A method includes providing a structure having a substrate, a first dielectric layer over the substrate, one or more semiconductor channel layers over the first dielectric layer and connecting a first source/drain (S/D) feature and a second S/D feature, and a gate structure engaging the one or more semiconductor channel layers; etching the substrate from the backside of the structure to form a first trench exposing the first S/D feature and a second trench exposing the second S/D feature; forming an S/D contact in the first trench; etching at least a portion of the first dielectric layer resulting in a portion of the S/D contact protruding from the first dielectric layer at the backside of the structure; and depositing a seal layer over the S/D contact, wherein the seal layer caps an air gap between the gate structure and the seal layer.