In a method, a gate structure is formed over a substrate, and source/drain (S/D) features are formed in the substrate and interposed by the gate structure. At least one of the S/D features is formed by forming a first semiconductor material including physically discontinuous portions, forming a second semiconductor material over the first semiconductor material, and forming a third semiconductor material over the second semiconductor material. The second semiconductor material has a composition different from a composition of the first semiconductor material. The third semiconductor material has a composition different from the composition of the second semiconductor material.