Electron dynamics in silicon is investigated by means of improved momentum- and energy-balance equations including particle diffusion and heat flux. The resulting system of partial differential equations is numerically solved in a variety of field configurations including strong discontinuities, in order to enhance velocity overshoot effects. It is found that diffusion, usually neglected in previous studies, plays a major role, and considerably modifies the features of the velocity vs distance curve, leading to an increase of the carrier drift velocity in the low-field region, i.e. before experiencing the effect of the strong field. In addition, it is found that, in order to take full advantage of velocity overshoot effects in MOSFET's, a structure must be designed having the strongest possible field at the source-end of the channel, where carrier density is controlled by the gate. © 1985.