Switching faces improves high-speed HEMTs
Researcher Umesh Mishra from the University of California, Santa Barbara, opened the International Conference on Nitride Semiconductors by highlighting the performance of GaN HEMTs and detailing approaches to take these transistors to a new level.
He kicked off his talk at the Scottish Exhibition and Conference Centre in Glasgow, UK, by saying that LEDs had made great progress in recent times, and now it was the turn of the electronics sector to make significant commercial headway.
Mishra then provided a brief overview of where the RF performance of HEMTs stands today: 13.7 W/mm at 30 GHz, 10.5 W/mm at 40 GHz, and 2.4 W/mm at 60 GHz. He also reminded the audience of the records for the fastest devices – a cut-off frequency of 220 GHz and a maximum oscillation frequency of 400 GHz.
“I'm convinced that you will see 500 GHz in the next five years. It will happen,” added Mishra, who explained that only a few years ago such frequencies were unthinkable.
However, he pointed out that these devices have a major weakness – the gain falls fast as frequency rises. This stems from parasitic capacitances and resistances.
According to Mishra, the best way to address these issues is to switch from a conventional HEMT, which is produced using the gallium face, to a variant based on the nitrogen face. Making contacts to this novel device is easier, and drastically reduces the contact resistance.
One downside of HEMTs with a nitrogen face is that they have traps at the interface between AlGaN and GaN. However, by doping this region, it is possible to lift these trap levels out of harms way.
Mishra's team have built MBE-grown nitrogen-face devices with optically defined gates that produce 5.7 W/mm at 10 GHz. And MOCVD-grown variants on off-cut SiC deliver 20.7 W/mm at 4 GHz, and 16.7 W/mm at 10 GHz.
The team has started to move to higher speeds, which is where the benefits of the nitrogen-face really come into play. Initial results include a cut-off frequency of 163 GHz.
The West-coast academic also spoke about the promise of GaN transistors for digital applications – his team has produced a device with an InAlN back barrier and a 60 nm gate length that produces 2.77 mA/mm.
Activities at Transphorm, the company that Mishra co-founded in 2007, were briefly covered towards the end of the presentation. Normally-off HEMTs produced by this start-up are converted into a normally-on mode with an additional circuit, and can be used to make products for power switching. This includes three-phase inverters for photovoltaics that can operate at efficiencies of more than 98 percent.
