In this work, the influence of various emitter ledge thicknesses on the performances of InGaP/GaAs heterojunction bipolar transistors is investigated based on the simulation data. The undesired surface channel phenomenon at the exposed base surface between base contact and emitter ledge is comprehensively analyzed. Moreover, improper thickness of emitter ledge passivation would cause serious surface recombination at the edge of emitter ledge. If the emitter ledge is too thick, current will flow through the undepleted ledge, which increases the emitter-size effect. In contrast, if the emitter ledge is too thin, it may not effectively passivate the surface. Therefore, the thickness of emitter ledge is a critical and should be carefully considered. From simulated results, the optimum emitter ledge thickness of InGaP/GaAs HBT is 100-200 Å.