Abstract: The characteristics of a heterojunction bipolar transistor (HBT) depends closely on the properties of the material system and can be improved greatly by bandgap engineering. A novel HBT system consisting of GaAsSb/InP heterostructures and based on bandgap engineering has much better device characteristics than other material systems. Furthermore, the relative position of the bandedge plays a very important role in each of the HBTs. Recent results show that the actual properties of these GaAsSb/InP HBTs are consistent with the predictions of theoretical analysis.