Fujitsu claims to have made the first millimetre-wave GaN transmit-receive amplifier pair.
Aimed at broadband wireless comms between 70 and 100GHz, the HEMT-based devices are a 350mW output transmitter and a receiver with 25dB signal gain in the W-band and a 3.8dB noise figure.
"Both figures are the world's best-ever reported for a GaN-based integrated circuit," claimed Fujitsu. "Compared to GaAs-based technology, the GaN HEMT amplifier chipset is anticipated to offer output that is increased by approximately four-fold, and a noise figure decreased by 40%, while transmission ranges are expected to be extended by roughly three-fold."
10Gbit/s millimetre links are seen as an alternative to fibre-optic cabling.
"The transceiver amplifier will help make communications equipment for trunk lines and ultra-high-speed fixed wireless access higher in quality and more compact, ultimately making wireless broadband more practical," claimed the firm.
It suggests that combining its impulse radio with the new amplifiers will push range up to several tens of kilometres.
InP and GaAs-based technologies cannot compete, said Fujitsu, because in these less robust materials - breakdown voltage of GaN is 10x that of GaAs - leakage from the transmitter can damage the local receiver input.
"The amplifier unit's reception performance is on par with those using GaAs-based technology, but because it obviates the need for a protective circuit, the performance of the transceiver as a whole can be expected to improve," said the firm.
Fujitsu developed a GaN HEMT structure with 0.12µm long gates in 2006.
To increase gain and reduce signal to noise ratio in the amplifiers, parasitic capacitance was cut by tuning the 2006 gate structure and adjusting the thickness of the protective layer, said the firm.
Spurious oscillation from signal to signal coupling was seen a potential problem with the chip which is roughly the same size as the wavelength of the signal.
Cross-talk was reduced to manageable levels by closely surrounding signal conductors with ground planes, and extending this shielding all the way through the SiC substrate, which underlies the GaN layers, with rows of vias to a plane on the back of the wafer.
"3-D electromagnetic field analysis was employed to optimise the placement of via-holes that penetrate the top and bottom of the chip relative to signal paths and circuits," said Fujitsu. "For the first time ever in GaN HEMT circuitry, signal interference was successfully suppressed and a circuit that features high output and high signal-gain operated free from circuit oscillations."
Comparison of millimetre-wave receiver amplifier performance
Fujitsu's long-range millimetre-wave impulse radio concept.