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Thermal-boundary resistance (TBR) is present at the interfaces between different materials due to mismatch in phonon density of states. When GaN is grown on silicon or silicon carbide, or when chemical-vapor deposited diamond is grown on GaN, the TBR of the interface between the GaN epilayers and the substrate can contribute significantly to the overall thermal resistance of electronic devices. However, the buffer layer in an AlGaN/GaN high-electron mobility transistor (HEMT) offers a certain degree of heat spreading when placed above the thermal boundary potentially offering a reduction in overall thermal resistance if its thickness were optimized. We analyze heat flow in a typical AlGaN/GaN HEMT on different substrates and show that optimizing the buffer layer leads to lower thermal resistance of the electronic device.