Graded-base InGaAs/lnP HBTs were passivated by (NH4)(2)S solution. The current gain increased after passivation. A current gain degradation was found when the passivated HBT was exposed to air. The Gummel plots in forward and reverse modes were used to evaluate the properties of the base-emitter and base-collector junctions. The degradations of the base-emitter and base-collector junctions were found to be different. The current gain was independent on the emitter size after passivation compared to the emitter size dependent one before passivation. The size dependent current gain appeared when HBTs exposed to air for 10 days.