It was recently found that the silicon oxynitride prepared by oxidation of silicon-rich silicon nitride (SRN) has several important features. The high nitrogen and extremely low hydrogen content of this material allows it to have a high dielectric constant and a low trap density. The present work investigates in further detail the electrical reliability of this kind of gate dielectric films by studying the charge trapping and interface state generation induced by constant current stressing. Capacitance–voltage (C–V) measurements indicate that for oxidation temperatures of 850 and 950 °C, the interface trap generation is minimal because of the high nitrogen content at the interface. At a higher oxidation temperature of 1050 °C, a large flatband shift is found for constant current stressing. This observation can be explained by the significant reduction of the nitrogen content and the phase separation effect at this temperature as found by X-ray photoelectron spectroscopy study. In addition to the high nitrogen content, the Si atoms at the interface exist in the form of random bonding to oxygen and nitrogen atoms for samples oxidized at 850 and 950 °C. This structure reduces the interface bonding constraint and results in the low interface trap density. For heavily oxidized samples the trace amount of interface nitrogen atoms exist in the form of a highly constraint SiN₄ phase and the interface oxynitride layer is a random mixture of SiO₄ and SiN₄ phases, which consequently reduces the reliability against high energy electron stressing.