Transistor laser

Transistor laser is a semiconductor device that functions as a transistor with an electrical output and an optical output as opposed to the typical two electrical outputs. This optical output separates it from typical transistors and, because optical signals travel faster than electrical signals, has the potential to speed up computing immensely. Researchers who discovered the transistor laser developed a new model of Kirchhoff's current law to better model the behavior of simultaneous optical and electrical output.

The team credited with discovering the transistor laser was headed by Milton Feng and Nick Holonyak, Jr., and was based at the University of Illinois at Urbana-Champaign. Research into the transistor laser came about after Feng and Holonyak created the first light-emitting transistor in 2004. Feng and his team then modified the light-emitting transistor to focus the light it output into a laser beam. Their research was funded by DARPA. The paper written about the discovery of the transistor laser was ranked as a top five paper out of all of Applied Physics Letters’ history, and the transistor laser was called one of the top 100 discoveries by Discover.

The transistor laser functions like a typical transistor, but emits infrared light through one of its outputs rather than electricity. A reflective cavity within the device focuses the emitted light into a laser beam. The transistor laser is a heterojunction bipolar transistor (using different materials between the base and emitter regions) that employs a quantum well in its base region that causes emissions of infrared light. While all transistors emit some small amount of light during operation, the use of a quantum well increases the intensity of light output by as much as 40 times.

The laser output of the device works when the quantum well in the base region captures electrons that would normally be sent out through the electrical output. These electrons then undergo a process of radiative recombination, during which electrons and positively charged "holes" recombine in the base. While this process occurs in all transistors, it has an exceedingly short lifespan of only 30 picoseconds in the transistor laser, allowing for faster operation. Photons are then released through stimulated emission. Light bounces back and forth between reflective walls inside the 2.2 micrometer wide emitter that acts as a resonant cavity. Finally, light is emitted as a laser.

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