Monte Carlo calculations of base transit time in bipolar transistors

Abstract

Previous analytical treatments of low-field electron transport in the base of bipolar transistors have all made the implicit assumption that the velocity distributions are Gaussian. Using material and scattering parameters appropriate to GaAs and a simple model for the transistor base in which the collector is replaced by a step potential, we have found that near the collector these distributions are distorted, leading to a diffusion velocity exceeding the thermal velocity. We also investigated the effects of the base-emitter boundary condition and found that standard injection techniques can force non-equilibrium distributions. This becomes critical as the base lengths become sub-micron and boundary effects dominate. With use of a dead region, where the electron distributions could relax before injection, to overcome this problem we found close agreement with the analytical results of Herbert.