Structures for a field-effect transistor and methods of forming a structure for a field-effect transistor. A sacrificial layer is epitaxially grown on a bulk semiconductor substrate, a plurality of epitaxial semiconductor layers are epitaxially grown over the sacrificial layer, and the sacrificial layer and the plurality of epitaxial semiconductor layers are patterned to form a fin. A first portion of the first sacrificial layer is removed to form a first cavity arranged between the plurality of epitaxial semiconductor layers and the bulk semiconductor substrate, and a first dielectric material is deposited in the first cavity. A second portion of the first sacrificial layer, which is located adjacent to the first dielectric material in the first cavity, is removed to form a second cavity between the first fin and the bulk semiconductor substrate. A second dielectric material is deposited in the second cavity.