A transistor includes a channel region, a gate stack, and source and drain structures. The channel region comprises silicon germanium and has a first silicon-to-germanium ratio. The gate stack is over the channel region and comprises a silicon germanium oxide layer over the channel region, a high-κ dielectric layer over the silicon germanium oxide layer, and a gate electrode over the high-κ dielectric layer. The silicon germanium oxide layer has a second silicon-to-germanium ratio. The second silicon-to-germanium ratio is substantially the same as the first silicon-to-germanium ratio. The channel region is between the source and drain structures.