Impurity density of states in n-type Si inversion layers

We calculate the impurity density of states for electrons bound to impurity centers localized inside the oxide and at the interface of the metal-oxide-semiconductor system. The screening and the disorder effects are taken into account in the calculation. We show that for a given impurity concentration, electric field, and binding energy of experimental interest, the ground-state band remains separated from the unperturbed band and starts to overlap the lowest excited band.