A study of non-linearity effects of collector and base currents in SiGe heterojunction bipolar transistor

A material system that appears to have a tremendous advantage and compatible with Si technology is the Si-SiGe system. One problem in npn SiGe HBT is boron out-diffusion from the base. Boron dopant that out-diffuses into the emitter and collector during SiGe growth and subsequent heat treatment results in the formation of parasitic energy barriers at the emitter/base and base/collector junctions. These barriers suppress the injection of electrons from the emitter to the collector which result in reduced collector current. In this work, a study on the non-linearity effects of the collector and base currents of the SiGe HBT is performed using a high performance process and device simulator SILVACO. The understanding of these effects is very important in the fabrication of high speed devices. The characteristics shows that the more out-diffusion of boron (B) at the emitter/base and base/collector junctions, the more non-ideal the currents. The underlying explanation for these trends will be discussed.