Investigation on γ radiation effects of N-channel VDMOSFETs irradiated without electric field stress

The ionizing radiation response of a kind of commercial n-channel VDMOSFETs irradiated without external electric field stress is investigated. The effects of total dose, dose rate and H2 ambient on the radiation response, and the post-irradiation annealing behaviors of the devices have been explored. The devices are degraded obviously after 1200 Gy (Si) γ radiation. No appreciable true dose rate effect (or Enhanced Low Dose Rate Sensitivity, ELDRS) has been observed in the tested devices. The small and irregular variations in performance degradation with dose rate are believed to come from the variability in the radiation response of the devices. In addition, the ambient of 100% H2 accelerates the total dose degradation of the devices. The γ radiation-induced performance degradation contains significantly negative shift of threshold voltage (Vth) and remarkable increase in off-state leakage current (Ileak), which are mainly caused by the increase in radiation-induced oxide-trapped charges. No obvious change in interface states is observed during the whole irradiation experiments and the annealing processes. The bias condition adopted in this work during irradiations is probably the reason for these unexpected phenomena. Furthermore, the annealing results indicate that the radiation-induced oxide-trapped charges seem to possess relatively stable properties, and could not be fully compensated or neutralized at room temperature (RT) or high temperatures, or under positive high electric field stress (PHEFS) of +70 V.