2D simulation of the effects of transient enhanced boron out-diffusion from base of SiGe HBT due to an extrinsic base implant

Abstract

Transient enhanced diffusion of boron in SiGe HBTs is studied by comparing measurements of the temperature dependence of the collector current with the predictions of 2D process and device simulations. The collector current is chosen for modelling because it is extremely sensitive to very small amounts of out-diffusion from the SiGe base, and hence provides a rigorous test for the accuracy of the transient enhanced diffusion models. The SiGe HBT studied incorporates an ion implanted extrinsic base adjacent to the SiGe base, which allows the influence of the implantation damage on the boron diffusion to be studied. The process simulations show that point defects generated by the extrinsic base implant lead to a broadening of the basewidth around the perimeter of the emitter due to transient enhanced diffusion of boron from the SiGe base. This causes parasitic energy barriers to form, which in the worst case, extend laterally several microns from the edge of the extrinsic base. The electrical effect of the transient enhanced diffusion is a decrease in collector current as the emitter geometry is reduced. Transistors with different emitter geometries and undoped SiGe spacer thicknesses are studied and the collector/base reverse bias is varied to modulate the parasitic energy barrier at the collector/base junction. The trends in the measured collector current are in all cases well predicted by a simplified “plus one” transient enhanced diffusion model.