GaN-on-diamond makes RF progress

November 03, 2014 // By Peter Clarke
GaN-on-diamond makes RF progress
Brian Coppa discusses progress with gallium nitride as a material for radio frequency devices.

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/Vs 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.

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 percent 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).

Comparison of ower density for GaN-on-SiC and GaN-on-diamond. Source: Element Six.

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 of dollars in funding to enhance their capabilities in this area and 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 percent in operating junction temperature, along with a 300 percent 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 percent 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.

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