INVESTIGATION OF THE HIGH-FREQUENCY NOISE-FIGURE REDUCTION OF SIGE HETEROJUNCTION BIPOLAR-TRANSISTORS USING ACTUALISED PHYSICAL MODELS

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 po ssibilities to improve the (base resistance)(-1) x current gain produc t. Hence, the use of SiGe as the base layer material instead of silico n can result in a major improvement of the high frequency noise. In th is paper, the minimum noise figure reduction as a function of the germ anium fraction is addressed, using actualized physical models for the bandgap and the mobility. Two limiting cases will be discussed: a hete rojunction bipolar transistor with very low base resistance and still acceptable minimum noise figure and current gain, and a high gain hete rojunction transistor with superior noise figure for acceptable base r esistance values.