The surface effects, the (NH4)(2)S 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)(2)S 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 N-2 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.