An integrated circuit (IC) device includes first and second fin-type semiconductor active regions on a substrate. A plurality of first semiconductor patterns are provided, which are stacked on the first fin-type active region as a first plurality of spaced-apart channel regions of a first FINFET. A plurality of second semiconductor patterns are provided, which are stacked on the second fin-type active region as a second plurality of spaced-apart channel regions of a second FINFET. A first gate structure is provided on the plurality of first semiconductor patterns. This first gate structure includes a first material region, which at least partially fills spaces between the first plurality of spaced-apart channel regions. A second gate structure is also provided on the plurality of second semiconductor patterns. The second gate structure includes second and third material regions, which at least partially fill spaces between the second plurality of spaced-apart channel regions.