A semiconductor structure includes a first fin and a second fin protruding from a substrate, isolation features over the substrate to separate the first and the second fins, where a top surface of each of the first and the second fins is below a top surface of the isolation features, inner fin spacers disposed along inner sidewalls of the first and the second fins, where the inner fin spacers have a first height measured from a top surface of the isolation features, outer fin spacers disposed along outer sidewalls of the first and the second fins, where the outer fin spacers have a second height measured from the top surface of the isolation features that is less than the first height, and a source/drain (S/D) structure merging the first and the second fins, where the S/D structure includes an air gap having a top portion over the inner fin spacers.