We have investigated theoretically the cause of the substantial threshold voltage (V-th) shifts observed in Hf-related high-k gate stacks with p+poly-Si gates, by focusing on the ionic nature of HfO2. The oxygen vacancy (Vo) level in ionic HfO2 is located in a relatively higher pail of the band gap. This high position of the Vo level results in a significant elevation of the Fermi level for p+poly-Si gates, if the p+poly-Si gate is in contact with the high-k HfO2. Vo formation in the HfO2 induces a subsequent electron transfer across the interface, causing a substantial V-th shifts in p+poly-Si gate MISFETs. Moreover, our theory also systematically reproduces other recent experimental results. (c) 2005 Elsevier B.V. All rights reserved.