A semiconductor device includes a semiconductor active region having a vertical stack of multiple spaced-apart semiconductor channel regions thereon. A gate electrode extends on the active region and between the spaced-apart channel regions. A source/drain region contacts the spaced-apart channel regions. The source/drain region includes a stack of at least first, second and third epitaxial layers having different electrical characteristics. The first epitaxial layer contacts the active region and each of the spaced-apart channel regions. The second epitaxial layer contacts a first portion of an upper surface of the first epitaxial layer. The third epitaxial layer contacts a second portion of the upper surface of the first epitaxial layer. Each of the first, second and third epitaxial layers includes silicon germanium (SiGe) with unequal levels of germanium (Ge) therein. A level of germanium in the third epitaxial layer exceeds a level of germanium in the second epitaxial layer, which exceeds a level of germanium in the first epitaxial layer.