COMPARATIVE-ANALYSIS OF THE HIGH-FREQUENCY PERFORMANCE OF SI/SI1-XGEX HETEROJUNCTION BIPOLAR AND SI BIPOLAR-TRANSISTORS

This paper presents a model-based comparison of the high-frequency performance of Si/Si1-xGex heterojunction bipolar transistors (HBTs) and Si bipolarjunction transistors (BJTs), in which the structural parameters were designed for maximum f(T) almost-equal-to f(max). This model study shows: (1) the Si1-xGex HBT has a peak f(T)(=f(max)) of 64 GHz, which represents a 16.4% improvement over the Si BJT; (2) emitter charging time has a sizable effect on high-frequency performance, even at current densities as high as 80 kA cm-2; (3) compositional grading of the SiGe base, as well as the profile of the base doping, strongly influence f(T) and f(max). A Gaussian grading profile is found to exhibit the highest peak f(T)=f(max); a 30% higher peak cutoff frequency is predicted over a uniform doping profile; (4) the dependence of high-frequency performance upon collector design represents a trade-off between f(T), f(max) and BV(CBO); and (5) by decreasing emitter or base doping levels, Si1-xGexHBTs with f(T) exceeding 100 GHz can be designed. Alternatively, f(max) of 100 GHz may be achieved by increasing base doping and reducing extrinsic capacitances and resistances.