(1) Field of the Invention
The present invention relates to a method for producing a field effect type semiconductor device, more particularly to a method for producing a Schottky barrier type field effect transistor (FET) using a semiconductor layer of gallium arsenide (GaAs) as an active layer.
(2) Description of the Prior Art
Schottky barrier type FET's using a semiconductor layer consisting of GaAs as an active layer (GaAs FET's) have been coming into increasing use due to their high frequency, high power, and high efficiency. Such high-performance GaAs FET's may, for example, be formed by a recess structure, in which the part of the GaAs active layer where a gate electrode is arranged is thinned, and an offset structure, in which a source electrode is arranged close to a gate electrode.
The advantage of this recess and offset structure is that the series resistance between the source electrode and the gate electrode can be decreased, the transconductance improved, and high frequency and high power obtained. Further, by keeping the length of the side of the drain electrode long, the Schottky withstand voltage between the gate electrode and the drain electrode can be increased, so that the gate current is decreased during the operation thereof, with the result that electromigration is also prevented.
In the recess structure, one shortens the recess width and forms the gate electrode on the center portion thereof in order to improve the above characteristics. If a gate electrode is formed in the recess structure, however, the closer the widths of the recess and gate electrode, the greater the difficulty of extending the depletion layer, whereby a high electric field is formed and the Schottky withstand voltage between the gate electrode and drain electrode is reduced. Further, when the Schottky withstand voltage is reduced, the operating gate current of the FET is increased, whereby electromigration of the gate electrode occurs and the life of the FET is remarkably shortened.
As a method for improving the Schottky withstand voltage, there has been provided a method for lowering carrier concentration in an active layer. However, when the carrier concentration in the active layer is lowered the transconductance is decreased and an FET having high performance characteristics cannot be obtained.