Researchers Build Exciton-Based Devices
By using excitons, i.e. particles that emit photons as they decay, it may be possible to produce a new form of computing, better suited to fast communications. University of California at San Diego Professor Leonid Butov has built exciton-based transistors and the first computing devices to use excitons.
Alexander E. Braun, Senior Editor -- Semiconductor International, 9/22/2008
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Physicists at the University of California, San Diego (UCSD) and the University of California, Santa Barbara (UCSB) have demonstrated that excitons — particles that emit photons as they decay — could be used to usher in a new era of computing that is better suited to fast communications.
Although transistors use electrons to transport the signals necessary for computation, almost all communications devices use photons to send their data. Therefore, because electrons must be converted to photons, speed can be severely curtailed. UCSD Professor Leonid Butov and his group, working with their colleagues at UCSB, have built exciton-based transistors that they think could lay the foundation for a different kind of computer. As a way to demonstrate this, they have built the first computing devices to use excitons. Their transistors process signals using excitons, which, like electrons, can be controlled with electrical voltages; however, unlike electrons, these transform into photons at the circuit’s output. “This direct coupling of excitons to photons bridges a gap between computing and communications,” Butov said.
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| As excitons decay, photons are emitted, which could lead to the development of communications devices that do not need to convert electrons into photons. |
Excitons are bosonic particles that can be created by light in semiconductor materials such as gallium arsenide (GaAs), which can separate a negatively charged electron from a positively charged hole. When the pair is linked, the result is an exciton. When an exciton decays, the electron recombines with the hole and a flash of photons is released. The researchers have used a special kind of exciton, in which the electron and hole are confined to different quantum wells, separated by several nanometers.
This configuration creates an opportunity to control the exciton flow by applying a voltage supplied by electrodes. “Through our research of the basic physics of excitons,” Butov said, “we have been able to develop a method aimed at accurately and precisely controlling excitons in artificially created potential. We can make exciton ‘traps’ using this method, and confine and control them in situ to study their properties.”
The voltages create an energy spike that can stop the movement of excitons or allow them to flow. Once that energy barrier is removed, the exciton travels to the transistor output, where it transforms into light that can then be directly fed into a communication circuit, bypassing the requirement of first having to convert the signal. Because excitons are directly coupled to photons, this allows the linking of computation and communication. The researchers created simple circuits by joining exciton transistors to form several types of switches that accurately detect signals along one or several pathways. Because excitons are fast, the investigators have been able to demonstrate switching on the order of 200 psec.
Thus, while exciton-based computation is not expected to work at faster speeds than its electron-based counterpart, the advantage results when signals are sent between parts of a chip connected by an optical link or to another machine. Because electrons are not optically active, their use requires some sort of an interconnect, which results in slower operation speeds.
“In contrast to electrons, excitons are optically active,” Butov said. “They can be created by a photon and emit a photon, which means that the delay between signal processing and optical communications can be avoided. We have shown that excitons can be used very effectively for signal processing — they can do it as well as electrons can. Once we discover a way to effectively use excitons in transistors and ICs, we will be able to eliminate this bottleneck.”
While the concept has indeed been demonstrated, practical applications will have to wait until different semiconductor materials are tested. At present, GaAs excitonic circuits only work below 40°K, which is determined by the exciton’s binding energy. At higher temperatures, the electrons will not bind with their holes and excitons will not be created. The researchers expect that through the choice of different semiconductor materials, operating temperatures can be increased. “This will be the aim of the next phase of our research effort,” Butov said.
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