Analysis of HBT behavior after strong electrothermal stress

We present two-dimensional simulations of one-finger power InGaP-GaAs heterojunction bipolar transistors (HBTs) before and after both electrical and thermal stress aging. It is well known that GaAs-HBTs with InGaP emitter material can be improved in terms of reliability if the emitter material covers the complete p-doped base layer forming the so-called InGaP ledge outside the active emitter. We analyze the influence of the ledge thickness and of the surface charges on the device performance and its impact on reliability. The possibility of explaining device degradation mechanisms by means of numerical simulation is of high practical importance.