Patent Publication Number: US-9899424-B2

Title: Semiconductor device and electronic device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the invention disclosed in this specification and the like also relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the invention disclosed in this specification and the like relates to a semiconductor device, or an electronic device including a semiconductor device. 
     In this specification and the like, the term “semiconductor device” denotes a device that can function by utilizing semiconductor characteristics. A display device (e.g., a liquid crystal display device and a light-emitting display device), a lighting device, an electro-optical device, a power storage device, a memory device, a semiconductor circuit, an imaging device, an electronic device, and the like may include a semiconductor device. 
     2. Description of the Related Art 
     In recent years, transistors including an oxide semiconductor in a semiconductor layer where a channel is formed (hereinafter referred to as an “OS transistor”) have garnered attention. An oxide semiconductor layer can be formed by a sputtering method or the like, and thus can be used for a semiconductor layer of a transistor in a large display device, for example. In addition, OS transistors can be fabricated by improving parts of a production equipment for transistors including amorphous silicon in a semiconductor layer where a channel is formed; this can reduce the capital investment. 
     It is known that an OS transistor has an extremely low leakage current in an off state. For example, a low-power CPU and the like utilizing the extremely low leakage current of an OS transistor have been disclosed (see Patent Document 1). 
     REFERENCE 
     Patent Document 1: Japanese Published Patent Application No. 2012-257187 
     SUMMARY OF THE INVENTION 
     However, it is well known that fabrication of p-channel OS transistors is difficult. As a result, to construct a logic circuit using only OS transistors, the constructed logic circuit need to be a logic circuit including only n-channel transistors. 
     On the other hand, even if p-channel OS transistors could be fabricated, when both p-channel and n-channel transistors are fabricated separately on one substrate, the number of process steps needed for fabrication increase; this increases the fabrication cost and reduces the productivity. Therefore, thin-film transistors to be fabricated on one substrate are preferably transistors with the same conductivity type. However, the output voltage of a logic circuit constructed with transistors with the same conductivity type drops as much as the threshold voltages of the transistors. 
     An object of one embodiment of the present invention is to provide a semiconductor device or the like that allows high productivity. Another object of one embodiment of the present invention is to provide a semiconductor device or the like with low power consumption. Another object is to provide a highly reliable semiconductor device or the like. Another object of one embodiment of the present invention is to provide a semiconductor device or the like including a logic circuit including only n-channel transistors. Another object of one embodiment of the present invention is to provide a novel semiconductor device or the like. 
     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. 
     One embodiment of the present invention is a semiconductor device which includes a first transistor, a second transistor, a third transistor, a fourth transistor, and a capacitor. One of a source and a drain of the first transistor is electrically connected to a first wiring. A gate of the first transistor is electrically connected to the first wiring. The other of the source and the drain of the first transistor 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 is electrically connected to a second wiring. A gate of the second transistor is electrically connected to a gate of the fourth transistor. One of a source and a drain of the third transistor is electrically connected to a third wiring. The other of the source and the drain of the third transistor is electrically connected to one of a source and a drain of the fourth transistor. A gate of the third transistor is electrically connected to the other of the source and the drain of the first transistor. The other of the source and the drain of the fourth transistor is electrically connected to a fourth wiring. One electrode of the capacitor is electrically connected to the gate of the third transistor. The other electrode of the capacitor is electrically connected to the other of the source and the drain of the third transistor. 
     Another embodiment of the present invention is a semiconductor device which includes a first transistor, a second transistor, a third transistor, a fourth transistor, and a capacitor. Each of the first to fourth transistors includes a first gate and a second gate. The first gate of the first transistor is electrically connected to a first wiring. The second gate of the first transistor is electrically connected to the first wiring. One of a source and a drain of the first transistor is electrically connected to the first wiring. The other of the source and the drain of the first transistor 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 is electrically connected to a second wiring. The first gate of the second transistor is electrically connected to the second gate of the second transistor. One of a source and a drain of the third transistor is electrically connected to a third wiring. The first gate of the third transistor is electrically connected to the other of the source and the drain of the first transistor. The second gate of the third transistor is electrically connected to the other of the source and the drain of the third transistor. One of a source and a drain of the fourth transistor is electrically connected to the other of the source and the drain of the third transistor. The other of the source and the drain of the fourth transistor is electrically connected to a fourth wiring. The first gate of the fourth transistor is electrically connected to the second gate of the fourth transistor. The first gate of the second transistor is electrically connected to the first gate of the fourth transistor. One electrode of the capacitor is electrically connected to the first gate of the third transistor. The other electrode of the capacitor is electrically connected to the other of the source and the drain of the third transistor. One of the first gate and the second gate in each of the first to fourth transistors functions as a gate, and the other of the first gate and the second gate in each of the first to fourth transistors functions as a back gate. 
     Any of the above transistors preferably includes an oxide semiconductor in a semiconductor layer in which a channel is formed. 
     The channel length of the second transistor is preferably shorter than the channel length of the first transistor. In addition, the channel width of the second transistor is preferably longer than the channel width of the first transistor. 
     A semiconductor device or the like that allows high productivity can be provided. Alternatively, a semiconductor device or the like with low power consumption can be provided. Alternatively, a highly reliable semiconductor device or the like can be provided. Alternatively, a semiconductor device or the like including a logic circuit including only n-channel transistors can be provided. Alternatively, a novel semiconductor device or the like 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 achieve all the objects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1D  are circuit diagrams illustrating a semiconductor device; 
         FIG. 2  is a timing chart illustrating the operation of a semiconductor device; 
         FIGS. 3A and 3B  are circuit diagrams each illustrating an operation of a semiconductor device; 
         FIGS. 4A and 4B  are circuit diagrams each illustrating operation of a semiconductor device; 
         FIGS. 5A to 5D  are circuit diagrams each illustrating a semiconductor device; 
         FIG. 6  is a timing chart illustrating an operation of a semiconductor device; 
         FIGS. 7A and 7B  are circuit diagrams illustrating an operation of a semiconductor device; 
         FIG. 8  is a circuit diagram illustrating an operation of a semiconductor device; 
         FIGS. 9A to 9D  are circuit diagrams each illustrating a semiconductor device; 
       FIGS.  10 A 1  to  10 C 2  each illustrates a structure example of a transistor; 
       FIGS.  11 A 1  to  11 B 2  each illustrates a structure example of a transistor; 
       FIGS.  12 A 1  to  12 C 2  each illustrates a structure example of a transistor; 
         FIGS. 13A to 13C  each illustrates a structure example of a transistor; 
         FIGS. 14A to 14C  each illustrates a structure example of a transistor; 
         FIGS. 15A to 15C  each illustrates a structure example of a transistor; 
         FIGS. 16A to 16C  each illustrates a structure example of a transistor; 
         FIGS. 17A to 17C  each illustrates a structure example of a transistor; 
         FIGS. 18A and 18B  each illustrates a structure example of a transistor; 
         FIGS. 19A to 19C  each illustrates a structure example of a transistor; 
         FIGS. 20A and 20B  show energy band structures; 
         FIG. 21A  is a flowchart showing a manufacturing process example of an electronic component, and  FIG. 21B  is a schematic perspective view of the electronic component; 
         FIGS. 22A and 22B  each illustrate an example of an electronic device; and 
         FIG. 23  illustrates an example of an electronic device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below. Note that in the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description of such portions is not repeated in some cases. 
     The position, size, range, and the like of each component illustrated in the drawings and the like are not accurately represented in some cases to facilitate understanding of the invention. Therefore, the present invention is not necessarily limited to the position, size, range, or the like disclosed in the drawings and the like. 
     In the drawings, some components might not be illustrated to facilitate understanding of the invention. In addition, some hidden lines and the like might not be shown. 
     Note that ordinal numbers such as “first” and “second” in this specification and the like are used in order to avoid confusion among components and do not denote a priority or an order such as the order of steps or the stacking order. A term without an ordinal number in this specification and the like might be provided with an ordinal number in a claim in order to avoid confusion among components. In addition, a term with an ordinal number in this specification and the like might be provided with a different ordinal number in a claim. Moreover, a term with an ordinal number in this specification and the like might not be provided with any ordinal number in a claim. 
     In addition, in this specification and the like, the term such as an “electrode” or a “wiring” does not limit a function of a component. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa. Further, the term electrode or wiring can also mean a combination of a plurality of electrodes and wirings formed in an integrated manner. 
     Note that the term “over” or “under” in this specification and the like does not necessarily mean that a component is placed “directly on” or “directly below” and “directly in contact with” another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is on and in direct contact with the insulating layer A and can mean the case where another component is provided between the insulating layer A and the electrode B. 
     Furthermore, functions of the source and the drain might be switched depending on operation conditions, e.g., when a transistor having a different polarity is employed or a direction of current flow is changed in circuit operation; it is therefore difficult to determine the source/drain function of electrodes in a transistor. Thus, the terms “source” and “drain” are used interchangeably in this specification. 
     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 provided between elements having a connection relation illustrated in drawings and texts, without being limited to a predetermined connection relation, for example, the connection relation illustrated in the drawings and the texts. 
     Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an object having any electric function as long as electric signals can be transmitted and received between components that are connected through the object. Accordingly, even when the expression “electrically connected” is used in this specification, there may be a case in which no physical connection is made and a wiring is just extended in an actual circuit. 
     Note that the term “channel length” refers to, for example, a distance between a source (source region or source electrode) and a drain (drain region or drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed (also referred to as a “channel formation region”) in a top view of the transistor. Note that in a given transistor, channel lengths are not necessarily the same in all regions. 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 the channel formation region. 
     The term “channel width” refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other, or in the channel formation region. Note that in a given transistor, channel lengths are not necessarily the same in all regions. 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 the channel formation region. 
     Note that depending on transistor structures, a channel width in a region where a channel is actually formed (also referred to as an “effective channel width”) is different from a channel width shown in a top view of a transistor (also referred to as an “apparent channel width”) in some cases. For example, when a gate electrode covers the side surfaces of a semiconductor layer in a transistor, an effective channel width is greater than an apparent channel width; in some cases, its impact can be substantial. For example, when a gate electrode covers the side surfaces of a semiconductor in a miniaturized transistor, the proportion of a channel region formed in a side surface of a semiconductor increases. In that case, an effective channel width is greater than an apparent channel width. 
     In such a case, an effective channel width is difficult to measure in some cases. For example, to estimate an effective channel width from design parameters, the profile of the semiconductor need to be known beforehand. Therefore, when accurate profile information of the semiconductor is not available, accurate measurement of an effective channel width is difficult. 
     Therefore, in this specification, an apparent channel width is referred to as a surrounded channel width (SCW) in some cases. Further, in this specification, in the case where the term “channel width” is used, it may denote a surrounded channel width or an apparent channel width. Alternatively, in this specification, in some cases where the term “channel width” is used, it may denote an effective channel width. Note that a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by analyzing a cross-sectional TEM image and the like. 
     Note that the surrounded channel width may be used for calculation, when field effect mobility, a current value per channel width, and the like of a transistor are obtained by calculation. In that case, the obtained value may differ from the value calculated using an effective channel width. 
     Furthermore, unless otherwise specified, transistors described in this specification and the like are enhancement-type (normally-off-type) field effect transistors. 
     Note that an impurity in a semiconductor refers to, for example, elements other than the main components of the semiconductor. For example, an element with a concentration lower than 0.1 atomic % can be regarded as an impurity. Inclusion of impurities may cause an increase in density of states (DOS) in a semiconductor, and/or a decrease in the carrier mobility or the crystallinity. 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 13 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components of the oxide semiconductor; specifically, they include hydrogen (also included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. Entry of impurities such as hydrogen may form oxygen vacancies within an oxide semiconductor. 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, 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°. Accordingly, the term “parallel” includes the case where the angle formed between two straight lines is greater than or equal to −5° and less than or equal to 5°. In addition, the term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. The terms “perpendicular” and “orthogonal” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°. Accordingly, the terms “perpendicular” and “orthogonal” includes the case where the angle formed between two straight lines is greater than or equal to 85° and less than or equal to 95°. A term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°. 
     In the specification and the like, the terms “identical,” “the same,” “equal,” “uniform,” and the like (including synonyms thereof) used in describing calculation values and actual measurement values allow for a margin of error of 20% unless otherwise specified. 
     In this specification, in the case where an etching step is performed after a photolithography process, a resist mask formed in the photolithography process is removed after the etching step, unless otherwise specified. 
     In this specification and the like, a high power supply potential V DD  (hereinafter also simply referred to as V DD  or H potential) is a power supply potential higher than a low power supply potential V SS . The low power supply potential V SS  (hereinafter also simply referred to as V SS  or L potential) is a power supply potential lower than the high power supply potential V DD . In addition, a ground potential can be used as V DD  or V SS . For example, in the case where a ground potential is used as V DD , V SS  is lower than the ground potential, and in the case where a ground potential is used as V SS , V DD  is higher than the ground potential. 
     A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a ground potential (GND potential) or a source potential). Note that a “potential” is a relative concept, and a potential supplied to wirings or the like may be changed depending on a reference potential. Therefore, the terms “voltage” and “potential” can be used interchangeably in some cases. Note that in this specification and the like, V SS  is the reference voltage unless otherwise specified. 
     Note that the terms “film” and “layer” can be used interchangeably depending on the case or the circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases. 
     In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system. 
     (Embodiment 1) 
     In this embodiment, a semiconductor device  100  of one embodiment of the present invention will be described with reference to drawings.  FIG. 1A  is a circuit diagram illustrating a structure of the semiconductor device  100 . 
     &lt;Structure Example of Semiconductor Device  100 &gt; 
     The semiconductor device  100  illustrated in  FIG. 1A  includes transistors  111  to  114  and a capacitor  117 . The transistors  111  to  114  are n-channel transistors each having a source, a drain, a gate, and a back gate. 
     The gate and the back gate are positioned so that the channel formation region of a semiconductor layer is provided between the gate and the back gate. Thus, the back gate electrode can function in a manner similar to that of the gate electrode. Note that the back gate potential can be the same as the gate potential, the ground potential (GND potential), or an arbitrary potential. By changing the back gate potential independently of the gate potential, the threshold voltage of the transistor can be changed. In this specification and the like, the term “first gate” may refer to one of the gate and the back gate, and the term “second gate” may refer to the other of the gate and the back gate. 
     In the semiconductor device  100  illustrated in  FIG. 1A , first and second gates of the transistor  111  are electrically connected to a wiring  121 , one of a source and a drain of the transistor  111  is electrically connected to the wiring  121 , and the other of the source and the drain of the transistor  111  is electrically connected to a node  131 . In addition, one of a source and a drain of the transistor  112  is electrically connected to the node  131 , the other of the source and the drain of the transistor  112  is electrically connected to a wiring  122 , and first and second gates of the transistor  112  are electrically connected to a node  132 . In addition, one of a source and a drain of the transistor  113  is electrically connected to a wiring  123 , the other of the source and the drain of the transistor  113  is electrically connected to a node  133 , one of first and second gates of the transistor  113  is electrically connected to the node  133 , and the other of the first and second gates of the transistor  113  is electrically connected to the node  131 . Furthermore, one of a source and a drain of the transistor  114  is electrically connected to the node  133 , the other of the source and the drain of the transistor  114  is electrically connected to a wiring  124 , and first and second gates of the transistor  114  are electrically connected to the node  132 . Furthermore, one electrode of the capacitor  117  is electrically connected to the node  131 , and the other electrode of the capacitor  117  is electrically connected to the node  133 . Furthermore, the node  132  is electrically connected to a terminal  102 , and the node  133  is electrically connected to a terminal  105 . 
     Providing a back gate in addition to a gate enlarges the area where carriers flow when the transistor is on, in the film thickness direction; this increases the mobility of the carriers. As a result, the on-state current and the field-effect mobility of the transistor increases. Thus, a transistor with a back gate enables a reduction of the occupied area when compared to a transistor without a back gate, when the required on-state current is the same for both transistors. Furthermore, by surrounding the semiconductor layer with the gate and the back gate, effects of external electric fields on the channel formation region can be reduced, thereby improving the reliability of the semiconductor device. Note that back gates will be explained in later sections in more detail. 
     When the same potential is supplied to the wirings  121  and  123 , one of the wirings  121  and  123  may be omitted. When the same potential is supplied to the wirings  122  and  124 , one of the wirings  122  and  124  may be omitted. In a semiconductor device  100   a  illustrated in  FIG. 1B , one of the source and the drain of the transistor  111  and one of the source and the drain of the transistor  113  are electrically connected to the wiring  121 . Furthermore, the other of the source and the drain of the transistor  112  and the other of the source and the drain of the transistor  114  are electrically connected to the wiring  122 . 
     One of the first and second gates of the transistor  112  may be electrically connected to the wiring  122 , as in a semiconductor device  100   b  illustrated in  FIG. 1C . One of the first and second gates of transistor  114  may be electrically connected to the wiring  124 . One of the first and second gates of the transistor  111  and one of the first and second gates of the transistor  113  may be electrically connected to a wiring  125 . Note that the wiring  125  will be supplied with a potential such as V SS , or a potential equivalent to the potential supplied to wirings  122  and  124 . 
     Depending on necessity, some or all of the transistors included in the semiconductor device  100  may be transistors that do not utilize back gates.  FIG. 1D  shows a circuit diagram of a semiconductor device  100   c ; the semiconductor device  100   c  is a derivative of the semiconductor device  100 , and includes transistors that do not include back gates. 
     OS transistors are preferably used as the transistors  111  to  114 . The oxide semiconductor has a band gap of 2 eV or more; therefore, an OS transistor has an extremely small off-state current. Specifically, the off-state current per micrometer in channel width at room temperature (25° C.) and at a source-drain voltage of 3.5 V can be lower than 1×10 −20  A, lower than 1×10 −22  A, or lower than 1×10 −24  A. That is, the on/off ratio of the transistor can be greater than or equal to 20 digits and less than or equal to 150 digits. An OS transistor has a high dielectric breakdown voltage between its source and drain. Thus, with use of the OS transistor, a semiconductor device for high power application can be provided. 
     &lt;Operation Example of Semiconductor Device  100 &gt; 
     The semiconductor device  100  can function as an inverter circuit. Specifically, an L potential is output from the terminal  105  when an H potential is input to the terminal  102 , and an H potential is output from the terminal  105  when an L potential is input to the terminal  102 . 
     An operation example of the semiconductor device  100  is described with reference to a timing chart in  FIG. 2  and circuit diagrams in  FIGS. 3A and 3B , and  FIGS. 4A and 4B . In this example, it is assumed that the wirings  121  and  123  are supplied with an H potential (V DD ), and the wirings  122  and  124  are supplied with an L potential (V SS ). Furthermore, it is assumed that the threshold voltages of the transistors  111  to  114  are the same, which will be denoted as “V th ” in this specification and the like. In addition, it is assumed that V th  is higher than 0 volts and lower than (V DD −V SS )/2. 
     &lt;Period  151 : H Potential Input Period&gt; 
     In a period  151 , when an H potential is input to the terminal  102 , the potential at the node  132  becomes an H potential, thereby turning on the transistors  112  and  114 . After this, each of the potentials of the nodes  131  and  133  becomes an L potential, thereby turning off the transistor  113 . In addition, an L potential is output from the terminal  105  which is electrically connected to the node  133  (refer to  FIG. 3A ). 
     The transistors  111  and  112  are turned on simultaneously. Therefore, to bring the potential of the node  131  closer to an L potential, the on-state resistance (resistance between a source and a drain when a transistor is on) of the transistor  112  is preferably made lower than that of the transistor  111 . This can be achieved by making the channel length of the transistor  112  shorter than that of the transistor  111 , for example. Specifically, the channel length of the transistor  112  can be ½ or less, preferably ⅕ or less, more preferably 1/10 or less, and further preferably 1/20 or less of the channel length of the transistor  111 . This can also be achieved by making the channel width of the transistor  112  longer than that of the transistor  111 , for example. Specifically, the channel width of the transistor  112  can be longer than the channel width of the transistor  111  by 2 times or more, preferably 5 times or more, more preferably 10 times or more, and further preferably 20 times or more. Furthermore, the transistor  112  may be a transistor including a back gate, and the transistor  111  may be a transistor that does not include a back gate, for example. 
     &lt;Period  152 : L Potential Input Period&gt; 
     In a period  152 , when an L potential is input to the terminal  102 , the potential at the node  132  becomes an L potential, thereby turning off the transistors  112  and  114 . After that, a potential is supplied from the wiring  121  through the transistor  111  to the node  131 . At this time, the potential at the node  131  is V DD −V th  (refer to  FIG. 3B ). 
     The potential at the node  131  is higher than V th ; thus, the transistor  113  is turned on (refer to  FIG. 3B ). After that, a potential is supplied from the wiring  123  through the transistor  113  to the node  133  (refer to  FIG. 4A ). At this time, the potential at the node  133  is V DD −V th . 
     When a potential is supplied to the node  133 , the potential of the node  131  increases; this is because the nodes  133  and  131  are capacitively coupled with each other through the capacitor  117 . Specifically, the potential of the node  131  becomes 2×(V DD −V th ). In addition, the potential of the node  133  ultimately becomes equal to that of the wiring  123 . Accordingly, the potential of the node  131  rises to a potential that is close to 2×V DD −V th . Accordingly, the transistor  111  is turned off. Furthermore, an H potential (V DD ) is output from the terminal  105  (refer to  FIG. 4B ). 
     &lt;Modification Example 1&gt; 
       FIG. 5A  is a circuit diagram that illustrates a semiconductor device  110 , which has a different configuration from that of the semiconductor device  100 . The semiconductor device  110  includes the transistors  111  to  113  and the capacitor  117 . The semiconductor device  110  includes fewer transistors than the semiconductor device  100 ; thus, the area occupied by the semiconductor device  110  is smaller than that of the semiconductor device  100 . Note that in this description, focus will be put on the differences between the semiconductor devices  110  and  100 , to avoid repetition of description. 
     &lt;Configuration Example of Semiconductor Device  110 &gt; 
     In the semiconductor device  110  illustrated in  FIG. 5A , one of the source and the drain of the transistor  111  is electrically connected to the wiring  121 , the other of the source and the drain and the first gate of the transistor  111  are electrically connected to the node  131 , and the second gate of the transistor  111  is electrically connected to the node  133 . In addition, in the transistor  112 , one of the source and the drain is electrically connected to the node  131 , the other of the source and the drain is electrically connected to the wiring  122 , and the first and second gates are electrically connected to the terminal  102 . Furthermore, in the transistor  113 , one of the source and the drain is electrically connected to a terminal  106 , the other of the source and the drain is electrically connected to the node  133 , and the first and second gates are electrically connected to the wiring  123 . One electrode of the capacitor  117  is electrically connected to the node  131 , and the other electrode of the capacitor  117  is electrically connected to the node  133 . The node  131  is electrically connected to the terminal  105 . 
     When the same potential is supplied to both the wirings  121  and  123 , one of the wirings  121  and  123  may be omitted. In a semiconductor device  110   a  illustrated in  FIG. 5B , the first and second gates of the transistor  113  are electrically connected to the wiring  121 . 
     As in a semiconductor device  110   b  shown in  FIG. 5C , the first and second gates of the transistor  112  may be electrically connected to the wiring  122 . One of the first and second gates of the transistor  113  may be electrically connected to the wiring  124 . Note that the wiring  124  is supplied with V SS . 
     Depending on necessity, at least one of the transistors  112  and  113  may be a transistor that does not include a back gate. A semiconductor device  110   c  is a modification example of the semiconductor device  110 , in which both of the transistors  112  and  113  are transistors that do not include back gates. The circuit diagram of the semiconductor device  110   c  is shown in  FIG. 5D . 
     &lt;Operation Example of Semiconductor Device  110 &gt; 
     The semiconductor device  110  can function as an inverter circuit. Specifically, when an H potential is input to the terminal  102 , an L potential is output from the terminal  105 . When an L potential is input to the terminal  102 , an H potential is output from the terminal  105 . 
     Different potentials are supplied to the terminals  102  and  106 . Specifically, when an H potential is supplied to the terminal  102 , an L potential is supplied to the terminal  106 . When an L potential is supplied to the terminal  102 , an H potential is supplied to the terminal  106 . 
     An operation example of the semiconductor device  110  is described with reference to a timing chart in  FIG. 6  and circuit diagrams in  FIGS. 7A and 7B  and  FIG. 8 . 
     &lt;Period  151 : H Potential Input Period&gt; 
     In the period  151 , an H potential is input to the terminal  102 , and an L potential is input to the terminal  106 . As a result, the transistors  112  and  113  are turned on, and an L potential is supplied to the nodes  131  and  133 . In addition, the terminal  105  electrically connected to the node  131  outputs an L potential (refer to  FIG. 7A ). 
     Note that in the semiconductor device  110 , the transistors  111  and  112  are not turned on at the same time. Therefore, there are no restrictions on the on-state resistance of the transistors  111  and  112 , which exist in the case of the semiconductor device  100 . 
     &lt;Period  152 : L Potential Input Period&gt; 
     In the period  152 , an L potential is input to the terminal  102 , and an H potential is input to the terminal  106 . As a result, the transistor  112  is turned off. Furthermore, a potential is supplied from the terminal  106  through the transistor  113  to the node  133 . At this time, an H potential (V DD ) is being supplied from the wiring  123  to the gate of the transistor  113 ; therefore, the potential of the node  131  becomes V DD −V th  (refer to  FIG. 7B ). 
     Furthermore, the potential of the node  133  is higher than V th , which turns on the transistor  111  (refer to  FIG. 7B ). As a result, a potential is supplied from the wiring  121  through the transistor  111  to the node  131 . 
     When a potential is supplied to the node  131 , the potential of the node  133  rises. The node  133  is electrically connected to the node  131  through the capacitor  117 . Ultimately, the potential of the node  133  rises to the vicinity of 2×V DD −V th . Accordingly, the transistor  113  is turned off. In addition, an H potential (V DD ) is output from the terminal  105  (refer to  FIG. 8 ). 
     &lt;Modification Example 2&gt; 
       FIG. 9A  shows the circuit diagram of a semiconductor device with further reduction in the number of transistors from the semiconductor device  110 . A semiconductor device  120  illustrated in  FIG. 9A  includes the transistors  111  and  112 . The semiconductor device  120  includes fewer transistors than the semiconductor device  110 ; thus the area occupied by the semiconductor device  120  is smaller than that of the semiconductor device  110 . 
     &lt;Configuration Example of Semiconductor Device  120 &gt; 
     In the transistor  111  of the semiconductor device  120  illustrated in  FIG. 9A , one of the source and the drain is electrically connected to the wiring  125 , the other of the source and the drain and the first gate are electrically connected to the node  131 , and the second gate is electrically connected to the terminal  103 . Furthermore, in the transistor  112 , one of the source and the drain is electrically connected to the node  131 , the other of the source and the drain is connected to the wiring  123 , one of the first and second gates is electrically connected to the terminal  102 , and the other of the first and second gates is electrically connected to the terminal  104 . In addition, the node  131  is electrically connected to the terminal  105 . 
     One of the first and second gates of the transistor  112  may be electrically connected to the wiring  123 , as in a semiconductor device  120   a  illustrated in  FIG. 9B . The semiconductor device  120   a  enables a reduction in the number of input terminals compared to the semiconductor device  120 , thereby allowing a higher productivity of the semiconductor device. 
     &lt;Operation Example of the Semiconductor Device  120 &gt; 
     The semiconductor device  120  can function as an inverter circuit. Specifically, when an H potential is input to the terminals  102  and  104 , and an L potential is input to the terminal  103 , an L potential is output from the terminal  105 . In addition, when an L potential is input to the terminals  102  and  104 , and an H potential is input to the terminal  103 , V DD −V th  is output from the terminal  105 . Note that a potential that is greater than or equal to V DD +V th  may be input to the terminal  103  to output an H potential from the terminal  105 . 
     &lt;Modification Example 3&gt; 
       FIG. 9C  shows the circuit diagram of a semiconductor device with further reduction in the number of transistors from the semiconductor device  110 . A semiconductor device  130  illustrated in  FIG. 9C  includes the transistors  111  and  112 . The semiconductor device  130  includes fewer transistors than the semiconductor device  110 ; thus the area occupied by the semiconductor device  130  may be smaller than that of the semiconductor device  110 . 
     &lt;Configuration Example of Semiconductor Device  130 &gt; 
     In the semiconductor device illustrated in  FIG. 9C , one of the source and the drain of the transistor  111  is electrically connected to the wiring  125 , the other of the source and the drain of the transistor  111  is electrically connected to the node  131 , one of the first and second gates of the transistor  111  is electrically connected to the terminal  101 , and the other of the first and second gates of the transistor  111  is electrically connected to the terminal  103 . In addition, one of the source and the drain of the transistor  112  is electrically connected to the node  131 , the other of the source and the drain of the transistor  112  is electrically connected to the wiring  123 , one of the first and second gates of the transistor  112  is electrically connected to the terminal  102 , and the other of the first and second gates of the transistor  112  is electrically connected to the terminal  104 . In addition, the node  131  is electrically connected to the terminal  105 . 
     As in a semiconductor device  130   a  shown in  FIG. 9D , one of the first and second gates of the transistor  111  may be electrically connected to the wiring  123 . One of the first and second gates of the transistor  112  may be electrically connected to the wiring  123 . The semiconductor device  130   a  enables a reduction in the number of input terminals compared to the semiconductor device  130 , thereby allowing a higher productivity of the semiconductor device. 
     &lt;Operation Example of Semiconductor Device  130 &gt; 
     The semiconductor device  130  can function as an inverter circuit. Specifically, when an H potential is input to the terminals  102  and  104 , and an L potential is input to the terminals  101  and  103 , an L potential is output from the terminal  105 . In addition, when an L potential is input to the terminals  102  and  104 , and an H potential is input to the terminals  101  and  103 , V DD −V th  is output from the terminal  105 . Note that to output an H potential from the terminal  105 , a potential greater than or equal to V DD +V th  may be input to the terminals  101  and  103 . 
     This embodiment can be implemented in an appropriate combination with any of the structures described in the other embodiments. 
     (Embodiment 2) 
     In this embodiment, examples of a transistor that can be used for the semiconductor device described in the above embodiments are described. 
     The semiconductor device in one embodiment of the present invention can be manufactured by using a transistor with any of various structures, such as a bottom-gate transistor and a top-gate transistor. Therefore, a material for a semiconductor layer or the structure of a transistor can be easily changed to comply with the existing production line. 
     [Bottom-Gate Transistor] 
     FIG.  10 A 1  is a cross-sectional view of a channel-protective transistor  410  that is a kind of bottom-gate transistor. The transistor  410  includes an electrode  246  over a substrate  271  with an insulating layer  272  positioned therebetween. The transistor  410  includes a semiconductor layer  242  over the electrode  246  with an insulating layer  226  positioned therebetween. The electrode  246  can function as a gate electrode. The insulating layer  226  can function as a gate insulating layer. 
     The transistor  410  includes an insulating layer  225  over a channel formation region in the semiconductor layer  242 . The transistor  410  includes an electrode  244   a  and an electrode  244   b  which are partly in contact with the semiconductor layer  242  and over the insulating layer  226 . Part of the electrode  244   a  and part of the electrode  244   b  are formed over the insulating layer  225 . 
     The insulating layer  225  can function as a channel protective layer. With the insulating layer  225  provided over the channel formation region, the semiconductor layer  242  can be prevented from being exposed at the time of forming the electrodes  244   a  and  244   b . Thus, the channel formation region in the semiconductor layer  242  can be prevented from being etched at the time of forming the electrodes  244   a  and  244   b . According to one embodiment of the present invention, a transistor with favorable electrical characteristics can be provided. 
     The transistor  410  includes an insulating layer  228  over the electrode  244   a , the electrode  244   b , and the insulating layer  225  and further includes an insulating layer  229  over the insulating layer  228 . 
     In the case where an oxide semiconductor is used for the semiconductor layer  242 , a material that is capable of removing oxygen from part of the semiconductor layer  242  to generate oxygen vacancies is preferably used at least for regions of the electrodes  244   a  and  244   b  that are in contact with the semiconductor layer  242 . The carrier concentration of the regions of the semiconductor layer  242  in which oxygen vacancies are generated is increased, so that the regions become n-type regions (n +  layers). Accordingly, the regions can function as a source region and a drain region. Examples of the material that is capable of removing oxygen from the oxide semiconductor to generate oxygen vacancies include tungsten and titanium. 
     Formation of the source region and the drain region in the semiconductor layer  242  makes it possible to reduce contact resistance between the semiconductor layer  242  and the electrode  244   a  and contact resistance between the semiconductor layer  242  and the electrode  244   b . Accordingly, the electrical characteristics of the transistor, such as field-effect mobility and threshold voltage, can be favorable. 
     In the case where a semiconductor such as silicon is used for the semiconductor layer  242 , a layer that functions as an n-type semiconductor or a p-type semiconductor is preferably provided between the semiconductor layer  242  and the electrode  244   a  and between the semiconductor layer  242  and the electrode  244   b . The layer that functions as an n-type semiconductor or a p-type semiconductor can function as a source region or a drain region in the transistor. 
     The insulating layer  229  is preferably formed using a material that can prevent or reduce diffusion of impurities into the transistor from the outside. The formation of the insulating layer  229  can be omitted as necessary. 
     When an oxide semiconductor is used for the semiconductor layer  242 , heat treatment may be performed before and/or after the insulating layer  229  is formed. The heat treatment can fill oxygen vacancies in the semiconductor layer  242  by diffusing oxygen contained in the insulating layer  229  or other insulating layers into the semiconductor layer  242 . Alternatively, the insulating layer  229  is formed while being heated, so that oxygen vacancies in the semiconductor layer  242  can be filled. 
     A transistor  411  in FIG.  10 A 2  is different from the transistor  410  in that an electrode  223  that can function as a back gate is provided over the insulating layer  229 . The electrode  223  can be formed using a material and a method similar to those of the electrode  246 . 
     &lt;Back Gate&gt; 
     In general, the back gate is formed using a conductive layer and positioned so that the channel formation region in the semiconductor layer is positioned between the gate and the back gate. Thus, the back gate can function like the gate. The potential of the back gate may be the same as that of the gate or may be a GND potential or a given potential. By changing the potential of the back gate independently of the potential of the gate, the threshold voltage of the transistor can be changed. 
     The electrode  246  and the electrode  223  can each function as a gate. Thus, the insulating layers  226 ,  225 ,  228 , and  229  can each function as a gate insulating layer. The electrode  223  may be provided between the insulating layers  228  and  229 . 
     In the case where one of the electrode  246  and the electrode  223  is simply referred to as a “gate” or “gate electrode,” the other is referred to as a “back gate” or “back gate electrode.” For example, in the transistor  411 , in the case where the electrode  223  is referred to as a “gate electrode,” the electrode  246  is referred to as a “back gate electrode.” In the case where the electrode  223  is used as a “gate electrode,” the transistor  411  can be regarded as a kind of top-gate transistor. Alternatively, one of the electrode  246  and the electrode  223  may be referred to as a “first gate” or “first gate electrode,” and the other may be referred to as a “second gate” or “second gate electrode.” 
     By providing the electrode  246  and the electrode  223  with the semiconductor layer  242  positioned therebetween and setting the potentials of the electrode  246  and the electrode  223  to be the same, a region of the semiconductor layer  242  through which carriers flow is enlarged in the film thickness direction; thus, the amount of transferred carriers is increased. As a result, the on-state current and field-effect mobility of the transistor  411  are increased. 
     Therefore, the transistor  411  has high on-state current for occupation area. That is, the area occupied by the transistor  411  can be small for required on-state current. According to one embodiment of the present invention, the area occupied by a transistor can be reduced. Therefore, according to one embodiment of the present invention, a highly integrated semiconductor device can be provided. 
     Furthermore, the gate and the back gate are formed using conductive layers and thus each have a function of preventing an electric field generated outside the transistor from influencing the semiconductor layer in which the channel is formed (in particular, an electric field blocking function against static electricity and the like). When the back gate is formed larger than the semiconductor layer such that the semiconductor layer is covered with the back gate, the electric field blocking function can be enhanced. 
     Since the electrode  246  (gate) and the electrode  223  (back gate) each have a function of blocking an electric field from the outside, electric charge of charged particles and the like generated on the insulating layer  272  side or above the electrode  223  do not influence the channel formation region in the semiconductor layer  242 . Thus, degradation induced by a stress test (e.g., a negative gate bias temperature (NGBT) stress test where negative charge is applied to a gate (this stress test is also referred to as NBT or NBTS)) can be reduced. Furthermore, variation in gate voltage (rising voltage) at which on-state current starts flowing at different drain voltages can be reduced. Note that this effect is obtained when the electrodes  246  and  223  have the same potential or different potentials. 
     Before and after a positive GBT (PGBT) stress test where positive electric charge is applied to a gate (this stress test is also referred to as PBT or PBTS)), a transistor including a back gate has a smaller change in threshold voltage than a transistor including no back gate. 
     The BT stress test such as NGBT or PGBT is a kind of accelerated test and can evaluate, in a short time, a change by long-term use (i.e., a change over time) in characteristics of transistors. In particular, the amount of change in threshold voltage of the transistor before and after the BT stress test is an important indicator to examine the reliability of the transistor. As the change in the threshold voltage is smaller, the transistor has higher reliability. 
     By providing the electrodes  246  and  223  and setting the potentials of the electrodes  246  and  223  to be the same, the amount of change in threshold voltage is reduced. Accordingly, variation in electrical characteristics among a plurality of transistors is also reduced. 
     When the back gate is formed using a light-blocking conductive film, light can be prevented from entering the semiconductor layer from the back gate side. Therefore, light deterioration of the semiconductor layer can be prevented and deterioration in electrical characteristics of the transistor, such as a shift of the threshold voltage, can be prevented. 
     According to one embodiment of the present invention, a transistor with high reliability can be provided. Moreover, a semiconductor device with high reliability can be provided. 
     FIG.  10 B 1  is a cross-sectional view of a channel-protective transistor  420  that is a kind of bottom-gate transistor. The transistor  420  has substantially the same structure as the transistor  410  but is different from the transistor  410  in that the insulating layer  225  covers the semiconductor layer  242 . With the insulating layer  225 , the semiconductor layer  242  can be prevented from being exposed at the time of forming the electrodes  244   a  and  244   b . Thus, the semiconductor layer  242  can be prevented from being reduced in thickness at the time of forming the electrodes  244   a  and  244   b.    
     The semiconductor layer  242  is electrically connected to the electrode  244   a  in an opening formed by selectively removing part of the insulating layer  225  that overlaps with the semiconductor layer  242 . The semiconductor layer  242  is electrically connected to the electrode  244   b  in another opening formed by selectively removing part of the insulating layer  225  that overlaps with the semiconductor layer  242 . A region of the insulating layer  225  that overlaps with the channel formation region can function as a channel protective layer. 
     A transistor  421  in FIG.  10 B 2  is different from the transistor  420  in that the electrode  223  that can function as a back gate is provided over the insulating layer  229 . 
     The distance between the electrodes  244   a  and  246  and the distance between the electrodes  244   b  and  246  in the transistors  420  and  421  are longer than those in the transistors  410  and  411 . Thus, parasitic capacitance generated between the electrodes  244   a  and  246  can be reduced. Furthermore, parasitic capacitance generated between the electrodes  244   b  and  246  can be reduced. According to one embodiment of the present invention, a transistor with favorable electrical characteristics can be provided. 
     A transistor  425  in FIG.  10 C 1  is a channel-etched transistor that is a kind of bottom-gate transistor. In the transistor  425 , the insulating layer  225  is not provided and the electrodes  244   a  and  244   b  are formed to be in contact with the semiconductor layer  242 . Thus, part of the semiconductor layer  242  that is exposed when the electrodes  244   a  and  244   b  are formed is etched in some cases. However, since the insulating layer  225  is not provided, productivity of the transistor can be increased. 
     A transistor  426  in FIG.  10 C 2  is different from the transistor  425  in that the electrode  223  that can function as a back gate is provided over the insulating layer  229 . 
     [Top-Gate Transistor] 
     FIG.  11 A 1  is a cross-sectional view of a transistor  430  that is a kind of top-gate transistor. The transistor  430  includes the semiconductor layer  242  over the substrate  271  with the insulating layer  272  positioned therebetween, the electrodes  244   a  and  244   b  that are over the semiconductor layer  242  and the insulating layer  272  and in contact with part of the semiconductor layer  242 , the insulating layer  226  over the semiconductor layer  242  and the electrodes  244   a  and  244   b , and the electrode  246  over the insulating layer  226 . 
     Since the electrode  246  overlaps with neither the electrode  244   a  nor the electrode  244   b  in the transistor  430 , parasitic capacitance generated between the electrodes  246  and  244   a  and parasitic capacitance generated between the electrodes  246  and  244   b  can be reduced. After the formation of the electrode  246 , an impurity  255  is introduced into the semiconductor layer  242  using the electrode  246  as a mask, so that an impurity region can be formed in the semiconductor layer  242  in a self-aligned manner (see FIG.  11 A 3 ). According to one embodiment of the present invention, a transistor with favorable electrical characteristics can be provided. 
     The impurity  255  can be introduced with an ion implantation apparatus, an ion doping apparatus, or a plasma treatment apparatus. 
     As the impurity  255 , for example, at least one kind of element of Group 13 elements and Group 15 elements can be used. In the case where an oxide semiconductor is used for the semiconductor layer  242 , it is possible to use at least one kind of element of a rare gas, hydrogen, and nitrogen as the impurity  255 . 
     A transistor  431  in FIG.  11 A 2  is different from the transistor  430  in that the electrode  223  and an insulating layer  227  are included. The transistor  431  includes the electrode  223  formed over the insulating layer  272  and the insulating layer  227  formed over the electrode  223 . The electrode  223  can function as a back gate. Thus, the insulating layer  227  can function as a gate insulating layer. The insulating layer  227  can be formed using a material and a method similar to those of the insulating layer  226 . 
     Like the transistor  411 , the transistor  431  has high on-state current for occupation area. That is, the area occupied by the transistor  431  can be small for required on-state current. According to one embodiment of the present invention, the area occupied by a transistor can be reduced. Therefore, according to one embodiment of the present invention, a highly integrated semiconductor device can be provided. 
     A transistor  440  illustrated in FIG.  11 B 1  is a kind of top-gate transistor. The transistor  440  is different from the transistor  430  in that the semiconductor layer  242  is formed after the formation of the electrodes  244   a  and  244   b . A transistor  441  illustrated in FIG.  11 B 2  is different from the transistor  440  in that the electrode  223  and the insulating layer  227  are included. Thus, in the transistors  440  and  441 , part of the semiconductor layer  242  is formed over the electrode  244   a  and another part of the semiconductor layer  242  is formed over the electrode  244   b.    
     Like the transistor  411 , the transistor  441  has high on-state current for occupation area. That is, the area occupied by the transistor  441  can be small for required on-state current. According to one embodiment of the present invention, the area occupied by a transistor can be reduced. Therefore, according to one embodiment of the present invention, a highly integrated semiconductor device can be provided. The transistor  441  as well as the transistor  411  has a high on-state current for the area occupied thereby. That is, the area occupied by the transistor  441  can be small for required on-state current. With one embodiment of the present invention, the area occupied by a transistor can be reduced. Therefore, with one embodiment of the present invention, a semiconductor device having a high degree of integration can be provided. 
     A transistor  442  illustrated in FIG.  12 A 1  is a kind of top-gate transistor. The transistor  442  includes the electrodes  244   a  and  244   b  over the insulating layer  229 . The electrodes  244   a  and  244   b  are electrically connected to the semiconductor layer  242  in openings formed in the insulating layers  228  and  229 . 
     Part of the insulating layer  226  that does not overlap with the electrode  246  is removed. The insulating layer  226  included in the transistor  442  is partly extended across the ends of the electrode  246 . 
     The impurity  255  is added to the semiconductor layer  242  using the electrode  246  and the insulating layer  226  as masks, so that an impurity region can be formed in the semiconductor layer  242  in a self-aligned manner (see FIG.  12 A 3 ). 
     At this time, the impurity  255  is not added to the semiconductor layer  242  in a region that overlaps with the electrode  246 , and the impurity  255  is added to the semiconductor layer  242  in a region that does not overlap with the electrode  246 . The semiconductor layer  242  in a region to which the impurity  255  is added through the insulating layer  226  has a lower impurity concentration than the semiconductor layer  242  in a region to which the impurity  255  is added without the insulating layer  226 . Thus, a lightly doped drain (LDD) region is formed in the semiconductor layer  242  in a region adjacent to the electrode  246 . 
     A transistor  443  in FIG.  12 A 2  is different from the transistor  442  in that the electrode  223  is provided below the semiconductor layer  242 . The electrode  223  and the semiconductor layer  242  overlap with each other with the insulating layer  272  positioned therebetween. The electrode  223  can function as a back gate electrode. 
     As in a transistor  444  in FIG.  12 B 1  and a transistor  445  in FIG.  12 B 2 , the insulating layer  226  in a region that does not overlap with the electrode  246  may be wholly removed. Alternatively, as in a transistor  446  in FIG.  12 C 1  and a transistor  447  in FIG.  12 C 2 , the insulating layer  226  except for the openings may be left without being removed. 
     In the transistors  444  to  447 , after the formation of the electrode  246 , the impurity  255  is added to the semiconductor layer  242  using the electrode  246  as a mask, so that an impurity region can be formed in the semiconductor layer  242  in a self-aligned manner. 
     [S-Channel Transistor] 
       FIGS. 13A to 13C  illustrate an example of the structure of a transistor using an oxide semiconductor for the semiconductor layer  242 . In a transistor  450  illustrated in  FIGS. 13A to 13C , a semiconductor layer  242   b  is formed over a semiconductor layer  242   a , and a semiconductor layer  242   c  covers a top surface and a side surface of the semiconductor layer  242   b  and a side surface of the semiconductor layer  242   a .  FIG. 13A  is a top view of the transistor  450 .  FIG. 13B  is a cross-sectional view (in the channel length direction) taken along dashed-dotted line X 1 -X 2  in  FIG. 13A .  FIG. 13C  is a cross-sectional view (in the channel width direction) taken along dashed-dotted line Y 1 -Y 2  in  FIG. 13A . 
     The transistor  450  includes an electrode  243  functioning as a gate electrode. The electrode  243  can be formed using a material and a method similar to those of the electrode  246 . The electrode  243  is formed using two stacked conductive layers in this embodiment. 
     Each of the semiconductor layer  242   a , the semiconductor layer  242   b , and the semiconductor layer  242   c  is formed using a material containing either In or Ga or both of them. Typical examples are an In—Ga oxide (an oxide containing In and Ga), an In—Zn oxide (an oxide containing In and Zn), and an In-M-Zn oxide (an oxide containing In, an element M, and Zn, where the element M is one or more kinds of elements selected from Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf and has a higher strength of bonding with oxygen than In has). 
     The semiconductor layer  242   a  and the semiconductor layer  242   c  are preferably formed using a material containing one or more kinds of metal elements contained in the semiconductor layer  242   b . With such a material, interface states at interfaces between the semiconductor layer  242   a  and the semiconductor layer  242   b  and between the semiconductor layer  242   c  and the semiconductor layer  242   b  are less likely to be generated. Accordingly, carriers are less likely to be scattered or captured at the interfaces, which results in an improvement in field-effect mobility of the transistor. Furthermore, variation in the threshold voltage of the transistor can be reduced. Thus, a semiconductor device having favorable electrical characteristics can be obtained. 
     Each of the thicknesses of the semiconductor layer  242   a  and the semiconductor layer  242   c  is greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm. The thickness of the semiconductor layer  242   b  is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, more preferably greater than or equal to 3 nm and less than or equal to 50 nm. 
     In the case where the semiconductor layer  242   b  is an In-M-Zn oxide and the semiconductor layers  242   a  and  242   c  are each an In-M-Zn oxide, the semiconductor layers  242   a  and  242   c  each have an atomic ratio of In:M:Zn=x 1 :y 1 :z 1 , and the semiconductor layer  242   b  has an atomic ratio of In:M:Zn=x 2 :y 2 :z 2 . In that case, the compositions of the semiconductor layers  242   a ,  242   c , and  242   b  can be determined so that y 1 /x 1  is larger than y 2 /x 2 . It is preferable that the compositions of the semiconductor layers  242   a ,  242   c , and  242   b  are determined so that y 1 /x 1  is 1.5 times or more as large as y 2 /x 2 . It is further preferable that the compositions of the semiconductor layers  242   a ,  242   c , and  242   b  are determined so that y 1 /x 1  is twice or more as large as y 2 /x 2 . It is still further preferable that the semiconductor layers  242   a ,  242   c , and  242   b  are determined so that y 1 /x 1  is three times or more as large as y 2 /x 2 . It is preferable that y 1  be larger than or equal to x 1  because the transistor can have stable electrical characteristics. However, when y 1  is three times or more as large as x 1 , the field-effect mobility of the transistor is decreased; accordingly, y 1  is preferably smaller than three times x 1 . When the semiconductor layers  242   a  and  242   c  have the above compositions, the semiconductor layers  242   a  and  242   c  can each be a layer in which oxygen vacancies are less likely to be generated than in the semiconductor layer  242   b.    
     In the case where the semiconductor layers  242   a  and  242   c  are each an In-M-Zn oxide and the summation of In and the element M is assumed to be 100 atomic %, the atomic percentages of In and the element M are preferably as follows: the percentage of In is lower than 50 atomic % and the percentage of M is higher than or equal to 50 atomic %. The percentages of In and M are more preferably as follows: the percentage of In is lower than 25 atomic % and the percentage of M is higher than or equal to 75 atomic %. In the case where the semiconductor layer  242   b  is an In-M-Zn oxide and the summation of In and M is assumed to be 100 atomic %, the atomic percentages of In and the element M are preferably more than or equal to 25 atomic % and less than 75 atomic %, respectively, more preferably more than or equal to 34 atomic % and less than 66 atomic %, respectively. 
     For example, an In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:3:2, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6, an In—Ga oxide that is formed using a target having an atomic ratio of In:Ga=1:9, gallium oxide, or the like can be used for each of the semiconductor layers  242   a  and  242   c  containing In or Ga. Furthermore, an In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=3:1:2, 1:1:1, 5:5:6, 5:1:7, or 4:2:4.1 can be used for the semiconductor layer  242   b . Note that the atomic ratio of each of the semiconductor layers  242   a ,  242   b , and  242   c  may vary within a range of ±20% of any of the above atomic ratios as an error. 
     In order to give stable electrical characteristics to the transistor including the semiconductor layer  242   b , it is preferable that impurities and oxygen vacancies in the semiconductor layer  242   b  be reduced to highly purify the semiconductor layer so that the semiconductor layer  242   b  can be a highly-purified intrinsic or substantially highly-purified intrinsic semiconductor layer. Furthermore, it is preferable that at least the channel formation region in the semiconductor layer  242   b  be a semiconductor layer that can be regarded as a highly-purified intrinsic or substantially highly-purified intrinsic semiconductor layer. 
     Note that the substantially highly-purified intrinsic oxide semiconductor layer refers to an oxide semiconductor layer in which carrier density is higher than or equal to 1×10 −9 /cm 3  and lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , more preferably lower than 1×10 10 /cm 3 . 
       FIGS. 14A to 14C  illustrate an example of the structure of a transistor using an oxide semiconductor for the semiconductor layer  242 . In a transistor  422  illustrated in  FIGS. 14A to 14C , the semiconductor layer  242   b  is formed over the semiconductor layer  242   a . The transistor  422  is a kind of bottom-gate transistor having a back gate.  FIG. 14A  is a top view of the transistor  422 .  FIG. 14B  is a cross-sectional view (in the channel length direction) taken along dashed-dotted line X 1 -X 2  in  FIG. 14A .  FIG. 14C  is a cross-sectional view (in the channel width direction) taken along dash-dot line Y 1 -Y 2  in  FIG. 14A . 
     The electrode  223  provided over the insulating layer  229  is electrically connected to the electrode  246  in an opening  247   a  and an opening  247   b  provided in the insulating layers  226 ,  228 , and  229 . Thus, the same potential is applied to the electrodes  223  and  246 . Furthermore, either or both of the openings  247   a  and  247   b  may be omitted. In the case where neither the opening  247   a  nor the opening  247   b  are provided, different potentials can be applied to the electrodes  223  and  246 . 
     [Energy Band Structure of Oxide Semiconductor] 
     Here, the function and effect of the semiconductor layer  242  that is a stacked layer including the semiconductor layers  242   a ,  242   b , and  242   c  are described with energy band structure diagrams shown in  FIGS. 20A and 20B .  FIG. 20A  is the energy band structure diagram of a portion along dashed-dotted line D 1 -D 2  in  FIG. 13B . That is,  FIG. 20A  shows the energy band structure of a channel formation region of the transistor  450 . 
     In  FIG. 20A , Ec 382 , Ec 383   a , Ec 383   b , Ec 383   c , and Ec 386  indicate the energy of the conduction band minimum of the insulating layer  272 , the semiconductor layer  242   a , the semiconductor layer  242   b , the semiconductor layer  242   c , and the insulating layer  226 , respectively. 
     Here, a difference in energy between the vacuum level and the conduction band minimum (the difference is also referred to as electron affinity) corresponds to a value obtained by subtracting an energy gap from a difference in energy between the vacuum level and the valence band maximum (the difference is also referred to as an ionization potential). Note that the energy gap can be measured using a spectroscopic ellipsometer (e.g., UT-300 manufactured by HORIBA JOBIN YVON S.A.S.). The difference in energy between the vacuum level and the valence band maximum can be measured using an ultraviolet photoelectron spectroscopy (UPS) device (e.g., VersaProbe manufactured by ULVAC-PHI, Inc.). 
     Note that an In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:3:2 has an energy gap of approximately 3.5 eV and an electron affinity of approximately 4.5 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:3:4 has an energy gap of approximately 3.4 eV and an electron affinity of approximately 4.5 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:3:6 has an energy gap of approximately 3.3 eV and an electron affinity of approximately 4.5 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:6:2 has an energy gap of approximately 3.9 eV and an electron affinity of approximately 4.3 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:6:8 has an energy gap of approximately 3.5 eV and an electron affinity of approximately 4.4 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:6:10 has an energy gap of approximately 3.5 eV and an electron affinity of approximately 4.5 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=1:1:1 has an energy gap of approximately 3.2 eV and an electron affinity of approximately 4.7 eV. An In—Ga—Zn oxide that is formed using a target having an atomic ratio of In:Ga:Zn=3:1:2 has an energy gap of approximately 2.8 eV and an electron affinity of approximately 5.0 eV. 
     Since the insulating layer  272  and the insulating layer  226  are insulators, Ec 382  and Ec 386  are closer to the vacuum level (have smaller electron affinity) than Ec 383   a , Ec 383   b , and Ec 383   c.    
     Furthermore, Ec 383   a  is closer to the vacuum level than Ec 383   b . Specifically, Ec 383   a  is preferably located closer to the vacuum level than Ec 383   b  by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     Furthermore, Ec 383   c  is closer to the vacuum level than Ec 383   b . Specifically, Ec 383   c  is preferably located closer to the vacuum level than Ec 383   b  by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     In the vicinity of an interface between the semiconductor layer  242   a  and the semiconductor layer  242   b  and the vicinity of an interface between the semiconductor layer  242   b  and the semiconductor layer  242   c , mixed regions are formed; thus, the energy of the conduction band minimum continuously changes. In other words, no state or few states exist at these interfaces. 
     Accordingly, electrons transfer mainly through the semiconductor layer  242   b  in the layered structure having the above energy band structure. Therefore, even when states exist at the interface between the semiconductor layer  242   a  and the insulating layer  272  or at the interface between the semiconductor layer  242   c  and the insulating layer  226 , the states hardly influence transfer of electrons. In addition, states do not exist or hardly exist at the interface between the semiconductor layer  242   a  and the semiconductor layer  242   b  and at the interface between the semiconductor layer  242   c  and the semiconductor layer  242   b ; thus, transfer of electrons are not prohibited in the regions. Accordingly, high field-effect mobility can be obtained in the transistor having the above layered structure of the oxide semiconductors. 
     Note that although trap states  390  due to impurities or defects might be formed in the vicinity of the interface between the semiconductor layer  242   a  and the insulating layer  272  and in the vicinity of the interface between the semiconductor layer  242   c  and the insulating layer  226  as shown in  FIG. 20A , the semiconductor layer  242   b  can be apart from the trap states owing to the existence of the semiconductor layer  242   a  and the semiconductor layer  242   c.    
     In particular, in the transistor  450  described in this embodiment, an upper surface and a side surface of the semiconductor layer  242   b  are in contact with the semiconductor layer  242   c , and a bottom surface of the semiconductor layer  242   b  is in contact with the semiconductor layer  242   a . In this manner, the semiconductor layer  242   b  is surrounded by the semiconductor layer  242   a  and the semiconductor layer  242   c , so that the influence of the trap state can be further reduced. 
     However, in the case where an energy difference between Ec 383   a  or Ec 383   c  and Ec 383   b  is small, electrons in the semiconductor layer  242   b  might reach the trap states by passing over the energy gap. The electrons are trapped by the trap states, which generates a negative fixed charge at the interface with the insulating layer, causing the threshold voltage of the transistor to be shifted in a positive direction. 
     Therefore, each of the energy differences between Ec 383   a  and Ec 383   b  and between Ec 383   c  and Ec 383   b  is preferably set to be larger than or equal to 0.1 eV, more preferably larger than or equal to 0.15 eV because a change in the threshold voltage of the transistor can be reduced and the transistor can have favorable electrical characteristics. 
     Each of the band gaps of the semiconductor layers  242   a  and  242   c  is preferably larger than that of the semiconductor layer  242   b.    
       FIG. 20B  is the energy band structure diagram of a portion along dashed-dotted line D 3 -D 4  in  FIG. 14B .  FIG. 20B  shows the energy band structure of a channel formation region of the transistor  422 . 
     In  FIG. 20B , Ec 387  represents the energy of the conduction band minimum of the insulating layer  228 . The semiconductor layer  242  is formed using two layers, the semiconductor layers  242   a  and  242   b ; thus, the transistor can be manufactured with improved productivity. Since the semiconductor layer  242   c  is not provided, the transistor including the two semiconductor layers is easily affected by the trap states  390  but can have higher field-effect mobility than a transistor including one semiconductor layer as the semiconductor layer  242 . 
     According to one embodiment of the present invention, a transistor with a small variation in electrical characteristics can be provided. Accordingly, a semiconductor device with a small variation in electrical characteristics can be provided. According to one embodiment of the present invention, a transistor with high reliability can be provided. Accordingly, a semiconductor device with high reliability can be provided. 
     Note that an oxide semiconductor has an energy gap as wide as 2 eV or more and high visible-light transmittance. In a transistor obtained by processing an oxide semiconductor under appropriate conditions, the off-state current at ambient temperature (e.g., 25° C.) can be less than or equal to 100 zA (1×10 −19  A), less than or equal to 10 zA (1×10 −20  A), or less than or equal to 1 zA (1×10 −21  A). Therefore, a low-power semiconductor device can be provided. 
     According to one embodiment of the present invention, a low-power transistor can be provided. Accordingly, a display element or a semiconductor device such as a display device with low power consumption can be provided. Moreover, a display element or a semiconductor device such as a display device with high reliability can be provided. 
     The transistor  450  in  FIGS. 13A to 13C  is described again. The semiconductor layer  242   b  is provided over a projecting portion of the insulating layer  272 , so that the electrode  243  can also cover a side surface of the semiconductor layer  242   b . Thus, the transistor  450  has a structure in which the semiconductor layer  242   b  can be electrically surrounded by the electric field of the electrode  243 . The structure of a transistor in which a semiconductor layer including a channel is electrically surrounded by the electric field of a conductive film in this manner is called a surrounded channel (s-channel) structure. A transistor having an s-channel structure is referred to as an s-channel transistor. 
     In the s-channel structure, a channel can be formed in the whole (bulk) of the semiconductor layer  242   b . In the s-channel structure, the drain current of the transistor is increased, so that higher on-state current can be obtained. Furthermore, the entire channel formation region of the semiconductor layer  242   b  can be depleted by the electric field of the electrode  243 . Accordingly, the off-state current of the transistor with an s-channel structure can be further reduced. 
     When the projecting portion of the insulating layer  272  is increased in height and the channel width is shortened, the effects of the s-channel structure to increase the on-state current and reduce the off-state current can be enhanced. Part of the semiconductor layer  242   a  exposed in the formation of the semiconductor layer  242   b  may be removed. In that case, the side surfaces of the semiconductor layer  242   a  and  242   b  may be aligned to each other. 
     As in a transistor  451  in  FIGS. 15A to 15C , the electrode  223  may be provided below the semiconductor layer  242  with an insulating layer positioned therebetween.  FIG. 15A  is a top view of the transistor  451 .  FIG. 15B  is a cross-sectional view taken along dashed-dotted line X 1 -X 2  in  FIG. 15A .  FIG. 15C  is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2  in  FIG. 15A . 
     As in a transistor  452  in  FIGS. 16A to 16C , an insulating layer  275  may be provided above the electrode  243  and a layer  214  may be provided over the insulating layer  275 .  FIG. 16A  is a top view of the transistor  452 .  FIG. 16B  is a cross-sectional view taken along dashed-dotted line X 1 -X 2  in  FIG. 16A .  FIG. 16C  is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2  in  FIG. 16A . 
     Although the layer  214  is provided over the insulating layer  275  in  FIGS. 16A to 16C , the layer  214  may be provided over the insulating layer  228  or  229 . When the layer  214  is formed using a light-blocking material, the change in characteristics or decrease in reliability of the transistor that is caused by light irradiation can be prevented. When the layer  214  is formed at least larger than the semiconductor layer  242   b  such that the semiconductor layer  242   b  is covered with the layer  214 , the above effects can be enhanced. The layer  214  can be formed using an organic material, an inorganic material, or a metal material. In the case where the layer  214  is formed using a conductive material, the layer  214  may be supplied with voltage or may be set to an electrically floating state. 
       FIGS. 17A to 17C  illustrate an example of a transistor with an s-channel structure. A transistor  448  illustrated in  FIGS. 17A to 17C  has almost the same structure as the transistor  447 . In the transistor  448 , the semiconductor layer  242  is formed over the projecting portion of the insulating layer  272 . The transistor  448  is a kind of top-gate transistor having a back gate electrode.  FIG. 17A  is a top view of the transistor  448 .  FIG. 17B  is a cross-sectional view taken along dashed-dotted line X 1 -X 2  in  FIG. 17A .  FIG. 17C  is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2  in  FIG. 17A . 
       FIGS. 17A to 17C  illustrate an example in which an inorganic semiconductor layer such as a silicon layer is used as the semiconductor layer  242  in the transistor  448 . In  FIGS. 17A to 17C , the semiconductor layer  242  includes a semiconductor layer  242   i  in a region overlapping with the gate electrode  246 , two semiconductor layers  242   t , and two semiconductor layers  242   u . The semiconductor layer  242   i  is positioned between the two semiconductor layers  242   t . The semiconductor layer  242   i  and the two semiconductor layers  242   t  are positioned between the two semiconductor layers  242   u.    
     A channel is formed in the semiconductor layer  242   i  when the transistor  448  is on. Therefore, the semiconductor layer  242   i  serves as a channel formation region. The semiconductor layers  242   t  serve as low concentration impurity regions (i.e., LDD). The semiconductor layers  242   u  serve as high concentration impurity regions. Note that one or both of the two semiconductor layers  242   t  are not necessarily provided. One of the two semiconductor layers  242   u  serves as a source region, and the other semiconductor layer  242   u  serves as a drain region. 
     The electrode  244   a  provided over the insulating layer  229  is electrically connected to one of the semiconductor layers  242   u  in an opening  247   c  formed in the insulating layers  226 ,  228 , and  229 . The electrode  244   b  provided over the insulating layer  229  is electrically connected to the other of the semiconductor layers  242   u  in an opening  247   d  formed in the insulating layers  226 ,  228 , and  229 . 
     The electrode  243  provided over the insulating layer  226  is electrically connected to the electrode  223  in an opening  247   a  and an opening  247   b  formed in the insulating layers  226  and  272 . Accordingly, the same potential is applied to the electrodes  223  and  243 . Either or both of the openings  247   a  and  247   b  may be omitted. In the case where neither the opening  247   a  nor the opening  247   b  are provided, different potentials can be applied to the electrodes  223  and  243 . 
       FIGS. 18A and 18B  illustrate another example of an s-channel transistor.  FIG. 18A  is a plan view of a transistor  473 .  FIG. 18B  illustrates a cross-sectional view taken along dashed-dotted line L 1 -L 2  and a cross-sectional view taken along dashed-dotted line W 1 -W 2  in  FIG. 18A . In  FIG. 18B , the cross-sectional view along L 1 -L 2  is taken in the channel length direction of the transistor  473  and the cross-sectional view along W 1 -W 2  is taken in the channel width direction of the transistor  473 . 
     The transistor  473  includes the semiconductor layer  242 , the insulating layer  226 , the electrode  246 , and the electrodes  244   a  and  244   b . The electrode  246  can function as a gate electrode. The insulating layer  226  can function as a gate insulating layer. The electrode  244   a  can function as one of a source electrode and a drain electrode. The electrode  244   b  can function as the other of the source electrode and the drain electrode. The transistor  473  is provided over the substrate  271  with the insulating layer  272  and an insulating layer  273  positioned therebetween. 
     In  FIG. 18B , the insulating layer  273  is provided over the substrate  271 , and the insulating layer  272  is provided over the insulating layer  273 . The insulating layer  272  has a projecting portion, and island-shaped semiconductor layers  242   a  and  242   b  are formed over the projecting portion. The electrodes  244   a  and  244   b  are provided over the semiconductor layer  242   b . A region of the semiconductor layer  242   b  that overlaps with the electrode  244   a  can function as one of a source and a drain of the transistor  473 . A region of the semiconductor layer  242   b  that overlaps with the electrode  244   b  can function as the other of the source and the drain of the transistor  473 . Thus, a region  269  of the semiconductor layer  242   b  that is located between the electrodes  244   a  and  244   b  can function as a channel formation region. 
     An oxide semiconductor layer  274  is provided over the electrodes  244   a  and  244   b , and an insulating layer  275  is provided over the oxide semiconductor layer  274 . An opening is provided in regions of the oxide semiconductor layer  274  and the insulating layer  275  that overlap with the region  269 , and the semiconductor layer  242   c  is provided along the side and bottom surfaces of the opening. In the opening, the insulating layer  226  is provided along the side and bottom surfaces of the opening with the oxide semiconductor layer  242   c  positioned therebetween. In the opening, the electrode  246  is provided along the side and bottom surfaces of the opening with the semiconductor layer  242   c  and the insulating layer  226  positioned therebetween. 
     Note that the opening is wider than the semiconductor layer  242   a  and the semiconductor layer  242   b  in the cross section in the channel width direction. Accordingly, the side surfaces of the semiconductor layer  242   a  and the semiconductor layer  242   b  in the region  269  are covered with the semiconductor layer  242   c . The side surfaces of the semiconductor layer  242   a  and the semiconductor layer  242   b  in regions other than the region  269  are covered with the oxide semiconductor layer  274 . 
     An insulating layer  276  is provided over the insulating layer  275 , and an insulating layer  277  is provided over the insulating layer  276 . Electrodes  289   a ,  289   b , and  289   c  are provided over the insulating layer  277 . The electrode  289   a  is electrically connected to the electrode  244   a  through a contact plug  288   a  in an opening obtained by removing parts of the insulating layer  277 , the insulating layer  276 , the insulating layer  275 , and the oxide semiconductor layer  274 . The electrode  289   b  is electrically connected to the electrode  244   b  through a contact plug  288   b  in an opening obtained by removing parts of the insulating layer  277 , the insulating layer  276 , the insulating layer  275 , and the oxide semiconductor layer  274 . The electrode  289   c  is electrically connected to the electrode  246  through a contact plug  288   c  in an opening obtained by removing parts of the insulating layer  277  and the insulating layer  276 . 
     As illustrated in  FIG. 18B , in the transistor  473  in the channel width direction, the electrode  246  covers the semiconductor layer  242   b . When the insulating layer  272  has the projecting portion, the side surfaces of the semiconductor layer  242   b  can be covered with the electrode  246 . 
     A transistor  474  in  FIGS. 19A and 19B  is different from the transistor  473  in that the electrode  223  functioning as a back gate electrode is provided between the insulating layers  272  and  273 .  FIG. 19A  is a plan view of the transistor  474 .  FIG. 19B  illustrates a cross-sectional view taken along dashed-dotted line L 1 -L 2  and a cross-sectional view taken along dashed-dotted line W 1 -W 2  in  FIG. 19A . Note that the electrode  223  may be provided between the substrate  271  and the insulating layer  273 . 
     The electrode  246  and the electrode  223  can each function as a gate electrode. Thus, the insulating layers  272  and  226  can each function as a gate insulating layer. 
     By providing the electrode  246  and the electrode  223  with the semiconductor layer  242  positioned therebetween and setting the potentials of the electrode  246  and the electrode  223  to be the same, a region of the semiconductor layer  242  through which carriers flow is enlarged in the film thickness direction; thus, the amount of transferred carriers is increased. As a result, the on-state current and field-effect mobility of the transistor  474  are increased. 
     Furthermore, for example, as illustrated in  FIG. 19C , the insulating layer  281  may be formed over the electrode  223 , the insulating layer  282  may be formed over the insulating layer  281 , and the insulating layer  272  may be formed over the insulating layer  282 . The insulating layers  281  and  282  can be formed using a material and a method similar to those of the insulating layer  272 . 
     Note that when the insulating layer  282  is formed using hafnium oxide, aluminum oxide, tantalum oxide, aluminum silicate, or the like, the insulating layer  282  can function as a charge trap layer. The threshold voltage of the transistor can be changed by injecting electrons into the insulating layer  282 . For example, electrons may be injected into the insulating layer  282  with the use of the tunnel effect. By applying positive voltage to the electrode  223 , tunnel electrons can be injected into the insulating layer  282 . 
     &lt;Deposition Method&gt; 
     Conductive layers, insulating layers, and semiconductor layers used for electrodes and the like described in this specification and the like can be formed by CVD, vapor deposition, sputtering, or the like. CVD can be typically classified into plasma-enhanced CVD (PECVD) using plasma, thermal CVD (TCVD) using heat, and the like. CVD can be further classified into metal CVD (MCVD) and metal organic CVD (MOCVD) according to a source gas to be used. 
     Furthermore, the vapor deposition can be typically classified into resistance heating vapor deposition, electron beam vapor deposition, molecular beam epitaxy (MBE), pulsed laser deposition (PLD), ion beam assisted deposition (IAD), atomic layer deposition (ALD), and the like. 
     A high-quality film can be formed by PECVD at a relatively low temperature. By using a deposition method that does not use plasma for deposition, such as MOCVD or vapor deposition, a film can be formed with few defects because damage is not easily caused on a surface on which the film is formed. 
     Sputtering can be typically classified into DC sputtering, magnetron sputtering, RF sputtering, ion beam sputtering, electron cyclotron resonance (ECR) sputtering, facing-target sputtering, and the like. 
     In facing-target sputtering, plasma is confined between targets; thus, plasma damage to a substrate can be reduced. Furthermore, step coverage can be improved because the incident angle of a sputtered particle to the substrate can be made smaller depending on the inclination of the target. 
     Unlike in a deposition method in which particles ejected from a target or the like are deposited, in CVD and ALD, a film is formed by reaction at a surface of an object. Thus, CVD and ALD enable favorable step coverage almost regardless of the shape of an object. In particular, ALD enables excellent step coverage and excellent thickness uniformity and can be favorably used for covering a surface of an opening portion with a high aspect ratio, for example. On the other hand, ALD has a relatively low deposition rate; thus, it is sometimes preferable to combine ALD with another deposition method with a high deposition rate, such as CVD. 
     When CVD or ALD is used, the composition of a film to be formed can be controlled with a flow rate ratio of source gases. For example, by CVD or ALD, a film with a certain composition can be formed depending on a flow rate ratio of the source gases. Moreover, with CVD or ALD, by changing the flow rate ratio of the source gases while forming the film, a film whose composition is continuously changed can be formed. In the case where the film is formed while changing the flow rate ratio of the source gases, as compared to the case where the film is formed using a plurality of deposition chambers, time taken for the film formation can be reduced because time taken for transfer and pressure adjustment is omitted. Thus, transistors or semiconductor devices can be manufactured with improved productivity. 
     &lt;Substrate&gt; 
     There is no significant limitation on a material used for the substrate  271 . The material may be determined depending on the purpose; for example, whether it has a light-transmitting property or heat resistance that can withstand heat treatment is taken into consideration for the determination. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. Alternatively, a semiconductor substrate, a flexible substrate, an attachment film, a base film, or the like may be used as the substrate  271 . 
     As the semiconductor substrate, a semiconductor substrate of silicon, germanium, or the like or a compound semiconductor substrate of silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, or gallium oxide, or the like is used, for example. The semiconductor substrate may be a single-crystal semiconductor substrate or a polycrystalline semiconductor substrate. 
     As materials of the flexible substrate, the attachment film, the base material film, and the like, the following materials can be used, for example: polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), polytetrafluoroethylene (PTFE), polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, aramid, an epoxy resin, and an acrylic resin. 
     The flexible substrate used as the substrate  271  preferably has a lower coefficient of linear expansion because a lower coefficient of linear expansion suppresses deformation due to an environment. The flexible substrate used as the substrate  271  is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10 −3 /K, lower than or equal to 5×10 −5 /K, or lower than or equal to 1×10 −5 /K. In particular, aramid is preferably used for the flexible substrate because of its low coefficient of linear expansion. 
     &lt;Insulating Layer&gt; 
     The insulating layers  272 ,  226 ,  225 ,  228 , and  229  can be formed using a single layer or a stack of layers of one or more materials selected from aluminum nitride, aluminum oxide, aluminum nitride oxide, aluminum oxynitride, magnesium oxide, silicon nitride, silicon oxide, silicon nitride oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, aluminum silicate, or the like. Alternatively, a material in which two or more materials selected from an oxide material, a nitride material, an oxynitride material, and a nitride oxide material are mixed may be used. 
     Note that in this specification, a nitride oxide refers to a compound that includes more nitrogen than oxygen. An oxynitride refers to a compound that includes more oxygen than nitrogen. The content of each element can be measured by Rutherford backscattering spectrometry (RBS), for example. 
     It is particularly preferable that the insulating layers  272  and  229  be formed using an insulating material that is relatively impermeable to impurities. The insulating layers  272  and  229  may each be formed using, for example, a single layer or a stack of layers of an insulating material containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. Examples of such an insulating material that is relatively impermeable to impurities include aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, and silicon nitride. The insulating layer  272  or  229  may be formed using indium tin zinc oxide (In—Sn—Zn oxide) having an excellent insulating property, or the like. 
     When the insulating material that is relatively impermeable to impurities is used for the insulating layer  272 , impurity diffusion from the substrate  271  side can be suppressed, and the reliability of the transistor can be improved. When the insulating material that is relatively impermeable to impurities is used for the insulating layer  229 , impurity diffusion from the insulating layer  229  side can be suppressed, and the reliability of the transistor can be improved. 
     A plurality of insulating layers formed using such materials may be stacked to form any of the insulating layers  272 ,  226 ,  225 ,  228 , and  229 . The formation method of the insulating layers  272 ,  226 ,  225 ,  228 , and  229  is not particularly limited, and a formation method such as sputtering, CVD, MBE, PLD, ALD, or spin coating can be used. 
     For example, in the case where aluminum oxide is formed by thermal CVD, two kinds of gases, e.g., H 2 O as an oxidizer and a source material gas which is obtained by vaporizing a solvent and liquid containing an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH 3 ) 3 . Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate). 
     When an oxide semiconductor is used for the semiconductor layer  242 , the hydrogen concentration in the insulating layers is preferably lowered in order to prevent an increase in hydrogen concentration in the semiconductor layer  242 . It is particularly preferable to lower the hydrogen concentration of the insulating layer in contact with the semiconductor layer  242 . Specifically, the hydrogen concentration in the insulating layer that is measured by SIMS is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , 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 . Furthermore, the nitrogen concentration in the insulating layers is preferably lowered in order to prevent an increase in the nitrogen concentration in the semiconductor layer  242 . It is particularly preferable to lower the nitrogen concentration of the insulating layer in contact with the semiconductor layer  242 . Specifically, the nitrogen concentration in the insulating layer that is 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 . 
     The concentration measured by SIMS analysis might include a variation within a range of ±40%. 
     When an oxide semiconductor is used for the semiconductor layer  242 , the insulating layers are preferably formed using insulating layers from which oxygen is released by heating. It is particularly preferable that the insulating layer in contact with the semiconductor layer  242  be an insulating layer from which oxygen is released by heating. For example, the amount of oxygen released from the insulating layer that is converted into oxygen atoms is preferably greater than or equal to 1.0×10 18  atoms/cm 3 , more preferably greater than or equal to 1.0×10 19  atoms/cm 3 , still more preferably greater than or equal to 1.0×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS) analysis. In the TDS analysis, heat treatment is performed so that the surface temperature of the insulating layer is higher than or equal to 100° C. and lower than or equal to 700° C., preferably higher than or equal to 100° C. and lower than or equal to 500° C. Note that in this specification and the like, oxygen released by heating is also referred to as excess oxygen. 
     The insulating layer containing excess oxygen can be formed by performing treatment for adding oxygen to an insulating layer. The treatment for adding oxygen can be performed by heat treatment in an oxygen atmosphere or can be performed with an ion implantation apparatus, an ion doping apparatus, or a plasma treatment apparatus. As a gas for adding oxygen, an oxygen gas of  16 O 2 ,  18 O 2 , or the like, a nitrous oxide gas, an ozone gas, or the like can be used. In this specification, the treatment for adding oxygen is also referred to as oxygen doping treatment. 
     The formation of an insulating layer by sputtering in an atmosphere including oxygen enables introduction of oxygen into the insulating layer. 
     In general, a capacitor has a structure in which a dielectric is sandwiched between two electrodes that face each other; as the thickness of the dielectric becomes smaller (as the distance between the two facing electrodes becomes shorter) or as the dielectric constant of the dielectric becomes higher, capacitance becomes higher. However, if the thickness of the dielectric is reduced in order to increase the capacitance of the capacitor, because of a tunnel effect or the like, current unintentionally flowing between the two electrodes (leakage current) tends to increase and the withstand voltage of the capacitor tends to be lower. 
     A portion where a gate electrode, a gate insulating layer, and a semiconductor layer of a transistor overlap with each other functions as a capacitor (hereinafter also referred to as a gate capacitor). A channel is formed in a region of the semiconductor layer that overlaps with the gate electrode with the gate insulating layer provided therebetween. That is, the gate electrode and a channel formation region function as two electrodes of the capacitor. Furthermore, the gate insulating layer functions as a dielectric of the capacitor. Although it is preferable that the capacitance of the gate capacitor be as high as possible, a reduction in the thickness of the gate insulating layer for increasing the capacitance increases the probability of occurrence of an increase in leakage current or a reduction in withstand voltage. 
     In the case where a high-k material such as hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate to which nitrogen is added (HfSi x O y N z  (x&gt;0, y&gt;0, z&gt;0)), hafnium aluminate to which nitrogen is added (HfAl x O y N z  (x&gt;0, y&gt;0, z&gt;0)), hafnium oxide, or yttrium oxide is used as a dielectric, even if the thickness of the dielectric is made thick, sufficient capacitance of the capacitor can be ensured. 
     For example, in the case where a high-k material with a high dielectric constant is used as the dielectric, even when the dielectric is made thick, a capacitance equivalent to that in the case of using silicon oxide as the dielectric can be obtained. This enables a reduction in leakage current between the two electrodes of the capacitor. The dielectric may have a layered structure of the high-k material and another insulating material. 
     The insulating layer  275  has a flat surface. As the insulating layer  275 , an organic material having heat resistance, such as polyimide, an acrylic-based resin, a benzocyclobutene-based resin, polyamide, or an epoxy-based resin, as well as the above insulating materials can be used. Other than such organic materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. Note that a plurality of insulating layers formed using these materials may be stacked to form the insulating layer  275 . 
     Note that the siloxane-based resin corresponds to a resin including an Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may include an organic group (e.g., an alkyl group or an aryl group) or a fluoro group as a substituent. The organic group may include a fluoro group. 
     There is no particular limitation on the method for forming the insulating layer  275 ; depending on a material thereof, any of the following methods may be used: sputtering, an SOG method, spin coating, dipping, spray coating, a droplet discharging method (e.g., an ink-jet method), a printing method (e.g., screen printing or offset printing), or the like. 
     The sample surface may be subjected to CMP treatment. The CMP treatment can reduce unevenness of the surface, so that coverage with an insulating layer or a conductive layer to be formed later can be increased. 
     &lt;Semiconductor Layer&gt; 
     A single-crystal semiconductor, a polycrystalline semiconductor, a microcrystalline semiconductor, an amorphous semiconductor, or the like can be used for the semiconductor layer  242 . As a semiconductor material, silicon, germanium, or the like can be used. Alternatively, a compound semiconductor of silicon germanium, silicon carbide, gallium arsenide, an oxide semiconductor, a nitride semiconductor, or the like; an organic semiconductor; or the like can be used. 
     In the case of using an organic semiconductor for the semiconductor layer  242 , a low molecular organic material having an aromatic ring, a π-electron conjugated conductive polymer, or the like can be used. For example, rubrene, tetracene, pentacene, perylenediimide, tetracyanoquinodimethane, polythiophene, polyacetylene, or polyparaphenylene vinylene can be used. 
     As described above, the band gap of an oxide semiconductor is 2 eV or wider; thus, when the oxide semiconductor is used for the semiconductor layer  242 , a transistor with an extremely low off-state current can be provided. An OS transistor has high withstand voltage between its source and drain. Thus, a transistor with high reliability can be provided. Furthermore, a display device, a semiconductor device, or the like with high reliability can be provided. 
     In this embodiment, the case where an oxide semiconductor is used for the semiconductor layer  242  is described. For the oxide semiconductor used for the semiconductor layer  242 , an oxide semiconductor containing, for example, indium (In) is preferably used. An oxide semiconductor can have a high carrier mobility (electron mobility) by containing indium, for example. An oxide semiconductor 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, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like. Note that two or more of the above elements may be used in combination as the element M. The element M has a high bonding energy with oxygen, for example. The element M increases the energy gap of the oxide semiconductor, for example. Furthermore, the oxide semiconductor preferably contains zinc. When the oxide semiconductor contains zinc, the oxide semiconductor is easily crystallized in some cases. 
     Note that the oxide semiconductor used for the semiconductor layer  242  is not limited to the oxide containing indium. The oxide semiconductor may be, for example, an oxide which does not contain indium and contains zinc, an oxide which does not contain indium and contains gallium, or an oxide which does not contain indium and contains tin, e.g., a zinc tin oxide, a gallium tin oxide, or gallium oxide. 
     For example, in the case where an InGaZnO X  (X&gt;0) film is formed as the semiconductor layer  242  by thermal CVD, trimethylindium (In(CH 3 ) 3 ), trimethylgallium (Ga(CH 3 ) 3 ), and dimethylzinc (Zn(CH 3 ) 2 ) are used. Without limitation to the above combination, triethylgallium (Ga(C 2 H 5 ) 3 ) can be used instead of trimethylgallium, and diethylzinc (Zn(C 2 H 5 ) 2 ) can be used instead of dimethylzinc. 
     For example, in the case where an InGaZnO X  film (X&gt;0) is formed as the semiconductor layer  242  by ALD, an In(CH 3 ) 3  gas and an O 3  gas are sequentially introduced more than once to form an InO 2  layer, subsequently a Ga(CH 3 ) 3  gas and an O 3  gas are sequentially introduced more than once to form a GaO layer, and then a Zn(CH 3 ) 2  gas and an O 3  gas are sequentially introduced more than once to form a ZnO layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an InGaO 2  layer, an InZnO 2  layer, a GaInO layer, a ZnInO layer, or a GaZnO layer may be formed by using these gases. Note that although an H 2 O gas which is obtained by bubbling water with an inert gas such as Ar may be used instead of an O 3  gas, it is preferable to use an O 3  gas, which does not contain H. Furthermore, instead of an In(CH 3 ) 3  gas, an In(C 2 H 5 ) 3  gas or tris(acetylacetonato)indium may be used. Note that tris(acetylacetonato)indium is also referred to as In(acac) 3 . In addition, instead of a Ga(CH 3 ) 3  gas, a Ga(C 2 H 5 ) 3  gas or tris(acetylacetonato)gallium may be used. Note that tris(acetylacetonato)gallium is also referred to as Ga(acac) 3 . Furthermore, a Zn(CH 3 ) 2  gas or zinc acetate may be used. However, the deposition gas is not limited to these. 
     In the case where the oxide semiconductor is formed by sputtering, a target containing indium is preferably used in order to reduce the number of particles. In addition, if an oxide target having a high atomic ratio of the element M is used, the conductivity of the target might be decreased. In the case where a target containing indium is used, the conductivity of the target can be increased and DC discharge or AC discharge is facilitated; thus, deposition over a large substrate can be easily performed. Thus, semiconductor devices can be manufactured with improved productivity. 
     As described above, in the case where the oxide semiconductor is formed by sputtering, the atomic ratio of In to M and Zn contained in the target may be 3:1:1, 3:1:2, 3:1:4, 1:1:0.5, 1:1:1, 1:1:2, 1:4:4, 5:1:7, or 4:2:4.1, for example. 
     When the oxide semiconductor is formed by sputtering, an oxide semiconductor having an atomic ratio different from the atomic ratio of the target might be formed. In particular, the atomic ratio of zinc in the deposited film is smaller than the atomic ratio in the target in some cases. Specifically, the proportion of zinc in the film is approximately 40 to 90 atomic % of that of zinc in the target in some cases. 
     In order to give stable electrical characteristics to the OS transistor, it is preferable that impurities and oxygen vacancies in the oxide semiconductor layer be reduced to highly purify the oxide semiconductor layer so that the semiconductor layer  242  can be regarded as a highly-purified intrinsic or substantially highly-purified intrinsic oxide semiconductor layer. Furthermore, it is preferable that at least the channel formation region in the semiconductor layer  242  can be regarded as a highly-purified intrinsic or substantially highly-purified intrinsic oxide semiconductor layer. 
     When an oxide semiconductor is used for the semiconductor layer  242 , a c-axis aligned crystalline oxide semiconductor (CAAC-OS) is preferably used. The CAAC-OS is an oxide semiconductor having a plurality of c-axis aligned crystal parts. 
     In the oxide semiconductor layer used as the semiconductor layer  242 , a region where CAAC is not formed preferably accounts for less than 20% of the whole oxide semiconductor layer. 
     The CAAC-OS has dielectric anisotropy. Specifically, the CAAC-OS has a higher dielectric constant in the c-axis direction than in the a-axis direction and the b-axis direction. In a transistor in which a CAAC-OS is used for a semiconductor layer where a channel is formed and a gate electrode is positioned in the c-axis direction, the dielectric constant in the c-axis direction is high; thus, an electric field generated from the gate electrode easily reaches the entire CAAC-OS. The subthreshold swing value (S value) can be made small. In addition, in the transistor in which a CAAC-OS is used for the semiconductor layer, an increase in the S value due to miniaturization is less likely to occur. 
     Moreover, since the dielectric constant in the a-axis direction and the b-axis direction of an CAAC-OS is low, the influence of an electric field generated between a source and a drain is reduced. Thus, a channel length modulation effect, a short-channel effect, or the like is less likely to occur, and the reliability of the transistor can be increased. 
     Here, the channel length modulation effect is a phenomenon in which, when the drain voltage is higher than the threshold voltage, a depletion layer expands from the drain side, so that the effective channel length is decreased. The short-channel effect is a phenomenon in which a short channel length leads to deterioration in electrical characteristics, such as a decrease in threshold voltage. The more a transistor is miniaturized, the more deterioration in electrical characteristics caused by the phenomena is likely to occur. 
     Note that after the oxide semiconductor layer is formed, oxygen doping treatment may be performed. In order to further decrease impurities such as water or hydrogen in the oxide semiconductor layer to highly purify the oxide semiconductor layer, heat treatment is preferably performed. 
     For example, the oxide semiconductor layer is subjected to heat treatment in a reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a rare gas, or the like, an oxidizing atmosphere, or ultra-dry air (the moisture amount is 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, more preferably 10 ppb or less, in the case where measurement is performed by a dew point meter in a cavity ring down laser spectroscopy (CRDS) system). Note that the oxidizing atmosphere refers to an atmosphere including an oxidizing gas such as oxygen, ozone, or nitrogen oxide at 10 ppm or more. The inert gas atmosphere refers to an atmosphere including the oxidizing gas at less than 10 ppm and being filled with nitrogen or a rare gas. 
     By the heat treatment, at the same time as the release of the impurities, oxygen contained in the insulating layer  226  is diffused into the oxide semiconductor layer and oxygen vacancies in the oxide semiconductor layer can be reduced. Note that the heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more in order to compensate for desorbed oxygen. The heat treatment may be performed at any time after the oxide semiconductor layer is formed. 
     There is no particular limitation on a heat treatment apparatus used for the heat treatment, and the apparatus may be an apparatus that heats an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an electric furnace, or a rapid thermal annealing (RTA) apparatus such as a lamp rapid thermal annealing (LRTA) apparatus or a gas rapid thermal annealing (GRTA) apparatus can be used. The LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. The GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. 
     The heat treatment is performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C. The treatment time is shorter than or equal to 24 hours. Heat treatment for over 24 hours is not preferable because productivity is reduced. 
     &lt;Electrode&gt; 
     As a conductive material for forming the electrodes  246 ,  223 ,  244   a ,  244   b ,  287 ,  297 ,  289   a ,  289   b ,  292   a , and  292   b , a material containing one or more metal elements selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium, or the like can be used. Alternatively, a semiconductor having a high electric conductivity typified by polycrystalline silicon including an impurity element such as phosphorus, or silicide such as nickel silicide may be used. A plurality of stacked conductive layers formed using these materials may be used as the electrode. 
     The conductive material for forming the electrodes  246 ,  223 ,  244   a ,  244   b ,  287 ,  297 ,  289   a ,  289   b ,  292   a , and  292   b  can also be formed using a conductive material containing oxygen, such as indium tin oxide (ITO), indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, indium gallium zinc oxide, or indium tin oxide to which silicon is added, or a conductive material containing nitrogen, such as titanium nitride or tantalum nitride. It is also possible to use a layered structure formed using a material containing the above metal element and a conductive material containing oxygen. It is also possible to use a layered structure formed using a material containing the above metal element and a conductive material containing nitrogen. It is also possible to use a layered structure formed using a material containing the above metal element, a conductive material containing oxygen, and a conductive material containing nitrogen. There is no particular limitation on the formation method of the conductive material, and any of a variety of formation methods such as an evaporation method, a CVD method, a sputtering method, and a spin coating method can be employed. 
     &lt;Contact Plug&gt; 
     A conductive material with high embeddability, such as tungsten or polysilicon, can be used for the contact plugs  288   a ,  288   b ,  288   c ,  298   a , and  298   b , for example. A side surface and a bottom surface of the material may be covered with a barrier layer (a diffusion prevention layer) of a titanium layer, a titanium nitride layer, or a stacked layer of these layers. In that case, the barrier layer is regarded as part of the contact plug in some cases. 
     According to one embodiment of the present invention, a transistor with favorable electrical characteristics can be provided. Furthermore, according to one embodiment of the present invention, a highly integrated semiconductor device can be provided. 
     This embodiment can be combined with any of the structures described in the other embodiments as appropriate. 
     (Embodiment 3) 
     In this embodiment, examples in which the semiconductor device described in the above embodiment is used in an electronic component and examples of an electronic device including the electronic component are described with reference to  FIGS. 21A and 21B  and  FIGS. 22A and 22B . Note that the electronic component is also referred to as a semiconductor package or an IC package. The 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 is completed when the semiconductor device described in any of the above embodiments is combined with components other than the semiconductor device in an assembly process (post-process). 
     The post-process is described with reference to a flow chart in  FIG. 21A . After an element substrate including the semiconductor device described in any of the above embodiments is completed in a pre-process, a back surface grinding step in which a back surface (a surface where a semiconductor device and the like are not formed) of the element substrate is ground is performed (Step S 1 ). When the element substrate is thinned by grinding, warpage or the like of the element substrate is reduced, so that the size of the electronic component can be reduced. 
     Next, the element substrate is divided into a plurality of chips in a dicing step (Step S 2 ). Then, the divided chips are separately picked up to be bonded to a lead frame in a die bonding step (Step S 3 ). To bond a chip and a lead frame in the die bonding step, a method such as resin bonding or tape-automated bonding is selected as appropriate depending on products. Note that the chip may be bonded to an interposer instead of the lead frame. 
     Next, a wire bonding step for electrically connecting a lead of the lead frame and an electrode on the chip through a metal wire is performed (Step S 4 ). As the metal wire, a silver wire or a gold wire can be used. Ball bonding or wedge bonding can be used as the wire bonding. 
     The wire-bonded chip is subjected to a molding step of sealing the chip with an epoxy resin or the like (Step S 5 ). Through the molding step, the inside of the electronic component is filled with a resin, so that a circuit portion incorporated in the chip and wire for connecting the chip to the lead can be protected from external mechanical force, and deterioration of characteristics (decrease in reliability) due to moisture or dust can be reduced. 
     Subsequently, the lead of the lead frame is plated in a lead plating step (Step S 6 ). This plating process prevents rust of the lead and facilitates soldering at the time of mounting the chip on a printed wiring board in a later step. Then, the lead is cut and processed in a formation step (Step S 7 ). 
     Next, a printing (marking) step is performed on a surface of the package (Step S 8 ). After a testing step (Step S 9 ) for checking whether an external shape is good and whether there is malfunction, for example, the electronic component is completed. 
     The above electronic component can include the transistor described in any of the above embodiments. Thus, the electronic component including the semiconductor device in which the frequency of malfunction in a high-temperature environment is reduced and manufacturing cost is reduced can be obtained. Since the electronic component includes the semiconductor device in which the frequency of malfunction in a high-temperature environment is reduced and manufacturing cost is reduced, limit of the electronic component on a usage environment is relieved and the size of the electronic component is reduced. 
       FIG. 21B  is a perspective schematic diagram of a completed electronic component.  FIG. 21B  is a perspective schematic diagram illustrating a quad flat package (QFP) as an example of the electronic component. An electronic component  700  in  FIG. 21B  includes a lead  705  and a semiconductor device  703 . As the semiconductor device  703 , the semiconductor device described in any of the above embodiments can be used. 
     The electronic component  700  in  FIG. 21B  is mounted on a printed circuit board  702 , for example. A plurality of electronic components  700  that are combined and electrically connected to each other over the printed circuit 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. 
     Next, application examples of the electronic components that are applied to a driver circuit for driving an inverter, a motor, or the like, which is provided in a vehicle driven with power from a fixed power source (e.g., a bicycle), are described with reference to  FIGS. 22A and 22B . 
       FIG. 22A  illustrates an electric bicycle  1010  as an application example. The electric bicycle  1010  obtains power when current flows through a motor  1011 . The electric bicycle  1010  includes a storage battery  1012  for supplying current to the motor  1011  and a driver circuit  1013  for driving the motor. Note that although a pedal is illustrated in  FIG. 22A , the pedal is not necessarily provided. 
     A circuit board provided with an electronic component including the semiconductor device described in any of the above embodiments is incorporated in the driver circuit  1013 . Thus, an electric bicycle including a smaller electronic component can be obtained. In addition, a low-power electric bicycle with a long cruising distance can be obtained. Moreover, a highly reliable electric bicycle can be obtained. 
       FIG. 22B  illustrates an electric car  1020  as another application example. The electric car  1020  obtains power when current flows through a motor  1021 . The electric car  1020  includes a storage battery  1022  for supplying current to the motor  1021  and a driver circuit  1023  for driving the motor. 
     A circuit board provided with an electronic component including the semiconductor device described in any of the above embodiments is incorporated in the driver circuit  1023 . Thus, an electric car including a smaller electronic component can be obtained. In addition, a low-power electric car with a long cruising distance can be obtained. Moreover, a highly reliable electric car can be obtained. 
     An electronic component including the semiconductor device described in any of the above embodiments is applicable not only to electric vehicles (EV) but also to hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and the like. 
     As described above, a circuit broad provided with an electronic component including the semiconductor device described in any of the above embodiments is incorporated in each of the electronic devices described in this embodiment. Thus, an electronic device including a smaller electronic component can be obtained. In addition, a low-power electronic device can be obtained. Moreover, a highly reliable electronic device can be obtained. 
     This embodiment can be combined with any of the structures described in the other embodiments as appropriate. 
     (Embodiment 4) 
     The semiconductor device in one embodiment of the present invention can be used for control circuits in a variety of electronic devices.  FIG. 23  illustrates specific examples of the electronic device including the semiconductor device in one embodiment of the present invention. 
     Examples of the electronic device including the semiconductor device in one embodiment of the present invention are as follows: display devices of televisions, monitors, and the like; lighting devices; desktop personal computers and laptop personal computers; word processors; image reproduction devices which reproduce still images and moving images stored in recording media such as digital versatile discs (DVD); portable CD players; portable radios; tape recorders; headphone stereos; stereos; table clocks; wall clocks; cordless phone handsets; transceivers; mobile phones; car phones; portable game machines; tablet terminals; large-sized game machines such as pachinko machines; calculators; portable information terminals; electronic notebooks; e-book readers; electronic translators; audio input devices; video cameras; digital still cameras; electric shavers; high-frequency heating appliances such as microwave ovens; electric rice cookers; electric washing machines; electric vacuum cleaners; water heaters; electric fans; hair dryers; air-conditioning systems such as air conditioners, humidifiers, and dehumidifiers; dishwashers; dish dryers; clothes dryers; futon dryers; electric refrigerators; electric freezers; electric refrigerator-freezers; freezers for preserving DNA; flashlights; tools such as a chain saw; smoke detectors; and medical equipment such as dialyzers. Other examples are as follows: industrial equipment such as guide lights, traffic lights, conveyor belts, elevators, escalators, industrial robots, power storage systems, and power storage devices for leveling the amount of power supply and smart grid. 
     In addition, moving objects driven by electric motors using power from a power storage device are also included in the category of electronic devices. Examples of the moving objects are electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats, ships, submarines, helicopters, aircrafts, rockets, artificial satellites, space probes, planetary probes, and spacecrafts. 
       FIG. 23  illustrates examples of electronic devices. In  FIG. 23 , a display device  8000  is an example of an electronic device including a semiconductor device  8004  in one embodiment of the present invention. Specifically, the display device  8000  corresponds to a display device for TV broadcast reception and includes a housing  8001 , a display portion  8002 , speaker portions  8003 , the semiconductor device  8004 , a power storage device  8005 , and the like. The semiconductor device  8004  in one embodiment of the present invention is provided in the housing  8001 . The semiconductor device  8004  can control driving of a cooling fan in the display device  8000 , emission luminance adjustment, and the like. The display device  8000  can receive power from a commercial power source. Alternatively, the display device  8000  can use power stored in the power storage device  8005 . 
     A display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoretic display device, a digital micromirror device (DMD), a plasma display panel (PDP), or a field emission display (FED) can be used for the display portion  8002 . 
     Note that the display device includes, in its category, all of information display devices for personal computers, advertisement displays, and the like, in addition to TV broadcast reception. 
     In  FIG. 23 , an installation lighting device  8100  is an example of an electronic device including a semiconductor device  8103  in one embodiment of the present invention. Specifically, the lighting device  8100  includes a housing  8101 , a light source  8102 , the semiconductor device  8103 , a power storage device  8105 , and the like. Although  FIG. 23  illustrates the case where the semiconductor device  8103  is provided in a ceiling  8104  on which the housing  8101  and the light source  8102  are installed, the semiconductor device  8103  may be provided in the housing  8101 . The semiconductor device  8103  can control emission luminance or the like of the light source  8102 . The lighting device  8100  can also receive power from a commercial power source. Alternatively, the lighting device  8100  can use power stored in the power storage device. 
     Although  FIG. 23  illustrates the installation lighting device  8100  provided in the ceiling  8104 , the semiconductor device in one embodiment of the present invention can be used in an installation lighting device provided in, for example, a wall  8405 , a floor  8406 , or a window  8407  other than the ceiling  8104 . Alternatively, the semiconductor device in one embodiment of the present invention can be used in a tabletop lighting device or the like. 
     As the light source  8102 , an artificial light source which emits light artificially by using power can be used. Specifically, an incandescent lamp, a discharge lamp such as a fluorescent lamp, and light-emitting elements such as an LED and an organic EL element are given as examples of the artificial light source. 
     In  FIG. 23 , an air conditioner including an indoor unit  8200  and an outdoor unit  8204  is an example of an electronic device including a semiconductor device  8203  in one embodiment of the present invention. Specifically, the indoor unit  8200  includes a housing  8201 , an air outlet  8202 , the semiconductor device  8203 , a power storage device  8205 , and the like. Although  FIG. 23  illustrates the case where the semiconductor device  8203  is provided in the indoor unit  8200 , the semiconductor device  8203  may be provided in the outdoor unit  8204 . Alternatively, the semiconductor devices  8203  may be provided in both the indoor unit  8200  and the outdoor unit  8204 . The semiconductor device  8203  can control operation of a motor used for a compressor in the air conditioner. The air conditioner can receive power from a commercial power source. Alternatively, the air conditioner can use power stored in the power storage device  8205 . 
     Note that although the split-type air conditioner including the indoor unit and the outdoor unit is illustrated in  FIG. 23 , the semiconductor device in one embodiment of the present invention can be used in an air conditioner in which the functions of an indoor unit and an outdoor unit are integrated in one housing. 
     In  FIG. 23 , an electric refrigerator-freezer  8300  is an example of an electronic device including a semiconductor device  8304  in one embodiment of the present invention. Specifically, the electric refrigerator-freezer  8300  includes a housing  8301 , a refrigerator door  8302 , a freezer door  8303 , the semiconductor device  8304 , a power storage device  8305 , and the like. The semiconductor device  8304  provided in the housing  8301  in  FIG. 23  can control operation of a motor used for a compressor in the electric refrigerator-freezer  8300 . The electric refrigerator-freezer  8300  can receive power from a commercial power source. Alternatively, the electric refrigerator-freezer  8300  can use power stored in the power storage device  8305 . 
     Note that among the electronic devices described above, a high-frequency heating appliance such as a microwave oven and an electronic device such as an electric rice cooker require high power in a short time. In addition, it is necessary to stable control high power for a certain period of time. With the use of a semiconductor device in one embodiment of the present invention, a highly reliable electronic device can be obtained. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     This application is based on Japanese Patent Application serial No. 2015-163532 filed with Japan Patent Office on Aug. 21, 2015, the entire contents of which are hereby incorporated by reference.