This paper presents a computational study by commercial TCAD of the potential electrical and thermal properties of n-p-n heterojunction bipolar transistors (HBTs) fabricated on relaxed Si1-yGey virtual substrates'. A full energy balance model coupled with a thermionic emission transport model is employed in two-dimensional numerical device simulations of vertical Si1-yGey/Si1-xGex/Si1-yGey HBT structures, having different combinations of Ge composition, x and y. The dependences of DC, AC and self-heating characteristics of virtual substrate HBTs (VS HBTs) on alloy composition are investigated in details. It is found that symmetrical VS HBTs generally exhibit higher current drive capabilities compared with equivalent HBTs formed pseudomorphically on Si substrates, but at the expense of a lower f(1) and, a decreased f(max). In addition, simulated results show that self-heating effects become increasingly significant for VS HBTs, substantially degrading device electrical parameters such as the early voltage. (C) 2003 Elsevier Ltd. All rights reserved.