Near-junction heat spreaders for hot spot thermal management of high power density electronic devices

Many high power (opto-) electronic devices such as transistors, diodes, and lasers suffer from significant hot spot temperature rises due to the high heat fluxes generated in their active area, which limits their performance, reliability, and lifetime. Employing high thermal conductivity materials near the heat source, known as near-junction heat spreaders, offers a low-cost and effective thermal management approach. Here, we present analytical heat spreader models and a methodology to evaluate their performance. Experimental demonstration of near-junction diamond heat spreaders on vertical GaN PiN diodes revealed significantly reduced spreading resistances, along with very low temperature gradients across the device. The findings in this work provide design guidelines and demonstrate excellent prospects, especially for the devices on low thermal conductivity substrates. The theoretical analysis of optimized diamond heat spreaders shows an 86% reduction of spreading resistance for GaN devices and 98% for Ga2O3 devices. In addition, our results show that a 3 μm-thick layer of high-quality CVD-deposited diamond heat spreaders on GaN-on-Si devices can provide better heat spreading than GaN-on-SiC devices and perform similar to GaN-on-diamond devices, highlighting the significant potential of heat spreaders as an effective and low-cost thermal management approach. INTRODUCTION

Impact of Strain and Alloy Disorder on the Electronic Properties of III-Nitride Based Two-dimensional Electron Gases

Pirouz Sohi

Over the past 20 years, III-nitrides (GaN, AlN, InN and their alloys) have proven to be an excellent material group for electronic devices, in particular, for high electron mobility transistors (HEMTs) operating at high frequency and high power. This is mainly thanks to the wide band gap of III-nitrides and the spontaneous polarization along the technologically relevant (0001) direction. Heteroepitaxy of III-nitride heterostructures enables the formation of two-dimensional electron gases (2DEGs) with high carrier densities (> 10^13 cm^-2) and high electron mobilities (2000 cm^2V^-1s^-1). The heterostructures usually consist of AlGaN, InAlN or AlN barriers grown epitaxially on GaN buffers. Devices based on these structures exhibit excellent performance and have reached technological maturity, enabling notably the commercialization of AlGaN/GaN HEMTs. Despite the success of III-nitride HEMTs thus far, the full potential of the material group for electronics has not yet been unleashed. The aim of this thesis is to investigate two novel heterostructure designs and to determine the physical mechanisms limiting their electronic properties. This was done by epitaxial growth followed by characterization of material and electronic properties. The first studied heterostructure was AlN/GaN/AlN where the GaN channel is fully strained to AlN. In order to achieve pseudomorphic growth of GaN on AlN, the onset of strain relaxation, i.e., the critical thickness, is of paramount importance. Furthermore, the impact of growth parameters such as temperature and initial dislocation density need to be considered. Over the past years several, research groups have reported on the electronic properties of ultra-thin GaN channels on AlN. Interestingly, all reported 2DEGs suffer from a low electron mobility with values usually below 600 cm^2V^-1s^-1. In this thesis, the strain relaxation and critical thickness of GaN on AlN were determined. Furthermore, the origin of low electron mobility for AlN/GaN/AlN heterostructures and in general thin-GaN channels grown on AlN were systematically studied. The second heterostructure that was investigated during this thesis was based on InGaN channels. In this case, the 2DEG is formed in an In-rich InGaN layer. The low electron effective mass of InN (0.05 m0) compared to GaN (0.2 m0) should theoretically give rise to a higher electron mobility. However, for In-rich channels alloy disorder scattering is particularity strong in III-nitrides. This has been shown both by optical measurements and theoretical calculations based on the localization landscape theory. Surprisingly, high electron mobilities have been reported in literature for high In content InGaN channels. In these cases, the electron mobility appears not to be limited by alloy disorder scattering. During this thesis, the electron mobility of InGaN channels was determined as a function of In content. Furthermore, the unintentional growth of GaN interlayers is proposed as the possible origin for the reported high electron mobilities in literature.

EPFL2019

Near-junction microfluidic cooling for high power-density GaN-on-Si electronics: A wafer, device, packaging, and system-level investigation

Remco Franciscus Peter van Erp

Electrification of the energy section, from generation to end-use, plays an essential role in reducing global CO2 emission. Innovations in power electronics are required to increase conversion efficiency and power density. Gallium nitride (GaN) transistors have emerged over the last decade as a viable alternative to silicon-based devices to address these needs and pave the way for power integrated circuits that replace traditional discrete devices. However, integration of high power components causes thermal challenges that ultimately limits integration density. Microfluidic cooling is a promising candidate to overcome these limitations. By integrating microchannels directly in a chip, and passing coolant through the device, over 1 kW/cm2 of heat can be extracted. While this approach has been studied for silicon ICs, GaN RF devices, an in-depth study on the application to GaN-on-Si power devices is lacking. In this thesis, we explore the possibility of improving thermal performance of GaN-on-Si power devices, by turning the silicon substrate from a low-cost carrier into a high-performance heat sink. 4 distinct levels of integration are investigated, and at each level, we benchmark the cooling performance and efficiency, and provide a breakdown of which components form the bottleneck in heat transfer. On a system level, the usage of a hierarchical microfluidic cold-plate structure to locally extract heat from a GaN-based converter with 20 individual heat sources in an efficient manner. We show that well-designed microfluidic heat sinks can provide system-level advantages such as higher power density and low pumping power despite the small channel dimensions. At the packaging level, we integrating cooling directly inside the silicon substrate of a GaN-on-Si power IC to eliminate interface and packaging thermal resistances, combined with new packaging concepts based on additive manufacturing and coolant-delivery PCBs. Using in-chip liquid cooling, a 7.5-fold increase in power density was obtained compared to commercially available systems. To reduce convective thermal resistance, a new GaN device manufacturing method is presented where microfluidic cooling and electronics are co-designed, resulting in a monolithically-integrated 3-dimensional manifold microchannel heat sink within the silicon substrate. Heat fluxes up to 1.7 kW/cm2 could be extracted within a compact form factor at low pressure drops and limited pumping power. To facilitate the commercialization, we developed a GaN-on-Si substrates with epitaxially-integrated cooling. This substrate can be considered as a drop-in replacement for traditional GaN-on-Si wafers onto which devices can be fabricated, that offers the benefits of microfluidic cooling without requiring a change in foundry process. Finally, a comparison is made between single-phase water cooling and two-phase flow boiling using R1233ZD(E) as a refrigerant. A benchmark is presented that shows comparable cooling efficiencies for single-phase cooling using co-designed manifold microchannel heat sinks. The benchmark and breakdown of thermal resistance presented in each chapter function as a reference for deciding which level of microfluidic cooling is required for a given application. The high cooling performance demonstrated in this thesis confirms that microfluidic cooling for GaN ICs may become an interesting thermal management technology for certain demanding applications.

EPFL2022

Near-junction microfluidic cooling for high power-density GaN-on-Si electronics: A wafer, device, packaging, and system-level investigation

Remco Franciscus Peter van Erp

EPFL2022

Impact of Strain and Alloy Disorder on the Electronic Properties of III-Nitride Based Two-dimensional Electron Gases

Pirouz Sohi

EPFL2019