Investigation of the high frequency noise figure reduction of SiGe heterojunction bipolar transistors using actualised physical models

Abstract

High frequency noise figure reduction is one of the major requirements for analog circuits. The introduction of strained SiGe alloys as the base layer material in heterojunction bipolar transistors opens new possibilities to improve the (base resistance) −1 × current gain product. Hence, the use of SiGe as the base layer material instead of silicon can result in a major improvement of the high frequency noise. In this paper, the minimum noise figure reduction as a function of the germanium fraction is addressed, using actualized physical models for the bandgap and the mobility. Two limiting cases will be discussed: a heterojunction bipolar transistor with very low base resistance and still acceptable minimum noise figure and current gain, and a high gain heterojunction transistor with superior noise figure for acceptable base resistance values.