Transistor sensitive to one electron.

Transistor sensitive to one electron

In this day, of transistors doing prodigious things, who remembers vacuum tubes? Electronic circuit elements are getting finer and finer and more and more precise. Now there is a transistor so precise that it will respond to a voltage change equivalent to a single electron.

According to a statement by AT&T Bell Labs in Murray Hill, N.J., where Ted Fulton and Gerry Dolan developed the device, the best previous miniaturized field-effect transistors (FETs) require thousands of electrons to elicit the same response. The one-electron transistors, in Bell Labs' estimation, "show potential to change the way we think about a future generation of integrated circuits, because they use infinitesimal amounts of power and space; have an intrinsic speed of less than a pico-second;... and use the smallest possible amount of charge transfer."

Transistors most often work as switches. An input current either comes out through the output terminal or not, depending on the state of a part of the transistor called the gate. These new transistors work best when made of superconducting metals--the old-fashioned, very low-temperature kind, not the modern high-temperature ones. The devices consist of two tunnel junctions separated by an "island" of metal only a few hundred atoms across. In a tunnel junction, two superconducting electrodes are separated by an insulating barrier only a few atoms thick, and current passes through the junction by the phenomenon known as quantum mechanical tunneling. The junctions and the island between them lie on an insulator, above a conducting substrate. A voltage -- that is, an electric field -- applied across this insulator forms the gate and controls whether or not current flows between the two junctions.

In the new devices, an electric field equivalent to a single electron will turn the current on or off. "If you charge the gate capacitance up to the equivalent of a single electron, 10.sup.-16 of a farad," says Fulton, "the device goes through its entire cycle." That is, it goes from full current to no current. "It's really cyclic in one electron." Furthermore, the cycle can be subdivided, so that the transistor is sensitive to changes as small as the equivalent of 1 percent of an electron. This leads to the suggestion of an early possible application as electrometers for measuring extremely minute electric fields.

New fabrication techniques developed by Dolan allow the microscopic transistors to be made less than one-twentieth of a micron across. The researchers use electron-beam lithography to make a pattern on an organic film layer. Then they deposit the electrodes and the barrier. Fulton and Greg Blonder, who heads the Bell Labs department where the work was done, stress that only the old-fashioned metal superconductros, which require refrigeration by liquid helium, can be used. Nobody has yet succeeded in making tunnel junctions with the new high-temperature superconductors, they say, and furthermore, the new materials do not have the stability and uniformity of properties required for this application.

At the moment the new devices are experimental, although there is a potential for applying them in computer circuitry. However, says Blonder, "You will be a lot older before you see it."