Flashy Transistors: Electronic workhorses also shed light

Researchers have discovered that the transistor, long the star of electronics, has a yet-untapped talent–emitting light.

SPOTLIGHT. This microscope image reveals a bipolar transistor’s infrared-light emissions (white spots). Feng, et al./Applied Physics Letters

With that newfound capability, the transistor could also become a stellar device for optical uses, such as computer displays, high-speed telecommunications, and light-based microcircuits, says Milton Feng of the University of Illinois at Urbana-Champaign.

Feng specializes in the world’s fastest transistors, which are called heterojunction bipolar transistors. These devices pass large flows of electric current through small regions of a microchip.

Developers of bipolar transistors have long been aware that the current flows must generate some light, comments Russell D. Dupuis of the Georgia Institute of Technology in Atlanta. “Everyone thought that was bad,” he notes. Some electrons get snagged by atoms in the semiconductor and then shed energy in the form of photons, so light emission signifies loss of electric current.

Aware of the high currents in their superfast transistors, Feng and his Illinois colleagues Nicholas Holonyak Jr. and Walid M. Hafez checked whether the components yield appreciable light.

In the Jan. 5 Applied Physics Letters, they report substantial emissions of infrared light.

An electronic component closely related to transistors is already widely used as a lamp. That’s the light-emitting diode, or LED, which is becoming increasingly prevalent in items ranging from on-off switches to taillights and traffic signals. Because the bipolar light-emitting transistors are active both optically and electrically, they’re unlikely to shine as brightly as do their diode cousins, which have only optical output, Feng says.

On the other hand, the new transistor simultaneously controls the electric power that drives a lamp and serves as the lamp itself. In that way, the new components may reduce the complexity and cost of light-emitting microtechnologies, while also eliminating extra devices and wires that tend to reduce how quickly signals can be switched on and off or changed in intensity.

Feng estimates that the new transistors will eventually transmit optical data through fibers at 10 times the maximum speeds currently possible with LEDs or other optical components known as laser diodes. Fine-tuning the light-emitting capabilities of bipolar transistors might also lead to transistors that double as lasers, providing engineers with yet more options for mixing photons and electrons, Dupuis says.

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