High injection effects on noise characteristics of SiBJTs and SiGeHBTs

We present a physically based comparison of the current spectral densities in a SiGe heterojunction bipolar transistors (HBT) and a Si bipolar junctio n transistor (BJT) of identical geometry and doping levels, based on the di rect evaluation of emitter, base and collector current fluctuations. An ens emble Monte Carlo (EMC) simulator self-consistently coupled with a 2D Poiss on solver has been employed for the calculations. In the studied bias range , the largest reduction of the RF current noise values in the HBT as compar ed with the BJT derives from the spectral density of base current fluctuati ons, S-JB, and from the spectra of the cross-correlation between emitter an d base current fluctuations, S-JBJE. This is due to the fact that the base current in the HBT is strongly reduced as a consequence of the lower gap of the SiGe base. At low injection, the collector spectral density S-JC exhib its a typical shot noise response while S-JB is governed by thermal noise. At high injection, the presence of hot carriers in the base-collector junct ion (which are less important in the HBT than in the BJT due to the SiGe/Si hetero-interface); the high carrier concentration in the base and the base push-out provokes the deviation of S-JC from the pure shot behavior. Under these conditions, the S-JB term can be neglected in the total noise analys is of the HBT for lower values of J(C), than in the BJT due to the Ge conte nt benefits. (C) 2001 Elsevier Science Ltd. All rights reserved.