The passivation of self-aligned InGaAs/InP heterostructure bipolar transistors (HBTs) with graded base by the combination of S and low-temperature deposited SiN, was investigated. Base current was found to decrease after the passivation. Collector current significantly increases at low base-emitter voltages. The increase is attributed to the leakage current of the base-collector diode. The current gain was found to increase. When annealing was performed at 300 degreesC for 5 min, the base current decreases further and the collector currentdecreases. The leakage of collector current was found to be suppressed. The current gain was further improved and the leakage current can affect the Gummel plots. The leakage source was identified to be the interface between the semiconductor and the SiN, layer. The leakage current can be decreased by the annealing process.