A method includes providing a substrate having a first region and a second region, forming a fin protruding from the first region, where the fin includes a first SiGe layer and a stack alternating Si layers and second SiGe layers disposed over the first SiGe layer and the first SiGe layer has a first concentration of Ge and each of the second SiGe layers has a second concentration of Ge that is greater than the first concentration, recessing the fin to form an S/D recess, recessing the first SiGe layer and the second SiGe layers exposed in the S/D recess, where the second SiGe layers are recessed more than the first SiGe layer, forming an S/D feature in the S/D recess, removing the recessed first SiGe layer and the second SiGe layers to form openings, and forming a metal gate structure over the fin and in the openings.