The present disclosure describes a method for forming gate spacer structures with air-gaps to reduce the parasitic capacitance between the transistor's gate structures and the source/drain contacts. In some embodiments, the method includes forming a gate structure on a substrate and a spacer stack on sidewall surfaces of the gate structure where the spacer stack comprises an inner spacer layer in contact with the gate structure, a sacrificial spacer layer on the inner spacer layer, and an outer spacer layer on the sacrificial spacer layer. The method further includes removing the sacrificial spacer layer to form an opening between the inner and outer spacer layers, depositing a polymer material on top surfaces of the inner and outer spacer layers, etching top sidewall surfaces of the inner and outer spacer layers to form a tapered top portion, and depositing a seal material.