TOKYO : Fujitsu Limited and Fujitsu Laboratories Ltd. today announced that they have developed a crystal structure that both increases current and voltage in gallium-nitride (GaN)(1) high electron mobility transistors (HEMT)(2), effectively tripling the output power of transistors used for transmitters in the microwave band. The GaN HEMT technology can serve as a power amplifier for equipment such as weather radar – by applying the developed technology to this area, it is expected that the observation range of the radar will be expanded by 2.3 times, enabling early detection of cumulonimbus clouds that can develop into torrential rainstorms.

To expand the observation range of equipment like radar, it is essential to increase the output power of the transistors used in power amplifiers. With conventional technology, however, applying high voltage could easily damage the crystals that compose a transistor. Therefore, it was technically difficult to increase current and voltage simultaneously, which is required to realize high-output power GaN HEMTs.

Fujitsu and Fujitsu Laboratories have now developed a crystal structure that improves operating voltage by dispersing the applied voltage to the transistor, and thereby prevents crystal damage (patent pending). This technology has enabled Fujitsu to successfully achieve the world’s highest power density at 19.9 watts per millimeter of gate width for GaN HEMT employing indium-aluminum-gallium nitride (InAlGaN) barrier layer.

This research was partially supported by Innovative Science and Technology Initiative for Security, established by the Acquisition, Technology & Logistics Agency (ATLA) of the Japanese Ministry of Defense.

Details of this technology will be announced at the International Symposium on Growth of III-Nitrides (ISGN-7), an international conference about nitride semiconductor crystal growth, held in Warsaw, Poland, from August 5-10.

Development Background

In recent years, GaN HEMTs have been widely used as high-frequency power amplifiers in long-distance radio wave applications, such as radars and wireless communications. It is also expected that they will be used for weather radars to accurately observe localized torrential rainfall, as well as in millimeter-waveband wireless communications for fifth-generation mobile communications (5G). The outreach of microwaves from the microwave and millimeter-wave bands used for radar and wireless communications can be extended by increasing the output power of the high-frequency GaN HEMT power amplifiers used for transmitter. This allows for expanded radar observation range as well as longer distance and higher capacity communications.

Fujitsu Laboratories has been conducting research on GaN HEMTs since the early 2000’s, and currently provides the aluminum-gallium nitride (AlGaN) HEMTs used in a variety of areas(3). Recently, Fujitsu Laboratories has been conducting research on indium-aluminum-gallium nitride (InAlGaN) HEMTs as a new generation GaN HEMT technology, which enables high current operation as high-density electrons become available. Accordingly, Fujitsu and Fujitsu Laboratories have developed a crystal structure that achieves both high current and high voltage simultaneously.

Issues

In order to improve the output power of a transistor, it is required to realize both high current and high voltage operation. Research is ongoing for indium-aluminum-gallium nitride (InAlGaN) HEMTs for the next generation GaN HEMT that would contribute to increased current, as InAlGaN HEMTs can increase electron density within the transistor. When high voltage is applied, however, an excessive amount of voltage becomes concentrated on a part of the electron supply layer, damaging the crystals within transistors. Consequently, these transistors had a serious issue whereby their operating voltage could not be increased