An integrated circuit device includes first and second conductive structures spaced apart from one another on a substrate along a first direction. The first and second conductive structures extend in a second direction substantially perpendicular to the first direction. A contact plug is interposed between the first and second conductive structures and is separated therefrom along the first direction by respective air gaps on opposite sides of the contact plug. The air gaps define first and second air spacers that electrically insulate the contact plug from the first and second conductive structures, respectively. An upper insulation layer covers the first and second air spacers and the first and second conductive structures. The air spacers may sufficiently reduce the loading capacitance between the conductive structures. Related fabrication methods are also discussed.