Embodiments are directed to a method of forming a semiconductor device and resulting structures that reduce shallow trench isolation (STI) undercutting, floating gates, and gate voids without degrading epitaxy quality. The method includes forming a first and second semiconductor fin on a substrate. A buffer layer is formed on a surface of the substrate between the first and second semiconductor fins and a semiconducting layer is formed on the buffer layer. The buffer layer is selectively removed and replaced with a dielectric layer. A first gate is formed over a first channel region of the first semiconductor fin and a second gate is formed over a second channel region of the first semiconductor fin. Source and drain epitaxy regions are selectively formed on surfaces of the first gate.