Thermal management of GaN-on-diamond high electron mobility transistors: Effect of the nanostructure in the diamond near nucleation region

The integration of diamond in ultra-high power GaN HEMT devices has demonstrated to be a very promising strategy to increase the device lifetime and their thermal management. Typically polycrystalline diamond films rather than single crystal diamond are used for this purpose, however for this material the thermal transport in the near-nucleation site is strongly affected by the small grain size and the accumulation of defects in this region. Here we modeled the phonon thermal transport in diamond, including the effect of the polycrystalline structure, showing that its thermal conductivity exhibits very different properties to those observed in single crystal diamond; namely, the thermal conductivity is severely reduced, the grain structure may induce anisotropy in the heat conduction and also a strong variation of the thermal conductivity from the nucleation and following the diamond growth direction is observed. All these features are included in a full thermal model of a GaN high power amplifier, showing their impact on the thermal management of the device. We show that including the full description of the polycrystalline diamond thermal conductivity is fundamental to accurately assess the thermal management of these devices, and thus to optimize their design.