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The effects of post-NH3 plasma nitridation on device's hot-carrier instability and low-frequency noise in the Hf-based high-κ/metal-gate n-channel metal-oxide-semiconductor field-effect transistors (nMOSFETs) with gadolinium (Gd) cap layers are investigated. With postnitridation, the direct-current and 1/f noise characteristics can be improved apparently. Moreover, a hot-carrier stressing-induced threshold voltage shift can be also suppressed despite of a similar transconductance degradation when comparing with that in the device without nitridation. With the charge-pumping and low-frequency noise measurements, we find that the bulk- and interfacial-trap densities can be reduced with nitrogen incorporation. The reduction of bulk and interfacial traps can be contributed to the suppression of Gd diffusion into a high-κ layer. In this paper, appropriate post-NH3 plasma nitridation can improve the device performance and reliability and low-frequency noise for a gate-first high-κ/metal-gate nMOSFET with a Gd cap layer.