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The soft threshold lucky drift model of impact ionisation in semiconductors due to Marsland (1987) has been developed by replacing an approximation with a more accurate power series approximation. The model has been fitted to measure ionisation coefficients and the new results give better agreement to the soft threshold lucky drift model due to Ridley (1987), especially for InP and GaAs. Further agreement for Si is obtained when the same experimental data is used. The variation of mean free paths and the 'softness of threshold' factor due to the choice of different experimental data and different ionisation threshold energies is estimated in silicon. The lucky drift model with the power series approximation is also fitted to ionisation coefficients measured in Al x Ga 1-x As and Ge and a chemical trend in the mean free path is observed for all the materials reported.
John Marsland (1990) studied this question.