Science / Medicine : New Plastic Transistor 1,000 Times as Fast as Earlier Organic Models
A newly developed transistor made of plastic operates at least 1,000 times as fast as previous transistors based on organic materials and is considered a significant step in the development of so-called molecular electronic devices.
Industrial scientists caution that molecular electronic devices, such as the new transistor developed at the Cavendish Laboratory in Cambridge, England, are still too immature for everyday use. But “one would have to conclude that they have some very exciting results,” said chemist Ron Elsenbaumer of the Allied-Signal Research Laboratory in Morristown, N.J.
Many researchers believe that molecular electronic devices are the wave of the future for many types of electronic devices, such as computers, because inorganic, silicon-based components cannot be made much smaller. The problem, according to physicist Phillip Seiden of IBM Corp., is that if any more components are crammed onto silicon chips, the components will short-circuit.
This happens because the silicon-based devices emit minute electromagnetic fields that impinge on nearby components, affecting their operation or altering stored data. Organic molecules are less susceptible to such electromagnetic interference and can be stacked virtually one on top of another.
In the fastest super-computers, Seiden said, speed of operation is limited primarily by the amount of time it takes an electronic signal to travel from one component to the next. If molecular electronic devices allow components to be placed closer together, computing speeds will be greatly enhanced and the size of the computers will be reduced. Such a compact computer might, for example, serve as the “brain” for an unmanned spacecraft to make it self-sufficient on long missions, such as to a distant star.
Because of the great potential of molecular electronic devices, U.S. corporations such as Allied-Signal, IBM, Westinghouse, and Hughes Aircraft are investing more than $100 million a year on research, according to the trade journal Chemical Week.
Within the last 15 years, chemists have developed a number of organic polymers that, like silicon, are semiconductors. That is, they are normally insulators, but they become conductors when they are “doped” with small quantities of electrically charged atoms. Many such doped polymers can conduct electricity as well as metal wires.
A transistor of the type made by the British researchers is an electrical gate that allows a current to pass from one metal electrode (the source) to a second electrode (the drain). The current flow is regulated by a third electrode (the gate), which is made from a semi-conducting material.
Normally, the gate is an insulator and no current passes from the source to the drain. But when a small current is passed into the semiconductor, it becomes conducting and allows a large current to pass from source to drain. The net effect is that a small current applied to the gate is amplified into a much larger current.
Chemist Mark Wrighton of the Massachusetts Institute of Technology has made transistors in which the gate is an organic polymer that undergoes a chemical reaction when a small current is applied. That chemical reaction, akin to doping, makes the polymer conducting. But because a chemical reaction must occur, the transistors require milliseconds to operate--fast in human terms, but slow in electronic terms.
But chemist Richard H. Friend and his Cavendish Laboratory colleagues have found a way of processing a polymer called polyacetylene so that no chemical reaction is required to convert it into a conductor. Instead, they reported last week in Nature, the application of a small magnetic field instantaneously rearranges bonds in the polymer so it becomes conducting. The transistor thus operates much faster than those developed by Wrighton--in millionths of a second.
Interestingly, the polyacetylene, which is normally transparent, becomes opaque when a magnetic field is applied.