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We have performed an experimental study of the effects of ionizing radiation and bias-temperature stress on Si MOS devices with HfSiON gate dielectrics. We compare the responses of homogeneous high-Si 3 N 4 films and low-Si 3 N 4 films that contain crystalline HfO 2 . We observe that the low-Si 3 N 4 films are more sensitive to ionizing radiation than the high-Si 3 N 4 films. In particular, the low-Si 3 N 4 film that includes crystalline HfO 2 is especially vulnerable to electron trapping due to substrate injection under positive irradiation bias. Both film types exhibit reduced radiation-induced charge trapping relative to previous Hf silicates. The high-Si 3 N 4 film exhibits 16 less radiation-induced net oxide-trap charge density than earlier Hf silicate films processed without nitride. We also find that these devices are relatively robust against bias-temperature stress instabilities. Consistent with the radiation response, the low-Si 3 N 4 devices also display elevated levels of charge trapping relative to the high-Si 3 N 4 devices during bias-temperature stress.