Comparative analysis of the high-frequency performance of Si/Si 1− xGe x heterojunction bipolar and Si bipolar transistors

Abstract

This paper presents a model-based comparison of the high-frequency performance of Si/Si 1− x Ge x heterojunction bipolar transistors (HBTs) and Si bipolar junction transistors (BJTs), in which the structural parameters were designed for maximum f T≈ f max. This model study shows: (1) the Si 1− x Ge x 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 Tand 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 maxand BV CBO; and (5) by decreasing emitter or base doping levels, Si 1− x Ge x HBTs 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.