We've had light-emitting diodes and lasers for more than 40 years, transistors for almost 60. But until now, we haven't had a single device that can take an electrical input and simultaneously output both an electrical signal and an optical signal.
The emergence of the transistor laser has taken a long time, but it's not because of lack of interest--but because of a dearth of ideas. Researchers have measured light emission from transistors before. In the early 1980s, a research group at the California Institute of Technology, in Pasadena, led by graduate student Joseph Katz even fabricated a few experimental devices they called translasers. Using a wire, they integrated a transistor with a laser diode to fashion a device that could produce both electrical signals and laser beams, though not both simultaneously.
In 1992, researchers at the Interuniversity MicroElectronics Center, in Leuven, Belgium, built an indium-gallium-arsenide bipolar transistor that emitted light when cooled to liquid nitrogen temperatures. Some groups have even reported that certain transistors emit light at room temperature. However, a transistor working as a transistor has never before shifted its operation into stimulated emission of light--that is, into generating an output laser signal--while simultaneously delivering an electrical signal with gain.
Now, based on our recent work at the University of Illinois, there is such a dual-output device, and a sound reason to envision a future where high-speed computing meets the next generation of broadband communications.
The ability to send and receive signals at the equivalent of three DVDs worth of data--100 billion bits--per second could turn those herky-jerky teleconferences between offices in Tokyo and New York City into high-resolution events. Grandparents in Montreal could watch a granddaughter in a school play in St. Louis through a video cellphone. Supercomputer grids crunching the test data from the world's most advanced particle accelerators might produce results in minutes instead of days. And if you think Internet searching is fast now, wait until servers with transistor-laser-equipped microprocessors pluck answers to the most obscure queries from the hundreds of billions of video, audio, and text files expected to be available online in the next few years.
We're talking instant gratification for the culturally insatiable, real-time entertainment on demand: you'll order Enrico Caruso singing an aria from Rigoletto, the third episode of "The Honeymooners," and the full text of Gravity's Rainbow, and download it all in less time than it takes you to speak requests into your set-top box.
To grasp the profound shift in computing and communications that the transistor laser could make possible, consider how the transceivers in today's optical-fiber communications networks work. First, a computer sends an electrical signal to an optical transmitter, where the signal is converted into pulses of light. The transmitter contains a laser and an electrical driver, which uses the source data carried in the electrical signal to modulate the laser beam, turning it on and off to generate 1s and 0s that travel on the beam through glass fiber. At the end of the fiber, a photodetector reads and converts the data encoded in the photons back into electrical data.
Now imagine implementing a similar process inside a chip, at the transistor level. We can use transistor lasers to convert electrical signals into optical signals and vice versa. In future electro-optical processors, transistor lasers could help route photons through circuits made of waveguides specially designed to take advantage of light's high speed and practically lossless efficiencies over short distances.
Instead of using relatively slow wires to connect chips stacked together in packages, we could use transistor lasers as optical interconnects. These would let data flow instantaneously to and from memory chips, graphics processors, and microprocessors, supercharging weather forecasting and online banking, security checks and telesurgery, airline reservation systems and video games, just about any application.
And then there are the applications we can't even imagine yet. After all, when one of us--Nick Holonyak Jr.--invented the first practical light-emitting diode in the early 1960s, no one could have guessed that it would wind up in traffic lights and key-chain fobs, revolutionize the traditional lighting industry, and become the basis of a global optoelectronics industry worth billions [see photo, " Laser Leaders"].
That industry today is growing at a phenomenal pace. Reed Electronics Research, in Oxon, England, projects that the worldwide market for LEDs will grow from $5.9 billion this year to $9.8 billion in 2008; the market for laser diodes will rise from $4.8 billion to $7.9 billion. The transistor laser that our group is developing could have an equally profound technological and economic impact.
To understand why our transistor laser is uniquely suited to emitting both electrical and optical signals, you need to understand both the workings of the transistor and the basic light-emitting diode, the workhorse of optoelectronics. And to understand the LED, you need to know a little about band gaps and crystals.
In a semiconductor crystal, the arrangement of atoms results in distinct bands of closely spaced energy levels; these determine the energy states of the crystal's electrons. Generally, only two bands matter: the valence band, which contains the energy levels normally occupied by electrons, and the band just above it, called the conduction band. Electrons energetic enough to reach the conduction band are free to accelerate under the influence of an electric field, thereby constituting current. The difference in energy between the top of the valence band and the bottom of the conduction band is known as the band gap, with typical energies ranging in wavelength from the infrared through the visible.
Normally, electrons occupy the valence band, but jolt them with the right dose of heat, light, or voltage, and they will hop to the conduction band, leaving behind a hole, which is basically the absence of an electron in the crystal lattice.
This electron-hole pair is ephemeral, however; sooner or later, the electron falls back to the valence band and recombines with a hole. Because energy is always conserved, this recombination of an electron and a hole is accompanied by the release of energy. In the case of an LED or a laser, a photon is released, whose energy matches the difference between the conduction band and the valence band--the band-gap energy.
But in addition to energy, electrons also have momentum. In indirect-band-gap materials such as silicon and germanium, the minimum energy in the conduction band and the maximum energy in the valence band occur at different values of electron momentum.
Because of that, an electron in the conduction band can recombine with a hole in the valence band to produce a photon only if a source of momentum of the right magnitude, such as a vibration in the crystal lattice--a phonon--is generated and assists in conserving momentum in the process. This is a low-probability effect, and defects and heat-generating phonons do the work, yielding very little light. Indeed, so few photons came out of Bardeen and Brattain's primordial device that they probably wouldn't have detected it even if they had tried.