Self-aligned recessed gate process

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT A semi-insulating GaAs wafer is used as the substrate 10 in carrying out the process of FIGS.
1-7.
The wafer is cut so that the deposition surface 12 is in the (100) crystallographic plane and the gate is aligned so that, when etched as illustrated in FIG.
6, a recessed channel is formed with inwardly-sloping sides, as described hereinafter.
Referring to FIG.
1, following customary wafer cleaning procedures and inspection, a first dielectric layer 14 of silicon nitride (Si.
sub.
3 N.
sub.
4), having a thickness of 1,000-5,000 .
ANG.
, is deposited over the entire surface of the substrate 12.
This deposition step is followed by a first photolithography masking step to define a channel region 17.
The channel region is oriented to align the gate parallel to the <011> direction on the seed face of the substrate.
Silicon ions are then implanted through opening 17 in the photoresist 16 and through layer 14 into the GaAs substrate to produce an N- (e.
g.
, 3.
times.
10.
sup.
17 average dopant concentration) implant 18 in the channel region.
The ion implant energies are controlled to produce an active layer having a total effective thickness greater than is used in planar self-aligned enhancement-mode GaAsFETs, e.
g.
, on the order of 2,000-3,000 .
ANG.
, anticipating recessing the gate in FIGS.
6 and 7 to a depth of, e.
g.
, 800-1,000 .
ANG.
.
The foregoing dimensions may be adjusted as needed for a particular application.
Upon completion of the N- implant, photoresist layer 16 is removed.
Proceeding to FIG.
2, a second photolithography step is performed to provide a photoresist mask 20 defining a pair of openings 22 spaced apart about the center of the N- implant 18 and approximately aligned with the periphery of the N- implant.
A second implant step is performed to implant silicon ions to a depth approximately twice the depth of the N- implant 18 and to an N+ (e.
g.
, 2.
times.
10.
sup.
18) average dopant concentration to form source and drain implant regions 24, 26