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We have demonstrated that oxynitridation using radical-oxygen (radical-O) and radical-nitrogen (radical-N) improves reverse narrow channel effects (RNCE) and reliability in sub-1.5-nm-thick gate-SiO2 FETs with narrow channel and shallow-trench isolation (STI), suitable for high-density SRAM and logic devices. The STI formation followed by oxidation for the gate-dielectric causes various orientations of the Si surface, and thus, thermal oxidation forms the partial thin SiO2 and causes RNCE and reliability degradation. Oxidation using radical-O forms uniform SiO2 on Si and Si surfaces and suppresses RNCE in a sub-1.5 nm-thick gate-SiO2 FET with STI. Nitrifying the SiO2 using radical-N increases the physical thickness while maintaining the oxide equivalent thickness on both Si and Si surfaces, thus producing a low-leakage and highly reliable sub-1.5 nm-thick gate-SiON.