A Reevaluation of Worst-Case Postirradiation Response for Hardened MOS Transistors

The "worst-case" postirradiation response of Sandia hardened n-channel transistors following Co-60 exposure to total dose levels of system interest is demonstrated to occur for zero-volt bias during radiation, and positive bias during a subsequent anneal. This observation is explained in terms of oxide-trapped and interface-state charge buildup and anneal. Additional results are presented which suggest that, for future technologies with very thin gate oxides, worst-case device leakage during irradiation may well occur for zero-volt irradiations. These results highlight the importance of periodically reevaluating the response of MOS devices during and after irradiation to determine worst-case test conditions, particularly as technologies advance and gate insulators become thinner.