Device structures for a field-effect transistor with a body contact and methods of forming such device structures. An opening is formed that extends through a device layer of a silicon-on-insulator (SOI) substrate and into a buried oxide layer of the silicon-on-insulator substrate. The buried oxide layer is laterally etched at the location of the opening to define a cavity in the buried oxide layer. The cavity is located partially beneath a section of the device layer, and the cavity is filled with a semiconductor material to form a body contact. A well is formed in the section of the device layer, and the body contact is coupled with a portion of the well.