The degradation of Si/SiOx/HfSiON/TaN field effect transistors under negative bias temperature stress was investigated for devices annealed in H-2/N-2 or D-2/N-2 ambient. A clear H-2/D-2 isotopic effect on device degradation was observed at low electric fields, typically below 5 MV/cm. The isotopic effect tended to disappear at higher electric field. These results are explained by assuming that fast interface states, which contribute significantly to device degradation at low electric fields, are related to the (dispersive) transport of H+ or D+ in the gate dielectric stack. Slow bulk traps, which are responsible for device degradation at high electric fields, are most likely not related to the transport/trapping of hydrogen species, but are rather induced by the trapping of holes injected from the channel during the electrical stress. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2135429] All rights reserved.