At the Advanced Power Electronics Conference (APEC) in Fort Worth in mid-March, much of the buzz on the tradeshow floor focused on three announcements of super-efficient MOSFET devices made from gallium nitride (GaN) or silicon carbide (SiC).
International Rectifier showed the performance of dc-dc modules it had previously announced and demonstrated 600-V devices that spokesman Mike Briere said would be available later this year. Cree, with a good decade of experience with SiC in its LEDs, demonstrated 1200-V SiC MOSFETs with 80-mΩ channel resistance, switching 20 A at 32 kHz. (The parts soon to be available will be rated at 1200 V. The MOSFETs in the demo actually were rated for 1700 V.)
Transphorm, a startup that had been operating in stealth mode up to showtime, announced that it was sampling 600-V GaN switches and diodes. For a demo, Transphorm was running a 220- to 400-V boost converter at 100 kHz and achieving 99% efficiency, according to spokesman Carl Blake. Transphorm’s largest financial backer, Google, has said that it looks to GaN as one aspect of its efforts to reduce datacenter electricity costs, giving a big boost to Transphorm’s credibility.
EPC, which has been shipping GaN devices for the past year, was absent from the conference. Subsequently, CEO Alex Lidow said that EPC was well into the second generation of its 40- and 100-V eEPC FETS and offered an appnote detailing performance improvements (see http://epc-co.com/epc/documents/product-training/Characteristics_of_Second_Generation_eGaN_FETs.pdf).
Rohm, another supplier of SiC MOSFETs, was at the conference but not speaking for publication. Nevertheless, the company would announce new SiC Schottky barrier diodes (SBDs) by the end of the month. (Rohm is well known for a very conservative approach to new product announcements.)
So why are power-supply engineers (and their customers in the data business) excited about SiC and GaN? Advanced semiconductor materials, such as SiC and GaN, can significantly reduce switching and conduction losses, raise blocking voltages and switching frequencies, and enhance the stability of electrical characteristics over a wider temperature range.
This comes at a price premium, because these wafers are smaller than the wafers used to make silicon power devices, and the fab processes are more difficult. But the industry seems confident that the availability of commercial products will drive demand, increasing volumes. In turn, this will drive manufacturing breakthroughs, ultimately leading to SiC and GaN prices closer to those for silicon products.
Like SiC, GaN is a wide-band-gap semiconductor material with high heat capacity and thermal conductivity. In building MOSFET devices and diodes, it is applied as an epitaxial layer on top of silicon carbide.
While MOSFET devices were the center of attention at APEC this year, SBDs have been breaking trail for MOSFETs since around the middle of the last decade. According to Rohm’s David Doan, SiC SBDs have lower reverse recovery time (RRT) and forward voltage drop (Vf) than silicon fast recovery (fast recovery epitaxial), ultra-fast recovery, and super-fast recovery diodes.
In terms of breakdown voltage (VBR), SiC SBDs, with a maximum VBR of 600 V, fall below silicon standard-recovery, fast recovery epitaxial, and ultra-fast diodes (1 kV). They also match silicon super-fast diodes and exceed plain-vanilla and silicon SBDs.
From a materials science standpoint, the advantages of SiC over silicon include three times the band gap and thermal coefficient and 10 times the dielectric breakdown field. SiC has also demonstrated temperature stability over a wide operating range. This simplifies the parallel connection of multiple devices and prevents thermal runaway.
Finally, what happened to gallium arsenide (GaAs)? Although GaAs power diodes have been available for several years, GaAs was not conspicuous at APEC this year. According to Rohm, its band gap, breakdown field, and thermal conductivity are all lower than SiC.
To hear directly from the companies that were at the conference, visit Engineering TV’s APEC coverage at www.engineeringtv.com/playlist/APEC-2011.