Collector-emitter offset voltage in heterojunction bipolar transistors

Abstract

Collector-emitter offset voltage in heterojunction bipolar transistors is studied in detail. Based on an in-depth analysis of junction characteristics at zero collector current, a general model for offset voltage applicable to homojunction, single heterojunction and double heterojunction bipolar transistors is obtained. For an abrupt SHBT, the conduction band discontinuity at emitter-base heterojunction is found to be the main cause of large offset voltages. For DHBT's it is found that even when emitter and collector base junctions are identical in all respects, they will have an offset voltage with a magnitude dependent on the current gain of the transistor. Appropriate grading of emitter-base junction is shown to reduce the offset voltage in HBT's to values obtained in homojunction bipolar transistors.