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1. Thermal analysis of an α-Ga2O3 MOSFET using micro-Raman spectroscopy

   https://doi.org/10.1063/5.0164095  

   Karim, Anwarul; Song, Yiwen; Shoemaker, Daniel C; Jeon, Dae-Woo; Park, Ji-Hyeon; Mun, Jae Kyoung; Lee, Hun Ki; Choi, Sukwon (November 2023, Applied Physics Letters)

   The ultra-wide bandgap (UWBG) energy (∼5.4 eV) of α-phase Ga2O3 offers the potential to achieve higher power switching performance and efficiency than today's power electronic devices. However, a major challenge to the development of the α-Ga2O3 power electronics is overheating, which can degrade the device performance and cause reliability issues. In this study, thermal characterization of an α-Ga2O3 MOSFET was performed using micro-Raman thermometry to understand the device self-heating behavior. The α-Ga2O3 MOSFET exhibits a channel temperature rise that is more than two times higher than that of a GaN high electron mobility transistor (HEMT). This is mainly because of the low thermal conductivity of α-Ga2O3 (11.9 ± 1.0 W/mK at room temperature), which was determined via laser-based pump-probe experiments. A hypothetical device structure was constructed via simulation that transfer-bonds the α-Ga2O3 epitaxial structure over a high thermal conductivity substrate. Modeling results suggest that the device thermal resistance can be reduced to a level comparable to or even better than those of today's GaN HEMTs using this strategy combined with thinning of the α-Ga2O3 buffer layer. The outcomes of this work suggest that device-level thermal management is essential to the successful deployment of UWBG α-Ga2O3 devices. 

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2. Transient Thermal Management of a β-Ga₂O₃ MOSFET Using a Double-Side Diamond Cooling Approach

   https://doi.org/10.1109/TED.2023.3244134  

   Kim, Samuel H.; Shoemaker, Daniel; Green, Andrew J.; Chabak, Kelson D.; Liddy, Kyle J.; Graham, Samuel; Choi, Sukwon (February 2023, IEEE Transactions on Electron Devices)

   β-phase gallium oxide ( β-Ga2O3) has drawn significant attention due to its large critical electric field strength and the availability of low-cost high-quality melt-grown substrates. Both aspects are advantages over gallium nitride (GaN) and silicon carbide (SiC) based power switching devices. However, because of the poor thermal conductivity of β-Ga2O3, device-level thermal management is critical to avoid performance degradation and component failure due to overheating. In addition, for high-frequency operation, the low thermal diffusivity of β-Ga2O3 results in a long thermal time constant, which hinders the use of previously developed thermal solutions for devices based on relatively high thermal conductivity materials (e.g., GaN transistors). This work investigates a double-side diamond-cooled β-Ga2O3 device architecture and provides guidelines to maximize the device’s thermal performance under both direct current (dc) and high-frequency switching operation. Under high-frequency operation, the use of a β-Ga2O3 composite substrate (bottom-side cooling) must be augmented by a diamond passivation overlayer (top-side cooling) because of the low thermal diffusivity of β-Ga2O3. 

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3. Progress and challenges in the development of ultra-wide bandgap semiconductor α-Ga2O3 toward realizing power device applications

   https://doi.org/10.1063/5.0126698  

   Oshima, Yuichi; Ahmadi, Elaheh (December 2022, Applied Physics Letters)

   Ultra-wide-bandgap (UWBG) semiconductors, such as Ga2O3 and diamond, have been attracting increasing attention owing to their potential to realize high-performance power devices with high breakdown voltage and low on-resistance beyond those of SiC and GaN. Among numerous UWBG semiconductors, this work focuses on the corundum-structured α-Ga2O3, which is a metastable polymorph of Ga2O3. The large bandgap energy of 5.3 eV, a large degree of freedom in band engineering, and availability of isomorphic p-type oxides to form a hetero p–n junction make α-Ga2O3 an attractive candidate for power device applications. Promising preliminary prototype device structures have been demonstrated without advanced edge termination despite the high dislocation density in the epilayers owing to the absence of native substrates and lattice-matched foreign substrates. In this Perspective, we present an overview of the research and development of α-Ga2O3 for power device applications and discuss future research directions. 

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4. High thermal conductivity in wafer-scale cubic silicon carbide crystals

   https://doi.org/10.1038/s41467-022-34943-w  

   Cheng, Zhe; Liang, Jianbo; Kawamura, Keisuke; Zhou, Hao; Asamura, Hidetoshi; Uratani, Hiroki; Tiwari, Janak; Graham, Samuel; Ohno, Yutaka; Nagai, Yasuyoshi; et al (December 2022, Nature Communications)

   Abstract High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as both active functional materials and thermal management materials. We report an isotropic high thermal conductivity exceeding 500 W m −1 K −1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-standing puzzle that the literature values of thermal conductivity for 3C-SiC are lower than the structurally more complex 6H-SiC. We show that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which avoids the exceptionally strong defect-phonon scatterings. Moreover, 3C-SiC is a SiC polytype which can be epitaxially grown on Si. We show that the measured 3C-SiC-Si thermal boundary conductance is among the highest for semiconductor interfaces. These findings provide insights for fundamental phonon transport mechanisms, and suggest that 3C-SiC is an excellent wide-bandgap semiconductor for applications of next-generation power electronics as both active components and substrates. 

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5. Implications of Interfacial Thermal Transport on the Self-Heating of GaN-on-SiC High Electron Mobility Transistors

   https://doi.org/10.1109/TED.2023.3303125  

   Shoemaker, Daniel C.; Song, Yiwen; Kang, Kyuhwe; Schuette, Michael L.; Tweedie, James S.; Sheppard, Scott T.; McIlwaine, Nathaniel S.; Maria, Jon-Paul; Choi, Sukwon (August 2023, IEEE Transactions on Electron Devices)

   Gallium nitride (GaN) high electron mobility transistors (HEMTs) are key components enabling today’s wireless communication systems. However, overheating concerns hinder today’s commercial GaN HEMTs from reaching their full potential. Therefore, it is necessary to characterize the respective thermally resistive components that comprise the device’s thermal resistance and determine their contributions to the channel temperature rise. In this work, the thermal conductivity of the GaN channel/buffer layer and the effective thermal boundary resistance (TBR) of the GaN/substrate interface of a GaN-on-SiC wafer were measured using a frequency-domain thermoreflectance technique. The results were validated by both experiments and modeling of a transmission line measurement (TLM) structure fabricated on the GaN-on-SiC wafer. The limiting GaN/substrate thermal boundary conductance (TBC) beyond which there is no influence on the device temperature rise was then quantified for different device configurations. It was determined that this limiting TBC is a function of the substrate material, the direction in which heat primarily flows, and the channel temperature. The outcomes of this work provide device engineers with guidance in the design of epitaxial GaN wafers that will help minimize the device’s thermal resistance. 

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