Theoretical small-signal performance of Si/SiGe/Si HBT

Application of the Monte Carlo technique to analyze electron and hole transport in bulk Si/sub 0.8/Ge/sub 0.2/ and strained Si/sub 0.8/Ge/sub 0.2//Si is discussed. The computed minority- and majority-carrier transport properties were used in a comprehensive small-signal model to evaluate the high-frequency performance of a state-of-the-art n-p-n heterostructure bipolar transistors (HBT) fabricated with SiGe as the base material. The valence band discontinuity of a SiGe-base HBT reverses the degradation in emitter injection efficiency caused by bandgap narrowing in the base, and permits a higher ratio of base doping to emitter doping than would be practical for a bipolar transistor. Any degradative effect of increased base doping on electron and hole mobilities is offset by improved transport in the strained SiGe base, resulting in a marked decrease in the base resistance and base transit time. Compared to the Si BJT, the use of Si/sub 0.8/Ge/sub 0.2/ for the base region of an HBT leads to significant improvements in low-frequency common emitter current gain, low-frequency unilateral power gain, and maximum oscillation frequency.<
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