Patent Publication Number: US-9893194-B2

Title: Method for manufacturing semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment relates to a semiconductor device. 
     2. Description of the Related Art 
     The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device. A silicon-based semiconductor material is widely known as a material for a semiconductor applicable to a transistor. 
     As another material, an oxide semiconductor has been attracting attention. For example, a transistor including an amorphous oxide semiconductor layer containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1. 
     Techniques for improving carrier mobility by stacking oxide semiconductor layers are disclosed in Patent Documents 2 and 3. 
     It is known that a transistor including an oxide semiconductor layer has an extremely small leakage current when the transistor is off. For example, a low-power-consumption CPU utilizing the small leakage current characteristic of a transistor including an oxide semiconductor layer is disclosed (see Patent Document 4). 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
     [Patent Document 2] Japanese Published Patent Application No. 2011-124360 
     [Patent Document 3] Japanese Published Patent Application No. 2011-138934 
     [Patent Document 4] Japanese Published Patent Application No. 2012-257187 
     [Patent Document 5] Japanese Published Patent Application No. 2012-074692 
     SUMMARY OF THE INVENTION 
     Miniaturization of transistors has been progressing with an increase in the degree of integration of circuits. The miniaturization of a transistor may cause deterioration of electrical characteristics, such as on-state current, off-state current, threshold, and an S value (subthreshold swing), of the transistor (see Patent Document 5). 
     In view of the above, an object is to provide a semiconductor device whose threshold is corrected; a semiconductor device in which deterioration of electrical characteristics which becomes more noticeable as miniaturization progresses can be suppressed: a highly integrated semiconductor device: a semiconductor device in which deterioration of on-state current characteristics is reduced; a semiconductor device with low power consumption; a highly reliable semiconductor device; a semiconductor device capable of holding data even when power supply is stopped; or a semiconductor device with good characteristics. Objects other than the above object are apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a method for manufacturing a semiconductor device including a first semiconductor device and a second semiconductor device each of which includes a first semiconductor, an electrode which is electrically in contact with the first semiconductor, and a first gate electrode, and a charge trap layer between the first gate electrode and the first semiconductor. The state where a first potential difference between the first gate electrode and the electrode of the first semiconductor device is different from a second potential difference between the first gate electrode and the electrode of the second semiconductor device is kept at higher than or equal to 150° C. and lower than or equal to 300° C. for one minute to one hour to make the thresholds of the first semiconductor device and the second semiconductor device different from each other. 
     One embodiment of the present invention can provide a semiconductor device whose threshold is corrected; a semiconductor device in which deterioration of electrical characteristics which becomes more noticeable as miniaturization progresses can be suppressed; a highly integrated semiconductor device; a semiconductor device in which deterioration of on-state current characteristics is reduced; a semiconductor device with low power consumption: a highly reliable semiconductor device; a semiconductor device capable of holding data even when power supply is stopped; or a semiconductor device with good characteristics, or can solve an object which is apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  illustrate examples of a semiconductor device of an embodiment. 
         FIGS. 2A to 2C  illustrate examples of a semiconductor device of an embodiment. 
         FIGS. 3A to 3D  illustrate band diagram examples of a semiconductor device of an embodiment. 
         FIG. 4A  schematically shows characteristics of a semiconductor device of an embodiment and  FIG. 4B  illustrates an example of a circuit in which the semiconductor device is used. 
         FIGS. 5A and 5B  show an example of a display device of an embodiment. 
         FIGS. 6A to 6C  show an example of a manufacturing process of a semiconductor device. 
         FIGS. 7A and 7B  show examples of a display device of an embodiment and a microprocessor of an embodiment, respectively. 
         FIGS. 8A and 8B  show examples of a memory element of an embodiment. 
         FIGS. 9A and 9B  show examples of a memory element of an embodiment. 
         FIGS. 10A and 10B  show examples of a memory element of an embodiment. 
         FIGS. 11A and 11B  show examples of a memory element of an embodiment. 
         FIGS. 12A to 12C  illustrate examples of a manufacturing process of a semiconductor device. 
         FIGS. 13A to 13C  illustrate an example of a top view and cross-sectional views of a transistor. 
         FIGS. 14A and 14B  illustrate examples of a schematic band diagram of stacked semiconductor layers. 
         FIGS. 15A to 15C  illustrate an example of a top view and cross-sectional views of a transistor. 
         FIGS. 16A to 16D  illustrate an example of a method for manufacturing a transistor. 
         FIGS. 17A to 17C  illustrate an example of a method for manufacturing a transistor. 
         FIGS. 18A to 18C  illustrate an example of a top view and cross-sectional views of a transistor. 
         FIGS. 19A to 19F  illustrate examples of an electronic device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that a variety of modifications can be made to the modes and details without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the following description of the embodiments. 
     Note that in the structures described below, the same portions or portions having similar functions are denoted by the same reference numerals in common in different drawings and repetitive description thereof will be omitted. 
     Note that functions of a “source” and a “drain” of a transistor are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification. 
     Note that in this specification and the like, ordinal numbers such as “first” and “second” are used in order to avoid confusion among components and do not limit the components numerically. 
     Embodiment 1 
     In this embodiment, a structure and operation principles of a semiconductor device including a semiconductor layer, a charge trap layer, and a gate electrode, and a circuit to which the semiconductor device is used will be described.  FIG. 1A  illustrates a semiconductor device including a semiconductor layer  101 , a charge trap layer  102 , and a gate electrode  103 . The charge trap layer  102  can serve as a gate insulating layer. 
     Here, the charge trap layer  102  may be a stacked body that includes a first insulating layer  102   a  and a second insulating layer  102   b  as illustrated in  FIG. 1B , for example. Alternatively, the charge trap layer  102  may be a stacked body that includes the first insulating layer  102   a , the second insulating layer  102   b , and a third insulating layer  102   c  as illustrated in  FIG. 1C , or a stacked body including four or more insulating layers. Alternatively, the charge trap layer  102  may include an electrically insulated conductive layer  102   d  in an insulator  102   e  as illustrated in  FIG. 1D . The insulator  102   e  may be composed of a plurality of insulating layers. 
     Alternatively, a semiconductor device may include, as in  FIG. 2A , the semiconductor layer  101 , the charge trap layer  102 , the gate electrode  103 , a gate insulating layer  104 , a gate electrode  105 . 
     Here, the charge trap layer  102  may be a stacked body that includes a first insulating layer  102   a  and a second insulating layer  102   b  as illustrated in  FIG. 2B , for example. Alternatively, the charge trap layer  102  may be a stacked body that includes the first insulating layer  102   a , the second insulating layer  102   b , and a third insulating layer  102   c  as illustrated in  FIG. 2C , or a stacked body including four or more insulating layers. Alternatively, the charge trap layer  102  may include an electrically insulated conductive layer in an insulator as illustrated in  FIG. 1D . 
       FIG. 3A  is an example of a band diagram between points A and B in the semiconductor device illustrated in  FIG. 1B . In  FIGS. 3A to 3D , Ec and Ev denote the conduction band minimum and the valence band maximum, respectively. In  FIG. 3A , the potential of the gate electrode  103  is equal to the potential of a source electrode or drain electrode (not illustrated). 
     In this example, the band gap of the first insulating layer  102   a  is larger than that of the second insulating layer  102   b , and the electron affinity of the first insulating layer  102   a  is smaller than that of the second insulating layer  102   b ; however, the present invention is not limited to this example. 
     Electron trap states  106  exist at the interface between the first insulating layer  102   a  and the second insulating layer  102   b  or inside the second insulating layer  102   b .  FIG. 3B  shows the state where the potential of the gate electrode  103  is higher than the potential of the source or drain electrode. The potential of the gate electrode  103  may be higher than the potential of the source or drain electrode by 1 V or more. The potential of the gate electrode  103  may be lower than the highest potential applied to the gate electrode  105  after this process. In the process, the difference between the potential of the gate electrode  103  and the potential of the source or drain electrode is typically less than 4 V. 
     Electrons  107  that exist in the semiconductor layer  101  move toward the gate electrode  103  having a higher potential. Some of the electrons  107  moving from the semiconductor layer  101  toward the gate electrode  103  are trapped by the electron trap states  106 . 
     There are some processes to enable the electrons  107  to go over the barrier of the first insulating layer  102   a  to reach the second insulating layer  102   b . The first is a process by the tunnel effect. The thinner the first insulating layer  102   a  is, the more prominent the tunnel effect is. Note that electrons trapped by the electron trap states  106  might sometimes return to the semiconductor layer  101  by the tunnel effect. 
     Even when the first insulating layer  102   a  is relatively thick, the tunnel effect (Fowler-Nordheim tunnel effect) can be obtained by applying an appropriate voltage to the gate electrode  103 . In the case of the Fowler-Nordheim tunnel effect, a tunnel current increases with the square of the electric field between the gate electrode  103  and the semiconductor layer  101 . 
     The second is the process that the electrons  107  hop from trap states to trap states in the band gap such as defect states in the first insulating layer  102   a  to reach the second insulating layer  102   b . This is a conduction mechanism called Poole-Frenkel conduction, in which as the absolute temperature is higher and trap states are shallower, the electric conductivity is higher. 
     The third is the process that the electrons  107  go over the barrier of the first insulating layer  102   a  by thermal excitation. The distribution of electrons existing in the semiconductor layer  101  follows the Fermi-Dirac distribution: in general, the proportion of electrons having high energy is larger as the temperature is higher. Assuming that the density of electrons having energy 3 eV higher than the Fermi level at 300 K (27° C.) is 1, for example, the density is 6×10 16  at 450 K (177° C.), 1.5×10 25  at 600 K (327° C.), and 1.6×10 30  at 750 K (477° C.). 
     The movement of the electrons  107  toward the gate electrode  103  by going over the barrier of the first insulating layer  102   a  occurs by the above three processes or the combination of these processes. In particular, the second and the third processes indicate that current increases exponentially as the temperature is higher. 
     Also, the Fowler-Nordheim tunnel effect is more likely to occur as the density of electrons in a thin part (a high-energy portion) of a barrier layer of the first insulating layer  102   a  is higher; thus, a higher temperature is better. 
     Note that in most cases, current generated by the conduction mechanism is weak in particular when the potential difference between the gate electrode  103  and the semiconductor layer  101  is small (5 V or lower). However, by taking a long time for the process, the needed number of electrons can be trapped by the electron trap states  106 . As a result, the charge trap layer  102  is negatively charged. 
     In other words, the potential of the gate electrode  103  is kept higher than that of the source or drain electrode at a high temperature (a temperature higher than the operating temperature or the storage temperature of the semiconductor device, or higher than or equal to 125° C. and lower than or equal to 450° C. for example higher than or equal to 150° C. and lower than or equal to 300° C.) for one second or longer, for example, one minute or longer. As a result, the needed number of electrons moves from the semiconductor layer  101  toward the gate electrode  103  and some of them are trapped by the electron trap states  106 . The temperature of the process for trapping electrons is referred to as process temperature below. 
     Here, the amount of electrons trapped by the electron trap states  106  can be controlled by the potential of the gate electrode  103 . When a certain number of electrons is trapped by the electron trap states  106 , due to the electric charge, the electric field of the gate electrode  103  is blocked and a channel formed in the semiconductor layer  101  disappears. 
     The total number of electrons trapped by the electron trap states  106  increases linearly at first, and then, the rate of increase gradually decreases, and the total number of electrons converges at a certain value. The convergence value depends on the potential of the gate electrode  103 . As the potential is higher, the number of trapped electrons is more likely to be large; however, it never exceeds the total number of electron trap states  106 . 
     The electrons trapped by the electron trap states  106  are required not to transfer from the charge trap layer  102  to the other regions. For this, each thickness of the first and second insulating layers  102   a  and  102   b  is preferably set at a thickness at which the tunnel effect is not a problem. For example, the physical thickness is preferably more than 1 nm. 
     However, electron transfer is hindered if the thickness of the first insulating layer  102   a  is too large; thus, 30 nm or less is preferable. Furthermore, if the thickness of the first and second insulating layers  102   a  and  102   b  is too large as compared with the channel length of the semiconductor device, the subthreshold value is increased to degrade the off-state characteristics. For this reason, the channel length is more than or equal to four times, typically more than or equal to ten times as large as the equivalent silicon oxide thickness of the first and second insulating layers  102   a  and  102   b . Note that when a so-called High-K material is used, the equivalent silicon oxide thickness is less than the physical thickness. 
     The thickness of the first insulating layer  102   a  is preferably more than or equal to 10 nm and less than or equal to 20 nm, and the equivalent silicon oxide thickness of the second insulating layer  102   b  is more than or equal to 1 nm and less than or equal to 25 nm. 
     Another method is to set the operating temperature or the storage temperature of the semiconductor device at a temperature that is lower enough than the process temperature. For example, the process temperature is set at 300° C., and the semiconductor device is stored at 120° C. or lower. The probability that electrons go over a 3 eV-barrier when the temperature is 120° C. is less than a one hundred-thousandth that when the temperature is 300° C. In this way, although electrons easily go over a barrier to be trapped by the electron trap states  106  during the process at 300° C., the electrons are difficult to go over the barrier during storage at 120° C. and are kept trapped by the electron trap states  106  for a long time. 
     It is also effective that the effective mass of a hole is extremely large or is substantially localized in the semiconductor layer  101 . In this case, the injection of holes from the semiconductor layer  101  to the first and second insulating layers  102   a  and  102   b  does not occur and consequently a phenomenon in which electrons trapped by the electron trap states  106  bond to holes and disappear does not occur. 
     A material showing Poole-Frenkel conduction may be used for the second insulating layer  102   b . The Poole-Frenkel conduction is, as described above, electron hopping conduction between defect states and the like in a material. A material including a large number of defect states or including deep defect states has low electric conductivity and consequently can hold electrons trapped by the electron trap states  106  for a long time. 
     Circuit design or material selection may be made so that no voltage at which electrons trapped in the first and second insulating layers  102   a  and  102   b  are released is applied. For example, in a material whose effective mass of holes is extremely large or is substantially localized, such as an In—Ga—Zn-based oxide semiconductor, a channel is formed when the potential of the gate electrode  103  is higher than that of the source or drain electrode; however, when the potential of the gate electrode  103  is lower than that of the source or drain electrode, the material shows characteristics similar to an insulator. In this case, the electric field between the gate electrode  103  and the semiconductor layer  101  is extremely small and consequently the Fowler-Nordheim tunnel effect or electron conduction according to the Poole-Frenkel conduction is significantly decreased. 
     To hold electrons trapped by electron trap states inside the second insulating layer  102   b  or at the interface with another insulating layer, it is effective that the charge trap layer  102  is formed using three insulating layers as illustrated in  FIG. 1C , that the electron affinity of the third insulating layer  102   c  is smaller than that of the second insulating layer  102   b , and that the bandgap of the third insulating layer  102   c  is larger than that of the second insulating layer  102   b.    
     In this case, if the physical thickness of the third insulating layer  102   c  is large enough, electrons trapped by the electron trap states  106  can be held even when the second insulating layer  102   b  has a small thickness. As a material of the third insulating layer  102   c , the same material as or a material similar to that of the first insulating layer  102   a  can be used. Alternatively, a material whose constituent elements are the same as those of the second insulating layer  102   b  but number of electron trap states is small enough may be used. The number (density) of electron trap states depends on the formation method. 
     Note that when the electrically insulated conductive layer  102   d  is surrounded by the insulator  102   e  as illustrated in  FIG. 1D , electrons are trapped in the conductive layer  102   d  according to the above principle.  FIGS. 3C and 3D  illustrate the examples. In  FIG. 3C , the potential of the gate electrode  103  is equal to that of the source or drain electrode. 
       FIG. 3D  shows the state where the potential of the gate electrode  103  is higher than the potential of the source or drain electrode. Electrons that exist in the semiconductor layer  101  move toward the gate electrode  103  having a higher potential. Some of the electrons moving from the semiconductor layer  101  toward the gate electrode  103  are trapped in the conductive layer  102   d . In other word, in the semiconductor device illustrated in  FIG. 1D , the conductive layer  102   d  functions as the electron trap state  106  in the semiconductor device in  FIG. 1B . 
     When the work function of the conductive layer  102   d  is large, an energy barrier to the insulator  102   e  is high and consequently the trapped electrons can be prevented from moving out from the conductive layer  102   d.    
     In the above structure, each of the first to third insulating layers  102   a  to  102   c  may be composed of a plurality of insulating layers. A plurality of insulating layers containing the same constituting elements and formed by different formation methods may be used. 
     For example, when the first and second insulating layers  102   a  and  102   b  are formed using insulating layers composed of the same constituting elements (e.g., hafnium oxide), the first insulating layer  102   a  may be formed by a chemical vapor deposition (CVD) method such as a metal organic chemical vapor deposition (MOCVD) method, a metal chemical deposition method, an atomic layer deposition (ALD) method, or a plasma-enhanced chemical vapor deposition (PECVD) method, an evaporation method, a pulsed laser deposition (PLD) method, or the like, and the second insulating layer  102   b  may be formed by sputtering. 
     Note that the first and second insulating layers  102   a  and  102   b  may be formed by different CVD methods. 
     In general, an insulating layer formed by sputtering includes more defects and stronger electron trapping characteristics than an insulating layer formed by CVD. From this reason, the second insulating layer  102   b  may be formed by sputtering and the third insulating layer  102   c  may be formed by CVD when the second and third insulating layers  102   b  and  102   c  contain the same constituent elements. 
     When the second insulating layer  102   b  is formed using a plurality of insulating layers containing the same constituent elements, one insulating layer may be formed by sputtering and another insulating layer may be formed by CVD. 
     As described above, the threshold of a semiconductor device is increased by the trap of electrons in the charge trap layer  102 . In particular, when the semiconductor layer  101  is formed using a wide bandgap material, a source-drain current (cut-off current. Icut) when the potential of the gate electrode  103  is equal to the potential of the source electrode can be significantly decreased. 
     For example, the Icut density (a current value per micrometer of a channel width) of an In—Ga—Zn-based oxide whose bandgap is 3.2 eV is 1 zA/μm (1×10 −21  A/μm) or less, typically 1 yA/μm (1×10 −24  A/μm) or less. 
       FIG. 4A  schematically shows dependence of current per micrometer of channel width (Id) between source and drain electrodes on the potential of the gate electrode  103  (Vg) at room temperature, before and after electron trap in the electron trap layer  102 . Note that each potential of the source electrode is 0 V and the potential of the drain electrode is +1 V. Although current smaller than 1 fA cannot be measured directly, it can be estimated from a value measured by another method, the subthreshold value, and the like. 
     As indicated by a curve  108 , the threshold of the semiconductor device is Vth1 at first. After electron trapping, the threshold is increased (shifts in the positive direction) to become Vth2. As a result, Icut per micrometer of a channel width becomes 1 aA/μm (1×10 −18  A/μm) or less, for example, greater than or equal to 1 zA/μm and less than or equal to 1 yA/μm. 
       FIG. 4B  illustrates a circuit in which charge stored in a capacitor  111  is controlled by a transistor  110 . Leakage current between electrodes of the capacitor  111  is ignored. The capacitance of the capacitor  111  is 1 fF, the potential of the capacitor  111  on the transistor  110  side is +1 V, and the potential of Vd is 0 V. 
     The curve  108  in  FIG. 4A  denotes the Id-Vd characteristics of the transistor  110  and the channel width is 0.1 m, in which case the Icut is approximately 1 fA and the resistivity of the transistor  110  at this time is approximately 1×10 15 Ω. Accordingly, the time constant of a circuit composed of the transistor  110  and the capacitor  111  is approximately one second. This means that most of the charge stored in the capacitor  111  is lost in approximately one second. 
     The curve  109  in  FIG. 4A  denotes the Id-Vd characteristics of the transistor  110  and the channel width is 0.1 μm, in which case the Icut is approximately 1 yA and the resistivity of the transistor  110  at this time is approximately 1×10 24 Ω. Accordingly, the time constant of the circuit composed of the transistor  110  and the capacitor  111  is approximately 1×10 9  seconds (=approximately 31 years). This means that one-third of the charge stored in the capacitor  111  is left after 10 years. 
     From this, charge can be held for 10 years in a simple circuit composed of a transistor and a capacitor. This can be applied to various kinds of memory devices. 
     The range of the threshold increase depends on the density of electrons trapped by the charge trap layer  102 . For example, in the semiconductor device illustrated in  FIG. 1B , in the case where electrons are trapped only at the interface between the first insulating layer  102   a  and the second insulating layer  102   b , the threshold is increased by Q/C, where Q is the surface density of trapped electrons and C is the dielectric constant of the first insulating layer  102   a.    
     As described above, the potential of the gate electrode  103  determines the value at which the number of trapped electrons converges. Accordingly, the degree of the threshold increase can be adjusted by the potential of the gate electrode  103 . 
     As an example, a case in which the potential of the gate electrode  103  is set higher than the potentials of the source electrode and the drain electrode by 1.5 V and the temperature is set at 150° C. to 250° C., typically 200° C.±20° C. is considered. Assuming that the threshold of the semiconductor device before electrons are trapped in the charge trap layer  102  (first threshold, Vth1) is +1.1 V, a channel is formed in the semiconductor layer  101  at first and electrons are trapped in the charge trap layer  102 . Then, the number of trapped electrons in the charge trap layer  102  increases, and the channel disappears. At this stage, trap of electrons in the charge trap layer  102  stops. 
     In this case, because the channel disappears when the potential of the gate electrode  103  is higher than the potentials of the source electrode and the drain electrode by +1.5 V, the threshold voltage becomes +1.5 V. In other words, the threshold value is increased by 0.4 V owing to the electrons trapped in the charge trap layer  102 . The threshold value changed by the electrons trapped in the charge trap layer  102  in the above manner is referred to as a second threshold value (Vth2). This process by which a threshold can be adjusted to an appropriate value is also referred to as threshold adjustment process. 
     By utilizing these characteristics, the thresholds of a plurality of semiconductor devices which are initially largely different from each other can converge at values within an appropriate range. For example, if three semiconductor devices with the first threshold voltages of +1.2 V, +1.1 V, and +0.9 V are subjected to the process under above-described conditions, trap of electrons does not make the threshold voltage to become significantly higher than +1.5 V in each semiconductor device; the second threshold voltage of each semiconductor device can become approximately +1.5 V. In this case, the number of trapped electrons in the charge trap layer  102  (e.g., the surface density of electrons) varies among the three semiconductor devices. 
     Note that the number of electrons trapped in the charge trap layer  102  also depends on the length of time for the threshold adjustment process and thus a desired threshold can be obtained by adjusting time for the threshold adjustment process. 
     Any of a variety of materials can be used for the gate electrode  103 . For example, a conductive layer of Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used. The gate electrode  103  may have a stacked-layer structure of any of these materials. Alternatively, a conductive layer containing nitrogen may be used for the gate electrode  103 . For example, a titanium nitride layer over which a tungsten layer is stacked, a tungsten nitride layer over which a tungsten layer is stacked, a tantalum nitride layer over which a tungsten layer is stacked, or the like can be used as the gate electrode  103 . 
     Note that the work function of the gate electrode  103  that faces the semiconductor layer  101  is one factor determining the threshold of the semiconductor device; in general, as the work function of a material is smaller, the threshold becomes lower. However, as described above, the threshold can be adjusted by adjusting the number of trapped electrons in the charge trap layer  102 ; accordingly, the range of choices for the material of the gate electrode  103  can be widened. 
     Any of a variety of materials can be used for the semiconductor layer  101 . For example, other than silicon, germanium, and silicon germanium, any of a variety of oxide semiconductors described later can be used. 
     Any of a variety of materials can be used for the first insulating layer  102   a . For example, an insulating layer containing one or more kinds selected from magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, and tantalum oxide can be used. 
     The second insulating layer  102   b  can be an insulating layer containing one or more kinds selected from hafnium oxide, aluminum oxide, tantalum oxide, aluminum silicate, silicon nitride, and the like, for example. 
     The third insulating layer  102   c  can be an insulating layer containing one or more kinds selected from magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, and tantalum oxide, for example. 
     The conductive layer  102   d  can be formed using any kind of materials. For example, a conductive layer of Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, Pt, Pd, or the like can be used. The conductive layer  102   d  may have a stacked-layer structure of any of these materials. Alternatively, a conductive layer containing nitrogen may be used as the conductive layer  102   d.    
     In particular, as a material having a large work function, a metal of the platinum group such as platinum or palladium: a nitride such as indium nitride, zinc nitride, In—Zn-based oxynitride, In—Ga-based oxynitride, or In—Ga—Zn-based oxynitride; or the like may be used. 
     Any of a variety of materials can be used for the insulator  102   e . For example, silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, or tantalum oxide can be used. 
     Thus, the semiconductor device in which a necessary number of electrons are trapped in the charge trap layer  102  is the same as a normal MOS semiconductor device. When the semiconductor device is used as a MOS semiconductor device, it is preferable that the potential of the gate electrode  103  be always set at the lowest potential in the circuit. 
     The threshold corrected by the above method is determined by the potential difference between the semiconductor layer  101  and the gate electrode  103  during the process. Thus, the thresholds of a plurality of transistors can be appropriate values depending on the intended use of the transistors, by using different potential differences for the threshold adjustment process. 
     A pixel  120  in  FIG. 5A  includes a transistor  121 , a transistor  122 , and a light-emitting element  123 . Another transistor, a capacitor, or the like is also included as necessary. Each of the transistors  121  and  122  includes a charge trap layer  102 . After a circuit is formed, the threshold is corrected in the above-described manner. Note that as shown in the drawing, a threshold-corrected transistor is represented by a symbol that is different from the symbol for a normal transistor to show that the threshold-corrected transistor contains electrons in the charge trap layer  102 . In this example, a transistor including the gate electrodes  103  and  105  as in  FIGS. 2A to 2C  is used. 
     Here, the threshold of the transistor  121  is different from that of the transistor  122 . For example, the off-state current of the transistor  121  is preferably smaller (the off-state resistance is higher) than that of the transistor  122 , that is, the threshold of the transistor  121  is preferably larger than that of the transistor  122 . This is because change in gate potential of the transistor  122  needs to be suppressed. The threshold of the transistor  122  is preferably less changed, but the off-state resistance of the transistor  122  does not need to be high. It is sometimes preferable that the threshold be smaller in view of suppressing potential change of a data line Pm (reducing power consumption). The potentials of the gate electrodes  103  in the transistors are thus different from each other in the threshold adjustment process. 
     The pixels  120  having the structure shown in  FIG. 5A  are arranged in a matrix display device. For example, the pixel  120  in the n-th row and the m-th column is connected to the data line Pm, a selection line Qn, a threshold correction line Rm, a threshold correction line Sn, and a power supply line  124 . Note that the threshold correction line Rm and the threshold correction line Sn are at right angles to each other. 
     The threshold correction can be performed as follows. First, the potentials of the data line Pm and the power supply line  124  are set to 0 V. The potential of the selection line Qn may also be set to 0 V or may be held at an appropriate potential for an appropriate period so that the transistor  121  can be in an on state. Then, the potentials of the threshold correction line Rm and Sn are set to appropriate potentials and held for an appropriate period in appropriate temperatures. In this way, the threshold is corrected to an appropriate value. 
     Note that the thresholds of the transistors  121  and  122  before the threshold adjustment process may be 0 V or lower or may be values for making Icut flow appropriately. With such a transistor, the potential of the gate electrode  105  of the transistor  122  can be the same as the potential of the data line Pm even in the threshold adjustment process. 
     In order to make the threshold of the transistor  121  higher than the threshold of the transistor  122 , the potential of the threshold correction line Sn during the threshold adjustment process is set higher than the potential of the threshold correction line Rm. For example, the potentials of the threshold correction lines Sn and Rm are +2 V and +1 V, respectively. The potentials of the threshold correction lines Sn and Rm may be +2 V and 0 V, respectively. In that case, the threshold of the transistor  122  does not change from the initial value. 
     Note that the threshold correction line Rm is not necessarily provided in the case where the threshold adjustment process for the transistor  122  is unnecessary. The threshold correction line Rm (or the corresponding line) may be provided even when the threshold adjustment process for the transistor  122  is not performed. In practical use, the threshold of the transistor  122  can be corrected by applying an appropriate potential to the threshold correction line Rm (or the corresponding line). 
     The potentials of the threshold correction lines Rm and Sn may be changed successively or gradually, and they may be changed independently or in synchronization with each other, for example. For example, in a first period, the potentials of the threshold correction lines Rm and Sn are set to +1 V. In the following second period, the potential of the threshold correction line Sn is changed to +2 V, but the potential of the threshold correction line Rm remains at +1 V. Here, the potential of the selection line Qn may be one by which the transistor  121  is turned on in the first period and may be changed to 0 V in the second period. 
     In this process, when the thresholds of the transistors  121  and  122  are both, for example, 0 V before the threshold adjustment process, the thresholds of the transistors  121  and  122  become +1 V or lower in the first period. In the second period, the threshold of the transistor  122  is less changed and variation in thresholds of the transistor  122  is small, whereas the threshold of the transistor  121  is increased to +2 V or lower. 
     Note that a threshold correction line Rn may be parallel to the threshold correction line Sn as shown in  FIG. 5B . In this case, the threshold correction method can be similar to that in the case of  FIG. 5A . 
     When the threshold is adjusted to an appropriate value by making the charge trap layer trap electrons as described above, it is preferable to avoid further addition of electrons to the charge trap layer in normal use after that. For example, further addition of electrons means a further increase of threshold, resulting in circuit deterioration. 
     When a charge trap layer is in the vicinity of a semiconductor layer and a wiring or an electrode facing the semiconductor layer and having a potential higher than that of the semiconductor layer exists, there is a possibility that electrons are trapped by the charge trap layer also in normal use. 
     In the pixel  120  shown in  FIG. 5A or 5B , for example, this problem can be solved by setting the potential of the threshold correction line Rm and the like to lower than or equal to the lowest potential of the data line Pm after the threshold adjustment process. 
     For example, steps illustrated in  FIGS. 6A to 6C  can be performed. First, as illustrated in  FIG. 6A , initial characteristics of finished display devices are measured to select conforming items. Here, it is preferable that items without malfunctions that is caused by disconnection or the like and cannot be recovered be regarded as conforming items. Threshold anomalies are not the criterion for selection because thresholds have not been corrected yet. 
     Electrons are then injected as in  FIG. 6B . An appropriate number of electrons are trapped in the charge trap layer  102 . This operation is performed in the above-described manner. 
     Then, measurement is performed again as in  FIG. 6C . One criterion for conforming items is that the threshold is corrected as planned. At this stage, display devices with threshold anomalies may be regarded as nonconforming items and may again be subjected to electron injection. Conforming items are shipped. 
     The process for trapping electrons in the charge trap layer  102  is not limited to the above, and it is preferable that the display device be not exposed to temperatures of 125° C. or higher for one hour or more after the process. 
     Note that particularly in the semiconductor device having any of the structures of  FIGS. 2A to 2C , any potential can be used as the potential applied to the gate electrode  103  as long as the threshold adjustment process is not performed in normal use and is performed one time before shipment from the factory, for example. A potential which is not usually used can be used, for example. 
     For example, even when the thickness of the third insulating layer  102   c  is considerably larger than those of the first insulating layer  102   a  and the second insulating layer  102   b  in the semiconductor device in  FIG. 2C , the second insulating layer  102   b  can trap electrons by application of an appropriate potential to the gate electrode  103 . The thickness of the third insulating layer  102   c  may be increased so that the thickness of the charge trap layer  102  is 500 nm or more, for example. 
     Assume that when the equivalent silicon oxide thicknesses of the first to third insulating layers  102   a  to  102   c  are each 10 nm, for example, the threshold adjustment process is performed under the potential difference between the gate electrode  103  and the semiconductor layer  101  being 1.5 V. When the same threshold adjustment process is performed with the first and second insulating layers  102   a  and  102   b  each having an equivalent silicon oxide thickness of 10 nm and the third insulating layer  102   c  having an equivalent silicon oxide thickness of 1 μm, the potential difference between the gate electrode  103  and the semiconductor layer  101  is set at 150 V. 
     This example of including a plurality of transistors with different thresholds in one device can be employed in other devices without limitation to the above. Instead of the light-emitting element  123 , an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), an interferometric modulator display (IMOD) element, an electrowetting element, a piezo-ceramic element, or a carbon nanotube, which are display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by electromagnetic action, may be used. 
       FIG. 7A  schematically shows a display device  130 . The display device  130  includes a driver region  131 , a display region  132 , an external connection terminal (e.g., an FPC  133 ), and the like. When the display device  130  is of an active matrix type, for example, thin film transistors are used in the driver region  131  and the display region  132 . 
     Here, for example, the threshold of the thin film transistor used in the driver region  131  may be higher than that of the thin film transistor in the display region  132 . In that case, time and voltage of the threshold adjustment process are preferably different between the driver region  131  and the display region  132 . The threshold adjustment process may be performed on only thin film transistors used in the driver region  131 , without being performed on thin film transistors used in the display region  132 . The opposite is possible. 
       FIG. 7B  is an example of a microprocessor  140 . The microprocessor  140  includes a logical unit  141  (including registers  142 ), a first level cache  143 , a second level cache  144 , an I/O circuit  145 , and the like. A memory device including the circuit shown in  FIG. 4B  can be used as a memory device used for the registers  142 , the first level cache  143 , the second level cache  144 , and the like. 
       FIG. 8A  is an example of a memory element  150  used in the registers  142 . The memory element  150  includes a switch  151   a , a switch  151   b , a switch  151   c , an inverter  152   a , an inverter  152   b , an inverter  152   c , a transistor  153  having a structure similar to  FIG. 2A , and a capacitor  154 . The transistor  153  is a threshold-corrected transistor. 
     An example of the threshold correction is as follows: a state where other than the potential of the signal SIG5 such as the potentials of signals IN, OUT, SIG1, SIG2, SIG3, and SIG4 and the power supply potentials of the inverters  152   a  to  152   c  are all the same (first potential), but only the potential of a signal SIG5 is an appropriate potential (second potential) higher than the first potential is kept at higher than or equal to 150° C. and lower than or equal to 300° C. An appropriate amount of electrons are then trapped in the charge trap layer of the transistor  153  to adjust the threshold. 
     In a normal operation, data is held by the inverters  152   a  and  152   b  (output of one is connected to input of the other) while power is supplied from outside to the memory element  150 . Since the inverters  152   a  and  152   b  consume power, the power supply may be stopped to reduce power consumption as needed. In that case, data (charge) is saved to the capacitor  154  and then the transistor  153  is turned off. Power might be off for a maximum of a few seconds to a few years, and the off-state resistance of the transistor  153  needs to be sufficiently high (i.e., the threshold needs to be high). 
     The above is an example in which a semiconductor device having the structure of  FIG. 2A  is used. Alternatively, a semiconductor device having a structure of  FIG. 1A  may be used.  FIG. 8B  is an example of a memory element  155  using a semiconductor device having the structure of  FIG. 1A . The memory element  155  includes a switch  156   a , a switch  156   b , a switch  156   c , an inverter  157   a , an inverter  157   b , an inverter  157   c , a transistor  158  having a structure similar to  FIG. 1A , and a capacitor  159 . The transistor  158  is a threshold-corrected transistor. 
     An example of the threshold correction is as follows: a state where other than the potentials of the signal SIG4 such as the potentials of the signals IN, OUT, SIG1, SIG2, and SIG3, the power supply potentials of the inverters  157   a  to  157   c  are all the same (first potential), but only the potential of the signal SIG4 is an appropriate potential (second potential) higher than the first potential is kept at higher than or equal to 150° C. and lower than or equal to 300° C. An appropriate amount of electrons are then trapped in the charge trap layer of the transistor  158  to adjust the threshold. 
       FIG. 9A  is an example of the memory element  160  used in the first level cache  143 . The memory element  160  includes a transistor  161   a , a transistor  161   b , an inverter  162   a , an inverter  162   b , transistors  163   a  and  163   b  each having a structure similar to  FIG. 2A , and capacitors  164   a  and  164   b . The transistors  163   a  and  163   b  are threshold-corrected transistors. 
     An example of the threshold correction is as follows: a state where other than the potential of a threshold correction line TC such as the potentials of a bit line BL_a, a bit line BL_b, a word line WL, a backup control line WE, the power supply potentials of inverters  162   a  and  162   b  are all the same (first potential), but only the potential of the threshold correction line TC is an appropriate potential (second potential) higher than the first potential is kept at higher than or equal to 150° C. and lower than or equal to 300° C. An appropriate amount of electrons are then trapped in the charge trap layers of the transistors  163   a  and  163   b  to adjust the thresholds. 
     Note that the potential of the threshold correction line TC is fixed in a normal use at the lowest one in the circuit, for example. It is thus possible to connect the threshold correction line TC with the capacitors  164   a  and  164   b , for example. 
     In a normal operation, data is held by the inverters  162   a  and  162   b  (output of one is connected to input of the other) while power is supplied from outside to the memory element  160 . Since the inverters  162   a  and  162   b  consume power, the power supply may be stopped to reduce power consumption as needed. In that case, data (charge) is saved to the capacitors  164   a  and  164   b  and then the transistors  163   a  and  163   b  are turned off. Power might be off for a maximum of a few seconds to a few years, and the off-state resistance of the transistors  163   a  and  163   b  needs to be sufficiently high (i.e., the threshold needs to be high). 
     The above is an example in which a semiconductor device having a structure of  FIG. 2A  is used, and alternatively, a semiconductor device having a structure of  FIG. 1A  may be used.  FIG. 9B  is an example of a memory element  165  using a semiconductor device having a structure of  FIG. 1A . The memory element  165  includes a transistor  166   a , a transistor  166   b , an inverter  167   a , an inverter  167   b , transistors  168   a  and  168   b  each having a structure similar to  FIG. 1A , and capacitors  169   a  and  169   b . The transistors  168   a  and  168   b  are threshold-corrected transistors. 
     An example of the threshold correction is as follows: a state where other than the potential of the backup control line WE such as the potentials of the bit line BL_a, the bit line BL_b, the word line WL, the power supply potentials of the inverters  167   a  and  167   b  are all the same (first potential), but only the potential of the backup control line WE is an appropriate potential (second potential) higher than the first potential is kept at higher than or equal to 150° C. and lower than or equal to 300° C. An appropriate amount of electrons are then trapped in the charge trap layers of the transistors  168   a  and  168   b  to adjust the thresholds. 
       FIG. 10A  is an example of a memory element  170  used for the first level cache  143  or the second level cache  144 . The memory element  170  includes a transistor  171 , a transistor  172 , and a capacitor  173 . The transistor  171  is a threshold-corrected transistor having a structure similar to that in  FIG. 2A . 
     An example of the threshold correction is as follows: a state where other than the potential of the threshold correction line TC such as the potentials of the bit line BL, a write word line WWL, a read word line RWL are all the same (first potential), but only the potential of the threshold correction line TC is an appropriate potential (second potential) higher than the first potential is kept at higher than or equal to 150° C. and lower than or equal to 300° C. An appropriate amount of electrons are then trapped in the charge trap layer of the transistor  171  to adjust the threshold. 
     The above is an example in which a semiconductor device having a structure of  FIG. 2A  is used, and alternatively, a semiconductor device having a structure of  FIG. 1A  may be used.  FIG. 10B  is an example of a memory element  175  using a semiconductor device having a structure of FIG. A. The memory element  175  includes a transistor  176 , a transistor  177 , and a capacitor  178 . The transistor  176  is a threshold-corrected transistor having a structure similar to  FIG. 1A . 
     An example of the threshold correction is as follows: a state where other than the potential of a write word line WWL such as the potentials of the bit line BL, the read word line RWL are all the same (first potential), but only the potential of the write word line WWL is an appropriate potential (second potential) higher than the first potential is kept at higher than or equal to 150° C. and lower than or equal to 300° C. An appropriate number of electrons are then trapped in the charge trap layer of the transistor  176  to adjust the threshold. 
       FIG. 11A  is an example of a memory element  180  used for the first level cache  143  or the second level cache  144 . The memory element  180  includes a transistor  181 , a transistor  182 , a transistor  183 , and a capacitor  184 . The transistor  181  is a threshold-corrected transistor having a similar structure as  FIG. 2A . The threshold correction is preferably performed as that for the transistor  171  of the memory element  170  is performed. Note that the potential of the threshold correction line TC is fixed in a normal use at the lowest one in the circuit, for example. It is thus possible to connect the threshold correction line TC with the capacitor  184 , for example. 
     The above is an example in which a semiconductor device having a structure of  FIG. 2A  is used, and alternatively, a semiconductor device having a structure of  FIG. 1A  may be used.  FIG. 11B  is an example of a memory element  185  using a semiconductor device having a structure of FIG. A. The memory element  185  includes a transistor  186 , a transistor  187 , a transistor  188 , and a capacitor  189 . Although the capacitor  189  is connected to the source line SL, the capacitor  189  may be connected to other wirings. The transistor  186  is a threshold-corrected transistor having a similar structure as  FIG. 1A . The threshold correction is preferably performed as that for the transistor  176  of the memory element  175  is performed. 
     Because the memory element  170  stores data using the charge held by the capacitor  173 , the transistor  171  needs to have high off-state resistance. The memory element  170  needs to respond at suitable high speed, and the transistor  171  also needs to have low on-state resistance. The charge hold period of the capacitor  173  used in a cache is one minute or shorter, and the threshold of the transistor  171  is thus determined so that the memory element  170  can hold data for about one minute. This is also applied to the memory elements  175 ,  180 , and  185 . 
     Although the transistors  153 ,  163   a ,  163   b ,  171 , and  181  in the memory elements  150 ,  160 ,  170 , and  180  have the same function of holding charge in the capacitors  154 ,  164   a ,  164   b .  173 , and  184 , the charge hold periods are different from one another and thus the thresholds need to be different. 
     In most cases, the transistors  153 ,  163   a ,  163   b ,  171 , and  181  are formed in the same microprocessor  140  to have the same layer structure. It is preferable to adjust the threshold of each of these transistors by changing conditions of the threshold adjustment process. Here, their threshold correction lines or the like are preferably configured to have different potentials from one another for the same period or for different periods. 
     The same is applied to the transistors  158 ,  168   a ,  168   b ,  176 , and  186  used in the memory elements  155 ,  165 ,  175 , and  185 . 
     Note that threshold correction is not necessarily performed on all of the transistors  153 ,  158 ,  163   a ,  163   b ,  168   a ,  168   b ,  171 ,  176 ,  181 , and  186 . For example, threshold correction is not performed on the transistors  171 ,  176 ,  181 , and  186  but is performed on all or some of the transistors  153 ,  158 ,  163   a ,  163   b ,  168   a , and  168   b.    
       FIG. 12A  is an example of a semiconductor chip  190  including the microprocessor  140  as in  FIG. 7B . The semiconductor chip  190  includes a plurality of pads  191  and a device region  192 . When a plurality of semiconductor devices having the structure of  FIG. 2A  is included, for example, a threshold correction line which is connected to the gate electrode  103  can be individually provided. 
     For example, the signal SIG5 for correcting the threshold of the transistor  153  in the memory element  150  is input from a pad  191   a , the threshold correction line TC of the memory element  160  is connected to a pad  191   b , and the threshold correction line TC of the memory element  170  is connected to a pad  191   c . When different potentials are applied to the pads  191   a ,  191   b , and  191   c  or appropriate potentials are applied to all or some of the other pads, the thresholds of transistors in memory elements are different from one another. 
     An example of the threshold adjustment process is as follows: the semiconductor chip  190  is heated to 150° C. to 300° C., and a state where the potentials of the pads  191   a ,  191   b , and  191   c  are +1.5 V, +1.2 V, and +1.0 V, respectively, and the potentials of the other pads  191  is all 0 V is kept for longer than or equal to one minute and shorter than or equal to one hour. 
     Another example of the threshold adjustment process is as follows: a state where the semiconductor chip  190  is heated to 150° C. to 300° C., the potentials of the pads  191   a ,  191   b , and  191   c  are +1.0 V, and the potentials of the other pads  191  are all 0 V is kept for longer than or equal to one minute and shorter than or equal to one hour; then, a state where the potentials of the pads  191   a ,  191   b , and  191   c  are +1.5 V, +1.2 V, +1.0 V, respectively, and the potentials of the other pads  191  are all 0 V is kept for longer than or equal to one minute and shorter than or equal to one hour. 
     Another example of the threshold adjustment process is as follows: a state where the semiconductor chip  190  is heated to 150° C. to 300° C., the potentials of the pads  191   a .  191   b , and  191   c  are +1.5 V, +1.2 V, +1.0 V, respectively, and the potentials of the other pads  191  are all 0 V is kept for longer than or equal to one minute and shorter than or equal to one hour; then, a state where the potential of the pad  191   c  is +1.0 V and the potentials of the other pads  191  (including the pads  191   a  and  191   b ) are all 0 V is kept for longer than or equal to one minute and shorter than or equal to one hour. 
     Note that thresholds of transistors after the threshold adjustment process are different, but the transistors have substantially the same thermal history. 
     The timing of the threshold adjustment process is not limited to the above. For example, the process may be performed after a lead frame  193  is connected to the pads  191  by bonding wirings  194  as shown in  FIG. 12B . 
     Here, for example, only the pads  191   a ,  191   b , and  191   c  are not connected to the lead frame  193 , and all the pads need to be connected to the outside are connected to the lead frame  193 . Here, the potentials of the lead frame  193 , the pads  191   a ,  191   b , and  191   c  are 0 V, +1.5 V, +1.2 V, and +1.0 V, for example, which are appropriate potentials for threshold correction, to perform the threshold adjustment process. 
     Furthermore, the threshold adjustment process can be performed in the state shown in  FIG. 12C  where the lead frame  193  is divided into a plurality of leads  195  each of which are connected to the semiconductor chip. Here, all or some of the leads  195  is connected to all or part of the pads  191  by the bonding wirings  194 . Leads  195   a ,  195   b , and  195   c  are connected to the pads  191   a ,  191   b , and  191   c , respectively by the bonding wirings  194 . 
     Here, the potentials of the leads  195   a ,  195   b , and  195   c  are +1.5 V, +1.2 V, and +1.0 V, respectively, and all or some of the other leads  195  are 0 V for example, to perform the threshold adjustment process. 
     Then, the semiconductor chip  190  is packaged. Note that the threshold adjustment process can be performed even after the packaging in consideration of heat resistance of the package material. 
     In the above example, thresholds are corrected by making the charge trap layer  102  trap electrons. Alternatively, thresholds can be corrected by making the charge trap layer  102  trap holes. 
     Embodiment 2 
     In this embodiment, a semiconductor device which can be used for the transistor and the like described in Embodiment 1 will be described with reference to the drawing. Although a transistor in which a threshold correction gate electrode exists between a substrate and a semiconductor layer is described below, a transistor in which a semiconductor layer exists between a threshold correction gate electrode and a substrate may be used. 
       FIGS. 13A to 13C  are a top view and cross-sectional views which illustrate the transistor  450 .  FIG. 13A  is the top view.  FIG. 13B  illustrates a cross section taken along the dashed-dotted line A-B in  FIG. 13A .  FIG. 13C  illustrates a cross section taken along the dashed-dotted line C-D in  FIG. 13A . Note that for simplification of the drawing, some components in the top view in  FIG. 13A  are not illustrated. In some cases, the direction of the dashed-dotted line A-B is referred to as a channel length direction, and the direction of the dashed-dotted line C-D is referred to as a channel width direction. 
     The transistor  450  in  FIGS. 13A to 13C  includes a gate electrode  401  (corresponding to the gate electrode  103  in  FIG. 2C ) embedded in a substrate  400 ; a base insulating layer  402  including a projection and a depression over the substrate  400  and the gate electrode  401 ; an oxide semiconductor layer  404   a  and an oxide semiconductor layer  404   b  over the projection of the base insulating layer  402 ; a source electrode  406   a  and a drain electrode  406   b  over the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   b ; an oxide semiconductor layer  404   c  in contact with a bottom surface of the depression of the base insulating layer  402 , a side surface of the depression (projection) of the base insulating layer  402 , a side surface of the oxide semiconductor layer  404   a , a side surface and a top surface of the oxide semiconductor layer  404   b , the source electrode  406   a , and the drain electrode  406   b ; a gate insulating layer  408  (corresponding to the gate insulating layer  104  in  FIG. 2C ) over the oxide semiconductor layer  404   c ; a gate electrode  410  (corresponding to the gate electrode  105  in  FIG. 2C ) provided over and in contact with the gate insulating layer  408  and facing the top surface and the side surface of the oxide semiconductor layer  404   b , and an oxide insulating layer  412  over the source electrode  406   a , the drain electrode  406   b , and the gate electrode  410 . 
     In addition, the base insulating layer  402  includes a first insulating layer  402   a  (corresponding to the third insulating layer  102   c  in  FIG. 2C ), a second insulating layer  402   b  (corresponding to the second insulating layer  102   b  in  FIG. 2C ), and a third insulating layer  402   c  (corresponding to the first insulating layer  102   a  in  FIG. 2C ) and functions as the charge trap layer described in Embodiment 1. The oxide semiconductor layers  404   a ,  404   b , and  404   c  are collectively referred to as a multilayer semiconductor layer  404 . The multilayer semiconductor layer  404  corresponds to the semiconductor layer  101  in  FIG. 2C . 
     In the case where a material used as the second insulating layer  402   b  has a high relative dielectric constant, the second insulating layer  402   b  can be formed thick. For example, in the case of using hafnium oxide with a relative dielectric constant of 16, the second insulating layer  402   b  can be formed approximately four times as thick as the second insulating layer  402   b  using silicon oxide with a relative dielectric constant of 3.9. The increase in the thickness of the second insulating layer  402   b  is preferable in terms of preventing the leakage of trapped electrons. Note that each thickness of the first insulating layer  402   a  and the third insulating layer  402   c  is more than or equal to 1 nm and less than or equal to 30 nm, preferably more than or equal to 10 nm and less than or equal to 20 nm. The thickness of the second insulating layer  402   b  is more than or equal to 1 nm and less than or equal to 100 nm, preferably more than or equal to 5 nm and less than or equal to 20 nm. 
     Note that the channel length means the distance between a source (a source region, source electrode) and a drain (drain region, drain electrode) in a region where a semiconductor layer overlaps with a gate electrode in the top view. That is, the channel length in  FIG. 13A  is the distance between the source electrode  406   a  and the drain electrode  406   b  in the region where the oxide semiconductor layer  404   b  overlaps with the gate electrode  410 . A channel width means a length of a region where a source faces parallel to a drain and where a semiconductor layer and a gate electrode overlap with each other. That is, in  FIG. 13A , a channel width is a length of a portion where the source electrode  406   a  and the drain electrode  406   b  face each other and where the oxide semiconductor layer  404   b  and the gate electrode  410  overlap with each other. 
     When the base insulating layer  402  functions as an electron trap layer, electrons can be trapped in electron trap states existing at the interface between the third insulating layer  402   c  and the second insulating layer  402   b  or inside the second insulating layer  402   b  as described in Embodiment 1. The number of electrons trapped in the electron trap states can be adjusted by the potential of the gate electrode  401 . 
     The gate electrode  410  electrically covers the oxide semiconductor layer  404   b , increasing the on-state current. This transistor structure is referred to as a surrounded channel (s-channel) structure. In the s-channel structure, a current flows through an entire region of the oxide semiconductor layer  404   b  (bulk). Since a current flows through the oxide semiconductor layer  404   b , an adverse effect of interface scattering is unlikely to occur, leading to a high on-state current. Note that as the oxide semiconductor layer  404   b  is thicker, the on-state current can be increased. 
     In formation of a transistor with a short channel length and a short channel width, when an electrode, a semiconductor layer, or the like is processed at the same time when a resist mask is recessed, the electrode, the semiconductor layer, or the like has a rounded upper end portion (curved surface) in some cases. With this structure, the coverage with the gate insulating layer  408 , the gate electrode  410 , and the oxide insulating layer  412 , which are to be formed over the oxide semiconductor layer  404   b , can be improved. In addition, electric field concentration that might occur at end portions of the source electrode  406   a  and the drain electrode  406   b  can be reduced, which can suppress deterioration of the transistor. 
     By miniaturization of the transistor, a high degree of integration and a high density can be achieved. For example, the channel length of the transistor is less than or equal to 100 nm, preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, and still further preferably less than or equal to 20 nm, and the channel width of the transistor is less than or equal to 100 nm, preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, and still further preferably less than or equal to 20 nm. Even with such a small channel width, the transistor  450  can increase the on-state current by having the s-channel structure. 
     The substrate  400  is not limited to a simple supporting substrate, and may be a substrate where another device such as a transistor is formed. In that case, at least one of the gate electrode  410 , the source electrode  406   a , and the drain electrode  406   b  of the transistor  450  may be electrically connected to the above device. 
     The base insulating layer  402  can have a function of supplying oxygen to the multilayer semiconductor layer  404  as well as a function of preventing diffusion of impurities from the substrate  400 . In the case where the substrate  400  is provided with another device as described above, the base insulating layer  402  also has a function as an interlayer insulating layer. In that case, since the base insulating layer  402  has an uneven surface, the base insulating layer  402  is preferably subjected to planarization treatment such as chemical mechanical polishing (CMP) treatment so as to have a flat surface. 
     The multilayer semiconductor layer  404  in the channel formation region of the transistor  450  has a structure in which the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  are stacked in this order from the substrate  400  side. The oxide semiconductor layer  404   b  is surrounded by the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c . As in  FIG. 13C , the gate electrode  410  electrically covers the oxide semiconductor layer  404   b.    
     Here, for the oxide semiconductor layer  404   b , for example, an oxide semiconductor whose electron affinity (an energy difference between a vacuum level and the conduction band minimum) is higher than those of the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  is used. The electron affinity can be obtained by subtracting an energy difference between the conduction band minimum and the valence band maximum (what is called an energy gap) from an energy difference between the vacuum level and the valence band maximum (what is called an ionization potential). 
     The oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  each contain one or more kinds of metal elements forming the oxide semiconductor layer  404   b . For example, the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  are preferably formed using an oxide semiconductor whose conduction band minimum is closer to a vacuum level than that of the oxide semiconductor layer  404   b  by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. 
     In such a structure, when an electric field is applied to the gate electrode  410 , a channel is formed in the oxide semiconductor layer  404   b  where the conduction band minimum is the lowest in the multilayer semiconductor layer  404 . In other words, the oxide semiconductor layer  404   c  is formed between the oxide semiconductor layer  404   b  and the gate insulating layer  408 , whereby a structure in which the channel of the transistor is provided in a region that is not in contact with the gate insulating layer  408  is obtained. 
     Further, since the oxide semiconductor layer  404   a  contains one or more metal elements contained in the oxide semiconductor layer  404   b , an interface state is unlikely to be formed at the interface between the oxide semiconductor layer  404   b  and the oxide semiconductor layer  404   a , compared with the interface between the oxide semiconductor layer  404   b  and the base insulating layer  402  on the assumption that the oxide semiconductor layer  404   b  is in contact with the base insulating layer  402 . The interface state sometimes forms a channel, leading to a change in the threshold of the transistor. Thus, with the oxide semiconductor layer  404   a , a variation in the electrical characteristics of the transistor, such as threshold, can be reduced. Further, the reliability of the transistor can be improved. 
     Furthermore, since the oxide semiconductor layer  404   c  contains one or more metal elements contained in the oxide semiconductor layer  404   b , scattering of carriers is unlikely to occur at the interface between the oxide semiconductor layer  404   b  and the oxide semiconductor layer  404   c , compared with the interface between the oxide semiconductor layer  404   b  and the gate insulating layer  408  on the assumption that the oxide semiconductor layer  404   b  is in contact with the gate insulating layer  408 . Thus, with the oxide semiconductor layer  404   c , the field-effect mobility of the transistor can be increased. 
     For the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c , for example, a material containing Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that used for the oxide semiconductor layer  404   b  can be used. Specifically, an atomic ratio of any of the above metal elements in the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  is 1.5 times or more, preferably 2 times or more, further preferably 3 times or more as much as that in the oxide semiconductor layer  404   b . Any of the above metal elements is strongly bonded to oxygen and thus has a function of suppressing generation of an oxygen vacancy in the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c . That is, an oxygen vacancy is less likely to be generated in the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  than in the oxide semiconductor layer  404   b.    
     Note that when each of the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  is an In-M-Zn oxide containing at least indium, zinc, and M (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and when the oxide semiconductor layer  404   a  has an atomic ratio of In to M and Zn which is x 1 :y 1 :z 1 , the oxide semiconductor layer  404   b  has an atomic ratio of In to M and Zn which is x 2 :y 2 :z 2 , and the oxide semiconductor layer  404   c  has an atomic ratio of In to M and Zn which is x 3 :y 3 :z 3 , y 1 /x 1  and y 3 /x 3  is each preferably larger than y 2 /x 2 . Y 1 /x 1  and y 3 /x 3  is each 1.5 times or more, preferably 2 times or more, further preferably 3 times or more as large as y 2 /x 2 . At this time, when y 2  is greater than or equal to x 2  in the oxide semiconductor layer  404   b , the transistor can have stable electrical characteristics. However, when y 2  is 3 times or more as large as x 2 , the field-effect mobility of the transistor is reduced; accordingly, y 2  is preferably less than 3 times x 2 . 
     The proportion of In and M atoms In/M in the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  is preferably less than 1 and further preferably less than 0.33. In addition, the proportion of In and M atoms In/M in the oxide semiconductor layer  404   b  is preferably 0.34 or more and further preferably 0.5 or more. 
     The thicknesses of the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  are each greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm. The thickness of the oxide semiconductor layer  404   b  is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm. In addition, the oxide semiconductor layer  404   b  is preferably thicker than the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c.    
     For the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c , an oxide semiconductor containing indium, zinc, and gallium can be used, for example. Note that the oxide semiconductor layer  404   b  preferably contains indium because carrier mobility can be increased. 
     Note that stable electrical characteristics can be effectively imparted to a transistor using an oxide semiconductor layer, by reducing the concentration of impurities in the oxide semiconductor layer to make the oxide semiconductor layer intrinsic or substantially intrinsic. The term “substantially intrinsic” refers to the state where an oxide semiconductor layer has a carrier density lower than 1×10 17 /cm 3 , preferably lower than 1×10 15 /cm 3 , further preferably lower than 1×10 13 /cm .    
     In the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon, and a metal element other than main components of the oxide semiconductor layer are impurities. For example, hydrogen and nitrogen form donor levels to increase the carrier density. In addition, silicon in the oxide semiconductor layer forms an impurity level. The impurity level might become a trap, so that the electrical characteristics of the transistor might deteriorate. Accordingly, in the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  and at interfaces between these layers, the impurity concentration is preferably reduced. 
     In order to make the oxide semiconductor layer intrinsic or substantially intrinsic, in secondary ion mass spectrometry (SIMS), for example, the concentration of silicon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than 1×10 19  atoms/cm 3 , further preferably lower than 5×10 18  atoms/cm 3 , still further preferably lower than 1−10 18  atoms/cm 3 . Further, the concentration of hydrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than or equal to 2×10 20  atoms/cm 3 , further preferably lower than or equal to 5×10 19  atoms/cm 3 , still further preferably lower than or equal to 1×10 19  atoms/cm 3 , yet still further preferably lower than or equal to 5×10 18  atoms/cm 3 . Further, the concentration of nitrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than 5×10 19  atoms/cm 3 , further preferably lower than or equal to 5×10 18  atoms/cm 3 , still further preferably lower than or equal to 1×10 18  atoms/cm 3 , yet still further preferably lower than or equal to 5×10 17  atoms/cm 3 . 
     In the case where the oxide semiconductor layer includes crystals, high concentration of silicon or carbon might reduce the crystallinity of the oxide semiconductor layer. In order not to lower the crystallinity of the oxide semiconductor layer, for example, the concentration of silicon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer may be lower than 1×10 19  atoms/cm 3 , preferably lower than 5×10 18  atoms/cm 3 , further preferably lower than 1×10 18  atoms/cm 3 . Further, the concentration of carbon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer may be lower than 1×10 19  atoms/cm 3 , preferably lower than 5×10 18  atoms/cm 3 , further preferably lower than 1×10 18  atoms/cm 3 , for example. 
     A transistor in which the above-described highly purified oxide semiconductor layer is used for a channel formation region has an extremely low off-state current. In the case where the voltage between a source and a drain is set at about 0.1 V, 5 V, or 10 V, for example, the off-state current standardized on the channel width of the transistor can be as low as several yoctoamperes per micrometer to several zeptoamperes per micrometer. 
     Note that as the gate insulating layer of the transistor, an insulating layer containing silicon is used in many cases; thus, it is preferable that a region of the multilayer semiconductor layer, which serves as a channel, be not in contact with the gate insulating layer for the above-described reason. In the case where a channel is formed at the interface between the gate insulating layer and the multilayer semiconductor layer, scattering of carriers occurs at the interface, whereby the field-effect mobility of the transistor may be reduced. Also from the view of the above, it is preferable that the region of the multilayer semiconductor layer, which serves as a channel, be separated from the gate insulating layer. 
     Accordingly, with the multilayer semiconductor layer  404  having a stacked structure of the oxide semiconductor layers  404   a ,  404   b , and  404   c , a channel can be formed in the oxide semiconductor layer  404   b ; thus, the transistor can have high field-effect mobility and stable electrical characteristics. 
     Next, the band structure of the multilayer semiconductor layer  404  is described. For analyzing the band structure, a stacked film corresponding to the multilayer semiconductor layer  404  is formed. In the stacked film. In—Ga—Zn oxide with an energy gap of 3.5 eV is used for layers corresponding to the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c , and In—Ga—Zn oxide with an energy gap of 3.15 eV is used for a layer corresponding to the oxide semiconductor layer  404   b.    
     The thickness of each of the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  was 10 nm. The energy gap was measured with the use of a spectroscopic ellipsometer (UT-300 manufactured by HORIBA Jobin Yvon). Further, the energy difference between the vacuum level and the valence band maximum was measured using an ultraviolet photoelectron spectroscopy (UPS) device (VersaProbe, ULVAC-PHI, Inc.). 
       FIG. 14A  is part of a schematic band structure showing an energy difference (electron affinity) between the vacuum level and the conduction band minimum of each layer, which is calculated by subtracting the energy gap from the energy difference between the vacuum level and the valence band maximum.  FIG. 14A  is a band diagram showing the case where silicon oxide layers are provided in contact with the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c . Here, Evac represents energy of the vacuum level, EcI1 and EcI2 represent the conduction band minimum of the silicon oxide layer, EcS1 represents the conduction band minimum of the oxide semiconductor layer  404   a . EcS2 represents the conduction band minimum of the oxide semiconductor layer  404   b , and EcS3 represents the conduction band minimum of the oxide semiconductor layer  404   c.    
     As shown in  FIG. 14A , the conduction band minimum continuously varies among the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c . This can be understood also from the fact that the constituent elements are common among the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  and oxygen is easily diffused among the oxide semiconductor layers  404   a  to  404   c . Accordingly, the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  have a continuous physical property although they have different compositions in a stack. 
     The multilayer semiconductor layer  404  in which layers containing the same main components are stacked is formed to have not only a simple stacked-layer structure of the layers but also a continuous energy band (here, in particular, a well structure having a U shape in which the conduction band minimum continuously varies among the layers). In other words, the stacked-layer structure is formed such that there exist no impurities that form a defect level such as a trap center or a recombination center at each interface. If impurities exist between the stacked layers in the multilayer semiconductor layer, the continuity of the energy band is lost and carriers at the interface disappear by a trap or recombination. 
     Note that  FIG. 14A  shows the case where EcS1 and EcS3 are equal to each other; however, EcS1 and EcS3 may be different from each other. For example, part of the band structure in the case where EcS1 is higher than EcS3 is shown in  FIG. 14B . 
     For example, when EcS1 is equal to EcS3, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, 1:3:3, 1:3:4, 1:6:4, or 1:9:6 can be used for the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 or 3:1:2 can be used for the oxide semiconductor layer  404   b . Further, when EcS1 is higher than EcS3, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:6:4 or 1:9:6 can be used for the oxide semiconductor layer  404   a , an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 or 3:1:2 can be used for the oxide semiconductor layer  404   b , and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, 1:3:3, or 1:3:4 can be used for the oxide semiconductor layer  404   c , for example. 
     According to  FIGS. 14A and 14B , the oxide semiconductor layer  404   b  of the multilayer semiconductor layer  404  serves as a well, so that a channel is formed in the oxide semiconductor layer  404   b  in a transistor including the multilayer semiconductor layer  404 . Note that since the conduction band minimum continuously varies, the multilayer semiconductor layer  404  can also be referred to as a U-shaped well. Further, a channel formed to have such a structure can also be referred to as a buried channel. 
     Note that trap levels due to impurities or defects might be formed in the vicinity of the interface between the oxide semiconductor layers  404   a  and  404   c  and an insulating layer such as a silicon oxide layer. The oxide semiconductor layer  404   b  can be distanced away from the trap levels owing to existence of the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c . However, when the energy difference between EcS2 and EcS1 or between EcS2 and EcS3 is small, an electron in the oxide semiconductor layer  404   b  might reach the trap levels by passing over the oxide semiconductor layer  404   a  or the oxide semiconductor layer  404   c . When electrons serving as negative fixed charges are trapped in the trap levels, the threshold of the transistor shifts in the positive direction. 
     Thus, to reduce a variation in the threshold voltage of the transistor, energy differences between EcS2 and each of EcS1 and EcS3 are necessary. Each of the energy differences is preferably greater than or equal to 0.1 eV, further preferably greater than or equal to 0.15 eV. 
     The oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  preferably include crystal parts. In particular, when a crystal in which c-axes are aligned is used, the transistor can have stable electrical characteristics. 
     In the case where an In—Ga—Zn oxide is used for the multilayer semiconductor layer  404 , it is preferable that the oxide semiconductor layer  404   c  contain less In than the oxide semiconductor layer  404   b  so that diffusion of In to the gate insulating layer is prevented. 
     For the source electrode  406   a  and the drain electrode  406   b , a conductive material that can be bonded to oxygen is preferably used. For example, Al, Cr, Cu, Ta, Ti, Mo, or W can be used. Among the materials, in particular, it is preferable to use Ti which is easily bonded to oxygen or to use W with a high melting point, which allows subsequent process temperatures to be relatively high. Note that the conductive material that can be bonded to oxygen includes, in its category, a material to which oxygen is easily diffused. 
     When the conductive material that can be bonded to oxygen is in contact with a multilayer semiconductor layer, a phenomenon occurs in which oxygen in the multilayer semiconductor layer is diffused to the conductive material that can be bonded to oxygen. The phenomenon noticeably occurs when the temperature is high. Since the fabricating process of the transistor involves some heat treatment steps, the above phenomenon causes generation of oxygen vacancies in the vicinity of a region which is in the multilayer semiconductor layer and is in contact with the source electrode or the drain electrode. The oxygen vacancies bond to hydrogen that is slightly contained in the layer, whereby the region is changed to an n-type region. Thus, the n-type region can serve as a source or a drain of the transistor. 
     In the case of forming a transistor with an extremely short channel length, an n-type region which is formed by the generation of oxygen vacancies might extend in the channel length direction of the transistor, causing a short circuit. In that case, the electrical characteristics of the transistor change: for example, the threshold voltage shifts to cause a state in which on and off states of the transistor cannot be controlled with the gate voltage (conduction state). Accordingly, when a transistor with an extremely short channel length is formed, it is not always preferable that a conductive material that can be bonded to oxygen be used for a source electrode and a drain electrode. 
     In such a case, a conductive material which is less likely to be bonded to oxygen than the above material is preferably used for the source electrode  406   a  and the drain electrode  406   b . As the conductive material which is not easily bonded to oxygen, for example, a material containing tantalum nitride, titanium nitride, or ruthenium or the like can be used. Note that in the case where the conductive material is in contact with the oxide semiconductor layer  404   b , the conductive materials may be stacked with the above-described conductive material which is easily bonded to oxygen. 
     The first insulating layer  402   a , the third insulating layer  402   c , and the gate insulating layer  408  can be formed using an insulating layer containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. The second insulating layer  402   b  can be formed using an insulating layer containing one or more of hafnium oxide, aluminum oxide, aluminum silicate, and the like. Note that each thickness of the first insulating layer  402   a  and the third insulating layer  402   c  is more than or equal to 1 nm and less than or equal to 30 nm, preferably more than or equal to 10 nm and less than or equal to 20 nm. The thickness of the second insulating layer  402   b  is more than or equal to 1 nm and less than or equal to 20 nm, preferably more than or equal to 5 nm and less than or equal to 10 nm. 
     For the gate electrodes  401  and  410 , a conductive layer formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used. The gate electrodes  401  and  410  may be a stack including any of the above materials. Alternatively, a conductive layer containing nitrogen may be used for the gate electrodes  401  and  410 . For example, the gate electrodes  401  and  410  can be a stack in which a tungsten layer is formed over a titanium nitride layer, a stack in which a tungsten layer is formed over a tungsten nitride layer, or a stack in which a tungsten layer is formed over a tantalum nitride layer. 
     The oxide insulating layer  412  may be formed over the gate insulating layer  408  and the gate electrode  410 . The oxide insulating layer  412  can be formed using an insulating layer containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, and tantalum oxide. The oxide insulating layer  412  may be a stack including any of the above materials. 
     Here, the oxide insulating layer  412  preferably contains excess oxygen. An oxide insulating layer containing excess oxygen refers to an oxide insulating layer from which oxygen can be released by heat treatment or the like. The oxide insulating layer containing excess oxygen is preferably a layer in which the amount of released oxygen when converted into oxygen atoms is 1.0×10 19  atoms/cm 3  or more in thermal desorption spectroscopy analysis. Note that the temperature of the oxide insulating layer surface in the thermal desorption spectroscopy analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. Oxygen released from the oxide insulating layer can be diffused to the channel formation region in the multilayer semiconductor layer  404  through the gate insulating layer  408 , so that oxygen vacancies formed in the channel formation region can be filled with the oxygen. In this manner, stable electrical characteristics of the transistor can be achieved. 
     High integration of a semiconductor device requires miniaturization of a transistor. However, it is known that miniaturization of a transistor causes deterioration of the electrical characteristics of the transistor. When a channel width is shortened, an on-state current is decreased. 
     However, in the transistor of this embodiment, as described above, the oxide semiconductor layer  404   c  is formed so as to cover a region where a channel is formed in the oxide semiconductor layer  404   b , and a channel formation layer and the gate insulating layer are not in contact with each other. Accordingly, scattering of carriers at the interface between a channel formation layer and the gate insulating layer can be reduced and the on-state current of the transistor can be increased. 
     When the oxide semiconductor layer is formed to be intrinsic or substantially intrinsic, the field-effect mobility might be reduced because of a decrease in the number of carriers contained in the oxide semiconductor layer. However, in the transistor of this embodiment, a gate electric field is applied to the oxide semiconductor layer in the side surface direction in addition to the perpendicular direction. That is, the gate electric field is applied to the whole of the oxide semiconductor layer, whereby current flows in the bulk of the oxide semiconductor layer. Consequently, a change in the electrical characteristics can be suppressed owing to the highly purified intrinsic oxide semiconductor layer and the field-effect mobility of the transistor can be increased. 
     In the transistor of this embodiment, the oxide semiconductor layer  404   b  is formed over the oxide semiconductor layer  404   a , so that an interface state is less likely to be formed. In addition, impurities do not enter the oxide semiconductor layer  404   b  from above and below because the oxide semiconductor layer  404   b  is an intermediate layer in a three-layer structure. With the structure in which the oxide semiconductor layer  404   b  is surrounded by the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  (or the oxide semiconductor layer  404   b  is electrically surrounded by the gate electrode  410 ), on-state current of the transistor is increased as described above, and in addition, threshold voltage can be stabilized and an S value can be reduced. Thus. Icut can be reduced and power consumption can be reduced. Furthermore, the threshold of the transistor becomes stable; thus, long-term reliability of the semiconductor device can be improved. 
     A transistor  470  illustrated in  FIGS. 15A to 15C  can be used.  FIGS. 15A to 15C  are a top view and cross-sectional views which illustrate the transistor  470 .  FIG. 15A  is the top view.  FIG. 15B  illustrates a cross section taken along the dashed-dotted line A-B in  FIG. 15A .  FIG. 15C  illustrates a cross section taken along the dashed-dotted line C-D in  FIG. 15A . Note that for simplification of the drawing, some components are not illustrated in the top view in  FIG. 15A . 
     In the transistor  470 , the base insulating layer  402  is not etched because overetching of a conductive layer to be the source electrode  406   a  and the drain electrode  406   b  does not occur when the source electrode  406   a  and the drain electrode  406   b  are formed. 
     To prevent the base insulating layer  402  from being etched by overetching of the conductive layer, the etching rate of the base insulating layer  402  is preferably set (sufficiently) lower than the etching rate of the conductive layer. 
     In this embodiment, although the oxide semiconductor layer  404   b  is sandwiched between the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c , without limited to this structure, one embodiment of the present invention may have a structure in which only the oxide semiconductor layer  404   b  is provided without the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   c  and is electrically covered with the gate electrode. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 3 
     In this embodiment, a method for forming the transistor  450 , which is described in Embodiment 2 with reference to  FIGS. 13A to 13C , is described with reference to  FIGS. 16A to 16D  and  FIGS. 17A to 17C . 
     To form the gate electrode  401 , a plurality of linear grooves is formed on the substrate  400 , a conductive layer is deposited using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or an alloy material containing any of these as a main component, and is planarized and etched (see  FIG. 16A ). The conductive layer can be formed by sputtering, CVD, or the like. 
     For the substrate  400 , a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon-on-insulator (SOI) substrate, or the like may be used. Any of these substrates further provided with a semiconductor element thereover may be used. 
     Furthermore, the base insulating layer  402  composed of the first to third insulating layers  402   a  to  402   c  is formed (see  FIG. 16B ). 
     Oxygen may be added to the base insulating layer  402  by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. Adding oxygen enables the base insulating layer  402  to supply oxygen much easily to the multilayer semiconductor layer  404 . 
     Next, the oxide semiconductor layers  404   a  and  404   b  are formed over the base insulating layer  402  by sputtering, CVD (including MOCVD. ALD, or PECVD), vacuum evaporation, or pulse laser deposition (PLD) (see  FIG. 16C ). At this time, as illustrated, the base insulating layer  402  can be slightly over-etched. By over-etching of the base insulating layer  402 , the gate electrode  410  to be formed later can cover the oxide semiconductor layer  404   c  easily. 
     For processing the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   b  into island shapes, a layer to be a hard mask (e.g., a tungsten layer) and a resist mask are provided over the oxide semiconductor layer  404   b , and the layer to be a hard mask is etched to form a hard mask. The resist mask is removed and then the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   b  are etched using the hard mask as a mask. Then, the resist mask is removed. At this step, the hard mask is gradually reduced as the etching progresses; as a result, the edges of the hard mask is rounded to have a curved surface. Accordingly, the edges of the oxide semiconductor layer  404   b  is rounded to have a curved surface. This structure improves the coverage with the oxide semiconductor layer  404   c , the gate insulating layer  408 , the gate electrode  410 , and the oxide insulating layer  412 , which are to be formed over the oxide semiconductor layer  404   b , and can prevent shape defects such as disconnection. In addition, electric field concentration which might occur at end portions of the source electrode  406   a  and the drain electrode  406   b  can be reduced, which can reduce deterioration of the transistor. 
     In order to form continuous junction in a stacked-layer including the oxide semiconductor layers  404   a  and  404   b , or a stacked-layer including the oxide semiconductor layers  404   a ,  404   b , and  404   c  to be formed in a later step, the layers need to be formed successively without exposure to the air with the use of a multi-chamber deposition apparatus (e.g., a sputtering apparatus) including a load lock chamber. It is preferable that each chamber of the sputtering apparatus be able to be evacuated to a high vacuum (to about 5×10 −7  Pa to 1×10 −4  Pa) by an adsorption vacuum pump such as a cryopump and that the chamber be able to heat a substrate to 100° C. or higher, preferably 500° C. or higher so that water and the like acting as impurities of the oxide semiconductor can be removed as much as possible. Alternatively, a combination of a turbo molecular pump and a cold trap is preferably used to prevent back-flow of a gas containing a carbon component, moisture, or the like from an exhaust system into the chamber. 
     Not only high vacuum evacuation in a chamber but also increasing the purity of a sputtering gas is necessary to obtain a high-purity intrinsic oxide semiconductor. As an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor layer can be prevented as much as possible. 
     The materials described in Embodiment 2 can be used for the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  that is to be formed in a later step. For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:4 or 1:3:2 can be used for the oxide semiconductor layer  404   a , an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 can be used for the oxide semiconductor layer  404   b , and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:4 or 1:3:2 can be used for the oxide semiconductor layer  404   c.    
     An oxide semiconductor that can be used for each of the oxide semiconductor layers  404   a ,  404   b , and  404   c  preferably contains at least indium (In) or zinc (Zn). Both In and Zn are preferably contained. Furthermore, in order to reduce variations in electrical characteristics of the transistors including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and Zn. 
     As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), zirconium (Zr), and the like are used. As another stabilizer, lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given. 
     As the oxide semiconductor, for example, any of the following can be used: indium oxide, tin oxide, zinc oxide. In—Zn oxide, Sn—Zn oxide. Al—Zn oxide, Zn—Mg oxide, Sn—Mg oxide, In—Mg oxide, In—Ga oxide, In—Ga—Zn oxide, In—Al—Zn oxide, In—Sn—Zn oxide, Sn—Ga—Zn oxide, Al—Ga—Zn oxide, Sn—Al—Zn oxide, In—Hf—Zn oxide, In—La—Zn oxide, In—Ce—Zn oxide, In—Pr—Zn oxide, In—Nd—Zn oxide. In—Sm—Zn oxide, In—Eu—Zn oxide, In—Gd—Zn oxide, In—Tb—Zn oxide, In—Dy—Zn oxide, In—Ho—Zn oxide, In—Er—Zn oxide, In—Tm—Zn oxide, In—Yb—Zn oxide, In—Lu—Zn oxide, In—Sn—Ga—Zn oxide, In—Hf—Ga—Zn oxide, In—Al—Ga—Zn oxide, In—Sn—Al—Zn oxide, In—Sn—Hf—Zn oxide, or In—Hf—Al—Zn oxide. 
     For example, “In—Ga—Zn oxide” means an oxide containing In, Ga, and Zn as its main components. The In—Ga—Zn oxide may contain another metal element in addition to In, Ga, and Zn. Note that in this specification, a layer containing the In—Ga—Zn oxide is also referred to as an IGZO layer. 
     Alternatively, a material represented by InMO 3 (ZnO) m  (m&gt;0 is satisfied, and m is not an integer) may be used. Note that M represents one or more metal elements selected from Ga, Fe, Mn, or Co. A material represented by In 2 SnO 5 (ZnO) n  (n&gt;0, n is an integer) may be used. 
     Note that as described in Embodiment 2 in detail, materials are selected so that the oxide semiconductor layers  404   a  and  404   c  each have an electron affinity lower than that of the oxide semiconductor layer  404   b.    
     Note that the oxide semiconductor layer is preferably formed by a sputtering method. As a sputtering method, an RF sputtering method, a DC sputtering method, an AC sputtering method, or the like can be used. In particular, a DC sputtering method is preferably used because dust generated in the film formation can be reduced and the film thickness can be uniform. 
     When In—Ga—Zn oxide is used for the oxide semiconductor layers  404   a ,  404   b , and  404   c , a material whose atomic ratio of In to Ga and Zn is any of 1:1:1, 2:2:1, 3:1:2, 1:3:2, 1:3:4, 1:4:3, 1:5:4, 1:6:6, 2:1:3 1:6:4, 1:9:6, 1:1:4, and 1:1:2 is used so that the oxide semiconductor layers  404   a  and  404   c  each have an electron affinity lower than that of the oxide semiconductor layer  404   b.    
     Note that the expression “the composition of an oxide including In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c==1), is in the neighborhood of the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b, and c satisfy the following relation: (a−A) 2 +(b−B) 2 +(c−C) 2 ≦r 2 , and r may be 0.05, for example. The same applies to other oxides. 
     The indium content in the oxide semiconductor layer  404   b  is preferably higher than those in the oxide semiconductor layers  404   a  and  404   c . In an oxide semiconductor, the s orbital of heavy metal mainly contributes to carrier transfer, and when the proportion of In in the oxide semiconductor is increased, overlap of the s orbitals is likely to be increased. Therefore, an oxide having a composition in which the proportion of In is higher than that of Ga has higher mobility than an oxide having a composition in which the proportion of In is equal to or lower than that of Ga. Thus, with use of an oxide having a high indium content for the oxide semiconductor layer  404   b , a transistor having high mobility can be achieved. 
     Here, a structure of an oxide semiconductor layer will be described. 
     In this specification, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 100, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 50. The term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 800 and less than or equal to 1000, and accordingly includes the case where the angle is greater than or equal to 850 and less than or equal to 95°. 
     In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system. 
     An oxide semiconductor layer is classified roughly into a non-single-crystal oxide semiconductor layer and a single crystal oxide semiconductor layer. The non-single-crystal oxide semiconductor layer includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) layer, a polycrystalline oxide semiconductor layer, a microcrystalline oxide semiconductor layer, an amorphous oxide semiconductor layer, and the like. 
     First of all, a CAAC-OS layer is described. 
     The CAAC-OS layer is an oxide semiconductor layer including a plurality of crystal parts. Most of the crystal parts each fit inside a cube whose one side is less than 100 nm. Thus, the CAAC-OS layer may include a crystal part that fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. 
     In a transmission electron microscope (TEM) image of the CAAC-OS layer, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS layer, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     In the TEM image of the CAAC-OS layer observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflecting a surface over which the CAAC-OS layer is formed (hereinafter, a surface over which the CAAC-OS layer is formed is referred to as a formation surface) or a top surface of the CAAC-OS layer, and is arranged to be parallel to the formation surface or the top surface of the CAAC-OS layer. 
     In the TEM image of the CAAC-OS layer observed in a direction substantially perpendicular to the sample surface (planar TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity in arrangement of metal atoms between different crystal parts. 
     From the cross-sectional TEM image and the planar TEM image, orientation characteristics are found in the crystal parts in the CAAC-OS layer. 
     A CAAC-OS layer is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS layer including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS layer. 
     When the CAAC-OS layer is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. Analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of the sample surface as an axis (φ axis) with 2θ fixed at around 56°. When the sample is a single-crystal oxide semiconductor layer of InGaZnO 4 , six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. In contrast, when the sample is the CAAC-OS layer, a peak is not clearly observed. 
     The above results mean that in the CAAC-OS layer having c-axis alignment, the directions of a-axes and b-axes are different between crystal parts, but the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS layer or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is oriented in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, when the shape of the CAAC-OS layer is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS layer. 
     Furthermore, the degree of crystallinity in the CAAC-OS layer is not necessarily uniform. For example, if crystal growth leading to the CAAC-OS layer occurs from the vicinity of the top surface of the layer, the degree of the crystallinity in the vicinity of the top surface may be higher than that in the vicinity of the formation surface. Moreover, when an impurity is added to the CAAC-OS layer, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS layer varies depending on regions. 
     Note that when the CAAC-OS layer with an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak of 2θ may be observed at around 36, in addition to the peak of 2θ at around 310. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS layer. It is preferable that a peak of 2θ appear at around 31° and a peak of 2θ not appear at around 360. 
     The CAAC-OS layer is an oxide semiconductor layer having low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor layer, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor layer, such as silicon, disturbs the atomic arrangement of the oxide semiconductor layer by depriving the oxide semiconductor layer of oxygen and causes a decrease in crystallinity. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor layer and causes a decrease in crystallinity if contained in the oxide semiconductor layer. Note that the impurity contained in the oxide semiconductor layer might serve as a carrier trap or a carrier generation source. 
     The CAAC-OS layer is an oxide semiconductor layer having a low density of defect states. Oxygen vacancies in the oxide semiconductor layer may serve as carrier traps or carrier generation sources when hydrogen is captured therein. 
     The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor layer has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor including the oxide semiconductor layer rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor layer has few carrier traps. Accordingly, the transistor including the oxide semiconductor layer has little variation in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor layer takes a long time to be released, and thus may behave like fixed electric charge. Accordingly, the transistor which includes the oxide semiconductor layer having high impurity concentration and a high density of defect states can have unstable electrical characteristics. 
     In a transistor using the CAAC-OS layer, change in electrical characteristics due to irradiation with visible light or ultraviolet light is small. 
     Next, a microcrystalline oxide semiconductor layer will be described. 
     In a TEM image, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor layer in some cases. In most cases, the crystal part size in the microcrystalline oxide semiconductor is more than or equal to 1 nm and less than or equal to 100 nm, or more than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size more than or equal to 1 nm and less than or equal to 10 nm, or a size more than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor layer including nanocrystal is referred to as an nc-OS (nanocrystalline oxide semiconductor) layer. In a TEM image of the nc-OS layer, for example, a crystal grain boundary is not clearly found in some cases. 
     In the nc-OS layer, a microscopic region (for example, a region with a size more than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size more than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. There is no regularity of crystal orientation between different crystal parts in the nc-OS layer. Thus, the orientation of the whole layer is not observed. Accordingly, the nc-OS layer sometimes cannot be distinguished from an amorphous oxide semiconductor layer depending on an analysis method. For example, when the nc-OS layer is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Furthermore, a halo pattern is shown in an electron diffraction pattern (also referred to as a selected-area electron diffraction pattern) of the nc-OS layer obtained by using an electron beam having a probe diameter (e.g., more than or equal to 50 nm) larger than the diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS layer obtained by using an electron beam having a probe diameter (e.g., more than or equal to 1 nm and smaller than or equal to 30 nm) close to, or smaller than or equal to a diameter of a crystal part. In a nanobeam electron diffraction pattern of the nc-OS layer, regions with high luminance in a circular (ring) pattern may be shown, and a plurality of spots may be shown in the ring-like region. 
     The nc-OS layer is an oxide semiconductor layer that has high regularity as compared with an amorphous oxide semiconductor layer. For this reason, the nc-OS layer has a lower density of defect states than an amorphous oxide semiconductor layer. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS layer; hence, the nc-OS layer has a higher density of defect states than the CAAC-OS layer. 
     Note that an oxide semiconductor layer may be a stacked layer including two or more layers of an amorphous oxide semiconductor layer, a microcrystalline oxide semiconductor layer, and a CAAC-OS layer, for example. 
     For example, the CAAC-OS layer can be deposited by a sputtering method using a polycrystalline oxide semiconductor sputtering target. When ions collide with the sputtering target, a crystal region included in the sputtering target is sometimes separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) flakes off from the sputtering target. The flat-plate-like sputtered particle or pellet-like sputtered particle is electrically charged and thus reaches the substrate while maintaining its crystal state, without being aggregation in plasma, forming a CAAC-OS layer. 
     First heat treatment may be performed after the oxide semiconductor layer  404   b  is formed. The first heat treatment may be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., typically higher than or equal to 300° C. and lower than or equal to 500° C., in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, or a reduced pressure state. Alternatively, the first heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, in order to compensate desorbed oxygen. By the first heat treatment, the crystallinity of the oxide semiconductor layer  404   b  can be improved, and in addition, impurities such as hydrogen and water can be removed from the base insulating layer  402  and the oxide semiconductor layer  404   a . Note that the first heat treatment may be performed before etching for formation of the oxide semiconductor layer  404   b.    
     A first conductive layer to be the source electrode  406   a  and the drain electrode  406   b  is formed over the oxide semiconductor layers  404   a  and  404   b . For the first conductive layer, Al, Cr, Cu, Ta, Ti, Mo, W, or an alloy material containing any of these as a main component can be used. For example, a 100-nm-thick titanium layer is formed by a sputtering method or the like. Alternatively, a tungsten layer is formed by a CVD method or the like. 
     Then, the first conductive layer is etched so as to be divided over the oxide semiconductor layer  404   b  to form the source electrode  406   a  and the drain electrode  406   b  (see  FIG. 16D ). 
     Next, the oxide semiconductor layer  403   c  is formed over the oxide semiconductor layer  404   b , the source electrode  406   a , and the drain electrode  406   b.    
     Note that second heat treatment may be performed after the oxide semiconductor layer  403   c  is formed. The second heat treatment can be performed in a condition similar to that of the first heat treatment. The second heat treatment can remove impurities such as hydrogen and water from the oxide semiconductor layer  403   c . In addition, impurities such as hydrogen and water can be further removed from the oxide semiconductor layer  404   a  and  404   b.    
     Next, an insulating layer  407  to be the gate insulating layer  408  is formed over the oxide semiconductor layer  403   c  (see  FIG. 17A ). The insulating layer  407  can be formed by sputtering, CVD (including MOCVD, ALD, or PECVD), vacuum evaporation, PLD. 
     Then, a second conductive layer  409  to be the gate electrode  410  is formed over the insulating layer  407  (see  FIG. 17B ). For the second conductive layer  409 , Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or an alloy material containing any of these as its main component can be used. The second conductive layer  409  can be formed by a sputtering method, a CVD method, or the like. A stack including a conductive layer containing any of the above materials and a conductive layer containing nitrogen, or a conductive layer containing nitrogen may be used for the second conductive layer  409 . 
     After that, the second conductive layer  409  is selectively etched using a resist mask to form the gate electrode  410  (see  FIG. 17C ). Note that as shown in  FIG. 13C , the oxide semiconductor layer  404   b  is electrically surrounded by the gate electrode  410 . 
     Then, the insulating layer  407  is selectively etched using the resist mask or the gate electrode  410  as a mask to form the gate insulating layer  408 . 
     Subsequently, the oxide semiconductor layer  403   c  is etched using the resist mask or the gate electrode  410  as a mask to form the oxide semiconductor layer  404   c.    
     The top edge of the oxide semiconductor layer  404   c  is aligned with the bottom edge of the gate insulating layer  408 . The top edge of the gate insulating layer  408  is aligned with the bottom edge of the gate electrode  410 . Although the gate insulating layer  408  and the oxide semiconductor layer  404   c  are formed using the gate electrode  410  as a mask, the gate insulating layer  408  and the oxide semiconductor layer  404   c  may be formed before the second conductive layer  409  is formed. 
     Next, the oxide insulating layer  412  is formed over the source electrode  406   a , the drain electrode  406   b , and the gate electrode  410  (see  FIGS. 13B and 13C ). A material and a method for the oxide insulating layer  412  can be similar to those for the first insulating layer  402   a . The oxide insulating layer  412  may be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or an oxide insulating layer containing nitrogen. The oxide insulating layer  412  can be formed by sputtering, CVD (including MOCVD, ALD, or PECVD), vacuum evaporation, PLD. The oxide insulating layer  412  preferably contains excessive oxygen so as to be able to supply oxygen to the multilayer semiconductor layer  404 . 
     Next, third heat treatment may be performed. The third heat treatment can be performed under a condition similar to that of the first heat treatment. By the third heat treatment, excess oxygen is easily released from the base insulating layer  402 , the gate insulating layer  408 , and the oxide insulating layer  412 , so that oxygen vacancies in the multilayer semiconductor layer  404  can be reduced. 
     Next, fourth heat treatment is performed. In the fourth heat treatment, the potential of the gate electrode  401  is kept higher than that of the source or drain electrode at a high temperature higher than or equal to 125° C. and lower than or equal to 450° C., for example higher than or equal to 150° C. and lower than or equal to 300° C. for one second or longer, for example, one minute or longer. As a result, the needed number of electrons moves from the multilayer semiconductor layer  404  toward the gate electrode  401  and some of them are trapped by the electron trap states existing inside the second insulating layer  402   b  or at the interface with another insulating layer. By controlling the number of trapped electrons, the degree of the threshold increase can be controlled. 
     Through the above process, the transistor  450  illustrated in  FIGS. 13A to 13C  can be fabricated. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 4 
     In this embodiment, a transistor having a planar structure that can be used as the transistor described in Embodiment 1 will be described. 
       FIGS. 18A to 18C  are a top view and cross-sectional views illustrating a transistor of one embodiment disclosed in this specification.  FIG. 18B  illustrates a cross section taken along the dashed-dotted line A-B in  FIG. 18A .  FIG. 18C  illustrates a cross section taken along the dashed-dotted line C-D in  FIG. 18A . Note that for simplification of the drawing, some components in the top view in  FIG. 18A  are not illustrated. In some cases, the direction of the dashed-dotted line A-B is referred to as a channel length direction, and the direction of the dashed-dotted line C-D is referred to as a channel width direction. 
     A transistor  460  illustrated in  FIGS. 18A to 18C  includes the base insulating layer  402  over the substrate  400 ; the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   b  over the base insulating layer  402 ; the source electrode  406   a  and the drain electrode  406   b  over the oxide semiconductor layer  404   a  and the oxide semiconductor layer  404   b ; the oxide semiconductor layer  404   c  in contact with the oxide semiconductor layer  404   b , the source electrode  406   a , and the drain electrode  406   b ; the gate insulating layer  408  over the oxide semiconductor layer  404   c ; the gate electrode  410  (corresponding to the gate electrode  103  in  FIG. 1B ) over the gate insulating layer  408 ; and the oxide insulating layer  412  over the source electrode  406   a , the drain electrode  406   b , and the gate electrode  410 . The gate insulating layer  408  functions as the charge trap layer described in Embodiment 1, and thus includes a first insulating layer  408   a  (corresponding to the first insulating layer  102   a  in  FIG. 1B ) and a second insulating layer  408   b  (corresponding to the second insulating layer  102   b  in  FIG. 1B ). In addition, the multilayer semiconductor layer  404  includes the oxide semiconductor layers  404   a ,  404   b , and  404   c  and corresponds to the semiconductor layer  101  in  FIG. 1B . 
     The transistor  450  in Embodiment 2 is different from the transistor  460  in this embodiment in that each of the channel length and the channel width is more than or equal to twice, typically ten times as large as the thickness of the multilayer semiconductor layer  404 . 
     Note that the channel length means the distance between a source (a source region, source electrode) and a drain (drain region, drain electrode) in a region where a semiconductor layer overlaps with a gate electrode in the top view. That is, the channel length in  FIG. 18A  is the distance between the source electrode  406   a  and the drain electrode  406   b  in the region where the oxide semiconductor layer  404   b  overlaps with the gate electrode  410 . A channel width means a length of a portion where a source and a drain face and is parallel to each other and where a semiconductor layer and a gate electrode overlap with each other. In  FIG. 18A , a channel width is thus a length of a portion where the source electrode  406   a  and the drain electrode  406   b  face each other and where the oxide semiconductor layer  404   b  and the gate electrode  410  overlap with each other. 
     Although this embodiment has a structure in which the oxide semiconductor layer  404   b  is sandwiched between the oxide semiconductor layers  404   a  and  404   c , without limitation to this structure, a structure including only the oxide semiconductor layer  404   b  and not including the oxide semiconductor layers  404   a  and  404   c  is possible. Alternatively, a structure in which any one or two of the oxide semiconductor layer  404   a , the oxide semiconductor layer  404   b , and the oxide semiconductor layer  404   c  is/are provided is possible. 
     This embodiment can be combined with any of the other embodiments in this specification as appropriate. 
     Embodiment 5 
     The above-described semiconductor device can be used for display devices, laptops, or image reproducing devices provided with recording media (typically devices which reproduce the content of recording media such as DVDs (digital versatile disc) and have displays for displaying the reproduced images). Other than the above, as an electronic apparatuses which can use the semiconductor device according to an embodiment of the present invention, mobile phones, portable game machines, portable information terminals, e-book readers, video cameras, digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), vending machines, and the like can be given. Specific examples of such electronic apparatuses are shown in  FIGS. 19A to 19F . 
       FIG. 19A  illustrates a portable game machine including a housing  501 , a housing  502 , a display portion  503 , a display portion  504 , a microphone  505 , a speaker  506 , an operation key  507 , a stylus  508 , and the like. Although the portable game machine in  FIG. 19A  has the two display portions  503  and  504 , the number of display portions included in a portable game machine is not limited to this. 
       FIG. 19B  illustrates a portable data terminal including a first housing  511 , a second housing  512 , a first display portion  513 , a second display portion  514 , a joint  515 , an operation key  516 , and the like. The first display portion  513  is provided in the first housing  511 , and the second display portion  514  is provided in the second housing  512 . The first housing  511  and the second housing  512  are connected to each other with the joint  515 , and the angle between the first housing  511  and the second housing  512  can be changed with the joint  515 . An image on the first display portion  513  may be switched depending on the angle between the first housing  511  and the second housing  512  at the joint  515 . A display device with a position input function may be used as at least one of the first display portion  513  and the second display portion  514 . Note that the position input function can be added by provision of a touch panel in a display device. Alternatively, the position input function can be added by provision of a photoelectric conversion element called a photosensor in a pixel area of a display device. 
       FIG. 19C  illustrates a laptop personal computer, which includes a housing  521 , a display portion  522 , a keyboard  523 , a pointing device  524 , and the like. 
       FIG. 19D  illustrates the electric refrigerator-freezer including a housing  531 , a door for a refrigerator  532 , a door for a freezer  533 , and the like. 
       FIG. 19E  illustrates a video camera, which includes a first housing  541 , a second housing  542 , a display portion  543 , operation keys  544 , a lens  545 , a joint  546 , and the like. The operation keys  544  and the lens  545  are provided for the first housing  541 , and the display portion  543  is provided for the second housing  542 . The first housing  541  and the second housing  542  are connected to each other with the joint  546 , and the angle between the first housing  541  and the second housing  542  can be changed with the joint  546 . Images displayed on the display portion  543  may be switched in accordance with the angle at the joint  546  between the first housing  541  and the second housing  542 . 
       FIG. 19F  illustrates a passenger car including a car body  551 , wheels  552 , a dashboard  553 , lights  554 , and the like. 
     This embodiment can be combined with any of the other embodiments disclosed in this specification as appropriate. 
     This application is based on Japanese Patent Application serial no. 2013-189222 filed with Japan Patent Office on Sep. 12, 2013, the entire contents of which are hereby incorporated by reference.