The role of hydrogen in negative-bias temperature instability (NBTI) at Si/SiO2 and Si/SiOxNy interfaces was investigated by using molecular models of the interfaces and first-principles calculations. The results suggest that the hydrogen is likely to migrate as an electrically neutral atom rather than a proton. It was found that the hydrogen migration at the Si/SiOxNy interface lowers the hole-trapping reaction energy, while that at the Si/SiO2 interface makes it higher. This means that the more severe NBTI at the Si/SiOxNy. interface compared to that at the Si/SiO2 interface is caused by the migrated hydrogen. (C) 2003 Elsevier Science B.V. All rights reserved.