DETERMINATION OF BANDGAP NARROWING AND PARASITIC ENERGY BARRIERS IN SIGE HBTS INTEGRATED IN A BIPOLAR TECHNOLOGY

This paper describes a method for characterizing the bandgap narrowing and parasitic energy barrier in SiGe heterojunction bipolar transisto rs (HBT's), fabricated using a single-polysilicon self-aligned bipolar process. From a comprehensive study of the temperature dependence of the collector current, the bandgap narrowing in the base due to german ium has been dissociated from that due to the heavy dopant concentrati on, The same approach has been used to characterize the height and wid th of parasitic energy barriers which appear when boron out-diffusion from the SiGe base is present, The method has been applied to SiGe het erojunction bipolar transistors fabricated using a single polysilicon, self aligned, bipolar process, as well as mesa transistors, The exper imental results show that small geometry transistors have degraded col lector currents due to boron out-diffusion around the perimeter of the emitter, This behavior has been explained by accelerated boron diffus ion due to point defect generated during the extrinsic base implant, T he values of undoped SiGe spacer thickness needed to suppress the para sitic energy barrier are described, Finally, high-frequency results ar e reported, which correlate the frequency transition to these parasiti c energy barriers.