Optimization, analysis, and fabrication of SiGe heterojunction bipolar transistors

Abstract

This thesis deals with the optimization, analysis, and fabrication of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs).

Two vertical base profile optimization studes for improving the high-frequency performance of SiGe HBTs are presented. In the first study, the Ge profile is optimized for the minimum contribution of the emitter and base delay times to the transition frequency in the low-injection regime. A fixed Ge dose is used as the optimization constraint. Non-quasi-static effects at high frequencies are taking into account. It is shown that the graded Ge profile is more effective than the box Ge profile in minimizing the two delay time contributors for SiGe HBTs with today's typical emitter and base dimensions.

In the second optimization study, the base doping and Ge profiles are optimized for minimum base delay time in low- and high-injection regimes before the onset of Kirk effect. Fixed Ge dose, intrinsic base resistance, and base concentration near the emitter are adopted as optimization constraints. The effect of plasma-induced bandgap narrowing in high injection is considered. An iteration scheme for calculating the base delay time for a wide range of collector current densities is developed. It is shown that the retrograde base doping profile with graded Ge profiles gives the minimum base delay time in both low- and high-injection regimes.

An analysis of the retrograde portion of a base retrograde doping profile in a SiGe HBT is also performed. A closed-form analytical expression of the base delay time is derived with various physical effects taken into consideration. The relative importance of the physical effects is assessed. It is found that the adverse effect of the retrograde portion of the base retrograde doping profile on the base delay time is less pronounced than expected, especially when a high Ge grading exists across the base. It is also shown that the effect of the field dependency of the electron diffusivity needs to be considered when modelling the base delay time in the SiGe base with a high electric field.

Finally, SiGe HBTs are fabricated by high-dose Ge implantation, Si amorphization, and solid-phase epitaxy. The results from electrical measurements are presented. Although further work is required in this area, transistor action is observed in SiGe HBTs with Si amorphization used.