Investigation of Cryogenic Current-Voltage Anomalies in SiGe HBTs: Role of Base-Emitter Junction Inhomogeneities

Abstract

The anomalous current-voltage characteristics of cryogenic SiGe HBTs have been a topic of investigation for many years. Proposed explanations include quasiballistic transport of electrons across the base or tunneling from the emitter to the collector, but inconsistencies exist with these hypotheses. Although similar behavior occurs in Schottky junctions and has been attributed to lateral inhomogeneities in the junction potential, this explanation has not been considered for SiGe HBTs. Here, we experimentally investigate this hypothesis by characterizing the base-emitter builtin potential and its temperature dependence using both capacitance-voltage and current-voltage characteristics. We observe a marked discrepancy in the built-in potential obtained using these two methods at cryogenic temperatures, supporting the origin for the anomalous behavior of SiGe HBTs as arising from lateral inhomogeneities in the base-emitter junction potential. This work helps to identify the physical mechanisms limiting the cryogenic microwave noise performance of SiGe HBTs.