The current gain of heterojunction bipolar transistors (HBT's) can be effectively modulated through Schottky diodes that contact the emitter passivation ledge directly. The behavior of the gain modulation is determined by the degree of the emitter ledge depletion. If the ledge is fully depleted, HBT's current gain can be modulated in the whole base-emitter bias voltage (V BE) range up to 1.6 V. If the ledge is partially depleted, HBT's current gain can be modulated only in the low V BE range (<1.35 V). This discovery leads to a simple method for monitoring the effectiveness of HBT's emitter ledge passivation and offers new insights to the mechanism of HBT gain degradation. It also creates a four-terminal HBT with an extra ledge electrode biased to control and modulate device's current gain at microwave frequencies.