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The effect of fast components in the threshold-voltage shift (??Vth) induced by electrical stress on the lifetime of bias temperature instability (BTI) is investigated in metal-oxide-semiconductor field-effect transistors with high-k gate dielectrics using dc and pulsed measurements. The empirical results confirm that the initial fast ??Vth (??Vth,fast,ini.) does not contribute to long-term device degradation under operating voltage conditions. Therefore, ??Vth is corrected by eliminating the initial ??Vth,fast,ini. With this adjustment, the effect of measurement time (tm) on the corrected ??Vth (??Vth,corr.) is negligible. However, compared with the lifetime estimates made using ??Vth,corr., the dc measurements still tend toward overestimation under low voltage stress because of the ??Vth,fast,str. component induced by stress (not the initial ??Vth,fast,ini.)In conclusion, it may be stated that the ??Vth,fast,str. component could play a critical role in BTI tests conducted under operating voltage conditions.