Structures for a bidirectional switch and methods of forming such structures. A substrate contact is formed in a trench defined in a substrate. A substrate includes a trench and a substrate contact in the trench. A bidirectional switch, which is on the substrate, includes a first source/drain electrode, a second source/drain electrode, an extension region between the first source/drain electrode and the second source/drain electrode, and a gate structure. A substrate-bias switch, which is on the substrate, includes a gate structure, a first source/drain electrode coupled to the substrate contact, a second source/drain electrode coupled to the first source/drain electrode of the bidirectional switch, and an extension region laterally between the gate structure and the first source/drain electrode.