A semiconductor device includes a substrate and a fin feature over the substrate. The fin feature includes a first portion having a first semiconductor material and a second portion having a second semiconductor material over the first portion. The second semiconductor material is different from the first semiconductor material. The semiconductor device further includes an isolation feature over the substrate and over sides of the fin feature; a semiconductor oxide feature including the first semiconductor material and disposed on sidewalls of the first portion; and a gate stack disposed on the fin feature and the isolation feature. The gate stack includes a gate dielectric layer extending into recesses that are into a top portion of the semiconductor oxide feature and below the second portion of the fin feature.