Velocity-modulation and transit-time effects in InP/InGaAs HBTs

The base-collector capacitance C-bc and the collector transit time delay tau (c) govern the high-speed performance of modern heterojunction bipolar transistors (HBTs). Both are shown to be strongly modified by velocity modulation effects in InP/InGaAs HBTs: The carrier velocity in the collector depends on V-cb and I-c, causing a reduction of C-bc and tau (c) respectively. The current induced transit time modulation is shown to be conveniently expressed by a minor but important modification of the conventional transit time delay expression. Particle simulations are performed to assess the relevance of these effects.