The degradation dynamics of magnesium oxide (MgO) layers in MOS structures has been investigated. It is shown that the shift of the conduction characteristics caused by both ramped and constant voltage stresses is well described by power-law models and that the signature of charge trapping is still visible even after the dielectric breakdown (BD) of the insulating film. The occurrence of progressive BD in 20-nm thick films is also reported. © 2009 Elsevier B.V. All rights reserved.