Characterisation of NPN and PNP SiGe heterojunction bipolar transistors formed by Ge⁺ implantation

This work compares NPN and PNP SiGe HBTs fabricated simultaneously using Ge+ implantation in the base to achieve an average Ge concentration of 4 at.%. Electrical measurements are presented and discussed with TEM and SIMS analysis. The results show that the PNP HBTs give superior performance to the NPNs, as they have more ideal collector characteristics. The NPN HBTs exhibit collector-emitter leakage. Both types of Ge+ implanted device have non-ideal base currents. TEM images show that both NPN and PNP devices have a high density of defects in the Ge+ implanted area. The electrical results are explained by the opposing effect of the Ge+ implant on the diffusion coefficients of boron and arsenic, as seen in the SIMS profiles. The increased emitter diffusion in the NPN HBTs creates a narrower base region, which is more prone to collector-emitter leakage. The hairpin dislocations in the base are thought to be the cause of the base current ideality deterioration.