Abstract The surface effects, the (NH4)2S and low-temperature-deposited SiNx passivations of InP-based heterostructure bipolar transistors (HBTs) have been investigated. The surface recombination current of InP-based HBTs is related to the base structures. The (NH4)2S treatment for InGaAs and InP removes the natural oxide layer and results in sulfur-bonded surfaces. This can create surface-recombination-free InP-based HBTs. Degradation is found when the HBTs were exposed to air for 10 days. The low-temperature-deposited SiNx passivation of InGaAs/InP HBTs causes a drastic decrease in the base current and a significant increase in the current gain. The improvement in the HBT performance is attributed to the low deposition temperature and the effect of N2 plasma treatment in the initial deposition process. The SiNx passivation is found to be stable. S/SiNx passivation of InGaAs/InP HBTs results in a decrease in the base current and an increase in the current gain. The annealing process can cause the base current to decrease further and the current gain increase.