With the continuous development of wide-bandgap semiconductor devices toward higher power density, higher operating frequency, higher voltage, higher temperature, and higher reliability, substrate materials are required to provide not only mechanical support but also efficient heat dissipation, stable electrical transport, high crystalline compatibility, process scalability, and acceptable manufacturing cost. However, conventional single-material substrates usually fail to satisfy these requirements simultaneously. For example, high-quality single-crystal substrates may offer excellent electrical and crystalline properties but suffer from high cost and limited wafer size, whereas low-cost or highly thermally conductive substrates may be limited by lattice mismatch, thermal expansion mismatch, or poor compatibility with device fabrication. Therefore, composite substrates have become an important strategy for improving the performance and manufacturability of next-generation wide-bandgap semiconductor devices.
Composite substrates are constructed by integrating a high-quality functional layer with a supporting substrate through film transfer, heterogeneous integration, interface reconstruction, and precision surface processing. By separating the functions of the active layer and the supporting layer, this material platform can combine the advantages of different materials and achieve coordinated optimization of thermal, electrical, mechanical, and technological properties. In this review, the main fabrication routes and key technical challenges of composite substrates are systematically summarized. Particular attention is paid to three critical steps: high-quality exfoliation and transfer of functional films, surface-activated bonding for low-temperature heterogeneous integration, and high-precision planarization after transfer. These processes determine the thickness uniformity, defect density, interfacial bonding strength, thermal and electrical transport across the interface, and surface quality required for subsequent epitaxy or device fabrication.
On this basis, recent progress in several representative composite substrate systems is reviewed. For SiC/SiC composite substrates, including single-crystal SiC/polycrystalline SiC, prime-grade SiC/dummy-grade SiC, and different SiC polytype combinations, the main objective is to improve the utilization of high-quality SiC, reduce substrate cost, lower conduction resistance, and support high-performance power devices. For AlN ceramic-based composite substrates, the focus is on combining high thermal conductivity, electrical insulation, and mechanical stability, making them promising for GaN-based power and radio-frequency devices that require efficient heat spreading and large-size manufacturability. For SiC/Diamond composite substrates, the outstanding thermal conductivity of diamond is used to enhance heat extraction from high-power devices, while the SiC layer provides amore compatible platform for epitaxial growth and device processing. These examples show that composite substrates are not simple material stacks, but engineered substrate systems in which the functional layer, supporting layer, and interface must be co-designed.
Finally, the remaining challenges and future directions of composite substrates are discussed. Large-diameter uniform fabrication, controllable interface structure, low thermal and electrical boundary resistance, suppression of residual stress, and long-term reliability under high-temperature and high-power operation remain key issues for practical application. With further progress in film transfer, low-temperature bonding, interface engineering, and precision planarization, composite substrates are expected to play an increasingly important role in wide-bandgap semiconductor devices and related high-performance electronic systems.