A modified Gummel-Poon model has been developed for the pnp heterojunction bipolar transistor which matches thermionic emission-diffusion of holes across the emitter-base heterojunction with drift-diffusion transport across a graded base. The effects of a high current density on base pushout and the collector junction capacitance have been incorporated to describe the falloff in current gain and high frequency performance,The model's predictions have been compared with reported device measurements and results from a commercial, numerical device simulator for an InP-based device with good agreement found. The model provides an improved description of device performance that will be of use to device designers.