Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

https://iopscience.iop.org/article/10.1088/0268-1242/31/3/034001/pdf

Recent progress in Ga2O3 power devices

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Guest Editorial: The dawn of gallium oxide microelectronics

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.

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Perspective: Ga2O3 for ultra-high power rectifiers and MOSFETS

The thermal conductivities of β-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80–495 K using the time domain thermoreflectance method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0 ± 2.0 W/mK, while that along the [100] direction has the lowest value of 10.9 ± 1.0 W/mK. At high temperatures, the thermal conductivity follows a ∼1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the β-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations, which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

Anisotropic thermal conductivity in single crystal β-gallium oxide

The thermal conductivities of beta-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80-495K using the time domain thermoreflectance (TDTR) method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0+/-2.0 W/mK, while that along the [100] direction has the lowest value of 10.9+/-1.0 W/mK. At high temperatures, the thermal conductivity follows a ~1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the \b{eta}-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

Anisotropic Thermal Conductivity in Single Crystal beta-Gallium Oxide

In evaluating GaN high-electron mobility transistors (HEMTs) for high-power applications, it is crucial to consider the device-level breakdown characteristics. This letter replaces the conventional AlGaN barrier and common AlGaN backbarrier with unstrained AlN, and it assesses the breakdown voltage of AlN/GaN/AlN quantum well HEMTs for gate-drain spacings in the range of 0.27– $5.1~\mu \text{m}$ . The results are highlighted by a high breakdown voltage of 78 V for a gate-drain spacing of 390 nm, among the best reported for submicron-channel devices. In addition, small-signal RF measurements showed record performance for HEMTs on the AlN platform, with $\text {f}_{\text {t}}/\text {f}_{\text {max}} = {161}/{70}$ GHz. The cut-off frequency and corresponding drain bias are benchmarked against the state-of-the-art GaN HEMTs using the Johnson figure of merit, with measured devices highlighted by a JFoM value of 2.2 THz $\cdot $ V. These results illustrate the potential for AlN/GaN/AlN quantum well HEMTs as a future platform for high-power RF transistors.

High Breakdown Voltage in RF AlN/GaN/AlN Quantum Well HEMTs

This work demonstrates the high-frequency and high-power performance capacity of GaN high electron mobility transistors (HEMTs) on Si substrates. Using a T-gate and ${n}^{++}$ -GaN source/drain contacts, the InAlN/GaN HEMT with a gate length of 55 nm and a source-drain spacing of 175 nm shows a maximum drain current ${I}_{{D,MAX}}$ of 2.8 A/mm and a peak transconductance ${g}_{m}$ of 0.66 S/mm. The same HEMT exhibits a forward-current-gain cutoff frequency ${f}_{T}$ of 250 GHz and a maximum frequency of oscillation ${f}_{{MAX}}$ of 204 GHz. The ${I}_{{D,MAX}}$ , peak ${g}_{m}$ and ${f}_{T} -{f}_{{MAX}}$ product are among the best reported for GaN HEMTs on Si, which are very close to the state-of-the-art depletion-mode GaN HEMTs on SiC without a back barrier. Given the low cost of Si and the high compatibility with CMOS circuits, GaN HEMTs on Si prove to be particularly attractive for cost-sensitive applications.

GaN HEMTs on Si With Regrown Contacts and Cutoff/Maximum Oscillation Frequencies of 250/204 GHz

Power and RF electronics applications have spurred massive investment into a range of wide and ultrawide bandgap semiconductor devices which can switch large currents and voltages rapidly with low losses. However, the end systems using these devices are often limited by the parasitics of integrating and driving these chips from the silicon complementary metal–oxide-semiconductor-based design (CMOS) circuitry necessary for complex control logic. For that reason, implementation of CMOS logic directly in the wide bandgap platform has become a way for each maturing material to compete. This review examines potential CMOS monolithic and hybrid approaches in a variety of wide bandgap materials.

Austin Hickman

Papers

Anisotropic thermal conductivity in single crystal β-gallium oxide

Anisotropic Thermal Conductivity in Single Crystal beta-Gallium Oxide

High Breakdown Voltage in RF AlN/GaN/AlN Quantum Well HEMTs

GaN HEMTs on Si With Regrown Contacts and Cutoff/Maximum Oscillation Frequencies of 250/204 GHz

Prospects for Wide Bandgap and Ultrawide Bandgap CMOS Devices