A new device-first low-temperature bonded gallium nitride (GaN)-on-diamond
high-electronic mobility transistor (HEMT) technology with state-of-the-art,
radio frequency (RF) power performance is described. In this process, the
devices were first fabricated on a GaN-on-silicon carbide (SiC) epitaxial wafer
and were subsequently separated from the SiC and bonded onto a
high-thermal-conductivity diamond substrate. Thermal measurements showed that
the GaN-on-diamond devices maintained equivalent or lower junction temperatures
than their GaN-on-SiC counterparts while delivering more than three-times higher
RF power within the same active area. Such results demonstrate that the GaN
device transfer process is capable of preserving intrinsic transistor electrical
performance while taking advantage of the excellent thermal properties of
diamond substrates. Preliminary step-stress and room-temperature, steady-state
life testing shows that the low-temperature bonded GaN-on-diamond device has no
inherently reliability limiting factor. GaN-on-diamond is ideally suited to
wideband electronic warfare (EW) power amplifiers as they are the most thermally
challenging due to continuous wave (CW) operation and the reduced power-added
efficiency obtained with ultra-wide bandwidth circuit implementations.
High-performance 0.1-µm InAlN/GaN high electron-mobility transistors (HEMTs) have been successfully developed for power amplifiers operating at E-band (targeting 71-76 and 81-86-GHz bands). High maximum drain current of 1.75 A/mm and maximum extrinsic transconductance of 0.8 S/mm have been achieved for depletion-mode devices. Enhancement-mode HEMTs have also shown maximum drain current of 1.5 A/mm and maximum extrinsic transconductance of 1 S/mm. The selection of atomic layer deposition aluminum oxide (Al 2 O 3 ) for device passivation enables a two-terminal breakdown voltage of ∼25 V, excellent subthreshold characteristics as well as the pulsed-IV featuring little current collapse for both types of HEMTs. When biased at a drain voltage of 10 V, a first-pass two-stage power amplifier design based on 0.1-µm depletion-mode devices has demonstrated an output power of 1.43 W with 12.7% power-added efficiency at 86 GHz, a level of performance that has been attained previously only by state-of-the-art counterparts based on AlGaN/GaN HEMTs at a much higher drain bias and compression level.
We report the first demonstration of GaN-ondiamond 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). Finiteelement 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.
Under the DARPA-sponsored ICECool Applications program, a microchannel cooling system using a 50-50 ethylene glycol-water mixture was optimized for cooling a high-power GaN-on-Diamond Monolithic Microwave Integrated Circuit (MMIC). Automated multi-objective optimization of the microchannel passages yielded an optimized design with a predicted thermal resistance of 22.4 K·cm2/kW at a pressure drop of only 121.4 kPa for an inlet temperature of 40°C. These values were corroborated by a coupled thermofluid analysis that included a detailed treatment of both the gate region and microchannel cooling geometry. Several versions of prototype coolers were fabricated, with one set consisting of pairs of coolers joined at their heated faces. These cooler pairs were used in heat exchange tests to characterize the average thermal resistance and the flow performance of the coolers. The performance testing results were consistent with the analytic predictions. Based on the analytical and experimental results, the system may be operated at inlet temperatures as high as 65°C without exceeding the transistor junction temperatures of 240 °C required for 106 hour mean-time-to -failure. The higher inlet temperature ameliorates system penalties associated with rejection of waste heat to ambient heat sinks.
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