Ultrawide bandgap oxide semiconductors represent a critical class of materials for next-generation high-power and high-temperature electronic applications. Their unique electronic and thermal properties make them promising candidates for devices operating under extreme conditions where conventional semiconductors like silicon (Si) or even wide bandgap materials like silicon carbide (SiC) and gallium nitride (GaN) face limitations. Among these oxides, gallium oxide (Ga2O3) and aluminum oxide (Al2O3) stand out due to their exceptionally large bandgaps, high breakdown fields, and potential for scalable synthesis.

The electronic structure of these materials is defined by their ultrawide bandgaps, which significantly influence their electrical and optical behavior. Ga2O3 has a bandgap ranging from 4.5 to 4.9 eV, depending on its crystal phase, with the beta-phase (β-Ga2O3) being the most thermally stable and widely studied. Al2O3, particularly in its alpha-phase (α-Al2O3), possesses an even larger bandgap of approximately 8.8 eV. These large bandgaps result in extremely low intrinsic carrier concentrations at room temperature, reducing leakage currents and enabling high-temperature operation. The conduction band minima in these oxides are primarily derived from metal s-orbitals, leading to high electron effective masses and limited carrier mobility compared to SiC or GaN. However, the high breakdown electric field strengths compensate for this limitation, allowing these materials to handle significantly higher voltages.

Breakdown field strength is a critical parameter for power electronics, dictating the maximum electric field a material can withstand before catastrophic failure. Ga2O3 exhibits a theoretical breakdown field of around 8 MV/cm, surpassing SiC (3 MV/cm) and GaN (3.3 MV/cm). Al2O3, though less explored for active electronic devices due to its insulating nature, has an even higher intrinsic breakdown strength exceeding 10 MV/cm. These values translate to superior performance in high-voltage applications, as the Baliga figure of merit (BFOM), which quantifies a material's suitability for power devices, scales with the cube of the breakdown field. Ga2O3’s BFOM is approximately an order of magnitude higher than SiC and GaN, indicating its potential for more efficient power switching devices.

Thermal properties, however, present a challenge for these oxide semiconductors. Ga2O3 has a relatively low thermal conductivity of about 0.1-0.3 W/cm·K, significantly lower than SiC (4.9 W/cm·K) or GaN (2.3 W/cm·K). This limitation necessitates innovative thermal management strategies in device design to prevent overheating during high-power operation. Al2O3 fares slightly better with a thermal conductivity around 0.3-0.5 W/cm·K, but it remains inferior to traditional wide bandgap semiconductors. Despite this, the high breakdown fields and bandgap energies allow these oxides to operate at higher junction temperatures without suffering from intrinsic conduction effects.

High-power applications benefit greatly from the unique properties of ultrawide bandgap oxide semiconductors. Ga2O3-based Schottky barrier diodes and field-effect transistors have demonstrated remarkable performance in high-voltage rectification and switching. The material's ability to sustain high electric fields enables thinner drift regions compared to SiC or GaN, reducing on-resistance and improving efficiency. For instance, Ga2O3 devices have shown specific on-resistances nearly ten times lower than SiC counterparts at similar breakdown voltages. This advantage is particularly valuable in power converters, electric vehicle systems, and renewable energy inverters, where minimizing conduction losses is critical.

Al2O3, while less commonly used as an active semiconductor, plays a vital role as a dielectric in high-power and high-frequency devices. Its large bandgap and high breakdown field make it an excellent insulator for gate dielectrics in GaN and SiC transistors, reducing leakage currents and enhancing device reliability. Additionally, its compatibility with high-temperature processing allows integration into advanced device architectures requiring stable passivation layers.

Comparing these oxides with SiC and GaN reveals trade-offs between material properties and application suitability. SiC offers superior thermal conductivity and mature fabrication techniques, making it dominant in high-power modules today. GaN excels in high-frequency applications due to its high electron mobility and polarization-induced doping. However, Ga2O3’s combination of ultrawide bandgap, high breakdown field, and potential for low-cost production from melt-grown bulk crystals positions it as a disruptive technology for ultra-high-voltage applications exceeding 10 kV. Al2O3 complements these materials as an insulating layer but lacks the necessary electrical properties for active devices.

The future of ultrawide bandgap oxide semiconductors hinges on addressing their thermal limitations and advancing device engineering. Heterostructure designs incorporating thermal vias or bonded substrates could mitigate heat dissipation issues. Furthermore, improving material quality to reduce defects and trap states will enhance carrier mobility and device reliability. As research progresses, these materials are poised to enable breakthroughs in power electronics, aerospace systems, and other demanding applications where performance under extreme conditions is paramount. Their development represents a significant step beyond the capabilities of SiC and GaN, pushing the boundaries of semiconductor technology into uncharted territories of voltage and efficiency.