A semiconductor device structure includes a dielectric layer, a first source/drain feature in contact with the dielectric layer, wherein the first source/drain feature comprises a first sidewall, and a second source/drain feature in contact with the dielectric layer and adjacent to the first source/drain feature, wherein the second source/drain feature comprises a second sidewall. The structure also includes an insulating layer disposed over the dielectric layer and between the first sidewall and the second sidewall, wherein the insulating layer comprises a first surface facing the first sidewall, a second surface facing the second sidewall, a third surface connecting the first surface and the second surface, and a fourth surface opposite the third surface. The structure includes a sealing material disposed between the first sidewall and the first surface, wherein the sealing material, the first sidewall, the first surface, and the dielectric layer are exposed to an air gap.