Monte Carlo study of the influence of collector region velocity overshoot on the high-frequency performance of AlGaAs/GaAs heterojunction bipolar transistors

Detailed Monte Carlo calculations of electron transport in the base-collector junctions of heterojunction bipolar transistors are reported. In particular, the author focused on velocity overshoot in the collector region and its influence on collector transit time. Very high peak electron velocities (7-9×10⁷ cm/s) were observed over narrow regions just inside the collector (300-500 Å). However, the effects of velocity overshoot on reducing the total collector transit time were found to be marginal and the transit times were not very different from what would be predicted by assuming a constant saturated drift velocity of 10⁷ cm/s throughout the collector depletion layer. The calculations presented here confirm that Y. Yamauchi and T. Ishibashi (1986) greatly overestimate the contribution to transistor performance from velocity overshoot in the collector region