Monte Carlo calculations of base transit time in bipolar transistors

Abstract

Previous analytical treatments of low-field electron transpolt 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 wllector 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-emitler boundary condition and found that standard injection techniques can force non-equilibrium distributions. This becomes critical as the base lengths become submicron 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.

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Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

“…This would result in a filtering effect and would affect the electron distribution injected into the base. It has already been demonstrated by Mills et al [40] that the form of the injected distribution at the edge of a GaAs base can influence the transit times in short bases, and it is possible that similar effects might also occur here.…”

Modelling of the influence of velocity overshoot on the cut-off frequency in Si and GaAs heterojunction bipolar transistors

“…Work by Mills et al [17] suggested that the Gaussian velocity distribution injected into the base region in these types of simulation does not accurately model the injected distribution in a real transistor, leading to higher base transit times, most significantly in short bases (less than 500 Å). This is because the injected distribution does not have a sufficient distance in which to relax to the correct equilibrium distribution before it encounters the base-collector boundary.…”