First Light-Emitting Transistor

The inventor of the LED makes another optoelectronics breakthrough

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21 January 2004--When IEEE Medal of Honor winner Nick Holonyak Jr. invented the light-emitting diode in the early 1960s, it would have been difficult to guess that the device would become a mainstay of a global optoelectronics industry worth billions of dollars. Now almost 40 years later, Holonyak and his colleagues Milton Feng and Walid Hafez at the University of Illinois, Urbana-Champaign, have developed a light-emitting transistor or LET, a device that could have an equally profound impact.

A light-emitting diode is essentially a junction between a region of p-doped semiconductors having an excess of holes and an n-doped region having an excess of electrons. At the junction between the two regions, the electrons and holes can combine. Because electrons occupy a higher conduction band than holes, they have more energy, which must be released during the process of recombination.

In silicon, the energy is released in the form of momentum, which is absorbed by the lattice to create heat. But in other semiconductors, the energy can be released in the form of photons. If the band gap between the electrons and the holes is large enough, visible light is produced.

The new work focuses on a device closely related to the LED, the heterojunction bipolar transistor, which is widely used as an amplifier in communications devices such as cellphones. Essentially these transistors are two diodes back-to-back. The term heterojunction means that at least two semiconductors are used in the transistor, giving designers the potential to engineer the band gaps at the junctions within them.

The idea behind LETs is to choose materials carefully in order to create a band gap large enough to emit visible light when electrons and holes recombine.

Researchers have measured light emissions from transistors before. In 1992, a team at the Interuniversity MicroElectronics Center in Leuven, Belgium, built an indium-gallium-arsenide bipolar transistor that emitted light when cooled to liquid nitrogen temperatures. Many other groups have reported that transistor junctions can emit light at room temperature. "But nobody has been able to make a transistor that produces both electronic and optical signals at the same time," says Holonyak.

Instead, some groups have worked to minimize the potential for light emission, trying to improve the transistor’s electronic characteristics. Holonyak and Feng have turned this conventional thinking on its head, says Russell Dupuis, an electrical engineer and optoelectronics expert at the Georgia Institute of Technology in Atlanta. "They’ve given up a little transistor performance to gain some light-emitting performance," he says.

Holonyak and Feng chose a combination of indium-gallium-phosphide and gallium arsenide for their transistor, a mix that led to a device that pulsed infrared light at a wavelength of 885 nm in sync with the 1-MHz switching of the transistor.

Just how engineers might use this new optical output is hard to say, but then all the applications for LEDs were not immediately apparent, either. Dupuis thinks LETs could dramatically reduce the complexity of devices that currently employ both LEDs and ordinary transistors. "You now have light emission and light modulation in the same physics--that could turn out to be very useful," he says.

What’s more, Holonyak believes that LETs could shake up the way microelectronic circuits are designed and the way they are interconnected. "If you want to connect one part of the device to another, do you use the electronic output or the optical output?" he asks.

Holonyak’s group is now developing the device further. LEDs have been designed to emit different colors of light and even white light, and Holonyak says it should be possible to modify LETs in the same way. This could mean that LETs find wide application in flat-panel displays, which could reduce the complexity of the design. And if they can emit photons, could they also be designed to work with an optical input? "That’s just another of the many questions we’re asking ourselves," says Holonyak.

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