Patent Publication Number: US-9412739-B2

Title: Semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a machine, a process, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor, a semiconductor device, a memory device, a processor, a display device, a light-emitting device, a lighting device, or a power storage device. Alternatively, the present invention relates to a method for manufacturing a semiconductor, a semiconductor device, a memory device, a processor, a display device, a light emitting device, a lighting device, or a power storage device. Alternatively, the present invention relates to a method for driving a semiconductor device, a memory device, a processor, a display device, a light emitting device, a lighting device, or a power storage device. 
     The semiconductor device in this specification indicates all the devices that can operate by utilizing semiconductor characteristics. Note that a semiconductor device means a circuit having a semiconductor element (e.g., a transistor or a diode) and a device having the circuit, and the like. For example, an electronic circuit and a chip including an electronic circuit are all semiconductor devices. A memory device, a display device, a light emitting device, a lighting device, an electro-optical device, an electronic device, and the like include a semiconductor device in some cases. 
     2. Description of the Related Art 
     Much attention is focused on a semiconductor device that can retain data even after the interruption of power supply by using a combination of a transistor including semiconductor silicon (Si) in a region where a channel is formed (also referred to as a channel formation region) (this transistor is referred to as a Si transistor in the following description) and a transistor including an oxide semiconductor (preferably an oxide including In, Ga, and Zn) in its channel formation region (see Patent Document 1). A Si transistor is used in a variety of electronic circuits or electronic components. As a structural unit, an electronic circuit contains a cell (also referred to as a logic cell or a standard cell) such as an inverter circuit, a NAND circuit, and a flip-flop which are formed by arranging and wiring an n-channel Si transistor and a p-channel Si transistor (see Non-Patent Document 1). 
     On the other hand, transistors including an oxide semiconductor (preferably an oxide including In, Ga, and Zn) in their channel formation regions are known. It is known that a transistor including an oxide semiconductor in a channel formation region has extremely small off-state current because an oxide semiconductor has a wider band gap than silicon. For example, Patent Document 1 discloses a semiconductor device that can retain data even after interruption of power supply with use of such a transistor in a memory cell. 
     In recent years, demand for a circuit in which semiconductor elements such as a miniaturized transistor are integrated with high density has risen with increased performance and reductions in the size and weight of an electronic device. 
     PATENT DOCUMENT 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2011-187950 
         [Non-Patent Document 1] Neil H. E. Weste and David Money Harris, CMOS VLSI Design: A Circuits and Systems Perspective (4th Edition), Addison Wesley, p. 27, 2011. 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is at least one of the following: to provide a semiconductor device (cell) including a circuit having a small circuit area; to provide a semiconductor device (cell) including a circuit with low power consumption; to provide a semiconductor device (cell) including a circuit with high operation speed; to provide a small semiconductor device; to provide a semiconductor device with low power consumption; to provide a semiconductor device with improved operation speed; to provide a semiconductor device which can be manufactured at low cost; and providing a novel semiconductor device. 
     Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     (1) One embodiment of the present invention is a semiconductor device including a first transistor and a second transistor. The first transistor includes a first conductor, a first insulator over the first conductor, an oxide semiconductor over the first insulator, a second insulator over the oxide semiconductor, a second conductor over the second insulator, a third conductor, and a fourth conductor. The oxide semiconductor overlaps with the first conductor. The third conductor is in contact with the oxide semiconductor. The fourth conductor is in contact with the oxide semiconductor. The oxide semiconductor includes a first region positioned between the third conductor and the fourth conductor when viewed from above. The first region includes a second region overlapping with the first conductor. The first region includes a third region not overlapping with the first conductor. The first region includes a fourth region overlapping with the second conductor. The first region includes a fifth region not overlapping with the second conductor. The second region and the fifth region have a sixth region where they overlap with each other. The third region and the fourth region have a seventh region where they overlap with each other. The second transistor is a p-channel transistor. A layer in which the first transistor is provided and a layer in which the second transistor is provided are provided so as to overlap with each other. 
     (2) One embodiment of the present invention is a semiconductor device including a first transistor, a second transistor, and a third transistor. The first transistor includes a first conductor, a first insulator over the first conductor, an oxide semiconductor over the first insulator, a second insulator over the oxide semiconductor, a second conductor over the second insulator, a third conductor, and a fourth conductor. The oxide semiconductor overlaps with the first conductor. The third conductor is in contact with the oxide semiconductor. The fourth conductor is in contact with the oxide semiconductor. The oxide semiconductor includes a first region positioned between the third conductor and the fourth conductor when viewed from above. The first region includes a second region overlapping with the first conductor. The first region includes a third region not overlapping with the first conductor. The first region includes a fourth region overlapping with the second conductor. The first region includes a fifth region not overlapping with the second conductor. The second region and the fifth region have a sixth region where they overlap with each other. The third region and the fourth region have a seventh region where they overlap with each other. The second and the third transistors are p-channel transistors. A layer in which the first transistor is provided and a layer in which the second transistor and the third transistor are provided are provided so as to overlap with each other. A direction in which a gate of the second transistor extends, a direction in which a gate of the third transistor extends, a direction in which the first conductor extends, and a direction in which the second conductor extends are parallel to each other. A channel formation region of the second transistor and a channel formation region of the third transistor are aligned in the direction. The first conductor and the gate of the second transistor are electrically connected to each other through a first connection portion. The second conductor and the gate of the third transistor are electrically connected to each other through a second connection portion. The first region, the channel formation region of the second transistor, and the channel formation region of the third transistor are provided between the first connection portion and the second connection portion when viewed from above. One of the third conductor and the fourth conductor, one of a source and a drain of the second transistor, and one of a source and a drain of the third transistor are electrically connected to each other. 
     (3) One embodiment of the present invention is the semiconductor device according to the embodiment (1) or (2), in which the second region and the fourth region overlap with each other. 
     (4) One embodiment of the present invention is the semiconductor device according to the embodiment (1) or (2), in which the third region and the fifth region have an eighth region where they overlap with each other. The eighth region is narrower than the sixth region and the seventh region. 
     (5) One embodiment of the present invention is a semiconductor device including a first transistor and a second transistor. The first transistor includes a first conductor, a first insulator over the first conductor, an oxide semiconductor over the first insulator, a second insulator over the oxide semiconductor, a second conductor over the second insulator, a third conductor, and a fourth conductor. The oxide semiconductor overlaps with the first conductor. The third conductor is in contact with the oxide semiconductor. The fourth conductor is in contact with the oxide semiconductor. The oxide semiconductor includes a first region positioned between the third conductor and the fourth conductor when viewed from above. The first region overlaps with the first conductor and is covered with the second conductor. The second transistor is a p-channel transistor. A layer in which the first transistor is provided and a layer in which the second transistor is provided are provided so as to overlap with each other. 
     (6) One embodiment of the present invention is a semiconductor device including a first transistor, a second transistor, and a third transistor. The first transistor includes a first conductor, a first insulator over the first conductor, an oxide semiconductor over the first insulator, a second insulator over the oxide semiconductor, a second conductor over the second insulator, a third conductor, and a fourth conductor. The oxide semiconductor overlaps with the first conductor. The third conductor is in contact with the oxide semiconductor. /The fourth conductor is in contact with the oxide semiconductor. The oxide semiconductor includes a first region positioned between the third conductor and the fourth conductor when viewed from above. The first region overlaps with the first conductor and is covered with the second conductor. The second transistor and the third transistor are p-channel transistors. A layer in which the first transistor is provided and a layer in which the second transistor and the third transistor are provided are provided to overlap with each other. A direction in which a gate of the second transistor extends, a direction in which a gate of the third transistor extends, a direction in which the first conductor extends, a direction in which the second transistor extends are parallel. A channel formation region of the second transistor and a channel formation region of the third transistor are aligned in the direction. The channel formation region of the second transistor and a channel formation region of the third transistor are aligned in the direction. The first conductor and the gate of the second transistor are electrically connected to each other through a first connection portion. The second conductor and the gate of the third transistor are electrically connected to each other through the second connection portion. The first region, a channel formation region of the second transistor, and a channel formation region of the third transistor are provided between the first connection portion and the second connection portion when viewed from above. One of the third conductor and the fourth conductor is electrically connected to one of a source and a drain of the second transistor. The other of the source and the drain of the second transistor and one of a source and a drain of the third transistor are electrically connected to each other. 
     (7) One embodiment of the present invention is a semiconductor device including a first transistor, a second transistor, and a third transistor. The first transistor includes a first conductor, a first insulator over the first conductor, an oxide semiconductor over the first insulator, a second insulator over the oxide semiconductor, a second conductor over the second insulator, a third conductor, and a fourth conductor. The oxide semiconductor overlaps with the first conductor. The third conductor is in contact with the oxide semiconductor. The fourth conductor is in contact with the oxide semiconductor. The oxide semiconductor includes a first region positioned between the third conductor and the fourth conductor when viewed from above. The first region overlaps with the first conductor and is covered with the second conductor. The second transistor and the third transistor are p-channel transistors. A layer in which the first transistor is provided and a layer in which the second transistor and the third transistor are provided are provided to overlap with each other. A direction in which a gate of the second transistor extends, a direction in which a gate of the third transistor extends, and a direction in which the first conductor extends, and a direction in which the second conductor extends are parallel to a first direction. A channel formation region of the second transistor and a channel formation region of the third transistor are aligned perpendicular to the first direction. The first conductor and a gate of the second transistor are electrically connected to each other through a first connection portion. The second conductor and the gate of the third transistor are electrically connected to each other through a second connection portion. The first region, a channel formation region of the second transistor, and a channel formation region of the third transistor are provided between the first connection portion and the second connection portion when viewed from above. One of the third conductor and the fourth conductor and one of a source and a drain of the second transistor are electrically connected to each other. The other of the source and the drain of the second transistor and one of a source and a drain of the third transistor are electrically connected to each other. 
     (8) One embodiment of the present invention is the semiconductor device according to the embodiment (2) or (6), in which the channel width of the first transistor when viewed from above is larger than the channel widths of the second transistor and the third transistor when viewed from above. 
     (9) One embodiment of the present invention is the semiconductor device according to the embodiment (7), in which the channel length of the first transistor is larger than the channel lengths of the second transistor and the third transistor. 
     (10) One embodiment of the present invention is an RFID tag including the semiconductor device according to any one of the embodiments (1) to (9) and an antenna. 
     (11) One embodiment of the present invention is an electronic device including the semiconductor device according to any one of the embodiments (1) to (9) and a printed wiring board. 
     A semiconductor device (cell) including a circuit having a reduced circuit area can be provided. Alternatively, a semiconductor device (cell) including a circuit with low power consumption can be provided. Alternatively, a semiconductor device (cell) including a circuit with improved operation speed can be provided. Alternatively, a small semiconductor device can be provided. Alternatively, a semiconductor device with low power consumption can be provided. Alternatively, a semiconductor device with improved operation speed can be provided. Alternatively, a semiconductor device which can be manufactured at low cost can be provided. Alternatively, a novel semiconductor device can be provided. 
     Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all of these effects. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  are circuit diagrams of a semiconductor device (cell) according to one embodiment of the present invention. 
         FIGS. 2A to 2C  are top views of a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 4  is a cross-sectional view of semiconductor device (cell) according to one embodiment of the present invention. 
         FIGS. 5A to 5C  are top views of a semiconductor device (cell) of one embodiment of the present invention. 
         FIGS. 6A to 6C  are top views of a transistor. 
         FIG. 7  is a cross sectional view of a semiconductor device (cell) of one embodiment of the present invention. 
         FIGS. 8A to 8C  are top views of a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of a semiconductor device (cell) of one embodiment of the present invention. 
         FIGS. 10A to 10C  are top views of a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 12  is a cross-sectional view of a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 13  is a cross-sectional view of a semiconductor device (cell) according to one embodiment of the present invention. 
         FIGS. 14A and 14B  are a top view and a cross-sectional view of a transistor; 
         FIGS. 15A and 15B  are a top view and a cross-sectional view of a transistor. 
         FIGS. 16A and 16B  are circuit diagrams illustrating a semiconductor device (cell) of one embodiment of the present invention. 
         FIG. 17  is a block diagram illustrating a memory device according to one embodiment of the present invention. 
         FIG. 18  is a circuit diagram illustrating a memory cell. 
         FIG. 19  is a circuit diagram illustrating a memory cell. 
         FIG. 20  is a circuit diagram illustrating a memory cell. 
         FIG. 21  is a circuit diagram illustrating a memory cell. 
         FIG. 22  is a block diagram illustrating a CPU of one embodiment of the present invention. 
         FIG. 23  is a block diagram illustrating an RFID of one embodiment of the present invention. 
         FIGS. 24A and 24B  each illustrate an electronic device of one embodiment of the present invention. 
         FIGS. 25A to 25F  each illustrate an electronic device of one embodiment of the present invention. 
         FIGS. 26A to 26C  are a cross-sectional view of stacked semiconductor layers and band diagrams thereof. 
         FIGS. 27A and 27B  are a top view and a cross sectional view of a transistor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described in detail with the reference to the 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 description of the embodiments described below. In describing structures of the present 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 is sometimes exaggerated for simplicity. 
     A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a source potential or a ground potential (GND)). A voltage can be referred to as a potential and vice versa. 
     Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as the ordinal numbers used to specify one embodiment of the present invention. 
     In this specification, the phrase “A has a region with a concentration B” means, for example, “the concentration of the entire 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 the concentration in a region of A in the depth direction is B”, “the maximum value of the concentration in a region of A in the depth direction is B”, “the minimum value of the concentration in a region of A in the depth direction is B”, “a convergence value of the concentration in a region of A in the depth direction is B”, and “the concentration in a region in which a probable value of A can be 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” means, for example, “the size, the length, the thickness, the width, or the distance of the entire 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 in which a probable value of A can be obtained in measurement is B”. 
     A transistor includes three nodes (terminals) called a gate, a source, and a drain. A gate is a node that controls the conduction state of a transistor. Depending on the channel type of the transistor or levels of potentials applied to the nodes (terminals), one of nodes (an input node and an output node) functions as a source and the other functions as a drain. In general, in an n-channel transistor, a node to which a low potential is applied is referred to as a source, and a node to which a high potential is applied is referred to as a drain. In contrast, in a p-channel transistor, a node to which a low potential is applied is referred to as a drain, and a node to which a high potential is applied is referred to as a source. 
     In this specification and the like, to clarify a circuit configuration and circuit operation, one of two nodes (an input node and an output node) of a transistor is fixed as a source and the other is fixed as a drain in some cases. It is needless to say that, depending on a driving method, the magnitude relationship between potentials applied to three terminals of the transistor might be changed, and the source and the drain might be interchanged. Thus, in one embodiment of the present invention, the distinction between the source and drain of the transistor is not limited to that described in this specification and the drawings. 
     A conductor functioning as a gate of the transistor is referred to as a gate electrode. A conductor functioning as a source of the transistor is referred to as a source electrode. A conductor functioning as a drain of the transistor is referred to as a drain electrode. A region functioning as a source of the transistor is referred to as a source region. A region functioning as a drain of the transistor is referred to as a drain region. In this specification, a gate electrode is referred to as a gate, a drain electrode or a drain region is referred to as a drain, and a source electrode or a source region is referred to as a source in some cases. 
     The channel length refers to, for example, a distance between a source and a drain in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate overlap 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 overlap with each other, or a region where a channel is formed. 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 transistor structures, a channel width in a region where a channel is actually formed (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 higher than the proportion of a channel region formed in a top surface of a semiconductor 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, without accurate information on the shape of a semiconductor, it is difficult to measure an effective channel width accurately. 
     Note that 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 “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 includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     Note that the layout of circuit blocks in a drawing is the one for specifying the positional relationship in description. Thus, even when a drawing shows that different functions are achieved in different circuit blocks, an actual circuit or region may be configured so that the different functions are achieved in the same circuit block. Furthermore, the function of each circuit block in a drawing is specified for description. Thus, even when one circuit block is illustrated, an actual circuit or region may be configured so that processing which is illustrated as being performed in the one circuit block is performed in a plurality of circuit blocks. 
     Embodiment 1 
     In this embodiment, a semiconductor device of one embodiment of the present invention will be described with reference to drawings. 
       FIGS. 1A to 1D  illustrate an example of a circuit configuration of a semiconductor device of one embodiment of the present invention. 
     Semiconductor devices having circuit structures illustrated in  FIGS. 1A to 1D  may serve as components of various electronic circuits. Such components are referred to as a standard cell, a logic cell, or simply as a cell. In the following description, these semiconductor devices are referred to as “semiconductor devices (cells)” or simply “cells” in some cases. 
     A cell  500  illustrated in  FIG. 1A  includes a transistor  490 , a transistor  491   a , and a transistor  491   b . Signals A and B are input to the cell  500 , and the cell  500  outputs a signal Z. Power supply potentials V 1  and V 2  are supplied to the cell  500 . 
     The transistor  490  includes a first gate, a second gate, a source, and a drain. The first gate and the second gate are positioned so as to vertically sandwich the channel formation region. The channel formation region includes a region A overlapping with the first gate and not overlapping with the second gate when viewed from above, and a region B not overlapping with the first gate and overlapping with the second gate. In this specification, the transistor  490  with such a structure is represented by a symbol surrounded by a dashed-dotted line  FIG. 1A . 
     A gate of the transistor  491   a  and the first gate of the transistor  490  are electrically connected to each other, and a signal A is input thereto. A gate of the transistor  491   b  and the second gate of the transistor  490  are electrically connected to each other, and a signal B is input thereto. The transistors  491   a  and  491   b  are connected in parallel. That is, a source of the transistor  491   a  and a source of the transistor  491   b  are electrically connected to each other. A drain of the transistor  491   a  and a drain of the transistor  491   b  are electrically connected to each other. The sources of the transistors  491   a  and  491   b  are supplied with the power supply potential V 2 . The drains of the transistors  491   a  and  491   b  and the drain of the transistor  490  are electrically connected to each other, and output the signal Z. The source of the transistor  490  is supplied with the power supply potential V 1 . Though connection lines indicated by dotted lines in  FIG. 1A  are electrically connected to the power supply potentials V 1  and V 2  and an output terminal of the signal Z, another element such as a transistor may be provided therebetween. A connection line is also illustrated with a dotted line in other drawings. 
     The transistor  490  is turned on or off depending on the potential of the first gate and the potential of the second gate. When a potential difference Vgs 1  between the first gate and the source exceeds a voltage Vth 1 , a channel is formed (or carriers are induced) in some cases in the channel formation region that overlaps with the first gate when viewed from above. When a potential difference Vgs 2  between the second gate and the source exceeds a voltage Vth 2 , a channel is formed (or carriers are induced) in some cases in the channel formation region that overlaps with the second gate when viewed from above. As described above, the transistor  490  has at least two threshold voltages Vth 1  and Vth 2 . The transistor  490  is on only when Vgs 1 &gt;Vth 1  and Vgs 2 &gt;Vth 2  are satisfied. That is, it can be said that the transistor  490  has a function equivalent to that of a circuit in which two transistors, a transistor whose threshold voltage is Vth 1  and a transistor whose threshold voltage is Vth 2 , are connected in series. 
     As the transistor  490 , an n-channel transistor can be used. As the transistors  491   a  and  491   b , p-channel transistors can be used. The power supply potential V 1  may be a low power supply potential VSS. The power supply potential V 2  may be a high power supply potential VDD. 
     Specifically, a complementary metal oxide semiconductor (CMOS) circuit can be formed by using a p-channel transistor and an n-channel transistor. With the CMOS circuit, the power consumption of an electronic circuit can be reduced. 
     The structure of a cell  502  illustrated in  FIG. 1C  is partly different from that of the circuit illustrated in  FIG. 1A . Specifically, a wiring indicated by a dotted line in  FIG. 1A  is indicated by a solid line in  FIG. 1C . That is, in  FIG. 1C , the sources of the transistors  491   a  and  491   b  are supplied with the power supply potential V 2  without through a transistor. The drains of the transistors  491   a  and  491   b  output the signal Z without through a transistor. The source of the transistor  490  is supplied with the power supply potential V 1  without through a transistor. The drain of the transistor  490  outputs the signal Z without through a transistor. The cell  502  illustrated in  FIG. 1C  is a two-input NAND circuit. 
     The symbol surrounded by a dashed-dotted line in  FIGS. 1A and 1C  refers to transistors where the first gates are formed over the channel formation region, and the second gates are formed under the channel formation region. In the transistor  490 , the first gate may be placed on the source side and the second gate may be placed on the drain side; alternatively, the first gate may be placed on the drain side and the second gate may be placed on the source side. 
     In  FIGS. 1A and 1C , the transistor  490  can be manufactured in a smaller area than the case where a transistor with a threshold voltage of Vth 1  and a transistor with a threshold voltage of Vth 2  are connected in series as described later. As a result, the cell area can be reduced in some cases. 
     As an example of the transistor  490 , a transistor with a low drain current in an off state (also referred to as a leakage current) can be used. For example, the drain current in an off state is 1×10 −18  A or lower, preferably 1×10 −1  A or lower, further preferably 1×10 −24  A or lower at room temperature (approximately 25° C.), or 1×10 −15  A or lower, preferably 1×10 −18  A or lower, further preferably 1×10 −21  A or lower at 85° C. For example, a transistor including an oxide semiconductor (preferably an oxide including In, Ga, and Zn) in its channel formation region (hereinafter also referred to as an oxide semiconductor transistor in the following description) can be used. Consequently, leakage current of the cell can be reduced. 
     As an example of the transistors  491   a  and  491   b , a p-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, a p-channel Si transistor can be used. As an example of the transistor  490 , an n-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, an oxide semiconductor transistor can be used. As a result, the delay time of the cell can be shortened. 
     Note that “a transistor has a high switching speed” means that the time required to switch the transistor is short. For example, “a switching speed of a transistor” means time taken for the transistor to shift from a non-conduction state to a conduction state without load. The time can be regarded as time taken for an increase of a drain current of the transistor to compensate for an increase of charge accumulated in the gate capacitance in response to a potential applied to the gate. Alternatively, the time required to switch a transistor is expressed by 1/(2×f T ) in some cases, where f T  is the maximum frequency (cutoff frequency) at which current gain becomes 1 or more when the transistor is used as an amplifier. 
     A cell  501  illustrated in  FIG. 1B  includes a transistor  492 , a transistor  493   a , and a transistor  493   b . The signals A and B are input to the cell  501 , and the cell  501  outputs the signal Z. The power supply potentials V 1  and V 2  are supplied to the cell  501 . 
     The transistor  492  includes a first gate, a second gate, a source, and a drain. The first gate and the second gate are positioned so as to vertically sandwich the channel formation region. The channel formation region is covered with the first gate and the second gate when viewed from above. In this specification, the transistor  492  with such a structure is represented by a symbol surrounded by a dashed-dotted line  FIG. 1B . 
     A gate of the transistor  493   a  and the first gate of the transistor  492  are electrically connected to each other, and the signal A is input thereto. A gate of the transistor  493   b  and the second gate of the transistor  492  are electrically connected to each other, and the signal B is input thereto. The transistors  493   a  and  493   b  are connected in series. Thus, a drain of the transistor  493   b  and a source of the transistor  493   a  are electrically connected to each other. A source of the transistor  493   b  is supplied with the power supply potential V 2 . A drain of the transistor  493   a  outputs the signal Z. A source of the transistor  492  is supplied with the power supply potential V 1 . The drain of the transistor  492  outputs the signal Z. Though connection lines indicated by dotted lines in  FIG. 1B  are electrically connected to the power supply potentials V 1  and V 2  and an output terminal of the signal Z, another element such as a transistor may be provided therebetween. 
     The first gate and the second gate of the transistor  492  each function as an input terminal of a semiconductor device (cell). Therefore, a signal is input to the first gate and the second gate of the transistor  492  instead of a power source with a constant potential. For example, a signal output from another cell is input to the first gate and the second gate of the transistor  492 . For example, the first gate and the second gate of the transistor  492  are electrically connected to a signal wiring. 
     The transistor  492  is turned on or off depending on the potential of the potential of the first gate and the potential of the second gate. When a potential difference Vgs 1  between the first gate and the source exceeds a voltage Vth 1 , a channel is formed (or carriers are induced) in some cases in the channel formation region that overlaps with the first gate when viewed from above. When a potential difference Vgs 2  between the second gate and the source exceeds a voltage Vth 2 , a channel is formed (or carriers are induced) in the channel formation region that overlaps with the second gate when viewed from above. Thus, the transistor  492  has at least two threshold voltages, that is, Vth 1  and Vth 2 . The transistor  492  is on when Vgs 1 &gt;Vth 1  and Vgs 2 &gt;Vth 2  are satisfied. That is, it can be said that the transistor  492  has a function equivalent to that of a circuit in which two transistors, a transistor whose threshold voltage is Vth 1  and a transistor whose threshold voltage is Vth 2 , are connected in parallel. 
     Since the channel formation region of the transistor  492  is controlled by the first gate and the second gate, Vth 1  may depend on Vgs 2 , and Vth 2  may depend on Vgs 1 . For example, when gate capacitance per unit area between the first gate and the channel formation region is Cg 1  and gate capacitance per unit area between the second gate and the channel formation region is Cg 2 , the condition where the transistor  492  is turned on is represented by using Vth 0  as (Cg 1 ×Vgs 1 +Cg 2 ×Vgs 2 )/(Cg 1 +Cg 2 )&gt;Vth 0  in some cases. In that case, the threshold voltage Vth 1  regarding the first gate is represented as Vth 1  (Vgs 2 )=(1+Cg 2 /Cg 1 )×Vth 0 −Cg 2 /Cg 1 ×Vgs 2 , which indicates that the threshold voltage Vth 1  depends on Vgs 2 . The threshold voltage Vth 2  regarding the second gate is represented as Vth 2  (Vgs 1 )=(1+Cg 1 /Cg 2 )×Vth 0 −Cg 1 /Cg 2 ×Vgs 1 , which indicates that Vth 2  depends on Vgs 1 . In logic operation, the transistor  492  is turned on when (Vgs 1 , Vgs 2 )=(VSS, VDD), (VDD, VSS), or (VDD, VDD), and the transistor  492  is turned off when (Vgs 1 , Vgs 2 )=(VSS, VSS). 
     As the transistor  492 , an n-channel transistor can be used. As the transistors  493   a  and  493   b , p-channel transistors can be used. The power supply potential V 1  may be a low power supply potential VSS. The power supply potential V 2  may be a high power potential VDD. 
     Specifically, a complementary metal oxide semiconductor (CMOS) circuit can be formed by using a p-channel transistor and an n-channel transistor. With the CMOS circuit, the power consumption of an electronic circuit can be reduced. 
     The structure of a cell  503  illustrated in  FIG. 1D  is partly different from that of the circuit illustrated in  FIG. 1B . Specifically, a wiring indicated by a dotted line in  FIG. 1B  is indicated by a solid line in  FIG. 1D . That is, in  FIG. 1D , the source of the transistor  493   b  is supplied with a power supply potential V 2  without through a transistor. The drain of the transistor  493   a  outputs the signal Z without through a transistor. The source of the transistor  492  is supplied with the power supply potential V 1  without through a transistor. The drain of the transistor  492  outputs the signal Z without through a transistor. The cell  503  illustrated in  FIG. 1D  is a two-input NOR circuit. 
     In  FIGS. 1B and 1D , the transistor  493   b , the transistor  493   a , and the transistor  492  are arranged in this order in the cells  501  and  503 ; however, one embodiment of the present invention is not limited thereto. The transistor  493   a ,  493   b , and  492  may be arranged in this order in the cells  501  and  503 . That is, the gate of the transistor  493   b  and the first gate of the transistor  492  may be electrically connected to each other, and the gate of the transistor  493   a  and the second gate of the transistor  492  may be electrically connected to each other. 
     In  FIGS. 1B and 1D , the transistor  492  can be manufactured in a smaller area than the case where a transistor with a threshold voltage Vth 1  and a transistor with a threshold voltage is Vth 2  are connected in series as described later. As a result, the cell area can be reduced in some cases. 
     As an example of the transistor  492 , a transistor with a low drain current in an off state (also referred to as a leakage current) can be used. For example, the drain current in an off state is 1×10 −18  A or lower, preferably 1×10 −21  A or lower, further preferably 1×10 −24  A or lower at room temperature (approximately 25° C.), or 1×10 −15  A or lower, preferably 1×10 −18  A or lower, further preferably 1×10 −21  A or lower at 85° C. As an example, an oxide semiconductor transistor can be used. Consequently, leakage current of the cell can be reduced. 
     As an example of the transistors  493   a  and  493   b , a p-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, a p-channel Si transistor can be used. As an example of the transistor  492 , an n-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, an oxide semiconductor transistor can be used. As a result, the delay time of the cell can be shortened. 
     As the semiconductor device (cell) of one embodiment of the present invention, a k-input NAND circuit, a k-input AND circuit, a k-input NOR circuit, and a k-input OR circuit (k is an integer of 2 or more) are given. Alternatively, an XOR circuit, an XNOR circuit, an AND-NOR circuit, an OR-NAND circuit, an AND-OR-INV circuit, an OR-AND-INV circuit, a flip-flop, a settable flip-flop, a resettable flip-flop, a settable and resettable flip-flop, an adder circuit, a half adder circuit, a multiplexer, a demultiplexer, a register, a scan register, a retention register, an isolator, a decoder, and the like are given. 
     Examples of an electronic circuit including a semiconductor device (cell) include a CPU, a graphics processing unit (GPU), a digital signal processor (DSP), a microcontroller unit (MCU), a radio frequency identification (RF-ID), and a custom LSI. In these electronic circuits, a plurality of cells are arranged in a plurality of rows, and input/output terminals of the cells are connected to each other so as to function as the electronic circuits. 
     The semiconductor device (cell) may include a plurality of input terminals and one output terminal. 
     Next, an example of the structure of the semiconductor device (cell) of one embodiment of the present invention is described with reference to  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4 . 
     Note that in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4 , some components such as an insulator are omitted for easy understanding, and a conductor or the like formed in the same layer are shown with the same hatching pattern. 
       FIGS. 2A to 2C  are top views illustrating an example of the structure of the cell  502  illustrated in  FIG. 1C .  FIG. 2A  is a top view of a region including portions where the signals A, B, and Z are input or output.  FIG. 2B  is a top view of a region including the transistor  490 .  FIG. 2C  is a top view of a region including the transistors  491   a  and  491   b . In  FIG. 2B , the hatching pattern of a semiconductor  406   c  is omitted. 
       FIGS. 3 and 4  are cross-sectional views illustrating an example of the structure of the cell  502 . A cross section taken along dashed-dotted line A 1 -A 2  in  FIGS. 2A to 2C  is shown in  FIG. 3 . A cross section taken along dashed-dotted line B 1 -B 2  in  FIGS. 2A to 2C  is shown on the left side of  FIG. 4 . A cross section taken along dashed-dotted line C 1 -C 2  in  FIGS. 2A to 2C  is shown on the right side of  FIG. 4 . 
     The cell  502  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4  includes the transistor  490 , the transistor  491   a , and the transistor  491   b . These transistors are connected as necessary through a plurality of conductors, whereby the circuit illustrated in  FIG. 1C  is formed. Here, as an example, an oxide semiconductor transistor is used as the transistor  490 , and p-channel Si transistors are used as transistors  491   a  and  491   b.    
     Specifically, the cell  502  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4  includes a substrate  400 , the transistors  491   a  and  491   b , an insulator  460  over the transistors  491   a  and  491   b , an insulator  442  over the insulator  460 , the transistor  490  over the insulator  442 , an insulator  452  over the transistor  490 , an insulator  462  over the insulator  452 , conductors  470   a  to  470   e  over the insulator  462 , an insulator  464  over the insulator  462  and the conductors  470   a  to  470   e , and conductors  480   a  to  480   c  over the insulator  464 . One or a plurality of layers of insulators or conductors may be further provided over the insulator  464  and the conductors  480   a  to  480   c . Openings are provided in the insulators  460 ,  442 ,  432 ,  452 ,  462 , and  464  as necessary, and conductors are provided in the openings. 
     As illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4 , a layer including the transistor  490  and a layer including the transistors  491   a  and  491   b  are provided so as to overlap with each other in the cell  502 . In such a case, the cell  502  can be reduced in size. The transistor  490  and the transistor  491   a  or the transistor  491   b  may be arranged so as to overlap with each other. In such a case, the memory cell  502  can be reduced in size. 
     An expression “a transistor A and a transistor B overlap with each other” means that at least part of a gate, a drain, or a source of the transistor A overlaps with part of a gate, a drain, or a source of the transistor B. Moreover, the expression means that a region including the gate, the drain, and the source of the transistor A at least partly overlaps with a region including the gate, the drain, and the source of the transistor B. Furthermore, the expression also means that a region including any of components of the transistor A at least partly overlaps with a region including any of components of the transistor B. 
     The structure of the transistor  491   a  illustrated in  FIG. 3  is described. 
     The transistor  491   a  includes an insulator  412  over the substrate  400 ; a conductor  422   a  over the insulator  412 ; an insulator  418  in contact with a side surface of the conductor  422   a ; and regions  402   a ,  402   b , and  403  in the substrate  400 . The regions  402   a  and  402   b  do not overlap with the conductor  422   a  and the insulator  418 . The region  403  overlaps with the insulator  418 . 
     The insulator  412  serves as a gate insulator of the transistor  491   a . The conductor  422   a  serves as a gate of the transistor  491   a . The insulator  418  serves as a sidewall insulator (also referred to as a sidewall) of the conductor  422   a . The regions  402   a  and  402   b  serve as a source and a drain of the transistor  491   a . The region  403  serves as a lightly doped drain (LDD) region of the transistor  491   a.    
     The region  403  can be formed by adding an impurity using the conductor  422   a  as a mask. After that, the insulator  418  is formed and an impurity is added using the conductor  422   a  and the insulator  418  as masks, so that the regions  402   a  and  402   b  can be formed. Therefore, in the case where the regions  403 ,  402   a , and  402   b  are formed by adding the same kind of impurities, the region  403  has a lower impurity concentration than the regions  402   a  and  402   b.    
     When the transistor  491   a  includes the region  403 , a short-channel effect can be suppressed. Therefore, the structure of the transistor  491   a  is suitable for miniaturization. 
     The transistor  491   b  illustrated in  FIG. 2C  has a structure similar to that of the transistor  491   a . The transistor  491   b  includes a conductor  422   b  instead of the conductor  422   a . The transistor  491   b  includes regions  402   c  and  402   d  instead of the regions  402   a  and  402   b.    
     The transistors  491   a  and  491   b  is kept away from another transistor provided in the substrate  400  by an insulator  440  or the like. Although an example where the insulator  440  is formed by a shallow trench isolation (STI) method is shown, one embodiment of the present invention is not limited thereto. For example, instead of the insulator  440 , an insulator formed by a local oxidation of silicon (LOCOS) method may be used so that transistors are separated from each other. 
     The structure of the transistor  490  in  FIG. 3  will be described. 
     As illustrated in  FIG. 3 , the transistor  490  includes a conductor  421 , an insulator  432  over the conductor  421 , the semiconductor  406   a  over the insulator  432 , a semiconductor  406   b  over the semiconductor  406   a , the conductors  416   a  and  416   b  in contact with a top surface of the semiconductor  406   b , the semiconductor  406   c  in contact with a side surface of the semiconductor  406   a , top and side surfaces of the semiconductor  406   b , top and side surfaces of the conductor  416   a , and top and side surfaces of the conductor  416   b , the insulator  411  over the semiconductor  406   c , and the conductor  420  over the insulator  411 . 
     The conductor  421  serves as a second gate of the transistor  490 . The insulator  432  serves as a gate insulator of the transistor  490 . The conductor  416   a  and the conductor  416   b  serve as a source and a drain of the transistor  490 . The insulator  411  serves as a gate insulator of the transistor  490 . The conductor  420  serves as a first gate of the transistor  490 . The semiconductor  406   b  serves as a channel formation region. 
     The second gate of the transistor  490  may be expressed as the gate (the conductor  421 ). The first gate of the transistor  490  may be expressed as the gate (the conductor  420 ). 
     As illustrated in  FIG. 2B  and  FIG. 3 , in a region positioned between the conductor  416   a  and the conductor  416   b  when viewed from above, the semiconductor  406   b  includes a region A overlapping with the conductor  421  and not overlapping with the conductor  420 , and a region B not overlapping with the conductor  421  and overlapping with the conductor  420 . 
     That is, the channel formation region of the transistor  490  includes the region A overlapping with the gate (the conductor  421 ) and not overlapping with the gate (the conductor  420 ), and the region B not overlapping with the gate (the conductor  421 ) and overlapping with the gate (the conductor  420 ). 
     Moreover, as illustrated in  FIG. 2B  and  FIG. 3 , in the region positioned between the conductor  416   a  and the conductor  416   b  when viewed from above, the semiconductor  406   b  includes a region that overlaps with the conductor  421  and the conductor  420 . 
     That is, the channel formation region of the transistor  490  includes a region overlapping with the gate (the conductor  421 ) and the gate (the conductor  420 ). 
     In other words, in the transistor  490 , the conductor  421  and the conductor  420  overlap with each other when viewed from above. 
     In addition, the semiconductor  406   b  may include a region not overlapping with the conductors  420  and  421  in a region sandwiched between the conductor  416   a  and the conductor  416   b  when viewed from above. The region is preferably smaller than the region A. The region is preferably smaller than the region B. 
     That is, the channel formation region of the transistor  490  may include a region not overlapping with a gate (the conductor  421 ) and not overlapping with a gate (the conductor  420 ). 
     In other words, in the transistor  490 , the conductor  421  and the conductor  420  may be arranged with a space therebetween when viewed from above. The space is preferably smaller than the width of the conductor  421 . The space is preferably smaller than the width of the conductor  420 . 
     In the transistor  490 , an end portion of the conductor  421  and an end portion of the conductor  420  may be aligned when viewed from above. 
     With such a structure, the transistor  490  has a function similar to that of a circuit in which two transistors are connected in series, and the area of the transistor  490  can be smaller than that of the circuit in which two transistors having gates formed using conductors in one layer are connected in series. 
     Two gates adjacent to each other included in the transistor  490  each include a connection portion (not illustrated) for inputting a signal. These two connection portions may be arranged at both sides of the channel formation region when viewed from above. As a result, this structure is preferable because the connection portions of the transistor  490  can be arranged near the channel formation region, and the transistor  490  including the connection portions can be provided in a small region. 
     The insulators  432  and  411  that serve as gate insulators of the transistor  490  preferably have approximately the same thickness. In this case, a voltage applied to the conductor  421  and a voltage applied to the conductor  420  which are needed to turn on the transistor  490  can be approximately equal to each other. The thickness of the insulator  432  is 1/10 to 10 times, preferably ⅕ to 5 times, and further preferably ½ to 2 times of the thickness of the insulator  411 . 
     As illustrated in  FIG. 4 , the conductor  420  electrically surrounds the semiconductor  406   b  in the channel width direction, that is, surrounds not only the top surface but also the side surfaces of the semiconductor  406   b . Such a structure of a transistor is referred to as a surrounded channel (s-channel) structure. By employing the s-channel structure in the transistor, the subthreshold swing (also referred to as S value) of the transistor  490  can be decreased, whereby a short-channel effect of the transistor can be suppressed. Therefore, the s-channel structure is suitable for miniaturization. 
     Here, reduction in the area occupied by the transistor  490  is explained with reference to  FIGS. 5A to 5C . 
       FIG. 5A  is a top view illustrating an example of a configuration in which two transistors ( 490 _ 1 A and  490 _ 2 A) whose gates are formed using conductors in the same layer are connected in series. In  FIG. 5A , a distance DA between a source (a conductor s_ 1 ) of the transistor  490 _ 1 A and a drain (a conductor d_ 2 ) of the transistor  490 _ 2 A is an indicator of the size of the two transistors in the channel length direction. The distance DA is represented as the sum of a channel length L_ 1 A of the transistor  490 _ 1 A, a channel length L_ 2 A of the transistor  490 _ 2 A, and a distance  5 _ 12 A between a channel formation region of the transistor  490 _ 1 A and that of the transistor  490 _ 2 A. 
     The distance DA is estimated when the minimum space between the conductors that is determined by processing limitation in a fabrication process is F S , and the minimum width of the line of the conductors is F L . The channel length L_ 1 A is a distance between the source (the conductor s_ 1 ) and the drain (a conductor ds_ 12 ), and the minimum value is F S . The channel length L_ 2 A is a distance between a source (the conductor ds_ 12 ) and the drain (the conductor d_ 2 ), and the minimum value is F S . The distance S_ 12 A is the width of the conductor ds_ 12 , and the minimum value is F L . Therefore, the distance DA is more than or equal to 2F S +F L . 
       FIG. 5B  is a top view illustrating a structure example in which the conductors  421  and  420  overlap with each other in the transistor  490  when viewed from above. In  FIG. 5B , a distance between the source (the conductor  416   b ) of the transistor  490  and the drain (the conductor  416   a ) of the transistor  490  is an indicator of the size of the transistor  490  in the channel length direction. The distance is represented as the sum of a distance L_ 1 B between the source (the conductor  416   b ) of the transistor  490  and the gate (the conductor  420 ) of the transistor  490 , a distance L_ 2 B between the drain (the conductor  416   a ) and the gate (the conductor  421 ) of the transistor  490 , and a width S_ 12 B of a region where the conductor  421  and the conductor  420  overlap with each other when viewed from above. The transistor  490  may have a function similar to that of a series connection of a transistor whose channel length is L_ 1 B and a transistor whose channel length is L_ 2 B. 
     The channel lengths L_ 1 B and L_ 2 B can be smaller than F S  and F L  because L_ 1 B and L_ 2 B are determined by the end portions of conductors in different layers. Similarly, the distance S_ 12 B between two channel formation regions can be smaller than F S  or F L  because the distance S_ 12 B is determined by the end portions of conductors in different layers. Therefore, the transistor  490  illustrated in  FIG. 5B  can be positioned in a narrower area than two transistors connected in series illustrated in  FIG. 5A . The channel lengths L_ 1 B and L_ 2 B are preferably small within a range that achieves favorable characteristics of the transistors. For example, the channel lengths L_ 1 B and L_ 2 B are preferably smaller than F S  and F L . For example, the channel lengths L_ 1 B and L_ 2 B are less than or equal to 100 nm, preferably less than or equal to 60 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm. Accordingly, the on-state current of the transistor  490  can be increased. The width S_ 12 B is preferably smaller than or equal to F S  or F L . For example, the width S_ 12 B is less than or equal to 100 nm, preferably less than or equal to 60 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm. Thus, the area occupied by the transistor  490  can be reduced. 
       FIG. 5C  is a top view illustrating a structure example in which the conductors and  421  and  420  do not overlap with each other in the transistor  490  when viewed from above. In  FIG. 5C , a distance between the source (the conductor  416   b ) of the transistor  490  and a drain (the conductor  416   a ) of the transistor  490  is an indicator of the size of two transistors in the channel length direction. The distance is represented as the sum of a distance L_ 1 C between the source (the conductor  416   b ) of the transistor  490  and the end portion of the gate (the conductor  421 ) of the transistor  490  which is farther from the conductor  416   b , the distance L_ 2 C between the drain (the conductor  416   a ) of the transistor  490  and the end portion of the gate (the conductor  420 ) of the transistor  490  which is farther from the conductor  416   a , and the distance S_ 12 C between the conductor  421  and the conductor  420 . The transistor  490  may have a function similar to that of a series connection of a transistor whose channel length is L_ 1 C and a transistor whose channel length is L_ 2 C. 
     The channel lengths L_ 1 C and L_ 2 C can be smaller than F S  and F L  because L_ 1 C and L_ 2 C are determined by the end portions of conductors in different layers. Similarly, the distance S_ 12 C between two channel formation regions can be smaller than F S  or F L  because the distance S_ 12 C is determined by the end portions of conductors in different layers. Therefore, the transistor  490  illustrated in  FIG. 5C  can be positioned in a narrower area than two transistors connected in series illustrated in  FIG. 5A . The channel lengths L_ 1 C and L_ 2 C are preferably small within a range that achieves favorable characteristics of the transistors. For example, the channel lengths L_ 1 C and L_ 2 C are preferably smaller than F S  and F L . For example, the channel lengths L_ 1 C and L_ 2 C are less than or equal to 100 nm, preferably less than or equal to 60 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm. Accordingly, the on-state current of the transistor  490  can be increased. The width S_ 12 C is preferably smaller than or equal to F S  or F L . For example, the width S_ 12 C is less than or equal to 100 nm, preferably less than or equal to 60 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm. Thus, the area occupied by the transistor  490  can be reduced. 
     The semiconductor  406   b  of the transistor  490  illustrated in  FIG. 5C  may have high resistance in a region between the conductor  421  and the conductor  420  when viewed from above. As a result, the on-state current of the transistor  490  is decreased in some cases. Therefore, the distance S_ 12 C between two channel formation regions is preferably small. For example, the distance S_ 12 C is less than or equal to the sum of the thickness of the insulator  432  serving as a gate insulator and the thickness of the insulator  411  serving as a gate insulator, preferably less than or equal to the thickness of the insulator  432 , or the thickness of the insulator  411 . As a result, when the transistor  490  is turned on, carriers are induced and the resistance is lowered in the region of the semiconductor  406   b  between the conductor  421  and the conductor  420  by an electric field at end portions (fringes) of the conductors  421  and  420  serving as gates. Accordingly, the on-state current of the transistor  490  can be increased as compared to the case where the distance S_ 12 C is larger. 
     Thus, the area occupied by the transistor  490  can be reduced. As a result, the area of a semiconductor device (cell) can be reduced. 
     Here, the characteristics of the oxide semiconductor transistor and those of the transistor with an s-channel structure are described. 
     When the transistor  490  has an s-channel structure, the channel formation region can be easily controlled by a gate electric field from the side surface side of the semiconductor  406   b . The structure where the conductor  420  reaches below the semiconductor  406   b  is preferable because higher controllability can be achieved. Consequently, the subthreshold swing (also referred to as S value) of the transistor  490  can be decreased, so that the current of the transistor  490  in an off state can be decreased. 
     Accordingly, favorable electrical characteristics can be obtained even when the transistor is miniaturized. For example, the channel length of the transistor  490  is preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm and the channel width of the transistor  490  is preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm. With miniaturization of the transistor, the area of the semiconductor device (cell) can be reduced. 
     When the transistor  490  has an s-channel structure, a channel might be formed in the entire semiconductor  406   b  (bulk). Therefore, as the semiconductor  406   b  has a larger thickness, a channel formation region becomes larger. For example, the semiconductor  406   b  has a region with a thickness of greater than or equal to 20 nm, preferably greater than or equal to 40 nm, more preferably greater than or equal to 60 nm, still more preferably greater than or equal to 100 nm. Note that the semiconductor  406   b  has a region with a thickness of, for example, less than or equal to 300 nm, preferably less than or equal to 200 nm, more preferably less than or equal to 150 nm because the productivity of the semiconductor device might be decreased. With this structure in the s-channel structure, a large amount of current can flow between the source and the drain of the transistor, so that a high current in an on state (on-state current) can be achieved. 
     As a result, the switching speed of the transistor can be increased in some cases. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. 
     When the transistor  490  is an accumulation-type transistor whose majority carriers are electrons, an electric field extending from the source and the drain to the channel formation region is easily shielded within a short distance; thus, carriers can be easily controlled with a gate electric field even when the channel is short. Accordingly, favorable electrical characteristics can be obtained even when the transistor is miniaturized. 
     Unlike in the case of using a semiconductor substrate as a channel formation region, when the transistor  490  is formed over an insulating surface, parasitic capacitance is not formed between the gate and the body or the semiconductor substrate and thus, carriers can be easily controlled with a gate electric field. Accordingly, favorable electrical characteristics can be obtained even when the transistor is miniaturized. 
     In the transistor  490  illustrated in  FIG. 3 , the conductors  416   a  and  416   b  are not in contact with the side surfaces of the semiconductor  406   b . Thus, an electric field applied from the conductor  420  functioning as a gate to the side surfaces of the semiconductor  406   b  is less likely to be blocked by the conductor  416   a  and the conductor  416   b . Moreover, the conductors  416   a  and  416   b  are not in contact with a top surface of the insulator  432 . Thus, excess oxygen (oxygen) released from the insulator  432  is not consumed to oxidize the conductors  416   a  and  416   b . Accordingly, excess oxygen (oxygen) released from the insulator  432  can be efficiently used to reduce oxygen vacancies in the semiconductor  406   b.    
     At least part (or all) of the conductor  416   a  (and/or the conductor  416   b ) is in contact with at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor layer, e.g., the semiconductor  406   b . The contact portion of the semiconductor  406   b , in which donor levels are formed by entry of hydrogen into oxygen vacancy sites in some cases, includes an n-type conductive region. A state in which hydrogen enters sites of oxygen vacancies are denoted by V O H in some cases. Because of current flow in the n-type conductive region, a high on-state current can be achieved. 
     Furthermore, it is effective to reduce the concentration of impurities in the semiconductor  406   b  to make the oxide semiconductor intrinsic or substantially intrinsic. The term “substantially intrinsic” refers to the state where an oxide semiconductor layer has a carrier density lower than 1×10 17 /cm 3 , preferably lower than 1×10 15 /cm 3 , further preferably lower than 1×10 13 /cm 3 . In the oxide semiconductor, hydrogen, nitrogen, carbon, silicon, and metal elements that are not main components are impurities. For example, hydrogen and nitrogen form donor levels to increase the carrier density. 
     A transistor including a substantially intrinsic oxide semiconductor has a low carrier density and thus rarely has negative threshold voltage. In addition, because of few carrier traps in the oxide semiconductor, the transistor including the oxide semiconductor has small variation in electrical characteristics and high reliability. Moreover, a transistor including the oxide semiconductor enables an extremely low off-state current. 
     For example, the drain current at the time when the transistor including the oxide semiconductor is off can be less than or equal to 1×10 −18  A, preferably less than or equal to 1×10 −21  A, further preferably less than or equal to 1×10 −24  A at room temperature (approximately 25° C.); or less than or equal to 1×10 −15  A, preferably less than or equal to 1×10 −18  A, further preferably less than or equal to 1×10 −21  A at 85° C. The off state of a transistor refers to a state where the gate voltage is lower than the threshold voltage in an n-channel transistor. 
     Note that the above-described three-layer structure is an example. For example, a two-layer structure without the semiconductor  406   a  or the semiconductor  406   c  may be employed. A four-layer structure in which any one of the semiconductors described as examples of the semiconductor  406   a , the semiconductor  406   b , and the semiconductor  406   c  is provided under or over the semiconductor  406   a  or under or over the semiconductor  406   c  may be employed. An n-layer structure (n is an integer of 5 or more) in which any one of the semiconductors described as examples of the semiconductor  406   a , the semiconductor  406   b , and the semiconductor  406   c  is provided at two or more of the following positions: over the semiconductor  406   a , under the semiconductor  406   a , over the semiconductor  406   c , and under the semiconductor  406   c.    
     Next, a connection and an arrangement of the transistors  490 ,  491   a  and  491   b , are described. 
     The conductor  470   e  is supplied with the high power supply potential VDD. The conductor  470   b  is supplied with the low power supply potential VSS. The conductor  470   e  is formed in the same layer as the conductor  470   b . The conductor  470   e  is connected to the source (the region  402   a ) of the transistor  491   a  and the source (the region  402   c ) of the transistor  491   b  with a conductor provided therebetween. The conductor  470   b  is connected to the source (the conductor  416   b ) of the transistor  490  with a conductor provided therebetween. The conductor  480   a  is an output portion that outputs the signal Z. The conductor  480   a  is connected to the drain (the conductor  416   a ) of the transistor  490  with a conductor provided therebetween. The conductor  480   b  is an input portion to which the signal A is input. The conductor  480   b  is connected to the gate (the conductor  420 ) of the transistor  490  with a conductor provided therebetween. The conductor  480   c  is an input portion to which the signal B is input. The conductor  480   c  is connected to the gate (the conductor  421 ) of the transistor  490  with a conductor provided therebetween. 
     The gate (the conductor  422   a ) of the transistor  491   a  and the gate (the conductor  420 ) of the transistor  490  are electrically connected to each other through a first connection portion. The gate (the conductor  422   b ) of the transistor  491   b  and the gate (the conductor  421 ) of the transistor  490  are electrically connected to each other through a second connection portion. The drain (the conductor  416   a ) of the transistor  490 , the drain (the region  402   b ) of the transistor  491   a , and the drain (the region  402   d ) of the transistor  491   b  are electrically connected to each other. 
     In  FIG. 4 , the first connection portion is conductors  475   a ,  476   a ,  470   c , and/or  476   b . The second connection portion is a conductor  475   b.    
     In this specification, a connection portion of a conductor A and a conductor B is a portion for connecting the conductor A and the conductor B. For example, in the case where the conductor A and the conductor B are directly connected, a region where the conductor A is in contact with the conductor B is a connection portion. For example, in the case where the conductor A and the conductor B are connected to each other with the conductor C and/or the conductor D provided therebetween, the conductor C and/or the conductor D is a connection portion. 
     The direction in which the gate of the transistor  491   a  extends and the direction in which the gate of the transistor  491   b  extends are substantially parallel to each other. 
     A channel formation region of the transistor  491   a  and a channel formation region of the transistor  491   b  are aligned in this direction. 
     A gate of a transistor is provided to cross a region including a source, a channel formation region, or a drain. A gate extends in this crossing direction. The direction in which a gate extends is referred to as a channel width direction. A direction where current flows in a transistor is referred to as a channel length direction. A channel length direction is substantially perpendicular to a channel width direction. In the top views illustrated in  FIGS. 2A to 2C , the channel width direction is the direction of dashed-dotted line B 1 -B 2 , and the channel length direction is the direction of dashed-dotted line A 1 -A 2 . 
     The channel width direction of the transistor  491   a  and the channel width direction of the transistor  491   b  are substantially parallel to each other. 
     The channel formation region of the transistor  491   a  and the channel formation region of the transistor  491   b  are aligned in the channel width direction. 
     As a result, the transistor  491   a  and the transistor  491   b  including their connection portions can be provided in smaller regions. 
     The channel formation region of the transistor  490  is provided between the first connection portion and the second connection portion when viewed from above. Accordingly, the first connection portion and the second connection portion can be provided close to the channel formation region of the transistor  490 , so that the transistor  490  including the connection portions can be provided in a smaller region. 
     The direction in which the gate of the transistor  491   a  extends, the direction in which the gate of the transistor  491   b  extends, the direction in which the gate (the conductor  420 ) of the transistor  490  extends, and the direction in which the gate (the conductor  421 ) of the transistor  490  extends are substantially parallel to each other. 
     The region A of the transistor  490 , the region B of the transistor  490 , the channel formation region of the transistor  491   a , and the channel formation region of the transistor  491   b  are provided between the first connection portion and the second connection portion when viewed from above. 
     As a result, the transistors  491   a ,  491   b , and  490  including their connection portions can be positioned in smaller regions. Accordingly, the area of the cell  502  can be further reduced. 
     When the cell  502  is provided as described above, the area of the cell  502  does not increase in some cases even if the channel width of the transistor  490  is larger than the channel widths of the transistors  491   a  and  491   b . This is because the transistor  491   a  and the transistor  491   b  are arranged in the channel width direction, and the transistor  490  is stacked thereover. For example, the channel width of the transistor  490  is 1 to 5 times, preferably 1.5 to 3 times as large as that of the channel widths of the transistor  491   a  and the transistor  491   b.    
     When the channel width of the transistor  490  is larger than the channel widths of the transistor  491   a  and the transistor  491   b , the delay time of the cell  502  can be shortened and the operational performance thereof can be increased. In particular, a large channel width of the transistor  490  is effective in increasing the operational performance of the cell  502  in the case where the on-state current per channel width of the transistor  490  is lower than the on-state current per channel width of the transistors  491   a  and  491   b , or in the case where the field-effect mobility of the transistor  490  is lower than that of the transistors  491   a  and  491   b.    
     Next, the cross-sectional structure of the cell  502  illustrated in  FIGS. 3 and 4  is described in detail. 
     The substrate  400  is a single-crystal silicon substrate. The substrate  400  may be a semiconductor substrate including a single-material semiconductor of silicon, germanium, or the like or a compound semiconductor of silicon carbide, silicon germanium, gallium arsenide, gallium nitride, indium phosphide, zinc oxide, gallium oxide, or the like, for example. For the semiconductor substrate, an amorphous semiconductor or a crystalline semiconductor may be used, and examples of the crystalline semiconductor include a single crystal semiconductor, a polycrystalline semiconductor, and a microcrystalline semiconductor. Alternatively, the substrate  400  may be a glass substrate. Further alternatively, the substrate  400  may be an element substrate in which a semiconductor element is formed on a semiconductor substrate or a glass substrate. 
     The insulator  432  is preferably an insulator containing excess oxygen. 
     The insulator containing excess oxygen means an insulator from which oxygen is released by heat treatment, for example. Silicon oxide containing excess oxygen means silicon oxide from which oxygen can be released by heat treatment or the like, for example. Therefore, the insulator  432  is an insulator in which oxygen can be moved. In other words, the insulator  432  may be an insulator having an oxygen-transmitting property. For example, the insulator  432  may be an insulator having a higher oxygen-transmitting property than the semiconductor positioned over the insulator  432 . 
     The insulator containing excess oxygen has a function of reducing oxygen vacancies in the semiconductor positioned over the insulator, in some cases. Such oxygen vacancies form DOS in the semiconductor and serve as hole traps or the like. In addition, hydrogen comes into the site of such an oxygen vacancy and forms an electron serving as a carrier. Thus, by a reduction in oxygen vacancies in the semiconductor, the transistor can have stable electrical characteristics. 
     The insulator  442  is provided between the transistors  491   a  and  491   b  and the transistor  490 . As the insulator  442 , an oxide containing aluminum, e.g., aluminum oxide, is used. The insulator  442  blocks oxygen and hydrogen, and aluminum oxide whose density is lower than 3.2 g/cm 3  is preferable because it has a particularly high capability of blocking hydrogen. Alternatively, aluminum oxide with low crystallinity is preferable because its function of blocking hydrogen is particularly high. 
     For example, in the case where the transistor  491   a  and the transistor  491   b  are silicon transistors, electrical characteristics of the transistor may be improved because dangling bonds of silicon can be reduced by supplying hydrogen from the outside. The supply of hydrogen is performed by, for example, providing an insulator containing hydrogen in the vicinity of the Si transistor and performing heat treatment to diffuse and supply the hydrogen to the Si transistor. 
     An insulator containing hydrogen may release hydrogen, the amount of which is larger than or equal to 1×10 18  atoms/cm 3 , larger than or equal to 1×10 19  atoms/cm 3 , or larger than or equal to 1×10 20  atoms/cm 3  in TDS analysis (converted into the number of hydrogen atoms) in the range of a surface temperature of 100° C. to 700° C. or 100° C. to 500° C. 
     Out of hydrogen diffused from the insulator containing hydrogen, a small amount of hydrogen reaches the transistor  490  because the insulator  442  has a function of blocking hydrogen. Hydrogen serves as a carrier trap or a carrier generation source in an oxide semiconductor and causes deterioration of electrical characteristics of the transistor  490  in some cases. Therefore, blocking hydrogen by the insulator  442  is important to improve performance and reliability of the semiconductor device. 
     On the other hand, for example, by supplying oxygen to the transistor  490  from the outside, oxygen vacancies in the oxide semiconductor can be reduced; thus, electrical characteristics of the transistor are improved in some cases. The supply of oxygen may be performed by heat treatment under an atmosphere containing oxygen, for example. Alternatively, for example, an insulator containing excess oxygen (oxygen) is provided in the vicinity of the transistor  490  and heat treatment is performed, so that the oxygen may be diffused and supplied to the transistor  490 . Here, an insulator containing excess oxygen is used as the insulator  432 . 
     The diffused oxygen might reach the Si transistor through layers; however, since the insulator  442  has a function of blocking oxygen, the amount of oxygen which reaches the Si transistor is small. The entry of oxygen into silicon might be a factor of decreasing the crystallinity of silicon or inhibiting carrier movement. Therefore, blocking oxygen by the insulator  442  is important to improve performance and reliability of the semiconductor device. 
     The insulator  452  is preferably provided over the transistor  490 . The insulator  452  has a function of blocking oxygen and hydrogen. For the insulator  452 , the description of the insulator  442  is referred to, for example. Furthermore, for example, the insulator  452  has the capability of blocking oxygen and hydrogen more effectively than the semiconductor  406   a  and/or the semiconductor  406   c.    
     When the semiconductor device includes the insulator  452 , outward diffusion of oxygen from the transistor  490  can be suppressed. Consequently, excess oxygen (oxygen) contained in the insulator  432  and the like can be effectively supplied to the transistor  490 . Since the insulator  452  blocks entry of impurities including hydrogen from layers above the insulator  452  or the outside of the semiconductor device, deterioration of electrical characteristics of the transistor  490  due to entry of impurities can be suppressed. 
     Although in the above description, the insulator  442  and/or the insulator  452  is described separately from the transistor  490  for convenience, the insulator  442  and/or the insulator  452  may be part of the transistor  490 . 
     Note that in this embodiment, in a channel formation region and the like of the transistor  490 , an oxide semiconductor can be used, for example; however, one embodiment of the present invention is not limited to this example. For example, depending on cases or conditions, a channel formation region, the vicinity of the channel formation region, a source region, a drain region, or the like of the transistor  490  may be formed using a material containing Si (silicon), Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), or the like. 
     Note that in this specification and the like, a transistor such as the transistor  490  can be formed using any of a variety of substrates, for example. The type of a substrate is not limited to a certain type. As the substrate, a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, or the like can be used, for example. As an example of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, or the like can be given. Examples of the flexible substrate, the attachment film, the base material film, or the like are as follows: plastic typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES); a synthetic resin such as acrylic; polypropylene; polyester; polyvinyl fluoride; polyvinyl chloride; polyamide; polyimide; aramid; epoxy; an inorganic vapor deposition film; and paper. Specifically, the use of semiconductor substrates, single crystal substrates, SOI substrates, or the like enables the manufacture of small-sized transistors with a small variation in characteristics, size, shape, or the like and with high current capability. A circuit using such transistors achieves lower power consumption of the circuit or higher integration of the circuit. 
     Alternatively, a flexible substrate may be used as the substrate, and the transistor may be provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate and the transistor. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the transistor can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, or an organic resin film of polyimide or the like formed over a substrate can be used, for example. 
     In other words, a transistor may be formed using one substrate, and then transferred to another substrate. Examples of a substrate to which a transistor is transferred include, in addition to the above substrate over which the transistor can be formed, a paper substrate, a cellophane substrate, an aramid film substrate, a polyimide film substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), and the like), a leather substrate, and a rubber substrate. When such a substrate is used, a transistor with excellent properties or a transistor with low power consumption can be formed, a device with high durability, high heat resistance can be provided, or reduction in weight or thickness can be achieved. 
     Since an n-channel Si transistor is unnecessary in the semiconductor device (cell) described above, a process for forming the n-channel Si transistor is not needed, so that the manufacturing cost can be reduced in some cases. 
     Next, a different example of the structure of the semiconductor device (cell) of one embodiment of the present invention is described with reference to  FIGS. 6A to 6C  and  FIG. 7 . 
     Note that in  FIGS. 6A to 6C  and  FIG. 7 , some components such as an insulator are omitted for easy understanding, and a conductor or the like formed in the same layer are shown with the same hatching pattern. 
       FIGS. 6A to 6C  are top views illustrating an example of the structure of the cell  502  illustrated in  FIG. 1C .  FIG. 6A  is a top view of a region including input-output portions of the signals A, B and Z.  FIG. 6B  is a top view of a region including the transistor  490 , and  FIG. 6C  is a top view of a region including the transistors  491   a  and  491   b . In  FIG. 6B , the hatching pattern of the semiconductor  406   c  is omitted. 
       FIG. 7  is a cross-sectional view illustrating an example of the structure of the cell  502 . A cross section taken along dashed-dotted line A 1 -A 2  in  FIGS. 6A to 6C  is shown in on the left side of  FIG. 7 , and a cross section taken along dashed-dotted line B 1 -B 2  in  FIGS. 6A to 6C  is shown on the right side of  FIG. 7 . 
     The cell  502  illustrated in  FIGS. 6A to 6C  and  FIG. 7  includes the transistor  490 , the transistor  491   a , and the transistor  491   b . The transistors are connected as necessary through a plurality of conductors, whereby a circuit illustrated in  FIG. 1C  is formed. Here, as an example, an oxide semiconductor transistor is used as a transistor  490 , and p-channel Si transistors are used as the transistors  491   a  and  491   b.    
     Specifically, the cell  502  illustrated in  FIGS. 6A to 6C  and  FIG. 7  includes the substrate  400 , the transistors  491   a  and  491   b , the insulator  460  over the transistors  491   a  and  491   b , conductors  471   a  to  471   d  over the insulator  460 , an insulator  461  over the insulator  460  and the conductors  471   a  to  471   d , the insulator  442  over the insulator  461 , the transistor  490  over the insulator  442 , the insulator  452  over the transistor  490 , the insulator  462  over the insulator  452 , the conductors  470   a  to  470   d  over the insulator  462 , the insulator  464  over the insulator  462  and the conductors  470   a  to  470   d , the insulator  464  over the insulator  462  and the conductors  470   a  to  470   d , and the conductors  480   a  to  480   c  over the insulator  464 . Another one or plurality of layers of insulators or conductors may be further provided over the insulator  464  and the conductors  480   a  to  480   c . Openings are provided in the insulators  460 ,  461 ,  442 ,  432 ,  452 ,  462 , and  464  as necessary, and conductors are provided in the openings. 
     As illustrated in  FIGS. 6A to 6C  and  FIG. 7 , the layer including the transistor  490  and the layer including the transistors  491   a  and  491   b  are provided so as to overlap with each other in the cell  502 . In such a case, the cell  502  can be reduced in size. The transistor  490  and the transistors  491   a  or  491   b  are arranged so as to overlap with each other. In such a case, the memory cell  502  can be reduced in size. 
     The transistors  491   a  and  491   b  included in the cell  502  illustrated in  FIGS. 6A to 6C  and  FIG. 7  have the same structures as the transistors  491   a  and  491   b  included in the cell  502  in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4 . Thus, the description of the transistors  491   a  and  491   b  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4  can be referred to as appropriate. 
     The transistor  490  illustrated in  FIGS. 6A to 6C  and  FIG. 7  has the same structure as the transistor  490  illustrated in  FIGS. 2A to 2C ,  FIG. 3  and  FIG. 4 . Thus, the description of the transistor  490  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4  can be referred to as appropriate. 
     A connection and arrangement of the transistors  490 ,  491   a  and  491   b , and arrangement thereof are described. 
     The conductor  471   b  is supplied with the high power supply potential VDD. The conductor  470   b  is supplied with the low power supply potential VSS. The conductor  471   b  is formed in a different layer as the conductor  470   b . The conductor  471   b  is connected to the source (the region  402   a ) of the transistor  491   a  and the source (the region  402   c ) of the transistor  491   b  with a conductor provided therebetween. The conductor  470   b  is connected to the source of the transistor  490  (the conductor  416   b ) with a conductor provided therebetween. The conductor  480   a  is an output portion that outputs the signal Z. The conductor  480   a  is connected to the drain of the transistor  490  (the conductor  416   a ) with a conductor provided therebetween. The conductor  480   b  is an input portion to which the signal A is input. The conductor  480   b  is connected to the gate of the transistor  490  (the conductor  420 ) with a conductor positioned therebetween. The conductor  480   c  is an input portion to which the signal B is input. The conductor  480   c  is connected to the gate of the transistor  490  (the conductor  421 ) with a conductor positioned therebetween. 
     In the cell  502  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4 , the conductors  470   e  and  470   b  formed using the same layer are supplied with the power supply potentials VDD and VSS. On the other hand, in the cell  502  illustrated in  FIGS. 6A to 6C  and  FIG. 7 , the conductors  471   b  and  470   b  formed using different layers are supplied with the power supply potentials VDD and VSS. The conductors  471   b  and  470   b  to which the power supply potentials VDD and VSS are supplied are provided to overlap with each other in the cell  502  illustrated in  FIGS. 6A to 6C  and  FIG. 7 , so that the cell area can be reduced as compared to the cell  502  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4 . On the other hand, the number of layers from which the conductors are formed in the cell  502  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4  is smaller than that in the cell  502  illustrated in  FIGS. 6A to 6C  and  FIG. 7 , so that the number of manufacturing steps can be reduced. 
     The gate (the conductor  422   a ) of the transistor  491   a  and the gate (the conductor  420 ) of the transistor  490  are electrically connected to each other through a first connection portion. The gate (the conductor  422   b ) of the transistor  491   b  and the gate (the conductor  421 ) of the transistor  490  are electrically connected to each other through a second connection portion. The drain (the conductor  416   a ) of the transistor  490 , the drain (the region  402   b ) of the transistor  491   a , and the drain (the region  402   d ) of the transistor  491   a  are electrically connected to each other. 
     The first connection portion is the conductors  475   a ,  471   c ,  476   a ,  470   c  and/or  476   b . The second connection portion is the conductors  475   b ,  471   d , and/or a conductor  477   a.    
     The direction in which the gate of the transistor  491   a  extends and the direction in which the gate of the transistor  491   b  extends are substantially parallel to each other. 
     The channel formation region of the transistor  491   a  and the channel formation region of the transistor  491   b  are aligned in this direction. 
     The channel width direction of the transistor  491   a  and the channel width direction of the transistor  491   b  are substantially parallel to each other. 
     The channel formation region of the transistor  491   a  and the channel formation region of the transistor  491   b  are aligned in the channel width direction. 
     As a result, the transistor  491   a  and the transistor  491   b  including their connection portions can be provided in smaller regions. 
     The channel formation region of the transistor  490  is provided between the first connection portion and the second connection portion when viewed from above. Accordingly, the first connection portion and the second connection portion can be provided close to the channel formation region of the transistor  490 , so that the transistor  490  including the connection portions can be provided in a smaller region. 
     The direction in which the gate of the transistor  491   a  extends, the direction in which the gate of the transistor  491   b  extends, the direction in which the gate (the conductor  420 ) of the transistor  490  extends, and the direction in which the gate (the conductor  421 ) of the transistor  490  extends are substantially parallel to each other. 
     The region A of the transistor  490 , the region B of the transistor  490 , the channel formation region of the transistor  491   a , and the channel formation region of the transistor  491   b  are provided between the first connection portion and the second connection portion when viewed from above. 
     As a result, the transistors  491   a ,  491   b , and  490  including their connection portions can be positioned in smaller regions. Accordingly, the area of the cell  502  can be further reduced. 
     When the cell  502  is provided as described above, the area of the cell  502  does not increase in some cases even if the channel width of the transistor  490  is larger than the channel widths of the transistors  491   a  and  491   b . This is because the transistor  491   a  and the transistor  491   b  are arranged in the channel width direction, and the transistor  490  is stacked thereover. For example, the channel width of the transistor  490  is 1 to 5 times, preferably 1.5 to 3 times as large as that of the channel widths of the transistor  491   a  and the transistor  491   b.    
     When the channel width of the transistor  490  is larger than the channel widths of the transistor  491   a  and the transistor  491   b , the delay time of the cell  502  can be shortened and the operational performance thereof can be increased. In particular, a large channel width of the transistor  490  is effective in increasing the operational performance of the cell  502  in the case where the on-state current per channel width of the transistor  490  is lower than the on-state current per channel width of the transistors  491   a  and  491   b , or in the case where the field-effect mobility of the transistor  490  is lower than that of the transistors  491   a  and  491   b.    
     Next, a different example of a semiconductor device (cell) of one embodiment of the present invention is described with reference to  FIGS. 8A to 8C  and  FIG. 9 . 
     Note that in  FIGS. 8A to 8C  and  FIG. 9 , some components such as an insulator are omitted for easy understanding, and a conductor or the like formed in the same layer are shown with the same hatching pattern. 
       FIGS. 8A to 8C  are top views illustrating an example of the structure of the cell  503  illustrated in  FIG. 1D .  FIG. 8A  is a top view of a region including input-output portions of the signals A, B and Z.  FIG. 8B  is a top view of a region including the transistor  492 .  FIG. 8C  is a top view of a region including the transistors  493   a  and  493   b . In  FIG. 8B , the hatching pattern of the semiconductor  406   c  is omitted. 
       FIG. 9  is a cross-sectional view illustrating an example of the structure of the cell  503 . A cross section taken along dashed-dotted line A 1 -A 2  in  FIGS. 8A to 8C  is shown in on the left side of  FIG. 9 , a cross section taken along dashed-dotted line B 1 -B 2  in  FIGS. 8A to 8C  is shown on the right side of  FIG. 9 . 
     The cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9  includes the transistor  492 , the transistor  493   a , and the transistor  493   b . The transistors are connected as necessary through a plurality of conductors, whereby the circuit illustrated in  FIG. 1D  is formed. Here, as an example, an oxide semiconductor transistor is used as the transistor  492 , and p-channel Si transistors are used as the transistors  493   a  and  493   b.    
     Specifically, the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9  includes the substrate  400 , the transistors  493   a  and  493   b , the insulator  460  over the transistors  493   a  and  493   b , the insulator  442  over the insulator  460 , the transistor  492  over the insulator  442 , the insulator  452  over the transistor  492 , the insulator  462  over the insulator  452 , the conductors  470   a  to  470   e  over the insulator  462 , the insulator  464  over the insulator  462  and the conductors  470   a  to  470   e , and the conductors  480   a  to  480   c  over the insulator  464 . One or a plurality of layers of insulators or conductors may be further provided over the insulator  464  and the conductors  480   a  to  480   c . Openings are provided in the insulators  460 ,  442 ,  432 ,  452 ,  462 , and  464  as necessary, and conductors are provided in the openings. 
     As illustrated in  FIGS. 8A to 8C  and  FIG. 9 , the layer including the transistor  492  and the layer including the transistors  493   a  and  493   b  are provided so as to overlap with each other. In such a case, the cell  503  can be reduced in size. The transistor  492  and the transistor  493   a  or the transistor  493   b  are arranged so as to overlap with each other. In such a case, the cell  503  can be reduced in size. 
     For the transistors  493   a  and  493   b  included in the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9 , the description of the transistors  491   a  and  491   b  included in the cell  502  illustrated in  FIGS. 2A to 2C ,  FIG. 3 , and  FIG. 4  can be referred to as appropriate. 
     The structure of the transistor  492  in  FIG. 9  will be described. 
     As illustrated in  FIG. 9 , the transistor  492  includes a conductor  421 , an insulator  432  over the conductor  421 , the semiconductor  406   a  over the insulator  432 , a semiconductor  406   b  over the semiconductor  406   a , the conductors  416   a  and  416   b  in contact with a top surface of the semiconductor  406   b , the semiconductor  406   c  in contact with a side surface of the semiconductor  406   a , top and side surfaces of the semiconductor  406   b , top and side surfaces of the conductor  416   a , and top and side surfaces of the conductor  416   b , the insulator  411  over the semiconductor  406   c , and the conductor  420  over the insulator  411 . 
     The conductor  421  serves as the second gate of the transistor  492 . The insulator  432  serves as a gate insulator of the transistor  492 . The conductor  416   a  and the conductor  416   b  serve as a source and a drain of the transistor  492 . The insulator  411  serves as a gate insulator of the transistor  492 . The conductor  420  serves as a first gate of the transistor  492 . The semiconductor  406   b  serves as a channel formation region. 
     As illustrated in  FIGS. 8A to 8C  and  FIG. 9 , in the transistor  492 , a region sandwiched between the conductors  416   a  and  416   b  (also referred to as a channel formation region) included in the semiconductor  406   b  overlaps with the conductor  421  and is covered with the conductor  420  when viewed from above. 
     The transistor  492  has a function similar to that of a circuit in which two transistors are connected in parallel. In addition, two gates (the conductor  421  and the conductor  420 ) overlap with each other in the channel formation region when viewed from above, so that the area of transistor  492  can be smaller than that of the circuit in which two transistors whose gates are formed using conductors in the same layer are connected in parallel. 
     Two gates adjacent to each other included in the transistor  492  each include a connection portion (not illustrated) for inputting a signal. The two connection portions may be arranged at both sides of the channel formation region when viewed from above. As a result, the transistor  492  is preferable because the connection portions of the transistor  492  can be positioned close to the channel formation region, and the transistor  492  including the connection portions can be positioned in a smaller area. 
     The insulators  432  and  411  that serve as gate insulators of the transistor  492  preferably have approximately the same thickness. In this case, a voltage applied to the conductor  421  and a voltage applied to the conductor  420  which are needed to turn on the transistor  492  can be approximately equal to each other. The thickness of the insulator  432  is 1/10 to 10 times, preferably ⅕ to 5 times, and further preferably ½ to 2 times of the thickness of the insulator  411 . 
     The conductor  420  electrically surrounds the semiconductor  406   b  in the channel width direction, that is, surrounds not only the top surface but also the side surfaces of the semiconductor  406   b  (s-channel structure). Consequently, the subthreshold swing of the transistor  492  can be smaller, whereby a short-channel effect of the transistor can be suppressed. Therefore, such a structure is suitable for miniaturization. For the characteristics of the oxide semiconductor transistor and the characteristics of the transistor with the s-channel structure, the description of the transistor  490  can be referred to as appropriate. 
     Next, a connection and an arrangement between transistors  492 ,  493   a  and  493   b  are described. 
     The conductor  470   e  is supplied with the high power supply potential VDD. The conductor  470   b  is supplied with the low power supply potential VSS. The conductor  470   e  is formed in the same layer as the conductor  470   b . The conductor  470   e  is connected to the source (the region  402   c ) of the transistor  493   b  with a conductor provided therebetween. The conductor  470   b  is connected to the source (the conductor  416   b ) of the transistor  492  with a conductor provided therebetween. The conductor  480   a  is an output portion that outputs the signal Z. The conductor  480   a  is connected to the drain (the conductor  416   a ) of the transistor  492  with a conductor provided therebetween. The conductor  480   b  is an input portion to which the signal A is input. The conductor  480   b  is connected to the gate (the conductor  420 ) of the transistor  492  with a conductor provided therebetween. The conductor  480   c  is an input portion to which the signal B is input. The conductor  480   c  is connected to the gate (the conductor  421 ) of the transistor  492  with a conductor provided therebetween. 
     The gate (the conductor  422   a ) of the transistor  493   a  and the gate (the conductor  420 ) of the transistor  492  are electrically connected to each other through a first connection portion. The gate (the conductor  422   b ) of the transistor  493   b  and the gate (the conductor  421 ) of the transistor  492  are electrically connected to each other through a second connection portion. The drain (the conductor  416   a ) of the transistor  492  and the drain (the region  402   a ) of the transistor  493   a  are electrically connected to each other. The source of the transistor  493   a  and the drain of the transistor  493   b  share the region  402   b , and they are electrically connected to each other. 
     The first connection portion is the conductors  475   a ,  476   a ,  470   c , and/or  476   b . The second connection portion is the conductor  475   b.    
     The direction in which the gate of the transistor  493   a  extends and the direction in which the gate of the transistor  493   b  extends are substantially parallel to each other. 
     A channel formation region of the transistor  493   a  and a channel formation region of the transistor  493   b  are aligned in the direction. 
     The channel width direction of the transistor  493   a  and the channel width direction of the transistor  493   b  are substantially parallel to each other. 
     The channel formation region of the transistor  493   a  and the channel formation region of the transistor  493   b  are aligned in the channel width direction. 
     As a result, the transistors  493   a  and  493   b  including their connection portions can be positioned in smaller regions. Note that current of the transistor  493   a  and the transistor  493   b  may flow in the opposite direction. 
     The channel formation region of the transistor  492  is provided between the first connection portion and the second connection portion when viewed from above. Accordingly, the first connection portion and the second connection portion can be provided close to the channel formation region of the transistor  492 , so that the transistor  492  including the connection portions can be provided in a smaller region. 
     The direction in which the gate of the transistor  493   a  extends, the direction in which the gate of the transistor  493   b  extends, the direction in which the gate (the conductor  420 ) of the transistor  492  extends, and the direction in which the gate (the conductor  421 ) of the transistor  492  extends are substantially parallel to each other. 
     The region A of the transistor  492 , the region B of the transistor  492 , the channel formation region of the transistor  493   a , and the channel formation region of the transistor  493   b  are provided between the first connection portion and the second connection portion when viewed from above. 
     As a result, the transistors  493   a ,  493   b , and  492  including their connection portions can be positioned in smaller regions. Accordingly, the area of the cell  503  can be further reduced. 
     When the cell  503  is provided as described above, the area of the cell  503  does not increase in some cases even if the channel width of the transistor  492  is larger than the channel widths of the transistors  493   a  and  493   b . This is because the transistor  493   a  and the transistor  493   b  are arranged in the channel width direction, and the transistor  492  is stacked thereover. For example, the channel width of the transistor  492  is 1 to 5 times, preferably 1.5 to 3 times as large as that of the channel widths of the transistor  493   a  and the transistor  493   b.    
     When the channel width of the transistor  492  is larger than the channel widths of the transistor  493   a  and the transistor  493   b , the delay time of the cell  503  can be shortened and the operational performance thereof can be increased. In particular, a large channel width of the transistor  492  is effective in increasing the operational performance of the cell  503  in the case where the on-state current per channel width of the transistor  492  is lower than the on-state current per channel width of the transistors  493   a  and  493   b , or in the case where the field-effect mobility of the transistor  492  is lower than that of the transistors  493   a  and  493   b.    
     Next, a different example of the structure of the semiconductor device (cell) of one embodiment of the present invention is described with reference to  FIGS. 10A to 10C  and  FIG. 11 . 
     Note that in  FIGS. 10A to 10C  and  FIG. 11 , some components such as an insulator are omitted for easy understanding, and a conductor or the like formed in the same layer are shown with the same hatching pattern. 
       FIGS. 10A to 10C  are top views illustrating an example of the structure of the cell  503  illustrated in  FIG. 1D .  FIG. 10A  is a top view of a region including portions where the signals A, B, and Z are input or output.  FIG. 10B  is a top view of a region including the transistor  492 .  FIG. 10C  is a top view of a region including the transistors  493   a  and  493   b . In  FIG. 10B , the hatching pattern of the semiconductor  406   c  is omitted. 
       FIG. 11  is a cross-sectional view illustrating an example of the structure of the cell  503 . A cross section taken along dashed-dotted line A 1 -A 2  in  FIGS. 10A to 10C  is shown in on the left side of  FIG. 11 , a cross section taken along dashed-dotted line B 1 -B 2  in  FIGS. 10A to 10C  is shown on the right side of  FIG. 11 . 
     The cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11  includes the transistor  492 , the transistor  493   a , and the transistor  493   b . The transistors are connected as necessary through a plurality of conductors, whereby the circuit illustrated in  FIG. 1D  is formed. Here, as an example, an oxide semiconductor transistor is used as the transistor  492 , and p-channel Si transistors are used as the transistors  493   a  and  493   b.    
     Specifically, the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11  includes the substrate  400 , the transistors  493   a  and  493   b , the insulator  460  over the transistors  493   a  and  493   b , conductors  471   a  to  471   d  over the insulator  460 , the insulator  461  over the insulator  460  and the conductors  471   a  to  471   d , the insulator  442  over the insulator  461 , the transistor  492  over the insulator  442 , the insulator  452  over the transistor  492 , the insulator  462  over the insulator  452 , the conductors  470   a  to  470   d  over the insulator  462 , the insulator  464  over the insulator  462  and the conductors  470   a  to  470   d , and the conductors  480   a  to  480   c  over the insulator  464 . One or a plurality of layers of insulators or conductors may be further provided over the insulator  464  and the conductors  480   a  to  480   c . Openings are provided in the insulators  460 ,  461 ,  442 ,  432 ,  452 ,  462 , and  464  as necessary, and conductors are provided in the openings. 
     As illustrated in  FIGS. 10A to 10C  and  FIG. 11 , the layer including the transistor  492  and the layer including the transistors  493   a  and  493   b  are provided so as to overlap with each other. In such a case, the cell  503  can be reduced in size. The transistor  492  and the transistor  493   a  or the transistor  493   b  are arranged so as to overlap with each other. In such a case, the cell  503  can be reduced in size. 
     The transistors  493   a  and  493   b  illustrated in  FIGS. 10A to 10C  and  FIG. 11  have the same structures as the transistors  493   a  and  493   b  in  FIGS. 8A to 8C  and  FIG. 9 . Thus, the description of the transistors  493   a  and  493   b  illustrated in  FIGS. 8A to 8C , and  FIG. 9  can be referred to as appropriate. 
     The transistor  492  illustrated in  FIGS. 10A to 10C  and  FIG. 11  has the same structure as the transistor  492  illustrated in  FIGS. 8A to 8C  and  FIG. 9 . Thus, the description of the transistor  492  illustrated in  FIGS. 8A to 8C  and  FIG. 9  can be referred to as appropriate. 
     Next, a connection and an arrangement between transistors  492 ,  493   a  and  493   b  are described. 
     The conductor  471   b  is supplied with the high power supply potential VDD. The conductor  470   b  is supplied with the low power supply potential VSS. The conductor  471   b  is formed in a different layer as the conductor  470   b . The conductor  471   b  is connected to the source (the region  402   c ) of the transistor  493   b  with a conductor provided therebetween. The conductor  470   b  is connected to the source of the transistor  492  (the conductor  416   b ) with a conductor provided therebetween. The conductor  480   a  is an output portion that outputs the signal Z. The conductor  480   a  is connected to the drain of the transistor  492  (the conductor  416   a ) with a conductor provided therebetween. The conductor  480   b  is an input portion to which the signal A is input. The conductor  480   b  is connected to the gate of the transistor  492  (the conductor  420 ) with a conductor positioned therebetween. The conductor  480   c  is an input portion to which the signal B is input. The conductor  480   c  is connected to the gate (the conductor  421 ) of the transistor  492  with a conductor provided therebetween. 
     In the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9 , the conductors  470   e  and  470   b  formed using the same layer are supplied with the power supply potentials VDD and VSS. On the other hand, in the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11 , the conductors  471   b  and  470   b  formed using different layers are supplied with the power supply potentials VDD and VSS. The conductors  471   b  and  470   b  to which the power supply potentials VDD and VSS are supplied are provided to overlap with each other in the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11 , so that the cell area can be reduced as compared to the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9 . On the other hand, the number of layers from which the conductors are formed in the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9  is smaller than that in the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11 , so that the number of manufacturing steps can be reduced. 
     The gate (the conductor  422   a ) of the transistor  493   a  and the gate (the conductor  420 ) of the transistor  492  are electrically connected to each other through a first connection portion. The gate (the conductor  422   b ) of the transistor  493   b  and the gate (the conductor  421 ) of the transistor  492  are electrically connected to each other through a second connection portion. The drain (the conductor  416   a ) of the transistor  492  and the drain (the region  402   a ) of the transistor  493   a  are electrically connected to each other. The source of the transistor  493   a  and the drain of the transistor  493   b  share the region  402   b , and they are electrically connected to each other. 
     The first connection portion is the conductors  475   a ,  471   c ,  476   a ,  470   c  and/or  476   b . The second connection portion is the conductors  475   b ,  471   d , and/or a conductor  477   a.    
     The direction in which the gate of the transistor  493   a  extends and the direction in which the gate of the transistor  493   b  extends are substantially parallel to each other. 
     The channel formation region of the transistor  493   a  and the channel formation region of the transistor  493   b  are aligned perpendicular to the direction. 
     The channel width direction of the transistor  493   a  and the channel width direction of the transistor  493   b  are substantially parallel to each other. 
     The channel formation region of the transistor  493   a  and the channel formation region of the transistor  493   b  are aligned in the channel width direction. 
     As a result, the transistors  493   a  and  493   b  including their connection portions can be positioned in smaller regions. 
     The channel formation region of the transistor  492  is provided between the first connection portion and the second connection portion when viewed from above. Accordingly, the first connection portion and the second connection portion can be provided close to the channel formation region of the transistor  492 , so that the transistor  492  including the connection portions can be provided in a smaller region. 
     The direction in which the gate of the transistor  493   a  extends, the direction in which the gate of the transistor  493   b  extends, the direction in which the gate (the conductor  420 ) of the transistor  492  extends, and the direction in which the gate (the conductor  421 ) of the transistor  492  extends are substantially parallel to each other. 
     The region A of the transistor  492 , the region B of the transistor  492 , the channel formation region of the transistor  493   a , and the channel formation region of the transistor  493   b  are provided between the first connection portion and the second connection portion when viewed from above. 
     As a result, the transistors  493   a ,  493   b , and  492  including their connection portions can be positioned in smaller regions. Accordingly, the area of the cell  503  can be further reduced. 
     In the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9 , the gate of the transistor  493   a  and the gate of the transistor  493   b  are aligned in the channel width direction. The gates of the transistors  493   a  and  493   b  overlap with the first gate and the second gate of the transistor  492 , whereby the area of the cell  503  can be reduced. In the arrangement of the cell  503  illustrated in  FIGS. 8A to 8C  and  FIG. 9 , the area of the cell  503  is less affected even when the width of the transistor  492  is larger than the widths of the transistors  493   a  and  493   b . On the other hand, in the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11 , the gate of the transistor  493   a  and the gate of the transistor  493   b  are aligned in the channel length direction. In that case, the region  402   b  which is a connection portion of the transistor  493   a  and the transistor  493   b  can be made very small, whereby the area of the cell  503  can be reduced. In the arrangement of the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11 , the area of the cell  503  is less affected even when the channel length of the transistor  492  is increased. 
     As illustrated in  FIGS. 10A to 10C  and  FIG. 11 , the area of the cell  503  does not increase in some cases even if the channel length of the transistor  492  is larger than the channel lengths of the transistors  493   a  and  493   b . This is because the transistor  493   a  and the transistor  493   b  are arranged in the channel length direction, and the transistor  492  is stacked thereover. For example, the channel length of the transistor  492  is 1 to 5 times, preferably 1.5 to 3 times as large as that of the channel lengths of the transistor  493   a  and/or the transistor  493   b.    
     In general, as an object is formed in the later step or is formed in the upper side, the minimum feature size thereof is bigger. Therefore, the minimum feature line width of a layer of a first gate electrode and a second gate electrode of the transistor  492  and the minimum feature space width of a layer of the source electrode and the drain electrode of the transistor  492  are larger than the minimum feature line widths of the gate electrodes of the transistors  493   a  and  493   b  formed before the transistor  492  in some cases. In such a case, the channel length of the transistor  492  is larger than the channel lengths of the transistors  493   a  and  493   b . In such a case, the cell  503  illustrated in  FIGS. 10A to 10C  and  FIG. 11  has a preferable structure. 
     Next, a different example of the structure of the semiconductor device (cell) of one embodiment of the present invention is described with reference to  FIG. 12  and  FIG. 13 . 
       FIG. 12  is a cross-sectional view illustrating an example of the structure of the cell  502 . A cross section taken along dashed-dotted line A 1 -A 2  in  FIGS. 6A to 6C  is shown in on the left side of  FIG. 12 , and a cross section taken along dashed-dotted line B 1 -B 2  in  FIGS. 6A to 6C  is shown on the right side of  FIG. 12 . 
     Note that the structure of the transistors  491   a  and  491   b  is not limited to the structures illustrated in  FIG. 7  and the like. For example, a structure where the semiconductor substrate  400  has a projection (also referred to as a protrusion or a fin), like the transistors  491   a  and  491   b  illustrated in  FIG. 12 , may be used. In the structure of the transistors  491   a  and  491   b  illustrated in  FIG. 12 , an effective channel width with respect to the occupation area can be increased as compared with that illustrated in  FIG. 7  and the like. Thus, the on-state currents of the transistors  491   a  and  491   b  can be increased. In addition, the conductors  422   a  and  422   b  surround the projection of the substrate  400  in the channel width direction, whereby the channel formation region can be easily controlled by a gate electric field. As a result, a short-channel effect of the transistor can be suppressed. Therefore, the structure of the transistors  491   a  and  491   b  is suitable of miniaturization. 
       FIG. 13  is a cross-sectional view illustrating an example of the structure of the cell  502 . A cross section taken along dashed-dotted line A 1 -A 2  in  FIGS. 6A to 6C  is shown in on the left side of  FIG. 13 , and a cross section taken along dashed-dotted line B 1 -B 2  in  FIGS. 6A to 6C  is shown on the right side of  FIG. 13 . 
     Note that the structures of the transistors  491   a  and  491   b  is not limited to the structures illustrated in  FIG. 7  and the like. For example, as in the transistor  491   a  illustrated in  FIG. 13 , the insulator  440  may be provided over the substrate  400 . With the structure of the transistor  491   a  illustrated in  FIG. 13 , the transistors can be separated from each other more surely and thus, leakage current can be suppressed. In addition, parasitic capacitance generated between the substrate and the transistor and leakage current into the substrate can be reduced. As a result, leakage current of the transistors  491   a  and  491   b  can be reduced. In addition, the transistors  491   a  and  491   b  can operate at high speed and with low power consumption. 
     As described above, the structures of the cross sections of the semiconductor devices (cells) of one embodiment of the present invention illustrated in  FIG. 7 ,  FIG. 9 ,  FIG. 11 ,  FIG. 12 , and  FIG. 13  are partly different from that of the cell  502  illustrated in  FIG. 3  and  FIG. 4 . Therefore, the description of the cross section of the cell  502  illustrated in  FIG. 3  and  FIG. 4  can be referred to for the substrate and the insulator as appropriate. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 2 
     The transistor  490  can have a variety of structures. In this embodiment, only the transistor  490  and the region in the vicinity thereof are illustrated in  FIGS. 14A and 14B ,  FIGS. 15A and 15B , and  FIGS. 27A and 27B  for easy understanding. 
       FIG. 14A  is a top view illustrating a structure example of the transistor  490 .  FIG. 14B  is an example of a cross-sectional view taken along dashed-dotted line E 1 -E 2  and dashed-dotted line E 3 -E 4  in  FIG. 14A . Note that some components such as an insulator are omitted in  FIG. 14A  for easy understanding. In  FIG. 14A , the hatching pattern of the semiconductor  406   c  is omitted. 
     Although in the transistor  490  illustrated in  FIG. 3 , the structure example where the conductor  416   a  and the conductor  416   b  which function as a source and a drain are in contact with only a top surface of the semiconductor  406   b  is shown, the structure of the transistor  490  is not limited thereto. For example, as illustrated in  FIGS. 14A and 14B , the conductor  416   a  and the conductor  416   b  may be in contact with the top surface and the side surfaces of the semiconductor  406   b , the top surface of the insulator  432 , and the like. 
     Like the transistor  490  illustrated in  FIG. 3 , the transistor having the structure illustrated in  FIGS. 14A and 14B  has the structure in which the conductor  420  electrically surrounds the semiconductor  406   b  in the channel width direction and the side surfaces as well as the top surface of the semiconductor  406   b  are surrounded. This is the s-channel structure. For the s-channel structure, the description of the above embodiments can be referred to. With the s-channel structure, even a miniaturized transistor can have excellent electrical characteristics such as a high on-state current, a low subthreshold swing, and a low off-state current. 
     In the transistor having the structure illustrated in  FIGS. 14A and 14B , the conductors  416   a  and  416   b  are in contact with side surfaces of the semiconductor  406   a  and the top and side surfaces of the semiconductor  406   b . In addition, the semiconductor  406   c  is in contact with the side surfaces of the semiconductor  406   a , the top surface and side surfaces of the semiconductor  406   b , the top surface and side surfaces of the conductor  416   a , and the top surface and side surfaces of the conductor  416   b.    
     The semiconductor  406   b  in contact with the conductors  416   a  and  416   b , in which donor levels are formed by entry of hydrogen into oxygen vacancy sites in some cases, includes an n-type conductive region. A state in which hydrogen enters sites of oxygen vacancies are denoted by V O H in some cases. Because of current flow in the n-type conductive region, high on-state current can be obtained. 
       FIG. 15A  is a top view illustrating a structure example of the transistor  490 .  FIG. 15B  illustrates an example of a cross-sectional view taken along dashed-dotted line G 1 -G 2  and dashed-dotted line G 3 -G 4  in  FIG. 15A . Note that some components such as an insulator are omitted in  FIG. 15A  for easy understanding. 
     The transistor  490  in  FIGS. 15A and 15B  includes the conductor  421  over the insulator  442 ; the insulator  432  having a projection over the insulator  442  and the conductor  421 ; the semiconductor  406   a  over the projection of the insulator  432 ; the semiconductor  406   b  over the semiconductor  406   a ; the semiconductor  406   c  over the semiconductor  406   b ; the conductor  416   a  and the conductor  416   b  which are in contact with the semiconductor  406   a , the semiconductor  406   b , and the semiconductor  406   c  and which are arranged to be separated from each other; the insulator  411  over the semiconductor  406   c  and the conductor  416   a ; the conductor  420  over the insulator  411 ; the insulator  452  over the conductor  416   a , the conductor  416   b , the insulator  411 , and the conductor  420 ; and the insulator  462  over the insulator  452 . 
     The insulator  411  is in contact with at least side surfaces of the semiconductor  406   b  in the cross section taken along line G 3 -G 4 . The conductor  420  faces a top surface and the side surfaces of the semiconductor  406   b  with at least the insulator  411  provided therebetween in the cross section taken along line G 3 -G 4 . The conductor  421  faces a bottom surface of the semiconductor  406   b  with the insulator  432  provided therebetween. The insulator  432  does not necessarily include a projection. The semiconductor  406   c  is not necessarily provided. The insulator  452  is not necessarily provided. The insulator  462  is not necessarily provided. 
     The structure of the transistor  490  illustrated in  FIGS. 15A and 15B  is partly different from that of the transistor  490  in  FIGS. 14A and 14B . Specifically, the structures of the semiconductors  406   a  to  406   c  of the transistor  490  illustrated in  FIGS. 14A and 14B  are different from the structures of the semiconductors  406   a  to  406   c  of the transistor  490  in  FIGS. 15A and 15B . Thus, for the transistor  490  in  FIGS. 15A and 15B , the description of the transistor  490  in  FIGS. 14A and 14B  can be referred to as appropriate. 
       FIG. 27A  is a top view illustrating a structure example of the transistor  490 .  FIG. 27B  is an example of a cross-sectional view taken along dashed-dotted line F 1 -F 2  and dashed-dotted line F 3 -F 4  in  FIG. 27A . Note that some components such as an insulator are omitted in  FIG. 27A  for easy understanding. In  FIG. 27A , the hatching pattern of the semiconductor  406   c  is omitted. 
     The structure of the transistor  490  illustrated in  FIGS. 27A and 27B  is partly different from that of the transistor  490  in  FIGS. 14A and 14B . Specifically, in the transistor  490  illustrated in  FIGS. 27A and 27B , the insulators  417   a  and  417   b  are provided over the conductors  416   a  and  416   b , respectively. Therefore, in the transistor  490  illustrated in  FIGS. 27A and 27B , capacitance between the gate (the conductor  420 ) and the source or the drain (the conductor  416   a ) is reduced as compared to the transistor  490  illustrated in  FIGS. 14A and 14B . As a result, the transistor  490  whose operation speed is improved can be obtained. 
     The other portions of the transistor  490  illustrated in  FIGS. 27A and 27B  are the same as those of the transistor  490  illustrated in  FIGS. 14A and 14B . Thus, for the transistor  490  in  FIGS. 27A and 27B , the description of the transistor  490  in  FIGS. 14A and 14B  can be referred to as appropriate. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 3 
     A structure of an oxide semiconductor which can be used as the semiconductor  406   a , the semiconductor  406   b , the semiconductor  406   c , or the like is described below. In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system. 
     Oxide semiconductors are classified roughly into a single-crystal oxide semiconductor and a non-single-crystal oxide semiconductor. The non-single-crystal oxide semiconductor includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, an amorphous oxide semiconductor, and the like. 
     First, a CAAC-OS is described. 
     The CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts. 
     With a transmission electron microscope (TEM), a combined analysis image (high-resolution TEM image) of a bright-field image and a diffraction pattern of the CAAC-OS is observed, and a plurality of crystal parts can be clearly observed. However, in the high-resolution TEM image, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     In the high-resolution cross-sectional TEM image of the CAAC-OS observed in a direction substantially parallel to the sample surface, metal atoms arranged in a layered manner are seen in the crystal parts. Each metal atom layer has a morphology reflecting a surface over which the CAAC-OS is formed (hereinafter, a surface over which the CAAC-OS is formed is referred to as a formation surface) or a top surface of the CAAC-OS, and is arranged parallel to the formation surface or the top surface of the CAAC-OS. 
     In the high-resolution plan-view TEM image of the CAAC-OS observed in a direction substantially perpendicular to the sample surface, metal atoms arranged in a triangular or hexagonal configuration are seen in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     For example, when a local Fourier transform image of the high-resolution cross-sectional TEM image of a CAAC-OS is observed, the angle of the c-axis continuously and gradually changes from 14.3°, 16.6° to 26.4° in a plurality of adjacent regions each having a diameter of 4 nm in some cases. In addition, in the other plurality of adjacent regions, the direction of the c-axis is different from that of the c-axis which changes continuously. In that case, it is suggested that the other plurality of adjacent regions have a different grain. For example, in the other plurality of adjacent regions, the angle of the c-axis continuously and gradually changes from −18.3°, −17.6°, −15.9° in some cases. 
     Note that in an electron diffraction pattern of the CAAC-OS, spots (luminescent spots) having alignment are shown. For example, when electron diffraction with an electron beam having a diameter of 1 nm or more and 30 nm or less (such electron diffraction is also referred to as nanobeam electron diffraction) is performed on the top surface of the CAAC-OS, spots are observed. For example, spots corresponding to the vertices of a hexagon are observed. This is a diffraction pattern indicating c-axis alignment. 
     From the results of the high-resolution cross-sectional TEM image and the high-resolution plan-view TEM image, alignment is found in the crystal parts in the CAAC-OS. 
     Most of the crystal parts included in the CAAC-OS each fit inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. Note that when a plurality of crystal parts included in the CAAC-OS are connected to each other, one large crystal region is formed in some cases. For example, a crystal region with an area of 2500 nm 2  or more, 5 μm 2  or more, or 1000 μm 2  or more is observed in some cases in the high-resolution plan-view TEM image. 
     A CAAC-OS is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. 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 the formation surface or the top surface of the CAAC-OS. 
     On the other hand, when the CAAC-OS is analyzed by an in-plane method in which an X-ray is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single crystal oxide semiconductor of InGaZnO 4 , six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. In the case of a CAAC-OS, a peak is not clearly observed even when φ scan is performed with 2θ fixed at around 56°. 
     According to the above results, in the CAAC-OS having c-axis alignment, while the directions of a-axes and b-axes are irregularly oriented between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the high-resolution cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where the shape of the CAAC-OS is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS. 
     Distribution of c-axis aligned crystal parts in the CAAC-OS is not necessarily uniform. For example, in the case where crystal growth leading to the crystal parts of the CAAC-OS occurs from the vicinity of the top surface of the, the proportion of the c-axis aligned crystal parts in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS, a region to which the impurity is added may be altered and the proportion of the c-axis aligned crystal parts in the CAAC-OS might vary depending on regions. 
     Note that when the CAAC-OS with an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak may also be observed when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak at 2θ of around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°. 
     The CAAC-OS is an oxide semiconductor having low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor, such as silicon, disturbs the atomic arrangement of the oxide semiconductor by depriving the oxide semiconductor of oxygen and causes a decrease in crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and causes a decrease in crystallinity when it is contained in the oxide semiconductor. Note that the impurity contained in the oxide semiconductor might serve as a carrier trap or a carrier generation source. 
     The CAAC-OS is an oxide semiconductor having a low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein. 
     The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus can have a low carrier density. Therefore, a transistor including the oxide semiconductor rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has a low density of defect states, and thus has few carrier traps. Accordingly, the transistor including the oxide semiconductor has little variation in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor takes a long time to be released and might behave like fixed electric charge. Thus, the transistor including the oxide semiconductor having high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases. 
     With the use of the CAAC-OS in a transistor, variation in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small. 
     Next, a microcrystalline oxide semiconductor is described. 
     A microcrystalline oxide semiconductor has a region where a crystal part is observed in a high resolution TEM image and a region where a crystal part is not clearly observed in a high resolution TEM image. In most cases, a crystal part in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor including nanocrystal is referred to as an nc-OS (nanocrystalline oxide semiconductor). In a high resolution TEM image of the nc-OS, a grain boundary cannot be found clearly in the nc-OS sometimes for example. 
     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 crystal parts in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than the diameter of a crystal part, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a crystal part (the electron diffraction is also referred to as selected-area electron diffraction). Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS obtained by using an electron beam having a probe diameter close to, or smaller than the diameter of a crystal part. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots is shown in a ring-like region in some cases. 
     The nc-OS is an oxide semiconductor that has high regularity as compared to an amorphous oxide semiconductor. Thus, the nc-OS has a lower density of defect states than an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS. Hence, the nc-OS has a higher density of defect states than the CAAC-OS. 
     Thus, the nc-OS may have a higher carrier density than the CAAC-OS. The oxide semiconductor having a high carrier density may have high electron mobility. Thus, a transistor including the nc-OS may have high field-effect mobility. The nc-OS has a higher defect state density than the CAAC-OS, and thus may have a lot of carrier traps. Consequently, a transistor including the nc-OS has larger changes in electrical characteristics and lower reliability than a transistor including the CAAC-OS. The nc-OS can be formed easily as compared to the CAAC-OS because nc-OS can be formed even when a relatively large amount of impurities are included; thus, depending on the purpose, the nc-OS can be favorably used in some cases. Thus, a semiconductor device including the transistor including the nc-OS can be manufactured with high productivity in some cases. 
     Next, an amorphous oxide semiconductor is described. 
     The amorphous oxide semiconductor has disordered atomic arrangement and no crystal part. For example, the amorphous oxide semiconductor does not have a specific state as in quartz. 
     In the high-resolution TEM image of the amorphous oxide semiconductor, crystal parts cannot be found. 
     When the amorphous oxide semiconductor is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak showing a crystal plane does not appear. A halo pattern is shown in an electron diffraction pattern of the amorphous oxide semiconductor. Furthermore, a halo pattern is shown but a spot is not shown in a nanobeam electron diffraction pattern of the amorphous oxide semiconductor. 
     Note that an oxide semiconductor may have a structure having physical properties between the nc-OS and the amorphous oxide semiconductor. The oxide semiconductor having such a structure is specifically referred to as an amorphous-like oxide semiconductor (amorphous-like OS). 
     In a high-resolution TEM image of the amorphous-like OS, a void may be seen. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed. In the amorphous-like OS, crystallization by a slight amount of electron beam used for TEM observation occurs and growth of the crystal part is found sometimes. In contrast, crystallization by a slight amount of electron beam used for TEM observation is less observed in the nc-OS having good quality. 
     Note that the crystal part size in the amorphous-like OS and the nc-OS can be measured using high-resolution TEM images. For example, an InGaZnO 4  crystal has a layered structure in which two Ga—Zn—O layers are included between In—O layers. A unit cell of the InGaZnO 4  crystal has a structure in which nine layers of three In—O layers and six Ga—Zn—O layers are layered in the c-axis direction. Thus, 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. Thus, each of the lattice fringes having a distance therebetween of from 0.28 nm to 0.30 nm is regarded as corresponding to the a-b plane of the InGaZnO 4  crystal, focusing on the lattice fringes in the high-resolution TEM image. The maximum length in the region in which the lattice fringes are observed is regarded as the size of the crystal parts of the amorphous-like OS and the nc-OS. Note that the crystal part whose size is 0.8 nm or larger is selectively evaluated. 
     In the case where a change in average size of crystal parts (20-40 points) in the amorphous-like OS and the nc-OS is measured using the high-resolution TEM images, the crystal part size in the amorphous-like OS increases with an increase of the total amount of electron irradiation. Specifically, the case where the crystal part of approximately 1.2 nm at the start of TEM observation grows to a size of approximately 2.6 nm at the total amount of electron irradiation of 4.2×10 8  e − /nm 2  was observed. In contrast, the crystal part size in the good-quality nc-OS shows a little change from the start of electron irradiation to the total amount of electron irradiation of 4.2×10 8  e − /nm 2  regardless of the amount of electron irradiation. 
     Furthermore, by linear approximation of the change in the crystal part size in the amorphous-like OS and the nc-OS and extrapolation to the total amount of electron irradiation of 0 e − /nm 2 , the average size of the crystal part is found to be a positive value. This means that the crystal parts exist in the amorphous-like OS and the nc-OS before TEM observation. 
     Note that an oxide semiconductor may be a stacked film including two or more of an amorphous oxide semiconductor, a microcrystalline oxide semiconductor, and a CAAC-OS, for example. 
     In the case where the oxide semiconductor has a plurality of structures, the structures can be analyzed using nanobeam electron diffraction in some cases. For example, there is a method for measuring a transmission electron diffraction pattern of a substance by the transmission electron diffraction measurement apparatus. 
     For example, changes in the structure of a substance may be observed by changing the irradiation position of the electrons that are a nanobeam on the substance (or by scanning). At this time, when the substance is a CAAC-OS, a diffraction pattern indicating c-axis alignment is observed. When the substance is an nc-OS, a plurality of spots are observed in a ring-like region in a diffraction pattern. 
     Even when the substance is a CAAC-OS film, a diffraction pattern similar to that of an nc-OS film or the like is partly observed in some cases. Therefore, whether or not a CAAC-OS is favorable can be determined by the proportion of a region where a diffraction pattern of a CAAC-OS is observed in a predetermined area (also referred to as proportion of CAAC). In the case of a high quality CAAC-OS, for example, the proportion of CAAC is higher than or equal to 50%, preferably higher than or equal to 80%, further preferably higher than or equal to 90%, still further preferably higher than or equal to 95%. Note that the proportion of a region where a diffraction pattern different from that of a CAAC-OS is observed in a certain area is referred to as the proportion of non-CAAC. 
     As an example, a sample including a CAAC-OS obtained just after deposition (represented as “as-sputtered”) and a sample including a CAAC-OS subjected to heat treatment at 450° C. in an atmosphere containing oxygen were made, and transmission electron diffraction patterns were obtained by scanning top surfaces of the samples. Here, the proportion of CAAC was obtained in such a manner that diffraction patterns were observed by scanning for 60 seconds at a rate of 5 nm/second and the obtained diffraction patterns were converted into still images every 0.5 seconds. Note that as an electron beam, a beam with a probe diameter of 1 nm was used. The above measurement was performed on six samples. The proportion of CAAC was calculated using the average value of the six samples. 
     The measurement result of the proportion of CAAC in each sample shows that the proportion of CAAC of the CAAC-OS obtained just after the deposition was 75.7% (the proportion of non-CAAC was 24.3%). The proportion of CAAC of the CAAC-OS subjected to the heat treatment at 450° C. was 85.3% (the proportion of non-CAAC was 14.7%). These results show that the proportion of CAAC obtained after the heat treatment at 450° C. is higher than that obtained just after the deposition. That is, heat treatment at a high temperature (e.g., higher than or equal to 400° C.) reduces the proportion of non-CAAC (increases the proportion of CAAC). Furthermore, the above results also indicate that even when the temperature of the heat treatment is lower than 500° C., the CAAC-OS film can have a high proportion of CAAC. 
     Here, most of diffraction patterns different from that of a CAAC-OS are diffraction patterns similar to that of an nc-OS. Furthermore, an amorphous oxide semiconductor was not able to be observed in the measurement region. Therefore, the above results suggest that the region having a structure similar to that of an nc-OS is rearranged by the heat treatment owing to the influence of the structure of the adjacent region, whereby the region becomes CAAC. 
     Comparison between high-resolution plan-view TEM images of the CAAC-OS obtained just after the deposition and the CAAC-OS subjected to the heat treatment at 450° C. shows that CAAC-OS subjected to the heat treatment at 450° C. has more uniform film quality. That is, the heat treatment at a high temperature improves the film quality of the CAAC-OS film. 
     With such a measurement method, the structure of an oxide semiconductor having a plurality of structures can be analyzed in some cases. 
     The above oxide semiconductor can be used as the semiconductor  406   a , the semiconductor  406   b , the semiconductor  406   c , or the like. 
     Next, the other components of a semiconductor which can be used as the semiconductor  406   a , the semiconductor  406   b , the semiconductor  406   c , or the like are described. 
     The oxide semiconductor that can be used for the semiconductor  406   b  is an oxide semiconductor containing indium, for example. The semiconductor  406   b  can have high carrier mobility (electron mobility) by containing indium, for example. The semiconductor  406   b  preferably contains an element M. The element M is preferably aluminum, gallium, yttrium, tin, or the like. Other elements which can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, yttrium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and the like. 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 a 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  406   b  preferably contains zinc. When the oxide semiconductor contains zinc, the oxide semiconductor is easily to be crystallized, for example. 
     For the semiconductor  406   b , an oxide with a wide energy gap may be used. For example, the energy gap of the semiconductor  406   b  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, more preferably greater than or equal to 3 eV and less than or equal to 3.5 eV. 
     Avalanche breakdown or the like is less likely to occur in some cases in the transistor including an oxide semiconductor than in a general transistor including silicon or the like, because, for example, an oxide semiconductor has a wide bandgap and thus electrons are less likely to be excited, and the effective mass of a hole is large. Therefore, it may be possible to inhibit hot-carrier degradation due to avalanche breakdown, for example. Accordingly, the drain withstand voltage can be increased, so that the transistor can be driven at a higher drain voltage. Thus, in some cases, a higher voltage, that is, more states can be held by a floating node, increasing storage density. 
     The semiconductor  406   a , the semiconductor  406   b , and the semiconductor  406   c  preferably include at least indium. In the case of using an In-M-Zn oxide as the semiconductor  406   a , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be less than 50 atomic % and greater than or equal to 50 atomic %, respectively, or further preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. In the case of using an In-M-Zn oxide as the semiconductor  406   b , when summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be greater than or equal to 25 atomic % and less than 75 atomic %, respectively, or further preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively. In the case of using an In-M-Zn oxide as the semiconductor  406   c , when summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be less than 50 atomic % and greater than or equal to 50 atomic %, respectively, or further preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. Note that the semiconductor  406   c  may be an oxide that is a type the same as that of the semiconductor  406   a.    
     As the semiconductor  406   b , an oxide having an electron affinity higher than those of the semiconductors  406   a  and  406   c  is used. For example, as the semiconductor  406   b , an oxide having an electron affinity higher than those of the semiconductors  406   a  and  406   c  by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, more preferably 0.15 eV or higher and 0.4 eV or lower is used. Note that the electron affinity refers to an energy gap between the vacuum level and the bottom of the conduction band. 
     An indium gallium oxide has a small electron affinity and a high oxygen-blocking property. Therefore, the semiconductor  406   c  preferably includes an indium gallium oxide. The gallium atomic ratio [In/(In+Ga)] is, for example, higher than or equal to 70%, preferably higher than or equal to 80%, more preferably higher than or equal to 90%. 
     At this time, when an electric field is applied to a gate, a channel is formed in the semiconductor  406   b  having the highest electron affinity in the semiconductors  406   a ,  406   b , and  406   c . Thus, the field effect mobility of the transistor can be increased. Here, the semiconductor  406   b  and the semiconductor  406   c  have the common constituent elements and thus interface scattering hardly occurs therebetween. 
     Here, in some cases, there is a mixed region of the semiconductor  406   a  and the semiconductor  406   b  between the semiconductor  406   a  and the semiconductor  406   b . Furthermore, in some cases, there is a mixed region of the semiconductor  406   b  and the semiconductor  406   c  between the semiconductor  406   b  and the semiconductor  406   c . The mixed region has a low density of interface states. For that reason, the stack of the semiconductor  406   a , the semiconductor  406   b , and the semiconductor  406   c  has a band structure where energy at each interface and in the vicinity of the interface is changed continuously (continuous junction). Note that  FIG. 26A  is a cross-sectional view in which the semiconductor  406   a , the semiconductor  406   b , and the semiconductor  406   c  are stacked in this order.  FIG. 26B  shows energy (Ec) of the bottom of the conduction band corresponding to dashed-dotted line P 1 -P 2  in  FIG. 26A  when the semiconductor  406   c  has a higher electron affinity than the semiconductor  406   a .  FIG. 26C  shows the case where the semiconductor  406   c  has a lower electron affinity than the semiconductor  406   a.    
     At this time, electrons move mainly in the semiconductor  406   b , not in the semiconductor  406   a  and the semiconductor  406   c . As described above, when the interface state density at the interface between the semiconductor  406   a  and the semiconductor  406   b  and the interface state density at the interface between the semiconductor  406   b  and the semiconductor  406   c  are decreased, electron movement in the semiconductor  406   b  is less likely to be inhibited and the on-state current of the transistor can be increased. 
     For example, the semiconductor  406   a  and the semiconductor  406   c  include one or more elements other than oxygen included in the semiconductor  406   b . Since the semiconductor  406   a  and the semiconductor  406   c  each include one or more elements other than oxygen included in the semiconductor  406   b , an interface state is less likely to be formed at the interface between the semiconductor  406   a  and the semiconductor  406   b  and the interface between the semiconductor  406   b  and the semiconductor  406   c.    
     Furthermore, the semiconductors  406   a ,  406   b , and  406   c  preferably have no or a small amount of spinel crystal structures. Moreover, the semiconductors  406   a ,  406   b , and  406   c  are preferably CAAC-OS. 
     For example, when a CAAC-OS having a plurality of c-axis aligned crystal parts is used as the semiconductor  406   a , the semiconductor  406   b  formed thereover can have a region with favorable c-axis alignment even in the vicinity of the interface with the semiconductor  406   a.    
     In addition, by an increase in the CAAC proportion of the CAAC-OS, defects can be reduced, for example. Furthermore, for example, an area having a spinel structure can be reduced. Moreover, for example, carrier scattering can be reduced. In addition, the CAAC-OS can be a film having a high blocking property against impurities. Accordingly, when the CAAC proportion of each of the semiconductors  406   a  and  406   c  is increased, a favorable interface with the semiconductor  406   b  where the channel is formed can be formed, so that carrier scattering can be low. For example, the CAAC proportion of the semiconductor  406   a  and/or the semiconductor  406   c  may be higher than or equal to 10%, preferably higher than or equal to 20%, further preferably higher than or equal to 50%, still further preferably higher than or equal to 70%. Furthermore, mixing of impurities to the semiconductor  406   b  can be prevented; as a result, the impurity concentration of the semiconductor  406   b  can be reduced. 
     The semiconductor  406   b  is preferably a semiconductor in which oxygen vacancies are reduced. 
     For example, in the case were the semiconductor  406   b  contains oxygen vacancies (also denoted by V O ), donor levels are formed by entry of hydrogen into oxygen vacancy sites in some cases. A state in which hydrogen enters oxygen vacancy sites is denoted by V O H in the following description in some cases. V O H is a factor of decreasing the on-state current of the transistor because V O H scatters electrons. Note that oxygen vacancy sites become more stable by entry of oxygen than by entry of hydrogen. Thus, by decreasing oxygen vacancies in the semiconductor  406   b , the on-state current of the transistor can be increased in some cases. 
     To decrease oxygen vacancies in the semiconductor  406   b , for example, there is a method in which excess oxygen in the insulator  432  is moved to the semiconductor  406   b  through the semiconductor  406   a . In this case, the semiconductor  406   a  is preferably a layer having an oxygen-transmitting property (a layer through which oxygen passes or is transmitted). 
     Oxygen is released from the insulator  432  and taken into the semiconductor  406   a  by heat treatment or the like. In some cases, oxygen exists and is apart from atomics in the semiconductor  406   a , or exists and is bonded to oxygen or the like. As the density becomes lower, i.e., the number of spaces between the atoms becomes larger, the semiconductor  406   a  has a higher oxygen-transmitting property. For example, in the case where the semiconductor  406   a  has a layered crystal structure and oxygen movement in which oxygen crosses the layer is less likely to occur, the semiconductor  406   a  is preferably a layer having low crystallinity as appropriate. 
     The semiconductor  406   a  preferably has crystallinity such that excess oxygen (oxygen) is transmitted so that excess oxygen (oxygen) released from the insulator  432  reaches the semiconductor  406   b . For example, in the case where the semiconductor  406   a  is a CAAC-OS, a structure in which a space is partly provided in the layer is preferably employed because when the whole layer becomes CAAC, excess oxygen (oxygen) cannot be transmitted. For example, the proportion of CAAC of the semiconductor  406   a  is lower than 100%, preferably lower than 98%, further preferably lower than 95%, still further preferably lower than 90%. 
     Moreover, the thickness of the semiconductor  406   c  is preferably as small as possible to increase the on-state current of the transistor. The thickness of the semiconductor  406   c  is less than 10 nm, preferably less than or equal to 5 nm, more preferably less than or equal to 3 nm, for example. Meanwhile, the semiconductor  406   c  has a function of blocking entry of elements other than oxygen (such as hydrogen and silicon) included in the adjacent insulator into the semiconductor  406   b  where a channel is formed. For this reason, it is preferable that the semiconductor  406   c  have a certain thickness. The thickness of the semiconductor  406   c  is greater than or equal to 0.3 nm, preferably greater than or equal to 1 nm, more preferably greater than or equal to 2 nm, for example. The semiconductor  406   c  preferably has an oxygen blocking property to suppress outward diffusion of oxygen released from the insulator  432  and the like. 
     To improve reliability, preferably, the thickness of the semiconductor  406   a  is large and the thickness of the semiconductor  406   c  is small. For example, the semiconductor  406   a  has a region with a thickness of, for example, greater than or equal to 10 nm, preferably greater than or equal to 20 nm, more preferably greater than or equal to 40 nm, still more preferably greater than or equal to 60 nm. When the thickness of the semiconductor  406   a  is made large, a distance from an interface between the adjacent insulator and the semiconductor  406   a  to the semiconductor  406   b  in which a channel is formed can be large. Since the productivity of the semiconductor device might be decreased, the semiconductor  406   a  has a region with a thickness of, for example, less than or equal to 200 nm, preferably less than or equal to 120 nm, more preferably less than or equal to 80 nm. 
     For example, a region in which the concentration of silicon which is measured by secondary ion mass spectrometry (SIMS) is lower than 1×10 19  atoms/cm 3 , preferably lower than 5×10 18  atoms/cm 3 , or further preferably lower than 2×10 18  atoms/cm 3  is provided between the semiconductor  406   b  and the semiconductor  406   a . A region with a silicon concentration of lower than 1×10 19  atoms/cm 3 , preferably lower than 5×10 18  atoms/cm 3 , more preferably lower than 2×10 18  atoms/cm 3  which is measured by SIMS is provided between the semiconductor  406   b  and the semiconductor  406   c.    
     It is preferable to reduce the concentration of hydrogen in the semiconductor  406   a  and the semiconductor  406   c  in order to reduce the concentration of hydrogen in the semiconductor  406   b . The semiconductor  406   a  and the semiconductor  406   c  each have a region in which the concentration of hydrogen measured by SIMS is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , more preferably lower than or equal to 1×10 19  atoms/cm 3 , still more preferably lower than or equal to 5×10 18  atoms/cm 3 . It is preferable to reduce the concentration of nitrogen in the semiconductor  406   a  and the semiconductor  406   c  in order to reduce the concentration of nitrogen in the semiconductor  406   b . The semiconductor  406   a  and the semiconductor  406   c  each have a region in which the concentration of nitrogen measured by SIMS is lower than 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , more preferably lower than or equal to 1×10 18  atoms/cm 3 , still more preferably lower than or equal to 5×10 17  atoms/cm 3 . 
     In the case where gallium oxide is used for the semiconductor  406   c , indium in the semiconductor  406   b  can be prevented from being diffused into the gate insulator; thus, the leakage current of the transistor can be reduced. 
     For example, when an In—Ga—Zn oxide film formed by a sputtering method is used as each of the semiconductor  406   a  and  406   c , the semiconductor  406   a  and  406   c  can be deposited with the use of an In—Ga—Zn oxide target having an atomic ratio of In:Ga:Zn=1:3:2. The deposition conditions can be as follows: an argon gas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm) are used as the deposition gas; the pressure is 0.4 Pa; the substrate temperature is 200° C.; and the DC power is 0.5 kW. 
     Furthermore, when the semiconductor  406   b  is a CAAC-OS, the semiconductor  406   b  is preferably deposited with the use of a polycrystalline target containing an In—Ga—Zn oxide (In:Ga:Zn=1:1:1 [atomic ratio]). The deposition conditions can be as follows: an argon gas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm) are used as the deposition gas; the pressure is 0.4 Pa; the substrate temperature is 300° C.; and the DC power is 0.5 kW. 
     The structures and the other elements of the oxide semiconductor that can be used for the semiconductors  406   a ,  406   b , and  406   c  and the like have been described so far. By using the above-described oxide semiconductor in the semiconductors  406   a ,  406   b , and  406   c  and the like, the transistor  490  can have favorable electrical characteristics. For example, excellent subthreshold characteristics and an extremely low off-state current can be achieved. Moreover, a high on-state current and favorable switching speed can be achieved. Furthermore, high withstand voltage can be achieved. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 4 
     An example of the configuration of the semiconductor device (cell) of one embodiment of the present invention will be described with reference to  FIGS. 16A and 16B . 
       FIG. 16A  shows a configuration example of the semiconductor device (cell). The cell  504  includes a transistor  494 , the transistor  491   a , the transistor  491   b , and a transistor  491   c . The signals A, B, and C are input to the cell  504 , and the cell  504  outputs the signal Z. The power supply potentials V 1  and V 2  are supplied to the cell  504 . 
     The transistor  494  includes a first gate, a second gate, a third gate, a source, and a drain. The first gate and the second gate are positioned so as to vertically sandwich the channel formation region. The second gate and the third gate are positioned so as to vertically sandwich the channel formation region. The channel formation region includes a region overlapping with the first gate and not overlapping with the second gate and the third gate, a region not overlapping with the first gate and the third gate and overlapping with the second gate, and a region overlapping with the third gate and not overlapping with the first gate and the third gate. In this specification, the transistor  494  with such a structure is represented by a symbol surrounded by dashed-dotted line  FIG. 16A . 
     A gate of the transistor  491   a  and the first gate of the transistor  494  are electrically connected to each other, and the signal A is input thereto. The gate of the transistor  491   b  and the second gate of the transistor  494  are electrically connected to each other, and the signal B is input thereto. A gate of the transistor  491   c  and the third gate of the transistor  494  are electrically connected to each other, and the signal C is input thereto. The transistors  491   a ,  491   b , and  491   c  are connected in parallel. That is, the source of the transistor  491   a , the source of the transistor  491   b , and the source of the transistor  491   c  are electrically connected. The drain of the transistor  491   a , the drain of the transistor  491   b , and a drain of the transistor  491   c  are electrically connected. The sources of the transistors  491   a ,  491   b , and  491   c  are supplied with the power supply potential V 2 . A source of the transistor  494  is supplied with the power supply potential V 1 . The drains of the transistors  491   a ,  491   b , and  491   c  and the drain of the transistor  494  are electrically connected to each other, and output the signal Z. 
     The transistor  494  is turned on or off depending on the potential of the first gate, the potential of the second gate, and the potential of the third gate. When a potential difference Vgs 1  between the first gate and the source exceeds a voltage Vth 1 , a channel is formed (or carriers are induced) in some cases in the channel formation region that overlaps with the first gate when viewed from above. When a potential difference Vgs 2  between the second gate and the source exceeds a voltage Vth 2 , a channel is formed (or carriers are induced) in some cases in the channel formation region that overlaps with the second gate when viewed from above. When a potential difference Vgs 3  between the third gate and the source exceeds a voltage Vth 3 , a channel is formed (or carriers are induced) in some cases in the channel formation region that overlaps with the third gate when viewed from above. Thus, the transistor  494  has at least three threshold voltages, that is, Vth 1 , Vth 2 , and Vth 3 . The transistor  494  is on when Vgs 1 &gt;Vth 1 , Vgs 2 &gt;Vth 2 , and Vgs 3 &gt;Vth 3  are satisfied. That is, it can be said that the transistor  494  has a function equivalent to that of a circuit in which three transistors, a transistor whose threshold voltage is Vth 1 , a transistor whose threshold voltage is Vth 2 , and a transistor whose threshold voltage is Vth 3  are connected in parallel. 
     The cell  504  illustrated in  FIG. 16A  is a three-input NAND circuit. 
     As the transistor  494 , an n-channel transistor can be used. As the transistors  491   a ,  491   b , and  491   c , p-channel transistors can be used. The power supply potential V 1  may be the low power supply potential VSS. The power supply potential V 2  may be the high power supply potential VDD. 
     In the transistor  494 , at least two adjacent gates preferably vertically sandwich the channel formation region. In particular, in the transistor  494 , the first gate may be formed over the channel formation region, the second gate may be formed under the channel formation region, and the third gate may be formed over the channel formation region. In the transistor  494 , the first gate may be formed under the channel formation region, the second gate may be formed over the channel formation region, and the third gate may be formed under the channel formation region. 
     With such a structure, the area of the transistor  494  can be smaller than that of the circuit in which three transistors whose gates are formed using conductors in the same layer are connected in series. This is explainable because the transistor  494  has a similar structure to that of the transistor  490  in that the two adjacent gates are provided so as to vertically sandwich the channel region of the transistor  494 . Therefore, the description of the transistors  490  illustrated in  FIGS. 5A to 5C  can be referred to as appropriate. 
     As an example of the transistor  494 , a transistor with a low drain current in an off state (also referred to as a leakage current) can be used. For example, the drain current in an off state is 1×10 −18  A or lower, preferably 1×10 −21  A or lower, further preferably 1×10 −24  A or lower at room temperature (approximately 25° C.), or 1×10 −15  A or lower, preferably 1×10 −18  A or lower, further preferably 1×10 −21  A or lower at 85° C. For example, a transistor including an oxide semiconductor (preferably an oxide including In, Ga, and Zn) in its channel formation region (hereinafter also referred to as an oxide semiconductor transistor in the following description) can be used. Consequently, leakage current of the cell can be reduced. 
     As the transistors  491   a ,  491   b , and  491   c , p-channel transistors having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, a p-channel Si transistor can be used. As an example of the transistor  494 , an n-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, an oxide semiconductor transistor can be used. As a result, the delay time of the cell can be shortened. 
     As illustrated in  FIG. 16A , the semiconductor device (cell) of one embodiment of the present invention includes a series of semiconductor regions or channel formation regions, and transistors having a plurality of gates which face the semiconductor regions or channel formation regions. Two gates, or three gates or more may be included. At least two adjacent gates in the plurality of gates are preferably provided so as to vertically sandwich the channel formation region. 
       FIG. 16B  illustrates another example of a structure of a semiconductor device (cell). The cell  505  includes the transistor  492   a , the transistor  492   b , the transistor  493   a , the transistor  493   b , the transistor  493   c , and the transistor  493   d . The signals A, B, C, and D are input to the cell  505 , and the cell  505  outputs the signal Z. The power supply potentials V 1  and V 2  are supplied to the cell  505 . 
     The transistors  492   a  and  492   b  have the same structure and function as the transistor  492  surrounded by a dotted line in  FIG. 1D . 
     The gate of the transistor  493   a  and the first gate of the transistor  492   a  are electrically connected to each other, and the signal A is input thereto. The gate of the transistor  493   b  and the second gate of the transistor  492   a  are electrically connected to each other, and the signal B is input thereto. A gate of the transistor  493   c  and a first gate of the transistor  492   b  are electrically connected to each other, and the signal C is input thereto. A gate of the transistor  493   d  and a second gate of the transistor  492   b  are electrically connected to each other, and the signal D is input thereto. The transistors  493   b ,  493   a ,  493   d , and  493   c  are connected in series. The source of the transistor  493   b  is supplied with the power supply potential V 2 . That is, the source of the transistor  493   a  and a drain of the transistor  493   b  are electrically connected to each other. That is, a source of the transistor  493   d  and the drain of the transistor  493   a  are electrically connected to each other. That is, a source of the transistor  493   c  and a drain of the transistor  493   d  are electrically connected to each other. A source of the transistor  492   a  and a source of the transistor  492   b  are supplied with the power supply potential V 1 . The drains of the transistors  492   a  and  492   b  and the drain of the transistor  493   c  are electrically connected to each other, and output the signal Z. 
     The cell  505  illustrated in  FIG. 16B  is a four-input NOR circuit. 
     As the transistors  492   a  and  492   b , n-channel transistors can be used. As the transistors  493   a ,  493   b ,  493   c , and  493   d , p-channel transistors can be used. The power supply potential V 1  may be the low power supply potential VSS. The power supply potential V 2  may be the high power supply potential VDD. 
       FIG. 16B  shows the case where the transistors  493   b ,  493   a ,  493   d , and  493   c  are connected in series in this order from the side to which the power supply potential V 2  is connected in the cell  505 , but one embodiment of the present invention is not limited thereto. The four transistors  493   b ,  493   a ,  493   d , and  493   c  may be connected in series in any order. 
     The transistors  492   a  and  492   b  illustrated in  FIG. 16B  have a function similar to that of a circuit in which two transistors are connected in parallel like the transistor  492  described in the above embodiment. In addition, two gates (the conductor  421  and the conductor  420 ) overlap with each other in the channel formation region when viewed from above, so that the areas of transistors  492   a  and  492   b  can be smaller than the area of the circuit in which the two transistors whose gates are formed using conductors in the same layer are connected in parallel. As a result, the cell area can be reduced in some cases. 
     As an example of the transistors  492   a  and  492   b , a transistor with a low drain current in an off state (also referred to as a leakage current) can be used. For example, the drain current in an off state is 1×10 −18  A or lower, preferably 1×10 −21  A or lower, further preferably 1×10 −24  A or lower at room temperature (approximately 25° C.), or 1×10 −15  A or lower, preferably 1×10 −18  A or lower, further preferably 1×10 −21  A or lower at 85° C. For example, an oxide semiconductor transistor can be used. Consequently, leakage current of the cell can be reduced. 
     As an example of the transistor  493   a ,  493   b ,  493   c , and  493   d , a p-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, a p-channel Si transistor can be used. As an example of the transistors  492   a  and  492   b , an n-channel transistor having a high switching speed can be used. For example, the time required to switch the transistor is shorter than 10 ns, preferably shorter than 1 ns, and more preferably shorter than 0.1 ns. For example, an oxide semiconductor transistor can be used. As a result, the delay time of the cell can be shortened. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 5 
     An example of the configuration of the semiconductor device (cell) of one embodiment of the present invention will be described with reference to  FIG. 17 . 
       FIG. 17  illustrates a configuration example of a semiconductor device. A semiconductor device  600  illustrated in  FIG. 17  is an example of a semiconductor device that can function as a memory device. The semiconductor device  600  includes a memory cell array  610 , a row decoder  621 , a word line driver circuit  622 , a bit line driver circuit  630 , an output circuit  640 , a control logic circuit  660 , and a power supply circuit  670 . 
     The bit line driver circuit  630  includes a column decoder  631 , a precharge circuit  632 , a read circuit  633 , and a write circuit  634 . The precharge circuit  632  has a function of precharging bit lines. The read circuit  633  has a function of detecting potentials of the bit lines and reading data from memory cells. The read signals are output outside the semiconductor device  600  as digital data signals RDATA, through the output circuit  640   
     To the semiconductor device  600 , a low power supply voltage (VSS), a high power supply voltage (VDD), and the like are supplied from the outside as power supply voltages. 
     Control signals (CE, WE, RE), an address signal ADDR, a data signal WDATA, and the like are input to the semiconductor device  600  from the outside. ADDR is input to the row decoder  621  and the column decoder  631 , and WDATA is input to the write circuit  634 . 
     A semiconductor device (cell) of one embodiment of the present invention can be applied to the low decoder  621 , a word line driver circuit  622 , a bit line driver circuit  630 , the control logic circuit  660 , and the like. The semiconductor device (cell) can be also applied to a logic circuit that can be formed using a standard cell. As a result, the semiconductor device  600  that can be miniaturized, the semiconductor device  600  that consumes less power, the semiconductor device  600  with improved operation speed, or the semiconductor device  600  which can be manufactured at low cost can be provided. 
     A memory cell which is used for the memory cell array  610  may be Dynamic RAM (DRAM) cell or Static RAM (SRAM) cell which is a volatile memory formed using a Si transistor. For example, a memory cell including an oxide semiconductor transistor may be used. Examples of such a memory cell are described with reference to  FIG. 18 ,  FIG. 19 ,  FIG. 20 , and  FIG. 21 . 
       FIG. 18  is a circuit diagram illustrating a configuration example of a memory cell. A memory cell MC 3  includes a transistor Mos 3  and a capacitor Cap 3 . A node FN 3  is a data holding portion, and a terminal of the capacitor Cap 3  is connected thereto. The transistor Mos 3  functions as a switch connecting the node FN 3  to a wiring BL, and a gate of the transistor Mos 3  is connected to a wiring WL. A signal OSG is input to the wiring WL. The amount of charge which is accumulated in the capacitor Cap 3  through the transistor Mos 3  is controlled, retained, and determined, whereby the memory cell MC 3  can function as a memory cell. 
       FIG. 19  is a circuit diagram showing a configuration example of a memory cell. A memory cell MC 4  includes a transistor Mos 4 , a transistor M 104 , and a capacitor Cap 4 . The transistor Mos 4  whose data holding portion is a node FN 4  functions as a switch connecting the node FN 4  to a wiring BL, and a gate of the transistor Mos 4  is connected to a wiring WL. A signal OSG is input to the wiring WL. The capacitor Cap 4  connects a wiring WLC to the node FN 4 . The transistor M 104  is a p-channel transistor, and a gate, a source, and a drain thereof are connected to the node FN 4 , a wiring SL, and the wiring BL, respectively. The amount of charge which is accumulated in the capacitor Cap 4  (it is sometimes including an amount of charge accumulated in a parasitic capacitance accompanied with the node FN 4 ) is controlled and retained through the transistor Mos 4 . In addition, the amount of charge is determined on the basis of the state of the transistor M 104 . Thus, the memory cell MC 4  functions as a memory cell. A memory cell which performs reading can be selected by the wiring WLC connected to the capacitor Cap 4 . The transistor M 104  may be an n-channel transistor. In accordance with the conductivity type of the transistor M 104 , a voltage applied to the wirings BL, SL, and WLC is determined. 
       FIG. 20  is a circuit diagram showing a configuration example of a memory cell. A memory cell MC 5  includes a transistor Mos 5 , a transistor M 105 , a transistor M 106 , and a capacitor Cap 5 . The transistor Mos 5  whose data holding portion is a node FN 5  functions as a switch connecting the node FN 5  to a wiring BL, and a gate of the transistor Mos 5  is connected to a wiring WL. A signal OSG is input to the wiring WL. The capacitor Cap 5  connects the node FN 5  and VSS. The transistors M 105  and M 106  are connected in series, and the drain of the transistor M 105  is connected to the wiring BL, and the source of the transistor M 106  is connected to VSS. A gate of the transistor M 105  is connected to a wiring RWL, and a gate of the transistor M 106  is connected to the node FN 5 . The amount of charge which is accumulated in the capacitor Cap 5  (it is sometimes including an amount of charge accumulated in a parasitic capacitance accompanied with the node FN 5 ) is controlled and retained through the transistor Mos 5 . In addition, the amount of charge is determined on the basis of the state of the transistor M 106 . Thus, the memory cell MC 5  functions as a memory cell. A memory cell which performs reading can be selected by the transistor M 105 . The transistors M 105  and M 106  may be p-channel transistors. In accordance with the conductivity type of the transistors M 105  and M 106 , a voltage applied to the wirings BL, WL, RWL is determined. The capacitor Cap 5  is not necessarily provided. 
     A memory cell MC 1  illustrated in  FIG. 21  is an example of an SRAM cell in which backup operation can be performed. The memory cell MC 1  includes transistors M 101 , M 102 , Mos 1 , and Mos 2 , inverters INV 101  and INV 102 , and capacitors Cap 1  and Cap 2 . The memory cell MC 1  is connected to wirings WL, BL, BLB, and BRL. To the semiconductor device  100 , a low power supply voltage (VSS), and the like are supplied as the power supply voltage. 
     The inverters INV 101  and INV 102  constitute an inverter loop including the nodes NET 1  and NET 2 . The gates of the transistors M 101  and M 102  are connected to the wiring WL. The transistor M 101  functions as a switch for connecting the node NET 1  and a wiring BL, and the transistor M 102  functions as a switch for connecting the node NET 2  and a wiring BLB, respectively. This is a structure of an SRAM cell, which functions as a volatile memory cell. 
     In memory circuits for backup (Mos 1  and Cap 1 ) and (Mos 2  and Cap 2 ), nodes FN 1  and FN 2  are data holding portions. The node NET 1  is connected to (the Mos 1 , and the Cap 1 ), and the node NET 2  is connected to (the Mos 2  and the Cap 2 ). The transistor Mos 1  functions as a switch for connecting the Cap 1  and the NET 1 . The transistor Mos 2  functions as a switch for connecting the Cap 2  and the NET 2 . Gates of the transistors Mos 1  and Mos 2  are connected to a wiring BRL. A signal OSG is input to the wiring BRL. The amount of charge which is accumulated in the capacitors Cap 1  and Cap 2  through the transistors Mos 1  and Mos 2  is controlled, retained, and determined, whereby the memory cell MC 1  can function as a memory cell. The memory cell MC 1  can hold data for a long time without supply of power, so that the memory cell can be used for backup. 
     In the configuration examples of the memory cells illustrated in  FIGS. 18, 19, 20 and 21 , the transistors Mos 1 , Mos 2 , Mos 3 , Mos 4 , and Mos 5  are preferably oxide semiconductor transistors. As a result, since a leakage current (off-state current) flows between a source and a drain in an off-state is extremely low, fluctuation in the voltage of the nodes FN 1 , FN 2 , FN 3 , FN 4 , and FN 5  can be suppressed. Thus, the circuit made up of the Mos 1  and the Cap 1 , the circuit made up of the Mos 2  and the Cap 2 , a circuit made up of the Mos 3  and the Cap 3 , a circuit made up of the Mos 4  and the Cap 4 , a circuit made up of the Mos 5  and the Cap 5  can be operated as nonvolatile memory circuits or memory circuits capable of holding data for a long time without supply of power. All the n-channel transistors included in the memory cells may be oxide semiconductor transistors. As a result, a manufacturing process of an n-channel Si transistor can be omitted in some cases. In addition, the circuits other than the memory cell may be provided under the memory cell array. 
     The control logic circuit  660  illustrated in  FIG. 17  processes the signals (CE, WE, RE) input from the outside, and generates control signals and the like for the row decoder  621 , the column decoder  631 , and the power supply circuit  670 . 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  660  are not limited to those listed above, and other control signals may be input as necessary. 
     The read circuit  633  may include a sense amplifier in which the potential Vref and the bit line potential may be compared. Furthermore, the read circuit  633  may include a logic circuit for converting the read data into the output format. The write circuit  634  may include a logic circuit for converting the input data WDATA into the writing format. 
     VDD, VSS, or another power supply voltage is input to the power supply circuit  670 , and the power supply circuit  670  generates and outputs potentials necessary for reading operation and writing operation 
     Note that the decision whether the circuits and signals described above are used or not can be made as appropriate as needed. 
     In the case where the memory cell includes an oxide semiconductor transistor, the oxide semiconductor transistor can be formed at the same time as the oxide semiconductor transistor included in the semiconductor device (cell) of one embodiment of the present invention in some cases. The process cost is reduced as compared to the case where an element for a memory element different from the oxide semiconductor transistor is formed. 
     The circuits other than the memory cell array  610  may include an n-channel Si transistor and a p-channel Si transistor. In the case where the memory cell includes a region where an oxide semiconductor transistor is included and a Si transistor is not included, the circuits other than the memory cell array  610  may be stacked under the memory cell array  610 . This leads to the semiconductor device  600  with a reduced size. 
     All the n-channel transistors may be oxide semiconductor transistors. In such a case, the process can be simplified because the n-channel Si transistor is not needed, which enables increase in yield and reduction in the process cost. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 6 
     An example of the structure of a semiconductor device including the semiconductor device (cell) of one embodiment of the present invention will be described with reference to  FIG. 22 . 
     A semiconductor device  300  illustrated in  FIG. 22  includes a CPU core  301 , a power management unit  303 , and a peripheral circuit  302 . The power management unit  303  includes a power supply control unit  331  and a clock control unit  332 . The peripheral circuit  302  includes an instruction cache control unit  321 , an instruction cache  323 , a data cache control unit  322 , a data cache  324 , and a bus interface  325 . The CPU core  301  includes a control unit  311 , a fetch and decode unit  312 , an execution unit  314 , and a register file  313 . 
     The semiconductor device (cell) of one embodiment of the present invention can be applied to a logic circuit which can be formed using a standard cell. Therefore, the semiconductor device (cell) of one embodiment of the present invention can be applied to the power supply control unit  331 , the clock control unit  332 , the instruction cache control unit  321 , the instruction cache  323 , the data cache control unit  322 , the data cache  324 , the bus interface  325 , the control unit  311 , the fetch and decode unit  312 , the execution unit  314  and/or the register file  313 . As a result, the semiconductor device  300  that can be miniaturized, the semiconductor device  300  that consumes less power, the semiconductor device  300  with improved operation speed, or the semiconductor device  300  which can be manufactured at low cost can be provided. 
     The fetch and decode unit  312  has a function of acquiring an instruction from the main memory and the instruction cache  323  and decoding the instruction. 
     The control unit  311  has a function of controlling the timing of data transmission and reception between the fetch and decode unit  312 , the execution unit  314 , the register file  313 , and the outside of the CPU core  301  in accordance with fetched instructions or the like. 
     The execution unit  314  includes an arithmetic logic unit (ALU), a shifter, a multiplier, and the like. The execution unit  314  has a function of performing a variety of arithmetic operations such as four arithmetic operations and logic operations. 
     The register file  313  includes a plurality of registers including a general purpose register and stores data that is read from the main memory and the data cache  324 , the data output from the execution unit  314 , or the like. 
     The instruction cache  323  has a function of temporarily storing frequently used instruction. The data cache  324  has a function of temporarily storing frequently used data. The instruction cache control unit  321  controls the operation of the instruction cache  323 , and the like. The data cache control unit  322  controls the operation of the data cache  324 , and the like. 
     The bus interface  325  is connected to an external bus, and functions as a path for data between the semiconductor device  300  and devices outside the semiconductor device  300 . 
     The power management unit  303  performs control regarding electric power of the semiconductor device  300 . 
     A control signal from the CPU core  301 , an interrupt signal from the outside of the semiconductor device  300 , and the like are input to the power supply control unit  331 , and outputs power control signals. For example, the semiconductor device  300  has a plurality of power domains. In the case where the power switches are provided between the power source and each of the power domains, the power supply control unit  331  may have a function of controlling the operation of the power switches. For example, in the case where a voltage regulator is provided, the power supply control unit  331  may have a function of controlling the voltage regulator. The voltage regulator may be provided in the semiconductor device  300  or outside the semiconductor device  300 . Alternatively, only part of the voltage regulator (for example, an inductor coil) may be provided outside the semiconductor device  300 . 
     A control signal from the CPU core  301 , and a clock signal, an interrupt signal, and the like from the outside of the semiconductor device  300  are input to the clock control unit  332 , and an internal clock is output. For example, the semiconductor device  300  may perform a coarse-grained clock gating by which clock is controlled for each large block such as the CPU core  301  and the peripheral circuit  302 . Alternatively, the semiconductor device  300  may perform a fine-grained clock gating by which clock is controlled for each small block comprising fewer flip-flops. 
     The semiconductor device  300  having the above-described configuration can perform power gating. An example of the flow of the power gating operation will be described. 
     The power gating is a technique for stopping the supply of power supply voltage to one or more of various circuits included in the semiconductor device  300  in a period during which the semiconductor device  300  does not execute processing or the like. The power gating is a technique for reducing power consumption by reducing DC power that is consumed when the power supply voltage is supplied. In the power gating, necessary data in the semiconductor device  300  is saved when power is turned off. When the power is turned on, the saved data is restored and the instruction is executed again in the CPU core  301 . 
     First, the CPU core  301  sets the mode of power gating by setting the value in the register in the power supply control unit  331  in advance. The power gating is started, for example, by an instruction to the CPU core  301 . After the CPU core  301  decodes the instruction, the CPU core  301  transmits a control signal for turning off power to the power supply control unit  331 . Next, the power supply control unit  331  saves data stored in a register included in the semiconductor device  300 , the register file  313 , the instruction cache  323 , the data cache  324 , and the like or part of the data. Then, the power supply control unit  331  stops the supply of power supply voltage to one or more of various circuits included in the semiconductor device  300  by controlling the operation of power switches. On the other hand, an interrupt signal is input to the power management unit  303 , thereby starting the supply of the power supply voltage to the circuits. A counter may be provided in the power supply control unit  331  to be used to determine the timing of starting the supply of the power supply voltage regardless of input of an interrupt signal. Then, the semiconductor device  300  restores the saved data. After that, the instruction is executed again in the CPU core  301 . 
     A state retention register can be used as the register. As a result, data stored in the register can be saved in a state retention portion of the register without saving the data outside the semiconductor device  300 . The state retention portion may have a configuration to which the supply of power supply voltage is not stopped while power is off. The state retention portion may include a circuit comprising an oxide semiconductor transistor and a capacitor, and a memory circuit capable of holding data for a long time without supply of power may be used. With such a structure, power and time are not required as compared to the case where data stored in the register is saved in the memory other than the register. 
     The register file  313 , the instruction cache  323 , and/or the data cache  324  may include the memory cell including the oxide semiconductor transistor described in the above embodiments. For example, in the case were a SRAM cell in which backup operation can be performed is included, the register file  313 , the instruction cache  323 , and/or the data cache  324  can save the stored data in the memory circuit for backup. As another structure, the register file  313 , the instruction cache  323 , and/or the data cache  324  have a mode where they are supplied with a low power supply voltage at which data can be held (also referred to a low power supply voltage mode). At the time of power gating, the register file  313 , the instruction cache  323 , and/or the data cache  324  is shifted to the low power supply voltage mode instead of stopping the supply of power supply voltage to the register file  313 , the instruction cache  323 , and/or the data cache  324 , whereby the power consumption can be reduced. With such a structure, power and time can be reduced as compared to the case where data stored in the register file  313 , the instruction cache  323 , and/or the data cache  324  is saved outside the semiconductor device  300 . Alternatively, power and/or time can be reduced as compared to the case where the data stored in the instruction cache  323  and/or the data cache  324  is not saved at the time of power gating, and data and instruction are acquired from outside the semiconductor device  300  as needed after the power supply is turned on. 
     Note that the semiconductor device of one embodiment of the present invention can be used for not only a CPU but also a graphics processing unit (GPU), a programmable logic device (PLD), a digital signal processor (DSP), a microcontroller unit (MCU), a radio frequency identification (RFID) tag, a custom LSI, and the like. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 7 
     An example of the structure of a semiconductor device including the semiconductor device of one embodiment of the present invention will be described with reference to  FIG. 23 . 
     A semiconductor device  800  illustrated in  FIG. 23  is an example of the structure of an RFID tag. The RFID tag can be used for an individual authentication system in which an object or the like is recognized by reading the individual information, for example. The semiconductor device  800  includes an antenna  804 , a rectifier circuit  805 , a constant voltage circuit  806 , a demodulation circuit  807 , a modulation circuit  808 , a logic circuit  809 , a memory circuit  810 , and a ROM  811 . 
     The semiconductor device (cell) of one embodiment of the present invention can be applied to the logic circuit  809 , the memory circuit  810 , the ROM  811 , and the like. The semiconductor device (cell) can be also applied to a logic circuit that can be formed using a standard cell. As a result, the semiconductor device  600  that can be miniaturized, the semiconductor device  600  that consumes less power, the semiconductor device  600  with improved operation speed, or the semiconductor device  600  which can be manufactured at low cost can be provided. 
     The antenna  804  exchanges the radio signal  803  with the antenna  802  which is connected to the communication device  801 . The rectifier circuit  805  generates a stable potential by rectification, for example, half-wave voltage doubler rectification of an input alternating signal generated by reception of a radio signal at the antenna  804  and smoothing of the rectified signal with a capacitor provided in a later stage in the rectifier circuit  805 . Note that a limiter circuit may be provided on an input side or an output side of the rectifier circuit  805 . The limiter circuit controls electric power so that electric power which is higher than or equal to certain electric power is not input to a circuit in a later stage if the amplitude of the input alternating signal is high and an internal generation voltage is high. 
     The constant voltage circuit  806  generates a stable power supply voltage from a potential output from the rectifier circuit  805  and supplies the voltage to each circuit. Note that the constant voltage circuit  806  may include a reset signal generation circuit. The reset signal generation circuit is a circuit which generates a reset signal of the logic circuit  809  by utilizing rise of the stable power supply voltage. 
     The demodulation circuit  807  demodulates the input alternating signal by envelope detection and generates the demodulated signal. Further, the modulation circuit  808  performs modulation in accordance with data to be output from the antenna  804 . 
     The logic circuit  809  analyzes and processes the demodulated signal. The memory circuit  810  holds the input data and includes a row decoder, a column decoder, a memory cell, and the like. The ROM  811  is a circuit that stores an identification number (ID) or the like. 
     Note that as the data transmission method, there are an electromagnetic coupling method in which a pair of coils is provided so as to face each other and communicates with each other by mutual induction, an electromagnetic induction method in which communication is performed using an induction field, a radio wave method in which communication is performed using a radio wave, and the like. Any of these methods can be used in the semiconductor device  800  described in this embodiment. 
     Note that decision whether each circuit described above is provided or not can be made as appropriate as needed. 
     The transistors including an oxide semiconductor described in the above embodiment may be used as elements having a rectifying function included in the demodulation circuit  807 . Since the transistors have low off-state currents, the reverse currents of the elements having a rectifying function can be reduced, leading to excellent rectification efficiency. Furthermore, these oxide semiconductor transistors are formed at the same time as the oxide semiconductor transistor included in the semiconductor device (cell) of one embodiment of the present invention, whereby high performance of the semiconductor device  800  can be achieved without increase in the number of manufacturing steps. 
     All the n-channel transistors may be oxide semiconductor transistors. In such a case, the process can be simplified because the n-channel Si transistor is not needed, which enables increase in yield and reduction in the process cost. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 8 
     In this embodiment, application examples of the semiconductor device described in the above embodiments to an electronic component and to an electronic device including the electronic component will be described with reference to  FIGS. 24A and 24B , and  FIGS. 25A to 25F . 
       FIG. 24A  shows an example where the semiconductor device (cell) of one embodiment of the present invention is used to make an electronic component. Note that an electronic component is also referred to as semiconductor package or IC package. This electronic component has a plurality of standards and names depending on a terminal extraction direction and a terminal shape. Thus, examples of the electronic component are described in this embodiment. 
     The electronic component can be completed after an assembly process (post-process) by using a plurality of components that can be detached and attached from and to a printed wiring board in combination. An assembly process (post-process) is described with reference to  FIG. 24A . 
     After an element substrate obtained in the preceding process is completed (Step S 1 ), a back surface of the substrate is ground (Step S 2 ). By thinning the substrate at this stage, the warpage or the like of the substrate in the preceding process is reduced and the component is downsized. 
     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 a 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 wiringboard 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 ). 
     The above electronic component can include a semiconductor device (cell) according to one embodiment of the present invention. As a result, an electronic component that can be miniaturized, an electronic component which consumes less power, an electronic component with improved operation speed, or an electronic component with reduced cost can be provided. 
       FIG. 24B  is a perspective schematic diagram of the completed electronic component.  FIG. 24B  shows a perspective schematic diagram of a quad flat package (QFP) as an example of the electronic component. An electronic component  700  illustrated in  FIG. 24B  includes a lead  701  and a semiconductor device  703 . The electronic component  700  in  FIG. 24B  is, for example, mounted on a printed wiringboard  702 . The plurality of electronic components  700  are used in combination to be electrically connected to each other over the printed wiring board  702 ; thus, a circuit board on which the electronic components are mounted (a circuit board  704 ) is completed. The completed circuit board  704  is provided in an electronic device or the like. 
     The above-described electronic component can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices which 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 above-described electronic component are mobile phones, game machines including portable game machines, portable data terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type display devices (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), and vending machines.  FIGS. 25A to 25F  illustrate specific examples of these electronic devices. 
       FIG. 25A  illustrates a portable game machine including a housing  901 , a housing  902 , a display portion  903 , a display portion  904 , a microphone  905 , a speaker  906 , an operation key  907 , a stylus  908 , and the like. Although the portable game machine in  FIG. 25A  has the two display portions  903  and  904 , the number of display portions included in a portable game machine is not limited to this. 
       FIG. 25B  illustrates a portable data terminal including a first housing  911 , a second housing  912 , a first display portion  913 , a second display portion  914 , a joint  915 , an operation key  916 , and the like. The first display portion  913  is provided in the first housing  911 , and the second display portion  914  is provided in the second housing  912 . The first housing  911  and the second housing  912  are connected to each other with the joint  915 , and the angle between the first housing  911  and the second housing  912  can be changed with the joint  915 . An image on the first display portion  913  may be switched depending on the angle between the first housing  911  and the second housing  912  at the joint  915 . A display device with a position input function may be used as at least one of the first display portion  913  and the second display portion  914 . Note that the position input function can be added by provision of a touch panel in a display device. Alternatively, the position input function can be added by provision of a photoelectric conversion element called a photosensor in a pixel portion of a display device. 
       FIG. 25C  illustrates a laptop personal computer, which includes a housing  921 , a display portion  922 , a keyboard  923 , a pointing device  924 , and the like. 
       FIG. 25D  illustrates an example of a wrist-watch-type information terminal, which includes a housing  931 , a display portion  932 , a band  933 , a buckle  934 , operation buttons  935 , an input/output terminal  936 , and the like. The information terminal is capable of executing a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game. The display surface of the display portion  932  is bent, and images can be displayed on the bent display surface. The display portion  932  includes a touch sensor, and operation control can be performed by touching the screen with a finger, a stylus, or the like. The information terminal can employ near field communication, which is a communication method based on an existing communication standard. Moreover, the information terminal includes the input output terminal  936 , and data can be directly transmitted to and received from another information terminal via a connector. 
       FIG. 25E  illustrates a video camera, which includes a first housing  941 , a second housing  942 , a display portion  943 , operation keys  944 , a lens  945 , a joint  946 , and the like. The operation keys  944  and the lens  945  are provided for the first housing  941 , and the display portion  943  is provided for the second housing  942 . The first housing  941  and the second housing  942  are connected to each other with the joint  946 , and the angle between the first housing  941  and the second housing  942  can be changed with the joint  946 . Images displayed on the display portion  943  may be switched in accordance with the angle at the joint  946  between the first housing  941  and the second housing  942 . 
       FIG. 25F  illustrates an ordinary vehicle including a car body  951 , wheels  952 , a dashboard  953 , lights  954 , and the like 
     By applying an electronic component which enables miniaturization, low power consumption, or high speed operation including the semiconductor device (cell) of one embodiment the present invention to these electronic devices, a small-sized electronic device, an electronic device which consumes less power, an electronic device with improved operation speed, or an electronic device with reduced cost can be provided. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     In this specification, a “semiconductor” may have characteristics of an “insulator” in some cases when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” 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. 
     Further, a “semiconductor” includes characteristics of a “conductor” in some cases when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” 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. 
     In this specification, an impurity in a semiconductor refers to, for example, elements other than the main components of a semiconductor. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. When an impurity is contained, the density of states (DOS) may be formed in a semiconductor, the carrier mobility may be decreased, or the crystallinity may be decreased, for example. In the case where the semiconductor is an oxide semiconductor, examples of an impurity which changes 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; specifically, there are hydrogen (included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. In the case of an oxide semiconductor, oxygen vacancy may be formed by entry of impurities such as hydrogen. Further, in the case where the semiconductor is silicon, examples of an impurity which changes characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements. 
     In this specification, an insulator may be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing one or more of boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, and tantalum unless otherwise specified. A resin may be used as the insulator. For example, a resin containing polyimide, polyamide, acrylic, silicone, or the like may be used. The use of a resin does not need planarization treatment performed on a top surface of the insulator in some cases. By using a resin, a thick film can be formed in a short time; thus, the productivity can be increased. The insulator may be preferably formed to have a single-layer structure or a stacked-layer structure including an insulator containing aluminum oxide, silicon nitride oxide, silicon nitride, gallium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     In this specification, unless otherwise specified, a conductor may be formed to have, for example, a single-layer structure or a stacked-layer structure including a 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 film or a compound film of the above element may be used, for example, 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. 
     For example, in this specification and the like, when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected, the case where X and Y are functionally connected, and the case where X and Y are directly connected are included therein. Accordingly, another element may be interposed between elements having a connection relation shown in drawings and texts, without limiting to a predetermined connection relation, for example, the connection relation shown in the drawings and the texts. 
     Here, X and Y each denote an object (e.g., a device, an element, a circuit, a line, an electrode, a terminal, a conductive film, a layer, or the like). 
     For example, in the case where X and Y are electrically connected, one or more elements that enable electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, or a load) can be connected between X and Y. A switch is controlled to be on or off. That is, a switch is conducting or not conducting (is turned on or off) to determine whether current flows therethrough or not. Alternatively, the switch has a function of selecting and changing a current path. 
     For example, in the case where X and Y are functionally connected, one or more circuits that enable functional connection between X and Y (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit; a signal converter circuit such as a DA converter circuit, an AD converter circuit, or a gamma correction circuit; a potential level converter circuit such as a power supply circuit (e.g., a dc-dc converter, a step-up dc-dc converter, or a step-down dc-dc converter) or a level shifter circuit for changing the potential level of a signal; a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a memory circuit; and/or a control circuit) can be connected between X and Y. Note that for example, in the case where a signal output from X is transmitted to Y even when another circuit is interposed between X and Y, X and Y are functionally connected. 
     Note that when it is explicitly described that X and Y are electrically connected, the case where X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), the case where X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and the case where X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween) are included therein. That is, the explicit description “X and Y are electrically connected” is the same as the explicit description “X and Y are connected”. 
     For example, any of the following expressions can be used for the case where a source (or a first terminal or the like) of a transistor is electrically connected to X through (or not through) Z 1  and a drain (or a second terminal or the like) of the transistor is electrically connected to Y through (or not through) Z 2 , or the case where a source (or a first terminal or the like) of a transistor is directly connected to one part of Z 1  and another part of Z 1  is directly connected to X while a drain (or a second terminal or the like) of the transistor is directly connected to one part of Z 2  and another part of Z 2  is directly connected to Y. 
     Examples of the expressions include, “X, Y, a source (or a first terminal or the like) of a transistor, and a drain (or a second terminal or the like) of the transistor are electrically connected to each other, and X the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in this order”, “a source (or a first terminal or the like) of a transistor is electrically connected to X a drain (or a second terminal or the like) of the transistor is electrically connected to Y, and X the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in this order”, and “X is electrically connected to Y through a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor, and X the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are provided to be connected in this order”. When the connection order in a circuit structure is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope. Here, each of X, Y, Z 1 , and Z 2  denotes an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, a layer, or the like). 
     Note that a content (or may be part of the content) described in one embodiment may be applied to, combined with, or replaced by a different content (or may be part of the different content) described in the embodiment and/or a content (or may be part of the content) described in one or a plurality of different embodiments. 
     Note that in each embodiment, a content described in the embodiment is a content described with reference to a variety of diagrams or a content described with a text described in this specification. 
     Note that by combining a diagram (or may be part of the diagram) illustrated in one embodiment with another part of the diagram, a different diagram (or may be part of the different diagram) illustrated in the embodiment, and/or a diagram (or may be part of the diagram) illustrated in one or a plurality of different embodiments, much more diagrams can be formed. 
     Note that contents that are not specified in any drawing or text in the specification can be excluded from one embodiment of the invention. Alternatively, when the range of a value that is defined by the maximum and minimum values is described, part of the range is appropriately narrowed or part of the range is removed, whereby one embodiment of the invention excluding part of the range can be constructed. In this manner, it is possible to specify the technical scope of one embodiment of the present invention so that a conventional technology is excluded, for example. 
     As a specific example, a diagram of a circuit including a first transistor to a fifth transistor is illustrated. In that case, it can be specified that the circuit does not include a sixth transistor in the invention. It can be specified that the circuit does not include a capacitor in the invention. It can be specified that the circuit does not include a sixth transistor with a particular connection structure in the invention. It can be specified that the circuit does not include a capacitor with a particular connection structure in the invention. For example, it can be specified that a sixth transistor whose gate is connected to a gate of the third transistor is not included in the invention. For example, it can be specified that a capacitor whose first electrode is connected to the gate of the third transistor is not included in the invention. 
     As another specific example, a description of a value, “a voltage is preferably higher than or equal to 3 V and lower than or equal to 10 V” is given. In that case, for example, it can be specified that the case where the voltage is higher than or equal to −2 V and lower than or equal to 1 V is excluded from one embodiment of the invention. For example, it can be specified that the case where the voltage is higher than or equal to 13 V is excluded from one embodiment of the invention. Note that, for example, it can be specified that the voltage is higher than or equal to 5 V and lower than or equal to 8 V in the invention. For example, it can be specified that the voltage is approximately 9 V in the invention. For example, it can be specified that the voltage is higher than or equal to 3 V and lower than or equal to 10 V but is not 9 V in the invention. Note that even when the description “a value is preferably in a certain range” or “a value preferably satisfies a certain condition” is given, the value is not limited to the description. In other words, a description of a value that includes a term “preferable”, “preferably”, or the like does not necessarily limit the value. 
     As another specific example, a description “a voltage is preferred to be 10 V” is given. In that case, for example, it can be specified that the case where the voltage is higher than or equal to −2 V and lower than or equal to 1 V is excluded from one embodiment of the invention. For example, it can be specified that the case where the voltage is higher than or equal to 13 V is excluded from one embodiment of the invention. 
     As another specific example, a description “a film is an insulating film” is given to describe properties of a material. In that case, for example, it can be specified that the case where the insulating film is an organic insulating film is excluded from one embodiment of the invention. For example, it can be specified that the case where the insulating film is an inorganic insulating film is excluded from one embodiment of the invention. For example, it can be specified that the case where the insulating film is a conductive film is excluded from one embodiment of the invention. For example, it can be specified that the case where the insulating film is a semiconductor film is excluded from one embodiment of the invention. 
     As another specific example, the description of a stacked structure, “a film is provided between an A film and a B film” is given. In that case, for example, it can be specified that the case where the film is a stacked film of four or more layers is excluded from the invention. For example, it can be specified that the case where a conductive film is provided between the A film and the film is excluded from the invention. 
     Note that in this specification and the like, it might be possible for those skilled in the art to constitute one embodiment of the invention even when portions to which all the terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected are not specified. In other words, one embodiment of the invention can be clear even when connection portions are not specified. Further, in the case where a connection portion is disclosed in this specification and the like, it can be determined that one embodiment of the invention in which a connection portion is not specified is disclosed in this specification and the like, in some cases. In particular, in the case where the number of portions to which the terminal is connected might be plural, it is not necessary to specify the portions to which the terminal is connected. Therefore, it might be possible to constitute one embodiment of the invention by specifying only portions to which some of terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected. 
     Note that in this specification and the like, it might be possible for those skilled in the art to specify the invention when at least the connection portion of a circuit is specified. Alternatively, it might be possible for those skilled in the art to specify the invention when at least a function of a circuit is specified. In other words, when a function of a circuit is specified, one embodiment of the invention can be clear. Furthermore, it can be determined that one embodiment of the invention whose function is specified is disclosed in this specification and the like. Therefore, when a connection portion of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a function is not specified, and one embodiment of the invention can be constituted. Alternatively, when a function of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a connection portion is not specified, and one embodiment of the invention can be constituted. 
     Note that in this specification and the like, in a diagram or a text described in one embodiment, it is possible to take out part of the diagram or the text and constitute an embodiment of the invention. Thus, in the case where a diagram or a text related to a certain portion is described, the context taken out from part of the diagram or the text is also disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the invention is clear. Therefore, for example, in a diagram or text in which one or more active elements (e.g., transistors or diodes), wirings, passive elements (e.g., capacitors or resistors), conductive layers, insulating layers, semiconductor layers, organic materials, inorganic materials, components, devices, operating methods, manufacturing methods, or the like are described, part of the diagram or the text is taken out, and one embodiment of the invention can be constituted. For example, from a circuit diagram in which N circuit elements (e.g., transistors or capacitors; N is an integer) are provided, it is possible to constitute one embodiment of the invention by taking out M circuit elements (e.g., transistors or capacitors; M is an integer, where M&lt;N). As another example, it is possible to constitute one embodiment of the invention by taking out M layers (M is an integer, where M&lt;N) from a cross-sectional view in which N layers (N is an integer) are provided. As another example, it is possible to constitute one embodiment of the invention by taking out M elements (M is an integer, where M&lt;N) from a flow chart in which N elements (N is an integer) are provided. As another example, it is possible to take out some given elements from a sentence “A includes B, C, D, E, or F” and constitute one embodiment of the invention, for example, “A includes B and E”, “A includes E and F”, “A includes C, E, and F”, or “A includes B, C, D, and E”. 
     Note that in the case where at least one specific example is described in a diagram or a text described in one embodiment in this specification and the like, it will be readily appreciated by those skilled in the art that a broader concept of the specific example can be derived. Therefore, in the diagram or the text described in one embodiment, in the case where at least one specific example is described, a broader concept of the specific example is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the invention is clear. 
     Note that in this specification and the like, a content described in at least a diagram (which may be part of the diagram) is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. Therefore, when a certain content is described in a diagram, the content is disclosed as one embodiment of the invention even when the content is not described with a text, and one embodiment of the invention can be constituted. In a similar manner, part of a diagram, which is taken out from the diagram, is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. The embodiment of the invention is clear. 
     This application is based on Japanese Patent Application serial no. 2014-082256 filed with Japan Patent Office on Apr. 11, 2014, the entire contents of which are hereby incorporated by reference.