An analytic model is proposed to determine the effect of band offsets at heteroemitter interface on the current transport and 1/f noise in heterojunction bipolar transistors (HBTs). The proposed model uses the modified form of drift-diffusion formalism, which requires that the net recombination rates be proportional to the densities of other type carriers across the heterointerface. The numerical analysis of the current-voltage and 1/f noise characteristics of Npn AlGaAs/GaAs HBT and npn GaAs BJT demonstrates that the role of band offsets at heteroemitter interface in the overall current transport and 1/f noise is very important in HBTs at low forward biases. The junction resistance due to diffusing minority electrons is much stronger (weaker) at small (high) forward biases than that due to recombined electrons and holes across the heteroemitted space charge region in both Npn AlGaAs/GaAs HBTs and npn GaAs BJTs. (C) 2000 Elsevier Science Ltd. All rights reserved.