Oxide-Charge-Induced Impurity Level in Silicon Inversion Layers

We have observed a peak in the conductivity-versus-gate-voltage curves for $n$-channel silicon metal-oxide-semiconductor field-effect-transistor devices containing large oxide-charge densities, $4.7\times {10}^{11} {\mathrm{cm}}^{-2\mathrm{}}<~N_{\mathrm{ox}}<~1.1\times {10}^{12} {\mathrm{cm}}^{-2\mathrm{}}$. This peak is interpreted as an oxide-charge-induced impurity level within the lowest sub-band tail. It is found that the impurity level contains one electron state for each oxide charge situated at the Si-Si${\mathrm{O}}_2$ interface.