Collector Profile Design of SiGe-HBTs for Optimized Static and High-Frequency Performance

Experimental and numerical results on the leverage of a modified vertical collector profile on dc characteristics as well as on high-frequency performance of a SiGe-HBT are presented. It is shown that an increased collector doping near the base junction allows to improve essentially the saturation behavior in high-current mode compared to a flat low doped collector region. The additional doping peak retards the onset of the Kirk effect and the accumulation of charge carriers at the base-collector heterojunction while keeping the collector-base capacitances CCB relatively low. Both from experiments and numerical calculations by means of a non-local ionization model we realize that the additional doping peak does not deteriorate the breakdown behavior. The drop of transit frequencyfT and maximum oscillation frequency fmax can be driven to higher collector current densities. Furthermore, the suggested doping profile is well suited to compensate the outdiffusion of boron doping from the base.