Fujitsu Develops Gallium-Nitride High Electron Mobility Transistor

(Business News & Technology News, 10 Oct 2008)

Fujitsu Ltd and Fujitsu Laboratories Ltd have developed a new type of gallium nitride(GaN)-based high electron mobility transistor (HEMT) that features a new structure ideal for use in amplifiers for microwave and millimeter-wave transmissions for high-speed wireless communications. In a technological first, a novel transistor structure was developed that achieves high output of over 100W and enables power to be cut when the transistor is in standby mode.

Details of the new technology were presented at the International Symposium on Compound Semiconductors(ISCS), held in Rust, Germany last September. 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.

Fujitsu's newly-developed GaN HEMT transistor is said to be 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 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, 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, and also raises output and reliability.

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 percent. This results in the characteristic of being able to sustain high current densities when on, and power to be cut when on standby.

The three-layer structure that covers the AlN layer with n-type GaN reduces surface roughness, permitting breakdown voltages that are high and exceeding 300V, thereby resulting in an unprecedented combination of achieving high current densities and high breakdown voltages. 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 100W, the first time that both have been achieved simultaneously.

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