A heterojunction bipolar transistor is integrated on radio frequency (RF) dies of different sizes. The heterojunction bipolar transistor includes an emitter on a first-side of a semiconductor-on-insulator (SOI) layer of an SOI substrate. The emitter is accessed from the first-side while a collector is accessed from a second-side of the SOI substrate. One or more portions of a base of the heterojunction bipolar transistor is between the emitter and one or more portions of the collector. The heterojunction bipolar transistor also includes a compound semiconductor layer between the collector and the emitter. The compound semiconductor layer carries a charge between the emitter and the collector.