Portland, Ore. An experimental light-emitting nanotube (LEN) transistor that's said to achieve 10,000 times the electroluminescent efficiency of LEDs could put researchers closer to the goal of "computing with light." By emitting thousands of photons for the same energy expenditure as one photon emission in an LED, the unipolar carbon nanotube transistor could lead to optical silicon chips, said its creator, IBM Corp.
Light emission in solid-state LEDs occurs when separately injected electrons and holes recombine in an exotic material such as gallium arsenide. The resultant drop in energy causes a single photon to be emitted to compensate.
IBM had earlier reported an ambipolar nanotube transistor, billed at the time as the world's smallest solid-state emitter, for which hot carriers electrons and holes were injected separately into the source and drain (see www.eetimes.com/showArticle.jhtml?articleID=18308486).
The new technique induces electroluminescence from a single type of carrier an exciton using a unipolar nanotube transistor that the company says is three orders of magnitude more efficient than the ambipolar transistor.
"Our carbon nanotube transistor creates 10,000 times more photon flux than large-area LEDs and a thousandfold more photons per unit time and area than our ambipolar nanotube transistor," said IBM researcher Jia Chen, who performed the work at IBM's T.J. Watson Research Center (Yorktown Heights, N.Y.). Collaborating with Chen were researchers Vasili Perebeinos, Marcus Freitag, James Tsang and Phaedon Avouris, as well as Duke University professor Jie Liu and his student Qiang Fu.
The excitons used in the unipolar transistors are highly efficient, Chen said. "Almost every carrier makes an electron-hole pair that can recombine to emit a photon. In our previous device it was very difficult to balance the number of the electrons and holes being injected, so comparatively few carriers achieved high enough energy to recombine."
IBM believes light-emitting nanotube transistors will revolutionize the communications industry by enabling silicon devices to perform both electronic and optical signal processing operations. Eventually all-optical silicon chips could result, but in the meantime silicon chips could perform electrical-to-optical conversions, reducing the need for separate devices made from exotic materials, IBM researchers believe.
"Today information to be transmitted over optical fibers must at some point be converted from an electrical signal," Chen said. "This work shows that silicon, combined with nanotubes, can be both a good electrical and a good optical material."
By contrast, gallium arsenide and other optical materials are difficult to fabricate alongside silicon, she said.
IBM predicts that arrays of light-emitting nanotubes could someday replace bulk optics and that individual LENs will enable "computing with light" on hybrid optoelectronic silicon chips.
IBM's technique produces 10,000 times more photons than LEDs, for the same energy consumed, by achieving a higher energy state for its charge carriers. The trick, according to IBM, was to suspend a carbon nanotube transistor channel over its gate. When the suspended nanotube was electrically stimulated, it produced superhighenergy excitons electrically neutral pairs of excited electrons and holes that were at a distance from one another but were bound by coulombic interactions. The excitons immediately recombined in the transistor channel, emitting an infrared photon to shed their extra energy.