Dec. 8, 1998 San Francisco -- Bell Labs researchers have developed an improved class of gallium arsenide-based transistors that may lead to cellular phones with longer battery lives and more powerful wireless base stations. Eventually, these transistors also may lead to the next generation of ultra-fast computers.
The research findings, announced Dec. 7 at the IEEE International Electron Devices Meeting (IEDM), describe how the researchers made the first highly stable gallium arsenide-based metal oxide semiconductor field effect transistor (MOSFET). The silicon version of MOSFETs is used in today's computers, digital signal processors and memory modules.
The major challenge in making gallium arsenide MOSFETs is perfecting the thin gate oxide material that's directly below the transistor's gate electrode, which receives electrical signal and acts as the transistor's switch by turning it on and off. Beneath the gate oxide is the channel that's made from gallium arsenide. As current flows, electrons move along the channel, between the source and the drain. For this reason, the channel is known as the transistor's active region, and it must have a smooth, defect-free interface with the gate oxide layer to operate properly. That's because the gate oxide acts as an insulator by protecting the channel from the electrode. Otherwise, a short circuit would occur.
For more than 30 years, researchers have been trying to make gallium arsenide-based gate oxides. Two years ago, also at IEDM, Bell Labs researchers announced the first gallium arsenide MOSFET by precisely depositing the gate oxide -- atomic layer by atomic layer -- on the channel surface, using a process known as molecular beam epitaxy.
In the latest research, the researchers have improved the stability and electrical characteristics of the gate oxide, comprised of gallium oxide and gadolinium oxide. As a result, the current decreases only 1.5 percent after 150 hours of operation under extreme conditions. In contrast, the previously best research results reported a 22 percent drop over a three-hour period.
"After we further improve the gate oxide material, gallium arsenide MOSFET devices may be very attractive for various applications, such as cellular phones, wireless base stations and potentially microprocessors in computers," said researcher Ming-Hwei Hong of Bell Labs, which is the research and development arm of Lucent Technologies.
Currently, most wireless applications use gallium arsenide metal-semiconductor field-effect transistors (MESFETs), which lack a gate oxide, but which are necessary because higher frequencies are attainable with gallium arsenide compared to silicon. (That's because electrons can travel five to six times faster in gallium arsenide-based transistors.) However, because the insulating gate oxide layer is not present, the current always is on, and cellular phones and base stations require two voltage sources.
Because of the lower power consumption of gallium arsenide MOSFETs, the talk time for cellular phones would be extended. Meanwhile, wireless base stations would be more powerful and more efficient.
Because of design limitations, it's more likely that gallium arsenide MOSFETs first would be used in wireless applications, as opposed to computer applications, such as microprocessors. That's because only a few thousand transistors are needed for a cellular phone chip, compared to tens of thousands in microprocessors. Besides Hong, the research team includes Yu-Chi Wang, Jenn-Ming Kuo, Joe Mannaerts, Raynien Kwo, Huan-Shang Tsai, James Krajewski, Young-Kai Chen, and Alfred Cho.
Lucent Technologies, headquartered in Murray Hill, N.J., designs, builds and delivers a wide range of public and private networks, communications systems and software, data networking systems, business telephone systems and microelectronics components. Bell Laboratories is the research and development arm for the company. For more information on Lucent Technologies, visit the company's web site at http://www.lucent.com.
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