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Light-emitting transistor unveiled

08 Jan 2004

Researchers in the US claim to have fabricated a transistor that bridges the worlds of optics and electronics.

Scientists at the University of Illinois at Urbana-Champaign, US, have made a light-emitting transistor. Grown using MOCVD, Nick Holonyak Jr and Milton Feng’s device consists of InGaP, InGaAs and GaAs layers and based on a GaAs substrate. (Applied Physics Letters 84 151)

“We have demonstrated light emission from the base layer of a heterojunction bipolar transistor, and showed that the light intensity can be controlled by varying the base current," said Holonyak, whose work was critical in producing the first practical light-emitting diode and the first semiconductor laser to operate in the visible spectrum.

"This work is still in the early stage, so it is not yet possible to say what all the applications will be," Holonyak said. "But a light-emitting transistor opens up a rich domain of integrated circuitry and high-speed signal processing that involves both electrical signals and optical signals."

A transistor usually has two ports: one for input and one for output. "Our new device has three ports: an input, an electrical output and an optical output," said Feng. "This means that we can interconnect optical and electrical signals for display or communication purposes."

Feng also came up with the world's fastest bipolar transistor, an InP device operating at 509 GHz.

"In a bipolar device, there are two kinds of injected carriers: negatively charged electrons and positively charged holes," Holonyak said. "Some of these carriers will recombine rapidly, supported by a base current that is essential for the normal transistor function."

The recombination process in InGaP and GaAs creates infrared photons, which in the past has been regarded as a waste current generating unwanted heat, explained Holonyak. "We've shown that for a certain type of transistor, the base current creates light that can be modulated at transistor speed."

Although the recombination process is the same as that which occurs in light-emitting diodes, the photons in light-emitting transistors are generated under much higher speed conditions.

So far, the researchers have demonstrated the modulation of light emission in phase with a base current in transistors operating at a frequency of 1 MHz. Much higher speeds are considered certain.

"At such speeds, optical interconnects could replace electrical wiring between electronic components on a circuit board," Feng said.

Michael Hatcher is editor of Compound Semiconductor magazine.

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