Device structures, fabrication methods, and design structures for a bipolar junction transistor. A first terminal of the bipolar junction transistor is formed from a section of a device layer of a semiconductor-on-insulator wafer. An intrinsic base of the bipolar junction transistor is formed from an epitaxially-grown section of a first semiconductor layer, which is coextensive with a sidewall of the section of the device layer. A second terminal of the bipolar junction transistor is formed from a second semiconductor layer that is coextensive with the epitaxially-grown section of the first semiconductor layer. The epitaxially-grown section of a first semiconductor layer defines a first junction with the section of the device layer, and the second semiconductor layer defines a second junction with the epitaxially-grown section of the first semiconductor layer.