The electron mobility in the inversion layer of a MOSFET, formed on the (100) silicon surface, is calculated by using a Monte Carlo approach which takes into account size quantization, acoustic phonon scattering, intervalley phonon scattering and surface roughness scattering. Degeneracy is also considered because it is important at higher normal effective fields (high gate voltages). The main emphasis is placed on the influence of the specific autocovariance function, used to describe the surface roughness, on the electron mobility. It is found that the electron mobility calculated with roughness scattering rates based on the exponential function shows a good agreement with experiments. Device simulation of a MOSFET is carried out to demonstrate the usefulness of the present model, where 3D electron states are taken into account in addtion to the 2D electron states.