Determining the effectiveness of HBT emitter ledge passivation by using an on-ledge Schottky diode potentiometer

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 (HBTs). The function and mechanism of this on-ledge potentiometer are carefully analyzed and modeled. With this apparatus, the emitter ledge potential (V/sub Ledge/) can be measured as a function of the base-emitter bias voltage V/sub BE/. By relating V/sub Ledge/ to V/sub BE/. We are capable of quantifying the extent of the ledge depletion down to a few angstroms (<10 /spl Aring/) 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.