Semiconductor devices and methods are provided to fabricate FET devices. For example, a semiconductor device can include a functional gate structure on a channel region of a fin structure; and a source/drain region on each side of the functional gate structure. The functional gate structure has an insulator material abutting a portion of the sidewalls of the functional gate structure and the source drain region and the top surface of the fin, with a top surface of the insulator material in contact with a bottom surface of the first spacer layer. The functional gate structure further includes a dielectric top layer. The dielectric top layer seals an air gap between the top surface of the insulator material and the dielectric top layer.