Methods of forming a field-effect transistor and structures for a field-effect transistor. A gate structure is formed that overlaps with a channel region beneath a top surface of a semiconductor fin. The semiconductor fin is etched with an anisotropic etching process to form a cavity having a sidewall with a planar section extending vertically toward the top surface of the semiconductor fin and adjacent to the channel region in the semiconductor fin. The semiconductor fin is then etched with an isotropic etching process that widens the cavity at the top surface while preserving verticality of the planar section.