Semiconductor devices and methods of forming the same include forming a second dielectric layer on sidewalls of a channel region of a semiconductor fin. The semiconductor fin is surrounded at a fin base by a first dielectric layer. The first dielectric layer is recessed to form a gap in the channel region of the semiconductor fin between the first dielectric layer and the second dielectric layer. Material from the semiconductor fin is etched away at the gap to separate the semiconductor fin from an underlying surface in the channel region. A gate stack is formed in the channel region that completely encircles the semiconductor fin.