A Fin Field-Effect Transistor (FinFET) includes a semiconductor layer over a substrate, wherein the semiconductor layer forms a channel of the FinFET. A first silicon germanium oxide layer is over the substrate, wherein the first silicon germanium oxide layer has a first germanium percentage. A second silicon germanium oxide layer is over the first silicon germanium oxide layer. The second silicon germanium oxide layer has a second germanium percentage greater than the first germanium percentage. A gate dielectric is on sidewalls and a top surface of the semiconductor layer. A gate electrode is over the gate dielectric.