An integrated circuit device includes: a plurality of channel regions spaced apart from each other in an active region; a plurality of source/drain regions; an insulating structure on the active region, the insulating structure defining a plurality of gate spaces; a first gate stack structure in a first of the gate spaces, the first gate stack structure including a first work function metal-containing layer; and an isolation stack structure in a second of the gate spaces that is adjacent the first of the gate spaces, the isolation stack structure having a different stack structure from the first gate stack structure and being configured to electrically isolate a portion of the active region.