As applications in NEVs (New Energy Vehicles), power electronics, 5G communications, radar, and high-voltage power grids evolve, electronic devices are advancing toward higher voltages, higher frequencies, and more demanding operating environments. Wide bandgap semiconductors, represented by SiC and GaN, have significantly driven the development of high-efficiency power electronics and RF technologies. However, as applications further expand into high-voltage and high-frequency domains, their material performance is gradually approaching new bottlenecks.
SiC and GaN have a critical electric field of approximately 3 MV/cm. In local power grid applications of 1 to 30 kV and regional power grid applications of 30 to 150 kV, utilizing these materials for power conversion requires thicker epitaxial layers to withstand high voltages, which in turn may introduce additional resistance and thermal losses. Consequently, ultrawide bandgap (UWB) semiconductors with higher critical electric fields have begun to attract attention.
Three Types of Materials Become Research Focus
Recently, researchers from the University of Padua in Italy published a review paper titled "Ultrawide Bandgap Semiconductors: State of the Art and Technological Challenges" in "IEEE Transactions on Electron Devices", systematically summarizing the research progress and technological challenges of UWB semiconductors. The paper focuses on three types of materials: β-Ga₂O₃, diamond, and AlN, providing a horizontal comparison from materials and devices to applications.
In terms of material parameters, all three exhibit potential that is distinctly different from traditional wide bandgap semiconductors. The critical electric fields provided in the paper are approximately 8 MV/cm for β-Ga₂O₃, 10 MV/cm for diamond, and 15 MV/cm for AlN. A higher critical electric field implies that devices are expected to reduce the active layer thickness under the same voltage withstand requirements, thereby providing new design space for high-voltage and high-power devices. The paper also points out that the theoretical Baliga's figure of merit for the three materials is about 2 to 19 times that of GaN.
However, material performance does not mean that devices can be directly realized. Therefore, the paper does not stop at parameter comparison but further examines the development of the three materials from crystal growth and epitaxy to diodes, power transistors, and RF devices, evaluating them using metrics such as breakdown voltage, specific on-resistance, power figure of merit, and cutoff frequency.
β-Ga₂O₃: Prominent Advantages in High Electric Field and Large-Size Substrates
β-Ga₂O₃ is currently one of the most thoroughly researched UWB materials. The paper points out that its bandgap is approximately 4.85 eV, and its critical electric field can reach up to 8 MV/cm. Meanwhile, native substrates can already be obtained through various crystal growth methods, with sizes advancing to 6 inches and even 8 inches.
Figure 1. Typical structures of vertical γ-Ga₂O₃ SBDs. (a) Simple SBD. (b) SBD with FP. (c) SBD with edge termination structure, which can ultimately be coupled with FP. (d) Trench MOS SBD. (e) HJD. (f) SBD with junction termination extension structure. (g) HSJ.
This is a crucial advantage of β-Ga₂O₃ over some other UWB materials: native substrates can reduce reliance on heteroepitaxy and help mitigate interface and thermal management issues. However, β-Ga₂O₃ itself has a relatively low thermal conductivity, and p-type doping remains challenging, which will affect its further development toward high-power devices.
Diamond: Engineering Challenges Behind Ultra-High Thermal Conductivity
Diamond, on the other hand, represents another material route. The paper points out that diamond possesses a breakdown electric field of approximately 10 MV/cm and an ultra-high thermal conductivity of about 20 W·cm⁻¹·K⁻¹. Meanwhile, boron doping can relatively easily achieve p-type conductivity, giving it unique potential in high-voltage, high-power, and high-temperature electronic devices.
Figure 2. Schematic cross-sections of field-effect transistor (FET) structures in diamond. (a) O-terminated MESFET. (b) O-terminated depletion-mode MOSFET. (c) O-terminated enhancement-mode MOSFET. (d) O-terminated p-channel JFET. (e) H-terminated two-dimensional hole gas (2DHG) MESFET. (f) H-terminated two-dimensional hole gas (2DHG) MOSFET.
However, the main issues currently facing diamond are not insufficient material performance per se, but rather material preparation and device engineering. HPHT and MPCVD are the primary substrate preparation routes. HPHT can obtain high-purity, low-defect insulating substrates, but with limited sizes; CVD, on the other hand, can form conductive substrates more suitable for vertical devices, but with a relatively higher dislocation density. The paper also points out that n-type doping in diamond has a high activation energy, which limits its room-temperature conductivity.
AlN: Coexistence of High Critical Electric Field and RF Potential
AlN possesses the highest theoretical critical electric field among the three types of materials, with the value given in the paper at approximately 15 MV/cm. Meanwhile, its relatively high thermal conductivity and ability to form heterostructures with materials like GaN provide it with development space in both power and RF devices.
Figure 3. (a) Schematic cross-section. (b) AlN mole fraction and simulated band diagram of the undoped DPD p-n diode reported in [199]. (c) Cross-section of an AlN MOSFET with graded ohmic contacts. (d) Cross-section of an AlN XHEMT with Si negative doping. (a) and (b) reprinted from T. Kumabe et al., IEEE TED 2024.
Currently, AlN devices have achieved kilovolt-level breakdown voltages, with the effective breakdown electric field of some devices reaching 1 to 3 MV/cm. However, the paper also points out that AlN device technology is still in an early stage, and issues such as ohmic contact quality, p-type doping, and vertical device structures remain prominent. On the other hand, AlN/GaN/AlN heterostructures have already demonstrated good RF performance, reflecting the potential of combining AlN with mature GaN technology.
From "Material Performance" to "Device Realizability"
Looking at the full text, the paper's true focus is not simply to determine which UWB material has the best performance, but to attempt to answer a more practical question: How far are these theoretically excellent materials from becoming truly competitive electronic devices?
The answer is not simple. All three types of materials have demonstrated high breakdown electric fields and potential in power or RF devices, but at the same time, they are constrained by issues such as substrate size, crystal quality, doping, ohmic contacts, thermal management, and device structures. In other words, the advantages in material parameters can only form true industrial value if they are further translated into stable, manufacturable, and integrable device performance.
The paper ultimately concludes that β-Ga₂O₃, diamond, and AlN are currently still in a relatively early stage of research, but all three exhibit unique advantages in critical electric field, thermal conductivity, and integration with other material systems. What is more likely to emerge in the future is not one material completely replacing the others, but a coexistence pattern formed by the three UWB systems around different application scenarios.
This also constitutes the core judgment of the paper: The competition in UWB semiconductors is gradually shifting from pure material performance competition to a comprehensive competition among materials, device structures, and manufacturing technologies. For β-Ga₂O₃, diamond, and AlN, the real breakthrough needed in the next stage is how to further translate the material potential already demonstrated into repeatable, manufacturable, and integrable device performance.
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