Spatial analysis of the electron transit time in a silicon/germanium heterojunction bipolar transistor by drift-diffusion, hydrodynamic, and full-band Monte Carlo device simulation

Transit times and cutoff frequency of a silicon/germanium heterojunction bipolar transistor (SiGe HBT) are investigated by consistent drift-diffusion (DD), hydrodynamic (HD), and full-band Monte Carlo (FB-MC) simulations. Good agreement of all three transport models is found for the collector transit time. The quasiballistic transport in the base is well described by the HD model and it yields the same transit time as the FB-MC model, whereas the DD model yields a much larger transit time, because it does not include any velocity overshoot effects at all. Surprisingly, in the emitter region the FB-MC model yields the largest transit time leading to a peak cutoff frequency for the special device structure under investigation which is even smaller than the DD peak value. The strong anisotropy of the strained band structure in the base, which is not captured in full detail by the DD and HD model, is identified as a possible reason for this unexpected behavior.