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We combine first-principles calculations and statistical modeling of hydrogen motion to investigate interface trap formation and post-irradiation annealing processes in the Si-SiO2 system. The dependence of the interface trap density on the temperature and bias voltage is explained on the basis of analytical modeling and statistical simulations based on Monte Carlo and master equation approaches. We suggest that the interface-trap buildup and annealing dynamics are primarily controlled by processes associated with hydrogen transport toward and along the interface, hydrogen reactions with passivated and unpassivated dangling bonds, and differences in most-favorable charge states for hydrogen-related species in SiO2 and Si. The dramatic change of the interface-trap formation dynamics as a function of postirradiation annealing bias in MOS devices at a temperature of ∼150°C is explained as a result of competition between the direct depassivation of the interfacial Si-H bonds by protons and the passivation of interfacial dangling bond defects by neutral molecular hydrogen formed near the interface and in the Si. We show that the transport of different hydrogen-related species in the interfacial region and their mutual transformations are responsible for significant postirradiation accumulation of hydrogen near the interface. We also discuss how this approach and these results may be employed for further understanding of enhanced low-dose rate sensitivity (ELDRS).