Review of low-frequency noise behaviour of polysilicon emitter bipolar junction transistors

Abstract

For many analogue integrated circuit applications, the polysilicon emitter bipolar junction transistor (PE-BJT) is still the preferred choice because of its higher operational frequency and lower noise performance characteristics compared to MOS transistors of similar active areas and at similar biasing currents. The authors briefly review why bipolar transistors are still of great interest and in widespread use in analogue integrated circuits. The review includes a comparison between BJTs and MOSFETs using a simple small-signal equivalent circuit with important parameters that can be used to compare these two technologies. An extensive review of the popular theories used to explain low-frequency noise results is presented. However, in almost all instances, these theories have not been fully tested. After a review of the effects of different processing technologies and conditions on the noise performance of PE-BJTs, a summary of some of the key technological steps and device parameters and their effects on noise is presented. The effects of temperature and emitter geometries scaling is reviewed. It is shown that dispersion of the low-frequency noise in ultrasmall geometries is a serious issue since the rate of increase of the noise dispersion is faster than that of the noise itself as the emitter geometry is scaled to smaller values. The same effect, but to a greater extent, is also observed in MOSFETs. Finally, some ideas for future research on PE-BJTs, some of which are also applicable to SiGe or SiGeC heterojunction bipolar transistors and MOSFETs, are presented after the conclusions.