FUJITSU has developed a gallium-nitride (GaN) power amplifier with the world’s highest output performance for W-band wireless transmissions.
The GaN high-electron mobility transistor power amplifier covers the W-band (75 to 110GHz), allowing high-capacity wireless transmissions over a radius of several kilometres.
W-band networks are used in areas where fibre-optic cable is difficult to lay. These networks bring high-speed connectivity and coverage rapidly to areas, allowing event organisers or disaster response units to set up temporary but fast communications capabilities.
Thanks to the wide-frequency band, W-band networks can achieve wireless speeds of several gigabits per second. By comparison, today’s mobile phones use frequencies in the 0.8 to 2.0GHz range, which is 50 times narrower than W-band, with the proportionate reduction in speed.
However, to get good long-distance coverage in these frequencies, the power amplifier’s output power must also be scaled up. To transmit over several kilometres, the antenna would require a power amplifier capable of high output in the order of several watts. Current power amplifiers for high-frequency transmissions in the 30-300GHz bands, built using gallium arsenide or CMOS semiconductors, are limited to an output of about 0.1W.
The Fujitsu breakthrough is a power amplifier for W-band transmissions, using GaN-HEMT technology to allow high output at 100GHz. The new power amplifier has 1.8 times increased output performance compared to its predecessors, allowing an increase of over 30 percent in transmission range when used in a high-speed wireless network.
Previously, GaN-HEMT power amplifiers have achieved high output performance in the microwave range of 3 to 30GHz, but faced performance ramp-down in the W-band range.
The Fujitsu GaN-HEMT device has a unique structure which is capable of increasing output in the millimetre band. The researchers used a layer of indium-aluminium-gallium-nitride (InAlGaN), and double-layer silicon nitride (SiN) passivation film to increase current density by a factor of about 1.4, resulting in 3.0 W of output power from a transistor per 1-mm of gate width, at a high frequency of 100 GHz.
In order to successfully design a power amplifier with high output performance, Fujitsu precisely measured and modelled the characteristics of GaN-HEMT during high-frequency operation. The simulation guided the researchers to design a circuit where pairs of GaN-HEMTs were grouped together into compact, high-gain units with low power loss.
In order to maximize the power from these units, GaN-HEMTs were connected in a series by the interstage circuit where the signal lines and the device layouts were carefully laid out.
Using a model of these compact, high-gain units, Fujitsu then conducted simulations to optimise the distributor and combiner matching circuits between the units, and their layouts and signal lines, resulting in a high-amplitude power amplifier.
Fujitsu now plans to apply this power-amplifier technology to high-capacity long-range wireless communications, and to implement high-speed wireless communications systems that can be deployed quickly during special events or disasters.