Abstract-This paper describes a method for characterizing the bandgap narrowing and parasitic energy barrier in SiGe heterojunction bipolar transistors (HBT's), fabricated using a singlepolysilicon self-aligned bipolar process. From a comprehensive study of the temperature dependence of the collector current, the bandgap narrowing in the base due to germanium has been dissociated from that due to the heavy dopant concentration. The same approach has been used to characterize the height and width of parasitic energy barriers which appear when boron out-diffusion from the SiGe base is present. The method has been applied to SiGe heterojunction bipolar transistors fabricated using a single polysilicon, self aligned, bipolar process, as well as mesa transistors. The experimental results show that small geometry transistors have degraded collector currents due to boron out-diffusion around the perimeter of the emitter. This behavior has been explained by accelerated boron diffusion due to point defect generated during the extrinsic base implant. The values of undoped SiGe spacer thickness needed to suppress the parasitic energy barrier are described. Finally, high-frequency results are reported, which correlate the frequency transition to these parasitic energy barriers.

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In this paper we present an investigation of the static performance over the 300K-80K temperature range of pseudo-heterojunction bipolar transistors using an advanced single-polysilicon CMOS compatible self-aligned structure and epitaxial growth for the base and the low doped emitter spacer. These devices exhibit ideal collector currents and non-ideal base currents. By analysing the base leakage current, we have been able to identify the main critical fabrication steps. The bandgap narrowing in the base has been deduced from the temperature dependence of the collector current and the effect of a parasitic boron spike in the base doping profile on the low temperature performance of the transistor has been studied.