||Presented in this paper is a quantum capacitance-voltage (C-V) modeling in depletion and inversion, incorporating the gate-depletion effect. The model enables fast and accurate extraction of the electrical thickness of gate oxide in deep submicron MOSFETs. The main quantum effect consists of the inversion capacitance of two-dimensional (2-D) electrons masking the true gate-oxide thickness, t(OX). The quantum mechanical and gate depletion effects necessitate 6-10 Angstrom equivalent oxide thickness correction, which is important for a t(OX) of 4 run or less. The classical C-V analysis is compared with the quantum results in the light of the data, highlighting the difference between the models. The model is shown in good agreement with experiments and also with numerically calculated results.