Patent Publication Number: US-8975115-B2

Title: Semiconductor device and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/949,329, filed Jul. 24, 2013, now allowed, which is a divisional of U.S. application Ser. No. 13/185,779, filed Jul. 19, 2011, now U.S. Pat. No. 8,519,387, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2010-167383 on Jul. 26, 2010, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. 
     In this specification, a semiconductor device means a general device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and an electronic device are all semiconductor devices. 
     2. Description of the Related Art 
     A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as integrated circuits (ICs) or image display devices (display devices). 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). 
     A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of fluctuation in electric characteristics, although the transistor including an oxide semiconductor can operate at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, the threshold voltage of the transistor fluctuates after a bias-temperature stress test (BT test). Note that in this specification, a threshold voltage refers to a gate voltage which is needed to turn on a transistor. A gate voltage refers to a potential difference between a source electrode and a gate electrode when the potential of the source electrode is used as a reference potential. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
       
    
     SUMMARY OF THE INVENTION 
     Fluctuation in the threshold voltage due to a BT test of the transistor including an oxide semiconductor remarkably reduces the reliability of the transistor including an oxide semiconductor. An object of one embodiment of the present invention is to improve the reliability of a semiconductor device including an oxide semiconductor. 
     One embodiment of the present invention is a semiconductor device or a method for manufacturing the semiconductor device, which is based on a technical idea that an insulating layer from which oxygen is released by heating is used as an insulating layer in contact with a channel region of an oxide semiconductor layer, and an insulating layer which releases a smaller amount of oxygen than the insulating layer in contact with the channel region is used as an insulating layer in contact with a source region and a drain region of the oxide semiconductor layer. 
     One embodiment of the present invention is a semiconductor device or a method for manufacturing the semiconductor device. The semiconductor device includes an insulating layer including a first region and a second region; and an oxide semiconductor layer which is in contact with the first region and the second region and includes a channel region, a source region, and a drain region. The channel region of the oxide semiconductor layer is in contact with the first region. The source region and the drain region of the oxide semiconductor layer are in contact with the second region. The first region is an insulating layer from which oxygen is released by heating. The second region is an insulating layer which releases a smaller amount of oxygen than the first region. 
     To release oxygen by heating means that the released amount of oxygen which is converted to oxygen atoms is greater than or equal to 1×10 18  atoms/cm 3 , preferably greater than or equal to 3×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS). 
     Supply of oxygen to the channel region from the first region that is the insulating layer in contact with the channel region can reduce an interface state density between the channel region and the first region. As a result, it is possible to sufficiently suppress trapping of charge or the like which can be generated due to the operation of a semiconductor device, or the like, at an interface between the first region and the channel region. 
     Further, charge is caused due to oxygen deficiency in the channel region in some cases. In general, part of oxygen deficiency in a channel region serves as a donor and generates electrons that are carriers. As a result, the threshold voltage of the transistor shifts in the negative direction. A sufficient amount of oxygen is released to the channel region from the first region that is the insulating layer in contact with the channel region, whereby oxygen deficiency in the channel region, which causes the shift of the threshold voltage in the negative direction, can be compensated. 
     In other words, when oxygen deficiency is caused in the channel region, it is difficult to suppress charge trapping at the interface between the channel region and the first region that is an insulating layer in contact with the channel region. However, by providing an insulating layer from which oxygen is released by heating as the first region, the interface state density between the channel region and the first region and the oxygen deficiency in the channel region can be reduced, and influence by the charge trapping at the interface between the channel region and the first region can be reduced. 
     In addition, the source region and the drain region are provided in contact with the second region which releases a smaller amount of oxygen than the first region so that oxygen is not supplied to the source region and the drain region. This structure is employed, considering that the part of oxygen deficiency in the oxide semiconductor layer generates electrons that are carriers. In other words, this is based on a technical idea that is to suppress the increase of the resistance of the source region and the drain region which is caused by the reduction of oxygen deficiency by supply of oxygen. For example, an insulating layer which releases oxygen of less than 1×10 18  atoms/cm 3  in TDS analysis can be used as the second region in contact with the source region and the drain region. 
     Thus, the effect of one embodiment of the present invention is attributed to the insulating layer from which oxygen is released by heating and the insulating layer which releases a smaller amount of oxygen than the insulating layer. 
     Owing to an effect of suppressing the charge trapping at the interface of the channel region of the oxide semiconductor layer and suppressing the increase of the resistance of the source region and the drain region, it is possible to suppress a malfunction such as the reduction of an on-state current of the transistor, to which the reduction of current flowing through the source region and the drain region in comparison with the channel region which is caused by the increase of the resistance of the source region and the drain region is attributed. In addition, a malfunction such as the increase of an off-state current of the transistor including the oxide semiconductor and the shift of the threshold voltage can be suppressed. Further, the reliability of the semiconductor device can be improved. 
     Note that the insulating layer from which oxygen is released by heating preferably has an enough thickness with respect to the oxide semiconductor layer. This is because when the thickness of the insulating layer from which oxygen is released by heating is smaller than that of the oxide semiconductor layer, oxygen is not sufficiently supplied to the oxide semiconductor layer in some cases. 
     One embodiment of the present invention is a semiconductor device or a method for manufacturing the semiconductor device. The semiconductor device includes an insulating layer including a first region and a second region; an oxide semiconductor layer which is provided in contact with the first region and the second region and includes a channel region, a source region, and a drain region; a gate insulating layer in contact with the oxide semiconductor layer; and a gate electrode in contact with the gate insulating layer. The channel region of the oxide semiconductor layer is provided in contact with the first region. The source region and the drain region of the oxide semiconductor layer are provided in contact with the second region. The first region is an insulating layer from which oxygen is released by heating. The second region is an insulating layer which releases a smaller amount of oxygen than the first region. Note that the first region may be formed using a material having constituent elements which are the same or different from those of the material of the second region or a material whose two or more constituent elements are the same as those of the material of the second region. 
     In the above structure, the insulating layer from which oxygen is released by heating may be oxygen-excess silicon oxide (SiO X  (X&gt;2)). In the oxygen-excess silicon oxide (SiO X  (X&gt;2)), the number of oxygen atoms per unit volume is more than twice the number of silicon atoms per unit volume. The number of silicon atoms and the number of oxygen atoms per unit volume are measured by Rutherford backscattering spectrometry. 
     In the above structure, as the insulating layer from which oxygen is released by heating, silicon oxide, silicon oxynitride, or aluminum oxide may be used. In addition, as the insulating layer which releases a smaller amount of oxygen than the first region, silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, or aluminum oxynitride may be used. The first region may be formed using a material having different constituent elements from the material of the second region. For example, silicon oxide is used for the insulating layer from which oxygen is released by heating, whereas silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, or aluminum oxynitride may be used for the insulating layer which releases a smaller amount of oxygen than the first region. For example, in the case where silicon oxide is used for the first region, aluminum oxide, whose oxygen diffusion coefficient is less than that of silicon oxide at an arbitrary temperature, is preferably used for the second region. The second region whose oxygen diffusion coefficient is less than that of the first region is provided, whereby the amount of oxygen released from the first region and diffused into the second region can be reduced. 
     In this specification, silicon oxynitride refers to a substance that contains more oxygen than nitrogen. For example, silicon oxynitride includes oxygen, nitrogen, silicon, and hydrogen at concentrations of greater than or equal to 50 at. % and less than or equal to 70 at. %, greater than or equal to 0.5 at. % and less than or equal to 15 at. %, greater than or equal to 25 at. % and less than or equal to 35 at. %, and greater than or equal to 0 at. % and less than or equal to 10 at. %, respectively. In addition, silicon nitride oxide refers to a substance that contains more nitrogen than oxygen. For example, silicon nitride oxide contains oxygen, nitrogen, silicon, and hydrogen at concentrations of greater than or equal to 5 at. % and less than or equal to 30 at. %, greater than or equal to 20 at. % and less than or equal to 55 at. %, greater than or equal to 25 at. % and less than or equal to 35 at. %, and greater than or equal to 10 at. % and less than or equal to 25 at. %, respectively. Note that percentages of the aforementioned elements are obtained in the cases where measurement is performed using Rutherford backscattering spectrometry (RBS) or hydrogen forward scattering (HFS). In addition, the total of the percentages of the constituent elements does not exceed 100 at. %. Aluminum oxynitride refers to a substance that contains more oxygen than nitrogen. 
     In the above structure, a surface of the first region and a surface of the second region are preferably aligned with each other. In other words, the first region preferably has the same thickness as the second region. In addition, at the vicinity of the boundary between the first region and the second region, the surface of the first region and the surface of the second region are preferably formed continuously. 
     Alternatively, it is possible to omit the second region in the above structure. In that case, a first insulating layer may be selectively provided over a substrate, and the first insulating layer may be used as the insulating layer from which oxygen is released by heating. Alternatively, a second insulating layer may be provided over the substrate, the first insulating may be selectively provided over the second insulating layer, and the first insulating layer may be used as the insulating layer from which oxygen is released by heating. 
     That is, one embodiment of the present invention is a semiconductor device or a method for manufacturing the semiconductor device. The semiconductor device includes a first insulating layer selectively provided over a substrate or a second insulating layer provided over the substrate; an oxide semiconductor layer which is provided in contact with the first insulating, and the substrate or the second insulating layer and includes a channel region, a source region, and a drain region; a gate insulating layer provided in contact with the oxide semiconductor layer; and a gate electrode provided in contact with the gate insulating layer. The channel region of the oxide semiconductor layer is provided in contact with the first insulating layer. The source region and the drain region of the oxide semiconductor layer are provided in contact with the substrate or the second insulating layer. The first insulating layer is an insulating layer from which oxygen is released by heating. 
     In the above structure, the insulating layer from which oxygen is released by heating may be oxygen-excess silicon oxide (SiO X  (X&gt;2)). In the oxygen-excess silicon oxide (SiO X  (X&gt;2)), the number of oxygen atoms per unit volume is more than twice the number of silicon atoms per unit volume. The number of silicon atoms and the number of oxygen atoms per unit volume are measured by Rutherford backscattering spectrometry. 
     In the above structure, as the insulating layer from which oxygen is released by heating, silicon oxide, silicon oxynitride, or aluminum oxide may be used. 
     In the above structure, the amount of oxygen released from the substrate or the second insulating layer is preferably smaller than that from the first insulating layer. 
     In the above structure, as the second insulating layer, silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, or aluminum oxynitride may be used. 
     In the above structure, an insulating layer from which oxygen is released by heating is preferably used as the gate insulating layer. Alternatively, silicon oxide in which the number of oxygen atoms is more than twice the number of silicon atoms per unit volume is preferably used as the gate insulating layer. 
     In the above structure, an interlayer insulating layer may be further provided over the gate electrode, and a wiring which is in contact with the oxide semiconductor layer through an opening portion provided in the interlayer insulating layer may be further provided over the interlayer insulating layer. 
     In the above structure, the source region and the drain region are obtained by reducing the resistance of the oxide semiconductor layer. In other words, the source region and the drain region are formed by reducing the resistance of part of the oxide semiconductor layer. At the same time, the channel region is formed in the oxide semiconductor layer. 
     In the above structure, the insulating layer from which oxygen is released by heating is preferably formed by a sputtering method. Alternatively, the insulating layer from which oxygen is released by heating is preferably formed by a sputtering method using oxygen or a mixed gas of oxygen and argon. 
     In the above structure, the oxide semiconductor layer is preferably formed by a sputtering method. 
     In the above structure, after the oxide semiconductor layer is formed, heat treatment is preferably performed at a temperature higher than or equal to 100° C. and lower than or equal to 650° C. 
     In the above structure, the source region and the drain region may be formed by reducing the resistance of part of the oxide semiconductor layer with the use of the gate electrode as a mask. In that case, the channel region is formed in part of the oxide semiconductor layer which is covered with the gate electrode. 
     In the above structure, the channel length L of the transistor can be greater than or equal to 10 nm and less than or equal to 10 μm, for example, 0.1 μm to 0.5 μm. Needless to say, the channel length L may be greater than or equal to 10 μm. The channel width W can be greater than or equal to 10 μm. 
     According to one embodiment of the present invention, an insulating layer from which oxygen is released by heating is provided as an insulating layer in contact with a channel region of an oxide semiconductor layer, and an insulating layer which releases a smaller amount of oxygen than the insulating layer in contact with the channel region is provided as an insulating layer in contact with a source region and a drain region of the oxide semiconductor layer. As a result, a transistor having a small off-state current, small variation in the threshold voltage, a large on-state current, and stable electrical characteristics can be provided. 
     According to one embodiment of the present invention, a semiconductor device including a highly reliable transistor having favorable electric characteristics is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are cross-sectional views each illustrating one embodiment of a semiconductor device; 
         FIGS. 2A to 2D  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device; 
         FIGS. 3A to 3D  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device; 
         FIGS. 4A to 4D  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device; 
         FIGS. 5A and 5B  are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device; 
         FIG. 6  is a cross-sectional view illustrating one embodiment of a semiconductor device; 
         FIGS. 7A to 7C  each illustrate one embodiment of a semiconductor device; 
         FIGS. 8A to 8F  each illustrate an electronic appliance as a semiconductor device; and 
         FIG. 9A to 9C  are a top view and cross-sectional views illustrating one embodiment of a semiconductor device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, 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. Therefore, 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. Note that the same hatch 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, an embodiment of a semiconductor device and a method for manufacturing the semiconductor device will be described with reference to  FIGS. 1A and 1B ,  FIGS. 2A to 2D ,  FIGS. 3A to 3D ,  FIGS. 4A to 4D , and  FIGS. 5A and 5B . 
       FIGS. 1A and 1B  each illustrate a cross-sectional view of a coplanar transistor  155 , which is one mode of a top-gate transistor, as an example of a semiconductor device according to an embodiment of the present invention. 
     The transistor  155  illustrated in  FIG. 1A  includes an insulating layer  103 , an oxide semiconductor layer  106 , a gate insulating layer  112 , and a gate electrode  114 , which are over a substrate  100 . The insulating layer  103  includes a first region  101  and a second region  102 . The oxide semiconductor layer  106  of the transistor  155  includes a channel region  126 , a source region  122   a , and a drain region  122   b . The channel region  126 , the source region  122   a , and the drain region  122   b  are included in the same layer. 
     The oxide semiconductor layer  106  is provided in contact with the first region  101  and the second region  102 . The channel region  126  of the oxide semiconductor layer  106  is provided in contact with the first region  101 . The source region  122   a  and the drain region  122   b  of the oxide semiconductor layer  106  are provided in contact with the second region  102 . The gate insulating layer  112  is provided in contact with the oxide semiconductor layer  106 . The gate electrode  114  is provided in contact with the gate insulating layer  112 . An interlayer insulating layer  124  is provided over the gate electrode  114 . The source region  122   a  and the drain region  122   b  are electrically connected to a wiring  108   a  and a wiring  108   b , respectively, through the interlayer insulating layer  124 . The wiring  108   a  and the wiring  108   b  function as a source electrode and a drain electrode. Note that the width of the gate insulating layer  112  is the same as that of the gate electrode  114  in  FIG. 1A ; however, an embodiment of the present invention is not limited thereto. Instead of the gate insulating layer  112 , a gate insulating layer  113  may be provided over the insulating layer  103  and the oxide semiconductor layer  106  as illustrated in  FIG. 1B . Note that the gate insulating layer  113  may be formed using the same method and the same material as those of the gate insulating layer  112 . The gate insulating layer  113  can be substituted for the gate insulating layer  112  in this specification as appropriate. 
     As the material of the first region  101 , silicon oxide, silicon oxynitride, aluminum oxide, a mixed material of any of them, or the like may be used. Oxygen is released from the first region  101  by heating. To release oxygen by heating means that the released amount of oxygen which is converted to oxygen atoms is greater than or equal to 1×10 18  atoms/cm 3 , preferably greater than or equal to 3×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS). Alternatively, as the material of the first region  101 , oxygen-excess silicon oxide (SiO X  (X&gt;2)) may be used. In the oxygen-excess silicon oxide (SiO X  (X&gt;2)), the number of oxygen atoms per unit volume is more than twice the number of silicon atoms per unit volume. The number of silicon atoms and the number of oxygen atoms per unit volume are measured by Rutherford backscattering spectrometry. 
     As the material of the second region  102 , silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, or aluminum oxynitride may be used. The second region  102  is an insulating layer which releases a smaller amount of oxygen than the first region  101 . Note that the first region  101  may be formed using a material having constituent elements which are the same as or different from those of the material of the second region  102  or a material whose two or more constituent elements are the same as those of the material of the second region  102 . In the case where the first region  101  is formed using the material having the same constituent elements as the material of the second region  102  or the a material whose two or more constituent elements are the same as those of the material of the second region  102 , a material whose number of oxygen atoms per unit volume is smaller than that per unit volume of the first region  101  may be used as the material of the second region  102 . For example, silicon oxide (SiO X  (X&gt;2)) in which the number of oxygen atoms is more than twice the number of silicon atoms per unit volume may be used as the material of the first region  101 , whereas silicon oxide (SiO X  (X≦2)) whose number of oxygen atoms per unit volume is smaller than the number of oxygen atoms per unit volume of the first region  101  may be used as the material of the second region  102 . Alternatively, silicon oxynitride whose number of oxygen atoms per unit volume is smaller than the number of oxygen atoms per unit volume of the first region  101  may be used as the material of the second region  102 . Further alternatively, an organic insulating material which can be formed by wet process, such as an acrylic resin, polyimide, a benzocyclobutene resin, polyamide, or an epoxy resin, may be used as the material of the second region  102 . Other than such organic insulating materials, an inorganic insulating material which can be formed by wet process, such as a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), or borophosphosilicate glass (BPSG), may be used. At a predetermined temperature (e.g., any of temperatures in the range of 100° C. to 650° C.), the oxygen diffusion coefficient of the second region  102  is preferably lower than that of the first region  101 . Thus, the amount of oxygen which is released from the first region  101  and diffused into the second region  102  can be reduced. 
     The insulating layer  103  including the first region  101  and the second region  102  may be formed by stacking the above material and silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, a mixed material thereof, or the like. In the case where the insulating layer  103  is formed with a stacked structure, a layer in contact with the oxide semiconductor layer  106  is preferably formed using the material of the first region  101  and the material of the second region  102 . Note that the insulating layer  103  functions as a base layer of the transistor  155 . 
     As a material used for the oxide semiconductor layer  106 , an In—Sn—Ga—Zn—O-based material, which is a four-component metal oxide; an In—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or a Sn—Al—Zn—O-based material, which are three-component metal oxides; 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, which are two-component metal oxides; an In—O-based material; a Sn—O-based material; a Zn—O-based material; or the like can be used. In addition, any of the above materials may contain silicon oxide. Here, for example, an In—Ga—Zn—O-based material means an oxide layer containing indium (In), gallium (Ga), and zinc (Zn), and there is no particular limitation on the composition ratio. Further, the In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga, and Zn. 
     In the case where an In—Zn—O-based material is used as the oxide semiconductor layer  106 , the atomic ratio is set so that In/Zn is in greater than or equal to 0.5 and less than or equal to 50, preferably greater than or equal to 1 and less than or equal to 20, more preferably greater than or equal to 1.5 and less than or equal to 15. When the atomic ratio of Zn is in the above range, the field effect mobility of the transistor can be improved. Here, when the atomic ratio of the compound is In:Zn:O=X:Y:Z, it is preferable to satisfy the relation of Z&gt;1.5X+Y. 
     Alternatively, the oxide semiconductor layer  106  can be formed using a thin film formed using a material represented by the chemical formula, InMO 3 (ZnO) m  (m&gt;0). Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. 
     The channel region  126  and the first region  101  are in contact with each other, whereby the interface state density between the first region  101  and the channel region  126  and oxygen deficiency in the channel region  126  can be reduced. As a result, charge or the like which can be generated due to the operation of the semiconductor device or the like can be sufficiently prevented from being trapped at the interface between the first region  101  and the channel region  126 . 
     Further, charge is generated due to oxygen deficiency of the channel region  126  in some cases. In general, part of oxygen deficiency in a channel region serves as a donor and generates electrons that are carriers. As a result, the threshold voltage of the transistor shifts in the negative direction. A sufficient amount of oxygen is released to the channel region  126  from the first region  101  that is an insulating layer in contact with the channel region  126 , whereby the oxygen deficiency in the channel region  126 , which causes the shift of the threshold voltage in the negative direction, can be compensated. 
     In other words, when oxygen deficiency is caused in the channel region  126 , it is difficult to suppress charge trapping at the interface between the channel region  126  and the first region  101  that is an insulating layer in contact with the channel region  126 . However, by providing an insulating layer from which oxygen is released by heating as the first region  101 , the interface state density between the channel region  126  and the first region  101  and the oxygen deficiency in the channel region  126  can be reduced, and influence by the charge trapping at the interface between the channel region  126  and the first region  101  can be reduced. 
     In addition, the source region  122   a  and the drain region  122   b  are provided in contact with the second region  102  which releases a smaller amount of oxygen than the first region  101 , so that oxygen is not supplied to the source region  122   a  and the drain region  122   b . This is based on the fact that part of the oxygen deficiency in the oxide semiconductor layer generates electrons that are carriers. In other words, this is based on a technical idea that is to suppress the increase of the resistance of the source region  122   a  and the drain region  122   b  which is caused by the reduction of oxygen deficiency by supply of oxygen. For example, an insulating layer which releases oxygen of less than 1×10 18  atoms/cm 3  in TDS analysis can be used as the second region  102  in contact with the source region  122   a  and the drain region  122   b.    
     Owing to an effect of suppressing the charge trapping at the interface of the channel region  126  of the oxide semiconductor layer and suppressing the increase of the resistance of the source region  122   a  and the drain region  122   b , it is possible to suppress a malfunction such as the reduction of an on-state current of the transistor  155 , to which the reduction of current flowing through the source region  122   a  and the drain region  122   b  which is caused by increasing the resistance of the source region  122   a  and the drain region  122   b  is attributed. In addition, a malfunction such as the increase of an off-state current of the transistor  155  including the oxide semiconductor and the shift of the threshold voltage can be suppressed. Further, the reliability of the semiconductor device can be improved. 
     The gate insulating layer  112  may include a structure (e.g., a material) similar to that of the first region  101 . In other words, the gate insulating layer  112  may be an insulating layer from which oxygen is released by heating. Alternatively, a material having a high dielectric constant, such as hafnium oxide or aluminum oxide, may be used for the gate insulating layer  112  considering the function of the gate insulating layer of the transistor. In addition, considering a gate withstand voltage or the interface state with the oxide semiconductor, a material having a high dielectric constant, such as hafnium oxide or aluminum oxide, may be stacked on silicon oxide, silicon oxynitride, or silicon nitride. 
     The gate electrode  114  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, nitride of any of these metal materials, or an alloy material which contains any of these metal materials as its main component. Note that the gate electrode  114  may have a single-layer structure or a stacked structure. 
     Further, the interlayer insulating layer  124  may be provided over the transistor  155 . The interlayer insulating layer  124  may include a structure (e.g., a material) similar to that of the second region  102 . In addition, an opening portion may be formed in the interlayer insulating layer  124  in order to obtain electrical connection of the wiring  108   a  and the wiring  108   b  with the source region  122   a  and the drain region  122   b.    
     As the conductive layer used for the wiring  108   a  and the wiring  108   b , for example, a metal layer containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride layer containing any of the above elements as its component (e.g., a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) can be used. A high-melting-point metal layer of Ti, Mo, W, or the like or a metal nitride layer of any of these elements (a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) may be stacked on one of or both a bottom side and a top side of a metal layer of Al, Cu, or the like. 
     In the transistor  155 , 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. 
     Examples of a manufacturing process of the transistor  155  in  FIG. 1A  will be described below with reference to  FIGS. 2A to 2D ,  FIGS. 3A to 3D ,  FIGS. 4A to 4D , and  FIGS. 5A and 5B . 
     First, an example of a manufacturing process of the transistor  155  illustrated in  FIG. 1A  will be described with reference to  FIGS. 2A to 2D  and  FIGS. 3A to 3D . 
     A first insulating layer  131  is formed over the substrate  100  (see  FIG. 2A ), and the first insulating layer  131  is processed by a method such as photolithography to form the island-shaped first region  101  (see  FIG. 2B ). A photomask used in the formation of the first region  101  can be the same as a photomask used in the formation of the gate electrode. Oxygen is released from the first region  101  by heating. As the material of the first region  101 , oxygen-excess silicon oxide (SiO X  (X&gt;2)) may be used. 
     There is no particular limitation on the property of a material and the like of the substrate  100  as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used as the substrate  100 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate  100 . Still alternatively, any of these substrates further provided with a semiconductor element may be used as the substrate  100 . 
     A flexible substrate may alternatively be used as the substrate  100 . In the case where a transistor is provided over the flexible substrate, the transistor may be directly formed on the flexible substrate. Alternatively, the transistor may be formed over a different substrate and then separated to be transferred to the flexible substrate that is the substrate  100 . Note that in order to separate the transistor from the substrate and transfer it to the flexible substrate, a separation layer is preferably formed between the different substrate and the transistor. 
     As a formation method of the first insulating layer  131  which is to be the first region  101 , a plasma CVD method or a sputtering method can be employed, for example. The insulating layer from which oxygen is released by heating is preferably formed by a sputtering method. 
     In order to form the insulating layer from which oxygen is released by heating by a sputtering method, in the case where oxygen or a mixed gas of oxygen and a rare gas (e.g., argon) is used as a deposition gas, the proportion of oxygen is preferably set higher. For example, the concentration of oxygen in the whole gas is preferably set to be higher than or equal to 6% and lower than 100%. 
     As the material of the first insulating layer  131  which is to be the first region  101 , silicon oxide, silicon oxynitride, aluminum oxide, a mixed material of any of them, or the like may be used. 
     For example, as the first insulating layer  131 , silicon oxide is formed by an RF sputtering method in the following conditions: quartz (preferably synthesized quartz) is used as a target; a substrate temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); oxygen or oxygen and argon are used as a deposition gas; and a proportion of O 2 /(O 2 +Ar) in the deposition gas is greater than or equal to 1% and less than or equal to 100% (preferably greater than or equal to 6% and less than or equal to 100%). 
     The thickness of each of the first insulating layer  131  and the first region  101  is preferably greater than or equal to 50 nm and more preferably greater than or equal to 200 nm. The first insulating layer  131  and the first region  101  have large thicknesses, so that the amount of oxygen released from the first region  101  can be increased. 
     Next, a second insulating layer  132  is formed over the substrate  100  and the first region  101  (see  FIG. 2C ). After that, the second insulating layer  132  is processed until a surface of the first region  101  is exposed, so that the insulating layer  103  in which the second region  102  is in contact with the first region  101  is formed (see  FIG. 2D ). The second region  102  is an insulating layer which releases a smaller amount of oxygen than the first region  101 . Note that in processing of the second insulating layer  132 , a surface of the first region  101  may be processed at the same time so that part of the first region  101  is removed. 
     As a formation method for the second insulating layer  132 , for example, a plasma CVD method, a sputtering method, or the like can be used. 
     As the material of the second insulating layer  132 , silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, or aluminum oxynitride may be used. 
     For example, a silicon nitride film is formed by a plasma CVD method as the second insulating layer  132 . Alternatively, as the second insulating layer  132 , silicon oxide may be formed by a plasma CVD method. 
     After the above process, the surface of the first region  101  and the surface of the second region  102  are preferably aligned with each other. For example, the second insulating layer  132  is subjected to polishing treatment such as chemical mechanical polishing (CMP) or etching treatment until the surface of the first region  101  is exposed, so that the insulating layer  103 , in which the second region  102  is in contact with the first region  101  and the surface of the first region  101  and the surface of the second region  102  are aligned with each other, can be formed. The alignment of the surface of the first region  101  and the surface of the second region  102  can lead to prevention of disconnection of the oxide semiconductor layer to be formed thereover. This effect is enhanced when the oxide semiconductor layer is thin. Prevention of the disconnection of the oxide semiconductor layer can lead to prevention of disconnection of the source region and the drain region, thereby suppressing reduction of the on-state current. In addition, disconnection of the gate insulating layer to be formed over the oxide semiconductor layer can be prevented. Prevention of the disconnection of the gate insulating layer can suppress the increase of leakage current and the reduction of a dielectric strength voltage. 
     Note that the thickness of the second region  102 , that is, the thickness of the insulating layer  103  is the same as that of the first insulating layer  131  and that of the first region  101 . Specifically, the thickness of the second region  102 , that is, the thickness of the insulating layer  103  is preferably greater than or equal to 50 nm, more preferably greater than or equal to 200 nm. Note that the thickness of the first insulating layer  131  subjected to the polishing treatment or etching treatment may be smaller than that of the just-formed first insulating layer  131 . 
     Alternatively, the insulating layer  103  including the first region  101  and the second region  102  may be formed by stacking the above material and silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, a mixed material thereof, or the like. In the case where the insulating layer  103  is formed with a stacked structure, a layer in contact with the oxide semiconductor layer  106  is preferably formed using the material of the first region  101  and the material of the second region  102 . Note that the insulating layer  103  functions as a base layer of the transistor  155 . 
     Note that the first region  101  is formed and then the second region  102  is formed here; however, the formation order of the first region  101  and the second region  102  may be inverted, that is, the second region  102  may be formed and then the first region  101  may be formed. In that case, after the second region  102  is selectively formed, the first insulating layer  131  is formed over the entire surface, and the first insulating layer  131  is subjected to polishing treatment or etching treatment until the surface of the second region  102  is exposed, so that the insulating layer  103  in which the second region  102  is in contact with the first region  101  can be formed. 
     Next, an oxide semiconductor layer is formed over the insulating layer  103  and processed to form the island-shaped oxide semiconductor layer  106  (see  FIG. 3A ). The oxide semiconductor layer  106  is formed to be in contact with the first region  101  and the second region  102 . 
     For example, the oxide semiconductor layer  106  can be formed by a sputtering method, a vacuum evaporation method, a pulse laser deposition method, a CVD method, or the like. The thickness of the oxide semiconductor layer  106  is preferably greater than or equal to 3 nm and less than or equal to 50 nm. This is because when the oxide semiconductor layer  106  is too thick (e.g., 100 nm or more), there is a possibility that the short channel effect has a large influence and the transistor with a small size is normally on. Here, “normally on” means a state where a channel exists without application of a voltage to a gate electrode and a current flows through the transistor. 
     In this embodiment, the oxide semiconductor layer  106  is formed by a sputtering method with the use of an In—Ga—Zn—O-based oxide target. 
     As the In—Ga—Zn—O-based oxide target, for example, an oxide target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [molar ratio] can be used. Note that it is not necessary to limit the material and the composition ratio of the target to the above. For example, an oxide target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2 [molar ratio] can be used. 
     The relative density of the oxide target is higher than or equal to 90% and lower than or equal to 100%, preferably higher than or equal to 95% to lower than or equal 100%. This is because, with the use of the metal oxide target with a high relative density, the dense oxide semiconductor layer can be formed. 
     The deposition atmosphere may be a rare gas (typically argon) atmosphere, an oxygen atmosphere, a mixed atmosphere containing a rare gas and oxygen, or the like. Moreover, it is preferably performed under an atmosphere using a high-purity gas in which impurities such as hydrogen, water, a hydroxyl group, and hydride are sufficiently removed so that entry of hydrogen, water, a hydroxyl group, and hydride into the oxide semiconductor layer can be prevented. 
     For example, the oxide semiconductor layer  106  can be formed as follows. 
     An example of the deposition conditions is as follows: the distance between the substrate and the target is 60 mm; the pressure is 0.4 Pa; the direct-current (DC) power is 0.5 kW; and the deposition atmosphere is a mixed atmosphere containing argon and oxygen (the flow rate of the oxygen is 33%). Note that a pulse DC sputtering method is preferable because powder substances (also referred to as particles or dust) generated in deposition can be reduced and the film thickness can be uniform. 
     At this time, the substrate temperature is set to higher than or equal to 100° C. and lower than or equal to 450° C., preferably higher than or equal to 150° C. and lower than or equal to 250° C., whereby oxygen is released from the first region  101 , so that oxygen deficiency of a portion of the oxide semiconductor layer  106  in contact with the first region  101  (a portion to be the channel region  126 ) can be reduced, and the interface state density between the oxide semiconductor layer  106  and the first region  101  can be reduced. 
     When portions of the oxide semiconductor layer  106  which are not in contact with the first region  101  (portions to be the source region  122   a  and the drain region  122   b ) are in contact with the second region  102  which releases a smaller amount of oxygen than the first region  101 , the increase of the resistance of the portions of the oxide semiconductor layer  106  can be suppressed. 
     Note that before the oxide semiconductor layer  106  is formed by a sputtering method, a substance attached to a surface where the oxide semiconductor layer  106  is to be formed (e.g., a surface of the insulating layer  103 ) may be removed by reverse sputtering in which an argon gas is introduced and plasma is generated. Here, the reverse sputtering is a method by which ions collide with a surface to be processed so that the surface is modified, in contrast to normal sputtering by which ions collide with a sputtering target. An example of a method for making ions collide with a surface to be processed is a method in which high-frequency voltage is applied to the surface side in a rare atmosphere so that plasma is generated near the object to be processed. Note that, instead of a rare gas atmosphere, an atmosphere of nitrogen, oxygen, or the like may be used. 
     The oxide semiconductor layer  106  can be processed by etching after a mask having a desired shape is formed over the oxide semiconductor layer. The mask may be formed by a method such as photolithography or an ink-jet method. Alternatively, the mask may be formed by an ink-jet method or the like. 
     For the etching of the oxide semiconductor layer, either wet etching or dry etching may be employed. Needless to say, both of them may be employed in combination. 
     After that, heat treatment (first heat treatment) is preferably performed on the oxide semiconductor layer  106 . By the first heat treatment, excessive hydrogen (including water and a hydroxyl group) in the oxide semiconductor layer  106  can be removed. The temperature of the first heat treatment is higher than or equal to 100° C. and lower than or equal to 650° C. or lower than the strain point of the substrate, preferably higher than or equal to 250° C. and lower than or equal to 600° C. The atmosphere of the first heat treatment is an oxidation gas atmosphere or an inert gas atmosphere. 
     Note that the inert gas contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and preferably does not contain water, hydrogen, or the like. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is set to 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). The inert gas atmosphere is an atmosphere that contains an inert gas as its main component and contains a reactive gas at a concentration less than 10 ppm. 
     Note that the oxidation gas is oxygen, ozone, nitrogen dioxide, or the like, and it is preferable that the oxidation gas does not contain water, hydrogen, and the like. For example, the purity of oxygen, ozone, or nitrogen dioxide introduced into a heat treatment apparatus is set to 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). As the oxidation gas atmosphere, an atmosphere in which an oxidation gas is mixed with an inert gas may be used, and the oxidation gas of at least 10 ppm is contained. 
     By the first heat treatment, oxygen is released from the first region  101 , so that the interface state density between the first region  101  and the portion of the oxide semiconductor layer  106  which is in contact with the first region  101  (the portion to be the channel region  126 ) can be reduced, and the oxygen deficiency in the portion of the oxide semiconductor layer  106  which is in contact with the first region  101  can be reduced. By the above reduction of the interface state density, the fluctuation in the threshold voltage after a BT test can be reduced. Further, in general, it is known that part of the oxygen deficiency in an oxide semiconductor layer serves as a donor and generates electrons that are carriers. By the generation of electrons in the oxide semiconductor layer  106 , the threshold voltage of the transistor  155  shifts in the negative direction, so that the transistor  155  tends to be normally on. The oxygen deficiency in the oxide semiconductor layer  106  is compensated, whereby the amount of the shift of the threshold voltage in the negative direction can be reduced. 
     In addition, the portions of the oxide semiconductor layer  106  which are not in contact with the first region  101  (the portions to be the source region  122   a  and the drain region  122   b ) are in contact with the second region  102  which releases a smaller amount of oxygen than the first region  101 , whereby the increase of the resistance of the portions of the oxide semiconductor layer  106  can be suppressed. 
     The heat treatment can be performed in such a manner that, for example, an object is introduced into an electric furnace in which a resistance heating element or the like is used and heated at 350° C. in a nitrogen atmosphere for an hour. During the heat treatment, the oxide semiconductor layer is not exposed to the air to prevent the entry of water and hydrogen. 
     Note that the heat treatment apparatus is not limited to an electric furnace, and may include a device for heating an object by heat conduction or heat radiation from a medium such as a heated gas. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the high temperature gas, used is an inert gas which does not react with an object to be processed in heat treatment, for example, nitrogen or a rare gas such as argon. 
     For example, as the first heat treatment, a GRTA process may be performed as follows. The object is put in an inert gas atmosphere that has been heated, heated for several minutes, and taken out from the inert gas atmosphere. The GRTA process enables high-temperature heat treatment for a short time. Moreover, the GRTA process can be employed even when the temperature exceeds the upper temperature limit of the object. Note that the inert gas atmosphere may be switched to an atmosphere containing an oxidation gas during the treatment. This is because by performing the first heat treatment in an atmosphere containing the oxidation gas, oxygen deficiency in the oxide semiconductor layer  106  can be compensated and defect levels in an energy gap due to the oxygen deficiency can be reduced. 
     The above heat treatment (first heat treatment) can be referred to as dehydration treatment, dehydrogenation treatment, or the like because of its advantageous effect of removing hydrogen, water, and the like. In addition, the above heat treatment can also be referred to as treatment for supplying oxygen because of its advantageous effect of supplying oxygen from the insulating layer, a heat treatment atmosphere, or the like. The dehydration treatment, dehydrogenation treatment, or treatment for supplying oxygen may be performed at the timing, for example, after the oxide semiconductor layer is processed to have an island shape. Such dehydration treatment, dehydrogenation treatment, or treatment for supplying oxygen may be performed once or plural times. 
     Note that the case is described here where the first heat treatment is performed after the oxide semiconductor layer  106  is processed to have an island shape; however, one embodiment of the present invention is not limited thereto. The oxide semiconductor layer  106  may be processed after the first heat treatment. 
     Next, an insulating layer is formed to be in contact with the oxide semiconductor layer  106 , and a conductive layer is formed to be in contact with the insulating layer. The insulating layer and the conductive layer are processed to the same pattern by photolithography to form the gate insulating layer  112  and the gate electrode  114  (see  FIG. 3B ). In other words, the gate electrode  114  and the gate insulating layer  112  can be formed using the same mask. Alternatively, the gate electrode  114  may be formed and then, the gate insulating layer  112  may be formed using the gate electrode  114  as a mask. 
     The gate insulating layer  112  may includes a structure (e.g., a material) similar to that of the first region  101 . Alternatively, a material having a high dielectric constant, such as hafnium oxide or aluminum oxide, may be used for the gate insulating layer  112  considering the function of the gate insulating layer of the transistor. In addition, considering a gate withstand voltage or the interface state with the oxide semiconductor, a material having a high dielectric constant, such as hafnium oxide or aluminum oxide, may be stacked on silicon oxide, silicon oxynitride, or silicon nitride. The total thickness of the gate insulating layer  112  is preferably greater than or equal to 1 nm and less than or equal to 300 nm, more preferably greater than or equal to 5 nm and less than or equal to 50 nm. As the thickness of the gate insulating layer is larger, a short channel effect is enhanced more and the threshold voltage tends to shift more in the negative side. In addition, it is found that when the thickness of the gate insulating layer is less than or equal to 5 nm, leakage due to a tunnel current is increased. 
     Second heat treatment is preferably performed after the gate insulating layer  112  is formed. The temperature of the second heat treatment is higher than or equal to 100° C. and lower than or equal to 650° C. or lower than the strain point of the substrate, preferably higher than or equal to 250° 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 oxidation gas or an inert gas. It is preferable that the atmosphere of an oxidation gas or an inert gas do not contain water, hydrogen, and the like. Further, the purity of the gas introduced into a heat treatment apparatus is preferably set to 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). 
     The second heat treatment is performed while the oxide semiconductor layer  106  is in contact with the first region  101  and the gate insulating layer  112 . Thus, oxygen, which is one of main components of the oxide semiconductor, can be supplied from the first region  101  and the gate insulating layer  112  which contain oxygen, to the oxide semiconductor layer  106 . Accordingly, oxygen deficiency in the oxide semiconductor layer  106 , the interface state density between the first region  101  and the oxide semiconductor layer  106 , and the interface state density between the oxide semiconductor layer 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. The second heat treatment may be performed plural times. 
     The gate electrode  114  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, nitride of any of these metal materials, or an alloy material which contains any of these metal materials as its main component. Note that the gate electrode  114  may have a single-layer structure or a stacked structure. 
     Next, the resistance of the oxide semiconductor layer  106  is reduced using the gate electrode  114  as a mask, so that the source region  122   a  and the drain region  122   b  are formed. A region under the gate electrode  114  where the resistance is not reduced becomes the channel region  126  (see  FIG. 3C ). As a method for reducing the resistance, argon plasma treatment, hydrogen plasma treatment, ammonia plasma treatment, and the like can be given. At this time, the channel length L of the transistor is determined by the width of the gate electrode  114 . By patterning using the gate electrode  114  as the mask in this manner, the source region  122   a  and the drain region  122   b  do not overlap with the gate electrode  114  and parasitic capacitance in the region is not generated; therefore, the operation speed of the transistor can be increased. 
     Next, the interlayer insulating layer  124  is formed and an opening portion is provided in a region of the interlayer insulating layer  124  which overlaps with the source region  122   a  and the drain region  122   b . Then, a conductive layer is formed and processed to form the wiring  108   a  and the wiring  108   b  (see  FIG. 3D ). 
     As the conductive layer used for the wiring  108   a  and the wiring  108   b , for example, a metal layer containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride layer containing any of the above elements as its component (e.g., a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) can be used. A high-melting-point metal layer of Ti, Mo, W, or the like or a metal nitride layer of any of these elements (a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) may be stacked on one of or both a bottom side and a top side of a metal layer of Al, Cu, or the like. 
     Alternatively, the conductive layer used for the wiring  108   a  and the wiring  108   b  may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In 2 O 3  or the like), tin oxide (SnO 2  or the like), zinc oxide (ZnO or the like), indium oxide-tin oxide alloy (In 2 O 3 —SnO 2  or the like, which is abbreviated to ITO), indium oxide-zinc oxide alloy (In 2 O 3 —ZnO or the like), or any of these metal oxide materials in which silicon oxide is contained can be used. 
     The conductive layer can be processed by etching with the use of a resist mask. Ultraviolet, a KrF laser light, an ArF laser light, or the like is preferably used for light exposure for forming a resist mask for the etching. 
     Through the above process, the transistor  155  is formed. 
     Next, an example of a manufacturing process of the insulating layer  103  is described with reference to  FIGS. 4A to 4D . First, through the same process as  FIGS. 2A to 2C , the first region  101  is formed over the substrate  100 , and the second insulating layer  132  is formed over the substrate  100  and the first region  101  (see  FIG. 4A ). Then, a third insulating layer  133  is formed over the second insulating layer  132  (see  FIG. 4B ). As the third insulating layer  133 , a planarization insulating layer can be used. For example, as a material of the third insulating layer  133 , an organic insulating material which can be formed by wet process, such as an acrylic resin, polyimide, a benzocyclobutene resin, polyamide, or an epoxy resin can be used. Other than such organic insulating materials, it is possible to use an inorganic insulating material which can be formed by wet process, such as a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), or borophosphosilicate glass (BPSG). 
     The third insulating layer  133  can be formed, depending on the material, by a spin coating method, a dipping method, spray coating, a droplet discharge method (e.g., an inkjet method, screen printing, or offset printing), roll coating, curtain coating, knife coating, or the like. 
     Then, the third insulating layer  133  and the second insulating layer  132  are subjected to etching treatment. An etchant whose etching selection ratio of the third insulating layer  133  and the second insulating layer  132  is 1:1 or close to 1:1 is used for the etching treatment. Accordingly, an etching rate of the third insulating layer  133  can be substantially the same as an etching rate of the second insulating layer  132  (see  FIG. 4C ). Note that the third insulating layer  133  and the second insulating layer  132  may be etched by either dry etching or wet etching. 
     Then, the third insulating layer  133  and the second insulating layer  132  are subjected to the etching treatment until the surface of the first region  101  is exposed, so that the insulating layer  103 , in which the second region  102  is in contact with the first region  101 , and the surface of the first region  101  and the surface of the second region  102  are aligned with each other can be formed (see  FIG. 4D ). The alignment of the surface of the first region  101  and the surface of the second region  102  can lead to prevention of disconnection of the oxide semiconductor layer to be formed thereover. This effect is enhanced when the oxide semiconductor layer  106  is thin. Prevention of the disconnection of the oxide semiconductor layer  106  can prevent disconnection of the source region  122   a  and the drain region  122   b  and can suppress the reduction of the on-state current. In addition, disconnection of the gate insulating layer  112  to be formed over the oxide semiconductor layer  106  can be prevented. Prevention of the disconnection of the gate insulating layer  112  can suppress the increase of leakage current and the reduction of a dielectric strength voltage. 
     Note that the first region  101  is formed and then the second region  102  is formed here; however, the formation order of the first region  101  and the second region  102  may be inverted, that is, the second region  102  may be formed and then the first region  101  may be formed. In that case, after the second region  102  is selectively formed, the first insulating layer  131  is formed over the entire surface, and the third insulating layer  133  is formed over the first insulating layer  131 . Then, the third insulating layer  133  and the first insulating layer  131  are subjected to polishing treatment or etching treatment until the surface of the second region  102  is exposed, so that the insulating layer  103  in which the second region  102  is in contact with the first region  101  and the surface of the first region  101  and the surface of the second region  102  are aligned with each other can be formed. Also in that case, the third insulating layer  133  and the first insulating layer  131  may be etched by either dry etching or wet etching. 
     Here, the second insulating layer  132  and the third insulating layer  133  are formed; however, the second insulating layer  132  may be formed with the use of the same material and the same method as those of the third insulating layer  133 , so that the second insulating layer  132  having a even surface may be formed. In other words, as illustrated in  FIG. 5A , the second insulating layer  132  having the even surface may be formed over the substrate  100  and the first region  101  with the use of the same material and the same method as those of the third insulating layer  133 . The second insulating layer  132  having the even surface is subjected to etching treatment until the surface of the first region  101  is exposed, so that the second region  102  can be formed as illustrated in  FIG. 5B . As a result, the insulating layer  103  in which the surface of the first region  101  and the surface of the second region  102  are aligned with each other can be formed. Also in that case, the material used as the second region  102  is an insulating layer which releases a smaller amount of oxygen than the first region  101 . Through the manufacturing method illustrated in  FIGS. 5A and 5B , the number of depositions for forming the insulating layer  103  is smaller than that of the manufacturing method illustrated in  FIGS. 4A to 4D  and processing becomes easier. 
     The subsequent steps can be the same as those of  FIG. 3A to 3D . 
     According to this embodiment, the first region  101  from which oxygen is released by heating is provided as an insulating layer in contact with the channel region  126  of the oxide semiconductor layer  106 , and the second region  102  which releases a smaller amount of oxygen than the first region  101  is provided as the insulating layer in contact with the source region  122   a  and the drain region  122   b  of the oxide semiconductor layer  106 . As a result, a transistor having a small off-state current, small variation in the threshold voltage, a large on-state current, and stable electrical characteristics can be provided. 
     According to this embodiment, a semiconductor device including a highly reliable transistor having favorable electric characteristics is provided. 
     The methods and structures described in this embodiment can be combined as appropriate with any of the methods and structures described in the other embodiments. 
     Embodiment 2 
     In this embodiment, an embodiment of a semiconductor device will be described with reference to  FIG. 6 .  FIG. 6  illustrates a cross section of a transistor  156  having a structure which is different from that of the transistor  155  illustrated in  FIG. 1A . The transistor  156  illustrated in  FIG. 6  is different from the transistor  155  illustrated in  FIG. 1A  in that the second region  102  is not provided. 
     The transistor  156  in  FIG. 6  includes a first insulating layer  104 , an oxide semiconductor layer  106 , a gate insulating layer  112 , and a gate electrode  114 , which are over a substrate  100 . The transistor  156  includes, in the oxide semiconductor layer  106 , a channel region  126 , a source region  122   a , and a drain region  122   b . The channel region  126 , the source region  122   a , and the drain region  122   b  are provided in the same layer. 
     A second insulating layer  105  may be provided below the transistor  156 . The second insulating layer  105  functions as a base layer of the transistor  156 . 
     The first insulating layer  104  is selectively provided over the substrate  100  or the second insulating layer  105  provided over the substrate  100 . The oxide semiconductor layer  106  is provided over the first insulating layer  104 . The oxide semiconductor layer  106  is provided in contact with the first insulating layer  104  and the substrate  100  or the second insulating layer  105  provided over the substrate  100 . The channel region  126  of the oxide semiconductor layer  106  is provided in contact with the first insulating layer  104 . The source region  122   a  and the drain region  122   b  of the oxide semiconductor layer  106  are provided in contact with the substrate  100  or the second insulating layer  105  provided over the substrate  100 . 
     The gate insulating layer  112  is provided in contact with the oxide semiconductor layer  106 . The gate electrode  114  is provided in contact with the gate insulating layer  112 . An interlayer insulating layer  124  is provided over the gate electrode  114 . A wiring  108   a  and a wiring  108   b  are electrically connected to the source region  122   a  and the drain region  122   b , respectively, through the interlayer insulating layer  124 . The wiring  108   a  and the wiring  108   b  function as a source electrode and a drain electrode. 
     The material of the first insulating layer  104  can have the same structure as the material of the first region  101  described in Embodiment 1. In other words, as the material of the first insulating layer  104 , silicon oxide, silicon oxynitride, aluminum oxide, a mixed material of any of them, or the like may be used. Oxygen is released from the first insulating layer  104  by heating. To release oxygen by heating means that the released amount of oxygen which is converted to oxygen atoms is greater than or equal to 1×10 18  atoms/cm 3 , preferably greater than or equal to 3×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS). Alternatively, as the material of the first insulating layer  104 , oxygen-excess silicon oxide (SiO X  (X&gt;2)) may be used. In the oxygen-excess silicon oxide (SiO X  (X&gt;2)), the number of oxygen atoms per unit volume is more than twice the number of silicon atoms per unit volume. The number of silicon atoms and the number of oxygen atoms per unit volume are measured by Rutherford backscattering spectrometry. 
     In the case where the second insulating layer  105  is provided, the material of the second insulating layer  105  can have the same structure as the material of the second region  102  described in Embodiment 1. In other words, as the material of the second insulating layer  105 , silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, aluminum oxynitride, or the like may be used. The second insulating layer  105  is an insulating layer which releases a smaller amount of oxygen than the first insulating layer  104 . 
     In addition, the first insulating layer  104  and/or the second insulating layer  105  may be formed by stacking the above material and silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, a mixed material thereof, or the like. In the case where the first insulating layer  104  and/or the second insulating layer  105  are/is formed with a stacked structure, a layer in contact with the oxide semiconductor layer  106  may be formed using the material of the first insulating layer  104  or the material of the second insulating layer  105 . 
     The channel region  126  and the first insulating layer  104  are in contact with each other, whereby the interface state density between the first insulating layer  104  and the channel region  126  and oxygen deficiency in the channel region  126  can be reduced. Owing to the reduction of the interface state density, the shift of the threshold voltage in the negative direction after a BT test can be suppressed. Alternatively, generation of carriers can be suppressed, so that normally-off characteristics can be obtained. 
     In addition, the source region  122   a  and the drain region  122   b  are in contact with the substrate  100  or the second insulating layer  105 , whereby the increase of the resistance of the source region  122   a  and the drain region  122   b  can be prevented, so that a semiconductor device including the highly reliable transistor  156  with good electrical characteristics can be provided. 
     There is no particular limitation on the property of a material and the like of the substrate  100  as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used as the substrate  100 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate  100 . Still alternatively, any of these substrates further provided with a semiconductor element may be used as the substrate  100 . 
     A flexible substrate may alternatively be used as the substrate  100 . In the case where a transistor is provided over the flexible substrate, the transistor may be directly formed on the flexible substrate. Alternatively, the transistor may be formed over a different substrate and then separated to be transferred to the flexible substrate that is the substrate  100 . Note that in order to separate the transistor from the substrate and transfer it to the flexible substrate, a separation layer is preferably formed between the different substrate and the transistor. 
     Note that in the case where the second insulating layer  105  is not provided, a substrate formed of a material which releases a smaller amount of oxygen than the first insulating layer  104  is preferably used as the substrate  100 . For example, in the case where the second insulating layer  105  is not provided, a glass substrate, a ceramics substrate, a quartz substrate, a sapphire substrate, an SOI substrate, or the like is preferably used as the substrate  100 . 
     A manufacturing process of the transistor  156  is described. The second insulating layer  105  is formed over the entire surface of the substrate  100  and the first insulating layer  104  is selectively formed over the second insulating layer  105 . Oxygen is released from the first insulating layer  104  by heating. As the material of the first insulating layer  104 , oxygen-excess silicon oxide (SiO X  (X&gt;2)) may be used. In order to improve a coverage with the oxide semiconductor layer  106  to be formed later, the edge portion of the first insulating layer  104  is preferably formed to have a slope. In addition, a photomask used in the formation of the first insulating layer  104  can be the same as the photomask used in the formation of the gate electrode  114 . 
     The subsequent steps can be the same as those described in Embodiment 1. 
     Since a step for aligning the surfaces of the insulating layers can be omitted in this embodiment, the transistor  156  can be provided by a simple method with low cost and high throughput. 
     According to this embodiment, the first insulating layer  104  from which oxygen is released by heating is provided as an insulating layer in contact with the channel region  126  of the oxide semiconductor layer  106 , and the substrate  100  or the second insulating layer  105  which releases a smaller amount of oxygen than the first insulating layer  104  is provided as a substrate or an insulating layer in contact with the source region  122   a  and the drain region  122   b  of the oxide semiconductor layer  106 . As a result, a transistor having a small off-state current, small variation in the threshold voltage, a large on-state current, and stable electrical characteristics can be provided. 
     According to this embodiment, a highly reliable semiconductor device having good electric characteristics is provided. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 3 
     In this embodiment, an embodiment of a semiconductor device will be described with reference to  FIGS. 9A to 9C .  FIG. 9A  is a top view of a transistor.  FIG. 9B  illustrates a cross-sectional structure corresponding to an alternate long and short dashed line A-B in  FIG. 9A . 
     A transistor illustrated in  FIG. 9B  includes an insulating layer  103 , an oxide semiconductor layer  136 , a gate insulating layer  112 , a gate electrode  114 , a sidewall insulating layer  130 , a source electrode  116   a , and a drain electrode  116   b  over a substrate  100 . The insulating layer  103  includes a first region  101  and a second region  102 . The transistor illustrated in  FIG. 9B  includes a channel region  126 , a source region  122   a, a  drain region  122   b , an offset region  123   a , and an offset region  123   b  in the oxide semiconductor layer  136 . The channel region  126 , the source region  122   a , the drain region  122   b , the offset region  123   a , and the offset region  123   b  are provided in the same layer. 
     The offset region  123   a  and the offset region  123   b  have lower resistance than the channel region  126  and have higher resistance than the source region  122   a  and the drain region  122   b . The width of the offset region  123   a  or the offset region  123   b  is also referred to as Loff, which is illustrated in  FIG. 9A . With the Loff, a short-channel effect of the transistor is reduced. Therefore, in the case of using a minute transistor in which a short-channel effect is notably enhanced, the structure illustrated in  FIG. 9B  (also referred to as a Loff structure) is favorable. In addition, by employing the Loff structure, degradation of the transistor, such as hot-carrier degradation, can be reduced. 
     The oxide semiconductor layer  136  is provided in contact with the first region  101  and the second region  102 . The channel region  126  of the oxide semiconductor layer  136  is provided in contact with the first region  101 . The source region  122   a , the drain region  122   b , the offset region  123   a , and the offset region  123   b  are provided in contact with the second region  102 . The offset region  123   a  and the offset region  123   b  are closer to the channel region  126  than the source region  122   a  and the drain region  122   b  are. 
     The gate insulating layer  112  is provided in contact with the channel region  126 , the offset region  123   a , and the offset region  123   b . The sidewall insulating layer  130  is provided in the periphery of the gate electrode  114 . The gate electrode  114  and the sidewall insulating layer  130  are provided in contact with the gate insulating layer  112 . An interlayer insulating layer  124  is provided over the gate electrode  114  and the sidewall insulating layer  130 . The source electrode  116   a  and the drain electrode  116   b  are provided in contact with the source region  122   a  and the drain region  122   b , respectively. A wiring  108   a  and a wiring  108   b  are electrically connected to the source electrode  116   a  and the drain electrode  116   b  through the interlayer insulating layer  124 . 
     As a conductive layer used for the source electrode  116   a  and the drain electrode  116   b , for example, a metal layer containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride layer containing any of the above elements as its component (e.g., a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) can be used. A high-melting-point metal layer of Ti, Mo, W, or the like or a metal nitride layer of any of these elements (a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) may be stacked on one of or both a bottom side and a top side of a metal layer of Al, Cu, or the like. 
     In addition, the offset region  123   a  and the offset region  123   b  are provided in contact with the second region  102  which releases a smaller amount of oxygen than the first region  101 , so that oxygen is not supplied to the offset region  123   a  and the offset region  123   b.    
     The offset region  123   a  and the offset region  123   b  are not regions whose resistance is particularly reduced, and are distinguished from the channel region  126  depending on which region of the insulating layer  103  is in contact with the offset region  123   a  and the offset region  123   b . That is, the offset region  123   a  and the offset region  123   b  are regions of the oxide semiconductor layer  136  which are not in contact with the insulating layer from which oxygen is released by heating. 
     The transistor described in this embodiment includes the offset regions, whereby the highly reliable transistor having better electrical characteristics can be provided. 
     Note that the offset regions are not necessarily provided. For example, a transistor illustrated in  FIG. 9C  has a different structure from the transistor of  FIG. 9B  in that offset regions are not provided. 
     According to this embodiment, a semiconductor device including a highly reliable transistor having good electrical characteristics can be provided. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 4 
     A semiconductor device having a display function (also referred to as a display device) can be manufactured using the transistor described in Embodiment 1, Embodiment 2, or Embodiment 3. Moreover, some or all of the driver circuits which include the transistor can be formed over a substrate where the pixel portion is formed, whereby a system-on-panel can be obtained. 
     In  FIG. 7A , a sealant  205  is provided so as 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. 7A , a scan line driver circuit  204  and a signal line driver circuit  203  each are formed using a single crystal semiconductor layer or a polycrystalline semiconductor layer over a substrate prepared separately, and 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 , each of which is separately formed, and the pixel portion  202 , from flexible printed circuits (FPCs)  218   a  and  218   b.    
     In  FIGS. 7B and 7C , 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. 7B and 7C , the signal line driver circuit  203  is formed using a single crystal semiconductor layer or a polycrystalline semiconductor layer over a substrate prepared separately, and mounted in a region different from the region surrounded by the sealant  205  over the first substrate  201 . In  FIGS. 7B and 7C , various signals and potentials are supplied from a FPC  218  to the signal line driver circuit  203  which is separately formed, the scan line driver circuit  204 , and the pixel portion  202 . 
     Although  FIGS. 7B and 7C  each show the example in which the signal line driver circuit  203  is formed separately and mounted on the first substrate  201 , one embodiment of 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. 7A  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. 7B  shows an example in which the signal line driver circuit  203  is mounted by a COG method.  FIG. 7C  shows an example in which the signal line driver circuit  203  is mounted by a TAB method. 
     The display device includes in its category a panel in which a display element is sealed and a module in which an IC such as a controller is mounted on the panel. 
     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 a connector such as an FPC, a TAB tape, or TCP; a module in which a printed wiring board is provided in the end of a TAB tape or TCP; and a module in which an integrated circuit (IC) is mounted directly on a display element by a COG method. 
     Further, the pixel portion and the scan line driver circuit which are provided over the first substrate  201  include a plurality of transistors, to which the transistor whose example is described in Embodiment 1, Embodiment 2, or Embodiment 3 can be applied. 
     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. 
     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 appears only in a narrow temperature range, a liquid crystal composition in which a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes a liquid crystal exhibiting a blue phase and a chiral agent has a short response time of 1 msec or less, has optical isotropy, which makes the alignment process unneeded, and has a small viewing angle dependence. 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. 
     The specific resistivity of the liquid crystal material is 1×10 9  Ω·cm or more, preferably 1×10 11  Ω·cm or more, more preferably 1×10 12  Ω·cm or more. 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 including an oxide semiconductor layer used in this embodiment, 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. Accordingly, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption. 
     The field-effect mobility of the transistor including an 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, since the transistors can be separately provided in a driver circuit portion and a pixel portion over one substrate, 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. Some examples are given as the vertical alignment mode. For example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an advanced super view (ASV) mode, and the like can be used. 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. An embodiment of 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. Owing to such a mechanism, this light-emitting element is referred to as 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. 
     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 device 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. 
     The display device performs display by transmitting light from a light source or a display element. Therefore, the substrate and the thin films such as the insulating layer and the conductive layer provided for the pixel portion where light is transmitted have light-transmitting properties with respect to light in the visible-light wavelength range. 
     The first electrode layer and the second electrode layer (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. 
     As described above, by using any of the transistors exemplified in Embodiment 1, Embodiment 2 or Embodiment 3, the highly reliable semiconductor device can be provided. Note that the transistors described in Embodiment 1, Embodiment 2 or Embodiment 3 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 semiconductor device having an image sensor function of reading information of an object. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments. 
     Embodiment 5 
     A semiconductor device that is one embodiment of the present invention can be applied to a variety of electronic appliances (including game machines). Examples of electronic appliances 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 portable information terminal, 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 the above embodiment will be described. 
       FIG. 8A  illustrates a laptop personal computer which includes a main body  301 , a housing  302 , a display portion  303 , a keyboard  304 , and the like. The semiconductor device described in any of Embodiments 1 to 4 is used, whereby a highly reliable laptop personal computer can be provided. 
       FIG. 8B  illustrates a personal digital assistant (PDA) which includes a main body  311  provided with a display portion  313 , an external interface  315 , an operation button  314 , and the like. A stylus  312  is included as an accessory for operation. The semiconductor device described in any of Embodiments 1 to 4 is used, whereby the highly reliable personal digital assistant (PDA) can be provided. 
       FIG. 8C  illustrates an example of an e-book reader. 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. 8C ) and graphics can be displayed on a display portion on the left side (the display portion  324  in  FIG. 8C ). The semiconductor device described in any of Embodiments 1 to 4 is used, whereby the highly reliable e-book reader can be provided. 
       FIG. 8C  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 a 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. 8D  illustrates a portable information terminal, 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 lens  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 portable information terminal, an external memory slot  341 , and the like. Further, an antenna is incorporated in the housing  331 . The semiconductor device described in any of Embodiments 1 to 4 is used, whereby the highly reliable portable information terminal can be provided. 
     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. 8D . 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. 
     In the display panel  332 , the display direction can be changed as appropriate depending on a usage pattern. Further, the mobile phone is provided with the camera lens  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. 8D  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 an AC adapter and various types of cables such as 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. 8E  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. The semiconductor device described in any of Embodiments 1 to 4 is used, whereby the highly reliable digital video camera can be provided. 
       FIG. 8F  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 semiconductor device described in any of Embodiments 1 to 4 is used, whereby the highly reliable television set  360  can be provided. 
     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. With the use of the receiver, general television broadcasting can be received. Moreover, when the television set 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. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments. 
     This application is based on Japanese Patent Application serial no. 2010-167383 filed with Japan Patent Office on Jul. 26, 2010, the entire contents of which are hereby incorporated by reference.