In order to get insight into the mechanisms that govern the characteristics of the gate oxide in metal-oxide-semiconductor (MOS) structures with irradiation and annealing at low temperature by ionizing irradiation, the shift of the threshold voltage of MOS field-effect transistors was measured at 90 K during and after irradiation. The creation of self-trapped holes is sufficient to explain the behavior of the net positive electric charge detected in the oxide at this temperature, the majority of the positive charge being due to these hole polarons. The experimental activation energy for a self-trapped hole is found to be close to recent theoretical estimations. In the light of the isochronal annealing results, the reaction of holes with atomic hydrogen is discussed together with the possible influence of the distribution of the positively charged defects and polarons within the oxide. The net oxide charge growth at medium dose rates (50 krad/h) indicates the possible existence of negatively charged centers created by irradiation. At low temperature (90 K) the transport of holes by hopping into different self-trapped hole sites, within the framework of the approximate continuous-time random-walk theory, was used to model the threshold voltage radiation response. Both the ionizing dose and dose rate have an influence on the trapped positive charge. The experimental results obtained here agree quantitatively with the predictive model developed. Experimental power-law dependences are contained in this model, which also provides an experimental determination of the stochastic transport parameter α in thin thermal oxides.

DOI:https://doi.org/10.1103/PhysRevB.55.7641