Gallium-Nitride (GaN) MMIC circuits have been developed that achieve much greater RF power density compared to GaAs based predecessors. Even greater power densities are possible from these wideband gap devices. However, the requirement to maintain reliable junction temperatures limits both linear and areal power densities. Under the DARPA-sponsored NJTT and ICECool Applications programs, chip-scale thermal management solutions are being developed that combine GaN-on-Diamond with microchannel embedded cooling solutions that enable reliable MMIC operation at heat flux levels of >1kW/cm2 .

We report recent progress on GaN-on-diamond high electron mobility transistors (HEMTs) fabricated using a low-temperature device-transfer process. The devices were first fabricated on a GaN-on-SiC epitaxial wafer and were subsequently separated from the SiC and bonded onto a high-thermal-conductivity diamond substrate at low temperature. The resulting $12 \times 50~\mu \text{m}$ GaN-on-diamond HEMTs demonstrated the state-of-the-art electrical characteristics, including a maximum drain current density of 1.2 A/mm and a peak transconductance of 390 mS/mm. CW load-pull measurements at 10 GHz yielded an RF output power density of 11 W/mm with 51% associated power-added efficiency. Device measurements show that the GaN-on-diamond devices maintained slightly lower channel temperatures than their GaN-on-SiC counterparts while delivering 3.6 times higher RF power within the same active area. These results demonstrate that the GaN device-transfer process is capable of preserving intrinsic GaN-on-SiC transistor electrical performance while taking advantage of the excellent thermal properties of diamond substrates.

We report recent progress on GaN-on-diamond high-electron-mobility transistors (HEMTs) fabricated by low-temperature device transfer. The devices were first fabricated on a GaN-on-SiC epitaxial wafer and were subsequently separated from the SiC and bonded onto a high-thermal-conductivity diamond substrate. The resulting GaN-on-diamond HEMTs demonstrated state-of-the-art electrical characteristics, including a maximum drain current density of 1.2A/mm and a peak transconductance of 390mS/mm. CW loadpull measurements at 10GHz gave an output power density of 11.0W/mm with 51% associated power-added efficiency. Thermal measurements showed the GaN-on-diamond devices maintained equivalent or lower junction temperatures than their GaN-on-SiC counterparts while dissipating 3 times higher power within the same active area. Such results demonstrate that the GaN device transfer process is capable of preserving transistor electrical performance while taking advantage of the excellent thermal properties of diamond substrates.

Self-heating effects severely limit the performance of high-power gallium nitride (GaN) high-electron-mobility transistors (HEMTs). High thermal resistances within micrometers of the transistor junction often dominate the junction temperature rise and fundamentally restrict the device power handling capability. The use of high-thermal-conductivity diamond near the junction can address this thermal limitation, but this approach requires careful attention to the quality of the thermal interface between the GaN and diamond. Here we use time-domain thermoreflectance (TDTR) to measure thermal resistances of thin silicon nitride (SiN) films with varying thicknesses on both diamond and GaN. Measurement of these two sets of samples provides an estimate for the thermal resistance between the GaN and diamond since the SiN film can be used as a bonding layer between the two materials. The effective resistances of the SiN film and bottom interface (SiN/diamond or SiN/GaN) range from 22 to 37 m2 K GW-1 for both sets of samples. Our findings suggest the possibility of achieving 22 m2 K GW-1 as the GaN/diamond thermal interface resistance.

We report the first demonstration of GaN-on-diamond RF power transistors produced by low-temperature substrate bonding technology. GaN high-electron-mobility transistors (HEMTs) are lifted from the original SiC substrate post fabrication and transferred onto high-quality polycrystalline diamond with thermal conductivity of 1,800-2,000 W/mK. Resulting GaN-on-diamond HEMTs demonstrated DC current density of 1.0A/mm transconductance of 330mS/mm and RF output power density of 6.0W/mm at 10GHz (CW). Finite-element thermal modeling indicates GaN-on-diamond technology based on low-temperature substrate bonding is capable of 3X increased power per area compared to conventional GaN-on-SiC devices.