Semiconductor transistor devices including alternatively stacked source/drain regions

A semiconductor device comprising:

• an active region extending in a first direction on a substrate;
• a plurality of channel layers on the active region and spaced apart from each other in a vertical direction that is perpendicular to an upper surface of the substrate;
• a gate structure traversing the active region and the plurality of channel layers and extending in a second direction, wherein the plurality of channel layers are in the gate structure;
• and
• a source/drain region on the active region adjacent to a side of the gate structure and contacting the plurality of channel layers,
• wherein the source/drain region comprises:
• a first epitaxial layer on the active region, the first epitaxial layer contacting the plurality of channel layers, comprising a first upper surface and having a first germanium (Ge) concentration;
• a second epitaxial layer on the first upper surface of the first epitaxial layer, the second epitaxial layer comprising a second upper surface and having a second Ge concentration that is higher than the first Ge concentration;
• a third epitaxial layer on the second upper surface of the second epitaxial layer, the third epitaxial layer comprising a third upper surface and having a third Ge concentration that is lower than the second Ge concentration;
• a fourth epitaxial layer on the third upper surface of the third epitaxial layer, the fourth epitaxial layer comprising a fourth upper surface and having a fourth Ge concentration that is higher than the third Ge concentration;
• a fifth epitaxial layer on the fourth upper surface of the fourth epitaxial layer, the fifth epitaxial layer comprising a fifth upper surface and having a fifth Ge concentration that is lower than the fourth Ge concentration;
• and
• a sixth epitaxial layer on the fifth upper surface of the fifth epitaxial layer and in a space defined by the fifth upper surface of the fifth epitaxial layer, the sixth epitaxial layer comprising a sixth upper surface and having a sixth Ge concentration that is higher than the fifth Ge concentration, and
• wherein the first, second, third, fourth and fifth upper surfaces are curved toward the substrate.