Germanium heterojunction gate GaAs FET

Heterojunction gate GaAs FETs (HJFET) have been developed using p-type epitaxial Ge gates on 1×10¹⁷cm^-3n-type GaAs channels. The Ge films were deposited epitaxially by electron beam evaporation in an oil free vacuum system on heated GaAs substrates (0.2 µm thick 1×10¹⁷cm^-3n-type epitaxial GaAs grown on a buffer layer on semi-insulating GaAs). An aluminum overlay on the Ge was used to lower the gate resistance. Interdiffusion at the Ge/GaAs and Al/Ge/GaAs interfaces was examined by Auger Electron Spectroscopy (AES) sputter profiling techniques. P/N junctions formed by mesa etching the Ge layer exhibit high reverse breakdown voltages (-18V), moderately low reverse current leakage (10^-4A/cm²at -5V), and an ideality factor of 1.4. The high breakdown voltage and low reverse current may provide improved reliability for devices where high gate to drain bias is important. Initial HJFETs were fabricated with relaxed geometries having gate lengths between 2 and 10 µm and gate widths of 250 µm which resulted in dc transconductance of 10-12 mmhos. At 1.8 GHz a noise figure of 5 dB with 4.5 dB associated gain was obtained from an 8 µm gate length HJFET.