Patent Publication Number: US-9887291-B2

Title: Semiconductor device, display device including the semiconductor device, display module including the display device, and electronic device including the semiconductor device, the display device, or the display module

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the present invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, or a manufacturing method thereof. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each an embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device. 
     2. Description of the Related Art 
     Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention (e.g., Patent Document 1). 
     Furthermore, for example, Patent document 2 discloses a semiconductor device in which, to reduce oxygen vacancy in an oxide semiconductor layer, an insulating film which releases oxygen by heating is used as a base insulating layer of the oxide semiconductor layer where a channel is formed. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165529 
         [Patent Document 2] Japanese Published Patent Application No. 2012-009836 
       
    
     SUMMARY OF THE INVENTION 
     In the case where a transistor is manufactured using an oxide semiconductor film for a channel region, oxygen vacancy formed in the oxide semiconductor film adversely affects the transistor characteristics; therefore, the oxygen vacancy causes a problem. For example, formation of oxygen vacancies in a channel region of an oxide semiconductor film causes carrier generation due to the oxygen vacancies. The carrier supply source generated in the oxide semiconductor film causes a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor including the oxide semiconductor film. Further, there is a problem in that electrical characteristics fluctuate among the transistors. Therefore, it is preferable that the amount of oxygen vacancy in the channel region of the oxide semiconductor film be as small as possible. 
     In view of the above problem, an object of one embodiment of the present invention is to inhibit a change in electrical characteristics and to improve reliability in a semiconductor device using a transistor including an oxide semiconductor. Another object of one embodiment of the present invention is to provide a semiconductor device with low power consumption. Another object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel display device. 
     Note that the description of the above object does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like. 
     One embodiment of the present invention is a semiconductor device provided with a transistor. The transistor includes a gate electrode, a first insulating film over the gate electrode, a second insulating film over the first insulating film, an oxide semiconductor film over the second insulating film, a first buffer film over the oxide semiconductor film, a second buffer film over the oxide semiconductor film, a source electrode electrically connected with the oxide semiconductor film, and a drain electrode electrically connected with the oxide semiconductor film. The source electrode is electrically connected with the oxide semiconductor film through the first buffer film. The drain electrode is electrically connected with the oxide semiconductor film through the second buffer film. 
     Another embodiment of the present invention is a semiconductor device provided with a transistor. The transistor includes a gate electrode, a first insulating film over the gate electrode, a second insulating film over the first insulating film, an oxide semiconductor film over the second insulating film, a first buffer film over the oxide semiconductor film, a second buffer film over the oxide semiconductor film, a source electrode electrically connected with the oxide semiconductor film, and a drain electrode electrically connected with the oxide semiconductor film. A third insulating film is provided over the transistor. The third insulating film includes oxygen. The source electrode is electrically connected with the oxide semiconductor film through the first buffer film. The drain electrode is electrically connected with the oxide semiconductor film through the second buffer film. 
     Another embodiment of the present invention is a semiconductor device provided with a transistor. The transistor includes a gate electrode, a first insulating film over the gate electrode, a second insulating film over the first insulating film, an oxide semiconductor film over the second insulating film, a third insulating film over the oxide semiconductor film, a first buffer film that is over the oxide semiconductor film and in contact with the third insulating film, a second buffer film that is over the oxide semiconductor film and in contact with the third insulating film, a source electrode electrically connected with the oxide semiconductor film, and a drain electrode electrically connected with the oxide semiconductor film. The third insulating film includes oxygen. The source electrode is electrically connected with the oxide semiconductor film through the first buffer film. The drain electrode is electrically connected with the oxide semiconductor film through the second buffer film. 
     In any of the above structures, a region where an end portion of the first buffer film is aligned with an end portion of the source electrode is provided, and a region where an end portion of the second buffer film is aligned with an end portion of the drain electrode is provided. In any of the above structures, the first buffer film and the second buffer film include at least one of indium, zinc, titanium, aluminum, tungsten, tantalum, and molybdenum. In any of the above structures, the first buffer film and the second buffer film preferably include oxygen and tantalum. 
     In any of the above structures, the third insulating film preferably includes oxygen, nitrogen, and silicon. 
     In any of the above structures, the oxide semiconductor film preferably includes O, In, Zn, and M (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). In any of the above structures, it is preferable that the oxide semiconductor film include a crystal part, and the crystal part have c-axis alignment. 
     Another embodiment of the present invention is a display device including the semiconductor device according to any of the above structures and a display element. Another embodiment of the present invention is a display module including the display device and a touch sensor. Another embodiment of the present invention is an electronic appliance including the semiconductor device according to any of the above structures, the display device, or the display module, and an operation key or a battery. 
     According to one object of one embodiment of the present invention, a change in electrical characteristics can be inhibited and reliability can be improved in a semiconductor device using a transistor including an oxide semiconductor. Alternatively, according to one embodiment of the present invention, a semiconductor device with low power consumption can be provided. According to one embodiment of the present invention, a novel semiconductor device can be provided. According to one embodiment of the present invention, a novel display device can be provided. 
     Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily 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 to 1C  are a top view and cross-sectional views illustrating one embodiment of a semiconductor device. 
         FIGS. 2A to 2C  are cross-sectional views each illustrating one embodiment of a semiconductor device. 
         FIGS. 3A to 3C  are a top view and cross-sectional views illustrating one embodiment of a semiconductor device. 
         FIGS. 4A to 4C  are a top view and cross-sectional views illustrating one embodiment of a semiconductor device. 
         FIGS. 5A to 5C  are a top view and cross-sectional views illustrating one embodiment of a semiconductor device. 
         FIGS. 6A to 6D  are cross-sectional views each illustrating one embodiment of a semiconductor device. 
         FIGS. 7A and 7B  are band diagrams. 
         FIGS. 8A to 8D  are cross-sectional views illustrating an example of the manufacturing process of a semiconductor device. 
         FIGS. 9A to 9C  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device. 
         FIGS. 10A to 10C  are cross-sectional views of an example of a manufacturing process of a semiconductor device. 
         FIGS. 11A to 11D  are cross-sectional views of an example of a manufacturing process of a semiconductor device. 
         FIGS. 12A to 12D  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device. 
         FIGS. 13A to 13C  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device. 
         FIGS. 14A and 14B  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device. 
         FIGS. 15A to 15D  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device. 
         FIGS. 16A to 16D  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device. 
         FIGS. 17A to 17C  are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and  FIG. 17D  is a cross-sectional schematic view of a CAAC-OS. 
         FIGS. 18A to 18D  are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS. 
         FIGS. 19A to 19C  show structure analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD. 
         FIGS. 20A and 20B  show electron diffraction patterns of a CAAC-OS. 
         FIG. 21  is a top view illustrating one embodiment of a display device. 
         FIG. 22  is a cross-sectional view illustrating one embodiment of a display device. 
         FIG. 23  is a cross-sectional view illustrating one embodiment of a display device. 
         FIGS. 24A to 24C  are a block diagram and circuit diagrams illustrating a display device. 
         FIG. 25  illustrates a display module. 
         FIGS. 26A to 26H  illustrate electronic devices. 
         FIG. 27  shows a change in crystal part of an In—Ga—Zn oxide induced by electron irradiation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments will be described with reference to drawings. However, the embodiments can be implemented with various modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments. 
     In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to shapes or values shown in the drawings. 
     Note that in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically. 
     Note that in this specification, terms for describing arrangement, such as “over” “above”, “under”, and “below”, are used for convenience in describing a positional relation between components with reference to drawings. Further, the positional relation between components is changed as appropriate in accordance with a direction in which each component is described. Thus, there is no limitation on terms used in this specification, and description can be made appropriately depending on the situation. 
     In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. In addition, the transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow through the drain region, the channel region, and the source region. Note that in this specification and the like, a channel region refers to a region through which current mainly flows. 
     Further, functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like. 
     Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring. 
     Note that in this specification and the like, a “silicon oxynitride film” refers to a film that includes oxygen at a higher proportion than nitrogen, and a “silicon nitride oxide film” refers to a film that includes nitrogen at a higher proportion than oxygen. 
     In this specification and the like, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     Embodiment 1 
     In this embodiment, a semiconductor device of one embodiment of the present invention is described with reference to  FIGS. 1A to 1C ,  FIGS. 2A to 2C ,  FIGS. 3A to 3C ,  FIGS. 4A to 4C ,  FIGS. 5A to 5C ,  FIGS. 6A to 6D ,  FIGS. 7A and 7B ,  FIGS. 8A to 8D ,  FIGS. 9A to 9C ,  FIGS. 10A to 10C ,  FIGS. 11A to 11D ,  FIGS. 12A to 12D ,  FIGS. 13A to 13C ,  FIGS. 14A and 14B ,  FIGS. 15A to 15D , and  FIGS. 16A to 16D . 
     &lt;Structure Example 1 of Semiconductor Device&gt; 
       FIG. 1A  is a top view of a transistor  100  that is a semiconductor device of one embodiment of the present invention.  FIG. 1B  is a cross-sectional view taken along a dashed dotted line X 1 -X 2  in  FIG. 1A , and  FIG. 1C  is a cross-sectional view taken along a dashed dotted line Y 1 -Y 2  in  FIG. 1A . Note that in  FIG. 1A , some components of the transistor  100  (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. Furthermore, the direction of the dashed dotted line X 1 -X 1  may be called a channel length direction, and the direction of the dashed dotted line Y 1 -Y 2  may be called a channel width direction. As in  FIG. 1A , some components are not illustrated in some cases in top views of transistors described below. 
     The transistor  100  includes a conductive film  104  serving as a gate electrode over a substrate  102 , an insulating film  106  (also referred to as a first insulating film) over the substrate  102  and the conductive film  104 , an insulating film  107  (also referred to as a second insulating film) over the insulating film  106 , an oxide semiconductor film  108  over the insulating film  107 , buffer films  113   a  and  113   b  over the oxide semiconductor film  108 , and conductive films  112   a  and  112   b  serving as a source electrode and a drain electrode electrically connected with the oxide semiconductor film  108 . The conductive film  112   a  is electrically connected with the oxide semiconductor film  108  through the buffer film  113   a , and the conductive film  112   b  is electrically connected with the oxide semiconductor film  108  through the buffer film  113   b . Furthermore, the insulating films  106  and  107  serve as gate insulating films of the transistor  100 . 
     The buffer films  113   a  and  113   b  are preferably conductors or semiconductors so that the conductive films  112   a  and  112   b  are electrically connected with the oxide semiconductor film  108 . 
     Over the transistor  100 , specifically, over the conductive films  112   a  and  112   b  and the oxide semiconductor film  108 , insulating films  114 ,  116 , and  118  are provided. The insulating films  114 ,  116 , and  118  serve as protective insulating films for the transistor  100 . 
     When oxygen vacancy is formed in the oxide semiconductor film  108  included in the transistor  100 , electrons serving as carriers are generated; as a result, the transistor  100  tends to be normally-on. Therefore, to obtain stable transistor characteristics, it is important to reduce oxygen vacancy in the oxide semiconductor film  108 . In a structure of a transistor of one embodiment of the present invention, a film which suppresses release of oxygen (hereinafter described as a buffer film  131  for convenience) is formed over the oxide semiconductor film  108 , and excess oxygen is introduced into the oxide semiconductor film  108  through the buffer film  131 . Thus, oxygen vacancies in the oxide semiconductor film  108  can be filled. As a method for introducing the excess oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be used, for example. 
     When excess oxygen is directly introduced into the oxide semiconductor film  108  without using the buffer film  131 , the oxide semiconductor film  108  might be damaged, leading to a decrease in reliability. In addition, when excess oxygen is directly introduced into the oxide semiconductor film  108  without using the buffer film  131 , oxygen cannot be efficiently supplied to the oxide semiconductor film  108  in some cases. However, in the semiconductor device in one embodiment of the present invention, the buffer film  131  is formed over the oxide semiconductor film  108 , and oxygen is introduced into the oxide semiconductor film  108  through the buffer film  131 , whereby damage to the oxide semiconductor film  108  can be relieved and oxygen can be efficiently supplied to the oxide semiconductor film  108 . 
     Thus, excess oxygen is introduced into the oxide semiconductor film  108  through the buffer film  131 , whereby the buffer film  131  serves as a buffer of the semiconductor film  108 . In addition, when the buffer films  113   a  and  113   b  which are formed by processing the buffer film  131  are provided between the oxide semiconductor film  108  and the conductive films  112   a  and  112   b  that serve as a source electrode and a drain electrode, the constituent elements of the conductive films  112   a  and  112   b  can be prevented from diffusing into the oxide semiconductor film  108 . In the case where the conductive film to be the conductive films  112   a  and  112   b  and the buffer film  131  are processed by the same process, the conductive film  112   a , the conductive film  112   b , the buffer film  113   a , and the buffer film  113   b  have substantially the same shape. 
     Note that in this specification and the like, the case where the conductive films  112   a  and  112   b  and the buffer films  113   a  and  113   b  have the same shape includes a case where the areas of the buffer films  113   a  and  113   b  are greater than or equal to 80% and less than or equal to 120% of the areas of the conductive films  112   a  and  112   b.    
     Here, an enlarged view of the transistor  100  in  FIG. 1B  is illustrated in  FIG. 2A .  FIG. 2B  and  FIG. 2C  each illustrate a modification example of  FIG. 2A .  FIG. 2A  illustrates a cross-sectional view of the vicinity of the oxide semiconductor film  108 , the buffer films  113   a  and  113   b , and the conductive films  112   a  and  112   b  of the transistor  100  in  FIG. 1B . 
     In the cross-sectional view of  FIG. 2A , there are a region where an end portion of the buffer film  113   a  is aligned with an end portion of the conductive film  112   a , and a region where an end portion of the buffer film  113   b  is aligned with an end portion of the conductive film  112   b . In the cross-sectional view of  FIG. 2B , there are a region where the end portion of the buffer film  113   a  is positioned on the outer side than the end portion of the conductive film  112   a , and a region where the end portion of the buffer film  113   b  is positioned on the outer side than the end portion of the conductive film  112   b . In the cross-sectional view of  FIG. 2C , there are a region where the end portion of the buffer film  113   a  is positioned on the inner side than the end portion of the conductive film  112   a , and a region where the end portion of the buffer film  113   b  is positioned on the inner side than the end portion of the conductive film  112   b.    
     As illustrated in  FIG. 2B , in the case where the conductive films  112   a  and  112   b  and the buffer films  113   a  and  113   b  are processed by the same process, the areas of the conductive films  112   a  and  112   b  may be smaller than those of the buffer films  113   a  and  113   b  owing to a difference in etching rate therebetween or the like. The structure illustrated in  FIG. 2B  is preferable because the coverage with the insulating films  114 ,  116 , and  118  can be increased. As illustrated in  FIG. 2C , in the case where the conductive films  112   a  and  112   b  and the buffer films  113   a  and  113   b  are processed by the same process, the areas of the conductive films  112   a  and  112   b  may be larger than those of the buffer films  113   a  and  113   b . With structures illustrated in  FIGS. 2B and 2C , the channel length of the transistor  100  can be shorter or longer than a distance between the conductive film  112   a  serving as a source electrode and the conductive film  112   b  serving as a drain electrode. 
     Other constituent elements of the semiconductor device of this embodiment are described below in detail. 
     &lt;Substrate&gt; 
     There is no particular limitation on the property of a material and the like of the substrate  102  as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like may be used as the substrate  102 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate  102 . Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate  102 . In the case where a glass substrate is used as the substrate  102 , a glass substrate having any of the following sizes can be used: 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), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured. 
     Note that in this specification and the like, a transistor can be formed using a variety of substrates. The type of a substrate is not limited to a certain type. As the substrate, a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, or the like can be used, for example. Examples of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, or the like can be given. Examples of the flexible substrate, the attachment film, the base material film, or the like are as follows: plastic typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES); a synthetic resin such as acrylic; polypropylene; polyester; polyvinyl fluoride; polyvinyl chloride; polyamide; polyimide; aramid; epoxy; an inorganic vapor deposition film; and paper. Specifically, the use of semiconductor substrates, single crystal substrates, SOI substrates, or the like enables the manufacture of small-sized transistors with a small variation in characteristics, size, shape, or the like and with high current capability. A circuit using such transistors achieves lower power consumption of the circuit or higher integration of the circuit. 
     Alternatively, a flexible substrate may be used as the substrate, and the transistor may be provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate and the transistor. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the transistor can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack of inorganic films, which are a tungsten film and a silicon oxide film, or an organic resin film of polyimide or the like formed over a substrate can be used, for example. 
     In other words, a transistor may be formed using one substrate, and then transferred to another substrate. Examples of a substrate to which a transistor is transferred include, in addition to the above substrate over which the transistor can be formed, a paper substrate, a cellophane substrate, an aramid film substrate, a polyimide film substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), and the like), a leather substrate, and a rubber substrate. When such a substrate is used, a transistor with excellent properties or a transistor with low power consumption can be formed, a device with high durability, high heat resistance can be provided, or reduction in weight or thickness can be achieved. 
     &lt;Conductive Film&gt; 
     The conductive film  104  serving as a gate electrode and the conductive films  112   a  and  112   b  serving as a source electrode and a drain electrode can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal element as its component; an alloy including a combination of any of these elements; or the like. 
     Furthermore, the conductive films  104 ,  112   a , and  112   b  may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used. 
     The conductive films  104 ,  112   a , and  112   b  can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. 
     A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films  104 ,  112   a , and  112   b . Use of a Cu—X alloy film enables the manufacturing cost to be reduced because wet etching process can be used in the processing. 
     &lt;Gate Insulating Film&gt; 
     As each of the insulating films  106  and  107  serving as a gate insulating film of the transistor  100 , an insulating layer including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that instead of a stacked structure of the insulating films  106  and  107 , an insulating film of a single layer formed using a material selected from the above or an insulating film of three or more layers may be used. 
     The insulating film  106  serves as a blocking film which inhibits penetration of oxygen. For example, in the case where excess oxygen is supplied to the insulating film  107 , the insulating film  114 , the insulating film  116 , and/or the oxide semiconductor film  108 , the insulating film  106  can inhibit penetration of oxygen. 
     Note that the insulating film  107  that is in contact with the oxide semiconductor film  108  serving as a channel region of the transistor  100  is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film  107  is an insulating film which is capable of releasing oxygen. In order to provide the oxygen excess region in the insulating film  107 , the insulating film  107  is formed in an oxygen atmosphere, for example. Alternatively, the oxygen excess region may be formed by introduction of oxygen into the insulating film  107  after the deposition. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed. 
     In the case where hafnium oxide is used for the insulating film  107 , the following effect is attained. Hafnium oxide has a higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, by using hafnium oxide or aluminum oxide, a physical thickness can be made larger than silicon oxide; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide with a crystalline structure has higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with a low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited to the above examples. 
     In this embodiment, a silicon nitride film is formed as the insulating film  106 , and a silicon oxide film is formed as the insulating film  107 . The silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the silicon nitride film is included in the gate insulating film of the transistor  100 , the physical thickness of the insulating film can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor  100  and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor  100 . 
     &lt;Oxide Semiconductor Film&gt; 
     The oxide semiconductor film  108  contains O, In, Zn, and M (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). Typically, In—Ga oxide, In—Zn oxide, or In-M-Zn oxide can be used for the oxide semiconductor film  108 . It is particularly preferable to use an In-M-Zn oxide for the semiconductor film  108 . 
     In the case where the oxide semiconductor film  108  includes In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In≧M and Zn≧M. As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that the atomic ratios of metal elements in the formed oxide semiconductor film  108  vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error. 
     Note that in the case where the oxide semiconductor film  108  is an In-M-Zn oxide, the proportion of In and the proportion of M, not taking Zn and O into consideration, are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, further preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively. 
     The energy gap of the oxide semiconductor film  108  is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistor  100  can be reduced. 
     The thickness of the oxide semiconductor film  108  is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm. 
     An oxide semiconductor film with low carrier density is used as the oxide semiconductor film  108 . For example, the oxide semiconductor film  108  has carrier density which 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 is higher than or equal to 1×10 −9 /cm 3 . 
     Note that, without limitation to the compositions and materials described above, a material with an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Further, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the oxide semiconductor film  108  be set to be appropriate. 
     Note that it is preferable to use, as the oxide semiconductor film  108 , an oxide semiconductor film in which the impurity concentration is low and density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which impurity concentration is low and density of defect states is low (the amount of oxygen vacancy is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has few carrier traps in some cases. Further, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10 6  μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10 −13  A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. 
     Accordingly, the transistor in which the channel region is formed in the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small variation in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, and the like are given. 
     Hydrogen included in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). 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 element causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen be reduced as much as possible in the oxide semiconductor film  108 . Specifically, in the oxide semiconductor film  108 , the concentration of hydrogen which is measured by secondary 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 , further 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 , further preferably lower than or equal to 5×10 17  atoms/cm 3 , further preferably lower than or equal to 1×10 16  atoms/cm 3 . 
     When silicon or carbon that is one of elements belonging to Group 14 is contained in the oxide semiconductor film  108 , oxygen vacancies are increased in the oxide semiconductor film  108 , and the oxide semiconductor film  108  becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the oxide semiconductor film  108  or the concentration of silicon or carbon (the concentration is measured by SIMS) in the vicinity of an interface with the oxide semiconductor film  108  is set to be lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     In addition, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film  108 , which is measured by SIMS, is lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film  108 . 
     Furthermore, when including nitrogen, the oxide semiconductor film  108  easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set, for example, lower than or equal to 5×10 18  atoms/cm 3 . 
     The oxide semiconductor film  108  may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure described later, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect levels, whereas CAAC-OS has the lowest density of defect levels. 
     The oxide semiconductor film  108  may have an amorphous structure, for example. The oxide semiconductor films having the amorphous structure each have disordered atomic arrangement and no crystalline component, for example. Alternatively, the oxide films having an amorphous structure have, for example, an absolutely amorphous structure and no crystal part. 
     Note that the oxide semiconductor film  108  may be a mixed film including two or more of the following: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a region of CAAC-OS, and a region having a single-crystal structure. The mixed film has a single-layer structure including, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Furthermore, in some cases, the mixed film has a stacked-layer structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure. 
     &lt;Buffer Film&gt; 
     The buffer films  113   a  and  113   b  include at least one of indium, zinc, titanium, aluminum, tungsten, tantalum, and molybdenum. For example, a conductive material such as an alloy including any of the metal elements, an alloy including any of the metal elements in combination, a metal oxide including any of the metal elements, a metal nitride including any of the metal elements, or a metal nitride oxide including any of the metal elements is used. As the buffer films  113   a  and  113   b , a film containing indium oxide, zinc oxide, titanium oxide, aluminum oxide, tungsten oxide, tantalum oxide, molybdenum oxide, or an In—Ga—Zn oxide can be used. Further, a film containing indium oxynitride, zinc oxynitride, titanium oxynitride, aluminum oxynitride, tungsten oxynitride, tantalum oxynitride, molybdenum oxynitride, or In—Ga—Zn oxynitride can be used. In addition, a film containing indium nitride, zinc nitride, titanium nitride, aluminum nitride, tungsten nitride, tantalum nitride, or molybdenum nitride can be used. 
     &lt;Protective Insulating Film&gt; 
     The insulating films  114 ,  116 , and  118  serve as a protective insulating film. 
     The insulating films  114  and  116  contain oxygen. Furthermore, the insulating film  114  is an insulating film which is permeable to oxygen. Note that the insulating film  114  also serves as a film which relieves damage to the oxide semiconductor film  108  at the time of forming the insulating film  116  in a later step. 
     A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the insulating film  114 . 
     In addition, it is preferable that the number of defects in the insulating film  114  be small and typically, the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×10 17  spins/cm 3  by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating film  114  is high, oxygen is bonded to the defects and the amount of oxygen that penetrates the insulating film  114  is decreased. 
     Note that all oxygen entering the insulating film  114  from the outside does not move to the outside of the insulating film  114  and some oxygen remains in the insulating film  114 . Furthermore, movement of oxygen occurs in the insulating film  114  in some cases in such a manner that oxygen enters the insulating film  114  and oxygen included in the insulating film  114  moves to the outside of the insulating film  114 . When an oxide insulating film which is permeable to oxygen is formed as the insulating film  114 , oxygen released from the insulating film  116  provided over the insulating film  114  can be moved to the oxide semiconductor film  108  through the insulating film  114 . 
     The insulating film  114  and the insulating film  116  are formed using an oxide insulating film that contains oxygen at a higher proportion than oxygen in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film containing more oxygen than that in the stoichiometric composition. The oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition is an oxide insulating film in which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10 19  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3  in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. 
     A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used as the insulating film  116 . 
     It is preferable that the number of defects in the insulating film  116  be small, and typically the spin density corresponding to a signal which appears at g=2.001 due to a dangling bond of silicon, be lower than 1.5×10 18  spins/cm 3 , more preferably lower than or equal to 1×10 18  spins/cm 3  by ESR measurement. Note that the insulating film  116  is provided more apart from the oxide semiconductor film  108  than the insulating film  114  is; thus, the insulating film  116  may have higher density of defects than the insulating film  114 . 
     Note that the insulating film  114  can be formed using an oxide insulating film having a low density of states due to nitrogen oxide 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 film. A silicon oxynitride film that releases less nitrogen oxide, an aluminum oxynitride film that releases less nitrogen oxide, or the like can be used as the oxide insulating film in which the density of states due to nitrogen oxide is low between E v   _   os  and E c   _   os . 
     Note that a silicon oxynitride film that releases a small amount of nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in thermal desorption spectroscopy analysis; the amount of released ammonia is typically greater than or equal to 1×10 18 /cm 3  and less than or equal to 5×10 19 /cm 3 . Note that the amount of released ammonia is the amount of ammonia released by heat treatment in which the surface temperature of a film becomes 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 (NO x ; x is greater than or equal to 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2), typically NO 2  or NO, forms levels in the insulating film  114 , for example. The level is positioned in the energy gap of the oxide semiconductor film  108 . Therefore, when nitrogen oxide is diffused to the interface between the insulating film  114  and the oxide semiconductor film  108 , an electron is trapped by the level on the insulating film  114  side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film  114  and the oxide semiconductor film  108 ; thus, the threshold voltage of the transistor is shifted in the positive direction. 
     Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide included in the insulating film  114  reacts with ammonia included in the insulating film  116  in heat treatment, nitrogen oxide included in the insulating film  114  is reduced. Therefore, an electron is hardly trapped at the interface between the insulating film  114  and the oxide semiconductor film  108 . 
     By using, for the insulating film  114 , the oxide insulating film having a low density of states due to nitrogen oxide between E v   _   os  and E c   _   os , the shift in the threshold voltage of the transistor can be reduced, which leads to a smaller change in the electrical characteristics of the transistor. 
     Note that in an ESR spectrum at 100 K or lower of the insulating film  114 , by heat treatment of a manufacturing process of the transistor, typically heat treatment at a temperature higher than or equal to 300° C. and lower than the strain point of the substrate, a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10 18  spins/cm 3 , typically higher than or equal to 1×10 17  spins/cm 3  and lower than 1×10 18  spins/cm 3 . 
     In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO x ; x is greater than or equal to 0 and smaller than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. In other words, the lower the total spin density of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is. 
     The nitrogen concentration of the oxide insulating film having a low density of states due to nitrogen oxide between E v   _   os  and E c   _   os  measured by SIMS is lower than or equal to 6×10 20  atoms/cm 3 . 
     The oxide insulating film in which the density of states due to nitrogen oxide is low between E v   _   os  and E c   _   os  is formed by a PECVD method at a substrate temperature of higher than or equal to 220° C., higher than or equal to 280° C., or higher than or equal to 350° C. using silane and nitrogen oxide, whereby a dense and hard film can be formed. 
     Furthermore, the insulating films  114  and  116  can be formed using insulating films formed of the same kinds of materials; thus, a boundary between the insulating films  114  and  116  cannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating films  114  and  116  is shown by a dashed line. Although a two-layer structure of the insulating films  114  and  116  is described in this embodiment, the present invention is not limited to this. For example, a single-layer structure of the insulating film  114  may be used. 
     The insulating film  118  contains nitrogen. Alternatively, the insulating film  118  contains nitrogen and silicon. The insulating film  118  has a function of blocking oxygen, hydrogen, water, an alkali metal, an alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor film  108 , outward diffusion of oxygen included in the insulating films  114  and  116 , and entry of hydrogen, water, or the like into the oxide semiconductor film  108  from the outside by providing the insulating film  118 . A nitride insulating film, for example, can be used as the insulating film  118 . The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like. Note that instead of the nitride insulating film having a blocking effect against oxygen, hydrogen, water, alkali metal, alkaline earth metal, and the like, an oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, may be provided. As the oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, and a hafnium oxynitride film can be given. 
     Alternatively, the insulating film  118  is preferably a blocking film which releases a small amount of oxygen and/or can inhibit permeation of oxygen. When the insulating film  118  is provided over the insulating films  114  and  116 , outward diffusion of excess oxygen contained in the insulating film  114  and/or the insulating film  116  can be inhibited. 
     Although a variety of films such as the conductive films, the insulating films, and the oxide semiconductor films which are described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal CVD method or an atomic layer deposition (ALD) method. As an example of a thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method can be given. 
     A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film. 
     Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at a time so that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and react with each other in the vicinity of the substrate or over the substrate. 
     Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer; then the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET. 
     The variety of films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films in this embodiment can be formed by a thermal CVD method such as an MOCVD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH 3 ) 3 . The chemical formula of trimethylgallium is Ga(CH 3 ) 3 . The chemical formula of dimethylzinc is Zn(CH 3 ) 2 . Without limitation to the above combination, triethylgallium (chemical formula: Ga(C 2 H 5 ) 3 ) can be used instead of trimethylgallium and diethylzinc (chemical formula: Zn(C 2 H 5 ) 2 ) can be used instead of dimethylzinc. 
     For example, in the case where a hafnium oxide film is formed using a deposition apparatus using an ALD method, two kinds of gases, i.e., ozone (O 3 ) as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (a hafnium alkoxide, or tetrakis(dimethylamide)hafnium (TDMAH)) are used. The chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH 3 ) 2 ] 4 . Examples of another material liquid include tetrakis(ethylmethylamide)hafnium. 
     For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H 2 O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH 3 ) 3 . Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate). 
     For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine contained in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O 2  or dinitrogen monoxide) are supplied to react with the adsorbate. 
     For example, in the case where a tungsten film is formed using a deposition apparatus employing ALD, a WF 6  gas and a B 2 H 6  gas are sequentially introduced plural times to form an initial tungsten film, and then a WF 6  gas and an H 2  gas are introduced at a time, so that a tungsten film is formed. Note that an SiH 4  gas may be used instead of a B 2 H 6  gas. 
     For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed using a deposition apparatus employing ALD, an In(CH 3 ) 3  gas and an O 3  gas are sequentially introduced plural times to form an InO 2  layer, a Ga(CH 3 ) 3  gas and an O 3  gas are introduced at a time to form a GaO layer, and then a Zn(CH 3 ) 2  gas and an O 3  gas are introduced at a time 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 In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing of these gases. Note that although an H 2 O gas which is obtained by bubbling 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. Further, instead of an In(CH 3 ) 3  gas, an In(C 2 H 5 ) 3  gas may be used. Instead of a Ga(CH 3 ) 3  gas, a Ga(C 2 H 5 ) 3  gas may be used. Furthermore, a Zn(CH 3 ) 2  gas may be used. 
     &lt;Structure Example 2 of Semiconductor Device&gt; 
     A structure example different from that of the transistor  100  in  FIGS. 1A to 1C  is described with reference to  FIGS. 3A to 3C . Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases. 
       FIG. 3A  is a top view of a transistor  150  that is a semiconductor device of one embodiment of the present invention.  FIG. 3B  is a cross-sectional view taken along dashed-dotted line X 1 -X 2  illustrated in  FIG. 3A , and  FIG. 3C  is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2  illustrated in  FIG. 3A . 
     The transistor  150  includes the conductive film  104  serving as a gate electrode over the substrate  102 , the insulating film  106  over the substrate  102  and the conductive film  104 , the insulating film  107  over the insulating film  106 , the oxide semiconductor film  108  over the insulating film  107 , the insulating film  114  over the oxide semiconductor film  108 , the insulating film  116  over the insulating film  114 , a buffer film  113   a  which is over the oxide semiconductor film  108  and in contact with the insulating films  114  and  116 , a buffer film  113   b  which is over the oxide semiconductor film  108  and in contact with the insulating films  114  and  116 , an opening  141   a  provided in the insulating films  114  and  116 , the conductive film  112   a  serving as a source electrode electrically connected with the oxide semiconductor film  108  through the buffer film  113   a , an opening  141   b  provided in the insulating films  114  and  116 , and the conductive film  112   b  serving as a drain electrode electrically connected with the oxide semiconductor film  108  through the buffer film  113   b . Over the transistor  150 , specifically, over the conductive films  112   a  and  112   b  and the insulating film  116 , the insulating film  118  is provided. The insulating film  114  and the insulating film  116  serve as protective insulating films for the oxide semiconductor film  108 . The insulating film  118  serves as a protective insulating film for the transistor  150 . Furthermore, the insulating films  106  and  107  serve as gate insulating films of the transistor  150 . 
     Although the transistor  100  described above has a channel-etched structure, the transistor  150  in  FIGS. 3A to 3C  has a channel-protective structure. Thus, either the channel-etched structure or the channel-protective structure can be applied to the semiconductor device of one embodiment of the present invention. 
     In the structure of the transistor  150 , the insulating films  114  and  116  are provided over the oxide semiconductor film  108 , and a film which suppresses release of oxygen (buffer film  131 ) is provided over the insulating films  114  and  116 . Excess oxygen is introduced into the oxide semiconductor film  108  through the buffer film  131  and the insulating films  114  and  116 , whereby oxygen vacancies in the oxide semiconductor film  108  can be filled. As a method for introducing the excess oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be used, for example. In addition, excess oxygen is introduced into the insulating films  114  and  116  as well as into the oxide semiconductor film  108 . Excess oxygen is introduced into the insulating films  114  and  116 , whereby excess oxygen contained in the insulating films  114  and  116  can be moved to the oxide semiconductor film  108  in heat treatment or the like in a later step. 
     &lt;Structure Example 3 of Semiconductor Device&gt; 
     A structure example different from that of the transistor  150  in  FIGS. 3A to 3C  is described with reference to  FIGS. 4A to 4C . Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases. 
       FIG. 4A  is a top view of a transistor  160  that is a semiconductor device of one embodiment of the present invention.  FIG. 4B  is a cross-sectional view taken along a dashed dotted line X 1 -X 2  in  FIG. 4A , and  FIG. 4C  is a cross-sectional view taken along a dashed dotted line Y 1 -Y 2  in  FIG. 4A . 
     The transistor  160  includes the conductive film  104  serving as a gate electrode over the substrate  102 , the insulating film  106  over the substrate  102  and the conductive film  104 , the insulating film  107  over the insulating film  106 , the oxide semiconductor film  108  over the insulating film  107 , the insulating film  114  over the oxide semiconductor film  108 , the insulating film  116  over the insulating film  114 , the buffer film  113   a  which is over the oxide semiconductor film  108  and in contact with the insulating films  114  and  116 , the buffer film  113   b  which is over the oxide semiconductor film  108  and in contact with the insulating films  114  and  116 , the conductive film  112   a  serving as a source electrode electrically connected with the oxide semiconductor film  108  through the buffer film  113   a , and the conductive film  112   b  serving as a drain electrode electrically connected with the oxide semiconductor film  108  through the buffer film  113   b . Over the transistor  160 , specifically, over the conductive films  112   a  and  112   b  and the insulating film  116 , the insulating film  118  is provided. The insulating film  114  and the insulating film  116  serve as protective insulating films for the oxide semiconductor film  108 . The insulating film  118  serves as a protective insulating film for the transistor  160 . Furthermore, the insulating films  106  and  107  serve as gate insulating films of the transistor  160 . 
     The transistor  160  is different from the transistor  150  in  FIGS. 3A to 3C  in the shapes of the insulating films  114  and  116 . Specifically, the insulating films  114  and  116  of the transistor  160  have island shapes and are provided over a channel region of the oxide semiconductor film  108 . The other components are the same as those of the transistor  150 , and the effect similar to that in the case of the transistor  150  is obtained. 
     &lt;Structure Example 4 of Semiconductor Device&gt; 
     A structure example different from that of the transistor  100  in  FIGS. 1A to 1C  is described with reference to  FIGS. 5A to 5C . Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases. 
       FIG. 5A  is a top view of a transistor  170  that is a semiconductor device of one embodiment of the present invention.  FIG. 5B  is a cross-sectional view taken along a dashed dotted line X 1 -X 2  in  FIG. 5A , and  FIG. 5C  is a cross-sectional view taken along a dashed dotted line Y 1 -Y 2  in  FIG. 5A . 
     The transistor  170  includes the conductive film  104  serving as a first gate electrode over the substrate  102 , the insulating film  106  over the substrate  102  and the conductive film  104 , the insulating film  107  over the insulating film  106 , the oxide semiconductor film  108  over the insulating film  107 , the buffer films  113   a  and  113   b  over the oxide semiconductor film  108 , the conductive films  112   a  and  112   b  serving as a source electrode and a drain electrode electrically connected with the oxide semiconductor film  108 , the insulating film  114  over the oxide semiconductor film  108  and the conductive films  112   a  and  112   b , the insulating film  116  over the insulating film  114 , the insulating film  118  over the insulating film  116 , and conductive films  120   a  and  120   b  over the insulating film  118 . The insulating films  114 ,  116 , and  118  serve as protective insulating films for the transistor  170 . The conductive film  120   a  is electrically connected with the conductive film  112   b  through an opening  142   c  provided in the insulating films  114 ,  116 , and  118 . The conductive film  120   b  is formed to overlap the oxide semiconductor film  108 . The conductive film  112   a  is electrically connected with the oxide semiconductor film  108  through the buffer film  113   a , and the conductive film  112   b  is electrically connected with the oxide semiconductor film  108  through the buffer film  113   b . Furthermore, the insulating films  106  and  107  serve as first gate insulating films of the transistor  170 . 
     The insulating films  114 ,  116 , and  118  in the transistor  170  function as second gate insulating films of the transistor  170 . The conductive film  120   a  in the transistor  170  serves as, for example, a pixel electrode used for a display device. The conductive film  120   b  in the transistor  170  serves as a second gate electrode (also referred to as a back gate electrode). 
     As illustrated in  FIG. 5C , the conductive film  120   b  is connected to the conductive film  104  functioning as a gate electrode through openings  142   a  and  142   b  provided in the insulating films  106 ,  107 ,  114 ,  116 , and  118 . Accordingly, the conductive film  120   b  and the conductive film  104  are supplied with the same potential. 
     Note that although the structure in which the openings  142   a  and  142   b  are provided so that the conductive film  120   b  and the conductive film  104  are connected to each other is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which only one of the openings  142   a  and  142   b  is provided so that the conductive film  120   b  and the conductive film  104  are connected to each other, or a structure in which the openings  142   a  and  142   b  are not provided and the conductive film  120   b  and the conductive film  104  are not connected to each other may be employed. Note that in the case where the conductive film  120   b  and the conductive film  104  are not connected to each other, it is possible to apply different potentials to the conductive film  120   b  and the conductive film  104 . 
     As illustrated in  FIG. 5B , the oxide semiconductor film  108  is positioned to face each of the conductive film  104  functioning as a gate electrode and the conductive film  120   b  serving as a second gate electrode, and is sandwiched between the two conductive films serving as gate electrodes. The lengths in the channel length direction and the channel width direction of the conductive film  120   b  serving as a second gate electrode are longer than those in the channel length direction and the channel width direction of the oxide semiconductor film  108 . The whole oxide semiconductor film  108  is covered with the conductive film  120   b  with the insulating films  114 ,  116 , and  118  positioned therebetween. Since the conductive film  120   b  serving as a second gate electrode is connected to the conductive film  104  serving as a gate electrode through the openings  142   a  and  142   b  provided in the insulating films  106 ,  107 ,  114 ,  116 , and  118 , a side surface of the oxide semiconductor film  108  in the channel width direction faces the conductive film  120   b  serving as a second gate electrode with the insulating films  114 ,  116 , and  118  positioned therebetween. 
     In other words, in the channel width direction of the transistor  170 , the conductive film  104  serving as a gate electrode and the conductive film  120   b  serving as a second gate electrode are connected to each other through the openings provided in the insulating films  106  and  107  serving as gate insulating films, and the insulating films  114 ,  116 , and  118  serving as second gate insulating films; and the conductive film  104  and the conductive film  120   b  surround the oxide semiconductor film  108  with the insulating films  106  and  107  serving as gate insulating films, and the insulating films  114 ,  116 , and  118  serving as second gate insulating films positioned therebetween. 
     Such a structure makes it possible that the oxide semiconductor film  108  included in the transistor  170  is electrically surrounded by electric fields of the conductive film  104  serving as a gate electrode and the conductive film  120   b  serving as a second gate electrode. A device structure of a transistor, like that of the transistor  170 , in which electric fields of a gate electrode and a second gate electrode electrically surround an oxide semiconductor film where a channel region is formed can be referred to as a surrounded channel (s-channel) structure. 
     Since the transistor  170  has the s-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film  108  by the conductive film  104  serving as a gate electrode; therefore, the current drive capability of the transistor  170  can be improved and high on-state current characteristics can be obtained. Since the on-state current can be increased, it is possible to reduce the size of the transistor  170 . In addition, since the oxide semiconductor film  108  of the transistor  170  is surrounded by the conductive film  104  serving as a gate electrode and the conductive film  120   b  serving as a second gate electrode, the mechanical strength of the oxide semiconductor  108  can be increased. 
     &lt;Structure Example 5 of Semiconductor Device&gt; 
     Structure examples different from that of the transistor  100  in  FIGS. 1A to 1C  are described with reference to  FIGS. 6A to 6D . Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases. 
       FIGS. 6A and 6B  each illustrate a cross-sectional view of a modification example of the transistor  100  in  FIGS. 1B and 1C .  FIGS. 6C and 6D  each illustrate a cross-sectional view of a modification example of the transistor  100  in  FIGS. 1B and 1C . 
     A transistor  100 C in  FIGS. 6A and 6B  has the same structure as the transistor  100  in  FIGS. 1B and 1C  except that the oxide semiconductor film  108  has a three-layer structure. Specifically, the oxide semiconductor film  108  of the transistor  100 C includes an oxide semiconductor film  108   a , an oxide semiconductor film  108   b , and an oxide semiconductor film  108   c.    
     A transistor  100 D in  FIGS. 6C and 6D  has the same structure as the transistor  100  in  FIGS. 1B and 1C  except that the oxide semiconductor film  108  has a two-layer structure. Specifically, the oxide semiconductor film  108  of the transistor  100 D includes the oxide semiconductor film  108   a  and the oxide semiconductor film  108   b.    
     Here, a band structure including the oxide semiconductor films  108   a ,  108   b , and  108   c  and insulating films in contact with the oxide semiconductor film  108  is described with reference to  FIGS. 7A and 7B . 
       FIG. 7A  shows an example of a band structure in the thickness direction of a stack including the insulating film  107 , the oxide semiconductor films  108   a ,  108   b , and  108   c , and the insulating film  114 .  FIG. 7B  shows an example of a band structure in the thickness direction of a stack including the insulating film  107 , the oxide semiconductor films  108   a ,  108   b , and  108   c , and the insulating film  114 . For easy understanding, the conduction band minimum (Ec) of each of the insulating film  107 , the oxide semiconductor films  108   a ,  108   b , and  108   c , and the insulating film  114  is shown in the band structures. 
     In  FIG. 7A , a silicon oxide film is used as each of the insulating films  107  and  114 , an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1 is used as the oxide semiconductor film  108   a , an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:4:5 is used as the oxide semiconductor film  108   b , and an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:3:6 is used as the oxide semiconductor film  108   c.    
     In the band structure of  FIG. 7B , a silicon oxide film is used as each of the insulating films  107  and  114 , an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1 is used as the oxide semiconductor film  108   a , and a metal oxide film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:3:6 is used as the oxide semiconductor film  108   b.    
     As illustrated in  FIGS. 7A and 7B , the conduction band minimum smoothly varies between the oxide semiconductor film  108   a  and the oxide semiconductor film  108   b . In other words, the energy level at the bottom of the conduction band is continuously varied or continuously connected. To obtain such a band structure, there exist no impurity, which forms a defect state such as a trap center or a recombination center for the oxide semiconductor, at the interface between the oxide semiconductor film  108   a  and the oxide semiconductor film  108   b.    
     To form a continuous junction between the oxide semiconductor film  108   a  and the oxide semiconductor film  108   b , it is necessary to form the films successively without exposure to the air by using a multi-chamber deposition apparatus (sputtering apparatus) provided with a load lock chamber. 
     With the band structure of  FIG. 7A  or  FIG. 7B , the oxide semiconductor film  108   a  serves as a well, and a channel region is formed in the oxide semiconductor film  108   a  in the transistor with the stacked-layer structure. 
     By providing the oxide semiconductor film  108   b  and/or the oxide semiconductor film  108   c , the oxide semiconductor film  108   a  can be distanced away from trap states. 
     In addition, the trap states might be more distant from the vacuum level than the conduction band minimum (Ec) of the oxide semiconductor film  108   a  serving as a channel region, so that electrons are likely to be accumulated in the trap states. When the electrons are accumulated in the trap states, the electrons become negative fixed electric charge, so that the threshold voltage of the transistor is shifted in the positive direction. Therefore, it is preferable that the trap states be closer to the vacuum level than the conduction band minimum (Ec) of the oxide semiconductor film  108   a . Such a structure inhibits accumulation of electrons in the trap states. As a result, the on-state current and the field-effect mobility of the transistor can be increased. 
     In  FIGS. 7A and 7B , the energy level of the bottom of the conduction band of each of the oxide semiconductor films  108   b  and  108   c  is closer to the vacuum level than that of the oxide semiconductor film  108   a . Typically, an energy difference between the bottom of the conduction band of the oxide semiconductor film  108   a  and the bottom of the conduction band of each of the oxide semiconductor films  108   b  and  108   c  is greater than or equal to 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2 eV or less than or equal to 1 eV. That is, the difference between the electron affinity of each of the oxide semiconductor films  108   b  and  108   c  and the electron affinity of the oxide semiconductor film  108   a  is greater than or equal to 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2 eV or less than or equal to 1 eV. 
     In such a structure, the oxide semiconductor film  108   a  serves as a main path of current and serves as a channel region. In addition, since the oxide semiconductor films  108   b  and  108   c  each include one or more metal elements included in the oxide semiconductor film  108   a  in which a channel region is formed, interface scattering is less likely to occur at the interface between the oxide semiconductor film  108   a  and the oxide semiconductor film  108   b . Thus, the transistor can have high field-effect mobility because the movement of carriers is not hindered at the interface. 
     To prevent each of the oxide semiconductor films  108   b  and  108   c  from serving as part of a channel region, a material having sufficiently low conductivity is used for the oxide semiconductor films  108   b  and  108   c . Alternatively, a material which has a smaller electron affinity (a difference in energy level between the vacuum level and the conduction band minimum) than the oxide semiconductor film  108   a  and has a difference in the conduction band minimum from the oxide semiconductor film  108   a  (band offset) is used for the oxide semiconductor films  108   b  and  108   c . Furthermore, to inhibit generation of a difference between threshold voltages due to the value of the drain voltage, it is preferable to form the oxide semiconductor films  108   b  and  108   c  using a material whose conduction band minimum is closer to the vacuum level than that of the oxide semiconductor film  108   a  is by more than 0.2 eV, preferably 0.5 eV or more. 
     It is preferable that the oxide semiconductor films  108   b  and  108   c  not have a spinel crystal structure. This is because if the oxide semiconductor films  108   b  and  108   c  have a spinel crystal structure, a constituent element of the conductive films  112   a  and  112   b  might be diffused into the oxide semiconductor film  108   a  at the interface between the spinel crystal structure and another region. Note that each of the oxide semiconductor film  108   b  and  108   c  is preferably a CAAC-OS, which is described later, in which case a higher blocking property against constituent elements of the conductive films  112   a  and  112   b , e.g., copper elements, is obtained. 
     The thickness of each of the oxide semiconductor films  108   b  and  108   c  is greater than or equal to a thickness that is capable of inhibiting diffusion of the constituent element of the conductive films  112   a  and  112   b  into the oxide semiconductor film  108   a , and less than a thickness that inhibits supply of oxygen from the insulating film  114  to the oxide semiconductor film  108   a . For example, when the thickness of each of the oxide semiconductor films  108   b  and  108   c  is greater than or equal to 10 nm, the constituent elements of the conductive films  112   a  and  112   b  can be prevented from diffusing into the oxide semiconductor film  108   a . When the thickness of each of the oxide semiconductor films  108   b  and  108   c  is less than or equal to 100 nm, oxygen can be effectively supplied from the insulating films  114  and  116  to the oxide semiconductor film  108   a.    
     When the oxide semiconductor films  108   b  and  108   c  are each an In-M-Zn oxide in which the atomic ratio of the element M (M is Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf) is higher than that of In, the energy gap of each of the oxide semiconductor films  108   b  and  108   c  can be large and the electron affinity thereof can be small. Therefore, a difference in electron affinity between the oxide semiconductor film  108   a  and each of the oxide semiconductor films  108   b  and  108   c  may be controlled by the proportion of the element M. Furthermore, oxygen vacancy is less likely to be generated in the metal oxide film in which the atomic ratio of Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf is higher than that of In because Ti, Ga, Y, Zr, La, Ce, Nd, Sn, and Hf each are a metal element that is strongly bonded to oxygen. 
     When an In-M-Zn oxide is used for the oxide semiconductor films  108   b  and  108   c , the proportions of In and M, not taking Zn and O into consideration, is preferably as follows: the atomic percentage of In is less than 50 atomic % and the atomic percentage of M is greater than or equal to 50 atomic %; further preferably, the atomic percentage of In is less than 25 atomic % and the atomic percentage of M is greater than or equal to 75 atomic %. Alternatively, a gallium oxide film may be used as each of the oxide semiconductor films  108   b  and  108   c.    
     Furthermore, in the case where each of the oxide semiconductor films  108   a ,  108   b , and  108   c  is an In-M-Zn oxide, the proportion of M atoms in each of the oxide semiconductor films  108   b  and  108   c  is higher than that in the oxide semiconductor film  108   a . Typically, the proportion of M in each of the oxide semiconductor films  108   b  and  108   c  is 1.5 or more times, preferably twice or more, more preferably three or more times that in the oxide semiconductor film  108   a.    
     Furthermore, in the case where the oxide semiconductor films  108   a ,  108   b , and  108   c  are each an In-M-Zn oxide, when the oxide semiconductor film  108   a  has an atomic ratio of In:M:Zn=x 1 :y 1 :z 1  and the oxide semiconductor films  108   b  and  108   c  each have an atomic ratio of In:M:Zn=x 2 :y 2 :z 2 , y 2 /x 2  is larger than y 1 /x 1 , preferably y 2 /x 2  is 1.5 or more times as large as y 1 /x 1 , further preferably, y 2 /x 2  is two or more times as large as y 1 /x 1 , still further preferably y 2 /x 2  is three or more times or four or more times as large as y 1 /x 1 . At this time, y 1  is preferably greater than or equal to x 1  in the oxide semiconductor film  108   a , because stable electrical characteristics of a transistor can be achieved. However, when y 1  is three or more times as large as x 1 , the field-effect mobility of the transistor including the oxide semiconductor film  108   a  is reduced. Accordingly, y 1  is preferably smaller than three times x 1 . 
     In the case where the oxide semiconductor film  108   a  is an In-M-Zn oxide and a target having the atomic ratio of metal elements of In:M:Zn=x 1 :y 1 :z 1  is used for depositing the oxide semiconductor film  108   a , x 1 /y 1  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6, and z 1 /y 1  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z 1 /y 1  is greater than or equal to 1 and less than or equal to 6, a CAAC-OS to be described later is easily formed as the oxide semiconductor film  108   a . Typical examples of the atomic ratio of the metal elements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, and the like. 
     In the case where the oxide semiconductor films  108   b  and  108   c  are each an In-M-Zn oxide and a target having an atomic ratio of metal elements of In:M:Zn=x 2 :y 2 :z 2  is used for depositing the oxide semiconductor films  108   b  and  108   c , x 2 /y 2  is preferably less than x 1 /y 1 , and z 2 /y 2  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. When the atomic ratio of M with respect to indium is high, the energy gap of the oxide semiconductor films  108   b  and  108   c  can be large and the electron affinity thereof can be small; therefore, y 2 /x 2  is preferably higher than or equal to 3 or higher than or equal to 4. Typical examples of the atomic ratio of the metal elements of the target include In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:5, In:M:Zn=1:3:6, In:M:Zn=1:4:2, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:5:5. 
     Furthermore, in the case where the oxide semiconductor films  108   b  and  108   c  are each an In-M oxide, when a divalent metal element (e.g., zinc) is not included as M, the oxide semiconductor films  108   b  and  108   c  which do not include a spinel crystal structure can be formed. As the oxide semiconductor films  108   b  and  108   c , for example, an In—Ga oxide film can be used. The In—Ga oxide can be formed by a sputtering method using an In—Ga metal oxide target (In:Ga=7:93), for example. To deposit the oxide semiconductor films  108   b  and  108   c  by a sputtering method using DC discharge, on the assumption that an atomic ratio of In:M is x:y, it is preferable that y/(x+y) be less than or equal to 0.96, further preferably less than or equal to 0.95, for example, 0.93. 
     In each of the oxide semiconductor films  108   a ,  108   b , and  108   c , the proportions of the atoms in the above atomic ratio vary within a range of ±40% as an error. 
     The structures of the transistors of this embodiment can be freely combined with each other. 
     &lt;Method 1 for Manufacturing Semiconductor Device&gt; 
     Next, a method for manufacturing the transistor  100 , which is a semiconductor device of one embodiment of the present invention, is described below in detail using  FIGS. 8A to 8D ,  FIGS. 9A to 9C , and  FIGS. 10A to 10C . 
     The films included in the transistor  100  (i.e., the insulating film, the oxide semiconductor film, the conductive film, and the like) can be formed by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, and a pulsed laser deposition (PLD) method. Alternatively, a coating method or a printing method can be used. Although a sputtering method and a plasma-enhanced chemical vapor deposition (PECVD) method are typical examples of the film formation method, a thermal CVD method, or an atomic layer deposition (ALD) method may be used. As an example of the thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method is given, for example. 
     Deposition by the thermal CVD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and a source gas and an oxidizer are supplied to the chamber at a time and react with each other in the vicinity of the substrate or over the substrate. Thus, no plasma is generated in the deposition; therefore, the thermal CVD method has an advantage that no defect due to plasma damage is caused. 
     Deposition by the ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching switching valves (also referred to as high-speed valves). In such a case, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first single-atomic layer; then the second source gas is introduced to react with the first single-atomic layer; as a result, a second single-atomic layer is stacked over the first single-atomic layer, so that a thin film is formed. 
     The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute transistor. 
     First, a conductive film is formed over the substrate  102  and processed through a lithography process and an etching process, whereby the conductive film  104  serving as a gate electrode is formed. Then, the insulating films  106  and  107  serving as gate insulating films are formed over the conductive film  104  (see  FIG. 8A ). 
     The conductive film  104  serving as a gate electrode can be formed by a sputtering method, a CVD method, a vacuum evaporation method, or a PLD method. Alternatively, a coating method or a printing method can be used. Although typical deposition methods are a sputtering method and PECVD method, a thermal CVD method, such as an MOCVD method, or an ALD method described above may be used. 
     In this embodiment, a glass substrate is used as the substrate  102 , and as the conductive film  104  serving as a gate electrode, a 100-nm-thick tungsten film is formed by a sputtering method. 
     The insulating films  106  and  107  serving as gate insulating films can be formed by a sputtering method, a PECVD method, a thermal CVD method, a vacuum evaporation method, a PLD method, or the like. In this embodiment, a 400-nm-thick silicon nitride film as the insulating film  106  and a 50-nm-thick silicon oxynitride film as the insulating film  107  are formed by a PECVD method. 
     Note that the insulating film  106  can have a stacked-layer structure of silicon nitride films. Specifically, the insulating film  106  can have a three-layer stacked-layer structure of a first silicon nitride film, a second silicon nitride film, and a third silicon nitride film. An example of the three-layer stacked-layer structure is as follows. 
     For example, the first silicon nitride film can be formed to have a thickness of 50 nm under the condition where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as a source gas to a reaction chamber of a PECVD apparatus; the pressure in the reaction chamber is controlled to 100 Pa, and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source. 
     The second silicon nitride film can be formed to have a thickness of 300 nm under the condition where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus; the pressure in the reaction chamber is controlled to 100 Pa, and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source. 
     The third silicon nitride film can be formed to have a thickness of 50 nm under the condition where silane at a flow rate of 200 sccm and nitrogen at a flow rate of 5000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus; the pressure in the reaction chamber is controlled to 100 Pa, and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source. 
     Note that the first silicon nitride film, the second silicon nitride film, and the third silicon nitride film can be each formed at a substrate temperature of 350° C. 
     When the insulating film  106  has the three-layer stacked-layer structure of silicon nitride films, for example, in the case where a conductive film including Cu is used as the conductive film  104 , the following effect can be obtained. 
     The first silicon nitride film can inhibit diffusion of a copper (Cu) element from the conductive film  104 . The second silicon nitride film has a function of releasing hydrogen and can improve withstand voltage of the insulating film serving as a gate insulating film. The third silicon nitride film releases a small amount of hydrogen and can inhibit diffusion of hydrogen released from the second silicon nitride film. 
     The insulating film  107  is preferably an insulating film including oxygen to improve characteristics of an interface with the oxide semiconductor film  108  formed later. 
     Next, the oxide semiconductor film  108  is formed over the insulating film  107  (see  FIG. 8B ). 
     In this embodiment, an oxide semiconductor film is formed by a sputtering method using an In—Ga—Zn metal oxide target (having an atomic ratio of In:Ga:Zn=1:1:1.2), a mask is formed over the oxide semiconductor film through a lithography process, and the oxide semiconductor film is processed into a desired region, whereby the oxide semiconductor film  108  having an island shape is formed. 
     After the oxide semiconductor film  108  is formed, heat treatment may be performed at a temperature higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 200° C. and lower than or equal to 450° C., further preferably higher than or equal to 300° C. and lower than or equal to 450° C. The heat treatment performed here serves as one kind of treatment for increasing the purity of the oxide semiconductor film and can reduce hydrogen, water, and the like included in the oxide semiconductor film  108 . Note that the heat treatment for the purpose of reducing hydrogen, water, and the like may be performed before the oxide semiconductor film  108  is processed into an island shape. 
     An electric furnace, an RTA apparatus, or the like can be used for the heat treatment performed on the oxide semiconductor film  108 . 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. Therefore, the heat treatment time can be shortened. 
     Note that the heat treatment performed on the oxide semiconductor film  108  may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or less, preferably 1 ppm or less, further preferably 10 ppb or less), or a rare gas (argon, helium, or the like). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like. Furthermore, after heat treatment 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 film and oxygen can be supplied to the oxide semiconductor film at the same time. Consequently, the amount of oxygen vacancies in the oxide semiconductor film can be reduced. 
     In the case where the oxide semiconductor film  108  is formed by a sputtering method, as a sputtering gas, a rare gas (typically argon), oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased. 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 which 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  108  can be minimized. 
     In the case where the oxide semiconductor film  108  is formed by a sputtering method, a chamber in a sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10 −7  Pa to 1×10 −4  Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity for the oxide semiconductor film  108 , as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas including carbon or hydrogen from an exhaust system to the inside of the chamber. 
     Next, a buffer film  131  serving as a film which suppresses release of oxygen is formed over the insulating film  107  and the oxide semiconductor film  108 . Next, oxygen  141  is added to the oxide semiconductor film  108  through the buffer film  131  (see  FIG. 8C ). 
     The buffer film  131  that inhibits release of oxygen includes at least one of indium, zinc, titanium, aluminum, tungsten, tantalum, and molybdenum. For example, a conductive material such as an alloy including any of the metal elements, an alloy including any of the metal elements in combination, a metal oxide including any of the metal elements, a metal nitride including any of the metal elements, or a metal nitride oxide including any of the metal elements is used. 
     The buffer film  131  can be formed using, for example, a tantalum nitride film, a titanium film, an indium tin oxide (ITO) film, an aluminum film, or an oxide semiconductor film (e.g., an In—Ga—Zn oxide film having an atomic ratio of In:Ga:Zn=1:4:5). 
     The thickness of the buffer film  131  can be greater than or equal to 1 nm and less than or equal to 20 nm, or greater than or equal to 2 nm and less than or equal to 10 nm. In this embodiment, a 5-nm-thick tantalum nitride film is formed using a sputtering apparatus as the buffer film  131 . 
     As a method for adding oxygen  141  to the oxide semiconductor film  108  through the buffer film  131 , an ion doping method, an ion implantation method, plasma treatment, or the like is given. The plasma treatment may be performed, for example, in such a manner that the substrate is set on the parallel-plate cathode side in a dry etching apparatus or an ashing apparatus, and an RF power is supplied so that a bias is applied to the substrate side. The bias application to the substrate side is preferable because the oxygen  141  can be efficiently introduced into the oxide semiconductor film  108 . Since the buffer film  131  is provided over the oxide semiconductor film  108 , damage to the oxide semiconductor film  108  can be relieved when the oxygen  141  is added. Furthermore, the buffer film  131  serves as a protective film for preventing oxygen from being released from the oxide semiconductor film  108 . Thus, a larger amount of oxygen can be added to the oxide semiconductor film  108 . In addition, oxygen can be added to the vicinity of the interface between the oxide semiconductor film  108  and the buffer film  131 . 
     In the case where oxygen is introduced by plasma treatment, by making oxygen excited by a microwave to generate high-density oxygen plasma, the amount of oxygen introduced into the oxide semiconductor film  108  can be increased. 
     Note that by the addition of the oxygen  141 , the buffer film  131  becomes the buffer film  131   x  formed of oxide or oxynitride of metal (indium, zinc, titanium, aluminum, tungsten, tantalum, or molybdenum). For example, the buffer film  131   x  is a film containing indium oxide, zinc oxide, titanium oxide, aluminum oxide, tungsten oxide, tantalum oxide, molybdenum oxide or In—Ga—Zn oxide. Alternatively, the buffer film  131   x  is a film containing indium oxynitride, zinc oxynitride, titanium oxynitride, aluminum oxynitride, tungsten oxynitride, tantalum oxynitride, molybdenum oxynitride, or In—Ga—Zn oxynitride. Note that the buffer film  131   x  is preferably a conductor or a semiconductor (see  FIG. 8D ). 
     Next, a conductive film  112  is formed over the buffer film  131   x  (see  FIG. 9A ). 
     The conductive film  112  can be formed using any of the materials that can be used for the conductive films  112   a  and  112   b  described above. In this embodiment, as the conductive film  112 , a three-layer stacked structure of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film is formed with a sputtering apparatus. 
     Then, masks  140   a  and  140   b  are formed over the conductive film  112  (see  FIG. 9B ). 
     The masks  140   a  and  140   b  are formed in such a manner that a photosensitive resin is applied with a spin coater apparatus and light exposure and development are performed on desired regions through a lithography process. Note that the photosensitive resin may be a negative-type or positive-type photosensitive resin. Alternatively, the masks  140   a  and  140   b  may be formed by an inkjet method. When the masks  140   a  and  140   b  are formed by an inkjet method, manufacturing cost can be reduced because a photomask is not used. 
     Next, the conductive film  112  and the buffer film  131   x  are etched from above the masks  140   a  and  140   b , using the etching gas  142  (see  FIG. 9C ). 
     Note that in this embodiment, the etching gas  142  is generated and the conductive film  112  and the buffer film  131   x  are etched by dry etching. Note that the method for etching the conductive film  112  and the buffer film  131   x  is not limited thereto. For example, the conductive film  112  and the buffer film  131   x  may be etched by wet etching using the etching gas  142  as a chemical solution. Alternatively, the conductive film  112  and the buffer film  131   x  may be etched by a combination of dry etching and wet etching. 
     Next, the masks  140   a  and  140   b  are removed, whereby the conductive films  112   a  and  112   b  serving as a source electrode and a drain electrode, the buffer film  113   a  under the conductive film  112   a , and the buffer film  113   b  under the conductive film  112   b  are formed (see  FIG. 10A ). 
     The buffer film  131   x  is separated into the buffer films  113   a  and  113   b  so that they have island shapes when the conductive films  112   a  and  112   b  are processed. 
     After the conductive films  112   a  and  112   b  and the buffer films  113   a  and  113   b  are formed, a surface of the oxide semiconductor film  108  (on a back channel side) may be cleaned. The cleaning may be performed, for example, using a chemical solution such as phosphoric acid. The cleaning using a chemical solution such as a phosphoric acid can remove impurities (e.g., an element included in the conductive films  112   a  and  112   b , an element included in the buffer films  113   a  and  113   b , or the like) attached to the surface of the oxide semiconductor film  108 . 
     Note that a recessed portion might be formed in part of the oxide semiconductor film  108  at the step of forming the conductive films  112   a  and  112   b  and the buffer films  113   a  and  113   b  and/or the cleaning step. 
     Through the steps, the transistor  100  is formed. 
     Next, over the oxide semiconductor film  108  and the conductive films  112   a  and  112   b , the insulating films  114  and  116  serving as protective insulating films of the transistor  100  are formed (see  FIG. 10B ). 
     Note that after the insulating film  114  is formed, the insulating film  116  is preferably formed in succession without exposure to the air. After the insulating film  114  is formed, the insulating film  116  is formed in succession by adjusting at least one of the flow rate of a source gas, pressure, a high-frequency power, and a substrate temperature without exposure to the air, whereby the concentration of impurities attributed to the atmospheric component at the interface between the insulating film  114  and the insulating film  116  can be reduced and oxygen in the insulating films  114  and  116  can be moved to the oxide semiconductor film  108 ; accordingly, the amount of oxygen vacancy in the oxide semiconductor film  108  can be reduced. 
     For example, as the insulating film  114 , a silicon oxynitride film can be formed by a PECVD method. In this case, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include dinitrogen monoxide and nitrogen dioxide. An insulating film including nitrogen and having a small number of defects can be formed as the insulating film  114  by a PECVD method under the conditions where the ratio of the oxidizing gas to the deposition gas is higher than 20 times and lower than 100 times, preferably higher than or equal to 40 times and lower than or equal to 80 times and the pressure in a treatment chamber is lower than 100 Pa, preferably lower than or equal to 50 Pa. 
     In this embodiment, a silicon oxynitride film is formed as the insulating film  114  by a PECVD method under the conditions where the substrate  102  is held at a temperature of 220° C., silane at a flow rate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are used as a source gas, the pressure in the treatment chamber is 20 Pa, and a high-frequency power of 100 W at 13.56 MHz (1.6×10 −2  W/cm 2  as the power density) is supplied to parallel-plate electrodes. 
     As the insulating film  116 , a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in a treatment chamber of the PECVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., preferably higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm 2  and less than or equal to 0.5 W/cm 2 , preferably greater than or equal to 0.25 W/cm 2  and less than or equal to 0.35 W/cm 2  is supplied to an electrode provided in the treatment chamber. 
     As the deposition conditions of the insulating film  116 , the high-frequency power having the above power density is supplied to a reaction chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; thus, the oxygen content in the insulating film  116  becomes higher than that in the stoichiometric composition. On the other hand, in the film formed at a substrate temperature within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the film is released by heat treatment in a later step. Thus, it is possible to form an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition and from which part of oxygen is released by heating. 
     Note that the insulating film  114  serves as a protective film for the oxide semiconductor film  108  in the step of forming the insulating film  116 . Therefore, the insulating film  116  can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film  108  is reduced. 
     Note that in the deposition conditions of the insulating film  116 , when the flow rate of the deposition gas containing silicon with respect to the oxidizing gas is increased, the amount of defects in the insulating film  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 which 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. 
     Heat treatment may be performed after the insulating films  114  and  116  are formed. The heat treatment can reduce nitrogen oxide included in the insulating films  114  and  116 . Alternatively, part of oxygen included in the insulating films  114  and  116  can be moved to the oxide semiconductor film  108 , so that the amount of oxygen vacancy included in the oxide semiconductor film  108  can be reduced. 
     The temperature of the heat treatment performed on the insulating films  114  and  116  is typically higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., further preferably higher than or equal to 320° C. and lower than or equal to 370° C. 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). It is preferable that hydrogen, water, and the like not be contained in the nitrogen, the oxygen, the ultra-dry air, or the rare gas. An electric furnace, an RTA apparatus, or the like can be used for the heat treatment. 
     In this embodiment, the heat treatment is performed at 350° C. for 1 hour in an atmosphere of nitrogen and oxygen. 
     Next, the insulating film  118  serving as a protective insulating film for the transistor  100  is formed over the insulating film  116  (see  FIG. 10C ). 
     In the case where the insulating film  118  is formed by a PECVD method, the substrate temperature is preferably set to higher than or equal to 300° C. and lower than or equal to 400° C., more preferably higher than or equal to 320° C. and lower than or equal to 370° C., so that a dense film can be formed. 
     For example, in the case where a silicon nitride film is formed by a PECVD method as the insulating film  118 , a deposition gas containing silicon, nitrogen, and ammonia are preferably used as a source gas. A small amount of ammonia compared to the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species cleave a bond between silicon and hydrogen which are included in a deposition gas containing silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen is few, can be formed. On the other hand, when the amount of ammonia with respect to nitrogen is large, decomposition of a deposition gas containing silicon and decomposition of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Therefore, in the source gas, a flow rate ratio of the nitrogen to the ammonia is set to be greater than or equal to 5 and less than or equal to 50, preferably greater than or equal to 10 and less than or equal to 50. 
     In this embodiment, with the use of a PECVD apparatus, a 50-nm-thick silicon nitride film is formed as the insulating film  118  using silane, nitrogen, and ammonia as a source gas. The flow rate of silane is 50 sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate of ammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and high-frequency power of 1000 W is supplied to parallel-plate electrodes with a 27.12 MHz high-frequency power source. Note that the PECVD apparatus is a parallel-plate PECVD apparatus in which the electrode area is 6000 cm 2 , and the power per unit area (power density) into which the supplied power is converted is 1.7×10 −1  W/cm 2 . 
     Heat treatment may be performed after the formation of the insulating film  118 . The heat treatment is performed typically at a temperature of higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., more preferably higher than or equal to 320° C. and lower than or equal to 370° C. When the heat treatment is performed, the amount of hydrogen and water in the insulating films  114  and  116  is reduced and accordingly the generation of defects in the oxide semiconductor film  108  described above is inhibited. 
     Through the above manufacturing steps, the semiconductor device shown in  FIGS. 1A to 1C  can be manufactured. 
     &lt;Method 2 for Manufacturing Semiconductor Device&gt; 
     Next, a method for manufacturing the transistor which is different from that described in &lt;Method 1 for manufacturing semiconductor device&gt; is described below in detail with reference to  FIGS. 11A to 11D . 
     First, the steps up to the step in  FIG. 10B  are performed and then the buffer film  131  is formed over the insulating film  116  (see  FIG. 11A ). 
     Next, the oxygen  141  is added to the insulating films  114  and  116  and the oxide semiconductor film  108  through the buffer film  131  (see  FIG. 11B ). 
     Note that by the addition of the oxygen  141 , the buffer film  131  becomes the buffer film  131   x  formed of oxide or oxynitride of metal (indium, zinc, titanium, aluminum, tungsten, tantalum, or molybdenum) (see  FIG. 11C ). 
     Next, the insulating film  118  is formed over the buffer film  131   x  (see  FIG. 11D ). 
     Note that the insulating film  118  is formed after the buffer film  131   x  formed in  FIG. 11C  is removed, whereby the transistor  100  illustrated in  FIGS. 1A to 1C  may be formed. In this manufacturing method, the oxygen  141  can be added to the oxide semiconductor film  108  twice, and excess oxygen can be added to the insulating film  114  in contact with the oxide semiconductor film  108 , and/or the insulating film  116  in contact with the insulating film  114 . Therefore, oxygen vacancies in the oxide semiconductor film  108  can be further reduced. 
     &lt;Method 3 for Manufacturing Semiconductor Device&gt; 
     Next, a method for manufacturing the transistor  150  in  FIGS. 3A to 3C  that is a semiconductor device of one embodiment of the present invention is described below in detail with reference to  FIGS. 12A to 12D ,  FIGS. 13A to 13C , and  FIGS. 14A and 14B . 
     First, the steps up to the step in  FIG. 8B  are performed, and then the insulating films  114  and  116  are formed over the insulating film  107  and the oxide semiconductor film  108  (see  FIG. 12A ). 
     Next, a mask is formed over the insulating film  116  through a lithography process, and the openings  141   a  and  141   b  are formed in desired regions of the insulating films  114  and  116 . Note that the openings  141   a  and  141   b  reach the oxide semiconductor film  108  (see  FIG. 12B ). 
     Next, the buffer film  131  serving as a film which suppresses release of oxygen is formed over the oxide semiconductor film  108  and the insulating film  116  to cover the openings  141   a  and  141   b . Then, the oxygen  141  is added to the oxide semiconductor film  108  through the buffer film  131 , and the insulating films  114  and  116  (see  FIG. 12C ). 
     Note that by the addition of the oxygen  141 , the buffer film  131  becomes the buffer film  131   x  formed of oxide or oxynitride of metal (indium, zinc, titanium, aluminum, tungsten, tantalum, or molybdenum) (see  FIG. 12D ). 
     Next, the conductive film  112  is formed over the buffer film  131   x  (see  FIG. 13A ). 
     Next, the masks  140   a  and  140   b  are formed over the conductive film  112  (see  FIG. 13B ). 
     Next, the conductive film  112  and the buffer film  131   x  are etched from above the masks  140   a  and  140   b , using the etching gas  142  (see  FIG. 13C ). 
     Next, the masks  140   a  and  140   b  are removed, whereby the conductive films  112   a  and  112   b  serving as a source electrode and a drain electrode, the buffer film  113   a  under the conductive film  112   a , and the buffer film  113   b  under the conductive film  112   b  are formed (see  FIG. 14A ). 
     The buffer film  131   x  is separated into buffer films  113   a  and  113   b  so that they have island shapes when the conductive films  112   a  and  112   b  are processed. 
     Next, the insulating film  118  is formed over the insulating film  116  and the conductive films  112   a  and  112   b  (see  FIG. 14B ). 
     Through the above process, the transistor  150  illustrated in  FIGS. 3A to 3C  can be manufactured. 
     &lt;Method 4 for Manufacturing Semiconductor Device&gt; 
     Next, a method for manufacturing the transistor  170  that is a semiconductor device of one embodiment of the present invention is described below in detail with reference to  FIGS. 15A to 15D  and  FIGS. 16A to 16D . 
       FIGS. 15A and 15C  and  FIGS. 16A and 16C  are each a cross-sectional view in the channel length direction of the transistor  170  and  FIGS. 15B and 15D  and  FIGS. 16B and 16D  are each a cross-sectional view in the channel width direction of the transistor  170 . 
     First, the steps up to the step in  FIG. 10C  are performed (see  FIGS. 15A  and  15 B). 
     Next, a mask is formed over the insulating film  118  through a lithography process, and the opening  142   c  is formed in a desired region in the insulating films  114 ,  116 , and  118 . In addition, a mask is formed over the insulating film  118  through a lithography process, and the openings  142   a  and  142   b  are formed in desired regions in the insulating films  106 ,  107 ,  114 ,  116 , and  118 . Note that the opening  142   c  reaches the conductive film  112   b . The openings  142   a  and  142   b  reach the conductive film  104  (see  FIGS. 15C and 15D ). 
     Note that the openings  142   a  and  142   b  and the opening  142   c  may be formed in the same step or may be formed by different steps. In the case where the openings  142   a  and  142   b  and the opening  142   c  are formed in the same step, for example, a gray-tone mask or a half-tone mask can be used. 
     Next, a conductive film  120  is formed over the insulating film  118  to cover the openings  142   a ,  142   b , and  142   c  (see  FIGS. 16A and 16B ). 
     For the conductive film  120 , for example, a material including one of indium (In), zinc (Zn), and tin (Sn) can be used. In particular, for the conductive film  120 , a light-transmitting conductive material such as indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added (ITSO) can be used. The conductive film  120  can be formed by a sputtering method, for example. In this embodiment, a 110-nm-thick ITSO film is formed by a sputtering method. 
     Next, a mask is formed over the conductive film  120  through a lithography process, and the conductive film  120  is processed into desired regions to form the conductive films  120   a  and  120   b  (see  FIGS. 16C and 16D ). 
     Through the above process, the transistor  170  illustrated in  FIGS. 5A to 5C  can be manufactured. 
     The structure and method described in this embodiment can be implemented by being combined as appropriate with any of the other structures and methods described in the other embodiments. 
     Embodiment 2 
     In this embodiment, an oxide semiconductor included in a semiconductor device of one embodiment of the present invention is described below in detail. 
     &lt;Oxide Semiconductor Structure&gt; 
     First, a structure of an oxide semiconductor is described below. 
     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. 
     From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS. 
     It is known that an amorphous structure is generally defined as being metastable and unfixed, and being isotropic and having no non-uniform structure. In other words, an amorphous structure has a flexible bond angle and a short-range order but does not have a long-range order. 
     This means that an inherently stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor film that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. Note that an a-like OS has a periodic structure in a microscopic region, but at the same time has a void and has an unstable structure. For this reason, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor. 
     &lt;CAAC-OS&gt; 
     First, a CAAC-OS is described. 
     A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets). 
     In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     The CAAC-OS observed with a TEM is described below.  FIG. 17A  shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be obtained with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd. 
       FIG. 17B  is an enlarged Cs-corrected high-resolution TEM image of a region ( 1 ) in  FIG. 17A .  FIG. 17B  shows that metal atoms are arranged in a layered manner in a pellet. Each metal atom layer has a configuration reflecting unevenness of a surface over which a CAAC-OS film is formed (hereinafter, the surface is referred to as a formation surface) or a top surface of the CAAC-OS, and is arranged parallel to the formation surface or the top surface of the CAAC-OS. 
     As shown in  FIG. 17B , the CAAC-OS has a characteristic atomic arrangement. The characteristic atomic arrangement is denoted by an auxiliary line in  FIG. 17C .  FIGS. 17B and 17C  prove that the size of a pellet is approximately 1 nm to 3 nm, and the size of a space caused by tilt of the pellets is approximately 0.8 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). Furthermore, a CAAC-OS can be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC). 
     Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets  5100  of a CAAC-OS over a substrate  5120  is illustrated by such a structure in which bricks or blocks are stacked (see  FIG. 17D ). The part in which the pellets are tilted as observed in  FIG. 17C  corresponds to a region  5161  shown in  FIG. 17D . 
       FIG. 18A  shows a Cs-corrected high-resolution TEM image of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface.  FIGS. 18B, 18C, and 18D  are enlarged Cs-corrected high-resolution TEM images of regions ( 1 ), ( 2 ), and ( 3 ) in  FIG. 18A , respectively.  FIGS. 18B, 18C, and 18D  indicate that metal atoms are arranged in a triangular, quadrangular, or hexagonal configuration in a pellet. However, there is no regularity of arrangement of metal atoms between different pellets. 
     Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in  FIG. 19A . This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. 
     Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°. 
     On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. In the case of the CAAC-OS, when analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector of the sample surface as an axis (φ axis), as shown in  FIG. 19B , a peak is not clearly observed. In contrast, in the case of a single crystal oxide semiconductor of InGaZnO 4 , when φ scan is performed with 2θ fixed at around 56°, as shown in  FIG. 19C , six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are irregularly oriented in the CAAC-OS. 
     Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO 4  crystal in a direction parallel to the sample surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in  FIG. 20A  can be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO 4  crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile,  FIG. 20B  shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in  FIG. 20B , a ring-like diffraction pattern is observed. Thus, the electron diffraction also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in  FIG. 20B  is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO 4  crystal. The second ring in  FIG. 20B  is considered to be derived from the (110) plane and the like. 
     As described above, the CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies). 
     Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity. 
     The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. Impurities contained in the oxide semiconductor might serve as carrier traps or carrier generation sources, for example. Furthermore, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein. 
     The CAAC-OS having small amounts of impurities and oxygen vacancies is an oxide semiconductor with low carrier density (specifically, 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 is higher than or equal to 1×10 −9 /cm 3 ). Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics. 
     &lt;nc-OS&gt; 
     Next, an nc-OS is described. 
     An nc-OS has a region in which a crystal part is observed and a region in which a crystal part is not clearly observed in a high-resolution TEM image. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, or greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part with a size greater than or equal to 10 nm and less than or equal to 100 nm is referred to as a microcrystalline oxide semiconductor in some cases. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description. 
     In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is subjected to structural analysis by an out-of-plane method using an X-ray having a diameter larger than the size of a pellet, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet. Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is applied. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS layer, a plurality of spots is shown in a ring-like region in some cases. 
     Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC). 
     The nc-OS is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS. 
     &lt;a-like OS&gt; 
     An a-like OS is an oxide semiconductor having a structure intermediate between the nc-OS and the amorphous oxide semiconductor. 
     In a high-resolution TEM image of the a-like OS, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed. 
     The a-like OS has an unstable structure because it contains a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below. 
     An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C) are prepared as samples subjected to electron irradiation. Each of the samples is an In—Ga—Zn oxide. 
     First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts. 
     Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of the InGaZnO 4  crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO 4 . Each of lattice fringes corresponds to the a-b plane of the InGaZnO 4  crystal. 
       FIG. 27  shows change in the average size of crystal parts (at 22 points to 45 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe.  FIG. 27  indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose. Specifically, as shown by ( 1 ) in  FIG. 27 , a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 2.6 nm at a cumulative electron dose of 4.2×10 8  e − /nm 2 . In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10 8  e − /nm 2 . Specifically, as shown by ( 2 ) and ( 3 ) in  FIG. 27 , the average crystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively, regardless of the cumulative electron dose. 
     In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS. 
     The a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor. 
     For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO 4  with a rhombohedral crystal structure is 6.357 g/cm 3 . Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm 3  and lower than 5.9 g/cm 3 . For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm 3  and lower than 6.3 g/cm 3 . 
     Note that single crystals with the same composition do not exist in some cases. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density. 
     As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example. 
     The structure and method described in this embodiment can be implemented by being combined as appropriate with any of the other structures and methods described in the other embodiments. 
     Embodiment 3 
     In this embodiment, an example of a display device that includes any of the transistors described in the embodiment above is described below with reference to  FIGS. 21 to 23 . 
       FIG. 21  is a top view of an example of a display device. A display device  700  illustrated in  FIG. 21  includes a pixel portion  702  provided over a first substrate  701 ; a source driver circuit portion  704  and a gate driver circuit portion  706  provided over the first substrate  701 ; a sealant  712  provided to surround the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706 ; and a second substrate  705  provided to face the first substrate  701 . The first substrate  701  and the second substrate  705  are sealed with the sealant  712 . That is, the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  are sealed with the first substrate  701 , the sealant  712 , and the second substrate  705 . Although not illustrated in  FIG. 21 , a display element is provided between the first substrate  701  and the second substrate  705 . 
     In the display device  700 , a flexible printed circuit (FPC) terminal portion  708  electrically connected to the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  is provided in a region different from the region which is surrounded by the sealant  712  and positioned over the first substrate  701 . Furthermore, an FPC  716  is connected to the FPC terminal portion  708 , and a variety of signals and the like are supplied to the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  through the FPC  716 . Furthermore, a signal line  710  is connected to the pixel portion  702 , the source driver circuit portion  704 , the gate driver circuit portion  706 , and the FPC terminal portion  708 . Various signals and the like are applied to the pixel portion  702 , the source driver circuit portion  704 , the gate driver circuit portion  706 , and the FPC terminal portion  708  via the signal line  710  from the FPC  716 . 
     A plurality of gate driver circuit portions  706  may be provided in the display device  700 . An example of the display device  700  in which the source driver circuit portion  704  and the gate driver circuit portion  706  are formed over the first substrate  701  where the pixel portion  702  is also formed is described; however, the structure is not limited thereto. For example, only the gate driver circuit portion  706  may be formed over the first substrate  701  or only the source driver circuit portion  704  may be formed over the first substrate  701 . In this case, a substrate where a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver-circuit substrate formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be mounted on the first substrate  701 . Note that there is no particular limitation on the method of connecting a separately prepared driver circuit substrate, and a chip on glass (COG) method, a wire bonding method, or the like can be used. 
     The pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portion  706  included in the display device  700  include a plurality of transistors. As the plurality of transistors, any of the transistors that are the semiconductor devices of embodiments of the present invention can be used. 
     The display device  700  can include any of a variety of elements. Examples of the element include a liquid crystal element, an electroluminescence (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, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator (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. In addition, a display device including display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by electrical or magnetic effect may be included. Note that examples of display devices having EL elements include an EL display. Examples of display devices including electron emitters are a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter 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). An example of a display device including electronic ink or electrophoretic elements is electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of 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. 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. 
     As a display method in the display device  700 , a progressive method, an interlace method, or the like can be employed. Further, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, a display unit may be composed of four pixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel. Alternatively, a display unit may be composed of two of color elements among R, G, and B as in PenTile layout. The two colors may differ among display units. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Further, the size of a display region may be different depending on respective dots of the color components. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display. 
     A coloring layer (also referred to as a color filter) may be used in order to obtain a full-color display device in which white light (W) for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp) is used. As the coloring layer, red (R), green (G), blue (B), yellow (Y), or the like may be combined as appropriate, for example. With the use of the coloring layer, higher color reproductivity can be obtained than in the case without the coloring layer. In this case, by providing a region with the coloring layer and a region without the coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without the coloring layer, a decrease in luminance due to the coloring layer can be suppressed, and 20% to 30% of power consumption can be reduced in some cases when an image is displayed brightly. Note that in the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light of their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption can be further reduced as compared to the case of using the coloring layer in some cases. 
     In this embodiment, a structure including a liquid crystal element and an EL element as display elements is described with reference to  FIG. 22  and  FIG. 23 . Note that  FIG. 22  is a cross-sectional view along dashed-dotted line Q-R shown in  FIG. 21 , and shows a structure including a liquid crystal element as a display element, whereas  FIG. 23  is a cross-sectional view along the dashed-dotted line Q-R shown in  FIG. 21  and shows a structure including an EL element as a display element. 
     Common portions between  FIG. 22  and  FIG. 23  are described first, and then different portions are described. 
     &lt;Common Portions in Display Devices&gt; 
     The display device  700  illustrated in  FIG. 22  and  FIG. 23  include a lead wiring portion  711 , the pixel portion  702 , the source driver circuit portion  704 , and the FPC terminal portion  708 . Note that the lead wiring portion  711  includes a signal line  710 . The pixel portion  702  includes a transistor  750  and a capacitor  790 . The source driver circuit portion  704  includes a transistor  752 . 
     Any of the transistors described above can be used as the transistors  750  and  752 . 
     The transistors used in this embodiment each include an oxide semiconductor film which is highly purified and in which formation of oxygen vacancies is suppressed. In the transistor, the current in an off state (off-state current) can be made small. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption. 
     In addition, the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high speed operation. For example, with such a transistor which can operate at high speed used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced. In addition, the transistor which can operate at high speed can be used also in the pixel portion, whereby a high-quality image can be provided. 
     The capacitor  790  includes a dielectric between a pair of electrodes. Specifically, a conductive film which is formed using the same step as a conductive film serving as a gate electrode of the transistor  750  is used as one electrode of the capacitor  790 , and a conductive film serving as a source electrode or a drain electrode of the transistor  750  is used as the other electrode of the capacitor  790 . Furthermore, an insulating film serving as a gate insulating film of the transistor  750  is used as the dielectric between the pair of electrodes. 
     In  FIG. 22  and  FIG. 23 , insulating films  764 ,  766 , and  768  and a planarization insulating film  770  are formed over the transistor  750 , the transistor  752 , and the capacitor  790 . 
     The insulating films  764 ,  766 , and  768  can be formed using materials and methods similar to those of the insulating films  114 ,  116 , and  118  described in the above embodiment, respectively. The planarization insulating film  770  can be formed using a heat-resistant organic material, such as a polyimide resin, an acrylic resin, a polyimide amide resin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin. Note that the planarization insulating film  770  may be formed by stacking a plurality of insulating films formed using these materials. Alternatively, a structure without the planarization insulating film  770  may be employed. 
     The signal line  710  is formed in the same process as conductive films serving as a source electrode and a drain electrode of the transistor  750  or  752 . Note that the signal line  710  may be formed using a conductive film which is formed in a different process as a source electrode and a drain electrode of the transistor  750  or  752 , e.g., a conductive film serving as a gate electrode may be used. In the case where the signal line  710  is formed using a material containing a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen. 
     The FPC terminal portion  708  includes a connection electrode  760 , an anisotropic conductive film  780 , and the FPC  716 . Note that the connection electrode  760  is formed in the same process as conductive films serving as a source electrode and a drain electrode of the transistor  750  or  752 . The connection electrode  760  is electrically connected with a terminal included in the FPC  716  through the anisotropic conductive film  780 . 
     For example, a glass substrate can be used as the first substrate  701  and the second substrate  705 . As the first substrate  701  and the second substrate  705 , a flexible substrate may be used. Examples of the flexible substrate include a plastic substrate. 
     A structure  778  is provided between the first substrate  701  and the second substrate  705 . The structure  778  is a columnar spacer obtained by selective etching of an insulating film and is provided to control the thickness (cell gap) between the first substrate  701  and the second substrate  705 . Alternatively, a spherical spacer may be used as the structure  778 . Although the structure in which the structure body  778  is provided on the first substrate  701  side is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which the structure body  778  is provided on the second substrate  705  side, or a structure in which both of the first substrate  701  and the second substrate  705  are provided with the structure body  778  may be employed. 
     Furthermore, a light-blocking film  738  serving as a black matrix, a coloring film  736  serving as a color filter, and an insulating film  734  in contact with the light-blocking film  738  and the coloring film  736  are provided on the second substrate  705  side. 
     &lt;Structure Example of Display Device Using Liquid Crystal Element as Display Element&gt; 
     The display device  700  illustrated in  FIG. 22  includes a liquid crystal element  775 . The liquid crystal element  775  includes the conductive film  772 , a conductive film  774 , and a liquid crystal layer  776 . The conductive film  774  is provided on the second substrate  705  side and serves as a counter electrode. The display device  700  in  FIG. 22  is capable of displaying an image in such a manner that transmission or non-transmission is controlled by change in the alignment state of the liquid crystal layer  776  depending on a voltage applied to the conductive film  772  and the conductive film  774 . 
     The conductive film  772  is connected to the conductive films serving as a source electrode and a drain electrode included in the transistor  750 . The conductive film  772  is formed over the planarization insulating film  770  to function as a pixel electrode, i.e., one electrode of the display element. The conductive film  772  has a function of a reflective electrode. The display device  700  in  FIG. 22  is what is called a reflective color liquid crystal display device in which external light is reflected by the conductive film  772  to display an image through the coloring film  736 . 
     A conductive film that transmits visible light or a conductive film that reflects visible light can be used for the conductive film  772 . For example, a material including one kind selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material including aluminum or silver may be used for the conductive film that reflects visible light. In this embodiment, the conductive film that reflects visible light is used for the conductive film  772 . 
     In the case where a conductive film which reflects visible light is used as the conductive film  772 , the conductive film may have a stacked-layer structure. For example, a 100-nm-thick aluminum film is formed as the bottom layer, and a 30-nm-thick silver alloy film (e.g., an alloy film including silver, palladium, and copper) is formed as the top layer. Such a structure makes it possible to obtain the following effects. 
     (1) Adhesion between the base film and the conductive film  772  can be improved. 
     (2) The aluminum film and the silver alloy film can be collectively etched depending on a chemical solution. 
     (3) The conductive film  772  can have a favorable cross-sectional shape (e.g., a tapered shape). 
     The reason for (3) is as follows: the etching rate of the aluminum film with the chemical solution is lower than that of the silver alloy film, or etching of the aluminum film that is the bottom layer is developed faster than that of the silver alloy film because when the aluminum film that is the bottom layer is exposed after the etching of the silver alloy film that is the top layer, electrons are extracted from metal that is less noble than the silver alloy film, i.e., aluminum that is metal having a high ionization tendency, and thus etching of the silver alloy film is suppressed. 
     Note that projections and depressions are provided in part of the planarization insulating film  770  of the pixel portion  702  in the display device  700  in  FIG. 22 . The projections and depressions can be formed in such a manner that the planarization insulating film  770  is formed using an organic resin film or the like, and projections and depressions are formed on the surface of the organic resin film. The conductive film  772  reflecting as a reflective electrode is formed along the projections and depressions. Therefore, when external light is incident on the conductive film  772 , the light is reflected diffusely at the surface of the conductive film  772 , whereby visibility can be improved. 
     Note that the display device  700  illustrated in  FIG. 22  is a reflective color liquid crystal display device given as an example, but a display type is not limited thereto. For example, a transmissive color liquid crystal display device in which the conductive film  772  is a conductive film that transmits visible light may be used. In the case of a transmissive color liquid crystal display device, projections and depressions are not necessarily provided on the planarization insulating film  770 . 
     Although not illustrated in  FIG. 22 , an alignment film may be provided on a side of the conductive film  772  in contact with the liquid crystal layer  776  and on a side of the conductive film  774  in contact with the liquid crystal layer  776 . Although not illustrated in  FIG. 22 , an optical member (an optical substrate) and the like such as a polarizing member, a retardation member, or an anti-reflection member may be provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source. 
     In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has a short response time and has optical isotropy. In addition, the liquid crystal composition which includes liquid crystal exhibiting a blue phase does not need alignment treatment and has a small viewing angle dependence. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. 
     In the case where a liquid crystal element is used as the display element, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     Further, a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, or the like can be employed. 
     &lt;Display Device Using Light-Emitting Element as Display Element&gt; 
     The display device  700  illustrated in  FIG. 23  includes a light-emitting element  782 . The light-emitting element  782  includes a conductive film  784 , an EL layer  786 , and a conductive film  788 . The display device  700  shown in  FIG. 23  is capable of displaying an image by light emission from the EL layer  786  included in the light-emitting element  782 . 
     The conductive film  784  is connected to the conductive films serving as a source electrode and a drain electrode included in the transistor  750 . The conductive film  784  is formed over the planarization insulating film  770  to function as a pixel electrode, i.e., one electrode of the display element. A conductive film which transmits visible light or a conductive film which reflects visible light can be used for the conductive film  784 . For example, a material including one kind selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material including aluminum or silver may be used for the conductive film that reflects visible light. 
     In the display device  700  shown in  FIG. 23 , an insulating film  730  is provided over the planarization insulating film  770  and the conductive film  784 . The insulating film  730  covers part of the conductive film  784 . Note that the light-emitting element  782  has a top emission structure. Therefore, the conductive film  788  has a light-transmitting property and transmits light emitted from the EL layer  786 . Although the top emission structure is described as an example in this embodiment, the structure is not limited thereto. A bottom-emission structure in which light is emitted to the conductive film  784  side, or a dual-emission structure in which light is emitted to both the conductive film  784  side and the conductive film  788  side may be employed. 
     The coloring film  736  is provided to overlap with the light-emitting element  782 , and the light-blocking film  738  is provided to overlap with the insulating film  730  and to be included in the lead wiring portion  711  and in the source driver circuit portion  704 . The coloring film  736  and the light-blocking film  738  are covered with the insulating film  734 . A space between the light-emitting element  782  and the insulating film  734  is filled with the sealing film  732 . Although a structure with the coloring film  736  is described as the display device  700  shown in  FIG. 23 , the structure is not limited thereto. In the case where the EL layer  786  is formed by a separate coloring method, the coloring film  736  is not necessarily provided. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments. 
     Embodiment 4 
     In this embodiment, a display device that includes a semiconductor device of one embodiment of the present invention is described with reference to  FIGS. 24A to 24C . 
     The display device illustrated in  FIG. 24A  includes a region including pixels of display elements (hereinafter the region is referred to as a pixel portion  502 ), a circuit portion being provided outside the pixel portion  502  and including a circuit for driving the pixels (hereinafter the portion is referred to as a driver circuit portion  504 ), circuits each having a function of protecting an element (hereinafter the circuits are referred to as protection circuits  506 ), and a terminal portion  507 . Note that the protection circuits  506  are not necessarily provided. 
     A part or the whole of the driver circuit portion  504  is preferably formed over a substrate over which the pixel portion  502  is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of the driver circuit portion  504  is not formed over the substrate over which the pixel portion  502  is formed, the part or the whole of the driver circuit portion  504  can be mounted by COG or tape automated bonding (TAB). 
     The pixel portion  502  includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (hereinafter, such circuits are referred to as pixel circuits  501 ). The driver circuit portion  504  includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver  504   a ) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver  504   b ). 
     The gate driver  504   a  includes a shift register or the like. The gate driver  504   a  receives a signal for driving the shift register through the terminal portion  507  and outputs a signal. For example, the gate driver  504   a  receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver  504   a  has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as scan lines GL_ 1  to GL_X). Note that a plurality of gate drivers  504   a  may be provided to control the scan lines GL_ 1  to GL_X separately. Alternatively, the gate driver  504   a  has a function of supplying an initialization signal. Without being limited thereto, the gate driver  504   a  can supply another signal. 
     The source driver  504   b  includes a shift register or the like. The source driver  504   b  receives a signal (video signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion  507 . The source driver  504   b  has a function of generating a data signal to be written to the pixel circuit  501  which is based on the video signal. In addition, the source driver  504   b  has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Further, the source driver  504   b  has a function of controlling the potentials of wirings supplied with data signals (hereinafter such wirings are referred to as data lines DL_ 1  to DL_Y). Alternatively, the source driver  504   b  has a function of supplying an initialization signal. Without being limited thereto, the source driver  504   b  can supply another signal. 
     The source driver  504   b  includes a plurality of analog switches or the like, for example. The source driver  504   b  can output, as the data signals, signals obtained by time-dividing the video signal by sequentially turning on the plurality of analog switches. The source driver  504   b  may include a shift register or the like. 
     A pulse signal and a data signal are input to each of the plurality of pixel circuits  501  through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits  501  are controlled by the gate driver  504   a . For example, to the pixel circuit  501  in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the gate driver  504   a  through the scan line GL_m, and a data signal is input from the source driver  504   b  through the data line DL_n in accordance with the potential of the scan line GL_m. 
     The protection circuit  506  shown in  FIG. 24A  is connected to, for example, the scan line GL between the gate driver  504   a  and the pixel circuit  501 . Alternatively, the protection circuit  506  is connected to the data line DL between the source driver  504   b  and the pixel circuit  501 . Alternatively, the protection circuit  506  can be connected to a wiring between the gate driver  504   a  and the terminal portion  507 . Alternatively, the protection circuit  506  can be connected to a wiring between the source driver  504   b  and the terminal portion  507 . Note that the terminal portion  507  means a portion having terminals for inputting power, control signals, and video signals to the display device from external circuits. 
     The protection circuit  506  is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit. 
     As illustrated in  FIG. 24A , the protection circuits  506  are provided for the pixel portion  502  and the driver circuit portion  504 , so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits  506  is not limited to that, and for example, the protection circuit  506  may be configured to be connected to the gate driver  504   a  or the protection circuit  506  may be configured to be connected to the source driver  504   b . Alternatively, the protection circuit  506  may be configured to be connected to the terminal portion  507 . 
     In the non-limiting example illustrated in  FIG. 24A , the driver circuit portion  504  includes the gate driver  504   a  and the source driver  504   b . For example, only the gate driver  504   a  may be formed and a separately prepared substrate where a source driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted. 
     Each of the plurality of pixel circuits  501  in  FIG. 24A  can have the structure illustrated in  FIG. 24B , for example. 
     The pixel circuit  501  illustrated in  FIG. 24B  includes the liquid crystal element  570 , the transistor  550 , and the capacitor  560 . As the transistor  550 , any of the transistors described in the above embodiment, for example, can be used. 
     The potential of one of a pair of electrodes of the liquid crystal element  570  is set in accordance with the specifications of the pixel circuit  501  as appropriate. The alignment state of the liquid crystal element  570  depends on written data. A common potential may be supplied to one of the pair of electrodes of the liquid crystal element  570  included in each of the plurality of pixel circuits  501 . Further, the potential supplied to one of the pair of electrodes of the liquid crystal element  570  in the pixel circuit  501  in one row may be different from the potential supplied to one of the pair of electrodes of the liquid crystal element  570  in the pixel circuit  501  in another row. 
     As examples of a driving method of the display device including the liquid crystal element  570 , any of the following modes can be given: 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, a transverse bend alignment (TBA) mode, and the like. Other examples of the driving method of the display device include ECB (electrically controlled birefringence) mode, PDLC (polymer dispersed liquid crystal) mode, PNLC (polymer network liquid crystal) mode, and a guest-host mode. Note that the present invention is not limited to this, and various liquid crystal elements and driving methods can be used as a liquid crystal element and a driving method thereof. 
     In the pixel circuit  501  in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor  550  is electrically connected with the data line DL_n, and the other is electrically connected with the other of the pair of electrodes of the liquid crystal element  570 . A gate electrode of the transistor  550  is electrically connected with the scan line GL_m. The transistor  550  has a function of controlling whether to write a data signal by being turned on or off. 
     One of a pair of electrodes of the capacitor  560  is electrically connected with a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other is electrically connected with the other of the pair of electrodes of the liquid crystal element  570 . The potential of the potential supply line VL is set in accordance with the specifications of the pixel circuit  501  as appropriate. The capacitor  560  serves as a storage capacitor for storing written data. 
     For example, in the display device including the pixel circuit  501  in  FIG. 24B , the pixel circuits  501  are sequentially selected row by row by the gate driver  504   a  illustrated in  FIG. 24A , whereby the transistors  550  are turned on and a data signal is written. 
     When the transistors  550  are turned off, the pixel circuits  501  in which the data has been written are brought into a holding state. This operation is sequentially performed row by row; thus, an image is displayed. 
     Each of the plurality of pixel circuits  501  in  FIG. 24A  can have the structure shown in  FIG. 24C , for example. 
     The pixel circuit  501  illustrated in  FIG. 24C  includes transistors  552  and  554 , a capacitor  562 , and a light-emitting element  572 . Any of the transistors described in the above embodiment, for example, can be used as one or both of the transistors  552  and  554 . 
     One of a source electrode and a drain electrode of the transistor  552  is electrically connected with a wiring to which a data signal is supplied (hereinafter referred to as a signal line DL_n). A gate electrode of the transistor  552  is electrically connected with a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m). 
     The transistor  552  has a function of controlling whether to write a data signal by being turned on or off. 
     One of a pair of electrodes of the capacitor  562  is electrically connected with a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected with the other of the source electrode and the drain electrode of the transistor  552 . 
     The capacitor  562  serves as a storage capacitor for storing written data. 
     One of a source electrode and a drain electrode of the transistor  554  is electrically connected with the potential supply line VL_a. Further, a gate electrode of the transistor  554  is electrically connected with the other of the source electrode and the drain electrode of the transistor  552 . 
     One of an anode and a cathode of the light-emitting element  572  is electrically connected with a potential supply line VL_b, and the other is electrically connected with the other of the source electrode and the drain electrode of the transistor  554 . 
     As the light-emitting element  572 , 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  572  is not limited to an organic EL element; an inorganic EL element including an inorganic material may be used. 
     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  501  in  FIG. 24C , the pixel circuits  501  are sequentially selected row by row by the gate driver  504   a  illustrated in  FIG. 24A , whereby the transistors  552  are turned on and a data signal is written. 
     When the transistors  552  are turned off, the pixel circuits  501  in which the data has been written are brought into a holding state. Further, the amount of current flowing between the source electrode and the drain electrode of the transistor  554  is controlled in accordance with the potential of the written data signal. The light-emitting element  572  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. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments. 
     Embodiment 5 
     In this embodiment, a display module and electronic appliances that include a semiconductor device of one embodiment of the present invention are described with reference to  FIG. 25  and  FIGS. 26A to 26H . 
     In a display module  8000  illustrated in  FIG. 25 , a touch panel  8004  connected to an FPC  8003 , a display panel  8006  connected to an FPC  8005 , a backlight unit  8007 , a frame  8009 , a printed board  8010 , and a battery  8011  are provided between an upper cover  8001  and a lower cover  8002 . 
     The semiconductor device of one embodiment of the present invention can be used for, for example, the display panel  8006 . 
     The shapes and sizes of the upper cover  8001  and the lower cover  8002  can be changed as appropriate in accordance with the sizes of the touch panel  8004  and the display panel  8006 . 
     The touch panel  8004  can be a resistive touch panel or a capacitive touch panel and may be formed so as to overlap with the display panel  8006 . A counter substrate (sealing substrate) of the display panel  8006  can have a touch panel function. A photosensor may be provided in each pixel of the display panel  8006  so that the touch panel  8004  can function as an optical touch panel. 
     The backlight  8007  includes a light source  8008 . Note that although a structure in which the light sources  8008  are provided over the backlight  8007  is illustrated in  FIG. 25 , one embodiment of the present invention is not limited to this structure. For example, a structure in which the light source  8008  is provided at an end portion of the backlight  8007  and a light diffusion plate is further provided may be employed. Note that the backlight  8007  need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed. 
     The frame  8009  protects the display panel  8006  and serves as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board  8010 . The frame  8009  can serve as a radiator plate. 
     The printed board  8010  is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery  8011  provided separately may be used. The battery  8011  can be omitted in the case of using a commercial power source. 
     The display module  8000  may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet. 
       FIGS. 26A to 26H  illustrate electronic appliances. These electronic appliances can include a housing  9000 , a display portion  9001 , a speaker  9003 , an LED lamp  9004 , operation keys  9005  (including a power switch or an operation switch), a connection terminal  9006 , a sensor  9007  (a sensor having a function of measuring or sensing 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. 
       FIG. 26A  illustrates a mobile computer that can include a switch  9009 , an infrared port  9010 , and the like in addition to the above components.  FIG. 26B  illustrates a portable image reproducing device (e.g., a DVD player) that is provided with a memory medium and can include a second display portion  9002 , a memory medium reading portion  9011 , and the like in addition to the above components.  FIG. 26C  illustrates a goggle-type display that can include the second display portion  9002 , a support  9012 , an earphone  9013 , and the like in addition to the above components.  FIG. 26D  illustrates a portable game machine that can include the memory medium reading portion  9011  and the like in addition to the above components.  FIG. 26E  illustrates a digital camera that has a television reception function and can include an antenna  9014 , a shutter button  9015 , an image receiving portion  9016 , and the like in addition to the above components.  FIG. 26F  illustrates a portable game machine that can include the second display portion  9002 , the memory medium reading portion  9011 , and the like in addition to the above components.  FIG. 26G  illustrates a television receiver, which can include a tuner, an image processing portion, and the like in addition to the above objects.  FIG. 26H  illustrates a portable television receiver that can include a charger  9017  capable of transmitting and receiving signals, and the like in addition to the above components. 
     The electronic appliances illustrated in  FIGS. 26A to 26H  can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling a process 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, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Furthermore, the electronic appliance including a plurality of display portions can have a function of displaying image data mainly on one display portion while displaying text data on another display portion, a function of displaying a three-dimensional image by displaying images on a plurality of display portions with a parallax taken into account, or the like. Furthermore, the electronic appliance including an image receiving portion can have a function of shooting a still image, a function of taking a moving image, a function of automatically or manually correcting a shot image, a function of storing a shot image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like. Note that functions that can be provided for the electronic appliances illustrated in  FIGS. 26A to 26H  are not limited to those described above, and the electronic appliances can have a variety of functions. 
     Electronic devices described in this embodiment are characterized by having a display portion for displaying some sort of information. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic device which does not have a display portion. 
     The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments. 
     This application is based on Japanese Patent Application serial no. 2014-056774 filed with the Japan Patent Office on Mar. 19, 2014, the entire contents of which are hereby incorporated by reference.