Hot carrier degradation experiments are compared to the results of a two-dimensional simulation of hot electron and hole emission into the oxide, taking into account carrier emission which does not lead to gate currents. A new degradation mechanism, due to trapping of electrons on trapped holes, is shown to be the dominant generator of negative effective charge. Hole trapping produces a sharp peak of positive effective charge close to threshold. A simple transistor degradation model based on the calculated carrier emissions gives a very good description of the experimental degradation behavior