Improved design of the gallium arsenide 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 fTcalculated. More than a 35-percent improvement in fTwas predicted when the ratio of the metal (Schottky) gate width to the space between gate fingers was varied. More than a doubling of fTcould be obtained when the source and drain doping was increased to produce an n⁺-n-n⁺configuration. A 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-percent improvement in high-frequency performance was predicted for this structure, operating at low gate voltages due to improved gmas well. 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.