Methods of forming a field-effect transistor and structures for a field effect-transistor. A sidewall spacer is formed adjacent to a sidewall of a gate structure of the field-effect transistor and a dielectric cap is formed over the gate structure and the sidewall spacer. A cut is formed that extends through the dielectric cap, the gate structure, and the sidewall spacer. After forming the cut, the sidewall spacer is removed from beneath the dielectric cap to define a cavity, and a dielectric material is deposited in the cut and in the cavity. The dielectric material encapsulates a portion of the cavity to define an airgap spacer.