Fujitsu plans GaN-on-Si production this year
Fujitsu is to produce GaN power transistors this year, and has demonstrated a 2.5kW server power supply using GaN-on-silicon devices.
GaN power transistors are fast that silicon types and have low on-resistance.
“Compared with conventional silicon-based power devices, GaN-based power devices feature lower on-resistance and the ability to perform high-frequency operations,” said Fujitsu. “Since these characteristics improve the conversion efficiency of power supply units and make them more compact, this technology is ideal not just for servers but for a broad range of applications including solar inverters, battery chargers or electric vehicles.”
However, traditionally they had to be grown on expensive sapphire or silicon carbide substrates, pricing them out of PSU use.
Now several firms have found ways to make them on cheaper silicon substrates – International Rectifier was first, using its transistors inside its own products. Other firms are start-up EpiGan and Plessey, the latter which is attempting to use GaN-on-Si for lighting LEDs.
Growing GaN on silicon requires the use of lattice matching buffer layers that can impart stress to the wafer causing it to bow – an effect which is exacerbated in larger wafers.
Consistently flat 8in GaN-on-Si wafers are proving a tough nut to crack amongst firms hoping to use the technology. Fujitsu is playing it safe and going into production on 6in wafers at its Aizu-Wakamatsu plant.
The firm began work on GaN technology in 2009 and provided samples to PSU firms in 2011. “Since then, we have worked on optimising the devices for use in PSUs and will start volume production of the GaN power devices by the second half of 2013,” said the firm.
Fujitsu should be good at this as, rather than use mosfet or bipolar structures, the HEMT (high electron mobility transistor) structure appears to be dominating for PSU use, and HEMTs were invented by Fujitsu.
“Fujitsu Semiconductor collaborated closely with Fujitsu Laboratories on several key technical initiatives to achieve this technological progress, including the development of the process technology for growing high-quality GaN crystals on a silicon substrate,” said the firm. “The collaboration also delivered device technologies, such as optimising the design of electrodes to control the rise of on-resistance during switching, and devising a circuit layout for power supply units that can support high-speed switching of GaN-based devices.”
In the demonstration server PSU, the firm used GaN in the power factor correction front end where it increased overall efficeincy by 0.2% at high power compared with an all-silicon version.