Substrate hot electron stress was applied on n(+)-ringed n-channel MOS capacitors with TiN/Hf-silicate based gate stacks to study the role of O vacancy induced deep bulk defects in trapping and transport. For the incident carrier energies above the calculated 0 vacancy formation threshold, applied on MOS devices with the thick high-kappa layer, both the flatband voltage shift due to electron trapping at the deep levels and the increase in leakage current during stress follow t(n) (n approximate to 0.4) power law dependence. Negative-U transitions to the deep levels are shown to be possibly responsible for the strong correlation observed between the slow transient trapping and the trap-assisted tunneling. (c) 2006 Elsevier Ltd. All rights reserved.