- Transistor Displays Laser Operation
By incorporating quantum wells into the active region of a light-emitting transistor, scientists at the University of Illinois at Urbana-Champaign have demonstrated laser operation from an InGaP/GaAs/InGaAs heterojunction bipolar transistor. This type of laser, they suggest, potentially could have a direct modulation frequency of several hundred gigahertz -- impossible to obtain with the conventional laser diode.
Researchers have reported laser operation from a heterojunction bipolar transistor that incorporates quantum wells into its active region.
The investigators, led by Nick Holonyak Jr. and Milton Feng, constructed the device in the same way as a conventional heterojunction bipolar transistor but with a modified base region to improve radiative recombination. The base current, which is essential for the normal transistor function, has until recently been regarded as a waste current that generates unwanted heat. They had shown that for a certain type of transistor, the base current creates light that can be modulated at transistor speed. To achieve laser operation, they layered the transistor structure and assembled it into an optical waveguide and cavity.
A transistor usually has two signal ports, one for input and one for output. The new device has three: an input, and electrical and optical outputs. It can be used simultaneously as a transistor and as a laser by biasing the device above a certain threshold, and it displays laser emission at ~960 nm with a current threshold at ~8 mA.
The group reported the laser running in quasi-CW and pulsed mode, but according to Feng, it more recently has displayed CW operation. He also believes that room-temperature operation is achievable. The reported device required cooling to approximately 200 K.
Feng noted that the work is "still very much in its infancy," with much more to be learned about transistor lasers, including how to separate and optimize the electrical and laser output. Nevertheless, it points to a future in which ultrafast transistor lasers could extend the modulation bandwidth of a semiconductor light source far beyond what is possible today. Current heterojunction bipolar transistors run at several hundred gigahertz, with much greater base current densities than are needed for laser operation, he said, suggesting the potential of similarly high direct modulation frequencies from transistor lasers.
Used as optoelectronic interconnects, such lasers could facilitate faster signal processing, higher-speed devices and large-capacity seamless communications, as well as a new generation of higher-performance electrical and optical integrated circuits. The availability of an optical port that is synchronized with the electrical ports opens various possibilities in the integration of transistors and lasers on a chip set to overcome the limitations in the speed of copper interconnects, Feng explained. Currently, such integration is complex and costly.
"We think the transistor laser can be the technology that enables this type of integration," he said. "The transistor laser is the building block of the integrated circuit. That is the beauty of it. The transistor laser can be operated as a transistor or a laser, and can be simultaneously used as both a transistor and a laser."
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