Inversion charge redistribution model of the high-frequency MOS capacitance

Abstract

The high-frequency semiconductor capacitance in an MOS structure is ordinarily calculated by a depletion-charge analysis approach assuming that there is no response of the inversion layer charge to the a.c. signal. The more realistic model, in which the inversion charge is allowed to be spatially redistributed at the high frequency, is treated here by the solution of the Poisson-Boltzmann equation incorporating an appropriate quasi-Fermi level for the inversion layer carriers. The inclusion of this effect leads to a much faster saturation of the capacitance and increases the value at strong inversion by 2–5 per cent for silicon at room temperature. It also predicts a shallow minimum less than 1 per cent below the asymptotic value. Agreement with experiment is shown to be excellent. An analytic expression for the asymptotic value is given.