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Brad Petersen
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1066 ECE Building
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Urbana, IL 61801
Phone: (217) 244-6376
bradp@illinois.edu

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megd@illinois.edu

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Redefining electrical current law with the transistor laser

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By Liz Ahlberg, U of I News Bureau
May 13, 2010

  • ECE Professors Milton Feng and Nick Holonyak Jr. have been reexamining a major current law.
  • They have found that the transistor laser does not follow Kirchoff's current law for equality of charge input and output.
  • They have expanded Kirchoff's law from a current law to a current-energy law.

Milton  Feng
Milton Feng

While the laws of physics weren’t made to be broken, sometimes they need revision. A major current law has been rewritten thanks to the three-port transistor laser, developed by ECE Professors Milton Feng and Nick Holonyak, Jr.

With the transistor laser, researchers can explore the behavior of photons, electrons, and semiconductors. The device could shape the future of high-speed signal processing, integrated circuits, optical communications, supercomputing, and other applications. However, harnessing these capabilities hinges on a clear understanding of the physics of the device, and data the transistor laser generated does not fit neatly within established circuit laws governing electrical currents.

“We were puzzled,” said Feng, who holds the Nick Holonyak Jr. Chair in Electrical and Computer Engineering. “How did that work? Is it violating Kirchhoff’s law? How can the law accommodate a further output signal, a photon, or optical signal?”

Kirchhoff’s current law, described by Gustav Kirchhoff in 1845, states that charge input at a node is equal to the charge output. In other words, all the electrical energy going in must go out again. On a basic bipolar transistor, with ports for electrical input and output, the law applies straightforwardly. The transistor laser adds a third port for optical output, emitting light.

Nick  Holonyak, Jr
Nick Holonyak, Jr

This posed a conundrum for researchers working with the laser: How were they to apply the laws of conservation of charge and conservation of energy with two forms of energy output?

“The optical signal is connected and related to the electrical signals, but until now it’s been dismissed in a transistor,” said Holonyak, who holds the John Bardeen Endowed Chair in Electrical and Computer Engineering and Physics. “Kirchhoff’s law takes care of balancing the charge, but it doesn’t take care of balancing the energies. The question is, how do you put it all together and represent it in circuit language?”

The unique properties of the transistor laser required Holonyak, Feng and graduate student Han Wui Then to reexamine and modify the law to account for photon particles as well as electrons, effectively expanding it from a current law to a current-energy law. They published their model and supporting data in the Journal of Applied Physics, available online May 10.

“The previous law had to do with the particles—electrons coming out at a given point. But it was never about energy conservation as it was normally known and used,” Feng said. “This is the first time we see how energy is involved in the conservation process.”

Simulations based on the modified law fit data collected from the transistor laser, allowing researchers to predict the bandwidth, speed, and other properties for integrated circuits, according to Feng. With accurate simulations, the team can continue exploring applications in integrated circuits and supercomputing.

“This fits so well, it’s amazing,” Feng said. “The microwave transistor laser model is very accurate for predicting frequency-dependent electrical and optical properties. The experimental data are very convincing.”

The Army Research Office supported this work.

Editor's note: media inquiries should be directed to Brad Petersen, Director of Communications, at bradp@illinois.edu or (217) 244-6376.

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