Transistor laser breaks the law

Charles T. Troy, charlie.troy@photonics.com

A three-port quantum well transistor laser developed by Milton Feng and Nick Holonyak Jr. at the University of Illinois at Urbana-Champaign is rewriting a major current law. Thanks to this device, the behavior of photons, electrons and semiconductors can be better explored. In fact, the researchers say it could shape the future of supercomputing, high-speed signal processing, integrated circuits, optical communications and other applications.

Harnessing these capabilities, however, depends on understanding the physics of the device; data generated by the laser did not fit 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?”

The law in question, described by physicist Gustav R. Kirchhoff in 1845, states that charge input at a node is equal to the charge output; i.e., all 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. Photons for the optical signal are generated when electrons and holes recombine in the base, an intrinsic feature of transistors.

A reversible option

The transistor laser employs a quantum well and a resonator in the base to control electron-hole recombination and electrical gain. By blocking the laser resonator with white paste, the researchers converted the device into an ordinary transistor. Because the process is reversible, they could compare collector characteristics when the device was functioning as a normal transistor and when it was functioning as a transistor laser, something never before possible.

This posed a conundrum: How could they apply the laws of conservation of charge and 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 chair in electrical and computer engineering and physics at the university. “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 unusual properties of the transistor laser required Feng, Holonyak and graduate student Han Wui Then to re-examine 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 May 2010 issue of the Journal of Applied Physics.

Milton Feng and Nick Holonyak Jr. of the University of Illinois. Courtesy of L. Brian Stauffer, University of Illinois at Urbana-Champaign.

“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, enabling researchers to predict the bandwidth, speed and other properties for integrated circuits, Feng said. 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 work was supported by the US Army Research Office.