Kavli Affiliate: Austin J. Minnich
| First 5 Authors: Nachiket R. Naik, Bekari Gabritchidze, Justin H. Chen, Jacob Kooi, Kieran A. Cleary
| Summary:
The deviations of cryogenic collector current-voltage characteristics of SiGe
heterojunction bipolar transistors (HBTs) from ideal drift-diffusion theory
have been a topic of investigation for many years. Recent work indicates that
direct tunneling across the base contributes to the non-ideal current in
highly-scaled devices. However, cryogenic discrepancies have been observed even
in older-generation devices for which direct tunneling is negligible,
suggesting another mechanism may also contribute. Although similar non-ideal
current-voltage characteristics have been observed in Schottky junctions and
were 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 collector
current 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 suggest that lateral inhomogeneities in the base-emitter junction
potential may contribute to the cryogenic non-idealities. This work helps to
identify the physical mechanisms limiting the cryogenic microwave noise
performance of SiGe HBTs.
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