Gallium Oxide has undergone rapid technological maturation over the last decade, pushing it to the forefront of ultra-wide band gap semiconductor technologies. Maximizing the potential for a new semiconductor system requires a concerted effort by the community to address technical barriers which limit performance. Due to the favorable intrinsic material properties of gallium oxide, namely, critical field strength, widely tunable conductivity, mobility, and melt-based bulk growth, the major targeted application space is power electronics where high performance is expected at low cost. This Roadmap presents the current state-of-the-art and future challenges in 15 different topics identified by a large number of people active within the gallium oxide research community. Addressing these challenges will enhance the state-ofthe-art device performance and allow us to design efficient, high-power, commercially scalable microelectronic systems using the newest semiconductor platform. © 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0060327

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β-Gallium oxide power electronics

This review introduces relevant nanoscale thermal transport processes that impact thermal abatement in power electronics applications. Specifically, we highlight the importance of nanoscale thermal transport mechanisms at each layer in material hierarchies that make up modern electronic devices. This includes those mechanisms that impact thermal transport through: (1) substrates, (2) interfaces and two-dimensional materials, and (3) heat spreading materials. For each material layer, we provide examples of recent works that (1) demonstrate improvements in thermal performance and/or (2) improve our understanding of the relevance of nanoscale thermal transport across material junctions. We end our discussion by highlighting several additional applications that have benefited from a consideration of nanoscale thermal transport phenomena, including radio frequency (RF) electronics and neuromorphic computing. [DOI: 10.1115/1.4049293]

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Applications and Impacts of Nanoscale Thermal Transport in Electronics Packaging

Gallium nitride (GaN) is one of the front-runner materials among the so-called wide bandgap semiconductors that can provide devices having high breakdown voltages and are capable of performing efficiently even at high temperatures. The wide bandgap, however, naturally leads to a high density of surface states on bare GaN-based devices or interface states along insulator/semiconductor interfaces distributed over a wide energy range. These electronic states can lead to instabilities and other problems when not appropriately managed. In this Tutorial, we intend to provide a pedagogical presentation of the models of electronic states, their effects on device performance, and the presently accepted approaches to minimize their effects such as surface passivation and insulated gate technologies. We also re-evaluate standard characterization methods and discuss their possible pitfalls and current limitations in probing electronic states located deep within the bandgap. We then introduce our own photo-assisted capacitance–voltage (C–V) technique, which is capable of identifying and examining near mid-gap interface states. Finally, we attempt to propose some directions to which some audience can venture for future development.

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Controlling surface/interface states in GaN-based transistors: Surface model, insulated gate, and surface passivation

Ultra-wide bandgap β-gallium oxide (Ga2O3) devices are of considerable interest with potential applications in both power electronics and radio frequency devices. However, current Ga2O3 device technologies are limited by the material's low intrinsic electron mobility and thermal conductivity. The former problem can be addressed by employing modulation-doped β-(AlxGa1−x)2O3/Ga2O3 heterostructures in the device architecture. In this work, (AlxGa1−x)2O3/Ga2O3 modulation-doped field effect transistors (MODFETs) have been investigated from a thermal perspective. Thermoreflectance thermal imaging was used to characterize the self-heating of the MODFETs. The (Al0.18Ga0.82)2O3 thermal conductivity (3.1–3.6 W/mK) was determined using a frequency domain thermoreflectance technique. Electro-thermal modeling was used to discern the effect of design parameters such as substrate orientation and channel length on the device self-heating behavior. Various thermal management schemes were evaluated using the electro-thermal device model. From an electro-thermal co-design perspective, the improvement in electrical performance followed by the mitigation of self-heating was also studied. For example, by employing a Ga2O3-on-SiC composite wafer, which was fabricated in this work, a 50% increase in power handling capability can be achieved as compared to a homoepitaxial device. Furthermore, flip-chip heterointegration and double-sided cooling approaches can lead to more than 2× improvement in the power handling capability. Using an augmented double-sided cooling design that includes nanocrystalline diamond passivation, a 5× improvement in the power handling capability can be accomplished, indicating the potential of the technology upon implementation of a suitable thermal management scheme.

Electro-thermal co-design of β-(AlxGa1-x)2O3/Ga2O3 modulation doped field effect transistors

Fundamental research and development of ultra-wide bandgap (UWBG) semiconductor devices are under way to realize next-generation power conversion and wireless communication systems. Devices based on aluminum gallium nitride (AlxGa1−xN, x is the Al composition), β-phase gallium oxide (β-Ga2O3), and diamond give promise to the development of power switching devices and radio frequency power amplifiers with higher performance and efficiency than commercial wide bandgap semiconductor devices based on gallium nitride (GaN) and silicon carbide (SiC). However, one of the most critical challenges for the successful deployment of UWBG device technologies is to overcome adverse thermal effects that impact the device performance and reliability. Overheating of UWBG devices originates from the projected high power density operation and poor intrinsic thermal properties of AlxGa1−xN and β-Ga2O3. This Perspective delineates the need and process for the “electro-thermal co-design” of laterally configured UWBG electronic devices and provides a comprehensive review of current state-of-the-art thermal characterization methods, device thermal modeling practices, and both device- and package-level thermal management solutions.

A perspective on the electro-thermal co-design of ultra-wide bandgap lateral devices

Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V) resulted in an ∼44% larger rise in peak junction temperature compared to that for a lower VDS (∼7.5 V) condition. The difference arises primarily due to reduction in the heat generating volume when operating under partially pinched-off (i.e., high VDS) conditions. Self-heating amplifies with this reduction as heating now takes place primarily over length scales less than the mean free path of the phonons tasked with energy dissipation. Being less efficient, the subcontinuum transport restricts thermal transport away from the device hot-spot causing a net increase in channel temperature. Taken together, even purely thermally driven device mean-time-to-failure is not, therefore, based on power dissipation alone as both bias dependence and subcontinuum thermal transport influence device lifetime.

Nanoscale electro-thermal interactions in AlGaN/GaN high electron mobility transistors

To increase the reliability and the maximum performance of AlGaN/GaN high electron mobility transistors (HEMTs), gate field plates are frequently used with surface passivation. Although significant research has been done to understand the electrical effects of gate field plates on devices, their thermal effects are still not fully understood. For this purpose, electrothermal simulations are performed on devices with and without gate field plates having different thicknesses of Si3N4 surface passivation at two different biasing conditions. These simulations prove that more than 8% reduction of maximum temperature can be achieved with the use of gate field plates on devices operated at ${P}={4}$ W/mm. Field plates, when used in multifinger device configurations, have a greater impact on the outermost fingers’ temperatures and can be used to achieve temperature uniformity. Surface passivation studies suggest that while thick passivation layers (~200 nm in case of Si3N4) eliminate the thermal advantages of the field plate technology, very thin passivation layers (~45 nm) cause an increase in the electric field and a decrease in the breakdown voltage. Thus, significant thermal advantages can be achieved when gate field plates are introduced following a field plate length and passivation thickness optimization based on the device biasing conditions.

The Effects of Gate-Connected Field Plates on Hotspot Temperatures of AlGaN/GaN HEMTs

The performance of AlGaN/GaN HEMTs (High Electron Mobility Transistors), frequently used in high voltage and frequency power applications, can be enhanced with the use of field-plate technology. Experimental characterization of the device maximum temperature is extremely important for device reliability, yet it is difficult in devices with the field-plates due to geometrical constraints. To compare the temperature measurement performance and limits of current experimental methods: micro-Raman thermography and Thermoreflectance Thermal Imaging (TTI) when used on field-plated devices, a numerical thermal model is built. The model accounts for the size effects in the channel that occur due to extremely small heat generation region in devices. The peak temperatures of the devices with and without field-plates, at various power densities obtained from micro-Raman and TTI measurements will be simulated using the numerical model. As a result of this, maximum temperature measurement capability of micro-Raman and TTI for field-plated devices will be identified and proper techniques will be suggested.

Canberk Dundar

Papers

Nanoscale electro-thermal interactions in AlGaN/GaN high electron mobility transistors

The Effects of Gate-Connected Field Plates on Hotspot Temperatures of AlGaN/GaN HEMTs

Thermal Characterization of Field Plated AlGaN/GaN HEMTs