an active region extending in a first direction on a substrate;

a plurality of channel layers vertically spaced apart from each other on the active region;

a gate structure extending in a second direction and intersecting the active region and the plurality of channel layers on the substrate, the gate structure surrounding the plurality of channel layers;

a source/drain region on the active region on at least one side of the gate structure and in contact with the plurality of channel layers; and

a contact plug connected to the source/drain region and extending in a vertical direction,

wherein:

the source/drain region includes a first epitaxial layer on side surfaces of the plurality of channel layers and including a first impurity; a second epitaxial layer on the first epitaxial layer and including the first impurity and a second impurity; and a third epitaxial layer on the second epitaxial layer and including the first impurity,

in a horizontal sectional view of a plane at a height level of one of the plurality of channel layers, the second epitaxial layer includes a peripheral portion having a thickness measured in the first direction that increases along the second direction,

a sum of thicknesses of the first epitaxial layer and the second epitaxial layer is about 2 nm to about 5 nm, and

the second epitaxial layer has a maximum thickness within a range from about 35% to less than 100% of the sum of the thicknesses of the first epitaxial layer and the second epitaxial layer.