*4.4. Challenges of Capping GaN HEMTs with Diamond*

The capping of the HEMT devices with a diamond film is the technically simplest approach. In theory, the GaN HEMTs can be capped with the diamond film after the passivation steps without any changes in the fabrication procedure. However, due to the nature of the diamond CVD process, the diamond-capped HEMTs face some of the same issues as the GaN-on-diamond wafers.

On one side, the passivation layer, though preventing the degradation of III-nitride layers in the harsh CVD environment, contributes with a non-negligible *TBR* which hampers the flow of heat towards the diamond heat spreader. On the other side, the quality—and hence the κ—of the diamond film close to the interface also play a critical role. In this sense, the discussion presented in Sections 4.1.2 and 4.1.3 holds valid for the capping diamond approach. In addition, the top AlGaN barrier layer features a low κ (Table 2) and introduces a non-negligible *R*th between the hot spot and the capping diamond layer.

Finally, thermal stresses will inevitably accumulate at the GaN/diamond interface due to the *CTE*s mismatch. Any change in the stress−strain state in AlGaN/GaN heterostructures, especially in the fully strained pseudomorphically grown AlGaN barrier layer, would have a significant impact on the 2DEG characteristics. Therefore, the proper understanding of the impact the stress-strain state induced by the diamond CVD on the 2DEG characteristics will require a thorough evaluation of the barrier layer stress.

Nevertheless, these limitations did not prevent Fujitsu from developing diamond capped GaN HEMTs with improved heat dissipating capabilities, meaning that capping HEMTs with diamond may provide a valuable way of improving the thermal management of these devices.
