The size of a transistor's channel -- the space between its electrodes -- influences its output current and switching speed.

In these new molecular-scale transistors, fabricated by a multidisciplinary team of Bell Labs researchers, the length of one molecule defines the channel's physical dimension; it is more than a factor of ten smaller than anything that has been demonstrated even with the most advanced lithography techniques.

The breakthrough is reported in the October 18th issue of the journal Nature.

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 subside 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.

A schematic drawing of a molecular-scale transistor, roughly a million times smaller than a grain of sand. Graphic by Lucent Technologies

Bell Labs scientists Hendrik Schon, Zhenan Bao and Hong Meng have now succeeded in fabricating molecular-scale transistors that rival conventional silicon transistors in performance, using a class of organic (carbon-based) semiconductor material known as thiols.

"When we tested them, they behaved extremely well as both amplifiers and switches," said Schon, an experimental physicist who was the lead researcher.

Using the tiny transistors, which are roughly a million times smaller than a grain of sand, the team 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 the simple circuit suggests that molecular-scale transistors could one day be used in microprocessors and memory chips, squeezing thousands of times as many transistors onto each chip than is possible today.

The main challenges in making molecular-scale transistors 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 in which each electrode is shared by many 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."

"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 inherent in other nanofabrication approaches."

The chemical self-assembly technique is relatively easy and inexpensive, but is key to reducing the transistor's channel length. The channel length of the experimental transistors is between one and two nanometers (billionths of a meter), an order of magnitude smaller than any transistor channel created before.

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 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.

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