Device structures, fabrication methods, and design structures for tunnel field-effect transistors. A drain comprised of a first semiconductor material having a first band gap and a source comprised of a second semiconductor material having a second band gap are formed. A tunnel barrier is formed between the source and the drain. The second semiconductor material exhibits a broken-gap energy band alignment with the first semiconductor material. The tunnel barrier is comprised of a third semiconductor material with a third band gap larger than the first band gap and larger than the second band gap. The third band gap is configured to bend under an external bias to assist in aligning a first energy band of the first semiconductor material with a second energy band of the second semiconductor material.