Submicrometre Al0.5Ga0.5As heterojunction gate GaAs f.e.t.

Preliminary results on the d. c. and microwave performance of Al0.5Ga0.5 As gate heterojunction GaAs field-effect transistors (h.j.f.e.t.) with submicrometre gate lengths are reported. The structure is of a 3-layer sandwich formed by an n-type 1017 cm3 Sn-doped active layer on a Cr-doped substrate, a p-type 1015 cm¿3 Ge-doped Al0.5Ga0.5As gate layer and a p+-type 5 × 1013 cm-3 Ge-doped GaAs `cap´ layer on the top of the gate. Selective etching of the p+ GaAs and p-Al0.5Ga0.5As provides the necessary overhang for self-aligned device fabrication. Experimental devices with sub-micrometre gate lengths and with a gate periphery of 300 ¿m have exhibited a d. c. transconductance of 20 to 30 mS and an m.a.g. of 9.5 dB at 8 GHz. To our knowledge, this is the first time a submicrometre gate h.j.f.e.t. has been fabricated to give amplification of power at X-band.