Radiation induced charge trapping in ultrathin HfO2 -based n-channel MOSFETs is characterized as a function of dielectric thickness and irradiation bias following exposure to 10 keV X-rays and/or constant voltage stress. Positive and negative oxide-trap charges are observed, depending on irradiation and bias stress conditions. No significant interface-trap buildup is found in these devices under these irradiation and stress conditions. Enhanced oxide-charge trapping occurs in some cases for simultaneous application of constant voltage stress and irradiation, relative to either type of stress applied separately. Room temperature annealing at positive bias after irradiation of transistors with thicker gate dielectric films leads to positive oxide-trapped charge annihilation and/or neutralization in these devices, and net electron trapping. The oxide thickness dependence of the radiation response confirms the extreme radiation tolerance of thin HfO2 dielectric layers of relevance to device applications, and suggests that hole traps in the thicker layers are located in the bulk of the dielectric. A revised methodology is developed to estimate the net effective charge trapping efficiency, fot, for high-kappa dielectric films. As a result, estimates of fot for Hf silicate capacitors and Al2O3 transistors in previous work are reduced by up to 18%.