## 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-Diamond Devices
Simulation studies demonstrated that in terms of device temperature rise, an HEMT-on-diamond with a GaN/diamond TBR of <30 m\({}^{2}\) K/GW could outperform an HEMT-on-SiC even with zero GaN/SiC TBR. Therefore, further reduction of the GaN/diamond TBR could enhance the cooling of HEMT-on-diamond devices [73]. In summary, owing to the substantial mismatch in the phonon DOS, a GaN/diamond interface with no interlayer typically has an extremely high TBR, even higher than 200 m\({}^{2}\) K/GW, which is significantly higher than the intrinsic thermal resistance of GaN active layers and diamond substrates, therefore dictating the total thermal resistance of GaN-on-diamond devices. To reduce the high TBR of GaN/diamond interfaces, interlayers, such as Si, SiC, SiN, and AlN, are typically used to enhance the bonding between GaN and diamond to facilitate phonon transmission, although the interlayer itself, with lower thermal conduc \begin{table}
\begin{tabular}{c c c c c c} \hline
**Interface** & **Growth Method** & **Interlayer** & **TBR (m\({}^{2}\) K/GW)** & **Measurement Method** & **Reference** \\ \hline GaN/Diamond & Direct bonding & None & \(\sim\)220 & TTR & Waller [99] \\ AlGaN/Diamond & MPCVD & None & \(\sim\)107 & TTR & Field [100] \\ AlGaN/Diamond & MPCVD & 10 nm SiC & \(\sim\)30 & TTR & Field [100] \\ GaN/Diamond & HFCVD & 46 nm SiN\({}_{\rm x}\) & \(\sim\)52.8 & TDTR & Siddique [101] \\ AlN/Diamond & MPCVD & None & \(\sim\)16 & TTR & Mandala [102] \\ GaN/Diamond & MPCVD & None & \(\sim\)61.1 & TTR & Zhou [103] \\ GaN/Diamond & MPCVD & 5 nm AlN & \(\sim\)15.9 & TTR & Zhou [103] \\ GaN/Diamond & MPCVD & 5 nm SiN & \(\sim\)6.5 & TTR & Zhou [103] \\ GaN/Diamond & MPCVD & None & \(\sim\)41.4 & TDTR & Yates [104] \\ GaN/Diamond & MPCVD & 5 nm AlN & \(\sim\)18.2 & TDTR & Yates [104] \\ GaN/Diamond & MPCVD & 5 nm SiN & \(\sim\)9.5 & TDTR & Yates [104] \\ GaN/Diamond & Simulation & 5 nm Si\({}_{3}\)N\({}_{4}\) & \(\sim\)4.58 & DFT & Huang [105] \\ GaN/Diamond & Simulation & 5 nm AlN & \(\sim\)5.04 & DFT & Huang [105] \\ GaN/Diamond & Simulation & 5 nm Si & \(\sim\)8.48 & DFT & Huang [105] \\ GaN/Diamond & MPCVD & 100 nm SiN & \(\sim\)38.5 & TDTR & Jia [106] \\ GaN/Diamond & MPCVD & 100 nm AlN & \(\sim\)56.4 & TDTR & Jia [106] \\ GaN/Diamond & MPCVD & 28–100 nm SiN\({}_{\rm x}\) & 12–50 & TTR & Sun [107] \\ GaN/Diamond & HFCVD & 31 nm SiN & \(\sim\)31.8 & TDTR & Cho [108] \\ GaN/Diamond & MPCVD & 22 nm SiN & \(\sim\)19.8 & TDTR & Cho [108] \\ GaN/Diamond & CVD & 25 nm dielectric & \(\sim\)27 & Raman & Pomeroy [109] \\ GaN/Diamond & CVD & 50 nm dielectric & \(\sim\)35.7 & Raman & Pomeroy [109] \\ GaN/Diamond & MPCVD & 1 nm Si\({}_{3}\)N\({}_{4}\) & \(\sim\)3.1 & TTR & Malakoutian [110] \\ GaN/Diamond & MPCVD & 100 nm SiN\({}_{\rm x}\) & \(\sim\)40.5 & TDTR & Jia [111] \\ GaN/Diamond & MPCVD & 100 nm SiN\({}_{\rm x}\) & \(\sim\)32.2 & TDTR & Jia [111] \\ GaN/Diamond & MPCVD & periodic term & \(\sim\)32.2 & TDTR & Jia [111] \\ GaN/Diamond & MPCVD & 80 nm SiN\({}_{\rm x}\) & \(\sim\)38.8 & TDTR & Jia [111] \\ GaN/Diamond & MPCVD & 70 nm SiN\({}_{\rm x}\) & \(\sim\)83 & TTR & Wang [112] \\ GaN/Diamond & MPCVD & 35 nm SiN\({}_{\rm x}\) & \(\sim\)26 & TTR & Wang [112] \\ GaN/Diamond & MPCVD & 40 nm SiN\({}_{\rm x}\) & \(\sim\)26 & TTR & Sun [113] \\ GaN/Diamond & MPCVD & 40 nm SiN\({}_{\rm x}\) & \(\sim\)33 & TTR & Sun [113] \\ GaN/Diamond & SAB & Si & \(\sim\)19.2 & TDTR & Cheng [114] \\ GaN/Diamond & SAD & Si & \(\sim\)10.9 & TDTR & Cheng [114] \\ GaN/Diamond & CVD & 50 nm dielectric & \(\sim\)18 & Raman & Dumka [115] \\ GaN/Diamond & Simulation & None & \(\sim\)3 & DMM & Zhou [103] \\ GaN/Diamond & Simulation & None & 16–120 & MD & Tao [117] \\ \hline \end{tabular}
\end{table} 
Table 2: TBR of GaN/diamond interfaces with different interlayers and different growth or integration methods. TBRs predicted using simulation methods are also shown for comparison. tivity, introduces additional extrinsic thermal resistance. Figure 9a shows the effects of the interlayer type on the TBR of the GaN/diamond interfaces.