A device comprises a plurality of nanosheets, source/drain stressors, and a gate structure wrapping around the nanosheets. The nanosheets extend in a first direction above a semiconductor substrate and are arranged in a second direction substantially perpendicular to the first direction. The source/drain stressors are on either side of the nanosheets. Each of the source/drain stressors comprises a first epitaxial layer and a second epitaxial layer over the first epitaxial layer. The first and second epitaxial layers are made of a Group IV element and a Group V element. An atomic ratio of the Group V element to the Group IV element in the second epitaxial layer is greater than an atomic ratio of the Group V element to the Group IV element in the first epitaxial layer.