Bias dependence of capacitances in modulation-doped FET's at 4 GHz

Scattering-parameter measurements were made on 1.0- µm gate normally-on and 1.6-µm gate normally-off modulation-doped field-effect transistors (MOD/FET's) as a function of bias at 4 GHz. A maximum oscillation frequency of 38 GHz, which is about 8 GHz larger than that for a comparable GaAs MESFET, due to larger transconductances for the MODFET's, was obtained. The input capacitance was found to vary more than expected with gate bias, while the feedback capacitance was nearly independent of bias in the saturation regime. Output capacitance increases as V
gs
approached the pinchoff voltage. These results should be of great importance in large-signal modeling of integrated circuits based on modulation-doped FET's.