Oct. 18, 2001 MURRAY HILL, N. J. - Scientists from Lucent Technologies' Bell Labs have created organic transistors with a single-molecule channel length, setting the stage for a new class of inexpensive and easily assembled molecular electronics based on compounds of carbon.
The scientific breakthrough is being reported in the October 18th issue of the journal Nature.
Scientists have been looking for alternatives to conventional silicon electronics because they anticipate that the continuing miniaturization of silicon-based integrated circuits will subside in approximately a decade because of fundamental physical limits.
A transistor is a three-electrode semiconductor device, conventionally made of an inorganic semiconductor like silicon. It amplifies electrical signals and acts as an electronic switch. The transistor's channel is the space between two of its electrodes that influences the transistor's current output and switching speed.
Bell Labs scientists Hendrik Schon, Zhenan Bao and Hong Meng have now fabricated molecular-scale organic transistors, made out of compounds of carbon, which can rival silicon transistors in performance.
The multidisciplinary Bell Labs team used the tiny organic transistors, which are roughly a million times smaller than a grain of sand, to build a voltage inverter - an electronic circuit that converts a "0" to a "1' or vice versa. Voltage inverters are standard components found in all computer chips.
Researchers have sought for decades to create molecular-scale transistors, in which single molecules are responsible for the transistor action. The Bell Labs team built their transistors of gold and a class of organic semiconductor material known as thiols. The amplification and switching properties of the transistors are comparable to those of silicon transistors.
"When we tested them, they behaved extremely well as both amplifiers and switches," said Schon, an experimental physicist who was the lead researcher.
Though still a prototype, the demonstration of a simple circuit indicates that molecular-scale transistors could one day be used in computer microprocessors and memory chips, which could squeeze thousands of times as many transistors as are found in today's circuits into the same amount of space.
"This is a beautiful, simple and clever approach," said Professor Paul Weiss of the Pennsylvania State University, an expert in molecular electronics. "It circumvents many of the difficulties with other nanofabrication approaches."
For now, scientists continue to work on resolving the practical issues that stand in the way of commercially available molecular electronics.
Bell Labs' transistor legacy
Bell Labs has a long and illustrious connection with transistors. William Shockley, John Bardeen and Walter Brattain invented the transistor at Bell Labs in 1947. Their invention spawned the digital age and earned them the Nobel Prize for Physics in 1956. Over the years, Bell Labs scientists made many important contributions as transistors became smaller, faster and more powerful. The technology curve has culminated with the latest development of molecular-scale transistors.
"The molecular-scale transistors that we have developed may very well serve as the historical 'bookend' to the transistor legacy started by Bell Labs in 1947," said Federico Capasso, physical research vice president at Bell Labs.
The scientific and technical challenges of molecular assembly
The main challenges in making molecular-scale transistors are fabricating electrodes separated that are separated by only a few molecules and attaching electrical contacts to the tiny devices. The Bell Labs researchers were able to overcome these hurdles by using a self-assembly technique and a clever design in which the electrodes were shared by many molecular-scale transistors.
"We solved the contact problem by letting one layer of organic molecules self-assemble on one electrode first, and then placing the second electrode above it," said Bao, an organic chemist. "For the self assembly, we simply make a solution of the organic semiconductor, pour it on the base, and the molecules do the work of finding the electrodes and attaching themselves."
The chemical self-assembly technique is easy and relatively inexpensive and determines the transistor channel length. The channel length of the Bell Labs molecular-scale transistors is 1 to 2 nanometers, less than a tenth the size of any channel that has been created, even with the most advanced lithography techniques. (Nanometers are a unit of measurement one-billionth of a meter long that are used to specify molecular sizes.) By way of comparison, a leading chip manufacturer announced a conventional 20-nanometer silicon transistor this past summer.
"This work shows the value of long-term research. Although there may be no practical applications for a decade, it could lead to a new paradigm in electronics," said Cherry Murray, Lucent's senior vice president of physical sciences research. "Gaps remain in the theoretical understanding of these tiny devices, however, and they would need to be closed before industrial applications would be possible."
"This is an extraordinary piece of work," said Professor Peter Littlewood, head of theory of condensed matter physics research at the University of Cambridge, U.K., and a former Bell Labs researcher. "Theoreticians will have a great deal of trouble explaining it."
Bell Labs research continues to push the frontiers of technology. With approximately 16,000 employees in 16 countries, Bell Labs is the leading source of new communications technologies. Bell Labs has generated more than 28,000 patents since 1925 and has played a pivotal role in inventing or perfecting key communications technologies, including transistors, digital networking and signal processing, lasers and fiber-optic communications systems, communications satellites, cellular telephony, electronic switching of calls, touch-tone dialing, and modems. Bell Labs scientists have received six Nobel Prizes in Physics, nine U.S. Medals of Science and six U.S. Medals of Technology. For more information about Bell Labs, visit its Web site at http://www.bell-labs.com.
Lucent Technologies, headquartered in Murray Hill, N.J., USA, designs and delivers networks for the world's largest communications service providers. Backed by Bell Labs research and development, Lucent relies on its strengths in mobility, optical, data and voice networking technologies as well as software and services to develop next-generation networks. The company's systems, services and software are designed to help customers quickly deploy and better manage their networks and create new, revenue-generating services that help businesses and consumers. For more information on Lucent Technologies, visit its Web site at http://www.lucent.com.
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