A semiconductor device is provided including a fin active region on a substrate. The fin active region includes a lower region, a middle region, and an upper region. The middle region has lateral surfaces with a slope less steep than the lateral surfaces of the upper region. An isolation region is on a lateral surface of the lower region of the fin active region. A gate electrode structure is provided. A gate dielectric structure having an oxidation oxide layer and a deposition oxide layer, while having a thickness greater than half a width of the upper region of the fin active region is provided. The deposition oxide layer is between the gate electrode structure and the fin active region and the gate electrode structure and the isolation region, and the oxidation oxide layer is between the fin active region and the deposition oxide layer.