SPICE MODELING OF IMPACT IONIZATION EFFECTS IN SILICON BIPOLAR-TRANSISTORS

A nonlocal, energy based impact ionisation model for bipolar transisto rs is implemented into a general purpose circuit simulator. With respe ct 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 transistor s, where non-negligible nonstationary transport effects take place as a consequence of the strong spatial variations in the electric field a t 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 c urrent. Neglecting the influence of the reverse base current on the ba se resistance can result in an underestimation of the degradation of b oth DC and switching performance of bipolar transistors due to impact ionisation. The implemented models are validated by comparison with ex perimental results obtained from devices of two different technologies .