Device structures and fabrication methods for a bipolar junction transistor. A trench isolation region surrounds an active region that includes a collector. A base layer includes a first section and a second section that are located over the active region. An emitter is positioned on the first section of the base layer, and an extrinsic base layer is positioned on the second section of the base layer. The extrinsic base layer has a side surface adjacent to the emitter. The side surface of the extrinsic base layer is inclined relative to a top surface of the base layer in a direction away from the emitter.