A device comprises a substrate comprising a first portion and a second portion separated by an isolation region, a first gate structure over the first portion, a first drain/source region and a second drain/source region in the first portion and on opposite sides of the first gate structure, wherein the first drain/source region and the second drain/source have concave surfaces, a second gate structure over the second portion and a third drain/source region and a fourth drain/source region in the second portion and on opposite sides of the second gate structure, wherein the third drain/source region and the fourth drain/source have the concave surfaces.