Implementation of nonlocal model for impact-ionization current in bipolar circuit simulation and application to SiGe HBT design optimization

A nonlocal characterization of impact-ionization current is implemented in a compact but physical bipolar transistor model for predictive circuit simulation. The charge-based model, which is applicable to SiGe-base HBT´s as well as Si BJT´s, provides at each bias point, including ones in quasisaturation, the electric field distribution E(x) in the epi-collector, and a simplified form of the energy-balance equation enables characterization of carrier temperature T_e(x) from E(x). Numerical spatial integration of the T_e -dependent ionization rate yields the impact-ionization current as post-processing in the model routine. The nonlocal model is verified by applications to two advanced bipolar (HBT and BJT) technologies in which the device breakdown voltages, which are underestimated by the local-field model, are predicted. The utility of the nonlocal model in assessing design tradeoffs involving impact ionization (i.e., device breakdown versus circuit performance) is demonstrated by simulations based on the two mentioned technologies