A transistor model defines the carrier mobility as a combination of both drift-diffusion mobility and ballistic mobility. The ballistic mobility is calculated based on the assumption that the kinetic energy of carriers near an injection point is no greater than the potential energy difference of carriers near that injection point. The abruptness of the onset of velocity saturation, as well as the asymptotic velocity associated therewith is made dependent on the degree to which the velocity is ballistically limited. The model further takes into account the inertial effects on the velocity and/or charge flux associated with carriers. The model computes the mobility and hence the velocity of carriers in accordance with their positions in the channel both along the direction of the current flow as well as the direction perpendicular to the current flow.