A new method is proposed for estimating transit times in heterojunction bipolar transistors (HBT’s) which are fabricated from semiconductor materials in which the conduction band can be represented by a two-valley model. The transition times for exchange of electrons between the conduction band valleys are treated as phenomenological parameters and are shown to be linked by the electric field in the device. Incorporation of the transition rates into the continuity equations for upper and lower valley electrons yields a set of equations, which can be solved under transient conditions to yield directly the transit times of carriers across the base and the base-collector space-charge region. Results from this approach are compared with Monte Carlo calculations and shown to exhibit good agreement. © 1990 IEEE
