Pre-existing electron and hole trap energy levels, lying deep within the bulk high-κ bandgap, are experimentally observed from low temperature measurements for the first time in metal organic chemical vapor deposited (MOCVD) TiN/HfSixOy based gate stacks. Their role in trapping under constant voltage and substrate hot hole stress conditions is investigated. Mixed degradation due to both electron and hole trapping in the deep pre-existing bulk traps, which induces a turn-around effect in fiatband voltage shift (ΔVFB), is observed for n-channel MOS capacitors under gate injection. Bulk hole trap generation, which causes ΔV FB to follow tn power law dependence, is observed for p+-ringed p-channel MOS capacitors under substrate hot hole injection. Activation energies of the stress induced traps and pre-existing deep defects are experimentally found to be different. copyright The Electrochemical Society.