Thermal Simulations in GaN HEMTs Considering the Coupling Effects of Ballistic-Diffusive Transport and Thermal Spreading

The self-heating effect during the operation of gallium nitride (GaN)-based high electron mobility transistors (HEMTs) can result in severe heat-dissipation issues, such as local hot spots, limiting the electrical performance and decreasing the lifetime of GaN devices. To develop effective thermal management strategies, accurate prediction of temperature fields in the near-junction region is essential, where phonon ballistic-diffusive transport due to the microscale layer size and the thermal spreading effect caused by small heat sources are coupled. In this work, the heat conduction characteristics in the near-junction region of GaN HEMT are systematically investigated by finite-element analyses with a GaN thermal conductivity model considering the coupling effects of ballistic-diffusive heat conduction and heat spreading. The simulation results show that, as the thickness of the GaN epitaxial layer increases, the junction temperature initially decreases and then increases, consistent with the rules under the condition considering ballistic-diffusive transport only. In addition, the optimum thickness of the GaN layer will increase with an increase in the interfacial thermal resistance (ITR). However, when considering the coupling effects, the optimal thickness of the GaN layer in the single-gate HEMT decreases significantly to $8.0~\mu \text{m}$ , which is smaller than that in the condition without considering the coupling effects ( $17~\mu \text{m}$ ). Two typical methods for near-junction thermal management, i.e., using a diamond substrate with high thermal conductivity and adopting nanocrystalline diamond capping (NDC) layers, are also investigated and compared. The investigation shows that both can effectively improve the temperature field in the near-junction region. Specifically, the application of the diamond substrate performs worse than the silicon carbide (SiC) substrate for the condition where the GaN layer thickness is equal to $1.2~\mu \text{m}$ due to the high ITR existing in GaN-on-diamond HEMTs. However, with the increase of the thickness of the GaN layer, the high thermal conductivity of the diamond substrate could further decrease the junction temperature and outperform SiC when the GaN layer thickness is greater than $2~\mu \text{m}$ . The adoption of the NDC layer can decrease the thermal resistance ratio of the GaN layer by about 9%, compared to that in the case of a diamond substrate, by enhancing the horizontal heat transport above the AlGaN layer. Different from the results in the literature, there is also an optimal thickness for the NDC layer. The comparison between these two methods shows that the utilization of the NDC layer is more effective in reducing the junction temperature than the use of the diamond substrate, especially when the ITR is high.