Embodiments of the invention are directed to a method of fabricating a semiconductor device. A non-limiting example of the method includes forming a fin over a substrate. The fin includes an upper fin region and a lower fin region. The lower fin region is physically coupled to the upper fin region and the substrate. A portion of the fin is removed to form a fin tunnel configured to physically separate the upper fin region from the lower fin region. A gate structure is formed and configured to fill the fin tunnel and cover a top surface, a bottom surface, a first sidewall, and a second sidewall of the upper fin region.