1. Field of the Invention
The present invention relates to a method of producing a compound semiconductor device, more particularly to a method of producing a MESFET (Metal Semiconductor FET) of, for example, GaAs, and having a recess portion.
2. Description of the Related Art
A GaAs MESFET is widely used in the fields of satellite communication and ground micro-wave communication, etc.
FIGS. 1A to 1E are cross-sectional step views explaining a conventional method of producing a GaAs MESFET having a recessed portion.
As shown in FIG. 1A, a resist film mask 2 is selectively formed on a semi-insulating (SI-) GaAs substrate 1, and after patterning with mask 2, silicon ions (Si.sup.+) are implanted into the exposed portions of the substrate 1 to form an n-GaAs active layer 3 having a uniform depth.
Then, as shown in FIG. 1B, the patterned resist film mask 2 is removed, a resist film mask 4 is formed over the n-GaAs active layer 3 portion or other portions, and silicon ions are implanted in exposed portions at both sides to form an n.sup.+ -GaAs contact layer.
As shown in FIG. 1C, after removing the resist film mask 4 and applying a heat treatment to activate the ion-implanted portions, a silicon dioxide (SiO.sub.2) film 6 is formed on the n.sup.+ GaAs contact layer 5, except for portions thereof on which source and drain electrodes 7 of AuGe/Ni/Au are formed.
Then, as shown in FIG. 1D, a third resist film mask 8 having an opening is formed on the n.sup.+ GaAs contact layer 5 and the source and drain electrodes 7, and the SiO.sub.2 film 6 covering a recess formation portion is etched by a hydrogen fluoride type solution, which etching is continued until a recess portion 9 is formed.
Then, as shown in FIG. 1E a gate electrode 10 of Al is formed is formed by evaporating aluminum (Al) from above the resist film mask 8, and then removing the resist film mask 8 together with unwanted Al by a lift-off process.
The recess portion 9 is provided to enable a thinning of the n GaAs active layer having a low resistivity by etching, to obtain a desired current value, thereby to separate the gate electrode from a position in which a surface potential exists, and disperse the electric field concentration to obtain a high breakdown voltage.
Nevertheless as explained above, when the recess portion is formed by a wet etching process, since the etching is carried out through a gate window having a thickness of 1 .mu.m or less, the etching quantity is not constant and is deviated, and the deviation of a saturated current, for example, a drain current (Id.sub.ss), becomes large.
Accordingly, the present inventors investigated other processes involving the step of preforming a recess portion.
FIGS. 2A and 2B are cross-sectional step views of one such process.
This process comprises the steps of forming a recess portion 19 on a GaAs substrate 11 and forming an n GaAs active layer 13 by an ion-implanting process, as shown in FIG. 2A. A GaAs MESFET then can be realized by forming an n.sup.+ GaAs contact layer 15, SiO.sub.2 layer 16, source and drain electrodes 17, and a gate electrode 20, as shown in FIG. 2B, by using the afore-mentioned conventional processes.
This method of producing the GaAs MESFET is advantageous in that an improvement of the breakdown voltage due to the recess structure and deviation of the saturated current can be reduced but when the gate voltage approaches a pinch-off voltage, the mutual conductance (Gm) is compressed and it is difficult to realize a high power and high efficiency operation. The compression of the Gm means that Gm becomes small at a low current area and the gain is reduced.