[en] Using an instantaneous-terminal-voltage (ITV) measurement technique, the variation of the oxide field in an MOS capacitor irradiated by X-rays without bias is examined. It is shown that the oxide field during irradiation depends on the device leakage resistance which can be estimated by the newly reported charge-then-decay method, and the flat-band voltage shift due to irradiation is consistent with the field so found, Interestingly, it is found that the device leakage resistance during irradiation for an MOS(p) capacitor is generally smaller than that for an MOS(n) capacitor. The different radiation sensitivities between an MOS(p) and an MOS(n) capacitor should account primarily for the dopant-dependent oxide resistances found in this work instead of the initial work function differences alone. An equivalent circuit is given to describe the dependence of the oxide field during irradiation on the device resistance, and to explain the observations. (author)