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We have investigated, theoretically the cause of the substantial threshold voltage (Vth) shifts observed in Hf-related high-k gate stacks with p+poly-Si and p-metal gates, by focusing on the ionic nature of HfO2. The oxygen vacancy (Vo) level in ionic HfO2 is located in a relatively higher part of the band gap. This high position of the VO results in a significant elevation of the Fermi level for p+poly-Si and p-metal gates, if Si can interact with high-k HfO2. Vo formation in the HfO2 induces a subsequent electron transfer from HfO2 to gates, causing a substantial Vth shifts in MISFETs.