Semiconductor devices

A semiconductor device, comprising:

• 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;
• and
• 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,
• wherein:
• the source/drain region includes a first epitaxial layer in contact with side surfaces of the plurality of channel layers and the active region and including silicon-germanium (SiGe) and boron (B);
• and
• a second epitaxial layer on the first epitaxial layer and including silicon-germanium (SiGe), boron (B), and carbon (C), and
• the second epitaxial layer covers end portions of the first epitaxial layer in the second direction.

A semiconductor device, comprising:

• an active region extending in a first direction on a substrate;
• a gate structure extending in a second direction, perpendicular to the first direction, and intersecting the active region;
• and
• a source/drain region on a recessed region of the active region at both sides of the gate structure, the source/drain region including a plurality of epitaxial layers,
• wherein:
• the plurality of epitaxial layers of the source/drain region include:
• a first epitaxial layer covering an inner wall of the recessed region of the active region;
• and
• a second epitaxial layer on the first epitaxial layer, in the recessed region of the active region,
• the first epitaxial layer and the second epitaxial layer include silicon-germanium (SiGe) having different compositions,
• each of the first epitaxial layer and the second epitaxial layer includes a first impurity including boron (B),
• the second epitaxial layer further includes a second impurity including carbon (C), and
• at least one of the first epitaxial layer and the second epitaxial layer includes different portions having different thicknesses in the first direction.