Switching performance is simulated for GaAs/GaAlAs heterojunction bipolar transistors (HBT's) by combining a realistic physical device model that involves numerical solutions for carrier transport equations and Poisson's equation with our own circuit simulator that enables direct access to the device model embedded in arbitrary circuits. Based on simulated results for five-stage ring oscillators, discussion is given as to how the switching performance depends on the circuit configuration such as current mode logic (CML) without and with emitter follower, and direct-coupled transistor logic (DCTL), inclusion or exclusion of external base areas, and choice for single-or double-heterojunction transistors.