A semiconductor device includes a plurality of nanostructures extending in a first direction above a semiconductor substrate and arranged in a second direction substantially perpendicular to the first direction and a gate structure extending in a third direction perpendicular to both the first and second directions, the gate structure surrounding each of the plurality of nano structures. Each of the plurality of nanostructures has an outer region having a composition different from a composition of an inner region of each of the plurality of the nanostructures. The gate structure includes a plurality of high-k gate dielectric layers respectively surrounding the plurality of nanostructures, a work function layer surrounding each of the plurality of high-k gate dielectric layers and a fill metal layer surrounding the work function layer.