Structures for a nanosheet field-effect transistor and methods for forming a structure for a nanosheet field-effect transistor. A body feature is formed that includes a sacrificial layer arranged vertically between the first and second nanosheet channel layers. The sacrificial layer is laterally recessed at a sidewall of the body feature to expose respective portions of the first and second nanosheet channel layers. A sacrificial spacer is formed by oxidizing a portion of the sacrificial layer at the sidewall of the body feature. Sections of a semiconductor material are epitaxially grown on the exposed portions of the first and second nanosheet channel layers to narrow a gap vertically separating the first and second nanosheet channel layers. The sacrificial spacer is removed to form a cavity between the sections of the semiconductor material and the sacrificial layer. A dielectric spacer is conformally deposited in the cavity.