## 3 Effects of TBR on Thermal Management in GaN-on-SiC and GaN-on-Diamond Devices  
### Importance of Thermal Management in GaN-Based Semiconductor Devices  
Wide-bandgap semiconductors include various materials, such as GaN, Silicon carbide (SiC), gallium oxide (Ga\({}_{2}\)O\({}_{3}\)), and diamond [71]. Among these semiconductors, GaN has gained significant attention in recent years and, particularly, GaN-based high-electron-mobility transistors (HEMTs) have been used widely for RF frequency applications, such as 5G cellular networks, satellite communication, and radar systems [72].  
Initially, AlGaAs/GaAs heterostructures served as the primary channel materials in HEMTs. In recent years, AlGaN/GaN HEMTs have been adopted widely for RF frequency applications. These GaN-based HEMTs have shown significantly higher output powers, thereby surpassing traditional GaAs-based devices by an order of magnitude. Several techniques have been developed to improve the electrical device performance of GaN-based HEMTs, including the mobility, transconductance, cut-off frequency, and output power. However, further improvements have been hindered by significant challenges in thermal management [73]. The performance and reliability of GaN-based HEMTs are limited by the high channel temperature induced by SHE in the device channel [74]. Consequently, to prevent significant thermal damage and ensure long-term reliability, the power dissipation of the current GaN-based HEMTs is reduced for operation at 5-6 W/mm in functional systems, although the theoretical power density can exceed 40 W/mm [75].  
The GaN-based HEMTs are conventionally grown on substrates, such as Si, sapphire, and SiC. Si substrates are preferred because of their low-cost mass production. Sapphire substrates are typically employed for GaN-based LEDs. However, the low thermal conductivity of sapphire limits its suitability for high-power applications. SiC substrates are significantly more expensive than Si and sapphire substrates. However, the smaller lattice mismatch (3.8%) and smaller thermal expansion coefficient mismatch (3.2%) between GaN and SiC enable the growth of GaN-based structures with good crystalline quality, making SiC widely used substrates for power electronic applications. The thermal conductivity of the substrate materials is a key factor to consider in the fabrication of GaN-based semiconductors. The thermal conductivity of the substrate material plays a crucial role in limiting the output power densities of GaN-based HEMTs because the joule heat generated by the SHE is primarily dissipated through the substrate. The thermal conductivities of Si, sapphire, and SiC are approximately 130, 30, and 490 W/mK, respectively [76]. Accordingly, SiC substrates are conventionally used for fabricating high-power AlGaN/GaN HEMTs. Currently, GaN-on-SiC devices are the standard for high-power RF applications [77]. Nevertheless, SiC substrates remain inadequate for fully harnessing the total potential of GaN. Among all known natural materials, diamond exhibits the highest thermal conductivity, ranging between 800 and 2000 W/mK, depending on the growth conditions. Diamond has attracted significant interest as a substrate material owing to its exceptional ability to efficiently dissipate heat, surpassing that of SiC [78]. The GaN-on-diamond shows a three-fold increase in the power density and lower junction temperatures compared with that of a GaN-on-SiC device. However, when GaN is integrated with these substrate materials, the presence of a high TBR at the GaN/substrate interface, which could exceed the intrinsic thermal resistance of the substrate materials, could offset the advantages offered by these high-thermal-conductivity substrates. It has been reported that up to 50% of the channel temperature in AlGaN/GaN HEMTs derives from the TBR of GaN/SiC interfaces for high-quality heteroepitaxy [79]. Therefore, several advanced experimental techniques have been developed to lower the TBR and fully exploit the high thermal conductivity of these substrates.