A method includes forming a semiconductor fin over a substrate, etching the semiconductor fin to form a recess, wherein the recess extends into the substrate, and forming a source/drain region in the recess, wherein forming the source/drain region includes epitaxially growing a first semiconductor material on sidewalls of the recess, wherein the first semiconductor material includes silicon germanium, wherein the first semiconductor material has a first germanium concentration from 10 to 40 atomic percent, epitaxially growing a second semiconductor material over the first semiconductor material, the second semiconductor material including silicon germanium, wherein the second semiconductor material has a second germanium concentration that is greater than the first germanium concentration, and epitaxially growing a third semiconductor material over the second semiconductor material, the third semiconductor material including silicon germanium, wherein the third semiconductor material has a third germanium concentration that is smaller than the second germanium concentration.