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Via hole etching raises wideband GaN power

Spanning from the C-band to the X-band with its HEMT technology, Fujitsu's latest efficient PA is set to benefit broadband wireless and radar.

Fujitsu has produced the highest efficiency high-output 5 GHz bandwidth hybrid amplifier, helped by via holes in a GaN transistor that ground the device.

Researchers at Fujitsu Laboratories have made a PA using this hybrid approach with 6.5 W output at 7 GHz and 4.1 W output at 12 GHz.

The device operates with 40 percent and 26 percent efficiency at these frequencies respectively. It was unveiled at the Compound Semiconductor IC Symposium in Monterey, California (CSICS), in October by Fujitsu s Yusuke Inoue.

By keeping its HEMT and capacitor separate the Japanese conglomerate says that its performance “significantly exceeds” other GaN hybrid amplifiers that integrate the components onto the same MMIC die.

In the Fujitsu approach inductance in the wires used to make connections between the HEMT and capacitor and ground the PA normally reduces overall amplification efficiency.

However Inoue and his colleagues have developed a dry-etch technique that can effectively etch via hole connections through the GaN chip and counter this effect.

“The inductance for a via hole is determined by wafer thickness and its diameter,” Hisao Shigematsu, leader of the GaN circuit design group that Inoue works in, told compoundsemiconductor.net. “We can make its inductance much smaller than that of a wire.”

The Fujitsu team made its ground connection with via holes and used a matching circuit to reduce the other wires effect on the PA s efficiency.

They chose to separate the HEMT and capacitor as the breakdown voltage of capacitors produced on GaN MMICs is presently too low for ultra-wideband (UWB) amplifiers.

The final PA consequently benefits from combining a 0.5 µm gate-length HEMT made using Fujitsu s highly reliable GaN transistor technology, and a 200 V-rated capacitor.

Existing UWB PAs feature in radar systems that employ different wavelengths to identify multiple targets and minimize the effect of adverse weather conditions. They are also used in broadband communications systems to expand transmission capacities.

As well as boosting output and efficiency, GaN amplifiers should demand less cooling and be smaller than existing GaAs products. The PA presented at CSICS measured just 6.0 mm x 6.6 mm.

Development of these devices will continue at Fujitsu, targeting commercialization “in a few years”, according to Shigematsu.

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