## 4 Conclusions and Outlook  
Starting with the SHE-induced temperature rise problem, the effects of TBR on thermal management in GaN-based semiconductor devices were reviewed in this study. The simulation and measurement methods for the TBR were also reviewed. With the rapid dimensional scaling and development of advanced technologies, such as GAAFET and 3D IC architectures, the SHE has become more pronounced and significantly affects the performance and reliability of semiconductor devices. The feature sizes of these semiconductor devices have reached a point where they are comparable with or even smaller than the mean free paths (MFPs) of the heat carriers. Consequently, the overall thermal resistance of these deeply scaled semiconductor devices is primarily determined by TBR. For GaN-based semiconductor devices, such as GaN HEMTs, SiC and diamond are good candidates for thermal management because of their ultrahigh thermal conductivities. However, the presence of a high TBR at GaN/SiC and GaN/diamond interfaces could offset the advantages of their high thermal conductivities. Currently, up to 50% of the channel temperature in GaN-based HEMTs is ascribed to the TBR between GaN and SiC or diamond substrates. Therefore, several advanced techniques have been developed to lower the TBR and fully exploit the high thermal conductivity of SiC and diamond substrates. Here, we comprehensively summarize the experimental and theoretical TBR values reported in previous studies. For GaN-on-SiC devices, AlN is typically used as a transition layer to relieve stress owing to the large lattice mismatch and thermal expansion coefficients between the GaN layer and SiC substrate. We found that the effective TBRs of GaN/SiC interfaces differed significantly in the range of 5-100 m\({}^{2}\) K/GW. Furthermore, the TBRs of GaN/SiC interfaces measured by different research groups showed no dependence on the thickness of the AlN transition layer, although the same growth methods were used. This finding indicates that the intrinsic thermal conductivity of the AlN transition layer makes a small contribution to the overall thermal resistance of GaN-on-SiC devices. In comparison, point defects, dislocations, and other disorders introduced by AlN transition layers related to growth techniques significantly affect the TBR values by scattering phonons. Therefore, a thin AlN transition layer with fewer interfacial defects and disorders is the optimal choice for reducing TBR. However, optimizing the growth conditions to minimize the TBR remains a significant challenge. As an alternative to MOCVD, room-temperature SAB is a promising method for reducing TBR by directly bonding SiC and diamond without an interlayer. An extremely low TBR of \(\sim\)4.3 m\({}^{2}\) K/GW has been achieved using the SAB method, which is close to the values predicted by DMM. However, even though the TBR of the GaN/SiC interface is reduced to zero, the SiC substrate is less competitive than the diamond substrate with a low TBR in terms of the device temperature rise. Therefore, the TBR of the GaN/diamond interface should be reduced to further unlock the potential of GaN-based semiconductor devices. For GaN-on-diamond devices, Si, SiC, SiN, and AlN interlayers have been used to enhance bonding between the GaN and diamond substrates to facilitate phonon transport across the interface. The superior candidate for reducing the TBR is SiN because of the smoother interface and stronger Si-C bond formed between diamond and SiN. Differing from the AlN interlayer in the GaN-on-SiC devices, the effective TBRs were found to scale with the thickness of the SiN interlayer in the GaN-on-diamond devices. This finding is attributed to the low thermal conductivity of the amorphous SiN interlayer (\(\sim\)1 W/mK). Therefore, reducing the thickness of the SiN interlayer is an effective method to reduce the TBR of the GaN/diamond interface.