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

## TL;DR

This study investigates the cause of cryogenic current-voltage anomalies in SiGe HBTs, proposing that base-emitter junction inhomogeneities, rather than direct tunneling, contribute to non-ideal behaviors at low temperatures.

## Contribution

It introduces the hypothesis that base-emitter junction inhomogeneities explain cryogenic anomalies in SiGe HBTs, supported by experimental characterization and theoretical analysis.

## Key findings

- Temperature-dependent ideality factor supports inhomogeneity hypothesis
- Built-in potential measurements align with Gaussian barrier distribution theory
- Inhomogeneities likely limit cryogenic microwave noise performance

## Abstract

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.