Device lifetimes are correlated with substrate currents for n-MOSFETs after static and dynamic stress. Good agreement between a simple theory and the experimental results is found in both static and dynamic cases. However, the damage is larger for dynamic than for static stress which requires a modification of duty cycle calculations. The results are explained by a model in which chargaable traps in the oxide close to the interface are important. These can modify the injection of holes into the oxide depending on stress time structure and potential conditions.