Defect properties of four types of hafnium defects in SiO2, viz, silicon substitution by hafnium (HfSi), oxygen substitution by hafnium (HfO), hafnium interstitial (Hfi) and oxygen vacancy in the vicinity of hafnium (VOHf) have been studied within the framework of density functional theory (DFT) using HSE06 functional, to understand their role in bias temperature instability (BTI). The calculated formation energies predict that Hfi (+4 charge) is the most stable defect in n-type MOS (NMOS) transistor followed by HfSi (0), VOHf in (+4) and HfO (+2) defects, whereas, in p-type MOS (PMOS), it is HfSi (0) followed by Hfi (+4), VOHf (0) and HfO (+1). Hence, the defects more likely to be formed in SiO2 are HfSi in both PMOS, NMOS and Hfi, VOHf in the NMOS transistor. Our charge transition levels (CTL) calculations show that the VOHf defect complex is likely to have a higher capture rate for electrons than Hfi and cause PBTI. However, HfSi defect will not trap any hole or electron and remains neutral during BTI conditions.