Simulations of non-uniform, nonlinear collector doping profiles for SiGe HBTs

Abstract

In this work, the authors examine the effects of the shape of the collector doping profile in current-generation SiGe npn heterojunction bipolar transistors. The n-type doping profile in the region between the p-type SiGe base and the heavily doped n-type subcollector is simulated by various analytic functions and included in one-dimensional computer simulations. The results are examined in terms of RF performance versus breakdown. It is determined that altering the shape of the profile as compared to a uniform profile enables an increase in performance only in certain doping regimes. At high-integrated collector dose (low breakdown), profiles with their dose concentrated nearer the collector–subcollector junction outperform those that have their dose more evenly spread throughout the collector region. However at low-integrated collector dose (high breakdown) this advantage disappears. In fact the trend actually reverses in some cases such that for high-breakdown devices a uniform collector profile may be preferred. The advantage in FT of one profile over another can be as much as 15 GHz at a given breakdown voltage.