BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods of fabricating field-effect transistors, and more specifically to a method of fabricating a high-power field-effect transistor of a recessed gate structure on a compound semiconductor substrate for use in the microwave region. 
     2. Description of the Related Art 
     For high power and microwave applications, recessed gate field-effect transistors are currently used. This type of field-effect transistor has the gate electrode formed in a slot etched in a compound semiconductor substrate, such as GaAs, between the source and drain electrodes. The use of the same mask for etching the slot and depositing metal to form the gate electrode results in the gate metal being placed in the center of the slot. Use of a recessed gate structure has the advantages that the extra channel thickness on each side of the gate reduces parasitic resistances between the gate and the source and drain and that the position of the gate below the substrate surface does not restrict the ability of the gate to modulate the source-drain current under positive gate bias despite the shrinkage of the depletion region that occurs under positive bias. In addition, low noise is another demand which requires small gate length. 
     According to a prior art method, a silicon dioxide layer is formed on a semi-insulating substrate and then etched to form a small recess, which is then sputtered with metal. However, if this recess opens upwards with an aspect ratio equal to or greater than unity and one side of the opening is smaller than 0.2 micrometers, difficulty arises to completely fill the gate metal into the recess. This results in the filling gate metal having an undesirable “void”, which in turn causes the gate to increase its resistance and weaken its structural integrity, 
     In order to overcome this problem, one prior-art solution employs a tapered sidewall forming process during the fabrication of a recessed-gate field-effect transistor, as shown in FIGS. 1A to  1 D. According to this approach, an AlGaAs layer  2  and a GaAs layer  3  are successively formed on a GaAs substrate  1 , as shown in FIG.  1 A. On the GaAs layer  3  is deposited an oxide layer  4  which is then etched to form a hole  7 . Using the oxide layer  4  as a photoresist, the GaAs layer  3  and AlGaAs layer  2  are isotropically wet-etched to form a recess  8  whose sidewalls slope down from the bottom of layer  4 . An oxide layer  5  is then grown on the layer  4  so that it fills in the recess  8 , leaving a tapered small recess  9 , as illustrated in FIG.  1 B. The upper oxide layer  5  is then dry-etched to create a hole  10  as illustrated in FIG.  1 C. Because of the presence of the tapered recess  9 , the dry etching process results in the hole  10  having a bevelled edge  10   a . Metal is then sputtered as shown in FIG. 1D to form a gate electrode  6 . Selected areas of the oxide layers  4  and  5  are removed by a patterned etching process to allow the subsequent metalization process to form the source and drain electrodes. 
     However, it is found that when the horizontal aspect ratio of hole  10  becomes equal to or greater than 2, a “void” still occurs in the filling gate metal as indicated by numeral  11  in FIG.  1 D. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method of fabricating a recessed-gate field-effect transistor without producing a void in the filling gate metal. 
     Another object of the present invention is provide a method of fabricating a recessed-gate field-effect transistor which ensures high precision control on the gate length. 
     According to the present invention, there is provided a method of fabricating a field-effect transistor comprising the steps of forming a masking layer having an opening therein on laminated compound semiconductor layers, removing a portion of the laminated layers using an etching solution acting through the opening and creating a gate-forming recess having sidewalls tapering in a direction away from the masking layer, filling the gate-forming recess with gate metal and forming a gate electrode, and forming a recess around the gate electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIGS. 1A to  1 D are cross-sectional views illustrating prior art steps of fabricating a field-effect transistor of a recessed gate structure; 
     FIGS. 2A to  2 F are cross-sectional views illustrating steps of fabricating a field-effect transistor of a recessed gate structure according to a first embodiment of the present invention; and 
     FIGS. 3A to  3 F are cross-sectional views illustrating steps of fabricating a field-effect transistor of a recessed gate structure according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In FIGS. 2A to  2 F, there is shown a method of fabricating a recessed-gate field effect transistor according to a first embodiment of the present invention. 
     In FIG. 2A, the starting material is a GaAs substrate  21  of ( 100 ) orientation on which is formed an 80-nm thick i-GaAs layer  22 . This layer serves as a buffer for a channel layer  23  of i-In 0.15 Ga 0.85 As with a thickness of 15 nm. On the channel layer  23  is a 40-nm thick electron supply layer  24  of n-Al 0.2 Ga 0.8 As and an 80-nm thick contact layer  25  of n-GaAs. All of these layers are grown using the metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) method. The layers formed on the substrate  21  are epitaxially aligned to crystal plane ( 100 ) and their cross-sectional view reveals ( 011 ) crystal plane perpendicular to crystal plane ( 100 ). A photoresist  26  is lithographically deposited on a portion of the surface of n-GaAs layer  25  where a gate electrode is to be produced. Photoresist  26  is formed with a hole  26 A with a horizontal dimension of 0.1 μm. 
     In FIG. 2B, the portion of the n-GaAs layer  25  which is exposed to the outside through the hole  26 A and the underlying portion of the lower layer  24  are subject to a mixed solution of sulphuric acid and hydrogen peroxide. The wet-etching process of this mixed solutions is isotropic to GaAs and has no etching capability to the photoresist  26 . Therefore, the hole  26 A maintains its original dimensions, while the exposed portion of the n-GaAs layer  25  and the underlying portion of the lower n-AlGaAs layer  24  are isotropically wet-etched. As a result, there is formed a recess  25 A having an undercut profile, where its sidewalls taper in a direction away from the photoresist  26  at an angle approximately 54 ° to the horizontal, following a direction parallel to crystal planes ( 111 ). This isotropic etching continues until the horizontal dimensions at the bottom of recess  25 A equal the horizontal dimensions (i.e., 0.1 μm) of hole  26 A. Although the volume of recess  25 A increases as the etching process continues after its bottom dimensions are equal to the horizontal dimensions of the hole  26 A, the bottom dimensions are maintained constant, thus giving an allowance to the stop timing of the etching process. The gate (channel) length of the device can therefore be precisely determined, 
     In FIG. 2C, the photoresist  26  is removed and tungsten silicide (Wsi) is sputtered on the surface of the device to form a conductive layer  27  to establish a Schottky barrier contact with the underlying material. On the layer  26  is sputtered a laminated conductive layer  28  of TiN, Pt and Au. The upper conductive layer  28  is then etched through a photoresist, not shown, by using the ion-milling method and the lower conductive layer  27  is dry-etched by a mixture of SF 6  and CF 4  gases to produce a gate electrode  30 , as shown in FIG.  2 D. 
     In FIG. 2E, a photoresist  31  is formed on the n-GaAs layer  25  and then the n-GaAs layer  25  is selectively and isotropically wet-etched through the photoresist  31  to create a recess  32  around the gate electrode  30 . Photoresist  31  is then removed and ohmic metal is sputtered on selected areas of the device to create a source electrode  33  and a drain electrode  34  as illustrated in FIG.  2 F. 
     It is seen that the recess  25 A can be completely filled with gate metal and assure that no void can occur in the gate metal. A gate electrode of desired structural integrity can be obtained. 
     In the first embodiment, the etching agent is one that acts nonselectively on the layers  25  and  24 . Therefore, these layers could be formed of a single layer of n-GaAs. 
     A second embodiment of this invention is shown in FIGS. 3A to  3 F. FIG. 3A shows that the starting structure of the second embodiment is the same as that shown in FIG. 2A, except that the photoresist layer  26  has a hole  26 B which is dimensioned by taking account of an etching agent that acts selectively on the layers  25  and  24 . 
     In the second embodiment, the etching solution is a mixture of citric acid with a concentration of 50 weight percent and hydrogen peroxide with a concentration of 30 weight percent, the mixture ratio in volume of citric acid to hydrogen peroxide being preferably 3 to 1. The mixed solution, which is preferably maintained at a temperature of 5 to 8° C., has no etching characteristic to the photoresist  26  but exhibits anisotropic etching characteristic to the n-GaAs layer  25 . However, it has no etching capability to the underlying n-AlGaAs layer  24 . The n-GaAs layer  25  is thus anisotropically wet-etched in a direction perpendicular to crystal plane ( 100 ). The mixed etching solution has a low etch rate on crystal plane ( 111 )B. As a result, this crystal plane is exposed, forming a recess  25 B whose sidewalls downwardly taper at approximately 54° to the horizontal, as illustrated in FIG.  3 B. 
     Since the mixed solution has no etching capability on the n-AlGaAs layer  24 , the etching process is continued until this layer reveals its upper surface. Thus, the dimensions at the bottom of recess  25 B are determined by the dimensions of the hole  26 B and the thickness of n-GaAs layer  25 . Thus, the gate length of the device can be precisely controlled. 
     In FIG. 3C, the photoresist  26  is removed and a tungsten silicide layer  27  and a TiN—Pt—Au layer  28  are successively formed. These conductive layers etched in the same manner as that described previously to produce a gate electrode  40 , as shown in FIG.  3 D. 
     In FIG. 3E, a photoresist  41  is formed on the n-GaAs layer  25  and then the n-GaAs layer  25  is selectively and isotropically wet-etched through the photoresist  41 . A recess  42  is created around the gate electrode  40 . Photoresist  41  is then removed and ohmic metal is sputtered on selected areas of the device to create a source electrode  43  and a drain electrode  44  as illustrated in FIG.  3 F. 
     Because of the high selectivity of the mixed solution of citric acid and hydrogen peroxide to the GaAs layer  25  with respect to the underlying AlGaAs layer  24 , the threshold voltage (V th ) of the recessed-gate field-effect transistor can be precisely controlled.
The present invention provides a method of fabricating a field-effect transistor comprising the steps of forming a masking layer having an opening therein on laminated compound semiconductor layers, removing a portion of the laminated layers using an etching solution acting through the opening and creating a gate-forming recess having sidewalls tapering in a direction away from the masking layer, filling the gate-forming recess with gate metal and forming a gate electrode, and forming a recess around the gate electrode.