SUMMARY

The development of high-power GaN HEMTs has created a significant opportunity for thermal engineers to better understand and manipulate nanoscale heat transfer mechanisms. This is because current limitations in high-power GaN technologies exist due to thermal effects in the device structures. The localized joule heating in the HEMT device concentrates an extremely large heat flux in thin device layers. While much effort has gone into growing higher quality material to aid in both electronic and thermal properties, there is still significant research needed into understanding the role of thermal interface resistances inherent in these devices. Many times the thermal boundary resistance (TBR) between the GaN and substrate can be a limiting factor in heat transfer away from the initial generation. Depending on the application different substrates can be used during HEMT fabrication. Power devices are often grown on silicon substrates in order to take advantage of large area wafers and reduce cost. High-power RF devices however attempt to place high thermal conductivity substrates (such as diamond) close to the heat generation to remove heat as quickly as possible. In order to thoroughly investigate the impact interfacial layers have in these devices, experimental methods are used to evaluate the TBR for a series of GaN-on-Si wafers with varying interlayers. A complete analysis of material strain, TBR, and finally thermal device impact is presented. To examine the effectiveness of using a high thermally conductive material as a device substrate, CVD diamond is grown on GaN using either an AlN or SiN interlayer and the TBR is evaluated as compared to a material system with no interlayer. Finally, the concept of thermal interface engineering through a phonon bridge is explored using the Debye temperature ratios as the guiding principle on a series of samples consisting of a GaN/AlN/Al and diamond/AlN/Al structure.