A semiconductor device has a semiconductor substrate. A silicon germanium layer is disposed on the semiconductor substrate. The silicon germanium layer has a first silicon-to-germanium ratio. A first gate structure is disposed on the silicon germanium layer, and the first gate structure includes an interfacial layer on the silicon germanium layer. The interface layer has a second silicon-to-germanium ratio substantially the same as the first silicon-to-germanium ratio of the silicon germanium layer. The first gate structure also includes a high-dielectric layer on the interfacial layer and a first gate electrode on the high-κ dielectric layer.