A semiconductor device and a method of manufacturing a semiconductor device, the device including gate structures on a substrate; source/drain layers on portions of the substrate that are adjacent the gate structures, respectively; first contact plugs contacting upper surfaces of the source/drain layers, respectively; a second contact plug contacting one of the gate structures, a sidewall of the second contact plug being covered by an insulating spacer; and a third contact plug commonly contacting an upper surface of at least one of the gate structures and at least one of the first contact plugs, at least a portion of a sidewall of the third contact plug not being covered by an insulating spacer.