Patent Publication Number: US-2022231644-A1

Title: Semiconductor device

Description:
TECHNICAL FIELD 
     One embodiment of the present invention relates to a semiconductor device. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. Thus, a semiconductor element such as a transistor or a diode and a circuit including a semiconductor element are semiconductor devices. A display device, a light-emitting device, a lighting device, an electro-optical device, a communication device, an electronic device, and the like may include a semiconductor element or a semiconductor circuit. Therefore, a display device, a light-emitting device, a lighting device, an electro-optical device, an imaging device, a communication device, an electronic device, and the like are referred to as a semiconductor device in some cases. 
     BACKGROUND ART 
     The fifth-generation mobile communication system (5G) has been introduced, and communication delay is one of big problems in providing the communication environment. The communication delay not only spoils the advantage of 5G but also makes it difficult to provide comfortable service in an environment for looking ahead to Internet of Things (IoT). As a measure to eliminate the communication delay, edge computing has been examined. The edge computing is achieved between a central station and a user terminal via at least a relay station. The relay stations are installed at places close to the user terminals, and the number of installed relay stations is larger than that of central stations. Such edge computing is expected to reduce communication delay caused between the central station and the user terminal. 
     As described above, unlike the central station, the relay station needs to be installed close to the user terminal, but it is difficult to install relay stations that require new construction in consideration of installation places and installation cost. Thus, installing the relay stations in existing buildings (e.g., office buildings or commercial buildings) is considered. 
     A 5G compatible semiconductor device is manufactured using a semiconductor containing one kind of element such as Si as its main component or a compound semiconductor containing a plurality of kinds of elements such as Ga and As as its main components. Furthermore, an oxide semiconductor, which is one kind of metal oxide, has attracted attention. 
     A CAAC (c-axis aligned crystalline) structure and an nc (nanocrystalline) structure, which are neither single crystal nor amorphous, have been found in an oxide semiconductor (see Non-Patent Document 1 and Non-Patent Document 2). 
     Non-Patent Document 1 and Non-Patent Document 2 each disclose a technique for manufacturing a transistor using an oxide semiconductor having a CAAC structure. 
     REFERENCES 
     Non-Patent Documents 
     [Non-Patent Document 1] S. Yamazaki et al., “SID Symposium Digest of Technical Papers”, 2012, volume 43, issue 1, pp. 183-186 
     [Non-Patent Document 2] S. Yamazaki et al., “Japanese Journal of Applied Physics”, 2014, volume 53, Number 4S, pp. 04ED18-1-04ED18-10 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the case where a communication device is installed as a relay station in an existing building, unlike the central station that is installed in a vast site area, the communication device needs to be reduced in size. Thus, one object of the present invention is to provide a communication device that functions as a relay station and is reduced in size. Such a communication device can be referred to as a semiconductor device or an electronic device. 
     Note that the description of these objects does not preclude the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than these are apparent from the description of the specification, the drawings, the claims, and the like, and objects other than these can be derived from the description of the specification, the drawings, the claims, and the like. 
     Means for Solving the Problems 
     In view of the foregoing, one of features of the present invention is as follows: an operational amplifier is included between an input terminal of a phase shifter and an output terminal of the phase shifter; a transistor electrically connected to a first input terminal of the operational amplifier, a first resistor electrically connected to a second input terminal of the operational amplifier, and a second resistor electrically connected to the second input terminal of the operational amplifier are included; a gate of the transistor is electrically connected to the first terminal; one of a source and a drain of the transistor is electrically connected to the input terminal of the phase shifter; the other of the source and the drain of the transistor is electrically connected to the first input terminal of the operational amplifier; the first resistor is electrically connected to the input terminal of the phase shifter; the second resistor is electrically connected to the output terminal of the phase shifter; and at least a transistor included in the operational amplifier has a region overlapping with the transistor. Since at least the transistor included in the operational amplifier has the region overlapping with the transistor, a phase shifter that is reduced in size can be provided. 
     In view of the foregoing, one of features of the present invention is as follows: an operational amplifier is included between an input terminal of a phase shifter and an output terminal of the phase shifter; a transistor electrically connected to a first input terminal of the operational amplifier, a memory element electrically connected to a gate of the first transistor, a first resistor electrically connected to a second input terminal of the operational amplifier, and a second resistor electrically connected to the second input terminal of the operational amplifier are included; a gate of the transistor is electrically connected to the first terminal; one of a source and a drain of the transistor is electrically connected to the input terminal of the phase shifter; the other of the source and the drain of the transistor is electrically connected to the first input terminal of the operational amplifier; the first resistor is electrically connected to the input terminal of the phase shifter; the second resistor is electrically connected to the output terminal of the phase shifter; and at least a transistor included in the operational amplifier has a region overlapping with the transistor. Since at least the transistor included in the operational amplifier has the region overlapping with the first transistor, a phase shifter that is reduced in size can be provided. 
     In view of the foregoing, in the present invention, the memory element may include a first transistor and a first capacitor, the first transistor may include an oxide semiconductor film, and at least the transistor included in the operational amplifier may have a region overlapping with the first transistor. 
     Effect of the Invention 
     According to the present invention, a communication device that functions as a relay station and is reduced in size can be provided. 
     Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not have to have all these effects. Note that effects other than these are apparent from the description of the specification, the drawings, the claims, and the like and effects other than these can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  and  FIG. 1B  are diagrams illustrating semiconductor devices of the present invention. 
         FIG. 2A  and  FIG. 2B  are diagrams illustrating semiconductor devices of the present invention. 
         FIG. 3A  and  FIG. 3B  are diagrams illustrating semiconductor devices of the present invention. 
         FIG. 4  is a diagram illustrating a semiconductor device. 
         FIG. 5  is a diagram illustrating a semiconductor device of the present invention. 
         FIG. 6  is a diagram illustrating a semiconductor device of the present invention. 
         FIG. 7A ,  FIG. 7B , and  FIG. 7C  are diagrams illustrating a semiconductor device of the present invention. 
         FIG. 8A ,  FIG. 8B , and  FIG. 8C  are diagrams illustrating a semiconductor device of the present invention. 
         FIG. 9A ,  FIG. 9B , and  FIG. 9C  are diagrams illustrating a semiconductor device of the present invention. 
         FIG. 10A ,  FIG. 10B , and  FIG. 10C  are diagrams illustrating an oxide semiconductor of the present invention. 
         FIG. 11A  and  FIG. 11B  are diagrams illustrating a semiconductor device of the present invention. 
         FIG. 12A  and  FIG. 12B  are diagrams illustrating a semiconductor device of the present invention. 
         FIG. 13  is a diagram illustrating application of the present invention. 
         FIG. 14A ,  FIG. 14B ,  FIG. 14C ,  FIG. 14D ,  FIG. 14E , and  FIG. 14F  are diagrams illustrating application of the present invention. 
         FIG. 15  is a diagram illustrating application of the present invention. 
         FIG. 16  is a diagram illustrating application of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments are described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it is readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Thus, the present invention should not be construed as being limited to the description in the following embodiments. 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 thereof is not repeated. 
     In addition, the position, size, range, and the like of each component illustrated in the drawings and the like do not represent the actual position, size, range, and the like in some cases for easy understanding of the invention. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like disclosed in drawings and the like. For example, in an actual manufacturing process, a resist mask or the like might be unintentionally reduced in size by treatment such as etching, which is not reflected in the drawings in some cases for easy understanding. 
     Furthermore, in a top view (also referred to as a plan view), a perspective view, or the like, the description of some components might be omitted for easy understanding of the drawings. 
     In addition, in this specification and the like, the terms “electrode” and “wiring” do not functionally limit these components. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa. Furthermore, the term “electrode” or “wiring” also includes the case where a plurality of “electrodes” or “wirings” are formed in an integrated manner, for example. 
     In this specification and the like, a “terminal” in an electric circuit refers to a portion that inputs or outputs current, inputs or outputs voltage, or receives or transmits a signal. Accordingly, part of a wiring or an electrode functions as a terminal in some cases. 
     Note that the term “over” or “under” in this specification and the like does not necessarily mean that a component is placed directly over and in contact with or directly under and in contact with another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is formed on and in direct contact with the insulating layer A, and does not exclude the case where another component is provided between the insulating layer A and the electrode B. 
     In addition, functions of a source and a drain are interchanged with each other depending on operation conditions and the like, for example, when a transistor of different polarity is employed or when the current direction is changed in a circuit operation; therefore, it is difficult to define which is the source or the drain. Thus, the terms “source” and “drain” can be interchangeably used in this specification. 
     In this specification and the like, the expression “electrically connected” includes the case where components are directly connected to each other and the case where components are connected through an “object having any electric function”. Here, there is no particular limitation on the “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Thus, even when the expression “electrically connected” is used, there is a case where no physical connection portion is made and a wiring is just extended in an actual circuit. 
     Furthermore, in this specification and the like, “parallel” indicates a state where two straight lines are placed at an angle of greater than or equal to −10° and less than or equal to 10°, for example. Accordingly, the case where the angle is greater than or equal to −5° and less than or equal to 5° is also included. Moreover, “perpendicular” and “orthogonal” indicate a state where two straight lines are placed at an angle of greater than or equal to 80° and less than or equal to 100°, for example. Accordingly, the case where the angle is greater than or equal to 85° and less than or equal to 95° is also included. 
     In this specification and the like, the terms “identical”, “same”, “equal”, “uniform”, and the like used in describing calculation values and measurement values allow for a margin of error of ±20% unless otherwise specified. 
     In addition, a voltage refers to a potential difference between a certain potential and a reference potential (e.g., a ground potential or a source potential) in many cases. Therefore, the terms “voltage” and “potential” can be replaced with each other in many cases. In this specification and the like, the terms “voltage” and “potential” can be replaced with each other unless otherwise specified. 
     Note that even a “semiconductor” has characteristics of an “insulator” when conductivity is sufficiently low, for example. Thus, a “semiconductor” can be replaced with an “insulator”. In that case, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other because a border therebetween is not clear. Accordingly, a “semiconductor” and an “insulator” described in this specification can be replaced with each other in some cases. 
     Furthermore, a “semiconductor” has characteristics of a “conductor” when conductivity is sufficiently high, for example. Thus, a “semiconductor” can be replaced with a “conductor”. In that case, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other because a border therebetween is not clear. Accordingly, a “semiconductor” and a “conductor” in this specification can be replaced with each other in some cases. 
     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 the priority or the 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 the scope of claims in order to avoid confusion among components. Furthermore, a term with an ordinal number in this specification and the like might be provided with a different ordinal number in the scope of claims. Furthermore, even when a term is provided with an ordinal number in this specification and the like, the ordinal number might be omitted in the scope of claims and the like. 
     Note that in this specification and the like, an “on state” of a transistor refers to a state in which a source and a drain of the transistor are electrically short-circuited (also referred to as a “conduction state”). Furthermore, an “off state” of the transistor refers to a state in which the source and the drain of the transistor are electrically disconnected (also referred to as a “non-conduction state”). 
     In addition, in this specification and the like, an “on-state current” sometimes refers to a current that flows between a source and a drain when a transistor is in an on state. Furthermore, an “off-state current” sometimes refers to a current that flows between a source and a drain when a transistor is in an off state. 
     In this specification and the like, a high power supply potential VDD (hereinafter, also simply referred to as “VDD”, “H potential”, or “H”) is a power supply potential higher than a low power supply potential VSS (hereinafter, also simply referred to as “VSS”, “L potential”, or “L”). VSS refers to a power supply potential at a potential lower than VDD. A ground potential (hereinafter, also simply referred to as “GND” or “GND potential”) can be used as VDD or VSS. For example, in the case where VDD is a ground potential, VSS is a potential lower than the ground potential, and in the case where VSS is a ground potential, VDD is a potential higher than the ground potential. 
     In addition, in this specification and the like, a gate refers to part or the whole of a gate electrode and a gate wiring. A gate wiring refers to a wiring for electrically connecting at least one gate electrode of a transistor to another electrode or another wiring. 
     Furthermore, in this specification and the like, a source refers to part or all of a source region, a source electrode, or a source wiring. A source region refers to a region in a semiconductor layer, where the resistivity is lower than or equal to a given value. A source electrode refers to part of a conductive layer that is connected to a source region. A source wiring refers to a wiring for electrically connecting at least one source electrode of a transistor to another electrode or another wiring. 
     Moreover, in this specification and the like, a drain refers to part or all of a drain region, a drain electrode, or a drain wiring. A drain region refers to a region in a semiconductor layer, where the resistivity is lower than or equal to a given value. A drain electrode refers to part of a conductive layer that is connected to a drain region. A drain wiring refers to a wiring for electrically connecting at least one drain electrode of a transistor to another electrode or another wiring. 
     In the drawings and the like, for easy understanding of the potentials of a wiring, an electrode and the like, “H” representing an H potential or “L” representing an L potential is sometimes written near the wiring, the electrode, and the like. In addition, enclosed “H” or “L” is sometimes written near a wiring, an electrode, and the like whose potential changes. Moreover, a symbol “×” is sometimes written on a transistor in an off state. 
     Embodiment 1 
     In this embodiment, a phase shifter included in a communication device of a relay station is described. 
     The 5G communication frequency band includes a frequency band up to 6 GHz and a frequency band from approximately 30 GHz to approximately 100 GHz. In Japan, there is a movement to enable use of both the frequency bands, and a specific frequency among the frequency bands is assigned to each carrier. Thus, it is expected that different frequencies are used by user terminals. Furthermore, as for other countries, a variety of frequency bands are used in accordance with the circumstances of the countries. In view of the above, the communication device must correspond to a number of frequencies. 
     Similarly, it is preferable that the phase shifter also have a structure that corresponds to a number of frequencies. First, a general phase shifter is described with reference to  FIG. 4 . A phase shifter  6000  illustrated in  FIG. 4  includes an input terminal Vin and an output terminal Vout. The phase shifter  6000  includes an operational amplifier  6001 , a resistor R 1  to a resistor R 3 , and a capacitor C 1 . The resistor R 1  is provided between the input terminal Vin of the phase shifter  6000  and a non-inverting input terminal (a + side terminal or a first input terminal) of the operational amplifier  6001  and is electrically connected to the input terminal Vin and the non-inverting input terminal. That is, a signal from the input terminal Vin is input to the operational amplifier  6001  through the resistor R 1 . 
     The resistor R 2  is provided between the input terminal Vin of the phase shifter  6000  and an inverting input terminal (a − side terminal or a second input terminal) of the operational amplifier  6001  and is electrically connected to the input terminal Vin and the inverting input terminal. That is, a signal from the input terminal Vin is input to the operational amplifier  6001  through the resistor R 2 . 
     The resistor R 3  is provided between the second input terminal of the operational amplifier  6001  and the output terminal Vout of the phase shifter  6000  and is electrically connected to the second input terminal and the output terminal Vout. That is, the resistor R 3  is placed in a feedback circuit. Note that the output terminal Vout of the phase shifter  6000  is equal to an output terminal of the operational amplifier  6001 . 
     The capacitor C 1  is electrically connected to the resistor R 1  and the first input terminal. 
     In such a circuit, the phase of the output terminal Vout with respect to the input terminal Vin in the phase shifter  6000  satisfies the following formula. Note that each sign in the formula is as follows. ω represents an angular frequency. θ represents the change amount of a phase shift of an input waveform and an output waveform. C 1  represents the capacitance of the capacitor C 1  illustrated in  FIG. 4 . R 1  represents the resistance value of the resistor R 1  illustrated in  FIG. 4 . R represents the resistance value of the resistor R 2  illustrated in  FIG. 4 , and the resistance value of the resistor R 3  illustrated in  FIG. 4  is equal to the resistance value of the resistor R 2 . 
     
       
         
           
             
               
                 
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                         R 
                       
                       
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                         - 
                         
                           
                             ω 
                             2 
                           
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                             C 
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                             2 
                           
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                   [ 
                   
                     Formula 
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     The frequency f and the angular frequency ω have a relation ω=2πf. According to the relation and the above (Formula 1), θ=90° when f=½πC 1 R 1 , and the phase of the input terminal Vin shifts by 90°. 
     As described above, in consideration of the use circumstance of the 5G communication frequency band, in the phase shifter, the phases of many frequency waves need to shift by 90°. Moreover, they also need to shift by a degree other than 90°. From the above (Formula 1), the phases of many frequency waves probably can shift by 90° by change of the value of However, in the structure of  FIG. 4 , the value of C 1 R 1  is determined in the design stage; thus, it is difficult to change the value of C 1 R 1  after the completion of the phase shifter. Thus, the present inventors have reached a novel phase shifter  1100  illustrated in  FIG. 1A . Note that in  FIG. 1A  and  FIG. 1B , resistors placed at the same positions as those of  FIG. 4  are denoted by the same reference numerals as those of  FIG. 4 . 
     Note that the phase shifter can function as a communication device, a semiconductor device, or an electronic device. That is, the phase shifter can be referred to as a communication device, a semiconductor device, or an electronic device. It can also be said that a communication device, a semiconductor device, or an electronic device placed in a relay station includes the phase shifter. 
     The phase shifter  1100  illustrated in  FIG. 1A  is different from that in  FIG. 4  in including a transistor Tr 1 . That is, the phase shifter  1100  illustrated in  FIG. 1A  includes an operational amplifier  1001 , the transistor Tr 1  electrically connected to a first input terminal of the operational amplifier  1001 , and the resistor R 2  and the resistor R 3  electrically connected to a second input terminal of the operational amplifier  1001 . A gate of the transistor Tr 1  is electrically connected to a terminal Vres. One of a source and a drain of the transistor Tr 1  is electrically connected to the first input terminal of the operational amplifier  1001 , and the capacitor C 1  electrically connected to the one and the first input terminal is included. 
     The other of the source and the drain of the transistor Tr 1  is electrically connected to the input terminal Vin of the phase shifter  1100 . The resistor R 2  is also electrically connected to the input terminal Vin. The resistor R 3  is electrically connected to a second output terminal of the operational amplifier  1001  and is also electrically connected to an output terminal of the operational amplifier  1001 . That is, the resistor R 3  is placed in a feedback circuit. The output terminal of the operational amplifier  1001  is equal to the output terminal Vout of the phase shifter  1100 . 
     The gate of the transistor Tr 1  is electrically connected to the terminal Vres, and a signal for controlling the gate is supplied from the terminal Vres. In accordance with the signal, the transistor Tr 1  can be in an on state or an off state. In the transistor Tr 1 , a resistance in an off state (off resistance) is higher than a resistance in an on state (on resistance). Furthermore, the on resistance can be a variety of resistance values in accordance with the vd-Id linearity of the transistor Tr 1 . That is, the on resistance of the transistor Tr 1  can be controlled by the signal from the terminal Vres. When the on resistance of the transistor Tr 1  functions as a variable resistance, the phase shifter  1100  can correspond to a variety of frequencies used for the 5G. Thus, the phase shifter  1100  that corresponds to a variety of frequencies used for the 5G can be provided. 
     In  FIG. 1A , the transistor Tr 1  includes four terminals. That is, the transistor Tr 1  includes a terminal for a back gate. Supply of a signal to the terminal for the back gate enables, for example, the control of the threshold value of the transistor Tr 1 . Note that the transistor Tr 1  in the phase shifter  1100  needs to have at least three terminals. Thus,  FIG. 1B  illustrates a structure example including a transistor Tr 11  including three terminals instead of the transistor Tr 1 . In  FIG. 1B , the other structures are similar to those in  FIG. 1A . 
     In  FIG. 1A  or  FIG. 1B , when the on resistance value of the transistor Tr 1  or the transistor Tr 11  is high, the capacitor C 1  can be smaller. Alternatively, the capacitor C 1  can be omitted. When the capacitor C 1  can be smaller or omitted, the phase shifter  1100  can be further reduced in size. 
     An active layer of the above-described transistor may be silicon or an oxide semiconductor. When silicon is used, it is expected that the operation of the phase shifter becomes fast because silicon has high mobility. When an oxide semiconductor is used, the capacitor can be omitted as described above because the off-state current of the transistor can be low. This contributes to a reduction in area of the phase shifter. Instead of the above-described capacitor, the gate capacitance of the above-described transistor can be used. 
     In the case where silicon is used for the active layer of the above-described transistor, there is no great necessity for the control of the threshold value as compared to the case of the transistor including an oxide semiconductor in its active layer. Thus, the transistor including silicon in its active layer can have a three-terminal structure as illustrated in  FIG. 1B . 
     Modification Example 1 
     Modification examples of the phase shifters illustrated in  FIG. 1A  and  FIG. 1B  are described with reference to  FIG. 2A  and  FIG. 2B , respectively. A phase shifter  1200  in  FIG. 2A  and  FIG. 2B  has a structure in which a transistor Tr 2  and a capacitor C 2  are added to the phase shifter  1100  in  FIG. 1A  and  FIG. 1B . Note that resistors in  FIG. 2A  and  FIG. 2B  placed at the same positions as those of  FIG. 4  are denoted by the same reference numerals as those of  FIG. 4 . 
     The transistor Tr 2  and the capacitor C 2  that are added to  FIG. 2A  function as a memory element M 1 . The memory element M 1  can retain electric charge or the like in the capacitor C 2 . In the memory element M 1 , a transistor including an oxide semiconductor film having a low off-state current (an OS transistor) is preferably used as the transistor Tr 2 . This is because the retention time of the electric charge or the like in the capacitor C 2  can be longer. 
     The phase shifter  1200  illustrated in  FIG. 2A  includes an operational amplifier  1021 , the transistor Tr 1  electrically connected to a first input terminal of the operational amplifier  1021 , and the resistor R 2  and the resistor R 3  electrically connected to a second input terminal of the operational amplifier  1021 . A gate of the transistor Tr 1  is electrically connected to the memory element M 1 . The memory element M 1  includes the transistor Tr 2  and the capacitor C 2 . The gate of the transistor Tr 1  is electrically connected to one of a source and a drain of the transistor Tr 2 . The gate of the transistor Tr 1  is electrically connected to the capacitor C 2 . A gate of the transistor Tr 2  is electrically connected to a terminal SET. The other of the source and the drain of the transistor Tr 2  is electrically connected to the terminal Vres. 
     The gate of the transistor Tr 1  is electrically connected to the memory element M 1 . The transistor Tr 1  is controlled by the memory element M 1 . The memory element M 1  can retain a potential with which the transistor Tr 1  can be brought into an on state. Additional signals do not need to be supplied to the terminal SET and the like during the retention; thus, the power consumption of the phase shifter  1200  can be reduced. 
     The one of the source and the drain of the transistor Tr 1  is electrically connected to the first input terminal of the operational amplifier  1021 , and the other is electrically connected to the input terminal Vin of the phase shifter  1200 . The resistor R 2  is also electrically connected to the input terminal Vin. The resistor R 3  is electrically connected to a second output terminal of the operational amplifier  1021  and is also electrically connected to an output terminal of the operational amplifier  1021 . That is, the resistor R 3  is placed in a feedback circuit. The output terminal of the operational amplifier  1021  is equal to the output terminal Vout of the phase shifter  1100 . 
     Furthermore, in  FIG. 2A , the transistor Tr 1  and the transistor Tr 2  each including four terminals are included.  FIG. 2B  illustrates a case where the transistor Tr 11  and a transistor Tr 21  each including three terminals are included. The other structures are similar to those in  FIG. 2A . 
     In  FIG. 2A  or  FIG. 2B , when the on resistance value of the transistor Tr 1  or the transistor Tr 11  is high, the capacitor C 1  can be smaller. Alternatively, the capacitor C 1  can be omitted. When the capacitor C 1  can be smaller or omitted, the phase shifter  1200  can be further reduced in size. 
     Owing to the memory element M 1  illustrated in  FIG. 2A  and  FIG. 2B , the power consumption of the phase shifter  1200  can be reduced. 
     Modification Example 2 
     Modification examples of the phase shifters illustrated in  FIG. 2A  and  FIG. 2B  are described with reference to  FIG. 3A  and  FIG. 3B , respectively.  FIG. 3A  and  FIG. 3B  are structure examples in which transistors are used also for the resistor on the negative feedback side. That is, phase shifters  1300  in  FIG. 3A  and  FIG. 3B  have a structure in which a transistor Tr 3  to a transistor Tr 5  and a capacitor C 3  are added to each of  FIG. 2A  and  FIG. 2B . 
     The phase shifter  1300  illustrated in  FIG. 3A  includes an operational amplifier  1031 , the transistor Tr 1  electrically connected to a first input terminal of the operational amplifier  1031 , and the transistor Tr 3  and the transistor Tr 4  electrically connected to a second input terminal of the operational amplifier  1031 . Although the transistor Tr 1  and the memory element M 1  are similar to those of  FIG. 2A , the following ordinal numbers are used in  FIG. 3A : the terminal Vres in  FIG. 2A  is referred to as a terminal Vres 1 , and the terminal SET in  FIG. 2A  is referred to as a terminal SET 1 . Gates of the transistor Tr 3  and the transistor Tr 4  are both electrically connected to one of a source and a drain of the transistor Tr 5 . One of a source and a drain of the transistor Tr 3  is electrically connected to the input terminal Vin of the phase shifter  1300 . The other of the source and the drain of the transistor Tr 3  is electrically connected to one of a source and a drain of the transistor Tr 4  and is electrically connected to the second input terminal of the operational amplifier  1031 . The other of the source and the drain of the transistor Tr 4  is electrically connected to an output terminal of the phase shifter  1300 . 
     A gate of the transistor Tr 5  is electrically connected to a terminal SET 2 . The one of the source and the drain of the transistor Tr 5  is electrically connected to the capacitor C 3 . The other of the source and the drain of the transistor Tr 5  is electrically connected to a terminal Vres 2 . 
     Moreover,  FIG. 3A  includes the transistors Tr 1  to Tr 5  each including four terminals.  FIG. 3B  illustrates a case where transistors Tr 11 , Tr 21 , Tr 31 , Tr 41 , and Tr 51  each including three terminals are included. The other structures are similar to those in  FIG. 3A . 
     In  FIG. 3A  or  FIG. 3B , when the on resistance value of the transistor Tr 1  or the transistor Tr 11  is high, the capacitor C 1  can be smaller. Alternatively, the capacitor C 1  can be omitted. When the capacitor C 1  can be smaller or omitted, the phase shifter  1300  can be further reduced in size. 
     In  FIG. 3A  or  FIG. 3B , when the on resistance value of the transistor Tr 5  or the transistor Tr 51  is high, the capacitor C 3  can be smaller. Alternatively, the capacitor C 3  can be omitted. When the capacitor C 3  can be smaller or omitted, the phase shifter  1300  can be further reduced in size. 
     The phase shifter  1300  illustrated in  FIG. 3A  and  FIG. 3B  makes it possible to broaden an applicable frequency band. 
     &lt;Structure of Semiconductor Device&gt; 
       FIG. 5  illustrates part of a cross-sectional structure of a semiconductor device. The semiconductor device illustrated in  FIG. 5  is an example in which a transistor  550 , a transistor  500 , and a capacitor  600  are included and stacked in this order, and owing to such a stacked-layer structure, the semiconductor device such as the phase shifter can be reduced in size. The transistor  500  includes a conductor  592  functioning as a back gate terminal and a conductor  593  functioning as a gate terminal. The conductor  592  functioning as a back gate terminal can be omitted. 
     The above-described operational amplifier can include the transistor  550  including silicon in its active layer, and the transistors Tr 1  to Tr 5  can include the transistor  500  including an oxide semiconductor in its active layer. At least the transistor  550  included in the operational amplifier includes a region overlapping with the transistor  500 . Thus, the phase shifter can be reduced in size. Moreover, the above-described capacitors C 1  to C 3  can include the capacitor  600 . The area of the capacitor  600  is reduced, and thus the phase shifter can be further reduced in size. 
     The transistor  500  including an oxide semiconductor in its active layer can be referred to as an OS transistor. The transistor  500  has an extremely low off-state current. Thus, a data voltage or electric charge written to a storage node through the transistor  500  can be retained in the storage node for a long time. When the above effect of the OS transistor is used for the memory element M 1  including the capacitor C 2  in the phase shifter illustrated in  FIG. 2A  and  FIG. 2B , a data voltage or electric charge can be retained in the capacitor C 2  for a long time. The power consumption of the semiconductor device such as the phase shifter can be reduced. 
     The transistor  550  is provided over a substrate  311  and includes a conductor  316 , an insulator  315 , a semiconductor region  313  composed of part of the substrate  311 , and a low-resistance region  314   a  and a low-resistance region  314   b.  The conductor  316  functions as a gate of the transistor  550 . The insulator  315  functions as a gate insulating film of the transistor  550 . The region  313  functions as a channel formation region. The low-resistance region  314   a  and the low-resistance region  314   b  function as one and the other of a source region and a drain region. The transistor  550  including silicon in its active layer has higher mobility than an OS transistor; thus, the transistor  550  can contribute to high speed operation of the operational amplifier. 
     &lt;Modification Example of each Transistor&gt; 
       FIG. 7A  and  FIG. 7B  illustrate a modification example of the above-described transistor  500 , and  FIG. 7C  illustrates a modification example of the above-described transistor  550 .  FIG. 7A  is a cross-sectional view of the transistor  500  in the channel length direction and  FIG. 7B  is a cross-sectional view of the transistor  500  in the channel width direction. 
       FIG. 7C  is a cross-sectional view of the transistor  550  in the channel width direction. In the transistor  550 , an upper portion of the semiconductor region  313  is covered with the conductor  316  with the insulator  315  therebetween. The transistor  550  is what is called a Fin-type transistor. Such a Fin-type transistor  550  can have an increased effective channel width, and thus have improved on-state characteristics. In addition, since contribution of an electric field from a gate electrode can be increased, the off-state characteristics of the transistor  550  can be improved. The improvement of the on-state characteristics and the off-state characteristics can contribute to high speed operation of the operational amplifier. 
     Note that as for the polarity, the transistor  550  can be either a p-channel transistor or an n-channel transistor. 
     The semiconductor region  313  preferably includes single crystal silicon as a semiconductor material. Alternatively, the region may be formed using a material containing Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), or the like. A structure may be employed in which silicon whose effective mass is controlled by applying stress to the crystal lattice and changing the lattice spacing is used. Alternatively, the transistor  550  may be a high electron mobility transistor (HEMT) with GaAs and GaAlAs, or the like. 
     The low-resistance region  314   a  and the low-resistance region  314   b  can be obtained by adding an element which imparts n-type conductivity, such as arsenic or phosphorus, or an element which imparts p-type conductivity, such as boron, to the semiconductor material used for the above-described semiconductor region  313 . The addition of the element can reduce the resistance. 
     For the conductor  316  functioning as a gate electrode, a semiconductor material such as silicon containing an element which imparts n-type conductivity, such as arsenic or phosphorus, or an element which imparts p-type conductivity, such as boron, or a conductive material such as a metal material, an alloy material, or a metal oxide material can be used. 
     Note that since the work function of a conductor depends on the material of the conductor, the threshold voltage of the transistor  550  can be controlled by selecting the material of the conductor. To control the threshold voltage, a nitride conductor material such as titanium nitride or tantalum nitride is preferably used for the conductor  316 . Furthermore, in order to ensure both conductivity and embeddability, it is preferable to use a stacked layer of metal materials such as tungsten and aluminum for the conductor  316 , and it is further preferable to use tungsten in terms of heat resistance. 
     The transistor  550  may be formed using an SOI (Silicon On Insulator) substrate, for example. 
     As the SOI substrate, the following substrate may be used: an SIMOX (Separation by Implanted Oxygen) substrate which is formed in such a manner that after an oxygen ion is implanted into a mirror-polished wafer, an oxide layer is formed at a certain depth from the surface and defects generated in a surface layer are eliminated by high-temperature annealing; or an SOI substrate formed by using a Smart-Cut method in which a semiconductor substrate is cleaved by utilizing growth of a minute void, which is formed by implantation of a hydrogen ion, due to thermal treatment, by using an ELTRAN method (a registered trademark: Epitaxial Layer Transfer), or the like. A transistor formed using a single crystal substrate contains a single crystal semiconductor in a channel formation region. 
     Note that the transistor  550  illustrated in  FIG. 7C  is an example and the configuration is not limited thereto; an appropriate transistor can be used in accordance with a circuit configuration or a driving method. 
     For example, in the case where the semiconductor device is a single-polarity circuit using only OS transistors (which represents a circuit with transistors having the same polarity, e.g., only n-channel transistors), the transistor  550  may have a structure similar to the structure of the transistor  500 .  FIG. 6  illustrates an example of a single-polarity circuit formed using the transistor  550  and the transistor  500  that are both OS transistors. Note that the details of the transistor  500  are described later. 
     In  FIG. 5  and  FIG. 6 , an insulator  320 , an insulator  322 , an insulator  324 , and an insulator  326  are stacked sequentially to cover the transistor  550 . Each insulator is placed to remain an insulating property between conductors placed over and under the insulator. 
     For the insulator  320 , the insulator  322 , the insulator  324 , and the insulator  326 , silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, or the like is used, for example. 
     Note that in this specification, silicon oxynitride refers to a material that contains oxygen at a higher proportion than nitrogen, and silicon nitride oxide refers to a material that contains nitrogen at a higher proportion than oxygen. Furthermore, in this specification, aluminum oxynitride refers to a material that contains oxygen at a higher proportion than nitrogen, and aluminum nitride oxide refers to a material that contains nitrogen at a higher proportion than oxygen. 
     The insulator  322  may have a function of a planarization film for eliminating a level difference caused by the transistor  550  or the like provided below the insulator  322 . For example, a top surface of the insulator  322  may be planarized by planarization treatment using a chemical mechanical polishing (CMP) method or the like to increase planarity. 
     Furthermore, the insulator  324  preferably includes a material having a high barrier property that prevents hydrogen and impurities from diffusing from the substrate  311 , the transistor  550 , or the like into the transistor  500  and a region where the transistor  500  is provided. 
     For the film having a barrier property against hydrogen, silicon nitride formed by a CVD method can be used, for example. Here, diffusion of hydrogen to a transistor including an oxide semiconductor, such as the transistor  500 , degrades the characteristics of the transistor in some cases. Therefore, a film that inhibits hydrogen diffusion, that is, a film having a high barrier property is preferably provided between the transistor  500  and the transistor  550 . 
     The film that inhibits hydrogen diffusion may refer to a film that releases a little amount of hydrogen. The amount of released hydrogen can be measured by thermal desorption spectroscopy (TDS). For example, when the preferable amount of hydrogen released from the insulator  324  is considered on the basis of TDS analysis, the amount of hydrogen released from the insulator  324  that is converted into hydrogen atoms per unit area of the insulator  324  is less than or equal to 10×10 15  atoms/cm 2 , preferably less than or equal to 5×10 15  atoms/cm 2 . It is found that the insulator  324  is a film having a high barrier property. Note that in the TDS analysis, the film surface temperature of the insulator  324  is in a range of 50° C. to 500° C. 
     Note that the permittivity of the insulator  326  is preferably lower than that of the insulator  324 . This is because when a material with a low permittivity is used for an interlayer film, parasitic capacitance generated between wirings can be reduced. The relative permittivity of the insulator  326  is preferably 0.7 times or less, further preferably 0.6 times or less the relative permittivity of the insulator  324 . For example, the relative permittivity of the insulator  326  is preferably lower than 4, further preferably lower than 3. 
     Moreover, a conductor  328 , a conductor  330 , and the like are embedded in the insulator  320 , the insulator  322 , the insulator  324 , and the insulator  326 . The conductor  328  and the conductor  330  can be referred to as a plug. Through the conductor  328  and the conductor  330 , electrical connection between the transistor  550  and the transistor  500  and electrical connection between the transistor  550  and the capacitor  600  become possible. Since the conductor  328  and the conductor  330  enable the electrical connection, it is found that the conductor  328  and the conductor  330  function as a wiring or part of a wiring. 
     As a material for each of the plugs and wirings (the conductor  328 , the conductor  330 , and the like), a single layer or a stacked layer of a conductive material such as a metal material, an alloy material, a metal nitride material, or a metal oxide material can be used. It is further preferable to use a material with a high melting point that has both heat resistance and conductivity, such as tungsten or molybdenum. Alternatively, it is preferable to form the plugs and wirings with a low-resistance conductive material such as aluminum or copper. The use of a low-resistance conductive material can reduce wiring resistance. Each of the plugs and wirings (the conductor  328 , the conductor  330 , and the like) preferably has a stacked-layer structure of a high-melting-point material and a low-resistant conductive material. 
     A wiring layer may be provided over the insulator  326  and the conductor  330 . For example, in  FIG. 5 , an insulator  350 , an insulator  352 , and an insulator  354  are provided to be stacked in this order. Furthermore, a conductor  356  is formed in the insulator  350 , the insulator  352 , and the insulator  354 . The conductor  356  has a function of a plug or a wiring that is connected to the transistor  550 . Note that the conductor  356  can be provided using a material similar to those for the conductor  328  and the conductor  330 . 
     Note that as the insulator  350 , like the insulator  324 , an insulator having a barrier property against hydrogen is preferably used. Furthermore, the conductor  356  preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator  350  having a barrier property against hydrogen. With the structure, it can be considered that the transistor  550  and the transistor  500  are placed in different regions that are separated by a barrier layer, and hydrogen can be inhibited from diffusing from the transistor  550  and a region in the vicinity of the transistor  550  into the transistor  500 , in particular, the oxide semiconductor film included in the transistor  500 . 
     Note that for the conductor having a barrier property against hydrogen, tantalum nitride is preferably used, for example. In addition, by stacking tantalum nitride and tungsten, which has high conductivity, the diffusion of hydrogen from the transistor  550  can be inhibited while the conductivity as a wiring is kept. In that case, a configuration in which a tantalum nitride layer having a barrier property against hydrogen is in contact with the insulator  350  having a barrier property against hydrogen is preferable. This is because the barrier property against hydrogen is increased. 
     A wiring layer may be provided over the insulator  354  and the conductor  356 . For example, in  FIG. 5 , an insulator  360 , an insulator  362 , and an insulator  364  are provided to be stacked in this order. Furthermore, a conductor  366  is formed in the insulator  360 , the insulator  362 , and the insulator  364 . The conductor  366  has a function of a plug or a wiring. Note that the conductor  366  can be provided using a material similar to those for the conductor  328  and the conductor  330 . 
     Note that for example, as the insulator  360 , like the insulator  324 , an insulator having a barrier property against hydrogen is preferably used. Furthermore, the conductor  366  preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator  360  having a barrier property against hydrogen. With this structure, the transistor  550  and the transistor  500  can be separated by a barrier layer, so that diffusion of hydrogen from the transistor  550  into the transistor  500  can be inhibited. 
     A wiring layer may be provided over the insulator  364  and the conductor  366 . For example, in  FIG. 5 , an insulator  370 , an insulator  372 , and an insulator  374  are provided to be stacked in this order. Furthermore, a conductor  376  is formed in the insulator  370 , the insulator  372 , and the insulator  374 . The conductor  376  has a function of a plug or a wiring. Note that the conductor  376  can be provided using a material similar to those for the conductor  328  and the conductor  330 . 
     Note that for example, as the insulator  370 , like the insulator  324 , an insulator having a barrier property against hydrogen is preferably used. Furthermore, the conductor  376  preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator  370  having a barrier property against hydrogen. With this structure, the transistor  550  and the transistor  500  can be separated by a barrier layer, so that diffusion of hydrogen from the transistor  550  into the transistor  500  can be inhibited. 
     A wiring layer may be provided over the insulator  374  and the conductor  376 . For example, in  FIG. 5 , an insulator  380 , an insulator  382 , and an insulator  384  are provided to be stacked in this order. Furthermore, a conductor  386  is formed in the insulator  380 , the insulator  382 , and the insulator  384 . The conductor  386  has a function of a plug or a wiring. Note that the conductor  386  can be provided using a material similar to those for the conductor  328  and the conductor  330 . 
     Any number of layers may be placed as the layers forming the plugs. 
     Note that for example, as the insulator  380 , like the insulator  324 , an insulator having a barrier property against hydrogen is preferably used. Furthermore, the conductor  386  preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator  380  having a barrier property against hydrogen. With this structure, the transistor  550  and the transistor  500  can be separated by a barrier layer, so that diffusion of hydrogen from the transistor  550  into the transistor  500  can be inhibited. 
     Although the wiring layer including the conductor  356 , the wiring layer including the conductor  366 , the wiring layer including the conductor  376 , and the wiring layer including the conductor  386  are described in the above, the structure can be regarded as the same as a structure of four stacked wiring layers each including the conductor  356 . With four or more wiring layers each of which is similar to the wiring layer including the conductor  356 , the above-described barrier property can be increased. In contrast, the structure of the semiconductor device is not limited to the above-described stacked-layer structure and the like. For example, the number of wiring layers similar to the wiring layer including the conductor  356  may be less than or equal to three. When the number of stacked layers is small, the cost can be reduced. 
     An insulator  510 , an insulator  512 , an insulator  514 , and an insulator  516  are stacked sequentially over the insulator  384 . A substance having a barrier property against oxygen or hydrogen is preferably used for any of the insulator  510 , the insulator  512 , the insulator  514 , and the insulator  516 . 
     For example, for the insulator  510  and the insulator  514 , it is preferable to use a film having a barrier property that inhibits diffusion of hydrogen or impurities from the substrate  311 , a region where the transistor  550  is provided, or the like into the region where the transistor  500  is provided. Therefore, a material similar to that for the insulator  324  can be used. 
     For the film having a barrier property against hydrogen, silicon nitride formed by a CVD method can be used, for example. Here, diffusion of hydrogen to a semiconductor element including an oxide semiconductor, such as the transistor  500 , degrades the characteristics of the semiconductor element in some cases. Therefore, a film that inhibits hydrogen diffusion is preferably provided between the transistor  500  and the transistor  550 . 
     In addition, for the film having a barrier property against hydrogen, a metal oxide such as aluminum oxide, hafnium oxide, or tantalum oxide is preferably used for the insulator  510  and the insulator  514 , for example. 
     In particular, aluminum oxide has an excellent blocking effect that prevents the passage of both oxygen and impurities such as hydrogen or moisture which are factors of change in electrical characteristics of the transistor. Accordingly, aluminum oxide can prevent mixing of impurities such as hydrogen or moisture into the transistor  500  in the manufacturing process and after the manufacturing of the transistor. In addition, release of oxygen from the oxide included in the transistor  500  can be inhibited. Therefore, aluminum oxide is suitably used for the protective film of the transistor  500 . 
     In addition, for the insulator  512  and the insulator  516 , a material similar to that for the insulator  320  can be used, for example. Furthermore, when a material with a comparatively low permittivity is used for these insulators, parasitic capacitance generated between wirings can be reduced. A silicon oxide film, a silicon oxynitride film, or the like can be used for the insulator  512  and the insulator  516 , for example. 
     Furthermore, a conductor  518 , a conductor included in the transistor  500  (a conductor  503  for example), and the like are embedded in the insulator  510 , the insulator  512 , the insulator  514 , and the insulator  516 . Note that the conductor  518  has a function of a plug or a wiring that is connected to the capacitor  600  or the transistor  550 . The conductor  518  can be provided using a material similar to those for the conductor  328  and the conductor  330 . 
     In particular, the conductor  518  in a region in contact with the insulator  510  and the insulator  514  is preferably a conductor having a barrier property against oxygen, hydrogen, and water. With this structure, the transistor  550  and the transistor  500  can be separated by a layer having a barrier property against oxygen, hydrogen, and water; thus, diffusion of hydrogen from the transistor  550  into the transistor  500  can be inhibited. 
     The transistor  500  is provided above the insulator  516 . 
     As illustrated in  FIG. 7A  and  FIG. 7B , the transistor  500  includes the conductor  503  positioned to be embedded in the insulator  514  and the insulator  516 ; an insulator  520  positioned over the insulator  516  and the conductor  503 ; an insulator  522  positioned over the insulator  520 ; an insulator  524  positioned over the insulator  522 ; an oxide  530   a  positioned over the insulator  524 ; an oxide  530   b  positioned over the oxide  530   a;  a conductor  542   a  and a conductor  542   b  positioned apart from each other over the oxide  530   b;  an insulator  580  that is positioned over the conductor  542   a  and the conductor  542   b  and is provided with an opening formed to overlap with a region between the conductor  542   a  and the conductor  542   b;  an insulator  545  positioned on a bottom surface and a side surface of an opening; and a conductor  560  positioned on a formation surface of the insulator  545 . 
     In addition, as illustrated in  FIG. 7A  and  FIG. 7B , an insulator  544  is preferably positioned between the insulator  580  and the oxide  530   a,  the oxide  530   b,  the conductor  542   a,  and the conductor  542   b.  Furthermore, as illustrated in  FIG. 7A  and  FIG. 7B , the conductor  560  preferably includes a conductor  560   a  provided on the inner side of the insulator  545  and a conductor  560   b  provided to be embedded on the inner side of the conductor  560   a.  Moreover, as illustrated in  FIG. 7A  and  FIG. 7B , an insulator  574  is preferably positioned over the insulator  580 , the conductor  560 , and the insulator  545 . 
     Note that in this specification and the like, the oxide  530   a  and the oxide  530   b  are sometimes collectively referred to as an oxide  530 . 
     Although a structure in which the two layers of the oxide  530   a  and the oxide  530   b  are stacked in the transistor  500  is described, the present invention is not limited thereto. For example, it is possible to employ a structure in which a single layer of the oxide  530   b  or a stacked-layer structure of three or more layers is provided. 
     Furthermore, although the conductor  560  is shown to have a stacked-layer structure of two layers in the transistor  500 , the present invention is not limited thereto. For example, the conductor  560  may have a single-layer structure or a stacked-layer structure of three or more layers. Note that the transistors  500  illustrated in  FIG. 5 ,  FIG. 6 , and  FIG. 7A  are examples, and the structures are not limited thereto; an appropriate transistor can be used in accordance with a circuit configuration or a driving method. 
     Here, the conductor  560  functions as a gate electrode of the transistor, and the conductor  542   a  and the conductor  542   b  each function as a source electrode or a drain electrode. As described above, the conductor  560  is formed to be embedded in the opening of the insulator  580 . The opening can be formed in a region sandwiched between the conductor  542   a  and the conductor  542   b.  The positions of the conductor  560 , the conductor  542   a,  and the conductor  542   b  with respect to the opening of the insulator  580  can be selected in a self-aligned manner. That is, in the transistor  500 , the gate electrode can be positioned between the source electrode and the drain electrode in a self-aligned manner. Therefore, the conductor  560  can be formed without an alignment margin, resulting in a reduction in the area occupied by the transistor  500 . Accordingly, miniaturization and high integration of the semiconductor device can be achieved. 
     In addition, since the conductor  560  is formed in the region between the conductor  542   a  and the conductor  542   b  in a self-aligned manner, the conductor  560  does not have a region overlapping with the conductor  542   a  or the conductor  542   b.  Thus, parasitic capacitance formed between the conductor  560  and each of the conductor  542   a  and the conductor  542   b  can be reduced. As a result, the switching speed of the transistor  500  can be improved, and the phase shifter can have high frequency characteristics. 
     The conductor  560  functions as a first gate (also referred to as top gate) electrode in some cases. The conductor  503  functions as a second gate (also referred to as bottom gate) electrode in some cases. In that case, the threshold voltage of the transistor  500  can be controlled by changing a potential applied to the conductor  503  independently of a potential applied to the conductor  560 . In particular, the threshold voltage of the transistor  500  can be higher and the off-state current can be reduced by applying a negative potential to the conductor  503 . Thus, a drain current at the time when a potential applied to the conductor  560  is 0 V can be lower in the case where a negative potential is applied to the conductor  503  than in the case where a negative potential is not applied to the conductor  503 . 
     The conductor  503  is positioned to overlap with the oxide  530  and the conductor  560 . Thus, in the case where potentials are applied to the conductor  560  and the conductor  503 , an electric field generated from the conductor  560  and an electric field generated from the conductor  503  are connected, so that a channel formation region formed in the oxide  530  can be covered. 
     In this specification and the like, a transistor structure in which a channel formation region is electrically surrounded by electric fields of a pair of gate electrodes (a first gate electrode and a second gate electrode) is referred to as a surrounded channel (S-channel) structure. The S-channel structure disclosed in this specification and the like is different from a Fin-type structure and a planar structure. With the S-channel structure, resistance to a short-channel effect can be enhanced, that is, a transistor in which a short-channel effect is unlikely to occur can be provided. 
     In addition, the conductor  503  has a structure similar to that of the conductor  518 ; a conductor  503   a  is formed in contact with an inner wall of an opening in the insulator  514  and the insulator  516 , and a conductor  503   b  is formed on the inner side. Note that although the transistor  500  having a structure in which the conductor  503   a  and the conductor  503   b  are stacked is illustrated, the present invention is not limited thereto. For example, the conductor  503  may be provided as a single layer or to have a stacked-layer structure of three or more layers. 
     Here, for the conductor  503   a,  a conductive material that has a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, and a copper atom and a function of making the impurities less likely to pass is preferably used. Alternatively, it is preferable to use a conductive material that has a function of inhibiting diffusion of oxygen (e.g., at least one of oxygen atoms, oxygen molecules, and the like) and a function of making oxygen less likely to pass. Note that in this specification, a function of inhibiting diffusion of impurities or oxygen means a function of inhibiting diffusion of any one or all of the impurities and oxygen. 
     For example, when the conductor  503   a  has a function of inhibiting diffusion of oxygen, a reduction in conductivity of the conductor  503   b  due to oxidation can be inhibited. 
     In addition, in the case where the conductor  503  also functions as a wiring, a conductive material with high conductivity that contains tungsten, copper, or aluminum as its main component is preferably used for the conductor  503   b.  Note that although the conductor  503  has a stacked layer of the conductor  503   a  and the conductor  503   b  in this embodiment, the conductor  503  may have a single-layer structure. 
     The insulator  520 , the insulator  522 , and the insulator  524  have a function of a second gate insulating film. 
     Here, as the insulator  524  that is in contact with the oxide  530 , an insulator that contains oxygen more than oxygen in the stoichiometric composition is preferably used. Such oxygen is easily released from the film by heating. In this specification and the like, oxygen released by heating is sometimes referred to as “excess oxygen”. That is, a region containing excess oxygen (also referred to as an excess-oxygen region) is preferably formed in the insulator  524 . When such an insulator containing excess oxygen is provided in contact with the oxide  530 , oxygen vacancies (Vo) in the oxide  530  can be reduced and the reliability of the transistor  500  can be improved. When hydrogen enters the oxygen vacancies in the oxide  530 , such defects (hereinafter, referred to as VoH in some cases) serve as donors and generate electrons serving as carriers in some cases. In other cases, bonding of part of hydrogen to oxygen bonded to a metal atom generates electrons serving as carriers. Thus, a transistor including an oxide semiconductor that contains a large amount of hydrogen is likely to have normally-on characteristics. Moreover, hydrogen in an oxide semiconductor is easily transferred by a stress such as heat or an electric field; thus, a large amount of hydrogen in an oxide semiconductor might reduce the reliability of the transistor. In one embodiment of the present invention, VoH in the oxide  530  is preferably reduced as much as possible so that the oxide  530  becomes a highly purified intrinsic or substantially highly purified intrinsic oxide. It is effective to remove impurities such as moisture and hydrogen in an oxide semiconductor (sometimes described as “dehydration” or “dehydrogenation treatment”) and to compensate for oxygen vacancies by supplying oxygen to the oxide semiconductor (sometimes described as “oxygen adding treatment”) in order to obtain an oxide semiconductor whose VoH is sufficiently reduced. When an oxide semiconductor with sufficiently reduced impurities such as VoH is used for a channel formation region of a transistor, the transistor can have stable electrical characteristics. 
     The surface of the insulator  524  may be etched so that oxygen is easily released. When the oxide semiconductor is patterned, the surface is etched in some cases; the surface of the insulator  524  is referred to as an overetched surface in some cases. 
     As the insulator including an excess-oxygen region, specifically, an oxide material that releases part of oxygen by heating is preferably used. An oxide that releases oxygen by heating is an oxide film in which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 18  atoms/cm 3 , preferably greater than or equal to 1.0×10 19  atoms/cm 3 , further preferably greater than or equal to 2.0×10 19  atoms/cm 3  or greater than or equal to 3.0×10 20  atoms/cm 3  in TDS (Thermal Desorption Spectroscopy) analysis. Note that the temperature of the film surface in the TDS analysis is preferably within the range of 100° C. to 700° C., or 100° C. to 400° C. 
     One or more of heat treatment, microwave treatment, and RF treatment may be performed in a state in which the insulator including the excess-oxygen region and the oxide  530  are in contact with each other. By the treatment, water or hydrogen in the oxide  530  can be removed. For example, in the oxide  530 , dehydrogenation can be performed when a reaction in which a bond of VoH is cut occurs, i.e., a reaction of VoH→Vo+H occurs. Part of hydrogen generated at this time is bonded to oxygen to be H 2 O, and removed from the oxide  530  or an insulator near the oxide  530  in some cases. Some hydrogen may be gettered into the conductor  542   a  and the conductor  542   b  in some cases. 
     For the microwave treatment, for example, an apparatus including a power supply that generates high-density plasma or an apparatus including a power supply that applies RF to the substrate side is suitably used. For example, the use of an oxygen-containing gas and high-density plasma enables high-density oxygen radicals to be generated, and application of the RF to the substrate side allows the oxygen radicals generated by the high-density plasma to be efficiently introduced into the oxide  530  or an insulator in the vicinity of the oxide  530 . The pressure in the microwave treatment is higher than or equal to 133 Pa, preferably higher than or equal to 200 Pa, further preferably higher than or equal to 400 Pa. As a gas introduced into an apparatus for performing the microwave treatment, for example, oxygen and argon are used and the oxygen flow rate (O 2 /(O 2 +Ar)) is lower than or equal to 50%, preferably higher than or equal to 10% and lower than or equal to 30%. 
     In a manufacturing process of the transistor  500 , heat treatment is preferably performed with the surface of the oxide  530  exposed. The heat treatment is performed at higher than or equal to 100° C. and lower than or equal to 450° C., preferably higher than or equal to 350° C. and lower than or equal to 400° C., for example. Note that the heat treatment is performed in a nitrogen gas or inert gas atmosphere, or an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more. For example, the heat treatment is preferably performed in an oxygen atmosphere. Accordingly, oxygen can be supplied to the oxide  530  to reduce oxygen vacancies (Vo). The heat treatment may be performed under reduced pressure. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in a nitrogen gas or 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 released oxygen. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more, and then another heat treatment is successively performed in a nitrogen gas or inert gas atmosphere. 
     Note that the oxygen adding treatment performed on the oxide  530  can promote a reaction in which oxygen vacancies in the oxide  530  are filled with supplied oxygen, i.e., a reaction of Vo+O→null. Furthermore, hydrogen remaining in the oxide  530  reacts with supplied oxygen, so that the hydrogen can be removed as H 2 O (dehydration). This can inhibit recombination of hydrogen remaining in the oxide  530  with oxygen vacancies and formation of VoH. 
     In addition, in the case where the insulator  524  includes an excess-oxygen region, it is preferable that the insulator  522  have a function of inhibiting diffusion of oxygen (e.g., an oxygen atom, an oxygen molecule, or the like) (through which oxygen is unlikely to pass). 
     When the insulator  522  has a function of inhibiting diffusion of oxygen or impurities, oxygen contained in the oxide  530  does not diffuse into the insulator  520  side, which is preferable. Furthermore, the conductor  503  can be inhibited from reacting with oxygen contained in the insulator  524  or the oxide  530 . 
     For the insulator  522 , a single layer or stacked layers of an insulator containing what is called a high-k material such as aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO 3 ), or (Ba,Sr)TiO 3  (BST) are preferably used, for example. As miniaturization and high integration of transistors progress, a problem such as leakage current might arise because of a thinner gate insulating film. When a high-k material is used for an insulator functioning as the gate insulating film, a gate potential during transistor operation can be reduced while the physical thickness is maintained. 
     It is particularly preferable to use an insulator containing an oxide of one or both of aluminum and hafnium, which is an insulating material having a function of inhibiting diffusion of impurities, oxygen, and the like (through which oxygen is unlikely to pass). Aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), or the like is preferably used as the insulator containing an oxide of one or both of aluminum and hafnium. In the case where the insulator  522  is formed using such a material, the insulator  522  functions as a layer that inhibits release of oxygen from the oxide  530  and mixing of impurities such as hydrogen from the periphery of the transistor  500  into the oxide  530 . 
     Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxide may be added to these insulators, for example. Alternatively, these insulators may be subjected to nitriding treatment. The insulator over which silicon oxide, silicon oxynitride, or silicon nitride is stacked may be used. 
     In addition, it is preferable that the insulator  520  be thermally stable. For example, silicon oxide and silicon oxynitride are suitable because they are thermally stable. Furthermore, when an insulator that is a high-k material is combined with silicon oxide or silicon oxynitride, the insulator  520  having a stacked-layer structure that has thermal stability and a high relative permittivity can be obtained. 
     Note that in the transistor  500  in  FIG. 7A  and  FIG. 7B , the insulator  520 , the insulator  522 , and the insulator  524  are illustrated as the second gate insulating film having a stacked-layer structure of three layers; however, the second gate insulating film may be a single layer or may have a stacked-layer structure of two layers or four or more layers. In such cases, without limitation to a stacked-layer structure formed of the same material, a stacked-layer structure formed of different materials may be employed. 
     In the transistor  500 , a metal oxide functioning as an oxide semiconductor is preferably used as the oxide  530  including a channel formation region. For example, as the oxide  530 , a metal oxide such as an In—M—Zn oxide (the element M is one or more kinds selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like) is preferably used. 
     The metal oxide functioning as an oxide semiconductor may be formed by a sputtering method or an ALD (Atomic Layer Deposition) method. Note that the metal oxide functioning as an oxide semiconductor is described in detail in another embodiment. 
     The metal oxide functioning as the channel formation region in the oxide  530  has a band gap of preferably 2 eV or higher, further preferably 2.5 eV or higher. With the use of a metal oxide having such a wide bandgap, the off-state current of the transistor can be reduced. 
     When the oxide  530  includes the oxide  530   a  under the oxide  530   b,  it is possible to inhibit diffusion of impurities into the oxide  530   b  from the components formed below the oxide  530   a.    
     Note that the oxide  530  preferably has a stacked-layer structure of a plurality of oxide layers that differ in the atomic ratio of metal atoms. Specifically, the atomic ratio of the element M to the constituent elements in the metal oxide used as the oxide  530   a  is preferably higher than the atomic ratio of the element M to the constituent elements in the metal oxide used as the oxide  530   b.  In addition, the atomic ratio of the element M to In in the metal oxide used as the oxide  530   a  is preferably higher than the atomic ratio of the element M to In in the metal oxide used as the oxide  530   b.  Furthermore, the atomic ratio of In to the element M in the metal oxide used as the oxide  530   b  is preferably higher than the atomic ratio of In to the element M in the metal oxide used as the oxide  530   a.    
     The energy of the conduction band minimum of the oxide  530   a  is preferably higher than the energy of the conduction band minimum of the oxide  530   b.  In other words, the electron affinity of the oxide  530   a  is preferably smaller than the electron affinity of the oxide  530   b.    
     Here, the energy level of the conduction band minimum gently changes at a junction portion of the oxide  530   a  and the oxide  530   b.  In other words, the energy level of the conduction band minimum at the junction portion of the oxide  530   a  and the oxide  530   b  continuously changes or is continuously connected. This can be obtained by decreasing the density of defect states in a mixed layer formed at the interface between the oxide  530   a  and the oxide  530   b.    
     Specifically, when the oxide  530   a  and the oxide  530   b  contain a common element (as a main component) in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, in the case where the oxide  530   b  is an In—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like is used as the oxide  530   a.    
     At this time, the oxide  530   b  serves as a main carrier path. When the oxide  530   a  has the above-described structure, the density of defect states at the interface between the oxide  530   a  and the oxide  530   b  can be made low. Thus, the influence of interface scattering on carrier conduction is small, and the transistor  500  can have a high on-state current. 
     The conductor  542   a  and the conductor  542   b  functioning as the source electrode and the drain electrode are provided over the oxide  530   b.  For the conductor  542   a  and conductor  542   b,  it is preferable to use a metal element selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium, ruthenium, iridium, strontium, and lanthanum; an alloy containing any of the above metal elements; an alloy containing a combination of the above metal elements; or the like. For example, it is preferable to use tantalum nitride, titanium nitride, tungsten, a nitride containing titanium and aluminum, a nitride containing tantalum and aluminum, ruthenium oxide, ruthenium nitride, an oxide containing strontium and ruthenium, an oxide containing lanthanum and nickel, or the like. In addition, tantalum nitride, titanium nitride, a nitride containing titanium and aluminum, a nitride containing tantalum and aluminum, ruthenium oxide, ruthenium nitride, an oxide containing strontium and ruthenium, and an oxide containing lanthanum and nickel are preferable because they are oxidation-resistant conductive materials or materials that retain their conductivity even after absorbing oxygen. Furthermore, a metal nitride film of tantalum nitride or the like is preferable because it has a barrier property against hydrogen or oxygen. 
     In addition, although the conductor  542   a  and the conductor  542   b  each having a single-layer structure are illustrated in  FIG. 7A , a stacked-layer structure of two or more layers may be employed. For example, it is preferable to stack a tantalum nitride film and a tungsten film. Alternatively, a titanium film and an aluminum film may be stacked. Alternatively, a two-layer structure where an aluminum film is stacked over a tungsten film, a two-layer structure where a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure where a copper film is stacked over a titanium film, or a two-layer structure where a copper film is stacked over a tungsten film may be employed. 
     Other examples include a three-layer structure where a titanium film or a titanium nitride film is formed, an aluminum film or a copper film is stacked over the titanium film or the titanium nitride film, and a titanium film or a titanium nitride film is formed over the aluminum film or the copper film; and a three-layer structure where a molybdenum film or a molybdenum nitride film is formed, an aluminum film or a copper film is stacked over the molybdenum film or the molybdenum nitride film, and a molybdenum film or a molybdenum nitride film is formed over the aluminum film or the copper film. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used. 
     In addition, as illustrated in  FIG. 7A , a region  543   a  and a region  543   b  are sometimes formed as low-resistance regions at an interface between the oxide  530  and the conductor  542   a  (the conductor  542   b ) and in the vicinity of the interface. In that case, the region  543   a  functions as one of a source region and a drain region, and the region  543   b  functions as the other of the source region and the drain region. Furthermore, the channel formation region is formed in a region between the region  543   a  and the region  543   b.    
     When the conductor  542   a  (the conductor  542   b ) is provided to be in contact with the oxide  530 , the oxygen concentration in the region  543   a  (the region  543   b ) sometimes decreases. In addition, a metal compound layer that contains the metal contained in the conductor  542   a  (the conductor  542   b ) and the component of the oxide  530  is sometimes formed in the region  543   a  (the region  543   b ). In such a case, the carrier density of the region  543   a  (the region  543   b ) increases, and the region  543   a  (the region  543   b ) becomes a low-resistance region. 
     The insulator  544  is provided to cover the conductor  542   a  and the conductor  542   b  and inhibits oxidation of the conductor  542   a  and the conductor  542   b.  At this time, the insulator  544  may be provided to cover a side surface of the oxide  530  and to be in contact with the insulator  524 . 
     A metal oxide containing one kind or two or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, neodymium, lanthanum, magnesium, and the like can be used as the insulator  544 . Alternatively, silicon nitride oxide, silicon nitride, or the like can be used for the insulator  544 . 
     It is particularly preferable to use an insulator containing an oxide of one or both of aluminum and hafnium, such as aluminum oxide, hafnium oxide, or an oxide containing aluminum and hafnium (hafnium aluminate), as the insulator  544 . In particular, hafnium aluminate has higher heat resistance than a hafnium oxide film. Therefore, hafnium aluminate is preferable because it is unlikely to be crystallized by heat treatment in a later step. Note that the insulator  544  is not an essential component when the conductor  542   a  and the conductor  542   b  are oxidation-resistant materials or do not significantly lose their conductivity even after absorbing oxygen. Design is appropriately set in consideration of required transistor characteristics. 
     When the insulator  544  is included, diffusion of impurities such as water and hydrogen contained in the insulator  580  into the oxide  530   b  through the insulator  545  can be inhibited. Furthermore, oxidation of the conductor  560  due to excess oxygen contained in the insulator  580  can be inhibited. 
     The insulator  545  functions as a first gate insulating film. Like the insulator  524 , the insulator  545  is preferably formed using an insulator that contains excess oxygen and releases oxygen by heating. 
     Specifically, silicon oxide containing excess oxygen, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, or porous silicon oxide can be used. In particular, silicon oxide and silicon oxynitride are preferable because they are thermally stable. 
     When an insulator containing excess oxygen is provided as the insulator  545 , oxygen can be effectively supplied from the insulator  545  to the channel formation region of the oxide  530   b.  Furthermore, as in the insulator  524 , the concentration of impurities such as water or hydrogen in the insulator  545  is preferably reduced. The thickness of the insulator  545  is preferably greater than or equal to 1 nm and less than or equal to 20 nm. 
     Furthermore, to efficiently supply excess oxygen contained in the insulator  545  to the oxide  530 , a metal oxide may be provided between the insulator  545  and the conductor  560 . The metal oxide preferably inhibits diffusion of oxygen from the insulator  545  to the conductor  560 . Providing the metal oxide that inhibits diffusion of oxygen inhibits diffusion of excess oxygen from the insulator  545  to the conductor  560 . That is, reduction in the amount of excess oxygen supplied to the oxide  530  can be inhibited. Moreover, oxidation of the conductor  560  due to excess oxygen can be inhibited. For the metal oxide, a material that can be used for the insulator  544  is used. 
     Note that the insulator  545  may have a stacked-layer structure like the second gate insulating film. As miniaturization and high integration of transistors progress, a problem such as leakage current might arise because of a thinner gate insulating film. For that reason, when the insulator functioning as the gate insulating film has a stacked-layer structure of a high-k material and a thermally stable material, a gate potential during transistor operation can be reduced while the physical thickness is maintained. Furthermore, the stacked-layer structure can be thermally stable and have a high relative permittivity. 
     Although the conductor  560  that functions as the first gate electrode and has a two-layer structure is illustrated in  FIG. 7A  and  FIG. 7B , a single-layer structure or a stacked-layer structure of three or more layers may be employed. 
     For the conductor  560   a,  it is preferable to use a conductive material having a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, a nitrogen molecule, a nitrogen oxide molecule (N 2 O, NO, NO 2 , and the like), and a copper atom. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like). When the conductor  560   a  has a function of inhibiting diffusion of oxygen, it is possible to inhibit a reduction in conductivity of the conductor  560   b  due to oxidation caused by oxygen contained in the insulator  545 . As a conductive material having a function of inhibiting diffusion of oxygen, for example, tantalum, tantalum nitride, ruthenium, ruthenium oxide, or the like is preferably used. For the conductor  560   a,  the oxide semiconductor that can be used as the oxide  530  can be used. In that case, when the conductor  560   b  is deposited by a sputtering method, the conductor  560   a  can have a reduced value of electrical resistance to be a conductor. Such a conductor can be referred to as an OC (Oxide Conductor) electrode. 
     In addition, a conductive material containing tungsten, copper, or aluminum as its main component is preferably used for the conductor  560   b.  Furthermore, the conductor  560   b  also functions as a wiring and thus a conductor having high conductivity is preferably used as the conductor  560   b.  For example, a conductive material containing tungsten, copper, or aluminum as its main component can be used. The conductor  560   b  may have a stacked-layer structure, for example, a stacked-layer structure of any of the above conductive materials and titanium or titanium nitride. 
     The insulator  580  is provided over the conductor  542   a  and the conductor  542   b  with the insulator  544  therebetween. The insulator  580  preferably includes an excess-oxygen region. For example, the insulator  580  preferably contains silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, resin, or the like. In particular, silicon oxide and silicon oxynitride are preferable because they are thermally stable. In particular, silicon oxide and porous silicon oxide are preferable because an excess-oxygen region can be easily formed in a later step. 
     The insulator  580  preferably includes an excess-oxygen region. When the insulator  580  that releases oxygen by heating is provided, oxygen in the insulator  580  can be efficiently supplied to the oxide  530 . Note that the concentration of impurities such as water or hydrogen in the insulator  580  is preferably reduced. 
     The opening of the insulator  580  is formed to overlap with the region between the conductor  542   a  and the conductor  542   b.  Accordingly, the conductor  560  is formed to be embedded in the opening of the insulator  580  and the region between the conductor  542   a  and the conductor  542   b.    
     The gate length needs to be short for miniaturization of the semiconductor device, but it is necessary to prevent a reduction in conductivity of the conductor  560 . When the conductor  560  is made thick to achieve this, the conductor  560  might have a shape with a high aspect ratio. In this embodiment, the conductor  560  is provided to be embedded in the opening of the insulator  580 ; thus, even when the conductor  560  has a shape with a high aspect ratio, the conductor  560  can be formed without collapsing during the process. 
     The insulator  574  is preferably provided in contact with a top surface of the insulator  580 , a top surface of the conductor  560 , and a top surface of the insulator  545 . When the insulator  574  is deposited by a sputtering method, excess-oxygen regions can be provided in the insulator  545  and the insulator  580 . Accordingly, oxygen can be supplied from the excess-oxygen regions to the oxide  530 . 
     For example, a metal oxide containing one kind or two or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium, and the like can be used as the insulator  574 . 
     In particular, aluminum oxide has a high barrier property, and even a thin aluminum oxide film having a thickness of greater than or equal to 0.5 nm and less than or equal to 3.0 nm can inhibit diffusion of hydrogen and nitrogen. Accordingly, aluminum oxide deposited by a sputtering method serves as an oxygen supply source and can also have a function of a barrier film against impurities such as hydrogen. 
     In addition, an insulator  581  functioning as an interlayer film is preferably provided over the insulator  574 . As in the insulator  524  or the like, the concentration of impurities such as water or hydrogen in the insulator  581  is preferably reduced. 
     Furthermore, a conductor  540   a  and a conductor  540   b  are positioned in openings formed in the insulator  581 , the insulator  574 , the insulator  580 , and the insulator  544 . The conductor  540   a  and the conductor  540   b  are provided to face each other with the conductor  560  therebetween. The structures of the conductor  540   a  and the conductor  540   b  are similar to a structure of a conductor  546  and a conductor  548  that are described later. 
     An insulator  582  is provided over the insulator  581 . A substance having a barrier property against oxygen or hydrogen is preferably used for the insulator  582 . Therefore, a material similar to that for the insulator  514  can be used for the insulator  582 . For the insulator  582 , a metal oxide such as aluminum oxide, hafnium oxide, or tantalum oxide is preferably used, for example. 
     In particular, aluminum oxide has an excellent blocking effect that prevents the passage of both oxygen and impurities such as hydrogen and moisture which are factors of change in electrical characteristics of the transistor. Accordingly, aluminum oxide can prevent mixing of impurities such as hydrogen and moisture into the transistor  500  in the manufacturing process and after the manufacturing of the transistor. In addition, release of oxygen from the oxide included in the transistor  500  can be inhibited. Therefore, aluminum oxide is suitably used for the protective film of the transistor  500 . 
     In addition, an insulator  586  is provided over the insulator  582 . For the insulator  586 , a material similar to that for the insulator  320  can be used. Furthermore, when a material with a comparatively low permittivity is used for these insulators, parasitic capacitance generated between wirings can be reduced. A silicon oxide film, a silicon oxynitride film, or the like can be used for the insulator  586 , for example. 
     Furthermore, the conductor  546 , the conductor  548 , and the like are embedded in the insulator  520 , the insulator  522 , the insulator  524 , the insulator  544 , the insulator  580 , the insulator  574 , the insulator  581 , the insulator  582 , and the insulator  586 . 
     The conductor  546  and the conductor  548  have functions of plugs or wirings that are connected to the capacitor  600 , the transistor  500 , or the transistor  550 . The conductor  546  and the conductor  548  can be provided using materials similar to those for the conductor  328  and the conductor  330 . 
     After the transistor  500  is formed, an opening may be formed to surround the transistor  500  and an insulator having a high barrier property against hydrogen or water may be formed to cover the opening. Surrounding the transistor  500  with the insulator having a high barrier property can prevent entry of moisture and hydrogen from the outside. Alternatively, a plurality of transistors  500  may be collectively surrounded by the insulator having a high barrier property against hydrogen or water. When an opening is formed to surround the transistor  500 , for example, the formation of an opening reaching the insulator  522  or the insulator  514  and the formation of the insulator having a high barrier property in contact with the insulator  522  or the insulator  514  are suitable because these formation steps can also serve as part of the manufacturing steps of the transistor  500 . The insulator having a high barrier property against hydrogen or water is formed using a material similar to that for the insulator  522  or the insulator  514 , for example. 
     Next, the capacitor  600  is provided above the transistor  500 . The capacitor  600  includes a conductor  610 , a conductor  620 , and an insulator  630 . 
     In addition, a conductor  612  may be provided over the conductor  546  and the conductor  548 . The conductor  612  has a function of a plug or a wiring that is connected to the transistor  500 . The conductor  610  has a function of an electrode of the capacitor  600 . Note that the conductor  612  and the conductor  610  can be formed at the same time. 
     For the conductor  612  and the conductor  610 , a metal film containing an element selected from molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, and scandium; a metal nitride film containing the above element as its component (a tantalum nitride film, a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film); or the like can be used. Alternatively, it is possible to use a conductive material such as indium tin oxide, 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, or indium tin oxide to which silicon oxide is added. 
     Although the conductor  612  and the conductor  610  each having a single-layer structure are illustrated in this embodiment, the structure is not limited thereto; a stacked-layer structure of two or more layers may be employed. For example, between a conductor having a barrier property and a conductor having high conductivity, a conductor that is highly adhesive to the conductor having a barrier property and the conductor having high conductivity may be formed. 
     The conductor  620  is provided to overlap with the conductor  610  with the insulator  630  therebetween. Note that a conductive material such as a metal material, an alloy material, or a metal oxide material can be used for the conductor  620 . It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. In addition, in the case where the conductor  620  is formed concurrently with another component such as a conductor, Cu (copper), Al (aluminum), or the like, which is a low-resistance metal material, is used. 
     An insulator  640  is provided over the conductor  620  and the insulator  630 . The insulator  640  can be provided using a material similar to that for the insulator  320 . In addition, the insulator  640  may function as a planarization film that covers an uneven shape therebelow. 
     With the use of this structure, a semiconductor device using a transistor including an oxide semiconductor can be miniaturized or highly integrated. 
     Examples of a substrate that can be used for the semiconductor device of one embodiment of the present invention include a glass substrate, a quartz substrate, a sapphire substrate, a ceramic substrate, a metal substrate (e.g., a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, and a substrate including tungsten foil), a semiconductor substrate (e.g., a single crystal semiconductor substrate, a polycrystalline semiconductor substrate, and a compound semiconductor substrate), and a SOI (Silicon on Insulator) substrate. Alternatively, a plastic substrate having heat resistance to the processing temperature in this embodiment may be used. Examples of a glass substrate include a barium borosilicate glass substrate, an aluminosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. Alternatively, crystallized glass or the like can be used. 
     Alternatively, a flexible substrate, an attachment film, paper including a fibrous material, a base film, or the like can be used as the substrate. As examples of the flexible substrate, the attachment film, the base material film, and the like, the following can be given. Examples include plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Another example is a synthetic resin such as acrylic. Other examples are polypropylene, polyester, polyvinyl fluoride, and polyvinyl chloride. Other examples are polyamide, polyimide, an aramid resin, an epoxy resin, an inorganic vapor deposition film, and paper. In particular, the use of a semiconductor substrate, a single crystal substrate, an SOI substrate, or the like enables the manufacture of small-sized transistors with a small variation in characteristics, size, shape, or the like and with high current capability. When a circuit is formed with such transistors, lower power consumption of the circuit or higher integration of the circuit can be achieved. 
     A flexible substrate may be used as the substrate, and a transistor, a resistor, a capacitor, and/or the like may be formed directly over the flexible substrate. Alternatively, a separation layer may be provided between the substrate and the transistor, the resistor, the capacitor, and/or the like. After part or the whole of a semiconductor device is completed over the separation layer, the separation layer can be used for separation from the substrate and transfer to another substrate. In such a case, the transistor, the resistor, the capacitor, and/or the like can be transferred to a substrate having low heat resistance or a flexible substrate. As the separation layer, a stack-layer structure of a tungsten film and a silicon oxide film that are inorganic films, a structure in which an organic resin film of polyimide or the like is formed over a substrate, or a silicon film containing hydrogen can be used, for example. 
     That is, a semiconductor device may be formed over one substrate and then transferred to another substrate. Examples of a substrate to which a semiconductor device is transferred include, in addition to the above-described substrates over which transistors can be formed, a paper substrate, a cellophane substrate, an aramid film substrate, a polyimide film substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupro, rayon, or regenerated polyester), or the like), a leather substrate, and a rubber substrate. With the use of any of these substrates, a flexible semiconductor device or a highly durable semiconductor device can be manufactured, high heat resistance can be provided, or a reduction in weight or thickness can be achieved. 
     Providing a semiconductor device over a flexible substrate can suppress an increase in weight and can produce a non-breakable semiconductor device. 
     Modification Example 1 of Transistor 
     A transistor  500 A illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C  is a modification example of the transistor  500  having the structure illustrated in  FIG. 7A  and  FIG. 7B .  FIG. 8A  is a top view of the transistor  500 A,  FIG. 8B  is a cross-sectional view of the transistor  500 A in the channel length direction, and  FIG. 8C  is a cross-sectional view of the transistor  500 A in the channel width direction. Note that for clarity of the drawing, some components are not illustrated in the top view of  FIG. 8A . The structure illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C  can also be used for other transistors such as the transistor  550  included in the semiconductor device of one embodiment of the present invention. 
     The transistor  500 A having the structure illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C  is different from the transistor  500  having the structure illustrated in  FIG. 7A  and  FIG. 7B  in that an insulator  552 , an insulator  513 , and an insulator  404  are included. Furthermore, the transistor  500 A having the structure illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C  is different from the transistor  500  having the structure illustrated in  FIG. 7A  and  FIG. 7B  in that the insulator  552  is provided in contact with a side surface of the conductor  540   a  and a side surface of the conductor  540   b.  Moreover, the transistor  500 A having the structure illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C  is different from the transistor  500  having the structure illustrated in  FIG. 7A  and  FIG. 7B  in that the insulator  520  is not included. 
     In the transistor  500 A having the structure illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C , the insulator  513  is provided over the insulator  512 . The insulator  404  is provided over the insulator  574  and the insulator  513 . 
     In the transistor  500 A having the structure illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C , the insulator  514 , the insulator  516 , the insulator  522 , the insulator  524 , the insulator  544 , the insulator  580 , and the insulator  574  are patterned and covered with the insulator  404 . That is, the insulator  404  is in contact with a top surface of the insulator  574 , a side surface of the insulator  574 , a side surface of the insulator  580 , a side surface of the insulator  544 , a side surface of the insulator  524 , a side surface of the insulator  522 , a side surface of the insulator  516 , a side surface of the insulator  514 , and a top surface of the insulator  513 . Thus, the oxide  530  and the like are isolated from the outside by the insulator  404  and the insulator  513 . 
     The insulator  513  and the insulator  404  preferably have high capability of inhibiting diffusion of hydrogen (e.g., at least one of a hydrogen atom, a hydrogen molecule, and the like) or a water molecule. For example, for the insulator  513  and the insulator  404 , silicon nitride or silicon nitride oxide that is a material having a high hydrogen barrier property is preferably used. This can inhibit diffusion of hydrogen or the like into the oxide  530 , thereby suppressing the degradation of the characteristics of the transistor  500 A. Consequently, the reliability of the semiconductor device of one embodiment of the present invention can be increased. 
     The insulator  552  is provided in contact with the insulator  581 , the insulator  404 , the insulator  574 , the insulator  580 , and the insulator  544 . The insulator  552  preferably has a function of inhibiting diffusion of hydrogen or water molecules. For example, for the insulator  552 , an insulator such as silicon nitride, aluminum oxide, or silicon nitride oxide that is a material having a high hydrogen barrier property is preferably used. In particular, it is preferable to use silicon nitride as the insulator  552  because of its high hydrogen barrier property. The use of a material having a high hydrogen barrier property for the insulator  552  can inhibit diffusion of impurities such as water and hydrogen from the insulator  580  and the like into the oxide  530  through the conductor  540   a  and the conductor  540   b.  Furthermore, oxygen contained in the insulator  580  can be inhibited from being absorbed by the conductor  540   a  and the conductor  540   b.  As described above, the reliability of the semiconductor device of one embodiment of the present invention can be increased. 
     Modification Example 2 of Transistor 
     A structure example of a transistor  500 B is described with reference to  FIG. 9A , FIG. 
       9 B, and  FIG. 9C .  FIG. 9A  is a top view of the transistor  500 B.  FIG. 9B  is a cross-sectional view of a portion indicated by dashed-dotted line L 1 -L 2  in  FIG. 9A .  FIG. 9C  is a cross-sectional view of a portion indicated by a dashed-dotted line W 1 -W 2  in  FIG. 9A . Note that for clarity of the drawing, some components are not illustrated in the top view of  FIG. 9A . 
     The transistor  500 B is a modification example of the transistor  500  and can be replaced with the transistor  500 . Therefore, differences of the transistor  500 B from the transistor  500  are mainly described to avoid repeated description. 
     The conductor  560  functioning as a first gate electrode includes the conductor  560   a  and the conductor  560   b  over the conductor  560   a.  For the conductor  560   a,  a conductive material that has a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, and a copper atom is preferably used. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like). 
     When the conductor  560   a  has a function of inhibiting oxygen diffusion, the range of choices for the material of the conductor  560   b  can be extended. That is, the conductor  560   a  inhibits oxidation of the conductor  560   b,  thereby preventing the decrease in conductivity. 
     The insulator  544  is preferably provided to cover the top surface and the side surface of the conductor  560  and a side surface of the insulator  545 . For the insulator  544 , an insulating material having a function of inhibiting diffusion of oxygen and impurities such as water and hydrogen is preferably used. For example, aluminum oxide or hafnium oxide is preferably used. Moreover, it is possible to use, for example, a metal oxide such as magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, or tantalum oxide, silicon nitride oxide, silicon nitride, or the like. 
     The insulator  544  can inhibit oxidation of the conductor  560 . Moreover, the insulator  544  can inhibit diffusion of impurities such as water and hydrogen contained in the insulator  580  into the transistor  500 B. 
     The transistor  500 B has the conductor  560  overlapping part of the conductor  542   a  and part of the conductor  542   b,  and thus tends to have larger parasitic capacitance than the transistor  500 . Consequently, the transistor  500 B tends to have a lower operating frequency than the transistor  500 . However, the transistor  500 B does not require steps of providing an opening in the insulator  580  and the like and embedding the conductor  560 , the insulator  545 , and the like in the opening; hence, the productivity of the transistor  500 B is higher than that of the transistor  500 . 
     The composition, structure, method, and the like described in this embodiment can be used in combination as appropriate with the compositions, structures, methods, and the like described in the other embodiments, the example, and the like. Although the example of the phase shifter that is placed in a relay station is described, the phase shifter that is reduced in size may be used for an electronic device in a central station. Moreover, the phase shifter that is reduced in size may be used for a portable terminal. 
     Embodiment 2 
     In this embodiment, an oxide semiconductor, which is one kind of metal oxide, is described. 
     The metal oxide preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more kinds selected from boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, cobalt, and the like may be contained. 
     &lt;Classification of Crystal Structure&gt; 
     First, classifications of the crystal structures of an oxide semiconductor are described with reference to  FIG. 10A .  FIG. 10A  is a diagram showing classifications of crystal structures of an oxide semiconductor, typically IGZO (metal oxide containing In, Ga, and Zn). 
     As shown in  FIG. 10A , an oxide semiconductor is roughly classified into “Amorphous”, “Crystalline”, and “Crystal”. The term “Amorphous” includes completely amorphous. The term “Crystalline” includes CAAC (c-axis-aligned crystalline), nc (nanocrystalline), and CAC (cloud-aligned composite) (excluding single crystal and poly crystal). Note that the term “Crystalline” excludes single crystal, poly crystal, and completely amorphous. The term “Crystal” includes single crystal and poly crystal. 
     Note that the structures in the thick frame in  FIG. 10A  are in an intermediate state between “Amorphous” and “Crystal”, and belong to a new crystalline phase. That is, these structures are completely different from “Amorphous”, which is energetically unstable, and “Crystal”. 
     A crystal structure of a film or a substrate can be analyzed with an X-ray diffraction (XRD) spectrum. Here,  FIG. 10B  shows an XRD spectrum, which is obtained by GIXD (Grazing-Incidence XRD) measurement, of a CAAC-IGZO film classified into “Crystalline”. Note that a GIXD method is also referred to as a thin film method or a Seemann-Bohlin method. The XRD spectrum that is shown in  FIG. 10B  and obtained by GIXD measurement is hereinafter simply referred to as an XRD spectrum. The CAAC-IGZO film in  FIG. 10B  has a composition in the vicinity of In:Ga:Zn=4:2:3 [atomic ratio]. The CAAC-IGZO film in  FIG. 10B  has a thickness of 500 nm. 
     As shown in  FIG. 10B , a clear peak indicating crystallinity is detected in the XRD spectrum of the CAAC-IGZO film. Specifically, a peak indicating c-axis alignment is detected at 2θ of around 31° in the XRD spectrum of the CAAC-IGZO film. As shown in  FIG. 10B , the peak at 2θ of around 31° is asymmetric with respect to the axis of the angle at which the peak intensity is detected. 
     A crystal structure of a film or a substrate can also be evaluated with a diffraction pattern obtained by a nanobeam electron diffraction method (NBED) (such a pattern is also referred to as a nanobeam electron diffraction pattern).  FIG. 10C  shows a diffraction pattern of the CAAC-IGZO film.  FIG. 10C  shows a diffraction pattern obtained with the NBED method in which an electron beam is incident in the direction parallel to the substrate. The composition of the CAAC-IGZO film in  FIG. 10C  is In:Ga:Zn=4:2:3 [atomic ratio] or the neighborhood thereof In the nanobeam electron diffraction method, electron diffraction is performed with a probe diameter of 1 nm. 
     As shown in  FIG. 10C , a plurality of spots indicating c-axis alignment are observed in the diffraction pattern of the CAAC-IGZO film. 
     &lt;&lt;Structure of Oxide Semiconductor&gt;&gt; 
     Oxide semiconductors might be classified in a manner different from that in  FIG. 10A  when classified in terms of the crystal structure. Oxide semiconductors are classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor, for example. Examples of the non-single-crystal oxide semiconductor include the above-described CAAC-OS and nc-OS. Other examples of the non-single-crystal oxide semiconductor include a polycrystalline oxide semiconductor, an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor. 
     Here, the above-described CAAC-OS, nc-OS, and a-like OS are described in detail. 
     [CAAC-OS] 
     The CAAC-OS is an oxide semiconductor that has a plurality of crystal regions each of which has c-axis alignment in a particular direction. Note that the particular direction refers to the film thickness direction of a CAAC-OS film, the normal direction of the surface where the CAAC-OS film is formed, or the normal direction of the surface of the CAAC-OS film. The crystal region refers to a region having a periodic atomic arrangement. When an atomic arrangement is regarded as a lattice arrangement, the crystal region also refers to a region with a uniform lattice arrangement. The CAAC-OS has a region where a plurality of crystal regions are connected in the a-b plane direction, and the region has distortion in some cases. Note that the distortion refers to a portion where the direction of a lattice arrangement changes between a region with a uniform lattice arrangement and another region with a uniform lattice arrangement in a region where a plurality of crystal regions are connected. That is, the CAAC-OS is an oxide semiconductor having c-axis alignment and having no clear alignment in the a-b plane direction. 
     Note that each of the plurality of crystal regions is formed of one or more fine crystals (crystals each of which has a maximum diameter of less than 10 nm). In the case where the crystal region is formed of one fine crystal, the maximum diameter of the crystal region is less than 10 nm. In the case where the crystal region is formed of a large number of fine crystals, the size of the crystal region may be approximately several tens of nanometers. 
     In the case of an In—M—Zn oxide (the element M is one or more kinds selected from aluminum, gallium, yttrium, tin, titanium, and the like), the CAAC-OS tends to have a layered crystal structure (also referred to as a stacked-layer structure) in which a layer containing indium (In) and oxygen (hereinafter, an In layer) and a layer containing the element M, zinc (Zn), and oxygen (hereinafter, an (M,Zn) layer) are stacked. Indium and the element M can be replaced with each other. Therefore, indium may be contained in the (M,Zn) layer. In addition, the element M may be contained in the In layer. Note that Zn may be contained in the In layer. Such a layered structure is observed as a lattice image in a high-resolution TEM image, for example. 
     When the CAAC-OS film is subjected to structural analysis by out-of-plane XRD measurement with an XRD apparatus using θ/2θ scanning, for example, a peak indicating c-axis alignment is detected at 2θ of 31° or around 31°. Note that the position of the peak indicating c-axis alignment (the value of 2θ) may change depending on the kind, composition, or the like of the metal element contained in the CAAC-OS. 
     For example, a plurality of bright spots are observed in the electron diffraction pattern of the CAAC-OS film. Note that one spot and another spot are observed point-symmetrically with a spot of the incident electron beam passing through a sample (also referred to as a direct spot) as the symmetric center. 
     When the crystal region is observed from the particular direction, a lattice arrangement in the crystal region is basically a hexagonal lattice arrangement; however, a unit lattice is not always a regular hexagon and is a non-regular hexagon in some cases. A pentagonal lattice arrangement, a heptagonal lattice arrangement, and the like are included in the distortion in some cases. Note that a clear grain boundary cannot be observed even in the vicinity of the distortion in the CAAC-OS. That is, formation of a crystal grain boundary is inhibited by the distortion of lattice arrangement. This is probably because the CAAC-OS can tolerate distortion owing to a low density of arrangement of oxygen atoms in the a-b plane direction, an interatomic bond distance changed by substitution of a metal atom, and the like. 
     A crystal structure in which a clear grain boundary is observed is what is called polycrystal. It is highly probable that the grain boundary becomes a recombination center and captures carriers and thus decreases the on-state current and field-effect mobility of a transistor, for example. Thus, the CAAC-OS in which no clear grain boundary is observed is one of crystalline oxides having a crystal structure suitable for a semiconductor layer of a transistor. Note that Zn is preferably contained to form the CAAC-OS. For example, an In—Zn oxide and an In—Ga—Zn oxide are suitable because they can inhibit generation of a grain boundary as compared with an In oxide. 
     The CAAC-OS is an oxide semiconductor with high crystallinity in which no clear grain boundary is observed. Thus, in the CAAC-OS, reduction in electron mobility due to the grain boundary is unlikely to occur. Moreover, since the crystallinity of an oxide semiconductor might be decreased by entry of impurities, formation of defects, or the like, the CAAC-OS can be regarded as an oxide semiconductor that has small amounts of impurities and defects (e.g., oxygen vacancies). Thus, an oxide semiconductor including the CAAC-OS is physically stable. Therefore, the oxide semiconductor including the CAAC-OS is resistant to heat and has high reliability. In addition, the CAAC-OS is stable with respect to high temperature in the manufacturing process (what is called thermal budget). Accordingly, the use of the CAAC-OS for the OS transistor can extend the degree of freedom of the manufacturing process. 
     [nc-OS] 
     In the nc-OS, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. In other words, the nc-OS includes a fine crystal. Note that the size of the fine crystal is, for example, greater than or equal to 1 nm and less than or equal to 10 nm, particularly greater than or equal to 1 nm and less than or equal to 3 nm; thus, the fine crystal is also referred to as a nanocrystal. Furthermore, there is no regularity of crystal orientation between different nanocrystals in the nc-OS. Thus, the orientation in the whole film is not observed. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor with some analysis methods. For example, when an nc-OS film is subjected to structural analysis using out-of-plane XRD measurement with an XRD apparatus using θ/2θ scanning, a peak indicating crystallinity is not detected. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS film is subjected to electron diffraction (also referred to as selected-area electron diffraction) using an electron beam with a probe diameter larger than the diameter of a nanocrystal (e.g., larger than or equal to 50 nm). Meanwhile, in some cases, a plurality of spots in a ring-like region with a direct spot as the center are observed in the obtained electron diffraction pattern when the nc-OS film is subjected to electron diffraction (also referred to as nanobeam electron diffraction) using an electron beam with a probe diameter nearly equal to or smaller than the diameter of a nanocrystal (e.g., 1 nm or larger and 30 nm or smaller). 
     [a-like OS] 
     The a-like OS is an oxide semiconductor having a structure between those of the nc-OS and the amorphous oxide semiconductor. The a-like OS includes a void or a low-density region. That is, the a-like OS has low crystallinity as compared with the nc-OS and the CAAC-OS. Moreover, the a-like OS has higher hydrogen concentration in the film than the nc-OS and the CAAC-OS. 
     &lt;&lt;Structure of Oxide Semiconductor&gt;&gt; 
     Next, the above-described CAC-OS is described in detail. Note that the CAC-OS relates to the material composition. 
     [CAC-OS] 
     The CAC-OS refers to one composition of a material in which elements constituting a metal oxide are unevenly distributed with a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 3 nm, or a similar size, for example. Note that a state in which one or more metal elements are unevenly distributed and regions including the metal element(s) are mixed with a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 3 nm, or a similar size in a metal oxide is hereinafter referred to as a mosaic pattern or a patch-like pattern. 
     In addition, the CAC-OS has a composition in which materials are separated into a first region and a second region to form a mosaic pattern, and the first regions are distributed in the film (this composition is hereinafter also referred to as a cloud-like composition). That is, the CAC-OS is a composite metal oxide having a composition in which the first regions and the second regions are mixed. 
     Note that the atomic ratios of In, Ga, and Zn to the metal elements contained in the CAC-OS in an In—Ga—Zn oxide are denoted with [In], [Ga], and [Zn], respectively. For example, the first region in the CAC-OS in the In—Ga—Zn oxide has [In] higher than that in the composition of the CAC-OS film. Moreover, the second region has [Ga] higher than that in the composition of the CAC-OS film. For example, the first region has higher [In] and lower [Ga] than the second region. Moreover, the second region has higher [Ga] and lower [In] than the first region. 
     Specifically, the first region includes indium oxide, indium zinc oxide, or the like as its main component. The second region includes gallium oxide, gallium zinc oxide, or the like as its main component. That is, the first region can be referred to as a region containing In as its main component. The second region can be referred to as a region containing Ga as its main component. 
     Note that a clear boundary between the first region and the second region cannot be observed in some cases. 
     For example, energy dispersive X-ray spectroscopy (EDX) is used to obtain EDX mapping, and according to the EDX mapping, the CAC-OS in the In—Ga—Zn oxide has a structure in which the region containing In as its main component (the first region) and the region containing Ga as its main component (the second region) are unevenly distributed and mixed. 
     In the case where the CAC-OS is used for a transistor, a switching function (on/off switching function) can be given to the CAC-OS owing to the complementary action of the conductivity derived from the first region and the insulating property derived from the second region. A CAC-OS has a conducting function in part of the material and has an insulating function in another part of the material; as a whole, the CAC-OS has a function of a semiconductor. Separation of the conducting function and the insulating function can maximize each function. Accordingly, when the CAC-OS is used for a transistor, a high on-state current (Ion), high field-effect mobility (μ), and excellent switching operation can be achieved. 
     An oxide semiconductor has various structures with different properties. Two or more kinds among the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, the a-like OS, the CAC-OS, the nc-OS, and the CAAC-OS may be included in an oxide semiconductor of one embodiment of the present invention. 
     &lt;Transistor Including Oxide Semiconductor&gt; 
     Next, the case where the above oxide semiconductor is used for a transistor is described. 
     When the above oxide semiconductor is used for a transistor, a transistor with high field-effect mobility can be achieved. In addition, a transistor having high reliability can be achieved. 
     An oxide semiconductor with a low carrier concentration is preferably used for the transistor. For example, the carrier concentration of an oxide semiconductor is lower than or equal to 1×10 17  cm −3 , preferably lower than or equal to 1×10 15  cm −3 , further preferably lower than or equal to 1×10 13  cm −3 , still further preferably lower than or equal to 1×10 11  cm −3 , yet further preferably lower than 1×10 10  cm −3 , and higher than or equal to 1×10 −9  cm −3 . In order to reduce the carrier concentration of an oxide semiconductor film, the impurity concentration in the oxide semiconductor film is reduced so that the density of defect states can be reduced. In this specification and the like, a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state. Note that an oxide semiconductor having a low carrier concentration may be referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. 
     A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and thus has a low density of trap states in some cases. 
     Electric charge trapped by the trap states in the oxide semiconductor takes a long time to disappear and might behave like fixed electric charge. Thus, a transistor whose channel formation region is formed in an oxide semiconductor with a high density of trap states has unstable electrical characteristics in some cases. 
     Accordingly, in order to obtain stable electrical characteristics of a transistor, reducing the impurity concentration in an oxide semiconductor is effective. In order to reduce the impurity concentration in the oxide semiconductor, it is preferable that the impurity concentration in an adjacent film be also reduced. Examples of impurities include hydrogen, nitrogen, an alkali metal, an alkaline earth metal, iron, nickel, and silicon. 
     &lt;Impurity&gt; 
     Here, the influence of each impurity in the oxide semiconductor is described. 
     When silicon or carbon, which is one of Group 14 elements, is contained in the oxide semiconductor, defect states are formed in the oxide semiconductor. Thus, the concentration of silicon or carbon in the oxide semiconductor and the concentration of silicon or carbon in the vicinity of an interface with the oxide semiconductor (the concentration obtained by secondary ion mass spectrometry (SIMS)) are each set lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     When the oxide semiconductor contains an alkali metal or an alkaline earth metal, defect states are formed and carriers are generated in some cases. Thus, a transistor using an oxide semiconductor that contains an alkali metal or an alkaline earth metal is likely to have normally-on characteristics. Thus, the concentration of an alkali metal or an alkaline earth metal in the oxide semiconductor, which is obtained using SIMS, is lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . 
     Furthermore, when the oxide semiconductor contains nitrogen, the oxide semiconductor easily becomes n-type by generation of electrons serving as carriers and an increase in carrier concentration. As a result, a transistor using an oxide semiconductor containing nitrogen as a semiconductor is likely to have normally-on characteristics. When nitrogen is contained in the oxide semiconductor, a trap state is sometimes formed. This might make the electrical characteristics of the transistor unstable. Therefore, the concentration of nitrogen in the oxide semiconductor, which is obtained using SIMS, is set lower than 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , further preferably lower than or equal to 1×10 18  atoms/cm 3 , still further preferably lower than or equal to 5×10 17  atoms/cm 3 . 
     Hydrogen contained in the oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and thus forms an oxygen vacancy in some cases. Entry of hydrogen into the oxygen vacancy generates an electron serving as a carrier in some cases. Furthermore, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier in some cases. Thus, a transistor using an oxide semiconductor containing hydrogen is likely to have normally-on characteristics. Accordingly, hydrogen in the oxide semiconductor is preferably reduced as much as possible. Specifically, the hydrogen concentration in the oxide semiconductor, which is obtained using SIMS, is set lower than 1×10 20  atoms/cm 3 , preferably lower than 1×10 19  atoms/cm 3 , further preferably lower than 5×10 18  atoms/cm 3 , still further preferably lower than 1×10 18  atoms/cm 3 . 
     When an oxide semiconductor with sufficiently reduced impurities is used for the channel formation region of the transistor, stable electrical characteristics can be given. 
     The composition, structure, method, and the like described above in this embodiment can be used in combination as appropriate with the compositions, structures, methods, and the like described in the other embodiments, the example, and the like. 
     Embodiment 3 
     In this embodiment, application examples of the above-described semiconductor device are described. 
     [Semiconductor Wafer and Chip] 
       FIG. 11A  is a top view of a substrate  711  before dicing treatment. As the substrate  711 , a semiconductor substrate (also referred to as a “semiconductor wafer”) can be used, for example. A plurality of circuit regions  712  are provided over the substrate  711 . A semiconductor device, a CPU, an RF tag, an image sensor, or the like of one embodiment of the present invention can be provided in the circuit region  712 . 
     The plurality of circuit regions  712  are each surrounded by a separation region  713 . Separation lines (also referred to as “dicing lines”)  714  are set at a position overlapping with the separation regions  713 . The substrate  711  can be cut along the separation lines  714  into chips  715  including the circuit regions  712 .  FIG. 11B  is an enlarged view of the chip  715 . 
     A conductive layer or a semiconductor layer may be provided in the separation regions  713 . Providing a conductive layer or a semiconductor layer in the separation regions  713  relieves ESD that might be caused in a dicing step, preventing a decrease in the yield of the dicing step. A dicing step is generally performed while letting pure water whose specific resistance is decreased by dissolution of a carbonic acid gas or the like flow to a cut portion, in order to cool down a substrate, remove swarf, and prevent electrification, for example. Providing a conductive layer or a semiconductor layer in the separation regions  713  allows a reduction in the usage of the pure water. Therefore, the cost of manufacturing semiconductor devices can be reduced. Moreover, semiconductor devices can be manufactured with improved productivity. 
     For the semiconductor layer provided in the separation regions  713 , a material having a band gap of 2.5 eV or more and 4.2 eV or less is preferably used, and a material having a band gap of preferably 2.7 eV or more and 3.5 eV or less is further preferably used. The use of such a material allows accumulated charges to be released slowly; thus, the rapid move of charges due to ESD can be suppressed and electrostatic breakdown is less likely to occur. 
     [Electronic Component] 
       FIG. 12A  and  FIG. 12B  show an example in which the chip  715  is used in an electronic component. Note that the electronic component is also referred to as a semiconductor package or an IC package. This electronic component has a plurality of standards and names depending on a terminal extraction direction and a terminal shape. 
     The electronic component is completed when the semiconductor device described in the above embodiment 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. 12A . After an element substrate including the semiconductor device described in the above embodiment 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 721 ). 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 (chips  715 ) in a dicing step (Step S 722 ). Then, the divided chips are separately picked up to be bonded to a lead frame in a die bonding step (Step S 723 ). 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 substrate instead of the lead frame. 
     Next, a wire bonding step of electrically connecting a lead of the lead frame and an electrode on the chip through a metal fine line (wire) is performed (Step S 724 ). A silver line or a gold line can be used as the metal fine line. Ball bonding or wedge bonding can be used as the wire bonding. 
     The wire-bonded chip is subjected to a sealing step (molding step) of sealing the chip with an epoxy resin or the like (Step S 725 ). Through the sealing step, the inside of the electronic component is filled with a resin, so that a circuit portion incorporated in the chip and a wire for connecting the chip to the lead can be protected from external mechanical force, and deterioration of characteristics (a 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 726 ). This plating process prevents rust of the lead and enables more reliable soldering at the time of mounting the electronic component on a printed circuit board in a later step. Then, the lead is cut and processed in a formation step (Step S 727 ). 
     Next, a printing (marking) step is performed on a surface of the package (Step S 728 ). After a testing step (Step S 729 ) for checking whether an external shape is good and whether there is a malfunction, for example, the electronic component is completed. 
       FIG. 12B  is a schematic perspective view of a completed electronic component.  FIG. 12B  is a schematic perspective view illustrating a QFP (Quad Flat Package) as an example of the electronic component. An electronic component  750  in  FIG. 12B  includes a lead  755  and a semiconductor device  753 . As the semiconductor device  753 , the semiconductor device described in the above embodiment or the like can be used. 
     The electronic component  750  in  FIG. 12B  is mounted on a printed circuit board  752 , for example. A plurality of electronic components  750  are combined and electrically connected to each other over the printed circuit board  752 ; thus, a substrate on which the electronic components are mounted (a circuit board  754 ) is completed. The completed circuit board  754  is provided in an electronic device or the like. 
     [Electronic Device] 
     Next, examples of electronic devices each including the semiconductor device of one embodiment of the present invention or the above-described electronic component are described with reference to  FIG. 13  and  FIG. 14A  to  FIG. 14F . 
     Examples of electronic devices including the semiconductor device of one embodiment of the present invention or the electronic component include display devices of televisions, monitors, and the like; lighting devices; desktop personal computers; laptop personal computers; word processors; image reproduction devices that reproduce still images and moving images stored in recording media such as DVDs (Digital Versatile Discs); portable CD players; 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 chain saws; smoke detectors; and medical equipment such as dialyzers. Other examples include 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 and the like driven by electric motors using electric power from the power storage devices are also included in the category of electronic devices. Examples of the moving objects include electric vehicles (EVs), hybrid electric vehicles (HEVs) that include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEVs), 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, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft. 
     The semiconductor device of one embodiment of the present invention or the electronic component can be used for a communication device or the like incorporated in any of the electronic devices. 
     The electronic devices may include a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), for example. 
     The electronic devices can have a variety of functions such as a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on a display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of executing a variety of software (programs), a wireless communication function, and a function of reading out a program or data stored in a recording medium, for example. 
       FIG. 13  and  FIGS. 14A to 14F  illustrate examples of electronic devices. In  FIG. 13 , a display device  8000  is an example of an electronic device including a semiconductor device  8004  of 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  of one embodiment of the present invention is provided in the housing  8001 . The semiconductor device  8004  can retain control data, a control program, or the like. The semiconductor device  8004  has a communication function, and the display device  8000  can function as an IoT device. The display device  8000  can receive electric power from a commercial power supply. Alternatively, the display device  8000  can use electric power stored in the power storage device  8005 . 
     A display device such as a liquid crystal display device, a light-emitting display device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), or an FED (Field Emission Display) 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 besides TV broadcast reception. 
     In  FIG. 13 , an installation lighting device  8100  is an example of an electronic device including a semiconductor device  8103  of 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. 13  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 retain data such as emission luminance of the light source  8102 , a control program, or the like. The semiconductor device  8103  has a communication function, and the lighting device  8100  can function as an IoT device. The lighting device  8100  can receive electric power from a commercial power supply. Alternatively, the lighting device  8100  can use electric power stored in the power storage device  8105 . 
     Although  FIG. 13  illustrates the installation lighting device  8100  provided in the ceiling  8104  as an example, the semiconductor device of one embodiment of the present invention can be used in an installation lighting device provided in, for example, a wall  8405 , a floor  8406 , a window  8407 , or the like other than the ceiling  8104 . Alternatively, the semiconductor device of 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 that emits light artificially by using electric power can be used. Specific examples of the artificial light source include an incandescent lamp, a discharge lamp such as a fluorescent lamp, and light-emitting elements such as an LED and an organic EL element. 
     In  FIG. 13 , 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  of 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. 13  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 retain control data, a control program, or the like of the air conditioner. The semiconductor device  8203  has a communication function, and the air conditioner can function as an IoT device. The air conditioner can receive electric power from a commercial power supply. Alternatively, the air conditioner can use electric power stored in the power storage device  8205 . 
     Although  FIG. 13  illustrates the split-type air conditioner including the indoor unit and the outdoor unit as an example, the semiconductor device of 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. 13 , an electric refrigerator-freezer  8300  is an example of an electronic device including a semiconductor device  8304  of 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. In  FIG. 13 , the power storage device  8305  is provided in the housing  8301 . The semiconductor device  8304  can retain control data, a control program, or the like of the electric refrigerator-freezer  8300 . The semiconductor device  8304  has a communication function, and the electric refrigerator-freezer  8300  can function as an IoT device. The electric refrigerator-freezer  8300  can receive electric power from a commercial power supply. Alternatively, the electric refrigerator-freezer  8300  can use electric power stored in the power storage device  8305 . 
       FIG. 14A  illustrates an example of a watch-type portable information terminal. A portable information terminal  6100  includes a housing  6101 , a display portion  6102 , a band  6103 , operation buttons  6105 , and the like. The portable information terminal  6100  further includes a secondary battery and the semiconductor device of one embodiment of the present invention or the electronic component. The portable information terminal  6100  including the semiconductor device of one embodiment of the present invention or the electronic component can function as an IoT device. 
       FIG. 14B  illustrates an example of a mobile phone. A portable information terminal  6200  includes a display portion  6202  incorporated in a housing  6201 , operation buttons  6203 , a speaker  6204 , a microphone  6205 , and the like. 
     The portable information terminal  6200  further includes a fingerprint sensor  6209  in a region overlapping with the display portion  6202 . The fingerprint sensor  6209  may be an organic optical sensor. Since a fingerprint differs between individuals, the fingerprint sensor  6209  can perform personal authentication when acquiring fingerprint patterns. As a light source for acquiring fingerprint patterns with the fingerprint sensor  6209 , light emitted from the display portion  6202  can be used. 
     The portable information terminal  6200  further includes a secondary battery and the semiconductor device of one embodiment of the present invention or the electronic component. The portable information terminal  6200  including the semiconductor device of one embodiment of the present invention or the electronic component can function as an IoT device. 
       FIG. 14C  illustrates an example of a cleaning robot. A cleaning robot  6300  includes a display portion  6302  placed on the top surface of a housing  6301 , a plurality of cameras  6303  placed on the side surface of the housing  6301 , a brush  6304 , operation buttons  6305 , a variety of sensors, and the like. Although a tire, an inlet, and the like are not illustrated, the cleaning robot  6300  is provided with the tire, the inlet, and the like. The cleaning robot  6300  is self-propelled, detects dust  6310 , and sucks up the dust through the inlet provided on the bottom surface. 
     For example, the cleaning robot  6300  can determine whether there is an obstacle such as a wall, furniture, or a step by analyzing images taken by the cameras  6303 . In the case where the cleaning robot  6300  detects an object that is likely to be caught in the brush  6304  (e.g., a wire) by image analysis, the rotation of the brush  6304  can be stopped. The cleaning robot  6300  further includes a secondary battery and the semiconductor device of one embodiment of the present invention or the electronic component. The cleaning robot  6300  including the semiconductor device of one embodiment of the present invention or the electronic component can function as an IoT device. 
       FIG. 14D  illustrates an example of a robot. A robot  6400  illustrated in  FIG. 14D  includes an arithmetic device  6409 , an illuminance sensor  6401 , a microphone  6402 , an upper camera  6403 , a speaker  6404 , a display portion  6405 , a lower camera  6406 , an obstacle sensor  6407 , and a moving mechanism  6408 . 
     The microphone  6402  has a function of detecting a speaking voice of a user, an environmental sound, and the like. The speaker  6404  has a function of outputting sound. The robot  6400  can communicate with a user using the microphone  6402  and the speaker  6404 . 
     The display portion  6405  has a function of displaying various kinds of information. The robot  6400  can display information desired by a user on the display portion  6405 . The display portion  6405  may be provided with a touch panel. Moreover, the display portion  6405  may be a detachable information terminal, in which case charging and data communication can be performed when the display portion  6405  is set at the home position of the robot  6400 . 
     The upper camera  6403  and the lower camera  6406  each have a function of taking an image of the surroundings of the robot  6400 . The obstacle sensor  6407  can detect an obstacle in the direction where the robot  6400  advances with the moving mechanism  6408 . The robot  6400  can move safely by recognizing the surroundings with the upper camera  6403 , the lower camera  6406 , and the obstacle sensor  6407 . 
     The robot  6400  further includes a secondary battery and the semiconductor device of one embodiment of the present invention or the electronic component. The robot  6400  including the semiconductor device of one embodiment of the present invention or the electronic component can function as an IoT device. 
       FIG. 14E  illustrates an example of a flying object. A flying object  6500  illustrated in  FIG. 14E  includes propellers  6501 , a camera  6502 , a battery  6503 , and the like and has a function of flying autonomously. 
     For example, image data taken by the camera  6502  is stored in an electronic component  6504 . The electronic component  6504  can analyze the image data to detect whether there is an obstacle in the way of the movement. Moreover, the electronic component  6504  can estimate the remaining battery level from a change in the power storage capacity of the battery  6503 . The flying object  6500  further includes the semiconductor device of one embodiment of the present invention or the electronic component. The flying object  6500  including the semiconductor device of one embodiment of the present invention or the electronic component can function as an IoT device. 
       FIG. 14F  illustrates an example of an automobile. An automobile  7160  includes an engine, tires, a brake, a steering gear, a camera, and the like. The automobile  7160  further includes the semiconductor device of one embodiment of the present invention or the electronic component. The automobile  7160  including the semiconductor device of one embodiment of the present invention or the electronic component can function as an IoT device. 
     The structures, the compositions, the methods, and the like described in this example can be combined as appropriate with any of the structures, the compositions, the methods, and the like described in the other embodiments and examples. 
     Embodiment 4 
     A normally-off CPU (also referred to as “Noff-CPU”) can be achieved by using the OS transistor described in this specification and the like. Note that the Noff-CPU is an integrated circuit including a normally-off transistor, which is in a non-conduction state (also referred to as an off state) even when a gate voltage is 0 V. 
     In the Noff-CPU, power supply to a circuit that does not need to operate can be stopped so that the circuit can be brought into a standby state. The circuit brought into the standby state because of the stop of power supply does not consume power. Thus, the power usage of the Noff-CPU can be minimized. Moreover, the Noff-CPU can retain data necessary for operation, such as setting conditions, for a long time even when power supply is stopped. The return from the standby state requires only restart of power supply to the circuit and does not require rewriting of setting conditions or the like. In other words, high-speed return from the standby state is possible. As described here, the power consumption of the Noff-CPU can be reduced without a significant decrease in operation speed. 
     The Noff-CPU can be suitably used for a small-scale system such as an IoT end device (also referred to as an endpoint microcomputer)  803  in the IoT (Internet of Things) field, for example. 
       FIG. 15  shows a hierarchical structure of an IoT network and tendencies of required specifications.  FIG. 15  shows power consumption  804  and processing performance  805  as the required specifications. The hierarchical structure of the IoT network is roughly divided into a cloud field  801  at the top and an embedded field  802  at the bottom. The cloud field  801  includes a server, for example. The embedded field  802  includes a machine, an industrial robot, an in-vehicle device, and a home appliance, for example. 
     Higher processing performance is required rather than lower power consumption towards the top of the hierarchical structure. Thus, a high-performance CPU, a high-performance GPU, a large-scale SoC (System on a Chip), and the like are used in the cloud field  801 . Furthermore, lower power consumption is required rather than higher processing performance towards the bottom of the hierarchical structure, and the number of devices is explosively increased. The semiconductor device of one embodiment of the present invention can be suitably used for a communication device in the IoT end device that needs to have low power consumption. 
     Note that an endpoint refers to an end region of the embedded field  802 . Examples of a device used in the endpoint include microcomputers used in a factory, a home appliance, infrastructure, agriculture, and the like. 
       FIG. 16  is an image diagram showing factory automation as an application example of the endpoint microcomputer. A factory  884  is connected to a cloud  883  through Internet connection. The cloud  883  is connected to a home  881  and an office  882  through Internet connection. The Internet connection may be wired communication or wireless communication. In the case of wireless communication, for example, wireless communication based on a communication standard such as the fourth-generation mobile communications system (4G) or the fifth-generation mobile communications system (5G) is performed using the semiconductor device of one embodiment of the present invention for a communication device. The factory  884  may be connected to a factory  885  and a factory  886  through Internet connection. 
     The factory  884  includes a master device (control device)  831 . The master device  831  is connected to the cloud  883  and has a function of transmitting and receiving data. The master device  831  is connected to a plurality of industrial robots  842  included in an IoT end device  841  through an M2M (Machine to Machine) interface  832 . As the M2M interface  832 , for example, industrial Ethernet (Ethernet is a registered trademark), which is a kind of wired communication, or local 5G, which is a kind of wireless communication, may be used. 
     A manager of the factory can check the operational status or the like from the home  881  or the office  882  connected to the factory  884  through the cloud  883 . In addition, the manager can check wrong items and part shortage, instruct a storage space, and measure takt time, for example. 
     In recent years, IoT has been globally introduced into factories; this situation is called Smart Factory. Smart Factory has been reported to enable not only simple examination and inspection by an endpoint microcomputer but also detection of failures and prediction of abnormality, for example. 
     The total power consumption of a small-scale system such as an endpoint microcomputer during operation is often small, which enhances the power reduction effect in a standby state by the Noff-CPU. Although the embedded field of IoT sometimes requires quick response, the use of the Noff-CPU achieves high-speed return from a standby state. 
     The structures, the compositions, the methods, and the like described in this embodiment can be combined as appropriate with any of the structures, the compositions, the methods, and the like described in the other embodiments and examples, for example. 
     REFERENCE NUMERALS 
     C 1 : capacitor, C 2 : capacitor, C 3 : capacitor, M 1 : memory element, R 1 : resistor, R 2 : resistor, R 3 : resistor, Vres: terminal, Vin: input terminal, Vout: output terminal, SET: terminal, SET 1 : terminal, SET 2 : terminal, Tr 1 : transistor, Tr 2 : transistor, Tr 3 : transistor, Tr 4 : transistor, Tr 5 : transistor, Tr 11 : transistor, Tr 21 : transistor, Tr 31 : transistor, Tr 41 : transistor, Tr 51 : transistor, Vres 1 : terminal, Vres 2 : terminal,  311 : substrate,  313 : semiconductor region,  314   a:  low-resistance region,  314   b:  low-resistance region,  315 : insulator,  316 : conductor,  320 : insulator,  322 : insulator,  324 : insulator,  326 : insulator,  328 : conductor,  330 : conductor,  350 : insulator,  352 : insulator,  354 : insulator,  356 : conductor,  360 : insulator,  362 : insulator,  364 : insulator,  366 : conductor,  370 : insulator,  372 : insulator,  374 : insulator,  376 : conductor,  380 : insulator,  382 : insulator,  384 : insulator,  386 : conductor,  404 : insulator,  500 : transistor,  500 A: transistor,  500 B: transistor,  503 : conductor,  503   a:  conductor,  503   b:  conductor,  510 : insulator,  512 : insulator,  513 : insulator,  514 : insulator,  516 : insulator,  518 : conductor,  520 : insulator,  522 : insulator,  524 : insulator,  530 : oxide,  530   a:  oxide,  530   b:  oxide,  540   a:  conductor,  540   b:  conductor,  542   a:  conductor,  542   b:  conductor,  543   a:  region,  543   b:  region,  544 : insulator,  545 : insulator,  546 : conductor,  548 : conductor,  550 : transistor,  552 : insulator,  560 : conductor,  560   a:  conductor,  560   b:  conductor,  574 : insulator,  580 : insulator,  581 : insulator,  582 : insulator,  586 : insulator,  592 : conductor,  593 : conductor,  600 : capacitor,  610 : conductor,  612 : conductor,  620 : conductor,  630 : insulator,  640 : insulator,  711 : substrate,  712 : circuit region,  713 : separation region,  714 : separation line,  715 : chip,  750 : electronic component,  752 : printed circuit board,  753 : semiconductor device,  754 : circuit board,  755 : lead,  801 : cloud field,  802 : field,  804 : power consumption,  805 : processing performance,  831 : master device,  832 : interface,  841 : IoT end device,  842 : industrial robot,  881 : home,  882 : office,  883 : cloud,  884 : factory,  885 : factory,  886 : factory,  1001 : operational amplifier,  1021 : operational amplifier,  1031 : operational amplifier,  1100 : phase shifter,  1200 : phase shifter,  1300 : phase shifter,  6000 : phase shifter,  6001 : operational amplifier,  6100 : portable information terminal,  6101 : housing,  6102 : display portion,  6103 : band,  6105 : operation button,  6200 : portable information terminal,  6201 : housing,  6202 : display portion,  6203 : operation button,  6204 : speaker,  6205 : microphone,  6209 : fingerprint sensor,  6300 : cleaning robot,  6301 : housing,  6302 : display portion,  6303 : camera,  6304 : brush,  6305 : operation button,  6310 : dust,  6400 : robot,  6401 : illuminance sensor,  6402 : microphone,  6403 : upper camera,  6404 : speaker,  6405 : display portion,  6406 : lower camera,  6407 : obstacle sensor,  6408 : moving mechanism,  6409 : arithmetic device,  6500 : flying object,  6501 : propeller,  6502 : camera,  6503 : battery,  6504 : electronic component,  7160 : automobile,  8000 : display device,  8001 : housing,  8002 : display portion,  8003 : speaker portion,  8004 : semiconductor device,  8005 : power storage device,  8100 : lighting device,  8101 : housing,  8102 : light source,  8103 : semiconductor device,  8104 : ceiling,  8105 : power storage device,  8200 : indoor unit,  8201 : housing,  8202 : air outlet,  8203 : semiconductor device,  8204 : outdoor unit,  8205 : power storage device,  8300 : electric refrigerator-freezer,  8301 : housing,  8302 : refrigerator door,  8303 : freezer door,  8304 : semiconductor device,  8305 : power storage device,  8405 : sidewall,  8406 : floor,  8407 : window