Semiconductor devices and methods of forming the same are provided. Semiconductor devices may include a plurality of gate electrodes that are stacked on a substrate and are spaced apart from each other in a vertical direction and a channel region extending through the plurality of gate electrodes in the vertical direction. Each of the plurality of gate electrodes may include a first conductive layer defining a recess recessed toward the channel region, and a second conductive layer in the recess defined by the first conductive layer. A first concentration of impurities in the second conductive layer may be higher than a second concentration of the impurities in the first conductive layer, and the impurities may include nitrogen (N).