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We have investigated the time dependent dielectric breakdown (TDDB) characteristics for a high-k/metal gate pMOSFET under inversion stress. We found that electrons, injected from the cathode, being minority carriers in the gate leakage current play an important role in determining TDDB lifetime and found that the presence of oxygen vacancies in HfSiON determine the electron current mechanism in HfSiON. Since electrons from the cathode electrode flow as a tunneling current with an effective barrier height determined by the energy level of the oxygen vacancy in the HfSiON, the electron current is strongly dependent on the effective work function of the metal gate. This implies that a higher work function would be more effective in suppressing electron current, due to the elevated barrier height for electrons. Therefore the formation of a high work function metal gate is essential, not only to give low threshold voltage of pMOSFETs but also to achieve long TDDB lifetimes.