A Schottky diode, which contacts the emitter ledge directly, is used as a potentiometer to monitor the effectiveness of the emitter/base junction passivation in GaAs based heterojunction bipolar transistors (HBT's). The function and mechanism of this on-ledge potentiometer are carefully analyzed and modeled. With this apparatus, the emitter ledge potential (VLedge) can be measured as a function of the base-emitter bias voltage VBE. By relating VLedge to VBE, we are capable of quantifying the extent of the ledge depletion down to a few angstroms (<10 angstroms) in precision. The excellent detectivity of the potentiometer makes it a very powerful tool in the diagnosis of HBT problems in both operation and long-term reliability. These problems have not been detectable or distinguishable with prior techniques.