This study examined the positive bias temperature instability (PBTI) of n-type metal-oxide-semiconductor field effect transistor (nMOSFET) characteristics and the negative bias temperature instability (NBTI) of p-type metal-oxide-semiconductor field effect transistor (pMOSFET) characteristics of HfO2 gate dielectrics with a metal gate. The interface trap charge (Delta N-it) and bulk oxide trap charge (Delta N-ot) densities were separately estimated to determine the defects responsible for the threshold voltage (V-th) shift during the BTI stress. The contribution of Delta N-it to the V-th shift for nMOSFET PBTI and pMOSFET NBTI was similar to 5 and similar to 30%, respectively. In addition, the estimated activation energy (E-a) of Delta N-it and Delta N-ot was 0.10 and 0.03 eV, respectively, for nMOSFET PBTI, and 0.11 and 0.17 eV, respectively, for pMOSFET NBTI. It was confirmed that the main degradation mechanism of nMOSFET PBTI is related to the generation of oxide trap charges. However, the degradation under pMOSFET NBTI stress is due to the generation of both interface states and oxide trap charges. By measuring the stress and recovery characteristics under various bias conditions, it was also found that the electron trapping/detrapping characteristics are dominant in the nMOSFET, whereas the generation of both hole trapping/detrapping and interface states competitively occurs in the pMOSFET.