FIG. 1: In this rendition of the Quantum Interference Effect Transistor (QuIET) by artist Helen Giesel, the central benzene ring consists of six carbon atoms (green), accompanied by atoms of hydrogen -(purple) and sulfur (yellow). The three structures of gold-colored atoms represent the three metallic leads of the transistor.
As most tech-savvy folks know, the ongoing shrinkage of electronic components cannot continue forever. At some point, conventional transistors will succumb to quantum effects, rendering them useless in integrated circuits. So here's a novel idea: why not use those same quantum effects to facilitate molecular-sized transistors? That's just what a team of physicists at the University of Arizona (www.arizona.edu) is working on. Research on the Quantum Interference Effect Transistor (QuIET) aims to overcome the limitations placed on conventional transistors by using the wave properties of electrons to turn current flow on and off.
“All transistors in current technology, and almost all proposed transistors, regulate current flow by raising and lowering an energy barrier,” says UA physicist Charles A. Stafford. “Using electricity to raise and lower energy barriers has worked for a century of switches, but that approach is about to hit the wall.”
Transistor elements in current ICs measure about 65 nanometers (1 nm is a billionth of a meter) across, and shrinking them smaller than about 25 nm is impractical. Three years ago, UA physicists Stafford, Sumit Mazumdar, and David Cardamone began thinking about how to use quantum mechanics to regulate the flow of current through a single-molecule transistor measuring as small as 1 nm.
The trick is to use the wavelike nature of individual electrons. As with sound and water waves, electron waves interfere with each other. When they are in phase, the interference is constructive, resulting in greater amplitude. When they are out of phase, they interfere destructively, causing the amplitude to decrease.
“Our approach is more finesse than brute force,” says Cardamone. “We don't put up a wall to stop current. We regulate how electron waves combine to turn the transistor on or off.” The initial iteration of this idea uses a benzene molecule, which consists of six carbon atoms in a ring (see Fig. 1). Other researchers have succeeded in attaching two leads to such a molecule, but the UA team expects three-lead devices to be just around the corner. That will allow one lead to act as a valve for current flowing between the other two.
“In classical physics, currents through each arm of the ring would just add,” Stafford explains. “But quantum mechanically, the two electron waves interfere with each other destructively, so no current gets through. That's the ‘off’ state of the transistor.” The transistor is turned on by changing the phase of the electron waves so that they interfere constructively.
According to Stafford, “It took a while to go from the idea of how this could work to developing realistic calculations of this kind of system. Within a few weeks, we were able to do the simplest kind of quantum chemical calculations that neglect interactions between different electrons. But it took some time to put in all the electron interactions that demonstrate that this really is a very robust device.” The team has determined that the QuIET has characteristics very similar to conventional field-effect and bipolar-junction transistors.
The QuIET research could result in much smaller, more powerful computers and other devices with the ability to process many more channels of high-resolution audio and video than today's music products can manage. Another possibility is the emergence of integrated devices that combine several advanced functions into one tiny unit with far more power than anything currently available. How ironic it is that a technology called QuIET could help make a joyous noise.