Abstract:
To provide a power MISFET using oxide semiconductor. A gate electrode, a source electrode, and a drain electrode are formed so as to interpose a semiconductor layer therebetween, and a region of the semiconductor layer where the gate electrode and the drain electrode do not overlap with each other is provided between the gate electrode and the drain electrode. The length of the region is from 0.5 μm to 5 μm. In such a power MISFET, a power source of 100 V or higher and a load are connected in series between the drain electrode and the source electrode, and a control signal is input to the gate electrode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    One embodiment of the present invention relates to a field-effect transistor (FET) using semiconductor, in particular, to a power-insulated-gate field-effect transistor (hereinafter, referred to as a power MISFET). 
         [0003]    2. Description of the Related Art 
         [0004]    A field effect transistor (FET) is a device in which regions called a source and a drain are provided in a semiconductor, each of the regions is connected to an electrode (a source electrode or a drain electrode), and a voltage is applied to the semiconductor via a gate electrode through an insulating film or a Schottky barrier so that the state of the semiconductor is controlled, whereby current flowing between the source electrode and the drain electrode is controlled. As a semiconductor used, a Group 14 element such as silicon or germanium, a compound such as gallium arsenide, indium phosphide, gallium nitride, zinc sulfide, or cadmium telluride, or the like can be used. 
         [0005]    In recent years, FETs in which an oxide such as zinc oxide or an indium gallium zinc oxide-based oxide (also referred to as In—Ga—Zn based oxide, IGZO) is used as a semiconductor have been reported (Patent Document 1 and Patent Document 2). FETs using such oxide semiconductor have relatively high mobility, and also materials of oxide semiconductors have a large band gap which is 3 eV or higher. 
       REFERENCE 
       [0000]    
       
         [Patent Document 1] United States Published Patent Application No. 2005/0199879 
         [Patent Document 2] United States Published Patent Application No. 2007/0194379 
       
     
       SUMMARY OF THE INVENTION 
       [0008]    An object of one embodiment of the present invention is to provide a power MISFET using an oxide semiconductor. Power MISFETs using silicon semiconductor have been already in practical use. However, when a withstand voltage is 60 V or lower, an on-state resistance is about 0.1Ω, whereas when a withstand voltage becomes higher, an on-state resistance is 1Ω or higher. For example, when an on-state resistance is 1Ω and on/off switching is performed with a current of 10 A, power of as much as 100 W is consumed, which is not practical. For such a usage of a high withstand voltage, an insulated gate bipolar transistor is used, however, its high frequency characteristics are inferior unfortunately. 
         [0009]    The inventor has focused on a withstand voltage of an oxide semiconductor having a band gap of 3 eV or higher. Thus, an object of one embodiment of the present invention to provide a highly-efficient power MISFET whose on-state resistance is 0.8Ω or lower, preferably 0.2Ω or lower even at a withstand voltage of 300 V, with use of such an oxide semiconductor. 
         [0010]    One embodiment of the present invention is a power MISFET including a gate insulating film interposed between a gate electrode and a semiconductor layer formed of an oxide semiconductor, a source electrode and a drain electrode formed in contact with the semiconductor layer, wherein the gate electrode has a region overlapping with the source electrode, not with the drain electrode, and the width of a region of the semiconductor layer which does not overlap with the drain electrode and the gate electrode is 0.5 μm to 5 μm. 
         [0011]    One embodiment of the present invention is a power MISFET including a gate insulating film interposed between a gate electrode and a semiconductor layer formed of an oxide semiconductor, a source electrode and a drain electrode formed in contact with the semiconductor layer, wherein the thickness of the semiconductor layer is uneven and the thickness of a portion in contact with the drain electrode is 0.5 μm to 5 μm. 
         [0012]    Between a drain electrode and a source electrode of such a power MISFET, a power of 100 V or higher and a load are connected in series, and a control signal is input to a gate electrode. Note that in any of the embodiments described above, the semiconductor layer may be interposed between the gate electrode and the back gate electrode. In addition, the gate electrode may be a region of a p-type single crystal silicon substrate or a region which is turned into a p-type by doping (p-type region), formed over an n-type single crystal silicon substrate. In addition, the gate electrode, the back gate electrode, the source electrode or the drain electrode may be formed using a metal or a conductive oxide. 
         [0013]    Further, the concentration of carriers originated from donor or acceptor in the semiconductor layer may be 1×10 12  cm −3  or lower, preferably 1×10 11  cm −3  or lower. Note that when a semiconductor is in contact with a conductor, carriers are injected from the conductor or absorbed into the conductor; therefore, it is difficult to know the original concentration of carriers, which is described below. Accordingly, actually, it is difficult to know the concentration of carries originated from donor or acceptor in a semiconductor layer of an MISFET. In this case, it is possible to know whether or not the carrier concentration is 1×10 12  cm −3  or lower in such a manner that a point of a semiconductor layer formed in the same manner as the semiconductor layer used in the MISFET, which is 10 μm or more away, preferably 100 μm or more away from a conductor is measured. 
         [0014]    As for the above description, the number of oxygen vacancies and the hydrogen concentration in a semiconductor layer are preferably as small as possible. This is because oxygen vacancies or mixed hydrogen become sources of carriers. Further, inclusion of hydrogen causes unstable operation of an MISFET. The hydrogen concentration is preferably 1×10 18  cm −3  or lower. 
         [0015]    In addition, the work function of the source electrode or the drain electrode is preferably lower than the sum of the electron affinity of the semiconductor layer and 0.3 eV (i.e., electron affinity +0.3 eV). Alternatively, the contact between the semiconductor layer and the source electrode or the drain electrode is preferably an ohmic contact. In addition, the work function of the drain electrode is preferably lower than that of the source electrode. 
         [0016]    Moreover, the work function of the gate electrode or the back gate electrode is preferably higher than that of the source electrode or the drain electrode by 0.3 eV or higher. Alternatively, the work function of the gate electrode or the back gate electrode is preferably higher than the sum of the electron affinity of the semiconductor layer and 0.6 eV (i.e., electron affinity +0.6 eV). 
         [0017]    Before description of the present invention, a conventional power MISFET is described.  FIG. 10A  is a diagram illustrating a principle of a conventional power MISFET using single crystal silicon. In other words, a source  502   a  and a drain  502   b  which are formed by diffusion of an n-type impurity are formed on a p-type single crystal silicon substrate  501 , and are provided with a source electrode  505   a  and a drain electrode  505   b  respectively. In addition, over the substrate, a gate electrode  504  and an insulator  506  are provided. 
         [0018]    These structural elements are similar to those of normal MISFETs; however, in addition to these elements, in the power MISFET, a drift region  503  is provided between the drain  502   b  and a channel region. The drift region  503  is provided in order to absorb a high voltage applied between the drain  502   b  and the gate electrode  504  in the MISFET, when the MISFET is turned off. 
         [0019]    In other words, when high voltage is applied, the drift region is depleted to be an insulator, so that an electric field of the withstand voltage or lower of silicon is applied and thus the MISFET is not broken. The width of the drift region needs 10 μm in an MISFET where the breakdown field strength of silicon is 0.3 MV/cm, and the withstand voltage is 300 V. 
         [0020]    On the other hand, when an MISFET is turned on, the drift region should have conductivity and thus should have an n-type conductivity; however, when the concentration of donor is too high, these regions cannot be depleted sufficiently. The concentration of donor is preferably 4×10 15  cm −3 . 
         [0021]    In addition, the resistivity of single crystal silicon in which the donor concentration is 4×10 15  cm −3  is 1 Ωcm or more. As illustrated in  FIG. 10A , when the drift region  503  is formed in a shallow portion of one surface of the substrate  501 , the resistance becomes high, while as illustrated in  FIG. 10B , when the drift region  503  is formed in the substrate itself and thus the cross-sectional area where current flows is made large, the resistance can be lowered. Even in that case, the resistance of the drift region is 1 Ωcm or more in order to ensure the withstand voltage of 300V. 
         [0022]    Further, the power MISFET needs many doping steps. In other words, an n-type impurity is added to a rear surface of a single crystal silicon substrate  501  having a weak n-type conductivity, so that the drain  502   b  is formed. Further, after the gate electrode  504  is formed, a p-type impurity is added from the front surface to form a p-type region  507  and an n-type impurity is added from the front surface to form the source  502   a . The impurity concentration of the drift region  503  is equal to that of the substrate. 
         [0023]    On the other hand, in an oxide semiconductor having a band gap of 3 eV or more, because the breakdown field strength is 3 MV/cm or more, the width corresponding to the drift region may be 1 μM. However, although in silicon semiconductor, the conductivity in an on-state can be ensured by diffusion of a slight amount of donors into a drift region, such a technique is not established for a general oxide semiconductor. 
         [0024]    It has been known that in an oxide semiconductor, hydrogen serves as a donor, and oxygen vacancy also becomes a factor of a donor. However, the inventor knows that hydrogen existing in an oxide semiconductor causes a big problem in reliability. On the other hand, a technique with which the concentrations of oxygen vacancies or other donor impurities can be controlled with accuracy has not found yet. 
         [0025]    Therefore, it is very difficult that a technique relating to silicon semiconductor is applied to an oxide semiconductor. In this point, the inventor has studied the operation of an MISFET using an oxide semiconductor from the basics, and thus found out that the following structure can give an objective withstand voltage to an MISFET, and allow sufficient current to flow on an on-state. 
         [0026]      FIG. 1A  illustrates an example of the structure. The power MISFET includes, for example, an i-type semiconductor layer  101  including an oxide semiconductor, a gate electrode  102   a , and a gate insulating film  104   a  interposed therebetween, a source electrode  103   a , and a drain electrode  103   b . Similarly to silicon semiconductors, a region corresponding to a drift region is provided between a channel region and the drain electrode  103   b . The length X of the region may be 1 μm long enough to ensure the withstand voltage of 300 V. In general, the length may be within the range of 0.5 μm to 5 μm. 
         [0027]    As the length X is longer, the withstand voltage becomes higher. However, when the X is too long, the electron concentration from the drain electrode  103   b  to the semiconductor layer  101  (in particular, to the thick semiconductor layer  101   b ) in an on state is insufficient, and thus the on-state resistance is high, and thereby a sufficient function of a transistor cannot obtained unfortunately. Accordingly, it is not preferred that the X is longer than 5 μm in general. 
         [0028]    However, if a problem is not caused when the on-state resistance is high (for example, when a current applied is enough small, and the like), a transistor can function well in some cases even when the X is longer than 5 μm. In general, the on-state resistance is preferably a tenth or lower of a load. 
         [0029]      FIG. 1B  illustrates a power MISFET which is further developed. In this FET, a portion of the semiconductor layer  101  in contact with the drain electrode  103   b  (semiconductor layer  101   b ) is formed thicker than a portion in contact with the source electrode  103   a  and a channel portion of a semiconductor layer  101   a , and the portion is a portion corresponding to the drift region. At this time, the thickness (the length X) of the semiconductor layer  101   b  may be 0.5 μm to 5 μm. 
         [0030]    In other words, the direction of flowing current is from right to left in  FIG. 1A , while the direction of flowing current in a region from the drain electrode  103   b  and the channel is from above to below in  FIG. 1B . In  FIG. 1A , the cross-sectional area of the portion where current flows is a product of the thickness of the semiconductor layer  101  and the width (length perpendicular to paper) of the semiconductor layer  101 , while in  FIG. 1B , it is a product of the width d 2  of the drain electrode and the width of the semiconductor layer  101  (or semiconductor layer  101   b ). 
         [0031]    In  FIG. 1A , the thickness of the portion corresponding to the drift region is limited by the thickness of the semiconductor layer  101 . The thickness of the portion corresponding to the drift region is the width d 2  of the drain electrode  103   b  in  FIG. 1B . As compared to the thickness of the semiconductor layer  101 , the width d 2  of the drain electrode  103   b  can be made large easily, and thus the resistance of the portion corresponding to the drift region is lowered. For example, the d 2  may be from 2 μm to 10 μm. 
         [0032]    If the d 2  is larger than 10 μm, the resistance itself per unit channel width of the region corresponding to the drift region is lowered; however, the channel width of the MISFET cannot be made large due to the restriction by a circuit layout, and as a result, it is difficult to lower the whole on-state resistance. 
         [0033]    In addition, when the thickness of the semiconductor layer  101  illustrated in  FIG. 1A  is large, the resistance between the source electrode  103   a  and the channel is increased, and thus the on-state resistance is increased. Thus, the thickness of the semiconductor layer  101  in  FIG. 1A  is preferably from 10 nm to 100 nm. For the same reason, the thickness of the semiconductor layer  101   a  in  FIG. 1B  is preferably from 10 nm to 100 nm. 
         [0034]    Moreover, in an FET not using a PN junction, when the thickness of a semiconductor layer in a channel portion is larger than a channel length, off-state leakage current is large. This effect relates to not only the thickness of the semiconductor layer but also an effective thickness of a gate insulating film (thickness×relative permittivity of the semiconductor layer/relative permittivity of the gate insulating film). 
         [0035]    When the sum of the thickness of the semiconductor layer in the channel portion and the effective thickness of the gate insulating film is one third or more of the channel length, in particular, in an FET having a channel length of 1 μm or lower, the threshold is extremely lowered and subthreshold characteristics are also worse. In other words, the off-state current between the source electrode and the drain electrode is increased. As described later, when a high voltage is applied to the source electrode and the drain electrode in such a state, a gate insulating film is subjected to the high voltage, so that the FET is broken. 
         [0036]    Although the details are omitted, the sum of the thickness of the semiconductor layer in the channel portion and the effective thickness of the gate insulating film may be one tenth or less, preferably one twentieth or less of the channel length. 
         [0037]    The power MISFETs illustrated in  FIG. 1A  and  FIG. 1B  each include the gate electrode  102   a  and further the back gate electrode  102   b  with the back gate insulating film  104   b  interposed therebetween, on the opposite side. The back gate electrode  102   b  may be supplied with a potential in synchronization with the gate electrode  102   a , or may be supplied with a constant potential constantly. In particular, in an off state, a potential lower than the potential of the source electrode  103   a  is supplied to the back gate electrode  102   b , which can effectively increase the withstand voltage. 
         [0038]      FIGS. 1A and 1B  are cross-sectional views of the power MISFETs, and examples of layouts when the power MISFET in  FIG. 1B  is seen from above are illustrated in  FIG. 2  to  FIG. 4 . In each of these examples, the channel width of the MISFET can be increased with use of a structure in which the source electrode  103   a  and the drain electrode  103   b  are provided in an interlocked manner. When the back gate electrode  102   b  is provided, it is formed between the source electrode  103   a  and the drain electrode  103   b.    
         [0039]    At this time, the interval d 1  between the back gate electrode  102   b  and the drain electrode  103   b  illustrated in  FIG. 1B  should be a value taken in consideration of the withstand voltage. In consideration of a breakdown voltage of an insulator between the back gate electrode  102   b  and the drain electrode  103   b , the d 1  is preferably 0.3 μm or more. On the other hand, the large interval d 1  is disadvantageous for integration, and thus the interval d 1  is preferably 2 μm or lower. 
         [0040]    Note that in order to lower the wiring resistance, the areas or thicknesses of the source electrode  103   a  and the drain electrode  103   b  can be made as large as possible. Additionally, a material with a high thermal conductance is preferably used also in view of a heat-dissipation property. In addition, as a material of the substrate, a metal or a semiconductor with a high heat-dissipation property is preferably used. 
         [0041]    The differences among  FIG. 2  to  FIG. 4  are how the semiconductor layer  101  overlaps with the source electrode  103   a , the back gate electrode  102   b , and the drain electrode  103   b . In the example of  FIG. 2 , the total of the portions of the semiconductor layer  101 , which does not overlap with the back gate electrode  102   b , the source electrode  103   a , and the drain electrode  103   b  are three or more rectangles (in this case, fourteen rectangles). On the other hand, in  FIG. 3 , the total of the portions are three or more substantially-U-shapes (in this case, seven substantially-U-shapes), and in  FIG. 4 , the portions are two complicated shapes. 
         [0042]    The reason why the resistance of the power MISFETs with such structures is sufficiently low is described. As for an oxide semiconductor, in particular, in an oxide semiconductor including zinc or indium, there are almost no reports on oxide semiconductors which have p-type conductivity. Accordingly, an FET using a PN junction like an FET using silicon has not been reported, and a conductor-semiconductor junction as disclosed in Patent Document 1 and Patent Document 2, where a conductor electrode is in contact with an n-type oxide semiconductor, has been used for forming a source or a drain. 
         [0043]    According to the knowledge of the inventor, when an i-type oxide semiconductor in which the concentration of carriers originated from donor is lowered by decreasing donors (in this specification, a semiconductor the carrier concentration of which is 10 12  cm −3  or lower is also referred to as an i-type oxide semiconductor) is used for an MISFET, the MISFET can have high reliability, a large on/off ratio, and a small subthreshold value. And operation of such an MISFET using an oxide semiconductor having a low donor concentration like this is studied below. 
         [0044]    In an MISFET where a source and a drain are formed using a conductor-semiconductor junction, when the carrier concentration in a semiconductor used is high, current (off-state current) flows between the source and the drain even in an off state. Therefore, an i-type semiconductor is obtained by lowering the carrier concentration in the semiconductor, so that the off-state current can be reduced. 
         [0045]    In a conductor-semiconductor junction, in general, an ohmic junction or a Schottky barrier junction is formed depending on the relation between a work function of a conductor and an electron affinity (or a Fermi level) of a semiconductor. For example, if an ideal conductor-semiconductor junction (i.e., no chemical reactions or no carrier traps at the junction interface) is formed by making a conductor with a work function of 3.9 eV in contact with a semiconductor with an electron affinity of 4.3 eV, electrons flow from the conductor into a region which is in the semiconductor and has a certain width. 
         [0046]    In that case, a region closer to a junction interface between the conductor and the semiconductor has a higher electron concentration, and the electron concentrations are 10 20  cm −3  at several nanometers from the interface of the conductor-semiconductor junction, 10 18  cm −3  at several tens of nanometers from the interface, 10 16  cm −3  at several hundreds of nanometers from the interface, and 10 14  cm −3  even at several micrometers from the interface according to rough calculation. That is, even when the semiconductor itself is an i-type semiconductor, a contact with a conductor produces a region with a high carrier concentration. Formation of such a region including many electrons in the vicinity of the interface of the conductor-semiconductor junction can make the conductor-semiconductor junction an ohmic junction. 
         [0047]    In contrast, for example, if an ideal conductor-semiconductor junction is formed by making a conductor with a work function of 4.9 eV in contact with a semiconductor with an electron affinity of 4.3 eV, electrons existing in a region which is in the semiconductor and has a certain width move to the conductor. In the region which the electrons have left, the electron concentration is, as is obvious, extremely low. The width of the region of the semiconductor from which electrons move depends on the electron concentration of the semiconductor; for example, when an original electron concentration of the semiconductor is 10 18  cm −3 , the width is about several tens of nanometers. 
         [0048]    The electron concentration in this portion becomes significantly low; accordingly, a barrier is formed at a junction interface between the conductor and the semiconductor in a band diagram. A conductor-semiconductor junction including such a barrier is referred to as a Schottky barrier junction. Electrons easily flow from the semiconductor to the conductor, whereas electrons are less likely to flow from the conductor to the semiconductor owing to the barrier. Therefore, rectification action is observed in the Schottky barrier junction. 
         [0049]    A similar phenomenon occurs even when a conductor is not in direct contact with a semiconductor. For example, even when an insulating film is provided between a semiconductor and a conductor, the electron concentration of the semiconductor is influenced by the conductor. Needless to say, the degree of the influence of the conductor depends on the thickness or the dielectric constant of the insulating film. When the thickness of the insulating film is increased or when the dielectric constant thereof is lowered, the influence of the conductor is reduced. 
         [0050]    Since it is preferred that a junction between a source electrode and a semiconductor or between a drain electrode and the semiconductor is formed so that current flows easily, a conductive material of the source electrode or the drain electrode is selected so that an ohmic junction is formed. For example, titanium and titanium nitride are given. When a junction between an electrode and a semiconductor is an ohmic junction, there are advantages of stable characteristics of an MISFET to be fabricated and of high percentage of non-defective products. 
         [0051]    As a material of a gate electrode, a material having action that eliminates electrons from a semiconductor is selected. For example, tungsten and platinum are given. Alternatively, a conductive oxide such as molybdenum oxide may be used. Some of conductive oxides have work functions of 5 eV or more. Because the conductivity of such materials are possibly inferior, they are preferably used in combination with a material with high conductivity in a stacked-layer. Alternatively, conductive nitride such as indium nitride or zinc nitride may be used. 
         [0052]    As described above, electrons intrude into the semiconductor layer, due to the contact with the conductor; however, for example, as in the power MISFET illustrated in  FIG. 1A , when the width X of the region corresponding to the drift region is 1 μm and there is no potential difference between the source electrode and the drain electrode, the electron concentration of the portion is estimated to about 1×10 16  cm −3 . This value is equal to or higher than the donor concentration of the drift region  503  illustrated in  FIGS. 10A and 10B . Needless to say, in an off state, the region having such a thickness is easily depleted, and a voltage applied between the drain electrode and the gate electrode is absorbed in this depleted portion. 
         [0053]      FIGS. 9A to 9C  illustrate schematically electron states of the power MISFET illustrated in  FIG. 1B .  FIG. 9A  illustrates an electron state of the semiconductor layer  101  where the potentials of the gate electrode  102   a , the back gate electrode  102   b , the source electrode  103   a , and the drain electrode  103   b  are equal. Electrons flow into the semiconductor layer  101  from the source electrode  103   a  and the drain electrode  103   b , and regions  110   a  and  110   b  with high electron concentrations are formed in the vicinity of the source electrode  103   a  and the drain electrode  103   b.    
         [0054]    In addition, when materials having high work functions are used for the gate electrode  102   a  and the back gate electrode  102   b , a function of removing electrons is obtained so that the electron concentrations are extremely small in the vicinity of the gate electrode  102   a  and the back gate electrode  102   b . The semiconductor layer  101  is thin in the vicinity of the source electrode  103   a , and is close to the gate electrode  102   a  and the back gate electrode  102   b , and thus the region  110   a  with high electron concentration is narrow. 
         [0055]    On the other hand, in the vicinity of the drain electrode  103   b , the semiconductor layer  101  is thick and is apart from the gate electrode  102   a  and the back gate electrode  102   b , and thus the region  110   b  with a high electron concentration is wider. In order to reduce the on-state resistance, preferably, the region  110   b  with a high electron concentration in the vicinity of the drain electrode  103   b  is large and the electron concentration of the region is high. For that, a material having a low work function is preferably used as the material of the drain electrode  103   b.    
         [0056]    On the other hand, the region  110   a  with a high electron concentration in the vicinity of the source electrode  103   a  is excessively expanded, and the high electron concentration is a factor of decreasing the withstand voltage in an off state, in particular. Therefore, as the material of the source electrode  103   a , a material having a work function higher than that of a material of the drain electrode  103   b  is preferably used. 
         [0057]    As illustrated in  FIG. 9B , when a load R is connected in series between the source electrode  103   a  and the drain electrode  103   b , and a high positive voltage (in the range of from 100 V to a withstand voltage) is applied between the source electrode  103   a  and the load R, electrons in the vicinity of the drain electrode  103   b  are absorbed by the drain electrode  103   b  and the region  110   b  with high electron concentration vanishes. On the other hand, the region  110   a  with high electron concentration in the vicinity of the source electrode  103   a  is pulled by a high positive potential of the drain electrode  103   b , and is tried unsuccessfully to expand to the drain electrode  103   b  side due to the hindrance of the gate electrode  102   a  and the back gate electrode  102   b , and thus the size of the regions  110   a  is hardly changed. 
         [0058]    Therefore, almost no current flows between the source electrode  103   a  and the drain electrode  103   b , and most of voltage applied to the circuit is absorbed between the drain electrode  103   b  and the gate electrode  102   a . If the semiconductor layer  101   b  and the gate insulating film  104   a  in this portion can withstand the voltage, the circuit and the MISFET are not broken. Even if the gate insulating film  104   a  is 100 nm thick or less, as long as the thickness of the semiconductor layer  101   b  is set such that it can withstand the voltage, the circuit and the MISFET are not broken. 
         [0059]    If the work functions of the gate electrodes  102   a  and  102   b  are not sufficiently high, the region  110   a  with high electron concentration in the vicinity of the source electrode  103   a  expands to the drain electrode  103   b  side, and some electrons moves into the drain electrode  103   b  from the source electrode  103   a . In that case, a high voltage is applied to the gate insulating film  104   a  and the element is broken unfortunately. Accordingly, the work functions of the gate electrode  102   a  and the back gate electrode  102   b  are important. 
         [0060]    In particular, the back gate electrode  102   b  is effective for prevention of electron movement from the source electrode  103   a  into the drain electrode  103   b . The gate electrode  102   a  preferably overlaps with the source electrode; however, the back gate electrode  102   b  is not needed to overlap with the source electrode. Note that when the potential of the back gate electrode  102   b  is in synchronization with the potential of the gate electrode  102   a , the on-state resistance is reduced when the back gate electrode  102   b  overlaps with the source electrode  103   a.    
         [0061]    In addition, in silicon semiconductor, a similar effect is obtained by a reverse PN junction between the source  502   a  and the channel region. In an oxide semiconductor, a PN junction cannot be employed and materials having high work functions are preferably used for the gate electrode  102   a  and the back gate electrode  102   b . Alternatively, in an off state, the potential(s) of either the gate electrode  102   a  or the back gate electrode  102   b , or the both is/are preferably lower than the source electrode  103   a  by one volt or higher. 
         [0062]    Next, as illustrated in  FIG. 9C , a positive potential is supplied to the gate electrode  102   a  (in some cases, also to back gate electrode  102   b ), so that the MISFET is turned on. In  FIG. 9C , the same potential as the gate electrode  102   a  is also supplied to the back gate electrode  102   b . As a result, a channel is formed in the semiconductor layer  101 , and thus the electron concentration of the whole semiconductor layer  101  is increased. 
         [0063]    It is noticeable that the region with high electron concentration expands to the vicinity of the drain electrode  103   b . This is because the resistance between the source electrode  103   a  and the drain electrode  103   b  is decreased, and the voltage therebetween is also dropped, so that electrons move into the semiconductor layer  101  (in particular, semiconductor layer  101   b ) from the drain electrode  103   b . With such a region with high electron concentration, the on-state resistance is decreased. 
         [0064]    As apparent from the above consideration, the withstand voltage of the MISFET illustrated in each of  FIGS. 1A and 1B  is sufficient. Additionally, the width of the region corresponding to the drift region is one tenth of a silicon semiconductor, so that the on-state resistance can be decreased. In addition, although there is a concern that the field effect mobility of the oxide semiconductor is as small as about one hundredth of a silicon semiconductor, on-state resistance of a power MISFET using a conventional silicon semiconductor mostly corresponds to resistance of a portion corresponding to a drift region, and there is little influence. Needless to say, if the field effect mobility is high, there is no adverse effect. 
         [0065]    In an estimation, in the MISFET illustrated in  FIG. 1B , when the channel length is 1 μm, the thickness of the gate insulating film is 25 nm (silicon oxide equivalent thickness), the field effect mobility is 10 cm 2 /Vs, and the threshold is +1V, the on-state resistance (when the gate voltage is 10 V and the drain voltage is 3 V) of the MISFET is 0.1Ω per meter of the channel width. On the other hand, when the width X of the drift region is 1 μm in order to withstand a voltage of 300 V and the width d 2  of the drain electrode  103   b  is 3 μm, the on-state resistance of the portion is 0.5Ω per meter of the channel width. Accordingly, the power MISFET with a channel width of 5 m has an on-state resistance of 0.12Ω. As for an MISFET having the structure illustrated in  FIG. 1B , the MISFET with a channel width of 5 m can be formed in a 5-mm square chip 
         [0066]    In the description of the above consideration, the semiconductor layer  101  (including the semiconductor layer  101   a  and the semiconductor layer  101   b ) is i-type. In order to obtain sufficient off-state resistance, the channel portion of the semiconductor layer  101  in the MISFET should be i-type; however, the region corresponding to the drift region is not necessarily i-type, and may include carriers originated from donor (or acceptor) that is a maximum or lower of the carrier concentration which is determined depending on a withstand voltage as needed. 
         [0067]    For example, when the thickness of the semiconductor layer  101   b  in  FIG. 1B  is 1 μm and the withstand voltage is set to 300 V, carriers originated from donor may be contained at a concentration of 1×10 17  cm −3  or lower. As described in this embodiment, because the semiconductor layer  101   b  and the semiconductor layer  101   a  are framed in different steps, the semiconductor layer  101   a  can have an i-type conductivity and the semiconductor layer  101   b  can have a weak n-type conductivity. 
         [0068]    An oxide semiconductor to be used preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. As a stabilizer for reducing variations in electrical characteristics of an MISFET using the oxide semiconductor, gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer. 
         [0069]    As another stabilizer, one or plural kinds of 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) may be contained. 
         [0070]    As the oxide semiconductor, for example, an indium oxide, a tin oxide, a zinc oxide, a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg—based oxide, a Sn—Mg—based oxide, an In—Mg—based oxide, or an In—Ga—based oxide, a three-component metal oxide such as an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn—based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn—based oxide, an In—La—Zn—based oxide, an In—Ce—Zn—based oxide, an In—Pr—Zn—based oxide, an In—Nd—Zn—based oxide, an In—Sm—Zn—based oxide, an In—Eu—Zn—based oxide, an In—Gd—Zn—based oxide, an In—Tb—Zn—based oxide, an In—Dy—Zn—based oxide, an In—Ho—Zn—based oxide, an In—Er—Zn—based oxide, an In—Tm—Zn—based oxide, an In—Yb—Zn—based oxide, or an In—Lu—Zn—based oxide, a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn—based oxide, an In—Al—Ga—Zn—based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn—based oxide, or an In—Hf—Al—Zn-based oxide can be used. 
         [0071]    Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main component, in which there is no particular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn. 
         [0072]    Alternatively, a material represented by InMO 3 (ZnO) m  (m&gt;0 is satisfied, and m is not an integer) may be used as an oxide semiconductor. Note that M represents one or more metal elements selected from the group of Ga, Fe, Mn, and Co. Alternatively, as the oxide semiconductor, a material expressed by a chemical formula, In 2 SnO 5 (ZnO) n  (n&gt;0, n is a natural number) may be used. 
         [0073]    For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or an oxide with an atomic ratio close to the above atomic ratios can be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide with an atomic ratio close to the above atomic ratios may be used. 
         [0074]    However, the composition is not limited to those described above, and a material having an appropriate composition may be used depending on necessary semiconductor characteristics (e.g., mobility, threshold voltage, and variation). In order to obtain necessary semiconductor characteristics, it is preferable that the carrier concentration, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like be set to be appropriate. 
         [0075]    For example, with the In—Sn—Zn-based oxide, a high mobility can be relatively easily obtained. Even when an In—Ga—Zn-based oxide is used, the mobility can be increased by a reduction in bulk defect density. 
         [0076]    Note that for example, 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 including 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. 
         [0077]    The oxide semiconductor may be either single crystal or non-single-crystal. In the latter case, the oxide semiconductor may be either amorphous or polycrystal. Further, the oxide semiconductor may have either an amorphous structure including a portion having crystallinity or a non-amorphous structure. 
         [0078]    In an oxide semiconductor in an amorphous state, a flat surface can be obtained relatively easily, so that when an MISFET is fabricated with the use of the oxide semiconductor, interface scattering can be reduced, and relatively high mobility can be obtained relatively easily. 
         [0079]    In an oxide semiconductor having crystallinity, defects in the bulk can be further reduced and when a surface flatness is improved, mobility higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (Ra) of 1 nm or less, preferably 0.3 nm or less, more preferably 0.1 nm or less. 
         [0080]    Note that Ra is obtained by expanding centerline average roughness, which is defined by JIS B 0601, into three dimensions so as to be applicable to a surface. Moreover, Ra can be expressed as average value of the absolute values of deviations from a reference surface to a specific surface. Ra can be measured using an atomic force microscope (AFM). 
         [0081]    As apparent from the above description, the power MISFET of one embodiment of the present invention has sufficient withstand voltage and low on-state resistance. In particular, the power MISFET of one embodiment of the present invention is different from a well-known power MISFET using silicon semiconductor, and there is no need to form a low concentration of donor region. The fabricating process can be thus simplified. 
         [0082]    Note that the work function of a conductor may be a value determined by an interface with a semiconductor in a simplest assumption; however, a complex physical property such as generation of a compound of the semiconductor and the conductor due to chemical reaction or a trap of electric charge or another element is often observed at the interface in reality. 
         [0083]    In the case where a first conductor layer with an extremely small thickness of several nanometers or less and a second conductor layer with a relatively large thickness are stacked over a semiconductor layer in this order, for example, the influence of the work function of the first conductor layer is considerably reduced. The same is applied to the gate electrode. Therefore, in application of the present invention, the design may be performed so that the work function or the like of each material at a position that is 5 nm away from an interface with the semiconductor layer satisfies favorable conditions in the present invention. 
         [0084]    The present invention is particularly effective for a semiconductor material in which substantially only one of an electron and a hole can be used as a carrier. In other words, a favorable result can be obtained, for example, in the case where the mobility of one of the electron and the hole is 1 cm 2 /Vs or higher whereas the mobility of the other is 0.01 cm 2 /Vs or lower, the other does not exist as a carrier, or the effective mass of one of the electron and the hole is 100 times or more as large as that of the other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0085]    In the accompanying drawings: 
           [0086]      FIGS. 1A and 1B  each illustrate an example of a power MISFET according to one embodiment of the present invention; 
           [0087]      FIG. 2  illustrates an example of a power MISFET according to one embodiment of the present invention; 
           [0088]      FIG. 3  illustrates an example of a power MISFET according to one embodiment of the present invention; 
           [0089]      FIG. 4  illustrates an example of a power MISFET according to one embodiment of the present invention; 
           [0090]      FIGS. 5A to 5D  illustrate a fabricating process of a power MISFET according to one embodiment of the present invention; 
           [0091]      FIGS. 6A to 6E  illustrate a fabricating process of a power MISFET according to one embodiment of the present invention; 
           [0092]      FIGS. 7A to 7D  illustrate a fabricating process of a power MISFET according to one embodiment of the present invention; 
           [0093]      FIG. 8  is a circuit diagram of a power MISFET according to one embodiment of the present invention; 
           [0094]      FIGS. 9A to 9C  each illustrate an electron state of a power MISFET according to one embodiment of the present invention; and 
           [0095]      FIGS. 10A and 10B  illustrate examples and operation of a conventional power MISFET. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0096]    Hereinafter, embodiments will be described with reference to drawings. However, the embodiments can be implemented with different modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention is not interpreted as being limited to the description of the embodiments below. Note that in structures described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description thereof is not repeated. 
       Embodiment 1 
       [0097]    In this embodiment, a fabricating process of the power MISFET illustrated in  FIG. 1B  is described with reference to  FIGS. 5A to 5D . As illustrated in  FIG. 5A , a thermal-oxide film  203  is formed on a surface of an n-type single crystal silicon substrate  201 . The thermal-oxide film  203  serves as a gate insulating film. The thickness may be from 20 nm to 100 nm. After that, a p-type impurity is diffused into an n-type single crystal silicon substrate  201  so that a p-type region  202  is formed. The impurity concentration of the p-type region is 2×10 20  cm −3  to 5×10 21  cm −3  and the semiconductor may be a degenerate p-type semiconductor. Note that before the thermal-oxide film  203  is formed, the p-type region  202  may be formed. 
         [0098]    Further, an oxide semiconductor film with a thickness of 10 nm to 50 nm is formed. A variety kinds of oxide semiconductors can be used for the oxide semiconductor film. For example, the oxide semiconductor film may be formed by a sputtering method with use of an oxide ceramic target including indium and zinc at equal ratios. The ratio between indium and zinc is not limited to the equal ratios, and may be set as appropriate by a practitioner depending on the purpose. Furthermore, the oxide semiconductor film is selectively etched so that a first semiconductor layer  204  is formed. 
         [0099]    Next, by a sputtering method, a plasma CVD, or the like, a back gate insulating film  205  is formed to have a thickness of 20 nm to 100 nm with use of a material of silicon oxide, silicon oxynitride, aluminum oxide, aluminum nitride, hafnium oxide, lanthanum oxide, yttrium oxide, or the like. In a portion of a region where the back gate insulating film  205  and the first semiconductor layer  204  overlap with each other, a first opening portion  206  is formed (see  FIG. 5B ). 
         [0100]    Note that appropriate heat treatment is preferably performed either after formation of the first semiconductor layer  204  or after formation of the back gate insulating film  205  or at both of the timings. This heat treatment is for reducing the hydrogen concentration or oxygen vacancies in the first semiconductor layer  204 , and if possible, the heat treatment is preferably performed right after formation of the first semiconductor layer  204 . 
         [0101]    Then, by a sputtering method, an oxide semiconductor film with a thickness of 0.5 μm to 5 μm is formed. The thickness may be determined in consideration of a withstand voltage of the power MISFET to be fabricated. In addition, in this embodiment, the composition of the oxide semiconductor film formed at this time is the same as that of the first semiconductor layer  204 . 
         [0102]    The oxide semiconductor film is selectively etched so that a second semiconductor layer  207  is formed. In the etching, the back gate insulating film  205  serves as an etching stopper, so that the first semiconductor layer  204  can be prevented from being etched. In addition, the back gate insulating film  205  and/or the thermal-oxide film  203  are/is selectively etched, so that a second opening portion  208  and a third opening portion  209  are formed. 
         [0103]    After that, by a sputtering method or the like, a film of a material having a high work function such as platinum, molybdenum oxide, indium nitride, zinc nitride, or the like is formed to have a thickness of 10 nm to 50 nm, and is selectively etched so that a film  210  of a material with a high work function is formed (see  FIG. 5C ). 
         [0104]    Moreover, then, by a sputtering method or the like, a film of a material with a low work function such as titanium, titanium nitride, or the like is formed to a thickness of 30 nm to 300 nm, then an aluminum film with a thickness of 300 nm to 10 μm is stacked thereover, and the stacked films are selectively are etched so that a source electrode  211   a , a back gate electrode  211   b , a drain electrode  211   c , and a connection electrode  211   d  are formed (see  FIG. 5D ). 
         [0105]    Through the above steps, the power MISFET is fabricated. In this embodiment, the p-type region  202  which is a degenerate p-type silicon with a work function of 5.2 eV is used as the gate electrode, in particular, the insulation between the source electrode  211   a  and the drain electrode  211   c  can be enhanced at the time of application of high voltage in an off state. 
       Embodiment 2 
       [0106]    In this embodiment, another method for fabricating the power MISFET will be described with reference to  FIGS. 6A to 6E . First, as illustrated in  FIG. 6A , a p-type impurity is diffused into a portion that is in a depth of 100 μm or more from a surface of an n-type single crystal silicon substrate  301 , so that a p-type region  302  is formed. The impurity concentration of the p-type region is 2×10 20  cm −3  to 5×10 21  cm −3  and the p-type region  302  may be a degenerate p-type semiconductor. The p-type region serves as a gate electrode of the MISFET. 
         [0107]    Note that the step is unnecessary if a p-type single crystal substrate doped with a high concentration of a p-type impurity is used instead of the n-type single crystal silicon substrate  301 . 
         [0108]    Next, a thermal-oxide film  303  is formed over a surface. A thermal-oxide film  303  serves as a gate insulating film. The thickness may be from 20 nm to 100 nm. Then, by the method described in Embodiment 1, an oxide semiconductor film which includes indium and zinc and has a thickness of 10 nm to 50 nm is formed. Further, a silicon nitride film with a thickness of 5 nm to 20 nm is formed thereover. 
         [0109]    The film formation is preferably performed without the surface of the oxide semiconductor film exposed to air. In other words, a structure in which film formation devices of the oxide semiconductor film and the silicon nitride film are connected to each other, and the silicon nitride film is formed following the step of forming the oxide semiconductor film without the substrate exposed to air, or a structure in which the oxide semiconductor film and the silicon nitride film are formed in the same film-formation chamber is preferable. In this manner, it is possible to prevent water or moisture from being absorbed into the oxide semiconductor film when the surface of the oxide semiconductor film is exposed to air. 
         [0110]    Then, these films are selectively etched so that a first semiconductor layer  304  and a barrier layer  305  are formed (see  FIG. 6A ). 
         [0111]    Next, with use of the barrier layer  305  as a mask, the thermal-oxide film  303  and the p-type region  302  are selectively etched so that shallow trenches  306   a  and  306   b  with a depth of 1 μm to 50 μm are formed (see  FIG. 6B ). After this etching, the etched surface may be thermally oxidized. 
         [0112]    Next, by a known plasma CVD method or the like which provides a favorable step coverage, an insulator  307  such as silicon oxide, silicon oxynitride, or the like is formed (see  FIG. 6C ). 
         [0113]    Then, the insulator  307  is polished by a surface planarization technique such as a chemical mechanical polishing (CMP) method. This step may be performed with the barrier layer  305  used as a stopper. After this step is finished, the barrier layer  305  is removed by a dry etching method or the like. In this manner, embedded insulators  307   a  and  307   b  are formed (see  FIG. 6D ). 
         [0114]    Then, by a sputtering method, an oxide semiconductor film which includes zinc oxide as a main component and has a thickness of 0.5 μm to 5 μm is formed. The thickness may be determined in consideration of a withstand voltage of the power MISFET to be fabricated. The oxide semiconductor film is selectively etched, so that a second semiconductor layer  308  is formed. By employing a method capable of selectively etching only zinc oxide, the first semiconductor layer  304  is hardly etched, so that the second semiconductor layer  308  can be formed. 
         [0115]    In addition, because the first semiconductor layer  304  and the second semiconductor layer  308  are formed using different materials, the properties thereof may be different. For example, the first semiconductor layer  304  may have an i-type conductivity, and the second semiconductor layer  308  may have a weak n-type conductivity (the concentration of carriers originated from donor is from 1×10 16  cm −3  to 1×10 17  cm −3 ). 
         [0116]    Next, in the embedded insulator  307   a , a contact hole reaching the p-type region  302  is formed. Then, by a sputtering method or the like, a film of a material with a low work function such as titanium, titanium nitride, or the like, and an aluminum film are stacked as a multilayer, and are selectively etched, so that a connection electrode  310   a , a source electrode  310   b , and a drain electrode  310   c  are formed. Note that in  FIG. 6E , a wiring extending from the drain electrode  310   c  is illustrated over the embedded insulator  307   b.    
         [0117]    As in Embodiment 1, the back gate insulating film  309  is formed, a contact hole is formed in a wiring portion extending from the drain electrode  310   c  over the embedded insulator  307   b , and then, by a sputtering method or the like, a film of a material having a high work function such as platinum, molybdenum oxide, indium nitride, zinc nitride, or the like and having a thickness of 10 nm to 50 nm and an aluminum film are stacked as a multilayer film, and are selectively etched, so that a back gate electrode  311   a  and a connection electrode  311   b  are formed (see  FIG. 6E ). Through the above steps, the power MISFET is fabricated. 
       Embodiment 3 
       [0118]    In this embodiment, another fabricating method of the power MISFET is described with reference to  FIGS. 7A to 7D  and  FIG. 8 . First, as illustrated in  FIG. 7A , a thermal-oxide film  403  is formed on a surface of n-type single crystal silicon substrate  401 . The thermal-oxide film  403  functions as a gate insulating film. The thickness may be from 20 nm to 100 nm. 
         [0119]    After that, a p-type impurity is diffused into an n-type single crystal silicon substrate  401  so that a p-type region  402  is formed. The impurity concentration of the p-type region is 2×10 20  cm −3  to 5×10 21  cm −3  and the semiconductor may be a degenerate p-type semiconductor. Note that before the thermal-oxide film  403  is formed, the p-type region  402  may be formed. 
         [0120]    Then, by a sputtering method, an oxide semiconductor film which includes indium and zinc and has a thickness of 0.5 μm to 5 μm is formed. The thickness may be determined in consideration of a withstand voltage of the power MISFET to be fabricated. In addition, the oxide semiconductor film is selectively etched so that a second semiconductor layer  404  is formed (see  FIG. 7A ). 
         [0121]    Further, an oxide semiconductor film which includes indium and zinc and has a thickness of 10 nm to 50 nm is formed. The oxide semiconductor film is selectively etched so that first semiconductor layers  405   a  and  405   b  are formed (see  FIG. 7B ). 
         [0122]    Then, by a sputtering method or the like, a film of a material with a low work function such as titanium, titanium nitride, or the like, and with a thickness of from 30 nm to 300 nm and an aluminum film with a thickness of from 300 nm to 10 μm are stacked as a multilayer, and are selectively etched, so that a first electrode  406   a , a second electrode  406   b , and a third electrode  406   c  are formed (see  FIG. 7C ). 
         [0123]    The first electrode  406   a  serves as a drain electrode of a diode, the second electrode  406   b  serves as a source electrode of the diode and a source electrode of the MISFET, and the third electrode  406   c  serves as a drain electrode of the MISFET. 
         [0124]    Next, by a sputtering method, a plasma CVD, or the like, a back gate insulating film  407  is formed to have a thickness of from 20 nm to 100 nm with use of a material of silicon oxide, silicon oxynitride, aluminum oxide, aluminum nitride, hafnium oxide, lanthanum oxide, yttrium oxide, or the like. In the back gate insulating film  407  over the first electrode  406   a , an opening portion is formed. 
         [0125]    Then, by a sputtering method or the like, a film of a material having a high work function such as platinum, molybdenum oxide, indium nitride, zinc nitride, or the like is formed to have a thickness of 10 nm to 50 nm and an aluminum film are stacked as a multilayer film, and are selectively etched, so that a back gate electrode  408   a  of the diode and a back gate electrode  408   b  are formed (see  FIG. 7D ). 
         [0126]    The first semiconductor layer  405   a , the first electrode  406   a , and the gate electrode  408   a  of the diode are in contact with each other to form the diode  409  as illustrated in  FIG. 7D . The forward direction of the diode  409  is from the first electrode  406   a  to the second electrode  406   b.    
         [0127]    As illustrated in the circuit diagram of  FIG. 8 , a plurality of such diodes are provided between the gate electrode and the second electrode  406   b , so that a gate protective diode column  411  is formed. With such a circuit, the power MISFET  410  can be prevented from being broken when a voltage exceeding a prescribed voltage is applied to the gate of the power MISFET  410 . 
         [0128]    This application is based on Japanese Patent Application serial no. 2010-212185 filed with Japan Patent Office on Sep. 22, 2010, the entire contents of which are hereby incorporated by reference.