27-10-2012, 05:10 PM
The Transistor Laser
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Introduction
For years we've been hearing about all the fantastic things computers will be able to do once they process data with light instead of electricity. The mysteries of the universe will be unlocked. A golden age of limitless computing power and bandwidth will usher in a techno-utopia. Don’t believe the hype.
Setting aside the question of whether an all-optical processor would even be desirable, optical computing schemes lack the photonic equivalent of the most fundamental computing element, the transistor. That device--first demonstrated in 1947, when John Bardeen and Walter H. Brattain stuck two cat-whisker wires onto a germanium base and showed power gain from one wire, called the emitter, to the other, called the collector--spawned the US $300-billion-per-year semiconductor industry. The transistor makes possible our digital lifestyle: cellphones and PCs, digital cameras and MP3 players, medical imaging systems and set-top boxes, supercomputers and the Internet, and more.
The transistor has given us much during the last six decades. Now it has given us light, and with that, the potential for much speedier broadband communications in both telecommunications networks and within and between chips--not in some remote sci-fi future, but perhaps within the next decade.
Our team at the University of Illinois at Urbana-Champaign has at its disposal an extraordinary prototype transistor that can switch on and off more than 700 billion times per second, faster than any other transistor in the world. On a hunch two years ago, we inspected in greater depth some samples of this transistor, which are made from indium phosphide and indium-gallium-arsenide, the same sort of semiconductor compounds used in today's light-emitting diodes and laser diodes.
Theory
Implementation:
The Transistor laser can be thought as two back to back diode separated by a thin connection layer. In this device the quantum well is a layer of Indium-gallium-arsenide no more than 10 nanometers thick. Inserted into the HBT (heterojunction bipolar transistor) base region, the quantum well acts like a special recombination center that governs the flow of charge from emitter to collector.
The development of the transistor laser has been going on for over twenty five years, but only recently two professors from University of Illinois named Milton Fang and Nick Holon yak were able to create a transistor that switched on and off faster than 700,000,000,000 times per second.
The development of the transistor laser has been going on for over twenty five years, but only recently two professors from University of Illinois named Milton Feng and Nick Holon yak were able to create a transistor that switched on and off faster than 700,000,000,000 times per second.
Methodology:
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.
Small-signal modeling of the transistor laser:
The base of a transistor laser and of the high-speed dynamics of this novel device. The modeling is based on analytically solving the continuity equation and the rate equations, which incorporate the virtual states as a conversion mechanism. Wide-band operation of the transistor laser along with the suppression of the relaxation oscillation frequency peak is demonstrated.
The transistor laser First introduced in [1], and it was predicted that this device could have a high-speed modulation bandwidth of up to 100 GHz [2]. In the transistor laser (TL), a quantum well (QW) is embedded in the base of the bipolar junction transistor (BJT) and acts as an optical collector. The stimulated recombination causes compression in the I-V characteristics of the transistor, and the current gain of the transistor (β=IC/IB) decreases (βstim < βspon). The interesting feature of the TL is the potential for an enhanced small-signal modulation bandwidth due to the reduced carrier lifetime in the base region. In [2], the authors present a model based on the charge control method and laser rate equations, which predicts a large intrinsic modulation band- width. However the model does not differentiate between the bulk carriers and the QW carriers in the carrier rate equation, and significantly over estimates the bandwidth. A more complete model would include the effects of the capture and escape lifetimes in the QW by using two-level rate equations [3]. Integrating the rate equations with the diffusion process in the base requires the introduction of the concept of virtual states localized in the QW, together with the quantum capture and escape effects [4]. In this work, a transistor laser model including diffusion, quantum capture and escape, and the laser rate equations is developed to calculate the minority carrier density distribution and the photon dynamics. We demonstrate that transistor lasers modulated by a small-signal base current are limited by the same relaxation oscillations that limit conventional semiconductor lasers, but with an improvement in carrier dynamic effects.
Applications
A Natural for Optoelectronic Integrated Circuits:
Since it was first demonstrated by John Bardeen and Walter H. Brattain in 1947, the transistor has spawned a $300 billion semiconductor industry and has made possible our digital way of life, enabling computers, cell phones, MP3 players and the Internet. Now it is poised to give us even more. As a source of coherent light, the transistor laser offers the potential for much faster broadband communications, both for long-haul telecommunications networks and for short-haul connections within and between chips.
Moreover, there are bound to be applications that we cannot even imagine. For example, when the first practical visible LED was invented in the early 1960s, no one could have guessed that it would wind up in traffic lights and key chain fobs and that it eventually could revolutionize traditional lighting, becoming the basis of a global optoelectronics industry worth billions of dollars.
The light-emitting transistor and the transistor laser could have equally profound technological and economic impacts.
Over the past decades, the number of transistors on an integrated circuit chip has doubled almost every 18 months — so predictably, in fact, that the effect was dubbed Moore’s law. However, the physical size of the chip has remained relatively unchanged because of the limitations of metal interconnects. Metals are too resistive, and they slow the operation of microprocessors. If we were to make larger chips, the transistors would have to interact over greater distances. This would result in more-resistive paths and longer delays, limiting the speed and increasing the crystalline real estate that requires cooling.