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

The base-collector capacitance C/sub bc/ and the collector transit time delay /spl 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/sub cb/ and I/sub c/, causing a reduction of C/sub bc/ and /spl 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.