Sub-ft gain resonance of InP/InGaAs-HBTs

Advanced npn-InP/InGaAs HBTs are often operated at high current levels for optimum high-speed performance. Because of velocity modulation effects, these transistors may operate in base-pushout although measurements of the cut-off frequency f_t indicate the opposite. We show that the low mobility of the holes has a strong effect on the transistor operation in this regime, which is only revealed from a dynamic analysis: The unilateral power gain peaks far below f_t followed by a -40 dB/dec roll-off. The effect was thoroughly analyzed and as a result, we present a simple equivalent circuit model that successfully describes transistors operating in pushout up to very high frequencies