We propose an analytical expression of the local carrier density inside silicon and SiO2 films assuming surface-accumulation and inversion conditions to simulate the capacitance of the modern MOS system. We assume a three-dimensional electron gas system and discuss the penetration of the electron wave function into SiO2 or high-k material films. It is demonstrated that this penetration slightly influences the capacitance-voltage characteristics. We show how the wave function penetration influences MOS capacitance under accumulation and inversion conditions by proposing advanced local carrier density models for electrons and holes. We also address the influence of the interlayer between the high-k material and the silicon substrate. (c) 2005 Elsevier Ltd. All rights reserved.