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This paper investigates the degradation mechanism of a nanoscale n-channel metal-oxide-semiconductor field-effect transistor (nMOSFET) that is subjected to off-state stress at high temperature and the impact of stress-induced defects on threshold voltage Vth during drain relaxation. Experimental results indicate that acceptor-like interface traps Nit, positive oxide charges Qox, and neutral electron traps were generated by the off-state stress. Although the Nit generated by the off-state stress caused an increase in Vth, it did not influence Vth during drain relaxation at a positive gate voltage. Drain relaxation filled the neutral electron traps and neutralized positive Qox's, which increased Vth and decreased the off-current significantly. This new observation suggests that the off -state stress-induced defects in a nanoscaled nMOSFET should be seriously taken in evaluating the reliability of inverter circuits.