Oxynitrides prepared by double nitridation in nitric oxide (NO) and nitrous oxide (N2O) are compared to the one with a single NO nitridation. Based on various hot-carrier stresses, harder oxide/Si interface, less charge trapping and generation of electron/hole traps in oxide, and larger chargeto-breakdown are observed for the doubly-nitrided gate dielectrics than the singly-nitrided one. By analyzing the nitrogen profiles in these oxynitrides, it is revealed that the involved mechanisms lie in the smaller distance of peak nitrogen concentration from the oxide/Si interface and the higher nitrogen content near the oxide=Si interface in the doublynitrided oxynitrides.