Bell Labs Scientists Build the World's Smallest Transistor

Nov. 9, 2001
Scientists from Lucent Technologies' Bell Labs, building upon their recent breakthrough in molecular-scale transistors, have now fabricated an individually addressable transistor whose channel consists of just one molecule

Scientists from Lucent Technologies' Bell Labs, building upon their recent breakthrough in molecular-scale transistors, have now fabricated an individually addressable transistor whose channel consists of just one molecule, a feat never previously accomplished.

The channel - the space between its electrodes - is where the transistor's electronic switching and amplification take place.

The tiny transistors are so small - only a billionth of a meter each - that approximately ten million of them would fit on the head of a pin. Made of an unconventional organic semiconductor material and using a novel fabrication technique, they may lead to smaller, faster and cheaper computer chips in the future.

Last month, the same Bell Labs team - physicist Hendrik Schon and chemists Zhenan Bao and Hong Meng - unveiled a transistor with a single-molecule channel length. But that device could only be fabricated as a matrix of a few thousand molecules that worked in tandem. Now, in a major advance, the team has succeeded in fabricating molecular-scale transistors that can be individually controlled.

The power of computer chips typically increases as the size of its transistors shrinks. Bell Labs' single-molecule transistor is less than a tenth the size of any transistor made previously.

``This work pushes the miniaturization of electronics to its final frontier,'' said Federico Capasso, physics research vice president at Bell Labs. ``It may become the cornerstone of a new nanoelectronics era.'' The breakthrough is described in an article published by the journal Science on their Science Express web site (

Scientists have been looking for alternatives to conventional silicon electronics for many years because they anticipate that the continuing miniaturization of silicon-based integrated circuits will peter out in approximately a decade as fundamental physical limits are reached. Some of this research has been aimed at producing molecular-scale transistors, in which single molecules are responsible for the transistor action - switching and amplifying electrical signals.

Bell Labs' ``nanotransistors'' - so-called because they are approximately a nanometer, or one-billionth of a meter, in size - appear to rival conventional silicon transistors in performance. They are made using a class of organic (carbon-based) semiconductor material known as thiols. In addition to carbon, thiols contain hydrogen and sulfur.

The main challenges in making nanotransistors are fabricating electrodes 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.

They carved a notch into a silicon wafer and deposited a layer of gold at the bottom to function as one of the transistor's three electrodes. Then they dipped the wafer into a solution that contained a mixture of thiol molecules and some inert organic molecules, and let it dry. The purpose of adding the inert molecules was to dilute the concentration of thiols. As the solution evaporated from the wafer, a film exactly one molecule thick was left behind on the gold electrode. By carefully adjusting the ratio of the thiol to the inert molecules, the scientists were able to statistically ensure that just one active molecule was present in the area on top of the gold electrode. They then deposited another gold electrode on top of this film, while they built the transistor's third electrode on one side of the silicon notch.

``It is virtually impossible to attach three electrodes to a microscopically small molecule,'' said Bao. ``We overcame this problem by letting the molecule find these contacts and attach itself to them, a process called 'self-assembly.' '' The chemical self-assembly technique is relatively easy and inexpensive and, unlike silicon, does not require clean room technology. ``Our experiment shows that it is possible to realize transistor action in a single molecule without sophisticated fabrication procedures,'' said Schon.

Using two nanotransistors, the Bell Labs scientists built a voltage inverter, a standard electronic circuit module commonly used in computer chips that converts a ``0'' to a ``1'' or vice versa. Though just a prototype, the success of this simple circuit suggests that nanotransistors could one day be used in microprocessors and memory chips, squeezing thousands of times as many transistors onto each chip than is possible today.

David Goldhaber-Gordon, a professor at Stanford University, commented that the Bell Labs scientists ``have achieved several impressive advances toward nanoelectronics. The fabrication technique is particularly elegant in its simplicity.''

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 have made many of the important contributions that have helped make transistors smaller, faster and more powerful. The technology curve has culminated with the latest development of single-molecule nanotransistors.

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

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

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