See also: Fujitsu GaN HEMT delivers 320W in C band
Fujitsu Laboratories has invented a HEMT (high electron mobility transistor) that turns off without a negative gate bias, allowing circuits to draw 'zero' power without a supply switch.
"Thus far, no compound semiconductor transistor had been able to both generate 100 W of power output, and cut power without requiring the addition of negative voltage," said Fujitsu.
"A transistor structure was developed that achieves an output of over 100W and enables power to be cut when the transistor is in stand-by mode. In addition, because the design of the control circuits used in previous transistors has been very complex, they required more power to be consumed."
HEMTs, mosfet-like devices which can be made with compound semiconductors, were invented at Fujitsu Laboratories in 1979 by Dr Takashi Mimura. They now underpin much of today's fundamental wireless infrastructure.
Together, the addition of an aluminium nitride layer on top of the n-type GaN layer that tops conventional HEMTs (see diagram below), and the design of the gate electrode, which is formed after removing the AlN layer, increase the density of carrier electrons when on - increasing current output capability - and decreases the density of carrier electrons around the region below the gate electrode when off - preventing current from flowing even in the absence of a negative gate voltage.
If the topmost layer is made of AlN, the surface becomes cracked (see photo below) which degrades the breakdown voltage.
This issue has been resolved by adding a n-type GaN-based layer above the AlN, resulting in a three-layer cap structure that improves the surface roughness, raising output and reliability.
The firm aims to have the low-standby HEMT in wireless systems around 2010.
A conventional HEMT, and Fujitsu's HEMT (right) that switches off without a negative gate bias.
Fujitsu's new HEMT operates at higher voltages because surface cracking it reduced by a three layer cap structure (right)
Carrier transfer characteristics of the newly-developed GaN HEMT. The addition of an AlN layer increased the volume of electrons transmitted by 60%.
Drain current and breakdown voltage benchmarks of the low standby GaN HEMT