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The positive bias temperature instability (PBTI) characteristics of contact-etch-stop-layer (CESL)-strained HfO2 nMOSFET are thoroughly investigated. For the first time, the effects of CESL on an HfO2 dielectric are investigated for PBTI characteristics. A roughly 50% reduction of VTH shift can be achieved for the 300-nm CESL HfO2 nMOSFET after 1000-s PBTI stressing without obvious HfO2/Si interface degradation, as demonstrated by the negligible charge pumping current increase (< 4%). In addition, the HfO2 film of CESL devices has a deeper trapping level (0.83 eV), indicating that most of the shallow traps (0.75 eV) in as-deposited HfO2 film can be eliminated for CESL devices.