Both the self- and non-self-aligned graded-base InP/InGaAs/InP double heterostructure bipolar transistors (DHBTs) were passivated by room-temperature deposited SiNx. Current gains were found to increase significantly after SiNx passivation. The passivation resulted in an increase in the leakage current of base-collector junction. The larger leakage current in the non-self-aligned DHBT made the sudden increase of the collector current in the common-emitter I-V curve occur at smaller collector-emitter bias than that in the self-aligned DHBT, which has smaller leakage current. Further increase in the current gain was found after the DHBTs were exposed to air for 200 days. The leakage current in the base-collector diode was reduced. The sudden increase of the collector current in the non-self-aligned DHBT in the common-emitter I-V characteristics was also suppressed. Investigation of reverse Gummel plots showed that the surface recombination in the base-collector diode was suppressed by the passivation. The leakage current in the base-collector diode was attributed to the increase of the generation rate.