The reliability of ultra-thin silicon oxynitride gate dielectrics was investigated in terms of the Weibull slope of the t(bd) distributions, and the voltage acceleration factor. High field t(BD) statistics were extrapolated using a worst-case linear fit, to the low field region. The samples showed low voltage acceleration, which degrades reliability, but this is overcome by high Weibull slopes (beta), which improve reliability. The high beta values were investigated by monitoring the change in beta as gate current increased during CVS. We saw the Weibull slope change from approximate to1 at low current to 2 at high current, indicating that breakdown results from 2-trap conduction. Charge pumping measurements showed that the trap generation rate at the SiON/Si interface, where most of the Nitrogen resides was very low. Thus it was concluded that there is a much higher trap generation rate in the bulk than at the interface and this is responsible for the values of beta. (C) 2004 Elsevier B.V. All rights reserved.