Abstract:
A field effect transistor includes: a first nitride semiconductor layer having a plane perpendicular to a (0001) plane or a plane tilted with respect to the (0001) plane as a main surface; a second nitride semiconductor layer formed on the first nitride semiconductor layer and having a wider bandgap than the first nitride semiconductor layer; a third nitride semiconductor layer formed on the second nitride semiconductor layer; and a source electrode and a drain electrode formed so as to contact at least a part of the second nitride semiconductor layer or the third nitride semiconductor layer. A recess that exposes a part of the second nitride semiconductor layer is formed between the source electrode and the drain electrode in the third nitride semiconductor layer. A gate electrode is formed in the recess and an insulating film is formed between the third nitride semiconductor layer and the gate electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 on Patent Application No. 2007-111451 filed in Japan on Apr. 20, 2007, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a normally-off type nitride semiconductor field effect transistor that is used in a high breakdown voltage power transistor. 
     2. Background Art 
     A GaN (gallium nitride) based compound semiconductor has a high breakdown field and a high saturation electron velocity and therefore is a very attractive semiconductor material for a future power transistor that requires a low on-state resistance and a high breakdown voltage. In an AlGaN/GaN (aluminum gallium nitride/gallium nitride) heterostructure formed on a substrate having a (0001) plane as a main surface, charges are formed at the heterointerface by spontaneous polarization and piezoelectric polarization, and a sheet carrier concentration of 1×10 13  cm −2  or more is obtained even in an undoped state. A high current density HFET (Hetero-junction Field Effect Transistor) has been implemented by using this high concentration 2DEG (2-Dimensional Electron Gas) at the heterointerface. A conventional AlGaN/GaN HFET is a so-called normally-on type HFET in which a source-drain current flows when a drain voltage is applied with a gate electrode of 0 V. 
     From a standpoint of the safety in case of power outage, operation with a single circuit power supply, and the like, a power transistor to be used in practical applications should be a so-called normally-off type power transistor in which no current flows when a gate voltage is 0 V. A normally-on type power transistor has safety problems; for example, a circuit is damaged in case of power outage. Moreover, the normally-on type power transistor is operated by applying a positive voltage to a drain electrode and a negative voltage to a gate electrode. Therefore, two power supplies, that is, positive and negative power supplies, are required for the normally-on type power transistor. However, a normally-off type power transistor is operated by applying a positive voltage to both a drain electrode and a gate electrode. Therefore, the normally-off type power transistor can be operated with a single power supply. 
     It is desirable that, when a gate voltage is increased, a normally-off type FET has a sufficiently low gate leakage current until a drain current is saturated. In a conventional Schottky gate structure, however, a significant forward current flows at a gate voltage Vg of about +1 V, and therefore, a gate leakage current flows before a drain current is saturated. Accordingly, instead of the Schottky gate structure, a gate structure in which a gate leakage current is low even when a positive gate voltage is applied is required for the normally-off type FET. 
     Hereinafter, a conventional normally-off type GaN-based FET will be described. A method for forming a MIS (Metal Insulator Semiconductor)-type AlGaN/GaN HFET as a low gate leakage current element by forming an SiN (silicon nitride) insulating film under a gate electrode has been reported (e.g., see Japanese Laid-Open Patent Publication No. 2006-173294). Since SiN used as the insulating film has a wide bandgap, carriers are less likely to be transported between a gate metal and a semiconductor layer, and a gate leakage current can be reduced as compared to the Schottky gate structure. 
     It has been recently reported that, in such an AlGaN/GaN MIS-HFET, forming an insulating film of, e.g., SiN on an AlGaN barrier layer increases a sheet carrier concentration at the AlGaN/GaN heterointerface (e.g., see M. Higashiwaki, T. Matsui, “AlGaN/GaN Heterostructure Field-Effect Transistors with Current Gain Cut-off Frequency of 152 GHz on Sapphire Substrates,” J. J. Appl. Phys. vol. 44 (2005), L475; hereinafter, this document is referred to as Document 1). In Document 1, a high current MIS-HFET having excellent high frequency characteristics is implemented by making positive use of a high concentration 2DEG. 
     However, in order to implement both a low gate leakage current and a normally-off type operation, it is desirable that the sheet carrier concentration at the AlGaN/GaN interface does not increase in the MIS structure as well. In other words, in order to obtain a normally-off type FET while reducing a gate leakage current, it is necessary to compensate not only for polarized charges produced by a polarization effect but for increased sheet carriers in the MIS structure to deplete the AlGaN/GaN interface under a gate electrode. 
     A normally-off type AlGaN/GaN MIS-HFET in which a 2DEG at the AlGaN/GaN interface is depleted by implanting fluorine (F) ions into an AlGaN layer directly below a gate electrode by plasma processing and an SiN film is formed between the gate electrode and an AlGaN layer has been reported as a MIS-HFET that meets the above requirements (e.g., see R. Wang, Y. Cai, C. W. Tang, K. M. Lau, K. J. Chen, “Enhancement-Mode Si 3 N 4 /AlGaN/GaN MISHFETs,” IEEE Electron Device Lett., vol. 27, no. 10, pp. 793-795, October 2006). 
     However, detailed physical properties of the conventional normally-off type AlGaN/GaN MIS-HFET involving fluorine implantation have not been clarified. Moreover, the element is damaged by the plasma processing for fluorine implantation into the AlGaN layer, causing degradation in operation characteristics of the element or fluctuation for reliability through device processing. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, it is an object of the invention to obtain a low gate leakage current normally-off type Group-III nitride semiconductor field effect transistor without involving damage by plasma processing. 
     In order to achieve the above object, a field effect transistor of the invention is formed on a so-called nonpolar plane of a nitride semiconductor. This structure suppresses generation of charges from spontaneous polarization and piezoelectric polarization. Therefore, a normally-off type Group-III nitride semiconductor field effect transistor can be implemented. Moreover, since the nonpolar plane is used as a main surface, increase in sheet carriers in a channel layer from formation of an insulating film can be prevented. Therefore, shift of a threshold voltage to negative bias of the normally-off type field effect transistor can be prevented. 
     More specifically, a field effect transistor according to the invention includes: a first nitride semiconductor layer having a plane perpendicular to a (0001) plane or a plane tilted with respect to the (0001) plane as a main surface; a second nitride semiconductor layer formed on the first nitride semiconductor layer and having a wider bandgap than the first nitride semiconductor layer; a third nitride semiconductor layer formed on the second nitride semiconductor layer; a source electrode and a drain electrode formed so as to contact at least a part of the second nitride semiconductor layer or the third nitride semiconductor layer; a gate electrode provided between the source electrode and the drain electrode in the third nitride semiconductor layer and formed in a recess that exposes a part of the second nitride semiconductor layer; and an insulating film formed between the third nitride semiconductor layer and the gate electrode. 
     In the field effect transistor of the invention, the first nitride semiconductor layer has a plane perpendicular to the (0001) plane or a plane tilted with respect to the (0001) plane as a main surface. Therefore, generation of charges from spontaneous polarization and piezoelectric polarization can be suppressed. A normally-off type nitride semiconductor field effect transistor can thus be implemented. Moreover, since a plane perpendicular to the (0001) plane or a plane tilted with respect to the (0001) plane is used as a main surface of the first nitride semiconductor layer, increase in sheet carriers in a channel layer can be prevented when the insulating film is formed between the third nitride semiconductor layer and the gate electrode. Accordingly, reduction in a threshold voltage of the normally-off type field effect transistor can be prevented. Moreover, a normally-off type nitride semiconductor field effect transistor having a higher threshold voltage and excellent operation characteristics can be obtained by using a so-called gate recess structure. 
     In the field effect transistor of the invention, it is preferable that impurities providing n-type conductivity are doped in at least one of the second nitride semiconductor layer and the third nitride semiconductor layer. 
     By doping impurities providing n-type conductivity to the second nitride semiconductor layer, electrons serving as carriers can be supplied to the channel layer of the field effect transistor. Therefore, a series resistance of the transistor can be reduced. By doping impurities providing n-type conductivity to the third nitride semiconductor layer, a contact resistance with an ohmic electrode formed thereon can be reduced. With this structure, a normally-off type field effect transistor having a low series parasitic resistance can be implemented. 
     In the field effect transistor of the invention, it is preferable that the insulating film is formed so as to cover at least a part of each surface of the source electrode and the drain electrode. 
     This structure eliminates the need to remove the insulating film by a dry etching method before formation of ohmic electrodes, that is, the source electrode and the drain electrode. Therefore, a normally-off type nitride semiconductor field effect transistor can be obtained without involving damage by plasma processing. 
     It is preferable that the field effect transistor of the invention has normally-off characteristics. 
     This improves safety in case of, e.g., power outage and enables operation with a single circuit power supply. 
     In the field effect transistor of the invention, it is preferable that the insulating film is a single-layer film of SiN, SiO 2 , Al 2 O 3 , HfO 2 , or AlN, or a multi-layer film including at least two of SiN, SiO 2 , Al 2 O 3 , HfO 2 , and AlN. 
     A low gate leakage current normally-off type nitride semiconductor field effect transistor can thus be implemented. 
     In the field effect transistor of the invention, it is preferable that the plane perpendicular to the (0001) plane or the plane tilted with respect to the (0001) plane is a (11-20) plane, a (1-100) plane, a (1-101) plane, a (1-102) plane, a (11-22) plane, or a (11-24) plane. 
     By using such a nonpolar or semipolar plane as a main surface of each nitride semiconductor layer, generation of charges from spontaneous polarization or piezoelectric polarization as generated on the (0001) plane in the conventional example can be suppressed. As a result, a normally-off type nitride semiconductor field effect transistor can be implemented. 
     In the field effect transistor of the invention, it is preferable that the first nitride semiconductor layer is made of GaN, the second nitride semiconductor layer is made of Al x Ga 1-x N (0&lt;x&lt;1), and the third nitride semiconductor layer is made of GaN. 
     In this case, a 2DEG is formed at the heterointerface between the first nitride semiconductor layer and the second nitride semiconductor layer at a gate bias voltage higher than a threshold voltage, and electrons can travel through the 2DEG as a channel. Accordingly, a normally-off type nitride semiconductor field effect transistor having a low series resistance can be implemented. Moreover, since the third nitride semiconductor layer can be used as a low resistance GaN cap layer, a normally-off type nitride semiconductor field effect transistor having a low series parasitic resistance can be formed. 
     As has been described above, a low gate leakage current normally-off type nitride semiconductor field effect transistor can be implemented by the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a field effect transistor according to an embodiment of the invention; 
         FIG. 2A  is a graph showing current-voltage characteristics of a field effect transistor according to an embodiment of the invention and a comparative example by a logarithmic scale, and  FIG. 2B  is a graph showing current-voltage characteristics of a field effect transistor according to an embodiment of the invention and a comparative example by a linear scale; 
         FIG. 3  is a table showing a change in a sheet carrier concentration before and after formation of a SiN gate insulating film in an a-plane AlGaN/GaN HFET as a field effect transistor according to an embodiment of the invention and a c-plane AlGaN/GaN HFET of a comparative example; 
         FIG. 4  is a graph showing current-voltage characteristics of a field effect transistor according to an embodiment of the invention; 
         FIG. 5  is a graph showing transfer characteristics of a field effect transistor according to an embodiment of the invention; 
         FIG. 6  is a graph showing high frequency characteristics of a field effect transistor according to an embodiment of the invention; 
         FIGS. 7A ,  7 B, and  7 C are cross-sectional views sequentially illustrating the steps of a method for manufacturing a field effect transistor according to an embodiment of the invention; 
         FIGS. 8A ,  8 B, and  8 C are cross-sectional views sequentially illustrating the steps of a method for manufacturing a field effect transistor according to an embodiment of the invention; 
         FIG. 9  is a cross-sectional view illustrating a step of a method for manufacturing a field effect transistor according to an embodiment of the invention; 
         FIGS. 10A ,  10 B, and  10 C are cross-sectional views sequentially illustrating the steps of a method for manufacturing a field effect transistor according to a modification of an embodiment of the invention; 
         FIGS. 11A ,  11 B, and  11 C are cross-sectional views sequentially illustrating the steps of a method for manufacturing a field effect transistor according to a modification of an embodiment of the invention; 
         FIGS. 12A ,  12 B, and  12 C are cross-sectional views sequentially illustrating the steps of a method for manufacturing a field effect transistor according to a modification of an embodiment of the invention; 
         FIGS. 13A and 13B  are schematic conduction band diagrams illustrating a behavior of a sheet carrier concentration at the AlGaN/GaN interface of a c-plane AlGaN/GaN HFET of a conventional example, where  FIG. 13A  shows the case in which a gate electrode is provided directly on a c-plane AlGaN/GaN HFET, and  FIG. 13B  shows the case in which a SiN film is formed between a c-plane AlGaN/GaN HFET and a gate electrode; and 
         FIGS. 13C and 13D  are schematic conduction band diagrams illustrating a behavior of a sheet carrier concentration at the AlGaN/GaN interface of an a-plane AlGaN/GaN HFET, where  FIG. 13C  shows the case in which a gate electrode is provided directly on an a-plane AlGaN/GaN HFET in a comparative example, and  FIG. 13D  shows the case in which a SiN film is formed between an a-plane AlGaN/GaN HFET and a gate electrode in the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 
     A field effect transistor according to an embodiment of the invention will now be described with reference to the accompanying drawings. In the following description, an a-plane indicates a (11-20) plane, an r-plane indicates a (1-102) plane, and a c-plane indicates a (0001) plane unless otherwise mentioned. For convenience, a negative sign given to a Miller index of plane orientation indicates inversion of an index following the negative sign. 
       FIG. 1  shows a cross-sectional structure of an AlGaN/GaN heterojunction field effect transistor having an a-plane as a main surface (hereinafter, simply referred to as an a-plane AlGaN/GaN HFET) according to a first embodiment of the invention. 
     As shown in  FIG. 1 , for example, an AlN (aluminum nitride) buffer layer  102  having a thickness of 500 nm, an undoped GaN layer  103  having a thickness of 3 nm, an AlN spacer layer  104  having a thickness of 1 nm, an n-type Al x Ga 1-x N barrier layer  105  (0&lt;x&lt;1) having a thickness of 15 nm, and an n-type GaN cap layer  106  having a thickness of 50 nm are sequentially formed on a sapphire substrate  101  in this order by crystal growth. The sapphire substrate  101  has an r-plane as a main surface, and each of the buffer layer  102 , the undoped GaN layer  103 , the AlN spacer layer  104 , the n-type Al x Ga 1-x N barrier layer  105 , and the n-type GaN cap layer  106  has an a-plane as a main surface. 
     The n-type Al x Ga 1-x N barrier layer  105  herein has an Al composition of 0.25. The spacer layer  104  formed between the GaN layer  103  and the barrier layer  105  need not necessarily be provided. 
     A part of the cap layer  106  is removed by, e.g. a dry etching method to form a recess (opening) exposing the barrier layer  105  from the cap layer  106 . A SiN gate insulating film  107  having a thickness of 3 nm is formed so as to cover the side surface and the bottom surface of the recess. 
     A high resistance isolation layer  110  is formed at both ends of each layer from the cap layer  106  to the upper portion of the GaN layer  103  by implantation of, e.g., boron (B + ) ions. The isolation layer  110  may be formed by a mesa isolation method or a selective oxidation method. 
     A Ti/Al (titanium/aluminum) ohmic electrode (source-drain electrode)  109  is formed on both sides of the recess on the cap layer  106  at a distance from the recess. A PdSi (palladium silicon) gate electrode  108  is formed as a so-called recess gate in the recess in the cap layer  106  with the gate insulating film  107  interposed therebetween. 
     In this embodiment, the n-type AlGaN barrier layer  105  has a thickness of 15 nm. However, in the case where a thinner barrier layer  105  is formed so that a depletion layer formed under the gate electrode  108  reaches a channel of the GaN layer  103 , the channel can be pinched off without application of a gate voltage. In this case, a threshold voltage of the normally-off type AlGaN/GaN HFET can further be increased. Accordingly, the thickness of the barrier layer  105  is not limited to 15 nm, and the barrier layer  105  may be thinner than 15 nm. 
     The MIS gate structure is used in this embodiment. However, a MES (Metal Semiconductor) gate structure may be used as long as normally-off type characteristics can be obtained. 
     In this embodiment, an electrode contact resistance is reduced by forming the n-type GaN cap layer  106  having an a-plane as a main surface by doping n-type impurities at a high concentration, and forming the source/drain electrode  109  on the n-type GaN cap layer  106 . With this structure, a sufficiently low contact resistance of 2.7×10 −6  Ωcm 2  can be obtained between the source/drain electrode  109  and the cap layer  106 . Accordingly, a normally-off type a-plane AlGaN/GaN HFET having a low source-drain series resistance can be obtained. 
     The cap layer  106  may alternatively be formed by forming n-type doped Al 0.25 Ga 0.75 N/GaN periodically, for example, with seven periods, with a thickness of 50 nm, or may be an In x Al y Ga 1-x-y N (indium aluminum gallium nitride) layer (0&lt;x&lt;1, 0&lt;y&lt;1, and 0&lt;x+y&lt;1). In order to reduce a contact resistance, a so-called ohmic recess structure may be formed by forming a recess under the ohmic electrode  109  in the cap layer  106  so that the ohmic electrode  109  contacts the barrier layer  105 . 
     Typical dimensions of the field effect transistor of this embodiment are as follows: the gate recess has a width of 0.6 μm in the gate length direction; the upper portion of the gate electrode  108  has a gate length of 1.0 μm; and both ends of the upper portion of the gate electrode  108  in the gate length direction overlap the cap layer  106  with a width of 0.2 μm. In other words, the field effect transistor of this embodiment has a narrow recess structure. In this structure, a 2DEG layer is formed near a heterojunction at the interface between the GaN layer  103  and the spacer layer  104  under the gate electrode  108  when a gate bias voltage higher than the threshold voltage is applied. Therefore, electrons serving as carriers travel through a path formed by the source electrode  109 , the cap layer  106  under the source electrode  109 , the 2DEG layer, the cap layer  106  under the drain electrode  109 , and the drain electrode  109 . Accordingly, in an on state, electrons can travel only in the 2DEG layer as a channel, whereby a low on-state resistance field effect transistor can be implemented. Since the gate electrode  108  and the cap layer  106  contact each other with the SiN gate insulating film  107  interposed therebetween, no significant gate leakage current is generated even when a forward voltage of 5 V or higher is applied. 
     More preferably, a channel resistance and a source resistance can be reduced by further reducing the width of the gate recess. A normally-off type a-plane AlGaN/GaN HFET having a low source-drain series resistance can thus be produced. 
       FIG. 2A  shows gate-drain current-voltage characteristics in the a-plane AlGaN/GaN HFET according to this embodiment by a logarithmic scale. It can be seen from  FIG. 2A  that the MIS type HFET of the invention has a reduced reverse leakage current and the reverse leakage current can be reduced to 10 −2  times at V g =−5 V as compared to a Schottky gate MES type HFET of a comparative example. 
       FIG. 2B  shows gate-drain forward current-voltage characteristics in the a-plane AlGaN/GaN HFET according to this embodiment by a linear scale. As shown in  FIG. 2B , a forward rising voltage is about 0.5 V in the MES type HFET of the comparative example, while no significant gate leakage current is recognized in the MIS type HFET of the invention even when a forward voltage of 5 V is applied. A gate leakage current can thus be reduced by using a MIS gate structure. 
     The gate insulating film  107  is made of SiN in this embodiment. However, the gate insulating film may be made of any material such as SiO 2  (silicon dioxide), HfO 2  (hafnium dioxide), Al 2 O 3  (aluminum oxide), or AlN as long as the same effects can be obtained. The gate insulating film may be a multi-layer film made of a combination of at least two kinds of the above insulating films. 
       FIG. 3  shows a change in a sheet carrier concentration before and after formation of the SiN gate insulating film  107  in the a-plane AlGaN/GaN HFET of this embodiment and a c-plane AlGaN/GaN HFET of a comparative example. An Al composition is 0.25 and an AlGaN layer has a thickness of 25 nm in the embodiment and the comparative example. In a c-plane AlGaN/GaN HFET of Document 1, an Al composition is 0.4 and an AlGaN layer has a thickness of 6 nm. It can be seen from the table that, in the a-plane Al 0.25 Ga 0.75 N 25 nm/GaN MIS-HFET of the invention, the sheet carrier concentration hardly changes by formation of SiN. In the c-plane Al 0.25 Ga 0.75 N 25 nm/GaN MIS-HFET of the comparative example, on the other hand, the sheet carrier concentration is approximately doubled by formation of SiN. In the c-plane Al 0.4 Ga 0.6 N 6 nm/GaN MIS-HFET of Document 1, the sheet carrier concentration is increased to about four times by formation of SiN. 
     It has been reported that, in AlGaN/GaN MIS-HFETs having a c-plane as a main surface, a surface potential of an AlGaN layer (a barrier layer) is reduced and a sheet carrier concentration at the AlGaN/GaN heterointerface is increased when an insulating film such as SiN is formed on the surface of the AlGaN layer (Document 1). The same results were obtained by the experiments conducted by the inventors. A high current, high frequency MIS-HFET that includes a nitride semiconductor layer having a c-plane as a main surface has been reported by making positive use of this high sheet carrier concentration. In order to implement a normally-off type operation while reducing a gate leakage current, however, it is desirable that the sheet carrier concentration at the AlGaN/GaN interface does not increase in the MIS structure as well. 
     On the other hand, the sheet carrier concentration changes only slightly in the MIS-HFET that includes a nitride semiconductor layer having an a-plane as a main surface as in the invention. Therefore, both a low gate leakage current and a normally-off type operation can be implemented at the same time without taking into consideration a change in a threshold voltage by the MIS structure. 
     Hereinafter, the reason why a low gate leakage current and a normally-off type operation can be implemented at the same time will be described based on  FIGS. 13A through 13D . 
       FIGS. 13A through 13D  are schematic conduction band diagrams illustrating a behavior of a sheet carrier concentration at the AlGaN/GaN interface of an AlGaN/GaN HFET. As shown in  FIGS. 13A and 13B , it can be considered that one of the causes that increase a sheet carrier concentration in a c-plane AlGaN/GaN HFET having a SiN film on the surface of an AlGaN layer is as follows: Si becomes donor impurities by Si—N bonding at the SiN/AlGaN interface, whereby positive charges are formed and a surface potential is reduced. As a result, electrons are compensated for at the AlGaN/GaN interface, and the concentration of a 2-dimensional electron gas (2DEG) layer is increased. 
     As shown in  FIGS. 13C and 13D , however, in the case of the AlGaN/GaN HFET having an a-plane as a main surface according to the invention, Group-III atoms and Group-V atoms are located on the same plane. Accordingly, the number of dangling bonds of nitrogen at the topmost surface is one half of that on the c-plane. As a result, it is considered that the number of free electrons that are compensated for in the 2DEG layer by Si serving as donor impurities is reduced by Si—N bonding. Since increase in the sheet carrier concentration is reduced as compared to the case where the c-plane is used, a low gate leakage current and a normally-off type operation can be implemented at the same time without taking into consideration a change in a threshold voltage by the MIS structure. 
       FIG. 4  shows the relation between a drain current and a drain voltage of the field effect transistor of the embodiment. It can be seen from  FIG. 4  that the threshold voltage is +1.3 V and a normally-off type field effect transistor is obtained. 
       FIG. 5  shows transfer characteristics of the field effect transistor of the embodiment. As shown in  FIG. 5 , a maximum drain current I max  is 112 mA/mm and a maximum mutual conductance g mmax  is 47 mS/mm. 
       FIG. 6  shows frequency dependencies of a current gain and a maximum stable power gain (MSG) or maximum available power gain (MAG) calculated from a measured S parameter in the field effect transistor of the embodiment. It can be seen that, at a gate voltage V g  of 4.5 V and a drain-source voltage V ds  of 4.5 V, a current gain cutoff frequency (f T ) and a maximum oscillating frequency (f max ) in the field effect transistor are 2.3 GHz and 4.0 GHz, respectively. 
     In this embodiment, an a-plane (a (11-20) plane) is used as a main growth surface of each nitride semiconductor layer. However, a (1-100) plane, a (1-101) plane, a (1-102) plane, a (11-22) plane, or a (11-24) plane, or any other orientations may be used as long as the same effects can be obtained. 
     In this embodiment, a sapphire substrate having an r-plane as a main surface is used as the substrate  101 . However, the invention is not limited to sapphire, and the substrate may be made of silicon carbide (SiC), gallium nitride (GaN), silicon (Si), or the like and the substrate may have any orientation as long as the same effects can be obtained. 
     Hereinafter, a method for manufacturing an a-plane AlGaN/GaN heterojunction field effect transistor having the above structure will be described with reference to the figures. 
       FIGS. 7A through 7C ,  FIGS. 8A through 8C , and  FIG. 9  show cross-sectional structures sequentially illustrating the steps of a method for manufacturing a field effect transistor according to an embodiment of the invention. 
     As shown in  FIG. 7A , an AlN buffer layer  702  having a thickness of 500 nm, an undoped GaN layer  703  having a thickness of 3 μm, an AlN spacer layer  704  having a thickness of 1 nm, an Al 0.25 Ga 0.75 N barrier layer (electron supply layer)  705  having a thickness of 15 nm, and a Si-doped n-type GaN cap layer  706  having a thickness of 50 nm are sequentially epitaxially grown on a main surface of a sapphire substrate  701  by, for example, a MOCVD (Metal Organic Chemical Vapor Deposition) method. The substrate  701  has an r-plane as a main surface, and each of the buffer layer  702 , the GaN layer  703 , the spacer layer  704 , the barrier layer  705 , and the cap layer  706  has an a-plane as a main surface (a growth surface). 
     As shown in  FIG. 7B , a recess (a gate recess) is then formed by selectively etching the cap layer  706  so as to expose the barrier layer  705  by a dry etching method such as an ICP (Inductively Coupled Plasma) method. 
     As shown in  FIG. 7C , an isolation layer  710  is then formed on both side of the gate recess so that each isolation layer  710  is located at a distance from the gate recess. More specifically, the isolation layer  710  is formed by selectively implanting boron (B + ) ions from the cap layer  706  to the upper portion of the GaN layer  703  so as to selectively increase the resistance of each semiconductor layer. The process of increasing the resistance is not limited to the ion implantation method. For example, a selective thermal oxidation method may alternatively be used. 
     As shown in  FIG. 8A , a SiN gate insulating film  707  having a thickness of 3 nm is then formed on the whole surface including the wall surface and the bottom surface of the gate recess on the isolation layer  710  and the cap layer  706  by, for example, a plasma CVD (Chemical Vapor Deposition) method. The gate insulating film  707  may be formed by an MOCVD method instead of the plasma CVD method. 
     As shown in  FIG. 8B , the cap layer  706  is selectively exposed from the gate insulating film  707  by removing the gate insulating film  707  in an ohmic electrode formation region between the gate recess and each isolation layer  710  by a dry etching method. A contact resistance between an ohmic electrode to be formed and the cap layer  706  is thus reduced. 
     As shown in  FIG. 8C , a Ti/Al ohmic electrode (source/drain electrode)  709  is formed on the exposed cap layer  706  by, for example, a sputtering method and a lift-off method. A sintering process is then conducted. 
     As shown in  FIG. 9 , a PdSi gate electrode  708  is formed in the gate recess in the cap layer  706  and on the periphery of the gate recess with the gate insulating film  707  interposed therebetween by, for example, a sputtering method and a lift-off method. 
     The field effect transistor according to the embodiment of the invention is thus obtained. In this embodiment, the substrate  701  for epitaxial growth is a sapphire substrate having an r-plane as a main surface. However, the invention is not limited to sapphire having an r-plane as a main surface. For example, the substrate  701  may be made of SiC, GaN, Si, or the like and any orientation may be used as a main surface of the substrate  701  as long as the same effects as those obtained by sapphire having an r-plane as a main surface can be obtained. 
     (Modification of the Manufacturing Method) 
     Hereinafter, a method for manufacturing an a-plane AlGaN/GaN heterojunction field effect transistor according to a modification of the embodiment of the invention will be described with reference to the figures. 
       FIGS. 10A through 10C ,  FIGS. 11A through 11C , and  FIGS. 12A through 12C  show cross-sectional structures sequentially illustrating the steps of the method for manufacturing a field effect transistor according to the modification of the embodiment. 
     As shown in  FIG. 10A , an AlN buffer layer  802  having a thickness of 500 nm, an undoped GaN layer  803  having a thickness of 3 μm, an AlN spacer layer  804  having a thickness of 1 nm, and an Al 0.25 Ga 0.75 N barrier layer (electron supply layer)  805  having a thickness of 15 nm are sequentially epitaxially grown on a main surface of a sapphire substrate  801  by, for example, a MOCVD method. The substrate  801  has an r-plane as a main surface, and each of the buffer layer  802 , the GaN layer  803 , the spacer layer  804 , and the barrier layer  805  has an a-plane as a main surface (a growth surface). 
     As shown in  FIG. 10B , a SiO 2  mask film  811  having a thickness of 100 nm is then selectively formed in a gate recess formation region on the barrier layer  805 . 
     As shown in  FIG. 10C , a Si-doped n-type GaN cap layer  806  having an a-plane as a main surface is then formed with a thickness of 50 nm on the barrier layer  805  having the mask film  811  formed thereon by a MOCVD method. The cap layer  806  is grown on the barrier layer  805  except on the mask film  811 . 
     As shown in  FIG. 11A , a recess (a gate recess) exposing the barrier layer  805  is then formed in the cap layer  806  by removing the mask film  811  with hydrofluoric acid (HF) or the like. In this modification, when the gate recess structure is formed in the cap layer  806 , the exposed surface of the barrier layer  805  from the cap layer  806  is not subjected to damage from plasma processing that is conducted in a dry etching method. 
     As shown in  FIG. 11B , an isolation layer  810  is then formed on both side of the gate recess so that each isolation layer  810  is located at a distance from the gate recess. More specifically, the isolation layer  810  is formed by selectively implanting boron (B + ) ions from the cap layer  806  to the upper portion of the GaN layer  803  so as to selectively increase the resistance of each semiconductor layer. The process of increasing the resistance is not limited to the ion implantation method. For example, a selective thermal oxidation method may alternatively be used. 
     As shown in  FIG. 11C , a Ti/Al ohmic electrode (source/drain electrode)  809  is selectively formed in a region between the gate recess and each isolation layer  810  on the cap layer  806  by, for example, a sputtering method and a lift-off method. A sintering process is then conducted. 
     As shown in  FIG. 12A , a SiN gate insulating film  807  having a thickness of 3 nm is then formed on the whole surface including the wall surface and the bottom surface of the gate recess on the isolation layer  810 , the cap layer  806 , and the ohmic electrode  809  by, for example, a plasma CVD method. The gate insulating film  807  may be formed by an MOCVD method instead of the plasma CVD method. 
     As shown in  FIG. 12B , a PdSi gate electrode  808  is then formed in the gate recess in the cap layer  806  and on the periphery of the gate recess with the gate insulating film  807  interposed therebetween by, for example, a sputtering method and a lift-off method. 
     As shown in  FIG. 12C , a contact hole exposing the top surface of each ohmic electrode  809  is then selectively formed by removing the gate insulating film  807  on each ohmic electrode  809  by a dry etching method. 
     According to the manufacturing method of this modification, the barrier layer  805  is not subjected to damage from plasma processing when the gate recess exposing the barrier layer  805  is formed in the cap layer  806 . Therefore, a low gate leakage current normally-off type field effect transistor can be obtained. 
     In this modification, the substrate  801  for epitaxial growth is a sapphire substrate having an r-plane as a main surface. However, the invention is not limited to sapphire having an r-plane as a main surface. For example, the substrate  801  may be made of SiC, GaN, Si, or the like and any orientation may be used as a main surface of the substrate  801  as long as the same effects as those obtained by sapphire having an r-plane as a main surface can be obtained. 
     As has been described above, a low gate leakage current normally-off type nitride semiconductor field effect transistor that is applicable to, for example, a high power transistor can be implemented by the invention.