The semiconductor device including: a semiconductor layer extending in a first direction, the semiconductor layer including a pair of source/drain regions and a channel region, a gate extending on the semiconductor layer to cover the channel region, and a gate dielectric layer interposed between the channel region and the gate, a corner insulating spacer having a first surface and a second surface, the first surface extending in the second direction along a side wall of the gate, the first surface covering from a side portion of the gate dielectric layer to at least a portion of the side wall of the gate, and the second surface covering a portion of the semiconductor layer, and an outer portion insulating spacer covering the side wall of the gate above the corner insulating spacer, the outer portion insulating spacer having a smaller dielectric constant than the corner insulating spacer, may be provided.