A semiconductor device includes a gate electrode extending in a first direction on a substrate, a first active pattern extending in a second direction intersecting the first direction on the substrate to penetrate the gate electrode, the first active pattern including germanium, an epitaxial pattern on a side wall of the gate electrode, a first semiconductor oxide layer between the first active pattern and the gate electrode, and including a first semiconductor material, and a second semiconductor oxide layer between the gate electrode and the epitaxial pattern, and including a second semiconductor material. A concentration of germanium of the first semiconductor material may be less than a concentration of germanium of the first active pattern, and the concentration of germanium of the first semiconductor material may be different from a concentration of germanium of the second semiconductor material.