Device structures, design structures, and fabrication methods for a bipolar junction transistor. A first layer comprised of a first semiconductor material and a second layer comprised of a second semiconductor material are disposed on a substrate containing a first terminal of the bipolar junction transistor. The second layer is disposed on the first layer and a patterned etch mask is formed on the second layer. A trench extends through the pattern hardmask layer, the first layer, and the second layer and into the substrate. The trench defines a section of the first layer stacked with a section of the second layer. A selective etching process is used to narrow the section of the second layer relative to the section of the first layer to define a second terminal and to widen a portion of the trench in the substrate to undercut the section of the first layer.