A semiconductor device including a selectively nitrided gate insulating layer may be fabricated by a method that includes forming a first gate insulating layer on a substrate having a first region and a second region, performing a nitridation process on the first gate insulating layer, removing the first gate insulating layer from at least a portion of the first region to expose at least a portion of the substrate, forming a second gate insulating layer on at least the exposed portion of the first region of the substrate, thermally treating the first and second gate insulating layers in an oxygen atmosphere, forming a high-k dielectric on the first and second gate insulating layers, and forming a metal gate electrode on the high-k dielectric.