GaN has emerged as the material of choice for advanced power amplifier devices
for both industrial and defense applications but near-junction thermal barriers
severely limit the inherent capability of high-quality GaN materials. Recent
“embedded cooling” efforts, funded by Defense Advanced
Research Projects Agency Microsystems Technology Office (DARPA-MTO), have
focused on reduction of this near-junction thermal resistance, through the use
of diamond substrates and efficient removal of the dissipated power with
convective and evaporative microfluidics. An overview of the accomplishments of
the DARPA Near-Junction Thermal Transport (NJTT) program and recent results from
the on-going DARPA Intra-Chip Embedded Cooling (ICECool) program are provided.
It is shown that growth or bonding of diamond to GaN epitaxy has enabled a
3-5× increase in power handling capability per transistor unit area,
while use of microfluidic cooling has enabled heat fluxes of 30
kW/cm2 at the transistor level and 1 kW/cm2 at the
die-level, for a 3-6× improvement in the total RF output power of GaN
power amplifiers. These demonstrations provide near-term validation of the large
improvement in output power gained through embedded cooling and confirm the
potential for well above a 6× improvement in GaN power amplifier output
power to the electrical, rather than thermal, limits of GaN.