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The deviations of cryogenic collector current-voltage characteristics of SiGeheterojunction bipolar transistors (HBTs) from ideal drift-diffusion theoryhave been a topic of investigation for many years. Recent work indicates thatdirect tunneling across the base contributes to the non-ideal current inhighly-scaled devices. However, cryogenic discrepancies have been observed evenin older-generation devices for which direct tunneling is negligible,suggesting another mechanism may also contribute. Although similar non-idealcurrent-voltage characteristics have been observed in Schottky junctions andwere attributed to spatial inhomogeneities in the base-emitter junctionpotential, this explanation has not been considered for SiGe HBTs. Here, weexperimentally investigate this hypothesis by characterizing the collectorcurrent ideality factor and built-in potential of a SiGe HBT versus temperatureusing a cryogenic probe station. The temperature-dependence of the idealityfactor and the relation between the built-in potential as measured bycapacitance-voltage and current-voltage characteristics are in good qualitativeagreement with the predictions of a theory of electrical transport across ajunction with a Gaussian distribution of potential barrier heights. Theseobservations suggest that lateral inhomogeneities in the base-emitter junctionpotential may contribute to the cryogenic non-idealities. This work helps toidentify the physical mechanisms limiting the cryogenic microwave noiseperformance of SiGe HBTs.

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