A new physically-based semi-empirical equation for electron effective mobility in MOS inversion layers has been developed by accounting explicitly for surface roughness scattering and screened Coulomb scattering in addition to phonon scattering. The new semi-empirical model shows excellent agreement with experimentally measured effective mobility data from five different published sources for a wide range of effective transverse field, channel doping, fixed interface charge, longitudinal field and temperature. By accounting for screened Coulomb scattering due to doping impurities in the channel, our model describes very well the roll-off of effective mobility in the low field (threshold) region for a wide range of channel doping level (NA = 3.9 × 1015-7.7 × 1017 cm-3). We have also developed a local-field-dependent mobility model for electrons in the inversion layer for use in device simulators by applying the previously published method to this new semi-empirical equation for the effective mobility. The new local-field-dependent mobility model has been implemented in the PISCES 2-D device simulation program, and comparisons of calculated vs measured data show excellent agreement for ID-VG and ID-VD curves for different devices with Leff ranging from 0.5 to 1.2 μ.