Observation and Origin of Anomalous Early Recovery of Base Currents in Low-Dose-Rate γ-Ray-Irradiated PNP Transistors

We observe an anomalous early recovery behavior of base currents in silicon PNP transistors irradiated by 60Co γ-rays at a dose rate below 1 mrad(Si)/s, which, however, disappears for a higher dose rate. The physical origin of this effect is thoroughly investigated by combining methods of defect concentration measurement, first-principles calculation, and kinetic modeling. The concentrations of interface traps and oxide-trapped charges in these transistors are found to increase monotonically as the total dose increases. Meanwhile, first-principles calculations show that γ-induced concentrated carrier clusters in n-type Si can greatly promote the diffusion of nearby electrically active vacancy-oxygen (VO) defect complexes. Therefore, the observed anomalous behavior of base current is attributed to an irradiation-induced annealing of pre-existing defects in Si at room temperature because moving VO can encounter interstitial oxygen atoms and transform into electrically inactive VO2. A kinetic model is derived for the observable base current by combining this mechanism with the ionization-induced buildup of defects in the SiO2–Si structure. It can uniformly describe the anomalous behavior at low dose rates and the normal behavior at high dose rates by considering the difference in the time of carrier action on the defect diffusion. Our work demonstrates that low-dose-rate γ irradiation is an effective method to realize defect engineering in semiconductors.