The achievement of significant performance from GaN heterojunction field effect transistors was presented by researchers from Cornell University, the University of California at Santa Barbara, a team from APA Optics and the University of South Carolina, and Hughes Research Laboratory (now HRL).
The Cornell results included .15 µm gate undoped Al.3Ga.7N/GaN/sapphire piezoelectric HEMT's with t = 67 GHz and max = 140 GHz, .30 µm gate devices with 12.8 db gain, 1.4 W/mm power and 74% optimized power-added efficiency at 3 GHz. They presented data showing that the drain-source breakdown voltage rose nearly linearly with gate length, being 70 V for .33 µm gate length. They also presented a theoretical result explaining the dependence of the mobility of HEMT electrons on dislocation density and electron sheet density.
The UCSB results include 2 W/mm at 6 GHz with 33% power-added efficiency, 2.57 W/mm power at 31% power-added efficiency with 5.1 db gain at 10 GHz with .7 µm gate length, and 3 W/mm at 18 GHz with .25 µm gate length. With a 1 µm gate length, they had 225 V breakdown between drain and source.
S. Carolina and APA Optics used doped channel and doped barrier Al.25Ga.75N/GaN HFET's, with electron mobility of 1,400 cm2/V-s and electron sheet density of 1.5 x 1013/cm2. They obtain 2.3 W, for 1.28 mm periphery, at 1 GHz. With .25 µm gates they got t = 37.5 GHz, and max = 80 GHz. With .7 µm gates on SiC substrates they got 2.8 W/mm with 17% power-added efficiency at 10 GHz.
HRL used MBE grown materials, as opposed to the OMVPE materials used by the others above. They achieved ~ 1,000 cm2/V-s electron mobility in Al.15Ga.85/GaN structures. They obtained .5 W/mm, with 55% power added efficiency at 4 GHz, in spite of some problems with traps. They had succeeded in flip-chip bonding for improved thermal management.
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