Based on the physical process of holes trapped in oxide and interface trap buildup induced by proton, a unified physics-based model of oxide-trapped charge and interface trap charge in MOSFET after ionizing radiation exposure as a function of radiation dose is proposed. This model predicts that the oxide-trapped charge density and interface trap charge density induced by radiation rays would be linear in dose at low dose levelss, and would deviate from linear relationship and tend to saturation at moderate to high dose levels. The change of these two kinds of charge induced by ionizing radiation as a function of radiation dose can be well described by the expressions proposed in this model. At last, the correlation between these two kinds of charge is discussed and a linear dependence is suggested at low dose level. This model gives insights into the trap generation in gate oxide induced by radiation, and provides a more accurate predictive model for radiation damage in MOS devices operated in space ionizing radiation environment.