  Irradiation damage is a key physics issue for semiconductor devices under
extreme environments. For decades, the ionization-irradiation-induced damage in
transistors with silica-silicon structures under constant dose rate is modeled
by a uniform generation of $E'$ centers in the bulk silica region and their
irreversible conversion to $P_b$ centers at the silica-silicon interface. But,
the traditional model fails to explain experimentally observed dependence of
the defect concentrations on dose, especially at low dose rate. Here, we
propose that, the generation of $E'$ is decelerated due to the dispersive
diffusion of induced holes in the disordered silica and the conversion of $P_b$
is reversible due to recombination-enhanced defect reactions under irradiation.
It is shown that the derived analytic model based on these new understandings
can consistently explain the fundamental but puzzling dependence of the defect
concentrations on dose and dose rate in a wide range.
