In this paper we investigate the impact of interface charges at heterojunctions on the performance of heterostructure bipolar transistors (HBT). Interface charges can modify the limiting process for the carrier transport in a device. Therefore. intentional interface charges introduced by delta-doped layers are basic tools for interface engineering. An accurate modelling of heterointerfaces which includes thermionic-field emission, surface charges, and surface dipoles allows to analyse the electrical performance of some modern devices based on band gap and interface engineering. It is demonstrated that the limiting transport process at an abrupt heterojunction can be shifted from thermionic emission towards drift-diffusion due to the presence of interface charges. We will also show that controlling the number and polarity of interface charges enables to improve HBT device performances. (C) 2002 Elsevier Science Ltd. All rights reserved.