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

Abstract

The anomalous current-voltage characteristics of cryogenic SiGe heterojunction bipolar transistors (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 spatial inhomogeneities in the base-emitter junction potential, this explanation has not been considered for SiGe HBTs. Here, we experimentally investigate this hypothesis by characterizing the base-emitter junction ideality factor and built-in potential of a SiGe HBT versus temperature using a cryogenic probe station. The temperature-dependence of the ideality factor and the relation between the built-in potential as measured by capacitance-voltage and current-voltage characteristics are in good qualitative agreement with the predictions of a theory of electrical transport across a junction with a Gaussian distribution of potential barrier heights. These observations support the origin of cryogenic electrical anomalies in 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