By employing a high aluminum doping value, researchers at Nitres Inc., Goleta, Calif., have significantly enhanced the power output and power efficiency of high-electron-mobility transistors (HEMTs) based on gallium nitride (GaN).
The transistors are formed with a metal-organic, chemical-vapor-deposition system. They contain a high mole fraction of aluminum in the aluminum-gallium-nitride (AlGaN) layer—over 30%—rather than the 14% or so used in previous power HEMTs. This results in high-power microwave HEMTs with on-resistance values of about 3 Ω-mm and transconductances as high as 230 mA/mm. To provide topnotch heat removal, the devices were fabricated on silicon-carbide substrates, which allow the transistors to operate at higher power levels.
The epitaxial layer in the HEMTs consists of an insulating GaN buffer and a modulation-doped AlGaN layer. That AlGaN layer supplies a charge for the two-dimensional electron gas while providing a Schottky-gate barrier. Test devices fabricated by the company using optical lithography have gate lengths of about 0.5 to 0.6 µm. These devices were described last month in paper 16.7 at the IEEE International Electron Devices Meeting in Washington D.C.
A test device fabricated with a 50-µm-wide gate handles a maximum current of 66 mA, or about 1.32 A/mm. This is about 50% better than what's achievable with materials using 14% aluminum. The device has a small-signal current-gain cutoff frequency of 30 GHz and a power-gain cutoff frequency of 100 GHz when biased with a drain voltage of 30 V. Such high cutoff frequencies are in agreement with estimates based on the high electron velocity predicted for the material.
When the HEMTs were characterized with large signal swings at 8.2 GHz and with a 100-µm-wide gate, the researchers attained a power density of 9.1 W/mm, along with a 47% power-added efficiency. This power density is the highest for a HEMT FET to date, and it is nine times greater than gallium-arsenide-based FETs in the same frequency band. When biased for class-AB operation and tuned for efficiency, the device can deliver a power-added efficiency of 60% and a power density measuring 6.5 W/mm.
The researchers also fabricated a number of large-periphery devices and flip-chip bonded them onto aluminum-nitride carriers to minimize source inductance and deliver adequate thermal management (see the figure). A 2-mm-wide device was then able to reach an output power of 9.8 W at 8.2 GHz with 44% power-added efficiency. Such a device outperforms, in all aspects, previous 3-mm-wide AlGaN/GaN HEMTs that have a 14% aluminum doping.
Although a 4-mm device was fabricated and tested, it wasn't optimized for matching to the test system. Nevertheless, it was able to achieve a power density of of 11.7 W at 8.2 GHz with a 27% power-added efficiency.
The work was supported in part by funding from the Office of Naval Research and the U.S. Air Force. For more information, contact Nitres at (805) 967-9433, or point your browser to www.nitres.com.