The surface quantization effect below the gate insulator of MOSFETs is practically introduced into the device simulation by adding the quantum correction potential to the drift term of the current density equation. The quantum correction potential is calculated as a simple function of the distance from the surface by substituting Fang's trial function as the carrier density distribution into Ancon's density-gradient method. The effective mass perpendicular to the surface is adjusted to reproduce Schrodinger-Poisson calculations. Simulation results of the gate voltage dependence of the gate capacitance are compared with experimenta results. It is demonstrated that the present approach could be a robust tool by showing simulations combined with the energy transport effect. (C) 2002 Published by Elsevier Science Ltd.