Monte Carlo analysis of the noise behavior in Si bipolar junction transistors and SiGe heterojunction bipolar transistors at radio frequencies

We present a comparative analysis of the current spectral densities in a Si bipolar junction transistor (BJT) and a SiGe heterojunction bipolar transi stor (HBT) of identical geometry performed by means of an ensemble Monte Ca rlo simulator self consistently coupled with a two-dimensional Poisson solv er. We focus on the physical origin of the different noise sources in the t ransistors at rf when varying the injection level conditions. At low inject ion the spectral density of base current fluctuations, S-JB(0), is governed by thermal noise related to the base resistance, while the collector spect ral density, S-JC(0), reaches a typical shot noise response. At high curren t density the onset of high injection in the base and the base push-out pla y an important role in the noise behavior of both transistors. Thus, S-JC(0 ) deviates from the typical shot noise response. Hot carrier effects are al so present. In the HBT, these effects are less important than in the BJT du e to the SiGe/Si heterointerface, and S-JB(0) can be neglected in the overa ll noise analysis because of the Ge content benefits when the structure ent ers the high-injection regime. (C) 2001 American Institute of Physics.