In a disclosed semiconductor structure, a lateral bipolar junction transistor (BJT) has a base positioned laterally between a collector and an emitter. The base includes a semiconductor fin with a first portion that extends from a substrate through an isolation layer, a second portion on the first portion, and a third portion on the second portion. The collector and emitter are on the isolation layer and positioned laterally immediately adjacent to opposing sidewalls of the second portion of the semiconductor fin. In some embodiments, the BJT is a standard BJT where the semiconductor fin (i.e., the base), the collector, and the emitter are made of the same semiconductor material. In other embodiments, the BJT is a heterojunction bipolar transistor (HBT) where a section of the semiconductor fin (i.e., the base) is made of a different semiconductor material for improved performance. Also disclosed is a method of forming the structure.