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The behavior of an off-state leakage current induced by heavy-ion irradiation in deep-submicrometer NMOSFETs is comprehensively investigated by 3-D simulation in this paper. The results show that the off-state drain current is increased, which is mainly due to the positively charged damage region generated by the heavy-ion strike in the shallow-trench isolation (STI) region. As the channel length scales down, the off-state leakage collapse becomes more severe. The dependence of the off-state leakage current on the device channel length and width is studied, which gives the location of the most critical physical damage region in the STI trench oxide. Moreover, the impact of the gate bias during exposure to heavy ions on the device off-state behavior is also analyzed, indicating that a low operating voltage is beneficial to the circuit radiation hardening. At last, to suppress the off-state leakage collapse, some possible solutions are proposed.