Beta-phase gallium oxide (β-Ga₂O₃) is a cutting-edge ultrawide bandgap (UWBG) semiconductor, featuring a bandgap energy of around 4.8 eV and a highly critical electric field strength of about 8 MV/cm. These properties make it highly suitable for next-generation power electronics and deep ultraviolet optoelectronics. Key advantages of β-Ga₂O₃ include the availability of large-size single-crystal bulk native substrates produced from melt and the precise control of n-type doping during both bulk growth and thin-film epitaxy. A comprehensive understanding of the fundamental growth processes, control parameters, and underlying mechanisms is essential to enable scalable manufacturing of high-performance epitaxial structures. This review highlights recent advancements in the epitaxial growth of β-Ga₂O₃ through various techniques, including Molecular Beam Epitaxy (MBE), Metal-Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), Mist Chemical Vapor Deposition (Mist CVD), Pulsed Laser Deposition (PLD), and Low-Pressure Chemical Vapor Deposition (LPCVD). This review concentrates on the progress of Ga₂O₃ growth in achieving high growth rates, low defect densities, excellent crystalline quality, and high carrier mobilities through different approaches. It aims to advance the development of device-grade epitaxial Ga₂O₃ thin films and serves as a crucial resource for researchers and engineers focused on UWBG semiconductors and the future of power electronics.

During the past decade, Gallium Oxide (Ga₂O₃) has attracted intensive research interest as an ultra-wide-bandgap (UWBG) semiconductor due to its unique characteristics, such as a large bandgap of 4.5–4.9 eV, a high critical electric field of ~8 MV/cm, and a high Baliga’s figure of merit (BFOM). Unipolar β-Ga₂O₃ devices such as Schottky barrier diodes (SBDs) and field-effect transistors (FETs) have been demonstrated. Recently, there has been growing attention toward developing β-Ga₂O₃-based heterostructures and heterojunctions, which is mainly driven by the lack of p-type doping and the exploration of multidimensional device architectures to enhance power electronics’ performance. This paper will review the most recent advances in β-Ga₂O₃ heterostructures and heterojunctions for power electronics, including NiO_x/β-Ga₂O₃, β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃, and β-Ga₂O₃ heterojunctions/heterostructures with other wide- and ultra-wide-bandgap materials and the integration of two-dimensional (2D) materials with β-Ga₂O₃. Discussions of the deposition, fabrication, and operating principles of these heterostructures and heterojunctions and the associated device performance will be provided. This comprehensive review will serve as a critical reference for researchers engaged in materials science, wide- and ultra-wide-bandgap semiconductors, and power electronics and benefits the future study and development of β-Ga₂O₃-based heterostructures and heterojunctions and associated power electronics.

This letter reports the demonstration and electrical characterization of high-voltage AlN metal-semiconductor field-effect transistors (MESFETs) on single-crystal AlN substrates. Compared with AlN MESFETs on foreign substrates, the AlN-on-AlN MESFETs showed high breakdown voltages of over 2 kV for drain-to-gate spacing of 15 μm and one of the highest average breakdown fields among reported AlN MESFETs. Additionally, the devices also exhibited decent drain saturation current and on/off ratio without complicated regrown or graded contact layers, which are several times higher than those of reported AlN-on-sapphire MESEFTs. This work is beneficial for the future development of ultrawide bandgap AlN power electronics.

In this Letter, low-temperature (400 °C) chemical vapor deposition-grown boron nitride (BN) was investigated as the gate dielectric for AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MISHEMTs) on a Si substrate. Comprehensive characterizations using x-ray photoelectron spectroscopy, reflection electron energy loss spectroscopy, atomic force microscope, high-resolution transmission electron microscopy, and time-of-flight secondary ion mass spectrometry were conducted to analyze the deposited BN dielectric. Compared with conventional Schottky-gate HEMTs, the MISHEMTs exhibited significantly enhanced performance with 3 orders of magnitude lower reverse gate leakage current, a lower off-state current of 1 × 10−7 mA/mm, a higher on/off current ratio of 108, and lower on-resistance of 5.40 Ω mm. The frequency-dependent conductance measurement was performed to analyze the BN/HEMT interface, unveiling a low interface trap state density (Dit) on the order of 5 × 1011–6 × 1011 cm−2 eV−1. This work shows the effectiveness of low-temperature BN dielectrics and their potential for advancing GaN MISHEMTs toward high-performance power and RF electronics applications.

We report the results of the study of the acoustic and optical phonons in Si-doped AlN thin films grown by metal–organic chemical vapor deposition on sapphire substrates. The Brillouin–Mandelstam and Raman light scattering spectroscopies were used to measure the acoustic and optical phonon frequencies close to the Brillouin zone center. The optical phonon frequencies reveal non-monotonic changes, reflective of the variations in the thin film strain and dislocation densities with the addition of Si dopant atoms. The acoustic phonon velocity decreases monotonically with increasing Si dopant concentration, reducing by ∼300 m/s at the doping level of 3 × 1019 cm−3. The knowledge of the acoustic phonon velocities can be used for the optimization of the ultra-wide bandgap semiconductor heterostructures and for minimizing the thermal boundary resistance of high-power devices.

The ON-resistance ( $\textit{R}_{\text{ON}}\text{)}$ instability of vertical GaN PIN diodes under high-temperature (up to 400 $^{\circ}$ C) and forward voltages stress were comprehensively investigated. The turn-on voltages ( $\textit{V}_{\text{ON}}\text{)}$ of vertical GaN PIN diodes under high temperature and forward voltage stress were generally stable, while the instability of the $\textit{R}_{\text{ON}}$ of the devices was very pronounced under these stresses. With temperature ramping up from 25 $^{\circ}$ C to 400 $^{\circ}$ C and then cooling down to 25 $^{\circ}$ C, both the $\textit{R}_{\text{ON}}$ and ideality factor of the devices showed a similar trend of first decreasing and then increasing. The decrease in the ideality factor was attributed to the transition from Shockley–Read–Hall (SRH) recombination to carrier diffusion currents. During the long-time temperature stress, the $\textit{R}_{\text{ON}}$ was unstable and increased for several hours at $\sim$ 150 $^{\circ}$ C–200 $^{\circ}$ C, likely due to electrical-thermal effects and the activation of nitrogen vacancies (VN) in p-GaN. For the forward voltage stress, it was found that the shift of $\textit{R}_{\text{ON}}$ was voltage-dependent and decreased from 1.05 to 0.6 m $\Omega $ cm $^{\text{2}}$ with increasing forward voltage stress due to voltage-dependent carrier accumulation. Further, increased forward voltage stress will cause the subturn ON current due to high electric field-induced electron injection into the p-GaN layer. In addition, the device $\textit{R}_{\text{ON}}$ can also be increased by high reverse voltage biasing due to holes being driven out of the p-GaN. These results can serve as a critical reference for the reliability study of vertical GaN devices and provide guidance for the future development of robust GaN power electronics.