SPICE modelling of impact ionisation effects in silicon bipolar transistors

Abstract
A nonlocal, energy based impact ionisation model for bipolar transistors is implemented into a general purpose circuit simulator. With respect to conventional, either empirical or electric field based, models, the proposed approach enables a more physical and accurate description of impact ionisation effects in modern, high speed bipolar transistors, where non-negligible nonstationary transport effects take place as a consequence of the strong spatial variations in the electric field at the base–collector junction. The conventional base resistance model is also modified, to take into account the base resistance dependence on bias in the presence of an impact ionisation induced reverse base current. Neglecting the influence of the reverse base current on the base resistance can result in an underestimation of the degradation of both DC and switching performance of bipolar transistors due to impact ionisation. The implemented models are validated by comparison with experimental results obtained from devices of two different technologies.