Negative bias temperature instability (NBTI) in p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) with ultrathin oxynitride layers is investigated. The degradation of the threshold voltage and the drain current of the devices are more important when the nitrogen content at the Si/SiON interface is higher. A degradation model is developed, based on the generation of Si(3)equivalent toSi(.) (P-b0) centers during electrical stress. The model includes a Gaussian spread of dissociation energies of the P-b0 centers that is closely related to the interfacial strain at the Si/SiON interface. Comparison between the experimental results and the model suggests that the strain at the Si/SiON interface increases when the nitrogen content is higher, leading to a faster degradation of the electrical properties of the devices. (C) 2003 The Electrochemical Society.