BASE RESISTANCE AND EFFECTIVE BANDGAP REDUCTION IN N-P-N SI SI1-XGEX/SI HBTS WITH HEAVY BASE DOPING/

This paper presents a comprehensive study of the effects of heavy dopi ng and germanimn in the base on the de performance of Si/Si1-xGex/Si n pn Heterojunction Bipolar Transistors (HBT's). The lateral drift mobil ity of holes in heavily doped epitaxial SiGe bases affects the base sh eet resistance while the effective bandgap is crucial for the vertical minority carrier transport, The devices used in this study were Si1-x Gex npn HBT's with flat Ge and B profiles in the base grown by Rapid T hermal Chemical Vapor Deposition (RTCVD), Hall and drift lateral hole mobilities were measured in a wide range of dopings and Ge concentrati ons, The drift mobility was indirectly measured based on measured shee t resistivity and SIMS measurements, and no clear Ge dependence was fo und, The Hall scattering factor is less than unity and decreases with increasing Ge concentration, The effective bandgap narrowing, includin g doping and Ge effects, was extracted from the room temperature colle ctor 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 observed in silicon, due to the effect of a lower densi ty of states, A model for the collector current enhancement with respe ct to Si devices versus base sheet resistance is presented.