Mechanical and Civil Engineering Seminar: PhD Thesis Defense

Abstract:

Silicon-germanium heterojunction bipolar transistors (HBTs) are widely used for high-speed communications and radar systems owing to the their low-cost and competitive performance relative to III-V compound semiconductor devices. Due to the higher cost and lower yield of III-V high electron mobility transistors (HEMTs) based on InGaAs quantum wells, SiGe HBTs operating at cryogenic temperatures are of significant interest for radio astronomy and quantum computing. However, their microwave noise performance has long been observed to be poorer than those of HEMTs. As a result, the physical mechanisms governing the cryogenic DC, microwave and noise performance of SiGe HBTS have been a topic of investigation for many years. Improved understanding of these mechanisms may ultimately allow for the realization of HBTs with noise performance rivaling those of HEMTs yet with lower cost, improved compatibility and integration with CMOS processes, and high yield.

This thesis uses theoretical and experimental methods to examine cryogenic charge transport phenomena in SiGe HBTs which affect the microwave noise performance. A particular focus is on the anomalous electrical characteristics at cryogenic temperatures, in which pronounced deviations from the ideal drift-diffusion theory are observed. Various explanations for the observed anomalous cryogenic I-V behavior have been postulated, such as quasi-ballistic transport and electron tunneling, among others. Despite a number of works on this topic over the past three decades, none of the explanations has been unambiguously confirmed or excluded.

The first contribution from this thesis is a study of the quasiballistic transport hypothesis using an exact, semi-analytic solution of the Boltzmann equation. Several prior studies have claimed quasiballistic electron transport across the base as the origin of cryogenic non-ideal current-voltage characteristics. Specifically, the observation of temperature independent DC performance below $\sim 80$ K has been attributed partly to quasiballistic transport resulting in a presumed increase in electron temperature, but this hypothesis has been examined only using empirical models which leave ambiguity. We overcome this limitation by adapting an exact, semi-analytic solution to the Boltzmann equation based on an asymptotic expansion approach to describe electron transport across the base region of an HBT. With this exact solution, we computed macroscopic electrical properties such as collector current and transconductance which could be directly compared with experiments. We find that the computed transport characteristics are inconsistent with experiment, with the calculated transconductance following the ideal drift-diffusion inverse temperature dependence. This finding implies that quasiballistic electron transport is unlikely to be the origin of cryogenic non-ideal I-V characteristics.

Next, we study a previously unexplored explanation, the presence of lateral spatial inhomogeneities in the base-emitter junction potential height, as the origin for the observed non-ideal cryogenic current-voltage anomalies in SiGe HBTs. While this phenomenon has been established as the origin for similar cryogenic I-V anomalies observed in Schottky diodes, this possibility has not yet been considered for SiGe HBTs. We experimentally investigate this hypothesis by characterizing the base-emitter built-in 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, a signature consistent with the presence of lateral inhomogeneities in the junction potential. We hypothesize that these inhomogeneities arise from clustering of Ge as a result of aggressive doping of modern devices, and propose future directions that allow direct probing of these inhomogeneities.

Finally, we explore the potential improvements in the minimum achievable noise temperature of HBT amplifiers by considering the effects of shot-noise correlation. We first model the expected reduction in cryogenic noise temperature of a state-of-the-art transistor as a result of shot-noise correlation. We then quantify the accuracy of the present noise measurement techniques that allow us to exploit the benefits of shot-noise correlation, and propose modifications to the noise measurement setup that will permit an unambiguous experimental determination of the magnitude of the effect.