Applied Field and Total Dose Dependence of Trapped Charge Buildup in MOS Devices

A rate equation for charge buildup which includes carrier sweep out, geminate recombination, hole/ electron trapping, and effects of internal fields is developed. The first moment of the resulting charge distribution is calculated to yield the midgap voltage shift as a function of irradiation time. The initial midgap voltage shift per dose and the maximum midgap voltage shift are derived. The field dependence of these quantities is shown to be a consequence of the field dependence of the hole/electron capture cross sections and geminate recombination escape probability The results of this formulation show that the E-1/2 decrease in the midgap shift per dose with field described in the literature is due to the decrease of the hole capture cross section with increasing applied field. The theory is validated by comparison with experimental results obtained on 225 A thermal oxide on p-type silicon test capacitors irradiated under bias at room temperature.