 |
|
Scientists break light modulation speed record -- twice
June 16, 2009CHAMPAIGN, Ill. - Researchers have constructed a light-emitting transistor that has set a new record with a signal-processing modulation speed of 4.3 gigahertz, breaking the previous record of 1.7 gigahertz held by a light-emitting diode.
But, the researchers didn't stop there. By internally connecting the base and collector of a light-emitting transistor, they created a new form of light-emitting diode, which modulates at up to 7 gigahertz, breaking the speed record once again.
In a pair of papers published in the June 15 issue of Applied Physics Letters, researchers at the University of Illinois and at U. of I. licensee Quantum Electro Opto Systems in Melaka, Malaysia, report the fabrication and testing of the new high-speed light-emitting transistor and the new "tilted-charge" light-emitting diode.
"Simple in design and construction, the tilted-charge light-emitting diode offers an attractive alternative for use in high-speed signal processing, optical communication systems and integrated optoelectronics," said Nick Holonyak Jr., a John Bardeen Chair Professor of Electrical and Computer Engineering and Physics at Illinois, and a co-author of both papers.
The modulation speed of either a light-emitting diode or a light-emitting transistor is limited by the rate at which electrons and holes (the minus and plus charges - the carriers of current) recombine. The recombination lifetime is important in determining device speed.
With a usual "slow" recombination process, the speed of a light-emitting diode is limited to approximately 1.7 gigahertz, which corresponds to a carrier lifetime of 100 picoseconds. For more than 40 years, scientists thought breaking the 100-picosecond barrier was impossible.
Recombination speeds of less than 100 picoseconds are not readily achieved in light-emitting diodes because equal number densities of electrons and holes are injected into the active region to preserve charge neutrality, said Holonyak, who invented the first practical visible light-emitting diode more than 40 years ago.
These charges become stuck, stacked-up waiting to recombine, Holonyak said. To achieve high recombination speeds, an extremely high injection level and a very high charge population are required in light-emitting diodes. These conditions are not necessary in transistors, however.
"Unlike a diode, a transistor does not store charge," said Milton Feng, the Holonyak Chair Professor of Electrical and Computer Engineering, and a co-author of the two papers. "Charges are delivered to the transistor's quantum well active region, where they either recombine almost instantly, or they are kept moving on out of the device. The charges do not become stacked-up, waiting to recombine with their oppositely charged twins."
To increase the modulation speed of their light-emitting transistor, the researchers reduced the emitter size, increased the so-called collector thickness (the third terminal region), and utilized a special internal common collector design. These changes resulted in a faster signal at a very low current level, and at low heat dissipation.
Having a "fast" recombination process, the modulation speed of the light-emitting transistor was measured at 4.3 gigahertz, which corresponds to a recombination lifetime of 37 picoseconds, well under the "100-picosecond barrier."
"In the light-emitting transistor, the third terminal - the collector - effectively 'tilts' the charge and removes carriers with slower recombination lifetimes," said Holonyak, who also is a professor in the university's Center for Advanced Study, one of the highest forms of campus recognition.
"As opposed to the charge 'pile-up' condition found in a normal diode, the dynamic 'tilted' charge flow condition in the transistor base is maintained with the collector in competition with the base recombination process," Holonyak said. "If the charge doesn't recombine and generate a photon fast enough, it is swept away by the current in the collector."
By preventing the build-up of "slow" charges in the base, the "fast" picosecond recombination dynamics also provided the basis for the researchers' light-emitting transistor rewired internally as a new type of light-emitting diode.
The tilted-charge light-emitting diode achieved a record-breaking modulation speed of 7 gigahertz, corresponding to a recombination lifetime of 23 picoseconds.
"The tilted-charge light-emitting diode is simple to make, low cost, and easy to package and use," Holonyak said.
Because of the tilted base population in the device, current flow, which is a function of the slope of the charge distribution, makes possible high current densities without requiring extreme carrier densities.
"That's the trick of the transistor," Holonyak said. "And now we've incorporated it into a diode. The physics has been there all along. It just wasn't recognized."
University of Illinois at Urbana-Champaign
"Simple in design and construction, the tilted-charge light-emitting diode offers an attractive alternative for use in high-speed signal processing, optical communication systems and integrated optoelectronics," said Nick Holonyak Jr., a John Bardeen Chair Professor of Electrical and Computer Engineering and Physics at Illinois, and a co-author of both papers.
The modulation speed of either a light-emitting diode or a light-emitting transistor is limited by the rate at which electrons and holes (the minus and plus charges - the carriers of current) recombine. The recombination lifetime is important in determining device speed.
With a usual "slow" recombination process, the speed of a light-emitting diode is limited to approximately 1.7 gigahertz, which corresponds to a carrier lifetime of 100 picoseconds. For more than 40 years, scientists thought breaking the 100-picosecond barrier was impossible.
Recombination speeds of less than 100 picoseconds are not readily achieved in light-emitting diodes because equal number densities of electrons and holes are injected into the active region to preserve charge neutrality, said Holonyak, who invented the first practical visible light-emitting diode more than 40 years ago.
These charges become stuck, stacked-up waiting to recombine, Holonyak said. To achieve high recombination speeds, an extremely high injection level and a very high charge population are required in light-emitting diodes. These conditions are not necessary in transistors, however.
"Unlike a diode, a transistor does not store charge," said Milton Feng, the Holonyak Chair Professor of Electrical and Computer Engineering, and a co-author of the two papers. "Charges are delivered to the transistor's quantum well active region, where they either recombine almost instantly, or they are kept moving on out of the device. The charges do not become stacked-up, waiting to recombine with their oppositely charged twins."
To increase the modulation speed of their light-emitting transistor, the researchers reduced the emitter size, increased the so-called collector thickness (the third terminal region), and utilized a special internal common collector design. These changes resulted in a faster signal at a very low current level, and at low heat dissipation.
Having a "fast" recombination process, the modulation speed of the light-emitting transistor was measured at 4.3 gigahertz, which corresponds to a recombination lifetime of 37 picoseconds, well under the "100-picosecond barrier."
"In the light-emitting transistor, the third terminal - the collector - effectively 'tilts' the charge and removes carriers with slower recombination lifetimes," said Holonyak, who also is a professor in the university's Center for Advanced Study, one of the highest forms of campus recognition.
"As opposed to the charge 'pile-up' condition found in a normal diode, the dynamic 'tilted' charge flow condition in the transistor base is maintained with the collector in competition with the base recombination process," Holonyak said. "If the charge doesn't recombine and generate a photon fast enough, it is swept away by the current in the collector."
By preventing the build-up of "slow" charges in the base, the "fast" picosecond recombination dynamics also provided the basis for the researchers' light-emitting transistor rewired internally as a new type of light-emitting diode.
The tilted-charge light-emitting diode achieved a record-breaking modulation speed of 7 gigahertz, corresponding to a recombination lifetime of 23 picoseconds.
"The tilted-charge light-emitting diode is simple to make, low cost, and easy to package and use," Holonyak said.
Because of the tilted base population in the device, current flow, which is a function of the slope of the charge distribution, makes possible high current densities without requiring extreme carrier densities.
"That's the trick of the transistor," Holonyak said. "And now we've incorporated it into a diode. The physics has been there all along. It just wasn't recognized."
University of Illinois at Urbana-Champaign
Light-emitting Diode Current Events and Light-emitting Diode News RSSSRI International and Showa Denko K.K. Announce Breakthrough Performance in Organic Light-Emitting Diode (OLED) Devices for Solid-State Lighting Applications
SRI International, an independent nonprofit research institute, and Showa Denko K.K. (SDK), a Japan-based chemical industry company in partnership with Itochu Plastics Inc. (CIPS), have achieved record-breaking results using SRI's new cavity organic light-emitting diode (COLED) technology and SDK's light-emitting polymers to produce a highly efficient light source that could one day replace incandescent and fluorescent light bulbs.
University of Miami physicist develops battery using new source of energy
Researchers at the University of Miami and at the Universities of Tokyo and Tohoku, Japan, have been able to prove the existence of a "spin battery," a battery that is "charged" by applying a large magnetic field to nano-magnets in a device called a magnetic tunnel junction (MTJ).
UConn chemists find secret to increasing luminescence efficiency of carbon nanotubes
Chemists at the University of Connecticut have found a way to greatly increase the luminescence efficiency of single-walled carbon nanotubes, a discovery that could have significant applications in medical imaging and other areas.
Bright idea illuminates LED standards
The lack of common measurement methods among light-emitting diode (LED) and lighting manufacturers has affected the commercialization of solid-state lighting products.
Duke team explains a longtime visual puzzler in new way
A team of neuroscientists at Duke University Medical Center has suggested an entirely new way to explain a puzzling visual phenomenon called the flash-lag effect.
Visualizing atomic-scale acoustic wavesin nanostructures
Acoustic waves play many everyday roles - from communication between people to ultrasound imaging. Now the highest frequency acoustic waves in materials, with nearly atomic-scale wavelengths, promise to be useful probes of nanostructures such as LED lights.
New coating is virtual black hole for reflections
Researchers have created an anti-reflective coating that allows light to travel through it, but lets almost none bounce off its surface. At least 10 times more effective than the coating on sunglasses or computer monitors, the material, which is made of silica nanorods, may be used to channel light into solar cells or allow more photons to surge through the surface of a light-emitting diode (LED).
Nanoscientists Provide New Picture of Semiconductor Material
For almost a decade, scientists thought they understood the surface structure of cubic gallium nitride, a promising new crystalline semiconductor.
Shadow technique improves measurement of micro holes
Sometimes seeing a shadow can be as good or better than seeing the real thing. A new measurement method developed by researchers working at the National Institute of Standards and Technology (NIST) is a case in point
More Light-emitting Diode Current Events and Light-emitting Diode News Articles
 |
Light-Emitting Diodes by E. Fred Schubert (Author) Revised and fully up-dated, the second edition of this graduate textbook offers a comprehensive explanation of the technology and physics of LEDs such as infrared, visible-spectrum, ultraviolet, and white LEDs made from III-V semiconductors. Elementary properties such as electrical and optical characteristics are reviewed, followed by the analysis of advanced device structures. With nine additional chapters, the treatment of LEDs has been vastly expanded, including new material on device packaging, reflectors, UV LEDs, III-V nitride materials, solid-state sources for illumination applications, and junction temperature. Radiative and non-radiative recombination dynamics, methods for improving light extraction, high-efficiency and high-power device designs, white-light emitters with... |
 |
Super Ultra-Bright Red 5mm LED 6000 mcd by Parts Express High-intensity 6000 mcd ultra-bright LED. This T1-3/4 (5 mm) LED is water clear in its off state and lights very bright red at only 20 mA. 30° viewing angle. Typical forward voltage: 1.8~2.4 V. |
 |
Introduction to Light Emitting Diode Technology and Applications by Gilbert Held (Author) Recent improvements in LED technology have made them as ubiquitous as cell phones. In fact, LEDs light up almost all cell phones screens. The technology’s myriad applications and low energy use have made it nearly impossible to get through daily chores without coming in contact with LEDs. Probable advances include increased ability of the technology to support more efficient lighting and enhanced communications. With balanced coverage of the basics and future developments, Introduction to Light Emitting Diode Technology and Applications takes you on a tour of the LED evolution. The book begins with a brief history of the effort to enable the device that generates light through modern organic LEDs and reviews the fundamentals and principles of light prior to a... |
 |
Flex Neck Reading Light (Baseball) by LightWedge The popular Flex Neck™ Reading Light now comes in fun colors and patterns! Perfect for kids of all ages, the Flex Neck clips anywhere and points two bright LEDs wherever you need light the most. And for readers on-the-go, the head snaps into the clip to form a secure loop. |
 |
Super Ultra-Bright White 5mm LED 18000 mcd by Parts Express When only the best and the brightest LEDs will do for your project, select Super Ultra-Bright LEDs. On average, one of these LEDs is about twice as bright as its standard Ultra-Bright counterpart. Each Super Bright LED is designed with a clear, 5 mm diameter lens (T 1-3/4). |
 |
Brilliant!: Shuji Nakamura And the Revolution in Lighting Technology by Bob Johnstone (Author) A revolution in the way we use artificial lighting is underway, one that is every bit as sweeping and significant as Edison’s invention of the light bulb. The technology of light emitting diodes (LEDs) is ready for widespread implementation. Its impacts will include a reduction in energy consumption for electric lighting by up to 80 percent. Brilliant! tells the story of Shuji Nakamura, a gifted Japanese engineer who came out of nowhere to stun the world with his announcement that he had created the last piece in the puzzle needed for manufacturing solid-state white lights. The invention of this holy-grail product, which promises to make Edison’s light bulb obsolete, had eluded the best minds at the top electronic firms for twenty-five years. Until his startling announcement,... |
 |
Ultra-Bright White 5mm LED 5000 mcd by Parts Express Select only ultra-bright LEDs for your next project. A high intensity design offers you the ability to dial in just the right amount of light for your application. Choose from a wide variety of colors. Each ultra-bright LED features a clear lens, and a 5 mm diameter (T1-3/4). |
 |
LightWedge Harry Potter LUMOS Book Light by LightWedge The Harry Potter LUMOS Book Light brings the magic of the LightWedgento Harry Potter fans of all ages! The Harry Potter LUMOS Book Light features colorful Harry Potter graphics and seven interchangeable switch covers that allow Harry Potter fans to customize their magical LightWedge. Recipient of the prestigious Oppenheim Toy Portfolio Platinum Seal Award. |
 |
LED Light bulb for automotive applications, 24LED 1156 12V, Amber by Cyron The brightest automotive LED bulb in the market. Automotive LED bulbs are becoming more popular than ever since many new cars come with LED clusters from the factory. LED bulbs may not function properly as turn signals in all automotive applications. Please consult with your automotive specialist. |
 |
LED Exit Light with Battery Backup by Simkar This fixture combines a modern, low profile design with LED energy saving technology. This exit sign comes with 2 faces and a back plate standard. The energy efficient red LED have a life of 25+ years and use less than 4 watts of energy (compared to 30-40 watts in incandescent exit sign fixtures). The battery backup operates for a minimum of 90 minutes in emergency situations. The fixture may be side of top mounted. |
| © 2009 BrightSurf.com |