Room-temperature deposited SiNx passivation of InGaAs/InP heterostructure bipolar transistors (HBTs) with graded base was investigated. The passivation indicated that the surface recombination significantly affected the HBT performances. Both the base current and the base current ideality factor were found to decrease drastically after the SiNx, passivation. The passivation also resulted in the increase of the current gain. The ideality factor of the surface recombination current of 2 was identified by the analyses of emitter-base diodes with different sizes. The emitter-size-dependent current gain showed that the improvement was due to the reduction of surface recombination. The SiNx passivation also suppressed the dependence on the perimeter-to-area ratio of the emitter junction.