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

Transient enhanced diffusion of boron in SiGe HBTs is studied by comparing measurements of the temperature dependence of the collector current with th e predictions of 2D process and device simulations. The collector current i s chosen for modelling because it is extremely sensitive to very small amou nts 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 b ase, which allows the influence of the implantation damage on the boron dif fusion to be studied. The process simulations show that point defects gener ated by the extrinsic base implant lead to a broadening of the basewidth ar ound the perimeter of the emitter due to transient enhanced diffusion of bo ron from the SiGe base. This causes parasitic energy barriers to form, whic h in the worst case, extend laterally several microns from the edge of the extrinsic base. The electrical effect of the transient enhanced diffusion i s a decrease in collector current as the emitter geometry is reduced. Trans istors with different emitter geometries and undoped SiGe spacer thicknesse s 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. (C) 1998 Elsevier Science L td. All rights reserved.