Modelling of the influence of velocity overshoot on the cut-off frequency in Si and GaAs heterojunction bipolar transistors

We present self-consistent Monte Carlo simulations of the collectors of Si and GaAs heterojunction bipolar transistors (HBTs) with wide (700 nm) collectors such as are used in power transistors. Recent experimental data on peak cut-off frequencies f(T) of 76 GHz for GaAs/AlGaAs HBTs have provided strong direct evidence for extended velocity overshoot in the collector under base push-out conditions and there are theoretical results using fast semi-analytic techniques confirming this explanation. Our aim is to investigate how these regions of extended velocity overshoot might form and how they would affect f(T) at high current densities in such structures. Differences in velocity-field characteristics and velocity overshoot between Si and GaAs collectors have large effects which are quantified by calculating f(T). Comparisons are also made with data from the semi-analytic results.