Structures that include bipolar junction transistors and methods of forming such structures. The structure comprises a semiconductor layer, a substrate, and a dielectric layer disposed between the semiconductor layer and the substrate. The structure further comprises a first bipolar junction transistor including a first collector in the substrate, a first emitter, and a first base layer. The first base layer extends through the dielectric layer from the first emitter to the first collector. The structure further comprises a second bipolar junction transistor including a second collector in the substrate, a second emitter, and a second base layer. The second base layer extends through the dielectric layer from the second emitter to the second collector. The second base layer is connected to the first base layer by a section of the semiconductor layer to define a base line.