A field effect transistor includes a substrate comprising a fin structure. The field effect transistor further includes an isolation structure in the substrate. The field effect transistor further includes a source/drain (S/D) recess cavity below a top surface of the substrate. The S/D recess cavity is between the fin structure and the isolation structure. The field effect transistor further includes a strained structure in the S/D recess cavity. The strain structure includes a lower portion. The lower portion includes a first strained layer, wherein the first strained layer is in direct contact with the isolation structure, and a dielectric layer, wherein the dielectric layer is in direct contact with the substrate, and the first strained layer is in direct contact with the dielectric layer. The strained structure further includes an upper portion comprising a second strained layer overlying the first strained layer.