Lighthearted Transistor: Electronic workhorse moonlights as laser

Transistors have long served as the building blocks of microelectronics. More recently, microchip lasers have been emerging as cornerstones of light-based circuitry, or photonics. Now, engineers have melded the two types of components into one miniature device that both amplifies electric current and emits a narrow beam of single-wavelength light.

LIGHTING THE WAY. Magnified view of transistor laser (left) shows infrared laser’s glow. Diagram (right) shows that laser light (wavy arrows) radiates from just one region of the transistor, while triangular probes monitor properties. Walter, et al. / Applied Physics Letters

“It’s simultaneously a transistor and a laser. It’s a transistor laser,” says co-developer Nick Holonyak Jr. of the University of Illinois at Urbana-Champaign. He previously contributed to the invention of various optoelectronics gadgets, including the first practical light-emitting diode, or LED.

The transistor laser is “a major technology breakthrough in high-speed optoelectronics,” comments K.C. Wang of HR Laboratories in Malibu, Calif.

The novel hybrid opens the way to significant speedups of computer circuits and telecommunication channels, its developers say. For instance, it may boost the rate at which light signals in an optical fiber can be turned on and off to represent digital 1s and 0s.

By easing integration of electronic and photonic elements, the new device may also bring about enhanced performance of consumer products and industrial equipment.

Holonyak and his colleagues Milton Feng, Gabriel Walter, and Richard Chan describe their dual-action component in the Nov. 15 Applied Physics Letters. Although the team currently must chill its microdevice to –73°C to achieve laser action, “we believe we’ll be [using the device] at room temperature very soon,” Feng says.

In creating the new transistor laser, Holonyak builds upon a finding that he and his colleagues published in January (SN: 1/10/04, p. 21: Available to subscribers at Flashy Transistors: Electronic workhorses also shed light). They revealed then that a type of superfast, high-current transistor known as a heterojunction bipolar transistor can emit useful amounts of infrared light.

Such a transistor consists of layers of exotic semiconductor compounds, such as gallium arsenide, stacked on a microchip. To make a transistor that pumps out even more infrared light, the team inserted an extra set of layers into the stack, Feng explains.

Those extra layers, known as a quantum well, promote pooling of positive charges—which are actually vacancies within the electron clouds that surround atoms. As electrons from the current flowing through the transistor spill into those vacancies, the positive charges vanish in flashes of light.

Other upper and lower sets of added layers confine the light and channel it to the stack’s reflective edges. The edges bounce the waves back and forth, prompting even more light emission, which builds up to a laser beam.

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