This research is concerned with theoretical and computational investigations of the flow of electrons and holes in semiconductor devices based on the hydrodynamic model. Numerical simulations of both 1-d and 2-d semiconductor devices are proposed, for both subsonic and subsonic/supersonic flow of charge carriers. The hydrodynamic shock waves will be analyzed theoretically and computationally. The physical importance of the hydrodynamic model for simulating 2-d Si and GaAs MESFETs will be demonstrated.