A full-band Monte Carlo simulation of the high-field electron transport in the ZnS phosphor layer of an alternating-current thin-film electroluminescent device is performed. The simulation includes a nonlocal empirical pseudopotential band structure for ZnS and the relevant scattering mechanisms for electrons in the first four conduction bands, including band-to-band impact ionization and impact excitation of Mn 2ϩ luminescent centers. The steady-state electron energy distribution in the ZnS layer is computed for phosphor fields from 1 to 2 MV/cm. The simulation reveals a substantial fraction of electrons with energies in excess of the Mn 2ϩ impact excitation threshold. The computed impact excitation yield for carriers transiting the phosphor layer exhibits an approximately linear increase with increasing phosphor field above threshold. The onset of Mn 2ϩ impact excitation coincides with the onset of band-to-band impact ionization of electron-hole pairs which prevents electron runaway at high electric fields.

Band-to-band impact ionization by hot electrons and holes is an important process in high-field transport in semiconductors, leading to carrier multiplication and avalanche breakdown. Here we perform first principles calculations for the respective microscopic scattering rates of both electrons and holes in various wide band gap semiconductors. The impact ionization rates themselves are calculated directly from the electronic band structure derived from empirical pseudopotential calculations for cubic GaN, ZnS, and SrS. In comparison with the electron rates, a cutoff in the hole rate is found due to the relatively narrow valence bandwidths in these wide band gap semiconductors, which correspondingly reduces hole initiated carrier multiplication.

The impact ionization rate in ZnS is calculated using a nonlocal empirical pseudopotential band structure and compared to previous results using a local calculation. The two resulting rates are then compared and simple fit formulas are presented. These are included in an ensemble Monte Carlo simulation of electron transport in bulk ZnS. The calculated impact ionization rate is then compared to experimental impact ionization coefficient data; reasonable agreement between the experimental data and the calculated impact ionization rate is obtained with an appropriate choice of optical deformation potentials.

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