Physically Based Comparison of Current Noise Analysis of Si BJT's and SiGe HBT's

Abstract

We present the first physically based conclusions of noise reduction in a HBT as compared with an identical BJT, based on the direct comparison of emitter, base and collector current fluctuations. In the HBT (as compared to the BJT), the largest reduction of the RF values of the spectral density of current fluctuations derives from the SJB and the SJBJE terms (22 % and 31 %, respectively). This is due to the fact that the base current in the HBT (mainly formed by holes crossing to the emitter) is strongly reduced as a consequence of the lower gap of the SiGe base. When studying the dc dependence of the different noise spectra, SJE(0), SJC(0) and SJEJC(0) undergo a pronounced rise in BJT and HBT as JC increases. However, the SJB(0) term can be neglected in the total noise analysis in the HBT for lower values of JC.