A semiconductor device includes a substrate and a fin feature over the substrate. The fin feature includes a first portion of a first semiconductor material and a second portion of a second semiconductor material disposed over the first portion. The second semiconductor material is different from the first semiconductor material. The semiconductor device further includes a semiconductor oxide feature disposed on sidewalls of the first portion and a gate stack disposed on the fin feature. The gate stack includes an interfacial layer over a top surface and sidewalls of the second portion and a gate dielectric layer over the interfacial layer and sidewalls of the semiconductor oxide feature. A portion of the gate dielectric layer is below the interfacial layer.