An on-ledge Schottky potentiometer for the diagnosis of HBT emitter passivation

A Schottky diode, which contacts the emitter passivation ledge directly, is used as a potentiometer to monitor the effectiveness of the emitter/base junction passivation in AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The function and mechanism of this on-ledge potentiometer are carefully examined and analyzed. With this on-ledge Schottky diode, the emitter ledge potential (V/sub Ledge/) can be measured directly as a function of the base-emitter bias voltage (VBE) By relating V/sub Ledge/ to V/sub BE/, We are able to quantify the extent of the ledge depletion down to a few angstroms (<10 /spl Aring/) in precision. The outstanding detectivity of the on-ledge potentiometer makes itself a very powerful tool in the diagnosis of HBT problems in both device operation and long-term reliability. These problems are not detectable or distinguishable with prior monitoring techniques.