Ion-implanted microwave field-effect transistors in GaAs

Microwave field effect transistors have been fabricated in gallium arsenide by using sulfur ion implantation directly into semi insulating Cr doped substrates to produce the channel region, eliminating the need for growth of an epitaxial layer. This implantation method has been used to produce 0·25 μm thick, n-type layers with uniform thickness and carrier concentration, and carrier mobility ranging from 2410 to 3620 cm²/V sec in different samples. Because of the uniformity, FET's fabricated in these layers have exhibited reproducibility of transconductance and pinchoff voltage from device to device on a wafer to better than ±10 per cent. Cr doped GaAs of commonly available quality was found to be satisfactory for FET fabrication, although minimum Cr compensation is desirable to obtain highest mobility. S parameter measurements of microwave characteristics indicated a projected f_max = 20 GHz but transducer gain cutoff occurred at approximately 7 GHz because of impedance mismatch and package parasitics.