Impact ionization processes define the breakdown characteristics of semiconductor devices. An accurate description of such phenomenon, however, is limited to very sophisticated device simulators such as Monte Carlo. A new physical model for the impact ionization process is presented, which accounts for dead space effects and high energy carrier transport at a Drift Diffusion level. Such model allows to define universal impact ionization coefficients which are device-geometry independent. By using available experimental data these parameters have been calculated for In0.53Ga0.47As.