SiGe-base PNP transistors fabricated with n-type UHV/CVD LTE in a `No Dt´ process

Experimental results are presented on the use of N-type ultrahigh-vacuum/chemical vapor deposition (UHV/CVD) low-temperature epitaxy (LTE) to deposit thin (45 nm), heavily doped (1×10¹⁹ cm^-3) SiGe films to form the base of PNP transistors. To take full advantage of epitaxial base technology, the thermal cycles following the base deposition that cause dopant diffusion and relaxation of highly strained layers must be eliminated. This objective is met by a novel process using PECVD insulators and UHV/CVD LTE emitter deposition to limit the temperature following the base deposition to 550°C. This is essentially a `No Dt´ process in the sense that the effective dopant diffusion length Dt is negligible at this temperature. An advanced double-polysilicon bipolar structure was modified to fabricate non-self-aligned small-geometry transistors. Both DC and AC measurements were used to characterize the devices, confirming the presence of a large valence band offset at the base-collector junction. The resulting barrier to minority carrier transport caused additional charge storage in the neutral base and limited the peak cutoff frequency to 15 GHz independent of collector doping. The results demonstrate the impact of the valence band offset of SiGe heterojunctions on the performance of PNP transistors