Numerical simulation of the permeable base transistor

Predictions of improved high frequency performance of the gallium arsenide permeable base transistor (PBT) have been made by the exact diffusion-drift, two-dimensional numerical analysis of several PBT designs. In this study, both the device geometry and/or the impurity doping profile were varied and the corresponding unity-current-gain frequency, (f sub T), calculated. More than a 35% improvement in (f sub T) was predicted when the ration of the metal (Schottky) gate width to the space between gate fingers was varied. More than a doubling of (f sub T) could be obtained when the source and drain doping was increased to produce and n(+) n n(+) configuration. Each device design was analyzed to determine the change in mobile charge density with gate bias in the major areas of the structure in order to compute its contribution to the device input capacitance. In this way information was obtained on design details for reduced capacitance and transconductance change for improved (f sub T). A new UPBT structure was investigated where the semiconductor material above the gate electrode was removed in order to reduce this region's contribution to the capacitance. A 100% improvement in high frequency performance was predicted for this structure, operating at low gate voltages. Less gain was obtained at higher voltages. Control of the surface states on the exposed walls of this device must be provided to obtain this advantage.