Sandia's quantum mechanical transistor may increase computer speed and sensor accuracy

QUANTUM MECHANIC: Sandia National Labs physicist Jerry Simmons inspects the end of a cryogenic sample holder for performing electrical measurements on the DELTT quantum mechanical transistor. Once loaded with DELTTs (in circular cases, foreground), the holder is lowered into a cryostat that varies in temperature from 0.3K to 300K, allowing transistor performances to be tested at different temperatures. Download 150dpi JPEG image, 'quantum.jpg', 1.2Mb

ALBUQUERQUE, N.M. -- Improvements in the transistor of the future may not rely on decreasing its size but rather on a radical change in operation made possible by a quantum mechanical transistor created at Sandia National Laboratories.

The quantum mechanical transistor is the equivalent of turning on a light bulb without closing a switch: Electrons "tunnel" from path to path through a barrier that, according to classical physics, is impenetrable.

The process takes place with extreme rapidity.

The term "tunneling" may bring to mind moles or the highway department, but physicists use it to describe an effect in which particles, like electrons, appear in places where by rights they should not be able to go. In effect, they have tunneled under an energy barrier the same way cars use a tunnel to appear at a new location without having to drive over an impossibly high summit. The atomic-scale effect is explained only by quantum mechanical principles.

A real device, not just theory
"We have demonstrated real circuits that work and are easily fabricated," says Jerry Simmons, leader of the Sandia development team. "It is not ready to be sold yet, but it is a significant advance."

The device, dubbed DELTT (Double Electron Layer Tunneling Transistor), offers promise of significant improvements in the speed of computers and in the accuracy of sensors. Sandia is a laboratory of the Department of Energy (DOE).

According to Paul Berger, professor of electrical and computer engineering at the University of Delaware, "I was impressed by the multitude of possible uses of the DELTT device, as well as by its simplicity of connectivity." Berger chaired the session at the International Electron Devices Meeting in Washington, D.C. late last year in which the first paper on the device was delivered.

Sandia has applied for one patent and is preparing others on the device.

Photograph of the DELTT transistor, seen from above. The semiconductor epitaxial layers, which contain the two layers of electrons, are sufficiently thin (0.25 microns) that light can penetrate them, rendering gates on both sides of the device visible. The top and back depletion gates allow independent contact to the two electron layers, while the top control gate turns the transistor on and off. (Source and drain contacts are outside the photo margins.) Download 150dpi JPEG image, 'quantum2.jpg', 544K

A trillion operations a second?
The very fast device may run at a trillion operations a second, as have other, more primitive tunneling devices. This is roughly ten times the speed of the fastest transistor circuits currently in use. Actual speed has not yet been measured, says Simmons, because it is "not easy to measure such high speeds, which are near the limits of what can be measured with conventional equipment." The extremely fast device also runs at extremely low power -- tens of millivolts and microamps -- as compared with the few volts and milliamps needed by transistors currently in use.

That electrons travel so rapidly in the DELTT gallium arsenide transistor means that normal electrical processes that slow down transmission of information -- like scattering of electrons by crystal imperfections that behave much like potholes in the electrons' pathway -- can be minimized.

From Sandia's point of view as a national defense laboratory, the device might someday be put to work in satellites and smart missiles to process information faster, with less payload and much lower power consumption than today's devices.

Because the device is also tunable, it could act like an energy spectrometer on a chip. This could allow sensitive detection of chemical and biological species like nerve gas and anthrax, helping to combat bio- or chem-terrorists by allowing more rapid and reliable detection of minute concentrations of toxic materials at customs and airports, on the battlefield and via airborne platforms.

Other possible uses of the modified structure would be as an optical detector in the far infrared. Incoming photons would be used to add energy needed for a transition between electron states, causing the device to turn on in only a narrow energy band.

Actual use of the transistor by industry may be years in the future because of other engineering problems to be tackled. These include questions of temperature -- the device now works only at temperatures at or below 77 degrees Kelvin, though rapid improvement, using existing technology, indicates it should be operating at room temperature by next year, says Simmons.

Another problem involves designing millions of such circuits on a chip, as is currently done with ordinary transistors. Because of the Sandia device's multifunctionality -- it has three positions, off-on-off, instead of the normal transistors' two on-or-off states -- the same amount of work can be performed with significantly fewer transistors, and chips would have to be completely redesigned.

Schematic of the structure of the DELTT transistor. Electrical contact is made to the top quantum well (QW) only by the source, and to the bottom quantum well only by the drain, in both cases accomplished by using the top and back depletion gates to remove electrons from the quantum well one does NOT wish to contact. Tunneling between the two quantum wells occurs only when electrons in both wells have identical momentum and energy, which can be controlled by the top control gate. The total thickness of the semiconductor layers is kept less than 2 microns, allowing the back depletion gate to be brought close to the quantum wells and the entire device to be made small. At left is shown the energy band diagram of the structure, containing the two quantum wells separated by a thin tunnel barrier. Download 150dpi JPEG image, 'quantum3.jpg', 160K

How it works
The technique relies in part upon the dual wave-particle nature of matter. In the device, two gallium arsenide layers, each only 150 angstroms thick, are separated by a 125 angstrom aluminum-gallium- arsenide barrier -- the equivalent of the yards of two houses separated by a sturdy fence. Ordinarily, gallium arsenide electrons in one yard do not have the energy to climb the fence to reach the other yard. But the tiny thickness of the barrier causes the electrons to behave like waves, which can poke into the barrier.

When an electron is adjusted to have the same energy and momentum states in both regions -- something that can be done by applying a voltage to these regions -- it can pass from one region to the other without any scattering, as occurs in normal electron motion due to crystal imperfections. In effect, they tunnel under the barrier fence.

Previous attempts at building tunneling transistors had been made by researchers who created layers side by side on a surface. This proved too difficult a task for current technology to manufacture accurately at 1,000 angstroms or the even smaller dimensions necessary. A novel design change allowed Sandia researchers to stack all DELTT layers vertically, using molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), which enabled single-atom layers to be grown. MBE and CVD are the same processes used to make, among other products, semiconductor lasers for compact disc players, and are readily available technologies.

The project is funded by Sandia's Laboratory-Directed Research and Development office, which funds speculative, defense-related projects, and DOE's Defense Programs. Sandia is a multiprogram DOE laboratory, operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national security, energy, and environmental technologies and economic competitiveness.

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