Abstract: A semiconductor device includes an active region, a plurality of channel layers spaced apart from each other on the active region, a gate structure including a gate dielectric layer and a gate electrode, and source/drain regions on both sides of the gate structure. The gate structure includes an upper portion and lower portions. A first lower portion of the lower portions has a first lower surface, a first upper surface, and first and second side surfaces. Each of the first and second side surfaces includes a first inclined portion sloped at a first acute angle from the first lower surface and a second inclined portion sloped at a second acute angle from the first upper surface. The gate dielectric layer includes portions disposed between the gate electrode and the plurality of channel layers and between the gate electrode and the source/drain regions.