A method for fabricating an NMOS transistor includes providing a substrate; forming a gate dielectric layer structure on the substrate and forming a gate electrode on the gate dielectric layer structure. The method further includes performing a fluorine ion implantation below the gate dielectric layer and an annealing process in an atmosphere comprising hydrogen or hydrogen plasma. The method also includes forming a source region and a drain region on both sides of the gate electrode before or after the fluorine ion implantation.