Fit of the experimental data of ZnS : Mn by a modified lucky-drift formula has been performed using the least square algorithm. The fit agrees well with the experimental data only at high field. The best fitting parameters at high field are the mean free path of order 102.74 Å and Keldysh factor,p₀ = 0.0138. A generalized Keldysh formula has been used, due to introduction of a soft threshold factor. The soft lucky-drift theory can also be used to calculate the impact ionization coefficients of high electron energy of ZnS : Mn without losing its physical significance compared to full band-structure Monte Carlo calculation with a remarkably reduced amount of computer resources.

The curvature on semi-log plot of experimental impact ionization coefficient against the inverse of electric field is different from what is observed for other materials at low electric fields due to impact ionization of deep level impurities.