In an embodiment, a device includes: a substrate; a first semiconductor layer extending from the substrate, the first semiconductor layer including silicon; a second semiconductor layer on the first semiconductor layer, the second semiconductor layer including silicon germanium, edge portions of the second semiconductor layer having a first germanium concentration, a center portion of the second semiconductor layer having a second germanium concentration, the second germanium concentration being less than the first germanium concentration, the edge portions of the second semiconductor layer including sides and a top surface of the second semiconductor layer; a gate stack on the second semiconductor layer; lightly doped source/drain regions in the second semiconductor layer, the lightly doped source/drain regions being adjacent the gate stack; and source and drain regions extending into the lightly doped source/drain regions.