Embodiments of the invention are directed to a method of fabricating a field effect transistor device, wherein the fabrication operations include forming a channel region over a substrate, forming a gate region over a top surface and along sidewalls of the channel region, and forming a source or drain (S/D) region over the substrate. A bottom encapsulated air-gap is formed over the substrate, and a first portion of the bottom encapsulated air-gap is positioned between the gate region and the S/D region. The first portion of the bottom encapsulated air-gap is further positioned below the top surface of the channel region.