The influence of parasitic resistances on the f/sub T/-optimisation of high-speed SiGe-HBTs

We study the influence of parasitic series resistances on the cut-off frequency of high-speed SiGe hetero-junction bipolar transistors. Due to coupling of the parasitic resistances with the internal collector-base capacitance, significant extra delay time is introduced. This extra delay will cause saturation, or even a decrease of f/sub T/ at higher collector doping levels. In addition, we study the optimisation of an n-cap emitter profile, which is only possible when the collector delay is reduced to a minimum, and the series resistances are properly included.