Defect Dynamic Model of the Synergistic Effect in Neutron- and γ-Ray-Irradiated Silicon NPN Transistors.

A defect dynamic model is proposed for the positive synergistic effect of neutron- and γ-ray-irradiated silicon NPN transistors. The model considers a γ-ray-induced transformation and annihilation of the neutron-induced divacancy defects in the p-type base region of the transistor. The derived model of the base current predicts a growth function of the γ-ray dose that approaches exponentially an asymptotic value, which depends linearly on the neutron-induced initial displacement damage (DD) and a linear decay function of the dose whose slope depends quadratically on the initial DD. Variable fluence and dose neutron-γ-ray irradiation experiments are carried out, and we find all of the novel dose and fluence dependence predicted by the proposed model are confirmed by the measured data. Our work, hence, identifies that the defect evolution under γ-ray irradiation, rather than the widely believed interface Coulomb interaction, is the dominating mechanism of the synergistic effect. Our work also paves the way for the modification of displacement defects in silicon by γ-ray irradiation.