A unified analytical charge transport model of HBTs, which is applicable for a wide variety of emitter-base (e-b) structures of HBTs, viz. abrupt or graded heterojunctions and p-n junctions displaced into wider or narrower band gap material is proposed. This is a thermionic field diffusion model, which considers thermionic emission and tunneling at the e-b heterojunction and diffusion in the quasi-neutral base regions. The Fermi-level splitting is considered while calculating the space charge region (SCR) recombination currents, which in turn is taken into account to calculate excess electron concentration at the edge of the depletion region in the base side of the e-b p-n junction. Closed form analytical expressions for the terminal currents are obtained, which has been implemented in a circuit simulator. The accuracy of the model is established by comparison with numerical simulation results and experimental data. (C) 2004 Elsevier Ltd. All rights reserved.