Patent Publication Number: US-11024730-B2

Title: Nitride semiconductor device and manufacturing method for the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2018/006034, filed on Feb. 20, 2018 and designated the U.S., the entire contents of which are incorporated herein by reference. The International Application PCT/JP2018/006034 is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2017-071862, filed on Mar. 31, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a nitride semiconductor device and a manufacturing method for the same. 
     BACKGROUND 
     In order to realize high output of a compound semiconductor device, a HEMT using a nitride semiconductor which may generate highly-concentrated two-dimensional electron gases having high piezoelectric polarization and spontaneous polarization has been actively developed. In recent years, instead of an electron supply layer (barrier layer) of AlGaN of the related art, an In-based nitride semiconductor such as InAlGaN having higher spontaneous polarization or high piezoelectric polarization for a GaN channel has been examined to be used for an electron supply layer. A barrier layer of the In-based nitride semiconductor having high spontaneous polarization may derive highly-concentrated two-dimensional electron gases even if the layer is thin, and thus attracts attention as a material having both high output property and high frequency property. Such a high two-dimensional electron gas density realizes a high current density. In a HEMT using GaN as a channel material, a high voltage operation is possible due to a high dielectric breakdown voltage. Japanese Laid-open Patent Publication No. 2016-105499 and Japanese Laid-open Patent Publication No. 2004-327892 are examples of the related art. 
     In recent years, further micronizing of a nitride semiconductor device has been progressing. In a nitride semiconductor device for high frequency use only, having a fine gate electrode, a drain leakage current bypassing a gate depletion layer in an OFF state occurs. The leakage current reduces controllability for a drain current due to application of a gate voltage, and thus reduces efficiency of a nitride semiconductor device. A fixed leakage current in an OFF state also impedes a high voltage operation of the device. In order to cope with this problem, many research institutes are developing a technique of reducing a leakage current using a so-called back-barrier layer. 
     However, even in a case where a back-barrier layer is used, it is difficult to suppress a short channel effect, for example, in a high voltage operation by sufficiently reducing a leakage current. In light of the above description, it is desirable to sufficiently suppress the short channel effect. 
     SUMMARY 
     According to an aspect of the embodiments, a nitride semiconductor device includes a first nitride semiconductor layer; a back-barrier layer that contains InGaN provided on the first nitride semiconductor layer; and a second nitride semiconductor layer that is provided on the back-barrier layer, wherein, in the back-barrier layer, in a thickness direction, an In composition increases at a first interface with the first nitride semiconductor layer, and the In composition is continuously reduced toward a second interface with the second nitride semiconductor layer. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A, 1B, 1C, and 1D  are schematic sectional views illustrating a manufacturing method  1  for an InAlGaN/GaN HEMT according to a first embodiment in a process order; 
         FIGS. 2A, 2B, 2C, and 2D  are subsequent to  FIG. 1D , and are schematic sectional views illustrating the manufacturing method  1  for the InAlGaN/GaN HEMT according to the first embodiment in a process order; 
         FIGS. 3A, 3B, and 3C  are subsequent to  FIG. 2D , and are schematic sectional views illustrating the manufacturing method  1  for the InAlGaN/GaN HEMT according to the first embodiment in a process order; 
         FIGS. 4A, 4B, and 4C  are schematic sectional views illustrating principal processes of a manufacturing method  2  for an InAlGaN/GaN HEMT according to the first embodiment in order; 
         FIG. 5  is a characteristic diagram illustrating a band state of a conductor in an InAlGaN/GaN HEMT of Comparative Example 2; 
         FIGS. 6A and 6B  are simulation diagrams illustrating electron distributions in InAlGaN/GaN HEMTs of Comparative Examples 1 and 2; 
         FIGS. 7A and 7B  are schematic diagrams illustrating an In composition of a back-barrier layer along with a schematic configuration of the InAlGaN/GaN HEMT according to the first embodiment (manufacturing method  2 ), 
         FIGS. 8A and 8B  are schematic diagrams illustrating an electron distribution in the InAlGaN/GaN HEMT according to the first embodiment along with an electron distribution of Comparative Example 2; 
         FIG. 9  is a characteristic diagram illustrating I-V characteristics of the InAlGaN/GaN HEMT according to the first embodiment along with I-V characteristics in Comparative Example 2; 
         FIGS. 10A and 10B  are characteristic diagrams illustrating I-V characteristics of the InAlGaN/GaN HEMT according to the first embodiment based on comparison with Comparative Example 2 in which a back-barrier layer is provided; 
         FIG. 11  is a characteristic diagram illustrating dependency of the back-barrier layer on a distance from a hetero interface in the I-V characteristics in the InAlGaN/GaN HEMT according to the first embodiment; 
         FIG. 12  is a characteristic diagram illustrating another example of an In composition of the InAlGaN/GaN HEMT according to the first embodiment; 
         FIGS. 13A, 13B, 13C, and 13D  are schematic sectional views illustrating a manufacturing method  1  for an InAlGaN/GaN HEMT according to a second embodiment in a process order; 
         FIGS. 14A, 14B, 14C, and 14D  are subsequent to  FIG. 13D , and are schematic sectional views illustrating the manufacturing method  1  for the InAlGaN/GaN HEMT according to the second embodiment in a process order; 
         FIGS. 15A, 15B, and 15C  are subsequent to  FIG. 14D , and are schematic sectional views illustrating the manufacturing method  1  for the InAlGaN/GaN HEMT according to the second embodiment in a process order; 
         FIGS. 16A, 16B, and 16C  are schematic sectional views illustrating principal processes of a manufacturing method  2  for an InAlGaN/GaN HEMT according to the second embodiment; 
         FIG. 17  is a characteristic diagram illustrating a band state of a conductor in an InAlGaN/GaN HEMT of a comparative example; 
         FIGS. 18A and 18B  are schematic diagrams illustrating an Al composition of a back-barrier layer along with a schematic configuration of the InAlGaN/GaN HEMT according to the second embodiment (manufacturing method  2 ); 
         FIGS. 19A and 19B  are schematic diagrams illustrating an electron distribution in the InAlGaN/GaN HEMT according to the second embodiment along with an electron distribution of the comparative example; 
         FIG. 20  is a schematic diagram illustrating an electron distribution in the InAlGaN/GaN HEMT according to the second embodiment along with an electron distribution of the comparative example; 
         FIG. 21  is a characteristic diagram illustrating another example of an Al composition of the InAlGaN/GaN HEMT according to the second embodiment; 
         FIGS. 22A and 22B  are schematic diagrams illustrating a group III element composition (a sum of an In composition and an Al composition) of a back-barrier layer along with a schematic configuration of an InAlGaN/GaN HEMT according to a modification example of the second embodiment (manufacturing method  2 ); 
         FIG. 23  is a connection block diagram illustrating a schematic configuration of a power source device according to a third embodiment; and 
         FIG. 24  is a connection block diagram illustrating a schematic configuration of a high frequency amplifier according to a fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     In the present embodiment, an InAlGaN/GaN HEMT will be described as a nitride semiconductor device. 
     Manufacturing Method  1   
       FIGS. 1A to 1D ,  FIGS. 2A to 2D , and  FIGS. 3A to 3C  are schematic sectional views illustrating a manufacturing method  1  for an InAlGaN/GaN HEMT according to the present embodiment in a process order. 
     First, as illustrated in  FIG. 1A , a nitride semiconductor laminated structure  2  is formed on, for example, a SiC substrate  1  as a growth substrate. As the growth substrate, instead of the SiC substrate, a Si substrate, a sapphire substrate, a GaN substrate, or the like may be used. The substrate may be semi-insulating or conductive. 
     The nitride semiconductor laminated structure  2  is configured to include a buffer layer  2   a , a back-barrier layer  2   b , an electron transit layer  2   c , an intermediate layer  2   d , an electron supply layer (barrier layer)  2   e , and a cap layer  2   f.    
     In an epitaxial crystal for a completed InAlGaN/GaN HEMT, a two-dimensional electron gas (2DEG) is generated around an interface of the electron transit layer  2   c  with the electron supply layer  2   e  (more specifically, the intermediate layer  2   d ). The 2DEG is generated based on a polarization difference between a compound semiconductor (herein, GaN) of the electron transit layer  2   c  and a compound semiconductor (herein, InAlGaN) of the electron supply layer  2   e.    
     Specifically, each of the following compound semiconductors grows on the SiC substrate  1  according to, for example, a metal organic vapor phase epitaxy (MOVPE) method. Instead of the MOVPE method, a molecular beam epitaxy (MBE) method may be used. 
     Intentionally undoped (i)-GaN at a thickness of approximately 2000 nm, InGaN at a thickness of 5 nm or less, for example, approximately 2 nm, i-GaN at a thickness of approximately 30 nm, AlN at a thickness of approximately 1 nm, n-InAlGaN at a thickness of approximately 10 nm, and n-GaN at a thickness of approximately 1 nm sequentially grow on the SiC substrate  1 . Consequently, the buffer layer  2   a , the back-barrier layer  2   b , the electron transit layer  2   c , the intermediate layer  2   d , the electron supply layer  2   e , and the cap layer  2   f  are formed. AlGaN or the like may be used to form the buffer layer  2   a  instead of GaN. 
     As a raw material gas of GaN forming the buffer layer  2   a , the electron transit layer  2   c , and the cap layer  2   f , a mixed gas of a trimethyl gallium (TMG) gas and a NH 3  gas is used. As a raw material gas of InGaN forming the back-barrier layer  2   b , a mixed gas of a trimethyl indium (TMI) gas, a TMG gas, and a NH 3  gas is used. As a raw material gas of AlN forming the intermediate layer  2   d , a mixed gas of a trimethyl aluminum (TMA) gas and a NH 3  gas is used. As a raw material gas of InAlGaN forming the electron supply layer  2   e , a mixed gas of a TMI gas, a TMA gas, a TMG gas, and a NH 3  gas is used. The supply of a TMG gas which is a Ga source, a TMI gas which is an In source, and a TMA gas which is an Al source and flow rates thereof are set as appropriate according to growing nitride semiconductor layers. A flow rate of an ammonia gas which is a common raw material is set to approximately 100 ccm to 10 LM. A growing pressure is set to approximately 50 Torr to 300 Torr, and a growing temperature is set to approximately 1000° C. to 1200° C. 
     In the present embodiment, in order to cause InGaN forming the back-barrier layer  2   b  to grow, a flow rate of a TMI gas is adjusted to provide such a composition gradient portion in which an In composition increases from 0% to the maximum value at an interface with the buffer layer  2   a , and the In composition is continuously reduced toward an upper surface (an interface with the electron transit layer  2   c  when the electron transit layer  2   c  is formed), and becomes 0% at the upper surface. The maximum value of the In composition is set to, for example, approximately 10%. 
     When the back-barrier layer  2   b  is formed, preferably, a temperature is reduced right before the back-barrier layer  2   b  is formed, and a temperature is increased right after the back-barrier layer  2   b  is formed. Consequently, a lower surface and a partial upper part of an upper surface of the back-barrier layer  2   b  contain carbon (C), and thus a carbon concentration of the lower surface side is higher than that of the upper surface side. C forms an acceptor level, and may thus increase a low potential of a conduction band of the lower surface of the back-barrier layer  2   b  with a high carbon concentration. Thus, electron diffusion to the buffer layer  2   a  side is reduced, and thus there is an effect of suppressing a short channel effect and reducing a drain leakage current. 
     In order to cause InAlGaN forming the electron supply layer  2   e  and GaN forming the cap layer  2   f  to grow as an n type, for example, a SiH 4  gas containing, for example, Si as an n-type impurity is added to a raw material gas at a predetermined flow rate, and thus GaN and AlGaN is doped with Si. A doping concentration of Si is approximately 1×10 18 /cm 3  to approximately 1×10 20 /cm 3 , for example, approximately 5×10 18 /cm 3 . 
     Next, as illustrated in  FIG. 1B , element isolation regions  3  are formed. 
     Specifically, for example, argon (Ar) is injected into element isolation locations of the nitride semiconductor laminated structure  2 . Consequently, the element isolation regions  3  are formed in the nitride semiconductor laminated structure  2  and a surface layer portion of the SiC substrate  1 . An active region is defined on the nitride semiconductor laminated structure  2  by the element isolation regions  3 . 
     Element isolation may be performed according to, for example, a shallow trench isolation (STI) method instead of the injection method. In this case, for example, a chlorine-based etching gas is used for dry etching of the nitride semiconductor laminated structure  2 . 
     Next, as illustrated in  FIG. 1C , a resist mask  11  is formed. 
     Specifically, a resist is coated on a surface of the nitride semiconductor laminated structure  2 , and the resist is processed through lithography such that openings  11   a  and  11   b  to which the nitride semiconductor laminated structure  2  corresponding to locations where electrodes are scheduled to be formed is exposed are formed. Consequently, the resist mask  11  having the openings  11   a  and  11   b  is formed on the nitride semiconductor laminated structure  2 . 
     Next, as illustrated in  FIG. 1D , electrode grooves  2 A and  2 B are formed at locations (electrode formation scheduled locations) where a source electrode and a drain electrode are scheduled to be formed on the surface of the nitride semiconductor laminated structure  2 . 
     Specifically, the electrode formation scheduled locations are subjected to dry etching up to the middle of the electron supply layer  2   e  through the cap layer  2   f  by using the resist mask  11 , so as to be removed. Consequently, the electrode grooves  2 A and  2 B to which the electrode formation scheduled locations on the surface of the electron supply layer  2   e  are exposed are formed. Regarding etching conditions, an inert gas such as Ar and a chlorine-based gas such as Cl 2  are used as etching gases, and, for example, Cl 2  is set to a flow rate of 30 sccm, a pressure of 2 Pa, and RF supply power of 20 W. The electrode grooves  2 A and  2 B may be etched to penetrate through the cap layer  2   f  and the electron supply layer  2   e  up to the middle of the intermediate layer  2   d.    
     The resist mask is removed by a heated organic solvent. 
     Next, as illustrated in  FIG. 2A , a source electrode  4  and a drain electrode  5  are formed. 
     First, for example, two-layer resist masks having an eaves structure suitable for a deposition method and a lift-off method are formed on the nitride semiconductor laminated structure  2 . Specifically, a resist mask  12  having openings  12   a  and  12   b  wider than the electrode grooves  2 A and  2 B and a resist mask  13  having openings  13   a  and  13   b  with a width equivalent to that of the electrode grooves  2 A and  2 B on the resist mask  12  are formed. 
     For example, Ti/Al as an electrode material is accumulated in the openings  12   a  and  13   a  to which the electrode grooves  2 A and  2 B are exposed and on the resist masks including the openings  12   b  and  13   b  according to, for example, a deposition method by using the resist masks  12  and  13 . A thickness of Ti is approximately 20 nm, and a thickness of Al is approximately 200 nm. The resist masks  12  and  13  and Ti/Al accumulated thereon are removed according to a lift-off method. Thereafter, the SiC substrate  1  is subjected to heat treatment in, for example, a nitrogen atmosphere at a temperature of approximately 400° C. to 1000° C., for example, approximately 550° C., and thus remaining Ti/Al is subjected to ohmic contact with the electron supply layer  2   e . The source electrode  4  and the drain electrode  5  in which the electrode grooves  2 A and  2 B are buried with parts of the electrode materials are formed. 
     Next, as illustrated in  FIG. 2B , a protection insulating film  6  is formed. 
     Specifically, for example, silicon nitride (SiN) is accumulated at a thickness of, for example, approximately 50 nm on the entire surface of the nitride semiconductor laminated structure  2  including the upper part of the source electrode  4  and the upper part of the drain electrode  5  by using a plasma CVD method or the like. A refractive index of SiN for light having a wavelength of 633 nm is approximately 2.0, and SiN is stoichiometric SiN. Through the above-described process, the protection insulating film  6  is formed. 
     Next, as illustrated in  FIG. 2C , a resist mask  14  is formed. 
     Specifically, an electron beam resist is coated on the entire surface of the protection insulating film  6  according to a spin coating method or the like. As the electron beam resist, for example, a resist which is a single layer and has the product name “PMGI” (manufactured by MicroChem Corporation of U.S.A.) is used. An electron beam is incident to the coated electron beam resist at a length of, for example, 0.1 μm in a current direction, so that the electron beam resist is exposed to light, and an opening  14   a  is formed through developing, and thus the resist mask  14  having the opening  14   a  is formed. 
     Next, as illustrated in  FIG. 2D , an opening  6   a  is formed in the protection insulating film  6 . 
     Specifically, the protection insulating film  6  is subjected to dry etching by using the resist mask  14  by using, for example, SF 6  as an etching gas. Through the above-described process, the opening  6   a  is formed in the protection insulating film  6 . 
     Next, as illustrated in  FIG. 3A , resist masks  15 ,  16 ,  17  are formed. 
     Specifically, first, an electron beam resist is coated on the protection insulating film  6 . The electron beam resist is formed of three layers, and the product name “PMMA” (manufactured by MicroChem Corporation of U.S.A.) is used for a lower layer resist, the product name “PMGI” (manufactured by MicroChem Corporation of U.S.A.) is used for an intermediate layer resist, and the product name “ZEP520” (manufactured by Zeon Corporation of Japan) is used for an upper layer resist. An electron beam is incident to a gate electrode formation scheduled region of the upper layer resist at a length of, for example, 0.8 μm in a current direction, so that the upper layer resist is exposed to light. After electron beam lithography, an opening having a length of, for example, 0.8 μm is formed in the upper layer resist by using, for example, the product name “ZEP-SD” (manufactured by Zeon Corporation of Japan) as a developer. The intermediate layer resist in a region which is set back from an opening end of the upper layer resist in an ohmic electrode direction by 0.5 μm is removed by using, for example, the product name “NMD-W” (manufactured by Tokyo Ohka Kogyo Co., Ltd.). Next, an electron beam is incident to an opening central part (including the opening  6   a  of the protection insulating film  6 ) of the upper layer resist and the intermediate layer resist at a length of, for example, 0.1 μm in a current direction, so that the opening central part is exposed to light. After electron beam lithography, an opening having a length of, for example, 0.15 μm which is wider than the opening  6   a  of the protection insulating film  6  is formed in the lower layer resist by using, for example, the product name “ZMD-B” (manufactured by Tokyo Ohka Kogyo Co., Ltd.) as a developer. Through the above-described process, the resist masks  15 ,  16 , and  17  provided with openings  15   a ,  16   a , and  17   a  are formed. 
     Next, as illustrated in  FIG. 3B , a gate electrode  7  is formed. 
     Specifically, Ni/Au as an electrode material is accumulated on the resist mask  15  including the opening  6   a  in the openings  16   a  and  17   a  according to, for example, a deposition method by using the resist masks  15 ,  16 , and  17 . A thickness of Ni is approximately 10 nm, and a thickness of Au is approximately 300 nm. Through the above-described process, the gate electrode  7  in which the opening  6   a  is buried with a part of the electrode material is formed on the protection insulating film  6 . The gate electrode  7  is integrally formed of a first portion burying the opening  6   a  of the protection insulating film  6 , a second portion stranding on the protection insulating film  6  so as to be wider than the opening  6   a , and a third portion wider than the second portion on the second portion. 
     Next, as illustrated in  FIG. 3C , the resist masks  15 ,  16 , and  17  and Ni/Au (not illustrated) accumulated on the resist mask  17  are removed according to a lift-off method using a heated organic solvent. 
     Thereafter, the InAlGaN/GaN HEMT according to the present embodiment is formed through various processes of forming wirings connected to the source electrode  4 , the drain electrode  5 , and the gate electrode  7 . 
     Manufacturing Method  2   
       FIGS. 4A, 4B, and 4C  are schematic sectional views illustrating principal processes of a manufacturing method  2  for an InAlGaN/GaN HEMT according to the first embodiment. 
     In a manufacturing method  2 , in the same manner as in the manufacturing method  1 , the respective processes in  FIGS. 1A to 1D  and  FIGS. 2A to 2D  are performed. In this case, the opening  6   a  is formed in the protection insulating film  6  on the nitride semiconductor laminated structure  2 . 
     Next, as illustrated in  FIG. 4A , resist masks  18 ,  16 , and  17  are formed. 
     Specifically, first, an electron beam resist is coated on the protection insulating film  6 . The electron beam resist is formed of three layers, and the product name “PMMA” (manufactured by MicroChem Corporation of U.S.A.) is used for a lower layer resist, the product name “PMGI” (manufactured by MicroChem Corporation of U.S.A.) is used for an intermediate layer resist, and the product name “ZEP520” (manufactured by Zeon Corporation of Japan) is used for an upper layer resist. An electron beam is incident to a gate electrode formation scheduled region of the upper layer resist at a length of, for example, 0.8 μm in a current direction, so that the upper layer resist is exposed to light. After electron beam lithography, an opening having a length of, for example, 0.8 μm is formed in the upper layer resist by using, for example, the product name “ZEP-SD” (manufactured by Zeon Corporation of Japan) as a developer. The intermediate layer resist in a region which is set back from an opening end of the upper layer resist in an ohmic electrode direction by 0.5 μm is removed by using, for example, the product name “NMD-W” (manufactured by Tokyo Ohka Kogyo Co., Ltd.). Next, an electron beam is incident to an opening central part (including the opening  6   a  of the protection insulating film  6 ) of the upper layer resist and the intermediate layer resist at a length of, for example, 0.1 μm in a current direction, so that the opening central part is exposed to light. After electron beam lithography, an opening having a length of, for example, 0.1 μm which is equivalent to that of the opening  6   a  of the protection insulating film  6  is formed in the lower layer resist by using, for example, the product name “ZMD-B” (manufactured by Tokyo Ohka Kogyo Co., Ltd.) as a developer. Through the processes, the resist masks  18 ,  16 , and  17  provided with openings  18   a ,  16   a , and  17   a  are formed. 
     Next, as illustrated in  FIG. 4B , a gate electrode  8  is formed. 
     Specifically, Ni/Au as an electrode material is accumulated on the resist mask  18  including the opening  6   a  in the openings  16   a  and  17   a  according to, for example, a deposition method by using the resist masks  18 ,  16 , and  17 . A thickness of Ni is approximately 10 nm, and a thickness of Au is approximately 300 nm. Through the above-described process, the gate electrode  8  in which the opening  6   a  is buried with a part of the electrode material is formed on the protection insulating film  6 . The gate electrode  8  is integrally formed of a first portion burying the opening  6   a  of the protection insulating film  6  and extending over the protection insulating film  6 , and a second portion wider than the first portion thereon. 
     Next, as illustrated in  FIG. 4C , the resist masks  18 ,  16 , and  17  and Ni/Au (not illustrated) accumulated on the resist mask  17  are removed according to a lift-off method using a heated organic solvent. 
     Thereafter, the InAlGaN/GaN HEMT according to the present embodiment is formed through various processes of forming wirings connected to the source electrode  4 , the drain electrode  5 , and the gate electrode  8 . 
     Hereinafter, an advantageous effect achieved by the InAlGaN/GaN HEMT according to the present embodiment will be described based on comparison with various comparative examples. 
     Comparative Example 1 corresponds to an InAlGaN/GaN HEMT not provided with a back-barrier layer. 
     Comparative Example 2 corresponds to an InAlGaN/GaN HEMT in which a back-barrier layer of InGaN is provided in inter-i-GaN (for example, between a buffer layer and an electron transit layer). In Comparative Example 2, the back-barrier layer has an In composition which increases stepwise (rapidly) at an interface with the buffer layer and an interface with the electron transit layer, the In composition being a substantially fixed value of 10% in a thickness direction between both of the interfaces. 
       FIG. 5  illustrates a band state of a conductor in the InAlGaN/GaN HEMT of Comparative Example 2. In this case, a barrier for an electron is formed due to the curvature of a conduction band caused by inverse piezoelectric charge, and discontinuity of conduction band potentials between i-GaN of the buffer layer and the electron transit layer and InGaN of the back-barrier layer. 
       FIGS. 6A and 6B  illustrate results of examining electron distributions of the InAlGaN/GaN HEMTs of Comparative Examples 1 and 2.  FIG. 6A  illustrates an electron distribution in Comparative Example 1, and  FIG. 6B  illustrates an electron distribution in Comparative Example 2. It may be seen that an electron concentration in the buffer layer or the electron transit layer is suppressed in Comparative Example 2 more than in Comparative Example 1. 
     However, it is understood that an electron is distributed at a high concentration even in the comparative example in which the back-barrier layer is provided. This is a cause to reduce a pinch-off characteristic of an InAlGaN/GaN HEMT. In Comparative Example 2, a barrier for an electron is formed due to the curvature of a conduction band caused by inverse piezoelectric charge of the back-barrier layer, and discontinuity of conduction band potentials between GaN of the buffer layer and the electron transit layer and InGaN of the back-barrier layer. In the present embodiment, the reason why an electron is distributed at a high concentration despite the barrier being formed is concluded to be that a sufficient confinement effect for the electron is not exhibited without an increase in a potential of the entire conduction band in the buffer layer or the electron transit layer. 
       FIGS. 7A and 7B  are schematic diagrams illustrating an In composition of a back-barrier layer along with a schematic configuration of the InAlGaN/GaN HEMT according to the present embodiment (manufacturing method  2 ).  FIG. 7A  illustrates a schematic configuration of the InAlGaN/GaN HEMT, and  FIG. 7B  illustrates an In composition of the back-barrier layer. 
     As illustrated in  FIG. 7A , in the InAlGaN/GaN HEMT, the nitride semiconductor laminated structure  2  is provided with a negative charge layer (herein, the back-barrier layer  2   b ) having negative charge on a lower surface thereof in inter-i-GaN of the buffer layer (or the electron transit layer) under a 2DEG. A portion in contact with a lower surface of the negative charge layer and a portion in contact with an upper surface of the negative charge layer are made of an identical material (i-GaN in  FIGS. 7A and 7B ) having an identical composition proportion. For example, the InAlGaN/GaN HEMT has remaining negative charge as a result of negative charge and positive charge canceling out each other at a position into which the back-barrier layer  2   b  of InGaN is inserted. The presence of the negative charge may be checked according to a CV method or a microscopic EB method. The presence of the negative charge may be checked according to other methods. 
     In Comparative Example 2, negative fixed charge (spontaneous polarization charge and piezoelectric polarization charge) is generated in an interface of the back-barrier layer with the buffer layer, and positive fixed charge (spontaneous polarization charge and piezoelectric polarization charge) is generated in an interface of the back-barrier layer with the electron transit layer. Amounts of the negative fixed charge and the positive fixed charge are the same as each other, both thereof cancel out each other, and thus electric charge neutrality occurs. 
     In contrast, in the present embodiment, the back-barrier layer  2   b  has an In composition as illustrated in  FIG. 7B . For example, in the thickness direction, the back-barrier layer  2   b  has a composition gradient portion in which the In composition increases from 0% to the maximum value (for example, 10%) at the interface with the buffer layer  2   a , and the In composition is continuously reduced toward the interface with the electron transit layer  2   c , and becomes 0% at the interface. In the back-barrier layer  2   b , the In composition continuously (gradually) changes at the interface with the electron transit layer  2   c , and thus part of the positive fixed charge is lost. Thus, in the back-barrier layer  2   b , the electric charge neutrality between the interface with the electron transit layer  2   c  and the interface with the buffer layer  2   a  collapses, and, as a result, an amount of the negative fixed charge is excessive. A potential of a conduction band of i-GaN in the buffer layer  2   a  increases due to the negative fixed charge. Consequently, a sufficient confinement effect for an electron may be achieved, and thus suppression of a short channel effect is realized. 
     On the other hand, a general back-barrier layer has a large thickness such as 100 nm, and there is a problem in that current collapse increases due to the presence of the back-barrier layer. In the present embodiment, the back-barrier layer  2   b  has an In composition distribution as described above, and may generate a sufficient amount of negative fixed charge even though the back-barrier layer  2   b  is thinner than a general back-barrier layer. Thus, current collapse may be reduced. For example, the back-barrier layer  2   b  in the present embodiment has a thickness of approximately 5 nm. Consequently, a pinch-off characteristic is improved by the back-barrier layer  2   b , and thus current collapse is alleviated. Regarding a mechanism of reducing current collapse, a thick back-barrier layer is used in the related art. In a case where, for example, AlGaN is used for a back-barrier layer, it is hard to reduce a trap in a crystal, and thus current collapse caused by the trap is problematic. The influence of the trap increases in proportion to a trap total amount (a product between a trap concentration and a trap thickness). 
       FIGS. 8A and 8B  are schematic diagrams illustrating an electron distribution in the InAlGaN/GaN HEMT according to the present embodiment along with an electron distribution of Comparative Example 2.  FIG. 8A  illustrates Comparative Example 2, and  FIG. 8B  illustrates the present embodiment. 
     In Comparative Example 2, a high electron concentration region which is distributed to be bent directly under the gate electrode is recognized, but, in the present embodiment, it may be seen that formation of the electron concentration region is suppressed. 
       FIG. 9  is a characteristic diagram illustrating I-V characteristics of the InAlGaN/GaN HEMT according to the present embodiment along with I-V characteristics in Comparative Example 2. 
     In the present embodiment, it may be clearly seen that a short channel effect is suppressed compared with the comparative example. As illustrated in  FIG. 9 , in the first embodiment, a threshold value returns to a positive side. The shift of the threshold value to the positive side suppresses a short channel effect, and achieves, for example, an effect of blocking the threshold value to the negative side. 
       FIGS. 10A and 10B  are characteristic diagrams illustrating I-V characteristics corresponding to different gate lengths in the InAlGaN/GaN HEMT according to the present embodiment based on comparison with Comparative Example 2 in which a back-barrier layer is provided.  FIG. 10A  illustrates Comparative Example 2, and  FIG. 10B  illustrates the present embodiment. 
     In the present embodiment, a short channel effect is suppressed compared with Comparative Example 2. 
     In the present embodiment, it is assumed that an In composition in the vicinity of the interface of the back-barrier layer  2   b  with the buffer layer  2   a , for example, the maximum value of the In composition in the back-barrier layer  2   b  is 5% to 20%, for example, 10%. In a case where the maximum value is smaller than 5%, suppression of the short channel effect, for example, a drain leakage current reduction effect wears off, and, in a case where the maximum value is greater than 20%, carriers are induced in the back-barrier layer  2   b  and have conductivity. Therefore, the above range is preferable. 
     The back-barrier layer  2   b  contains carbon (C), and a lower surface side has a carbon concentration higher than that of an upper surface side. With this configuration, a conduction band potential of i-GaN of the buffer layer  2   a  is increased, and thus back-barrier property is improved. 
     The back-barrier layer  2   b  has an In composition which increases stepwise (rapidly) at an interface with the buffer layer  2   a  in a thickness direction, and is continuously reduced from the interface with the buffer layer  2   a  toward an interface with the electron transit layer  2   c , and becomes 0% at the interface. With this configuration, the back-barrier layer  2   b  has a larger amount of negative charge in the interface with the buffer layer  2   a  than an amount of positive charge in the interface with the electron transit layer  2   c  by, for example, 10% or less. Consequently, a conduction band potential of i-GaN of the buffer layer  2   a  sufficiently increases, and this contributes to suppression of a short channel effect. 
     The back-barrier layer  2   b  is preferably located within a distance of approximately 40 nm from the upper surface of the electron transit layer  2   c  which is a hetero interface (for example, a thickness of the electron transit layer  2   c  is approximately 40 nm or less). 
       FIG. 11  is a characteristic diagram illustrating dependency of the back-barrier layer on a distance from an InAlGaN/GaN hetero interface in the I-V characteristics in the InAlGaN/GaN HEMT according to the present embodiment. 
     In this simulation, a principal region (sub-threshold slope) of a characteristic curve of when a drain current enters an ON state from an OFF state is focused, and slopes of characteristic curves corresponding to the different distances are compared with a slope in Comparative Example 1 (non-B.B.) in which a back-barrier layer is not provided. It may be seen that the slopes in the principal region of the characteristic curves corresponding to the distances of 10 nm to 40 nm are greater than the slope in the non-B.B., and a pinch-off characteristic higher than in the non-B.B. is obtained. In contrast, the slope of the characteristic curve corresponding to the distance of 50 nm is substantially the same as the slope in the non-B.B., and a pinch-off characteristic is not sufficient. The slope in I-V characteristics indicates a current/voltage gradient in a small current region of approximately 10 −4  A/mm or less. The gradient indicates a voltage used to change a current value by one digit, and indicates channel controllability using a gate depletion layer. As the slope becomes larger, a device having more favorable switching characteristics or low power consumption property is obtained. Generally, the slope decreases in a case where a short channel effect is great, and a drain leakage current is large. The distance is preferably equal to or less than approximately 40 nm from the description. 
     As described above, according to the present embodiment, an InAlGaN/GaN HEMT having high reliability which realizes sufficient suppression of a short channel effect is implemented. 
     In the present embodiment, the back-barrier layer  2   b  of InGaN may be formed to have an In composition including a portion where an In composition is uniform, for example, as illustrated in  FIG. 12 . 
     In this case, in order to cause InGaN forming the back-barrier layer  2   b  to grow, a flow rate of a TMI gas is adjusted to provide such a composition gradient portion in which an In composition increases from 0% to the maximum value at an interface with the buffer layer  2   a , the maximum value is maintained for a predetermined time, and then the In composition is continuously reduced toward an upper surface (an interface with the electron transit layer  2   c  when the electron transit layer  2   c  is formed), and becomes 0% at the upper surface. The maximum value of the In composition is set to, for example, approximately 10%. 
     Consequently, the back-barrier layer  2   b  has the composition gradient portion in which the In composition increases to the maximum value (for example, 10%) from 0% at the interface with the buffer layer  2   a  in the thickness direction, the maximum value is maintained by a slight thickness, and the In composition is continuously reduced toward the interface with the electron transit layer  2   c , and becomes 0% at the interface. 
     Even in this case, an electron distribution as illustrated in  FIG. 8B  is obtained, and formation of a high electron concentration region which is distributed to be bent directly under the gate electrode as illustrated in  FIG. 8A  is suppressed, and thus a short channel effect may be sufficiently suppressed. 
     Second Embodiment 
     In the present embodiment, an InAlGaN/GaN HEMT will be described as a nitride semiconductor device in the same manner as in the first embodiment, but there is a difference from the first embodiment in terms of a material of a back-barrier layer. 
     Manufacturing Method  1   
       FIGS. 13A to 13D ,  FIGS. 14A to 14D , and  FIGS. 15A to 15C  are schematic sectional views illustrating a manufacturing method  1  for an InAlGaN/GaN HEMT according to the present embodiment in a process order. 
     First, as illustrated in  FIG. 13A , a nitride semiconductor laminated structure  21  is formed on, for example, a SiC substrate  1  as a growth substrate. As the growth substrate, instead of the SiC substrate, a Si substrate, a sapphire substrate, a GaN substrate, or the like may be used. The substrate may be semi-insulating or conductive. 
     The nitride semiconductor laminated structure  21  is configured to include a buffer layer  2   a , a back-barrier layer  22 , an electron transit layer (barrier layer)  2   c , an intermediate layer  2   d , an electron supply layer  2   e , and a cap layer  2   f.    
     In a completed AlGaN/GaN HEMT, a two-dimensional electron gas (2DEG) is generated around an interface of the electron transit layer  2   c  with the electron supply layer  2   e  (more specifically, the intermediate layer  2   d ). The 2DEG is generated based on a polarization difference between a compound semiconductor (herein, GaN) of the electron transit layer  2   c  and a compound semiconductor (herein, InAlGaN) of the electron supply layer  2   e.    
     Specifically, each of the following compound semiconductors grows on the SiC substrate  1  according to, for example, a metal organic vapor phase epitaxy (MOVPE) method. Instead of the MOVPE method, a molecular beam epitaxy (MBE) method may be used. 
     Intentionally undoped (i)-GaN at a thickness of approximately 2000 nm, AlGaN at a thickness of 5 nm or less, for example, approximately 2 nm, i-GaN at a thickness of approximately 30 nm, AlN at a thickness of approximately 1 nm, n-InAlGaN at a thickness of approximately 10 nm, and n-GaN at a thickness of approximately 1 nm sequentially grow on the SiC substrate  1 . Consequently, the buffer layer  2   a , the back-barrier layer  22 , the electron transit layer  2   c , the intermediate layer  2   d , the electron supply layer  2   e , and the cap layer  2   f  are formed. AlGaN or the like may be used to form the buffer layer  2   a  instead of GaN. 
     As a raw material gas of GaN forming the buffer layer  2   a , the electron transit layer  2   c , and the cap layer  2   f , a mixed gas of a TMG gas and a NH 3  gas is used. As a raw material gas of AlGaN forming the back-barrier layer  22 , a mixed gas of a TMA gas, a TMG gas, and a NH 3  gas is used. As a raw material gas of AlN forming the intermediate layer  2   d , a mixed gas of a TMA gas and a NH 3  gas is used. As a raw material gas of InAlGaN forming the electron supply layer  2   e , a mixed gas of a TMI gas, a TMA gas, a TMG gas, and a NH 3  gas is used. The supply of a TMG gas which is a Ga source, a TMI gas which is an In source, and a TMA gas which is an Al source and flow rates thereof are set as appropriate according to growing nitride semiconductor layers. A flow rate of an ammonia gas which is a common raw material is set to approximately 100 ccm to 10 LM. A growing pressure is set to approximately 50 Torr to 300 Torr, and a growing temperature is set to approximately 1000° C. to 1200° C. 
     In the present embodiment, in order to cause AlGaN forming the back-barrier layer  22  to grow, a flow rate of a TMA gas is adjusted such that an Al composition continuously increases from 0% to the maximum value at an interface with the buffer layer  2   a  toward an upper surface (an interface with the electron transit layer  2   c  when the electron transit layer  2   c  is formed) from an interface with the buffer layer  2   a , and is reduced to 0% at the upper surface. The maximum value of the Al composition is set to, for example, approximately 30%. 
     When the nitride semiconductor laminated structure  21  is formed, preferably, a temperature is changed or a V-III ratio is adjusted. For example, a temperature is reduced right before a lower surface of the back-barrier layer  22  is formed, and a temperature is increased right after an upper surface of the back-barrier layer  22  is formed. Consequently, the back-barrier layer  22  contains carbon (C), and a carbon concentration of the lower surface side is higher than that of the upper surface side. 
     In order to cause InAlGaN forming the electron supply layer  2   e  and GaN forming the cap layer  2   f  to grow as an n type, for example, a SiH 4  gas containing, for example, Si as an n-type impurity is added to a raw material gas at a predetermined flow rate, and thus GaN and InAlGaN is doped with Si. A doping concentration of Si is approximately 1×10 18 /cm 3  to approximately 1×10 20 /cm 3 , for example, approximately 5×10 18 /cm 3 . 
     Next, as illustrated in  FIG. 13B , element isolation regions  3  are formed. 
     Specifically, for example, argon (Ar) is injected into element isolation locations of the nitride semiconductor laminated structure  21 . Consequently, the element isolation regions  3  are formed in the nitride semiconductor laminated structure  21  and a surface layer portion of the SiC substrate  1 . An active region is defined on the nitride semiconductor laminated structure  21  by the element isolation regions  3 . 
     Element isolation may be performed according to, for example, a shallow trench isolation (STI) method instead of the injection method. In this case, for example, a chlorine-based etching gas is used for dry etching of the nitride semiconductor laminated structure  21 . 
     Next, as illustrated in  FIG. 13C , a resist mask  11  is formed. 
     Specifically, a resist is coated on a surface of the nitride semiconductor laminated structure  21 , and the resist is processed through lithography such that openings  11   a  and  11   b  to which the nitride semiconductor laminated structure  21  corresponding to locations where electrodes are scheduled to be formed is exposed are formed. Consequently, the resist mask  11  having the openings  11   a  and  11   b  is formed on the nitride semiconductor laminated structure  21 . 
     Next, as illustrated in  FIG. 13D , electrode grooves  2 A and  2 B are formed at locations (electrode formation scheduled locations) where a source electrode and a drain electrode are scheduled to be formed on the surface of the nitride semiconductor laminated structure  21 . 
     Specifically, the electrode formation scheduled locations are subjected to dry etching up to the middle of the electron supply layer  2   e  through the cap layer  2   f  by using the resist mask  11 , so as to be removed. Consequently, the electrode grooves  2 A and  2 B to which the electrode formation scheduled locations on the surface of the electron supply layer  2   e  are exposed are formed. Regarding etching conditions, an inert gas such as Ar and a chlorine-based gas such as Cl 2  are used as etching gases, and, for example, Cl 2  is set to a flow rate of 30 sccm, a pressure of 2 Pa, and RF supply power of 20 W. The electrode grooves  2 A and  2 B may be etched to penetrate through the cap layer  2   f  and the electron supply layer  2   e  up to the middle of the intermediate layer  2   d.    
     The resist mask is removed by a heated organic solvent. 
     Next, as illustrated in  FIG. 14A , a source electrode  4  and a drain electrode  5  are formed. 
     First, for example, two-layer resist masks having an eaves structure suitable for a deposition method and a lift-off method are formed on the nitride semiconductor laminated structure  21 . Specifically, a resist mask  12  having openings  12   a  and  12   b  wider than the electrode grooves  2 A and  2 B and a resist mask  13  having openings  13   a  and  13   b  with a width equivalent to that of the electrode grooves  2 A and  2 B on the resist mask  12  are formed. 
     For example, Ti/Al as an electrode material is accumulated in the openings  12   a  and  13   a  to which the electrode grooves  2 A and  2 B are exposed and on the resist masks including the openings  12   b  and  13   b  according to, for example, a deposition method by using the resist masks  12  and  13 . A thickness of Ti is approximately 20 nm, and a thickness of Al is approximately 200 nm. The resist masks  12  and  13  and Ti/Al accumulated thereon are removed according to a lift-off method. Thereafter, the SiC substrate  1  is subjected to heat treatment in, for example, a nitrogen atmosphere at a temperature of approximately 400° C. to 1000° C., for example, approximately 550° C., and thus remaining Ti/Al is subjected to ohmic contact with the electron supply layer  2   e . The source electrode  4  and the drain electrode  5  in which the electrode grooves  2 A and  2 B are buried with parts of the electrode materials are formed. 
     Next, as illustrated in  FIG. 14B , a protection insulating film  6  is formed. 
     Specifically, for example, silicon nitride (SiN) is accumulated at a thickness of, for example, approximately 50 nm on the entire surface of the nitride semiconductor laminated structure  21  including the upper part of the source electrode  4  and the upper part of the drain electrode  5  by using a plasma CVD method or the like. A refractive index of SiN for light having a wavelength of 633 nm is approximately 2.0, and SiN is stoichiometric SiN. Through the above-described process, the protection insulating film  6  is formed. 
     Next, as illustrated in  FIG. 14C , a resist mask  14  is formed. 
     Specifically, an electron beam resist is coated on the entire surface of the protection insulating film  6  according to a spin coating method or the like. As the electron beam resist, for example, a resist which is a single layer and has the product name “PMGI” (manufactured by MicroChem Corporation of U.S.A.) is used. An electron beam is incident to the coated electron beam resist at a length of, for example, 0.1 μm in a current direction, so that the electron beam resist is exposed to light, and an opening  14   a  is formed through developing. Through the above-described process, the resist mask  14  having the opening  14   a  is formed. 
     Next, as illustrated in  FIG. 14D , an opening  6   a  is formed in the protection insulating film  6 . 
     Specifically, the protection insulating film  6  is subjected to dry etching by using the resist mask  14  by using, for example, SF 6  as an etching gas. Through the above-described process, the opening  6   a  is formed in the protection insulating film  6 . 
     Next, as illustrated in  FIG. 15A , resist masks  15 ,  16 ,  17  are formed. 
     Specifically, first, an electron beam resist is coated on the protection insulating film  6 . The electron beam resist is formed of three layers, and the product name “PMMA” (manufactured by MicroChem Corporation of U.S.A.) is used for a lower layer resist, the product name “PMGI” (manufactured by MicroChem Corporation of U.S.A.) is used for an intermediate layer resist, and the product name “ZEP520” (manufactured by Zeon Corporation of Japan) is used for an upper layer resist. An electron beam is incident to a gate electrode formation scheduled region of the upper layer resist at a length of, for example, 0.8 μm in a current direction, so that the upper layer resist is exposed to light. After electron beam lithography, an opening having a length of, for example, 0.8 μm is formed in the upper layer resist by using, for example, the product name “ZEP-SD” (manufactured by Zeon Corporation of Japan) as a developer. The intermediate layer resist in a region which is set back from an opening end of the upper layer resist in an ohmic electrode direction by 0.5 μm is removed by using, for example, the product name “NMD-W” (manufactured by Tokyo Ohka Kogyo Co., Ltd.). Next, an electron beam is incident to an opening central part (including the opening  6   a  of the protection insulating film  6 ) of the upper layer resist and the intermediate layer resist at a length of, for example, 0.15 μm in a current direction, so that the opening central part is exposed to light. After electron beam lithography, an opening having a length of, for example, 0.15 μm which is wider than the opening  6   a  of the protection insulating film  6  is formed in the lower layer resist by using, for example, the product name “ZMD-B” (manufactured by Tokyo Ohka Kogyo Co., Ltd.) as a developer. Through the above-described processes, the resist masks  15 ,  16 , and  17  provided with openings  15   a ,  16   a , and  17   a  are formed. 
     Next, as illustrated in  FIG. 15B , a gate electrode  7  is formed. 
     Specifically, for example, Ni/Au as an electrode material is accumulated on the resist mask  15  including the opening  6   a  in the openings  16   a  and  17   a  according to, for example, a deposition method by using the resist masks  15 ,  16 , and  17 . A thickness of Ni is approximately 10 nm, and a thickness of Au is approximately 300 nm. Through the above-described process, the gate electrode  7  in which the opening  6   a  is buried with a part of the electrode material is formed on the protection insulating film  6 . The gate electrode  7  is integrally formed of a first portion burying the opening  6   a  of the protection insulating film  6 , a second portion stranding on the protection insulating film  6  so as to be wider than the opening  6   a , and a third portion wider than the second portion on the second portion. 
     Next, as illustrated in  FIG. 15C , the resist masks  15 ,  16 , and  17  and Ni/Au (not illustrated) accumulated on the resist mask  17  are removed according to a lift-off method using a heated organic solvent. 
     Thereafter, the InAlGaN/GaN HEMT according to the present embodiment is formed through various processes of forming wirings connected to the source electrode  4 , the drain electrode  5 , and the gate electrode  7 . 
     Manufacturing Method  2   
       FIGS. 16A, 16B, and 16C  are schematic sectional views illustrating principal processes of a manufacturing method  2  for an InAlGaN/GaN HEMT according to the present embodiment. 
     In a manufacturing method  2 , in the same manner as in the manufacturing method  1 , the respective processes in  FIGS. 13A to 13D  and  FIGS. 14A to 14D  are performed. In this case, the opening  6   a  is formed in the protection insulating film  6  on the nitride semiconductor laminated structure  21 . 
     Next, as illustrated in  FIG. 16A , resist masks  18 ,  16 , and  17  are formed. 
     Specifically, first, an electron beam resist is coated on the protection insulating film  6 . The electron beam resist is formed of three layers, and the product name “PMMA” (manufactured by MicroChem Corporation of U.S.A.) is used for a lower layer resist, the product name “PMGI” (manufactured by MicroChem Corporation of U.S.A.) is used for an intermediate layer resist, and the product name “ZEP520” (manufactured by Zeon Corporation of Japan) is used for an upper layer resist. An electron beam is incident to a gate electrode formation scheduled region of the upper layer resist at a length of, for example, 0.8 μm in a current direction, so that the upper layer resist is exposed to light. After electron beam lithography, for example, an opening having a length of, for example, 0.8 μm is formed in the upper layer resist by using, for example, the product name “ZEP-SD” (manufactured by Zeon Corporation of Japan) as a developer. The intermediate layer resist in a region which is set back from an opening end of the upper layer resist in an ohmic electrode direction by 0.5 μm is removed by using, for example, the product name “NMD-W” (manufactured by Tokyo Ohka Kogyo Co., Ltd.). Next, an electron beam is incident to an opening central part (including the opening  6   a  of the protection insulating film  6 ) of the upper layer resist and the intermediate layer resist at a length of, for example, 0.1 μm in a current direction, so that the opening central part is exposed to light. After electron beam lithography, for example, an opening having a length of, for example, 0.1 μm which is equivalent to that of the opening  6   a  of the protection insulating film  6  is formed in the lower layer resist by using, for example, the product name “ZMD-B” (manufactured by Tokyo Ohka Kogyo Co., Ltd.) as a developer. Through the processes, the resist masks  18 ,  16 , and  17  provided with openings  18   a ,  16   a , and  17   a  are formed. 
     Next, as illustrated in  FIG. 16B , a gate electrode  8  is formed. 
     Specifically, Ni/Au as an electrode material is accumulated on the resist mask  18  including the opening  6   a  in the openings  16   a  and  17   a  according to, for example, a deposition method by using the resist masks  18 ,  16 , and  17 . A thickness of Ni is approximately 10 nm, and a thickness of Au is approximately 300 nm. Through the above-described process, the gate electrode  8  in which the opening  6   a  is buried with a part of the electrode material is formed on the protection insulating film  6 . The gate electrode  8  is integrally formed of a first portion burying the opening  6   a  of the protection insulating film  6  and extending over the protection insulating film  6 , and a second portion wider than the first portion thereon. 
     Next, as illustrated in  FIG. 16C , the resist masks  18 ,  16 , and  17  and Ni/Au (not illustrated) accumulated on the resist mask  17  are removed according to a lift-off method using a heated organic solvent. 
     Thereafter, the InAlGaN/GaN HEMT according to the present embodiment is formed through various processes of forming wirings connected to the source electrode  4 , the drain electrode  5 , and the gate electrode  8 . 
     Hereinafter, an advantageous effect achieved by the InAlGaN/GaN HEMT according to the present embodiment will be described based on comparison with a comparative example. 
     The comparative example corresponds to an InAlGaN/GaN HEMT in which a back-barrier layer of AlGaN is provided in inter-i-GaN (for example, between a buffer layer and an electron transit layer). In the comparative example, the back-barrier layer has an Al composition which increases stepwise (rapidly) at an interface with the buffer layer and an interface with the electron transit layer, the Al composition being a substantially fixed value of 30% in a thickness direction between both of the interfaces. 
       FIG. 17  illustrates a band state of a conductor in the InAlGaN/GaN HEMT of the comparative example. In this case, a barrier for an electron is formed due to the curvature of a conduction band caused by inverse piezoelectric charge, and discontinuity of conduction band potentials between i-GaN of the buffer layer and the electron transit layer and AlGaN of the back-barrier layer. 
     In the comparative example, a barrier for an electron is formed due to discontinuity of conduction band potentials between GaN of the buffer layer and the electron transit layer and AlGaN of the back-barrier layer. In the present embodiment, the reason why an electron is distributed at a high concentration despite the barrier being formed is concluded to be that a sufficient confinement effect for the electron is not exhibited without an increase in a potential of the entire conduction band in the buffer layer or the electron transit layer. 
       FIGS. 18A and 18B  are schematic diagrams illustrating an Al composition of a back-barrier layer along with a schematic configuration of the InAlGaN/GaN HEMT according to the present embodiment (manufacturing method  2 ).  FIG. 18A  illustrates a schematic configuration of the InAlGaN/GaN HEMT, and  FIG. 18B  illustrates an Al composition of the back-barrier layer. 
     As illustrated in  FIG. 18A , in the InAlGaN/GaN HEMT, the nitride semiconductor laminated structure  21  is provided with a negative charge layer (herein, the back-barrier layer  22 ) having negative charge on an upper surface thereof in inter-i-GaN of the electron transit layer (or the buffer layer) under a 2DEG. A portion in contact with a lower surface of the negative charge layer and a portion in contact with an upper surface of the negative charge layer are made of an identical material (i-GaN in  FIGS. 18A and 18B ) having an identical composition proportion. For example, the InAlGaN/GaN HEMT has remaining negative charge as a result of negative charge and positive charge canceling out each other at a position into which the back-barrier layer  22  of AlGaN is inserted. The presence of the negative charge may be checked according to a CV method or a microscopic EB method. 
     In the comparative example, positive fixed charge (spontaneous polarization charge and piezoelectric polarization charge) is generated in an interface of the back-barrier layer with the buffer layer, and negative fixed charge (spontaneous polarization charge and piezoelectric polarization charge) is generated in an interface of the back-barrier layer with the electron transit layer. Amounts of the negative fixed charge and the positive fixed charge are the same as each other, both thereof cancel out each other, and thus electric charge neutrality occurs. 
     In contrast, in the present embodiment, the back-barrier layer  22  has an Al composition as illustrated in  FIG. 18B . For example, in the back-barrier layer  22 , the Al composition continuously increases toward the interface with the electron transit layer  2   c  from the interface with the buffer layer  2   a  in the thickness direction, becomes the maximum value (for example, 30%) at the interface with the electron transit layer  2   c , and the Al composition is reduced from 30% to 0% at the interface with the electron transit layer  2   c . In the back-barrier layer  22 , the Al composition continuously (gradually) changes at the interface with the buffer layer  2   a , and thus part of the positive fixed charge is lost. Thus, in the back-barrier layer  22 , the electric charge neutrality between the interface with the electron transit layer  2   c  and the interface with the buffer layer  2   a  collapses, and, as a result, an amount of the negative fixed charge is excessive. A potential of a conduction band of i-GaN in the electron transit layer  2   c  increases due to the negative fixed charge. Consequently, a sufficient confinement effect for an electron may be achieved, and thus suppression of a short channel effect is realized. 
     In the present embodiment, the back-barrier layer  22  has an Al composition distribution as described above, and may generate a sufficient amount of negative fixed charge even though the back-barrier layer  22  is thinner than a general back-barrier layer. Thus, current collapse may be reduced. For example, the back-barrier layer  22  in the present embodiment has a thickness of approximately 5 nm. Consequently, a pinch-off characteristic is improved by the back-barrier layer  22 . 
       FIGS. 19A and 19B  are schematic diagrams illustrating an electron distribution in the InAlGaN/GaN HEMT according to the present embodiment along with an electron distribution of the comparative example.  FIG. 19A  illustrates the comparative example, and  FIG. 19B  illustrates the present embodiment. 
     In the comparative example, a high electron concentration region which is distributed to be bent directly under the gate electrode is recognized, but, in the present embodiment, it may be seen that formation of the electron concentration region is suppressed. 
       FIG. 20  is a schematic diagram illustrating I-V characteristics of the InAlGaN/GaN HEMT according to the present embodiment along with I-V characteristics of the comparative example. A dashed line indicates the comparative example, and a solid line indicates the present embodiment. 
     In the comparative example, a high electron concentration region which is distributed to be bent directly under the gate electrode is recognized, but, in the present embodiment, it may be seen that formation of the electron concentration region is suppressed. 
     In the present embodiment, it is assumed that an Al composition in the vicinity of the interface of the back-barrier layer  22  with the buffer layer  2   a , for example, the maximum value of the Al composition in the back-barrier layer  22  is 5% to 35%, for example, 30%. In a case where the maximum value is smaller than 5%, the effect of the back-barrier layer is reduced, and, in a case where the maximum value is greater than 35%, crystal quality deteriorates, and a trap concentration increases. Therefore, the above range is preferable. 
     Also in the present embodiment, in the same manner as in the first embodiment, preferably, the back-barrier layer  22  contains carbon (C), and a lower surface side has a carbon concentration higher than that of an upper surface side. 
     The back-barrier layer  22  has a larger amount of negative charge in the interface with the electron transit layer  2   c  than an amount of positive charge in the interface with the buffer layer  2   a  by, for example, 10% or less. 
     The back-barrier layer  22  is preferably located within a distance of approximately 40 nm from the upper surface of the electron transit layer  2   c  which is a hetero interface (for example, a thickness of the electron transit layer  2   c  is approximately 40 nm or less). 
     As described above, according to the present embodiment, an InAlGaN/GaN HEMT having high reliability which realizes sufficient suppression of a short channel effect is implemented. In the InAlGaN/GaN HEMT, an increase in current collapse due to the presence of a back-barrier layer may be suppressed, and thus the current collapse may be alleviated. 
     In the present embodiment, the back-barrier layer  22  of AlGaN may be formed to have an Al composition including a portion where an Al composition is uniform, for example, as illustrated in  FIG. 21 . 
     In this case, in order to cause AlGaN forming the back-barrier layer  22  to grow, a flow rate of a TMA gas is adjusted such that an Al composition continuously increases from 0% to the maximum value toward an upper surface (an interface with the electron transit layer  2   c  when the electron transit layer  2   c  is formed) from an interface with the buffer layer  2   a , the maximum value is maintained for a predetermined time, and then the Al composition is reduced to 0% at the interface with the electron transit layer  2   c . The maximum value of the Al composition is set to, for example, approximately 30%. 
     Consequently, in the back-barrier layer  22 , the Al composition continuously increases toward the interface with the electron transit layer  2   c  from the interface with the buffer layer  2   a  in the thickness direction, becomes the maximum value (for example, 30%) at the interface with the electron transit layer  2   c , the maximum value is maintained by a slight thickness, and the Al composition is reduced from the maximum value to 0% at the interface with the electron transit layer  2   c.    
     Even in this case, an electron distribution as illustrated in  FIG. 19B  is obtained, and formation of a high electron concentration region which is distributed to be bent directly under the gate electrode as illustrated in  FIG. 19A  is suppressed, and thus a short channel effect may be sufficiently suppressed. 
     Modification Example 
     Hereinafter, a description will be made of a modification example of the second embodiment. In the present modification example, a back-barrier layer made of InAlGaN is formed instead of a back-barrier layer made of AlGaN in the second embodiment. 
     In the present modification example, in either manufacturing method  1  or  2 , InAlGaN is accumulated instead of AlGaN forming the back-barrier layer  22  according to an MOVPE method. 
     In the present modification example, in order to cause InAlGaN forming the back-barrier layer to grow, flow rates of a TMA gas and a TMI gas are adjusted such that a sum of an In composition and an Al composition continuously increases from 0% toward an upper surface (an interface with the electron transit layer  2   c  when the electron transit layer  2   c  is formed) from an interface with the buffer layer  2   a , becomes the maximum value at the upper surface, and is reduced stepwise (rapidly) to 0% at the upper surface. The maximum value of the sum of the In composition and the Al composition is set to, for example, approximately 80%. 
       FIGS. 22A and 22B  are schematic diagrams illustrating a group III element composition (a sum of an In composition and an Al composition) of a back-barrier layer along with a schematic configuration of an InAlGaN/GaN HEMT according to the modification example of the present embodiment (manufacturing method  2 ).  FIG. 22A  illustrates a schematic configuration of the InAlGaN/GaN HEMT, and  FIG. 22B  illustrates a group III element composition of a back-barrier layer. 
     As illustrated in  FIG. 22A , in the InAlGaN/GaN HEMT, a nitride semiconductor laminated structure  23  is provided with a negative charge layer (herein, a back-barrier layer  24 ) having negative charge on an upper surface thereof in inter-i-GaN of the electron transit layer (or the buffer layer) under a 2DEG. A portion in contact with a lower surface of the negative charge layer and a portion in contact with an upper surface of the negative charge layer are made of an identical material (i-GaN in  FIGS. 22A and 22B ) of an identical composition proportion. For example, the InAlGaN/GaN HEMT has remaining negative charge as a result of negative charge and positive charge canceling out each other at a position into which the back-barrier layer  24  of InAlGaN is inserted. The presence of the negative charge may be checked according to a CV method or a microscopic EB method. 
     In the present modification example, the back-barrier layer  24  has a distribution of a group III element composition (a sum of an In composition and an Al composition) as illustrated in  FIG. 22B . For example, in the back-barrier layer  24 , the group III element composition continuously increases toward the interface with the electron transit layer  2   c  from the interface with the buffer layer  2   a  in the thickness direction, becomes the maximum value (for example, 80%) at the interface with the electron transit layer  2   c , and the group III element composition is reduced from 80% to 0% at the interface with the electron transit layer  2   c . In the back-barrier layer  24 , the group III element composition continuously (gradually) changes at the interface with the buffer layer  2   a , and thus part of the positive fixed charge is lost. Thus, in the back-barrier layer  24 , the electric charge neutrality between the interface with the electron transit layer  2   c  and the interface with the buffer layer  2   a  collapses, and, as a result, an amount of the negative fixed charge is excessive. A potential of a conduction band of i-GaN in the electron transit layer  2   c  increases due to the negative fixed charge. Consequently, a sufficient confinement effect for an electron may be achieved, and thus suppression of a short channel effect is realized. 
     In the present modification example, the back-barrier layer  24  has the distribution of the group III element composition as described above, and may generate a sufficient amount of negative fixed charge even though the back-barrier layer  24  is thinner than a general back-barrier layer. Thus, current collapse may be reduced. For example, the back-barrier layer  24  in the present modification example has a thickness of approximately 5 nm. Consequently, a pinch-off characteristic is improved by the back-barrier layer  24 , and thus current collapse is alleviated. 
     In the present modification example, it is assumed that a group III element composition in the vicinity of the interface of the back-barrier layer  24  with the buffer layer  2   a , for example, the maximum value of the sum of the In composition and the Al composition in the back-barrier layer  24  is 50% to 80%, for example, 80%. In a case where the maximum value is smaller than 50%, the effect of the back-barrier layer is reduced, and a drain leakage current increases, and, in a case where the maximum value is greater than 80%, crystal quality deteriorates, and a trap is generated. Therefore, the above range is preferable. 
     Also in the present modification example, in the same manner as in the second embodiment, preferably, the back-barrier layer  24  contains carbon (C), and a lower surface side has a carbon concentration higher than that of an upper surface side. 
     The back-barrier layer  24  has a larger amount of negative charge in the interface with the electron transit layer  2   c  than an amount of positive charge in the interface with the buffer layer  2   a  by, for example, 10% or less. 
     The back-barrier layer  24  is preferably located within a distance of approximately 40 nm from the upper surface of the electron transit layer  2   c  which is a hetero interface (for example, a thickness of the electron transit layer  2   c  is approximately 40 nm or less). 
     As described above, according to the present modification example, an InAlGaN/GaN HEMT having high reliability which realizes sufficient suppression of a short channel effect is implemented. In the InAlGaN/GaN HEMT, an increase in current collapse due to the presence of a back-barrier layer may be suppressed, and thus the current collapse may be alleviated. 
     In the present modification example, the back-barrier layer  24  of InAlGaN may be formed to have a group III element composition including a portion where a group III element composition (a sum of an In composition and an Al composition) is uniform, for example, as illustrated in  FIG. 21 . 
     In this case, in order to cause InAlGaN forming the back-barrier layer  24  to grow, flow rates of a TMI gas and a TMA gas are adjusted such that a group III element composition continuously increases from 0% to the maximum value at an interface with the buffer layer  2   a  toward an upper surface (an interface with the electron transit layer  2   c  when the electron transit layer  2   c  is formed) from an interface with the buffer layer  2   a , the maximum value is maintained for a predetermined time, and then the group III element composition is reduced to 0% at the interface with the electron transit layer  2   c . The maximum value of the group III element composition is set to, for example, approximately 80%. 
     Consequently, in the back-barrier layer  24 , the group III element composition continuously increases toward the interface with the electron transit layer  2   c  from the interface with the buffer layer  2   a  in the thickness direction, becomes the maximum value (for example, 80%) at the interface with the electron transit layer  2   c , the maximum value is maintained by a slight thickness, and the group III element composition is reduced from the maximum value to 0% at the interface with the electron transit layer  2   c.    
     Even in this case, an electron distribution as illustrated in  FIG. 19B  is obtained, and formation of a high electron concentration region which is distributed to be bent directly under the gate electrode as illustrated in  FIG. 19A  is suppressed, and thus a short channel effect may be sufficiently suppressed. 
     In the first and second embodiments and the modification example of the second embodiment, as a nitride semiconductor device, an InAlGaN/GaN HEMT has been described, but not only the InAlGaN/GaN HEMT but also, for example, an AlGaN/GaN HEMT, an InAlN/GaN HEMT, or an AlN/GaN HEMT may be employed. 
     Third Embodiment 
     In the present embodiment, a description will be made of a power source device to which a single InAlGaN/GaN HEMT selected from the first and second embodiments and the modification example is applied. 
       FIG. 23  is a connection block diagram illustrating a schematic configuration of a power source device according to a third embodiment. 
     The power source device according to the present embodiment is configured to include a high-voltage primary circuit  31 , a low-voltage secondary circuit  32 , and a transformer  33  disposed between the primary circuit  31  and the secondary circuit  32 . 
     The primary circuit  31  is configured to include an AC power source  34 , a so-called bridge rectification circuit  35 , and a plurality of (for example, four) switching elements  36   a ,  36   b ,  36   c , and  36   d . The bridge rectification circuit  35  includes a switching element  36   e.    
     The secondary circuit  32  is configured to include a plurality of (for example, three) switching elements  37   a ,  37   b , and  37   c.    
     In the present embodiment, each of the switching elements  36   a ,  36   b ,  36   c ,  36   d , and  36   e  of the primary circuit  31  is configured with a single InAlGaN/GaN HEMT selected from the first and second embodiments and the modification example. On the other hand, each of the switching elements  37   a ,  37   b , and  37   c  of the secondary circuit  32  is configured with a general MIS FET using silicon. 
     In the present embodiment, the InAlGaN/GaN HEMT which has high reliability and realizes both of suppression of a short channel effect and alleviation of current collapse is applied to a high-voltage circuit. Consequently, a large-power power source circuit having high reliability is implemented. 
     Fourth Embodiment 
     In the present embodiment, a description will be made of a high frequency amplifier to which a single InAlGaN/GaN HEMT selected from the first and second embodiments and the modification example is applied. 
       FIG. 24  is a connection block diagram illustrating a schematic configuration of a high frequency amplifier according to a fourth embodiment. 
     The high frequency amplifier according to the present embodiment is configured to include a digital pre-distortion circuit  41 , mixers  42   a  and  42   b , and a power amplifier  43 . 
     The digital pre-distortion circuit  41  compensates for nonlinear distortion of an input signal. The mixer  42   a  mixes the input signal of which the nonlinear distortion is compensated for with an AC signal. The power amplifier  43  amplifies the input signal mixed with the AC signal, and has a single InAlGaN/GaN HEMT selected from the first and second embodiments and the modification example. In  FIG. 24 , a signal on the output side is mixed with an AC signal in the mixer  42   b  through, for example, switching of a switch, and is sent to the digital pre-distortion circuit  41 . 
     In the present embodiment, the InAlGaN/GaN HEMT which has high reliability and realizes both of suppression of a short channel effect and alleviation of current collapse is applied to a high frequency amplifier. Consequently, a high frequency amplifier having high reliability and a high breakdown voltage is implemented. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.