A method for fabricating a bipolar transistor includes forming a vertical sequence of semiconductor layers, forming an implant mask on the last formed semiconductor layer, and implanting dopant ions into a portion of one or more of the semiconductor layers. The sequence of semiconductor layers includes a collector layer, a base layer that is in contact with the collector layer, and an emitter layer that is in contact with the base layer. The implanting uses a process in which the implant mask stops dopant ions from penetrating into a portion of the sequence of layers.