Third-generation semiconductor devices such as gallium nitride (GaN) and silicon carbide (SiC) have gradually become well-known due to their rapid development in fields like new energy vehicles and consumer electronics. However, the emergence of new application scenarios such as artificial intelligence, data centers, and drones has revealed certain bottlenecks in the existing material systems. In this context, fourth-generation semiconductor materials represented by gallium oxide (Ga₂O₃) have begun to emerge. These materials possess wider bandgaps, relatively smaller dielectric constants, high breakdown field strengths, and certain advantages in material stability. However, gallium oxide's low thermal conductivity (10–27 W/m·K) makes it highly susceptible to uneven temperature distribution during device operation, thereby affecting device performance and lifespan. Therefore, thermal management is crucial for the development of gallium oxide devices.
Gallium oxide (Ga₂O₃) is an ultra-wide bandgap semiconductor material with five crystalline phases: α, β, γ, ε, and δ. It typically exists in the β-phase (this article discusses β-phase Ga₂O₃). Its bandgap width is approximately 4.85 eV, higher than silicon carbide's 3.2 eV and gallium nitride's 3.39 eV. A wider bandgap means electrons require more energy to transition from the valence band to the conduction band, enabling gallium oxide to operate stably in high-temperature and high-voltage environments. As shown in the table below, gallium oxide has an extremely high breakdown electric field (theoretical value up to 8 MV/cm), which is more than 20 times that of silicon and significantly higher than silicon carbide and gallium nitride. This means that under the same voltage, gallium oxide devices can be made very thin, enabling system miniaturization and lightweighting-key requirements in fields such as new energy vehicles and drones. Additionally, compared to silicon carbide and gallium nitride, gallium oxide is one of the few crystal materials that can be grown using the atmospheric-pressure melt method. Currently, multiple institutions worldwide have achieved the preparation of 6-inch gallium oxide wafers.
Thermal Management Solutions
1. Heterogeneous Integration (Current Most Effective Solution)
Using heterogeneous integration methods to transfer single-crystal gallium oxide thin films onto high-thermal-conductivity substrates to form gallium oxide heterogeneous integrated wafers is an effective way to address the bottleneck of low thermal conductivity in gallium oxide.
A team has successfully developed an ion implantation-bonding and exfoliation technology for gallium oxide wafers. This technology involves implanting ions into the gallium oxide wafer to form an extremely thin damaged layer inside, bonding it to a silicon carbide substrate, and then annealing to precisely exfoliate and transfer the thin film along the damaged layer, achieving heterogeneous integration of the gallium oxide thin film with a high-thermal-conductivity substrate. Based on this technology, the Ga₂O₃-on-SiC devices exhibit significantly improved thermal transport capabilities. The thermal conductivity of the heterogeneous integrated gallium oxide thin film reaches 9.0 W/m·K, doubling compared to before annealing, while the interfacial thermal resistance is reduced to one-third of its original value. After high-temperature annealing, the thermal diffusion rate of the silicon carbide-based gallium oxide heterogeneous integrated wafer approaches that of bulk silicon carbide material, far exceeding that of bulk gallium oxide material.
2. Collaborative Thermal Design at the Device and Packaging Levels
1. Substrate Thinning
Thermal resistance is a key parameter that measures the difficulty of heat conduction in a material and is directly proportional to the material's thickness. Substrate thinning technology shortens the heat conduction path, allowing heat generated in the active region to dissipate, thereby maintaining the temperature stability of β-Ga₂O₃ devices during operation and preventing performance degradation caused by excessive temperatures.
Seki et al. reduced the thermal resistance of β-Ga₂O₃ Schottky barrier diodes by one-third by decreasing the substrate thickness from 250 μm to 100 μm.
2. Junction-Side Cooling
Bottom Cooling: Bottom cooling is the most common method, where the β-Ga₂O₃ material stack is integrated with a high-thermal-conductivity heterogeneous substrate, and a constant-temperature heat sink or convective boundary conditions are applied to the bottom of the substrate. Heat generated is dissipated through the low-thermal-conductivity β-Ga₂O₃ layer, the β-Ga₂O₃/substrate thermal interface, and the substrate to the heat sink. Bottom cooling is suitable for high-thermal-resistance semiconductors (e.g., silicon carbide and diamond) but may not achieve efficient thermal management for low-thermal-resistance semiconductors (e.g., gallium nitride).
Top Cooling: Top cooling employs an interconnect structure where source, drain, and gate contact pads are connected to a high-thermal-conductivity substrate via micro-bumps and encapsulated with a polymer-based underfill material. Heat is conducted from the β-Ga₂O₃ device to the high-thermal-conductivity material (metal pads and bump connections) and then to the substrate and heat sink with fixed or convective boundary conditions. In this approach, heat is directly conducted from the device junction to the package rather than through the device body. Top cooling significantly reduces junction-to-case thermal resistance, thereby improving power density. This cooling method is suitable for low-thermal-resistance semiconductors.
Double-Sided Cooling: Double-sided cooling combines the advantages of bottom and top cooling by dissipating heat from both sides of the chip, providing better thermal management. It is highly suitable for ultra-wide bandgap materials with low thermal resistance.
3. Microfluidic Cooling
Microfluidic cooling methods include microchannel cooling and jet impingement cooling. By constructing microchannel structures near the β-Ga₂O₃ device, the small size of the microchannels increases the heat exchange area between the coolant and the channel walls, enabling rapid heat transfer from the device to the coolant, which is then carried away. Jet impingement cooling involves directing coolant at high speed directly onto the surface of the β-Ga₂O₃ device, creating intense convective heat transfer in localized areas to quickly remove heat.