TOKYO — Nippon Telegraph and Telephone Corp. (NTT) has developed a diamond semiconductor device that operates at 81 GHz frequency, more than twice the speed of earlier devices. The advance promises to make amplification in the millimeter-wave band from 30 to 300 GHz possible for the first time, NTT claimed.
Diamond is expected to be the next generation semiconductor material because of its high thermal conductivity, high breakdown voltage and high carrier mobility. Together, these characteristics makes diamond semiconductors most suitable for high frequency, high power devices.
But diamond semiconductors are prone to defects and impurities that have hindered development of prototype devices with performance close to the expected, theoretical performance of diamond semiconductor devices.
NTT Basic Research Laboratory said it developed a diamond semiconductor thin-film layer in April 2002 that overcomes some of these problems. NTT researchers found that crystal defects, carbon graphite and impurities hurt performance. They eliminated crystal defects and graphite and reduced impurities to 1/20th of previous materials using a high purity gas to grow the diamond layer at lower temperatures of 650-750 degrees C.
The resulting diamond layer showed a carrier mobility at 1300 cm2/Vs, about 20 times higher when compared to previous prototypes, NTT said.
The NTT lab has been in collaboration with the University of Ulm in Germany, which had already succeeded in fabricating FET devices, to develop a diamond semiconductor device using its diamond thin-film layer.
The joint team formed T-shaped gates on the diamond layer, which is on a 3-mm2 diamond substrate. The gate width, which determines the performance of devices, is 0.2 micron. The device operated continuously at 81 GHz. Once the peripherals technology is established, NTT researchers said they expect to boost the output power of the device as high as 30 W/mm, the level required for practical use.
NTT is now working to further decrease impurities to improve the quality of diamond crystal. It is targeting devices with an operating frequency of 200 GHz and an output power of 30 W/mm.
The diamond devices are expected to be in demand to replace with the vacuum tubes that are used in the high frequency, high-power applications such as receivers and transmitters at digital TV broadcasting stations.
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