The InP/GaAsSb/InP double heterostructure bipolar transistors with 20-nm-thick base layer are passivated by low-temperature deposited SiNx. The SiNx passivation results in an increase of the current gain, seen from common-emitter I–V characteristics. An improvement of the Early voltage is also observed after the passivation. The study of the forward Gummel plots shows that the collector current keeps unchanged, while the base current decreases after SiNx passivation. A decrease of the base current ideality factor is found by the SiNx passivation. The passivation also results in an improvement of the base–collector junction. In contrast, the leakage current of the base–collector junction increases tow orders after passivation.