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Fujitsu claims normally-off GaN HEMT first

A carrier-injecting AlN layer enables reliable 126W GaN transistors for economical high-power, high-frequency wireless communication.

Japanese wide-bandgap pioneer Fujitsu has opened the way for the first 100 W-plus III-V enhancement-mode transistor in the market, targeting microwave and millimeter wave base stations.

Fujitsu unveiled the HEMT, slated for use in high-capacity wireless communication by 2010, at the International Symposium on Compound Semiconductors in Rust, Germany on September 23. Toshihiro Ohki of Fujitsu Laboratories introduced the normally-off device, which boasts 126 W power output and a 336 V breakdown voltage.

“Our new technology enables us to use the GaN HEMT in enhancement-mode without using special growth and process techniques,” said Toshihide Kikkawa, research fellow in Fujitsu's advanced device laboratory. “Reliability can be easily obtained,” Kikkawa told compoundsemiconductor.net.

With the conglomerate s first GaN HEMTs finding their way into base stations this year, Ohki and Kikkawa s team were able to boost these devices power to target millimeter-wave operation.

To do this, they introduced an AlN layer into their MOCVD-grown transistor s cap structure and modified its gate design. The piezoelectric properties of AlN mean that it induces positive charge at the interface with an n-type GaN layer beneath it, helping boost power output.

“This positive charge produces additional carriers in the channel layer,” Kikkawa said. “As a result, two-dimensional electron gas density is increased.”

AlN can not be left as the topmost layer in the cap structure, as its rough surface lowers the transistor s breakdown voltage. The final cap structure is therefore a single n-type AlN layer between two n-type GaN layers.

The results presented in Rust show that this three-layer cap boosts the volume of electrons transmitted through the device by 60 percent. The final HEMT also avoids the negative voltage needed to turn off other devices, instead boasting a 0.25 V positive threshold voltage.

Fujitsu calls the end result “an unprecedented combination of high current densities and high breakdown voltages”.

High power and normally-off characteristics makes the transistor ideal for energy-efficient and economical high-data rate communications in gigahertz frequencies and even beyond, according to the company. It says millimeter wave communications in particular have great potential thanks to good directionality, fast data transfer and the extensive bandwidth that is currently unused in that range.

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