Device structures and fabrication methods for a bipolar junction transistor. The device structure includes a substrate and a trench isolation region in the substrate. The trench isolation region surrounds an active region of the substrate. The device structure further includes a collector in the active region of the substrate, a base layer having a first section positioned on the active region and a second section oriented at an angle relative to the first section, an emitter positioned on the first section of the base layer, and an extrinsic base layer positioned over the trench isolation region and adjacent to the emitter. The second section of the base layer is laterally positioned between the extrinsic base layer and the emitter.