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June 23, 2008

Exciton communication and transistors could be the basis of new faster computers


An circuit that uses excitons for computing flashes light as the particles decay to release photons. Credit: Leonid Butov/UCSD

Particles called excitons that emit a flash of light as they decay could be used for a new form of computing better suited to fast communication, physicists at UC San Diego have demonstrated. Switching times on the order of 200 picoseconds have been demonstrated so far. (A picosecond is one trillionth of a second). While exciton computation itself may not be faster than electron-based circuits, the speed will come when sending signals to another machine, or between different parts of a chip that are connected by an optical link. The gallium arsenide excitonic circuits will only work at frigid temperatures - below 40 degrees Kelvin (or -390 degrees Fahrenheit), a limit determined by the binding energy of the excitons. The operating temperature can be increased by choosing different semiconductor materials.

Integrated circuits, assemblies of transistors that are the building blocks for all electronic devices, currently use electrons to ferry the signals needed for computation. But almost all communications devices use light, or photons, to send signals. The need to convert the signalling language from electrons to photons limits the speed of electronic devices.

Leonid Butov, a professor of physics at UCSD, and his colleagues at UCSD and UC Santa Barbara have built several exciton-based transistors that could be the basis of a new type of computer, they report this week in an advance online version of the journal Science. The circuits they have assembled are the first computing devices to use excitons.

"Our transistors process signals using exitons, which like electrons can be controlled with electrical voltages but unlike electrons transform into photons at the output of the circuit,” Butov said. “This direct coupling of excitons to photons bridges a gap between computing and communications."


Excitons are pairs of negatively charged electrons and positively charged “holes” that are created by light in a semiconductor such as gallium arsenide. When the electron and hole recombine, the exciton decays and releases its energy as a flash of light.

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