Abstract:
A field-effect transistor includes: a carrier supply layer supplying carriers; a Schottky contact layer forming a Schottky barrier; and an intermediate layer formed between the carrier supply layer and the Schottky contact layer. Here, the intermediate layer has an electron affinity which is higher than an electron affinity of the carrier supply layer but lower than an electron affinity of the Schottky contact layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The present invention relates to a field-effect transistor (FET), especially to an FET for use in a high-frequency communication device.  
         [0003]     (2) Related Art  
         [0004]     Conventionally, a Metal-Insulator-Semiconductor Field-Effect Transistor (MISFET), which is an insulated-gate FET, has been utilized in an electronic device such as a silicon semiconductor integrated circuit device.  
         [0005]     When compared with a silicon (Si) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) of the same size, an FET formed using gallium arsenide (GaAs) or a combination of GaAs and aluminum gallium arsenide (Al x Ga 1-x As) provides higher-speed performance, and is more suitable for a microwave amplifier and a high-speed logic IC. This is because group III-V compound semiconductors such as GaAs, InP and GaN have a higher (approximately five- to six-fold) electron mobility than Si. An FET formed using a semiconductor with such a high electron mobility achieves excellent high-frequency characteristics.  
         [0006]     However, a MOSFET or MISFET formed using a group III-V compound semiconductor such as GaAs has a problem regarding interface states. To be specific, interface states are formed at an interface between an insulating film and a compound semiconductor. Because such interface states lower mutual conductance (gm), the MOSFET or MISFET can not achieve good performance.  
         [0007]     For this reason, a Metal Semiconductor Field-Effect Transistor (MESFET) or a High Electron Mobility Transistor (HEMT) formed using a compound semiconductor such as GaAs is used for a constituent of an electronic device such as a high-frequency amplifier and an ultrahigh-speed integrated circuit device. For example, a MESFET is used as an amplifying element in a mobile communication device such as a mobile telephone, and a HEMT is used as an amplifying element in a receiving antenna for satellite broadcasting.  
         [0008]     In particular, a HEMT provides high-speed performance and low noise figure, since two-dimensional electron gas (2DEG) which is produced in a channel layer made of high-purity InGaAs and has a high electron mobility is used as carriers. To be specific, a layer in which electrons are generated (carrier supply layer) and a layer in which electrons move (the channel layer) are separated from each other. Therefore, electrons can move stably at high speed from a source to a drain without colliding with impurities, thereby reducing noise figure.  
         [0009]     Here, to increase a surface charge density of the 2DEG accumulated in the channel layer, an energy barrier formed at an interface between the channel layer and the carrier supply layer needs to be made as high as possible.  
         [0010]     In the HEMT, a Schottky contact layer and an ohmic contact layer are formed, in the stated order, on the carrier supply layer during a wafer epitaxial growth. The ohmic contact layer is partially removed, so as to partially expose the Schottky contact layer. A gate electrode is provided on the exposed surface of the Schottky contact layer. Furthermore, a source electrode and a drain electrode are formed on the ohmic contact layer so as to oppose each other with the gate electrode therebetween.  
         [0011]     As described above, the Schottky contact layer, on which the gate electrode is formed, is sandwiched between the carrier supply layer and the ohmic contact layer. Here, a high Schottky barrier is formed between the gate electrode and the Schottky contact layer. A height Φ B  of the Schottky barrier is defined as a difference between an electron affinity φ s  of the Schottky contact layer and an electron affinity φ m  of a metal forming the gate electrode, as indicated by the following formula (1): 
 
Φ B =φ m −φ s   (1) 
 
         [0012]     If the Schottky barrier Φ B  is sufficiently high, electrons in the carrier supply layer and the 2DEG accumulated in the channel layer are difficult to overcome or pass through the Schottky barrier Φ B  to flow into the gate electrode. This can reduce leakage currents caused when a reverse bias is applied, and improve resistance to dielectric breakdown.  
         [0013]     The Schottky contact layer forms a path of electric currents flowing between the source electrode and the drain electrode. In detail, electrons need to go through the Schottky contact layer in order to reach the channel layer from the source electrode, and in order to reach the drain electrode through the channel layer. Therefore, while forming a high Schottky barrier (high resistance) with respect to the gate electrode as stated above, the Schottky contact layer is required to provide a lowest possible resistance with respect to the source electrode and the drain electrode. To meet these contradictory needs, the ohmic contact layer is formed between the Schottky contact layer and the source and drain electrodes. As a result, an ohmic contact, which has a low resistance, is formed between each of the source and drain electrodes and the ohmic contact layer. Hence, a resistance between the source and rain electrodes is reduced.  
         [0014]     Furthermore, the Schottky contact layer is conventionally made of the same material as the carrier supply layer, which is AlGaAs. Therefore, the Schottky barrier ΦB is relatively high in the HEMT including the Schottky contact layer made of AlGaAs. This results in reduction in leakage currents caused when the gate electrode is reverse-biased.  
         [0015]     However, the HEMT including the Schottky contact layer made of AlGaAs has a problem of frequency dispersion of drain currents. In other words, a maximum drain current density decreases when a high-frequency signal is applied. Frequency dispersion of drain conductance causes a degree of amplification of a circuit to vary in accordance with an operating frequency. This makes it impossible to amplify a pulse appropriately.  
         [0016]     A possible reason for such frequency dispersion of drain currents is that AlGaAs has a high interface state density. AlGaAs forms many interface states, that is to say, electron trap levels, at an interface between different layers. Electrons are trapped by the electron trap levels at the interface. Each of the electron trap levels releases trapped electrons into a conduction band after a lifetime of the electron trap level. Here, the lifetime of the interface states formed by AlGaAs is relatively long. If an FET with such many electron trap levels having a long lifetime operates at a frequency corresponding to the lifetime, the electron trap levels significantly affects frequency performance of the FET.  
         [0017]     For example, some electron trap levels of a GaAs substrate have a long lifetime of several milliseconds. If an operating frequency of several KHz is set as a criteria, there is a large difference in performance of the FET between when operating at frequencies lower and higher than the criteria. This causes frequency dispersion of drain conductance.  
         [0018]     The problem of the frequency dispersion can be solved by a technique disclosed in Japanese patent application H15-032038. This technique uses InGaP for a Schottky contact layer, instead of AlGaAs.  
         [0019]      FIG. 5  illustrates a cross-section of an FET  50  disclosed in the patent document. As shown in  FIG. 5 , the FET  50  is formed by providing an AlGaAs buffer layer  61 , an n-AlGaAs barrier layer  62 , an undoped InGaAs channel layer  63 , an n-AlGaAs carrier supply layer  64 , an undoped InGaP Schottky contact layer  65 , and an ohmic contact layer  66  which has an opening so that part of the Schottky contact layer  65  is exposed, in the stated order, on a GaAs semiconductor substrate  60 .  
         [0020]     Furthermore, a gate electrode  67  is formed so as to be in contact with the Schottky contact layer  65 , and a source electrode  68  and a drain electrode  69  are each formed so as to be in contact with the ohmic contact layer  66 .  
         [0021]     The Schottky contact layer  65  is made of InGaP, which has a lower interface state density than AlGaAs. Therefore, the number of interface states formed at an interface between the Schottky contact layer  65  and the gate electrode  67  can be reduced. This can reduce frequency dispersion of drain currents.  
         [0022]     However, the conventional FET  50  has the following drawback. A discontinuity in energy of a conduction band (hereinafter referred to as a band discontinuity) occurs at an interface between the AlGaAs carrier supply layer  64  and the InGaP Schottky contact layer  65 . This band discontinuity blocks electrons passing through the interface between the carrier supply layer  64  and the Schottky contact layer  65 . Therefore, a source resistance Rs of the FET  50  increases, and mutual conductance (gm), which is one of the properties of the FET  50 , accordingly decreases. As a result, the FET  50  can not obtain a sufficient current drive power.  
         [0023]     It is known that the amount of a band discontinuity at an interface between different layers is generally determined by materials of the layers, and that, if the layers are each made of mixed crystal, a composition of each of the layers determines the amount of the band discontinuity. Here, the mutual conductance (gm) is calculated by the following formula (2): 
 
 gm=gmi /(1+ Rs×gmi )  (2) 
 
         [0024]     In the formula (2), gm, gmi, and Rs respectively denote mutual conductance, intrinsic mutual conductance, and a source resistance. If the carrier supply layer  64  is made of Al 0.25 Ga 0.75 As, as an example, a band discontinuity of approximately 0.3 eV occurs at the interface between the AlGaAs carrier supply layer  64  and the InGaP Schottky contact layer  65 , as shown in  FIG. 6 .  
         [0025]     In  FIG. 6 , an energy level of a conduction band is plotted on the vertical axis, and the layers composing the FET  50  are plotted on the horizontal axis in the order of the lamination on the substrate  60 .  
       SUMMARY OF THE INVENTION  
       [0026]     In view of the above problem, an object of the present invention is to provide an FET that has a lower source resistance Rs than in the related art.  
         [0027]     The above object can be achieved by a field-effect transistor including: a carrier supply layer supplying carriers; a Schottky contact layer forming a Schottky barrier; and an intermediate layer formed between the carrier supply layer and the Schottky contact layer. Here, the intermediate layer has an electron affinity which is higher than an electron affinity of the carrier supply layer but lower than an electron affinity of the Schottky contact layer.  
         [0028]     According to this construction, a source resistance of the FET can be reduced for the following reason. If the intermediate layer is configured to have a constant electron affinity, a band discontinuity (ΔEc) at an interface between the intermediate layer and the Schottky contact layer and a band discontinuity (ΔEc) at an interface between the intermediate layer and the carrier supply layer are each smaller than a band discontinuity to be generated at the interface between the Schottky contact layer and the carrier supply layer when the intermediate layer is not provided. As a result, electrons can move smoothly through the Schottky contact layer and the carrier supply layer, to a channel layer adjacent to the carrier supply layer.  
         [0029]     Here, an electron affinity indicates an energy required to excite electrons from a bottom of a conduction band to a vacuum level.  
         [0030]     An electron affinity (χ) is calculated by the following formula (3), where EVAC denotes a vacuum level, and Ec denotes an energy of a conduction band: 
 
χ= EVAC−Ec   (3) 
 
         [0031]     According to the formula (3), when an electron affinity decreases, an energy (Ec) of a conduction band increases. On the other hand, when an electron affinity increases, an energy (Ec) of a conduction band decreases. Since a vacuum level for each layer is the same, a difference in electron affinity is equal to the amount of a band discontinuity (ΔEc).  
         [0032]     Here, the field-effect transistor may further include: a low resistance layer formed on the Schottky contact layer, where the low resistance layer has an electron affinity which is different, by less than 0.1 eV, from the electron affinity of the Schottky contact layer; an ohmic contact layer formed on the low resistance layer; and an electrode formed on the ohmic contact layer.  
         [0033]     According to this construction, the amount of a band discontinuity at an interface between the Schottky contact layer and the low resistance layer is less than 0.1 eV. Here, the band discontinuity of less than 0.1 eV is so small that an energy barrier does not exist virtually at the interface. Therefore, electrons can go through the interface from or to the low resistance layer smoothly. This can improve performance of the FET.  
         [0034]     Here, the electron affinity of the intermediate layer may gradually increase from an interface between the intermediate layer and the carrier supply layer to an interface between the intermediate layer and the Schottky contact layer.  
         [0035]     According to this construction, the electron affinity continuously and gradually increases in the intermediate layer to prevent a band discontinuity. Therefore, an energy barrier is not formed against electrons going through the intermediate layer. This can dramatically reduce the source resistance.  
         [0036]     Here, a difference in electron affinity at the interface between the intermediate layer and the Schottky contact layer may be less than 0.1 eV.  
         [0037]     According to this construction, the intermediate layer and the Schottky contact layer have approximately the same electron affinity at the interface therebetween. In other words, the amount of the band discontinuity can be less than 0.1 eV, which is so low that an energy barrier does not exist virtually. This can significantly lower a source resistance at the interface between the intermediate layer and the Schottky contact layer for electrons moving from or to the intermediate layer.  
         [0038]     Here, the Schottky contact layer may be made of InGaP, the carrier supply layer may be made of Al 0.25 Ga 0.75 As, and the intermediate layer may be made of Al x Ga 1-x As, where 0≦x≦0.25, x denoting a composition ratio. Furthermore, the composition ratio x may gradually increase from the interface between the intermediate layer and the Schottky contact layer to the interface between the intermediate layer and the carrier supply layer.  
         [0039]     According to this construction, the Schottky contact layer is made of InGaP, and the carrier supply layer is made of AlGaAs. Furthermore, the electron affinity gradually increases in the intermediate layer from the interface between the intermediate layer and the carrier supply layer to the interface between the intermediate layer and the Schottky contact layer. This can disperse a band discontinuity between the carrier supply layer and the Schottky contact layer. Therefore, an increase in source resistance caused by the band discontinuity between the Schottky contact layer and the carrier supply layer can be effectively prevented. As a result, even if InGaP, which has a lower interface state density than AlGaAs, is used to form the Schottky contact layer in order to reduce frequency dispersion, a source resistance does not increase.  
         [0040]     Here, the composition ratio x may gradually increase from 0 at the interface between the intermediate layer and the Schottky contact layer, to 0.25 at the interface between the intermediate layer and the carrier supply layer.  
         [0041]     According to this construction, there is virtually no barrier between the intermediate layer and the carrier supply layer. This means no band discontinuity at the interface between the intermediate layer and the carrier supply layer, and a band discontinuity of only a small amount at the interface between the intermediate layer and the Schottky contact layer. Consequently, the source resistance of the FET can be dramatically lowered.  
         [0042]     Here, the Schottky contact layer may be made of InGaP, the carrier supply layer may be made of Al 0.25 Ga 0.75 As, and the intermediate layer may be made of Al x Ga 1-x As, where x is a composition ratio and takes any value in a range no less than 0 but less than 0.25.  
         [0043]     According to this construction, the Schottky contact layer is made of a different compound semiconductor from the carrier supply layer. In detail, the Schottky contact layer is made of InGaP, which has a lower interface state density than AlGaAs, to reduce frequency dispersion. The carrier supply layer is made of AlGaAs. In addition, the intermediate layer is formed between the AlGaAs carrier supply layer and the InGaP Schottky contact layer. The intermediate layer is made of Al x Ga 1-x As, where 0≦x&lt;0.25, and an electron affinity of the intermediate layer is smaller than that of the Schottky contact layer and larger than that of the carrier supply layer. In this way, frequency dispersion of drain currents can be effectively reduced without increasing the source resistance. This is because the band discontinuity at each of the interface between the intermediate layer and the Schottky contact layer, and the interface between the intermediate layer and the carrier supply layer is made smaller than a band discontinuity between the Schottky contact layer and the carrier supply layer when the intermediate layer is not formed.  
         [0044]     Furthermore, this intermediate layer in which the composition ratio x is constant can be manufactured in a simpler method than the intermediate layer in which the composition ratio x is continuously increased.  
         [0045]     Here, the object can be also achieved by an field-effect transistor including: a carrier supply layer supplying carriers; and a Schottky contact layer provided on the carrier supply layer, the Schottky contact layer forming a Schottky barrier. Here, the carrier supply layer is divided into a first portion and a second portion, where the first portion is closer to the Schottky contact layer than the second portion is, and a composition of a compound semiconductor is different between the first portion and the second portion, so that a difference in electron affinity is smaller between the first portion and the Schottky contact layer, than between the second portion and the Schottky contact layer.  
         [0046]     According to this construction, a difference in electron affinity is smaller between the first portion and the Schottky contact layer than between the second portion and the Schottky contact layer. Therefore, the amount of the band discontinuity at the interface between the carrier supply layer and the Schottky contact layer can be reduced, thereby effectively lowering the source resistance of the FET.  
         [0047]     Here, the second portion may be made of Al 0.25 Ga 0.75 As, the first portion may be made of Al x Ga 1-x As, where x denotes a composition ratio. The composition ratio x may gradually increase from 0 at an interface between the first portion and the Schottky contact layer, to 0.25 at an interface between the first portion and the second portion.  
         [0048]     According to this construction, an electron affinity gradually increases in the first portion from an interface between the first portion and the second portion to the interface between the first portion and the Schottky contact layer. Thus, an energy of a conduction band gradually decreases in the first portion. This effectively prevents the band discontinuity at the interface between the carrier supply layer and the Schottky contact layer, and therefore significantly lowers the source resistance of the FET. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]     These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention.  
         [0050]     In the drawings:  
         [0051]      FIG. 1  is a cross-sectional view illustrating an FET  1  relating to a first embodiment;  
         [0052]      FIG. 2  illustrates an energy change pattern for the FET  1  relating to the first embodiment;  
         [0053]      FIG. 3  is a cross-sectional view illustrating a semiconductor lamination  31 ;  
         [0054]      FIG. 4  illustrates an energy change pattern for an FET relating to a second embodiment;  
         [0055]      FIG. 5  is a cross-sectional view illustrating a conventional FET  50 ;  
         [0056]      FIG. 6  illustrates an energy change pattern for the conventional FET  50 ;  
         [0057]      FIG. 7  shows favorable material, thickness, composition ratio, and carrier concentration for each layer of the FET  1  relating to the first embodiment;  
         [0058]      FIG. 8  shows conditions for forming the semiconductor lamination  31  by crystal growth;  
         [0059]      FIG. 9  shows conditions for forming each layer of the semiconductor lamination  31  by crystal growth; and  
         [0060]      FIG. 10  shows modification examples for a substrate, a carrier supply layer, and an intermediate layer, and a Schottky contact layer to realize the energy change pattern shown in  FIG. 2  or  4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]     The following describes embodiments of the present invention with reference to the attached figures.  
       First Embodiment  
       [0000]     Construction  
         [0062]      FIG. 1  is a cross-sectional view illustrating an FET  1  relating to a first embodiment. As shown in  FIG. 1 , the FET  1  is formed in such a manner that a buffer layer  11 , a barrier layer  12 , a channel layer  13 , a carrier supply layer  14 , an intermediate layer  15 , a Schottky contact layer  16 , a low resistance layer  17 , and an ohmic contact layer  18  are formed in this order on a GaAs compound semiconductor substrate  10 . In addition, a source electrode  21  and a drain electrode  22  are formed on the ohmic contact layer  18 , and a gate electrode  20  is formed on the Schottky contact layer  16 .  
         [0063]     The ohmic contact layer  18  and the low resistance layer  17  are partially removed so that part of the Schottky contact layer  16  is exposed, except for areas in which the source electrode  21  and the drain electrode  22  are formed. Thus, an opening  30  is formed in the ohmic contact layer  18  and the low resistance layer  17 . The gate electrode  20  is formed in the opening  30  so as to be in contact with the Schottky contact layer  16 .  
         [0064]     Here, a silicon nitride film  19  is applied on a wall and a bottom of the opening  30  and a surface of the ohmic contact layer  18 .  
         [0065]     The buffer layer  11  is provided to improve a crystal structure of upper layers (especially the channel layer  13 ). The barrier layer  12  supplies carriers with the channel layer  13 , and prevents leakage of carriers to the buffer layer  11 .  
         [0066]     The channel layer  13  is a region in which carriers move. The carrier supply layer  14  generates carriers. The Schottky contact layer  16  forms a high Schottky barrier to reduce leakage currents caused when the gate electrode  20  is reverse-biased.  
         [0067]     The low resistance layer  17  and the ohmic contact layer  18  are provided to form an ohmic contact between the Schottky contact layer  16  and each of the source electrode  21  and the drain electrode  22 . The low resistance layer  17  is provided to lower a resistance between the source electrode  21  and the drain electrode  22 .  
         [0068]     The intermediate layer  15  is an undoped layer originally to control a threshold voltage, which, when applied to a gate electrode, causes electric currents to start flowing between a drain electrode and a source electrode. According to the first embodiment, however, a composition ratio of the intermediate layer  15  is continuously changed, to reduce the amount of a band discontinuity between the carrier supply layer  14  and the Schottky contact layer  16 , and therefore lowers a source resistance Rs.  
         [0069]      FIG. 7  shows favorable material, thickness, composition ratio, and carrier concentration of each of the layers  11  to  18 .  
         [0070]     In  FIG. 7 , a value of 3. E+18 shown in a section of CARRIER CONCENTRATION, for example, indicates a carrier concentration of 3.0×10 18  cm −3 .  
         [0071]     As seen from  FIG. 7 , the channel layer  13  is made of indium gallium arsenide, the carrier supply layer  14  and the intermediate layer  15  are each made of aluminum gallium arsenide, and the Schottky contact layer is made of indium gallium phosphide.  
         [0072]     The carrier supply layer  14  and the intermediate layer  15  are both made of AlGaAs, but different from each other in terms of a ratio between Al and Ga. In detail, the carrier supply layer  14  is made of Al 0.25 Ga 0.75 As, and the intermediate layer  15  is made of Al x Ga 1-x As, where a composition ratio x changes from 0 (GaAs) to  0 . 25  (Al 0.25 Ga 0.75 As) in the intermediate layer  15 .  
         [0073]     Here, the composition ratio x is set to 0 at the interface between the intermediate layer  15  and the Schottky contact layer  16 , and to 0.25 at the interface between the intermediate layer  15  and the carrier supply layer  14 . Furthermore, the composition ratio x increases from 0 to 0.25 gradually, continuously, and linearly. To sum up, a portion of the intermediate layer  15  which is in contact with the Schottky contact layer  16  is made of GaAs, and a portion in contact with the carrier supply layer  14  is made of Al 0.25 Ga 0.75 As. Here, since the carrier supply layer  14  is made of Al 0.25 Ga 0.75 As as mentioned above, a band discontinuity does not exist at the interface between the intermediate layer  15  and the carrier supply layer  14 .  
         [0074]     On the other hand, a band discontinuity occurs at the interface between the Schottky contact layer  16  and the intermediate layer  15 , because of a difference in composition. However, the amount of this band discontinuity (ΔEc) is very small as mentioned later.  
         [0075]      FIG. 2  illustrates an energy change pattern for the FET  1  relating to the first embodiment. In  FIG. 2 , an energy of a conduction band of each of the layers  13  to  18  shown in  FIG. 1  is plotted along the vertical axis, and the layers  18  to  13  are plotted along the horizontal axis in this order.  
         [0076]     As mentioned above, when the intermediate layer  15  is made of Al x Ga 1-x As, the composition ratio x is gradually increased from 0 to 0.25 from the interface between the intermediate layer  15  and the Schottky contact layer  16  to the interface between the intermediate layer  15  and the carrier supply layer  14 . Thus, the electron affinity gradually decreases in the intermediate layer  15 , thereby increasing the energy of the conduction band continuously and gradually, from the interface between the intermediate layer  15  and the Schottky contact layer  16  to the interface between the intermediate layer  15  and the carrier supply layer  14 . As a result, a band discontinuity does not occur at the interface between the intermediate layer  15  and the carrier supply layer  14 , and a very small band discontinuity of 0.1 eV or less occurs at the interface between the intermediate layer  15  and the Schottky contact layer  16 , as shown in  FIG. 2 . Therefore, electrons are hardly blocked when going thorough the interface between the Schottky contact layer  16  and the intermediate layer  15 . This lowers the source resistance Rs.  
         [0077]     Since the ratio of Al is constant at 0.25 in the carrier supply layer  14 , a surface charge density of 2DEG accumulated in the channel layer  13  does not decrease despite the gradual change in energy of the conduction band of the intermediate layer  15 . Furthermore, since the Schottky contact layer  16  is made of undoped InGaP, which achieves lattice matching with GaAs, the gradual change in energy of the conduction band of the intermediate layer  15  has little impact on the height of the Schottky barrier between the Schottky contact layer  16  and the gate electrode  20 .  
         [0078]     The first embodiment is especially effective in lowering the source resistance Rs of the FET  1 , when a non-alloy ohmic electrode structure is used to form an ohmic contact between the ohmic contact layer  18  and each of the source electrode  21  and the drain electrode  22 , in order to solve problems of an alloying process, such as a difficulty in control and a rough metal surface. To be specific, an ohmic contact is formed between the ohmic contact layer  18  and each of the source electrode  21  and the drain electrode  22  without using an alloying process, that is, a thermal treatment.  
         [0000]     Manufacturing Method  
         [0079]     The following part describes a manufacturing method for the FET  1  relating to the first embodiment.  
         [0080]      FIG. 3  is a cross-sectional view illustrating a semiconductor lamination  31 . As shown in  FIG. 3 , the semiconductor lamination  31  includes the buffer layer  11 , the barrier layer  12 , the channel layer  13 , the carrier supply layer  14 , the intermediate layer  15 , the Schottky contact layer  16 , the low resistance layer  17  and the ohmic contact layer  18 . The layers  11  to  18  are formed by crystal growth in the stated order on the semiconductor substrate  10  to manufacture the FET  1 .  FIG. 8  shows conditions for the formation of the semiconductor lamination  31  by crystal growth.  
         [0081]      FIG. 9  shows conditions for forming each layer by crystal growth. When forming the intermediate layer  15  by crystal epitaxial growth, a gas flow ratio of each of TMGa and TMA 1  is adjusted so that, when the intermediate layer  15  is made of Al x Ga 1-x As, the composition ratio x linearly increases from 0 to 0.25, from the interface between the intermediate layer  15  and the Schottky contact layer  16  to the interface between the intermediate layer  15  and the carrier supply layer  14 .  
         [0082]     After the semiconductor lamination  31  is completed by crystal growth, a resist mask (not shown in  FIG. 3 ) is partially formed on the ohmic contact layer  18 . Subsequently, the ohmic contact layer  18  and the low resistance layer  17  are partially etched using a mixture of phosphoric acid and hydrogen peroxide solution. Thus, the opening  30  is formed in the semiconductor lamination  31 . Here, the Schottky contact layer  16  plays a role of stopping the etching.  
         [0083]     After this, the silicon nitride film  19  is applied using plasma chemical vapor deposition (CVD), on the wall and bottom of the opening  30  and the surface of the ohmic contact layer  18 . Subsequently, areas of the silicon nitride film  19  in which the gate electrode  20 , the source electrode  21 , and the drain electrode  22  are to be formed are removed by dry etching.  
         [0084]     After this, WSi, Ti, Pt and Au are applied on the resulting surface in the stated order by sputtering, to form a lamination film (not shown in  FIG. 3 ).  
         [0085]     After this, the lamination film is partially removed by dry etching except for areas in which the gate electrode  20 , the source electrode  21  and the drain electrode  22  are to be formed. In this way, the FET  1  shown in  FIG. 1  is completed.  
       Second Embodiment  
       [0000]     Construction  
         [0086]     An FET relating to a second embodiment is different from the FET  1  relating to the first embodiment in that the composition ratio x is constant at 0.15 in the entire intermediate layer  15 . Except for this, the FET relating to the second embodiment is the same as the FET  1 . Therefore, an explanation of a construction of the FET relating to the second embodiment is omitted.  
         [0087]     As described above, the composition ratio x is constant in the entire intermediate layer  15  in the second embodiment. Therefore, a manufacturing method for the FET relating to the second embodiment can be simplified when compared with that for the FET  1 .  
         [0088]      FIG. 4  shows an energy change pattern for the FET relating to the second embodiment. As seen from  FIG. 4 , a band discontinuity (ΔEc1) of approximately 0.2 eV exists at the interface between the intermediate layer  15  and the Schottky contact layer  16 , and a band discontinuity (ΔEc2) of approximately 0.1 eV exists at the interface between the intermediate layer  15  and the carrier supply layer  14 . This is because the composition ratio x is discontinuous over the interface between the intermediate layer  15  and the Schottky contact layer  16 , and over the interface between the intermediate layer  15  and the carrier supply layer  14 .  
         [0089]     The following formula (4) indicates a relation between ΔEc1, ΔEc2, and ΔEc that is the amount of the band discontinuity at the interface between the Schottky contact layer  65  and the carrier supply layer  64  of the conventional FET  50  (shown in  FIG. 6  illustrating an energy change pattern for the conventional FET  50 ). It should be noted that the formula (4) is not true when the electron affinity X is changed continuously in the intermediate layer  15  as in the first embodiment. 
 
Δ Ec 1+Δ Ec 2=Δ Ec   (4) 
 
         [0090]     According to the second embodiment, the composition ratio x is constant in the entire intermediate layer  15  between the carrier supply layer  14  and the Schottky contact layer  16 . In this case, the sum of the amounts of the band discontinuities at the interface between the intermediate layer  15  and the Schottky contact layer  16 , and at the interface between the intermediate layer  15  and the carrier supply layer  14  is equal to the amount (ΔEc) of the band discontinuity which is observed at the interface between the Schottky contact layer  65  and the carrier supply layer  64 , without an intermediate layer therebetween.  
         [0091]     In other words, the single band discontinuity (ΔEc) is divided into two smaller band discontinuities (ΔEc1 and ΔEc2) by the intermediate layer  15  according to the second embodiment. The band discontinuities (ΔEc1 and ΔEc2) are each less likely to block movement of electrons.  
         [0092]     It should be noted that the FET relating to the second embodiment has a larger source resistance Rs than the FET  1  relating to the first embodiment. However, each of the band discontinuities (ΔEc1 and ΔEc2) is smaller (ΔEc1=approximately 0.2 eV, and ΔEc2=approximately 0.1 eV) than the band discontinuity at the interface between the Schottky contact layer  65  and the carrier supply layer  64  (ΔEc=approximately 0.3 eV as shown in  FIG. 6 ). Accordingly, the FET relating to the second embodiment has a lower source resistance Rs than the conventional FET  50 .  
         [0093]     According to the second embodiment, the composition ratio x is constant (at 0.15 in the second embodiment) in the intermediate layer  15  between the Schottky contact layer  16  and the carrier supply layer  14 , so that the electron affinity of the intermediate layer  15  is smaller than that of the Schottky contact layer  16 , but larger than that of the carrier supply layer  14 . As a result, the FET relating to the second embodiment has a lower source resistance Rs than a FET which does not include an intermediate layer.  
       MODIFICATION EXAMPLES  
       [0094]     (1) According to the first and second embodiments, the substrate  10  is made of GaAs, the carrier supply layer  14  and the intermediate layer  15  are made of AlGaAs, and the Schottky contact layer  16  is made of InGaP. However, the present invention is not limited to such.  
         [0095]      FIG. 10  shows alternative compound semiconductor materials for the substrate  10 , the carrier supply layer  14 , the intermediate layer  15 , and the Schottky contact layer  16 . The examples 1 and 2 shown in  FIG. 10  enable the energy change pattern shown in one of  FIGS. 2 and 4  to be achieved, and therefore can lower the source resistance Rs.  
         [0096]     (2) According to the first embodiment, the composition ratio x in the Al x Ga 1-x As intermediate layer  15  is changed linearly from the interface between the intermediate layer  15  and the Schottky contact layer  16  to the interface between the intermediate layer  15  and the carrier supply layer  14 . However, the present invention is not limited to such.  
         [0097]     The composition ratio x can be changed in any manner in the intermediate layer  15 , as long as the composition ratio x continuously (gradually) increases from 0 to 0.25, from the interface between the intermediate layer  15  and the Schottky contact layer  16  to the interface between the intermediate layer  15  and the carrier supply layer  14 .  
         [0098]     (3) According to the second embodiment, the composition ratio x is set to 0.15 in the intermediate layer  15 , but not limited to such.  
         [0099]     The composition ratio x in the intermediate layer  15  can be set to any value in a range from no less than 0 to less than 0.25. In other words, the composition ratio x in the intermediate layer  15  is set, so that the energy of the conduction band of the intermediate layer  15 , which is determined in accordance with the composition ratio x, is larger than an energy of a conduction band of the Schottky contact layer  16  at the interface between the intermediate layer  15  and the Schottky contact layer  16 , and smaller than an energy of a conduction band of the carrier supply layer  14  at the interface between the intermediate layer  15  and the carrier supply layer  14 .  
         [0100]     (4) According to the first embodiment, a portion of the intermediate layer  15  in the vicinity of the carrier supply layer  14  has the same composition as the carrier supply layer  14 , and can generate carriers. Therefore, it can be said that part of the intermediate layer  15  is virtually an extended part of the carrier supply layer  14 . Considering this, the first and second embodiments may be modified in the following manner. To start with, the intermediate layer  15  is not provided between the Schottky contact layer  16  and the carrier supply layer  14 . Furthermore, the ratio of Al in a portion of the carrier supply layer  14  which is contiguous to the Schottky contact layer  16  is made different from the ratio of Al in the remaining portion of the carrier supply layer  14 . This can achieve a smaller band discontinuity at an interface between the carrier supply layer  14  and the Schottky contact layer  16  than in the related art.  
         [0101]     For example, an upper half of the carrier supply layer  14 , which is closer to the Schottky contact layer  16 , may be constituted as the contiguous portion. Here, the upper half is made of Al x Ga 1-x As, where the composition ratio x gradually increases from 0 to 0.25, from the interface between the carrier supply layer  14  and the Schottky contact layer  16  to the boundary between the upper and lower halves in the carrier supply layer  14 . Here, the lower half of the carrier supply layer  14  may be made of Al 0.25 Ga 0.75 As.  
         [0102]     In this way, a difference in electron affinity is smaller between the upper half and the Schottky contact layer  16  than between the lower half and the Schottky contact layer  16 . Furthermore, the electron affinity gradually increases in the upper half towards the interface between the carrier supply layer  14  and the Schottky contact layer  16 . Accordingly, the energy of the conduction band gradually decreases in the upper half towards the interface between the carrier supply layer  14  and the Schottky contact layer  16 . As a result, a band discontinuity at the interface between the carrier supply layer  14  and the Schottky contact layer  16  can be effectively reduced. This results in a significant decrease in source resistance Rs.  
         [0103]     Note that the contiguous portion can be any part of the carrier supply layer  14 , and not limited to the upper half.  
         [0104]     The composition ratio x may not be changed in the contiguous portion, but fixed to any value between 0 and 0.25.  
         [0105]     (5) The present invention includes any combination of the first and second embodiments and the above-described modification examples.  
         [0106]     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.