A semiconductor device includes source and a drain above a substrate and spaced apart along a first direction, and a semiconductor channel extending between the source and the drain. The semiconductor device further includes gate spacers, an interfacial layer, and a metal gate structure. The gate spacers are disposed on the semiconductor channel and spaced apart by a spacer-to-spacer distance along the first direction. The interfacial layer is on the semiconductor channel. The interfacial layer extends a length along the first direction, and the length is less than a minimum of the spacer-to-spacer distance along the first direction. The metal gate structure is over the interfacial layer.