to 1×10 13 /cm 3 , still further preferably lower than or equal to 1×10 11 /cm 3 . 
     Note that, without limitation to the compositions and materials described above, a material with an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Furthermore, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the first oxide semiconductor film  108   a  and the second oxide semiconductor film  108   b  be set to be appropriate. 
     Note that it is preferable to use, as the first oxide semiconductor film  108   a  and the second oxide semiconductor film  108   b , an oxide semiconductor film in which the impurity concentration is low and the density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancy is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and thus has a low density of trap states in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10 6  μm and a channel length of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10 −13  A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. 
     Accordingly, the transistor in which the channel region is formed in the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small change in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, and the like are given. 
     Hydrogen included in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen be reduced as much as possible in the oxide semiconductor film  108 . Specifically, in the oxide semiconductor film  108 , the concentration of hydrogen which is measured by SIMS is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , further preferably lower than or equal to 1×10 19  atoms/cm 3 , further preferably lower than or equal to 5×10 18  atoms/cm 3 , further preferably lower than or equal to 1×10 18  atoms/cm 3 , further preferably lower than or equal to 5×10 17  atoms/cm 3 , and further preferably lower than or equal to 1×10 16  atoms/cm 3 . 
     The first oxide semiconductor film  108   a  preferably has a region in which hydrogen concentration is smaller than that in the second oxide semiconductor film  108   b . A semiconductor device including the first oxide semiconductor film  108   a  having the region in which hydrogen concentration is smaller than that in the second oxide semiconductor film  108   b  can be increased in reliability. 
     When silicon or carbon that is one of elements belonging to Group 14 is included in the first oxide semiconductor film  108   a , oxygen vacancy is increased in the first oxide semiconductor film  108   a , and the first oxide semiconductor film  108   a  becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the first oxide semiconductor film  108   a  or the concentration of silicon or carbon (the concentration is measured by SIMS) in the vicinity of an interface with the first oxide semiconductor film  108   a  is set to be lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     In addition, the concentration of alkali metal or alkaline earth metal of the first oxide semiconductor film  108   a , which is measured by SIMS, is lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the first oxide semiconductor film  108   a.    
     Furthermore, when including nitrogen, the first oxide semiconductor film  108   a  easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set to be, for example, lower than or equal to 5×10 18  atoms/cm 3 . 
     Each of the first and second oxide semiconductor films  108   a  and  108   b  may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states. 
     &lt;&lt;Insulating Film Functioning as Protective Insulating Film of Transistor&gt;&gt; 
     The insulating films  114  and  116  each have a function of supplying oxygen to the oxide semiconductor film  108 . The insulating film  118  has a function of a protective insulating film of the transistor  100 . The insulating films  114  and  116  include oxygen. Furthermore, the insulating film  114  is an insulating film which can transmit oxygen. The insulating film  114  also serves as a film which relieves damage to the oxide semiconductor film  108  at the time of forming the insulating film  116  in a later step. 
     A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the insulating film  114 . 
     In addition, it is preferable that the number of defects in the insulating film  114  be small and typically, the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×10 17  spins/cm 3  by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating film  114  is high, oxygen is bonded to the defects and the amount of oxygen that transmits the insulating film  114  is decreased. 
     Note that all oxygen entering the insulating film  114  from the outside does not move to the outside of the insulating film  114  and some oxygen remains in the insulating film  114 . Furthermore, movement of oxygen occurs in the insulating film  114  in some cases in such a manner that oxygen enters the insulating film  114  and oxygen included in the insulating film  114  moves to the outside of the insulating film  114 . When an oxide insulating film which can transmit oxygen is formed as the insulating film  114 , oxygen released from the insulating film  116  provided over the insulating film  114  can be moved to the oxide semiconductor film  108  through the insulating film  114 . 
     Note that the insulating film  114  can be formed using an oxide insulating film having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the energy of the valence band maximum (E v_os ) and the energy of the conduction band minimum (E c_os ) of the oxide semiconductor film. A silicon oxynitride film that releases less nitrogen oxide, an aluminum oxynitride film that releases less nitrogen oxide, and the like can be used as the above oxide insulating film. 
     Note that a silicon oxynitride film that releases less nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in TDS analysis; the amount of released ammonia is typically greater than or equal to 1×10 18 /cm 3  and less than or equal to 5×10 19 /cm 3 . Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of a film becomes higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C. 
     Nitrogen oxide (NO x ; x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2), typically NO 2  or NO, forms levels in the insulating film  114 , for example. The level is positioned in the energy gap of the oxide semiconductor film  108 . Therefore, when nitrogen oxide is diffused to the interface between the insulating film  114  and the oxide semiconductor film  108 , an electron is in some cases trapped by the level on the insulating film  114  side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film  114  and the oxide semiconductor film  108 ; thus, the threshold voltage of the transistor is shifted in the positive direction. 
     Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide included in the insulating film  114  reacts with ammonia included in the insulating film  116  in heat treatment, nitrogen oxide included in the insulating film  114  is reduced. Therefore, an electron is hardly trapped at the vicinity of the interface between the insulating film  114  and the oxide semiconductor film  108 . 
     By using such an oxide insulating film, the insulating film  114  can reduce the shift in the threshold voltage of the transistor, which leads to a smaller change in the electrical characteristics of the transistor. 
     Note that in an ESR spectrum at 100 K or lower of the insulating film  114 , by heat treatment of a manufacturing process of the transistor, typically heat treatment at a temperature higher than or equal to 300° C. and lower than 350° C., a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10 18  spins/cm 3 , typically higher than or equal to 1×10 17  spins/cm 3  and lower than 1×10 18  spins/cm 3 . 
     In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO x ; x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. In other words, the lower the total spin density of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is. 
     The concentration of nitrogen of the above oxide insulating film measured by SIMS is lower than or equal to 6×10 20  atoms/cm 3 . 
     The above oxide insulating film is formed by a PECVD method at a film surface temperature higher than or equal to 220° C. and lower than or equal to 350° C. using silane and dinitrogen monoxide, whereby a dense and hard film can be formed. 
     The insulating film  116  is formed using an oxide insulating film that contains oxygen in excess of that in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film including oxygen in excess of that in the stoichiometric composition. The oxide insulating film including oxygen in excess of that in the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 10  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3  in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. 
     A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used as the insulating film  116 . 
     It is preferable that the number of defects in the insulating film  116  be small, and typically the spin density corresponding to a signal which appears at g=2.001 due to a dangling bond of silicon be lower than 1.5×10 18  spins/cm 3 , preferably lower than or equal to 1×10 18  spins/cm 3  by ESR measurement. Note that the insulating film  116  is provided more apart from the oxide semiconductor film  108  than the insulating film  114  is; thus, the insulating film  116  may have higher density of defects than the insulating film  114 . 
     Furthermore, the insulating films  114  and  116  can be formed using insulating films formed of the same kinds of materials; thus, a boundary between the insulating films  114  and  116  cannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating films  114  and  116  is shown by a dashed line. Although a two-layer structure of the insulating films  114  and  116  is described in this embodiment, the present invention is not limited to this. For example, a single-layer structure of the insulating film  114  may be employed. 
     The insulating film  118  includes nitrogen. Alternatively, the insulating film  118  includes nitrogen and silicon. The insulating film  118  has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor film  108 , outward diffusion of oxygen included in the insulating films  114  and  116 , and entry of hydrogen, water, or the like into the oxide semiconductor film  108  from the outside by providing the insulating film  118 . A nitride insulating film, for example, can be used as the insulating film  118 . The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like. Note that instead of the nitride insulating film having a blocking effect against oxygen, hydrogen, water, alkali metal, alkaline earth metal, and the like, an oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like may be provided. As the oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, a hafnium oxynitride film, and the like can be given. 
     Although the variety of films such as the conductive films, the insulating films, and the oxide semiconductor films which are described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal CVD method. Examples of the thermal CVD method include a metal organic chemical vapor deposition (MOCVD) method and an atomic layer deposition (ALD) method. 
     A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film. 
     Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at a time so that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and react with each other in the vicinity of the substrate or over the substrate. 
     Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer; then the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET. 
     The variety of films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films in this embodiment can be formed by a thermal CVD method such as an MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH 3 ) 3 . The chemical formula of trimethylgallium is Ga(CH 3 ) 3 . The chemical formula of dimethylzinc is Zn(CH 3 ) 2 . Without limitation to the above combination, triethylgallium (chemical formula: Ga(C 2 H 5 ) 3 ) can be used instead of trimethylgallium and diethylzinc (chemical formula: Zn(C 2 H 5 ) 2 ) can be used instead of dimethylzinc. 
     For example, in the case where a hafnium oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, that is, ozone (O 3 ) as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (e.g., a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH 3 ) 2 ] 4 . Examples of another material liquid include tetrakis(ethylmethylamide)hafnium. 
     For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H 2 O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH 3 ) 3 . Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate). 
     For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine included in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O 2  or dinitrogen monoxide) are supplied to react with the adsorbate. 
     For example, in the case where a tungsten film is formed using a deposition apparatus using an ALD method, a WF 6  gas and a B 2 H 6  gas are sequentially introduced plural times to form an initial tungsten film, and then a WF 6  gas and an H 2  gas are used, so that a tungsten film is formed. Note that an SiH 4  gas may be used instead of a B 2 H 6  gas. 
     For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed using a deposition apparatus using an ALD method, an In(CH 3 ) 3  gas and an O 3  gas) are sequentially introduced plural times to form an InO layer, a GaO layer is formed using a Ga(CH 3 ) 3  gas and an O 3  gas), and then a ZnO layer is formed using a Zn(CH 3 ) 2  gas and an O 3  gas). Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases. Note that although an H 2 O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O 3  gas), it is preferable to use an O 3  gas), which does not contain H. Furthermore, instead of an In(CH 3 ) 3  gas, an In(C 2 H 5 ) 3  gas may be used. Instead of a Ga(CH 3 ) 3  gas, a Ga(C 2 H 5 ) 3  gas may be used. Furthermore, a Zn(CH 3 ) 2  gas may be used. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 4 
     In this embodiment, structures of a transistor that can be used in the display panel of one embodiment of the present invention will be described with reference to  FIGS. 19A to 19C . 
     &lt;Structure Example of Semiconductor Device&gt; 
       FIG. 19A  is a top view of the transistor  100 .  FIG. 19B  is a cross-sectional view taken along the cutting plane line X 1 -X 2  in  FIG. 19A , and  FIG. 19C  is a cross-sectional view taken along the cutting plane line Y 1 -Y 2  in  FIG. 19A . Note that in  FIG. 19A , some components of the transistor  100  (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. Furthermore, the direction of the cutting plane line X 1 -X 2  may be called a channel length direction, and the direction of the cutting plane line Y 1 -Y 2  may be called a channel width direction. As in  FIG. 19A , some components are not illustrated in some cases in top views of transistors described below. 
     The transistor  100  can be used for the display panel described in Embodiment 1. 
     For example, when the transistor  100  is used as the transistor MD, the substrate  102 , the conductive film  104 , a stacked film of the insulating film  106  and the insulating film  107 , the oxide semiconductor film  108 , the conductive film  112   a , the conductive film  112   b , a stacked film of the insulating film  114  and the insulating film  116 , the insulating film  118 , and a conductive film  120   b  can be referred to as the second insulating film  501 C, the conductive film  504 , the insulating film  506 , the semiconductor film  508 , the conductive film  512 A, the conductive film  512 B, the insulating film  516 , the insulating film  518 , and the conductive film  524 , respectively. 
     The transistor  100  includes a conductive film  104  functioning as a first gate electrode over a substrate  102 , an insulating film  106  over the substrate  102  and the conductive film  104 , an insulating film  107  over the insulating film  106 , an oxide semiconductor film  108  over the insulating film  107 , and conductive films  112   a  and  112   b  functioning as source and drain electrodes electrically connected to the oxide semiconductor film  108 , the insulating films  114  and  116  over the oxide semiconductor film  108  and the conductive films  112   a  and  112   b , a conductive film  120   a  that is over the insulating film  116  and electrically connected to the conductive film  112   b , the conductive film  120   b  over the insulating film  116 , and the insulating film  118  over the insulating film  116  and the conductive films  120   a  and  120   b.    
     The insulating films  106  and  107  function as a first gate insulating film of the transistor  100 . The insulating films  114  and  116  function as a second gate insulating film of the transistor  100 . The insulating film  118  serves as a protective insulating film of the transistor  100 . In this specification and the like, the insulating films  106  and  107  are collectively referred to as a first insulating film, the insulating films  114  and  116  are collectively referred to as a second insulating film, and the insulating film  118  is referred to as a third insulating film in some cases. 
     The conductive film  120   b  can be used as a second gate electrode of the transistor  100 . 
     In the case where the transistor  100  is used in a pixel portion of a display panel, the conductive film  120   a  can be used as an electrode of a display element, or the like. 
     The oxide semiconductor film  108  includes the oxide semiconductor film  108   b  (on the conductive film  104  side) that serves as a first gate electrode, and an oxide semiconductor film  108   c  over the oxide semiconductor film  108   b . The oxide semiconductor films  108   b  and  108   c  contain In, M (M is Al, Ga, Y, or Sn), and Zn. 
     The oxide semiconductor film  108   b  preferably includes a region in which the atomic proportion of In is larger than the atomic proportion of M, for example. The oxide semiconductor film  108   c  preferably includes a region in which the atomic proportion of In is smaller than that in the oxide semiconductor film  108   b.    
     The oxide semiconductor film  108   b  including the 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  100 . Specifically, the field-effect mobility of the transistor  100  can exceed 10 cm 2 /Vs, preferably exceed 30 cm 2 /Vs. 
     For example, the use of the transistor with high field-effect mobility for a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame. 
     On the other hand, the oxide semiconductor film  108   b  including the region in which the atomic proportion of In is larger than that of M makes it easier to change electrical characteristics of the transistor  100  in light irradiation. However, in the semiconductor device of one embodiment of the present invention, the oxide semiconductor film  108   c  is formed over the oxide semiconductor film  108   b . Furthermore, the oxide semiconductor film  108   c  including the region in which the atomic proportion of In is smaller than that in the oxide semiconductor film  108   b  has larger Eg than the oxide semiconductor film  108   b . For this reason, the oxide semiconductor film  108  which is a layered structure of the oxide semiconductor film  108   b  and the oxide semiconductor film  108   c  has high resistance to a negative bias stress test with light irradiation. 
     Impurities such as hydrogen or moisture entering the channel region of the oxide semiconductor film  108 , particularly the oxide semiconductor film  108   b  adversely affect the transistor characteristics and therefore cause a problem. Moreover, it is preferable that the amount of impurities such as hydrogen or moisture in the channel region of the oxide semiconductor film  108   b  be as small as possible. Furthermore, oxygen vacancies formed in the channel region in the oxide semiconductor film  108   b  adversely affect the transistor characteristics and therefore cause a problem. For example, oxygen vacancies formed in the channel region in the oxide semiconductor film  108   b  are bonded to hydrogen to serve as a carrier supply source. The carrier supply source generated in the channel region in the oxide semiconductor film  108   b  causes a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor  100  including the oxide semiconductor film  108   b . Therefore, it is preferable that the amount of oxygen vacancies in the channel region of the oxide semiconductor film  108   b  be as small as possible. 
     In view of this, one embodiment of the present invention is a structure in which insulating films in contact with the oxide semiconductor film  108 , specifically the insulating film  107  formed under the oxide semiconductor film  108  and the insulating films  114  and  116  formed over the oxide semiconductor film  108  include excess oxygen. Oxygen or excess oxygen is transferred from the insulating film  107  and the insulating films  114  and  116  to the oxide semiconductor film  108 , whereby the oxygen vacancies in the oxide semiconductor film can be reduced. As a result, a change in electrical characteristics of the transistor  100 , particularly a change in the transistor  100  due to light irradiation, can be reduced. 
     In one embodiment of the present invention, a manufacturing method is used in which the number of manufacturing steps is not increased or an increase in the number of manufacturing steps is extremely small, because the insulating film  107  and the insulating films  114  and  116  are made to contain excess oxygen. Thus, the transistor  100  can be manufactured with high yield. 
     Specifically, in a step of forming the oxide semiconductor film  108   b , the oxide semiconductor film  108   b  is formed by a sputtering method in an atmosphere containing an oxygen gas, whereby oxygen or excess oxygen is added to the insulating film  107  over which the oxide semiconductor film  108   b  is formed. 
     Furthermore, in a step of forming the conductive films  120   a  and  120   b , the conductive films  120   a  and  120   b  are formed by a sputtering method in an atmosphere containing an oxygen gas, whereby oxygen or excess oxygen is added to the insulating film  116  over which the conductive films  120   a  and  120   b  are formed. Note that in some cases, oxygen or excess oxygen is added also to the insulating film  114  and the oxide semiconductor film  108  under the insulating film  116  when oxygen or excess oxygen is added to the insulating film  116 . 
     &lt;Oxide Conductor&gt; 
     Next, an oxide conductor is described. In a step of forming the conductive films  120   a  and  120   b , the conductive films  120   a  and  120   b  serve as a protective film for suppressing release of oxygen from the insulating films  114  and  116 . The conductive films  120   a  and  120   b  serve as semiconductors before a step of forming the insulating film  118  and serve as conductors after the step of forming the insulating film  118 . 
     To allow the conductive films  120   a  and  120   b  to serve as conductors, an oxygen vacancy is formed in the conductive films  120   a  and  120   b  and hydrogen is added from the insulating film  118  to the oxygen vacancy, whereby a donor level is formed in the vicinity of the conduction band. As a result, the conductivity of each of the conductive films  120   a  and  120   b  is increased, so that the oxide semiconductor film becomes a conductor. The conductive films  120   a  and  120   b  having become conductors can each be referred to as 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;Components of the Semiconductor Device&gt; 
     Components of the semiconductor device of this embodiment will be described below in detail. 
     As materials described below, materials described in Embodiment 3 can be used. 
     The material that can be used for the substrate  102  described in Embodiment 3 can be used for the substrate  102  in this embodiment. Furthermore, the materials that can be used for the insulating films  106  and  107  described in Embodiment 3 can be used for the insulating films  106  and  107  in this embodiment. 
     In addition, the materials that can be used for the conductive films functioning as the gate electrode, the source electrode, and the drain electrode described in Embodiment 3 can be used for the conductive films functioning as the first gate electrode, the source electrode, and the drain electrode in this embodiment. 
     &lt;&lt;Oxide Semiconductor Film&gt;&gt; 
     The oxide semiconductor film  108  can be formed using the materials described above. 
     In the case where the oxide semiconductor film  108   b  includes In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In &gt;M. The atomic ratio between 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. 
     In the case where the oxide semiconductor film  108   c  is In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In-M-Zn oxide satisfy In≤M. The atomic ratio of metal elements in such a sputtering target is, for example, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, or In:M:Zn=1:4:5. 
     In the case where the oxide semiconductor films  108   b  and  108   c  are formed of In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor films  108   b  and  108   c  having crystallinity. Note that the atomic ratios of metal elements in each of the formed oxide semiconductor films  108   b  and  108   c  vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error. For example, when a sputtering target of the oxide semiconductor film  108   b  with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the oxide semiconductor film  108   b  may be 4:2:3 or in the vicinity of 4:2:3. 
     The energy gap of the oxide semiconductor film  108  is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. The use of an oxide semiconductor having a wide energy gap can reduce off-state current of the transistor  100 . In particular, an oxide semiconductor film having an energy gap more than or equal to 2 eV, preferably more than or equal to 2 eV and less than or equal to 3.0 eV is preferably used as the oxide semiconductor film  108   b , and an oxide semiconductor film having an energy gap more than or equal to 2.5 eV and less than or equal to 3.5 eV is preferably used as the oxide semiconductor film  108   c . Furthermore, the oxide semiconductor film  108   c  preferably has a higher energy gap than the oxide semiconductor film  108   b.    
     Each thickness of the oxide semiconductor film  108   b  and the oxide semiconductor film  108   c  is more than or equal to 3 nm and less than or equal to 200 nm, preferably more than or equal to 3 nm and less than or equal to 100 nm, more preferably more than or equal to 3 nm and less than or equal to 50 nm. 
     An oxide semiconductor film with low carrier density is used as the oxide semiconductor film  108   c . For example, the carrier density of the oxide semiconductor film  108   c  is lower than or equal to 1×10 17 /cm 3 , preferably lower than or equal to 1×10 15 /cm 3 , further preferably lower than or equal to 1×10 13 /cm 3 , still further preferably lower than or equal to 1×10 11 /cm 3 . 
     Note that, without limitation to the compositions and materials described above, a material with an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Furthermore, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the oxide semiconductor film  108   b  and the oxide semiconductor film  108   c  be set to be appropriate. 
     Note that it is preferable to use, as the oxide semiconductor film  108   b  and the oxide semiconductor film  108   c , an oxide semiconductor film in which the impurity concentration is low and the density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancy is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and thus has a low density of trap states in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10 6  μm and a channel length of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10 −13  A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. 
     Accordingly, the transistor in which the channel region is formed in the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small change in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, and alkaline earth metal are given. 
     Hydrogen included in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen be reduced as much as possible in the oxide semiconductor film  108 . Specifically, in the oxide semiconductor film  108 , the concentration of hydrogen which is measured by SIMS is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , further preferably lower than or equal to 1×10 19  atoms/cm 3 , further preferably lower than or equal to 5×10 18  atoms/cm 3 , further preferably lower than or equal to 1×10 18  atoms/cm 3 , further preferably lower than or equal to 5×10 17  atoms/cm 3 , and further preferably lower than or equal to 1×10 16  atoms/cm 3 . 
     The oxide semiconductor film  108   b  preferably includes a region in which hydrogen concentration is smaller than that in the oxide semiconductor film  108   c . A semiconductor device including the oxide semiconductor film  108   b  having the region in which hydrogen concentration is smaller than that in the oxide semiconductor film  108   c  can be increased in reliability. 
     When silicon or carbon that is one of elements belonging to Group 14 is included in the oxide semiconductor film  108   b , oxygen vacancy is increased in the oxide semiconductor film  108   b , and the oxide semiconductor film  108   b  becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the oxide semiconductor film  108   b  or the concentration of silicon or carbon (the concentration is measured by SIMS) in the vicinity of an interface with the oxide semiconductor film  108   b  is set to be lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     In addition, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film  108   b , which is measured by SIMS, is lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film  108   b.    
     Furthermore, when including nitrogen, the oxide semiconductor film  108   b  easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set to be, for example, lower than or equal to 5×10 18  atoms/cm 3 . 
     The oxide semiconductor film  108   b  and the oxide semiconductor film  108   c  may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states. 
     &lt;&lt;Insulating Films Functioning as Second Gate Insulating Film&gt;&gt; 
     The insulating films  114  and  116  function as a second gate insulating film of the transistor  100 . In addition, the insulating films  114  and  116  each have a function of supplying oxygen to the oxide semiconductor film  108 . That is, the insulating films  114  and  116  contain oxygen. Furthermore, the insulating film  114  is an insulating film which can transmit oxygen. Note that the insulating film  114  also serves as a film which relieves damage to the oxide semiconductor film  108  at the time of forming the insulating film  116  in a later step. 
     For example, the insulating films  114  and  116  described in Embodiment 3 can be used as the insulating films  114  and  116  in this embodiment. 
     &lt;&lt;Oxide Semiconductor Film Functioning as Conductive Film, Oxide Semiconductor Film Functioning as Second Gate Electrode&gt;&gt; 
     The material of the oxide semiconductor film  108  described above can be used for the conductive film  120   a  and the conductive film  120   b  functioning as the second gate electrode. 
     That is, the conductive film  120   a  and the conductive film  120   b  functioning as a second gate electrode contain a metal element which is the same as that contained in the oxide semiconductor film  108  (the oxide semiconductor film  108   b  and the oxide semiconductor film  108   c ). For example, the conductive film  120   b  functioning as a second gate electrode and the oxide semiconductor film  108  (the oxide semiconductor film  108   b  and the oxide semiconductor film  108   c ) contain the same metal element; thus, the manufacturing cost can be reduced. 
     For example, in the case where the conductive film  120   a  and the conductive film  120   b  functioning as a second gate electrode are each 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≥M The atomic ratio of metal elements in such a sputtering target is In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, or the like. 
     The conductive film  120   a  and the conductive film  120   b  functioning as a 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  120   a  and the conductive film  120   b  each have a stacked-layer structure, the composition of the sputtering target is not limited to that described above. 
     &lt;&lt;Insulating Film Functioning as Protective Insulating Film of Transistor&gt;&gt; 
     The insulating film  118  serves as a protective insulating film of the transistor  100 . 
     The insulating film  118  includes one or both of hydrogen and nitrogen. Alternatively, the insulating film  118  includes nitrogen and silicon. The insulating film  118  has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor film  108 , outward diffusion of oxygen included in the insulating films  114  and  116 , and entry of hydrogen, water, or the like into the oxide semiconductor film  108  from the outside by providing the insulating film  118 . 
     The insulating film  118  has a function of supplying one or both of hydrogen and nitrogen to the conductive film  120   a  and the conductive film  120   b  functioning as a second gate electrode. The insulating film  118  preferably includes hydrogen and has a function of supplying the hydrogen to the conductive films  120   a  and  120   b . The conductive films  120   a  and  120   b  supplied with hydrogen from the insulating film  118  function as conductors. 
     A nitride insulating film, for example, can be used as the insulating film  118 . The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like. 
     Although the variety of films such as the conductive films, the insulating films, and the oxide semiconductor films which are described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal CVD method. Examples of the thermal CVD method include an MOCVD method and an ALD method. Specifically, the methods described above can be used. 
     A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film. 
     Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at a time so that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and react with each other in the vicinity of the substrate or over the substrate. 
     Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer; then the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET. 
     The variety of films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films in this embodiment can be formed by a thermal CVD method such as an MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH 3 ) 3 . The chemical formula of trimethylgallium is Ga(CH 3 ) 3 . The chemical formula of dimethylzinc is Zn(CH 3 ) 2 . Without limitation to the above combination, triethylgallium (chemical formula: Ga(C 2 H 5 ) 3 ) can be used instead of trimethylgallium and diethylzinc (chemical formula: Zn(C 2 H 5 ) 2 ) can be used instead of dimethylzinc. 
     For example, in the case where a hafnium oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, that is, ozone (O 3 ) as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (e.g., a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH 3 ) 2 ] 4 . Examples of another material liquid include tetrakis(ethylmethylamide)hafnium. 
     For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H 2 O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH 3 ) 3 . Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate). 
     For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine included in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O 2  or dinitrogen monoxide) are supplied to react with the adsorbate. 
     For example, in the case where a tungsten film is formed using a deposition apparatus using an ALD method, a WF 6  gas and a B 2 H 6  gas are sequentially introduced plural times to form an initial tungsten film, and then a WF 6  gas and an H 2  gas are used, so that a tungsten film is formed. Note that an SiH 4  gas may be used instead of a B 2 H 6  gas. 
     For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed using a deposition apparatus using an ALD method, an In(CH 3 ) 3  gas and an O 3  gas) are sequentially introduced plural times to form an InO layer, a GaO layer is formed using a Ga(CH 3 ) 3  gas and an O 3  gas), and then a ZnO layer is formed using a Zn(CH 3 ) 2  gas and an O 3  gas). Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases. Note that although an H 2 O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O 3  gas), it is preferable to use an O 3  gas), which does not contain H. Furthermore, instead of an In(CH 3 ) 3  gas, an In(C 2 H 5 ) 3  gas may be used. Instead of a Ga(CH 3 ) 3  gas, a Ga(C 2 H 5 ) 3  gas may be used. Furthermore, a Zn(CH 3 ) 2  gas may be used. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 5 
     &lt;Composition of CAC-OS&gt; 
     Described below is the composition of a cloud aligned complementary oxide semiconductor (CAC-OS) applicable to a transistor disclosed in one embodiment of the present invention. 
     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 an active layer of a transistor is called an oxide semiconductor in some cases. In other words, an OS FET is a transistor including a metal oxide or an oxide semiconductor. 
     In this specification, a metal oxide in which regions functioning as a conductor and regions functioning as a dielectric are mixed and which functions as a semiconductor as a whole is defined as a CAC-OS or a CAC-metal oxide. 
     The CAC-OS has, for example, a composition in which elements included in an oxide semiconductor 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 0.5 nm and less than or equal to 3 nm, or a similar size. Note that in the following description of an oxide semiconductor, a state in which one or more elements are unevenly distributed and regions including the element(s) are mixed is referred to as a mosaic pattern or a patch-like pattern. The region has 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 0.5 nm and less than or equal to 3 nm, or a similar size. 
     The physical properties of a region including an unevenly distributed element are determined by the properties of the element. For example, a region including an unevenly distributed element which relatively tends to serve as an insulator among elements included in a metal oxide serves as a dielectric region. In contrast, a region including an unevenly distributed element which relatively tends to serve as a conductor among elements included in a metal oxide serves as a conductive region. A material in which conductive regions and dielectric regions are mixed to form a mosaic pattern serves as a semiconductor. 
     That is, a metal oxide in one embodiment of the present invention is a kind of matrix composite or metal matrix composite, in which materials having different physical properties are mixed. 
     Note that an oxide semiconductor preferably contains at least indium. In particular, indium and zinc are preferably contained. In addition, an element M (M is one or more of gallium, aluminum, silicon, boron, yttrium, copper, vanadium, beryllium, 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 (InO X1 , where X1 is a real number greater than 0) or indium zinc oxide (In X2 Zn Y2 O Z2 , where X2, Y2, and Z2 are real numbers greater than 0), and gallium oxide (GaO X3 , where X3 is a real number greater than 0), gallium zinc oxide (Ga X4 Zn Y4 O Z4 , where X4, Y4, and Z4 are real numbers greater than 0), or the like, and a mosaic pattern is formed. Then, InO X1  and In X2 Zn Y2 O Z2  forming the mosaic pattern are evenly distributed in the film. This composition is also referred to as a cloud-like composition. 
     That is, the CAC-OS is a composite oxide semiconductor 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 Min 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 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 an oxide semiconductor. 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, silicon, boron, yttrium, copper, vanadium, beryllium, 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 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. 
     &lt;Analysis of CAC-OS&gt; 
     Next, measurement results of an oxide semiconductor over a substrate by a variety of methods are described. 
     &lt;&lt;Structure of Samples and Formation Method Thereof&gt;&gt; 
     Nine samples of one embodiment of the present invention are described below. The samples are formed at different substrate temperatures and with different ratios of an oxygen gas flow rate in formation of the oxide semiconductor. Note that each sample includes a substrate and an oxide semiconductor over the substrate. 
     A method for forming the samples is described. 
     A glass substrate is used as the substrate. Over the glass substrate, a 100-nm-thick In—Ga—Zn oxide is formed as an oxide semiconductor with a sputtering apparatus. The formation conditions are as follows: the pressure in a chamber is 0.6 Pa, and an oxide target (with an atomic ratio of In:Ga:Zn=4:2:4.1) is used as a target. The oxide target provided in the sputtering apparatus is supplied with an AC power of 2500 W. 
     As for the conditions in the formation of the oxide of the nine samples, the substrate temperature is set to a temperature that is not increased by intentional heating (hereinafter such a temperature is also referred to as room temperature or R. T.), to 130° C., and to 170° C. The ratio of a flow rate of an oxygen gas to a flow rate of a mixed gas of Ar and oxygen (also referred to as an oxygen gas flow rate ratio) is set to 10%, 30%, and 100%. 
     &lt;&lt;Analysis by X-Ray Diffraction&gt;&gt; 
     In this section, results of X-ray diffraction (XRD) measurement performed on the nine samples are described. As an XRD apparatus, D8 ADVANCE manufactured by Bruker AXS is used. The conditions are as follows: scanning is performed by an out-of-plane method at θ/2θ, the scanning range is 15 deg. to 50 deg., the step width is 0.02 deg., and the scanning speed is 3.0 deg./min. 
       FIG. 29  shows XRD spectra measured by an out-of-plane method. In  FIG. 29 , the top row shows the measurement results of the samples formed at a substrate temperature of 170° C.; the middle row shows the measurement results of the samples formed at a substrate temperature of 130° C.; the bottom row shows the measurement results of the samples formed at a substrate temperature of R.T. The left column shows the measurement results of the samples formed with an oxygen gas flow rate ratio of 10%; the middle column shows the measurement results of the samples formed with an oxygen gas flow rate ratio of 30%; the right column shows the measurement results of the samples formed with an oxygen gas flow rate ratio of 100%. 
     In the XRD spectra shown in  FIG. 29 , the higher the substrate temperature at the time of formation is or the higher the oxygen gas flow rate ratio at the time of formation is, the higher the intensity of the peak at around 2θ=31° is. Note that it is found that the peak at around 2θ=31° is derived from a crystalline IGZO compound whose c-axes are aligned in a direction substantially perpendicular to a formation surface or a top surface of the crystalline IGZO compound (such a compound is also referred to as c-axis aligned crystalline (CAAC) IGZO). 
     As shown in the XRD spectra in  FIG. 29 , as the substrate temperature at the time of formation is lower or the oxygen gas flow rate ratio at the time of formation is lower, a peak becomes less clear. Accordingly, it is found that there are no alignment in the a-b plane direction and c-axis alignment in the measured areas of the samples that are formed at a lower substrate temperature or with a lower oxygen gas flow rate ratio. 
     &lt;&lt;Analysis with Electron Microscope&gt;&gt; 
     This section describes the observation and analysis results of the samples formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10% with a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM). An image obtained with an HAADF-STEM is also referred to as a TEM image. 
     Described are the results of image analysis of plan-view images and cross-sectional images obtained with an HAADF-STEM (also referred to as plan-view TEM images and cross-sectional TEM images, respectively). The TEM images are observed with a spherical aberration corrector function. The HAADF-STEM images are obtained using an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd. under the following conditions: the acceleration voltage is 200 kV, and irradiation with an electron beam with a diameter of approximately 0.1 nm is performed. 
       FIG. 30A  is a plan-view TEM image of the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10%.  FIG. 30B  is a cross-sectional TEM image of the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10%. 
     &lt;&lt;Analysis of Electron Diffraction Patterns&gt;&gt; 
     This section describes electron diffraction patterns obtained by irradiation of the sample formed at a substrate temperature of R.T. and an oxygen gas flow rate ratio of 10% with an electron beam with a probe diameter of 1 nm (also referred to as a nanobeam). 
     Electron diffraction patterns of points indicated by black dots a1, a2, a3, a4, and a5 in the plan-view TEM image in  FIG. 30A  of the sample formed at a substrate temperature of R.T. and an oxygen gas flow rate ratio of 10% are observed. Note that the electron diffraction patterns are observed while electron beam irradiation is performed at a constant rate for 35 seconds.  FIGS. 30C, 30D, 30E, 30F, and 30G  show the results of the points indicated by the black dots a1, a2, a3, a4, and a5, respectively. 
     In  FIGS. 30C, 30D, 30E, 30F, and 30G , regions with high luminance in a circular (ring) pattern can be shown. Furthermore, a plurality of spots can be shown in a ring-like shape. 
     Electron diffraction patterns of points indicated by black dots b1, b2, b3, b4, and b5 in the cross-sectional TEM image in  FIG. 30B  of the sample formed at a substrate temperature of R.T. and an oxygen gas flow rate ratio of 10% are observed.  FIGS. 30H, 30I, 30J, 30K, and 30L  show the results of the points indicated by the black dots b1, b2, b3, b4, and b5, respectively. 
     In  FIGS. 30H, 30I, 30J, 30K, and 30L , regions with high luminance in a ring pattern can be shown. Furthermore, a plurality of spots can be shown in a ring-like shape. 
     For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO 4  crystal in a direction parallel to the sample surface, a diffraction pattern including a spot derived from the (009) plane of the InGaZnO 4  crystal is obtained. That is, the CAAC-OS has c-axis alignment and the c-axes are aligned in the direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, a ring-like diffraction pattern is shown when an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. That is, it is found that the CAAC-OS has neither a-axis alignment nor b-axis alignment. 
     Furthermore, a diffraction pattern like a halo pattern is observed when an oxide semiconductor including a nanocrystal (a nanocrystalline oxide semiconductor (nc-OS)) is subjected to electron diffraction using an electron beam with a large probe diameter (e.g., 50 nm or larger). Meanwhile, bright spots are shown in a nanobeam electron diffraction pattern of the nc-OS obtained using an electron beam with a small probe diameter (e.g., smaller than 50 nm). Furthermore, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of bright spots are shown in a ring-like shape in some cases. 
     The electron diffraction pattern of the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10% has regions with high luminance in a ring pattern and a plurality of bright spots appear in the ring-like pattern. Accordingly, the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10% exhibits an electron diffraction pattern similar to that of the nc-OS and does not show alignment in the plane direction and the cross-sectional direction. 
     According to what is described above, an oxide semiconductor formed at a low substrate temperature or with a low oxygen gas flow rate ratio is likely to have characteristics distinctly different from those of an oxide semiconductor film having an amorphous structure and an oxide semiconductor film having a single crystal structure. 
     &lt;&lt;Elementary Analysis&gt;&gt; 
     This section describes the analysis results of elements included in the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10%. For the analysis, by energy dispersive X-ray spectroscopy (EDX), EDX mapping images are obtained. An energy dispersive X-ray spectrometer AnalysisStation JED-2300T manufactured by JEOL Ltd. is used as an elementary analysis apparatus in the EDX measurement. A Si drift detector is used to detect an X-ray emitted from the sample. 
     In the EDX measurement, an EDX spectrum of a point is obtained in such a manner that electron beam irradiation is performed on the point in a detection target region of a sample, and the energy of characteristic X-ray of the sample generated by the irradiation and its frequency are measured. In this embodiment, peaks of an EDX spectrum of the point are attributed to electron transition to the L shell in an In atom, electron transition to the K shell in a Ga atom, and electron transition to the K shell in a Zn atom and the K shell in an O atom, and the proportions of the atoms in the point are calculated. An EDX mapping image indicating distributions of proportions of atoms can be obtained through the process in an analysis target region of a sample. 
       FIGS. 31A to 31C  show EDX mapping images in a cross section of the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10%.  FIG. 31A  shows an EDX mapping image of Ga atoms. The proportion of the Ga atoms in all the atoms is 1.18 atomic % to 18.64 atomic %.  FIG. 31B  shows an EDX mapping image of In atoms. The proportion of the In atoms in all the atoms is 9.28 atomic % to 33.74 atomic %.  FIG. 31C  shows an EDX mapping image of Zn atoms. The proportion of the Zn atoms in all the atoms is 6.69 atomic % to 24.99 atomic %.  FIGS. 31A to 31C  show the same region in the cross section of the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10%. In the EDX mapping images, the proportion of an element is indicated by grayscale: the more measured atoms exist in a region, the brighter the region is; the less measured atoms exist in a region, the darker the region is. The magnification of the EDX mapping images in  FIGS. 31A to 31C  is 7200000 times. 
     The EDX mapping images in  FIGS. 31A to 31C  show relative distribution of brightness indicating that each element has a distribution in the sample formed at a substrate temperature of R.T. and with an oxygen gas flow rate ratio of 10%. Areas surrounded by solid lines and areas surrounded by dashed lines in  FIGS. 31A to 31C  are examined. 
     In  FIG. 31A , a relatively dark region occupies a large area in the area surrounded by the solid line, while a relatively bright region occupies a large area in the area surrounded by the dashed line. In  FIG. 31B , a relatively bright region occupies a large area in the area surrounded by the solid line, while a relatively dark region occupies a large area in the area surrounded by the dashed line. 
     That is, the areas surrounded by the solid lines are regions including a relatively large number of In atoms and the areas surrounded by the dashed lines are regions including a relatively small number of In atoms. In  FIG. 31C , the right portion of the area surrounded by the solid line is relatively bright and the left portion thereof is relatively dark. Thus, the area surrounded by the solid line is a region including In X2 Zn Y2 O Z2 , InO X1 , and the like as main components. 
     The area surrounded by the solid line is a region including a relatively small number of Ga atoms and the area surrounded by the dashed line is a region including a relatively large number of Ga atoms. In  FIG. 31C , the upper left portion of the area surrounded by the dashed line is relatively bright and the lower right portion thereof is relatively dark. Thus, the area surrounded by the dashed line is a region including GaO X3 , Ga X4 Zn Y4 O Z4 , and the like as main components. 
     Furthermore, as shown in  FIGS. 31A to 31C , the In atoms are relatively more uniformly distributed than the Ga atoms, and regions including InO X1  as a main component is seemingly joined to each other through a region including In X2 Zn Y2 O Z2  as a main component. Thus, the regions including In X2 Zn Y2 O Z2  and InO X1  as main components extend like a cloud. 
     An In—Ga—Zn oxide having a composition in which the regions including GaO X3  or the like as a main component and the regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are unevenly distributed and mixed can be referred to as a CAC-OS. 
     The crystal structure of the CAC-OS includes an nc structure. In an electron diffraction pattern of the CAC-OS with the nc structure, several or more bright spots appear in addition to bright sports derived from IGZO including a single crystal, a polycrystal, or a CAAC. Alternatively, the crystal structure is defined as having high luminance regions appearing in a ring pattern in addition to the several or more bright spots. 
     As shown in  FIGS. 31A to 31C , each of the regions including GaO X3  or the like as a main component and the regions including In X2 Zn Y2 O Z2  or InO X1  as a main component has a size of greater than or equal to 0.5 nm and less than or equal to 10 nm, or greater than or equal to 1 nm and less than or equal to 3 nm. Note that it is preferable that a diameter of a region including each metal element as a main component be greater than or equal to 1 nm and less than or equal to 2 nm in the EDX mapping images. 
     As described above, 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 exhibits. 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 InO X1  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 typified by a display. 
     At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     Embodiment 6 
     In this embodiment, a structure of an input/output device of one embodiment of the present invention will be described with reference to  FIG. 20 . 
       FIG. 20  is an exploded view of an input/output device  800  for illustrating the components. 
     The input/output device  800  includes a display panel  806  and a touch sensor  804  having a region overlapping with the display panel  806 . Note that the input/output device  800  can be referred to as a touch panel. 
     The input/output device  800  is provided with a driver circuit  810  for driving the touch sensor  804  and the display panel  806 , a battery  811  for supplying power to the driver circuit  810 , and a housing where the touch sensor  804 , the display panel  806 , the driver circuit  810 , and the battery  811  are stored. 
     &lt;&lt;Touch Sensor  804 &gt;&gt; 
     The touch sensor  804  includes a region overlapping with the display panel  806 . Note that an FPC  803  is electrically connected to the touch sensor  804 . 
     For the touch sensor  804 , a resistive touch sensor, a capacitive touch sensor, or a touch sensor using a photoelectric conversion element can be used, for example. 
     Note that the touch sensor  804  may be used as part of the display panel  806 . 
     &lt;&lt;Display Panel  806 &gt;&gt; 
     For example, the display panel described in Embodiment 1 can be used as the display panel  806 . Note that an FPC  805  is electrically connected to the display panel  806 . 
     &lt;&lt;Driver Circuit  810 &gt;&gt; 
     As the driver circuit  810 , a power supply circuit or a signal processing circuit can be used, for example. Power supplied to the battery or an external commercial power supply can be utilized. 
     The signal processing circuit has a function of outputting a video signal and a clock signal. 
     The power supply circuit has a function of supplying predetermined power. 
     &lt;&lt;Housing&gt;&gt; 
     An upper cover  801 , a lower cover  802  which fits the upper cover  801 , and a frame  809  which is stored in a region surrounded by the upper cover  801  and the lower cover  802  can be used for the housing, for example. 
     The frame  809  has a function of protecting the display panel  806 , and a function of blocking electromagnetic waves generated by the operation of the driver circuit  810  or a function of a radiator plate. 
     Metal, a resin, an elastomer, or the like can be used for the upper cover  801 , the lower cover  802 , or the frame  809 . 
     &lt;&lt;Battery  811 &gt;&gt; 
     The battery  811  has a function of supplying power. 
     Note that a member such as a polarizing plate, a retardation plate, or a prism sheet can be used for the input/output device  800 . 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 7 
     In this embodiment, a structure of an information processing device of one embodiment of the present invention will be described with reference to  FIGS. 21A and 21B ,  FIGS. 22A to 22C ,  FIGS. 23A and 23B , and  FIG. 24 . 
       FIG. 21A  is a block diagram illustrating a structure of an information processing device  200 .  FIG. 21B  is a perspective view illustrating an example of an external view of the information processing device  200 . 
       FIG. 22A  is a block diagram illustrating a configuration of a display portion  230 .  FIG. 22B  is a block diagram illustrating a configuration of a display portion  230 B.  FIG. 22C  is a circuit diagram illustrating a configuration of a pixel  232 ( i, j ). 
     &lt;Configuration Example of Information Processing Device&gt; 
     The information processing device  200  described in this embodiment includes an arithmetic device  210  and an input/output device  220  (see  FIG. 23A ). 
     The arithmetic device  210  is configured to receive positional information P 1  and supply image information V and control information. 
     The input/output device  220  is configured to supply the positional information P 1  and receive the image information V and the control information. 
     The input/output device  220  includes the display portion  230  that displays the image information V and an input portion  240  that supplies the positional information P 1 . 
     The display portion  230  includes a first display element and a second display element overlapping with the opening in the reflective film of the first display element. The display portion  230  further includes a first pixel circuit for driving the first display element and a second pixel circuit for driving the second display element. 
     The input portion  240  is configured to detect the position of a pointer and supply the positional information P 1  determined in accordance with the position. 
     The arithmetic device  210  is configured to determine the moving speed of the pointer in accordance with the positional information P 1 . 
     The arithmetic device  210  is configured to determine the contrast or brightness of the image information V in accordance with the moving speed. 
     The information processing device  200  described in this embodiment includes the input/output device  220  that supplies the positional information P 1  and receives the image information V and the arithmetic device  210  that receives the positional information P 1  and supplies the image information V. The arithmetic device  210  is configured to determine the contrast or brightness of the image information V in accordance with the moving speed of the positional information P 1 . 
     With this structure, eyestrain on a user caused when the display position of image information is moved can be reduced, that is, eye-friendly display can be achieved. Moreover, the power consumption can be reduced and excellent visibility can be provided even in a bright place exposed to direct sunlight, for example. Thus, the novel information processing device that is highly convenient or reliable can be provided. 
     &lt;Configuration&gt; 
     The information processing device of one embodiment of the present invention includes the arithmetic device  210  or the input/output device  220 . 
     &lt;&lt;Arithmetic Device  210 &gt;&gt; 
     The arithmetic device  210  includes an arithmetic portion  211  and a memory portion  212 . The arithmetic device  210  further includes a transmission path  214  and an input/output interface  215  (see  FIG. 21A ). 
     &lt;&lt;Arithmetic Portion  211 &gt;&gt; 
     The arithmetic portion  211  is configured to, for example, execute a program. For example, a CPU described in Embodiment 7 can be used. Thus, power consumption can be sufficiently reduced. 
     &lt;&lt;Memory Portion  212 &gt;&gt; 
     The memory portion  212  is configured to, for example, store the program executed by the arithmetic portion  211 , initial information, setting information, an image, or the like. 
     Specifically, a hard disk, a flash memory, a memory including a transistor including an oxide semiconductor, or the like can be used for the memory portion  212 . 
     &lt;&lt;Input/Output Interface  215 , Transmission Path  214 &gt;&gt; 
     The input/output interface  215  includes a terminal or a wiring and is configured to supply and receive information. For example, the input/output interface  215  can be electrically connected to the transmission path  214  and the input/output device  220 . 
     The transmission path  214  includes a wiring and is configured to supply and receive information. For example, the transmission path  214  can be electrically connected to the input/output interface  215 . In addition, the transmission path  214  can be electrically connected to the arithmetic portion  211  or the memory portion  212 . 
     &lt;&lt;Input/Output Device  220 &gt;&gt; 
     The input/output device  220  includes the display portion  230 , the input portion  240 , a sensor portion  250 , or a communication portion  290 . 
     &lt;&lt;Display Portion  230 &gt;&gt; 
     The display portion  230  includes a display region  231 , a driver circuit GD, and a driver circuit SD (see  FIG. 22A ). For example, the display panel described in Embodiment 1 can be used. Thus, low power consumption can be achieved. 
     The display region  231  includes a plurality of pixels  232 ( i ,  1 ) to  232  ( i, n ) arranged in the row direction, a plurality of pixels  232 ( 1 ,  j ) to  232  ( m, j ) arranged in the column direction, a scan line G(i) electrically connected to the pixels  232 ( i ,  1 ) to  232  ( i, n ), and a signal line S(j) electrically connected to the pixels  232 ( 1 ,  j ) to  232  ( m, j ). Note that i is an integer greater than or equal to 1 and less than or equal to m, j is an integer greater than or equal to 1 and less than or equal to n, and each of m and n is an integer greater than or equal to 1. 
     Note that the pixel  232 ( i, j ) is electrically connected to the scan line G 1 ( i ), the scan line G 2 ( i ), the signal line S(j), the wiring ANO, the wiring VCOM 1 , and the wiring VCOM 2  (see  FIG. 22C ). 
     Note that the scan line G 1 ( i ) includes the scan line G 1 ( i ) and the scan line G 2 ( i ) (see  FIGS. 22A and 22B ). 
     The display portion can include a plurality of driver circuits. For example, the display portion  230 B can include a driver circuit GDA and a driver circuit GDB (see  FIG. 22B ). 
     &lt;&lt;Driver Circuit GD&gt;&gt; 
     The driver circuit GD is configured to supply a selection signal in accordance with the control information. 
     For example, the driver circuit GD is configured to supply a selection signal to one scan line at a frequency of 30 Hz or higher, preferably 60 Hz or higher, in accordance with the control information. Accordingly, moving images can be smoothly displayed. 
     For example, the driver circuit GD is configured to supply a selection signal to one scan line at a frequency of lower than 30 Hz, preferably lower than 1 Hz, more preferably less than once per minute, in accordance with the control information. Accordingly, a still image can be displayed while flickering is suppressed. 
     For example, in the case where a plurality of driver circuits is provided, the driver circuits GDA and GDB may supply the selection signals at different frequencies. Specifically, the selection signal can be supplied at a higher frequency to a region on which moving images are smoothly displayed than to a region on which a still image is displayed in a state where flickering is suppressed. 
     &lt;&lt;Driver Circuit SD&gt;&gt; 
     The driver circuit SD is configured to supply an image signal in accordance with the image information V. 
     &lt;&lt;Pixel  232 ( i, j )&gt;&gt; 
     The pixel  232 ( i, j ) includes a first display element  235 LC and a second display element  235 EL overlapping with the opening in the reflective film of the first display element  235 LC. The pixel  232 ( i, j ) further includes a first pixel circuit for driving the first display element  235 LC and a second pixel circuit for driving the second display element  235 EL (see  FIG. 22C ). 
     &lt;&lt;First Display Element  235 LC&gt;&gt; 
     For example, a display element having a function of controlling light transmission can be used as the first display element  235 LC. Specifically, a polarizing plate and a liquid crystal element, a MEMS shutter display element, or the like can be used. 
     Specifically, a liquid crystal element driven in any of the following driving modes can be used: an in-plane switching (IPS) mode, a twisted nematic (TN) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, and the like. 
     In addition, a liquid crystal element that can be driven by, for example, a vertical alignment (VA) mode such as a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an electrically controlled birefringence (ECB) mode, a continuous pinwheel alignment (CPA) mode, or an advanced super view (ASV) mode can be used. 
     The first display element  235 LC includes a first electrode, a second electrode, and a liquid crystal layer. The liquid crystal layer contains a liquid crystal material whose orientation is controlled by voltage applied between the first electrode and the second electrode. For example, the orientation of the liquid crystal material can be controlled by an electric field in the thickness direction (also referred to as the vertical direction), the horizontal direction, or the diagonal direction of the liquid crystal layer. 
     For example, thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, polymer dispersed liquid crystal, ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or the like can be used. These liquid crystal materials exhibit a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. Alternatively, a liquid crystal material that exhibits a blue phase can be used. 
     &lt;&lt;Second Display Element  235 EL&gt;&gt; 
     A display element having a function of emitting light, such as an organic EL element, can be used as the second display element  235 EL. 
     Specifically, an organic EL element which emits white light can be used as the second display element  235 EL. Alternatively, an organic EL element which emits blue light, green light, or red light can be used as the second display element  235 EL. 
     &lt;&lt;Pixel Circuit&gt;&gt; 
     A pixel circuit including a circuit which is configured to drive the first display element  235 LC and/or the second display element  235 EL can be used. 
     For example, a pixel circuit which is electrically connected to the scan line G 1 ( i ), the scan line G 2 ( i ), the signal line S(j), the wiring ANO, the wiring VCOM 1 , and the wiring VCOM 2  and which drives a light-emitting element and an organic EL element is described (see  FIG. 22C ). 
     Alternatively, for example, a switch, a transistor, a diode, a resistor, a capacitor, or an inductor can be used in the pixel circuit. 
     For example, one or a plurality of transistors can be used as a switch. Alternatively, a plurality of transistors connected in parallel, in series, or in combination of parallel connection and series connection can be used as a switch. 
     For example, a capacitor may be formed by the first electrode of the first display element  235 LC and a conductive film having a region overlapping with the first electrode. 
     For example, the pixel circuit includes a transistor functioning as the switch SW 1 , the first display element  235 LC, and the capacitor C 1 . A gate electrode of the transistor is electrically connected to the scan line G 1  ( i ), and a first electrode of the transistor is electrically connected to the signal line S(j). A first electrode of the first display element  235 LC is electrically connected to a second electrode of the transistor, and a second electrode of the first display element  235 LC is electrically connected to the wiring VCOM 1 . A first electrode of the capacitor C 1  is electrically connected to the second electrode of the transistor, and a second electrode of the capacitor C 1  is electrically connected to the wiring VCOM 1 . 
     The pixel circuit includes the transistor functioning as the switch SW 2 . A gate electrode of the transistor is electrically connected to the scan line G 2 ( i ), a first electrode of the transistor is electrically connected to the signal line S(j). In addition, the pixel circuit includes the transistor M. A gate electrode of the transistor M is electrically connected to a second electrode of the transistor functioning as the switch SW 2 . A first electrode of the transistor M is electrically connected to the wiring ANO. In addition, the pixel circuit includes the capacitor C 2 . A first electrode of the capacitor C 2  is electrically connected to the second electrode of the transistor functioning as the switch SW 2 . A second electrode of the capacitor C 2  is electrically connected to the second electrode of the transistor M. In addition, the pixel circuit includes a second display element  235 EL. A first electrode and a second electrode of the second display element  235 EL are electrically connected to the second electrode of the transistor M and the wiring VCOM 2 , respectively. 
     &lt;&lt;Transistor&gt;&gt; 
     For example, a semiconductor film formed at the same step can be used for transistors in the driver circuit and the pixel circuit. 
     As the transistors in the driver circuit and the pixel circuit, bottom-gate transistors, top-gate transistors, or the like can be used. 
     For example, a manufacturing line for a bottom-gate transistor including amorphous silicon as a semiconductor can be easily remodeled into a manufacturing line for a bottom-gate transistor including an oxide semiconductor as a semiconductor. Furthermore, for example, a manufacturing line for a top-gate transistor including polysilicon as a semiconductor can be easily remodeled into a manufacturing line for a top-gate transistor including an oxide semiconductor as a semiconductor. 
     For example, a transistor including a semiconductor containing an element of Group 4 can be used. Specifically, a semiconductor containing silicon can be used for a semiconductor film. For example, single crystal silicon, polysilicon, microcrystalline silicon, or amorphous silicon can be used for the semiconductor of the transistor. 
     Note that the temperature for forming a transistor using polysilicon in a semiconductor is lower than the temperature for forming a transistor using single crystal silicon in a semiconductor. 
     In addition, the transistor using polysilicon in a semiconductor has higher field-effect mobility than the transistor using amorphous silicon in a semiconductor, and therefore a pixel including the transistor using polysilicon can have a high aperture ratio. Moreover, pixels arranged at a high density, a gate driver circuit, and a source driver circuit can be formed over the same substrate. As a result, the number of components included in an electronic device can be reduced. 
     In addition, the transistor using polysilicon in a semiconductor has higher reliability than the transistor using amorphous silicon in a semiconductor. 
     For example, a transistor including an oxide semiconductor can be used. Specifically, an oxide semiconductor containing indium or an oxide semiconductor containing indium, gallium, and zinc can be used for a semiconductor film. 
     For example, a transistor having a lower leakage current in an off state than a transistor that uses amorphous silicon for a semiconductor film can be used. Specifically, a transistor that uses an oxide semiconductor for a semiconductor film can be used. 
     A pixel circuit in the transistor that uses an oxide semiconductor for the semiconductor film can hold an image signal for a longer time than a pixel circuit in a transistor that uses amorphous silicon for a semiconductor film. Specifically, the selection signal can be supplied at a frequency of lower than 30 Hz, preferably lower than 1 Hz, more preferably less than once per minute while flickering is suppressed. Consequently, eyestrain on a user of the information processing device can be reduced, and power consumption for driving can be reduced. 
     Alternatively, for example, a transistor including a compound semiconductor can be used. Specifically, a semiconductor containing gallium arsenide can be used for a semiconductor film. 
     For example, a transistor including an organic semiconductor can be used. Specifically, an organic semiconductor containing any of polyacenes and graphene can be used for the semiconductor film. 
     &lt;&lt;Input Portion  240 &gt;&gt; 
     A variety of human interfaces or the like can be used as the input portion  240  (see  FIG. 21A ). 
     For example, a keyboard, a mouse, a touch sensor, a microphone, a camera, or the like can be used as the input portion  240 . Note that a touch sensor having a region overlapping with the display portion  230  can be used. An input/output device that includes the display portion  230  and a touch sensor having a region overlapping with the display portion  230  can be referred to as a touch panel. 
     For example, a user can make various gestures (e.g., tap, drag, swipe, and pinch in) using his/her finger as a pointer on the touch panel. 
     The arithmetic device  210 , for example, analyzes information on the position, track, or the like of the finger on the touch panel and determines that a specific gesture is supplied when the analysis results meet predetermined conditions. Therefore, the user can supply a certain operation instruction associated with a certain gesture by using the gesture. 
     For instance, the user can supply a “scrolling instruction” for changing a portion where image information is displayed by using a gesture of touching and moving his/her finger on the touch panel. 
     &lt;&lt;Sensor Portion  250 &gt;&gt; 
     The sensor portion  250  is configured to acquire information P 2  by measuring the surrounding state. 
     For example, a camera, an acceleration sensor, a direction sensor, a pressure sensor, a temperature sensor, a humidity sensor, an illuminance sensor, or a global positioning system (GPS) signal receiving circuit can be used as the sensor portion  250 . 
     For example, when the arithmetic device  210  determines that the ambient light level measured by an illuminance sensor of the sensor portion  250  is sufficiently higher than the predetermined illuminance, image information is displayed using the first display element  235 LC. When the arithmetic device  210  determines that it is dim, image information is displayed using the first display element  235 LC and the second display element  235 EL. When the arithmetic device  210  determines that it is dark, image information is displayed using the second display element  235 EL. 
     Specifically, an image is displayed with a reflective display element and/or a self-luminous display element depending on the ambient brightness. For example, a liquid crystal element and an organic EL element can be used as the reflective display element and the self-luminous display element, respectively. 
     Thus, image information can be displayed in such a manner that, for example, a reflective display element is used under strong ambient light, a reflective display element and a self-luminous display element are used in dim light, and a self-luminous display element is used in dark light. Thus, a novel display device with high visibility can be provided. A novel display device with low power consumption can be provided. A novel information processing device which is highly convenient or reliable can be provided. 
     For example, a sensor measuring chromaticity of ambient light, such as a CCD camera, can be used in the sensor portion  250 , white balance can be adjusted in accordance with the chromaticity of ambient light measured by the sensor portion  250 . 
     Specifically, the white balance is adjusted through first to third steps described below. 
     In the first step, imbalance disruption of white balance of ambient light is measured. 
     In the second step, the intensity of light of a color which is insufficient in an image to be displayed by the first display element using reflection of ambient light is estimated. 
     In the third step, ambient light is reflected by the first display element, and light is emitted from the second display element so that light of the insufficient color is supplemented, whereby the image is displayed. 
     In this manner, display can be performed with adjusted white balance by utilizing light reflected by the first display element and light emitted from the second display element. Thus, a novel information processing device which can display an image with low power consumption or with adjusted white balance and which is highly convenient and reliable can be provided. 
     &lt;&lt;Communication Portion  290 &gt;&gt; 
     The communication portion  290  is configured to supply and acquire information to/from a network. 
     &lt;&lt;Program&gt;&gt; 
     A program of one embodiment of the present invention will be described with reference to  FIGS. 23A and 23B  and  FIG. 24 . 
       FIG. 23A  is a flow chart showing main processing of the program of one embodiment of the present invention, and  FIG. 23B  is a flow chart showing interrupt processing. 
       FIG. 24  schematically illustrates a method for displaying image information on the display portion  230 . 
     The program of one embodiment of the present invention has the following steps (see  FIG. 23A ). 
     In a first step, setting is initialized (see S 1  in  FIG. 23A ). 
     For instance, predetermined image information and the second mode can be used for the initialization. 
     For example, a still image can be used as the predetermined image information. Alternatively, a mode in which the selection signal is supplied at a frequency of lower than 30 Hz, preferably lower than 1 Hz, more preferably less than once per minute can be used as the second mode. For example, in the case where the time is displayed on the information processing device on the second time scale, a mode in which the selection signal is supplied at a frequency of 1 Hz can be used as the second mode. In the case where the time is displayed on the information processing device on the minute time scale, a mode in which the selection signal is supplied once per minute can be used as the second mode. 
     In a second step, interrupt processing is allowed (see S 2  in  FIG. 23A ). Note that an arithmetic device allowed to execute the interrupt processing can perform the interrupt processing in parallel with the main processing. The arithmetic device which has returned from the interrupt processing to the main processing can reflect the results of the interrupt processing in the main processing. 
     The arithmetic device may execute the interrupt processing when a counter has an initial value, and the counter may be set at a value other than the initial value when the arithmetic device returns from the interrupt processing. Thus, the interrupt processing is ready to be executed after the program is started up. 
     In a third step, image information is displayed in a mode selected in the first step or the interrupt processing (see S 3  in  FIG. 23A ). 
     For instance, predetermined image information is displayed in the second mode, in accordance with the initialization. 
     Specifically, the predetermined image information is displayed in a mode in which the selection signal is supplied to one scan line at a frequency of lower than 30 Hz, preferably lower than 1 Hz, more preferably less than once per minute. 
     For example, the selection signal is supplied at Time T 1  so that first image information PIC 1  is displayed on the display portion  230  (see  FIG. 24 ). At Time T 2 , which is, for example, one second after Time T 1 , the selection signal is supplied so that the predetermined image information is displayed. 
     Alternatively, in the case where a predetermined event is not supplied in the interrupt processing, image information is displayed in the second mode. 
     For example, the selection signal is supplied at Time T 5  so that fourth image information PIC 4  is displayed on the display portion  230 . At Time T 6 , which is, for example, one second after Time T 5 , the selection signal is supplied so that the same image information is displayed. Note that the length of a period from Time T 5  to Time T 6  can be equal to that of a period from Time T 1  to Time T 2 . 
     For instance, in the case where the predetermined event is supplied in the interrupt processing, predetermined image information is displayed in the first mode. 
     Specifically, in the case where an event associated with a “page turning instruction” is supplied in the interrupt processing, image information is switched from one to another in a mode in which the selection signal is supplied to one scan line at a frequency of 30 Hz or higher, preferably 60 Hz or higher. 
     Alternatively, in the case where an event associated with the “scrolling instruction” is supplied in the interrupt processing, second image information PIC 2 , which includes part of the displayed first image information PIC 1  and the following part, is displayed in a mode in which the selection signal is supplied to one scan line at a frequency of 30 Hz or higher, preferably 60 Hz or higher. 
     Thus, for example, moving images in which images are gradually switched in accordance with the “page turning instruction” can be displayed smoothly. Alternatively, a moving image in which an image is gradually moved in accordance with the “scrolling instruction” can be displayed smoothly. 
     Specifically, the selection signal is supplied at Time T 3  after the event associated with the “scrolling instruction” is supplied so that the second image information PIC 2  whose display position and the like are changed from those of the first image information PIC 1  is displayed (see  FIG. 24 ). The selection signal is supplied at Time T 4  so that third image information PIC 3  whose display position and the like are changed from those of the second image information PIC 2  is displayed. Note that each of a period from Time T 2  to Time T 3 , a period from Time T 3  to Time T 4 , and a period from Time T 4  to Time T 5  is shorter than the period from Time T 1  to Time T 2 . 
     In the fourth step, the program moves to the fifth step when a termination instruction is supplied, and the program moves to the third step when the termination instruction is not supplied (see S 4  in  FIG. 23A ). 
     Note that in the interrupt processing, for example, the termination instruction can be supplied. 
     In the fifth step, the program terminates (see S 5  in  FIG. 23A ). 
     The interrupt processing includes sixth to ninth steps described below (see  FIG. 23B ). 
     In the sixth step, the processing proceeds to the seventh step when a predetermined event has been supplied during a predetermined period, whereas the processing proceeds to the eighth step when the predetermined event has not been supplied (see S 6  in  FIG. 23B ). 
     For example, the predetermined period is shorter than 0.5 seconds, preferably shorter than 0.1 seconds. 
     For example, the predetermined event can include an event associated with the termination instruction. 
     In the seventh step, the first mode is selected (see S 7  in  FIG. 23B ). 
     In the eighth step, the second mode is selected (see S 8  in  FIG. 23B ). 
     In the ninth step, the operation returns from the interrupt processing (see S 9  in  FIG. 23B ). 
     &lt;&lt;Predetermined Event&gt;&gt; 
     A variety of instructions can be associated with a variety of events. 
     The following instructions can be given as examples: “page-turning instruction” for switching displayed image information from one to another and “scroll instruction” for moving the display position of part of image information and displaying another part continuing from that part. 
     For example, the following events can be used: events supplied using a pointing device such as a mouse (e.g., “click” and “drag”) and events supplied to a touch panel with a finger or the like used as a pointer (e.g., “tap”, “drag”, and “swipe”). 
     For example, the position of a slide bar pointed by a pointer, the swipe speed, and the drag speed can be used as parameters of various instructions. 
     Specifically, a parameter that determines the page-turning speed or the like can be used to execute the “page-turning instruction,” and a parameter that determines the moving speed of the display position or the like can be used to execute the “scroll instruction.” 
     For example, the display brightness, contrast, or saturation may be changed in accordance with the page-turning speed and/or the scroll speed. 
     Specifically, in the case where the page-turning speed and/or the scroll speed are/is higher than the predetermined speed, the display brightness may be decreased in synchronization with the speed. 
     Alternatively, in the case where the page-turning speed and/or the scroll speed are/is higher than the predetermined speed, the contrast may be decreased in synchronization with the speed. 
     For example, the speed at which user&#39;s eyes cannot follow displayed images can be used as the predetermined speed. 
     The contrast can be reduced in such a manner that the gray level of a bright region (with a high gray level) included in image information is brought close to the gray level of a dark region (with a low gray level) included in the image information. 
     Alternatively, the contrast can be reduced in such a manner that the gray level of the dark region included in image information is brought close to the gray level of the bright region included in the image information. 
     Specifically, in the case where the page-turning speed and/or the scroll speed are/is higher than the predetermined speed, display may be performed such that the yellow tone is increased or the blue tone is decreased in synchronization with the speed. 
     Image information may be generated on the basis of information of the usage environment of the information processing device acquired by the sensor portion  250 . For example, user&#39;s favorite color can be used as the background color of the image information in accordance with the acquired ambient brightness or the like. 
     Thus, favorable environment can be provided for a user of the information processing device  200 . 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 8 
     In this embodiment, a semiconductor device (memory device) that can retain stored data even when not powered and that has an unlimited number of write cycles, and a CPU including the semiconductor device will be described. The CPU described in this embodiment can be used for the information processing device described in Embodiment 6, for example. 
     &lt;Memory Device&gt; 
     An example of a semiconductor device (memory device) which can retain stored data even when not powered and which has an unlimited number of write cycles is shown in  FIGS. 25A to 25C . Note that  FIG. 25B  is a circuit diagram of the structure in  FIG. 25A . 
     The semiconductor device illustrated in  FIGS. 25A and 25B  includes a transistor  3200  using a first semiconductor material, a transistor  3300  using a second semiconductor material, and a capacitor  3400 . 
     The first and second semiconductor materials preferably have different energy gaps. For example, the first semiconductor material can be a semiconductor material other than an oxide semiconductor (examples of such a semiconductor material include silicon (including strained silicon), germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, and an organic semiconductor), and the second semiconductor material can be an oxide semiconductor. A transistor using a material other than an oxide semiconductor, such as single crystal silicon, can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor has a low off-state current. 
     The transistor  3300  is a transistor in which a channel is formed in a semiconductor layer including an oxide semiconductor. Since the off-state current of the transistor  3300  is small, stored data can be retained for a long period. In other words, power consumption can be sufficiently reduced because a semiconductor memory device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided. 
     In  FIG. 25B , a first wiring  3001  is electrically connected to a source electrode of the transistor  3200 . A second wiring  3002  is electrically connected to a drain electrode of the transistor  3200 . A third wiring  3003  is electrically connected to one of a source electrode and a drain electrode of the transistor  3300 . A fourth wiring  3004  is electrically connected to a gate electrode of the transistor  3300 . A gate electrode of the transistor  3200  and the other of the source electrode and the drain electrode of the transistor  3300  are electrically connected to one electrode of the capacitor  3400 . A fifth wiring  3005  is electrically connected to the other electrode of the capacitor  3400 . 
     The semiconductor device in  FIG. 25A  has a feature that the potential of the gate electrode of the transistor  3200  can be retained, and thus enables writing, retaining, and reading of data as follows. 
     Writing and retaining of data are described. First, the potential of the fourth wiring  3004  is set to a potential at which the transistor  3300  is turned on, so that the transistor  3300  is turned on. Accordingly, the potential of the third wiring  3003  is supplied to the gate of the transistor  3200  and the capacitor  3400 . That is, a predetermined charge is supplied to the gate electrode of the transistor  3200  (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the fourth wiring  3004  is set to a potential at which the transistor  3300  is turned off, so that the transistor  3300  is turned off. Thus, the charge supplied to the gate electrode of the transistor  3200  is held (retaining). 
     Since the off-state current of the transistor  3300  is extremely small, the charge of the gate electrode of the transistor  3200  is retained for a long time. 
     Next, reading of data is described. An appropriate potential (a reading potential) is supplied to the fifth wiring  3005  while a predetermined potential (a constant potential) is supplied to the first wiring  3001 , whereby the potential of the second wiring  3002  varies depending on the amount of charge retained in the gate electrode of the transistor  3200 . This is because in the case of using an n-channel transistor as the transistor  3200 , an apparent threshold voltage V th_H  at the time when the high-level charge is given to the gate electrode of the transistor  3200  is lower than an apparent threshold voltage V th_L  at the time when the low-level charge is given to the gate electrode of the transistor  3200 . Here, an apparent threshold voltage refers to the potential of the fifth wiring  3005  which is needed to turn on the transistor  3200 . Thus, the potential of the fifth wiring  3005  is set to a potential V 0  which is between V th_H  and V th_L , whereby charge supplied to the gate electrode of the transistor  3200  can be determined. For example, in the case where the high-level charge is supplied to the gate electrode of the transistor  3200  in writing and the potential of the fifth wiring  3005  is V 0  (&gt;V th_H ), the transistor  3200  is turned on. On the other hand, in the case where the low-level charge is supplied to the gate electrode of the transistor  3200  in writing, even when the potential of the fifth wiring  3005  is V 0  (&lt;V th_L ), the transistor  3200  remains off. Thus, the data retained in the gate electrode of the transistor  3200  can be read by determining the potential of the second wiring  3002 . 
     Note that in the case where memory cells are arrayed, it is necessary that data of a desired memory cell be read. For example, the fifth wiring  3005  of memory cells from which data is not read may be supplied with a potential at which the transistor  3200  is turned off regardless of the potential supplied to the gate electrode, that is, a potential lower than V th_H , whereby only data of a desired memory cell can be read. 
     The semiconductor device illustrated in  FIG. 25C  differs from the semiconductor device illustrated in  FIG. 25A  in that the transistor  3200  is not provided. Also in this case, writing and retaining operation of data can be performed in a manner similar to the semiconductor device illustrated in  FIG. 25A . 
     Next, reading of data of the semiconductor device illustrated in  FIG. 25C  is described. When the transistor  3300  is turned on, the third wiring  3003  which is in a floating state and the capacitor  3400  are electrically connected to each other, and the charge is redistributed between the third wiring  3003  and the capacitor  3400 . As a result, the potential of the third wiring  3003  is changed. The amount of change in the potential of the third wiring  3003  varies depending on the potential of the one electrode of the capacitor  3400  (or the charge accumulated in the capacitor  3400 ). 
     For example, the potential of the third wiring  3003  after the charge redistribution is (C B ×V B0 +C×V)/(C B +C), where V is the potential of the one electrode of the capacitor  3400 , C is the capacitance of the capacitor  3400 , C B  is the capacitance component of the third wiring  3003 , and V B0  is the potential of the third wiring  3003  before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the one electrode of the capacitor  3400  is V 1  and V 0  (V 1 &gt;V 0 ), the potential of the bit line BL in the case of retaining the potential V 1  (=(C B ×V B0 +C×V 1 )/(C B +C)) is higher than the potential of the bit line BL in the case of retaining the potential V 0  (=(C B ×V B0 +C×V 0 )/(C B +C)). 
     Then, by comparing the potential of the third wiring  3003  with a predetermined potential, data can be read. 
     In this case, a transistor including the first semiconductor material may be used for a driver circuit for driving a memory cell, and a transistor including the second semiconductor material may be stacked over the driver circuit as the transistor  3300 . 
     When including a transistor in which a channel formation region is formed using an oxide semiconductor and which has an extremely small off-state current, the semiconductor device described in this embodiment can retain stored data for an extremely long time. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long time even when power is not supplied (note that a potential is preferably fixed). 
     Furthermore, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of a gate insulating film is not caused. That is, the semiconductor device described in this embodiment does not have a limit on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the state of the transistor (on or off), whereby high-speed operation can be easily achieved. 
     The above memory device can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), in addition to a central processing unit (CPU), and a radio frequency (RF) device, for example. 
     &lt;CPU&gt; 
     A CPU including the above memory device is described below. 
       FIG. 26  is a block diagram illustrating a configuration example of the CPU including the above memory device. 
     The CPU illustrated in  FIG. 26  includes, over a substrate  1190 , an arithmetic logic unit (ALU)  1191 , an ALU controller  1192 , an instruction decoder  1193 , an interrupt controller  1194 , a timing controller  1195 , a register  1196 , a register controller  1197 , a bus interface (BUS I/F)  1198 , a rewritable ROM  1199 , and a ROM interface (ROM I/F)  1189 . A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate  1190 . The ROM  1199  and the ROM interface  1189  may be provided over a separate chip. Needless to say, the CPU in  FIG. 26  is just an example in which the configuration is simplified, and an actual CPU may have a variety of configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU illustrated in  FIG. 26  or an arithmetic circuit is considered as one core; a plurality of the cores are included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be, for example, 8, 16, 32, or 64. 
     An instruction that is input to the CPU through the bus interface  1198  is input to the instruction decoder  1193  and decoded therein, and then, input to the ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195 . 
     The ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195  conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller  1192  generates signals for controlling the operation of the ALU  1191 . While the CPU is executing a program, the interrupt controller  1194  processes an interrupt request from an external input/output device or a peripheral circuit depending on its priority or a mask state. The register controller  1197  generates an address of the register  1196 , and reads/writes data from/to the register  1196  depending on the state of the CPU. 
     The timing controller  1195  generates signals for controlling operation timings of the ALU  1191 , the ALU controller  1192 , the instruction decoder  1193 , the interrupt controller  1194 , and the register controller  1197 . For example, the timing controller  1195  includes an internal clock generator for generating an internal clock signal on the basis of a reference clock signal, and supplies the internal clock signal to the above circuits. 
     In the CPU illustrated in  FIG. 26 , a memory cell is provided in the register  1196 . 
     In the CPU illustrated in  FIG. 26 , the register controller  1197  selects operation of retaining data in the register  1196  in accordance with an instruction from the ALU  1191 . That is, the register controller  1197  selects whether data is retained by a flip-flop or by a capacitor in the memory cell included in the register  1196 . When data retaining by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register  1196 . When data retaining by the capacitor is selected, the data is rewritten in the capacitor, and supply of the power supply voltage to the memory cell in the register  1196  can be stopped. 
       FIG. 27  is an example of a circuit diagram of a memory element that can be used for the register  1196 . A memory element  1200  includes a circuit  1201  in which stored data is volatile when power supply is stopped, a circuit  1202  in which stored data is nonvolatile even when power supply is stopped, a switch  1203 , a switch  1204 , a logic element  1206 , a capacitor  1207 , and a circuit  1220  having a selecting function. The circuit  1202  includes a capacitor  1208 , a transistor  1209 , and a transistor  1210 . Note that the memory element  1200  may further include another element such as a diode, a resistor, or an inductor, as needed. 
     Here, the above-described memory device can be used as the circuit  1202 . When supply of a power supply voltage to the memory element  1200  is stopped, a ground potential (0 V) or a potential at which the transistor  1209  in the circuit  1202  is turned off continues to be input to a gate of the transistor  1209 . For example, the gate of the transistor  1209  is grounded through a load such as a resistor. 
     Shown here is an example in which the switch  1203  is a transistor  1213  having one conductivity type (e.g., an n-channel transistor) and the switch  1204  is a transistor  1214  having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor). A first terminal of the switch  1203  corresponds to one of a source and a drain of the transistor  1213 , a second terminal of the switch  1203  corresponds to the other of the source and the drain of the transistor  1213 , and conduction or non-conduction between the first terminal and the second terminal of the switch  1203  (i.e., the on/off state of the transistor  1213 ) is selected by a control signal RD input to a gate of the transistor  1213 . A first terminal of the switch  1204  corresponds to one of a source and a drain of the transistor  1214 , a second terminal of the switch  1204  corresponds to the other of the source and the drain of the transistor  1214 , and conduction or non-conduction between the first terminal and the second terminal of the switch  1204  (i.e., the on/off state of the transistor  1214 ) is selected by the control signal RD input to a gate of the transistor  1214 . 
     One of a source and a drain of the transistor  1209  is electrically connected to one of a pair of electrodes of the capacitor  1208  and a gate of the transistor  1210 . Here, the connection portion is referred to as a node N 2 . One of a source and a drain of the transistor  1210  is electrically connected to a wiring that can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch  1203  (the one of the source and the drain of the transistor  1213 ). The second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) is electrically connected to the first terminal of the switch  1204  (the one of the source and the drain of the transistor  1214 ). The second terminal of the switch  1204  (the other of the source and the drain of the transistor  1214 ) is electrically connected to a wiring that can supply a power supply potential VDD. The second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ), the first terminal of the switch  1204  (the one of the source and the drain of the transistor  1214 ), an input terminal of the logic element  1206 , and one of a pair of electrodes of the capacitor  1207  are electrically connected to each other. Here, the connection portion is referred to as a node N 1 . The other of the pair of electrodes of the capacitor  1207  can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor  1207  can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor  1207  is electrically connected to the wiring that can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor  1208  can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor  1208  can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor  1208  is electrically connected to the wiring that can supply a low power supply potential (e.g., a GND line). 
     The capacitor  1207  and the capacitor  1208  are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized. 
     A control signal WE is input to a first gate (first gate electrode) of the transistor  1209 . As for each of the switch  1203  and the switch  1204 , a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD that is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state. 
     A signal corresponding to data retained in the circuit  1201  is input to the other of the source and the drain of the transistor  1209 .  FIG. 27  illustrates an example in which a signal output from the circuit  1201  is input to the other of the source and the drain of the transistor  1209 . The logic value of a signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) is inverted by the logic element  1206 , and the inverted signal is input to the circuit  1201  through the circuit  1220 . 
     In the example of  FIG. 27 , a signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) is input to the circuit  1201  through the logic element  1206  and the circuit  1220 ; however, one embodiment of the present invention is not limited thereto. The signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) may be input to the circuit  1201  without its logic value being inverted. For example, in the case where the circuit  1201  includes a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is retained, the signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) can be input to the node. 
     In  FIG. 27 , the transistors included in the memory element  1200  except for the transistor  1209  can each be a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or in the substrate  1190 . For example, the transistor can be a transistor whose channel is formed in a silicon layer or a silicon substrate. Alternatively, a transistor in which a channel is formed in an oxide semiconductor film can be used for all the transistors in the memory element  1200 . Further alternatively, in the memory element  1200 , a transistor in which a channel is formed in an oxide semiconductor film can be included besides the transistor  1209 , and a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or the substrate  1190  can be used for the rest of the transistors. 
     As the circuit  1201  in  FIG. 27 , for example, a flip-flop circuit can be used. As the logic element  1206 , for example, an inverter or a clocked inverter can be used. 
     In a period during which the memory element  1200  is not supplied with the power supply voltage, the semiconductor device described in this embodiment can retain data stored in the circuit  1201  by the capacitor  1208  that is provided in the circuit  1202 . 
     The off-state current of a transistor in which a channel is formed in an oxide semiconductor film is extremely small. For example, the off-state current of a transistor in which a channel is formed in an oxide semiconductor film is significantly smaller than that of a transistor in which a channel is formed in silicon having crystallinity. Thus, when the transistor in which a channel is formed in an oxide semiconductor film is used as the transistor  1209 , a signal is retained in the capacitor  1208  for a long time also in a period during which the power supply voltage is not supplied to the memory element  1200 . The memory element  1200  can accordingly retain the stored content (data) also in a period during which the supply of the power supply voltage is stopped. 
     Since the memory element performs pre-charge operation with the switch  1203  and the switch  1204 , the time required for the circuit  1201  to retain original data again after the supply of the power supply voltage is restarted can be shortened. 
     In the circuit  1202 , a signal retained by the capacitor  1208  is input to the gate of the transistor  1210 . Thus, after supply of the power supply voltage to the memory element  1200  is restarted, the state (the on state or the off state) of the transistor  1210  is determined in accordance with the signal retained by the capacitor  1208  and can be read from the circuit  1202 . Consequently, an original signal can be accurately read even when a potential corresponding to the signal retained by the capacitor  1208  changes to some degree. 
     By using the above-described memory element  1200  in a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Furthermore, shortly after the supply of the power supply voltage is restarted, the memory device can be returned to the same state as that before the power supply is stopped. Thus, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor, resulting in lower power consumption. 
     Although the memory element  1200  is used in a CPU in this embodiment, the memory element  1200  can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and an RF device. 
     At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     Embodiment 9 
     In this embodiment, a display module and electronic devices which include a reflective display device of one embodiment of the present invention will be described with reference to  FIGS. 28A to 28H . 
       FIGS. 28A to 28G  illustrate electronic devices. These electronic devices can include a housing  5000 , a display portion  5001 , a speaker  5003 , an LED lamp  5004 , operation keys  5005  (including a power switch and an operation switch), a connection terminal  5006 , a sensor  5007  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone  5008 , and the like. 
       FIG. 28A  illustrates a mobile computer which can include a switch  5009 , an infrared port  5010 , and the like in addition to the above components.  FIG. 28B  illustrates a portable image reproducing device (e.g., a DVD reproducing device) provided with a recording medium, and the portable image reproducing device can include a second display portion  5002 , a recording medium reading portion  5011 , and the like in addition to the above components.  FIG. 28C  illustrates a goggle-type display which can include the second display portion  5002 , a support portion  5012 , an earphone  5013 , and the like in addition to the above components.  FIG. 28D  illustrates a portable game console which can include the recording medium reading portion  5011  and the like in addition to the above components.  FIG. 28E  illustrates a digital camera with a television reception function, and the digital camera can include an antenna  5014 , a shutter button  5015 , an image receiving portion  5016 , and the like in addition to the above components.  FIG. 28F  illustrates a portable game console which can include the second display portion  5002 , the recording medium reading portion  5011 , and the like in addition to the above components.  FIG. 28G  illustrates a portable television receiver which can include a charger  5017  capable of transmitting and receiving signals, and the like in addition to the above components. 
     The electronic devices in  FIGS. 28A to 28G  can have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a recording medium and displaying it on the display portion. Furthermore, the electronic device including a plurality of display portions can have a function of displaying image information mainly on one display portion while displaying text information mainly on another display portion, a function of displaying a three-dimensional image by displaying images on a plurality of display portions with a parallax taken into account, or the like. Furthermore, the electronic device including an image receiving portion can have a function of shooting a still image, a function of taking moving images, a function of automatically or manually correcting a shot image, a function of storing a shot image in a recording medium (an external recording medium or a recording medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like. Note that functions of the electronic devices in  FIGS. 28A to 28G  are not limited thereto, and the electronic devices can have a variety of functions. 
       FIG. 28H  illustrates a smart watch, which includes a housing  7302 , a display panel  7304 , operation buttons  7311  and  7312 , a connection terminal  7313 , a band  7321 , a clasp  7322 , and the like. 
     The display panel  7304  mounted in the housing  7302  serving as a bezel includes a non-rectangular display region. The display panel  7304  may have a rectangular display region. The display panel  7304  can display an icon  7305  indicating time, another icon  7306 , and the like. 
     The smart watch in  FIG. 28H  can have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a recording medium and displaying it on the display portion. 
     The housing  7302  can include a speaker, a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), a microphone, and the like. Note that the smart watch can be manufactured using the light-emitting element for the display panel  7304 . 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     In this specification and the like, for example, when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected, the case where X and Y are functionally connected, and the case where X and Y are directly connected are included therein. Accordingly, another element may be interposed between elements having a connection relation shown in drawings and texts, without limiting to a predetermined connection relation, for example, the connection relation shown in the drawings and the texts. 
     Here, X and Y each denote an object (e.g., a device, an element, a circuit, a line, an electrode, a terminal, a conductive film, or a layer). 
     For example, in the case where X and Y are directly connected, an element that enables 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) is not connected between X and Y, and X and Y are connected without the element that enables 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) provided therebetween. 
     For example, in the case where X and Y are electrically connected, one or more elements that enable 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. A switch is controlled to be 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. Note that for example, in the case where a signal output from X is transmitted to Y even when another circuit is interposed between X and Y, X and Y are functionally connected. Note that 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 when it is explicitly described that X and Y are electrically connected, the case where X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), the case where X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and the case where X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween) are included therein. That is, 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”. 
     For example, any of the following expressions can be used for the case where a source (or a first terminal or the like) of a transistor is electrically connected to X through (or not through) Z1 and a drain (or a second terminal or the like) of the transistor is electrically connected to Y through (or not through) Z2, or the case where a source (or a first terminal or the like) of a transistor is directly connected to one part of Z1 and another part of Z1 is directly connected to X while a drain (or a second terminal or the like) of the transistor is directly connected to one part of Z2 and another part of Z2 is directly connected to Y. 
     Examples of the expressions include, “X, Y, a source (or a first terminal or the like) of a transistor, and a drain (or a second terminal or the like) of the transistor are electrically connected to each other, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in this order”, “a source (or a first terminal or the like) of a transistor is electrically connected to X, a drain (or a second terminal or the like) of the transistor is electrically connected to Y, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in this order”, and “X is electrically connected to Y through a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are provided to be connected in this order”. When the connection order in a circuit structure is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope. 
     Other examples of the expressions include, “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least a first connection path, the first connection path does not include a second connection path, the second connection path is a path between the source (or the first terminal or the like) of the transistor and a drain (or a second terminal or the like) of the transistor, Z1 is on the first connection path, the drain (or the second terminal or the like) of the transistor is electrically connected to Y through at least a third connection path, the third connection path does not include the second connection path, and Z2 is on the third connection path”. Another example of the expression is “a source (or a first terminal or the like) of a transistor is electrically connected to X at least with a first connection path through Z1, the first connection path does not include a second connection path, the second connection path includes a connection path through which the transistor is provided, a drain (or a second terminal or the like) of the transistor is electrically connected to Y at least with a third connection path through Z2, and the third connection path does not include the second connection path”. Still another example of the expression is “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least Z1 on a first electrical path, the first electrical path does not include a second electrical path, the second electrical path is an electrical path from the source (or the first terminal or the like) of the transistor to a drain (or a second terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor is electrically connected to Y through at least Z2 on a third electrical path, the third electrical path does not include a fourth electrical path, and the fourth electrical path is an electrical path from the drain (or the second terminal or the like) of the transistor to the source (or the first terminal or the like) of the transistor”. When the connection path in a circuit structure is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope. 
     Note that these expressions are examples and there is no limitation on the expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, and a layer). 
     Even when independent components are electrically connected to each other in a circuit diagram, one component has functions of a plurality of components in some cases. For example, when part of a wiring also serves as an electrode, one conductive film serves 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. 
     This application is based on Japanese Patent Application serial no. 2015-157369 filed with Japan Patent Office on Aug. 7, 2015 and Japanese Patent Application serial no. 2016-125619 filed with Japan Patent Office on Jun. 24, 2016, the entire contents of which are hereby incorporated by reference.