Technology achievements with respect to gallium nitride (GaN) continue to propel this material toward new horizons in the realm of RF devices. Its relatively high electron mobility (440 cm2/(V-s) at room temperature) elevates its footing for more switching and RF power applications at higher breakdown voltages, lower leakage currents, and higher frequencies than competitive semiconductors such as silicon and silicon carbide (SiC). Many research groups have been focusing on improving the heat dissipation properties of GaN through SiC substrates in order to increase its overall efficiency in high-power device applications, and positive developments have benefitted this technical field over the last year.
(Image: Element Six)
US defense contractors such as Raytheon and TriQuint Semiconductor have been leading the race in GaN-on-diamond device fabrication in order to improve the heat-spreading properties of GaN. GaN-on-diamond transistors can achieve areal power densities of nearly four times that of GaN-on-SiC substrate based devices, along with a 50% reduction in thermal barrier resistance between the gate junction and substrate. Ultimately, these properties bolster the commercialization potential of GaN via reduced material costs and cooling architecture for fabrication of GaN-on-diamond transistors in future military radar and commercial cellular and satellite products. GaN-on-diamond has been especially targeted for high-power-density RF devices such as high-electron mobility transistors (HEMTs).
Over the last three years, the GaN-on-diamond technology has been funded by the Defense Advanced Research Projects Agency under the US Department of Defense (DOD) Near Junction Thermal Transport (NJTT) program. Raytheon and TriQuint received millions in funding to enhance their capabilities in this area to develop devices that they could sell to branches of the DOD. Thus, it's a win-win for all parties involved. Last year, these companies announced reductions of nearly 45% in operating junction temperature, along with a 300% increase in the areal RF power density using GaN-on-diamond instead of SiC. In addition, TriQuint reported output power exceeding 5 W/mm with a power-added efficiency of 55% at a 28V drain voltage, which was another impressive accomplishment. These companies have been closing in on NJTT goals of minimizing the thermal boundary resistance between GaN and diamond by eliminating the AlGaN/AlN buffer layers under the GaN electrical transport layer in order to reduce costs and boost performance.
However, for GaN-on-diamond technology to reach its full potential, the quality, mass production, and manufacturing cost of diamond substrates or wafers must meet the challenge. One company in particular is rising to the occasion. A synthetic diamond materials firm called Element Six, a member of the De Beers Family of Companies, is the leader in the growth of diamond substrates. It has supplied the wafers for TriQuint and Raytheon to achieve GaN-on-diamond device milestones.
This past month, Element Six said it has developed a new thermal grade of diamond grown by chemical vapor deposition (CVD). The product, which has been given the trade name Diafilm TM130, has a thermal conductivity of at least 1,300 W/mK and isotropic heat spreading in both planar and through-plane directions, which is well beyond the capacity of materials such as copper. Element Six's solid thermal products are available up to 3 mm thick and in diameters up to 140 mm. They can be laser cut to any required size for customization. Metallization product solutions have enabled die bonding with low thermal barrier resistance that is compatible with soldering and brazing used in standard microelectronic packaging processes.
A robust diamond wafer supply chain is critical for the success of this technology, but very few players have been willing to take the risk to enter this market. The situation is similar to the initial landscape for SiC wafers, which was dominated for decades by Cree. GaN-on-diamond transistors can operate at lower temperatures to enhance overall electrical performance, leading to longer device lifetimes and enhanced reliability. This will inevitably foster more RF device-related defense and commercial applications over the next decade.