Semiconductor devices are provided including an active layer, a gate structure, a spacer, and a source/drain layer. The active layer is on the substrate and includes germanium. The active layer includes a first region having a first germanium concentration, and a second region on both sides of the first region. The second region has a top surface getting higher from a first portion of the second region adjacent to the first region toward a second portion of the second region far from the first region, and has a second germanium concentration less than the first germanium concentration. The gate structure is formed on the first region of the active layer. The spacer is formed on the second region of the active layer, and contacts a sidewall of the gate structure. The source/drain layer is adjacent to the second region of the active layer.