The unwanted high threshold voltage (Vt) is the major challenge for metal-gate/high-κ CMOS especially at small equivalent-oxide-thickness (EOT). We have investigated the high Vt issue that is due to flat-band voltage (Vfb) roll-off at smaller EOT. A mechanism of charged oxygen vacancies formed by interface reaction was proposed to explain the Vfb roll-off effect. This interface reaction can be decreased by inserting a thin interfacial SiON and using novel low temperature process. The self-aligned and gate-first metal-gate/high-κ CMOSFETs using these methods have achieved low Vt and good control of Vfb roll-off at small 0.6–1.2 nm EOT.