Patent Publication Number: US-9852778-B2

Title: Semiconductor device, memory device, and electronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/963,434, filed Dec. 9, 2015, now allowed, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2014-250551 on Dec. 11, 2014, both of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     One embodiment of the present invention relates to a semiconductor device, a memory device, and an electronic device. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a power storage device, an imaging device, a memory device, a processor, an electronic device, a method for driving any of them, a method for manufacturing any of them, a method for testing any of them, and a system including any of them. 
     BACKGROUND ART 
     In recent years, display devices have been used in various electronic devices such as television receivers, personal computer monitors, and smart phones, and the performance of the display devices has been improved to achieve higher definition, lower power consumption, and other various objectives. 
     In addition, semiconductor devices such as central processing units (CPUs), memories, and sensors have been used in various electronic devices such as personal computers, smart phones, and digital cameras. The performance of the semiconductor devices has also been improved to achieve miniaturization, lower power consumption, and other various objectives. 
     One of the ways that have been proposed to achieve higher performance such as higher definition, lower power consumption, and miniaturization is the use of an oxide semiconductor for a semiconductor layer (hereinafter also referred to as an active layer, a channel layer, or a channel formation region) of a transistor in a semiconductor device. An example of the transistor includes an oxide of indium, gallium, and zinc (hereinafter also referred to as an In—Ga—Zn oxide) for a channel layer (see Patent Document 1). 
     Patent Document 2 discloses a memory device including an oxide semiconductor transistor and a single crystal silicon transistor. 
     REFERENCES 
     Patent Documents 
     [Patent Document 1] Japanese Translation of PCT International Application No. H11-505377 
     [Patent Document 2] Japanese Published Patent Application No. 2012-256400 
     DISCLOSURE OF INVENTION 
     An object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a module including the novel semiconductor device. Another object of one embodiment of the present invention is to provide an electronic device using the module including the novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel semiconductor device, a novel memory device, a novel module, a novel electronic device, and the like. 
     Another object of one embodiment of the present invention is to provide a semiconductor device with a large storage capacity. Another object of one embodiment of the present invention is to provide a semiconductor device with a low power consumption. Another object of one embodiment of the present invention is to provide a highly reliable semiconductor device. 
     Note that the objects of one embodiment of the present invention are not limited to the above objects. The objects described above do not disturb the existence of other objects. The other objects are ones that are not described above and will be described below. The other objects will be apparent from and can be derived from the description of the specification, the drawings, and the like by those skilled in the art. One embodiment of the present invention solves at least one of the above objects and the other objects. One embodiment of the present invention need not solve all the aforementioned objects and the other objects. 
     (1) 
     One embodiment of the present invention is a semiconductor device including a first circuit. The first circuit includes a first input terminal, a first output terminal, a first transistor, and a second circuit. The second circuit includes a second to a (2n+1)th transistor, a first to an n-th capacitor, a first wiring, a first to an n-th storage node (n is an integer of 2 or more). A gate of a 2i-th transistor is electrically connected to the first wiring (i is an integer of 1 to n), a first terminal of the 2i-th transistor is electrically connected to a gate of a (2i+1)th transistor and a first terminal of an i-th capacitor through an i-th storage node. The first input terminal is electrically connected to a first terminal of the first transistor. A second terminal of the first transistor is electrically connected to a first terminal of a third transistor. A second terminal of a (2i−1)th transistor is electrically connected to a first terminal of the (2i+1)th transistor. A second terminal of the (2n+1)th transistor is electrically connected to the first output terminal. 
     (2) 
     Another embodiment of the present invention is a semiconductor device including a first circuit. The first circuit includes a first input terminal, a first output terminal, a first transistor, and a second circuit. The second circuit includes a second to a (2n+1)th transistor, a first to an n-th capacitor, a first wiring, a first to an n-th storage node (n is an integer of 2 or more). A gate of a 2i-th transistor is electrically connected to the first wiring (i is an integer of 1 to n), a first terminal of the 2i-th transistor is electrically connected to a gate of a (2i+1)th transistor and a first terminal of an i-th capacitor through an i-th storage node. The first input terminal is electrically connected to a first terminal of the third transistor. A second terminal of a (2i−1)th transistor is electrically connected to a first terminal of the (2i+1)th transistor. A second terminal of the (2n+1)th transistor is electrically connected to a first terminal of the first transistor. A second terminal of the first transistor is electrically connected to the first output terminal. 
     (3) 
     Another embodiment of the present invention is the semiconductor device according to (1) or (2), which includes a memory cell array and a plurality of first circuits. The memory cell array includes a plurality of second circuits. 
     (4) 
     Another embodiment of the present invention is the semiconductor device according to (3), which includes a plurality of memory cell arrays that overlap with each other. 
     (5) 
     Another embodiment of the present invention is the semiconductor device according to any one of (1) to (4), which includes a third circuit. The third circuit is configured to supply a potential to the first circuit and read a potential output from the first circuit. The first input terminal is electrically connected to the third circuit. The first output terminal is electrically connected to the third circuit. 
     (6) 
     Another embodiment of the present invention is the semiconductor device according to (5), in which the second circuit is over the third circuit. 
     (7) 
     Another embodiment of the present invention is the semiconductor device according to any one of (1) to (6), in which the first to the (2n+1)th transistor include an oxide semiconductor in a channel formation region. 
     (8) 
     Another embodiment of the present invention is the semiconductor device according to any one of (1) to (6), in which the first transistor includes silicon in a channel formation region and the second to the (2n+1)th transistor include an oxide semiconductor in a channel formation region. 
     (9) 
     Another embodiment of the present invention is the semiconductor device according to any one of (1) to (8), in which at least one of the first to the (2n+1)th transistor includes a back gate. 
     (10) 
     Another embodiment of the present invention is the semiconductor device according to any one of (1) to (9), in which the first to the n-th storage node are configured to retain M-bit data (M is an integer of 1 or more) as a potential and output, from the first output terminal, a potential corresponding to the potential maintained at the first to the n-th storage node. 
     According to one embodiment of the present invention, a novel semiconductor device can be provided. According to another embodiment of the present invention, a module including the novel semiconductor device can be provided. According to another embodiment of the present invention, an electronic device using the module including the novel semiconductor device can be provided. According to another embodiment of the present invention, a novel semiconductor device, a novel memory device, a novel module, a novel electronic device, and the like can be provided. 
     According to another embodiment of the present invention, a semiconductor device with a large storage capacity can be provided. According to another embodiment of the present invention, a semiconductor device with a low power consumption can be provided. According to another embodiment of the present invention, a highly reliable semiconductor device can be provided. 
     Note that the effects of one embodiment of the present invention are not limited to the above effects. The effects described above do not disturb the existence of other effects. The other effects are ones that are not described above and will be described below. The other effects will be apparent from and can be derived from the description of the specification, the drawings, and the like by those skilled in the art. One embodiment of the present invention has at least one of the above effects and the other effects. Accordingly, one embodiment of the present invention does not have the aforementioned effects in some cases. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a circuit diagram of a semiconductor device of one embodiment of the present invention; 
         FIG. 2  is a circuit diagram of a memory cell of one embodiment of the present invention; 
         FIGS. 3A and 3B  are timing charts showing an operation example of a memory cell of one embodiment of the present invention; 
         FIGS. 4A and 4B  are timing charts showing an operation example of a memory cell of one embodiment of the present invention; 
         FIG. 5  is a timing chart showing an operation example of a memory cell of one embodiment of the present invention; 
         FIGS. 6A and 6B  are a top view and a cross-sectional view of a memory cell of one embodiment of the present invention; 
         FIG. 7  is a circuit diagram of a memory cell of one embodiment of the present invention; 
         FIG. 8  is a timing chart showing an operation example of a semiconductor device of one embodiment of the present invention; 
         FIG. 9  is a timing chart showing an operation example of a semiconductor device of one embodiment of the present invention; 
         FIG. 10  is a top view of a semiconductor device of one embodiment of the present invention; 
         FIG. 11  is a cross-sectional view of a semiconductor device of one embodiment of the present invention; 
         FIG. 12  is a circuit diagram of a semiconductor device of one embodiment of the present invention; 
         FIG. 13  is a circuit diagram of a semiconductor device of one embodiment of the present invention; 
         FIG. 14  illustrates a semiconductor device of one embodiment of the present invention; 
         FIG. 15  illustrates a semiconductor device of one embodiment of the present invention; 
         FIG. 16  illustrates a semiconductor device of one embodiment of the present invention; 
         FIG. 17  illustrates a memory device of one embodiment of the present invention; 
         FIGS. 18A to 18D  illustrate structure examples of OS transistors: A, a top view; B, a cross-sectional view along dashed-dotted line y 1 -y 2 ; C, a cross-sectional view along dashed-dotted line x 1 -x 2 ; and D, a cross-sectional view along dashed-dotted line x 3 -x 4 ; 
         FIG. 19A  is an enlarged view of part of  FIG. 18B , and  FIG. 19B  is an energy band diagram of an OS transistor; 
         FIGS. 20A to 20C  are cross-sectional views each illustrating a structure example of an OS transistor; 
         FIGS. 21A to 21E  show structural analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD and selected-area electron diffraction patterns of a CAAC-OS; 
         FIGS. 22A to 22E  show a cross-sectional TEM image and plan-view TEM images of a CAAC-OS and images obtained through analysis thereof; 
         FIGS. 23A to 23D  show electron diffraction patterns and a cross-sectional TEM image of an nc-OS; 
         FIGS. 24A and 24B  show cross-sectional TEM images of an a-like OS; 
         FIG. 25  shows a change of crystal parts of an In—Ga—Zn oxide due to electron irradiation 
         FIGS. 26A and 26B  are a flow chart and a perspective view showing one embodiment of the present invention; and 
         FIGS. 27A to 27F  illustrate electronic devices of one embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described in detail with reference to drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. Furthermore, the present invention is not construed as being limited to the description of the embodiments. In describing structures of the invention with reference to the drawings, common reference numerals are used for the same portions in different drawings. Note that the same hatched pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases. 
     Note that the size, the thickness of films (layers), or regions in diagrams are exaggerated for clarity in some cases. 
     A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a ground potential (GND) or a source potential). A voltage can be referred to as a potential and vice versa. 
     In this specification, when a gate of an n-channel transistor (a p-channel transistor) has a high-level potential, the potential is high enough in a positive direction to bring a drain and a source of the n-channel transistor (the p-channel transistor) into conduction (out of conduction). In this specification, when a gate of an n-channel transistor (a p-channel transistor) has a low-level potential, the potential is high enough in a negative direction to bring a drain and a source of the n-channel transistor (the p-channel transistor) out of conduction (into conduction). 
     In this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components. Thus, the terms do not limit the number or order of components. For example, in the present specification and the like, a “first” component in one embodiment can be referred to as a “second” component in other embodiments or claims. Furthermore, in the present specification and the like, a “first” component in one embodiment can be referred to without the ordinal number in other embodiments or claims. 
     The same elements or elements having similar functions, elements formed using the same material, elements formed at the same time, or the like in the drawings are denoted by the same reference numerals in some cases, and the description thereof is not repeated in some cases. 
     Note that a “semiconductor” has characteristics of an “insulator” in some cases when, for example, the conductivity is sufficiently low. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because the border therebetween is not clear. Accordingly, a “semiconductor” in this specification can be called an “insulator” in some cases. Similarly, an “insulator” in this specification can be called a “semiconductor” in some cases. 
     Note that a “semiconductor” has characteristics of a “conductor” in some cases when, for example, the conductivity is sufficiently high. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because the border therebetween is not clear. Accordingly, a “semiconductor” in this specification can be called a “conductor” in some cases. Similarly, a “conductor” in this specification can be called a “semiconductor” in some cases. 
     Note that an impurity in a semiconductor refers to, for example, elements other than the main components of a semiconductor layer. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. When an impurity is contained, for example, the density of states (DOS) may be formed in a semiconductor, the carrier mobility may be decreased, or the crystallinity may be decreased. When the semiconductor is an oxide semiconductor, examples of an impurity that changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components, and specifically include, for example, hydrogen (also included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. In particular, oxygen vacancies may be formed by entry of impurities such as hydrogen. When the semiconductor is silicon, examples of an impurity that changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements. 
     In this specification, the phrase “A has a region with a concentration B” includes, for example, “the concentration of the entire region in a region of A in the depth direction is B”, “the average concentration in a region of A in the depth direction is B”, “the median value of a concentration in a region of A in the depth direction is B”, “the maximum value of a concentration in a region of A in the depth direction is B”, “the minimum value of a concentration in a region of A in the depth direction is B”, “a convergence value of a concentration in a region of A in the depth direction is B”, and “a concentration in a region of A in which a probable value is obtained in measurement is B”. 
     In this specification, the phrase “A has a region with a size B, a length B, a thickness B, a width B, or a distance B” includes, for example, “the size, the length, the thickness, the width, or the distance of the entire region in a region of A is B”, “the average value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the median value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the maximum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the minimum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “a convergence value of the size, the length, the thickness, the width, or the distance of a region of A is B”, and “the size, the length, the thickness, the width, or the distance of a region of A in which a probable value is obtained in measurement is B”. 
     In this specification and the like, in description of connections of a transistor, one of a source and a drain is referred to as “one of a source and a drain” (or a first electrode or a first terminal), and the other of the source and the drain is referred to as “the other of the source and the drain” (or a second electrode or a second terminal). This is because a source and a drain of a transistor are interchangeable depending on the structure, operation conditions, or the like of the transistor. Note that the source or the drain of the transistor can also be referred to as a source (or drain) terminal, a source (or drain) electrode, or the like as appropriate depending on the situation. 
     Note that the channel length refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a top view. In one transistor, channel widths in all regions do not necessarily have the same value. In other words, a channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, a channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     Note that depending on a transistor structure, a channel width in a region where a channel is formed actually (hereinafter referred to as an effective channel width) is different from a channel width shown in a top view of a transistor (hereinafter referred to as an apparent channel width) in some cases. For example, in a transistor having a three-dimensional structure, an effective channel width is greater than an apparent channel width shown in a top view of the transistor, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a three-dimensional structure, the proportion of a channel region formed in a side surface of a semiconductor is high in some cases. In that case, an effective channel width obtained when a channel is actually formed is greater than an apparent channel width shown in the top view. 
     In a transistor having a three-dimensional structure, an effective channel width is difficult to measure in some cases. For example, estimation of an effective channel width from a design value requires an assumption that the shape of a semiconductor is known. Therefore, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure an effective channel width accurately. 
     Therefore, in this specification, in a top view of a transistor, an apparent channel width that is a length of a portion where a source and a drain face each other in a region where a semiconductor and a gate electrode overlap with each other is referred to as a surrounded channel width (SCW) in some cases. Furthermore, in this specification, in the case where the term “channel width” is simply used, it may denote a surrounded channel width or an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may denote an effective channel width in some cases. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by obtaining and analyzing a cross-sectional TEM image and the like. 
     Note that in the case where the electric field mobility, the current value per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width is used for the calculation in some cases. In that case, the values might be different from those calculated by using an effective channel width. 
     In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. In addition, the terra “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. The term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°. 
     In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system. 
     Embodiment 1 
     An example of a semiconductor device of one embodiment of the present invention will be described below. 
     &lt;Memory Cell&gt; 
     Described first is an example of a memory cell which is a basic configuration of the semiconductor device of one embodiment of the present invention. 
     &lt;&lt;Configuration Example of Memory Cell&gt;&gt; 
       FIG. 2  illustrates a memory cell MC[i] (i is an integer of 1 or more). The memory cell MC[i] includes a transistor TRA[i], a transistor TRB[i], and a capacitor C[i] and is electrically connected to a wiring CNODE[i], a wiring WL, a wiring D[i], a wiring S[i], and a wiring WBL[i]. The transistor TRA[i] includes a back gate BGA[i], and the transistor TRB[i] includes a back gate BGB[i]. 
     A gate of the transistor TRA[i] is electrically connected to the wiring WL; one of a source and a drain of the transistor TRA[i] is electrically connected to the wiring WBL[i]; the capacitor C[i] is connected between the other of the source and the drain of the transistor TRA[i] and the wiring CNODE[i]; a gate of the transistor TRB[i] is electrically connected to the other of the source and the drain of the transistor TRA[i]; one of a source and a drain of the transistor TRB[i] is electrically connected to the wiring D[i]; and the other of the source and the drain of the transistor TRB[i] is electrically connected to the wiring S[i]. 
     Next, a structure of the memory cell MC[i] will be described.  FIG. 6A  is a top view of part of the memory cell MC[i], and  FIG. 6B  is a cross-sectional view along dashed-dotted lines A 1 -A 2  and A 3 -A 4  in  FIG. 6A . 
     Note that in  FIG. 6B , all regions that are not hatched and are not given reference numerals denoting a conductor, a semiconductor, an element, a circuit, and the like represent insulators. 
     The transistor TRA[i], the transistor TRB[i], and the wiring WL are provided in the same layer. Above the layer, the trench-shape capacitor C[i], the wiring CNODE[i], and the wiring WBL[i] are provided in this order. 
     The transistor TRA[i] includes the wiring WL[i], a semiconductor  101   a , a conductor  103   a , a conductor  103   b , and the back gate BGA[i]. The transistor TRB[i] includes a conductor  102 , a semiconductor  101   b , the wiring S[i], the wiring D[i], and the back gate BGB[i]. Note that the wiring S[i] and the wiring D[i] are not illustrated in  FIG. 6B  because they are not on dashed-dotted line A 1 -A 2 . 
     Above the conductor  103   a , a conductor  104   a , a conductor  105   a , a conductor  106 , and a conductor  107  are provided in order. The conductor  103   a  is electrically connected to the wiring WBL[i] through the conductors  104   a ,  105   a ,  106 , and  107 . 
     A conductor  104   b  is in contact with and electrically connected to the conductors  103   b  and  102 . The trench-shape capacitor C[i] is provided above the conductor  104   b , and the conductor  104   b  is electrically connected to one electrode of the capacitor C[i]. 
     A conductor  105   b  is provided above the capacitor C[i], and the other electrode of the capacitor C[i] is electrically connected to the wiring CNODE[i] through the conductor  105   b.    
     In particular, the semiconductors  101   a  and  101   b  are preferably oxide semiconductors containing at least one of indium, zinc, and gallium. A transistor using such an oxide semiconductor can have an extremely low off-state current. 
     According to an estimate using a 60-nm-technology, the number of memory cells MC[i] per unit area could be increased. That is, it is possible to reduce the area needed to increase one-bit storage capacity. For example, according to an estimate using the memory cell MC[i] in  FIG. 6A , the length of OX is 0.365 μm and the length of OY is 0.14 μm; thus, the memory cell MC[i] has an area of 0.0511 μm 2 . In the case where the memory cell MC[i] stores four bits, the area for each bit is 0.0127775 μm 2 . 
     Note that the structure of the memory device (memory cell) of one embodiment of the present invention is not limited to that illustrated in  FIGS. 6A and 6B . For example, in  FIG. 6B , an upper electrode of the capacitor C[i] can be led to serve as the wiring CNODE[i]. In that case, the conductors  105   b ,  106 , and  107  and the wiring CNODE[i] above the capacitor C[i] can be omitted, reducing the number of manufacturing processes of the memory device. 
     &lt;&lt;Memory Cell Operation Example&gt;&gt; 
     Next, writing and reading operations of the memory cell MC[i] will be described; here, 1-bit data is written to and read from each memory cell. The 1-bit data can handle two kinds of data “0” and “1” in a binary number. In the following description, in some cases, potentials indicating data “0” and “1” are represented by V 0  and V 1 , respectively, and data “0” and “1” are collectively referred to as two levels. 
       FIG. 3A  is a timing chart showing an operation of writing V 1  to the memory cell MC[i]. From Time T 0  to Time T 1 , the potentials of the wiring WL, the wiring CNODE[i], the node FN[i], the wiring S[i], and the wiring D[i] are Low (hereinafter also referred to as a low level or an L level), and the potentials of the wiring WBL[i] and the node FN[i] are V 0 . In particular, the low-level potential supplied to the wiring CNODE[i] is preferably a reference potential. 
     At Time T 1 , the potential of the wiring WL is High (hereinafter also referred to as a high level or an H level). Then, the drain and source of the transistor TRA[i] are electrically connected, so that the potential of the wiring WBL[i] is supplied to the node FN[i]. That is, in the case where the wiring WBL[i] has the potential V 1  from Time T 1  to Time T 2 , the node FN[i] also has the potential V 1 . At this time, a potential difference occurs between the node FN[i] and the wiring CNODE[i], and the potential difference is maintained in the capacitor C[i]. 
     At Time T 2 , the potential of the wiring WL is set at the low level, making no electrical conduction between the drain and source of the transistor TRA[i]. Even when the potential of the wiring WBL[i] is at the low level from Time T 2  to Time T 3 , the potential of the node FN[i] can be maintained at V 1  by the capacitor C[i]. 
       FIG. 3B  is a timing chart showing an operation of reading V 1  from the memory cell MC[i]. Time T 4  is after Time T 3  in the timing chart of  FIG. 3A . In other words, the potentials of the wiring WL, the wiring WBL[i], the wiring CNODE[i], the node FN[i], the wiring D[i], and the wiring S[i] at Time T 4  are those after Time T 3  in  FIG. 3A . 
     From Time T 5  to Time T 6 , a high-level potential is supplied to the wiring S[i]. At this time, the potential of the node FN[i] is V 1 , so that the transistor TRB[i] is turned on and a high-level potential is supplied to the wiring D[i]. Thus, the potential (V 1 ) of the node FN[i] can be read when the wiring D[i] of the memory cell MC[i] is electrically connected to a reading circuit. 
     When the reading operation is completed, a low-level potential is supplied to the wiring S[i] at Time T 6 . Since the potential of the node FN[i] is V 1 , the transistor TRB[i] is turned on and a low-level potential is supplied to the wiring D[i] from Time T 6  to Time T 7 . Note that at this time, the reading circuit electrically connected to the wiring D[i] of the memory cell MC[i] does not operate. 
       FIG. 4A  is a timing chart showing an operation of writing V 0  to the memory cell MC[i]. Time T 8  is after Time T 7  in the timing chart of  FIG. 3B . In other words, the potentials of the wiring WL, the wiring WBL[i], the wiring CNODE[i], the node FN[i], the wiring D[i], and the wiring S[i] at Time T 8  are those after Time T 7  in  FIG. 3B . 
     At Time T 9 , the potential of the wiring WL is set at the high level. Then, the drain and source of the transistor TRA[i] are electrically connected, so that the potential of the wiring WBL[i] is supplied to the node FN[i]. That is, in the case where the wiring WBL[i] has the potential V 0  from Time T 9  to Time T 10 , the node FN[i] also has the potential V 0 . At this time, a potential difference occurs between the node FN[i] and the wiring CNODE[i], and the potential difference is maintained in the capacitor C[i]. 
     At Time T 10 , the potential of the wiring WL is set at the low level, making no electrical conduction between the drain and source of the transistor TRA[i]. Even when the potential of the wiring WBL[i] is at the low level from Time T 10  to Time T 11 , the potential of the node FN[i] can be maintained at V 0  by the capacitor C[i]. 
       FIG. 4B  is a timing chart showing an operation of reading V 0  from the memory cell MC[i]. Time T 12  is after Time T 11  in the timing chart of  FIG. 4A . In other words, the potentials of the wiring WL, the wiring WBL[i], the wiring CNODE[i], the node FN[i], the wiring D[i], and the wiring S[i] at Time T 12  are those after Time T 11  in  FIG. 4A . 
     From Time T 13  to Time T 14 , a high-level potential is supplied to the wiring S[i]. At this time, the potential of the node FN[i] is V 0 , so that the transistor TRB[i] is turned off and a high-level potential is not supplied to the wiring D[i] from the wiring S[i]. Thus, the potential (V 0 ) of the node FN[i] can be read when the wiring D[i] of the memory cell MC[i] is electrically connected to a reading circuit. 
     When the reading operation is completed, a low-level potential is supplied to the wiring S[i] at Time T 14 . Since the potential of the node FN[i] is V 0 , the transistor TRB[i] is off and the potential of the wiring D[i] is maintained at the low level. Note that at this time, the reading circuit electrically connected to the wiring D[i] of the memory cell MC[i] does not operate. 
       FIG. 5  shows a timing chart in the case where a high-level potential is supplied to the wiring CNODE[i] of the memory cell MC[i]. From Time T 16  to Time T 17 , the potentials of the wiring WL, the wiring CNODE[i], the wiring S[i], and the wiring D[i] are at the low level, the wiring WBL[i] has the potential V 0 , and either the potential V 0  or V 1  is maintained at the node FN[i]. 
     From Time T 17  to Time T 18 , a high-level potential is supplied to the wiring CNODE[i]. At this time, the transistor TRA[i] is off, and the potential of the node FN[i] increases in accordance with the high-level potential supplied to the wiring CNODE[i] because the capacitor C[i] maintains charges. For example, when a high-level potential is supplied to the wiring CNODE[i] while the potential V 0  is maintained at the node FN[i], the node FN[i] has a potential of (V 0 +a potential corresponding to a coupling ratio). Hence, when a high-level potential is supplied to the wiring CNODE[i] while the potential V 1  is maintained at the node FN[i], the node FN[i] has a potential of (V 1 +a potential corresponding to a coupling ratio). That is, the transistor TRB[i] is turned on regardless of whether the potential V 0  or V 1  is maintained at the node FN[i]. 
     When a high-level potential is supplied to the wiring S[i] from Time T 17  to Time T 18 , a high-level potential can be supplied to the wiring D[i] because the transistor TRB[i] is on. 
     The memory cell of one embodiment of the present invention is not limited to be used alone but two or more memory cells may be connected in series. In that case, a memory cell to be read is selected by adjusting the potential of the wiring CNODE[i], a potential is supplied to the wiring S[i], and then a potential is read from the wiring D[i]. A configuration including two or more memory cells connected in series will be described later. 
     In addition, the memory cell of one embodiment of the present invention is not limited to two levels. Two or more levels such as four levels (two bits) or eight levels (three bits) can be selected in accordance with the configuration of the memory cell. 
     Note that by using an oxide semiconductor containing at least one of indium, zinc, and gallium for a channel formation region of the transistor TRA[i], the off-state current of the transistor TRA[i] can be reduced in some cases, allowing the potential of the node FN[i] to be maintained for a long time. Hereinafter, a transistor including the oxide semiconductor is also referred to as an oxide semiconductor (OS) transistor. 
     It is supposed that 1-bit data is retained in the node FN[i] for 10 years. In the case where the power source voltage is higher than or equal to 2 V and lower than or equal to 3.5 V, the storage capacitance of the node FN[i] is 21 fF, and the acceptable amount of change in stored potential is less than 0.5 V, the leakage current of the node FN[i] needs to be lower than 33×10 −24  A in order that a change in stored potential is less than the acceptable range for 10 years. In the case where the leakage current of other components is lower than the above and a leakage current occurs almost exclusively in OS transistors, each OS transistor with a channel width of 350 nm preferably has a leakage current per unit area of lower than 93×10 −24  A/μm. In the memory cell MC[i] having the above configuration, data can be retained at 85° C. for 10 years. 
     It is supposed that 4-bit data is retained in the node FN[i]. In the case where the power source voltage is higher than or equal to 2 V and lower than or equal to 3.5 V, the storage capacitance is 0.1 fF, the distribution width of a stored potential is less than 30 mV, and the acceptable amount of change in stored potential is less than 80 mV, the leakage current of the node FN[i] needs to be lower than 0.025×10 −24  A in order that a change in stored potential is less than the acceptable range for 10 years. In the case where the leakage current of other components is lower than the above and a leakage current occurs almost exclusively in OS transistors, each OS transistor with a channel width of 60 nm preferably has a leakage current per unit area of lower than 0.423×10 −24  A/μm. In the memory cell MC[i] having the above configuration, data can be retained at 85° C. for 10 years. 
     It is supposed that 8-bit data is retained in the node FN[i]. In the case where the power source voltage is higher than or equal to 2 V and lower than or equal to 3.5 V, the storage capacitance is 0.1 fF, the distribution width of a stored potential is less than 2 mV, and the acceptable amount of change in stored potential is less than 5 mV, the leakage current of the node FN[i] needs to be lower than 0.0016×10 −24  A in order that a change in stored potential is less than the acceptable range for 10 years. In the case where the leakage current of other components is lower than the above and a leakage current occurs almost exclusively in OS transistors, each OS transistor with a channel width of 60 nm preferably has a leakage current per unit area of lower than 0.026×10 −24  A/μm. In the memory cell MC[i] having the above configuration, data can be retained at 85° C. for 10 years. 
     Note that the memory cell of one embodiment of the present invention is not limited to the aforementioned memory cell MC[i]. Depending on the circumstances or case, a variety of semiconductors may be used for the channel formation regions, the source and drain regions, and the like of the transistors included in the memory cell MC[i], and the structures of the transistors may also be changed. 
     For example, as in a memory cell pMC[i] illustrated in  FIG. 7 , the n-channel transistors TRA[i] and TRB[i] in the memory cell MC[i] may be changed into p-channel transistors pTRA[i] and pTRB[i]. Alternatively, not all the n-channel transistors but some n-channel transistors may be changed into p-channel transistors. 
     &lt;Semiconductor Device&gt; 
     A semiconductor device of one embodiment of the present invention will be described below. 
     &lt;&lt;Configuration Example of Semiconductor Device&gt;&gt; 
       FIG. 1  illustrates an example of the semiconductor device in which the two memory cells MC[i] described in &lt;&lt;Configuration Example of Memory Cell&gt;&gt; are provided in the same layer. A semiconductor device  110  includes a circuit unit MCS and a circuit unit Si—C. The circuit unit MCS includes a transistor SW and a circuit unit MCC. The circuit unit MCC includes a memory cell MC[ 1 ] and a memory cell MC[ 2 ]. The memory cell MC[ 1 ] and the memory cell MC[ 2 ] have the same configuration as the memory cell MC[i] in  FIG. 2 . The memory cell MC[ 1 ] is electrically connected to a wiring CNODE[ 1 ], the wiring WL, and a wiring WBL[ 1 ] and is also electrically connected to a node NA through the transistor SW. The memory cell MC[ 2 ] is electrically connected to a wiring CNODE[ 2 ], the wiring WL, a wiring WBL[ 2 ], and a node NB. The circuit unit Si—C includes an output terminal and an input terminal. The output terminal is electrically connected to the node NA and the input terminal is electrically connected to the node NB. 
     The memory cell MC[ 1 ] includes a transistor TRA[ 1 ], a transistor TRB[ 1 ], and a capacitor C[ 1 ]. The transistor TRA[ 1 ] includes a back gate BGA[ 1 ] and the transistor TRB[ 1 ] includes a back gate BGB[ 1 ]. The memory cell MC[ 2 ] includes a transistor TRA[ 2 ], a transistor TRB[ 2 ], and a capacitor C[ 2 ]. The transistor TRA[ 2 ] includes a back gate BGA[ 2 ] and the transistor TRB[ 2 ] includes a back gate BGB[ 2 ]. 
     A gate of the transistor TRA[ 1 ] is electrically connected to the wiring WL; one of a source and a drain of the transistor TRA[ 1 ] is electrically connected to the wiring WBL[ 1 ]; the capacitor C[ 1 ] is connected between the other of the source and the drain of the transistor TRA[ 1 ] and the wiring CNODE[ 1 ]; a gate of the transistor TRB[ 1 ] is electrically connected to the other of the source and the drain of the transistor TRA[ 1 ]; one of a source and a drain of the transistor TRB[ 1 ] is electrically connected to one of a source and a drain of the transistor SW. 
     A gate of the transistor TRA[ 2 ] is electrically connected to the wiring WL; one of a source and a drain of the transistor TRA[ 2 ] is electrically connected to the wiring WBL[ 2 ]; the capacitor C[ 2 ] is connected between the other of the source and the drain of the transistor TRA[ 2 ] and the wiring CNODE[ 2 ]; a gate of the transistor TRB[ 2 ] is electrically connected to the other of the source and the drain of the transistor TRA[ 2 ]; one of a source and a drain of the transistor TRB[ 2 ] is electrically connected to the other of the source and the drain of the transistor TRB[ 1 ] in the memory cell MC[ 1 ]. 
     The other of the source and the drain of the transistor SW is electrically connected to the node NA; the other of the source and the drain of the transistor TRB[ 2 ] is electrically connected to the node NB; and a gate of the transistor SW is electrically connected to a wiring SWG. 
     Next, a structure of the semiconductor device  110  will be described.  FIG. 10  is a top view of part of the semiconductor device  110 , and  FIG. 11  is a cross-sectional view along dashed-dotted lines A 5 -A 6  and A 7 -A 8  in  FIG. 10 . 
     Note that in  FIG. 11 , all regions that are not hatched and are not given reference numerals denoting a conductor, a semiconductor, an element, a circuit, and the like represent insulators. 
     Note that the region of the circuit unit Si—C in  FIG. 11  is not a cross-sectional view, but illustrates that the circuit unit Si—C is provided under the layer including the transistors TRA[ 1 ], TRB[ 1 ], TRA[ 2 ], and TRB[ 2 ] (not illustrated in  FIG. 11 ) with an insulating film interposed therebetween. 
     Note that the transistor SW is not illustrated in  FIG. 11 . 
     The transistors TRA[ 1 ], TRB[ 1 ], TRA[ 2 ], and TRB[ 2 ], and the wiring WL are provided in the same layer. Above the layer, the trench-shape capacitors C[ 1 ] and C[ 2 ], the wirings CNODE[ 1 ] and CNODE[ 2 ], and the wirings WBL[ 1 ] and WBL[ 2 ] are provided in this order. 
     The transistor TRA[ 1 ] includes the wiring WL, the semiconductor  101   a , the conductor  103   a , the conductor  103   b , and the back gate BGA[ 1 ]. The transistor TRB[ 1 ] includes the conductor  102 , the semiconductor  101   b , and the back gate BGB[ 1 ]. The transistor TRA[ 2 ] includes the wiring WL, the semiconductor  101   a , the conductor  103   a , the conductor  103   b , and the back gate BGA[ 2 ]. The transistor TRB[ 2 ] includes the conductor  102 , the semiconductor  101   b , and the back gate BGB[ 2 ]. The other of the source and the drain of the transistor TRB[ 1 ] is electrically connected to one of the source and the drain of the transistor TRB[ 2 ] through a conductor  108 . 
     In the memory cell MC[ 1 ], above the conductor  103   a , the conductor  104   a , the conductor  105   a , the conductor  106 , and the conductor  107  are provided in order. The conductor  103   a  is electrically connected to the wiring WBL[ 1 ] through the conductors  104   a ,  105   a ,  106 , and  107 . That is, one of the source and the drain of the transistor TRA[ 1 ] is connected to the wiring WBL[ 1 ] through the conductors of the respective layers. 
     In the memory cell MC[ 1 ], the conductor  104   b  is in contact with and electrically connected to the conductors  103   b  and  102 . The trench-shape capacitor C[ 1 ] is provided above the conductor  104   b , and the conductor  104   b  is electrically connected to one electrode of the capacitor C[ 1 ]. The conductor  105   b  is provided above the capacitor C[ 1 ], and the other electrode of the capacitor C[ 1 ] is electrically connected to the wiring CNODE[ 1 ] through the conductor  105   b . That is, the wiring CNODE[ 1 ] is electrically connected to the other of the source and the drain of the transistor TRA[ 1 ] and the gate of the transistor TRB[ 1 ] through the capacitor C[ 1 ] and the conductors of the respective layers. 
     Unlike the memory cell MC[ 1 ], the memory cell MC[ 2 ] is not clearly illustrated in the cross-sectional view, but the memory cell MC[ 2 ] has a structure similar to that of the memory cell MC[ 1 ]. For example, the transistor TRA[ 2 ] is electrically connected to the wiring WBL[ 2 ] through the conductors of the respective layers as in the memory cell MC[ 1 ]. The wiring CNODE[ 2 ] is electrically connected to the other of the source and the drain of the transistor TRA[ 2 ] and the gate of the transistor TRB[ 2 ] through the capacitor C[ 2 ] and the conductors of the respective layers. Note that the wiring CNODE[ 1 ] and the wiring CNODE[ 2 ] are provided so as not to overlap with each other in the wiring layout. That is, the conductor  105   b  electrically connecting the wiring CNODE[ 1 ] to the capacitor C[ 1 ] in the memory cell MC[ 1 ] is shifted from the conductor between the wiring CNODE[ 2 ] and the capacitor C[ 2 ] in the A 5  direction of dashed-dotted line A 5 -A 6  in  FIG. 11 . Alternatively, the conductor  105   b  may be shifted in the A 6  direction of dashed-dotted line A 5 -A 6  in  FIG. 11  as long as the wiring CNODE[ 1 ] and the wiring CNODE[ 2 ] do not overlap with each other. 
     Note that the semiconductor device  110  illustrated in  FIGS. 1, 10, and 11  includes the transistors TRA[ 1 ], TRA[ 2 ], TRB[ 1 ], and TRB[ 2 ] each provided with a back gate; however, one embodiment of the present invention is not limited thereto. In the semiconductor device  110 , all of the transistors TRA[ 1 ], TRA[ 2 ], TRB[ 1 ], and TRB[ 2 ] do not need to have a back gate, or some of the transistors may have a back gate depending on the need or circumstances. 
     &lt;&lt;Operation Example of Semiconductor Device&gt;&gt; 
     Next, writing and reading operations of the semiconductor device  110  will be described; here, 2-bit data is written to and read from each memory cell. The 2-bit data can handle four kinds of data “00” to “11” in a binary number. In the following description, in some cases, potentials indicating data “00”, “01”, “10”, and “11” are represented by V 00 , V 01 , V 10 , and V 11 , respectively, and data “00” to “11” are collectively referred to as four levels. 
       FIG. 8  is a timing chart showing a writing operation in the semiconductor device  110 . From Time T 20  to Time T 21 , the potentials of the wiring WL, the wiring WBL[ 1 ], the wiring WBL[ 2 ], the wiring CNODE[ 1 ], the wiring CNODE[ 2 ], the node FN[ 1 ], the node FN[ 2 ], the wiring SWG, the node NA, and the node NB are Low (hereinafter also referred to as a low level or an L level). In particular, the low-level potentials supplied to the wirings CNODE[ 1 ] and CNODE[ 2 ] are each preferably a reference potential. 
     At Time T 21 , the potential of the wiring WL is High (hereinafter also referred to as a high level or an H level). Then, the drain and source of the transistor TRA[ 1 ] are electrically connected, so that the potential of the wiring WBL[ 1 ] is supplied to the node FN[ 1 ]. At this time, any of the potentials V 00  to V 11  is supplied to the wiring WBL[ 1 ], and the node FN[ 1 ] has the same potential, which is maintained by the capacitor C[ 1 ]. 
     At T 21 , the drain and source of the transistor TRA[ 2 ] are also electrically connected when the potential of the wiring WL is High, so that the potential of the wiring WBL[ 2 ] is supplied to the node FN[ 2 ]. In other words, when any of the potentials V 00  to V 11  is supplied to the wiring WBL[ 2 ] from Time T 21  to Time T 22 , the node FN[ 2 ] has the same potential, which is maintained by the capacitor C[ 2 ]. 
     In that case, the threshold voltage of the transistor TRB[ 1 ] is affected by the potential (any of the potentials V 00  to V 11 ) of the node FN[ 1 ] to be shifted in the negative direction effectively. The threshold voltage effectively shifted in the negative direction is determined by the potential of the node FN[ 1 ], and increases with the potential of the node FN[ 1 ] in the order of V 00 , V 01 , V 10 , and V 11 . When the reference potential is V 00 , the threshold voltage is not effectively shifted in the negative direction. In the timing chart of  FIG. 8 , the reference potential is V 00 . Therefore, the threshold voltage is not effectively shifted in the negative direction when the potential of the node FN[ 1 ] is V 00 . 
     In addition, the effective negative shift in threshold voltage, which occurs when the potential of the node FN[ 1 ] is V 01 , V 10 , or V 11 , determines the drain-source current and voltage of the transistor TRB[ 1 ]. For example, when the potential difference between the wiring CNODE[ 1 ] and one of the source and the drain of the transistor TRB[ 1 ] is V TRB1W , the drain-source current and voltage of the transistor TRB[ 1 ] at V TRB1W  of 0 V increase with the potential of the node FN[ 1 ] in the order of V 01 , V 10 , and V 11 . 
     On the basis of the same principle as the transistor TRB[ 1 ] and the node FN[ 1 ], the threshold voltage of the transistor TRB[ 2 ] is also affected by the node FN[ 2 ] to be shifted in the negative direction effectively. The threshold voltage effectively shifted in the negative direction is determined by the potential of the node FN[ 2 ], and increases with the potential of the node FN[ 2 ] in the order of V 00 , V 01 , V 10 , and V 11 . When the reference potential is V 00 , the threshold voltage is not effectively shifted in the negative direction. In the timing chart of  FIG. 8 , the reference potential is V 00 . Therefore, the threshold voltage is not effectively shifted in the negative direction when the potential of the node FN[ 2 ] is V 00 . 
     In addition, the effective negative shift in threshold voltage, which occurs when the potential of the node FN[ 2 ] is V 01 , V 10 , or V 11 , determines the drain-source current and voltage of the transistor TRB[ 2 ]. For example, when the potential difference between the wiring CNODE[ 2 ] and one of the source and the drain of the transistor TRB[ 2 ] is V TRB2W , the drain-source current and voltage of the transistor TRB[ 2 ] at V TRB2W  of 0 V increase with the potential of the node FN[ 2 ] in the order of V 01 , V 10 , and V 11 . 
     At Time T 22 , the potential of the wiring WL is set at the low level, making no electrical conduction between the drain and source of each of the transistors TRA[ 1 ] and TRA[ 2 ]. Even when the potentials of the wirings WBL[ 1 ] and WBL[ 2 ] are at the low level from Time T 22  to Time T 23 , the potential of the node FN[ 1 ] can be maintained at any value of V 00  to V 11  by the capacitor C[ 1 ], and the potential of the node FN[ 2 ] can be maintained at any value of V 00  to V 11  by the capacitor C[ 2 ]. 
     In particular, an oxide semiconductor containing at least one of indium, zinc, and gallium is preferably used for channel formation regions of the transistors TRA[ 1 ] and TRA[ 2 ]. The off-state currents of the transistors TRA[ 1 ] and TRA[ 2 ] including the oxide semiconductor can be reduced in some cases, allowing the potentials of the nodes FN[ 1 ] and [ 2 ] to be maintained for a long time. 
       FIG. 9  is a timing chart showing a reading operation of the semiconductor device  110 . Time T 24  is after Time T 23  in the timing chart of  FIG. 8 . In other words, the potentials of the wiring WL, the wiring WBL[ 1 ], the wiring WBL[ 2 ], the wiring CNODE[ 1 ], the wiring CNODE[ 2 ], the node FN[ 1 ], the node FN[ 2 ], the wiring SWG, the node NA, and the node NB at Time T 24  are those after Time T 23  in  FIG. 8 . 
     From Time T 25  to Time T 26 , the potential of the wiring SWG is set at the high level, so that the transistor SW is turned on. In addition, the potential of the node NA is at the high level, whereby a high-level potential is supplied to one of the source and the drain of the transistor TRB[ 1 ] through the transistor SW. 
     Since the wiring CNODE[ 1 ] has a low-level potential, the drain-source current and voltage of the transistor TRB[ 1 ] at V TRB1W  of 0 V are determined by the potential of the node FN[ 1 ]. That is, the voltage applied from the node NA to one of the source and the drain of the transistor TRB[ 1 ] is lowered to a voltage corresponding to the potential of the node FN[ 1 ] through the transistor TRB[ 1 ] and then is output to the other of the source and the drain of the transistor TRB[ 1 ]. Note that the voltage drop between the drain and the source of the transistor TRB[ 1 ] does not occur when the potential of the node FN[ 1 ] is V 11  because V 11  is at the high level. 
     When the potential of the wiring CNODE[ 2 ] is at the high level, the potential of the node FN[ 2 ] is increased by a potential corresponding to a coupling ratio because the capacitor C[ 2 ] intends to maintain the potential difference between the wiring CNODE[ 2 ] and the node FN[ 2 ]. 
     At that time, V TRB2W  has a potential capable of making the transistor TRB[ 2 ] on. Accordingly, the voltage input to one of the source and the drain of the transistor TRB[ 2 ] is output to the other of the source and the drain of the transistor TRB[ 2 ], i.e., the node NB without any change. In other words, the voltage lowered by the transistor TRB[ 1 ] in accordance with the potential of the capacitor C[ 1 ] is applied from the other of the source and the drain of the transistor TRB[ 1 ] to the node NB. 
     From Time T 26  to Time T 27 , the potentials of the wiring CNODE[ 2 ], the wiring SWG, and the node NA are set at the low level. 
     From Time T 27  to Time T 28 , the potential of the wiring SWG is set at the high level, so that the transistor SW is turned on. In addition, the potential of the node NA is set at the high level, whereby a high-level potential is supplied to one of the source and the drain of the transistor TRB[ 1 ] through the transistor SW. 
     When the potential of the wiring CNODE[ 1 ] is at the high level, the potential of the node FN[ 1 ] is increased by a potential corresponding to a coupling ratio because the capacitor C[ 1 ] intends to maintain the potential difference between the wiring CNODE[ 1 ] and the node FN[ 1 ]. 
     At that time, V TRB1W  has a potential capable of making the transistor TRB[ 1 ] on. Accordingly, the voltage input to one of the source and the drain of the transistor TRB[ 1 ] is output to the other of the source and the drain of the transistor TRB[ 1 ] without any change. In other words, the voltage from the node NA is input to one of the source and the drain of the transistor TRB[ 2 ]. 
     Since the wiring CNODE[ 2 ] has a low-level potential, the drain-source current and voltage of the transistor TRB[ 2 ] at V TRB2W  of 0 V are determined by the potential of the node FN[ 2 ]. That is, the voltage applied from the node NA to one of the source and the drain of the transistor TRB[ 2 ] is lowered to a voltage corresponding to the potential of the node FN[ 2 ] through the transistor TRB[ 2 ] and then is applied to the other of the source and the drain of the transistor TRB[ 1 ], i.e., the node NB. Note that the voltage drop between the drain and the source of the transistor TRB[ 2 ] does not occur when the potential of the node FN[ 2 ] is V 11  because V 11  is at the high level. 
     As described above, a high-level potential is input from the node NA to the memory cells MC[ 1 ] and MC[ 2 ] and the potentials of the wirings CNODE[ 1 ] and CNODE[ 2 ] are switched, so that either of a potential lowered between the source and the drain of the transistor TRB[ 1 ] or a potential lowered between the source and the drain of the transistor TRB[ 2 ] can be supplied to the circuit unit Si—C serving as a reading circuit. The potential difference between the source and the drain of the transistor TRB[ 1 ] is determined by the potential of the node FN[ 1 ] and the potential difference between the source and the drain of the transistor TRB[ 2 ] is determined by the potential of the node FN[ 2 ]. Hence, the potential maintained at the node FN[ 1 ] or the node FN[ 2 ] can be read by obtaining the voltage lowered between the source and the drain of the transistor TRB[ 1 ] or the voltage lowered between the source and the drain of the transistor TRB[ 2 ]. 
     The transistors TRA[ 1 ], TRB[ 1 ], TRA[ 2 ], and TRB[ 2 ] each preferably include an oxide semiconductor. In particular, an oxide semiconductor containing at least one of indium, zinc, and gallium is further preferably used for channel formation regions of the transistors. 
     One embodiment of the present invention is not limited to the aforementioned semiconductor device  110 . For example, as in a semiconductor device  160  illustrated in  FIG. 12 , p-channel transistors pTRA[ 1 ], pTRB[ 1 ], pTRA[ 2 ], and pTRB[ 2 ] may be used instead of the n-channel transistors TRA[ 1 ], TRB[ 1 ], TRA[ 2 ], and TRB[ 2 ] in the semiconductor device  110 . Alternatively, not all but some of the n-channel transistors may be changed into p-channel transistors. 
     The memory cell of one embodiment of the present invention does not necessarily have 2-bit storage capacity. The memory cell can have 1-bit, 4-bit, 8-bit, or  9 - or more-bit storage capacity in some cases by changing the components used in the memory cells MC[ 1 ] and MC[ 2 ] and the connection therebetween. 
     The position of the transistor SW of one embodiment of the present invention is not limited to that shown in the circuit diagram of  FIG. 1 . For example, the transistor SW may be provided on the node NB side or inside the circuit unit Si—C. In the case where Si is used for the channel formation region of the transistor SW, the effective cell area can be possibly made smaller than that using an oxide semiconductor. In the case where the circuit unit Si—C serving as a reading circuit has a large area, an oxide semiconductor may be preferably used for the channel formation region of the transistor SW. 
     The semiconductor device  110  illustrated in  FIG. 1  includes, but is not limited to, the two memory cells in one embodiment of the present invention. For example, three or more memory cells MC[i] may be connected in series as illustrated in  FIG. 13 , so that an n-bit semiconductor device  120  can be obtained (n is an integer of 1 or more, and in  FIG. 13 , i is an integer of 1 to n). The semiconductor device  120  includes a circuit unit MCS and the circuit unit Si—C. The circuit unit MCS includes the circuit unit MCC and the transistor SW. The circuit unit MCC includes memory cells MC[ 1 ] to MC[n]. The memory cell MC[i] includes the transistor TRA[i], the transistor TRB[i], and the capacitor C[i]. The transistor TRA[i] includes a back gate BGA[i] and the transistor TRB[i] includes a back gate BGB[i]. The memory cell MC[i] has the connection structure as illustrated in  FIG. 2 . The memory cell MC[i] is electrically connected to the wiring CNODE[i], the wiring WBL[i], and the wiring WL. The writing and reading operations of the semiconductor device  120  can be performed in a manner similar to those shown in the timing charts of  FIGS. 8 and 9 . 
     The semiconductor device of one embodiment of the present invention is not limited to the semiconductor devices  110  and  120  illustrated in  FIGS. 1 and 13 , respectively. For example, as illustrated in  FIG. 14 , the semiconductor device may have a memory cell array including two or more semiconductor devices  120 . 
     A semiconductor device  170  includes a circuit unit MCSS and the circuit unit Si—C. The circuit unit MCSS includes circuit units MCS[ 1 ] to MCS[m] (m is an integer of 2 or more). A circuit unit MCS[j] includes a circuit unit MCC[j] and a transistor SW[j] (in  FIG. 14 , j is an integer of 1 to m). A memory cell array MCA is constituted by the circuit units MCC[ 1 ] to MCC[m]. That is, the memory cell array MCA includes n memory cells in a row and m memory cells in a column, i.e., m×n memory cells in a matrix. Description is made here on the assumption that memory cells MC[ 1 , 1 ] to MC[m,n] each have a structure similar to that of the memory cell MC[i] in  FIG. 2  (in  FIG. 14 , although not illustrated, i is an integer of 1 to n as in  FIG. 13 ). 
     The semiconductor device  170  includes transistors SW[ 1 ] to SW[m], each of which corresponds to the transistor SW in the semiconductor device  120 . Gates of the transistors SW[ 1 ] to SW[m] are electrically connected to the respective wirings SWG[ 1 ] to SWG[m]. One of a source and a drain of each of the transistors SW[ 1 ] to SW[m] is electrically connected to the corresponding one of the memory cells MC[ 1 , 1 ] to MC[m, 1 ], and the other of the source and the drain of each of the transistors SW[ 1 ] to SW[m] is electrically connected to the corresponding one of the nodes NA[ 1 ] to NA[m]. The memory cells MC[ 1 , n] to MC[m, n] are electrically connected to the nodes NB[ 1 ] to NB[m], respectively. The nodes NA[ 1 ] to NA[m] and the nodes NB[ 1 ] to NB[m] are electrically connected to the circuit unit Si—C. 
     In a j-th row, memory cells MC[j, 1 ] to MC[j,n] are electrically connected in series to each other. For the connection of wirings CNODE[j, 1 ] to CNODE[j,n] and a wiring WL[j] in the j-th row, the description of the wirings CNODE[ 1 ] and CNODE[ 2 ] and the wiring WL is referred to. 
     Note that in the semiconductor device  170  in  FIG. 14 , the memory cell array MCA, the memory cell MC[ 1 , 1 ], the memory cell MC[m, 1 ], the memory cell MC[ 1 ,n], the memory cell MC[m,n], the wiring CNODE[ 1 , 1 ], the wiring CNODE[ 1 ,n], the wiring CNODE[m, 1 ], the wiring CNODE[m,n], the wiring WL[ 1 ], the wiring WL[m], the wiring WBL[ 1 ], the wiring WBL[n], the wiring SWG[ 1 ], the wiring SWG[m], the node NA[ 1 ], the node NA[m], the node NB[ 1 ], the node NB[m], the transistor SW[ 1 ], the transistor SW[m], the circuit unit MCSS, the circuit unit MCS[ 1 ], the circuit unit MCS[m], the circuit unit MCC[ 1 ], the circuit unit MCC[m], and the circuit unit Si—C are only illustrated and other wirings, components, block diagrams, and reference numerals are omitted. 
     In order to perform a writing operation in the semiconductor device  170 , any of the rows is selected by the wirings WL[ 1 ] to WL[m], and in a manner similar to that shown in the timing charts of  FIGS. 8 and 9 , a potential is supplied to any of the wirings WBL[ 1 ] to WBL[n]. 
     In order to perform a reading operation in the semiconductor device  170 , any of the rows is selected by the wirings SWG[ 1 ] to SWG[m] (here, the j-th row is selected), and in a manner similar to that shown in the timing charts of  FIGS. 8 and 9 , a potential is supplied to any of the wirings CNODE[j, 1 ] to CNODE[j,n] in the selected j-th row. 
     The memory cells MC[ 1 , 1 ] to MC[m,n] included in the memory cell array MCA in  FIG. 14  may have the pMC[i] structure illustrated in  FIG. 7  without limitation to the memory cell MC[i] structure illustrated in  FIG. 2 . 
     The configuration of the memory cell array of one embodiment of the present invention is not limited to that illustrated in  FIG. 14 . For example, the circuit units MCS are not necessarily provided in each row as illustrated in  FIG. 14 , but they may be provided in a matrix as illustrated in  FIG. 15 . 
     A semiconductor device  180  includes in circuit units MCS in a column and n circuit units MCS in a row, i.e., m×n circuit units MCS in a matrix. Here, circuit units MCS[ 1 , 1 ] to MCS[m,n] are arranged in a matrix. A circuit unit MCS[j,i] includes the memory cell MC[ 1 ] to a memory cell MC[k] each having a configuration similar to that of the memory cell MC[i] illustrated in  FIG. 2  (k is an integer of 2 or more). Although not illustrated in  FIG. 15 , the circuit unit Si—C is also included in the semiconductor device  180 . 
     In the j-th row and the i-th column, the circuit unit MCS[j,i] includes a transistor SWUM and a circuit unit MCC[j,i] (for the possible values of i and j in  FIG. 15 , the description of  FIG. 14  is referred to). One of a source and a drain of the transistor SWUM is electrically connected to the circuit unit MCC[j,i], a gate of the transistor SWUM is electrically connected to a wiring SWG[j,i], and the other of the source and the drain of the transistor SWUM is electrically connected to a node NA[j,i]. A node NB UM is electrically connected to the circuit unit MCC[j,i]. The node NA[j,i] and the node NB[j,i] are electrically connected to the circuit unit Si—C (not illustrated). 
     In the j-th row, circuit units MCC[j, 1 ] to MCC[j,n] are electrically connected to the wiring WL[j]. Wirings CNODE( 1 )[j] to CNODE(k)[j] are electrically connected to the circuit units MCC[j, 1 ] to MCC[j,n]. In particular, a wiring CNODE(h)[j] is electrically connected to a memory cell MC[h] included in each of the circuit units MCC[j, 1 ] to MCC[j,n] (h is an integer of 1 to k). 
     In the i-th column, circuit units MCC[ 1 ,i] to MCC[m,i] are electrically connected to wirings WBL( 1 )[i] to WBL(k)[i]. In particular, the wiring WBL(h)[i] is electrically connected to the memory cell MC[h] included in each of the circuit units MCC[ 1 ,i] to MCC[m,i]. 
     Note that in the semiconductor device  180  in  FIG. 15 , the circuit units MCS[ 1 , 1 ], MCS[m, 1 ], MCS[ 1 ,n], and MCS[m,n], the circuit units MCC[ 1 , 1 ], MCC[m, 1 ], MCC[ 1 ,n], and MCC[m,n], the transistors SW[ 1 ,l], SW[m, 1 ], SW[ 1 ,n], and SW[m,n], the wirings SWG[ 1 , 1 ], SWG[m, 1 ], SWG[ 1 ,n], and SWG[m,n], the nodes NA[ 1 , 1 ], NA[m, 1 ], NA[ 1 ,n], and NA[m,n], the nodes NB[ 1 , 1 ], NB[m, 1 ], NB[ 1 ,n], and NB[m,n], the wirings WL[ 1 ] and WL[m], the wirings WBL( 1 )[ 1 ], WBL(k)[ 1 ], WBL( 1 )[n], and WBL(k)[n], the wirings CNODE( 1 )[ 1 ], CNODE(k)[ 1 ], CNODE( 1 )[m], and CNODE(k)[m] are only illustrated and other wirings, components, block diagrams, and reference numerals are omitted. 
     In order to perform a writing operation in the semiconductor device  180 , any of the rows is selected by the wirings WL[ 1 ] to WL[m], and in a manner similar to that shown in the timing charts of  FIGS. 8 and 9 , a potential is supplied to any of the wirings WBL( 1 )[ 1 ] to WBL(k)[n]. 
     In order to perform a reading operation in the semiconductor device  180 , any of the rows is selected by the wirings SWG[ 1 , 1 ] to SWG[m,n] (here, the j-th row is selected), and in a manner similar to that shown in the timing charts of  FIGS. 8 and 9 , a potential is supplied to any of the wirings CNODE( 1 )[j] to CNODE(k)[j]. 
     The semiconductor device of one embodiment of the present invention is not limited to the semiconductor device  110  in  FIG. 1 , the semiconductor device  120  in  FIG. 13 , the semiconductor device  170  in  FIG. 14 , and the semiconductor device  180  in  FIG. 15 . For example, as illustrated in  FIG. 16 , the semiconductor device may have a stack of two or more memory cell arrays MCA. 
     A semiconductor device  190  includes a plurality of circuit units MCSS and the circuit unit Si—C. The circuit unit MCSS included in the semiconductor device  190  may have a configuration similar to that of the memory cell array MCA in the semiconductor device  170 . The circuit units MCSS are electrically connected to the circuit unit Si—C. 
     In the semiconductor device  190 , the memory cell arrays MCA in the circuit units MCSS overlap with each other. 
     The transistor SW in each circuit unit MCSS need not be stacked with the memory cell array MCA. In particular, in the case where the transistor SW is formed using a material different from that of the transistors TRA and TRB in the memory cell MC, the transistor SW is preferably provided outside the circuit unit MCSS. 
     The memory cell MC in the memory cell array MCA may have the configuration of the memory cell MC[i] in  FIG. 2  or the configuration of pMC[i] in  FIG. 7 . 
     Although the two layers of the circuit units MCSS are stacked in  FIG. 16 , one embodiment of the present invention is not limited thereto and three layers of the circuit units MCSS may be stacked. 
     Such a stack of the circuit units MCSS achieves a memory device with a larger storage capacity. 
     Note that an example described as one embodiment of the present invention in this embodiment can be combined with any of the other examples as appropriate. 
     In this embodiment, one embodiment of the present invention has been described. 
     Embodiments of the present invention are described in other embodiments. Note that one embodiment of the present invention is not limited to the above examples. In other words, various embodiments of the invention are described in this embodiment and the other embodiments, and one embodiment of the present invention is not limited to a particular embodiment. For example, as one embodiment of the present invention, an oxide semiconductor is used for the channel formation region of the transistor, which includes the semiconductors  101   a  and  101   b , the source and drain regions, and the like; however, one embodiment of the present invention is not limited to this example. Depending on the circumstances or case, a variety of semiconductors may be used for transistors in one embodiment of the present invention, the channel formation regions of the transistors, the source and drain regions of the transistors, and the like. Depending on the circumstances or case, transistors in one embodiment of the present invention, the channel formation regions of the transistors, the source and drain regions of the transistors, and the like may include, for example, at least one of silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, and an organic semiconductor. Depending on the circumstances or case, transistors in one embodiment of the present invention, the channel formation regions of the transistors, the source and drain regions of the transistors, and the like do not necessarily include an oxide semiconductor. Depending on the circumstances or case, the transistor including the semiconductors  101   a  and  101   b  and the like, the channel formation region of the transistor, the source and drain regions of the transistor, and the like do not necessarily include an oxide semiconductor. For example, an example of application to a memory cell is described as one embodiment of the present invention; however, one embodiment of the present invention is not limited thereto. Depending on the circumstances or case, for example, one embodiment of the present invention may be applied to a circuit having another function. Depending on the circumstances or case, one embodiment of the present invention is not necessarily applied to a memory cell. 
     This embodiment can be combined as appropriate with any of the other embodiments described in this specification. 
     Embodiment 2 
     A configuration example of a memory device of one embodiment of the present invention will be described with reference to  FIG. 17 . 
       FIG. 17  illustrates a configuration example of a memory device. A memory device  2600  includes a peripheral circuit  2601  and a memory cell array  2610  (denoted by Memory Cell Array in  FIG. 17 ). The peripheral circuit  2601  includes a row decoder  2621  (denoted by Row Decoder in  FIG. 17 ), a word line driver circuit  2622  (abbreviated to Word Line Driver Cir. in  FIG. 17 ), a bit line driver circuit  2630  (abbreviated to Bit Line Driver Cir. in  FIG. 17 ), an output circuit  2640  (abbreviated to Output Cir. in  FIG. 17 ), and a control logic circuit  2660  (abbreviated to Control Logic Cir. in  FIG. 17 ). 
     The bit line driver circuit  2630  includes a column decoder  2631  (denoted by Column Decoder in  FIG. 17 ), a precharge circuit  2632  (abbreviated to Precharge Cir. in  FIG. 17 ), a sense amplifier  2633  (abbreviated to Sense Amp. in  FIG. 17 ), and a write circuit  2634  (abbreviated to Write Cir. in  FIG. 17 ). The precharge circuit  2632  has a function of precharging the node NA, the wiring CNODE[i], and the like (not illustrated in  FIG. 17 ). The sense amplifier  2633  has a function of amplifying a data signal input from the node NB. The amplified data signal is output as a digital data signal RDATA from the memory device  2600  through the output circuit  2640 . 
     Note that the precharge circuit  2632  and the sense amplifier  2633  are not necessarily provided in the bit line driver circuit  2630  as shown in  FIG. 17 , but may be provided in the circuit unit Si—C which is under the memory cell array  2610  as described in Embodiment 1 (the circuit unit Si—C is not illustrated in  FIG. 17 ). 
     As power source voltages, a low power source voltage (VSS), a high power source voltage (VDD) for the peripheral circuit  2601 , and a high power source voltage (VIL) for the memory cell array  2610  are supplied to the memory device  2600  from the outside. 
     Control signals (CE, WE, and RE), an address signal ADDR, and a data signal WDATA are input to the memory device  2600  from the outside. ADDR is input to the row decoder  2621  and the column decoder  2631 , and WDATA is input to the write circuit  2634 . 
     The control logic circuit  2660  processes the signals (CE, WE, RE) input from the outside, and generates control signals for the row decoder  2621  and the column decoder  2631 . CE, WE, and RE are a chip enable signal, a write enable signal, and a read enable signal, respectively. Signals processed by the control logic circuit  2660  are not limited to those listed above, and other control signals may be input as necessary. 
     Note that whether each of the aforementioned circuits or signals is provided can be determined as appropriate and as needed. 
     When a p-channel Si transistor and a transistor including a channel formation region using an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) described in a later embodiment are used in the memory device  2600 , the memory device  2600  can be reduced in size. In addition, the memory device  2600  can be reduced in power consumption. Furthermore, the memory device  2600  can be increased in operation speed. In particular, by using only a p-channel transistor as the Si-transistor, manufacturing costs can be reduced. 
     Note that the configuration of this embodiment is not limited to that illustrated in  FIG. 17 . Depending on the circumstances or case, the structure of the memory device  2600  may be changed. For example, in the case where either the memory cell MC[i] or the memory cell pMC[i] shown in Embodiment 1 is used for the memory device  2600 , the number of wirings and the configuration of the peripheral circuit may be changed as appropriate in accordance with the memory cell. Also in the case where any of the semiconductor devices  120 ,  170 ,  180 , and  190  is used for the memory device  2600 , the number of wirings and the configuration of the peripheral circuit may be changed as appropriate in accordance with the semiconductor device. 
     This embodiment can be combined as appropriate with any of the other embodiments described in this specification. 
     Embodiment 3 
     OS transistors (transistors including an oxide semiconductor) will be described. 
     &lt;Structure Example 1 of OS Transistor&gt; 
       FIGS. 18A to 18D  illustrate a structure example of an OS transistor.  FIG. 18A  is a top view illustrating a structure example of an OS transistor.  FIG. 18B  is a cross-sectional view along dashed-dotted line y 1 -y 2  in  FIG. 18A .  FIG. 18C  is a cross-sectional view along dashed-dotted line x 1 -x 2  in  FIG. 18A .  FIG. 18D  is a cross-sectional view along dashed-dotted line x 3 -x 4  in  FIG. 18A . In some cases, the direction of dashed-dotted line y 1 -y 2  is referred to as a channel length direction, and the direction of dashed-dotted line x 1 -x 2  is referred to as a channel width direction. Accordingly,  FIG. 18B  illustrates a cross-sectional structure of the OS transistor in the channel length direction, and  FIGS. 18C and 18D  each illustrate a cross-sectional structure of the OS transistor in the channel width direction. Note that to clarify the device structure,  FIG. 18A  does not illustrate some components. 
     An OS transistor  581  is formed over an insulating surface, here, over an insulator  511 . The insulator  511  is formed over a surface of a substrate  510 . The OS transistor  581  is covered with an insulator  516 . Note that the insulator  516  can be regarded as a component of the OS transistor  581 . The OS transistor  581  includes an insulator  512 , an insulator  513 , an insulator  514 , an insulator  515 , semiconductors  521  to  523 , a conductor  530 , a conductor  531 , a conductor  532 , and a conductor  533 . Here, the semiconductors  521  to  523  are collectively referred to as a semiconductor region  520 . 
     The conductor  530  serves as a gate electrode and the conductor  533  serves as a back gate electrode. The conductors  531  and  532  each serve as a source or a drain electrode. The insulator  511  has a function of electrically isolating the substrate  510  from the conductor  533 . The insulator  515  is included in a gate insulator and the insulators  513  and  514  are included in a gate insulator on a back channel side. 
     Note that the channel length refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification and the like, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     As illustrated in  FIGS. 18B and 18C , the semiconductor region  520  includes a region where the semiconductors  521  to  523  are stacked in order. The insulator  515  covers this stacked region. The conductor  530  overlaps with the stacked region with the insulator  514  positioned therebetween. The conductors  531  and  532  are provided over a stack of the semiconductors  521  and  522 , and are in contact with a top surface of the stack and a side surface thereof in the channel length region. The stack of the semiconductors  521  and  522  and the conductors  531  and  532  is formed in an etching step using the same mask. 
     The semiconductor  523  is formed to cover the semiconductors  521  and  522  and the conductors  531  and  532 . The insulator  515  covers the semiconductor  523 . Here, the semiconductor  523  and the insulator  515  are etched using the same mask. 
     The conductor  530  is formed so as to surround, in the channel width direction, the stack of the semiconductors  521  to  523  with the insulator  515  positioned therebetween (see  FIG. 18C ). Therefore, a gate electric field is applied to this stacked region in the lateral direction as well as the vertical direction. In the OS transistor  581 , the gate electric field refers to an electric field generated by voltage applied to the conductor  530  (gate electrode layer). Such a structure of the transistor in which the whole stacked region of the semiconductors  521  to  523  can be electrically surrounded by the gate electric fields is referred to as a surrounded channel (s-channel) structure. This structure makes a channel in the whole semiconductor  522  (bulk) in some cases; thus, the OS transistor  581  can have a high on-state current. The OS transistor  581  with the s-channel structure has an improved high frequency property. Specifically, the cut-off frequency can be improved. 
     The s-channel structure, because of its high on-state current, is suitable for a semiconductor device such as large-scale integration (LSI) which requires a miniaturized transistor. Furthermore, the s-channel structure is suitable for a transistor that needs to operate at high frequency because of its high on-state current. A semiconductor device including the transistor can operate at high frequency. 
     With the miniaturized transistor, a highly integrated or small sized semiconductor device can be provided. The OS transistor preferably has, for example, a region where a channel length is greater than or equal to 10 nm and less than 1 μm, further preferably greater than or equal to 10 nm and less than 100 nm, still further preferably greater than or equal to 10 nm and less than 70 nm, yet still further preferably greater than or equal to 10 nm and less than 60 nm, and yet still further preferably greater than or equal to 10 nm and less than 30 nm. In addition, the transistor preferably has, for example, a region where a channel width is greater than or equal to 10 nm and less than 1 μm, further preferably greater than or equal to 10 nm and less than 100 nm, still further preferably greater than or equal to 10 nm and less than 70 nm, yet still further preferably greater than or equal to 10 nm and less than 60 nm, and yet still further preferably greater than or equal to 10 nm and less than 30 nm. 
     &lt;Insulator&gt; 
     Each of the insulators  511  to  516  includes a single layer or multilayer insulating film. The insulating film is formed using, for example, a material such as aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     Note that in this specification, an oxynitride refers to a compound that includes more oxygen than nitrogen, and a nitride oxide refers to a compound that includes more nitrogen than oxygen. In this specification and the like, an oxide used for an insulating material includes an oxide with a nitrogen concentration of less than 1 atomic %. 
     The insulators  514  and  515  are in contact with the semiconductor region  520  and therefore preferably contain an oxide. In particular, the insulators  514  and  515  preferably include an oxide material from which part of oxygen is released by heating. The insulators  514  and  515  preferably include an oxide containing oxygen more than that in the stoichiometric composition. By heating, part of oxygen is released from an oxide film containing oxygen more than that in the stoichiometric composition. Oxygen released from the insulators  514  and  515  is supplied to the semiconductor region  520  which is an oxide semiconductor, so that oxygen vacancies in the oxide semiconductor can be reduced. Consequently, changes in the electrical characteristics of the transistor can be reduced, improving the reliability of the transistor. 
     The oxide film containing oxygen more than that in the stoichiometric composition is, for example, an oxide film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 18  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS) analysis under the conditions where the temperature of the film surface is higher than or equal to 100° C. and lower than or equal to 700° C. 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. 
     The insulator  513  has a passivation function of preventing oxygen included in the insulator  514  from decreasing by being bonded to metal included in the conductor  533 . The insulator  516  has a passivation function of preventing oxygen included in the insulator  515  from decreasing. 
     The insulators  511 ,  513 , and  516  preferably have a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. The insulators  511 ,  513 , and  516  prevent outward diffusion of oxygen from the semiconductor region  520  and entry of hydrogen, water, or the like into the semiconductor region  520  from the outside. To have such a function, the insulators  511 ,  513 , and  516  include, for example, at least a single layer of an insulating film made of silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, or the like. 
     &lt;Conductor&gt; 
     The conductors  530 ,  531 ,  532 , and  533  each preferably have a single layer or a multilayer of a conductive film containing a low-resistance material selected from copper (Cu), tungsten (W), molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium (Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn), iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt), iridium (Ir), and strontium (Sr), an alloy of such a low-resistance material, or a compound containing such a material as its main component. It is particularly preferable to use a high-melting-point material which has both heat resistance and conductivity, such as tungsten or molybdenum. In addition, the conductors are preferably formed using a low-resistance conductive material such as aluminum or copper. The conductors are further preferably formed using a Cu—Mn alloy, in which case manganese oxide formed at the interface with an insulator containing oxygen has a function of preventing Cu diffusion. 
     The conductors  531  and  532  in the OS transistor  581  are formed using a hard mask used for forming the stack of the semiconductors  521  and  522 . Therefore, the conductors  531  and  532  do not have regions in contact with the side surfaces of the semiconductors  521  and  522 . For example, the semiconductors  521  and  522  and the conductors  531  and  532  can be formed through the following steps. A two-layer oxide semiconductor film including the semiconductors  521  and  522  is formed. A single-layer or multilayer conductive film is formed over the oxide semiconductor film. This conductive film is etched, so that a hard mask is formed. Using this hard mask, the two-layer oxide semiconductor film is etched to form the semiconductors  521  and  522 . Then, the hard mask is etched to form the conductors  531  and  532 . 
     &lt;Semiconductor&gt; 
     The semiconductor  522  is, for example, an oxide semiconductor containing indium (In). The semiconductor  522  has a high carrier mobility (electron mobility) when containing, for example, indium. The semiconductor  522  preferably contains an element M. The element M is preferably aluminum (Al), gallium (Ga), yttrium (Y), tin (Sn), or the like. Other elements that can be used as the element M include boron (B), silicon (Si), titanium (Ti), iron (Fe), nickel (Ni), germanium (Ge), zirconium (Zr), molybdenum (Mo), lanthanum (La), cerium (Ce), neodymium (Nd), hafnium (Hf), tantalum (Ta), and tungsten (W). Note that two or more of the above elements may be used in combination as the element M. The element M is an element having high bonding energy with oxygen, for example. The element M is an element whose bonding energy with oxygen is higher than that of indium. The element M is an element that can increase the energy gap of the oxide semiconductor, for example. Furthermore, the semiconductor  522  preferably contains zinc (Zn). The oxide semiconductor containing zinc is easily crystallized in some cases. 
     Note that the semiconductor  522  is not limited to the oxide semiconductor containing indium. The semiconductor  522  may be, for example, an oxide semiconductor which does not contain indium and contains zinc, an oxide semiconductor which does not contain indium and contains gallium, or an oxide semiconductor which does not contain indium and contains tin, e.g., a zinc tin oxide or a gallium tin oxide. The semiconductor  522  is formed using an oxide with a wide energy gap. For example, the energy gap of the semiconductor  522  is greater than or equal to 2.5 eV and less than or equal to 4.2 eV, preferably greater than or equal to 2.8 eV and less than or equal to 3.8 eV, and further preferably greater than or equal to 3 eV and less than or equal to 3.5 eV. The semiconductor region  520  is preferably formed using CAAC-OS which is described later. Alternatively, at least the semiconductor  522  is preferably formed using CAAC-OS. 
     The semiconductors  521  and  523  include, for example, one or more elements other than oxygen included in the semiconductor  522 . Since the semiconductors  521  and  523  each include one or more elements other than oxygen included in the semiconductor  522 , an interface state is less likely to be formed at the interface between the semiconductors  521  and  522  and the interface between the semiconductors  522  and  523 . 
     In the case where an In-M-Zn oxide is used as the semiconductor  521 , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be lower than 50 atomic % and higher than 50 atomic %, respectively, further preferably lower than 25 atomic % and higher than 75 atomic %, respectively. In the case where the semiconductor  521  is formed by a sputtering method, a sputtering target with the above composition, for example, a sputtering target containing In, M, and Zn at an atomic ratio of 1:3:2, is preferably used. 
     In the case where an In-M-Zn oxide is used as the semiconductor  522 , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be higher than 25 atomic % and lower than 75 atomic %, respectively, further preferably higher than 34 atomic % and lower than 66 atomic %, respectively. In the case where the semiconductor  522  is formed by a sputtering method, a sputtering target with the above composition, for example, a sputtering target containing In, M, and Zn at an atomic ratio of 1:1:1, 1:1:1.2, 2:1:3, 3:1:2, or 4:2:4.1, is preferably used. In particular, when a sputtering target containing In, Ga, and Zn at an atomic ratio of 4:2:4.1 is used, the semiconductor  522  may contain In, Ga, and Zn at an atomic ratio of around 4:2:3. 
     In the case where an In-M-Zn oxide is used as the semiconductor  523 , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be lower than 50 atomic % and higher than 50 atomic %, respectively, further preferably lower than 25 atomic % and higher than 75 atomic %, respectively. Note that the semiconductors  523  and  521  may be formed using the same type of oxide. Note that the semiconductor  521  and/or the semiconductor  523  do/does not necessarily contain indium in some cases. For example, the semiconductor  521  and/or the semiconductor  523  may be gallium oxide. 
     (Energy Band Structure) 
     A function and an effect of the semiconductor region  520  including the stack of the semiconductors  521 ,  522 , and  523  will be described with reference to  FIGS. 19A and 19B . FIG.  19 A is an enlarged view of the active layer (the channel portion) of the OS transistor  581  illustrated in  FIG. 18B .  FIG. 19B  shows an energy band diagram of the active layer of the OS transistor  581 , i.e., the energy band diagram of a portion along chain line z 1 -z 2  in  FIG. 19A . 
     In  FIG. 19B , Ec 514 , Ec 521 , Ec 522 , Ec 523 , and Ec 515  indicate the energy of the conduction band minimum of the insulator  514 , the semiconductor  521 , the semiconductor  522 , the semiconductor  523 , and the insulator  515 , respectively. 
     Here, a difference in energy between the vacuum level and the conduction band minimum (the difference is also referred to as electron affinity) corresponds to a value obtained by subtracting an energy gap from a difference in energy between the vacuum level and the valence band maximum (the difference is also referred to as ionization potential). The energy gap can be measured using a spectroscopic ellipsometer. The energy difference between the vacuum level and the valence band maximum can be measured using an ultraviolet photoelectron spectroscopy (UPS) device. 
     Since the insulators  514  and  515  are insulators, Ec 514  and Ec 515  are closer to the vacuum level than Ec 521 , Ec 522 , and Ec 523  (that is, the insulators  514  and  515  have a smaller electron affinity than the semiconductors  521  to  523 ). 
     As the semiconductor  522 , an oxide having an electron affinity higher than those of the semiconductors  521  and  523  is used. For example, as the semiconductor  522 , an oxide having an electron affinity higher than those of the semiconductors  521  and  523  by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, and further preferably 0.15 eV or higher and 0.4 eV or lower is used. Note that the electron affinity refers to an energy difference between the vacuum level and the conduction band minimum. 
     An indium gallium oxide has a small electron affinity and a high oxygen-blocking property. Therefore, the semiconductor  523  preferably contains an indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)] is, for example, higher than or equal to 70%, preferably higher than or equal to 80%, and further preferably higher than or equal to 90%. At this time, when a gate voltage is applied, a channel is formed in the semiconductor  522  having the highest electron affinity among the semiconductors  521  to  523 . 
     Here, in some cases, there is a mixed region of the semiconductors  521  and  522  between the semiconductor  521  and the semiconductor  522 . Furthermore, in some cases, there is a mixed region of the semiconductors  522  and  523  between the semiconductor  522  and the semiconductor  523 . The mixed region has a low interface state density. For that reason, the stack of the semiconductors  521 ,  522 , and  523  has a band structure where energy at each interface and in the vicinity of the interface is changed continuously (also referred to as a band structure with a continuous junction). 
     At this time, electrons move mainly in the semiconductor  522 , not in the semiconductors  521  and  523 . As described above, when the interface state density at the interface between the semiconductors  521  and  522  and the interface state density at the interface between the semiconductors  522  and  523  are decreased, electron movement in the semiconductor  522  is less likely to be inhibited and the on-sate current of the transistor can be increased. 
     As factors of inhibiting electron movement are decreased, the on-state current of the transistor can be increased. For example, in the case where there is no factor of inhibiting electron movement, electrons are assumed to be efficiently moved. Electron movement is inhibited, for example, in the case where physical unevenness in a channel formation region is large. The electron movement is also inhibited, for example, in the case where the density of defect states is high in a region where a channel is formed. 
     To increase the on-state current of the OS transistor  581 , for example, root mean square (RMS) roughness with a measurement area of 1 μm×1 μm of a top surface or a bottom surface of the semiconductor  522  (a formation surface; here, the semiconductor  521 ) is less than 1 nm, preferably less than 0.6 nm, further preferably less than 0.5 nm, and still further preferably less than 0.4 nm. The average surface roughness (also referred to as Ra) with the measurement area of 1 μm×1 μm is less than 1 nm, preferably less than 0.6 nm, further preferably less than 0.5 nm, and still further preferably less than 0.4 nm. The maximum difference (P−V) with the measurement area of 1 μm×1 μm is less than 10 nm, preferably less than 9 nm, further preferably less than 8 nm, and still further preferably less than 7 nm. Note that RMS roughness, Ra, and P−V can be measured using a scanning probe microscope. 
     For example, in the case where the semiconductor  522  contains oxygen vacancies (also denoted by V 0 ), donor levels are formed by entry of hydrogen into sites of oxygen vacancies in some cases. A state in which hydrogen enters sites of oxygen vacancies is denoted by V 0 H in the following description in some cases. V 0 H scatters electrons to be a factor of decreasing the on-state current of the transistor. Note that sites of oxygen vacancies are more stable by entry of oxygen than by entry of hydrogen. Thus, a decrease in oxygen vacancies in the semiconductor  522  results in an increase in the on-state current of the transistor in some cases. 
     For example, the hydrogen concentration at a certain depth in the semiconductor  522  or in a certain region of the semiconductor  522 , which is measured by secondary ion mass spectrometry (SIMS), is higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 2×10 20  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 19  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 19  atoms/cm 3 , and still further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 . 
     To decrease oxygen vacancies in the semiconductor  522 , for example, excess oxygen in the insulator  515  can be moved to the semiconductor  522  through the semiconductor  521 . In this case, the semiconductor  521  is preferably a layer having oxygen permeability (a layer through which oxygen can permeate). 
     In the case where the OS transistor  581  has an s-channel structure, a channel is formed in the whole of the semiconductor  522 . Therefore, as the semiconductor  522  has a larger thickness, a channel region becomes larger. In other words, the thicker the semiconductor  522  is, the higher the on-state current of the OS transistor  581  is. 
     Moreover, the thickness of the semiconductor  523  is preferably as small as possible to increase the on-state current of the OS transistor  581 . For example, the semiconductor  523  has a region with a thickness of less than 10 nm, preferably less than or equal to 5 nm, and further preferably less than or equal to 3 nm. Meanwhile, the semiconductor  523  has a function of blocking entry of elements other than oxygen (such as hydrogen and silicon) included in the adjacent insulator into the semiconductor  522  where a channel is formed. For this reason, it is preferable that the semiconductor  523  have a certain thickness. For example, the semiconductor  523  has a region with a thickness of greater than or equal to 0.3 nm, preferably greater than or equal to 1 nm, and further preferably greater than or equal to 2 nm. The semiconductor  523  preferably has an oxygen blocking property to suppress outward diffusion of oxygen released from the insulator  515  and the like. 
     To improve the reliability of the OS transistor  581 , preferably, the thickness of the semiconductor  521  is large and the thickness of the semiconductor  523  is small. For example, the semiconductor  521  has a region with a thickness of greater than or equal to 10 nm, preferably greater than or equal to 20 nm, further preferably greater than or equal to 40 nm, and still further preferably greater than or equal to 60 nm. When the thickness of the semiconductor  521  is made large, the distance from an interface between the adjacent insulator and the semiconductor  521  to the semiconductor  522  in which a channel is formed can be large. However, to prevent the productivity of the semiconductor device from decreasing, the semiconductor  521  has a region with a thickness of, for example, less than or equal to 200 nm, preferably less than or equal to 120 nm, and further preferably less than or equal to 80 nm. 
     In order that the OS transistor  581  have stable electrical characteristics, it is effective to make the semiconductor  522  intrinsic or substantially intrinsic by reducing the concentration of impurities in the semiconductor region  520 . Note that in this specification and the like, the expression “an oxide semiconductor is substantially intrinsic” refers to a state where an oxide semiconductor film has a carrier density lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , and further preferably lower than 1×10 10 /cm 3  and higher than or equal to 1×10 −9 /cm 3 . 
     In the oxide semiconductor, hydrogen, nitrogen, carbon, silicon, and a metal element other than a main component are impurities. For example, hydrogen and nitrogen form donor levels to increase the carrier density, and silicon forms impurity levels in the oxide semiconductor. The impurity levels serve as traps and might cause deterioration of the electric characteristics of the transistor. Therefore, it is preferable to reduce the concentration of the impurities in the semiconductors  521 ,  522 , and  523  and at interfaces between the semiconductors. 
     For example, a region with a silicon concentration of higher than or equal to 1×10 16  atoms/cm 3  and lower than 1×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than 5×10 18  atoms/cm 3 , and further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than 2×10 18  atoms/cm 3  is provided between the semiconductor  522  and the semiconductor  521 . In addition, a region with a silicon concentration of higher than or equal to 1×10 16  atoms/cm 3  and lower than 1×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than 5×10 18  atoms/cm 3 , and further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than 2×10 18  atoms/cm 3  is provided between the semiconductor  522  and the semiconductor  523 . The silicon concentration can be measured by, for example, SIMS. 
     It is preferable to reduce the concentration of hydrogen in the semiconductors  521  and  523  in order to reduce the concentration of hydrogen in the semiconductor  522 . The semiconductors  521  and  523  each have a region in which the concentration of hydrogen is higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 2×10 20  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 19  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 19  atoms/cm 3 , and still further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 . The hydrogen concentration can be measured by, for example, SIMS. 
     It is preferable to reduce the concentration of nitrogen in the semiconductors  521  and  523  in order to reduce the concentration of nitrogen in the semiconductor  522 . The semiconductors  521  and  523  each have a region in which the concentration of nitrogen is higher than or equal to 1×10 16  atoms/cm 3  and lower than 5×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 18  atoms/cm 3 , and still further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 17  atoms/cm 3 . The nitrogen concentration can be measured by SIMS. 
     A transistor in which the above highly purified oxide semiconductor is used for a channel formation region exhibits an extremely low off-state current. When a source-drain voltage is, for example, about 0.1 V, 5 V, or 10 V, the off-state current standardized on the channel width of the transistor can be as low as several yoctoamperes per micrometer to several zeptoamperes per micrometer. 
       FIGS. 18A to 18D  show an example of the three-layer semiconductor region  520 ; however, one embodiment of the present invention is not limited thereto. For example, the semiconductor region  520  may have a two-layer structure without the semiconductor  521  or the semiconductor  523 . Alternatively, the semiconductor region  520  may have a four-layer structure in which a semiconductor similar to any one of the semiconductors  521  to  523  is provided below or over the semiconductor  521  or below or over the semiconductor  523 . Alternatively, the semiconductor region  520  may have an n-layer structure (n is an integer of 5 or more) in which a semiconductor similar to any one of the semiconductors  521  to  523  is provided at two or more of the following positions: over the semiconductor  521 , below the semiconductor  521 , over the semiconductor  523 , and below the semiconductor  523 . 
     In the case where the OS transistor  581  does not include a back gate electrode, the conductor  533  need not be provided. In that case, the insulator  513  may be formed over the insulator  511  without the insulator  512 . 
     &lt;Substrate&gt; 
     As the substrate  510 , for example, an insulator substrate, a semiconductor substrate, or a conductor substrate may be used. Examples of the insulator substrate include a glass substrate, a quartz substrate, a sapphire substrate, a stabilized zirconia substrate (e.g., an yttria-stabilized zirconia substrate), and a resin substrate. Examples of the semiconductor substrate include a single material semiconductor substrate made of silicon, germanium, or the like; and a compound semiconductor substrate made of silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, or gallium oxide. The semiconductor substrate may be a bulk substrate or a silicon on insulator (SOI) substrate in which a semiconductor is provided over a semiconductor substrate with an insulating region positioned therebetween. Examples of the conductor substrate include a graphite substrate, a metal substrate, an alloy substrate, and a conductive resin substrate. A substrate including a metal nitride, a substrate including a metal oxide, or the like may also be used. Furthermore, an insulator substrate provided with a conductor or a semiconductor, a semiconductor substrate provided with a conductor or an insulator, a conductor substrate provided with a semiconductor or an insulator, or the like may also be used. Alternatively, any of these substrates over which an element is provided may be used. Examples of the element provided over the substrate include a capacitor, a resistor, a switching element, a light-emitting element, and a memory element. 
     Alternatively, a flexible substrate may be used as the substrate  510 . To provide a transistor over a flexible substrate, the transistor may be formed over a non-flexible substrate (e.g., a semiconductor substrate) and then separated and transferred to the substrate  510  which is a flexible substrate. In that case, a separation layer is preferably provided between the non-flexible substrate and the transistor. As the substrate  510 , a sheet, a film, or a foil containing a fiber may be used. The substrate  510  may have elasticity. The substrate  510  may have a property of returning to its original shape when bending or pulling is stopped. Alternatively, the substrate  510  may have a property of not returning to its original shape. The thickness of the substrate  510  is, for example, greater than or equal to 5 μm and less than or equal to 700 μm, preferably greater than or equal to 10 μm and less than or equal to 500 μm, and further preferably greater than or equal to 15 μm and less than or equal to 300 μm. When the substrate  510  has a small thickness, the weight of the semiconductor device can be reduced. When the substrate  510  has a small thickness, even in the case of using glass or the like, the substrate  510  may have elasticity or a property of returning to its original shape when bending or pulling is stopped. Therefore, an impact applied to the semiconductor device over the substrate  510 , which is caused by dropping or the like, can be reduced. That is, a robust semiconductor device can be provided. 
     For the substrate  510  which is a flexible substrate, for example, metal, an alloy, resin, glass, or fiber thereof can be used. The flexible substrate  510  preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate  510  is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10 −3 /K, lower than or equal to 5×10 −5 /K, or lower than or equal to 1×10 −5 /K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, acrylic, and polytetrafluoroethylene (PTFE). In particular, aramid is preferably used for the flexible substrate  510  because of its low coefficient of linear expansion. 
     &lt;Structure Example 2 of OS Transistor&gt; 
     In the OS transistor  581  illustrated in  FIG. 18A , the semiconductor  523  and the insulator  515  can be etched with the conductor  530  used as a mask.  FIG. 20A  illustrates a structure example of the OS transistor manufactured through such a process. In an OS transistor  582  illustrated in  FIG. 20A , edges of the semiconductor  523  and the insulator  515  are substantially aligned with an edge of the conductor  530 . The semiconductor  523  and the insulator  515  are positioned only under the conductor  530 . 
     &lt;Structure Example 3 of OS Transistor&gt; 
     An OS transistor  583  illustrated in  FIG. 20B  has a device structure in which a conductor  535  and a conductor  536  are added to the OS transistor  582 . A pair of electrodes serving as a source and a drain electrode of the OS transistor  583  include a stack of the conductors  535  and  531  and a stack of the conductors  536  and  532 . 
     Each of the conductors  535  and  536  includes a single-layer or multilayer conductor containing one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound may also be used, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     The conductors  535  and  536  may have a property of transmitting visible light. Alternatively, the conductors  535  and  536  may have a property of not transmitting visible light, ultraviolet light, infrared light, or X-rays by reflecting or absorbing it. In some cases, such a property can suppress a change in the electric characteristics of the OS transistor  583  due to stray light. 
     The conductors  535  and  536  may preferably be formed using a layer that does not form a Schottky barrier with the semiconductor  522  or the like. Accordingly, the on-state characteristics of the OS transistor  583  can be improved. 
     Note that the conductors  535  and  536  may preferably have higher resistance than the conductors  531  and  532 . The conductors  535  and  536  may preferably have a resistance lower than that of a channel of the OS transistor  583  (specifically, the semiconductor  522 ). For example, the conductors  535  and  536  may have a resistivity higher than or equal to 0.1 Ωcm and lower than or equal to 100 Ωcm, higher than or equal to 0.5 Ωcm and lower than or equal to 50 Ωcm, or higher than or equal to 1 Ωcm and lower than or equal to 10 Ωcm. The conductors  535  and  536  having a resistivity within the above range can reduce electric field concentration in a boundary portion between the channel and the drain. This suppresses a change in the electric characteristics of the OS transistor  583 . In addition, a punch-through current generated by an electric field from the drain can be reduced. Thus, a transistor with a small channel length can have favorable saturation characteristics. Note that in a circuit configuration where the source and the drain do not interchange, only one of the conductors  535  and  536  (e.g., the layer on the drain side) may preferably be provided. 
     &lt;Structure Example 4 of OS Transistor&gt; 
     In the OS transistor  581  illustrated in  FIGS. 18A to 18D , the conductors  531  and  532  may be in contact with side surfaces of the semiconductors  521  and  522 .  FIG. 20C  illustrates an example of such a structure. In an OS transistor  584  illustrated in  FIG. 20C , the conductors  531  and  532  are in contact with the side surfaces of the semiconductors  521  and  522 . 
     &lt;Heat Dissipation Method of OS Transistor&gt; 
     An oxide semiconductor containing indium, zinc, and gallium has a low thermal conductivity; hence, when a transistor including the oxide semiconductor continues to be driven, the reliability of the transistor decreases due to self-heating in some cases. Therefore, the transistor preferably has a structure allowing dissipation of heat generated by driving. 
     In  FIG. 20C , the area where the conductors  531  and  532  are in contact with the semiconductor region  520  is increased so that heat generated from the semiconductor region  520  is dissipated to the conductors  531  and  532 . Although not illustrated, when the area where the conductor  531  is in contact with the conductors  531  and  532  is increased, heat is also dissipated to the conductor  530 . 
     Embodiment 4 
     Described in this embodiment is a structure of an oxide semiconductor film capable of being used for the OS transistors described in the above embodiments. 
     &lt;Structure of Oxide Semiconductor&gt; 
     An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis-aligned and a-b-plane anchored crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor. 
     From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS. 
     An amorphous structure is generally thought to be isotropic and have no non-uniform structure, to be metastable and not have fixed positions of atoms, to have a flexible bond angle, and to have a short-range order but have no long-range order, for example. 
     In other words, a stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. In contrast, an a-like OS, which is not isotropic, has an unstable structure that contains a void. Because of its instability, an a-like OS is close to an amorphous oxide semiconductor in terms of physical properties. 
     &lt;CAAC-OS&gt; 
     First, a CAAC-OS is described. 
     A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets). 
     Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO 4  crystal that is classified into the space group R-3m is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2 of around 31° as shown in  FIG. 21A . This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to a surface over which the CAAC-OS film is formed (also referred to as a formation surface) or the top surface of the CAAC-OS film. Note that a peak sometimes appears at a 2θ of around 36° in addition to the peak at a 2θ of around 31°. The peak at a 2θ of around 36° is derived from a crystal structure that is classified into the space group Fd-3m; thus, this peak is preferably not exhibited in a CAAC-OS. 
     On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on the CAAC-OS in a direction parallel to the formation surface, a peak appears at a 2θ of around 56°. This peak is attributed to the (110) plane of the InGaZnO 4  crystal. When analysis (φ scan) is performed with  28  fixed at around 56° and with the sample rotated using a normal vector to the sample surface as an axis (φ axis), as shown in  FIG. 21B , a peak is not clearly observed. In contrast, in the case where single crystal InGaZnO 4  is subjected to φ scan with  28  fixed at around 56°, as shown in  FIG. 21C , six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are irregularly oriented in the CAAC-OS. 
     Next, a CAAC-OS analyzed by electron diffraction is described. 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 formation surface of the CAAC-OS, a diffraction pattern (also referred to as a selected-area electron diffraction pattern) shown in  FIG. 21D  can be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO 4  crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile,  FIG. 21E  shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in  FIG. 21E , a ring-like diffraction pattern is observed. Thus, the electron diffraction using an electron beam with a probe diameter of 300 nm also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular orientation. The first ring in  FIG. 21E  is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO 4  crystal. The second ring in  FIG. 21E  is considered to be derived from the (110) plane and the like. 
     In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, even in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed in some cases. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur. 
       FIG. 22A  shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be observed with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd. 
       FIG. 22A  shows pellets in which metal atoms are arranged in a layered manner.  FIG. 22A  proves that the size of a pellet is greater than or equal to 1 nm or greater than or equal to 3 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS can also be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC). A pellet reflects unevenness of a formation surface or a top surface of the CAAC-OS, and is parallel to the formation surface or the top surface of the CAAC-OS. 
       FIGS. 22B and 22C  show Cs-corrected high-resolution TEM images of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface.  FIGS. 22D and 22E  are images obtained through image processing of  FIGS. 22B and 22C . The method of image processing is as follows. The image in  FIG. 22B  is subjected to fast Fourier transform (FFT), so that an FFT image is obtained. Then, mask processing is performed such that a range of from 2.8 nm −1  to 5.0 nm −1  from the origin in the obtained FFT image remains. After the mask processing, the FFT image is processed by inverse fast Fourier transform (IFFT) to obtain a processed image. The image obtained in this manner is called an FFT filtering image. The FFT filtering image is a Cs-corrected high-resolution TEM image from which a periodic component is extracted, and shows a lattice arrangement. 
     In  FIG. 22D , a portion where a lattice arrangement is broken is denoted with a dashed line. A region surrounded by a dashed line is one pellet. The portion denoted with the dashed line is a junction of pellets. The dashed line draws a hexagon, which means that the pellet has a hexagonal shape. Note that the shape of the pellet is not always a regular hexagon but is a non-regular hexagon in many cases. 
     In  FIG. 22E , a dotted line denotes a portion between a region where a lattice arrangement is well aligned and another region where a lattice arrangement is well aligned, and dashed lines denote the directions of the lattice arrangements. A clear crystal grain boundary cannot be observed even in the vicinity of the dotted line. When a lattice point in the vicinity of the dotted line is regarded as a center and surrounding lattice points are joined, a distorted hexagon, pentagon, and/or heptagon can be formed, for example. That is, a lattice arrangement is distorted so that formation of a crystal grain boundary is inhibited. This is probably because the CAAC-OS can tolerate distortion owing to a low density of the atomic arrangement in an a-b plane direction, an interatomic bond distance changed by substitution of a metal element, and the like. 
     As described above, the CAAC-OS has c-axis alignment, its pellets (nanocrystals) are connected in an a-b plane direction, and the crystal structure has distortion. For this reason, the CAAC-OS can also be referred to as an oxide semiconductor including a c-axis-aligned a-b-plane-anchored (CAA) crystal. 
     The CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies). 
     Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity. 
     The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. Impurities included in the oxide semiconductor might serve as carrier traps or carrier generation sources, for example. For example, oxygen vacancy in the oxide semiconductor might serve as a carrier trap or serve as a carrier generation source when hydrogen is captured therein. 
     The CAAC-OS having small amounts of impurities and oxygen vacancy is an oxide semiconductor film with a low carrier density; specifically, lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , and further preferably lower than 1×10 10 /cm 3  and higher than or equal to 1×10 −9 /cm 3 ). Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics. 
     &lt;nc-OS&gt; 
     Next, an nc-OS is described. 
     Analysis of an nc-OS by XRD is described. When the structure of an nc-OS is analyzed by an out-of-plane method, a peak indicating orientation does not appear. That is, a crystal of an nc-OS does not have orientation. 
     For example, when an electron beam with a probe diameter of 50 nm is incident on a 34-nm-thick region of thinned nc-OS including an InGaZnO 4  crystal in a direction parallel to the formation surface, a ring-shaped diffraction pattern (a nanobeam electron diffraction pattern) shown in  FIG. 23A  is observed.  FIG. 23B  shows a diffraction pattern obtained when an electron beam with a probe diameter of 1 nm is incident on the same sample. As shown in  FIG. 23B , a plurality of spots are observed in a ring-like region. In other words, ordering in an nc-OS is not observed with an electron beam with a probe diameter of 50 nm but is observed with an electron beam with a probe diameter of 1 nm. 
     Furthermore, an electron diffraction pattern in which spots are arranged in an approximately hexagonal shape is observed in some cases as shown in  FIG. 23C  when an electron beam having a probe diameter of 1 nm is incident on a region with a thickness of less than 10 nm. This means that an nc-OS has a well-ordered region, i.e., a crystal, in the range of less than 10 nm in thickness. Note that an electron diffraction pattern having regularity is not observed in some regions because crystals are aligned in various directions. 
       FIG. 23D  shows a Cs-corrected high-resolution TEM image of a cross section of an nc-OS observed from the direction substantially parallel to the formation surface. In a high-resolution TEM image, an nc-OS has a region in which a crystal part is observed, such as the part indicated by additional lines in  FIG. 23D , and a region in which a crystal part is not clearly observed. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, or specifically, greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm is sometimes referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description. 
     As described above, in the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not ordered. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method. 
     Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC). 
     The nc-OS is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an a-like OS and an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS. 
     &lt;a-Like OS&gt; 
     An a-like OS has a structure intermediate between those of the nc-OS and the amorphous oxide semiconductor. 
       FIGS. 24A and 24B  are high-resolution cross-sectional TEM images of an a-like OS.  FIG. 24A  is the high-resolution cross-sectional TEM image of the a-like OS at the start of the electron irradiation.  FIG. 24B  is the high-resolution cross-sectional TEM image of a-like OS after the electron (e − ) irradiation at 4.3×10 8  e − /nm 2 .  FIGS. 24A and 24B  show that stripe-like bright regions extending vertically are observed in the a-like OS from the start of the electron irradiation. It can be also found that the shape of the bright region changes after the electron irradiation. Note that the bright region is presumably a void or a low-density region. 
     The a-like OS has an unstable structure because it contains a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below. 
     An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each of the samples is an In—Ga—Zn oxide. 
     First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts. 
     It is known that a unit cell of an InGaZnO 4  crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO 4  in the following description. Each of lattice fringes corresponds to the a-b plane of the InGaZnO 4  crystal. 
       FIG. 25  shows change in the average size of crystal parts (at 22 points to 30 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe.  FIG. 25  indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose in obtaining TEM images, for example. As shown in  FIG. 25 , a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 1.9 nm at a cumulative electron (e − ) dose of 4.2×10 8  e − /nm 2 . In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10 8  e − /nm 2 . As shown in  FIG. 25 , the crystal part sizes in an nc-OS and a CAAC-OS are approximately 1.3 nm and approximately 1.8 nm, respectively, regardless of the cumulative electron dose. For the electron beam irradiation and TEM observation, a Hitachi H-9000NAR transmission electron microscope was used. The conditions of electron beam irradiation were as follows: the accelerating voltage was 300 kV; the current density was 6.7×10 5  e − /(nm 2 ·s); and the diameter of irradiation region was 230 nm. 
     In this manner, growth of the crystal part in the a-like OS is sometimes induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS. 
     The a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor. 
     For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO 4  with a rhombohedral crystal structure is 6.357 g/cm 3 . Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm 3  and lower than 5.9 g/cm 3 . For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm 3  and lower than 6.3 g/cm 3 . 
     Note that in the case where an oxide semiconductor having a certain composition does not exist in a single crystal structure, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density. 
     As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more films of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example. 
     This embodiment can be combined as appropriate with any of the other embodiments described in this specification. 
     Embodiment 5 
     In this embodiment, application examples of the semiconductor device described in the foregoing embodiments to an electronic component and to an electronic device including the electronic component will be described with reference to  FIGS. 26A and 26B  and  FIGS. 27A to 27F . 
     &lt;Electronic Component&gt; 
       FIG. 26A  shows an example where the semiconductor device described in the foregoing embodiments is used to make an electronic component. Note that an electronic component is also referred to as semiconductor package or IC package. For the electronic component, there are various standards and names corresponding to the direction of terminals or the shape of terminals; hence, one example of the electronic component will be described in this embodiment. 
     A semiconductor device including the transistors as shown in Embodiment 1 is completed by integrating detachable components on a printed circuit board through the assembly process (post-process). 
     The post-process can be completed through steps shown in  FIG. 26A . Specifically, after an element substrate obtained in the wafer process is completed (Step S 1 ), a back surface of the substrate is ground (Step S 2 ). The substrate is thinned in this step to reduce warpage or the like of the substrate in the wafer process and to reduce the size of the component itself. 
     A dicing step of grinding the back surface of the substrate to separate the substrate into a plurality of chips is performed. Then, a die bonding step of individually picking up separate chips to be mounted on and bonded to a lead frame is performed (Step S 3 ). To bond a chip and a lead frame in the die bonding step, resin bonding, tape-automated bonding, or the like is selected as appropriate depending on products. Note that in the die bonding step, a chip may be mounted on and bonded to an interposer. 
     Next, wire bonding for electrically connecting a lead of the lead frame and an electrode on a chip through a metal wire is performed (Step S 4 ). As the metal wire, a silver wire or a gold wire can be used. For wire bonding, ball bonding or wedge bonding can be employed. 
     A wire-bonded chip is subjected to a molding step of sealing the chip with an epoxy resin or the like (Step S 5 ). With the molding step, the inside of the electronic component is filled with a resin, thereby reducing damage to the circuit portion and the wire embedded in the component caused by external mechanical force as well as reducing deterioration of characteristics due to moisture or dust. 
     Subsequently, the lead of the lead frame is plated. Then, the lead is cut and processed into a predetermined shape (Step S 6 ). With the plating process, corrosion of the lead can be prevented, and soldering for mounting the electronic component on a printed circuit board in a later step can be performed with higher reliability. 
     Next, printing process (marking) is performed on a surface of the package (Step S 7 ). Then, through a final test step (Step S 8 ), the electronic component is completed (Step S 9 ). 
     Since the electronic component described above includes the semiconductor device described in the foregoing embodiments, it is possible to obtain a highly reliable electronic component. 
       FIG. 26B  is a schematic perspective view of a completed electronic component.  FIG. 26B  shows a schematic perspective view of a quad flat package (QFP) as an example of the electronic component. An electronic component  700  illustrated in  FIG. 26B  includes a lead  701  and a circuit portion  703 . The electronic component  700  in  FIG. 26B  is, for example, mounted on a printed circuit board  702 . A combination of electronic components  700  electrically connected to each other over the printed circuit board  702  can be equipped in an electronic device. A completed circuit board  704  is provided in an electronic device or the like. 
     &lt;Electronic Device&gt; 
     Next, electronic devices including the aforementioned electronic component will be described. 
     The semiconductor device of one embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices that reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can be equipped with the semiconductor device of one embodiment of the present invention are cellular phones, game machines including portable game machines, portable information terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head-mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), vending machines, and medical devices.  FIGS. 27A to 27F  illustrate specific examples of these electronic devices. 
       FIG. 27A  illustrates a portable game machine which includes a housing  5001 , a housing  5002 , a display portion  5003 , a display portion  5004 , a microphone  5005 , a speaker  5006 , an operation key  5007 , a stylus  5008 , and the like. The semiconductor device of one embodiment of the present invention can be used for a variety of integrated circuits included in portable game machines. Although the portable game machine in  FIG. 27A  has the two display portions  5003  and  5004 , the number of display portions included in a portable game machine is not limited to two. 
       FIG. 27B  illustrates a portable information terminal which includes a first housing  5601 , a second housing  5602 , a first display portion  5603 , a second display portion  5604 , a joint  5605 , an operation key  5606 , and the like. The semiconductor device of one embodiment of the present invention can be used for a variety of integrated circuits included in portable information terminals. The first display portion  5603  is provided in the first housing  5601 , and the second display portion  5604  is provided in the second housing  5602 . The first housing  5601  and the second housing  5602  are connected to each other with the joint  5605 , and the angle between the first housing  5601  and the second housing  5602  can be changed with the joint  5605 . Images displayed on the first display portion  5603  may be switched in accordance with the angle at the joint  5605  between the first housing  5601  and the second housing  5602 . A display device with a position input function may be used as the first display portion  5603  and/or the second display portion  5604 . Note that the position input function can be added by providing a touch panel in a display device. Alternatively, the position input function can be added by providing a photoelectric conversion element called a photosensor in a pixel portion of a display device. 
       FIG. 27C  illustrates a laptop which includes a housing  5401 , a display portion  5402 , a keyboard  5403 , a pointing device  5404 , and the like. The semiconductor device of one embodiment of the present invention can be used for a variety of integrated circuits included in laptops. 
       FIG. 27D  illustrates an electric refrigerator-freezer which includes a housing  5301 , a refrigerator door  5302 , a freezer door  5303 , and the like. The semiconductor device of one embodiment of the present invention can be used for a variety of integrated circuits included in electric refrigerator-freezers. 
       FIG. 27E  illustrates a video camera which includes a first housing  5801 , a second housing  5802 , a display portion  5803 , operation keys  5804 , a lens  5805 , a joint  5806 , and the like. The semiconductor device of one embodiment of the present invention can be used for a variety of integrated circuits included in video cameras. The operation keys  5804  and the lens  5805  are provided in the first housing  5801 , and the display portion  5803  is provided in the second housing  5802 . The first housing  5801  and the second housing  5802  are connected to each other with the joint  5806 , and the angle between the first housing  5801  and the second housing  5802  can be changed with the joint  5806 . Images displayed on the display portion  5803  may be switched in accordance with the angle at the joint  5806  between the first housing  5801  and the second housing  5802 . 
       FIG. 27F  illustrates a car which includes a car body  5101 , wheels  5102 , a dashboard  5103 , lights  5104 , and the like. The semiconductor device of one embodiment of the present invention can be used for a variety of integrated circuits included in cars. 
     This application is based on Japanese Patent Application serial No. 2014-250551 filed with Japan Patent Office on Dec. 11, 2014, the entire contents of which are hereby incorporated by reference. 
     EXPLANATION OF REFERENCE 
     MC[i]: memory cell, MC[ 1 ]: memory cell, MC[ 2 ]: memory cell, MC[n]: memory cell, MC: memory cell, pMC[i]: memory cell, MC[ 1 , 1 ]: memory cell, MC[m, 1 ]: memory cell, MC[ 1 ,n]: memory cell, MC[m,n]: memory cell, MCC: circuit unit, MCC[ 1 ]: circuit unit, MCC[m]: circuit unit, MCC[ 1 , 1 ]: circuit unit, MCC[m, 1 ]: circuit unit, MCC[ 1 ,n]: circuit unit, MCC[m,n]: circuit unit, MCA: memory cell array, MCS: circuit unit, MCS[ 1 ]: circuit unit, MCS[m]: circuit unit, MCS[ 1 , 1 ]: circuit unit, MCS[m, 1 ]: circuit unit, MCS[ 1 ,n]: circuit unit, MCS[m,n]: circuit unit, MCSS: circuit unit, Si—C: circuit unit, TRA[i]: transistor, TRA[ 1 ]: transistor, TRA[ 2 ]: transistor, TRA[n]: transistor, TRB[i]: transistor, TRB[ 1 ]: transistor, TRB[ 2 ]: transistor, TRB[n]: transistor, BGA[i]: back gate, BGA[ 1 ]: back gate, BGA[ 2 ]: back gate, BGA[n]: back gate, BGB[i]: back gate, BGB[ 1 ]: back gate, BGB[ 2 ]: back gate, BGB[n]: back gate, pTRA[i]: transistor, pTRB[i]: transistor, SW: transistor, SW[ 1 ]: transistor, SW[m]: transistor, SW[ 1 , 1 ]: transistor, SW[m, 1 ]: transistor, SW[ 1 ,n]: transistor, SW[m,n]: transistor, C[i]: capacitor, C[ 1 ]: capacitor, C[ 2 ]: capacitor, C[n]: capacitor, FN[i]: node, FN[ 1 ]: node, FN[ 2 ]: node, FN[n]: node, CNODE[ 1 ]: wiring, CNODE[ 1 ]: wiring, CNODE[ 2 ]: wiring, CNODE[n]: wiring, CNODE[ 1 , 1 ]: wiring, CNODE[ 1 ,n]: wiring, CNODE[m, 1 ]: wiring, CNODE[m,n]: wiring, CNODE( 1 )[ 1 ]: wiring, CNODE(k)[ 1 ]: wiring, CNODE( 1 )[n]: wiring, CNODE(k)[n]: wiring, D[i]: wiring, S[i]: wiring, WBL[ 1 ]: wiring, WBL[ 1 ]: wiring, WBL[ 2 ]: wiring, WBL[n]: wiring, WBL( 1 )[ 1 ]: wiring, WBL(k)[ 1 ]: wiring, WBL( 1 )[n]: wiring, WBL(k)[n]: wiring, WL: wiring, WL[ 1 ]: wiring, WL[m]: wiring, SWG: wiring, SWG[ 1 ]: wiring, SWG[m]: wiring, SWG[ 1 , 1 ]: wiring, SWG[m, 1 ]: wiring, SWG[ 1 ,n]: wiring, SWG[m,n]: wiring, NA: node, NA[ 1 ]: node, NA[m]: node, NA[ 1 , 1 ]: node, NA[m, 1 ]: node, NA[ 1 ,n]: node, NA[m,n]: node, NB: node, NB[ 1 ]: node, NB[m]: node, NB[ 1 , 1 ]: node, NB[m, 1 ]: node, NB[ 1 ,n]: node, NB[m,n]: node,  101   a : semiconductor,  101   b : semiconductor,  102 : conductor,  103   a : conductor,  103   b : conductor,  104   a : conductor,  104   b : conductor,  105   a : conductor,  105   b : conductor,  106 : conductor,  107 : conductor,  108 : conductor,  110 : semiconductor device,  120 : semiconductor device,  160 : semiconductor device,  170 : semiconductor device,  180 : semiconductor device,  190 : semiconductor device,  510 : substrate,  511 : insulator,  512 : insulator,  513 : insulator,  514 : insulator,  515 : insulator,  516 : insulator,  520 : semiconductor region,  521 : semiconductor,  522 : semiconductor,  523 : semiconductor,  530 : conductor,  531 : conductor,  532 : conductor,  533 : conductor,  535 : conductor,  536 : conductor,  700 : electronic component,  701 : lead,  702 : printed circuit board,  703 : circuit portion,  704 : circuit board,  581 : OS transistor,  582 : OS transistor,  583 : OS transistor,  584 : OS transistor,  2600 : memory device,  2601 : peripheral circuit,  2610 : memory cell array,  2621 : row decoder,  2622 : word line driver circuit,  2630 : bit line driver circuit,  2631 : column decoder,  2632 : precharge circuit,  2633 : sense amplifier,  2634 : write circuit,  2640 : output circuit,  2660 : control logic circuit,  5001 : housing,  5002 : housing,  5003 : display portion,  5004 : display portion,  5005 : microphone,  5006 : speaker,  5007 : operation key,  5008 : stylus,  5101 : car body,  5102 : wheels,  5103 : dashboard,  5104 : light,  5301 : housing,  5302 : refrigerator door,  5303 : freezer door,  5401 : housing,  5402 : display portion,  5403 : keyboard,  5404 : pointing device,  5601 : first housing,  5602 : second housing,  5603 : first display portion,  5604 : second display portion,  5605 : joint,  5606 : operation key,  5801 : first housing,  5802 : second housing,  5803 : display portion,  5804 : operation key,  5805 : lens,  5806 : joint