Patent Publication Number: US-9837544-B2

Title: Semiconductor device having an oxide semiconductor layer

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device using an oxide semiconductor and a manufacturing method thereof. 
     In this specification, the semiconductor device indicates all the devices that can function by utilizing semiconductor characteristics. For example, a transistor is a semiconductor device. An electro-optical device such as a liquid crystal display device and a light-emitting device, a semiconductor circuit, an electronic appliance, and the like which include the transistor are also semiconductor devices. 
     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. Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or 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 material, an oxide semiconductor has been attracting attention. 
     For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 10 18 /cm 3  is disclosed (see Patent Document 1). 
     Further, a transistor using an oxide semiconductor operates faster than a transistor using amorphous silicon. The transistor using an oxide semiconductor can be included in a driver circuit, a high-speed memory circuit, and the like of a display device. 
     REFERENCE 
     [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
     DISCLOSURE OF INVENTION 
     For example, high speed operation is required for a transistor used for a driver circuit of a display device. In particular, as the definition of a display portion is increased, writing time of a display image is reduced. Therefore, for a transistor used in a driver circuit, a transistor having high on-state current and capable of high-speed operation is desired. 
     Thus, an object of one embodiment of the present invention is to provide a transistor having high on-state current and capable of high-speed operation and to provide a manufacturing method of the transistor. In addition, another object is to provide a semiconductor device including the transistor. 
     According to one embodiment of the present invention, a buffer layer having n-type conductivity is formed between an oxide semiconductor layer which is a channel formation region of a transistor and a metal layer which is a source electrode layer or a drain electrode layer; thus, parasitic resistance is reduced, resulting in improvement of on-state characteristics of the transistor. 
     One embodiment of the present invention disclosed in this specification is a semiconductor device including: an oxide semiconductor layer formed over an insulating layer; a source electrode layer and a drain electrode layer which overlap with part of the oxide semiconductor layer; a gate insulating layer in contact with part of the oxide semiconductor layer; and a gate electrode layer over the gate insulating layer. A buffer layer having n-type conductivity is formed between the source electrode layer and the oxide semiconductor layer and between the drain electrode layer and the oxide semiconductor layer. 
     For the above described source electrode layer and drain electrode layer, a single film containing an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten as its main component, an alloy film containing any of these elements, or a layered film including any of the films can be used. 
     Further, for the above described buffer layer, a metal oxide selected from indium oxide, indium tin oxide, indium zinc oxide, tin oxide, zinc oxide, and tin zinc oxide, or a material including the metal oxide which contains one or more elements selected from aluminum, gallium, and silicon can be used. With such a structure, the parasitic resistance between the source electrode layer and drain electrode layer and the oxide semiconductor layer can be reduced. 
     Another embodiment of the present invention disclosed in this specification is a method for manufacturing a semiconductor device including the following steps: forming an insulating layer over a substrate having an insulating surface; forming an island-shaped oxide semiconductor layer over the insulating layer; forming a buffer layer having n-type conductivity and a metal layer over the oxide semiconductor layer; forming a source region and a drain region which include the buffer layer and the metal layer so as to overlap with part of the oxide semiconductor layer by selectively etching the buffer layer and the metal layer; forming a gate insulating layer so as to cover the oxide semiconductor layer, the source region, and the drain region; and forming a gate electrode layer over the gate insulating layer so as to overlap with part of the oxide semiconductor layer. 
     According to one embodiment of the present invention, the parasitic resistance between the oxide semiconductor layer and the metal layer which is used as the source electrode layer and the drain electrode layer can be reduced. Consequently, a semiconductor device including a transistor having high on-state current and capable of high-speed operation and a manufacturing method of the semiconductor device can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a plan view of a semiconductor device and  FIGS. 1B and 1C  are cross-sectional views of the semiconductor device. 
         FIGS. 2A to 2E  are process cross-sectional views illustrating a manufacturing method of a semiconductor device. 
         FIGS. 3A to 3C  are plan views each illustrating one embodiment of a semiconductor device. 
         FIG. 4  is a cross-sectional view illustrating one embodiment of a semiconductor device. 
         FIG. 5  is a cross-sectional view illustrating one embodiment of a semiconductor device. 
         FIG. 6  is a cross-sectional view illustrating one embodiment of a semiconductor device. 
         FIGS. 7A to 7F  are diagrams each illustrating an electronic appliance. 
         FIGS. 8A to 8C  are cross-sectional views each illustrating a semiconductor device. 
         FIGS. 9A to 9C  are graphs each showing I-V characteristics of a transistor before and after a negative gate BT stress is applied. 
         FIG. 10  illustrates scientific calculation models and distribution of an electric field intensity when a negative gate BT stress is applied. 
         FIG. 11  illustrates I-V characteristics of a transistor and distribution of current density when a negative charge is fixed (at a drain voltage of +0.1 V). 
         FIG. 12  illustrates I-V characteristics of a transistor and distribution of current density when a negative charge is fixed (at a drain voltage of +3 V). 
         FIGS. 13A and 13B  are a cross-sectional TEM image and a schematic view of a transistor including a buffer layer. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below and it is easily understood by those skilled in the art that the mode and details can be changed variously. The present invention is not construed as being limited to description of the embodiments. In describing structures of the present invention with reference to the drawings, the same reference numerals are used in common for the same portions in different drawings. The same hatching pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases. 
     Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention. 
     Embodiment 1 
     In this embodiment, a semiconductor device and a manufacturing method of the semiconductor device each of which is one embodiment of the present invention will be described. 
     As an example of a semiconductor device which is one embodiment of the present invention, a plan view and cross-sectional views of a top-gate transistor are illustrated in  FIGS. 1A to 1C . Here,  FIG. 1A  is a top view,  FIG. 1B  is a cross-sectional view along A-B of  FIG. 1A , and  FIG. 1C  is a cross-sectional view along C-D of  FIG. 1A . A transistor  151  is referred to as a top-contact transistor as well as a top-gate transistor because a source region and a drain region of the transistor  151  are formed over and in contact with a semiconductor layer. 
     The transistor  151  in  FIGS. 1A to 1C  includes an insulating layer  102 , an oxide semiconductor layer  106 , a source electrode layer  108   a , a drain electrode layer  108   b , a gate insulating layer  112 , and a gate electrode layer  114 , which are over a substrate  100 . A buffer layer  107   a  is formed between the oxide semiconductor layer  106  and the source electrode layer  108   a . A buffer layer  107   b  is formed between the oxide semiconductor layer  106  and the drain electrode layer  108   b . Note that in this specification, the source electrode layer  108   a  and the buffer layer  107   a  are collectively called the source region and the drain electrode layer  108   b  and the buffer layer  107   b  are collectively called the drain region for simple description in some cases. The transistor using an oxide semiconductor layer in this embodiment is an n-channel transistor. 
     Note that the transistor  151  may have structures illustrated in cross-sectional views in  FIGS. 8A to 8C . In  FIG. 8A , a channel formation region of the oxide semiconductor layer  106  is formed to have a small thickness. In  FIG. 8B , the source electrode layer  108   a  and the drain electrode layer  108   b  are formed to provide steps over the buffer layer  107   a  and the buffer layer  107   b .  FIG. 8C  is a combination of  FIGS. 8A and 8B . Transistors having these structures have electric characteristics equivalent to that of a transistor having a structure in the cross-sectional view in  FIG. 1B . 
     For a material of the insulating layer  102 , silicon oxide; silicon oxynitride; aluminum oxide, gallium oxide, or gallium aluminum oxide which is expressed by Ga x Al 2-x O 3+y  (0≦x≦2 and 0&lt;y&lt;1, i. e., x is greater than or equal to 0 and less than or equal to 2 and y is greater than 0 and less than 1); a mixed material of any of them; or the like can be used. Alternatively, the insulating layer  102  may be a stacked layer of the above material and silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum nitride, a mixed material of any of them, or the like. For example, a layered structure of a silicon nitride layer and a silicon oxide layer is used for the insulating layer  102 , thereby preventing moisture from entering the transistor  151  from the substrate or the like. In the case where the insulating layer  102  is formed to have a layered structure, an oxide layer of silicon oxide, silicon oxynitride, aluminum oxide, a mixed material of any of them, or the like is preferably formed on a side where the insulating layer  102  is in contact with the oxide semiconductor layer  106 . Note that the insulating layer  102  functions as a base layer of the transistor  151 . 
     An oxide semiconductor to be used preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. As a stabilizer for reducing variation in electric characteristics of a transistor using the oxide semiconductor, gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer. 
     As another stabilizer, one or plural kinds of lanthanoid such as lantern (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), or lutetium (Lu) may be contained. 
     For example, the following can be used as the oxide semiconductor: indium oxide; tin oxide; zinc oxide; a two-component metal oxide material such as an In—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-based material, a Sn—Mg—O-based material, an In—Mg—O-based material, or an In—Ga—O-based material; a three-component metal oxide material such as an In—Ga—Zn—O-based material (also referred to as IGZO), an In—Al—Zn—O-based material, an In—Sn—Zn—O-based material, a Sn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, an In—Hf—Zn—O-based material, an In—La—Zn—O-based material, an In—Ce—Zn—O-based material, an In—Pr—Zn—O-based material, an In—Nd—Zn—O-based material, an In—Sm—Zn—O-based material, an In—Eu—Zn—O-based material, an In—Gd—Zn—O-based material, an In—Tb—Zn—O-based material, an In—Dy—Zn—O-based material, an In—Ho—Zn—O-based material, an In—Er—Zn—O-based material, an In—Tm—Zn—O-based material, an In—Yb—Zn—O-based material, or an In—Lu—Zn—O-based material; or a four-component metal oxide material such as an In—Sn—Ga—Zn—O-based material, an In—Hf—Ga—Zn—O-based material, an In—Al—Ga—Zn—O-based material, an In—Sn—Al—Zn—O-based material, an In—Sn—Hf—Zn—O-based material, or an In—Hf—Al—Zn—O-based material. 
     Note that here, for example, an “In—Ga—Zn—O-based material” means an oxide containing In, Ga, and Zn as its main component and there is no particular limitation on the ratio of In:Ga:Zn. Further, a metal element in addition to In, Ga, and Zn may be contained. 
     For the oxide semiconductor layer, a thin film using a material represented by the chemical formula, InMO 3 (ZnO) m  (m&gt;0, where m is not an integer), can be used. Here, M represents one or more metal elements selected from Zn, Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. 
     For example, an In—Ga—Zn—O-based material with an atomic ratio of In:Ga:Zn=1:1:1 (=⅓:⅓:⅓) or In:Ga:Zn=2:2:1 (=⅖:⅖:⅕), or an oxide with an atomic ratio close to the above atomic ratios can be used. Alternatively, an In—Sn—Zn—O-based material with an atomic ratio of In:Sn:Zn=1:1:1 (=⅓:⅓:⅓), In:Sn:Zn=2:1:3 (=⅓:⅙:½), or In:Sn:Zn=2:1:5 (=¼:⅛:⅝), or an oxide with an atomic ratio close to the above atomic ratios may be used. 
     The oxide semiconductor may be either single crystal or non-single-crystal. In the latter case, the oxide semiconductor may be either amorphous or polycrystal. Further, the oxide semiconductor may have either an amorphous structure including a portion having crystallinity or a non-amorphous structure. 
     In an oxide semiconductor in an amorphous state, a flat surface can be obtained with relative ease, so that when a transistor is manufactured with the use of the oxide semiconductor, interface scattering can be reduced, and relatively high mobility can be obtained with relative ease. 
     In an oxide semiconductor having crystallinity, bulk defects can be further reduced and when a surface flatness is improved, mobility higher than that of an oxide semiconductor layer in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (Ra) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably less than or equal to 0.1 nm. 
     When the oxide semiconductor layer and an oxide insulating layer which is a base are in contact with each other, the interface state between the insulating layer  102  and the oxide semiconductor layer  106  and oxygen deficiency in the oxide semiconductor layer  106  can be reduced. Change in the threshold voltage of the transistor  151  over time can be reduced due to the reduction of the interface state and the oxygen deficiency. 
     In the source region and the drain region in the transistor  151 , the buffer layers  107   a  and  107   b  having n-type conductivity are formed on the side which is in contact with the oxide semiconductor layer  106 . The parasitic resistance between each of the oxide semiconductor layer  106  and the source electrode layer  108   a , and the oxide semiconductor layer  106  and the drain electrode layer  108   b  can be reduced by providing the buffer layers. Consequently, on-state current of the transistor can be increased and frequency characteristics of a circuit can be improved. These effects are noticeable particularly when the channel length of the transistor is less than or equal to 5 μm. Here, the parasitic resistance is reduced mainly by a reduction of the contact resistance between the oxide semiconductor layer  106  and the source electrode layer  108   a  or the drain electrode layer  108   b.    
     For a material which can be used for the buffer layers  107   a  and  107   b  having n-type conductivity, metal oxide such as indium oxide (an In—O-based material), indium tin oxide (an In—Sn—O-based material), indium zinc oxide (an In—Zn—O-based material), tin oxide (a Sn—O-based material), zinc oxide (a Zn—O-based material), tin zinc oxide (a Sn—Zn—O-based material) is typically used. One or more elements selected from aluminum (Al), gallium (Ga), and silicon (Si) may be contained in the above metal oxide. Alternatively, titanium oxide (a Ti—O-based material), titanium niobium oxide (a Ti—Nb—O-based material), molybdenum oxide (a Mo—O-based material), tungsten oxide (a W—O-based material), magnesium oxide (a Mg—O-based material), calcium oxide (a Ca—O-based material), gallium oxide (a Ga—O-based material), or the like can be used. Nitrogen (N) may be contained in the above materials. 
     As a metal layer used for the source electrode layer  108   a  and the drain electrode layer  108   b , a metal film containing an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy film containing any of the above elements as its component (e.g., a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film); or a stacked layer of any of the above films can be used. For example, a structure may be used in which a film of a refractory metal such as titanium, molybdenum, or tungsten or a nitride film of any of them (e.g., a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) may be provided on one surface or both surfaces of a metal film such as an aluminum film or a copper film. 
     When a source electrode and a drain electrode are in contact with a gate insulating layer, it is preferable to use a metal film which is less likely to extract oxygen as the source electrode and the drain electrode in order to prevent a reduction in the withstand voltage of the gate insulating layer. For the metal film, molybdenum, tungsten, or the like can be used. Note that in the case where the source electrode and the drain electrode are a stacked layer, at least a side in contact with the gate insulating layer may be the metal film. 
     For the gate insulating layer  112 , silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, hafnium oxide, gallium oxide, or lanthanum oxide, or a mixed material thereof can be used. The gate insulating layer  112  can be formed by a plasma CVD method, a sputtering method, or the like. The structure of the gate insulating layer  112  is not limited to a single-layer structure, and may be a layered structure of a plurality of the above materials. 
     It is preferable that an insulating material containing the same kind of component as the oxide semiconductor layer be used for the gate insulating layer  112 . Such a material enables the state of the interface with the oxide semiconductor layer to be kept well. Here, containing “the same kind of component as the oxide semiconductor layer” means containing one or more elements selected from constituent elements of the oxide semiconductor layer. For example, in the case where the oxide semiconductor layer is formed using an In—Ga—Zn-based oxide semiconductor material, gallium oxide or the like is given as such an insulating material containing the same kind of component as the oxide semiconductor layer. 
     For the gate electrode layer  114 , a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium; a nitride of any of these materials; or an alloy material containing any of these materials as its main component can be used. The gate electrode layer  114  may have a single-layer structure or a layered structure of any of the above materials. 
     Although not illustrated, an insulating layer may be provided over the transistor  151 . A material similar to that of the insulating layer  102  can be used for this insulating layer. In order to electrically connect the source electrode layer  108   a  or the drain electrode layer  108   b  and a wiring, an opening may be formed in the gate insulating layer  112  and the like. A second gate electrode may further be provided below the oxide semiconductor layer  106 . Note that it is not always necessary but preferable to process the oxide semiconductor layer  106  into an island shape. 
     Next, an example of manufacturing steps of the transistor  151  illustrated in  FIGS. 1A to 1C  will be described. 
     First, the insulating layer  102  serving as a base film is formed over the substrate  100  having an insulating surface (see  FIG. 2A ). The insulating layer  102  has a function of preventing diffusion of an impurity element from the substrate  100 . The insulating layer  102  can be formed of a film selected from a silicon nitride film; a silicon oxide film; a silicon nitride oxide film; a silicon oxynitride film; and an aluminum oxide film, a gallium oxide film, or a gallium aluminum oxide film which is expressed by Ga x Al 2-x O 3+y  (0≦x≦2 and 0&lt;y&lt;1, i.e., x is greater than or equal to 0 and less than or equal to 2 and y is greater than 0 and less than 1). The structure of the base film is not limited to a single-layer structure, and may be a layered structure of films selected from the above films. 
     Here, a substrate having heat resistance enough to withstand at least heat treatment performed later can be used for the substrate  100 . For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like; a ceramic substrate; a quartz substrate; or a sapphire substrate can be used. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like; a compound semiconductor substrate made of silicon germanium or the like; an SOI substrate; or the like may be used as the substrate  100 . 
     A flexible substrate may alternatively be used as the substrate  100 . A substrate provided with a separation layer may alternatively be used as the substrate  100 . In the latter case, a transistor including an oxide semiconductor layer is manufactured in accordance with a method disclosed below and then the transistor is transferred to a flexible substrate. 
     Next, an oxide semiconductor film having a thickness of greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm is formed over the insulating layer  102 . 
     As described above, a film represented by the chemical formula, InMO 3 (ZnO) m  (m&gt;0, where m is not an integer), can be used as the oxide semiconductor film. Here, an In—Ga—Zn—O film is formed by a sputtering method. 
     As the film formation target used for a sputtering method, metal oxide with the following composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [molar ratio] is used. Alternatively, metal oxide with the composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2 [molar ratio] may be used. 
     In the case where an In—Zn—O-based material is used for an oxide semiconductor, a target to be used has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In 2 O 3 :ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In 2 O 3 :ZnO=10:1 to 1:2 in a molar ratio), more preferably In:Zn=1.5:1 to 15:1 in an atomic ratio (In 2 O 3 :ZnO=3:4 to 15:2 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O═X:Y:Z, the relation of Z&gt;1.5X+Y is satisfied. 
     Further in the case where an In—Sn—Zn—O-based material is used for the oxide semiconductor, a target to be used preferably has a composition ratio of In:Sn:Zn=2:1:3 in an atomic ratio. 
     The fill rate of the film formation target is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 100%. With the use of the film formation target with high fill rate, a dense oxide semiconductor film can be formed. 
     As the sputtering gas, a rare gas (typically, argon), oxygen, or a mixed gas of a rare gas and oxygen can be used. It is preferable to use a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, and hydride are removed as the sputtering gas. 
     The oxide semiconductor film is preferably formed in the state where the substrate is heated. The substrate is held in a film formation chamber kept under reduced pressure, and film formation is performed in the state where the substrate temperature is set to higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C.; thus, the impurity concentration in the oxide semiconductor film can be reduced. 
     In order to remove moisture remaining in the film formation chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the film formation chamber which is evacuated with a cryopump, a hydrogen atom, a compound containing a hydrogen atom such as water, a compound containing a carbon atom, and the like are evacuated, whereby the impurity concentration in the oxide semiconductor film formed in the film formation chamber can be reduced. 
     As one example of the film formation condition, the following is given: the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and the atmosphere is an oxygen atmosphere (the proportion of the oxygen flow is 100%). When a pulsed direct-current power source is used, powder substances (also referred to as particles or dust) that are generated in film formation can be reduced and the film thickness can be uniform. 
     Next, the oxide semiconductor film is processed into island-shaped oxide semiconductor layer  106  by a first photolithography step and an etching step (see  FIG. 2B ). 
     Note that a resist mask used in the photolithography step may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Here, the etching of the oxide semiconductor film may be either dry etching or wet etching. Alternatively, both of them may be used. As an etchant used for wet etching of the oxide semiconductor film, for example, a mixed solution of phosphoric acid, acetic acid, and nitric acid, or the like can be used. Alternatively, ITO-07N (produced by KANTO CHEMICAL CO., INC.) may be used. 
     Then, dehydration or dehydrogenation of the oxide semiconductor layer  106  is performed through first heat treatment. In this specification, the term “dehydration or dehydrogenation” refers to not only elimination of water or a hydrogen molecule but also elimination of a hydrogen atom, a hydroxyl group, or the like. 
     Through this heat treatment, excessive hydrogen (including water and a hydroxyl group) is removed, so that an impurity level in energy gap of the oxide semiconductor layer  106  can be reduced. The temperature of the heat treatment is higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 350° C. and lower than or equal to 500° C., more preferably higher than or equal to 390° C. and lower than or equal to 460° C. The length of time of the heat treatment may be about an hour as long as the temperature is in the above favorable range. The heat treatment can be performed through rapid thermal annealing (RTA) treatment in an atmosphere of an inert gas (such as nitrogen, helium, neon, or argon) at a temperature from higher than or equal to 500° C. and lower than or equal to 750° C. (or a temperature lower than or equal to the strain point of the glass substrate) for more than or equal to 1 minute and less than or equal to 10 minutes, preferably at 650° C. for more than or equal to 3 minutes and less than or equal to 6 minutes. A method for the heat treatment may be determined by the practitioner as appropriate. Note that the timing of heat treatment for the dehydration or dehydrogenation of the oxide semiconductor layer  106  is not limited to this timing and may be performed plural times, for example, before and after a photolithography step or a film formation process. In that case, the heat treatment may be performed in an atmosphere containing oxygen. 
     The heat treatment performed on the oxide semiconductor may be performed on the oxide semiconductor film which has not yet been processed into the island-shaped oxide semiconductor layer. In that case, a photolithography step is performed after the heat treatment. The heat treatment may be performed after the source electrode layer and the drain electrode layer are formed over the island-shaped oxide semiconductor layer as long as the oxide semiconductor is formed. 
     Next, a buffer layer and a metal layer are formed over the insulating layer  102  and the oxide semiconductor layer  106 . The above described materials can be used for the buffer layer and the metal layer. Here, a single layer of indium tin oxide and a single layer of tungsten are formed for the buffer layer and the metal layer respectively, by a sputtering method. 
     Next, a resist mask is formed over the metal layer by a second photolithography step. The metal layer and the buffer layer are selectively etched, so that the source region including the buffer layer  107   a  and the source electrode layer  108   a  and the drain region including the buffer layer  107   b  and the drain electrode layer  108   b  are formed. Then, the resist mask is removed (see  FIG. 2C ). 
     Note that the etching of the source region and the drain region is performed so that a portion of the oxide semiconductor layer  106  which is to be etched is minimized. However, it is difficult to obtain etching conditions under which only the metal layer and the buffer layer are etched. In some cases, the oxide semiconductor layer  106  is partly etched so as to have a groove portion (a recessed portion) by the etching of the metal layer and the buffer layer. The source electrode layer  108   a  and the drain electrode layer  108   b  may be formed to provide steps over the buffer layer  107   a  and the buffer layer  107   b.    
     Next, the gate insulating layer  112  is formed over the source region, the drain region, and the oxide semiconductor layer  106  (see  FIG. 2D ). Here, a layer of silicon oxide is formed for the gate insulating layer  112  by a sputtering method. 
     For the formation of the gate insulating layer  112 , a high-density plasma CVD method using microwaves (e.g., with a frequency of 2.45 GHz) is preferably employed because a high-quality insulating layer which is dense and has high breakdown voltage can be formed. The oxide semiconductor layer is formed in close contact with the high-quality gate insulating layer, whereby the interface state density can be reduced. 
     Moreover, it is possible to use as the gate insulating layer which is an insulating layer whose quality and characteristics of the interface with the oxide semiconductor layer are improved by heat treatment performed after the formation of the insulating layer. In any case, the gate insulating layer  112  is preferably formed using an insulating layer that can reduce the interface state density with the oxide semiconductor layer to form a favorable interface, as well as having favorable film quality. 
     Second heat treatment is preferably performed after the gate insulating layer  112  is formed. The second heat treatment is performed at temperature of higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 350° C. and lower than or equal to 600° C. or lower than the strain point of the substrate. 
     The second heat treatment may be performed under an atmosphere of an oxidizing gas or an inert gas. Note that it is preferable that water, hydrogen, and the like be not contained in the atmosphere of oxidizing gas or an inert gas. Further, the purity of the gas introduced into a heat treatment apparatus is preferably set to higher than or equal to 6N (99.9999%), more preferably higher than or equal to 7N (99.99999%) (that is, the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm). 
     The second heat treatment is performed while the oxide semiconductor layer  106  and the gate insulating layer  112  containing oxygen are in contact with each other. Thus, oxygen which is one of main components of the oxide semiconductor can be supplied from the gate insulating layer  112  containing oxygen to the oxide semiconductor layer  106 . Consequently, oxygen deficiency in the oxide semiconductor layer  106  and an interface state between the oxide semiconductor layer  106  and the gate insulating layer  112  can be reduced. At the same time, deficiency in the gate insulating layer  112  can also be reduced. 
     Note that there is no particular limitation on the timing of the second heat treatment as long as it is after the gate insulating layer  112  is formed. For example, the second heat treatment may be performed after the gate electrode layer  114  is formed. 
     Next, a conductive film is formed and then the gate electrode layer  114  is formed through a third photolithography step and an etching step (see  FIG. 2E ). Here, a stacked layer of tungsten and tantalum nitride is formed as the conductive film by a sputtering method. 
     Although not illustrated, an insulating layer may be formed over the gate insulating layer  112  and the gate electrode layer  114 . As the insulating layer, an inorganic insulating film such as a silicon oxide film; a silicon oxynitride film; an aluminum oxynitride film; or an aluminum oxide film, a gallium oxide film, or a gallium aluminum oxide film which is expressed by Ga x Al 2-x O 3+y  (0≦x≦2 and 0&lt;y&lt;1, i.e., x is greater than or equal to 0 and less than or equal to 2 and y is greater than 0 and less than 1) can be used. 
     A protective insulating layer for improving reliability may be further formed over the insulating layer. As the protective insulating layer, an inorganic insulating film such as a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film can be used. 
     A planarization insulating film may be formed over the insulating layer and the protective insulating layer in order to reduce surface unevenness due to the transistor. For the planarization insulating film, an organic material such as polyimide, acrylic, or benzocyclobutene can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. 
     Through the above steps, the transistor  151  is formed. 
     As described above, a semiconductor device having favorable characteristics can be provided. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Embodiment 2 
     A semiconductor device (also referred to as a display device) with a display function can be manufactured using the transistor an example of which is described in Embodiment 1. Some or all of driver circuits which include the transistor can be formed over a substrate where a pixel portion is formed, whereby a system-on-panel can be obtained. 
     In  FIG. 3A , a sealant  205  is provided to surround a pixel portion  202  provided over a first substrate  201 , and the pixel portion  202  is sealed with the sealant  205  between the first substrate  201  and a second substrate  206 . In  FIG. 3A , a scan line driver circuit  204  and a signal line driver circuit  203  each are formed using a single crystal semiconductor or a polycrystalline semiconductor mounted in a region different from the region surrounded by the sealant  205  over the first substrate  201 . Various signals and potentials are supplied to the signal line driver circuit  203  and the scan line driver circuit  204  or the pixel portion  202  through flexible printed circuits (FPCs)  218   a  and  218   b.    
     In  FIGS. 3B and 3C , the sealant  205  is provided to surround the pixel portion  202  and the scan line driver circuit  204  which are provided over the first substrate  201 . The second substrate  206  is provided over the pixel portion  202  and the scan line driver circuit  204 . Thus, the pixel portion  202  and the scan line driver circuit  204  are sealed together with a display element by the first substrate  201 , the sealant  205 , and the second substrate  206 . In  FIGS. 3B and 3C , the signal line driver circuit  203  is formed using a single crystal semiconductor or a polycrystalline semiconductor and mounted in a region different from the region surrounded by the sealant  205  over the first substrate  201 . In  FIGS. 3B and 3C , various signals and potentials are supplied to the signal line driver circuit  203  which is separately formed, the scan line driver circuit  204 , and the pixel portion  202  through an FPC  218 . 
     Although  FIGS. 3B and 3C  each illustrate the example in which the signal line driver circuit  203  is formed separately and mounted on the first substrate  201 , the present invention is not limited to this structure. The scan line driver circuit may be formed separately and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be formed separately and then mounted. 
     Note that a connection method of a separately formed driver circuit is not particularly limited, and a chip on glass (COG) method, a wire bonding method, a tape automated bonding (TAB) method, or the like can be used.  FIG. 3A  shows an example in which the signal line driver circuit  203  and the scan line driver circuit  204  are mounted by a COG method.  FIG. 3B  shows an example in which the signal line driver circuit  203  is mounted by a COG method.  FIG. 3C  shows an example in which the signal line driver circuit  203  is mounted by a TAB method. 
     Note that a display device in this specification means an image display device, a display device, or a light source (including a lighting device). The display device includes any of the following modules: a module provided with an FPC or a TAB tape; a module in which a printed wiring board is provided in the end of a TAB tape; and a module in which an integrated circuit (IC) is mounted directly on a display element by a COG method. 
     The pixel portion and the scan line driver circuit which are provided over the first substrate include a plurality of transistors and the transistor an example of which is described in Embodiment 1 can be used as one of the transistors. 
     As the display element provided in the display device, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. A light-emitting element includes, in its category, an element whose luminance is controlled by current or voltage, and specifically an inorganic electroluminescent (EL) element, an organic EL element, and the like. Furthermore, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used. 
     One embodiment of the display device is described with reference to  FIG. 4 ,  FIG. 5 , and  FIG. 6 .  FIG. 4 ,  FIG. 5 , and  FIG. 6  correspond to cross-sectional views along line M-N in  FIG. 3B . 
     As illustrated in  FIG. 4 ,  FIG. 5 , and  FIG. 6 , the semiconductor device includes a connection terminal electrode  215  and a terminal electrode  216 . The connection terminal electrode  215  and the terminal electrode  216  are electrically connected to a terminal included in the FPC  218  through an anisotropic conductive layer  219 . 
     The connection terminal electrode  215  is formed of the same conductive layer as a first electrode layer  230 . The terminal electrode  216  is formed of the same conductive layer as a source electrode and a drain electrode of a transistor  210  and a transistor  211 . 
     Each of the pixel portion  202  and the scan line driver circuit  204  provided over the first substrate  201  includes a plurality of transistors. In  FIG. 4 ,  FIG. 5 , and  FIG. 6 , the transistor  210  included in the pixel portion  202  and the transistor  211  included in the scan line driver circuit  204  are illustrated as an example. 
     In this embodiment, the transistor described in Embodiment 1 can be applied to the transistor  210  and the transistor  211 . The transistors  210  and  211  have excellent electric characteristics; thus, a display device having an excellent display capability can be provided. 
     The transistor  210  provided in the pixel portion  202  is electrically connected to the display element to form a display panel. There is no particular limitation on the kind of the display element as long as display can be performed, and various kinds of display elements can be employed. 
     An example of a liquid crystal display device using a liquid crystal element as the display element is illustrated in  FIG. 4 . In  FIG. 4 , a liquid crystal element  213  is a display element including the first electrode layer  230 , a second electrode layer  231 , and a liquid crystal layer  208 . Note that insulating layers  232  and  233  serving as alignment layers are provided so that the liquid crystal layer  208  is interposed therebetween. The second electrode layer  231  is formed on the second substrate  206  side. The first electrode layer  230  and the second electrode layer  231  are stacked with the liquid crystal layer  208  interposed therebetween. Although a structure is illustrated in which a color filter  237  for performing color display is provided on a counter substrate side, the color filter  237  may be provided on the substrate side where the transistors are formed. The color filter  237  is unnecessary in the case where color display is not performed. 
     Light-blocking layers  238   a  and  238   b  are provided in a lower layer of the transistors  210  and  211  respectively. The light-blocking layers  238   a  and  238   b  block light emitted from a back light or the like to channel formation regions of the transistors. There is no particular limitation on a material which can be used for the light-blocking layers as long as the material has a high light-blocking property. For example, in the case where a metal layer is used, the light-blocking layers can function as second gate electrodes. 
     In addition, a light-blocking layer  236  is formed in a region on the counter substrate side, which is over the transistors  210  and  211 . The light-blocking layer  236  blocks light emitted to the transistors. The light-blocking layer  236  can function as a black matrix in a display region; thus, display quality can be improved. 
     By blocking light emitted to the transistors  210  and  211  as described above, light deterioration phenomenon that is a problem of a transistor using an oxide semiconductor can be suppressed and deterioration in threshold voltage can be suppressed for a long time. 
     A spacer  235  is a columnar spacer obtained by selective etching of an insulating layer and is provided in order to control the thickness (a cell gap) of the liquid crystal layer  208 . Alternatively, a spherical spacer may be used. 
     In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     Alternatively, a liquid crystal exhibiting a blue phase for which an alignment layer is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase is generated only within a relatively narrow range of temperature, a liquid crystal composition containing a chiral agent at greater than or equal to 5 wt. %, a UV curable resin, and the like are mixed and which is irradiated with ultraviolet light is used for the liquid crystal layer in order to improve the range of temperature. This liquid crystal layer shows a blue phase with a wide range of temperature and has a short response time of 1 msec or less. Further, this liquid crystal layer is optically isotropic; therefore, alignment treatment is not necessary and viewing angle dependence is small. In addition, since an alignment layer does not need to be provided and rubbing treatment is unnecessary, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device can be reduced in the manufacturing process. Thus, productivity of the liquid crystal display device can be increased. Note that in the case where a blue phase is used, one embodiment of the present invention is not limited to the structure in  FIG. 4 , and a structure of a so-called horizontal electric field mode may be employed, in which the second electrode layer  231  is formed on the substrate side where the first electrode layer  230  is formed. 
     The specific resistivity of the liquid crystal material is greater than or equal to 1×10 9  Ω·cm, preferably greater than or equal to 1×10 11  Ω·cm, further preferably greater than or equal to 1×10 12  Ω·cm. The value of the specific resistivity in this specification is measured at 20° C. 
     The size of a storage capacitor provided in the liquid crystal display device is set in consideration of the leakage current of the transistor provided in the pixel portion or the like so that a charge can be held for a predetermined period. By using a transistor including a high-purity oxide semiconductor layer, it is enough to provide a storage capacitor having a capacitance that is less than or equal to ⅓, preferably less than or equal to ⅕ of a liquid crystal capacitance of each pixel. 
     In the transistor used in this embodiment, which uses a highly-purified oxide semiconductor layer, the current in an off state (the off-state current) can be made small. Therefore, an electrical signal such as an image signal can be held for a long period, and a writing interval can be set long when the power is on. Consequently, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption. 
     The field-effect mobility of the transistor including a highly-purified oxide semiconductor layer used in this embodiment can be relatively high, whereby high-speed operation is possible. Therefore, by using the transistor in a pixel portion of a liquid crystal display device, a high-quality image can be provided. In addition, a driver circuit portion can be formed over the same substrate by using the transistor, whereby the number of components of the liquid crystal display device can be reduced. 
     For the liquid crystal display device, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     A normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode is preferable. The vertical alignment mode is a method of controlling alignment of liquid crystal molecules of a liquid crystal display panel, in which liquid crystal molecules are aligned vertically to a panel surface when no voltage is applied. For example, an MVA (multi-domain vertical alignment) mode, a PVA (patterned vertical alignment) mode, or an ASV mode can be employed. Moreover, it is possible to use a method called domain multiplication or multi-domain design, in which a pixel is divided into some regions (subpixels) and molecules are aligned in different directions in their respective regions. 
     In the display device, a black matrix (a light-blocking layer), an optical member (an optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, and the like are provided as appropriate. For example, circular polarization may be obtained by using a polarizing substrate and a retardation substrate. In addition, a backlight, a side light, or the like may be used as a light source. 
     In addition, it is possible to employ a time-division display method (also called a field-sequential driving method) with the use of a plurality of light-emitting diodes (LEDs) as a backlight. By employing a field-sequential driving method, color display can be performed without using a color filter. 
     As a display method in the pixel portion, a progressive method, an interlace method, or the like can be employed. Further, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue respectively). For example, R, G, B, and W (W corresponds to white), or R, G, B, and one or more of yellow, cyan, magenta, and the like can be used. Further, the sizes of display regions may be different between respective dots of color elements. The present invention is not limited to the application to a display device for color display but can also be applied to a display device for monochrome display. 
     Alternatively, as the display element included in the display device, a light-emitting element utilizing electroluminescence can be used. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element. 
     In an organic EL element, by application of a voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, so that current flows. The carriers (electrons and holes) are recombined, and thus the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. An element which emits light with such a mechanism is called a current-excitation light-emitting element. 
     The inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an example of an organic EL element as a light-emitting element is described here. 
     In order to extract light emitted from the light-emitting element, it is acceptable as long as at least one of a pair of electrodes is transparent. The light-emitting element can have a top emission structure in which light emission is extracted through the surface opposite to the substrate; a bottom emission structure in which light emission is extracted through the surface on the substrate side; or a dual emission structure in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side, and a light-emitting element having any of these emission structures can be used. 
     An example of a light-emitting device in which a light-emitting element is used as the display element is illustrated in  FIG. 5 . A light-emitting element  243  which is a display element is electrically connected to the transistor  210  provided in the pixel portion  202 . The structure of the light-emitting element  243  is not limited to the layered structure including the first electrode layer  230 , an electroluminescent layer  241 , and the second electrode layer  231 , which is illustrated in  FIG. 5 . The structure of the light-emitting element  243  can be changed as appropriate depending on a direction in which light is extracted from the light-emitting element  243 , or the like. 
     A partition wall  240  can be formed using an organic insulating material or an inorganic insulating material. It is particularly preferable that the partition wall  240  be formed using a photosensitive resin material to have an opening over the first electrode layer  230  so that a sidewall of the opening is formed as a tilted surface with continuous curvature. 
     The electroluminescent layer  241  may be formed with either a single layer or a stacked layer of a plurality of layers. 
     A protective layer may be formed over the second electrode layer  231  and the partition wall  240  in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element  243 . As a protective layer, a silicon nitride layer, a silicon nitride oxide layer, a DLC layer, or the like can be formed. In a space sealed with the first substrate  201 , the second substrate  206 , and the sealant  205 , a filler  244  is provided and tightly sealed. In such a manner, it is preferable that the light-emitting element be packaged (sealed) with a protective film (such as a laminate film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the light-emitting element is not exposed to the outside air. 
     As the filler  244 , an ultraviolet curable resin or a thermosetting resin can be used in addition to an inert gas such as nitrogen or argon, and polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA), or the like can be used. 
     If necessary, an optical film such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter may be provided as appropriate for a light-emitting surface of the light-emitting element. Further, a polarizing plate or a circularly polarizing plate may be provided with an anti-reflection layer. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed. 
     Further, an electronic paper in which electronic ink is driven can be provided as the display device. The electronic paper is also called an electrophoretic display device (electrophoretic display) and has advantages in that it has the same level of readability as regular paper, it has less power consumption than other display devices, and it can be set to have a thin and light form. 
     An electrophoretic display device can have various modes. An electrophoretic display device contains a plurality of microcapsules dispersed in a solvent or a solute, each microcapsule containing first particles which are positively charged and second particles which are negatively charged. By applying an electric field to the microcapsules, the particles in the microcapsules move in opposite directions to each other and only the color of the particles gathering on one side is displayed. Note that the first particles and the second particles each contain pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless). 
     Thus, an electrophoretic display device is a display that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a high-electric field region. 
     A solution in which the above microcapsules are dispersed in a solvent is referred to as electronic ink. This electronic ink can be printed on a surface of glass, plastic, cloth, paper, or the like. Furthermore, by using a color filter or particles that have a pigment, color display can also be achieved. 
     Note that the first particles and the second particles in the microcapsules may each be formed of a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material, or formed of a composite material thereof. 
     As the electronic paper, a display device using a twisting ball display system can be used. The twisting ball display system refers to a method in which spherical particles each colored in black and white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed. 
       FIG. 6  illustrates active matrix electronic paper as one embodiment of a semiconductor device. The electronic paper in  FIG. 6  is an example of a display device using a twisting ball display system. 
     Between the first electrode layer  230  connected to the transistor  210  and the second electrode layer  231  provided on the second substrate  206 , spherical particles  253  each of which includes a black region  255   a , a white region  255   b , and a cavity  252  around the regions which is filled with liquid, are provided. A space around the spherical particles  253  is filled with a filler  254  such as a resin. The second electrode layer  231  corresponds to a common electrode (counter electrode). The second electrode layer  231  is electrically connected to a common potential line. 
     Note that in  FIGS. 4 to 6 , a flexible substrate as well as a glass substrate can be used as the first substrate  201  and the second substrate  206 . As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. In addition, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used. 
     An insulating layer  221  can be formed using an organic insulating material or an inorganic insulating material. Note that an organic insulating material having heat resistance, such as an acrylic resin, a polyimide, a benzocyclobutene-based resin, a polyamide, or an epoxy resin is preferably used as a planarizing insulating layer. As well as such organic insulating materials, it is possible to use a low-dielectric material (a low-k material), a siloxane-based resin, phosphorus silicate glass (PSG), boron phosphorus silicate glass (BPSG), or the like. Note that the insulating layer  221  may be formed by stacking a plurality of insulating layers formed using any of these materials. 
     There is no particular limitation on the method for forming the insulating layer  221 , and the insulating layer  221  can be formed, depending on a material thereof, by a sputtering method, a spin coating method, a dipping method, a spray coating method, a droplet discharging method (e.g., an ink-jet method, screen printing, or offset printing), a roll coating method, a curtain coating method, a knife coating method, or the like. 
     The first electrode layer  230  and the second electrode layer  231  (each of which may be called a pixel electrode layer, a common electrode layer, a counter electrode layer, or the like) for applying voltage to the display element may have light-transmitting properties or light-reflecting properties, which depends on the direction in which light is extracted, the position where the electrode layer is provided, the pattern structure of the electrode layer, and the like. 
     Either or both of the first electrode layer  230  and the second electrode layer  231  can be formed using a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon is added. 
     Either the first electrode layer  230  or the second electrode layer  231  can be formed using one or more materials selected from a metal such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag); an alloy of these metals; or a nitride of these metals. 
     A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) can be used for either of the first electrode layer  230  and the second electrode layer  231 . As the conductive macromolecule, a so-called p-electron conjugated conductive macromolecule can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, and a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof can be given. 
     Since the transistor is easily broken owing to static electricity or the like, a protective circuit for protecting the driver circuit is preferably provided. The protective circuit is preferably formed using a nonlinear element. 
     As described above, by using any of the transistors described in Embodiment 1, a display device having better display performance can be provided. Note that the transistors described in Embodiment 1 can be applied to not only semiconductor devices having the display functions described above but also semiconductor devices having a variety of functions, such as a power device which is mounted on a power supply circuit, a semiconductor integrated circuit such as an LSI and a memory and a semiconductor device having an image sensor function of reading information of an object. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Embodiment 3 
     A semiconductor device which is one embodiment of the present invention can be applied to a variety of electronic appliances (including game machines). Examples of electronic devices are a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game machine, a personal digital assistant, an audio reproducing device, and a large-sized game machine such as a pachinko machine. Examples of electronic appliances each including the semiconductor device described in Embodiment 1 or 2 will be described. 
       FIG. 7A  illustrates a notebook PC. The notebook PC includes a main body  301 , a housing  302 , a display portion  303 , a keyboard  304 , and the like. 
       FIG. 7B  illustrates a personal digital assistant (PDA). The personal digital assistant includes a display portion  313 , an external interface  315 , an operation button  314 , and the like in a main body  311 . A stylus  312  is included as an accessory for operation. 
       FIG. 7C  illustrates an example of an electronic book device. For example, an e-book reader  320  includes two housings, a housing  321  and a housing  322 . The housing  321  and the housing  322  are combined with a hinge  325  so that the e-book reader  320  can be opened and closed with the hinge  325  as an axis. With such a structure, the e-book reader  320  can operate like a paper book. 
     A display portion  323  and a display portion  324  are incorporated in the housing  321  and the housing  322  respectively. The display portion  323  and the display portion  324  may display one image or different images. When the display portion  323  and the display portion  324  display different images, for example, text can be displayed on a display portion on the right side (the display portion  323  in  FIG. 7C ) and photographs and pictures can be displayed on a display portion on the left side (the display portion  324  in  FIG. 7C ). 
       FIG. 7C  illustrates an example in which the housing  321  is provided with an operation portion and the like. For example, the housing  321  is provided with a power switch  326 , operation keys  327 , a speaker  328 , and the like. With the operation keys  327 , pages can be turned. Note that a keyboard, a pointing device, or the like may also be provided on the surface of the housing, on which the display portion is provided. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Further, the e-book reader  320  may have a function of an electronic dictionary. 
     The e-book reader  320  may send and receive data wirelessly. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server. 
       FIG. 7D  illustrates a mobile phone, which includes two housings, a housing  330  and a housing  331 . The housing  331  includes a display panel  332 , a speaker  333 , a microphone  334 , a pointing device  336 , a camera  337 , an external connection terminal  338 , and the like. In addition, the housing  330  includes a solar cell  340  having a function of charge of the personal digital assistant, an external memory slot  341 , and the like. Further, an antenna is incorporated in the housing  331 . 
     Further, the display panel  332  is provided with a touch panel. A plurality of operation keys  335  which are displayed as images is illustrated by dashed lines in  FIG. 7D . Note that the mobile phone includes a boosting circuit for raising a voltage output from the solar cell  340  to a voltage necessary for each circuit. 
     The display direction of the display panel  332  is changed as appropriate depending on a usage pattern. Further, the mobile phone is provided with the camera  337  on the same surface as the display panel  332 , and thus it can be used as a video phone. The speaker  333  and the microphone  334  can be used for videophone calls, recording and playing sound, and the like as well as voice calls. Moreover, the housings  330  and  331  in a state where they are opened as illustrated in  FIG. 7D  can be slid so that one overlaps the other; therefore, the size of the mobile phone can be reduced, which makes the mobile phone suitable for being carried. 
     The external connection terminal  338  can be connected to various types of cables such as a charging cable and a USB cable, and charging and data communication with a personal computer are possible. Moreover, a larger amount of data can be stored by inserting a recording medium to the external memory slot  341  and can be moved. 
     Further, in addition to the above functions, an infrared communication function, a television reception function, or the like may be provided. 
       FIG. 7E  illustrates a digital video camera, which includes a main body  351 , a display portion (A)  357 , an eyepiece  353 , an operation switch  354 , a display portion (B)  355 , a battery  356 , and the like. 
       FIG. 7F  illustrates an example of a television set. In a television set  360 , a display portion  363  is incorporated in a housing  361 . The display portion  363  can display images. Here, the housing  361  is supported by a stand  365 . 
     The television set  360  can be operated by an operation switch of the housing  361  or a separate remote controller. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller. 
     Note that the television set  360  is provided with a receiver, a modem, and the like. Moreover, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     EXAMPLE 
     In this example, transistors including an n-type buffer layer described in Embodiment 1 were manufactured. Results of comparison between the electric characteristics and reliability of the transistors and those of transistors having a conventional structure will be described. 
     Transistor whose structure corresponds to a structure in  FIG. 8A  were manufactured in accordance with a manufacturing method of Embodiment 1. The following were included in the transistors: a 300 nm-thick silicon oxide film as the insulating layer  102  which is a base layer; an In—Ga—Zn—O-based film whose channel formation region has a thickness of 25 nm as the oxide semiconductor layer  106 ; a 5 nm-thick indium tin oxide (hereinafter referred to as ITO) film containing silicon (Si) or a 5 nm-thick indium zinc oxide (hereinafter referred to as IZO) film as the buffer layers  107   a  and  107   b ; a 100 nm-thick tungsten film as the source electrode layer  108   a  and the drain electrode layer  108   b ; a 15 nm-thick silicon oxynitride film as the gate insulating layer  112 ; and a 15 nm-thick tantalum nitride film and a 135 nm-thick tungsten film are stacked in this order from the gate insulating layer  112  side as the gate electrode layer  114 . 
     Although not illustrated in  FIG. 8A , as an interlayer insulating film, a 300 nm-thick silicon oxide film was formed over the gate electrode layer  114  and the gate insulating layer  112  by a sputtering method. A 50 nm-thick titanium film, a 100 nm-thick aluminum film, and a 5 nm-thick titanium film were formed as wiring layers each of which is connected to the source electrode layer  108   a , the drain electrode layer  108   b , and the gate electrode layer  114  respectively, through contact holes formed in the interlayer insulating film. The wiring layers were formed as extraction electrodes which were used in obtaining the electric characteristics of the transistors. 
     First, parasitic resistance between the semiconductor layer and the source/drain electrodes was obtained by a transmission line model (TLM) method using data of L length dependence of on-resistance of the manufactured transistors (L=0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 2 μm, 3 μm, 5 μm, and 10 μm, W=10 μm). Field-effect mobility was obtained by the I-V characteristics (a drain voltage was 3 V and a gate voltage was −6 V to +6 V) of the manufactured transistors (L/W=0.8 μm/10 μm). The results are shown in Table 1. Here, values of the field effect mobility are the largest value when the gate voltage is in the above range. Twelve of the transistors were measured for each of three structures (the one which does not include a buffer layer, the one which includes a buffer layer using ITO containing Si, and the one which includes a buffer layer using IZO), and the medium value of the field effect mobility of the transistors were shown. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Buffer Layer 
               
            
           
           
               
               
               
               
            
               
                   
                 Not 
                 ITO containing 
                   
               
               
                   
                 included 
                 Si (5 nm) 
                 IZO (5 nm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Parasitic Resistance between 
                 860 
                 520 
                 540 
               
               
                 Semiconductor Layer and 
               
               
                 Source/Drain Electrodes [Ω] 
               
               
                 Field Effect Mobility [cm 2 / 
                 12.4 
                 13.5 
                 13.7 
               
               
                 Vs] 
               
               
                   
               
            
           
         
       
     
     From the results in Table 1, the parasitic resistance between the semiconductor layer and the source/drain electrodes of the transistors including an n-type buffer layer were reduced by about 40%. This shows that including an n-type buffer layer is effective. In addition, it was confirmed that on-state current characteristics was improved and the field effect mobility were improved by about 9% to 10% owing to the effect. 
     Moreover, a negative gate BT (bias-temperature) stress test was performed on transistors (L/W=3 μm/50 μm) manufactured in a manner similar to that of the above transistors. The test was performed at 150° C. for an hour under the following conditions: sources of the transistors were set to GND; +0.1 V was applied to drains of the transistors; +3 V (+2 MV/cm), −3 V (−2 MV/cm), +9 V (+6 MV/cm), or −9 V (−6 MV/cm) was applied to gates of the transistors. 
     No differences were seen between the transistors in which +3 V, −3 V, and +9 V were applied to the gates among the above tests, whether buffer layers were included or not. In contrast, in the transistors in which −9 V was applied, deterioration in on-state current (a current when the gate voltage was 3 V) of the transistor including a buffer layer was reduced as shown in  FIGS. 9A to 9C . The deterioration rates of the on-state current shown in  FIGS. 9A to 9C  are described in Table 2. Here, the on-state current in an early stage is defined as a current value when the gate voltage is 3 V. The on-state current after the negative gate BT stress test is defined as a current value when the gate voltage is 3 V−ΔVth (the difference between the threshold voltage in the early stage and the threshold voltage after the stress test). 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Buffer Layer 
               
            
           
           
               
               
               
               
            
               
                   
                 Not 
                 ITO containing 
                   
               
               
                   
                 included 
                 Si (5 nm) 
                 IZO (5 nm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Deterioration Rate 
                 54.4% 
                 3.3% 
                 12.8% 
               
               
                 when Drain Voltage = 0.1 V 
               
               
                 Deterioration Rate 
                 20.8% 
                 1.2% 
                  1.8% 
               
               
                 when Drain Voltage = 3 V 
               
               
                   
               
            
           
         
       
     
     These results show that a transistor which includes a buffer layer also has an effect of greatly reducing the deterioration rate of on-state current compared with a transistor which does not include a buffer layer. In addition, it is found that the deterioration rate of on-state current can be more suppressed when ITO containing Si is used as a buffer layer compared to the case where IZO is used. 
     Scientific calculation was performed in order to investigate the cause of the negative gate BT stress test result (suppression of a reduction in on-state current). 
     Models used in the scientific calculation have basically the structure illustrated in  FIG. 8A . Considering an actual device structure, a reduction in a thickness of a channel formation region in an oxide semiconductor layer was set to 5 nm in a transistor of a model which does not include a buffer layer; taper angle of edges was set to 30° and a reduction in a thickness of a channel formation region in an oxide semiconductor layer was set to 10 nm in a transistor of a model which includes a buffer layer according to the result of cross-sectional TEM observation of a transistor shown in  FIG. 13A . Note that  FIG. 13B  is a schematic view for describing  FIG. 13A . 
     The size of the transistors was assumed to be L/W= 3/50 μm; the thickness of a gate insulating film (silicon oxynitride), 15 nm; a band gap (Eg) of an oxide semiconductor, 3.15 eV; the electron affinity (χ), 4.3 eV; the dielectric constant, 15; and the electron mobility, 10 cm 2 /Vs. An oxide semiconductor layer serving as a channel formation region was assumed to be uniformly amorphous. An n-type buffer layer was assumed to be an oxide semiconductor including donors at high concentration (1×10 20 /cm 3 ). Values of the band gap and the electron affinity of the n-type buffer layer were set to the same value as those of the oxide semiconductor serving as a channel formation region and the value of the electron mobility was determined so that the resistivity was 3×10 −3  Ω·cm. The work function of a gate metal was 4.9 eV (the gate was assumed to be tungsten) and the work function of a source/drain metal was 4.5 eV (the source-drain metal was assumed to be molybdenum). A device simulation tool Sentaurus Device manufactured by Synopsys. Inc. was used for the calculation. 
       FIG. 10  shows diagrams of distribution of an electric field intensity in the case where a negative gate BT stress (a gate voltage was −9 V and a drain voltage was +0.1 V at 150° C.) was applied to the transistors. As a result of the scientific calculation, it was found that electric field concentration occurs in part of the oxide semiconductor layer as shown by arrows in the figure. Note that “OS” in the figure indicates an oxide semiconductor layer and “GI” in the figure indicates a gate insulating layer. 
     Next,  FIG. 11  and  FIG. 12  show calculation results in the case where an electron trap was assumed to be generated by a negative gate BT stress and a negative charge was fixed in a position where electric field concentration was generated (an interface between an oxide semiconductor layer and a gate insulating layer). 
     The surface density of the negative charge was 1×10 13  cm −2 . I-V characteristics in the state where the negative charge was fixed and the state where the negative charge was not fixed were calculated. Thus, whether a phenomenon of deterioration in on-state current at the time of performing the negative gate BT stress test reappeared or not was confirmed. Note that the negative charge was fixed to the same position on a source side so as to be symmetric with a drain side. 
     As a result, as shown in  FIG. 11 , on-state current was greatly reduced in a model which does not include a buffer layer when a drain voltage was +0.1 V, whereby the I-V characteristics in  FIG. 9A  was reproduced. Note that a reduction in on-state current was confirmed in a model which includes a buffer layer in  FIG. 11  but the amount of the reduction is very little. In view of this point, it can be said that the I-V characteristics in  FIGS. 9B and 9C  are reproduced. 
     In current density distribution diagrams in  FIG. 11  (a gate voltage is +3 V and a drain voltage is +0.1 V), an electron at an interface of a gate insulating layer is repelled by the negative charge and a depletion layer is formed surrounding the negative charge in the model which does not include a buffer layer. On-state current is reduced because this depletion layer exists in a path of current. On the other hand, in the model which includes a buffer layer, it is considered that an electron is supplied from an n-type buffer layer in the vicinity, so that a depletion layer is very small even when the negative charge exists; thus, a reduction of on-state current is suppressed. 
     In current density distribution diagrams in  FIG. 12  (a gate voltage is +3 V and a drain voltage is +3 V), it is found that current detours a depletion layer due to the negative charge and flows even in a model which does not include a buffer layer. When a drain voltage is high (in a saturation region), current flows in a position which is distant from an interface of a gate insulating layer due to the electric field of the drain; therefore, it can be considered that the current detours the negative charge and flows. Consequently, it can be said that a reduction in on-state current due to a negative gate BT stress is less likely to be caused when a drain voltage is high. 
     As described above, the phenomenon of the reduction in on-state current after performing the negative gate BT stress test shown in  FIGS. 9A to 9C  can be explained by the scientific calculation results. 
     This example can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     EXPLANATION OF REFERENCE 
     
         
           100 : substrate,  102 : insulating layer,  106 : oxide semiconductor layer,  107   a : buffer layer,  107   b : buffer layer,  108   a : source electrode layer,  108   b : drain electrode layer,  112 : gate insulating layer,  114 : gate electrode layer,  151 : transistor,  201 : substrate,  202 : pixel portion,  203 : signal line driver circuit,  204 : scan line driver circuit,  205 : sealant,  206 : substrate,  208 : liquid crystal layer,  210 : transistor,  211 : transistor,  213 : liquid crystal element,  215 : connection terminal electrode,  216 : terminal electrode,  218 : FPC,  218   a : FPC,  218   b : FPC,  219 : anisotropic conductive layer,  221 : insulating layer,  230 : electrode layer,  231 : electrode layer,  232 : insulating layer,  233 : insulating layer,  235 : spacer,  236 : light-blocking layer,  237 : color filter,  238   a : light-blocking layer,  238   b : light-blocking layer,  240 : partition,  241 : electroluminescent layer,  243 : light-emitting element,  244 : filler,  252 : cavity,  253 : spherical particle,  254 : filler,  255   a : black region,  255   b : white region,  301 : main body,  302 : housing,  303 : display portion,  304 : keyboard,  311 : main body,  312 : stylus,  313 : display portion,  314 : operation key,  315 : external interface,  320 : e-book reader,  321 : housing,  322 : housing,  323 : display portion,  324 : display portion,  325 : hinge,  326 : power switch,  327 : operation key,  328 : speaker,  330 : housing,  331 : housing,  332 : display panel,  333 : speaker,  334 : microphone,  335 : operation key,  336 : pointing device,  337 : camera,  338 : external connection terminal,  340 : solar cell,  341 : external memory slot,  351 : main body,  353 : eyepiece,  354 : operation switch,  355 : display portion (B),  356 : battery,  357 : display portion (A),  360 : television set,  361 : housing,  363 : display portion, and  365 : stand. 
       
    
     This application is based on Japanese Patent Application serial no. 2010-152050 filed with Japan Patent Office on Jul. 2, 2010 and Japanese Patent Application serial no. 2010-181523 filed with Japan Patent Office on Aug. 16, 2010, the entire contents of which are hereby incorporated by reference.