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Diamond substrate unleashes GaN potential

Posted: 30 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:GaN? diamond substrate? GaN-on-diamond? CVD? heat spreader?

Innovation in electronic deviceshigher complexity and power density in smaller sizesis slowed by significant hurdles in thermal management. While Gallium Nitride (GaN) offers an impressive set of intrinsic characteristics that help RF transistors set a new benchmark in performance, heating in channel when the RF chip operates near its peak power output degrades device lifetime. The true potential of GaN-based devices has yet to be unleashed.


To break through this barrier, Element Six has developed a higher thermal conductivity substrate that more effectively extracts heat out of a transistor-based device: directly deposited chemical vapour deposition (CVD) diamond.

Diamond is the best commercial heat-spreading material in the world. It can have a room temperature thermal conductivity up to 2,000Wm-1K-1, which is four-to-five times that of Silicon Carbide (SiC). As a substrate, diamond can be deposited to within hundreds of nanometres of the GaN channel, where it can efficiently extract heat out of the transistor based device.

With this development, GaN-on-diamond devices may replace the more commonly used traveling wave tubes and GaN-on-SiC devices for applications including defence radar, electronic warfare systems, cellular base stations and weather and communications satellites. Simulations, modelling and experiments all illustrate the promise of GaN-on-diamond. Researchers from various groups, including those assembled by the Defense Advanced Research Projects Agency (DARPA), have determined that such transistor-based devices can operate at reduced channel temperatures and are capable of delivering about three times the areal power density of GaN-on-SiC RF power amplifiers.

GaN-on-diamond wafers

GaN-on-diamond wafers (see figure 1) are formed by starting with an epitaxial stack on top of silicon that includes a 1.2?m thick proprietary transition layer, an 800nm-thick un-doped GaN buffer layer, a 17nm-thick Al0.26Ga0.74N Schottky barrier and a 2nm-thick GaN cap layer. A pair of additional layers is added by first removing the host silicon substrate and transition layers beneath the AlGaN/GaN epitaxy, then depositing a 35nm-thick proprietary dielectric onto the exposed AlGaN/GaN, and finally growing a 100?m-thick CVD diamond substrate onto the films.

GaN-on-diamond wafers

Figure 1: GaN-on-diamond structures are formed by bonding the GaN face to a temporary carrier, etching away the substrate and transition layers, depositing a 35nm-thick dielectric and then a 100?m diamond layer on the backside of GaN, and finally removing the temporary carrier.

The real test for GaN-on-diamond is whether it delivers improvements to transistor performance, which is demonstrated through the following two studies: the first compares GaN-on-diamond and GaN-on-silicon high electron mobility transistors (HEMTs); and the second compares device performance to GaN-on-SiC HEMTs, the industry's prevailing GaN technology.

Engineers at the U.S. Air Force Research laboratory (AFRL) independently investigated whether Element Six's GaN epi-flip and diamond deposition process is detrimental to GaN epitaxy and if it can cause any deterioration in device performance. Their examination involved the analysis of thousands of GaN-on-diamond and GaN-on-silicon HEMTs.

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