This paper reports on the effects of backgating on the sub-threshold characteristics of Si:SiGe n-channel heterojunction field effect transistors. The layer structure is optimised for small sub-threshold slope and negligible threshold voltage shift as a function of substrate bias. MEDICI simulations were carried out to minimize the body effect observed in the DC measurements of MOS-gated n-channel Si/SiGe HFETs. By doping profile engineering, we achieve virtually body effect free devices with a threshold voltage shift of 400 muV for a substrate bias changes of -2 V while maintaining a sub-threshold current slope near the theoretical limit. (C) 2002 Elsevier Science Ltd. All rights reserved.