Charge trapping in Hf-based high-κ dielectrics, used in MOS transistor gate stacks, causes threshold voltage shift (ΔV T) during constant voltage stress (CVS). It seemed to be very transient due to the presence of a large number of shallow traps and can be eliminated by applying a reverse direction electric field. Recent findings, however, suggest that deep traps significantly contribute to the reliability of various Hf-based high-κ gate stacks. The origin of the deep traps and their dependence on O vacancy formation during dielectric deposition is discussed. ΔV FB and leakage current dependence on these deep traps is also outlined. It is shown that trapping at deep levels inhibits fast ΔV T recovery. We have experimentally observed trap levels from low temperature measurements assuring the presence of O vacancies in Hf-silicate based films. Substrate hot electron (SHE) injection gives rise to significant electron trapping and slow post-stress recovery under 'negative bias' conditions, which confirms that O vacancy induced deep defects determine the transient behavior in Hf-silicate based high-κ gate dielectrics. It is further shown that negative-U transition to deep defects is responsible for trap assisted tunneling under substrate injection. A fraction of the injected electrons remains trapped at the deep defects and gives rise to significant ΔV FB. This has the potential to be the ultimate limiting factor for the long-term reliability of Hf-based high-κ gate dielectrics. Copyright The Electrochemical Society.