Effects of collector heterojunction displacement from its p–n junction on the unilateral power gain at 10 GHz in SiGe HBTs

Abstract

The design of SiGe heterojunction bipolar transistors for high frequency (10 GHz), high power applications has been investigated by numerical device modelling using a commercial simulator. For the SiGe base layer, boron outdiffusion can give rise to displacement of the base-collector p–n junction from the collector heterojunction, which contributes to the development of a parasitic barrier in the conduction band. In this paper, using a Gaussian distribution for the base's boron profile, we examine the effects of the device's epitaxial layer design on the device's unilateral power gain at 10 GHz and its relation to this barrier. Degradation in the device's microwave power gain was found to correlate with formation of the collector junction parasitic barrier, where the barrier height was found to depend upon the extent of the p–n junction displacement from the heterojunction, the Ge concentration in the base and the dc bias point. Trade-offs in device design to reduce its sensitivity to outdiffusion were examined including the use of undoped SiGe spacer layers, increased collector doping and the insertion of a thin, n+ layer at the collector junction.