The evolution and model of breakdown mechanisms of constant-voltage stress (CVS) and constant-current stress (CCS) are investigated and compared. The results show that for both stressing methods, the transient evolution of stress current determines the breakdown. For CVS, because the stress current decreases with time, the trapped charge and interface state density increases with stress time with a T-s(1/2betalambda) ' dependence. For CCS, due to the constant trapped charge generation rate, the trapped charge and interface state increases linearly with stress time. A passing current model for CVS has been developed to explain the breakdown mechanism and its dependence on trapped charge distribution and trapped charge density. (C) 2002 Published by Elsevier Science Ltd.