Structures that include a field effect-transistor and methods of forming a structure that includes a field-effect transistor. A semiconductor fin has an upper portion and a lower portion, and a trench isolation region surrounds the lower portion of the semiconductor fin. The trench isolation region has a top surface arranged above the lower portion of the semiconductor fin and arranged below the upper portion of the semiconductor fin. A dielectric layer arranged over the top surface of the trench isolation region. The dielectric layer is composed of a low-k dielectric material.