The Woodruff School

SUMMARY

The development of high-power GaN HEMTs has created opportunities for thermal engineers to better understand and manipulate nanoscale heat transfer mechanisms in these devices. Current limitations in high-power GaN technologies exist due to thermal resistances in the device structures which impact heat dissipation during operation. Localized joule heating in the HEMT device concentrates a large heat flux in thin device layers. Much effort has gone into growing high-quality material to aid in both electronic and thermal properties but there is still significant research needed into understanding the role of thermal interface resistances in these devices. Often, the thermal boundary resistance (TBR) between the GaN and substrate can be a limiting factor in heat transfer away from the initial generation. 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 and heat spreader a series of bulk CVD diamond samples are evaluated along with thin films that range from 1 to 13.9 µm in thickness. An in-depth study of the impact of the dielectric layer on the TBR for a GaN on diamond device is carried out with CVD diamond that 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, a study of thermal transport in vertical GaN-on-GaN PN diodes was undertaken by evaluating the impact of polyimide passivation on device characteristics and heating. Followed by measurements of the thermal properties of the GaN due to doping effects with TDTR. Finally, full field transient thermal imaging of the device as compared to a thermal and FEM model is used to provide insight into thermal time constants and proper duty cycle operation.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Luke Yates

TIME:	Monday, March 25, 2019, 3:00 p.m.

PLACE:	Love Building, 210

TITLE:	Experimental Methods to Inform Thermal Interface Engineering in High Power GaN Devices

COMMITTEE:	Dr. Samuel Graham, Chair (ME) Dr. Alan Doolittle (ECE) Dr. Satish Kumar (ME) Dr. Asegun Henry (ME) Dr. Shannon Yee (ME)

SUMMARY The development of high-power GaN HEMTs has created opportunities for thermal engineers to better understand and manipulate nanoscale heat transfer mechanisms in these devices. Current limitations in high-power GaN technologies exist due to thermal resistances in the device structures which impact heat dissipation during operation. Localized joule heating in the HEMT device concentrates a large heat flux in thin device layers. Much effort has gone into growing high-quality material to aid in both electronic and thermal properties but there is still significant research needed into understanding the role of thermal interface resistances in these devices. Often, the thermal boundary resistance (TBR) between the GaN and substrate can be a limiting factor in heat transfer away from the initial generation. 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 and heat spreader a series of bulk CVD diamond samples are evaluated along with thin films that range from 1 to 13.9 µm in thickness. An in-depth study of the impact of the dielectric layer on the TBR for a GaN on diamond device is carried out with CVD diamond that 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, a study of thermal transport in vertical GaN-on-GaN PN diodes was undertaken by evaluating the impact of polyimide passivation on device characteristics and heating. Followed by measurements of the thermal properties of the GaN due to doping effects with TDTR. Finally, full field transient thermal imaging of the device as compared to a thermal and FEM model is used to provide insight into thermal time constants and proper duty cycle operation.