Base Resistance and Effective Bandgap Reduction in n-p-n Si/Sil-,Ge,/Si HBT's with Heavy Base Doping

This paper presents a comprehensive study of the effects of heavy doping and germanium in the base on the dc performance of Si/Sil --2 Ge,/Si npn Heterojunction Bipolar Transistors (HBT's). The lateral drift mobility of holes in heavily doped epitaxial SiGe bases affects the base sheet resistance while the effective bandgap is crucial for the vertical minority carrier transport. The devices used in this study were Sil-,Ge, npn HBT's with flat Ge and B profiles in the base grown by Rapid Thermal Chemical Vapor Deposition (RTCVD). Hall and drift lateral hole mobilities were measured in a wide range of dopings and Ge concentrations. The drift mobility was indirectly measured based on measured sheet resistivity and SIMS measurements, and no clear Ge dependence was found. The Hall scattering factor is less than unity and decreases with increasing Ge concentration. The effective bandgap narrowing, including doping and Ge effects, was extracted from the room temperature collector current measurements over a wide range of Ge and heavy doping for the first time. We have observed bandgap narrowing due to heavy base doping which is, to first order, independent of Ge concentration, but less than that ob- served in silicon, due to the effect of a lower density of states. A model for the collector current enhancement with respect to Si devices versus base sheet resistance is presented.

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