A semiconductor device may include: a semiconductor substrate, a device isolating layer embedded within the semiconductor substrate and defining an active region, a channel region formed in the active region, a gate electrode disposed above the channel region, a gate insulating layer provided between the channel region and the gate electrode, and a silicon germanium epitaxial layer adjacent to the channel region within the active region and including a first epitaxial layer containing a first concentration of germanium, a second epitaxial layer containing a second concentration of germanium, higher than the first concentration, and a third epitaxial layer containing a third concentration of germanium, lower than the second concentration, the first to third epitaxial layers being sequentially stacked on one another in that order.