SCIENCE & ENGINEERING NEWS
San Diego, CA — A new breed of transistors takes advantage of so-called quantum effects that grow in importance as device dimensions scale down to a few nanometers. In this regime, the location of an individual electron is controlled. And electrons can tunnel through energy barriers that would bar their way in the world of classical physics. In the September IEEE Spectrum article, the author describes several types of quantum devices now under development.
The smaller quantum transistors are made, the better they perform. So device density depends mainly on the ever-growing ability of manufacturing processes to shrink feature sizes – even down to atomic-scale dimensions. Because only a handful of electrons operate the devices, switching frequencies approaching a terahertz are foreseeable.
One new quantum device is the double-electron-layer tunneling transistor (Deltt), built by researchers at Sandia National Laboratories in Albuquerque, N.M. Though Deltt developers are still in the early stages of exploration, they are pinning their hopes for high speed on performance already obtained with quantum devices similar to the Deltt, except that they have two terminals, compared to the Deltt’s three. Called resonant tunnel diodes, they have been shown to operate at up to 700 GHZ.
Other researchers are using the resonant tunnel diodes directly and pairing them with conventional transistors to boost the latter’s performance.
Another type of quantum device, the single-electron transistor, sometimes called the quantum dot transistor, shows great promise for nonvolatile memory, and research groups worldwide are developing them. The quantum dots are minuscule conducting islands to which individual electrons can be transferred. For quantum dot transistors to work at room temperature, the dots can be only a few nanometers in diameter.
Another new device for use in a flash (or nonvolatile) memory is the nanocrystal memory cell, a type of single-electron memory cell. It operates at room temperature and should prove to have faster read and write times than conventional nonvolatile memories.
Quantum cellular automata represent a fourth type of device. The automata are cells that contain four quantum dots arranged in a square. An extra electron resides on each dot of one diagonal: the diagonal that contains the electrons determines if the cell stores a logical 1 or a 0. By interacting with appropriately arranged neighboring cells, the cells perform all the necessary logic functions. The dots can be made of a metal like aluminum, but molecular arrangements are also possible. Visit http://www.spectrum.ieee.org for more information.