A theoretical model is proposed for the predictive simulation of the performance and reliability of heterojunction bipolar transistors (HBTs) operating at high temperatures and pressures at any levels of carrier injections. The model emphasizes the effects of heteroemitter interface band offsets and space charge region recombination on the carrier transport, current gain and I If noise in HBTs any level of carrier injections. The extended tight binding model (ETB) and statistical thermodynamic model are used to study the material properties of heteroemitter as a function of composition, temperature, strain, and pressure. The results are implemented in the developed extended drift-diffusion (EDD) model to study the current-voltage, current gain and I If noise characteristics of HBTs. The model suggests that decreasing (increasing) the emitter alloy composition increases (decreases) the heteroemitter space charge recombination current and base current and decreases (increases) the dc current gain of Npn HBTs. Numerical analysis indicates that the de emitter and base current-voltage, dc current gain, and 1/f noise characteristics of Npn-(Al, Ga)As/GaAs HBTs and npn-GaAs BJTs are dominated by the heteroemitter space charge region recombination process for small forward biases at T = 300 K. The model can be useful in predicting the performance of HBTs operating at high temperatures, pressures for any level of carrier injections. (C) 2001 Elsevier Science B.V. All rights reserved.