Semiconductor devices, and methods of fabricating the same, include forming device isolation regions in a substrate to define active regions, forming gate trenches in the substrate to expose the active regions and device isolation regions, conformally forming a preliminary gate insulating layer including silicon oxide on the active regions exposed in the grate trenches, nitriding the preliminary gate insulating layer using a radio-frequency bias having a frequency of about 13.56 MHz and power between about 100 W and about 300 W to form a nitrided preliminary gate insulating layer including silicon oxynitride, forming a gate electrode material layer on the nitride preliminary gate insulating layer, partially removing the nitrided preliminary gate insulating layer and the gate electrode material layer to respectively form a gate insulating layer and a gate electrode layer, and forming a gate capping layer on the gate electrode layer to fill the gate trenches.