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  7. Fujitsu Develops World's First Gallium-Nitride HEMT Able to Cut Power in Standby Mode and Achieve High Output

Fujitsu Develops World's First Gallium-Nitride HEMT Able to Cut Power in Standby Mode and Achieve High Output

- Achieves low power consumption and high output of over 100W -

Fujitsu Limited,Fujitsu Laboratories Ltd.

Tokyo and Kawasaki, Japan, October 10, 2008

Fujitsu Limited and Fujitsu Laboratories Ltd. today announced the development of a new type of gallium nitride (GaN)(1) -based high electron mobility transistor (HEMT)(2) that features a new structure ideal for use in amplifiers for microwave(3) and millimeter-wave(4) transmissions, frequency ranges for which usage is expected to grow. In a technological first, a novel transistor structure was developed that achieves high output of over 100 watts and enables power to be cut when the transistor is in standby mode. This technological advance will contribute to higher output and lower power consumption in microwave and millimeter-wave transmission amplifiers for high-speed wireless communications.

Details of the new technology were presented at the International Symposium on Compound Semiconductors (ISCS), held in Rust, Germany from September 21 - 24. Certain aspects of research for this new technology were conducted as part of the Research and Development Project for Expansion of Radio Spectrum Resources, sponsored by Japan's Ministry of Internal Affairs and Communications.

Background

As transmission speeds in next-generation wireless communications have become faster, wireless base stations that operate in the microwave frequency range at several gigahertz consume an ever-increasing amount of power. In addition to the fact that the millimeter wave frequency range above 30 GHz has a large amount of available bandwidth that is currently not being used, because it delivers high speed and good directionality, its potential for use in high-speed transmissions is significant. However, due to millimeter-wave frequencies being higher than frequencies for conventional wireless transmissions, it has been difficult to develop amplifiers for practical use that are both compact and economical, and thus the millimeter band is not yet widely used.

Amplifiers based on silicon semiconductors have low breakdown voltages(5), making it difficult for them to achieve both high frequencies and high power output. As a result, attention has shifted to compound semiconductors, which feature breakdown voltages that are high. For example, the GaN-based semiconductors have high innate breakdown voltages, making them appropriate for high-output applications. The development of high-output power amplifiers would make microwave and millimeter-wave transmissions possible, thereby enabling their use in high-speed wireless communications.

Technological Challenges

Amplifiers based on previous compound semiconductors required a negative voltage(6) to be added to the gate electrode even when the circuit was in standby mode, resulting in the problem of high power consumption. This is attributable to the fact that the transistors being used require negative voltage to be continuously applied to the circuit-power cutoff switch in order to turn off the electron flow (see Figure 1a). Thus far, no compound semiconductor transistor had been able to both generate 100 W of power output and cut power without requiring the addition of negative voltage. In addition, because the design of the control circuits used in previous transistors has been very complex, they required more power to be consumed.

Newly Developed Technology

Figure 1b shows the structure of Fujitsu's newly-developed GaN HEMT transistor, the world's first that does not require a negative voltage to turn off the circuit's power. Key features of this structure include the following:

  1. The addition of an aluminum nitride (AlN) layer on top of the n-type GaN layer increases the density(7) of carrier electrons when on, making this design naturally amenable to higher output.
  2. The design of the gate electrode, which was formed after removing the AlN layer, decreases the density of carrier electrons around the region below the gate electrode, preventing current from flowing even in the absence of a negative voltage.
  3. If the topmost layer is made of AlN, this results in a micro-cracked surface as shown in Figure 2a, which degrades the breakdown voltage. This issue has been resolved by adding a n-type GAN-based layer above the AlN layer, resulting in a three-layer cap structure that improves the surface roughness as shown in Figure 2b, and also raises output and reliability.
Figure 1: Structure of a GaN HEMT(a) Conventional structure(b) Structure of newly-developed HEMT from Fujitsu

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Figure 2: Image of atomic-scale surface roughness on GaN HEMT(a) 2-layer cap structure without uppermost layer(b) 3-layer cap structure newly developed by Fujitsu

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Results

The newly developed three-layer cap structure obviates the need for a negative voltage across the gate electrode and results in a GaN HEMT that allows power to be cut during standby. Fujitsu also verified that the addition of an AlN layer increased the volume of electrons transmitted by 60% (Figure 1b). This results in the characteristic of being able to sustain high current densities when on, and power to be cut when on standby (Figure 3). The three-layer structure that covers the AlN layer with n-type GaN reduces surface roughness, permitting breakdown voltages that are high and exceeding 300 volts, thereby resulting in an unprecedented combination of achieving high current densities and high breakdown voltages (Figure 4). This enables high-efficiency power amplification with low resistance and low power consumption. Implementation of this new transistor structure eliminates the need for power during standby, and has realized output exceeding 100 W, the first time that both have been achieved simultaneously.

Future Developments

Fujitsu will pursue commercialization of this GaN transistor technology that features high breakdown voltages, aiming to apply this technology in high-capacity wireless communications systems by around 2010.

Figure 3: Carrier transfer characteristics of the newly-developed GaN HEMT

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Figure 4: Maximum drain current and breakdown voltage benchmarks of the newly-developed GaN HEMT (transistor enables electrical current to be turned off without applying negative voltage)

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  • [1] Gallium nitride (GaN)

    A wide band-gap semiconductor that features stronger resistance against breakdowns that can occur due to high voltages, compared to conventional semiconductors based on silicon (Si) or gallium arsenide (GaAs).

  • [2] High electron mobility transistor (HEMT)

    Invented in 1979 by Takashi Mimura of Fujitsu Laboratories (currently a Fellow at Fujitsu Laboratories), this is a transistor made of compound semiconductors that feature excellent speed and noise characteristics. HEMTs are now widely used in satellite broadcast equipment, mobile phones, GPS navigation systems, broadband wireless access systems, and other core technologies fundamental to an information-driven society.

  • [3] Microwave

    The radio band from 30 MHz to 30 GHz.

  • [4] Millimeter wave

    The radio band from 30 GHz to 300 GHz. Not widely used yet compared to the band below 10 GHz.

  • [5] Breakdown voltage

    The maximum voltage that can be applied across a gate electrode and a drain electrode. Voltages in excess of this will cause the semiconductor to break down.

  • [6] Negative voltage

    With conventional GaN HEMT transistors, applying negative voltage to the gate enables the density of the electrons just below the gate to be zero, thereby enabling the transistor's electrical current to be turned off.

  • [7] Electron density

    The number per unit volume of electrons distributed on the GaN side at the interface of GaN and n-type AlGaN. These electrons are referred to as a two-dimensional electron gas. In HEMT transistors, a high electron density enables a high power output.

About Fujitsu

Fujitsu is a leading provider of IT-based business solutions for the global marketplace. With approximately 160,000 employees supporting customers in 70 countries, Fujitsu combines a worldwide corps of systems and services experts with highly reliable computing and communications products and advanced microelectronics to deliver added value to customers. Headquartered in Tokyo, Fujitsu Limited (TSE:6702) reported consolidated revenues of 5.3 trillion yen (US$53 billion) for the fiscal year ended March 31, 2008. For more information, please see: www.fujitsu.com.

About Fujitsu Laboratories

Founded in 1968 as a wholly owned subsidiary of Fujitsu Limited, Fujitsu Laboratories Limited is one of the premier research centers in the world. With a global network of laboratories in Japan, China, the United States and Europe, the organization conducts a wide range of basic and applied research in the areas of Multimedia, Personal Systems, Network, Peripherals, Advanced Materials and Electronic Devices. For more information, please see: http://jp.fujitsu.com/labs/en/

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Company and product names referenced herein are trademarks or registered trademarks of their respective owners. Information provided in this press release is accurate at time of publication and is subject to change without advance notice.

Date: 10 October, 2008
City: Tokyo and Kawasaki, Japan
Company: Fujitsu Limited, Fujitsu Laboratories Ltd., , , ,

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