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The negative bias temperature instability (NBTI) reliability of sub-1-nanometer equivalent oxide thickness (EOT) ZrO2 and HfO2 dielectrics with metal gate is investigated. The threshold voltage shift (ΔVTH) at identical NBTI over-drive stress conditions is observed to be lower in ZrO2 than in HfO2 field-effect transistors. Ring oscillator charge pumping is applied to determine interface trap generation (ΔNit) in the sub-1-nanometer EOT devices, with ZrO2 devices showing about one order of magnitude lower ΔNit than HfO2 device. However, the ΔNit contribution to the total ΔVTH is very limited in sub-1-nanometer EOT devices, as the recoverable component from the pre-existing bulk defects dominates the whole NBTI degradation. Pulsed Id-Vg technique is applied to analyze the pre-existing bulk defects in those sub-1-nanometer EOT devices, and lower pre-existing bulk defect density is shown in ZrO2, which decisively reduces NBTI in ZrO2 gate dielectric.