A method of manufacturing a semiconductor device includes forming a plurality of preliminary gate structures, forming a capping layer pattern on sidewalls of the plurality of preliminary gate structures, and forming a blocking layer on top surfaces of the plurality of preliminary gate structures and the capping layer pattern such that a void is formed therebetween. The method also includes removing the blocking layer and an upper portion of the capping layer pattern such that at least the upper sidewalls of the plurality of preliminary gate structures are exposed, and a lower portion of the capping layer pattern remains on lower sidewalls of the preliminary gate structures. The method further includes forming a conductive layer on at least the upper sidewalls of the plurality of preliminary gate structures, reacting the conductive layer with the preliminary gate structures, and forming an insulation layer having an air gap therein.