The effect of ionization threshold softness on the temperature dependence of the impact ionization coefficient

The effect of the ionization threshold softness on the temperature variation of impact ionization coefficients is examined theoretically. It is found that increasing the softness reduces the temperature dependence of ionization, because temperature induced heating or cooling of the carrier distribution results in a smaller change in the ionization scattering rate sampled. This may explain the wide variation in the temperature dependence of breakdown voltage reported in the literature and the difficulty in modelling accurately the temperature dependence of the ionization process.