Patent Publication Number: US-10777685-B2

Title: Transistor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention 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. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, a processor, or a driving method or manufacturing method thereof. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases. 
     2. Description of the Related Art 
     Silicon is known as a material used for a semiconductor layer of a transistor; either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, when silicon is used for a semiconductor layer of a transistor included in a large-sized display device, it is preferable to use amorphous silicon, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, when silicon is used for a semiconductor layer of a transistor included in a high-performance display device in which a driver circuit and a display portion are formed over one substrate, it is preferable to use polycrystalline silicon, which can achieve a transistor with high field-effect mobility. 
     Meanwhile, an oxide semiconductor has attracted attention recently as a material used for a semiconductor layer of a transistor. For example, a transistor using an amorphous oxide semiconductor containing indium, gallium, and zinc is known (see Patent Document 1). 
     An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a semiconductor layer of a transistor in a large display device. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized. A transistor including an oxide semiconductor has high field-effect mobility; therefore, a high-performance display device where a driver circuit and a display portion are formed over one substrate can be obtained. 
     It is also known that a transistor using an oxide semiconductor for a semiconductor layer has an extremely low leakage current in a non-conducting state (an off state). For example, a low-power CPU utilizing the feature of low leakage current of the transistor including an oxide semiconductor is disclosed (see Patent Document 2). 
     There are demands for improved integration density in a semiconductor device achieved by reducing the footprint of transistors, improved electrical characteristics of transistors, and the like. For example, a transistor proposed in Patent Document 3 has increased transistor integration density by including an arch-shaped gate. A CMOS transistor with an SOI structure proposed in Patent Document 4 achieves improved electrical characteristics with the use of distortion that is generated in a channel formation region owing to a difference between the thickness of an embedded oxide layer under an n-channel transistor and the thickness of an embedded oxide layer under a p-channel transistor. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
         [Patent Document 2] Japanese Published Patent Application No. 2012-257187 
         [Patent Document 3] Japanese Published Patent Application No. H10-125873 
         [Patent Document 4] Japanese Published Patent Application No. 2010-183065 
       
    
     SUMMARY OF THE INVENTION 
     On the other hand, the structures disclosed in Patent Document 3 and Patent Document 4 require high-temperature treatment performed at higher than 1000° C., which puts considerable restrictions on manufacture, such as limitation on the substrates that can be used. Specifically, the maximum process temperature of display devices is required to be 600° C. or lower, preferably 400° C. or lower. 
     An object of one embodiment of the present invention is to provide a transistor which has high electrical characteristics. Another object of one embodiment of the present invention is to provide a transistor with low power consumption. Another object of one embodiment of the present invention is to provide a transistor having high reliability. Another object of one embodiment of the present invention is to provide a transistor with a small footprint. Another object of one embodiment of the present invention is to provide a novel transistor. Another object of one embodiment of the present invention is to provide a semiconductor device including at least one of these transistors. 
     Note that the descriptions of these objects do not preclude the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     A transistor is provided over an insulating layer that has a projection. Over the projection, at least a channel formation region of a semiconductor layer is provided. This structure can reduce the footprint of the transistor. The transistor has a curved cross-sectional structure, which inhibits light that enters from the outside from reaching a channel formation region of the semiconductor layer. Accordingly, deterioration of the transistor due to external light can be reduced, whereby the transistor can have increased reliability. The projection can be obtained by utilizing the internal stress of the layer formed over the insulating layer. Alternatively, the projection can be obtained by placing, under the insulating layer, a structure body for providing the insulating layer with the projection. 
     One embodiment of the present invention is a transistor that includes first to third insulating layers, a first gate electrode, a second gate electrode, a first gate insulating layer, a second gate insulating layer, and a semiconductor layer. The first insulating layer has a projection. The second insulating layer is positioned over the first insulating layer. The first gate electrode is positioned over the second insulating layer. The first gate insulating layer is positioned over the first gate electrode. The semiconductor layer is positioned over the first gate insulating layer. The second gate insulating layer is positioned over the semiconductor layer. The second gate electrode is positioned over the second gate insulating layer. The third insulating layer has a region overlapping the second gate electrode and a region in contact with the semiconductor layer. The projection, the first gate electrode, the first gate insulating layer, the semiconductor layer, the second gate insulating layer, and the second gate electrode have regions overlapping each other. The first insulating layer includes an organic resin. 
     One embodiment of the present invention is a transistor that includes a first insulating layer, a second insulating layer, a first gate electrode, a second gate electrode, a first gate insulating layer, a second gate insulating layer, a source electrode, a drain electrode, and a semiconductor layer. The first insulating layer has a projection. The second insulating layer is positioned over the first insulating layer. The first gate electrode is positioned over the second insulating layer. The first gate insulating layer is positioned over the first gate electrode. The semiconductor layer is positioned over the first gate insulating layer. The source electrode has a region in contact with the semiconductor layer. The drain electrode has a region in contact with the semiconductor layer. The second gate insulating layer is positioned over the semiconductor layer, the source electrode, and the drain electrode. The second gate electrode is positioned over the second gate insulating layer. The projection, the first gate electrode, the first gate insulating layer, the semiconductor layer, the second gate insulating layer, and the second gate electrode have regions overlapping each other. The first insulating layer includes an organic resin. 
     The organic resin is preferably at least one of a polyimide, an acrylic resin, a benzocyclobutene resin, a polyamide, and an epoxy resin. The second insulating layer preferably includes an inorganic material. The third insulating layer preferably includes silicon and nitrogen. The semiconductor layer is preferably an oxide semiconductor layer. 
     According to one embodiment of the present invention, a transistor which has high electrical characteristics can be provided. A transistor with low power consumption can be provided. A transistor having high reliability can be provided. A transistor with a small footprint can be provided. A novel transistor can be provided. A semiconductor device including at least one of these transistors 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 achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a transistor. 
         FIG. 2  illustrates a transistor. 
         FIGS. 3A and 3B  illustrate a transistor. 
         FIGS. 4A and 4B  illustrate a transistor. 
         FIGS. 5A and 5B  each illustrate a transistor. 
         FIGS. 6A to 6C  illustrate manufacturing steps of a transistor. 
         FIGS. 7A to 7C  illustrate manufacturing steps of a transistor. 
         FIGS. 8A to 8C  illustrate manufacturing steps of a transistor. 
         FIGS. 9A and 9B  illustrate manufacturing steps of a transistor. 
         FIGS. 10A and 10B  illustrate manufacturing steps of a transistor. 
         FIGS. 11A and 11B  illustrate manufacturing steps of a transistor. 
         FIGS. 12A and 12B  illustrate a transistor. 
         FIG. 13  illustrates a transistor. 
         FIGS. 14A and 14B  illustrate manufacturing steps of a transistor. 
         FIGS. 15A and 15B  illustrate a transistor. 
         FIG. 16  illustrates a transistor. 
         FIGS. 17A and 17B  illustrate a transistor. 
         FIG. 18  illustrates a transistor. 
         FIGS. 19A and 19B  illustrate a transistor. 
         FIG. 20  illustrates a transistor. 
         FIGS. 21A and 21B  illustrate a transistor. 
         FIGS. 22A and 22B  each illustrate a transistor. 
         FIGS. 23A and 23B  illustrate manufacturing steps of a transistor. 
         FIGS. 24A and 24B  illustrate manufacturing steps of a transistor. 
         FIGS. 25A and 25B  illustrate manufacturing steps of a transistor. 
         FIGS. 26A and 26B  illustrate manufacturing steps of a transistor. 
         FIGS. 27A and 27B  illustrate a transistor. 
         FIG. 28  illustrates a transistor. 
         FIGS. 29A and 29B  illustrate a transistor. 
         FIG. 30  illustrates a transistor. 
         FIGS. 31A and 31B  illustrate a transistor. 
         FIG. 32  illustrates a transistor. 
         FIGS. 33A to 33C  each illustrate an atomic ratio range of an oxide of one embodiment of the present invention. 
         FIGS. 34A and 34B  illustrate a display device. 
         FIGS. 35A to 35C  are a block diagram and circuit diagrams illustrating one mode of a display device. 
         FIGS. 36A to 36D  illustrate manufacturing steps of a display device. 
         FIGS. 37A to 37D  illustrate manufacturing steps of a display device. 
         FIG. 38  illustrates a manufacturing step of a display device. 
         FIGS. 39A and 39B  illustrate manufacturing steps of a display device. 
         FIGS. 40A and 40B  illustrate manufacturing steps of a display device. 
         FIGS. 41A and 41B  illustrate manufacturing steps of a display device. 
         FIGS. 42A and 42B  illustrate manufacturing steps of a display device. 
         FIGS. 43A to 43C  each illustrate a display device. 
         FIG. 44  illustrates a display device. 
         FIG. 45  illustrates an example of a display module. 
         FIGS. 46A and 46B  each illustrate a structure example of a light-emitting element. 
         FIGS. 47A to 47G  each illustrate an example of an electronic device. 
         FIGS. 48A and 48B  illustrate an example of an electronic device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to the drawings. Note that the present invention is not limited to the following description. It will be readily appreciated by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the spirit and 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 of such portions is not repeated in some cases. 
     The position, size, range, and the like of each component illustrated in the drawings and the like are not accurately represented in some cases to facilitate understanding of the invention. Therefore, the present invention is not necessarily limited to the position, size, range, or the like disclosed in the drawings and the like. For example, in the actual manufacturing process, a layer, a resist mask, or the like may be unintentionally reduced in size by treatment such as etching, which is not illustrated in some cases for easy understanding. 
     Especially in a top view (also referred to as “plan view”), a perspective view, or the like, some components might not be illustrated for easy understanding of the invention. In addition, some hidden lines and the like might not be shown. 
     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 may be provided with an ordinal number in a claim in order to avoid confusion among components. A term with an ordinal number in this specification and the like may be provided with a different ordinal number in a claim. Moreover, a term with an ordinal number in this specification and the like might not be provided with any ordinal number in a claim and the like. 
     In this specification and the like, the term such as an “electrode” or a “wiring” does not limit a function of the component. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa. Furthermore, the term “electrode” or “wiring” can also mean that a plurality of “electrodes” or “wirings” are provided in an integrated manner. 
     Note that the term “over” or “under” in this specification and the like does not necessarily mean that a component is placed “directly over” or “directly under” and “directly in contact with” another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is on and in direct contact with the insulating layer A and can mean the case where another component is provided between the insulating layer A and the electrode B. 
     Functions of a source and a drain might be switched depending on operation conditions, for example, when a transistor having opposite polarity is employed or the direction of current flow is changed in circuit operation. Thus, it is difficult to define which is a source or a drain. Accordingly, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification. 
     Furthermore, in this specification and the like, an explicit description “X and Y are connected” means that X and Y are electrically connected, X and Y are functionally connected, and X and Y are directly connected. Accordingly, without being limited to a predetermined connection relation, for example, a connection relation shown in drawings or texts, another connection relation is included in the drawings or the texts. 
     In this specification and the like, the term “electrically connected” includes the case where components are connected through an object having any electric function. 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. Accordingly, even when the expression “electrically connected” is used in this specification, there is a case in which no physical connection is made and a wiring is just extended in an actual circuit. 
     Note that the channel length refers to, for example, the distance from a source (a source region or a source electrode) to a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap each other or a region where a channel is formed, in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap each other or a region where a channel is formed, in a top view of the transistor. In many cases, the extending direction of the channel length is orthogonal to the extending direction of the channel width. In one transistor, channel widths in all regions are not necessarily the same. In other words, the channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     Note that depending on transistor structures, a channel width in a region where a channel is actually formed (hereinafter referred to as “effective channel width”) is sometimes different from a channel width shown in a top view of a transistor (hereinafter referred to as “apparent channel width”). For example, in a transistor having a gate electrode covering a side surface of a semiconductor layer, an effective channel width is greater than an apparent channel width, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a gate electrode covering a side surface of a semiconductor, the proportion of a channel formation region formed on a side surface of a semiconductor is sometimes increased. In that case, an effective channel width is greater than an apparent channel width. 
     In such a case, an effective channel width is difficult to measure in some cases. For example, estimation of the effective channel width from a design value requires an assumption that the shape of a semiconductor is known. Accordingly, when the shape of a semiconductor is not known accurately, it is difficult to measure the effective channel width accurately. 
     Therefore, in this specification, an apparent channel width is referred to as a surrounded channel width (SCW) in some cases. Furthermore, when the term “channel width” is simply used in this specification, it may represent a surrounded channel width or an apparent channel width. Alternatively, when the term “channel width” is simply used in this specification, it may represent an effective channel width in some cases. Note that a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by analyzing a cross-sectional TEM image and the like. 
     Note that when field-effect mobility, a current per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width may be used for the calculation. In that case, the obtained value is sometimes different from the value obtained by using an effective channel width for the calculation. 
     Note that the term “impurity” in a semiconductor refers to, for example, an element other than the main components of the semiconductor. For example, an element with a concentration lower than 0.1 atomic % can be regarded as an impurity. Inclusion of impurities may cause an increase in density of states (DOS) in a semiconductor and/or a decrease in the carrier mobility or the crystallinity. In the case where the semiconductor is an oxide semiconductor, examples of an impurity which changes characteristics of the semiconductor include elements other than the main components of the oxide semiconductor, such as Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, Group 15 elements, and transition metals; there are hydrogen, lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. 
     In this specification, the term “parallel” indicates that the angle formed between two straight lines ranges from −10° to 10°, and accordingly also includes the case where the angle ranges from −5° to 5°. The term “substantially parallel” indicates that the angle formed between two straight lines ranges from −30° to 30°. The terms “perpendicular” and “orthogonal” indicate that the angle formed between two straight lines ranges from 80° to 100°, and accordingly also include the case where the angle ranges from 85° to 95°. The term “substantially perpendicular” indicates that the angle formed between two straight lines ranges from 60° to 120°. 
     In the specification and the like, the terms used in describing calculation values and actual measurement values, such as “identical”, “same”, “equal”, and “uniform”, (including synonyms thereof) allow for a margin of error of ±20% unless otherwise specified. 
     In this specification and the like, when an etching step (a removing step) is performed after a resist mask is formed by a photolithography method, the resist mask is removed after the etching step unless otherwise specified. 
     In this specification and the like, a high power supply potential VDD (also referred to as “VDD” or “H potential”) is a power supply potential higher than a low power supply potential VSS (also referred to as “VSS” or “L potential”). The low power supply potential VSS is a power supply potential lower than the high power supply potential VDD. A ground potential (also referred to as “GND” or a “GND potential”) can be used as VDD or VSS. For example, when the ground potential is used as VDD, VSS is lower than the ground potential, and when the ground potential is used as VSS, VDD is higher than the ground potential. 
     Note that the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, in some cases, the term “conductive film” can be used instead of the term “conductive layer”, or the term “insulating film” can be used instead of the term “insulating layer”. 
     In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor includes a channel formation region between the drain (a drain terminal, a drain region, or a drain electrode) and the source (a source terminal, a source region, or a source electrode), and current can flow between the source and the drain through the channel formation region. Note that in this specification and the like, a channel formation region refers to a region through which current mainly flows. 
     A transistor shown in this specification and the like refers to an enhancement-mode (normally-off) field-effect transistor, unless otherwise specified. Moreover, a transistor shown in this specification and the like is an n-channel transistor, unless otherwise specified. Therefore, the threshold voltage (also referred to as Vth) of a transistor is higher than 0 V, unless otherwise specified. 
     Note that Vth of a transistor having a backgate in this specification and the like refers to Vth obtained when the potential of the backgate is set equal to that of a source or a gate, unless otherwise specified. 
     Unless otherwise specified, the off-state current in this specification and the like refers to a drain current of a transistor in an off state (also referred to as non-conducting state and cutoff state). Unless otherwise specified, the off state of an n-channel transistor means that the voltage between its gate and source (Vgs: gate-source voltage) is lower than the threshold voltage Vth, and the off state of a p-channel transistor means that the gate-source voltage Vgs is higher than the threshold voltage Vth. For example, the off-state current of an n-channel transistor sometimes refers to a drain current that flows when the gate-source voltage Vgs is lower than the threshold voltage Vth. 
     The off-state current of a transistor depends on Vgs in some cases. Thus, the expression “the off-state current of a transistor is lower than or equal to I” may mean that there is Vgs with which the off-state current of the transistor becomes lower than or equal to I. Moreover, the off-state current of a transistor sometimes means the off-state current at given Vgs, at Vgs in a given range, or at Vgs with which sufficiently low off-state current is obtained, for example. 
     As an example, the assumption is made of an n-channel transistor where the threshold voltage Vth is 0.5 V and the drain current is 1×10 −9  A at Vgs of 0.5 V, 1×10 −13  A at Vgs of 0.1 V, 1×10 −19  A at Vgs of −0.5 V, and 1×10 −22  A at Vgs of −0.8 V. The drain current of the transistor is 1×10 −19  A or lower at Vgs of −0.5 V or at Vgs in the range of −0.8 V to −0.5 V; thus, it can be said that the off-state current of the transistor is 1×10 −19  A or lower. Since there is Vgs at which the drain current of the transistor is 1×10 −22  A or lower, it may be said that the off-state current of the transistor is 1×10 −22  A or lower. 
     The off-state current of a transistor depends on temperature in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at room temperature (RT), 60° C., 85° C., 95° C., or 125° C. Alternatively, the off-state current may be an off-state current at a temperature at which the reliability of a semiconductor device or the like including the transistor is ensured or a temperature at which the semiconductor device or the like is used (e.g., a temperature higher than or equal to 5° C. and lower than or equal to 35° C.). The description “the off-state current of a transistor is lower than or equal to I” may mean that there is Vgs at which the off-state current of the transistor at RT, 60° C., 85° C., 95° C., 125° C., a temperature at which the reliability of a semiconductor device or the like including the transistor is ensured, or a temperature at which the semiconductor device or the like is used (e.g., a temperature higher than or equal to 5° C. and lower than or equal to 35° C.) is lower than or equal to I. 
     The off-state current of a transistor sometimes depends on a voltage Vds between its drain and source. Unless otherwise specified, the off-state current in this specification may be an off-state current at Vds of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V. Alternatively, the off-state current may be an off-state current at Vds at which the required reliability of a semiconductor device or the like including the transistor is ensured or Vds at which a semiconductor device or the like including the transistor is used. The description “the off-state current of a transistor is lower than or equal to I” may mean that there is Vgs at which the off-state current of the transistor is lower than or equal to the current I at Vds of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, Vds at which the reliability of a semiconductor device or the like including the transistor is ensured, or Vds at which a semiconductor device or the like including the transistor is used. 
     In the above description of off-state current, a drain may be replaced with a source. That is, the off-state current sometimes refers to a current that flows through a source of a transistor in the off state. 
     In this specification and the like, the term “leakage current” sometimes expresses the same meaning as off-state current. In this specification and the like, the off-state current sometimes refers to current that flows between a source and a drain of a transistor in the off state, for example. 
     Embodiment 1 
     A transistor  100  of one embodiment of the present invention will be described with reference to drawings. 
     &lt;Structure Example of Transistor  100 &gt; 
       FIG. 1A  is a plan view of the transistor  100 .  FIG. 1B ,  FIG. 3A , and  FIG. 3B  are each a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 1A . Note that  FIG. 3B  illustrates only a substrate  101  and an insulating layer  102 .  FIG. 2  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 1A . 
     The transistor  100  described in this embodiment is a kind of top-gate transistor. The transistor  100  is provided over the substrate  101  with the insulating layer  102  and an insulating layer  103  provided therebetween. The transistor  100  includes an electrode  104 , an insulating layer  105 , a semiconductor layer  106 , an insulating layer  107 , an electrode  108 , an insulating layer  109 , and an insulating layer  110 . 
     Specifically, the insulating layer  102  has a projection and is provided over the substrate  101 . The insulating layer  103  is provided over the insulating layer  102 . The electrode  104  is provided over the insulating layer  103 . The insulating layer  105  is provided to cover the electrode  104 . The semiconductor layer  106  is provided over the insulating layer  105 . The insulating layer  107  is provided over the semiconductor layer  106 . The electrode  108  is provided over the insulating layer  107 . The insulating layer  109  is provided to cover the electrode  108 , the insulating layer  107 , and the semiconductor layer  106 . The insulating layer  109  has a region that is in contact with part of the semiconductor layer  106 . The insulating layer  110  is provided over the insulating layer  109 . 
     An electrode  112   a  and an electrode  112   b  are provided over the insulating layer  110 . The electrode  112   a  is electrically connected to part of the semiconductor layer  106  through an opening  111   a  that is formed by removing part of the insulating layer  110  and part of the insulating layer  109 . The electrode  112   b  is electrically connected to another part of the semiconductor layer  106  through an opening  111   b  that is formed by removing part of the insulating layer  110  and part of the insulating layer  109 . 
     The electrode  112   a  can serve as one of a source electrode and a drain electrode. The electrode  112   b  can function as the other of the source electrode and the drain electrode. An insulating layer  113  having a flat surface may be provided over the electrode  112   a , the electrode  112   b , and the insulating layer  110 . 
     In the transistor  100 , a region of the semiconductor layer  106  that is overlapped by the electrode  108  serves as a channel formation region. A region of the semiconductor layer  106  that is not overlapped by the electrode  108  serves as a source region or a drain region. Specifically, a region  106   c  illustrated in  FIG. 1B  serves as the channel formation region. A region  106   s  illustrated in  FIG. 1B  serves as one of the source region and the drain region. A region  106   d  illustrated in  FIG. 1B  serves as the other of the source region and the drain region. The channel length L of the transistor  100  is the length of the region of the semiconductor layer  106  that is overlapped by the electrode  108 , in a direction parallel to the direction in which carriers flow. 
     In some of  FIG. 2  and subsequent drawings, the region  106   c , the region  106   s , and the region  106   d  are not illustrated. 
     The semiconductor layer  106 , the electrode  104 , and the electrode  108  each include a region that overlaps the projection of the insulating layer  102 . Since the semiconductor layer  106 , the electrode  104 , and the electrode  108  overlap the projection of the insulating layer  102 , for example, the length B of the semiconductor layer  106  from the opening  111   a  to the opening  111   b  when seen in a plan view can be shorter than the actual length A of the semiconductor layer  106  from the opening  111   a  to the opening  111   b  (see  FIG. 1A  and  FIG. 3A ). In this manner, the cross-sectional shape of the transistor  100  is curved. The transistor  100  having such a curved shape occupies a smaller area. 
     Light that enters the transistor  100  from the outside is sometimes diffused, for example, to reach the channel formation region of the semiconductor layer  106 . The light reaching the channel formation region of the semiconductor layer  106  might cause a change in electrical characteristics of the transistor and a reduction in reliability of the transistor. The curved cross-sectional shape of the transistor  100  allows light  191  that enters the transistor  100  from the outside to be reflected in a direction such that the light goes away from the channel formation region of the semiconductor layer  106  (see  FIG. 3A ). Consequently, changes in the electrical characteristics of the transistor can be reduced to improve the reliability of the transistor. 
     In the transistor  100 , the channel formation region overlaps the projection of the insulating layer  102 . The top of the projection of the insulating layer  102  or the vicinity of the top has a radius of curvature R, which is preferably greater than or equal to one and less than or equal to 20 times the channel length L, further preferably greater than or equal to two and less than or equal to 10 times the channel length L, still further preferably greater than or equal to three and less than or equal to five times the channel length L (see  FIG. 3A ). 
     The projection of the insulating layer  102  has a height H, which is preferably greater than or equal to two and less than or equal to 20 times the thickness of the semiconductor layer  106 , further preferably greater than or equal to five and less than or equal to 10 times the thickness of the semiconductor layer  106  (see  FIG. 3A ). 
     Too steep an incline from the projection of the insulating layer  102  to a depression thereof might reduce the coverage with the layer that is formed over the insulating layer  102  in a later step. Accordingly, the maximum angle Om formed by a line  141  parallel to a surface of the substrate  101  and a tangent  142  of a side surface of the projection is preferably greater than or equal to 5° and less than or equal to 60°, further preferably greater than or equal to 5° and less than or equal to 45°, still further preferably greater than or equal to 5° and less than or equal to 20° (see  FIG. 3B ). 
     The projection of the insulating layer  102  can be formed by utilizing a difference between the mechanical strengths of the layers. For example, the insulating layer  102  is formed using a material whose Young&#39;s modulus is one tenth or less, preferably one fiftieth or less, further preferably one hundredth or less that of the electrode  104 . In addition, the insulating layer  102  is formed using a material whose Young&#39;s modulus is one tenth or less, preferably one fiftieth or less, further preferably one hundredth or less that of the insulating layer  103 . 
     Here, in the case where a given layer A consists of stacked layers, the Young&#39;s modulus of the layer A means that of the layer having a larger Young&#39;s modulus than any other layers included in the layer A. Alternatively, the Young&#39;s modulus of the layer A means the Young&#39;s modulus obtained by evaluating the layer A consisting of stacked layers as a single layer. 
     The insulating layer  105  is formed using a material having a compressive stress of greater than or equal to 200 MPa, preferably greater than or equal to 1000 MPa, further preferably greater than or equal to 1500 MPa. The electrode  104  is preferably formed using a material having compressive stress. When the electrode  104  is formed using a material having tensile stress, the absolute value of the stress of the electrode  104  is preferably smaller lower than or equal to the absolute value of the stress of the insulating layer  105 . 
     Here, in the case where a given layer A consists of stacked layers, the stress of the layer A means the sum of the stresses of all the layers included in the layer A. For example, in the case where the insulating layer  105  consists of two layers of an insulating layer  105   a  and an insulating layer  105   b , the stress of the insulating layer  105  means the sum of the stress of the insulating layer  105   a  and that of the insulating layer  105   b.    
     The thickness of the insulating layer  103  is preferably half or less, further preferably one fifth or less, still further preferably one tenth or less that of the insulating layer  105 . 
     The insulating layer  105  may be formed using an insulating material that is unlikely to transmit impurities, in which case the insulating layer  103  is not necessarily provided (see  FIGS. 4A and 4B ).  FIG. 4A  is a cross-sectional view corresponding to  FIG. 1B .  FIG. 4B  is a cross-sectional view corresponding to  FIG. 2 . 
     The semiconductor layer  106  is not limited to a single layer and may be a stack of a plurality of layers. For example, the semiconductor layer  106  may have a two-layer structure of a semiconductor layer  106 _ 1  and a semiconductor layer  106 _ 2  as illustrated in  FIG. 5A . Alternatively, the semiconductor layer  106  may have a three-layer structure of the semiconductor layer  106 _ 1 , the semiconductor layer  106 _ 2 , and a semiconductor layer  106 _ 3  as illustrated in  FIG. 5B , for example. Needless to say, the semiconductor layer  106  may be a stack of four or more layers. Note that  FIGS. 5A and 5B  are cross-sectional views corresponding to  FIG. 1B . 
     [Gate Electrode and Backgate Electrode] 
     The electrode  104  and the electrode  108  can each function as a gate electrode. In the case where one of the electrode  104  and the electrode  108  is referred to as a “gate electrode” or a “gate”, the other can be referred to as a “backgate electrode” or a “backgate”. For example, in the transistor  100 , in the case where the electrode  108  is referred to as a “gate electrode”, the electrode  104  is referred to as a “backgate electrode”. For example, in the transistor  100 , in the case where the electrode  104  is referred to as a “gate electrode”, the electrode  108  is referred to as a “backgate electrode”. 
     When the electrode  104  is used as the gate electrode, the transistor  100  can be considered as a bottom-gate transistor. In some cases, one of the electrodes  104  and  108  is referred to as “first gate electrode” and the other is referred to as “second gate electrode”. 
     The gate electrode and the backgate electrode are generally formed using conductive layers. The gate electrode and the backgate electrode are placed so that a channel formation region of a semiconductor layer is sandwiched therebetween. In other words, the gate electrode and the backgate electrode surround the semiconductor layer. This structure enables the semiconductor layer  106  in the transistor  100  to be electrically surrounded by electric fields of the electrode  108  serving as the gate electrode and the electrode  104  serving as the backgate electrode. Such a structure of a transistor in which electric fields of the gate electrode and the backgate electrode electrically surround the semiconductor layer where a channel is formed can be referred to as a surrounded-channel (S-channel) structure. 
     The backgate electrode can function in a manner similar to that of the gate electrode. The potential of the backgate electrode may be the same as that of the gate electrode or may be a ground potential or a predetermined potential. By changing the potential of the backgate electrode independently of the potential of the gate electrode, the threshold voltage of the transistor can be changed. 
     As described above, the electrode  108  can function as a gate electrode. Thus, the insulating layer  107  can function as a gate insulating layer. In addition, the electrode  104  can also function as a gate electrode. Accordingly, the insulating layer  105  can also function as a gate insulating layer. 
     By providing the electrode  104  and the electrode  108  so that the semiconductor layer  106  is located therebetween, and by setting the potentials of the electrode  104  and the electrode  108  to be the same, a region of the semiconductor layer  106  through which carriers flow is enlarged in the film thickness direction; thus, the amount of transferred carriers is increased. As a result, the on-state current and the field-effect mobility of the transistor are increased. 
     Accordingly, the transistor can have a large on-state current for its area. That is, the footprint of the transistor can be small for a required on-state current. Thus, a semiconductor device having a high degree of integration can be achieved. 
     Furthermore, the gate electrode and the backgate electrode are formed using conductive layers and thus each have a function of preventing an electric field generated outside the transistor from influencing the semiconductor layer in which the channel is formed (in particular, an electric field blocking function against static electricity and the like). When the backgate electrode is formed larger than the semiconductor layer to cover the semiconductor layer in the plan view, the electric field blocking function can be enhanced. 
     Since each of the electrodes  104  and  108  has a function of blocking an electric field from the outside, charges of charged particles and the like generated above the electrode  108  and below the electrode  104  do not influence the channel formation region of the semiconductor layer  106 . As a result, degradation of electrical characteristics by a stress test (e.g., a negative gate bias temperature (−GBT) stress test in which negative voltage is applied to a gate) is reduced. In addition, the electrodes  104  and  108  can block an electric field generated from the drain electrode so as not to affect the semiconductor layer. Thus, changes in the rising voltage of on-state current due to changes in drain voltage can be suppressed. Note that this effect is significant when a potential is supplied to the electrodes  104  and  108 . 
     A GBT stress test is a kind of acceleration test and can evaluate, in a short time, a change in transistor characteristics due to long-term use (i.e., a change over time). In particular, the amount of change in threshold voltage of a transistor between before and after the GBT stress test is an important indicator when examining the reliability of the transistor. If the amount of change in the threshold voltage between before and after the GBT stress test is small, the transistor has high reliability. 
     By providing the electrode  104  and the electrode  108  and setting the potentials of the electrode  104  and the electrode  108  to be the same, the change in threshold voltage is reduced. Accordingly, variations in electrical characteristics among a plurality of transistors are also reduced. 
     The transistor including a backgate electrode has a smaller change in threshold voltage between before and after a positive GBT stress test in which positive voltage is applied to a gate than a transistor including no backgate electrode. 
     In the case where light is incident on the backgate electrode side, when the backgate electrode is formed using a light-blocking conductive layer, light can be prevented from entering the semiconductor layer from the backgate electrode side. Thus, photodegradation of the semiconductor layer can be prevented and deterioration in electrical characteristics of the transistor, such as a change in the threshold voltage, can be prevented. 
     [Substrate] 
     As the substrate  101 , a glass substrate, a ceramic substrate, a flexible substrate that has heat resistance high enough to withstand a process temperature of this manufacturing process, or the like can be used. When the substrate does not need a light-transmitting property, a metal substrate, such as a stainless alloy substrate, with a surface provided with an insulating layer may be used. As the glass substrate, an alkali-free glass substrate of barium borosilicate glass, aluminoborosilicate glass, aluminosilicate glass, or the like can be used, for example. In addition, a quartz substrate, a sapphire substrate, or the like can be used. 
     As the substrate  101 , a glass substrate having any of the following sizes can be used: the 3rd generation (550 mm×650 mm), the 3.5th generation (600 mm×720 mm or 620 mm×750 mm), the 4th generation (680 mm×880 mm or 730 mm×920 mm), the 5th generation (1100 mm×1300 mm), the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm or 2450 mm×3050 mm), and the 10th generation (2950 mm×3400 mm). 
     When a flexible substrate is used as the substrate  101 , a transistor, a capacitor, and the like may be directly formed over the flexible substrate, or they may be formed over a manufacturing substrate, and then transferred onto the flexible substrate. To transfer the transistor, the capacitor, and the like from the manufacturing substrate to the flexible substrate, a separation layer is preferably provided between the manufacturing substrate and the transistor, the capacitor, and the like. 
     For the flexible substrate, metal, an alloy, resin, glass, or fiber thereof can be used, for example. The flexible substrate used as the substrate  101  preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate used as the substrate  101  is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10 −3 /K, lower than or equal to 5×10 −5 /K, or lower than or equal to 1×10 −5 /K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon and aramid), polyimide, polycarbonate, and acrylic resin. In particular, aramid is preferably used for the flexible substrate because of its low coefficient of linear expansion. 
     As the substrate  101 , a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like or a compound semiconductor substrate made of silicon germanium or the like can also be used. Alternatively, an SOI substrate, a semiconductor substrate on which a semiconductor element such as a strained transistor or a FIN-type transistor is provided, or the like can be used. Alternatively, gallium arsenide, aluminum gallium arsenide, indium gallium arsenide, gallium nitride, indium phosphide, silicon germanium, or the like that can be used for a high-electron-mobility transistor (HEMT) may be used. That is, the substrate  101  is not limited to a simple supporting substrate and may be a substrate where a device such as a transistor is formed. In that case, at least one of the gate, the source, and the drain of the transistor  100  may be electrically connected to the device. 
     [Insulating Layer] 
     The insulating layer  102  can be formed using an organic resin (an organic material) having heat resistance, such as a polyimide, an acrylic resin, a benzocyclobutene resin, a polyamide, or an epoxy resin. Depending on the purpose or intended use, the insulating layer  102  may be formed using a material and a method similar to those used for any of the insulating layer  103 , the insulating layer  105 , the insulating layer  107 , the insulating layer  109 , the insulating layer  110 , the insulating layer  113 , and the like, which will be described later. In addition, the insulating layer  102  may be a stack including the above material. 
     There is no particular limitation on the method for forming the insulating layer  102 , and any of the following methods which depend on a material thereof can be used: a sputtering method; an SOG method; spin coating; dipping; spray coating; a droplet discharging method (e.g., an ink-jet method); a printing method (e.g., screen printing or offset printing); or the like. When a liquid organic resin is used for an insulating layer, a step of baking the organic resin is commonly needed. When the baking step also serves as heat treatment for another layer, a transistor can be manufactured efficiently. 
     The insulating layers  103 ,  105 ,  107 ,  109 ,  110 , and  113  can be formed with a single layer or a stack of layers of one or more materials selected from aluminum nitride, aluminum oxide, aluminum nitride oxide, aluminum oxynitride, magnesium oxide, silicon nitride, silicon oxide, silicon nitride oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, aluminum silicate, and the like. Alternatively, a material in which two or more materials selected from an oxide material, a nitride material, an oxynitride material, and a nitride oxide material are mixed may be used. 
     Note that in this specification, a nitride oxide refers to a compound that contains more nitrogen than oxygen. An oxynitride refers to a compound that contains more oxygen than nitrogen. The content of each element can be measured by Rutherford backscattering spectrometry (RBS), for example. 
     In particular, the insulating layer  103  is preferably formed using an insulating material through which impurities do not easily pass. Furthermore, the insulating layer  109  and/or the insulating layer  110  are/is preferably formed using an insulating material through which impurities do not easily pass. Examples of an insulating material through which impurities do not easily pass include aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, and silicon nitride. 
     When the insulating material through which impurities do not easily pass is used for the insulating layer  103 , impurity diffusion from the substrate  101  side can be prevented, and the reliability of the transistor can be improved. When the insulating material through which impurities do not easily pass is used for the insulating layer  109  and/or the insulating layer  110 , impurity diffusion from the insulating layer  113  side can be prevented, and the reliability of the transistor can be improved. 
     When an oxide semiconductor layer is used as the semiconductor layer  106 , the insulating layer  103  is preferably formed using an insulating material into which oxygen is less likely to diffuse and/or be absorbed. Moreover, the insulating layer  109  and/or the insulating layer  110  are/is preferably formed using an insulating material into which oxygen is less likely to diffuse and/or be absorbed. In that case, diffusion of oxygen to the outside can be suppressed. 
     Note that the insulating layer  103 ,  109 , and/or  110  may be a stack of insulating layers formed with the above materials. 
     When an oxide semiconductor layer is used as the semiconductor layer  106 , the hydrogen concentration of the insulating layers is preferably low in order to prevent an increase in the hydrogen concentration of the semiconductor layer  106 . In particular, the hydrogen concentration of the insulating layer in contact with the semiconductor layer  106  is preferably low. In this embodiment, the hydrogen concentration of the insulating layers  105  and  107  is preferably low. Specifically, the hydrogen concentration of the insulating layers, which is measured by secondary ion mass spectrometry (SIMS), is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , further preferably lower than or equal to 1×10 19  atoms/cm 3 , still further preferably lower than or equal to 5×10 18  atoms/cm 3 . Furthermore, the nitrogen concentration of the insulating layer is preferably low in order to prevent an increase in the nitrogen concentration of the oxide semiconductor layer. Specifically, the nitrogen concentration of the insulating layer, which is measured by SIMS, is lower than 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , 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 . 
     At least one of the insulating layer  105  and the insulating layer  107  is preferably an insulating layer from which oxygen is released by heating. Specifically, it is preferable to use an insulating layer in which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 18  atoms/cm 3 , further preferably greater than or equal to 1.0×10 19  atoms/cm 3 , or still further preferably greater than or equal to 1.0×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS) where heat treatment is performed such that the surface temperature of the insulating layer ranges from 100° C. to 700° C., preferably from 100° C. to 500° C. Note that in this specification and the like, oxygen released by heating is also referred to as excess oxygen. 
     It is particularly preferred that the amount of defects in the insulating layer in contact with the oxide semiconductor layer be small. Typically, the spin density of a signal that appears at g=2.001 due to a dangling bond of silicon is preferably lower than or equal to 3×10 17  spins/cm 3  by electron spin resonance (ESR) measurement. If the amount of defects in the insulating layer is large, oxygen might be bonded to the defects, reducing excess oxygen. 
     It is particularly preferred that the insulating layer in contact with the oxide semiconductor layer be an oxide insulating layer in which the density of states due to nitrogen oxide (NO X , where X is greater than 0 and less than or equal to 2; typically NO or NO 2 ) is low. As the oxide insulating layer, a silicon oxynitride layer that releases less nitrogen oxide, an aluminum oxynitride layer that releases less nitrogen oxide, or the like can be used. An oxide insulating layer that releases less nitrogen oxide is a layer in which the amount of released ammonia is larger than the amount of released nitrogen oxide in TDS. The amount of released ammonia molecules is typically greater than or equal to 1×10 18  molecules/cm 3  and less than or equal to 5×10 19  molecules/cm 3 . Note that the amount of released ammonia molecules is the amount of ammonia molecules released by heat treatment which makes the surface temperature of the oxide insulating layer higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C. 
     Nitrogen oxide forms a level in the oxide semiconductor layer or the insulating layer. The level is positioned in the energy gap of the oxide semiconductor. When nitrogen oxide reaches the interface between the insulating layer and the oxide semiconductor layer, an electron can potentially be trapped by the level on the insulating layer side. As a result, the trapped electrons remain in the vicinity of the interface between the insulating layer and the oxide semiconductor layer; thus, the threshold voltage of the transistor is shifted in the positive direction. 
     Note that the density of states due to nitrogen oxide can sometimes be formed between the energy of the valence band maximum (E v_os ) and the energy of the conduction band minimum (E c_os ) of the oxide semiconductor layer. 
     Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide contained in the insulating layer reacts with ammonia contained in the insulating layer in heat treatment, the nitrogen oxide contained in the insulating layer is reduced. Consequently, electrons are hardly trapped at the interface between the insulating layer and the oxide semiconductor layer. 
     By using the above oxide insulating layer as the insulating layer in contact with the oxide semiconductor layer, a shift in the threshold voltage of the transistor can be reduced, which leads to reduced fluctuations in the electrical characteristics of the transistor. 
     The insulating layer containing excess oxygen can be formed by performing treatment for adding oxygen to an insulating layer. The treatment for adding oxygen can be performed by, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. The treatment for adding oxygen can be heat treatment under an oxidizing atmosphere, plasma treatment, inverse sputtering treatment, or the like. The plasma treatment under an oxidizing atmosphere is preferably performed using an apparatus including a power source for generating high-density plasma using microwaves, for example. Alternatively, a power source for applying a radio frequency (RF) to a substrate side may be provided. The use of high-density plasma generates oxygen radicals at a high density. Application of RF to the substrate side allows the oxygen radicals generated by the high-density plasma to be efficiently guided into a target layer. Alternatively, plasma treatment may be performed under an inert atmosphere and then plasma treatment may be performed under an oxidizing atmosphere to compensate released oxygen. By oxygen addition through inverse sputtering treatment, the effect of cleaning a sample surface can be expected. Meanwhile, damage might occur on the sample surface depending on treatment conditions. As a gas for adding oxygen, an oxygen gas of  16 O 2 ,  18 O 2 , or the like, a nitrous oxide gas, an ozone gas, or the like can be used. In this specification, the treatment for adding oxygen is also referred to as “oxygen doping treatment”. 
     Such oxygen doping treatment sometimes increases the crystallinity of the semiconductor layer. Furthermore, the oxygen doping treatment sometimes removes impurities contained in the target layer, such as hydrogen and water. That is, “oxygen doping treatment” can also be referred to as “impurity-removing treatment”. Specifically, plasma treatment performed using oxygen under a reduced pressure and an oxidizing atmosphere as the oxygen doping treatment cuts a bond involving hydrogen or water in a target insulating layer or a target semiconductor layer. This makes it easy for hydrogen and water in the target layer to be released. Thus, the plasma treatment as the oxygen doping treatment is preferably performed while heating is performed. Alternatively, heat treatment is preferably performed after the plasma treatment. When plasma treatment is performed after heat treatment and heat treatment is further performed, the impurity concentration in the target layer can be lowered. 
     The insulating layer  113  is preferably an insulating layer (hereinafter also referred to as a planarization layer) that has a function of covering unevenness and the like caused by the transistor or the like. The insulating layer  113  can be formed using, for example, an organic resin similar to that used for the insulating layer  102 . It is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like for the insulating layer  113 . 
     Note that a siloxane resin corresponds to a resin containing a Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane resin may include an organic group (e.g., an alkyl group or an aryl group) or a fluoro group as a substituent. In addition, the organic group may include a fluoro group. 
     Note that the insulating layer  113  is formed using an insulating material. Accordingly, the insulating layer  113  can be formed using any of the above-described inorganic materials and the above-described organic materials. The insulating layer  113  may be formed by stacking a plurality of insulating layers formed of an inorganic material and/or an organic material. 
     [Electrode] 
     As a conductive material for forming the electrodes  104 ,  112   a ,  112   b , and  108 , a material containing one or more metal elements selected from aluminum (Al), chromium (Cr), iron (Fe), copper (Cu), silver (Ag), gold (Au), platinum (Pt), tantalum (Ta), nickel (Ni), titanium (Ti), cobalt (Co), molybdenum (Mo), tungsten (W), hafnium (Hf), vanadium (V), niobium (Nb), manganese (Mn), magnesium (Mg), zirconium (Zr), beryllium (Be), and the like can be used. Moreover, a semiconductor with a high electric conductivity, typified by polycrystalline silicon containing an impurity element such as phosphorus, or silicide such as nickel silicide may be used. 
     A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used as the conductive material. A layer made of a Cu—X alloy can be processed with a wet etching process, resulting in lower manufacturing cost. 
     A conductive material containing the above metal element and oxygen may be used. A conductive material containing the above metal element and nitrogen may be used. For example, a conductive material containing nitrogen, such as titanium nitride or tantalum nitride, may be used. Indium tin oxide (ITO), indium zinc oxide, indium gallium zinc oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide to which silicon is added, or indium gallium zinc oxide containing nitrogen may be used. 
     A stack of a plurality of conductive layers formed with the above materials may be used. For example, a stacked-layer structure formed using a material containing the above metal element and a conductive material containing oxygen may be used. Alternatively, a stacked-layer structure formed using a material containing the above metal element and a conductive material containing nitrogen may be used. Further alternatively, a stacked-layer structure formed using a material containing the above metal element, a conductive material containing oxygen, and a conductive material containing nitrogen may be used. 
     When copper is used for the electrodes  112   a  and  112   b  to lower the resistance of the electrodes  112   a  and  112   b , a conductive material into which copper does not easily diffuse is preferably provided between the electrode  112   a  and the semiconductor layer  106  and between the electrode  112   b  and the semiconductor layer  106 . Since copper is likely to diffuse into a semiconductor layer, the operation of a semiconductor device might be unstable and the yield might be significantly reduced. When a conductive material into which copper does not easily diffuse is provided between the semiconductor layer and a wiring or an electrode that contains copper, the reliability of the transistor  100  can be increased. 
     Examples of a conductive material into which copper does not easily diffuse include a metal material having a higher melting point than copper (e.g., tungsten, titanium, and tantalum) and a nitride material thereof. Moreover, a wiring or an electrode containing copper may be covered with such a conductive material. When a wiring or an electrode containing copper is covered with or wrapped by a conductive material into which copper does not easily diffuse, the reliability of the transistor  100  can be further increased. 
     In the case where an oxide semiconductor layer is used as the semiconductor layer  106 , a conductive material that has a function of absorbing hydrogen when being subjected to heat treatment is preferably used for regions of the electrodes  112   a  and  112   b  in contact with the semiconductor layer  106 , in which case the hydrogen concentration of the semiconductor layer  106  can be reduced by heat treatment performed later. Examples of a conductive material with a function of absorbing hydrogen include titanium, indium zinc oxide, and indium tin oxide to which silicon is added. 
     [Semiconductor Layer] 
     The semiconductor layer  106  can be formed using an amorphous semiconductor, a microcrystalline semiconductor, a polycrystalline semiconductor, or the like. For example, amorphous silicon or microcrystalline germanium can be used. Alternatively, a compound semiconductor such as silicon carbide, gallium arsenide, an oxide semiconductor, or a nitride semiconductor, an organic semiconductor, or the like can be used. 
     In particular, an oxide semiconductor is preferably used for the semiconductor layer  106 . The band gap of an oxide semiconductor is greater than or equal to 2 eV; thus, when the oxide semiconductor is used for the semiconductor layer  106 , a transistor with an extremely low off-state current can be achieved. A transistor using an oxide semiconductor in the semiconductor layer where a channel is formed (also referred to as OS transistor) has high withstand voltage between its source and drain. Thus, a transistor with high reliability can be provided. A transistor with high output voltage and high withstand voltage can be provided. Furthermore, a semiconductor device or the like with high reliability can be provided. A semiconductor device with high output voltage and high withstand voltage can be provided. 
     In this embodiment, the case where an oxide semiconductor is used for the semiconductor layer  106  is described. 
     Note that in this specification and the like, an oxide semiconductor (OS) is a material that has separate portions: a conductive portion for providing a switching (on/off) function and a dielectric portion. The whole (the whole electric field) of the material serves as a semiconductor. A metal oxide, which is a material whose conductive components and dielectric components are separated at a nanoparticle level, also has a switching (on/off) function as a whole and thus is classified as an OS in this specification and the like. 
     An oxide semiconductor will be described below. An oxide semiconductor preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, an element M (M is one or more of aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like) may be contained. 
     [Structure of Oxide Semiconductor] 
     An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis-aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor. 
     The CAAC-OS has c-axis alignment, its nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion. Note that 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 the nanocrystals are connected. 
     The shape of the nanocrystal is basically a hexagon but is not always a regular hexagon and is a non-regular hexagon in some cases. A pentagonal lattice arrangement, a heptagonal lattice arrangement, or the like is included in the distortion in some cases. Note that a clear crystal grain boundary cannot be observed even in the vicinity of distortion in the CAAC-OS. That is, formation of a crystal grain boundary is inhibited by distortion of a lattice arrangement. This is probably because the CAAC-OS can tolerate distortion owing to a low density of arrangement of oxygen atoms in an a-b plane direction, a change in interatomic bond distance by substitution of a metal element, and the like. 
     The CAAC-OS tends to have a layered crystal structure (also referred to as a layered structure) in which a layer containing indium and oxygen (hereinafter, In layer) and a layer containing the element M, zinc, and oxygen (hereinafter, (M,Zn) layer) are stacked. Note that indium and the element M can be replaced with each other, and when the element M of the (M,Zn) layer is replaced with indium, the layer can also be referred to as an (In,M,Zn) layer. When indium of the In layer is replaced with the element M, the layer can also be referred to as an (In,M) layer. 
     In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different nanocrystals in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method. 
     The a-like OS has a structure intermediate between the structure of the nc-OS and the structure of the amorphous oxide semiconductor. The a-like OS has 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. 
     An oxide semiconductor can have various structures which show various different properties. Two or more of the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, the a-like OS, the nc-OS, and the CAAC-OS may be included in an oxide semiconductor that can be used in one embodiment of the present invention. 
     [CAC-OS] 
     Described here is the composition of a cloud aligned composite OS (CAC-OS) which can be used for a transistor according to one embodiment of the present invention. 
     The CAC-OS has, for example, a composition in which elements included in an oxide semiconductor are unevenly distributed. Materials including unevenly distributed elements each have a size of greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size. Note that in the following description of an oxide semiconductor, the state in which one or more metal elements are unevenly distributed in regions each having a size of greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size and the regions including the metal element(s) are mixed is referred to as a mosaic pattern or a patch-like pattern in some cases. 
     As an example of the CAC-OS, an In—Ga—Zn oxide with the CAC composition (such an In—Ga—Zn oxide may be particularly referred to as CAC-IGZO) will be described. The CAC-IGZO has a composition with a mosaic pattern in which materials are separated into indium oxide (InO X1 , where X 1  is a real number greater than 0) or indium zinc oxide (In X2 Zn Y2 O Z2 , where X 2 , Y 2 , and Z 2  are each a real number greater than 0) and gallium oxide (GaO X3 , where X 3  is a real number greater than 0) or gallium zinc oxide (Ga Y4 Zn Y4 O Z4 , where X 4 , Y 4 , and Z 4  are each a real number greater than 0), for example. Furthermore, InO X1  or In X2 Zn Y2 O Z2  forming the mosaic pattern is evenly distributed in the film. This composition is also referred to as a cloud-like composition. 
     That is, the CAC-OS is a composite oxide semiconductor with a composition in which a region including GaO X3  as a main component and a region including In X2 Zn Y2 O Z2  or InO X1  as a main component are mixed. Note that in this specification, for example, when the atomic ratio of In to the element M in a first region is larger than the atomic ratio of In to the element M in a second region, the first region has a higher In concentration than the second region. 
     Note that a compound containing In, Ga, Zn, and O is also known as IGZO. Typical examples of IGZO include a crystalline compound represented by InGaO 3 (ZnO) m1  (m1 is a natural number) and a crystalline compound represented by In (1+x0) Ga (1−x0) O 3 (ZnO) m0  (−1≤x 0 ≤1; m0 is a given number). 
     The above crystalline compound has a single-crystal structure, a polycrystalline structure, or a CAAC structure. Note that the CAAC structure is a crystal structure in which a plurality of IGZO nanocrystals have c-axis alignment and are connected in the a-b plane direction without alignment. 
     On the other hand, the CAC-OS relates to the material composition of an oxide semiconductor. In part of the material composition of a CAC-OS containing In, Ga, Zn, and O, nanoparticle regions including Ga as a main component and nanoparticle regions including In as a main component are observed. These nanoparticle regions are randomly dispersed in a mosaic pattern. Therefore, the crystal structure is a secondary element for the CAC-OS. 
     Note that the CAC-OS does not include a stacked structure of two or more films with different compositions. For example, a two-layer structure of a film including In as a main component and a film including Ga as a main component is not included. 
     A boundary between the region including GaO X3  as a main component and the region including In X2 Zn Y2 O Z2  or InO X1  as a main component is not clearly observed in some cases. 
     In part of the composition of a CAC-OS which contains, instead of gallium, one or more metal elements selected from aluminum, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like, nanoparticle regions including the metal element(s) as a main component(s) and nanoparticle regions including In as a main component are observed. These nanoparticle regions are randomly dispersed in a mosaic pattern. 
     The CAC-OS can be formed by a sputtering method under the conditions where a substrate is not heated intentionally, for example. In the case of forming the CAC-OS by a sputtering method, one or more gases selected from an inert gas (typically, argon), an oxygen gas, and a nitrogen gas may be used as a deposition gas. The percentage of the oxygen gas flow rate in the total flow rate of the deposition gas at the time of deposition is preferably as low as possible; for example, the percentage of the oxygen gas flow rate is preferably higher than or equal to 0% and lower than 30%, further preferably higher than or equal to 0% and lower than or equal to 10%. 
     The CAC-OS is characterized in that no clear peak is observed in measurement using θ/2θ scan by an out-of-plane method, which is an X-ray diffraction (XRD) measurement method. That is, X-ray diffraction shows no alignment in the a-b plane direction and the c-axis direction in a measured region. 
     In an electron diffraction pattern of the CAC-OS which is obtained by irradiation with an electron beam with a probe diameter of 1 nm (also referred to as a nanometer-sized electron beam), a ring-like region with high luminance and a plurality of bright spots in the ring-like region are observed. Therefore, the electron diffraction pattern indicates that the crystal structure of the CAC-OS includes a nanocrystal (nc) structure with no alignment in the plan-view direction and the cross-sectional direction. 
     For example, an energy dispersive X-ray spectroscopy (EDX) mapping image confirms that an In—Ga—Zn oxide with the CAC composition has a structure in which regions including GaO X3  as a main component and regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are unevenly distributed and mixed. 
     The CAC-OS has a structure and characteristics different from those of an IGZO compound in which metal elements are evenly distributed. That is, in the CAC-OS, regions including GaO X3  or the like as a main component and regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are phase-separated from each other in a mosaic pattern. 
     The conductivity of the region including In X2 Zn Y2 O Z2  or InO X1  as a main component is higher than that of the region including GaO X3  or the like as a main component. In other words, when carriers flow through the region including In X2 Zn Y2 O Z2  or InO X1  as a main component, the oxide semiconductor exhibits conductivity. Accordingly, when the regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are distributed in the oxide semiconductor like a cloud, a high field-effect mobility (μ) can be achieved. 
     In contrast, the insulating property of the region including GaO x3  or the like as a main component is higher than that of the region including In X2 Zn Y2 O Z2  or InO X1  as a main component. In other words, when the regions including GaO X3  or the like as a main component are distributed in the oxide semiconductor, leakage current can be suppressed and favorable switching operation can be achieved. 
     Accordingly, when a CAC-OS is used for a transistor, the insulating property derived from GaO X3  or the like and the conductivity derived from In X2 Zn Y2 O Z2  or InO X1  complement each other, whereby a high on-state current (Ion) and a high field-effect mobility (μ) can be achieved. 
     A semiconductor element including a CAC-OS has high reliability. The CAC-OS can be used for a variety of semiconductor devices typified by a display. 
     [Atomic Ratio of Oxide Semiconductor] 
     Next, preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide semiconductor that can be used in one embodiment of the present invention are described with reference to  FIGS. 33A to 33C . Note that the proportion of oxygen atoms is not shown in  FIGS. 33A to 33C . The terms of the atomic ratio of indium, the element M, and zinc contained in the oxide semiconductor are denoted by [In], [M], and [Zn], respectively. 
     In  FIGS. 33A to 33C , broken lines indicate a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):1 (where −1≤α≤1), a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α): 3 , a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):5. 
     Dashed-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn] is 5:1:β (where β≥0), a line where the atomic ratio [In]:[M]:[Zn] is 2:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:3:β, and a line where the atomic ratio [In]:[M]:[Zn] is 1:4:β. 
     Furthermore, an oxide semiconductor with the atomic ratio of [In]:[M]:[Zn]=0:2:1 or a neighborhood thereof in  FIGS. 33A to 33C  tends to have a spinel crystal structure. 
     A plurality of phases (e.g., two phases or three phases) exist in the oxide semiconductor in some cases. For example, with an atomic ratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinel crystal structure and a layered crystal structure are likely to exist. In addition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0, two phases of a bixbyite crystal structure and a layered crystal structure are likely to exist. In the case where a plurality of phases exist in the oxide semiconductor, a grain boundary might be formed between different crystal structures. 
     A region A in  FIG. 33A  represents examples of the preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide semiconductor. 
     The oxide semiconductor containing indium in a higher proportion can have higher carrier mobility (electron mobility). Therefore, an oxide semiconductor having a high content of indium has higher carrier mobility than that of an oxide semiconductor having a low content of indium. 
     In contrast, when the indium content and the zinc content in an oxide semiconductor become lower, carrier mobility becomes lower. Thus, with an atomic ratio [In]:[M]:[Zn] of 0:1:0 and the vicinity thereof (e.g., a region C in  FIG. 33C ), insulation performance becomes better. 
     Accordingly, an oxide semiconductor that can be used in one embodiment of the present invention preferably has an atomic ratio represented by the region A in  FIG. 33A . With the atomic ratio, a layered structure with high carrier mobility and a few grain boundaries is easily obtained. 
     An oxide semiconductor with an atomic ratio in the region A, particularly in a region B in  FIG. 33B , is excellent because the oxide semiconductor easily becomes a CAAC-OS and has high carrier mobility. 
     The CAAC-OS is an oxide semiconductor with high crystallinity. In contrast, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur because a clear grain boundary cannot be observed. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies). Thus, an oxide semiconductor including a CAAC-OS is physically stable. Therefore, the oxide semiconductor including a CAAC-OS is resistant to heat and has high reliability. 
     Note that the region B includes an atomic ratio [In]:[M]:[Zn] of 4:2:3 to 4:2:4.1 and the vicinity thereof. The vicinity includes an atomic ratio [In]:[M]:[Zn] of 5:3:4. Note that the region B includes an atomic ratio [In]:[M]:[Zn] of 5:1:6 and the vicinity thereof and an atomic ratio [In]:[M]:[Zn] of 5:1:7 and the vicinity thereof. 
     Note that the property of an oxide semiconductor is not uniquely determined by an atomic ratio. Even with the same atomic ratio, the property of an oxide semiconductor might be different depending on a formation condition. For example, in the case where the oxide semiconductor is deposited with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of a target is formed. In addition, [Zn] in the film might be smaller than [Zn] in the target depending on the substrate temperature in deposition. Thus, the illustrated regions each represent an atomic ratio with which an oxide semiconductor tends to have specific characteristics, and boundaries of the regions A to C are not clear. 
     [Transistor Including Oxide Semiconductor] 
     Next, the case where the oxide semiconductor is used for a transistor will be described. 
     Note that when the oxide semiconductor is used for a transistor, carrier scattering or the like at a grain boundary can be reduced; thus, the transistor can have high field-effect mobility. In addition, the transistor can have high reliability. 
     An oxide semiconductor with low carrier density is preferably used for the transistor. In order to reduce the carrier density of the 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. For example, an oxide semiconductor whose carrier density is lower than 8×10 11 /cm 3 , preferably lower than 1×10 11 /cm 3 , further preferably lower than 1×10 10 /cm 3 , and higher than or equal to 1×10 −9 /cm 3  is used. 
     A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases. 
     Charges trapped by the trap states in the oxide semiconductor take a long time to be released and may behave like fixed charges. Thus, a transistor whose channel formation region is formed in an oxide semiconductor having a high density of trap states has unstable electrical characteristics in some cases. 
     To obtain stable electrical characteristics of the transistor, it is effective to reduce the concentration of impurities in the oxide semiconductor. In addition, to reduce the concentration of impurities in the oxide semiconductor, the concentration of impurities in a film that is adjacent to the oxide semiconductor is preferably reduced. Examples of impurities include hydrogen, nitrogen, an alkali metal, an alkaline earth metal, iron, nickel, and silicon. 
     [Impurity in Oxide Semiconductor] 
     Here, the influence of impurities in the oxide semiconductor will be described. 
     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 including an oxide semiconductor that contains an alkali metal or an alkaline earth metal is likely to be normally-on. Therefore, it is preferable to reduce the concentration of an alkali metal or an alkaline earth metal in the oxide semiconductor. Specifically, the concentration of an alkali metal or an alkaline earth metal in the oxide semiconductor measured by secondary ion mass spectrometry (SIMS) is set lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . 
     Hydrogen contained in an oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy, in some cases. Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor that contains hydrogen is likely to be normally-on. Accordingly, it is preferable that hydrogen in the oxide semiconductor be reduced as much as possible. Specifically, the hydrogen concentration measured by 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 impurity concentration is used for a channel formation region in a transistor, the transistor can have stable electrical characteristics. 
     For example, in the case where an InGaZnO X  film (X&gt;0) is formed as the semiconductor layer  106  by a thermal CVD method, trimethylindium (In(CH 3 ) 3 ), trimethylgallium (Ga(CH 3 ) 3 ), and dimethylzinc (Zn(CH 3 ) 2 ) are used. Without limitation to the above combination, triethylgallium (Ga(C 2 H 5 ) 3 ) can be used instead of trimethylgallium, and diethylzinc (Zn(C 2 H 5 ) 2 ) can be used instead of dimethylzinc. 
     For example, in the case where an InGaZnO X  film (X&gt;0) is formed as the semiconductor layer  106  by an ALD method, an In(CH 3 ) 3  gas and an O 3  gas are sequentially introduced a plurality of times to form an InO 2  layer, a Ga(CH 3 ) 3  gas and an O 3  gas are sequentially introduced a plurality of times to form a GaO layer, and then a Zn(CH 3 ) 2  gas and an O 3  gas are sequentially introduced a plurality of times to form a ZnO layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an InGaO 2  layer, an InZnO 2  layer, a GaInO layer, a ZnInO layer, or a GaZnO layer may be formed using these gases. Note that although an H 2 O gas obtained by bubbling water with an inert gas such as Ar may be used instead of an O 3  gas, it is preferable to use an O 3  gas, which does not contain H. Instead of an In(CH 3 ) 3  gas, an In(C 2 H 5 ) 3  gas or tris(acetylacetonato)indium may be used. Note that tris(acetylacetonato)indium is also referred to as In(acac) 3 . Instead of a Ga(CH 3 ) 3  gas, a Ga(C 2 H 5 ) 3  gas or tris(acetylacetonato)gallium may be used. Note that tris(acetylacetonato)gallium is also referred to as Ga(acac) 3 . Furthermore, a Zn(CH 3 ) 2  gas or zinc acetate may be used. However, the deposition gas is not limited to the above. 
     In the case where the semiconductor layer  106  is formed by a sputtering method, a target containing indium is preferably used in order to reduce the number of particles. In addition, if an oxide target having a high atomic ratio of the element M is used, the conductivity of the target may be decreased. When a target containing indium is used, the conductivity of the target can be increased and DC discharge or AC discharge is facilitated; thus, deposition over a large substrate can be easily performed. Thus, semiconductor devices can be manufactured with improved productivity. 
     In the case where the semiconductor layer  106  is formed by a sputtering method, the atomic ratio of In to M and Zn (In:M:Zn) contained in the target is 3:1:1, 3:1:2, 3:1:4, 1:1:0.5, 1:1:1, 1:1:2, 1:1:1.2, 1:4:4, 4:2:4.1, 1:3:2, 1:3:4, 5:1:6, 5:1:8, or the like. 
     In the case where the semiconductor layer  106  is formed by a sputtering method, a film having an atomic ratio deviated from the atomic ratio of the target is formed in some cases. In particular, the atomic ratio of zinc in a deposited film is smaller than that in the target in some cases. Specifically, the film sometimes has an atomic ratio of zinc of 40 atomic % to 90 atomic % of the atomic ratio of zinc in the target. 
     When the semiconductor layer  106  is a stack including a plurality of layers as illustrated in  FIGS. 5A and 5B , an oxide semiconductor with a wide energy gap is preferably used for the semiconductor layer  106 _ 1 , for example. The energy gap of the semiconductor layer  106 _ 1  ranges, for example, from 2.5 eV to 4.2 eV, preferably from 2.8 eV to 3.8 eV, further preferably from 3 eV to 3.5 eV. 
     The semiconductor layers  106 _ 2  and  106 _ 3  are preferably formed using a material containing one or more kinds of metal elements, other than oxygen, contained in the semiconductor layer  106 _ 1 . With the use of such a material, interface states at interfaces between the semiconductor layer  106 _ 3  and the semiconductor layer  106 _ 1  and between the semiconductor layer  106 _ 2  and the semiconductor layer  106 _ 1  are less likely to be generated. Accordingly, carriers are not likely to be scattered or captured at the interfaces, which results in an improvement in field-effect mobility of the transistor. Furthermore, variations in threshold voltage of transistors can be reduced. Thus, a semiconductor device having favorable electrical characteristics can be obtained. 
     When the semiconductor layer  106 _ 1  is an In-M-Zn oxide (oxide containing In, the element M, and Zn) with an atomic ratio In:M:Zn of x 2 :y 2 :z 2  and each of the semiconductor layers  106 _ 2  and  106 _ 3  is an In-M-Zn oxide with an atomic ratio In:M:Zn of x 1 :y 1 :z 1 , y 1 /x 1  is preferably larger than y 2 /x 2 , further preferably greater than or equal to 1.5 times y 2 /x 2 , still further preferably greater than or equal to two times y 2 /x 2 , yet still further preferably greater than or equal to three times y 2 /x 2 . At this time, y 2  is preferably greater than or equal to x 2  in the semiconductor layer  106 _ 1 , in which case the transistor can have stable electrical characteristics. However, if y 2  is greater than or equal to five times x 2 , the field-effect mobility of the transistor is reduced; accordingly, y 2  is preferably less than five times x 2 . With the above structure, each of the semiconductor layers  106 _ 2  and  106 _ 3  can be a layer in which oxygen vacancy is less likely to be formed than in the semiconductor layer  106 _ 1 . 
     In the case where the semiconductor layer  106 _ 3  is an In-M-Zn oxide and the total content of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably less than 50 atomic % and greater than 50 atomic %, respectively, further preferably less than 25 atomic % and greater than 75 atomic %, respectively. In the case where the semiconductor layer  106 _ 1  is an In-M-Zn oxide and the total content of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably greater than 25 atomic % and less than 75 atomic %, respectively, further preferably greater than 34 atomic % and less than 66 atomic %, respectively. In the case where the semiconductor layer  106 _ 2  is an In-M-Zn oxide and the total content of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably less than 50 atomic % and greater than 50 atomic %, respectively, further preferably less than 25 atomic % and greater than 75 atomic %, respectively. Note that the semiconductor layers  106 _ 2  and  106 _ 3  may be formed using the same type of oxide. 
     For example, an In—Ga—Zn oxide formed using a target with an atomic ratio In:Ga:Zn of 1:3:2, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6 or an In—Ga oxide formed using a target with an atomic ratio In:Ga of 1:9 or 7:93 can be used for each of the semiconductor layers  106 _ 2  and  106 _ 3  containing In or Ga. Moreover, an In—Ga—Zn oxide formed using a target with an atomic ratio In:Ga:Zn of 1:1:1 or 3:1:2 can be used for the semiconductor layer  106 _ 1 , for example. Note that the atomic ratio of each of the semiconductor layers  106 _ 1  to  106 _ 3  may vary within a margin of ±40% of the corresponding atomic ratio. 
     For the semiconductor layer  106 _ 1 , an oxide having a higher electron affinity than the semiconductor layers  106 _ 2  and  106 _ 3  is preferably used. For example, the semiconductor layer  106 _ 1  may be an oxide having an electron affinity higher than that of each of the semiconductor layers  106 _ 2  and  106 _ 3  by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, further preferably 0.15 eV or higher and 0.4 eV or lower. Note that the electron affinity refers to an energy difference between the vacuum level and the conduction band minimum. 
     Indium gallium oxide has a small electron affinity and an excellent oxygen-blocking property. Therefore, the semiconductor layer  106 _ 2  preferably contains indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)] is, for example, higher than or equal to 70%, preferably higher than or equal to 80%, further preferably higher than or equal to 90%. 
     Note that the semiconductor layer  106 _ 2  and/or the semiconductor layer  106 _ 3  may be gallium oxide. For example, when gallium oxide is used for the semiconductor layer  106 _ 3 , a leakage current generated between the electrode  108  and the semiconductor layer  106  can be reduced. In other words, the off-state current of the transistor  100  can be reduced. 
     At this time, when a gate voltage is applied, a channel is formed in the semiconductor layer  106 _ 1  having the highest electron affinity among the semiconductor layers  106 _ 1  to  106 _ 3 . 
     In order to give stable electrical characteristics to the OS transistor, it is preferred that impurities and oxygen vacancies in the oxide semiconductor layer be reduced to highly purify the oxide semiconductor layer so that at least the semiconductor layer  106 _ 1  can be regarded as an intrinsic or substantially intrinsic oxide semiconductor layer. Furthermore, preferably at least the channel formation region of the semiconductor layer  106 _ 1  is regarded as an intrinsic or substantially intrinsic semiconductor layer. 
     [Film Formation Method] 
     The insulating layers, the conductive layers for forming electrodes or wirings, the semiconductor layers, or the like can be formed by a sputtering method, a spin coating method, a chemical vapor deposition (CVD) method (such as a thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method, a plasma-enhanced CVD (PECVD) method, a high-density plasma CVD method, a low-pressure CVD (LPCVD) method, or an atmospheric-pressure CVD (APCVD) method), an atomic layer deposition (ALD) method, a molecular beam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, a dipping method, a spray coating method, a droplet discharging method (such as an inkjet method), or a printing method (such as screen printing or offset printing). 
     By using a PECVD method, a high-quality film can be formed at a relatively low temperature. By using a deposition method that does not use plasma for deposition, such as an MOCVD method, an ALD method, or a thermal CVD method, damage is not easily caused on a surface on which the film is deposited. For example, a wiring, an electrode, an element (e.g., transistor or capacitor), or the like included in a semiconductor device may sometimes be charged up by receiving charges from plasma. In that case, accumulated charges might break the wiring, electrode, element, or the like included in the semiconductor device. Such plasma damage is not caused in the case of using a deposition method without using plasma, and thus the yield of a semiconductor device can be increased. In addition, since plasma damage does not occur in the deposition, a film with few defects can be obtained. 
     Unlike in a deposition method in which particles ejected from a target or the like are deposited, a film is formed by reaction at a surface of an object in a CVD method and an ALD method. Thus, a CVD method and an ALD method enable favorable step coverage almost regardless of the shape of the object. In particular, an ALD method enables excellent step coverage and excellent thickness uniformity and can be favorably used to cover a surface of an opening with a high aspect ratio, for example. On the other hand, an ALD method has a relatively low deposition rate; thus, it is sometimes preferable to combine an ALD method with another deposition method with a high deposition rate, such as a CVD method. 
     When a CVD method or an ALD method is used, the composition of a film to be formed can be controlled with a flow rate ratio of source gases. For example, by a CVD method or an ALD method, a film with a certain composition can be formed depending on a flow rate ratio of the source gases. Moreover, with a CVD method or an ALD method, by changing the flow rate ratio of the source gases while forming the film, a film whose composition is continuously changed can be formed. In the case where the film is formed while changing the flow rate ratio of the source gases, as compared to the case where the film is formed using a plurality of deposition chambers, the time taken for the film formation can be reduced because the time taken for transfer and pressure adjustment is omitted. Thus, semiconductor devices can be manufactured with improved productivity in some cases. 
     In the case where a film is formed by an ALD method, a gas that does not contain chlorine is preferably used as a material gas. 
     When an oxide semiconductor is formed by a sputtering method, each chamber of a sputtering apparatus is preferably evacuated to a high vacuum (to the degree of approximately 5×10 −7  Pa to 1×10 −4  Pa) by an adsorption vacuum pump such as a cryopump so that water and the like acting as impurities for the oxide semiconductor can be removed as much as possible. In particular, the partial pressure of gas molecules corresponding to H 2 O (gas molecules corresponding to m/z=18) in the chamber in the standby mode of the sputtering apparatus is preferably lower than or equal to 1×10 −4  Pa, further preferably lower than or equal to 5×10 −5  Pa. The deposition temperature is preferably higher than or equal to RT and lower than or equal to 500° C., further preferably higher than or equal to RT and lower than or equal to 300° C., still further preferably higher than or equal to RT and lower than or equal to 200° C. 
     In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used for a sputtering gas, a gas that is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film can be minimized. 
     In the case where the insulating layers, the conductive layers, the semiconductor layers, or the like are formed by a sputtering method using a sputtering gas containing oxygen, oxygen can be supplied to a layer over which these layers are formed. As the amount of oxygen contained in the sputtering gas increases, the amount of oxygen supplied to a layer over which these layers are formed tends to increase. 
     &lt;Example of Method for Manufacturing Transistor  100 &gt; 
     An example of a method for manufacturing the transistor  100  will be described with reference to  FIGS. 6A to 6C ,  FIGS. 7A to 7C ,  FIGS. 8A to 8C ,  FIGS. 9A and 9B ,  FIGS. 10A and 10B , and  FIGS. 11A and 11B . The cross-sectional views of  FIGS. 6A to 6C ,  FIGS. 7A to 7C ,  FIGS. 8A to 8C ,  FIGS. 9A and 9B ,  FIGS. 10A and 10B , and  FIGS. 11A and 11B  correspond to the cross section of the portion indicated by the dashed-dotted line X 1 -X 2  in  FIG. 1A . 
     [Step 1] 
     First, the insulating layer  102  is formed over the substrate  101  (see  FIG. 6A ). In this embodiment, aluminoborosilicate glass is used for the substrate  101 . In this embodiment, a 2-μm-thick polyimide layer is formed as the insulating layer  102 . Specifically, a liquid polyimide is applied onto the substrate  101  by a droplet discharge method and baking is performed at 400° C. in a nitrogen atmosphere for one hour. Note that the insulating layer  102  used in this embodiment has a Young&#39;s modulus of approximately larger than or equal to 3 GPa and smaller than or equal to 5 GPa after the baking. 
     [Step 2] 
     Next, the insulating layer  103  is formed (see  FIG. 6B ). In this embodiment, a 200-nm-thick silicon oxynitride layer is formed by a PECVD method as the insulating layer  103 . For example, the formation can be performed under the conditions where a silane gas at a flow rate of 75 sccm and a dinitrogen monoxide gas at a flow rate of 1200 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 70 Pa, the substrate temperature is controlled to 330° C., and a power of 120 W is supplied using a 27.12 MHz high-frequency power source. Note that the insulating layer  103  used in this embodiment has a Young&#39;s modulus of approximately larger than or equal to 60 GPa and smaller than or equal to 80 GPa. 
     As described above, the insulating layer  103  may be formed using an insulating material through which impurities do not easily pass. Furthermore, the insulating layer  103  may be formed using an insulating material into which oxygen is less likely to diffuse. In the case where an aluminum oxide layer is used as the insulating layer  103 , for example, the aluminum oxide layer may be formed by a DC sputtering method using an aluminum target or by an AC sputtering method using an aluminum oxide target. 
     [Step 3] 
     Next, a conductive layer  181  for forming the electrode  104  is formed (see  FIG. 6C ). In this embodiment, titanium is used for the conductive layer  181 . Specifically, a 100-nm-thick titanium layer is formed by a sputtering method. Note that the conductive layer  181  used in this embodiment has a Young&#39;s modulus of approximately larger than or equal to 100 GPa and smaller than or equal to 120 GPa. 
     [Step 4] 
     Next, a resist mask is formed (not illustrated). The resist mask can be formed by a photolithography method, a printing method, an inkjet method, or the like as appropriate. Formation of the resist mask by a printing method, an inkjet method, or the like needs no photomask; thus, the manufacturing cost can be reduced. 
     The formation of the resist mask by a photolithography method can be performed in such a manner that a photosensitive resist is irradiated with light through a photomask and a portion of the resist that has been exposed to light (or has not been exposed to light) is removed using a developing solution. Examples of light with which the photosensitive resist is irradiated include KrF excimer laser light, ArF excimer laser light, and extreme ultraviolet (EUV) light. Alternatively, a liquid immersion technique may be employed in which light exposure is performed with a portion between a substrate and a projection lens filled with liquid (e.g., water). An electron beam or an ion beam may be used instead of the above-mentioned light. Note that a photomask is not necessary in the case of using an electron beam or an ion beam. 
     With the use of the resist mask as a mask, part of the conductive layer  181  is selectively removed to form the electrode  104  (see  FIG. 7A ). The insulating layer  181  can be removed by a dry etching method, a wet etching method, or the like. Both the dry etching method and the wet etching method may be used. 
     The resist mask is removed after the portion of the conductive layer  181  is removed. The resist mask can be removed by a dry etching method such as ashing or a wet etching method using a dedicated stripper or the like. Both the dry etching method and the wet etching method may be used. 
     A side surface of the electrode  104  is preferably tapered in cross section. A taper angle θ of the side surface of the electrode  104  is preferably 20° or more and less than 90°, further preferably 30° or more and less than 80°, still further preferably 40° or more and less than 70°. Note that the taper angle θ refers to an angle formed by a side surface and a bottom surface of a layer having a tapered shape when the layer is seen from the cross-sectional direction (i.e., the direction of the plane perpendicular to the substrate surface). 
     The tapered shape of the side surface of the electrode  104  can prevent disconnection of a layer formed over the electrode  104  and improve the coverage. Moreover, the tapered shape of the side surface of the electrode  104  can relieve electric field concentration at an upper edge portion of the electrode  104 . Meanwhile, if the taper angle θ is too small, miniaturization of the transistor is sometimes difficult or variations in opening size, wiring width, or the like sometimes increase. 
     The side surface of the electrode  104  may have a step-like shape, in which case disconnection of a layer formed over the electrode  104  can be prevented and the coverage can be improved. As well as the side surface of the electrode  104 , an edge portion of any layer can have a tapered shape or a step-like shape, in which case disconnection of a layer covering the layer (disconnection caused by a step) can be prevented, resulting in favorable coverage. 
     [Step 5] 
     Then, the insulating layer  105  is formed by a PECVD method (see  FIG. 7B ). In this embodiment, the insulating layer  105  has a four-layer structure in which a first silicon nitride layer, a second silicon nitride layer, a third silicon nitride layer, and a silicon oxynitride layer are stacked in this order. 
     For example, the first silicon nitride layer can be formed to have a thickness of 50 nm under the conditions where a silane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, the substrate temperature is controlled to 350° C., and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source. 
     For example, the second silicon nitride layer can be formed to have a thickness of 300 nm under the conditions where a silane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, the substrate temperature is controlled to 350° C., and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source. 
     For example, the third silicon nitride layer can be formed to have a thickness of 50 nm under the conditions where a silane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, the substrate temperature is controlled to 350° C., and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source. 
     For example, the silicon oxynitride layer can be formed to have a thickness of 50 nm under the conditions where a silane gas at a flow rate of 20 sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 40 Pa, the substrate temperature is controlled to 350° C., and a power of 100 W is supplied using a 27.12 MHz high-frequency power source. 
     Note that the insulating layer  105  used in this embodiment has a compressive stress of approximately greater than or equal to 1000 MPa and less than or equal to 1200 MPa. 
     The insulating layer  102  has a smaller Young&#39;s modulus than the electrode  104  and the insulating layer  103  and thus more easily changes in shape than the electrode  104  and the insulating layer  103 . A region of the insulating layer  102  near the periphery of the electrode  104  is pressed owing to the compressive stress of the insulating layer  105 , so that a region of the insulating layer  102  overlapped by the electrode  104  changes into a convex shape. As a result, the insulating layer  102  has a projection. The projection is formed in a self-aligned manner in a position overlapped by the electrode  104 . 
     As the insulating layer  105 , an insulating layer containing excess oxygen may be used. The insulating layer  105  may be subjected to oxygen doping treatment. Moreover, heat treatment is preferably performed after formation of the insulating layer  105  to reduce hydrogen and moisture contained in the insulating layer  105 . Oxygen doping treatment may be performed after the heat treatment. For example, the oxygen doping treatment is performed while the substrate is heated to 350° C. and a gas containing argon and oxygen is excited at a frequency of 2.45 GHz. The heat treatment and the oxygen doping treatment may be repeated a plurality of times. 
     For example, the heat treatment is performed in an inert gas atmosphere containing nitrogen, a rare gas, or the like, an oxidizing atmosphere, or an ultra-dry air atmosphere (in which the moisture amount measured with a dew point meter in a cavity ring down laser spectroscopy (CRDS) system is 20 ppm (corresponding to a dew point of −55° C.) or lower, preferably 1 ppm or lower, further preferably 10 ppb or lower). Note that the oxidizing atmosphere refers to an atmosphere containing an oxidizing gas such as oxygen, ozone, or nitrogen oxide at 10 ppm or higher. The inert atmosphere refers to an atmosphere that contains the oxidizing gas at lower than 10 ppm and is filled with nitrogen or a rare gas. Although there is no particular limitation on the pressure during the heat treatment, the heat treatment is preferably performed under a reduced pressure. 
     The heat treatment is performed at temperatures of 150° C. or higher and lower than the strain point of the substrate, preferably from 200° C. to 500° C., further preferably from 250° C. to 400° C. The treatment time is shorter than or equal to 24 hours. Heat treatment for over 24 hours is not preferable because the productivity is reduced. 
     The heat treatment can be performed using an electric furnace, an RTA apparatus, or the like. With the use of an RTA apparatus, the heat treatment can be performed at a temperature higher than or equal to the strain point of the substrate if the heating time is short. Thus, the heating time can be shortened. The heat treatment may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which water content is 20 ppm or less, preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas (e.g., argon or helium). Note that the atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like. 
     [Step 6] 
     Next, a semiconductor layer  182  is formed (see  FIG. 7C ). Note that before the semiconductor layer  182  is formed, an oxygen gas may be supplied to generate plasma, in which case oxygen can be added to the insulating layer  105 , which is a surface where the semiconductor layer  182  is formed. 
     For the semiconductor layer  182 , it is preferable to use indium zinc oxide, indium gallium zinc oxide formed using a target with an atomic ratio In:Ga:Zn=5:1:7 or 4:2:4.1, or the like. 
     In this embodiment, for the semiconductor layer  182 , indium gallium zinc oxide is deposited by a sputtering method using a target with an atomic ratio In:Ga:Zn=4:2:4.1. Oxygen or a mixed gas of oxygen and a rare gas is used as a sputtering gas. In this embodiment, a mixed gas of oxygen and argon in which the flow rate ratio of oxygen is 10% is used as the sputtering gas. 
     When the flow rate ratio of oxygen in the sputtering gas is higher than or equal to 0% and lower than or equal to 30%, preferably higher than or equal to 5% and lower than or equal to 20%, an oxygen-deficient oxide semiconductor layer is formed. A transistor including an oxygen-deficient oxide semiconductor layer can have relatively high field-effect mobility. 
     At the time of the formation of the semiconductor layer  182 , part of oxygen contained in the sputtering gas is supplied to the insulating layer  105  in some cases. As the amount of oxygen contained in the sputtering gas increases, the amount of oxygen supplied to the insulating layer  105  increases. Part of oxygen supplied to the insulating layer  105  reacts with hydrogen left in the insulating layer  105  to produce water, and the water is released from the insulating layer  105  by heat treatment performed later. Thus, the hydrogen concentration in the insulating layer  105  can be reduced. Moreover, when the amount of excess oxygen in the insulating layer  105  is increased, oxygen can be supplied to the semiconductor layer  182  (that is to be the semiconductor layer  106 ) by heat treatment performed later. 
     When the semiconductor layer  106  is a stack of two layers or three layers as illustrated in  FIGS. 4A and 4B , an oxide semiconductor layer for forming the semiconductor layer  106 _ 1  is formed using the above-described material and method. 
     For an oxide semiconductor layer(s) for forming the semiconductor layer  106 _ 2  and/or the semiconductor layer  106 _ 3 , it is preferable to use an oxide semiconductor layer with high crystallinity, for example, a CAAC-OS layer. For example, in a subsequent etching step for forming the insulating layer  107  and the electrode  108 , an exposed portion of the oxide semiconductor layer is etched and the oxide semiconductor layer is damaged in some cases. An oxide semiconductor layer with high crystallinity is not likely to be etched in this etching step. Using an oxide semiconductor layer with high crystallinity as the semiconductor layer  106 _ 2  and/or the semiconductor layer  106 _ 3  reduces damage caused to the oxide semiconductor layer in the etching step; thus, the reliability of the transistor can be improved. 
     For the oxide semiconductor layer(s) for forming the semiconductor layer  106 _ 2  and/or the semiconductor layer  106 _ 3 , indium gallium zinc oxide is deposited by a sputtering method using a target with an atomic ratio In:Ga:Zn=1:1:1.2, for example. Oxygen or a mixed gas of oxygen and a rare gas is used as a sputtering gas. For example, a sputtering gas with an oxygen proportion of 100% is used. The flow rate ratio of oxygen in the sputtering gas for forming the semiconductor layer  106 _ 2  and/or the semiconductor layer  106 _ 3  is preferably 70% or more, further preferably 80% or more, still further preferably 100%. By increasing the proportion (flow rate ratio) of oxygen in the sputtering gas, the crystallinity of the oxide semiconductor layer can be increased. 
     By introducing an impurity element after formation of the semiconductor layer  182 , the threshold voltage of the transistor  100  can be changed. An impurity element can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment using a gas containing an impurity element, or the like. 
     After the semiconductor layer  182  is formed, heat treatment and/or oxygen doping treatment may be performed. Heat treatment and oxygen doping treatment may be repeated a plurality of times. 
     Furthermore, after heat treatment is performed in a nitrogen atmosphere or a rare gas atmosphere, heat treatment may be additionally performed in an oxygen atmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water, and the like can be released from the oxide semiconductor layer and oxygen can be supplied to the oxide semiconductor layer at the same time. Consequently, oxygen vacancies in the oxide semiconductor layer can be reduced. 
     [Step 7] 
     Then, a resist mask is formed by a photolithography method (not illustrated). With the use of the resist mask as a mask, part of the semiconductor layer  182  is selectively removed, so that the island-shaped semiconductor layer  106  is formed (see  FIG. 8A ). 
     The semiconductor layer  182  can be removed by a dry etching method, a wet etching method, or the like. Both the dry etching method and the wet etching method may be used. By introducing an impurity element after formation of the semiconductor layer  106 , the threshold voltage of the transistor  100  can be changed. 
     After the semiconductor layer  106  is formed, heat treatment and/or oxygen doping treatment may be performed. Heat treatment and oxygen doping treatment may be repeated. 
     In the case where the semiconductor layer  106  is a stack of two layers or three layers as illustrated in  FIGS. 5A and 5B , heat treatment and/or oxygen doping treatment may be performed after the semiconductor layer  106 _ 1  is formed, after the semiconductor layer  106 _ 1  and the semiconductor layer  106 _ 2  are formed, or after the semiconductor layers  106 _ 1  to  106 _ 3  are formed. Heat treatment and oxygen doping treatment may be repeated. 
     [Step 8] 
     Then, an insulating layer  183  to be the insulating layer  107  is formed (see  FIG. 8B ). The insulating layer  183  can be a silicon oxynitride layer formed by a PECVD method, for example. In this case, a deposition gas containing silicon and an oxidizing gas are preferably used as source gases. Typical examples of a deposition gas containing silicon include a silane gas, a disilane gas, a trisilane gas, and a silane fluoride gas. Examples of an oxidizing gas include a dinitrogen monoxide gas and a nitrogen dioxide gas. The flow rate of the oxidizing gas is greater than or equal to 20 and less than or equal to 5000 times that of the deposition gas, preferably greater than or equal to 40 and less than or equal to 100 times that of the deposition gas. 
     In this embodiment, the insulating layer  183  has a three-layer structure in which a first silicon oxynitride layer, a second silicon oxynitride layer, and a third silicon oxynitride layer are stacked in this order. 
     For example, the first silicon oxynitride layer can be formed to have a thickness of 30 nm under the conditions where a silane gas at a flow rate of 20 sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 200 Pa, the substrate temperature is controlled to 350° C., and a power of 100 W is supplied using a 27.12 MHz high-frequency power source. 
     For example, the second silicon oxynitride layer can be formed to have a thickness of 100 nm under the conditions where a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 200 Pa, the substrate temperature is controlled to 220° C., and a power of 1500 W is supplied using a 27.12 MHz high-frequency power source. 
     The second silicon oxynitride layer is preferably an insulating layer containing excess oxygen. Further, it is preferable that the amount of defects in the second silicon oxynitride layer be small. Typically, the spin density of a signal which appears at g=2.001 originating from a dangling bond of silicon is preferably lower than 1.5×10 18  spins/cm 3 , further preferably lower than or equal to 1×10 18  spins/cm 3  by ESR measurement. Note that the second silicon oxynitride layer is more distant from the semiconductor layer  106  than the first silicon oxynitride layer and thus may have a higher defect density than the first silicon oxynitride layer. 
     As the second silicon oxynitride layer, a silicon oxynitride layer may be formed under the conditions where the substrate is held at a temperature ranging from 180° C. to 400° C., the pressure in the reaction chamber into which source gases are introduced ranges from 100 Pa to 250 Pa, preferably from 100 Pa to 200 Pa, and a high-frequency power of 0.17 W/cm 2  to 0.5 W/cm 2 , preferably 0.25 W/cm 2  to 0.35 W/cm 2  is supplied to an electrode provided in the reaction chamber. 
     In formation of the second silicon oxynitride layer, when the high-frequency power with the above power density is supplied in the reaction chamber at the above pressure, decomposition efficiency of the source gases is increased in plasma. That is, oxygen radicals increase in the reaction chamber, and oxidation of the source gases proceeds. Thus, the oxygen content of the second silicon oxynitride layer can be higher than that in the stoichiometric composition. 
     In the insulating layer formed at a substrate temperature within the above range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the insulating layer is released by heat treatment in a later step. Thus, part of the released oxygen is supplied to the semiconductor layer  106 . 
     Note that in the formation conditions for the second silicon oxynitride layer, the flow rate of the deposition gas containing silicon relative to the oxidizing gas can be increased, whereby the amount of defects in the second silicon oxynitride layer can be reduced. Typically, it is possible to form an oxide insulating layer in which the amount of defects is small, i.e., the spin density of a signal that appears at g=2.001 originating from a dangling bond of silicon is lower than 6×10 17  spins/cm 3 , preferably lower than or equal to 3×10 17  spins/cm 3 , further preferably lower than or equal to 1.5×10 17  spins/cm 3  by ESR measurement. As a result, the reliability of the transistor can be improved. 
     Note that in the step of forming the second silicon oxynitride layer, the first silicon oxynitride layer functions as a protective layer for the semiconductor layer  106 . Therefore, the second silicon oxynitride layer can be formed using the high-frequency power having a high power density while damage to the semiconductor layer  106  is reduced. 
     For example, the third silicon oxynitride layer can be formed to have a thickness of 20 nm under the conditions where a silane gas at a flow rate of 20 sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 200 Pa, the substrate temperature is controlled to 350° C., and a power of 100 W is supplied using a 27.12 MHz high-frequency power source. 
     In the next step in which a conductive layer  184  is formed, the third silicon oxynitride layer functions as a protective layer for the second silicon oxynitride layer. 
     Heat treatment may be performed after Step 8. For example, heat treatment may be performed at 350° C. in a nitrogen atmosphere for one hour. Oxygen doping treatment may be performed after Step 8. The heat treatment and the oxygen doping treatment may be repeated alternately a plurality of times. 
     [Step 9] 
     Next, the conductive layer  184  for forming the electrode  108  is formed (see  FIG. 8C ). In this embodiment, indium gallium zinc oxide layers are used as the conductive layer  184 . Specifically, a two-layer stack formed using indium gallium zinc oxide is used as the conductive layer  184 . 
     First, a 10-nm-thick indium gallium zinc oxide layer is formed using a target with an atomic ratio In:Ga:Zn=4:2:4.1 and a sputtering gas with an oxygen proportion of 100%. Next, a 90-nm-thick indium gallium zinc oxide layer is formed using a target with an atomic ratio In:Ga:Zn=4:2:4.1 and a sputtering gas with an oxygen proportion of 10% and an argon proportion of 90%. 
     [Step 10] 
     Next, a resist mask is formed by a photolithography method (not illustrated). With the use of the resist mask as a mask, part of the conductive layer  184  is selectively removed, so that the electrode  108  is formed. At this time, with the use of the electrode  108  as a mask, part of the insulating layer  183  is also selectively removed to form the insulating layer  107  (see  FIG. 9A ). By Step 10, part of the semiconductor layer  106  is exposed. A channel is formed in a region of the semiconductor layer  106  which is overlapped by the electrode  108 . 
     The conductive layer  184  and the insulating layer  183  can be removed by a dry etching method, a wet etching method, or the like. Both the dry etching method and the wet etching method may be used. 
     [Step 11] 
     Then, impurities are introduced into the regions of the semiconductor layer  106  which are exposed in Step 8. The impurities may be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. Introducing impurities such as nitrogen into the regions can lower the resistance of the regions. 
     Moreover, the regions may be exposed to a plasma atmosphere of nitrogen or an inert gas. Exposing the regions to a plasma atmosphere causes a defect in the regions and thus can lower the resistance of the regions. 
     In the semiconductor layer  106 , the regions into which the impurities are introduced or the regions exposed to a plasma atmosphere can function as a source region and a drain region of the transistor. Moreover, in the semiconductor layer  106 , a region overlapped by the electrode  108  can function as a channel formation region. In other words, the source region and the drain region of the transistor can be formed in a self-aligned manner. 
     In this embodiment, the plasma treatment is performed in an atmosphere containing argon and nitrogen. 
     [Step 12] 
     Then, the insulating layer  109  is formed (see  FIG. 9B ). In this embodiment, a 100-nm-thick silicon nitride layer is used as the insulating layer  109 . 
     The silicon nitride layer used as the insulating layer  109  is formed by a PECVD method or the like. For example, the silicon nitride layer can be formed under the conditions where a silane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, the substrate temperature is controlled to 220° C., and a power of 1000 W is supplied using a 27.12 MHz high-frequency power source. 
     The silicon nitride layer is an insulating material through which impurities do not easily pass and thus can prevent diffusion of impurities from above to the semiconductor layer  106 . The regions of the semiconductor layer  106  that are in contact with the silicon nitride layer are supplied with an impurity such as hydrogen or nitrogen at the time of the formation of the silicon nitride layer to have reduced resistance. Thus, the regions that can serve as source and drain regions and that are described in Step 11 can have further reduced resistance. 
     [Step 13] 
     Next, the insulating layer  110  is formed (see  FIG. 9B ). In this embodiment, as the insulating layer  110 , a silicon oxynitride layer having a thickness of 300 nm is formed by a PECVD method. 
     For example, the silicon oxynitride layer used as the insulating layer  110  can be formed under the conditions where a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 200 Pa, the substrate temperature is controlled to 220° C., and a power of 1500 W is supplied using a 27.12 MHz high-frequency power source. 
     [Step 14] 
     Next, a resist mask is formed by a photolithography method (not illustrated). With the use of the resist mask as a mask, portions of the insulating layers  110  and  109  are selectively removed to form the openings  111   a  and  111   b  (see  FIG. 10A ). At this time, part of the semiconductor layer  106  is exposed. 
     The insulating layer  110  and the insulating layer  109  can be removed by a dry etching method, a wet etching method, or the like. Both the dry etching method and the wet etching method may be used. 
     [Step 15] 
     Next, a conductive layer  186  for forming the electrodes  112   a  and  112   b  is formed (see  FIG. 10B ). In this embodiment, a stack including titanium and copper is used for the conductive layer  186 . Specifically, a 10-nm-thick titanium layer and a 100-nm-thick copper layer are formed in this order by a sputtering method. 
     [Step 16] 
     Next, a resist mask is formed by a photolithography method (not illustrated). With the use of the resist mask as a mask, part of the conductive layer  186  is selectively removed to form the electrode  112   a  and the electrode  112   b  (see  FIG. 11A ). One of the electrode  112   a  and the electrode  112   b  can function as a source electrode, and the other can function as a drain electrode. 
     The conductive layer  186  can be removed by a dry etching method, a wet etching method, or the like. Both the dry etching method and the wet etching method may be used. 
     [Step 17] 
     Then, the insulating layer  113  having a flat surface is formed (see  FIG. 11B ). In this embodiment, a 1.5-μm-thick acrylic resin layer is formed as the insulating layer  113 . Note that the insulating layer  113  is not necessarily provided depending on the purpose or the intended use. 
     Through the above steps, the transistor  100  can be manufactured. In one embodiment of the present invention, the maximum process temperature in formation of the transistor  100  can be lower than or equal to 400° C. Thus, the transistor  100  can be manufactured with improved productivity. In addition, a semiconductor device including the transistor  100  can be manufactured with improved productivity. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 2 
     In this embodiment, a modification example of the transistor  100  described in Embodiment 1 will be described with reference to drawings. Note that differences from the transistor  100  are mainly described to avoid repeated description. Embodiment 1 can be referred to for the portions that are not described in this embodiment. 
     Modification Example 1 
       FIG. 12A  is a plan view of a transistor  100 A.  FIG. 12B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 12A .  FIG. 13  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 12A . 
     The transistor  100 A, having almost the same structure as the transistor  100 , is different from the transistor  100  in including a structure body  122  between the insulating layer  102  and the substrate  101 . In the transistor  100 A, the semiconductor layer  106 , the electrode  104 , and the electrode  108  each have a region overlapping the structure body  122 . 
     The insulating layer  102  covers the structure body  122 , thereby having a projection. The height or shape of the projection can be arbitrarily set by changing the size of the structure body  122 . The structure body  122  can be formed using any of the above-described insulating materials. Depending on the purpose or the intended use, the structure body  122  may be formed using a conductive material, a semiconductor material, or the like. 
     Providing the insulating layer  102  with the projection by the use of the structure body  122  makes the range of materials used for the transistor wider than that in the case of employing the method described in the above embodiment, which utilizes the difference in mechanical strength between the layers. Thus, the thickness, the Young&#39;s modulus, the stress, and the like of the insulating layer  102 , the insulating layer  103 , the electrode  104 , and the insulating layer  105  can be set relatively freely. 
     Next, an example of a manufacturing method of the transistor  100 A will be described. 
     [Step 1a] 
     Step 1a is performed instead of Step 1 described in Embodiment 1. In this embodiment, a photosensitive polyimide layer is formed over the substrate  101  and photolithography is employed to form the structure body  122  (see  FIG. 14A ). The use of the photosensitive polyimide layer makes it possible to form the structure body  122  without a resist mask. The height H S  of the structure body  122  can be adjusted by changing the thickness of the photosensitive polyimide layer. In this embodiment, the structure body  122  whose height H S  is 2.0 μm is formed. 
     Note that the structure body  122  may be formed using a non-photosensitive material. In that case, the structure body  122  is formed by a typical patterning method that uses a resist mask or the like, for example. 
     [Step 2a] 
     Next, the insulating layer  102  is formed (see  FIG. 14B ). In this embodiment, a polyimide layer whose thinnest portion not overlapping the structure body  122  has a thickness TB of 1.0 μm is formed as the insulating layer  102 . Specifically, a liquid polyimide is applied onto the substrate  101  and the structure body  122  by a droplet discharge method and baking is performed at 400° C. in a nitrogen atmosphere for one hour. 
     In the case where the insulating layer  102  is formed using a liquid material, the thickness T G  of the portion of the insulating layer  102  that overlaps the top of the structure body  122  and the thickness T B  depend on the viscosity and the applied amount of the material. The height H of the projection is obtained by subtracting the thickness T B  from the sum of the height H S  and the thickness T G . 
     The structure body  122  and the insulating layer  102  are preferably formed using materials having the same components. 
     The steps including and after Step 1b can be performed in a manner similar to that of the steps including and after Step 2 described in Embodiment 1. 
     Modification Example 2 
       FIG. 15A  is a plan view of a transistor  100 B.  FIG. 15B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 15A .  FIG. 16  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 15A . 
     The transistor  100 B has the structure of the transistor  100 A without the electrode  104 . The electrode  104  is not necessarily provided, as determined by required performance, purpose, or the like of the transistor. When the electrode  104  is not provided, the number of steps for manufacturing the transistor is reduced; thus, the manufacturing cost can be reduced. Moreover, the manufacturing yield of the transistor can be increased. 
     Modification Example 3 
       FIG. 17A  is a plan view of a transistor  100 C.  FIG. 17B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 17A .  FIG. 18  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 17A . 
     Providing a plurality of structure bodies  122  forms a depression in the insulating layer  102 . The transistor  100 C has a structure similar to that of the transistor  100 A. The transistor  100 C is different from the transistor  100 A in that the semiconductor layer  106 , the electrode  104 , and the electrode  108  each have a region overlapping the depression of the insulating layer  102 . The transistor  100 C has a region where the channel formation region overlaps the depression of the insulating layer  102 . The transistor  100 C also has a region where the channel formation region does not overlap the projection of the insulating layer  102 . 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 3 
     A transistor  150 , which has a structure different from the structures of the transistors  100 ,  100 A,  100 B, and  100 C in the above embodiments, will be described with reference to drawings. Note that differences from the transistor  100  are mainly described to avoid repeated description. The above embodiments can be referred to for the portions that are not described in this embodiment. 
     &lt;Structure Example of Transistor  150 &gt; 
       FIG. 19A  is a plan view of the transistor  150 .  FIG. 19B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 19A .  FIG. 20  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 19A . 
     The transistor  150  described in this embodiment is a kind of bottom-gate transistor. The transistor  150  is provided over the substrate  101  with the insulating layer  102  and the insulating layer  103  provided therebetween. The transistor  150  includes the electrode  104 , the insulating layer  105 , the semiconductor layer  106  (the semiconductor layer  106 _ 1  and the semiconductor layer  106 _ 2 ), the electrode  112   a , the electrode  112   b , an insulating layer  115 , an insulating layer  116 , an insulating layer  117 , and the electrode  108 . 
     The electrode  112   a  can serve as one of a source electrode and a drain electrode. The electrode  112   b  can function as the other of the source electrode and the drain electrode. The insulating layer  113  having a flat surface may be provided over the transistor  150 . 
     The insulating layer  105  may be formed using an insulating material that is unlikely to transmit impurities, in which case the insulating layer  103  is not necessarily provided (see  FIGS. 21A and 21B ).  FIG. 21A  is a cross-sectional view corresponding to  FIG. 19B .  FIG. 21B  is a cross-sectional view corresponding to  FIG. 20 . 
     In the example illustrated in  FIGS. 19A and 19B  and  FIG. 20 , the semiconductor layer  106  of the transistor  150  has a two-layer structure of the semiconductor layer  106 _ 1  and the semiconductor layer  106 _ 2 . The semiconductor layer  106  may be a single layer or a stack of three or more layers. For example, the semiconductor layer  106  may consist only of the semiconductor layer  106 _ 1  as illustrated in  FIG. 22A . Alternatively, the semiconductor layer  106  may have a three-layer structure of the semiconductor layer  106 _ 1 , the semiconductor layer  106 _ 2 , and the semiconductor layer  106 _ 3  as illustrated in  FIG. 22B , for example. Needless to say, the semiconductor layer  106  may be a stack of four or more layers. Note that  FIGS. 22A and 22B  are cross-sectional views corresponding to  FIG. 19B . 
     In the transistor  150 , a region of the semiconductor layer  106  that overlaps the electrode  104  and that is positioned between the electrode  112   a  and the electrode  112   b  in a plan view serves as a channel formation region. A region of the semiconductor layer  106  that is in contact with the electrode  112   a  serves as one of a source region and a drain region. A region of the semiconductor layer  106  that is in contact with the electrode  112   b  serves as the other of the source region and the drain region. Specifically, the region  106   c  illustrated in  FIG. 19B  serves as the channel formation region. The region  106   s  illustrated in  FIG. 19B  serves as one of the source region and the drain region. The region  106   d  illustrated in  FIG. 19B  serves as the other of the source region and the drain region. 
     In some of  FIG. 20  and subsequent drawings, the region  106   c , the region  106   s , and the region  106   d  are not illustrated. 
     The channel length L of the transistor  150  is the length from the end portion of the electrode  112   a  that overlaps the semiconductor layer  106  to the end portion of the electrode  112   b  that overlaps the semiconductor layer  106  (see  FIGS. 19A and 19B ). The channel formation region of the transistor  150  overlaps the projection of the insulating layer  102 . 
     &lt;Example of Method for Manufacturing Transistor  150 &gt; 
     An example of a method for manufacturing the transistor  150  will be described with reference to  FIGS. 23A and 23B ,  FIGS. 24A and 24B ,  FIGS. 25A and 25B , and  FIGS. 26A and 26B . The cross-sectional views of  FIGS. 23A and 23B ,  FIGS. 24A and 24B ,  FIGS. 25A and 25B , and  FIGS. 26A and 26B  correspond to the cross section of the portion indicated by the dashed-dotted line X 1 -X 2  in  FIG. 19A . Note that differences from the manufacturing method of the transistor  100  are mainly described to avoid repeated description. Embodiment 1, for example, can be referred to for the portions that are not described in this embodiment. 
     First, the steps up to Step 5 in the manufacturing method of the transistor  100  are performed. 
     [Step 1b] 
     Next, a semiconductor layer  182 _ 1  is formed, and a semiconductor layer  182 _ 2  is formed over the semiconductor layer  182 _ 1  (see  FIG. 23A ). Note that before the semiconductor layer  182 _ 1  is formed, an oxygen gas may be supplied to generate plasma, in which case oxygen can be added to the insulating layer  105 , which is a surface where the semiconductor layer  182 _ 1  is formed. 
     In this embodiment, for the semiconductor layer  182 _ 1 , indium gallium zinc oxide is deposited by a sputtering method using a target with an atomic ratio In:Ga:Zn=4:2:4.1. Oxygen or a mixed gas of oxygen and a rare gas is used as a sputtering gas. In this embodiment, a mixed gas of oxygen and argon in which the flow rate ratio of oxygen is 10% is used as the sputtering gas. 
     Then, for the semiconductor layer  182 _ 2 , indium gallium zinc oxide is deposited by a sputtering method using a target with an atomic ratio In:Ga:Zn=1:1:1.2. Oxygen or a mixed gas of oxygen and a rare gas is used as a sputtering gas. For example, a sputtering gas with an oxygen proportion of 100% is used. The flow rate ratio of oxygen in the sputtering gas for forming the semiconductor layer  182 _ 2  is preferably 70% or more, further preferably 80% or more, still further preferably 100%. By increasing the proportion (flow rate ratio) of oxygen in the sputtering gas, the crystallinity of the oxide semiconductor layer can be increased. 
     For the oxide semiconductor layer for forming the semiconductor layer  106 _ 2 , it is preferable to use an oxide semiconductor layer with high crystallinity. For example, in a subsequent etching step for forming the electrode  112   a  and the electrode  112   b , an exposed portion of the semiconductor layer  106  is etched and the semiconductor layer  106  is damaged in some cases. An oxide semiconductor layer with high crystallinity is not likely to be etched in this etching step. Using an oxide semiconductor layer with high crystallinity as the semiconductor layer  106 _ 2  can reduce damage caused to the oxide semiconductor layer in the etching step; thus, the reliability of the transistor can be improved. 
     By introducing an impurity element after formation of the semiconductor layer  182 _ 2 , the threshold voltage of the transistor  150  can be changed. An impurity element can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment using a gas containing an impurity element, or the like. 
     After the semiconductor layer  182 _ 2  is formed, heat treatment and/or oxygen doping treatment may be performed. Heat treatment and oxygen doping treatment may be repeated a plurality of times. 
     Furthermore, after heat treatment is performed in a nitrogen atmosphere or a rare gas atmosphere, heat treatment may be additionally performed in an oxygen atmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water, and the like can be released from the oxide semiconductor layer and oxygen can be supplied to the oxide semiconductor layer at the same time. Consequently, oxygen vacancies in the oxide semiconductor layer can be reduced. 
     [Step 2b] 
     Then, a resist mask is formed by a photolithography method as in Step 7 above (not illustrated). With the use of the resist mask as a mask, part of the semiconductor layer  182 _ 1  and part of the semiconductor layer  182 _ 2  are selectively removed, so that the island-shaped semiconductor layer  106 _ 1  and the island-shaped semiconductor layer  106 _ 2  are formed (see  FIG. 23B ). 
     [Step 3b] 
     Next, the conductive layer  186  for forming the electrodes  112   a  and  112   b  is formed as in Step 15 above (see  FIG. 24A ). In this embodiment, a stack including tungsten, aluminum, and titanium is used for the conductive layer  186 . Specifically, a 50-nm-thick tungsten layer, a 400-nm-thick aluminum layer, and a 100-nm-thick titanium layer are formed in this order by a sputtering method. 
     [Step 4b] 
     Next, a resist mask is formed by a photolithography method as in Step 16 above (not illustrated). With the use of the resist mask as a mask, part of the conductive layer  186  is selectively removed to form the electrode  112   a  and the electrode  112   b  (see  FIG. 24B ). One of the electrode  112   a  and the electrode  112   b  can function as a source electrode, and the other can function as a drain electrode. At this time, an exposed portion of the semiconductor layer  106 _ 2  is removed in some cases. 
     [Step 5b] 
     Next, the insulating layer  115 , the insulating layer  116 , and the insulating layer  117  are formed in this order (see  FIG. 25A ). 
     In this embodiment, a 30-nm-thick silicon oxynitride layer is formed as the insulating layer  115  under the conditions where a silane gas at a flow rate of 50 sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 20 Pa, the substrate temperature is controlled to 350° C., and a power of 130 W is supplied using a 27.12 MHz high-frequency power source. 
     A 400-nm-thick silicon oxynitride layer is formed as the insulating layer  116  under the conditions where a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 200 Pa, the substrate temperature is controlled to 220° C., and a power of 1500 W is supplied using a 27.12 MHz high-frequency power source. 
     The insulating layer  116  is preferably an insulating layer containing excess oxygen. Further, it is preferable that the amount of defects in the insulating layer  116  be small. Typically, the spin density of a signal which appears at g=2.001 originating from a dangling bond of silicon is preferably lower than 1.5×10 18  spins/cm 3 , further preferably lower than or equal to 1×10 18  spins/cm 3  by ESR measurement. Note that the insulating layer  116  is more distant from the semiconductor layer  106  than the insulating layer  115  and thus may have a higher defect density than the insulating layer  115 . 
     As the insulating layer  116 , a silicon oxide layer or a silicon oxynitride layer may be formed under the conditions where the substrate is held at a temperature ranging from 180° C. to 400° C., the pressure in the reaction chamber into which source gases are introduced ranges from 100 Pa to 250 Pa, preferably from 100 Pa to 200 Pa, and a high-frequency power of 0.17 W/cm 2  to 0.5 W/cm 2 , preferably 0.25 W/cm 2  to 0.35 W/cm 2  is supplied to an electrode provided in the reaction chamber. 
     In formation of the insulating layer  116 , when the high-frequency power with the above power density is supplied in the reaction chamber at the above pressure, decomposition efficiency of the source gases is increased in plasma. That is, oxygen radicals increase in the reaction chamber, and oxidation of the source gases proceeds. Thus, the oxygen content of the insulating layer  116  can be higher than that in the stoichiometric composition. 
     In the insulating layer formed at a substrate temperature within the above range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the insulating layer is released by heat treatment in a later step. Thus, it is possible to form an insulating layer which contains more oxygen than that in the stoichiometric composition and contains excess oxygen. 
     Note that in the formation conditions for the insulating layer  116 , the flow rate of the deposition gas containing silicon relative to the oxidizing gas can be increased, whereby the amount of defects in the insulating layer  116  can be reduced. Typically, it is possible to form an oxide insulating layer in which the amount of defects is small, i.e., the spin density of a signal that appears at g=2.001 originating from a dangling bond of silicon is lower than 6×10 17  spins/cm 3 , preferably lower than or equal to 3×10 17  spins/cm 3 , further preferably lower than or equal to 1.5×10 17  spins/cm 3  by ESR measurement. As a result, the reliability of the transistor can be improved. 
     Note that in the step of forming the insulating layer  116 , the insulating layer  115  functions as a protective layer for the semiconductor layer  106 . Therefore, the insulating layer  116  can be formed using the high-frequency power having a high power density while damage to the semiconductor layer  106  is reduced. 
     A 100-nm-thick silicon nitride layer is formed as the insulating layer  117  under the conditions where a silane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as source gases to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, the substrate temperature is controlled to 350° C., and a power of 1000 W is supplied using a 27.12 MHz high-frequency power source. 
     [Step 6b] 
     Then, the conductive layer  184  for forming the electrode  108  is formed as in Step 9 above (see  FIG. 25B ). In this embodiment, as the conductive layer  184 , a layer of indium tin oxide containing silicon is formed to a thickness of 100 nm by a sputtering method. 
     [Step 7b] 
     Next, a resist mask is formed by a photolithography method as in Step 10 above (not illustrated). With the use of the resist mask as a mask, part of the conductive layer  184  is selectively removed, so that the electrode  108  is formed (see  FIG. 26A ). 
     [Step 8b] 
     Then, the insulating layer  113  having a flat surface is formed (see  FIG. 26B ). In this embodiment, a 1.5-μm-thick acrylic resin layer is formed as the insulating layer  113 . Note that the insulating layer  113  is not necessarily provided depending on the purpose or the intended use. 
     Through the above steps, the transistor  150  can be manufactured. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 4 
     A modification example of the transistor  150  described in Embodiment 3 will be described in this embodiment with reference to drawings. Note that differences from the transistor  150  are mainly described to avoid repeated description. Embodiment 3 can be referred to for the portions that are not described in this embodiment. 
     Modification Example 1 
       FIG. 27A  is a plan view of a transistor  150 A.  FIG. 27B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 27A .  FIG. 28  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 27A . 
     The transistor  150 A, having almost the same structure as the transistor  150 , is different from the transistor  150  in including the structure body  122  between the insulating layer  102  and the substrate  101 . In the transistor  150 A, the semiconductor layer  106 , the electrode  104 , and the electrode  108  each have a region overlapping the structure body  122 . 
     The effect of the use of the structure body  122  and the manufacturing method of the structure body are described in Embodiment 2 and thus will not be described in this embodiment. 
     Modification Example 2 
       FIG. 29A  is a plan view of a transistor  150 B.  FIG. 29B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 29A .  FIG. 30  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 29A . 
     The transistor  150 B has the structure of the transistor  150 A without the electrode  108 . The electrode  108  is not necessarily provided, as determined by required performance, purpose, or the like of the transistor. When the electrode  108  is not provided, the number of steps for manufacturing the transistor is reduced; thus, the manufacturing cost can be reduced. Moreover, the manufacturing yield of the transistor can be increased. 
     Modification Example 3 
       FIG. 31A  is a plan view of a transistor  150 C.  FIG. 31B  is a cross-sectional view (in the channel length direction) of the portion indicated by dashed-dotted line X 1 -X 2  in  FIG. 31A .  FIG. 32  is a cross-sectional view (in the channel width direction) of the portion indicated by dashed-dotted line Y 1 -Y 2  in  FIG. 31A . 
     Providing a plurality of structure bodies  122  forms a depression in the insulating layer  102 . The transistor  150 C has a structure similar to that of the transistor  150 A. The transistor  150 C is different from the transistor  150 A in that the semiconductor layer  106 , the electrode  104 , and the electrode  108  each have a region overlapping the depression of the insulating layer  102 . The transistor  150 C has a region where the channel formation region overlaps the depression of the insulating layer  102 . The transistor  150 C also has a region where the channel formation region does not overlap the projection of the insulating layer  102 . 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 5 
     In this embodiment, a display device and a display module will be described as examples of a semiconductor device including any of the transistors disclosed in this specification and the like. 
     A structure example of a display device  200  is described with reference to drawings. Note that this embodiment mainly describes a light-emitting display device in which a light-emitting element is used as a display element. As the display device  200 , a display device having a top-emission structure is described as an example. Note that the display device  200  can be a display device having a bottom-emission structure or a dual-emission structure. 
     When a liquid crystal element is used as the display element, the display device  200  can be a liquid crystal display device. 
     &lt;Structure Example of Display Device&gt; 
       FIG. 34A  is a perspective view of the display device  200  to which a flexible printed circuit (FPC)  224  serving as an external electrode is connected.  FIG. 34B  is a cross-sectional view of the portion indicated by the dashed-dotted line A 1 -A 2  in  FIG. 34A . 
     The display device  200  described in this embodiment includes a display region  231 , a circuit  232 , and a circuit  233 . The display region  231  includes a plurality of pixels. One pixel includes a transistor  251  and a light-emitting element  225  that includes an electrode  215 , an EL layer  217 , and an electrode  218 . The transistor  251  is electrically connected to the light-emitting element  225 . The transistor  251  has a function of controlling the amount of light emitted from the light-emitting element  225 . The display device  200  includes a terminal electrode  216 . The light-emitting element  225  has a function of emitting white light, for example. 
     The FPC  224  and the terminal electrode  216  are electrically connected to each other through an anisotropic conductive layer  223 . Part of the terminal electrode  216  is electrically connected to the circuit  232  and/or the circuit  233 . Note that the terminal electrode  216  is formed using the same conductive layer as the conductive layer for forming the electrode  112   a  and the electrode  112   b.    
     The circuit  232  and the circuit  233  each include a plurality of transistors  252 . The circuit  232  and the circuit  233  each have a function of determining which of the light-emitting elements  225  in the display region  231  is supplied with a signal through the FPC  224 . 
     Any of the transistors disclosed in the above embodiments can be used as the transistor  251  and the transistor  252 . For example, the transistor  100 , the transistor  150 , or the like can be used. In addition, the insulating layer  113  is formed over the transistor  251  and the transistor  252 , and the electrode  215  is formed over the insulating layer  113 . The electrode  215  is electrically connected to a drain electrode of the transistor  251  through an opening formed in the insulating layer  113 . A partition  214  is formed over the electrode  215 , and the EL layer  217  and the electrode  218  are formed over the electrode  215  and the partition  214 . 
     In the display device  200 , a substrate  211  and a substrate  221  are attached to each other with a bonding layer  220  provided therebetween. 
     One surface of the substrate  211  is adjacent to the insulating layer  102  with a bonding layer  212  positioned therebetween. One surface of the substrate  221  is adjacent to an insulating layer  245  with a bonding layer  244  positioned therebetween. The one surface of the substrate  221  is provided with a light-blocking layer  264  with the insulating layer  245  positioned therebetween. The one surface of the substrate  221  is also provided with a coloring layer  266  and an overcoat layer  268  with the insulating layer  245  positioned therebetween. 
     The coloring layer  266  is provided to overlap the light-emitting element  225 . The coloring layer  266  has a function of transmitting light in a specific wavelength range. The white light emitted by the light-emitting element  225  and passing through the coloring layer  266  is converted into light in a specific wavelength range. When the coloring layer  266  that transmits light in the red wavelength range is used, the white light can be converted into red light. When the coloring layer  266  that transmits light in the green wavelength range is used, the white light can be converted into green light. When the coloring layer  266  that transmits light in the blue wavelength range is used, the white light can be converted into blue light. 
     In this manner, a pixel that emits light in a specific wavelength range can be formed. Full-color display can be achieved by making a pixel that emits red light, a pixel that emits green light, and a pixel that emits blue light collectively function as one pixel and by controlling the amount of light emitted from each pixel. Thus, these three pixels function as sub-pixels. The colors of light emitted by the three sub-pixels are not limited to the combination of red, green, and blue and may be yellow, cyan, and magenta. 
     Four sub-pixels may collectively function as one pixel. For example, a sub-pixel that emits white light may be added to the three sub-pixels that emit red light, green light, and blue light. The addition of the sub-pixel that controls white light can increase the luminance of the display region. When a larger number of sub-pixels emitting light of colors such as red, green, blue, yellow, cyan, and magenta are combined as appropriate and function as one pixel, the range of color reproduction can be increased. 
     The light emitted by the light-emitting element  225  is not limited to white light. For example, the light-emitting element  225  that emits red light, the light-emitting element  225  that emits green light, the light-emitting element  225  that emits blue light, and the like may be used, in which case the coloring layer  266  can be omitted. 
     The coloring layer  266  transmitting light in the red wavelength range may overlap the light-emitting element  225  emitting red light. The coloring layer  266  transmitting light in the green wavelength range may overlap the light-emitting element  225  emitting green light. The coloring layer  266  transmitting light in the blue wavelength range may overlap the light-emitting element  225  emitting blue light. The overlap between the light-emitting element  225  and the coloring layer  266  reduces reflection of external light, which increases the display quality of the display device. 
     Using the pixels arranged in a matrix of 1920×1080, the display device  200  can display an image with full high-definition (Full HD, also referred to as 2K resolution, 2K1K, 2K, or the like) quality. Using the pixels arranged in a matrix of 3840×2160, the display device  200  can display an image with Ultra HD (also referred to as 4K resolution, 4K2K, 4K, or the like) quality. Using the pixels arranged in a matrix of 7680×4320, the display device  200  can display an image with “Super Hi-Vision” (also referred to as 8K resolution, 8K4K, 8K, or the like) quality. Using a larger number of pixels, the display device  200  can display an image with 16K or 32K resolution. 
     A flexible material such as an organic resin can be used for the substrate  211  and the substrate  221 . In the case where the display device  200  has a bottom-emission structure or a dual-emission structure, a material having a light-transmitting property with respect to light emitted from the EL layer  217  is used for the substrate  211 . In the case where the display device  200  has a top-emission structure or a dual-emission structure, a material having a light-transmitting property with respect to light emitted from the EL layer  217  is used for the substrate  221 . 
     The thickness of each of the substrate  211  and the substrate  221  is preferably greater than or equal to 5 μm and less than or equal to 100 μm, further preferably greater than or equal to 10 μm and less than or equal to 50 μm. One or both of the substrate  211  and the substrate  221  may be a stacked-layer substrate that includes a plurality of layers. 
     It is preferable that the substrate  211  and the substrate  221  be formed using the same material and have the same thickness. However, depending on the purpose, the substrates  211  and  221  may be formed using different materials or have different thicknesses. 
     Examples of the flexible and visible-light-transmitting materials that can be used for the substrate  211  and the substrate  221  include a polyethylene terephthalate resin, a polyethylene naphthalate resin, a polyacrylonitrile resin, a polyimide resin, a polymethylmethacrylate resin, a polycarbonate resin, a polyethersulfone resin, a polyamide resin, a cycloolefin resin, a polystyrene resin, a polyamide imide resin, a polyvinylchloride resin, and polytetrafluoroethylene (PTFE). Furthermore, when a light-transmitting property is not necessary, a non-light-transmitting substrate may be used. For example, aluminum or the like may be used for the substrate  221  or the substrate  211 . 
     The thermal expansion coefficients of the substrate  221  and the substrate  211  are preferably less than or equal to 30 ppm/K, more preferably less than or equal to 10 ppm/K. In addition, on surfaces of the substrate  221  and the substrate  211 , a protective film having low water permeability may be formed in advance; examples of the protective film include a film containing nitrogen and silicon such as a silicon nitride film or a silicon oxynitride film and a film containing nitrogen and aluminum such as an aluminum nitride film. Note that a structure in which a fibrous body is impregnated with an organic resin (also called prepreg) may be used as the substrate  221  and the substrate  211 . 
     With such substrates, a non-breakable display device can be provided. Alternatively, a lightweight display device can be provided. Alternatively, an easily bendable display device can be provided. 
     &lt;Example of Pixel Circuit Configuration&gt; 
     Next, an example of a specific configuration of the display device  200  is described with reference to  FIGS. 35A to 35C .  FIG. 35A  is a block diagram illustrating the configuration of the display device  200 . The display device  200  includes the display region  231 , the circuit  232 , and the circuit  233 . The circuit  232  functions as a scan line driver circuit, for example, and the circuit  233  functions as a signal line driver circuit, for example. 
     The display device  200  includes m scan lines  235  which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the circuit  232 , and n signal lines  236  which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the circuit  233 . The display region  231  includes a plurality of pixels  230  arranged in a matrix of m rows by n columns. Note that m and n are each an integer of 1 or more. 
     Each of the scan lines  235  is electrically connected to the n pixels  230  in the corresponding row among the pixels  230  arranged in the display region  231 . Each of the signal lines  236  is electrically connected to the m pixels  230  in the corresponding column among the pixels  230 . 
     In some cases, the circuits  232  and  233  may be collectively called a driver circuit portion. The pixel  230  includes a pixel circuit  237  and the light-emitting element  225 . The pixel circuit  237  is a circuit that drives the light-emitting element  225 . A transistor included in the driver circuit portion and a transistor included in the pixel circuit  237  can be formed at the same time. That is, with the use of the transistor described in this specification and the like, part of the driver circuit portion or the entire driver circuit portion can be formed over a substrate where a pixel portion is formed, whereby a system-on-panel can be obtained. 
     Part of the driver circuit portion or the entire driver circuit portion may be formed over another substrate and electrically connected to the display device  200 . For example, part of the driver circuit portion or the entire driver circuit portion may be formed using a single crystal substrate and electrically connected to the display device  200 . 
       FIGS. 35B and 35C  illustrate circuit configurations that can be used for the pixels  230  in the display device illustrated in  FIG. 35A . 
     [Example of Pixel Circuit for Light-Emitting Display Device] 
     The pixel circuit  237  illustrated in  FIG. 35B  includes a transistor  431 , a capacitor  438 , a transistor  433 , and a transistor  434 . The pixel circuit  237  is electrically connected to the light-emitting element  225  that can function as a display element. 
     One of a source and a drain of the transistor  431  is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a signal line DL_n). A gate of the transistor  431  is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m). 
     The transistor  431  has a function of controlling whether to write a data signal to a node  435 . 
     One of a pair of electrodes of the capacitor  438  is electrically connected to the node  435 , and the other is electrically connected to a node  437 . The other of the source and the drain of the transistor  431  is electrically connected to the node  435 . 
     The capacitor  438  functions as a storage capacitor for storing data written to the node  435 . 
     One of a source and a drain of the transistor  433  is electrically connected to a potential supply line VL_a, and the other is electrically connected to the node  437 . A gate of the transistor  433  is electrically connected to the node  435 . 
     One of a source and a drain of the transistor  434  is electrically connected to a potential supply line V 0 , and the other of the source electrode and the drain electrode of the transistor  434  is electrically connected to the node  437 . A gate of the transistor  434  is electrically connected to the scan line GL_m. 
     One of an anode and a cathode of the light-emitting element  225  is electrically connected to a potential supply line VL_b, and the other is electrically connected to the node  437 . 
     As the light-emitting element  225 , an organic electroluminescent element (also referred to as an organic EL element) or the like can be used, for example. Note that the light-emitting element  225  is not limited to organic EL elements; an inorganic EL element including an inorganic material can be used, for example. 
     As a power supply potential, a potential on the relatively high potential side or a potential on the relatively low potential side can be used, for example. A power supply potential on the high potential side is referred to as a high power supply potential (also referred to as VDD), and a power supply potential on the low potential side is referred to as a low power supply potential (also referred to as VSS). A ground potential can be used as the high power supply potential or the low power supply potential. For example, in the case where a ground potential is used as the high power supply potential, the low power supply potential is a potential lower than the ground potential, and in the case where a ground potential is used as the low power supply potential, the high power supply potential is a potential higher than the ground potential. 
     A high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other, for example. 
     In the display device including the pixel circuit  237  in  FIG. 35B , the pixel circuits  237  are sequentially selected row by row by the circuit  232 , whereby the transistors  431  and the transistors  434  are turned on and a data signal is written to the nodes  435 . 
     When the transistors  431  and the transistors  434  are turned off, the pixel circuits  237  in which the data signal has been written to the nodes  435  are brought into a holding state. Furthermore, the amount of current flowing between the source and the drain of the transistor  433  is controlled in accordance with the potential of the data signal written to the node  435 . The light-emitting element  225  emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed. 
     [Example of Pixel Circuit for Liquid Crystal Display Device] 
     The pixel circuit  237  illustrated in  FIG. 35C  includes the transistor  431  and the capacitor  438 . The pixel circuit  237  is electrically connected to a liquid crystal element  432  that can function as a display element. 
     The potential of one of a pair of electrodes of the liquid crystal element  432  is set according to the specifications of the pixel circuits  237  as appropriate. The alignment state of the liquid crystal element  432  depends on data written to a node  436 . A common potential may be applied to one of the pair of electrodes of the liquid crystal element  432  included in each of the plurality of pixel circuits  237 . The potential supplied to one of the pair of electrodes of the liquid crystal element  432  in the pixel circuit  237  in one row may be different from the potential supplied to one of the pair of electrodes of the liquid crystal element  432  in the pixel circuit  237  in another row. 
     Examples of a method of driving the display device including the liquid crystal element  432  include a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, and a transverse bend alignment (TBA) mode. Other examples of the method of driving the display device include an electrically controlled birefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Note that one embodiment of the present invention is not limited thereto, and various liquid crystal elements and driving methods can be used. 
     The liquid crystal element  432  may be formed using a liquid crystal composition including liquid crystal exhibiting a blue phase and a chiral material. The liquid crystal exhibiting a blue phase has a short response time of 1 msec or less. Since the liquid crystal exhibiting a blue phase is optically isotropic, alignment treatment is not necessary and viewing angle dependence is small. 
     In the pixel circuit  237  in the m-th row and the n-th column, one of the source and the drain of the transistor  431  is electrically connected to the signal line DL_n, and the other is electrically connected to the node  436 . The gate of the transistor  431  is electrically connected to the scan line GL_m. The transistor  431  has a function of controlling whether to write a data signal to the node  436 . 
     One of the pair of electrodes of the capacitor  438  is electrically connected to a wiring to which a particular potential is supplied (a capacitor line CL), and the other is electrically connected to the node  436 . The other of the pair of electrodes of the liquid crystal element  432  is electrically connected to the node  436 . The potential of the capacitor line CL is set in accordance with the specifications of the pixel circuit  237  as appropriate. The capacitor  438  functions as a storage capacitor for storing data written to the node  436 . 
     For example, in the display device including the pixel circuit  237  in  FIG. 35C , the pixel circuits  237  are sequentially selected row by row by the circuit  232 , whereby the transistors  431  are turned on and a data signal is written to the nodes  436 . 
     When the transistors  431  are turned off, the pixel circuits  237  in which the data signal has been written to the nodes  436  are brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed on the display region  231 . 
     [Display Element] 
     The display device of one embodiment of the present invention can employ various modes and can include various display elements. Examples of the display elements include an electroluminescent (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a display element using micro electro mechanical systems (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, and a display element including a carbon nanotube, which are display media whose contrast, luminance, reflectance, transmittance, or the like is changed by electrical or magnetic action. Alternatively, quantum dots may be used as the display element. Note that examples of display devices having EL elements include an EL display. Examples of display devices including an electron emitter include a field emission display (FED), an SED-type flat panel display (SED: surface-conduction electron-emitter display), and the like. Examples of display devices including quantum dots include a quantum dot display. Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). Examples of a display device including electronic ink, Electronic Liquid Powder (registered trademark), or an electrophoretic element include electronic paper. The display device may be a plasma display panel (PDP). The display device may be a retina scanning type projection device. 
     In the case of a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption. 
     Note that in the case of using an LED, graphene or graphite may be provided under an electrode or a nitride semiconductor of the LED. Graphene or graphite may be a multilayer film in which a plurality of layers are stacked. As described above, provision of graphene or graphite enables easy formation of a nitride semiconductor film thereover, such as an n-type GaN semiconductor layer including crystals. Furthermore, a p-type GaN semiconductor layer including crystals or the like can be provided thereover, and thus the LED can be formed. Note that an AlN layer may be provided between the n-type GaN semiconductor layer including crystals and graphene or graphite. The GaN semiconductor layers included in the LED may be formed by metal organic chemical vapor deposition (MOCVD). Note that when the graphene is provided, the GaN semiconductor layers included in the LED can also be formed by a sputtering method. 
     &lt;Example of Method for Manufacturing Display Device&gt; 
     [Formation of Element Substrate] 
     First, a separation layer  242  is formed over the substrate  101 , and the insulating layer  102  is formed over the separation layer  242  (see  FIG. 36A ). The subsequent steps are performed as in the manufacturing method described in Embodiment 1, whereby the components up to the insulating layer  113  are formed. In this embodiment, a layer that releases hydrogen by absorbing light and generating heat is used as the separation layer  242 . An example of such a layer is a hydrogenated amorphous silicon (a-Si:H) layer. A hydrogenated amorphous silicon layer can be formed by a PECVD method using a deposition gas containing silane (SiH 4 ), for example. Alternatively, the separation layer  242  may be a silicon layer having crystallinity. In order that the separation layer  242  can contain much hydrogen, heat treatment may be performed in an atmosphere containing hydrogen after formation of the separation layer  242 . 
     The thickness of the separation layer  242  is preferably greater than or equal to 1 nm and less than or equal to 200 nm, further preferably greater than or equal to 5 nm and less than or equal to 100 nm, for example. 
     After the components up to the insulating layer  113  are formed, openings  129  are formed in regions that overlap the electrode  112   b . When the insulating layer  113  is formed using a photosensitive material, the opening  129  can be formed without a resist mask. A surface of the electrode  112   b  is partly exposed at the bottom portion of the opening  129 . 
     Next, the electrode  215  is formed over the insulating layer  113  (see  FIG. 36B ). The electrode  215  is preferably formed using a conductive material that efficiently reflects light emitted from the EL layer  217  formed later. Note that the electrode  215  may have a stacked-layer structure of a plurality of layers without limitation to a single-layer structure. For example, in the case where the electrode  215  is used as an anode, a layer in contact with the EL layer  217  may be a light-transmitting layer, such as an indium tin oxide layer, having a work function higher than that of the EL layer  217 , and a layer having high reflectance (e.g., aluminum, an alloy containing aluminum, or silver) may be provided in contact with the layer. 
     The electrode  215  can be formed in such a manner that a conductive layer to be the electrode  215  is formed over the insulating layer  113 , a resist mask is formed over the conductive layer, and a region of the conductive layer that is not covered with the resist mask is etched. The conductive layer can be etched by a dry etching method, a wet etching method, or both a dry etching method and a wet etching method. The resist mask can be formed by a photolithography method, a printing method, an inkjet method, or the like as appropriate. Formation of the resist mask by an inkjet method needs no photomask; thus, the manufacturing cost can be reduced. The resist mask is removed after the formation of the electrode  215 . 
     Next, the partition  214  is formed (see  FIG. 36C ). The partition  214  is provided in order to prevent an unintended electrical short-circuit between the light-emitting elements  225  in adjacent pixels and unintended light emission from the light-emitting elements  225 . In the case of using a metal mask for formation of the EL layer  217  described later, the partition  214  has a function of preventing the contact of the metal mask with the electrode  215 . The partition  214  can be formed of an organic resin such as an epoxy resin, an acrylic resin, or an imide resin or an inorganic material such as silicon oxide. The partition  214  is preferably formed so that its sidewall has a tapered shape or a tilted surface with a continuous curvature. The sidewall of the partition  214  having the above-described shape enables favorable coverage with the EL layer  217  and the electrode  218  formed later. 
     Then, the EL layer  217  is formed. A structure of the EL layer  217  is described in Embodiment 8. 
     The electrode  218  is used as a cathode in this embodiment, and thus is preferably formed using a material that has a low work function and can inject electrons into the EL layer  217 . As well as a single-layer of a metal having a low work function, a stack in which a metal material such as aluminum, a conductive oxide material such as indium tin oxide, or a semiconductor material is formed over a several-nanometer-thick buffer layer formed of an alkali metal or an alkaline earth metal having a low work function may be used as the electrode  218 . As the buffer layer, an oxide of an alkaline earth metal, a halide, a magnesium-silver alloy, or the like can also be used. 
     In the case where light emitted from the EL layer  217  is extracted through the electrode  218 , the electrode  218  preferably has a property of transmitting visible light. The light-emitting element  225  includes the electrode  215 , the EL layer  217 , and the electrode  218  (see  FIG. 36D ). 
     In this embodiment, the substrate  101  over which the transistor and the light-emitting element  225  are provided is referred to as an element substrate  271 . 
     [Formation of Counter Substrate] 
     A separation layer  243  and the insulating layer  245  are formed over a substrate  241  (see  FIG. 37A ). The substrate  241  can be formed using a material similar to that of the substrate  101 . The separation layer  243  can be formed using a material and a method similar to those of the separation layer  242 . The insulating layer  245  can be formed using a material and a method similar to those of the insulating layer  102 . 
     Next, the light-blocking layer  264  is formed over the insulating layer  245  (see  FIG. 37B ). After that, the coloring layer  266  is formed (see  FIG. 37C ). 
     The light-blocking layer  264  and the coloring layer  266  each are formed in a desired position with any of various materials by a printing method, an inkjet method, a photolithography method, or the like. 
     Next, the overcoat layer  268  is formed over the light-blocking layer  264  and the coloring layer  266  (see  FIG. 37D ). 
     For the overcoat layer  268 , an organic insulating layer of an acrylic resin, an epoxy resin, polyimide, or the like can be used. With the overcoat layer  268 , for example, an impurity or the like contained in the coloring layer  266  can be prevented from diffusing to the light-emitting element  225  side. Note that the overcoat layer  268  is not necessarily formed. 
     A light-transmitting conductive layer may be formed as the overcoat layer  268 . The light-transmitting conductive layer is formed as the overcoat layer  268 , so that the light emitted from the light-emitting element  225  can be transmitted through the overcoat layer  268 , while ionized impurities can be prevented from passing through the overcoat layer  268 . 
     The light-transmitting conductive layer can be formed using, for example, indium oxide, indium tin oxide (ITO), indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added. Graphene or a metal layer that is thin enough to have a light-transmitting property can also be used. 
     Through the above-described steps, the components such as the coloring layer  266  can be formed over the substrate  241 . In this embodiment, the substrate  241  over which the coloring layer  266  and the like are provided is referred to as a counter substrate  281 . 
     [Attachment of Element Substrate to Counter Substrate] 
     Next, the element substrate  271  is attached to the counter substrate  281  with the bonding layer  220  positioned therebetween such that the light-emitting element  225  included in the element substrate  271  faces the coloring layer  266  included in the counter substrate  281  (see  FIG. 38 ). 
     A photo-curable adhesive, a reactive curable adhesive, a thermosetting adhesive, or an anaerobic adhesive can be used as the bonding layer  220 . For example, an epoxy resin, an acrylic resin, or an imide resin can be used. In a top-emission structure, a drying agent (e.g., zeolite) having a size less than or equal to the wavelength of light or a filler (e.g., titanium oxide or zirconium) with a high refractive index is preferably mixed into the bonding layer  220 , in which case the efficiency of extracting light emitted from the EL layer  217  can be improved. 
     Next, the separation layer  243  is irradiated with laser light  272  through the substrate  241  (see  FIG. 39A ). The region of the separation layer  243  that is subject to separation may be entirely irradiated with the laser light  272  at one time. Alternatively, the region that is subject to separation may be divided into a plurality of regions and these regions may be irradiated with the laser light  272  one by one. The laser light  272  may be linear laser light. 
     The region that is subject to separation may be irradiated with the laser light  272  while the laser light  272  and/or the substrate  241  are/is relatively moved. In the case where the laser light  272  is linear laser light, the laser light  272  and/or the substrate  241  are/is moved relatively along the short-axis direction of the laser light  272 . 
     The irradiation with the laser light  272  heats the separation layer  243  and releases hydrogen from the separation layer  243 . At this time, the hydrogen is released in a gaseous state. The released gas remains in the vicinity of the interface between the separation layer  243  and the insulating layer  245  or in the vicinity of the interface between the separation layer  243  and the substrate  241 . This reduces adhesion, particularly adhesion between the separation layer  243  and the insulating layer  245 , which allows the substrate  241  and the separation layer  243  to be easily separated from the insulating layer  245 . 
     Part of the hydrogen contained in the separation layer  243  sometimes remains in the separation layer  243 . Thus, the separation layer  243  is embrittled and separation easily occurs in the separation layer  243  in some cases. 
     The laser light  272  is preferably light at least part of which passes through the substrate  241  and which has a wavelength such that the light is absorbed by the separation layer  243 . The laser light  272  is preferably light having a wavelength such that the light is absorbed by the insulating layer  245 . 
     Note that the insulating layer  245  sometimes absorbs part of the laser light  272 . Thus, an element such as a transistor is irradiated with the laser light  272  which is transmitted through the separation layer  243  and an adverse effect on the characteristics of the element can be suppressed. 
     The laser light  272  is preferably light in a wavelength range from visible light to ultraviolet light. For example, light having a wavelength of greater than or equal to 200 nm and less than or equal to 400 nm, preferably greater than or equal to 250 nm and less than or equal to 350 nm can be used. In particular, an excimer laser having a wavelength of 308 nm is preferably used because the productivity is increased. The excimer laser is preferable because the excimer laser can be used also for laser crystallization of low-temperature polysilicon (LTPS), so that the existing LTPS production line device can be used and new capital investment is not necessary. Alternatively, a solid-state UV laser (also referred to as a semiconductor UV laser), such as a UV laser having a wavelength of 355 nm which is the third harmonic of an Nd:YAG laser, may be used. A solid-state laser is preferable because it does not use a gas and thus the running cost can be reduced to approximately ⅓ of the case of using an excimer laser. Further alternatively, a pulsed laser such as a picosecond laser may be used. 
     Next, the substrate  241  and the separation layer  243  are separated from the insulating layer  245  (see  FIG. 39B ). For example, the separation of the substrate  241  and the separation layer  243  from the insulating layer  245  can be performed by applying tensile stress to the substrate  241  perpendicularly. Specifically, the substrate  241  and the separation layer  243  can be peeled by pulling up the substrate  241  by part of its suction-attached top surface. At this time, the separation trigger is preferably formed by inserting a sharp instrument such as a knife between the substrate  241  and the insulating layer  245 . 
     Then, the substrate  221  is attached to the insulating layer  245  with the adhesive layer  244  positioned therebetween (see  FIG. 40A ). Any of a variety of curable adhesives, e.g., photo-curable adhesives such as a UV curable adhesive, a reactive curable adhesive, a thermosetting adhesive, and an anaerobic adhesive, can be used for the adhesive layer  244 . Still alternatively, an adhesive sheet or the like may be used. 
     Next, the separation layer  242  is irradiated with the laser light  272  through the substrate  101  (see  FIG. 40B ). The region of the separation layer  242  that is subject to separation may be entirely irradiated with the laser light  272  at one time. Alternatively, the region that is subject to separation may be divided into a plurality of regions and these regions may be irradiated with the laser light  272  one by one. The laser light  272  may be linear laser light. 
     Next, the substrate  101  and the separation layer  242  are separated from the insulating layer  245  (see  FIG. 41A ). The separation of the substrate  101  and the separation layer  242  from the insulating layer  245  can be performed in a manner similar to that of the above-described separation of the substrate  241  and the separation layer  243  from the insulating layer  245 . 
     Next, the substrate  211  is attached to the insulating layer  245  with the bonding layer  212  positioned therebetween (see  FIG. 41B ). For the adhesive layer  212 , a material similar to that of the adhesive layer  244  can be used. For the substrate  211 , a material similar to that of the substrate  221  can be used. 
     After that, the insulating layer  113 , the adhesive layer  220 , the overcoat layer  268 , the coloring layer  266 , the insulating layer  245 , the adhesive layer  244 , and the substrate  221  are partly removed to expose the terminal electrode  216  (see  FIG. 42A ). 
     Then, the FPC  224  is electrically connected to the terminal electrode  216  through the anisotropic conductive layer  223  (see  FIG. 42B ). In the above-described manner, the display device  200  to which the FPC  224  is connected can be fabricated. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 6 
     This embodiment describes another example of a display device. In this embodiment, a description is made of portions different from the above embodiments to avoid repetition of the same description. Other embodiments and the like can be referred to for the portions that are not described in this embodiment. 
       FIGS. 43A to 43C  are plan views of display devices in this embodiment. In  FIG. 43A , a sealant  4005  is provided to surround the display region  231 . A region of the substrate  211  and a region of the substrate  221  overlap each other with the sealant  4005  positioned therebetween. A region of the substrate  211  and a region of the substrate  221  overlap each other with the display region  231  positioned therebetween. 
     Over the substrate  211  of the display device illustrated in  FIG. 43A , the circuits  232  and  233  formed using a single crystal semiconductor or a polycrystalline semiconductor over separately prepared substrates are mounted in regions other than the region surrounded by the sealant  4005 . The circuit  232  serves as a signal line driver circuit, for example. The circuit  233  serves as a scan line driver circuit, for example. 
     Signals and potentials are supplied from an FPC  224   a  and an FPC  224   b  to the circuit  232 , the circuit  233 , or the display region  231 . 
     In  FIGS. 43B and 43C , the sealant  4005  is provided to surround the display region  231  and the circuit  233  that are provided over the substrate  211 . A region of the substrate  211  and a region of the substrate  221  overlap each other with the sealant  4005  positioned therebetween. A region of the substrate  211  and a region of the substrate  221  overlap each other with the display region  231  positioned therebetween. A region of the substrate  211  and a region of the substrate  221  overlap each other with the circuit  233  positioned therebetween. 
     The display region  231  and the circuit  233  are sealed together with a display element by the substrate  211 , the sealant  4005 , and the substrate  221 . Over the substrate  211  in  FIGS. 43B and 43C , the circuit  232  that is formed using a single crystal semiconductor or a polycrystalline semiconductor over a separately prepared substrate is mounted in a region other than the region surrounded by the sealant  4005 . 
     In  FIG. 43A , the circuit  232  is mounted over the substrate  211  in a region between the display region  231  and the FPC  224   a . The circuit  233  is mounted over the substrate  211  in a region between the display region  231  and the FPC  224   b . In  FIG. 43B , the circuit  232  is mounted over the substrate  211  in a region between the display region  231  and the FPC  224 . In  FIG. 43C , the circuit  232  is mounted over the FPC  224 . 
     The circuit  232  is not necessarily formed separately and mounted over the substrate  211  or the FPC  224  as in the examples illustrated in  FIGS. 43B and 43C . For example, part of a circuit included in the circuit  232  may be provided over the substrate  211  in a region surrounded by the sealant  4005 . Alternatively, part of a circuit included in the circuit  232  may be mounted over the FPC  224 . 
     The connection method of a separately formed driver circuit is not particularly limited; wire bonding, a chip on glass (COG), a tape carrier package (TCP), a chip on film (COF), or the like can be used.  FIG. 43A  illustrates an example in which the circuit  232  and the circuit  233  are mounted by a COG.  FIG. 43B  illustrates an example in which the circuit  232  is mounted by a COG.  FIG. 43C  illustrates an example in which the circuit  232  is mounted by a TCP. 
     In some cases, the display device encompasses a panel in which a display element is sealed, and a module in which an IC or the like including a controller is mounted on the panel. 
       FIG. 44  shows an example of a cross-sectional structure of a display device including an EL element and a liquid crystal element in one pixel. Note that description is made mainly on portions different from the display device in  FIGS. 34A and 34B  to avoid repeated description.  FIG. 44  corresponds to a cross-sectional view taken along dashed-dotted line N 1 -N 2  in  FIG. 43B . As shown in  FIG. 44 , the display device has an electrode  4015 , and the electrode  4015  is electrically connected to a terminal included in the FPC  224  through the anisotropic conductive layer  223 . The electrode  4015  is electrically connected to a wiring  4014  in an opening that is provided in the insulating layer  4101 , the insulating layer  4102 , the insulating layer  102 , and the insulating layer  103 . 
     The electrode  4015  is formed using the same conductive layer as the electrode  4131 . The wiring  4014  is formed using the same conductive layer as the conductive layer for forming the electrodes  104  of a transistor  251   a , a transistor  251   b , and the transistor  252 . 
       FIG. 44  illustrates the transistors  251   a  and  251   b  that are included in the display region  231  and the transistor  252  that is included in the circuit  233 . The display device illustrated in  FIG. 44  includes the transistor  251   a , the transistor  251   b , the transistor  252 , a capacitor  4020   a , and a capacitor  4020   b  over the insulating layer  102 . The transistors  251   a  and  251   b  have the same structure as the transistor  251 . The capacitor  4020   a  has a region where the electrode  4021  and part of the electrode  112   a  or the electrode  112   b  of the transistor  251   a  overlap each other with an insulating layer positioned therebetween. The electrode  4021  is formed using the same conductive layer as the electrode  104  of the transistor  251   a . The capacitor  4020   b  has the same structure as the capacitor  4020   a . The transistor  251   a  has a function of driving the light-emitting element  225  and the transistor  251   b  has a function of driving a liquid crystal element  4013 . 
     The display device illustrated in  FIG. 44  includes the insulating layer  4102  below the insulating layer  102 . The display device includes, below the insulating layer  4102 , a reflective electrode  4130 , the insulating layer  4101 , the electrode  4131 , an alignment film  4032 , a liquid crystal layer  4008 , an alignment film  4033 , a spacer  4035 , an electrode  4031 , the overcoat layer  268 , the coloring layer  266 , the substrate  211 , and a polarizing plate  4134 . 
     The liquid crystal element  4013  includes the electrode  4131 , the electrode  4031 , and the liquid crystal layer  4008 . In the display device illustrated in  FIG. 44 , the alignment films  4032  and  4033  are provided so that the liquid crystal layer  4008  is positioned therebetween. A region of the electrode  4131  and a region of the electrode  4031  overlap each other with the liquid crystal layer  4008  positioned therebetween. The electrode  4131  has a region overlapped by the reflective electrode  4130 . In addition, the electrode  4131  is electrically connected to one of a source and a drain of the transistor  251   b  through the reflective electrode  4130 . 
     The spacer  4035  is a columnar spacer obtained by selective etching of an insulating layer and is provided in order to control the distance between the electrode  4131  and the electrode  4031  (a cell gap). Alternatively, a spherical spacer may be used as the spacer  4035 . 
     The display device illustrated in  FIG. 44  has functions as a light-emitting display device having a bottom emission structure and as a reflective liquid crystal display device. In the display device illustrated in  FIG. 44 , the coloring layer  266 , the light-blocking layer  264 , and the overcoat layer  268  are provided on the substrate  211  side. 
     Light  4520  emitted from the light-emitting element  225  is extracted through the substrate  211 . Light  4521  entering through the substrate  211  is reflected by the reflective electrode  4130  and extracted through the substrate  211 . When the light  4521  is transmitted through the coloring layer  266 , light in a specific wavelength range is absorbed, so that the light  4521  becomes light  4522  in a wavelength range different from that of the light  4521 . However, if the wavelength range of the entering light  4521  is within the wavelength range of light transmitted through the coloring layer  266 , the wavelength range of the light  4522  is almost the same as that of the light  4521 . 
     The insulating layers  4101  and  4102  can each be formed using a material and a method that are similar to those of the insulating layer  103  or the like. The reflective electrode  4130 , the electrode  4131 , and the electrode  4031  can each be formed using a material and a method that are similar to those of the electrode  104  or the like. In the display device illustrated in  FIG. 44 , the reflective electrode  4130  is formed using a conductive material having high light reflectivity and the electrodes  4131  and  4031  are formed using a conductive material having a light-transmitting property. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 7 
     In this embodiment, a display module is described as an example of a semiconductor device using the above-described transistor. In a display module  6000  illustrated in  FIG. 45 , a touch sensor  6004  connected to an FPC  6003 , a display panel  6006  connected to an FPC  6005 , a backlight unit  6007 , a frame  6009 , a printed circuit board  6010 , and a battery  6011  are provided between an upper cover  6001  and a lower cover  6002 . Note that the backlight unit  6007 , the battery  6011 , the touch sensor  6004 , and the like are not provided in some cases. 
     A semiconductor device of one embodiment of the present invention can be used, for example, for the touch sensor  6004 , the display panel  6006 , and an integrated circuit mounted on the printed circuit board  6010 . For example, the above-described display device can be used for the display panel  6006 . 
     The shapes and sizes of the upper cover  6001  and the lower cover  6002  can be changed as appropriate in accordance with the sizes of the touch sensor  6004 , the display panel  6006 , and the like. 
     The touch sensor  6004  can be a resistive touch panel or a capacitive touch panel and can overlap the display panel  6006 . A touch sensor function can be added to the display panel  6006 . For example, an electrode for a touch sensor can be provided in each pixel of the display panel  6006  so that a capacitive touch sensor function is added. Alternatively, a photosensor can be provided in each pixel of the display panel  6006  so that an optical touch sensor function is added. In the case where the touch sensor  6004  is not necessarily provided, the touch sensor  6004  can be omitted. 
     The backlight unit  6007  includes a light source  6008 . The light source  6008  may be provided at an end portion of the backlight unit  6007 , and a light diffusing plate may be used. When a light-emitting display device or the like is used for the display panel  6006 , the backlight unit  6007  can be omitted. 
     The frame  6009  protects the display panel  6006  and functions as an electromagnetic shield for blocking electromagnetic waves generated from the printed circuit board  6010  side. The frame  6009  may also function as a radiator plate. 
     The printed circuit board  6010  includes a power supply circuit, a signal processing circuit for outputting a video signal and a clock signal, and the like. As a power source for supplying power to the power supply circuit, the battery  6011  or a commercial power source may be used. Note that the battery  6011  can be omitted when a commercial power source is used as the power source. 
     The display module  6000  can be additionally provided with a polarizing plate, a retardation plate, a prism sheet, or the like. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments and the like. 
     Embodiment 8 
     In this embodiment, structure examples of a light-emitting element that can be used as the light-emitting element  225  will be described. Note that an EL layer  320  described in this embodiment corresponds to the EL layer  217  described in the above embodiment. 
     &lt;Structure of Light-Emitting Element&gt; 
     In a light-emitting element  330  illustrated in  FIG. 46A , the EL layer  320  is sandwiched between a pair of electrodes (electrodes  318  and  322 ). Note that the electrode  318  is used as an anode and the electrode  322  is used as a cathode as an example in the following description of this embodiment. 
     The EL layer  320  includes at least a light-emitting layer and may have a stacked-layer structure including a functional layer other than the light-emitting layer. As the functional layer other than the light-emitting layer, a layer containing a substance having a high hole-injection property, a substance having a high hole-transport property, a substance having a high electron-transport property, a substance having a high electron-injection property, a bipolar substance (a substance having high electron- and hole-transport properties), or the like can be used. Specifically, functional layers such as a hole-injection layer, a hole-transport layer, an electron-transport layer, and an electron-injection layer can be used in appropriate combination. 
     The light-emitting element  330  illustrated in  FIG. 46A  emits light when current flows by applying a potential difference between the electrode  318  and the electrode  322  and holes and electrons are recombined in the EL layer  320 . In other words, a light-emitting region is formed in the EL layer  320 . 
     In one embodiment of the present invention, light emitted from the light-emitting element  330  is extracted to the outside from the electrode  318  side or the electrode  322  side. Thus, one of the electrodes  318  and  322  is formed using a light-transmitting substance. 
     Note that a plurality of EL layers  320  may be stacked between the electrode  318  and the electrode  322  as in a light-emitting element  331  illustrated in  FIG. 46B . In the case where n (n is a natural number of 2 or more) layers are stacked, an electric charge generation layer  320   a  is preferably provided between an m-th EL layer  320  and an (m+1)th EL layer  320 . Note that m is a natural number greater than or equal to 1 and less than n. 
     The electric charge generation layer  320   a  can be formed using a composite material of an organic compound and a metal oxide, a metal oxide, or a composite material of an organic compound and an alkali metal, an alkaline earth metal, or a compound thereof; alternatively, these materials can be combined as appropriate. Examples of the composite material of an organic compound and a metal oxide include composite materials of an organic compound and a metal oxide such as vanadium oxide, molybdenum oxide, and tungsten oxide. As the organic compound, a variety of compounds can be used; for example, low molecular compounds such as an aromatic amine compound, a carbazole derivative, and an aromatic hydrocarbon and oligomers, dendrimers, and polymers of these low molecular compounds. Note that as the organic compound, it is preferable to use an organic compound that has a hole-transport property and has a hole mobility of 10 −6  cm 2 /Vs or higher. However, other substances may be used as long as their hole-transport properties are higher than their electron-transport properties. These materials used for the electric charge generation layer  320   a  have excellent carrier-injection properties and carrier-transport properties; thus, the light-emitting element  331  can be driven with low current and with low voltage. 
     Note that the electric charge generation layer  320   a  may be formed by a combination of a composite material of an organic compound and a metal oxide with another material. For example, the electric charge generation layer  320   a  may be formed by a combination of a layer containing the composite material of an organic compound and a metal oxide with a layer containing one compound selected from electron-donating substances and a compound having a high electron-transport property. Furthermore, the electric charge generation layer  320   a  may be formed by a combination of a layer containing the composite material of an organic compound and a metal oxide with a transparent conductive film. 
     The light-emitting element  331  having such a structure is unlikely to suffer the problem of energy transfer, quenching, or the like and has an expanded choice of materials, and thus can easily have both high emission efficiency and a long lifetime. Moreover, it is easy to obtain phosphorescence from one light-emitting layer and fluorescence from the other light-emitting layer. 
     The electric charge generation layer  320   a  has a function of injecting holes to one of the EL layers  320  that is in contact with the electric charge generation layer  320   a  and a function of injecting electrons to the other EL layer  320  that is in contact with the electric charge generation layer  320   a , when voltage is applied to the electrodes  318  and  322 . 
     The light-emitting element  331  illustrated in  FIG. 46B  can provide a variety of emission colors by changing the type of the light-emitting substance used for the EL layers  320 . In addition, a plurality of light-emitting substances having different emission colors may be used as the light-emitting substances, so that light emission having a broad spectrum or white light emission can be obtained. 
     In the case of obtaining white light emission using the light-emitting element  331  in  FIG. 46B , as for a combination of a plurality of EL layers, a structure for emitting white light including red light, blue light, and green light may be used. For example, the structure may include a light-emitting layer containing a blue fluorescent substance as a light-emitting substance and a light-emitting layer containing green and red phosphorescent substances as light-emitting substances. Alternatively, the structure may include a light-emitting layer emitting red light, a light-emitting layer emitting green light, and a light-emitting layer emitting blue light. Further alternatively, with a structure including light-emitting layers emitting light of complementary colors, white light emission can be obtained. In a stacked-layer element including two light-emitting layers in which light emitted from one of the light-emitting layers and light emitted from the other light-emitting layer have complementary colors to each other, the combinations of colors are as follows: blue and yellow, blue-green and red, and the like. 
     Note that in the structure of the above-described stacked-layer element, by providing the electric charge generation layer between the stacked light-emitting layers, the element can have a long lifetime in a high-luminance region while keeping the current density low. In addition, the voltage drop due to the resistance of the electrode material can be reduced, whereby uniform light emission in a large area is possible. 
     When the light-emitting element  330  or the light-emitting element  331  has a micro optical resonator (also referred to as microcavity) structure which allows light emitted from the EL layer  320  to resonate, lights with different wavelengths and narrowed spectra can be extracted even when one EL layer  320  is used for different light-emitting elements  330  or different light-emitting elements  331 . The light-emitting element  330  and the light-emitting element  331  described in this embodiment can be used as the light-emitting element  225 . 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 9 
     A transistor and/or a semiconductor device of one embodiment of the present invention can be used in a variety of electronic devices.  FIGS. 47A to 47G  and  FIGS. 48A and 48B  illustrate examples of electronic devices including the transistor and/or the semiconductor device of one embodiment of the present invention. 
     Examples of electronic devices including the semiconductor device of one embodiment of the present invention are display devices of televisions, monitors, and the like; lighting devices; desktop personal computers and laptop personal computers; word processors; image reproduction devices that reproduce still images and moving images stored in recording media such as digital versatile discs (DVD); 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 appliances; 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 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 (EV), hybrid electric vehicles (HEV) that include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats, ships, submarines, helicopters, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecrafts. 
     Electronic devices illustrated in  FIGS. 47A to 47G  include a housing  9000 , a display portion  9001 , a speaker  9003 , an operation key  9005  (including a power switch or an operation switch), a connection terminal  9006 , a sensor  9007  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone  9008 , and the like. 
     The electronic devices illustrated in  FIGS. 47A to 47G  have a variety of functions. For example, the electronic devices in  FIGS. 47A to 47G  can have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a recording medium and displaying it on the display portion. Note that functions of the electronic devices in  FIGS. 47A to 47G  are not limited to the above, and the electronic devices can have a variety of functions. Although not illustrated in  FIGS. 47A to 47G , the electronic devices may have a plurality of display portions. The electronic devices may be provided with a camera or the like and have a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like. 
       FIG. 47A  is a perspective view of a television device  9100 . The television device  9100  can include the display portion  9001  having a large screen size of, for example, 50 inches or more, or 100 inches or more. 
       FIG. 47B  is a perspective view of a portable information appliance  9101 . The portable information appliance  9101  functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information appliance can be used as a smartphone. Note that the portable information appliance  9101  may include the speaker  9003 , the connection terminal  9006 , the sensor  9007 , or the like. The portable information appliance  9101  can display letters and image information on its plurality of surfaces. For example, three operation buttons  9050  (also referred to as operation icons, or simply icons) can be displayed on one surface of the display portion  9001 . Furthermore, information  9051  indicated by dashed rectangles can be displayed on another surface of the display portion  9001 . Examples of the information  9051  include display indicating reception of an incoming email, social networking service (SNS) message, call, and the like; the title and sender of an email and SNS message; the date; the time; remaining battery; and the reception strength of an antenna. Instead of the information  9051 , the operation buttons  9050  or the like may be displayed on the position where the information  9051  is displayed. 
       FIG. 47C  is a perspective view of a portable information appliance  9102 . The portable information appliance  9102  has a function of displaying information on three or more surfaces of the display portion  9001 . Here, information  9052 , information  9053 , and information  9054  are displayed on different surfaces. For example, a user of the portable information appliance  9102  can see the display (here, the information  9053 ) with the portable information appliance  9102  put in a breast pocket of his/her clothes. Specifically, a caller&#39;s phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information appliance  9102 . Thus, the user can see the display without taking out the portable information appliance  9102  from the pocket and decide whether to answer the call. 
       FIG. 47D  is a perspective view of a watch-type portable information appliance  9200 . The portable information appliance  9200  is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of the display portion  9001  is curved, and images can be displayed on the curved display surface. The portable information appliance  9200  can employ near field communication based on an existing communication standard. In that case, for example, mutual communication between the portable information appliance  9200  and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. The portable information appliance  9200  includes the connection terminal  9006 , and data can be directly transmitted to and received from another information appliance via a connector. Power charging through the connection terminal  9006  is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal  9006 . 
       FIGS. 47E, 47F, and 47G  are perspective views of a foldable portable information appliance  9201  that is opened, that is shifted from the opened state to the folded state or from the folded state to the opened state, and that is folded, respectively. The portable information appliance  9201  is highly portable when folded. When the portable information appliance  9201  is opened, a seamless large display region is highly browsable. The display portion  9001  of the portable information appliance  9201  is supported by three housings  9000  joined by hinges  9055 . By folding the portable information appliance  9201  at a connection portion between two housings  9000  with the hinges  9055 , the portable information appliance  9201  can be reversibly changed in shape from an opened state to a folded state. For example, the portable information appliance  9201  can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm. 
       FIGS. 48A and 48B  show an example of an electronic device that is different from the electronic devices illustrated in  FIGS. 47A to 47G .  FIGS. 48A and 48B  are perspective views of a display device including a plurality of display panels. The plurality of display panels are wound in the perspective view of  FIG. 48A  and are unwound in the perspective view of  FIG. 48B . 
     A display device  9500  illustrated in  FIGS. 48A and 48B  includes a plurality of display panels  9501 , a hinge  9511 , and a bearing  9512 . Each of the display panels  9501  includes a display region  9502  and a light-transmitting region  9503 . 
     Each of the display panels  9501  is flexible. Two adjacent display panels  9501  are provided so as to partly overlap each other. For example, the light-transmitting regions  9503  of the two adjacent display panels  9501  can overlap each other. A display device having a large screen can be obtained with the plurality of display panels  9501 . The display device is highly versatile because the display panels  9501  can be wound depending on its use. 
     The display regions  9502  of the two adjacent display panels  9501  do not overlap each other in the example illustrated in  FIGS. 48A and 48B . However, one embodiment of the present invention is not limited to this example, and the display regions  9502  of the two adjacent display panels  9501  may overlap each other without any space so that a continuous display region  9502  is obtained, for example. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     This application is based on Japanese Patent Application serial no. 2016-115215 filed with Japan Patent Office on Jun. 9, 2016 and Japanese Patent Application serial no. 2016-115216 filed with Japan Patent Office on Jun. 9, 2016, the entire contents of which are hereby incorporated by reference.