Transistor Lasers Constantine Kapatos Moses Farley Vicki Kaiser Philip Furgala Melroy Machado. Courtesy hispamp3.com. The History of the Laser Transistor.
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Transistor LasersConstantine KapatosMoses FarleyVicki KaiserPhilip FurgalaMelroy Machado
The History of the Laser Transistor
Light was detected at the base of the transistor and the creation of the transistor laser had occurred.
How They Work
L.A.S.E.R.: Light Amplification by Stimulated Emission of Radiation
The transistor laser combines the functions of both a transistor and a laser by converting electrical input signals into two output signals, one electrical and one optical. Photons for the optical signal are generated when electrons and holes recombine in the base, an intrinsic feature of transistors.
The structure for the transistor laser is a Bipolar Junction Transmitter (BJT), which is a solid-state, semiconductor device which uses electrons and holes to carry the main electric current, and is often used in amplifying/switching applications like this laser.
It is essentially two back-to-back diodes separated by a thin, connecting base-layer.
When voltage is applied to the base-emitter junction, injected electrons from the emitter diffuse across the base. The base is thin enough that most of the electrons can pass through to the collector before recombining with holes in the p-type base.
The semiconductor compounds in the transistor laser are Gallium-Arsenide (GaAs) and Indium-Gallium-Phosphide (InGaP), III-VI compounds (from the periodic table).
GaAs and InGaP are direct band-gap materials, an electron that has been excited into the conduction band can easily fall back to the valence band through the creation of a photon (of little momentum) whose energy matches the band-gap energy.
The transistor laser light beam with a infrared wavelength labeled "hv" at the top is captured by CCD camera. The contact probes (dark shadow) on the Emitter, Base and Collector.
So, these materials will readily produce light (photons)….
A voltage at the emitter injects electrons into the base. In the well, more electrons combine with holes, a process which emits light.
The light is reflected off mirrors around the inside of the well to form a resonant cavity. Light is increasingly stimulated until a beam of laser light escapes.
The device can be switched on and off rapidly (billions of switches per second), and produces optical and electrical signals.
Electrons that don’t recombine with holes in the well or the base go into the collector, which exhibits a current gain.
The quantum well in the transistor laser acts as a recombination center that governs the flow of charge from the emitter to the collector. The quantum well takes in electrons from the base as they move from emitter(input) to collector(output), thus ‘trapping’ the electrons and quantizing energy levels.
This process decreases the current gain of the transistor by approximately 90%, but as seen in the previous figure, the recombination of electrons and holes is increased, thus increasing the photon production, thus increasing the strength of the outputted light from the base, as well as the electrical signal from the collector.
To turn this light into a laser beam, the edges of the transistor are modified:
The crystal is cut to make the opposite ends of the recombination region reflective, creating a resonant cavity, so the photons bounce between the reflective ends, stimulating the emission of additional photons that are in phase with the others generated in the region.
When the light-emitting transistor begins operating as a laser at a near-infrared wavelength of 1006 nm, the spontaneous signal scattered about in the crystal shifts to an intense directed signal - a coherent laser beam that can be toggled on and off 10 billion times per second. The point at which lasing (coherent radiation emission by the laser) begins, called the lasing threshold, depends on several factors, including current and ambient temperature. And only recently has the technology evolved such that we can operate transistor lasers at room temperature – thus making them possible for commercial usage.
How the Transistor laser is made:
The Transistor laser can be thought as two back to back diode separated by a thin connection layer, a base layer
Advantages and Disadvantages of Transistor lasers
Process data with light instead of electricity.
Faster broadband communication
Input electrical signals output electrical & optical
Integrate transistor lasers into devices and route out signals
Ways to exploit fast transistors that output signals in two different modes simultaneous
Potential Radiation Exposure
Future Uses of Transistor Lasers
-Used as optoelectronic interconnects
Holonyak, Nick Jr. and Feng, Milton. The Transistor Laser. February 2006. Spectrum, IEEE. 25 April 2006. <http://www.spectrum.ieee.org/feb06/2800/1>
Kloeppel, James E. Hidden Structure Revealed in Characteristics of Transistor Laser. 10 April 2006. Science Daily, Science Daily LCC. April 25 2006. <http://www.sciencedaily.com/releases/2006/04/060410164025.htm>
Kloeppel, James E. New transistor laser could lead to faster signal processing. 15 November 2004. News Bureau, University of Illinois at Urbana-Champaign. 25 April 2006. <http://www.photonics.com/content/news/2006/April/7/82059.aspx>