CONTINUING the electronics revolution well into the next century will require a radical departure from conventional electronic devices and circuits. One reason is that transistors do not work well if they are miniaturized beyond a certain size. Another is that every transistor must have wiring to each of its terminals. As more and more are packed into a small area, we get a nightmare of multi-level, crisscrossed wiring, in much the same way that population growth in a city leads to a roadway maze and traffic congestion -- the "Los Angeles problem." Finally, both the wiring and the transistors give off heat, and the problem of dissipating it eventually becomes a limiting factor.
To bypass these roadblocks, we need a radically new approach. One possibility is "quantum-effect electronics." According to the theory of quantum mechanics, an electron will display particle-like behavior when it is free to move in regions substantially larger than its wavelength and wave-like properties if it is confined in regions comparable to its wavelength. Conventional transistor dimensions are large compared with electronic wavelengths, so we can treat the electrons as classical particles. But in recent years -- thanks in part to X-ray lithography -- researchers have fabricated semiconductor structures so small that they enable scientists to see for the first time electrical characteristics that clearly manifest the wave-like properties of electrons. Exploiting such quantum effects may be the key to a new phase of the electronics revolution, since small size is a prerequisite rather than a problem. The smaller the structure or device, the more effectively it should function.
A human hair is about 75 micrometers (millionths of a meter) in diameter; the most advanced commercial transistors today have "channel lengths" -- the key length scale of the most widely used type of transistor -- that measure 0.75 micrometer. The quantum-effect electronic devices we are experimenting with in our lab at MIT have features measured not in micrometers but in nanometers (billionths of a meter). Some features of these devices are only 25 nanometers -- 3,000 times smaller than the diameter of a human hair, or a little more than 100 atoms across.
A transistor is a switch that turns a current on and off (or, in some cases, modulates the current). A conventional MOSFET (metal-oxide semiconductor field-effect transistor -- see illustration) consists of a source, gate and drain built on a semiconductor material such as silicon. Electrons can flow from source to drain through the channel underneath the gate only when sufficient positive voltage is applied to the gate. Thus the gate acts like the lever of a switch. One property of waves known as resonance makes it possible to construct a device that can function as a sort of quantum-effect transistor. When a wave is confined to a small space, it tends to reflect back and forth between the walls, creating what is called a "standing wave." For example, sound waves reflected from opposite walls of an organ pipe reinforce rather than cancel one another out. Electrons can be confined in much the same way, only instead of having walls of metal or wood, the walls of the potential well are energy barriers.
When the width of the well is narrow enough so that electrons form a standing wave, a phenomenon known as resonant tunneling becomes possible. If the energy of free electrons in the source corresponds to one of the energy levels in the well, electrons from the source can pass directly through one wall, across the well and through the other wall into the drain. Thus by controlling either the energy levels in the well or the energy of the electrons at the source, it should be possible to control the flow of electrons from source to drain. The device should function as a switch.
Henry Smith is director of the Submicron Structures Laboratory at Massachusetts Institute of Technology; Dimitri Antoniadis is director of MIT's Microsystems Technology Laboratories. This is excerpted from their article in the April issue of Technology Review.