Ranjit Pati
Ranjit Pati
“Molecular computing is a whole new paradigm of the twenty-first century.”

The Incredible Shrinking Transistor

by Marcia Goodrich

Ever since UNIVAC predicted a landslide victory for Eisenhower in 1951, people have been driven to make electronic devices faster, cheaper, and much, much smaller. Ranjit Pati and Paul Bergstrom are two of the scientists at Michigan Tech who are exploring tiny technologies that could power the next wave of computing and make the iPhone look as quaint and stodgy as a two-ton mainframe.

Smallest of the Switches, Largest of Possibilities

Ranjit Pati and his team have developed a model to explain the mechanism behind computing’s elusive Holy Grail, the single molecular switch. If borne out experimentally, his work could help explode Moore’s Law and revolutionize computing technology.

Moore’s Law predicts that the number of transistors that can be economically placed on an integrated circuit will double about every two years. But by 2020, Moore’s Law is expected to hit a brick wall, as manufacturing costs rise and transistors shrink beyond the reach of the laws of classical physics.

A solution lies in the fabled molecular switch. If molecules could replace the current generation of transistors, you could fit more than a trillion switches onto a centimetersquare chip. In 1999, a team of researchers at Yale University published a description of the first such switch, but scientists have been unable to replicate their discovery or explain how it worked. Now, Pati, an associate professor of physics, thinks his team may have explained the mystery.

Applying quantum physics, he and his group developed a computer model of an organometallic molecule firmly bound between two gold electrodes. Then he turned on the juice.

As the laws of physics would suggest, the current increased along with the voltage, until it rose to a miniscule 142 microamps. Then suddenly—and counterintuitively—it dropped, a mysterious phenomenon known as negative differential resistance (NDR).

Pati was astonished at what his analysis of the NDR revealed.

Up until the 142-microamp tipping point, the molecule’s cloud of electrons had been whizzing about the nucleus in equilibrium, like planets orbiting the sun. But under the bombardment of the higher voltage, that steady state fell apart, and the electrons were forced into a different equilibrium, a process known as “quantum phase transition.”

“I never thought this would happen,” Pati said. “I was really excited to see this beautiful result.”

What’s so beautiful about it? A molecule that can exhibit two different phases when subjected to electric fields has promise as a switch: one phase is the “zero” and the other the “one,” which form the foundation of digital electronics.

Ultimately, the objective is to mimic the human brain. Brains, which have billions of neurons, can do more than one thing at a time: rehash last night’s hockey game, for instance, while driving to a lunch date and scratching an itch—what Pati calls “massively parallel” activity. As well, all of the brain’s neurons aren’t used at the same time; some are in “sleep” mode. The idea is to have computers do the same things—store memory, retrieve information selectively, process information, and communicate, “to do what the human brain does,” said Pati. “Molecular computing is a whole new paradigm of the twenty-first century.”

Pati is working with other scientists to test the model experimentally. His results appear in the article “Origin of Negative Differential Resistance in a Strongly Coupled Single Molecule-metal Junction Device,” published June 16, 2008, in Physical Review Letters. An abstract and a PDF file of the article are available at http://link.aps.org/abstract/PRL/v100/e246801.