## 3 Effects of TBR on Thermal Management in GaN-on-SiC and GaN-on-Diamond Devices
### Effects of TBR on Thermal Management in GaN-on-SiC Devices  
[PERSON] et al. first compared the SHE of AlGaN-GaN HFETs grown on sapphire and 6H-SiC substrates. These authors found that heat dissipation strongly affects the device characteristics soon after the application of the source-drain voltage. Thermal resistance is determined primarily by the substrate rather than the AlGaN-GaN active device layer. However, the TBR between the active device layer and substrate was not measured in this study [80]. [PERSON] et al. [81] experimentally investigated and compared the TBR of GaN, Si, SiC, and sapphire substrates. Heterostructures of AlGaN/GaN were grown on different substrates using metal-organic chemical vapor deposition (MOCVD) systems. An AlN nucleation layer of unknown thickness was used. The effective TBR at the GaN/Si interface measured by employing Raman spectroscopy was \(\sim\)70 m\({}^{2}\) K/GW, which is in good agreement with the value measured by the transient interferometric mapping (TIM) method [82]. The effective TBR at the GaN/SiC interface was \(\sim\)120 m\({}^{2}\) K/GW. However, estimating the TBR at the GaN/sapphire interface is difficult because of the relatively low thermal conductivity of sapphire substrates. The authors assumed that the thermal expansion coefficients, substrate roughness, and defects related to the growth techniques influenced the TBR values. Moreover, they investigated the role of the TBR values for different substrates by calculating the steady-state temperature profiles in the cross-sections of the devices. Using the measured TBR, the highest surface temperature was obtained for GaN grown on sapphire (810 K), and the lowest value was obtained for the SiC substrate (420 K). Employing Raman spectroscopy, [PERSON] et al. [83] investigated the TBR of GaN devices grown on various substrates. The effective TBRs of both GaN/Si and GaN/SiC interfaces were determined as \(\sim\)33 m\({}^{2}\) K/GW. For the GaN device on the sapphire substrate, the TBR was estimated at approximately 120 m\({}^{2}\) K/GW. The determined effective TBR is a combined effect of the TBR because of the phonon mismatch and the reduced thermal conductivity of the GaN layer. Furthermore, the effects of the TBR on the temperature increase in ungated AlGaN/GaN field-effect devices were also investigated. The TBRs of the GaN/Si and GaN/SiC interfaces resulted in an increase in the GaN channel temperature by about 30% and 10% for GaN/SiC and GaN/Si devices, respectively, with respect to the case of negligible TBR at the interface. In comparison, despite the high TBR of GaN/sapphire,the influence of TBR on the temperature increase is much smaller at about 2-4%, which is attributed to the low thermal conductivity of the sapphire substrate. [PERSON] et al. [84] measured the effective TBRs of GaN on Si and SiC substrates with 38 nm and 36 nm thick AlN transition layers, respectively. The measured effective TBRs of the GaN/Si and GaN/SiC interfaces were \(\sim\)7.8 and \(\sim\)5.3 m\({}^{2}\) K/GW, respectively. An approximate solution to the phonon Boltzmann transport equation was used to present a comprehensive model for the effective resistance of the AlN film, indicating that a combination of point defects within the AlN transition layer and near-interfacial defects could be responsible for the effective TBR. [PERSON] et al. [85] conducted a systematic study on the impact of SiC substrate surface pretreatment on the crystalline quality of subsequently grown AlN nucleation layers and GaN buffer layers in an HEMT structure. These authors also investigated the effects of the structural properties on the TBR using Raman thermography, including the surface of the SiC substrate, as well as the crystalline quality, morphology, and thickness of the AlN nucleation layer.