Patent Publication Number: US-9893192-B2

Title: Semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to semiconductor devices. 
     In this specification and the like, the term “semiconductor device” means any kind of device that can function by utilizing semiconductor characteristics, and a transistor, a semiconductor circuit, an arithmetic device, a memory device, an imaging device, an electro-optical device, a power generation device (e.g., a thin film solar cell and an organic thin film solar cell), an electronic appliance, and the like are each one embodiment of semiconductor devices. 
     2. Description of the Related Art 
     A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic appliances such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). As semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials. 
     For example, a technique by which a transistor is formed using zinc oxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor is disclosed (see Patent Documents 1 and 2). 
     In recent years, demand for integrated circuits in which semiconductor elements such as miniaturized transistors are integrated with high density has risen with increased performance and reductions in the size and weight of electronic appliances. 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2007-123861 
     [Patent Document 2] Japanese Published Patent Application No. 2007-096055 
     SUMMARY OF THE INVENTION 
     The accuracy of processing a thin film included in a semiconductor device into a desired shape influences on electric characteristics of the semiconductor device. For example, in a transistor, the distance between a source and a drain greatly influences on electric characteristics of the transistor; thus highly accurate processing is needed. As the shape of a thin film to be processed is minuter, it is more difficult to process the film with high accuracy. 
     An object of one embodiment of the present invention is to provide a semiconductor device that is suitable for miniaturization. 
     Another object of one embodiment of the present invention is to provide a highly reliable semiconductor device. Another object of one embodiment of the present invention is to give favorable electrical characteristics to a semiconductor device including an oxide semiconductor. 
     Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a semiconductor device including: an island-shaped semiconductor layer over an insulating surface; a pair of electrodes in contact with a side surface of the semiconductor layer and overlapping with a part of a top surface of the semiconductor layer; an oxide layer located between the semiconductor layer and the electrode and in contact with a part of the top surface of the semiconductor layer and a part of a bottom surface of the electrode; a gate electrode overlapping with the semiconductor layer; and a gate insulating layer between the semiconductor layer and the gate electrode. In addition, the semiconductor layer includes an oxide semiconductor, and the pair of electrodes includes Al, Cr, Cu, Ta, Ti, Mo, or W. 
     In addition, preferably, the semiconductor layer and the oxide layer each include an In-M-Zn oxide (M is an element of Al, Ti, Ga, Y, Zr, La, Ce, Nd, and Hf), and the proportion of the element M to In included in the oxide layer is higher than the proportion of the element M to In included in the semiconductor layer. 
     Alternatively, in the oxide layer, the content of the element M is preferably three or more times as large as the content of In. 
     Alternatively, the oxide layer preferably includes any of gallium oxide, Ga—Zn oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and hafnium oxide. 
     The semiconductor layer preferably includes a crystal part. 
     Note that in this specification and the like, the content of a specified element contained in a compound is expressed by an atomic ratio unless otherwise described. The atomic ratio has a margin of error of ±20%. 
     According to one embodiment of the present invention, a semiconductor device that is suitable for miniaturization can be provided. A highly reliable semiconductor device can also be provided. Furthermore, a semiconductor device formed using an oxide semiconductor can have favorable electrical characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  illustrate a semiconductor device according to one embodiment of the present invention. 
         FIG. 2  illustrates a semiconductor device according to one embodiment of the present invention. 
         FIG. 3  illustrates a semiconductor device according to one embodiment of the present invention. 
         FIGS. 4A to 4C  each illustrate a semiconductor device according to one embodiment of the present invention. 
         FIGS. 5A to 5E  illustrate a method for manufacturing of a semiconductor device according to one embodiment of the present invention. 
         FIGS. 6A to 6C  each illustrate a semiconductor device according to one embodiment of the present invention. 
         FIGS. 7A and 7B  are a cross-sectional view and a circuit diagram, respectively, of a semiconductor device according to one embodiment of the present invention. 
         FIG. 8  is a circuit diagram of a semiconductor device according to one embodiment of the present invention. 
         FIG. 9  is a block diagram of a semiconductor device according to one embodiment of the present invention. 
         FIG. 10  is a circuit diagram illustrating a memory device according to one embodiment of the present invention. 
         FIGS. 11A to 11C  each illustrate an electronic appliance according to one embodiment of the present invention. 
         FIGS. 12A and 12B  are graphs showing sheet resistance measured in Example. 
         FIGS. 13A and 13B  are graphs showing sheet resistance measured in Example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below. 
     Note that in the structures of the present invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description of such portions is not repeated. In addition, the same hatching pattern is applied to portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     Note that in each drawing described in this specification, the size, the layer thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. 
     Note that in this specification and the like, ordinal numbers such as “first”, “second”, and the like are used in order to avoid confusion among components and do not limit the number. 
     A transistor is a kind of semiconductor elements and can achieve amplification of current or voltage, switching operation for controlling conduction or non-conduction, or the like. A transistor in this specification includes an insulated-gate field effect transistor (IGFET) and a thin film transistor (TFT). 
     Embodiment 1 
     In this embodiment, a structure and a manufacturing method of a transistor, which is an example of a semiconductor device of one embodiment of the present invention, are described with reference to drawings. 
     Structural Example 
       FIG. 1A  is a schematic top view of a transistor  100  described in this structural example.  FIGS. 1B and 1C  are schematic cross-sectional views taken along the section lines A-B and C-D in  FIG. 1A . Some components are not illustrated in  FIG. 1A  for clarity. 
     The transistor  100  is formed over a substrate  101 . The transistor  100  includes an island-shaped semiconductor layer  102 ; a pair of electrodes  104  in contact with the side surfaces of the semiconductor layer  102  and overlapping with a part of the top surface of the semiconductor layer  102 ; a pair of oxide layers  103  that is located between the semiconductor layer  102  and the electrodes  104  and is in contact with parts of the top surfaces of the semiconductor layer  102  and parts of the bottom surfaces of the electrode  104 ; a gate electrode  106  overlapping with the semiconductor layer  102 ; and a gate insulating layer  105  between the semiconductor layer  102  and the gate electrode  106 . 
     In addition, the semiconductor layer  102  includes a low-resistant region  111  around the side surface thereof that is in contact with the pair of electrodes  104 . Note that the boundary line between the low-resistant region  111  included in the semiconductor layer  102  and another region is not clear in some cases and thus the boundary line is illustrated with a broken line. 
     In addition, an insulating layer  107  is formed over the substrate  101 , and the semiconductor layer  102  is formed over the insulating layer  107 . An insulating layer  108  is formed to cover the gate insulating layer  105  and the gate electrode  106 . 
     One of the pair of electrodes  104  serves as a source electrode of the transistor  100 , and the other of the electrodes  104  serves as a drain electrode. 
     The semiconductor layer  102  contains an oxide semiconductor. The semiconductor layer  102  preferably contains at least indium (In) or zinc (Zn). Alternatively, both In and Zn are preferably contained. Preferably, the semiconductor layer  102  contains an oxide that is represented as an In-M-Zn oxide (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf). 
     The oxide layer  103  includes at least a metal oxide or a semiconductor oxide. Preferably, the oxide layer  103  includes an oxide that is represented as an In-M-Zn oxide (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf). Alternatively, the oxide layer  103  includes a gallium oxide or a Ga—Zn oxide. 
     For the oxide layer  103 , a material that hardly transmits oxygen or a material from which oxygen is hardly released can be used. The material used for the oxide layer  103  preferably exhibits semiconductor characteristics, but may be an insulator, for example, a material including silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, or hafnium oxide. 
     When an In-M-Zn oxide is used for the oxide layer  103 , the In-M-Zn oxide is preferably a material that contains M more than In. For example, an oxide that contains M three times, preferably five times, or further preferably eight times as much as In is preferably used. 
     Furthermore, when In-M-Zn oxides are used for both the semiconductor layer  102  and the oxide layer  103 , a material used for the oxide layer  103  preferably contains M more than M contained in a material used for the semiconductor layer  102 . For example, an oxide layer used for the oxide layer  103  contains M one and a half times, preferably two times, or further preferably three times as much as M contained in an oxide layer used for the semiconductor layer  102 . 
     The electrode  104  includes a conductive material easily bonded to oxygen at least at a portion in contact with the semiconductor layer  102 . Examples of the conductive material are Al, Cr, Cu, Ta, Ti, Mo, W, and the like. Alternatively, nitride of any of these conductive materials may be used. The use of W or Ti with relatively high melting point can raise the upper limit of the temperature in the manufacturing process of the transistor  100 , which is preferable. Note that the conductive material which is easily bonded to oxygen includes, in its category, a material to which oxygen is easily diffused. 
     When such a conductive material and an oxide semiconductor are in contact with each other, oxygen in the oxide semiconductor is partly diffused into the conductive material. Furthermore, when the conductive material and the oxide semiconductor in contact with each other are heated, oxygen is further diffused into the conductive material. Due to such movement of oxygen, oxygen vacancies are generated in a region in the vicinity of the interface between the semiconductor layer  102  and the electrodes  104 , so that such regions have n-type conductivity (the low-resistant regions  111  are formed). The low-resistant regions  111  can serve as a source and a drain of the transistor  100 . 
       FIG. 2  is an enlarged schematic view of a region indicated by the broken line in  FIG. 1B . In the top surface of the semiconductor layer  102 , the oxide layer  103  is formed between the semiconductor layer  102  and the electrode  104 , so that the semiconductor layer  102  is not in a direct contact with the electrode  104 . Therefore, as shown by an arrow in  FIG. 2 , the low-resistant region  111  formed in the semiconductor layer  102  is formed toward the inside of the semiconductor layer  102  in contact with the electrode  104  from the side surface of the semiconductor layer  102 . 
     Here, when an In-M-Zn based oxide containing a large amount of the element M is used for the oxide layer  103 , the M contained is strongly bonded to oxygen more than In is, and thus oxygen movement hardly occurs even when the oxide layer  103  is heated while being in contact with the electrode  104 . In addition, when a gallium oxide, a Ga—Zn oxide, or the like is used, gallium also has the same property. In other words, oxygen movement from the semiconductor layer  102  to the electrode  104  through the oxide layer  103  can be suppressed. 
     In addition, a layer containing at least one kind of the constituent elements of the semiconductor layer  102  is used for the oxide layer  103  formed in contact with the top surface of the semiconductor layer  102 ; therefore, an interface state is less likely to be produced between the oxide layer  103  and the semiconductor layer  102 . As a result, variation or fluctuation of electric characteristics such as a threshold voltage of the transistor can be reduced; thus, a highly reliable transistor can be formed. 
     The distance where the low-resistant region  111  extends toward the inside of the semiconductor layer  102  from the side surface of the semiconductor layer  102  (in other words, the width of the low-resistant region  111 ) can be controlled by selection of materials of the semiconductor layer  102  and the electrode  104  and under the conditions such as heat treatment conditions after the formation of the electrode  104 . 
     For example, an In—Ga—Zn based oxide containing In, Ga, and Zn at an atomic ratio of 1:1:1 is used for the semiconductor layer  102 , tungsten (W) is used for the electrode  104 , and heat treatment is performed under nitrogen atmosphere at 400° C. for one hour, so that the low-resistant region  111  can be formed at a portion ranging from the interface between the electrode  104  and the semiconductor layer  102  to 20 nm. 
     In addition, the oxide layer  103  has a sheet resistance five times, preferably ten times, further preferably fifty times as high as the low-resistant region  111 . When the low-resistant region  111  has sufficiently low resistance as compared with the oxide layer  103 , the oxide layer  103  does not serve as a source or a drain, and the low-resistant region  111  can serve as a source and a drain. As a result, the effective channel length of the transistor  100  can be determined based on the interval between the low-resistant regions  111 . Note that the oxide layer  103  may serve as a source and a drain depending on materials used for the oxide layer  103 . Even in this case, oxygen movement from the semiconductor layer  102  to the electrode  104  side through the oxide layer  103  can be suppressed. 
     Here, the channel length of the transistor will be described. The channel length means the distance between the source and the drain of the transistor in many cases. 
       FIG. 3  is a schematic cross-sectional view of a transistor not including the oxide layer  103 . The low-resistant region  111  is formed in the semiconductor layer  102 . The top surface of the semiconductor layer  102  is in contact with the electrode  104 , and the low-resistant region  111  is formed throughout the region of the semiconductor layer  102  that overlaps with the electrode  104 . Furthermore, in the semiconductor layer  102 , the low-resistant region  111  extends to a region (not overlapping with the electrode  104 ) on the inner side than the end portion of the electrode  104  over the semiconductor layer  102 . As a result, the distance L eff  between the pair of low-resistant regions  111  in the semiconductor layer  102  is shorter than the distance L between the pair of electrodes  104 . Here, L eff  corresponds to the effective channel length of the transistor. 
     In such a structure, it may be difficult to control the depth (distance) of the low-resistant region  111  in the semiconductor layer  102  because of variation of the shape of the electrode  104  at its end portion and the thinning degree of the semiconductor layer  102  in processing of the electrode  104 . Therefore, it is difficult to reduce variation of the effective channel length L eff  of the transistor and electric characteristics of minute transistor may vary in some cases. 
     Also in this structure, with miniaturization of the transistor, the distance L between the pair of electrodes  104  in the semiconductor layer  102  is extremely shortened, so that the low-resistant regions  111  in pairs overlaps with each other only to short-circuit the source and the drain electrically, leading to failure to obtain transistor characteristics. 
     In the transistor  100  illustrated in  FIGS. 1A to 1C , the degree of expansion of the low-resistant region  111  in the lateral direction does not depend on the shapes of the electrode  104  and the semiconductor layer  102 , leading to reduction in the variation. Therefore, the transistor  100  is more suitable for miniaturization. 
     Here, the effective channel length L eff  of the transistor can be controlled by changing the width in the channel direction of the island-shaped semiconductor layer  102 . 
     Three schematic cross-sectional views illustrated in  FIGS. 4A to 4C  are structural examples of transistors  100  where the widths in the channel directions of the semiconductor layers  102  are different. 
       FIG. 4A  illustrates an example where a width L s  in the channel length direction of the semiconductor layer  102  is designed such that the effective channel length L eff  is equal to the distance between the pair of electrodes  104 . 
       FIG. 4B  illustrates an example where the width L s  is shorter than that in  FIG. 4A  such that the effective channel length L eff  is shorter than the distance between the pair of electrodes  104 . Thus, the effective channel length L eff  of the transistor  100  can be shortened securely and the transistor  100  can exhibit high on-state current without short-circuiting between the source and the drain. 
       FIG. 4C  illustrates an example where the width L s  is longer than that in  FIG. 4A  such that the effective channel length L eff  is longer than the distance between the pair of electrodes  104 . With this structure, in a region of the semiconductor layer  102  other than the low-resistant region  111 , an off-set region without being subjected to an electric field of the gate (or with being more difficult to be subjected the electric field than the channel formation region is) can be formed securely without variations. As a result, the source-drain withstand voltage of the transistor can be improved, so that the transistor can exhibit high reliability. 
     In this manner, the width L of the semiconductor layer  102  in the transistor  100  in the channel length direction can be changed and thus the effective channel length L eff  can be formed into a desired length securely. 
     The effective channel length L eff  depends on the width L s  in the channel length direction of the semiconductor layer  102 . Thus, for example, even if the distance between the pair of electrodes  104  varies when the electrodes  104  are processed minutely, the electric characteristics of the transistor are hardly influenced. 
     Note that the low-resistant region  111  is derived from oxygen vacancies formed in the semiconductor layer  102  by oxygen movement from the semiconductor layer  102  to the electrodes  104 . The density of the oxygen vacancies in the semiconductor layer  102  is higher at a portion closer to the interface. Accordingly, the resistance is lower at a portion closer to the interface, and the resistance is higher at a portion more distant from the interface, so that a continuous distribution of the conductivity can be exhibited in some cases. For that reason, the boundary line between the low-resistant region  111  and a portion other than the low-resistant region  111  in the semiconductor layer  102  is sometimes not clear. 
     [Components] 
     Components of the transistor  100  are described below. 
     [Semiconductor Layer] 
     As an oxide semiconductor included in the semiconductor layer  102 , an oxide semiconductor having a wider band gap and a lower carrier density than that of silicon is preferably used, in which case off-state leakage current can be reduced. 
     Further, there is no particular limitation on the crystallinity of a semiconductor used for the semiconductor layer  102 , and an amorphous semiconductor or a semiconductor having crystallinity (a microcrystalline semiconductor, a polycrystalline semiconductor, a single crystal semiconductor, a semiconductor partly including crystal regions, or a semiconductor including crystal regions in the whole area) may be used. A semiconductor having crystallinity is preferably used, in which case deterioration of transistor characteristics can be reduced. 
     Note that details of a preferable mode and a formation method of an oxide semiconductor applicable to the semiconductor layer  102  are described in an embodiment below. 
     [Substrate] 
     There is no particular limitation on the property of a material and the like of the substrate  101  as long as the material has heat resistance high enough to withstand at least heat treatment in the process. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or an yttria-stabilized zirconia (YSZ) substrate may be used as the substrate  101 . 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 can be used as the substrate  101 . 
     Still alternatively, a component in which a semiconductor element is provided over any kind of semiconductor substrate or an SOI substrate may be used as the substrate  101 . In this case, the transistor  100  is formed over the substrate  101  with an interlayer insulating layer provided therebetween. The transistor  100  in this case may have a structure in which at least one of the gate electrode  106  and the pair of electrodes  104  is electrically connected to the above semiconductor element by a connection electrode buried in the interlayer insulating layer. The transistor  100  is provided over the semiconductor element with the interlayer insulating layer provided therebetween, which does not cause an increase in area due to provision of the transistor  100 . 
     Still alternatively, a flexible substrate such as a plastic substrate may be used as the substrate  101 , and the transistor  100  may be provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate  101  and the transistor  100 . The separation layer can be used when part or the whole of the transistor formed over the separation layer is formed and separated from the substrate  101  and transferred to another substrate. Thus, the transistor  100  can be transferred to a substrate having low heat resistance or a flexible substrate. 
     [Gate Electrode] 
     The gate electrode  106  can be formed using a metal selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metals as a component; an alloy containing any of these metals in combination; or the like. Furthermore, either or both metal of manganese and zirconium may be used. Alternatively, a semiconductor typified by polycrystalline silicon doped with an impurity element such as phosphorus, or a silicide such as a nickel silicide may be used. In addition, the gate electrode  106  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 containing aluminum and one or more metals selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium; or a nitride film of the alloy film may be used. 
     The gate electrode  106  can also be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal. 
     Further, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-based oxynitride semiconductor film, an In—Ga-based oxynitride semiconductor film, an In—Zn-based oxynitride semiconductor film, a Sn-based oxynitride semiconductor film, an In-based oxynitride semiconductor film, a film of metal nitride (such as InN or ZnN), or the like may be provided between the gate electrode  106  and the insulating layer  105 . These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which is higher than the electron affinity of an oxide semiconductor; thus, the threshold voltage of a transistor including the oxide semiconductor can be shifted in the positive direction. Accordingly, a switching element having what is called normally-off characteristics is realized. For example, as an In—Ga—Zn-based oxynitride semiconductor film, an In—Ga—Zn-based oxynitride semiconductor film having a higher nitrogen concentration than at least that of the semiconductor layer  102 , specifically an In—Ga—Zn-based oxynitride semiconductor film having a nitrogen concentration higher than or equal to 7 at. %, is used. 
     [Gate Insulating Layer] 
     The gate insulating layer  105  may be formed with a single layer or a stack using, for example, one or more of silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, Ga—Zn-based metal oxide, silicon nitride, and the like. 
     The gate insulating layer  105  may be formed using a high-k material such as hafnium silicate (HfSiO x ), hafnium silicate to which nitrogen is added (HfSi x O y N z ), hafnium aluminate to which nitrogen is added (HfAl x O y N z ), hafnium oxide, or yttrium oxide, so that gate leakage current of the transistor can be reduced. 
     [A Pair of Electrodes] 
     The above-described conductive material which is easily bonded to oxygen may be used for at least part of the pair of electrodes  104  that is in contact with the semiconductor layer  102 . Alternatively, a stacked-layer structure in which another conductive material is stacked over the above-described conductive material may be used. For example, a conductive material of nickel, yttrium, zirconium, silver, or the like, nitride of the conductive material, or a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used. Still alternatively, a stacked-layer structure of two or more layers formed of the conductive material which is easily bonded to oxygen can be used. 
     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 tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order, and the like can be given. 
     [Insulating Layer] 
     The insulating layer  107  serves as a barrier layer that prevents diffusion of impurities contained in the substrate  101 . 
     At least one of the insulating layers  107  and  108  is preferably formed using an oxide insulating film containing oxygen at a higher proportion than oxygen in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film containing oxygen at a higher proportion than oxygen in the stoichiometric composition. The oxide insulating film containing oxygen at a higher proportion than oxygen in 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 18  atoms/cm 3 , preferably greater than or equal to 3.0×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS) analysis. 
     With the use of such an insulating film as the insulating layer  107  or the insulating layer  108 , oxygen is supplied to the semiconductor layer  102  through heat treatment or the like in the manufacturing process, which makes it possible to reduce oxygen vacancies in the semiconductor layer  102 . In particular, such an insulating film is preferably used for the layer (e.g., the insulating layer  107 ) in contact with the semiconductor layer  102 . 
     As the insulating layer  107  or  108 , for example, silicon oxide, silicon oxynitride, or the like can be used. 
     An oxide film that can transmit oxygen may be provided below the insulating layer  108 . 
     As the oxide film that can transmit oxygen, a silicon oxide film, a silicon oxynitride film, or the like can be formed. Note that in this specification, “silicon oxynitride film” refers to a film that contains more oxygen than nitrogen, and “silicon nitride oxide film” refers to a film that contains more nitrogen than oxygen. 
     Alternatively, the insulating layer  108  may be formed using an insulating film having a blocking effect against oxygen, hydrogen, water, and the like. It is possible to prevent outward diffusion of oxygen from the semiconductor layer  102  and entry of hydrogen, water, or the like into the semiconductor layer  102  from the outside by providing the insulating layer  108 . As for the insulating film having a blocking effect against oxygen, hydrogen, water, and the like, a silicon nitride film, a silicon nitride oxide film, 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 as examples. 
     Note that the insulating layer  108  may have a stacked structure where an insulating film from which part of oxygen is released by heat treatment and an insulating film having a blocking effect against oxygen, hydrogen, water, and the like are stacked. The stacked structure of the insulating layer  108  can improve the blocking effect and the planarity of the upper portion of the transistor  100 . 
     In the case of forming a silicon oxide film or a silicon oxynitride film as the insulating layer  107  or the insulating layer  108  by a plasma CVD method, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon are silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas are oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide. 
     For example, a silicon oxide film or a silicon oxynitride film is formed under the conditions as follows: the substrate placed in a vacuum-evacuated treatment chamber of a plasma CVD apparatus is held at a temperature higher than or equal to 180° C. and lower than or equal to 260° C., preferably higher than or equal to 200° C. and lower than or equal to 240° C.; a source gas is supplied to the treatment chamber and the pressure in the treatment chamber is set 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; and high-frequency power higher than or equal to 0.17 W/cm 2  and lower than or equal to 0.5 W/cm 2 , preferably higher than or equal to 0.25 W/cm 2  and lower than or equal to 0.35 W/cm 2  is supplied to an electrode provided in the treatment chamber. 
     As the film formation conditions, the high-frequency power having the above power density is supplied to the treatment chamber having the above pressure, whereby the decomposition efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; therefore, oxygen is contained in the oxide insulating film at a higher proportion than oxygen in the stoichiometric composition. However, in the case where the substrate temperature is within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen is released by heating. 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. 
     In the case where the insulating layer  108  is formed using a silicon nitride film or a silicon nitride oxide film, a deposition gas containing silicon, an oxidizing gas, and a gas containing nitrogen are preferably used as a source gas. Typical examples of the deposition gas containing silicon are silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas are oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide. Examples of the gas containing nitrogen are nitrogen and ammonia. 
     The above is the description of the structural examples of the transistor  100 . 
     [Example of Manufacturing Method] 
     An example of a method for manufacturing the transistor  100  illustrated in  FIG. 1A to 1C  is described below with reference to the drawings.  FIGS. 5A to 5E  are schematic cross-sectional views of steps in the example of the manufacturing method described below. 
     [Formation of Insulating Layer] 
     First, the insulating layer  107  is formed over the substrate  101 . 
     The insulating layer  107  is formed by a sputtering method, a chemical vapor deposition (CVD) method, an evaporation method, or the like. 
     In order to make the insulating layer  107  contain excess oxygen, the insulating layer  107  may be formed in an oxygen atmosphere, for example. Alternatively, the insulating layer  107  may excessively contain oxygen in such a manner that oxygen is supplied into the insulating layer  107  which has been formed. Both the methods may be combined. 
     For example, oxygen (at least including any of oxygen radicals, oxygen atoms, and oxygen ions) is introduced into the insulating layer  107  which has been deposited, whereby a region excessively containing oxygen is formed. Oxygen can be supplied by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. 
     A gas containing oxygen can be used for supplying oxygen. As the gas containing oxygen, oxygen, dinitrogen monoxide, nitrogen dioxide, carbon dioxide, carbon monoxide, or the like can be used. Further, a dilution gas such as a rare gas may be contained in the gas containing oxygen in the oxygen supply treatment. 
     [Formation of Semiconductor Film and Oxide Film] 
     Next, a semiconductor film  122  and an oxide film  123  are formed over the insulating layer  107  ( FIG. 5A ). 
     The semiconductor film  122  and the oxide film  123  can be formed by a sputtering method, a CVD method, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, a pulsed laser deposition (PLD) method, or the like. Alternatively, a technique for formation of a thin film using a liquid material, such as a sol-gel method, a spray method, or a mist method, can be used. The semiconductor film  122  is preferably formed by a sputtering method. As a sputtering method, an RF sputtering method, a DC sputtering method, an AC sputtering method, or the like can be used. In particular, a DC sputtering method is preferably used because dust generated in the deposition can be reduced and the film thickness can be uniform. 
     Note that heat treatment may be performed after the formation of the semiconductor film  122 . The heat treatment may be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C., in an inert gas atmosphere, an atmosphere containing an oxidization gas at 10 ppm or more, or a reduced pressure state. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidization gas at 10 ppm or more, in order to compensate desorbed oxygen. By the heat treatment, oxygen is supplied from the insulating layer  107  to the semiconductor film  122  (or the semiconductor layer  102 ), which enables a reduction in oxygen vacancies in the oxide semiconductor included in the semiconductor layer  102 . Note that the heat treatment may be performed at any timing after the formation of the semiconductor film  122 . The heat treatment may be performed before the processing of the semiconductor film  122  or may be performed after the formation of the semiconductor layer  102  by the processing of the semiconductor film  122 . 
     The oxide film  123  is preferably formed successively after the formation of the semiconductor film  122  without exposing its surface to air. By the successive formation of the films, the interface states between the semiconductor layer  102  and the oxide layer  103  can be reduced. 
     [Formation of Semiconductor Layer and Oxide Layer] 
     Then, a resist mask is formed over the oxide film  123  by a photolithography process or the like, and unnecessary portions of the oxide film  123  and the semiconductor film  122  are removed by etching. After that, the resist mask is removed, and thus a stacked body of the island-shaped semiconductor layer  102  and the island-shaped oxide layer  133  is formed ( FIG. 5B ). 
     As light used to form the resist mask, light with an i-line (with a wavelength of 365 nm), light with a g-line (with a wavelength of 436 nm), light with an h-line (with a wavelength of 405 nm), or light in which the i-line, the g-line, and the h-line are mixed can be used. Alternatively, ultraviolet light, KrF laser light, ArF laser light, or the like can be used. Exposure may be performed by liquid immersion exposure technique. As the light for the exposure, extreme ultra-violet light (EUV) or X-rays may be used. Instead of the light for the exposure, an electron beam can be used. It is preferable to use extreme EUV, X-rays, or an electron beam because extremely minute processing can be performed. Note that in the case of performing exposure by scanning a beam such as an electron beam, a photomask is not needed. 
     [Formation of a Pair of Electrodes] 
     Next, a conductive film is formed over the insulating layer  107  and the oxide layer  133  so as to be in contact with the side surface of the semiconductor layer  102 . Then, a resist mask is formed over the conductive film by a photolithography process or the like. Then, unnecessary portions of the conductive film and the oxide layer  133  are removed by etching. The resist mask is removed, and thus the pair of electrodes  104  and the oxide layer  103  located between the electrode  104  and the semiconductor layer  102  are formed ( FIG. 5C ). 
     The conductive film is formed by a sputtering method, a vapor deposition method, a CVD method, or the like. 
     As illustrated in  FIG. 5C , a part of the upper portion of the semiconductor layer  102  is etched when the conductive film and the oxide layer  133  are etched, and thereby the portion not overlapping with the pair of electrodes  104  may be thinned. For this reason, the thickness of the semiconductor film serving as the semiconductor layer  102  is preferably large in light of the etching depth. 
     Note that the etching of the oxide layer  133  using the resist mask as the etching mask is described here, but the resist mask may be removed after the conductive film is etched and the pair of electrodes  104  may be used as the etching mask to etch the oxide layer  133 . 
     As illustrated in  FIG. 5C , after the formation of the conductive film, the end portion of the conductive film may be in contact with the end portion of the semiconductor layer  102 , so that a low-resistant region  121  may be formed in a part of the end portion of the semiconductor layer  102 . The low-resistant region  121  formed at this time does not expand so much (e.g., expands several nanometers) in the channel length direction and is formed in the vicinity of the interface between the conductive film or the electrode  104  and the semiconductor layer  102 . 
     [Formation of Gate Insulating Layer] 
     Then, the gate insulating layer  105  is formed over the semiconductor layer  102 , the oxide layer  103 , the insulating layer  107 , and the pair of electrodes  104 . 
     The gate insulating layer  105  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, a PLD method, or the like. In particular, it is preferable that the gate insulating layer  105  be formed by a CVD method, further preferably a plasma CVD method because coverage can be further improved. 
     In addition, after the formation of the gate insulating layer  105 , the oxygen supply treatment described above may be performed on the gate insulating layer  105 . Oxygen is supplied into the gate insulating layer  105  so that the gate insulating layer  105  can contain excess oxygen, and thus oxygen can be supplied into the semiconductor layer  102  in later heat treatment. 
     [Formation of Gate Electrode] 
     A conductive film is formed over the gate insulating layer  105 . A resist mask is formed over the conductive film by a photolithography process or the like and unnecessary portions of the conductive film are etched, and then the resist mask is removed. In this manner, the gate electrode  106  can be formed ( FIG. 5D ). 
     The conductive film serving as the gate electrode  106  can be formed by a sputtering method, a CVD method, a vapor deposition method, or the like, for example. 
     [Formation of Insulating Layer] 
     Next, the insulating layer  108  is formed over the gate insulating layer  105  and the gate electrode  106 . 
     The insulating layer  108  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, a PLD method, or the like. In particular, it is preferable that the insulating layer  108  be formed by a CVD method, further preferably a plasma CVD method because coverage can be excellent. 
     [Heat Treatment] 
     Next, heat treatment is performed. By the heat treatment, at an end portion of the semiconductor layer  102  in contact with the electrode  104 , part of oxygen moves into the electrode  104  from the semiconductor layer  102 , and oxygen vacancies are produced at the end portion of the semiconductor layer  102  along the channel length direction; therefore, the low-resistant region  111  is formed in the semiconductor layer  102  ( FIG. 5E ). 
     The width in the channel length direction and the distribution of conductivity of the low-resistant region  111  formed in the semiconductor layer  102  can be controlled with conditions such as the temperature, time, pressure, and atmosphere of heat treatment. The heat treatment may be performed under the above-described conditions. 
     In addition, oxygen released from the insulating layer  107  (or the gate insulating layer  105  or the insulating layer  108 ) is supplied into the semiconductor layer  102  by the heat treatment, and thus oxygen vacancies at least in a channel-formation region in the semiconductor layer  102  can be reduced. 
     Here, the heat treatment is performed after the insulating layer  108  is formed; however, the heat treatment can be performed to form the low-resistant region  111  in any step after the conductive film serving as the pair of electrodes  104  is formed. However, the heat treatment is preferably performed after the formation of the insulating layer  108  (at least after the formation of the gate insulating layer  105 ) in order to prevent oxygen from being released from the semiconductor layer  102  and oxygen vacancies from being produced in the channel-formation region by the heat treatment. 
     When the heat treatment is performed after the gate insulating layer  105  is formed, impurities such as water and hydrogen contained in the gate insulating layer  105  can be removed (dehydrated or dehydrogenated). 
     Through the above steps, the transistor  100  can be manufactured. 
     [Examples of Variations] 
     A structural example of a transistor that has a partly different structure from the transistor  100  described in the above structural example is described below. Note that description of the portions already described is omitted and only different portions are described. Even when positions and shapes of components are different from those in the above example, the same reference numerals are used as long as the components have the same functions as those in the above example, and detailed description thereof is omitted in some cases. 
     [Variation 1] 
       FIG. 6A  is a schematic cross-sectional view of the transistor  150  exemplified below. The transistor  150  is different from the transistor  100  mainly in that the gate electrode  106  is located on the substrate  101  side, not the semiconductor layer  102  side. The transistor  150  is a bottom-gate type transistor. 
     The transistor  150  includes the gate electrode  106  over the insulating layer  107 ; the gate insulating layer  105  covering the gate electrode  106 ; the semiconductor layer  102  having the low-resistant region  111  over the gate insulating layer  105 ; the pair of oxide layers  103  in contact with the top surface of the semiconductor layer  102 ; and the pair of electrodes  104  in contact with the top surface of the oxide layer  103  and the side surface of the semiconductor layer  102 . In addition, the insulating layer  108  is formed over the pair of electrodes  104  and the semiconductor layer  102 . 
     [Variation 2] 
       FIG. 6B  is a schematic cross-sectional view of a transistor  160  exemplified below. The transistor  160  is different from the transistors  100  and  150  mainly in that the transistor  160  includes two gate electrodes (a first gate electrode  106   a  and a second gate electrode  106   b ). 
     The transistor  160  includes the first gate electrode  106   a  over the insulating layer  107 ; a first gate insulating layer  105   a  covering the first gate electrode  106   a ; the semiconductor layer  102  having the low-resistant region  111  over the first gate insulating layer  105   a ; the pair of oxide layers  103  in contact with the top surface of the semiconductor layer  102 ; the pair of electrodes  104  in contact with the top surface of the oxide layer  103  and the side surface of the semiconductor layer  102 ; a second gate insulating layer  105   b  over the pair of electrodes  104  and the semiconductor layer  102 ; and the second gate electrode  106   b  overlapping with the semiconductor layer  102  over the second gate insulating layer  105   b . In addition, the insulating layer  108  is formed over the second gate insulating layer  105   b  and the second gate electrode  106   b.    
     Here, preferably, the first gate insulating layer  105   a  and the second gate insulating layer  105   b  are formed using the same material and have substantially the same thickness. 
     The first gate electrode  106   a  and the second gate electrode  106   b  may be supplied with different potentials or may be electrically connected and supplied with the same potential. The threshold voltage of the transistor  160  can be varied by supplying a constant potential to one of the first gate electrode  106   a  and the second gate electrode  106   b  and supplying a different potential to the other. 
     [Variation 3] 
       FIG. 6C  is a schematic cross-sectional diagram of a transistor  170  exemplified below. The transistor  170  is different from the transistor  100  mainly in that oxide layers are formed above and below the semiconductor layer  102 . 
     The transistor  170  includes an oxide layer  171  over the insulating layer  107 ; the semiconductor layer  102  having the low-resistant region  111  over the oxide layer  171 ; the pair of oxide layers  103  in contact with the top surface of the semiconductor layer  102 ; the pair of electrodes  104  in contact with the top surface of the oxide layer  103  and the side surface of the semiconductor layer  102 ; an oxide layer  172  over the pair of electrodes  104  and the semiconductor layer  102 ; the gate insulating layer  105  over the oxide layer  172 ; and the gate electrode  106  over the gate insulating layer  105 . In addition, the insulating layer  108  is formed over the pair of electrodes  104  and the gate electrode  106 . 
     The oxide layer  171  and the oxide layer  172  each contain at least one kind of the constituents of the semiconductor layer  102 . 
     Note that the boundary line between the semiconductor layer  102  and the oxide layer  171  and the boundary line between the semiconductor layer  102  and the oxide layer  172  are not clear in some cases. 
     For example, the oxide layer  171  and the oxide layer  172  contain In or Ga; typically, a material such as an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) that has an energy level of the conduction band minimum closer to the vacuum level than that of the semiconductor layer  102  is used. Typically, the difference in the conduction band minimum energy between the oxide layer  171  and the oxide layer  172  and the semiconductor layer  102  is preferably 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     For each of the oxide layer  171  and the oxide layer  172  between which the semiconductor layer  102  is sandwiched, an oxide that contains a larger amount of Ga serving as a stabilizer than that of the semiconductor layer  102  is used, which can suppress release of oxygen from the semiconductor layer  102 . 
     Therefore, oxygen movement from the oxide layer  171  and the oxide layer  172  to the electrode  104  is suppressed and thus almost no low-resistant regions are formed in the oxide layer  171  and the oxide layer  172 . Accordingly, even in the structure where the oxide layer  171  and oxide layer  172  are provided, the effective channel length of the transistor  170  can be controlled by the width in the channel length direction of the semiconductor layer  102 . 
     When an In—Ga—Zn-based oxide in which the atomic ratio of In to Ga and Zn is 1:1:1 or 3:1:2 is used for the semiconductor layer  102 , for example, an In—Ga—Zn-based oxide in which the atomic ratio of In to Ga and Zn is 1:3:2, 1:3:4, 1:3:6, 1:6:4, 1:6:8, 1:6:10, or 1:9:6 can be used for the oxide layer  171  or the oxide layer  172 . Note that the atomic ratios in the semiconductor layer  102 , the oxide layer  171 , and the oxide layer  172  each have a margin of error of ±20%. For the oxide layers  171  and  172 , materials with the same composition ratio or materials with different composition ratios may be used. 
     By using a material in which the energy level of the conduction band minimum is closer to the vacuum level than that of the semiconductor layer  102  is for the oxide layers  171  and  172 , a channel is mainly formed in the semiconductor layer  102 , so that the semiconductor layer  102  serves as a main current path. When the semiconductor layer  102  in which a channel is formed is sandwiched between the oxide layers  171  and  172  as described above, generation of interface states between these layers is suppressed, and thus reliability of electrical characteristics of the transistor is improved. 
     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 the required semiconductor characteristics of the 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 semiconductor layer  102 , the oxide layer  171 , and the oxide layer  172  be set to appropriate values. 
     In addition, in the transistor  170 , the oxide layer  172 , the gate insulating layer  105 , and the gate electrode  106  are processed with the use of the same photomask such that the end portion of the oxide layer  172  and the end portion of the gate insulating layer  105  substantially are aligned with the end portion of the gate electrode  106 . The insulating layer  108  is provided in contact with the side surface of the oxide layer  172  and the side surface of the gate insulating layer  105 . Such a structure can inhibit release of oxygen from the semiconductor layer  102  through the end portion of the oxide layer  172  and the end portion of the gate insulating layer  105 . 
     The above is the description of the example of variation. 
     Note that this embodiment can be combined with any of the other embodiments and examples described in this specification as appropriate. 
     Embodiment 2 
     An oxide semiconductor that can be favorably used for a semiconductor device of one embodiment of the present invention is described in this embodiment. 
     An oxide semiconductor has a wide energy gap of 3.0 eV or more. A transistor including an oxide semiconductor film obtained by processing of the oxide semiconductor in an appropriate condition and having extremely lowered carrier density of the oxide semiconductor can have much lower leakage current between a source and a drain in an off state (off-state current) than a conventional transistor including silicon. 
     An applicable oxide semiconductor preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. In addition, as a stabilizer for reducing variation in electric characteristics of the transistor using the oxide semiconductor, one or more selected from gallium (Ga), tin (Sn), hafnium (Hf), zirconium (Zr), titanium (Ti), scandium (Sc), yttrium (Y), and an lanthanoid (such as cerium (Ce), neodymium (Nd), or gadolinium (Gd), for example) is preferably contained. 
     As the oxide semiconductor, for example, any of the following can be used: indium oxide, tin oxide, zinc oxide, an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, an In—Ga-based oxide, an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, an In—Sc—Zn-based oxide, an In—Y—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide. 
     Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In, Ga, and Zn. The In—Ga—Z-based oxide may contain another metal element in addition to In, Ga, and Zn. 
     Alternatively, a material represented by InMO 3 (ZnO) m  (m&gt;0 is satisfied, and m is not an integer) may be used as an oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co, or the above-described element as a stabilizer. Alternatively, as the oxide semiconductor, a material represented by a chemical formula, In 2 SnO 5 (ZnO) n  (n&gt;0, n is a natural number) may be used. 
     For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, In:Ga:Zn=1:3:6, In:Ga:Zn=3:1:2, or In:Ga:Zn=2:1:3, or an oxide with an atomic ratio close to the above atomic ratios can be used. 
     When the oxide semiconductor film contains a large amount of hydrogen, the hydrogen and oxide semiconductor are bonded to each other, so that part of the hydrogen serves as a donor and causes generation of an electron which is a carrier. As a result, the threshold voltage of the transistor shifts in the negative direction. Therefore, it is preferable that, after formation of the oxide semiconductor film, dehydration treatment (dehydrogenation treatment) be performed to remove hydrogen or moisture from the oxide semiconductor film so that the oxide semiconductor film is highly purified to contain impurities as little as possible. 
     Note that oxygen in the oxide semiconductor film is also reduced by the dehydration treatment (dehydrogenation treatment) in some cases. Therefore, it is preferable that oxygen be added to the oxide semiconductor film to fill oxygen vacancies increased by the dehydration treatment (dehydrogenation treatment). In this specification and the like, supplying oxygen to an oxide semiconductor film may be expressed as oxygen adding treatment, or treatment for making the oxygen content of an oxide semiconductor film be in excess of that of the stoichiometric composition may be expressed as treatment for making an oxygen-excess state. 
     In this manner, hydrogen or moisture is removed from the oxide semiconductor film by the dehydration treatment (dehydrogenation treatment) and oxygen vacancies therein are filled by the oxygen adding treatment, whereby the oxide semiconductor film can be turned into an i-type (intrinsic) oxide semiconductor film or a substantially i-type (intrinsic) oxide semiconductor film which is extremely close to an i-type oxide semiconductor film. Note that “substantially intrinsic” means that the oxide semiconductor film contains extremely few (close to zero) carriers derived from a donor and has a carrier density of lower than or equal to 1×10 17 /cm 3 , lower than or equal to 1×10 16 /cm 3 , lower than or equal to 1×10 15 /cm 3 , lower than or equal to 1×10 14 /cm 3 , or lower than or equal to 1×10 13 /cm 3 . 
     Thus, the transistor including an i-type or substantially i-type oxide semiconductor film can have extremely favorable off-state current characteristics. For example, the drain current at the time when the transistor including an oxide semiconductor film is in an off-state can be less than or equal to 1×10 −18  A, preferably less than or equal to 1×10 −21  A, further preferably less than or equal to 1×10 −24  A at room temperature (about 25° C.); or less than or equal to 1×10 −15  A, preferably less than or equal to 1×10 −18  A, further preferably less than or equal to 1×10 −21  A at 85° C. Note that an off state of an n-channel transistor refers to a state where the gate voltage is sufficiently lower than the threshold voltage. Specifically, the transistor is in an off state when the gate voltage is lower than the threshold voltage by 1V or more, 2V or more, or 3V or more. 
     A structure of the oxide semiconductor film is described below. 
     An oxide semiconductor film is classified roughly into a single-crystal oxide semiconductor film and a non-single-crystal oxide semiconductor film. The non-single-crystal oxide semiconductor film includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, a polycrystalline oxide semiconductor film, a microcrystalline oxide semiconductor film, an amorphous oxide semiconductor film, and the like. 
     First, a CAAC-OS film is described. 
     In this specification, a 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°. In addition, 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°. 
     In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system. 
     The CAAC-OS film is one of oxide semiconductor films having a plurality of c-axis aligned crystal parts. 
     In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film. 
     Most of the crystal parts included in the CAAC-OS film each fit inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. Note that when a plurality of crystal parts included in the CAAC-OS film are connected to each other, one large crystal region is formed in some cases. For example, a crystal region with an area of 2500 nm 2  or more, 5 μm 2  or more, or 1000 μm 2  or more is observed in some cases in the plan TEM image. 
     A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS film 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 film. 
     On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. Here, analysis (θ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (θ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZnO 4 , six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when 0 scan is performed with 2θ fixed at around 56°. 
     According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. 
     In addition, distribution of c-axis aligned crystal parts in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the crystal parts of the CAAC-OS film occurs from the vicinity of the top surface of the film, the proportion of the c-axis aligned crystal parts in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, in the CAAC-OS film to which an impurity is added, a region to which the impurity is added may be deformed, and the proportion of the c-axis aligned crystal parts in the CAAC-OS film may vary depending on regions. 
     Note that when the CAAC-OS film with an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at 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 film. It is preferable that in the CAAC-OS film, a peak of 2θ appears at around 31° and a peak of 2θ do not appear at around 36°. 
     The CAAC-OS film is an oxide semiconductor film having low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor film, such as silicon, disturbs the atomic arrangement of the oxide semiconductor film by depriving the oxide semiconductor film of oxygen and causes a decrease in crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor film and causes a decrease in crystallinity when it is contained in the oxide semiconductor film. Note that the impurity contained in the oxide semiconductor film might serve as a carrier trap or a carrier generation source. 
     The CAAC-OS film is an oxide semiconductor film having a low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein. 
     The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor including the oxide semiconductor film rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has little variation in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released, and might behave like fixed electric charge. Thus, the transistor which includes the oxide semiconductor film having high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases. 
     With the use of the CAAC-OS film in a transistor, variation in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small. 
     Next, a microcrystalline oxide semiconductor film is described. 
     In an image obtained with the TEM, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor in some cases. In most cases, a crystal part in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor film including nanocrystal is referred to as an nc-OS (nanocrystalline oxide semiconductor) film. In an image obtained with TEM, a crystal grain cannot be found clearly in the nc-OS film in some cases. 
     In the nc-OS film, 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 order. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern appears in a selected-area electron diffraction pattern of the nc-OS film which is obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 50 nm) larger than the diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter close to, or smaller than the diameter of a crystal part. Furthermore, in a nanobeam electron diffraction pattern of the nc-OS film, 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 film, a plurality of spots is shown in a ring-like region in some cases. 
     The nc-OS film is an oxide semiconductor film that has high regularity as compared to an amorphous oxide semiconductor film. Therefore, the nc-OS film has a lower density of defect states than that of an amorphous oxide semiconductor film. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; hence, the nc-OS film has a higher density of defect states than that of the CAAC-OS film. 
     Note that an oxide semiconductor film may be a stacked film including two or more kinds of an amorphous oxide semiconductor film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example. 
     This embodiment can be combined with any of the other embodiments disclosed in this specification as appropriate. 
     Embodiment 3 
     In this embodiment, an example of a semiconductor device (memory device) that includes a transistor according to one embodiment of the present invention, that can hold stored data even when not powered, and that has an unlimited number of write cycles will be described with reference to drawings. 
       FIG. 7A  is a cross-sectional view of the semiconductor device, and  FIG. 7B  is a circuit diagram of the semiconductor device. 
     The semiconductor device illustrated in  FIGS. 7A and 7B  includes a transistor  3200  including a first semiconductor material in a lower portion, and a transistor  3300  including a second semiconductor material and a capacitor  3400  in an upper portion. Note that the transistor described in Embodiment 1 can be used as the transistor  3300 .  FIG. 7A  illustrates an example where the transistor  100  is used. 
     One electrode of the capacitor  3400  is formed using the same material as a source electrode layer and a drain electrode layer of the transistor  3300 , the other electrode of the capacitor  3400  is formed using the same material as a gate electrode layer of the transistor  3300 , and a dielectric of the capacitor  3400  is formed using the same material as the gate insulating layer of the transistor  3300 ; thus, the capacitor  3400  can be formed at the same time as the transistor  3300 . 
     Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material may be the oxide semiconductor described in Embodiment 1. A transistor including a material other than oxide semiconductor, such as single crystal silicon, can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor enables charge to be held for a long time owing to its electrical characteristics, that is, the low off-state current. 
     Although all the transistors are n-channel transistors here, it is needless to say that p-channel transistors can also be used. The specific structure of the semiconductor device, such as the material used for the semiconductor device and the structure of the semiconductor device, is not necessarily limited to those described here except for the use of the transistor described in Embodiment 1 that is formed using an oxide semiconductor for holding data. 
     The transistor  3200  in  FIG. 7A  includes a channel formation region provided in a substrate  3000  including a semiconductor material (such as crystalline silicon), impurity regions provided such that the channel formation region is sandwiched therebetween, intermetallic compound regions provided in contact with the impurity regions, a gate insulating layer provided over the channel formation region, and a gate electrode provided over the gate insulating layer. Note that a transistor whose source electrode and drain electrode are not illustrated explicitly in a drawing may be referred to as a transistor for the sake of convenience. In such a case, in description of connection relation of a transistor, a source region and a source electrode are collectively referred to as a “source electrode,” and a drain region and a drain electrode are collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region. 
     Further, an element isolation insulating layer  3100  is formed on the substrate  3000  so as to surround the transistor  3200 , and an insulating layer  3150  is formed so as to cover the transistor  3200 . Note that the element isolation insulating layer  3100  can be formed by an element isolation technique such as local oxidation of silicon (LOCOS) or shallow trench isolation (STI). 
     For example, in the case where the transistor  3200  is formed using a crystalline silicon substrate, the transistor  3200  can operate at high speed. Thus, when the transistor is used as a reading transistor, data can be read at a high speed. 
     The transistor  3300  is provided over the insulating layer  3150 , and one of the source electrode and the drain electrode thereof is extended so as to function as the one electrode of the capacitor  3400 . In addition, the one electrode of the capacitor  3400  is electrically connected to the gate electrode of the transistor  3200  through a connection wiring  3350 . 
     The transistor  3300  in  FIG. 7A  is a top-gate transistor in which a channel is formed in a semiconductor layer including an oxide semiconductor. Since the off-state current of the transistor  3300  is low, stored data can be retained for a long period owing to such a transistor. In other words, power consumption can be sufficiently reduced because a semiconductor device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided. 
     Further, an electrode  3250  overlaps with the transistor  3300  with the insulating layer  3150  provided therebetween. By supplying an appropriate potential to the electrode  3250  to be used as a second gate electrode, the threshold voltage of the transistor  3300  can be controlled. In addition, long-term reliability of the transistor  3300  can be improved. When the electrode operates with the same potential as that of the gate electrode of the transistor  3300 , on-state current can be increased. Note that the electrode  3250  is not necessarily provided. 
     The transistor  3300  and the capacitor  3400  can be formed over the substrate over which the transistor  3200  is formed as illustrated in  FIG. 7A , which enables the degree of the integration of the semiconductor device to be increased. 
     An example of a circuit configuration corresponding to  FIG. 7A  is illustrated in  FIG. 7B . 
     In  FIG. 7B , a first wiring  3001  is electrically connected to a source electrode layer of the transistor  3200 . A second wiring  3002  is electrically connected to a drain electrode layer of the transistor  3200 . A third wiring  3003  is electrically connected to the one of the source electrode and the drain electrode of the transistor  3300 . A fourth wiring  3004  is electrically connected to the gate electrode of the transistor  3300 . The gate electrode of the transistor  3200  and the other of the source electrode and the drain electrode of the transistor  3300  are electrically connected to the one electrode of the capacitor  3400 . A fifth wiring  3005  is electrically connected to the other electrode of the capacitor  3400 . Note that a component corresponding to the electrode  3250  is not illustrated. 
     The semiconductor device in  FIG. 7B  utilizes a feature that the potential of the gate electrode of the transistor  3200  can be held, and thus enables writing, storing, and reading of data as follows. 
     Writing and holding of data will be described. First, the potential of the fourth wiring  3004  is set to a potential at which the transistor  3300  is turned on, so that the transistor  3300  is turned on. Accordingly, the potential of the third wiring  3003  is supplied to the gate electrode of the transistor  3200  and the capacitor  3400 . That is, predetermined charge is supplied to the gate electrode of the transistor  3200  (writing). Here, charge for supplying either of two different potential levels (hereinafter referred to as low-level charge and high-level charge) is given. After that, the potential of the fourth wiring  3004  is set to a potential at which the transistor  3300  is turned off, so that the transistor  3300  is turned off. Thus, the charge supplied to the gate electrode of the transistor  3200  is retained (retaining). 
     Since the off-state current of the transistor  3300  is extremely low, the charge of the gate electrode of the transistor  3200  is retained for a long time. 
     Next, reading of data will be described. By supplying an appropriate potential (a reading potential) to the fifth wiring  3005  while supplying a predetermined potential (a constant potential) to the first wiring  3001 , the potential of the second wiring  3002  varies depending on the amount of charge held in the gate electrode of the transistor  3200 . This is because in general, when the transistor  3200  is an n-channel transistor, an apparent threshold voltage V th   _   H  in the case where a high-level charge is given to the gate electrode of the transistor  3200  is lower than an apparent threshold voltage V th   _   L  in the case where a low-level charge is given to the gate electrode of the transistor  3200 . Here, an apparent threshold voltage refers to the potential of the fifth wiring  3005  which is needed to turn on the transistor  3200 . Thus, by setting the potential of the fifth wiring  3005  to a potential V 0  which is between V th   _   H  and V th   _   L  (e.g., V 0 =ground potential GND), charge given to the gate electrode of the transistor  3200  can be determined. For example, in the case where the high-level charge is supplied in writing, when the potential of the fifth wiring  3005  is V 0  (&gt;V th   _   H ), the transistor  3200  is turned on. In the case where the low-level charge is supplied in writing, even when the potential of the fifth wiring  3005  is V 0  (&lt;V th   _   L ), the transistor  3200  remains off. Therefore, the data stored in the gate electrode can be read by determining the potential of the second wiring  3002 . 
     Note that in the case where memory cells are arrayed, only data of desired memory cells needs to be read. The fifth wiring  3005  in the case where data is not read may be supplied with a potential at which the transistor  3200  is turned off regardless of the state of the gate electrode, that is, a potential lower than V th   _   H . Alternatively, the fifth wiring  3005  may be supplied with a potential at which the transistor  3200  is turned on regardless of the state of the gate electrode layer, that is, a potential higher than V th   _   L . 
     When a transistor having a channel formation region formed using an oxide semiconductor and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be sufficiently reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed). 
     Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional nonvolatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating layer does not arise. In other words, the semiconductor device according to one embodiment of the present invention does not have a limit on the number of times of writing, which has been a problem in a conventional nonvolatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily achieved. 
     As described above, a miniaturized and highly integrated semiconductor device having favorable electrical characteristics can be provided. 
     Note that this embodiment can be combined with any of the other embodiments and examples described in this specification as appropriate. 
     Embodiment 4 
     In this embodiment, a semiconductor device that includes a transistor of one embodiment of the present invention, that can hold stored data even when not powered, that does not have a limitation on the number of write cycles, and that has a structure different from that described in Embodiment 3, is described. 
       FIG. 8  illustrates an example of a circuit configuration of the semiconductor device. In the semiconductor device, a first wiring  4500  is electrically connected to a source electrode of a transistor  4300 , a second wiring  4600  is electrically connected to a first gate electrode of the transistor  4300 , and a drain electrode of the transistor  4300  is electrically connected to a first terminal of a capacitor  4400 . Note that the transistor described in Embodiment 1 can be used as the transistor  4300  included in the semiconductor device. The first wiring  4500  can serve as a bit line and the second wiring  4600  can serve as a word line. 
     The semiconductor device (a memory cell  4250 ) can have a connection mode similar to that of the transistor  3300  and the capacitor  3400  illustrated in  FIGS. 7A and 7B . Thus, the capacitor  4400  can be formed through the same process and at the same time as the transistor  4300  in a manner similar to that of the capacitor  3400  described in Embodiment 3. 
     Next, writing and holding of data in the semiconductor device (a memory cell  4250 ) illustrated in  FIG. 8  will be described. 
     First, a potential at which the transistor  4300  is turned on is supplied to the second wiring  4600 , so that the transistor  4300  is turned on. Accordingly, the potential of the first wiring  4500  is supplied to one terminal of the capacitor  4400  (writing). After that, the potential of the second wiring  4600  is set to a potential at which the transistor  4300  is turned off, so that the transistor  4300  is turned off. Thus, the potential of the one terminal of the capacitor  4400  is held (holding). 
     In addition, the transistor  4300  including an oxide semiconductor has an extremely low off-state current. For that reason, the potential of the first terminal of the capacitor  4400  (or a charge accumulated in the capacitor  4400 ) can be held for an extremely long time by turning off the transistor  4300 . 
     Next, operation of data reading will be described. When the transistor  4300  is turned on, the first wiring  4500  which is in a floating state and the capacitor  4400  are electrically connected to each other, and the charge is redistributed between the first wiring  4500  and the capacitor  4400 . As a result, the potential of the first wiring  4500  is changed. The amount of change in potential of the first wiring  4500  varies depending on the potential of the first terminal of the capacitor  4400  (or the charge accumulated in the capacitor  4400 ). 
     For example, the potential of the first wiring  4500  after charge redistribution is (C B ×V B0 +C×V) (C B +C), where V is the potential of the first terminal of the capacitor  4400 , C is the capacitance of the capacitor  4400 , C B  is the capacitance component of the first wiring  4500 , and V B0  is the potential of the first wiring  4500  before the charge redistribution. Therefore, it can be found that assuming that the memory cell  4250  is in either of two states in which the potentials of the first terminal of the capacitor  4400  are V 1  and V 0  (V 1 &gt;V 0 ), the potential of the first wiring  4500  in the case of holding the potential V 1  (=(C B ×V B0 +C×V 1 ) (C B +C)) is higher than the potential of the first wiring  4500  in the case of holding the potential V 0  (=(C B ×V B0 +C×V 0 ) (C B +C)). 
     Then, by comparing the potential of the first wiring  4500  with a predetermined potential, data can be read. 
     As described above, the semiconductor device (the memory cell  4250 ) illustrated in  FIG. 8  can hold charge that is accumulated in the capacitor  4400  for a long time because the off-state current of the transistor  4300  is extremely low. In other words, power consumption can be sufficiently reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be stored for a long time even when power is not supplied. 
     A substrate over which a driver circuit for the memory cell  4250  is formed and the memory cell  4250  illustrated in  FIG. 8  are preferably stacked. When the memory cell  4250  and the driver circuit are stacked, the size of the semiconductor device can be reduced. Note that there is no limitation on the numbers of the memory cells  4250  and the driver circuits that are stacked. 
     It is preferable that a semiconductor material of a transistor included in the driver circuit be different from that of the transistor  4300 . For example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like can be used, and a single crystal semiconductor is preferably used. A transistor formed using such a semiconductor material can operate at higher speed than a transistor formed using an oxide semiconductor and is suitable for the driver circuit for the memory cell  4250 . 
     As described above, a miniaturized and highly integrated semiconductor device having favorable electrical characteristics can be provided. 
     Note that this embodiment can be combined with any of the other embodiments and examples described in this specification as appropriate. 
     Embodiment 5 
     In this embodiment, a CPU in which at least the transistor described in the above embodiment can be used and the memory device described in the above embodiment is included will be described. 
       FIG. 9  is a block diagram illustrating a configuration example of a CPU using at least the transistor described in Embodiment 1. 
     The CPU illustrated in  FIG. 9  includes an arithmetic logic unit (ALU)  1191 , an ALU controller  1192 , an instruction decoder  1193 , an interrupt controller  1194 , a timing controller  1195 , a register  1196 , a register controller  1197 , a bus interface (Bus I/F)  1198 , a rewritable ROM  1199 , and an ROM interface (ROM I/F)  1189  over a substrate  1190 . A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate  1190 . The ROM  1199  and the ROM interface  1189  may be provided over a separate chip. Obviously, the CPU illustrated in  FIG. 9  is just an example in which the configuration is simplified, and actual CPUs may have various configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU illustrated in  FIG. 9  or an arithmetic circuit is considered as one core; a plurality of the cores is included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be 8, 16, 32, or 64, for example. 
     An instruction that is input to the CPU through the bus interface  1198  is input to the instruction decoder  1193  and decoded therein, and then, input to the ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195 . 
     The ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195  conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller  1192  generates signals for controlling the operation of the ALU  1191 . While the CPU is executing a program, the interrupt controller  1194  processes an interrupt request from an external input/output device or a peripheral circuit depending on its priority or a mask state. The register controller  1197  generates an address of the register  1196 , and reads/writes data from/to the register  1196  depending on the state of the CPU. 
     The timing controller  1195  generates signals for controlling operation timings of the ALU  1191 , the ALU controller  1192 , the instruction decoder  1193 , the interrupt controller  1194 , and the register controller  1197 . For example, the timing controller  1195  includes an internal clock generator for generating an internal clock signal CLK 2  on the basis of a reference clock signal CLK 1 , and supplies the internal clock signal CLK 2  to the above circuits. 
     In the CPU illustrated in  FIG. 9 , a memory cell is provided in the register  1196 . For the memory cell of the register  1196 , any of the transistors described in the above embodiment can be used. 
     In the CPU illustrated in  FIG. 9 , the register controller  1197  selects operation of holding data in the register  1196  in accordance with an instruction from the ALU  1191 . That is, the register controller  1197  selects whether data is held by a flip-flop or by a capacitor in the memory cell included in the register  1196 . When data holding by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register  1196 . When data holding by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register  1196  can be stopped. 
       FIG. 10  is an example of a circuit diagram of a storage element that can be used as the register  1196 . A memory element  700  includes a circuit  701  in which stored data is volatile when power supply is stopped, a circuit  702  in which stored data is nonvolatile when power supply is stopped, a switch  703 , a switch  704 , a logic element  706 , a capacitor  707 , and a circuit  720  having a selecting function. The circuit  702  includes a capacitor  708 , a transistor  709 , and a transistor  710 . Note that the memory element  700  may further include another element such as a diode, a resistor, or an inductor, as needed. 
     Here, the memory device described in the above embodiment can be used as the circuit  702 . When supply of the power supply voltage to the memory element  700  is stopped, a ground potential (0 V) or a potential to turn off the transistor  709  in the circuit  702  continues to be input to a gate of the transistor  709 . For example, the gate of the transistor  709  is grounded through a load such as a resistor. 
     An example in which the switch  703  is a transistor  713  having one conductivity type (e.g., an n-channel transistor) and the switch  704  is a transistor  714  having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor) is described. Here, a first terminal of the switch  703  corresponds to one of a source and a drain of the transistor  713 , a second terminal of the switch  703  corresponds to the other of the source and the drain of the transistor  713 , and conduction or non-conduction between the first terminal and the second terminal of the switch  703  (i.e., the on/off state of the transistor  713 ) is selected by a control signal RD input to a gate of the transistor  713 . A first terminal of the switch  704  corresponds to one of a source and a drain of the transistor  714 , a second terminal of the switch  704  corresponds to the other of the source and the drain of the transistor  714 , and conduction or non-conduction between the first terminal and the second terminal of the switch  704  (i.e., the on/off state of the transistor  714 ) is selected by the control signal RD input to a gate of the transistor  714 . 
     One of a source and a drain of the transistor  709  is electrically connected to one of a pair of electrodes of the capacitor  708  and a gate of the transistor  710 . Here, the connection portion is referred to as a node M 2 . One of a source and a drain of the transistor  710  is electrically connected to a line which can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch  703  (the one of the source and the drain of the transistor  713 ). The second terminal of the switch  703  (the other of the source and the drain of the transistor  713 ) is electrically connected to the first terminal of the switch  704  (the one of the source and the drain of the transistor  714 ). The second terminal of the switch  704  (the other of the source and the drain of the transistor  714 ) is electrically connected to a line which can supply a power supply potential VDD is supplied. The second terminal of the switch  703  (the other of the source and the drain of the transistor  713 ), the first terminal of the switch  704  (the one of the source and the drain of the transistor  714 ), an input terminal of the logic element  706 , and one of a pair of electrodes of the capacitor  707  are electrically connected to each other. Here, the connection portion is referred to as a node M 1 . The other of the pair of electrodes of the capacitor  707  can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor  707  can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor  707  is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor  708  can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor  707  can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor  708  is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). 
     The capacitor  707  and the capacitor  708  are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring (line), or the like is actively utilized. 
     A control signal WE is input to the first gate (first gate electrode) of the transistor  709 . As for each of the switch  703  and the switch  704 , a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD which is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state. 
     A signal corresponding to data held in the circuit  701  is input to the other of the source and the drain of the transistor  709 .  FIG. 10  illustrates an example in which a signal output from the volatile memory circuit  701  is input to the other of the source and the drain of the transistor  709 . The logic value of a signal output from the second terminal of the switch  703  (the other of the source and the drain of the transistor  713 ) is inverted by the logic element  706 , and the inverted signal is input to the circuit  701  through the circuit  720 . 
     In the example of  FIG. 10 , a signal output from the second terminal of the switch  703  (the other of the source and the drain of the transistor  713 ) is input to the circuit  701  through the logic element  706  and the circuit  720 ; however, this embodiment is not limited thereto. The signal output from the second terminal of the switch  703  (the other of the source and the drain of the transistor  713 ) may be input to the circuit  701  without its logic value being inverted. For example, in the case where a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is held is provided in the circuit  701 , the signal output from the second terminal of the switch  703  (the other of the source and the drain of the transistor  713 ) can be input to the node. 
     As the transistor  709  in  FIG. 10 , the transistor described in Embodiment 1 can be used. As described in Embodiment 3, the transistor  709  preferably includes a second gate (second gate electrode). The control signal WE can be input to the first gate and the control signal WE 2  can be input to the second gate. The control signal WE 2  may be a signal having a constant potential. As the constant potential, for example, a ground potential GND or a potential lower than a source potential of the transistor  709  is selected. The control signal WE 2  is a potential signal for controlling the threshold voltage of the transistor  709 , and Icut of the transistor  709  can be further reduced. Note that as the transistor  709 , the transistor without the second gate can be used. 
     In addition, in  FIG. 10 , the transistors included in the memory element  700  except for the transistor  709  can each be a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or in the substrate  1190 . For example, the transistor can be a transistor in which a channel is formed in a silicon layer or a silicon substrate. Alternatively, a transistor in which a channel is formed in an oxide semiconductor layer can be used for all the transistors used for the memory element  700 . Further alternatively, in the memory element  700 , a transistor in which a channel is formed in an oxide semiconductor layer can be included besides the transistor  709 , and a transistor in which a channel is formed in a layer or the substrate  1190  including a semiconductor other than an oxide semiconductor can be used for the rest of the transistors. 
     As the circuit  701  in  FIG. 10 , for example, a flip-flop circuit can be used. As the logic element  706 , for example, an inverter, a clocked inverter, or the like can be used. 
     In the driving method of the semiconductor device of one embodiment of the present invention, in a period during which the memory element  700  is not supplied with the power supply voltage, data stored in the circuit  701  can be held by the capacitor  708  which is provided in the circuit  702 . 
     The off-state current of a transistor in which a channel is formed in an oxide semiconductor layer is extremely low. For example, the off-state current of a transistor whose channel is formed in an oxide semiconductor layer is much lower than that of a transistor whose channel is formed in crystalline silicon. Thus, when such a transistor including an oxide semiconductor is used for the transistor  709 , a signal held in the capacitor  708  is held for a long time also in a period during which the power supply voltage is not supplied to the memory element  700 . The memory element  700  can accordingly hold the stored content (data) also in a period during which the supply of the power supply voltage is stopped. 
     Since the switch  703  and the switch  704  are provided, the memory element performs a precharge operation; thus, the time required for the circuit  701  to hold original data again after the supply of the power supply voltage is restarted can be shortened. 
     In the circuit  702 , a signal held by the capacitor  708  is input to the gate of the transistor  710 . Therefore, after supply of the power supply voltage to the memory element  700  is restarted, the signal held by the capacitor  708  can be converted into the one corresponding to the state (the on state or the off state) of the transistor  710  to be read from the circuit  702 . Consequently, an original signal can be accurately read even when a potential corresponding to the signal held by the capacitor  708  fluctuates to some degree. 
     By applying the memory element  700  to a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Furthermore, shortly after the supply of the power supply voltage is restarted, the memory element can be returned to the same state as that before the power supply is stopped. Therefore, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor. Accordingly, power consumption can be suppressed. 
     Although an example in which the memory element  700  is used in a CPU is described in this embodiment, the storage element  700  can also be used in a digital signal processor (DSP), a custom LSI, an LSI such as a programmable logic device (PLD), and a radio frequency identification (RF-ID). 
     Note that this embodiment can be combined with any of the other embodiments and examples described in this specification as appropriate. 
     Embodiment 6 
     In this embodiment, examples of an electronic appliance which can include the transistor described in Embodiment 1, the memory device described in Embodiment 3 or 4, or the CPU and the like (including a DSP, a custom LSI, a PLD, and an RF-ID) described in Embodiment 5 is described. 
     The transistor described in Embodiment 1, the memory device described in Embodiment 3 or 4, and the CPU and the like described in Embodiment 5 can be applied to a variety of electronic appliances (including game machines). Examples of the electronic appliances include display devices of televisions, monitors, and the like, lighting devices, personal computers, word processors, image reproduction devices, portable audio players, radios, tape recorders, stereos, phones, cordless phones, mobile phones, car phones, transceivers, wireless devices, game machines, calculators, portable information terminals, electronic notebooks, e-book readers, electronic translators, audio input devices, video cameras, digital still cameras, electric shavers, IC chips, high-frequency heating appliances such as microwave ovens, electric rice cookers, electric washing machines, electric vacuum cleaners, air-conditioning systems such as air conditioners, dishwashers, dish dryers, clothes dryers, futon dryers, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, radiation counters, and medical equipments such as dialyzers and X-ray diagnostic equipments. In addition, the examples of the electronic appliances include alarm devices such as smoke detectors, heat detectors, gas alarm devices, and security alarm devices. Furthermore, the examples include industrial equipment such as guide lights, traffic lights, belt conveyors, elevators, escalators, industrial robots, and power storage systems. In addition, moving objects and the like driven by fuel engines and electric motors using power from non-aqueous secondary batteries are also included in the category of electronic appliances. Examples of the moving objects include electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats or ships, submarines, helicopters, aircrafts, rockets, artificial satellites, space probes, planetary probes, spacecrafts, and the like. Some specific examples of these electronic appliances are illustrated in  FIGS. 11A to 11C . 
     An alarm device  8100  illustrated in  FIG. 11A  is a residential fire alarm, and includes a sensor portion  8102  for smoke or heat and a microcomputer  8101 . The microcomputer  8101  is an example of an electronic appliance including the transistor, the memory device, or the CPU described in any of the above embodiments. 
     An air conditioner which includes an indoor unit  8200  and an outdoor unit  8204  illustrated in  FIG. 11A  is an example of an electronic appliance including the transistor, the memory device, the CPU, or the like described in any of the above embodiments. Specifically, the indoor unit  8200  includes a housing  8201 , an air outlet  8202 , a CPU  8203 , and the like. Although the CPU  8203  is provided in the indoor unit  8200  in  FIG. 11A , the CPU  8203  may be provided in the outdoor unit  8204 . Alternatively, the CPU  8203  may be provided in both the indoor unit  8200  and the outdoor unit  8204 . The transistor described in the above embodiment is used in the CPU in the air conditioner, whereby power consumption is reduced. 
     An electric refrigerator-freezer  8300  illustrated in  FIG. 11A  is an example of an electronic appliance including the transistor, the memory device, the CPU, or the like described in any of the above embodiments. Specifically, the electric refrigerator-freezer  8300  includes a housing  8301 , a door for a refrigerator  8302 , a door for a freezer  8303 , a CPU  8304 , and the like. In  FIG. 11A , the CPU  8304  is provided in the housing  8301 . Any of the transistors described in the above embodiments is used in the CPU  8304  of the electric refrigerator-freezer  8300 , whereby power consumption is reduced. 
       FIG. 11B  illustrates an example of an electric vehicle which is an example of an electronic appliance. An electric vehicle  9700  is equipped with a secondary battery  9701 . The output of the electric power of the secondary battery  9701  is adjusted by a circuit  9702  and the electric power is supplied to a driving device  9703 . The circuit  9702  is controlled by a processing unit  9704  including a ROM, a RAM, a CPU, or the like which is not illustrated. The transistor described in the above embodiment is used in the CPU in the electric vehicle  9700 , whereby power consumption is reduced. 
     The driving device  9703  includes a DC motor or an AC motor either alone or in combination with an internal-combustion engine. The processing unit  9704  outputs a control signal to the circuit  9702  based on input data such as data of operation (e.g., acceleration, deceleration, or stop) by a driver or data during driving (e.g., data on an upgrade or a downgrade, or data on a load on a driving wheel) of the electric vehicle  9700 . The control circuit  9702  adjusts the electric energy supplied from the secondary battery  9701  in accordance with the control signal of the processing unit  9704  to control the output of the driving device  9703 . In the case where the AC motor is mounted, although not illustrated, an inverter which converts direct current into alternate current is also incorporated. 
     Note that this embodiment can be combined with any of the other embodiments and examples described in this specification as appropriate. 
     Example 1 
     In this example, a sample where a conductive film was formed over an oxide semiconductor film was formed, and changes in conductivity of the oxide semiconductor with or without heat treatment were examined. The results are described below. 
     [Sample] 
     An about 300-nm-thick silicon oxide film was formed over a glass substrate by a sputtering method. Then, an about 50-nm-thick In—Ga—Zn oxide (hereinafter referred to as IGZO) was formed as an oxide semiconductor film over the silicon oxide film by a sputtering method. Note that the IGZO film was formed by a DC sputtering method with a sputtering target of IGZO containing In, Ga, and Zn at an atomic ratio of 1:1:1 and a deposition gas containing Ar and O 2  at a flow rate ratio of 2:1. Next, as a conductive film, an approximately 50-nm-thick tungsten film was formed over the IGZO film by a sputtering method. The tungsten film was formed by a DC sputtering method with a metal tungsten sputtering target and an Ar gas used as a deposition gas. 
     Next, the glass substrate was sectioned into two pieces, one sectioned part of the substrate was subjected to heat treatment at 400° C. under nitrogen atmosphere for one hour, and the other part was not subjected to heat treatment. 
     Then, tungsten films were removed from the two substrates by etching. The etching of the tungsten films were performed by dry etching using chlorine, carbon fluoride, and oxygen as an etching gas. 
     In this manner, a sample 1 that was subjected to heat treatment after the formation of the conductive film (hereinafter, referred to as the heated sample 1) and a sample 2 that was not subjected to heat treatment (hereinafter, referred to as the non-heated sample 2) were fabricated. 
     [Measurement of Sheet Resistance] 
     Next, sheet resistances of IGZO films with respect to etching depth were measured in such a manner that the measurement of sheet resistances and etching of the IGZO films were alternately performed on the samples repeatedly. The IGZO films were etched using a mixed solution of hydrogen peroxide water and ammonia. The remaining thicknesses of the IGZO films after the etching were measured using spectroscopic ellipsometry before and after the etching to obtain the depths to which the IGZO films were etched. 
     [Measurement Result] 
       FIG. 12A  shows measurement results of sheet resistances with respect to etching depths of the heated sample 1 and the non-heated sample 2. Note that the upper limit of a measurement apparatus used for the measurement of the sheet resistance is 6×10 6  ohms per square, and the sheet resistance over the limit cannot be measured. Accordingly, the sheet resistance values over the upper limit are shown by an open circle and an open triangle in  FIG. 12A . 
     As shown in  FIGS. 12A and 12B , the non-heated sample 2 shows extremely high resistance. On the other hand, it is confirmed that the heated sample 1 shows low resistance even in a region with a depth of about 20 nm from the surface of the IGZO film. 
     Only the vertical axis is a logarithmic axis in  FIG. 12A , while the vertical and horizontal axes are logarithmic axes in  FIG. 12B . In  FIG. 12B , when a film shows a uniform conductivity in the thickness direction, the sheet resistance value is a linear function of the film thickness, and thus a set of points along a straight line is shown in the log-log graph. Because plots of the sample 2 shows a straight-line distribution in  FIG. 12B , the uniform conductivity in the film thickness direction is shown in the case where the heat treatment is not performed. On the other hand, because the heated sample 1 does not show straight-line plots, it is suggested that the heated sample 1 has conductivity distribution where the conductivity changes continuously from the surface of the IGZO film in the depth direction. 
     Through the above, it is confirmed that the low-resistant region can be formed in the semiconductor film in the following manner: the conductive film is formed in contact with the semiconductor film including an oxide semiconductor, and heat treatment is performed thereon. 
     Moreover, it is also confirmed that the low-resistant region formed in the semiconductor film can have conductivity distribution where the conductivity is lowered as it is more distant from the contact surface with the conductive film. 
     Example 2 
     In this example, the conductivity of oxide semiconductor films having different compositions was examined by the method described in Example 1. The results are described below. 
     [Sample] 
     An about 50-nm-thick IGZO film was formed as a semiconductor film over a glass substrate by a sputtering method. Four targets, an IGZO containing In, Ga, and Zn at the atomic ratio of 1:1:1, an IGZO containing In, Ga, and Zn at the atomic ratio of 1:3:2, an IGZO containing In, Ga, and Zn at the atomic ratio of 1:6:4, and an IGZO containing In, Ga, and Zn at the atomic ratio of 1:9:6, were used to form IGZO films over different substrates. The formation of the IGZO films were performed by a DC sputtering using Ar and O 2  (the flow rate of Ar to O 2  is 2:1) as a deposition gas. Then, as conductive films, approximately 100-nm-thick tungsten films were formed over the IGZO films by a sputtering method. The tungsten films were formed by a DC sputtering method with a metal tungsten sputtering target and an Ar gas used as a deposition gas. 
     Then, the substrates with the IGZO films having different compositions were subjected to heat treatment at 400° C. under nitrogen atmosphere for one hour. 
     Then, the tungsten films were removed from the substrates by etching. The etching was performed under the conditions similar to those in Example 1. 
     In this manner, four samples were fabricated: a sample 3 was formed using the IGZO containing In, Ga, and Zn at the atomic ratio of 1:1:1 as a sputtering target; a sample 4 was formed using the IGZO containing In, Ga, and Zn at the atomic ratio of 1:3:2 as a sputtering target; a sample 5 was formed using the IGZO containing In, Ga, and Zn at the atomic ratio of 1:6:4 as a sputtering target; and a sample 6 was formed using the IGZO containing In, Ga, and Zn at the atomic ratio of 1:9:6 as a sputtering target. 
     [Measurement of Sheet Resistance] 
     Next, sheet resistances of the oxide semiconductor films in the samples with respect to etching depth were measured in a manner similar to that in Example 1. 
     [Measurement Result] 
       FIG. 13A  shows measurement results of sheet resistances with respect to etching depth in the samples. 
     As shown in  FIG. 13A , it is known that the content of Ga lower than the content of In shows lower resistance. Moreover, the low-resistant region expands to a region distant (deep) from the surface. Although not shown in the figure, because the samples 3 to 5 do not show straight-line plots in the log-log graph, the conductivity distribution in the depth direction where higher resistance is shown in a region more distant from the surface can be confirmed. 
       FIG. 13B  is a graph showing plotted values of sheet resistances at 0 nm of etching depth in the IGZO films in  FIG. 13A  with the atomic ratio (content) of Ga to In in the IGZO films. As is apparent from  FIG. 13B , as the content of Ga is increased with respect to the content of In, the sheet resistance values are increased exponentially. Specifically, when the content of Ga to In is increased three times with the sample 3 (Ga/In=1) regarded as the basis, the sheet resistance is increased ten or more times. 
     Through the above, it is confirmed that the material having the content of Ga to In higher than (preferably three or more times as high as) that of the semiconductor layer can be used for the oxide layer described in Embodiment 1 or the like. In particular, when an oxide layer having the content of Ga to In three or more times as high as that of the semiconductor layer is used as the oxide layer, the sheet resistance of the oxide layer can be increased by one digit higher than the sheet resistance of the semiconductor layer, and thereby can be sufficiently high resistance. 
     This application is based on Japanese Patent Application serial no. 2013-091211 filed with Japan Patent Office on Apr. 24, 2013, the entire contents of which are hereby incorporated by reference.