Patent Publication Number: US-9905695-B2

Title: Multi-layered oxide semiconductor transistor

Description:
This application is a continuation of U.S. application Ser. No. 14/272,767, filed on May 8, 2016 which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a storage device, an arithmetic unit, an imaging device, a method for driving any of them, or a method for manufacturing any of them. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. In some cases, a storage device, a display device, or an electronic device includes a semiconductor device. 
     BACKGROUND ART 
     Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another example, an oxide semiconductor has attracted attention. 
     For example, a transistor whose active layer includes an amorphous oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
       
    
     DISCLOSURE OF INVENTION 
     A high density of an integrated circuit requires miniaturization of a transistor, and a transistor having a simple structure and a simple manufacturing method of a transistor are required because the miniaturization increases the degree of difficulty of a manufacturing process. 
     In addition, it is known that miniaturization of a transistor is likely to cause deterioration of or variation in electrical characteristics of the transistor. In other words, miniaturization of a transistor is likely to cause a decrease in yield of an integrated circuit. 
     Thus, one object of one embodiment of the present invention is to provide a semiconductor device having a structure with which the device can be manufactured through a simple process even in the case of miniaturization. Another object is to provide a semiconductor device having a structure with which a decrease in a yield due to miniaturization can be suppressed. Another object of one embodiment of the present invention is to provide a semiconductor device in which deterioration of electrical characteristics which becomes more noticeable as the transistor is miniaturized can be suppressed. Another object is to provide a semiconductor device having a high degree of integration. Another object is to provide a semiconductor device in which deterioration of electrical characteristics is reduced. Another object is to provide a semiconductor device in which variation in electrical characteristics is reduced. Another object is to provide a semiconductor device with low power consumption. Another object is to provide a semiconductor device with high reliability. Another object is to provide a semiconductor device which can retain data even when power supply is stopped. Another object is to provide a method for manufacturing the semiconductor device. 
     Note that the descriptions of these objects do not disturb the existence of other objects. Note that in one embodiment of the present invention, there is no need to achieve all the objects. Other objects are 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 relates to a semiconductor device in which a source electrode layer or a drain electrode layer is formed on an upper surface of an oxide semiconductor layer. 
     Note that “side contact” in this specification means a state where a side surface of one element is in contact with part of the other element, so that electrical connection between the one element and the other element is obtained. 
     One embodiment of the present invention is a semiconductor device including a first oxide semiconductor layer over an insulating surface; a second oxide semiconductor layer over the first oxide semiconductor layer; a source electrode layer and a drain electrode layer which are over the second oxide semiconductor layer and whose side surfaces are provided on the same surface as side surfaces of the second oxide semiconductor layer; a third oxide semiconductor layer which is over the second oxide semiconductor layer and partly in contact with each of the source electrode layer and the drain electrode layer; a gate insulating film over the third oxide semiconductor layer; a gate electrode layer over the gate insulating film; and an insulating layer over the insulating surface, the source electrode layer, the drain electrode layer, and the gate electrode layer. In the insulating layer, a first opening where part of the second oxide semiconductor layer and part of the source electrode layer are exposed, a second opening where part of the second oxide semiconductor layer and part of the drain electrode layer are exposed, and a third opening where part of the gate electrode layer is exposed are formed. The second oxide semiconductor layer and the source electrode layer are electrically connected to a first wiring in the first opening. The second oxide semiconductor layer and the drain electrode layer are electrically connected to a second wiring in the second opening. The gate electrode layer is electrically connected to a third wiring in the third opening. 
     Note that in this specification and the like, ordinal numbers such as “first” and “second” are used in order to avoid confusion among components and do not limit the components numerically. 
     Further, a conduction band minimum of the first oxide semiconductor layer and a conduction band minimum of the third oxide semiconductor layer are preferably closer to a vacuum level than a conduction band minimum of the second oxide semiconductor layer by 0.05 eV or more and 2 eV or less. 
     It is preferable that the first oxide semiconductor layer, the second oxide semiconductor layer, and the third oxide semiconductor layer each include an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), and that an atomic ratio of M to In in each of the first oxide semiconductor layer and the third oxide semiconductor layer be higher than an atomic ratio of M to In in the second oxide semiconductor layer. 
     Each of the first oxide semiconductor layer, the second oxide semiconductor layer, and the third oxide semiconductor layer preferably includes a crystal in which c-axes are aligned. 
     Further, the source electrode layer and the drain electrode layer are each preferably formed of a single layer of Al, Cr, Cu, Ta, Ti, Mo, or W, a stacked film of any of these, or an alloy material containing any of these as its main component. 
     Another embodiment of the present invention is a method for manufacturing a semiconductor device comprising the steps of: forming a stacked film of a first oxide semiconductor film and a second oxide semiconductor film over an insulating surface; forming a first conductive film over the stacked film; forming a first resist mask over the first conductive film; selectively etching the first conductive film using the first resist mask as a mask to form a first conductive layer; selectively etching the stacked film using the first conductive layer as a mask and selectively etching the first conductive layer to divide the first conductive layer, thereby forming a stack of a first oxide semiconductor layer and a second oxide semiconductor layer and a source electrode layer and a drain electrode layer over the stack; forming a third oxide semiconductor film over the insulating surface, the stack, the source electrode layer, and the drain electrode layer; forming an oxide insulating film over the third oxide semiconductor film; forming a second conductive film over the oxide insulating film; forming a second resist mask over the second conductive film; selectively etching the second conductive film using the second resist mask as a mask to form a gate electrode layer; selectively etching the oxide insulating film and the third oxide semiconductor film using the gate electrode layer as a mask to form a gate insulating film and a third oxide semiconductor layer; forming an insulating layer over the insulating surface, the source electrode layer, the drain electrode layer, and the gate electrode layer; forming, in the insulating layer, a first opening where part of the second oxide semiconductor layer and part of the source electrode layer are exposed, a second opening where part of the second oxide semiconductor layer and part of the drain electrode layer are exposed, and a third opening where part of the gate electrode layer is exposed; and forming a first wiring electrically connected to the second oxide semiconductor layer and the source electrode layer in the first opening, a second wiring electrically connected to the second oxide semiconductor layer and the drain electrode layer in the second opening, and a third wiring electrically connected to the gate electrode layer in the third opening. 
     Further, the first oxide semiconductor layer and the third oxide semiconductor layer are each preferably formed using a material in which a conduction band minimum of the first oxide semiconductor layer and a conduction band minimum of the third oxide semiconductor layer are closer to a vacuum level than a conduction band minimum of the second oxide semiconductor layer is by 0.05 eV or more and 2 eV or less. 
     The first oxide semiconductor layer, the second oxide semiconductor layer, and the third oxide semiconductor layer are each preferably formed using an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), and that an atomic ratio of M to In in each of the first oxide semiconductor layer and the third oxide semiconductor layer be higher than an atomic ratio of M to In in the second oxide semiconductor layer. 
     For each of the first oxide semiconductor layer, the second oxide semiconductor layer, and the third oxide semiconductor layer, a material including a crystal in which c-axes are aligned is preferably used. 
     In the above structure, it is preferable that the source electrode layer and the drain electrode layer be each formed using a single layer of Al, Cr, Cu, Ta, Ti, Mo, or W, a stacked layer of any of these, or an alloy material containing any of these as its main component. 
     According to one embodiment of the present invention, a semiconductor device having a structure with which the device can be manufactured in a simple process even when the device is miniaturized can be provided. Alternatively, a semiconductor device having a structure which can prevent a decrease in yield caused by miniaturization can be provided. Alternatively, a semiconductor device in which a deterioration in electrical characteristics which becomes more noticeable as the transistor is miniaturized can be suppressed can be provided. Alternatively, a highly integrated semiconductor device can be provided. Alternatively, a semiconductor device in which deterioration in electrical characteristics is reduced can be provided. Alternatively, a semiconductor device in which variation in electrical characteristics is suppressed can be provided. Alternatively, a semiconductor device with low power consumption can be provided. Alternatively, a highly reliable semiconductor device can be provided. Alternatively, a semiconductor device in which data is retained even when not powered can be provided. Alternatively, a method for manufacturing the above semiconductor device can be provided. 
     Note that the descriptions of these effects do not disturb the existence of other effects. In one embodiment of the present invention, there is no need to obtain all the effects. Other effects are apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are a top view and a cross-sectional view of a transistor; 
         FIGS. 2A and 2B  are cross-sectional views of a transistor; 
         FIGS. 3A to 3C  are cross-sectional views of transistors; 
         FIGS. 4A to 4C  are cross-sectional views of transistors; 
         FIG. 5  is a cross-sectional view of a transistor; 
         FIGS. 6A and 6B  are cross-sectional views of transistors; 
         FIG. 7  is a cross-sectional view of a transistor; 
         FIGS. 8A to 8C  are cross-sectional views illustrating a method for manufacturing a transistor; 
         FIGS. 9A to 9C  are cross-sectional views illustrating a method for manufacturing a transistor; 
         FIGS. 10A and 10B  are cross-sectional views illustrating a method for manufacturing a transistor; 
         FIGS. 11A and 11B  are a cross-sectional view and a circuit diagram of a semiconductor device; 
         FIG. 12  is a circuit diagram of a semiconductor device; 
         FIGS. 13A to 13C  illustrate electronic devices to which semiconductor devices can be applied; 
         FIGS. 14A and 14B  are a top view and a cross-sectional view of a transistor; 
         FIGS. 15A and 15B  are a top view and a cross-sectional view of a transistor; 
         FIGS. 16A to 16D  show models used for calculation and calculation results thereof; and 
         FIGS. 17A and 17B  show Id-Vg characteristics of a transistor. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments are described in detail with reference to the drawings. Note that the present invention is not limited to the following description and it is readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention should not be limited to the descriptions of the embodiments below. Note that in 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 thereof is omitted in some cases. 
     Note that in this specification and the like, when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected, the case where X and Y are functionally connected, and the case where X and Y are directly connected are included therein. Here, each of X and Y denotes an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, a layer, or the like). Accordingly, a connection relation other than connection relations shown in the drawings and texts is also included, without being limited to a predetermined connection relation, for example, a connection relation shown in the drawings and texts. 
     In the case where X and Y are electrically connected, one or more elements (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, and a load) that enable an electrical connection between X and Y can be connected between X and Y, for example. Note that the switch is controlled to be turned on or off. That is, the switch has a function of determining whether current flows or not by being turned on or off (becoming an on state and an off state). Alternatively, the switch has a function of selecting and changing a current path. 
     In the case where X and Y are functionally connected, one or more circuits (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit; a signal converter circuit such as a DA converter circuit, an AD converter circuit, or a gamma correction circuit; a potential level converter circuit such as a power supply circuit (e.g., a step-up circuit or a step-down circuit) or a level shifter circuit for changing the potential level of a signal; a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a storage circuit; and a control circuit) that enable a functional connection between X and Y can be connected between X and Y, for example. Note that for example, in the case where a signal output from X is transmitted to Y even when another circuit is interposed between X and Y, X and Y are functionally connected. 
     Note that when it is explicitly described that X and Y are connected, the case where X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), the case where X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and the case where X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween) are included therein. That is, when it is explicitly described that “X and Y are electrically connected”, the description is the same as the case where it is explicitly only described that “X and Y are connected”. 
     Even when independent components are electrically connected to each other in a circuit diagram, one component has functions of a plurality of components in some cases. For example, when part of a wiring also functions as an electrode, one conductive film functions as the wiring and the electrode. Thus, an “electrical connection” in this specification includes in its category such a case where one conductive film has functions of a plurality of components. 
     Note that in this specification and the like, a transistor can be formed using any of a variety of substrates. The type of a substrate is not limited to a certain type. Examples of the substrate include a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, and a base material film. Examples of a glass substrate include a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, and a soda lime glass substrate. For a flexible substrate, a flexible synthetic resin such as plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES), or acrylic can be used, for example. Examples of an attachment film include attachment films formed using polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, and the like. Examples of a base film include a base film formed using polyester, polyamide, polyimide, an inorganic vapor deposition film, paper, and the like. Specifically, when a transistor is formed using a semiconductor substrate, a single crystal substrate, an SOI substrate, or the like, a transistor with few variations in characteristics, size, shape, or the like, high current supply capability, and a small size can be formed. By forming a circuit using such a transistor, power consumption of the circuit can be reduced or the circuit can be highly integrated. 
     Note that a transistor may be formed using one substrate, and then, the transistor may be transferred to another substrate. Examples of a substrate to which a transistor is transferred include, in addition to the above-described substrates over which transistors can be formed, a paper substrate, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), or the like), a leather substrate, a rubber substrate, and the like. With the use of such a substrate, a transistor with excellent properties, a transistor with low power consumption, or a device with high durability can be formed, high heat resistance can be provided, or a reduction in weight or thinning can be achieved. 
     Embodiment 1 
     In this embodiment, a semiconductor device of one embodiment of the present invention is described with reference to drawings. 
       FIGS. 1A and 1B  are a top view and a cross-sectional view of a transistor of one embodiment of the present invention.  FIG. 1A  is the top view.  FIG. 1B  illustrates a cross section taken along dashed-dotted line A 1 -A 2  in.  FIG. 1A .  FIG. 2A  is a cross-sectional view taken along dashed-dotted line A 3 -A 4  in  FIG. 1A .  FIG. 2B  is a cross-sectional view taken along dashed-dotted line A 5 -A 6  in  FIG. 1A . Note that for simplification of the drawing, some components are not illustrated in the top view in  FIG. 1A . In some cases, the direction of the dashed-dotted line A 1 -A 2  is referred to as a channel length direction, and the direction of the dashed-dotted line A 3 -A 4  is referred to as a channel width direction. 
     A transistor  100  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B  includes a base insulating film  120  over a substrate  110 , a stack in which a first oxide semiconductor layer  131  and a second oxide semiconductor layer  132  are formed in this order and which is over the base insulating film, a source electrode layer  140  and a drain electrode layer  150  over the second oxide semiconductor layer, a third oxide semiconductor layer  133  which is formed in contact with the base insulating film  120  and the stack and is partly in contact with each of the source electrode layer  140  and the drain electrode layer  150 , a gate insulating film  160  over the third oxide semiconductor layer, a gate electrode layer  170  over the gate insulating film, and an insulating layer  180  over the base insulating film  120 , the source electrode layer  140 , the drain electrode layer  150 , and the gate electrode layer  170 . 
     Note that functions of a “source” and a “drain” of a transistor are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Thus, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification. 
     An insulating layer  185  formed of an oxide may be formed over the insulating layer  180 . Note that the insulating layer  185  may be provided as needed and another insulating layer may be further provided thereover. The first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133  are collectively referred to as an oxide semiconductor layer  130 . 
     In the insulating layer  180 , a first opening  147  where the second oxide semiconductor layer  132  and the source electrode layer  140  are partly exposed is formed. Further, a second opening  157  where the second oxide semiconductor layer  132  and the drain electrode layer  150  are partly exposed is formed. Furthermore, a third opening  177  where the gate electrode layer  170  is partly exposed is formed. 
     In the first opening  147 , a side surface of the second oxide semiconductor layer  132  and a side surface of the source electrode layer  140  are provided on the same surface and are electrically connected to a first wiring  145 . In the second opening  157 , a side surface of the second oxide semiconductor layer  132  and a side surface of the drain electrode layer  150  are provided on the same surface and are electrically connected to a second wiring  155 . In the third opening  177 , the gate electrode layer  170  is electrically connected to a third wiring  175  by side contact. 
     Conventionally, electrical connection has been obtained by providing an opening in an insulating layer and the like formed on an electrode layer so that part of a wiring formed in the opening is in contact with part of the upper surface of the electrode layer. 
     However, as miniaturization of the transistor progresses, the degree of difficulty in manufacturing increases, which results in a defect in the opening provided in the insulating layer or the like, variation in the depth of the opening, and the like. Thus, contact resistance between the electrode layer and the wiring is likely to vary among elements. In other words, an increase in the degree of difficulty in manufacturing a miniaturized transistor is one factor of variation in the electrical characteristics of transistors. 
     On the other hand, in one embodiment of the present invention, part of an electrode layer exposed in an opening and part of a wiring formed in the opening are electrically connected to each other by side contact. Thus, variation in contact area between the electrode layer and the wiring can be less likely to occur. In other words, variation in contact resistance between the electrode layer and the wiring in elements can be suppressed, which enables reduction in variation in the electrical characteristics of a transistor which is caused by the variation. 
     Further, in the case where an opening is provided in an insulating layer to expose an electrode layer and the like, over-etching an electrode layer and the like to expose side surfaces of the electrode layer and the like in the opening is less difficult than exposing upper surfaces of the electrode layer and the like by controlling etching conditions strictly. In the case where an opening is formed so as to extend into the electrode layer, for example, the etching conditions can be freely selected even when the etching rate of the electrode layer is sufficiently lower than that of the insulating layer. Accordingly, the yield of the transistor can be improved. 
     In one embodiment of the present invention, it is preferable to employ a structure in which the first opening  147  and the second opening  157  reach the base insulating film  120  as illustrated in  FIG. 1B . The structure can be formed under etching conditions having a high degree of freedom and can reduce variation in electrical characteristics of a transistor and improve yield. Moreover, since a wiring in contact with a semiconductor layer functions as part of an electrode layer, contact resistance between the electrode layer and the semiconductor layer can be further reduced. 
     Further, when the gate electrode layer  170  is connected to the third wiring  175  by side contact as illustrated in  FIGS. 2A and 2B , variation in contact area between the electrode layer and the wiring can be less likely to occur and variation in contact resistance can be suppressed. Note that the bottom of the third opening  177  is positioned in a range D (in any of the gate insulating film  160 , the third oxide semiconductor layer  133 , and the base insulating film  120 ) in the drawing. 
     Note that the structures inside the first opening  147  and the second opening  157  are not limited to the example illustrated in  FIG. 1B . For example, as illustrated in  FIG. 3A , a structure may be employed in which upper surfaces of the source electrode layer  140  and the drain electrode layer  150  are partly exposed in the first opening  147  and the second opening  157 . When the etching rates of the source electrode layer  140  and the drain electrode layer  150  are sufficiently lower than that of the insulating layer  180 , the structure can be formed easily. 
     Alternatively, as illustrated in  FIG. 3B , a structure may be employed in which an upper surface of the second oxide semiconductor layer  132  is partly exposed in the first opening  147  and the second opening  157 . Further alternatively, although not illustrated, a structure may be employed in which an upper surface of the first oxide semiconductor layer  131  is partly exposed in the openings. When the etching rate of the second oxide semiconductor layer  132  and/or the etching rate of the first oxide semiconductor layer  131  are/is sufficiently lower than that of the insulating layer  180 , the structure can be formed easily. 
     Note that in the description of  FIGS. 3A and 3B , a layer whose upper surface is partly exposed may be partly etched in the film thickness direction. 
     Still further alternatively, as illustrated in  FIG. 3C , the bottoms of the first opening  147  and the second opening  157  may be positioned in the base insulating film  120 . When the etching rate of the insulating layer  180  is close to the etching rate of each of the source electrode layer  140 , the drain electrode layer  150 , the second oxide semiconductor layer  132 , the first oxide semiconductor layer  131 , and the base insulating film  120 , the structure can be formed easily. 
     Note that when the etching conditions can be controlled strictly, as illustrated in  FIGS. 14A and 14B , a structure may be employed in which upper surfaces of the source electrode layer  140  and the drain electrode layer  150  are partly exposed to be in contact with the first wiring  145  and the second wiring  155 . 
     Alternatively, in a transistor of one embodiment of the present invention, as illustrated in  FIGS. 15A and 15B , upper surface shapes of the third oxide semiconductor layer  133  and the gate insulating film  160  may be different from an upper surface shape of the gate electrode layer  170 . The structure illustrated in  FIGS. 15A and 15B  can reduce gate leakage current. Note that the structure can be applied to another transistor described in this embodiment. 
     Since the source electrode layer  140  and the drain electrode layer  150  are formed only over the oxide semiconductor layer in the transistor of one embodiment of the present invention, there is a concern that an effective channel width is shortened and thus the on-state current decreases slightly; however, application of gate electric field to a side portion of the oxide semiconductor layer is not blocked and thus gate electric field is applied to the entire oxide semiconductor layer, whereby the S value of the transistor can be decreased. The effect is confirmed by the scientific calculation described below. 
       FIG. 16A  is a top view of a model (a) assuming a transistor having a conventional structure, and the width of each of the source electrode layer  140  and the drain electrode layer  150  is larger than that of the oxide semiconductor layer.  FIG. 16B  is a top view of a model (b) assuming one embodiment of the present invention, and the width of each of the source electrode layer  140  and the drain electrode layer  150  is the same as that of the oxide semiconductor layer. 
       FIG. 16C  shows calculation results of current density distributions in cross sections of channel portions in the W width direction at a drain current of about 1E-12 [A] in the models. The left part of  FIG. 16C  shows calculation results of the model (a) and the current density is high around the center of a lower layer of the channel portion. In other words, current cannot be controlled around the center of the lower layer of the channel portion. On the other hand, the right part of  FIG. 16C  shows calculation results of the model (b) and the current density is high near an upper layer of the channel portion. This is because gate electric field is sufficiently applied from the side surface. 
     As shown in  FIG. 16D , from Id-Vg characteristics obtained by calculation using the above models, it is found that the S value of the model (b) assuming one embodiment of the present invention is extremely small as compared to the model (a). 
     Next, components of the transistor  100  of one embodiment of the present invention are described in detail. 
     The substrate  110  is not limited to a simple supporting substrate, and may be a substrate where another device such as a transistor is formed. In that case, one of the gate electrode layer  170 , the source electrode layer  140 , and the drain electrode layer  150  of the transistor  100  may be electrically connected to the above device. 
     The base insulating film  120  can have a function of supplying oxygen to the oxide semiconductor layer  130  as well as a function of preventing diffusion of impurities from the substrate  110 . For this reason, the base insulating film  120  is preferably an insulating film containing oxygen and further preferably, the base insulating film  120  is an insulating film containing oxygen in which the oxygen content is higher than that in the stoichiometric composition. In the case where the substrate  110  is provided with another device as described above, the base insulating film  120  also has a function as an interlayer insulating film. In that case, the base insulating film  120  is preferably subjected to planarization treatment such as chemical mechanical polishing (CMP) treatment so as to have a flat surface. 
     Further, in a region where a channel of the transistor  100  is formed, the oxide semiconductor layer  130  has a structure in which the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133  are stacked in this order from the substrate  110  side. Furthermore, in the first oxide semiconductor layer  131 , a region not overlapping with the second oxide semiconductor layer  132 , the source electrode layer  140 , and the drain electrode layer  150  is in contact with the third oxide semiconductor layer  133 , which means that the second oxide semiconductor layer  132  is surrounded by the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133 . 
     Here, for the second oxide semiconductor layer  132 , for example, an oxide semiconductor whose electron affinity (an energy difference between a vacuum level and the conduction band minimum) is higher than those of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  is used. The electron affinity can be obtained by subtracting an energy difference between the conduction band minimum and the valence band maximum (what is called an energy gap) from an energy difference between the vacuum level and the valence band maximum (what is called an ionization potential). 
     Although the case where the oxide semiconductor layer  130  is a stack of three layers is described in detail in this embodiment, the oxide semiconductor layer  130  may be a single layer or a stack of two layers or four or more layers. In the case where the oxide semiconductor layer  130  is a single layer, for example, a layer corresponding to the second oxide semiconductor layer  132  is used as illustrated in  FIG. 4A . In the case where the oxide semiconductor layer  130  is a stack of two layers, for example, a structure without the third oxide semiconductor layer  133  is used as illustrated in  FIG. 4B . In such a case, the second oxide semiconductor layer  132  and the first oxide semiconductor layer  131  can be replaced with each other. In the case where the oxide semiconductor layer  130  is a stack of three layers, a structure different from that in  FIGS. 1A and 1B , such as that in  FIG. 4C , can be employed. In the case of a stack of four or more layers, for example, a structure in which an oxide semiconductor layer is stacked over the three-layer stacked structure described in this embodiment or a structure in which an oxide semiconductor layer is provided between any of two layers in the three-layer stacked structure can be employed. 
     It is preferable that each of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  contain one or more kinds of metal elements forming the second oxide semiconductor layer  132  and be formed, for example, using an oxide semiconductor whose energy of the conduction band minimum is closer to the vacuum level than that of the second oxide semiconductor layer  132  by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     In such a structure, when an electric field is applied to the gate electrode layer  170 , a channel is formed in the second oxide semiconductor layer  132  whose conduction band minimum is the lowest in the oxide semiconductor layer  130 . In other words, the third oxide semiconductor layer  133  is formed between the second oxide semiconductor layer  132  and the gate insulating film  160 , whereby a structure in which the channel of the transistor is not in contact with the gate insulating film  160  is obtained. 
     Further, since the first oxide semiconductor layer  131  contains one or more metal elements contained in the second oxide semiconductor layer  132 , an interface state is less likely to be formed at the interface of the second oxide semiconductor layer  132  with the first oxide semiconductor layer  131  than at the interface with the base insulating film  120  on the assumption that the second oxide semiconductor layer  132  is in contact with the base insulating film  120 . The interface state sometimes forms a channel, leading to a change in the threshold voltage of the transistor. Thus, with the first oxide semiconductor layer  131 , variation in the electrical characteristics of the transistors, such as a threshold voltage, can be reduced. Further, the reliability of the transistor can be improved. 
     Furthermore, since the third oxide semiconductor layer  133  contains one or more metal elements contained in the second oxide semiconductor layer  132 , scattering of carriers is less likely to occur at the interface of the second oxide semiconductor layer  132  with the third oxide semiconductor layer  133  than at the interface with the gate insulating film  160  on the assumption that the second oxide semiconductor layer  132  is in contact with the gate insulating film  160 . Thus, with the third oxide semiconductor layer  133 , the field-effect mobility of the transistor can be increased. 
     For the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133 , for example, a material containing Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that used for the second oxide semiconductor layer  132  can be used. Specifically, an atomic ratio of any of the above metal elements in the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  is 1.5 times or more, preferably 2 times or more, further preferably 3 times or more as much as that in the second oxide semiconductor layer  132 . Any of the above elements is strongly bonded to oxygen and thus has a function of suppressing generation of an oxygen vacancy in an oxide semiconductor layer. That is, an oxygen vacancy is less likely to be generated in the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  than in the second oxide semiconductor layer  132 . 
     Note that when each of the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133  is an In-M-Zn oxide containing at least indium, zinc, and M (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and the first oxide semiconductor layer  131  has an atomic ratio of In to M and Zn which is x 1 :y 1 :z 1 , the second oxide semiconductor layer  132  has an atomic ratio of In to M and Zn which is x 2 :y 2 :z 2 , and the third oxide semiconductor layer  133  has an atomic ratio of In to M and Zn which is x 3 :y 3 :z 3 , each of y 1 /x 1  and y 3 /x 3  is preferably larger than y 2 /x 2 . Each of y 1 /x 1  and y 3 /x 3  is 1.5 times or more, preferably 2 times or more, further preferably 3 times or more as large as y 2 /x 2 . At this time, when y 2  is greater than or equal to x 2  in the second oxide semiconductor layer  132 , the transistor can have stable electrical characteristics. However, when y 2  is 3 times or more as large as x 2 , the field-effect mobility of the transistor is reduced; accordingly, y 2  is preferably less than 3 times x 2 . 
     Note that in this specification, an atomic ratio used for describing the composition of an oxide semiconductor layer can be also used as the atomic ratio of a base material. In the case where an oxide semiconductor layer is deposited by a sputtering method using an oxide semiconductor material as a target, the composition of the oxide semiconductor layer might be different from that of the target, which is a base material, depending on the kind or a ratio of a sputtering gas, the density of the target, or deposition conditions. Thus, in this specification, an atomic ratio used for describing the composition of an oxide semiconductor layer is also used as the atomic ratio of a base material. For example, in the case where a sputtering method is used for deposition, an In—Ga—Zn oxide film whose atomic ratio of In to Ga and Zn is 1:1:1 can be also understood as an In—Ga—Zn oxide film formed using an In—Ga—Zn oxide material whose atomic ratio of In to Ga and Zn is 1:1:1 as a target. 
     Further, in the case where Zn and O are not taken into consideration, the proportion of In and the proportion of M in each of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  are preferably less than 50 atomic % and greater than or equal to 50 atomic %, respectively, and further preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. In addition, in the case where Zn and O are not taken into consideration, the proportion of In and the proportion of M in the second oxide semiconductor layer  132  are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, and further preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively. 
     The thicknesses of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  are each greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm. The thickness of the second oxide semiconductor layer  132  is greater than or equal to 1 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm. 
     For each of the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133 , an oxide semiconductor containing indium, zinc, and gallium can be used, for example. Note that the second oxide semiconductor layer  132  preferably contains indium because carrier mobility can be increased. 
     Note that stable electrical characteristics can be effectively imparted to a transistor in which an oxide semiconductor layer serves as a channel by reducing the concentration of impurities in the oxide semiconductor layer to make the oxide semiconductor layer intrinsic or substantially intrinsic. The term “substantially intrinsic” refers to the state where an oxide semiconductor layer has a carrier density lower than 1×10 17 /cm 3 , preferably lower than 1×10 15 /cm 3 , further preferably lower than 1×10 13 /cm 3 . 
     Further, in the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon, and a metal element other than main components are impurities. For example, hydrogen and nitrogen form donor levels to increase the carrier density, and silicon forms impurity levels in the oxide semiconductor layer. The impurity levels serve as traps and might cause the electrical characteristics of the transistor to deteriorate. Thus, it is preferable to reduce the concentration of the impurities in the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133 , and at interfaces between the layers. 
     In order to make the oxide semiconductor layer intrinsic or substantially intrinsic, in SIMS (secondary ion mass spectrometry), for example, the concentration of silicon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than 1×10 19  atoms/cm 3 , further preferably lower than 5×10 18  atoms/cm 3 , still further preferably lower than 1×10 18  atoms/cm 3 . Further, the concentration of hydrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than or equal to 2×10 20  atoms/cm 3 , further preferably lower than or equal to 5×10 19  atoms/cm 3 , still further preferably lower than or equal to 1×10 19  atoms/cm 3 , yet still further preferably lower than or equal to 5×10 18  atoms/cm 3 . Further, the concentration of nitrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than 5×10 19  atoms/cm 3 , further preferably lower than or equal to 5×10 18  atoms/cm 3 , still further preferably lower than or equal to 1×10 18  atoms/cm 3 , yet still further preferably lower than or equal to 5×10 17  atoms/cm 3 . 
     In the case where the oxide semiconductor layer includes crystals, high concentration of silicon or carbon might reduce the crystallinity of the oxide semiconductor layer. In order not to lower the crystallinity of the oxide semiconductor layer, for example, the concentration of silicon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer may be lower than 1×10 19  atoms/cm 3 , preferably lower than 5×10 18  atoms/cm 3 , further preferably lower than 1×10 18  atoms/cm 3 . Further, the concentration of carbon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer may be lower than 1×10 19  atoms/cm 3 , preferably lower than 5×10 18  atoms/cm 3 , further preferably lower than 1×10 18  atoms/cm 3 , for example. 
     A transistor in which the above-described highly purified oxide semiconductor layer is used for a channel formation region has an extremely low off-state current. In the case where the voltage between a source and a drain is set to about 0.1 V, 5 V, or 10 V, for example, the off-state current standardized on the channel width of the transistor can be as low as several yoctoamperes per micrometer to several zeptoamperes per micrometer. 
     Note that as the gate insulating film of the transistor, an insulating film containing silicon is used in many cases; thus, it is preferable that, as in the transistor of one embodiment of the present invention, a region of the oxide semiconductor layer, which serves as a channel, be not in contact with the gate insulating film for the above-described reason. In the case where a channel is formed at the interface between the gate insulating film and the oxide semiconductor layer, scattering of carriers occurs at the interface, whereby the field-effect mobility of the transistor is reduced in some cases. Also from the view of the above, it is preferable that the region of the oxide semiconductor layer, which serves as a channel, be separated from the gate insulating film. 
     Accordingly, with the oxide semiconductor layer  130  having a stacked-layer structure including the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133 , a channel can be formed in the second oxide semiconductor layer  132 ; thus, the transistor can have a high field-effect mobility and stable electrical characteristics. 
     In the band structures of the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133 , the energy of the conduction band minimum continuously changes. This can be understood also from the fact that the compositions of the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133  are close to one another and oxygen is easily diffused among the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133 . Thus, the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133  have a continuous physical property although they have different compositions and form a stack. In the drawings, interfaces between the oxide semiconductor layers of the stack are indicated by dotted lines. 
     The oxide semiconductor layer  130  in which layers containing the same main components are stacked is formed to have not only a simple stacked-layer structure of the layers but also a continuous junction (here, in particular, a well structure having a U shape in which energies of the conduction band minimums successively vary between layers). In other words, the stacked-layer structure is formed such that there exists no impurity that forms a defect level such as a trap center or a recombination center at each interface. If impurities exist between the stacked oxide semiconductor layers, the continuity of the energy band is lost and carriers disappear by a trap or recombination. 
     For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:6:4, or 1:9:6 can be used for the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1, 5:5:6, or 3:1:2 can be used for the second oxide semiconductor layer  132 . 
     The second oxide semiconductor layer  132  of the oxide semiconductor layer  130  serves as a well, so that a channel is formed in the second oxide semiconductor layer  132  in a transistor including the oxide semiconductor layer  130 . Note that since the energies of the conduction band minimums continuously changes, the oxide semiconductor layer  130  can also be referred to as a U-shaped well. Further, a channel formed to have such a structure can also be referred to as a buried channel. 
     Note that trap levels due to impurities or defects might be formed in the vicinity of the interface between an insulating film such as a silicon oxide film and each of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133 . The second oxide semiconductor layer  132  can be distanced away from the trap levels owing to existence of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133 . 
     However, when the energy difference between the conduction band minimum of the first oxide semiconductor layer  131  and the conduction band minimum of the second oxide semiconductor layer  132  and the energy difference between the conduction band minimum of the third oxide semiconductor layer  133  and the conduction band minimum of the second oxide semiconductor layer  132  are small, an electron in the second oxide semiconductor layer  132  might reach the trap level by passing over the energy differences. When the electron is trapped in the trap level, a negative fixed charge is generated at the interface with the insulating film, whereby the threshold voltage of the transistor is shifted in the positive direction. 
     Thus, to reduce fluctuations in the threshold voltage of the transistor, energy differences of at least certain values between the conduction band minimum of the second oxide semiconductor layer  132  and the conduction band minimum of each of the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133  is necessary. Each of the energy differences is preferably greater than or equal to 0.1 eV, further preferably greater than or equal to 0.15 eV. 
     Note that the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , and the third oxide semiconductor layer  133  preferably include crystal parts. In particular, when crystals in which c-axes are aligned are used, the transistor can have stable electrical characteristics. 
     In the case where an In—Ga—Zn oxide is used for the oxide semiconductor layer  130 , it is preferable that the third oxide semiconductor layer  133  contain less In than the second oxide semiconductor layer  132  so that diffusion of In to the gate insulating film is prevented. 
     For the source electrode layer  140 , the drain electrode layer  150 , the first wiring  145 , the second wiring  155 , and the third wiring  175 , a conductive material which is easily bonded to oxygen is preferably used. For example, Al, Cr, Cu, Ta, Ti, Mo, or W can be used. Among the materials, it is particularly preferable to use Ti, which is easily bonded to oxygen, or W, which has a high melting point and thus allows subsequent process temperatures to be relatively high. Note that the conductive material which is easily bonded to oxygen includes, in its category, a material to which oxygen is easily diffused. Note that the first wiring  145 , the second wiring  155 , and the third wiring  175  may each be a stack such as Ti/Al/Ti. 
     In addition, a conductive material which is not easily bonded to oxygen may be used as needed. For example, it is possible to use a single layer formed of a material containing tantalum nitride, titanium nitride, gold, platinum, palladium, or ruthenium or a stack of the conductive material and the above conductive material which is easily bonded to oxygen. 
     When the conductive material which is easily bonded to oxygen is in contact with an oxide semiconductor layer, a phenomenon occurs in which oxygen in the oxide semiconductor layer is diffused into the conductive material which is easily bonded to oxygen. The phenomenon noticeably occurs when the temperature is high. Thus, by a heat treatment step in the manufacturing process of the transistor, oxygen vacancies are generated in a region of the oxide semiconductor layer, which is in the vicinity of the interface between the oxide semiconductor layer and each of the source electrode layer and the drain electrode layer. The oxygen vacancies are bonded to hydrogen slightly contained in the film, whereby the region is likely changed to an n-type. Thus, the n-type region can serve as a source or a drain of the transistor. 
     The n-type region is illustrated in an enlarged cross-sectional view of the transistor (showing part of a cross section in the channel length direction, which is near the source electrode layer  140 ) in  FIG. 5 . A boundary  135  indicated by a dotted line in the first oxide semiconductor layer  131  and the second oxide semiconductor layer  132  is a boundary between an intrinsic semiconductor region and an n-type semiconductor region. In the first oxide semiconductor layer  131  and the second oxide semiconductor layer  132 , a region near the source electrode layer  140  and the first wiring  145  becomes an n-type region. The boundary  135  is schematically illustrated here, but actually, the boundary is not clearly seen in some cases. Although  FIG. 5  shows that part of the boundary  135  extends in the lateral direction in the second oxide semiconductor layer  132 , a region in the first oxide semiconductor layer  131  and the second oxide semiconductor layer  132 , which is sandwiched between the source electrode layer  140  and the base insulating film  120 , becomes n-type entirely in the thickness direction, in some cases. 
     In one embodiment of the present invention, the first wiring  145  and the second wiring  155  are connected to the first oxide semiconductor layer  131  and the second oxide semiconductor layer  132  by side contact, an n-type region formed in the first oxide semiconductor layer  131  and the second oxide semiconductor layer  132  can be enlarged. The n-type region serves as a source (or a drain) of the transistor. When the n-type region is enlarged, the series resistance between a channel formation region and the source electrode (or the drain electrode) or between the channel formation region and the first wiring  145  (or the second wiring  155 ) can be reduced and the electrical characteristics of the transistor can be improved. 
     In the case of forming a transistor with an extremely short channel length, an n-type region which is formed by the generation of oxygen vacancies might extend in the channel length direction of the transistor. In that case, the electrical characteristics of the transistor change; for example, the threshold voltage is shifted, or on and off states of the transistor is hard to control with the gate voltage (in which case the transistor is turned on). Accordingly, when a transistor with an extremely short channel length is formed, it is not always preferable that a conductive material easily bonded to oxygen be used for a source electrode layer and a drain electrode layer. 
     In such a case, a conductive material which is less likely to be bonded to oxygen than the above material can be used for the source electrode layer  140  and the drain electrode layer  150 . As the conductive material which is not easily bonded to oxygen, for example, a material containing tantalum nitride, titanium nitride, gold, platinum, palladium, or ruthenium or the like can be used. Note that in the case where the conductive material is in contact with the second oxide semiconductor layer  132 , the source electrode layer  140  and the drain electrode layer  150  may each have a structure in which the conductive material which is not easily bonded to oxygen and the above-described conductive material that is easily bonded to oxygen are stacked. 
     The gate insulating film  160  can be formed using an insulating film containing one or more of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. The gate insulating film  160  may be a stack including any of the above materials. 
     For the gate electrode layer  170 , a conductive film formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used. The gate electrode layer may be a stack including any of the above materials. Alternatively, a conductive film containing nitrogen may be used for the gate electrode layer. 
     An aluminum oxide film is preferably contained in the gate insulating film  160  and the insulating layer  180  over the gate electrode layer  170 . The aluminum oxide film has a high shielding effect (blocking effect) of preventing penetration of both oxygen and impurities such as hydrogen and moisture. Accordingly, the aluminum oxide film can be suitably used as a protective film that prevents entry of an impurity such as hydrogen or moisture, which causes variation in the electrical characteristics of the transistor, into the oxide semiconductor layer  130 , release of oxygen, which is a main component material of the oxide semiconductor layer  130 , from the oxide semiconductor layer during and after the manufacturing process of the transistor, and unnecessary release of oxygen from the base insulating film  120 . Further, oxygen contained in the aluminum oxide film can be diffused into the oxide semiconductor layer. 
     Further, the insulating layer  185  is preferably formed over the insulating layer  180 . The insulating layer  185  can be formed using an insulating film containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. The insulating layer  185  may be a stack including any of the above materials. 
     Here, the insulating layer  185  preferably contains excess oxygen. An insulating layer containing excess oxygen refers to an insulating layer from which oxygen can be released by heat treatment or the like. The insulating layer containing excess oxygen is preferably a film in which the amount of released oxygen when converted into oxygen atoms is 1.0×10 19  atoms/cm 3  or more in thermal desorption spectroscopy analysis. Oxygen released from the insulating layer can be diffused into the channel formation region in the oxide semiconductor layer  130  through the gate insulating film  160 , so that oxygen vacancies formed in the channel formation region can be filled with the oxygen. In this manner, the electrical characteristics of the transistor can be stable. 
     High integration of a semiconductor device requires miniaturization of a transistor. However, it is known that miniaturization of a transistor causes deterioration of the electrical characteristics of the transistor. In particular, a reduction in on-state current, which is directly caused by a decrease in channel width, is significant. 
     However, in the transistor of one embodiment of the present invention, as described above, the third oxide semiconductor layer  133  is formed so as to cover the second oxide semiconductor layer  132  where a channel is formed and the channel formation layer and the gate insulating film are not in contact with each other. Accordingly, scattering of carriers at the interface between the second oxide semiconductor layer  132  where a channel is formed and the gate insulating film can be reduced and the field-effect mobility of the transistor can be increased. 
     The transistor of one embodiment of the present invention can have particular improved electrical characteristics when having a structure in which the length (W T ) of a top surface of the second oxide semiconductor layer  132  in the channel width direction is as large as or smaller than the thickness of the oxide semiconductor layer, as in a cross-sectional diagram in the channel width direction in  FIGS. 6A and 6B . Note that in the cross section in the channel width direction, the second oxide semiconductor layer  132  may have tapered side surfaces and an upper surface having a flat portion as illustrated in  FIG. 6A . Alternatively, as illustrated in  FIG. 6B , the second oxide semiconductor layer  132  may have tapered side surfaces and an upper surface having a curvature. 
     In the case where W T  is sufficiently small as in either of the transistors illustrated in  FIGS. 6A and 6B , for example, an electric field from the gate electrode layer  170  to the side surface of the second oxide semiconductor layer  132  is applied to the entire second oxide semiconductor layer  132 ; thus, a channel is formed equally in the side and top surfaces of the second oxide semiconductor layer  132 . 
     In the case where a channel region  137  as in either of  FIGS. 6A and 6B  is formed in the transistor, the channel width can be defined as the sum of W T  and the lengths of the side surfaces (W S1  and W S2 ) of the second oxide semiconductor layer  132  in the channel width direction (i.e., W T +W S1 +W S2 ), and on-state current flows in the transistor in accordance with the channel width. In the case where W T  is sufficiently small, current flows in the entire second oxide semiconductor layer  132 . 
     In other words, the transistor illustrated in  FIGS. 6A and 6B  has a higher on-state current than that of the conventional transistor because the transistor illustrated in  FIGS. 6A and 6B  has an effect of suppressing scattering of carriers and an effect of extending the effective channel width. 
     Note that in order to efficiently increase the on-state current of the transistor when W S1  and W S2  are represented by W S  (W S1 =W S2 =W S ), a relation 0.3W S ≦W T ≦3W S  (W T  is greater than or equal to 0.3W S  and less than or equal to 3W S ) is satisfied. Further, W T /W S  is preferably greater than or equal to 0.5 and less than or equal to 1.5, further preferably greater than or equal to 0.7 and less than or equal to 1.3. In the case where W T /W S &gt;3, the S value and the off-state current might be increased. 
     As described above, with the transistor of one embodiment of the present invention, sufficiently high on-state current can be obtained even when the transistor is miniaturized. 
     In the transistor of one embodiment of the present invention, the second oxide semiconductor layer  132  is formed over the first oxide semiconductor layer  131 , so that an interface state is less likely to be formed. In addition, impurities do not enter the second oxide semiconductor layer  132  from above and below because the second oxide semiconductor layer  132  is an intermediate layer in a three-layer structure. Since the second oxide semiconductor layer  132  is surrounded by the first oxide semiconductor layer  131  and the third oxide semiconductor layer  133 , not only can the on-state current of the transistor be increased but also the threshold voltage can be stabilized and the S value (subthreshold value) can be reduced. Thus, Icut (current when gate voltage VG is 0 V) can be reduced and power consumption can be reduced. Further, the threshold voltage of the transistor becomes stable; thus, long-term reliability of the semiconductor device can be improved. 
     The transistor of one embodiment of the present invention may include a conductive film  172  between the oxide semiconductor layer  130  and the substrate  110  as illustrated in  FIG. 7 . When the conductive film is used as a second gate electrode, the on-state current can be further increased and the threshold voltage can be controlled. In order to increase the on-state current, for example, the gate electrode layer  170  and the conductive film  172  are set to have the same potential, and the transistor is driven as a dual-gate transistor. Further, to control the threshold voltage, a fixed potential, which is different from a potential of the gate electrode layer  170 , is supplied to the conductive film  172 . 
     This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification. 
     Embodiment 2 
     In this embodiment, a method for manufacturing the transistor  100 , which is described in Embodiment 1 with reference to  FIGS. 1A and 1B , is described with reference to  FIGS. 9A to 9C ,  FIGS. 10A and 10B , and  FIGS. 11A and 11B . 
     For the substrate  110 , a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used. 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, a silicon-on-insulator (SOI) substrate, or the like can be used. Further alternatively, any of these substrates further provided with a semiconductor element can be used. 
     The base insulating film  120  can be formed by a plasma chemical vapor deposition (CVD) method, a sputtering method, or the like using an oxide insulating film of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or the like; a nitride insulating film of silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like; or a film in which any of the above materials are mixed. Alternatively, a stack including any of the above materials may be used, and at least an upper layer of the base insulating film  120  which is in contact with the oxide semiconductor layer  130  is preferably formed using a material containing excess oxygen that might serve as a supply source of oxygen to the oxide semiconductor layer  130 . 
     Oxygen may be added to the base insulating film  120  by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. Adding oxygen enables the base insulating film  120  to supply oxygen much easily to the oxide semiconductor layer  130 . 
     In the case where a surface of the substrate  110  is made of an insulator and there is no influence of impurity diffusion to the oxide semiconductor layer  130  to be formed later, the base insulating film  120  is not necessarily provided. 
     Next, a first oxide semiconductor film  331  to be the first oxide semiconductor layer  131  and a second oxide semiconductor film  332  to be the second oxide semiconductor layer  132  are deposited over the base insulating film  120  by a sputtering method, a CVD method, an MBE method, an atomic layer deposition (ALD) method, or a PLD method. 
     The first oxide semiconductor film  331  and the second oxide semiconductor film  332  are preferably stacked successively without exposure to the air with the use of a multi-chamber deposition apparatus (e.g., a sputtering apparatus) including a load lock chamber. It is preferable that each chamber of the sputtering apparatus be able to be evacuated to a high vacuum (to about 5×10 −7  Pa to 1×10 −4  Pa) by an adsorption vacuum pump such as a cryopump and that the chamber be able to heat a substrate over which a film is to be deposited to 100° C. or higher, preferably 500° C. or higher, so that water and the like acting as impurities of an oxide semiconductor are removed as much as possible. Alternatively, a combination of a turbo molecular pump and a cold trap is preferably used to prevent back-flow of a gas containing a carbon component, moisture, or the like from an exhaust system into the chamber. 
     Not only high vacuum evacuation of the chamber but also high purity of a sputtering gas is necessary to obtain a highly purified intrinsic oxide semiconductor. An oxygen gas or an argon gas used as the sputtering gas is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, so that entry of moisture and the like into the oxide semiconductor layer can be prevented as much as possible. 
     For the first oxide semiconductor film  331 , the second oxide semiconductor film  332 , and a third oxide semiconductor film  333  to be the third oxide semiconductor layer  133  formed in a later step, any of the materials described in Embodiment 1 can be used. For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4, 1:3:3, or 1:3:2 can be used for the first oxide semiconductor film  331 , an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1, 5:5:6, or 3:1:2 can be used for the second oxide semiconductor film  332 , and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4, 1:3:3, or 1:3:2 can be used for the third oxide semiconductor film  333 . 
     An oxide semiconductor that can be used for each of the first oxide semiconductor film  331 , the second oxide semiconductor film  332 , and the third oxide semiconductor film  333  preferably contains at least indium (In) or zinc (Zn). Alternatively, the oxide semiconductor preferably contains both In and Zn. In order to reduce variation in the electrical characteristics of the transistor including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and/or Zn. 
     Examples of a stabilizer include gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), and zirconium (Zr). Other examples of a stabilizer include lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). 
     As the oxide semiconductor, for example, any of the following can be used: indium oxide, tin oxide, zinc oxide, an In—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide. 
     Note that here, for example, an “In—Ga—Zn oxide” means an oxide containing In, Ga, and Zn as its main components. The In—Ga—Zn oxide may contain a metal element other than In, Ga, and Zn. Further, in this specification, a film formed using an In—Ga—Zn oxide is also referred to as an IGZO film. 
     Alternatively, a material represented by InAlO 3 (ZnO) m  (m&gt;0, where m is not an integer) may be used. Note that M represents one or more metal elements selected from Ga, Y, Zr, La, Ce, and Nd. Further alternatively, a material represented by In 2 SnO 5 (ZnO) n  (n&gt;0, where n is an integer) may be used. 
     Note that as described in Embodiment 1 in detail, the second oxide semiconductor layer  132  is formed so as to have an electron affinity higher than that of the first oxide semiconductor layer  131  and that of the third oxide semiconductor layer  133 . 
     The oxide semiconductor layers are each 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 the case of using an In—Ga—Zn oxide, a material whose atomic ratio of In to Ga and Zn is any of 1:1:1, 2:2:1, 2:2:3, 3:1:2, 5:5:6, 1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:4:3, 1:5:4, 1:6:6, 1:6:4, 1:9:6, 1:1:4, and 1:1:2 can be used for the first oxide semiconductor film  331 , the second oxide semiconductor film  332 , and/or the third oxide semiconductor film  333 . 
     Note that for example, in the case where the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in the neighborhood of the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1), a, b, and c satisfy the following relation: (a−A) 2 +(b−B) 2 +(c−C) 2 ≦r 2 , and r may be 0.05, for example. The same applies to other oxides. 
     The indium content of the second oxide semiconductor film  332  is preferably higher than the indium content of the first oxide semiconductor film  331  and the indium content of the third oxide semiconductor film  333 . In an oxide semiconductor, the s orbital of heavy metal mainly contributes to carrier transfer, and when the proportion of In in the oxide semiconductor is increased, overlap of the s orbitals is likely to be increased. Thus, an oxide having a composition in which the proportion of In is higher than that of Ga has higher mobility than an oxide having a composition in which the proportion of In is equal to or lower than that of Ga. For this reason, with the use of an oxide having a high indium content for the second oxide semiconductor film  332 , a transistor having high mobility can be achieved. 
     A structure of an oxide semiconductor film is described below. 
     Note that 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, a 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 includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system. 
     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. 
     The CAAC-OS film is one of oxide semiconductor films including a plurality of crystal parts, and most of the crystal parts 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. 
     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. 
     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 (0 scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ 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 φ 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-axes of the crystal are 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. 
     Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies 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 appear 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 reduction in crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor film and causes a reduction 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. 
     Further, the CAAC-OS film is an oxide semiconductor film having a low density of defect states. For example, an oxygen vacancy in the oxide semiconductor film serves as a carrier trap or a carrier generation source in some cases when hydrogen is captured therein. 
     The state in which the impurity concentration is low and the density of defect states is low (the number of oxygen vacancies is small) is referred to as a highly purified intrinsic state or a 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 small 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 that 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 a TEM, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor in some cases. In most cases, the size of a crystal part in the microcrystalline oxide semiconductor film 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. An oxide semiconductor film including nanocrystal (nc), which is 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 a nanocrystalline oxide semiconductor (nc-OS) film. In an image of the nc-OS film obtained with a TEM, for example, a crystal grain cannot be observed clearly in some cases. 
     In the nc-OS film, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Further, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; thus, the orientation in the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor film 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. Further, a halo pattern is observed in an electron diffraction pattern (also referred to as a selected-area electron diffraction pattern) of the nc-OS film 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 observed in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm) close to, or smaller than or equal to the diameter of a crystal part. In some cases, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are observed. Further, in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots are shown in a ring-like region in some cases. 
     Since an nc-OS film is an oxide semiconductor film having more regularity than an amorphous oxide semiconductor film, the nc-OS film has a lower density of defect states than the amorphous oxide semiconductor 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 a CAAC-OS film. 
     Note that an oxide semiconductor film may be a stacked film including two or more films of an amorphous oxide semiconductor film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example. 
     A CAAC-OS film can be deposited by a sputtering method with a polycrystalline oxide semiconductor sputtering target, for example. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along the a-b plane; in other words, a sputtered particle having a plane parallel to the a-b plane (a flat-plate-like sputtered particle or a pellet-like sputtered particle) might flake off from the target. In this case, the flat-plate-like or pellet-like sputtered particle is electrically charged and thus reaches a substrate while maintaining its crystal state without being aggregated in plasma, whereby a CAAC-OS film can be formed. 
     In the case where the second oxide semiconductor film  332  is formed using an In-M-Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd) and a sputtering target whose atomic ratio of In to M and Zn is a 1 :b 1 :c 1  is used for forming the second oxide semiconductor film  332 , a 1 /b 1  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6, and c 1 /b 1  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when c 1 /b 1  is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film is easily formed as the second oxide semiconductor film  332 . Typical examples of the atomic ratio of In to M and Zn of the target are 1:1:1, 3:1:2, and 5:5:6. 
     In the case where the first oxide semiconductor film  331  and the third oxide semiconductor film  333  are each formed using an In-M-Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd) and a sputtering target whose atomic ratio of In to M and Zn is a 2 :b 2 :c 2  is used for forming the first oxide semiconductor film  331  and the third oxide semiconductor film  333 , a 2 /b 2  is preferably less than a 1 /b 1 , and c 2 /b 2  is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when c 2 /b 2  is greater than or equal to 1 and less than or equal to 6, CAAC-OS films are easily formed as the first oxide semiconductor film  331  and the third oxide semiconductor film  333 . Typical examples of the atomic ratio of In to M and Zn of the target are 1:3:2, 1:3:3, 1:3:4, and 1:3:6. 
     First heat treatment may be performed after the second oxide semiconductor film  332  is formed. The first 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, in an atmosphere containing an oxidizing gas at 10 ppm or more, or under reduced pressure. Alternatively, the first heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more in order to compensate desorbed oxygen. By the first heat treatment, the crystallinity of the second oxide semiconductor film  332  can be improved, and in addition, impurities such as hydrogen and water can be removed from the base insulating film  120  and the first oxide semiconductor film  331 . Note that the first heat treatment may be performed after etching for formation of the first oxide semiconductor layer  131  and the second oxide semiconductor layer  132  which is described later. 
     Next, a first conductive film  340  is formed over the second oxide semiconductor film  332 . For the first conductive film  340 , Al, Cr, Cu, Ta, Ti, Mo, W, or an alloy material containing any of these as its main component can be used. For example, a tungsten film with a thickness of 5 nm to 25 nm is formed by a sputtering method, a CVD method, or the like. 
     Next, a first resist mask  400  is formed over the first conductive film  340  (see  FIG. 8A ). It is preferable that the first resist mask  400  be formed by a photolithography process using electron beam exposure, liquid immersion exposure, or EUV exposure, for example. With such a process, the first resist mask  400  having an extremely minute shape can be formed. 
     Next, the first conductive film  340  is selectively etched using the first resist mask  400  as a mask, so that a first conductive layer  341  having an upper surface shape similar to an upper surface shape of the first resist mask  400  is formed. 
     Here, the first conductive layer  341  is used as a hard mask. In an etching step, the shape of a resist mask is changed because of change in quality and reduction in thickness. Thus, when the second oxide semiconductor layer  132  and the first oxide semiconductor layer  131  are formed using only a resist mask, the second oxide semiconductor layer  132  and the first oxide semiconductor layer  131  reflect the changed shape of the resist mask and thus cannot have a desired shape. When the first conductive layer  341  is used as a hard mask, the second oxide semiconductor layer  132  and the first oxide semiconductor layer  131  can be formed to have a desired shape. 
     The second oxide semiconductor film  332  and the first oxide semiconductor film  331  are selectively etched, so that the second oxide semiconductor layer  132  and the first oxide semiconductor layer  131  are formed (see  FIG. 8B ). Note that the base insulating film  120  may be partly etched by over-etching the first oxide semiconductor film  331 . 
     Next, a second resist mask is formed over the first conductive layer  341  by a method similar to that of the first resist mask  400 . Then, the first conductive layer  341  is selectively etched using the second resist mask as a mask, so that the source electrode layer  140  and the drain electrode layer  150  are formed (see  FIG. 8C ). Note that the first conductive layer  341  may be over-etched so that the second oxide semiconductor layer  132  is partly etched. 
     Subsequently, the third oxide semiconductor film  333  to be the third oxide semiconductor layer  133  is formed over the first oxide semiconductor layer  131 , the second oxide semiconductor layer  132 , the source electrode layer  140 , and the drain electrode layer  150 . 
     Note that second heat treatment may be performed after the third oxide semiconductor film  333  is formed. The second heat treatment can be performed under the conditions similar to those of the first heat treatment. The second heat treatment can remove impurities such as hydrogen and water from the third oxide semiconductor film  333 , the first oxide semiconductor layer  131 , and the second oxide semiconductor layer  132 . 
     Next, an insulating film  360  to be the gate insulating film  160  is formed over the third oxide semiconductor film  333 . The insulating film  360  can be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or the like. The insulating film  360  may be a stack including any of the above materials. The insulating film  360  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, a PLD method, or the like. 
     Then, a second conductive film  370  to be the gate electrode layer  170  is formed over the insulating film  360  (see  FIG. 9A ). For the second conductive film  370 , Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or an alloy material containing any of these as its main component can be used. The second conductive film  370  can be formed by a sputtering method, a CVD method, or the like. A stack including a conductive film containing any of the above materials and a conductive film containing nitrogen, or a conductive film containing nitrogen may be used for the second conductive film  370 . 
     After that, a third resist mask is formed over the second conductive film  370 , and the second conductive film  370  is selectively etched using the third resist mask to form the gate electrode layer  170 . 
     Then, the insulating film  360  is selectively etched using the gate electrode layer  170  as a mask to form the gate insulating film  160 . 
     Subsequently, the third oxide semiconductor film  333  is etched using the gate electrode layer  170  or the gate insulating film  160  as a mask to form the third oxide semiconductor layer  133  (see  FIG. 9B ). 
     The second conductive film  370 , the insulating film  360 , and the third oxide semiconductor film  333  may be etched individually or successively. Further, either dry etching or wet etching may be used as the etching method, and an appropriate etching method may be selected individually. 
     Next, the insulating layer  180  and the insulating layer  185  are formed over the source electrode layer  140 , the drain electrode layer  150 , and the gate electrode layer  170  (see  FIG. 9C ). The insulating layer  180  and the insulating layer  185  can each be formed using a material and a method which are similar to those of the base insulating film  120 . In particular, aluminum oxide is preferably used for the insulating layer  180 . 
     Oxygen may be added to the insulating layer  180  and/or the insulating layer  185  by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. Adding oxygen enables the insulating layer  180  and/or the insulating layer  185  to supply oxygen much easily to the oxide semiconductor layer  130 . 
     Next, third heat treatment may be performed. The third heat treatment can be performed under conditions similar to those of the first heat treatment. By the third heat treatment, excess oxygen is easily released from the base insulating film  120 , the gate insulating film  160 , the insulating layer  180 , and the insulating layer  185 , so that oxygen vacancies in the oxide semiconductor layer  130  can be reduced. 
     Next, a fourth resist mask is formed over the insulating layer  185 , and the insulating layer  185 , the insulating layer  180 , the source electrode layer  140 , the drain electrode layer  150 , the second oxide semiconductor layer  132 , and the first oxide semiconductor layer  131  are selectively etched using the fourth resist mask, so that the first opening  147  and the second opening  157  are formed (see  FIG. 10A ). At this time, the third opening  177  illustrated in  FIG. 2A  is also formed. 
     Note that the insulating layer  185 , the insulating layer  180 , the source electrode layer  140 , the drain electrode layer  150 , the second oxide semiconductor layer  132 , and the first oxide semiconductor layer  131  may be etched individually or successively. Further, either dry etching or wet etching may be used as the etching method, and an appropriate etching method may be selected individually. 
     By controlling etching conditions at this time, transistors having different structures illustrated in  FIGS. 3A to 3C  can be formed. 
     After that, the first wiring  145  and the second wiring  155  are formed to cover the first opening  147  and the second opening  157 . The second oxide semiconductor layer  132  and the source electrode layer  140  are electrically connected to the first wiring  145 , and the second oxide semiconductor layer  132  and the drain electrode layer  150  are electrically connected to the second wiring  155  (see  FIG. 10B ). Further, at this time, the third wiring  175  is formed to cover the third opening  177  illustrated in  FIG. 2A  and is electrically connected to the gate electrode layer  170 . 
     Note that the first wiring  145 , the second wiring  155 , and the third wiring  175  can be formed using a material and a method similar to those of the source electrode layer  140 , the drain electrode layer  150 , or the gate electrode layer  170 . 
     Through the above process, the transistor  100  illustrated in  FIGS. 1A and 1B  can be fabricated. 
     A variety of films such as the metal film described in this embodiment can be formed typically by a sputtering method or a plasma CVD method; however, these films may be formed by another method such as a thermal CVD method. A metal organic chemical vapor deposition (MOCVD) method and an ALD method are given as examples of a thermal CVD method. 
     A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film. 
     Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at a time, the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and reaction is caused in the vicinity of the substrate or over the substrate. 
     Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). In such a case, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first source gas, and then a second source gas is introduced, whereby the source gases are not mixed. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Instead of the introduction of the inert gas, the first source gas may be exhausted by vacuum evacuation, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer and then, the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET. 
     In the case where a tungsten film is formed using a deposition apparatus employing ALD, for example, a WF 6  gas and a B 2 H 6  gas are sequentially introduced plural times to form an initial tungsten film, and then a WF 6  gas and an H 2  gas are introduced at a time, so that the tungsten film is formed. Note that an SiH 4  gas may be used instead of a B 2 H 6  gas. 
     This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification. 
     Embodiment 3 
     In this embodiment, an example of a semiconductor device (storage device) which includes the transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles is described with reference to drawings. 
       FIG. 11A  is a cross-sectional view of the semiconductor device, and  FIG. 11B  is a circuit diagram of the semiconductor device. 
     The semiconductor device illustrated in  FIGS. 11A and 11B  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  100  described in Embodiment 1 can be used as the transistor  3300 . 
     One electrode of the capacitor  3400  is formed using the same material as a wiring layer electrically connected to a source electrode layer or 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 insulating layer  180  and the insulating layer  185  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 preferably have different energy 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 an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor enables charge to be retained for a long time owing to its electrical characteristics, that is, the low off-state current. 
     Although both of the above transistors are n-channel transistors in the following description, it is needless to say that p-channel transistors can be used. The specific structure of the semiconductor device, such as a material used for the semiconductor device and the structure of the semiconductor device, needs not to be limited to that described here except for the use of the transistor described in Embodiment 1, which is formed using an oxide semiconductor for retaining data. 
     The transistor  3200  in  FIG. 11A  includes a channel formation region provided in a substrate  3000  containing a semiconductor material (such as crystalline silicon), impurity regions provided such that the channel formation region is provided therebetween, intermetallic compound regions in contact with the impurity regions, a gate insulating film provided over the channel formation region, and a gate electrode layer provided over the gate insulating film. Note that a transistor whose source electrode layer and drain electrode layer are not illustrated in a drawing may also be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode layer may be collectively referred to as a source electrode layer, and a drain region and a drain electrode layer may be collectively referred to as a drain electrode layer. That is, in this specification, the term “source electrode layer” might include a source region. 
     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). 
     In the case where the transistor  3200  is formed using a crystalline silicon substrate, for example, the transistor  3200  can operate at high speed. Thus, when the transistor is used as a reading transistor, data can be read at high speed. 
     The transistor  3300  is provided over the insulating layer  3150 , and the wiring layer electrically connected to the source electrode layer or the drain electrode layer of the transistor  3300  serves as the one electrode of the capacitor  3400 . Further, the one electrode of the capacitor  3400  is electrically connected to the gate electrode layer of the transistor  3200 . 
     The transistor  3300  in  FIG. 11A  is a top-gate transistor in which a channel is formed in an oxide semiconductor layer. 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, refresh operation becomes unnecessary or the frequency of the refresh operation in a semiconductor storage device can be extremely low, which leads to a sufficient reduction in power consumption. 
     Further, an electrode  3250  is provided so as to overlap 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. 11A , which enables the degree of the integration of the semiconductor device to be increased. 
     An example of a circuit configuration of the semiconductor device in  FIG. 11A  is illustrated in  FIG. 11B . 
     In  FIG. 11B , 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 one of the source electrode layer and the drain electrode layer of the transistor  3300 . A fourth wiring  3004  is electrically connected to the gate electrode layer of the transistor  3300 . The gate electrode layer of the transistor  3200  and the other of the source electrode layer and the drain electrode layer 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. 11B  utilizes a feature that the potential of the gate electrode layer of the transistor  3200  can be retained, and thus enables writing, retaining, and reading of data as follows. 
     Writing and retaining of data are 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 layer of the transistor  3200  and the capacitor  3400 . That is, a predetermined charge is supplied to the gate electrode layer of the transistor  3200  (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. 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 layer 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 layer of the transistor  3200  is retained for a long time. 
     Next, reading of data is described. An appropriate potential (a reading potential) is supplied to the fifth wiring  3005  while a predetermined potential (a constant potential) is supplied to the first wiring  3001 , whereby the potential of the second wiring  3002  varies depending on the amount of charge retained in the gate electrode layer of the transistor  3200 . This is because in general, in the case of using an n-channel transistor as the transistor  3200 , an apparent threshold voltage V th   _   H  at the time when the high-level charge is given to the gate electrode layer of the transistor  3200  is lower than an apparent threshold voltage V th   _   L  at the time when the low-level charge is given to the gate electrode layer 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, the potential of the fifth wiring  3005  is set to a potential V 0  which is between V th   _   H  and V th   _   L , whereby charge supplied to the gate electrode layer of the transistor  3200  can be determined. For example, in the case where the high-level charge is supplied in writing and 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  (V th   _   L ), the transistor  3200  remains off. Thus, the data retained in the gate electrode layer can be read by determining the potential of the second wiring  3002 . 
     Note that in the case where memory cells are arrayed, it is necessary that only data of a desired memory cell be able 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 layer, 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 including a transistor having a channel formation region formed using an oxide semiconductor and having an extremely low off-state current, the semiconductor device described in this embodiment can retain stored data for an extremely long time. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long time 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. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of a gate insulating film is unlikely to be caused. That is, the semiconductor device of the disclosed invention does not have a limit on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the state of the transistor (on or off), whereby high-speed operation can be easily achieved. 
     As described above, a miniaturized and highly-integrated semiconductor device having high electrical characteristics can be provided. 
     This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification. 
     Embodiment 4 
     In this embodiment, a semiconductor device including the transistor of one embodiment of the present invention, which can retain stored data even when not powered, which does not have a limit on the number of write cycles, and which has a structure different from that described in Embodiment 3, is described. 
       FIG. 12  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 layer of a transistor  4300 , a second wiring  4600  is electrically connected to a gate electrode layer of the transistor  4300 , and a drain electrode layer of the transistor  4300  is electrically connected to a first terminal of a capacitor  4400 . Note that the transistor  100  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. 11A and 11B . 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 retaining of data in the semiconductor device (the memory cell  4250 ) illustrated in  FIG. 12  are 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 the first 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 first terminal of the capacitor  4400  is retained (retaining). 
     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 retained for an extremely long time by turning off the transistor  4300 . 
     Next, reading of data is 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 the 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. Thus, it can be found that, assuming that the memory cell  4250  is in either of two states in which the potential of the first terminal of the capacitor  4400  is V 1  and V 0  (V 1 &gt;V 0 ), the potential of the first wiring  4500  in the case of retaining 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 retaining 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. 12  can retain 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, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained 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. 12  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 which 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, or gallium arsenide 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 high electrical characteristics can be provided. 
     This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification. 
     Embodiment 5 
     The transistor described in Embodiment 1 can be used in a semiconductor device such as a display device, a storage device, a CPU, a digital signal processor (DSP), an LSI such as a custom LSI or a programmable logic device (PLD), or a radio frequency identification (RF-ID). In this embodiment, electronic devices each including the semiconductor device will be described. 
     Examples of the electronic devices having the semiconductor devices 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 equipment such as dialyzers and X-ray diagnostic equipment. In addition, the examples of the electronic devices include alarm devices such as smoke detectors, heat detectors, gas alarm devices, and security alarm devices. Further, the examples of the electronic devices also 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 devices. 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, and spacecrafts. Some specific examples of these electronic devices are illustrated in  FIGS. 13A to 13C . 
     In a television set  8000  illustrated in  FIG. 13A , a display portion  8002  is incorporated in a housing  8001 . The display portion  8002  can display an image and a speaker portion  8003  can output sound. A storage device including the transistor of one embodiment of the present invention can be used for a driver circuit for operating the display portion  8002 . 
     The television set  8000  may also include a CPU  8004  for performing information communication or a memory. For the CPU  8004  and the memory, a CPU or a storage device including the transistor of one embodiment of the present invention can be used. 
     An alarm device  8100  illustrated in  FIG. 13A  is a residential fire alarm, which is an example of an electronic device including a sensor portion  8102  for smoke or heat and a microcomputer  8101 . Note that the microcomputer  8101  includes a storage device or a CPU including the transistor of one embodiment of the present invention. 
     An air conditioner which includes an indoor unit  8200  and an outdoor unit  8204  illustrated in  FIG. 13A  is an example of an electronic device including the transistor, the storage 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. 13A , 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 . By using any of the transistors of one embodiment of the present invention for the CPU in the air conditioner, a reduction in power consumption of the air conditioner can be achieved. 
     An electric refrigerator-freezer  8300  illustrated in  FIG. 13A  is an example of an electronic device including the transistor, the storage 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. 13A , the CPU  8304  is provided in the housing  8301 . When the transistor of one embodiment of the present invention is used for the CPU  8304  of the electric refrigerator-freezer  8300 , a reduction in power consumption of the electric refrigerator-freezer  8300  can be achieved. 
       FIGS. 13B and 13C  illustrate an example of an electric vehicle which is an example of an electronic device. 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. When the transistor of one embodiment of the present invention is used for the CPU in the electric vehicle  9700 , a reduction in power consumption of the electric vehicle  9700  can be achieved. 
     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  on the basis of 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 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 a direct current into an alternate current is also incorporated. 
     This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification. 
     EXAMPLE 
     In this example, the electrical characteristics of a transistor of one embodiment of the present invention are described. 
     First, a method for manufacturing a transistor is described. The transistor in this example has the structure illustrated in  FIGS. 15A and 15B . 
     As a substrate, a glass substrate was used, and a silicon oxynitride film was formed over the glass substrate by a plasma CVD method. 
     Next, a first oxide semiconductor film with a thickness of approximately 10 nm and a second oxide semiconductor film with a thickness of approximately 40 nm were formed in this order over the silicon oxynitride film by a sputtering method. Note that an IGZO film having a composition ratio of In:Ga:Zn=1:3:2 and an IGZO film having a composition ratio of In:Ga:Zn=1:1:1 or In:Ga:Zn=3:1:2 were used as the first oxide semiconductor film and the second oxide semiconductor film, respectively. 
     Next, a 15-nm-thick tungsten film and an organic resin were formed over the second oxide semiconductor film, a negative resist film was formed, exposure was performed on the resist film by scanning with an electron beam, and development treatment was performed, so that a first resist mask was formed. 
     Then, by using the first resist mask, the organic resin and the tungsten film were selectively etched. A dry etching apparatus using inductively coupled plasma (ICP) was used for the etching. 
     Next, the first resist mask and the organic resin were removed by ashing. Then, the first oxide semiconductor film and the second oxide semiconductor film were selectively etched using the tungsten film as a mask, so that a stack of a first oxide semiconductor layer, a second oxide semiconductor layer, and the tungsten film was formed. 
     Next, a second resist mask was formed over the tungsten film, and the tungsten film was selectively etched using the second resist mask, so that a source electrode layer and a drain electrode layer were formed. 
     Next, a 5-nm-thick third oxide semiconductor film was formed over the oxide semiconductor layers, the source electrode layer, and the drain electrode layer by a sputtering method. Note that an IGZO film having a composition ratio of In:Ga:Zn 1:3:2 was used as the third oxide semiconductor film. 
     Next, a 10-nm-thick silicon oxynitride film to be a gate insulating film was formed over the third oxide semiconductor film by a plasma CVD method. 
     Next, a 10-nm-thick titanium nitride film and a 10-nm-thick tungsten film were successively formed by a sputtering method. After that, a third resist mask was formed over the tungsten film. 
     Next, the titanium nitride film and the tungsten film were selectively etched using the third resist mask, so that a gate electrode layer was formed. 
     Next, a fourth resist mask was formed over the gate electrode layer and the gate insulating film, and the gate insulating film and the third oxide semiconductor film were selectively etched using the fourth resist mask, so that the gate insulating film and a third oxide semiconductor layer having shapes illustrated in  FIGS. 15A and 15B  were formed. 
     Next, an aluminum oxide film and a silicon oxynitride film were formed as insulating layers. 
     Through the above process, the transistor of one embodiment of the present invention (corresponding to the model (b) illustrated in  FIG. 16B ) was fabricated. 
     Further, a transistor having a conventional structure (corresponding to the model (a) illustrated in  FIG. 16A ) was also fabricated by changing part of the above process. 
     Next, electrical characteristics of the fabricated transistors are described. 
       FIG. 17A  shows Id-Vg characteristics of the transistor having the conventional structure. The composition ratio of the second oxide semiconductor layer of the transistor was In:Ga:Zn=1:1:1. The field-effect mobility of the transistor was approximately 14 cm 2 /Vs and the S value thereof was approximately 105 mV/decade; thus, favorable characteristics were obtained. 
       FIG. 17B  shows Id-Vg characteristics of the transistor of one embodiment of the present invention. The composition ratio of the second oxide semiconductor layer of the transistor was In:Ga:Zn=3:1:2. The field-effect mobility of the transistor was approximately 21 cm 2 /Vs and the S value thereof was approximately 90 mV/decade; thus, the obtained characteristics were more favorable than those of the transistor having the conventional structure. 
     Here, in the case where the composition ratio of the second oxide semiconductor layer used in the transistor having the conventional structure was In:Ga:Zn=3:1:2, a field-effect mobility of approximately 100 cm 2 Ns was obtained; however, favorable characteristics were not obtained (e.g., the threshold voltage was largely shifted in the negative direction). Further, in the case where the composition ratio of the second oxide semiconductor layer used in the transistor of one embodiment of the present invention was In:Ga:Zn=1:1:1, an on-state current and a field-effect mobility were lower than those in Id-Vg characteristics in  FIG. 17A . 
     In other words, it is found that when an appropriate material is selected for the oxide semiconductor layer, the transistor of one embodiment of the present invention can have more favorable electrical characteristics than the transistor having the conventional structure. 
     Note that this example can be combined with any of the embodiments in this specification as appropriate. 
     EXPLANATION OF REFERENCE 
     
         
           100 : transistor,  110 : substrate,  120 : base insulating film,  130 : oxide semiconductor layer,  131 : first oxide semiconductor layer,  132 : second oxide semiconductor layer,  133 : third oxide semiconductor layer,  135 : boundary,  137 : channel region,  140 : source electrode layer,  145 : wiring,  147 : first opening,  150 : drain electrode layer,  155 : wiring,  157 : second opening,  160 : gate insulating film,  170 : gate electrode layer,  172 : conductive film,  175 : wiring,  177 : third opening,  180 : insulating layer,  185 : insulating layer,  331 : first oxide semiconductor film,  332 : second oxide semiconductor film,  333 : third oxide semiconductor film,  340 : first conductive film,  341 : first conductive layer,  360 : insulating film,  370 : second conductive film,  400 : resist mask,  3000 : substrate,  3001 : wiring,  3002 : wiring,  3003 : wiring,  3004 : wiring,  3005 : wiring,  3100 : element isolation insulating layer,  3150 : insulating layer,  3200 : transistor,  3250 : electrode,  3300 : transistor,  3400 : capacitor,  4250 : memory cell,  4300 : transistor,  4400 : capacitor,  4500 : wiring,  4600 : wiring,  8000 : television set,  8001 : housing,  8002 : display portion,  8003 : speaker portion,  8004 : CPU,  8100 : alarm device,  8101 : microcomputer,  8102 : sensor portion,  8200 : indoor unit,  8201 : housing,  8202 : air outlet,  8203 : CPU,  8204 : outdoor unit,  8300 : electric refrigerator-freezer,  8301 : housing,  8302 : door for refrigerator,  8303 : door for freezer,  8304 : CPU,  9700 : electric vehicle,  9701 : secondary battery,  9702 : circuit,  9703 : driving device, and  9704 : processing unit. 
       
    
     This application is based on Japanese Patent Application serial no. 2013-099534 filed with Japan Patent Office on May 9, 2013, the entire contents of which are hereby incorporated by reference.