A semiconductor device may include a semiconductor substrate having a first region and a second region. The nitrogen-incorporated active region may be formed within the first region. A first gate electrode may be formed on the nitrogen-incorporated active region. A first gate dielectric layer may be interposed between the nitrogen-incorporated active region and the first gate electrode. The first gate dielectric layer may include a first dielectric layer and a second dielectric layer. The second dielectric layer may be a nitrogen contained dielectric layer. A second gate electrode may be formed on the second region. A second gate dielectric layer may be interposed between the second region and the second gate electrode. The first gate dielectric layer may have the same or substantially the same thickness as the second gate dielectric layer, and the nitrogen contained dielectric layer may contact with the nitrogen-incorporated active region.