A method of fabricating a semiconductor device includes forming an interface layer on a substrate, forming a first gate insulating layer having a first dielectric constant on the interface layer, forming a second gate insulating layer having a second dielectric constant smaller than the first dielectric constant on the first gate insulating layer, annealing the substrate, nitriding a resultant of the annealed first and second gate insulating layers to form a nitrided gate insulator, forming a work function control layer on the nitride gate insulator, and forming a metal gate electrode on the work function control layer. At least one of the work function control layer and the metal gate electrode is of or includes aluminum (Al).