Sponsors: DARPA NJTT Program (4 separate contracts), as well as separate contracts with Raytheon, Northrop Grumman, and BAE Systems
Students: Prof. Jungwan Cho (Sungkyunkwan University), Dr. Aditya Sood, Dr. Joe Katz

A variety of modern high-power electronic devices are based on high electron mobility transistors (HEMT) and generate enormous heat fluxes that can approach tens of kW/cm2. The overall power conversion capabilities are strongly limited by nanoscale heat transfer within and near active regions in the functional materials such as GaN and AlGaN. We are working with collaborators to develop novel composite substrates to dramatically reduce temperatures in these devices, including novel composites based on CVD diamond, which has the highest room temperature thermal conductivity. Stanford has a history of work on composite silicon-diamond substrates dating back 20 years, in particular focussing on the interface resistance at diamond boundaries.

For the resurgent interest in diamond composites for HEMT technology, we have developed a comprehensive optical and electrical measurement strategy to capture the thermal transport properties within the complex nanoscale multilayers in exploratory HEMT devices. Breakthrough measurement results include the anisotropic thermal conductivities in GaN functional layers, thermal resistances at diamond interfaces in novel HEMT composite substrates, as well as process-dependent properties of buffer layers. Measurement approaches include picosecond and nanosecond thermoreflectance, in which differing timescales are used to extract properties at varying depths within complex multilayers. We are also using narrow electrical heaters and thermometers, patterned using e-beam lithography down to widths as low as 50 nm, to extract the anisotropic conduction properties. Transmission electron microscopy yields details about interface and material quality, which serve as the basis for simulations of phonon transport. Simulations are also used to show the impact of the properties on HEMT device temperatures.

GaN-diamond scattering
Thermal resistance mechanisms for GaN-diamond composites
GaN-diamond composite for power HEMTs (courtesy Group4 Labs)