Amorphous Transistor Called Breakthrough in Integrated Circuits
A high-performance transistor based on a new operating principle that allows the transistor to control the flow of a much greater electric current than was hitherto possible has been developed by Energy Conversion Devices Inc. of Detroit, the company said today.
The new device, the company’s announcement said in Tokyo, “involves the first new fundamental transistor principle with wide device applications since the invention of . . . transistors in 1947.”
The device, called by the acronym DIFET, is viewed by the company as being particularly important because it makes transistors produced from relatively inexpensive thin-film amorphous silicon nearly as fast as those produced in the conventional manner from crystalline silicon. A major advantage of the new technology, the company said, is that it makes possible for the first time the construction of highly complex three-dimensional integrated circuits. When made with conventional materials, such circuits are restricted to two dimensions.
Long Sought Circuitry
Three-dimensional integrated circuits have long been sought after for an array of advanced computer, telecommunications and artificial intelligence applications.
The development of the new device, furthermore, marks a major milestone for an unconventional company that has already cornered a sizable share of the solar energy industry by pursuing applications of amorphous materials--a family of solids that had previously been considered worthless for electronics applications.
The development of high-performance transistors and other electronic devices based upon amorphous materials presages a revolution in the electronics industry that could be nearly as great as the revolution wrought by the development of the transistor itself, the company said.
The announcement of the new development was made in Tokyo, the company said, because most of its licenses for technology based on amorphous films have been granted to Japanese companies.
Device Still Unfamiliar
Scientists contacted by The Times who were not affiliated with Energy Conversion Devices were not familiar with the firm’s transistor work because it has not yet been presented at a scientific meeting or in a technical journal.
The company has been criticized in the past for the flamboyant manner in which it has announced new developments, but its claims have generally proven to be firmly grounded.
The transistor, in simplest terms, is an electronic gate. In one form of the transistor, for example, a small piece of material called a semiconductor--which conducts electricity, but not as well as metal--acts as a bridge between two sections of an electric circuit. Normally, this bridge does not allow current to pass.
However, when a small electric charge is applied to the bridge, its conductivity is increased and it allows a much larger current to pass through the circuit. A vacuum tube works on exactly the same principle, but it uses different materials and requires much more space.
Such an arrangement permits the electric current to be amplified, and is the basis for radios, stereos and other consumer electronics. Other types of transistors act as simple switches, simply blocking current flow or letting it pass. Such switches are fundamental to computers.
The conventional transistor, known as a bipolar junction transistor, “is fast but uses a lot of power,” according to Robert Nolan of Energy Conversion Devices. That power use creates heat, which is the bane of electrical engineers because it degrades the performance of all electronic components.
Scientists “partially got around this problem” by developing the metal oxide semiconductor field effect transistor, or MOSFET, Nolan said in a telephone interview. The MOSFET uses an electric field rather than current flow to activate the electronic gate.
The use of the field reduces heat output, but it also reduces the amount of current that can be controlled by the gate. That, in turn, reduces the speed of the devices.
The DIFET, or double injection field effect transistor, developed by Michael Hack and Wolodymyr Czubatyj of Energy Conversion Devices and Michael Shur of the University of Minnesota, “incorporates the most desirable features of both bipolar transistors and MOSFETs,” Nolan said.
High Reaction Speed
The DIFET exhibits the high reaction speed and high current flow of bipolar transistors and the low current use and heat output of the MOSFET. It accomplishes this by allowing two types of charge carriers to flow through the device rather than the single type that is characteristic of conventional devices.
Electric current in a transistor is normally carried by either electrons--small, negatively charged particles--or holes--positive charges created by the absence of an electron in a three-dimensional structure.
Hack, Czubatyj and Shur developed a transistor configuration that allows current to be carried through the device by both holes and electrons at the same time. This has the effect of greatly increasing the amount of current that can flow through the device.
Their device works well with conventional crystalline materials normally used in the electronic industry. However, the deeper significance of their development is that it makes transistors based on so-called amorphous materials competitive in performance with those based on crystalline materials.
That competitiveness opens up a whole new range of applications, including much larger integrated circuits and more compact electronic equipment.
Relatively Poor Conductors
Because semiconductors are relatively poor conductors of electricity, they are very sensitive to any structural defects or impurities that can impede the flow of electricity through an electronic device. The electronics industry has thus made wide use of semiconductors made from highly purified crystals, materials in which the atoms or molecules are arranged in a precise, orderly fashion that is free of such defects.
However, silicon crystals are expensive to produce and can only be grown (made) to limited sizes, the largest being perhaps 6 inches square. It is this factor, as much as the desire to produce compact electronic devices, that has spurred the micro-miniaturization of electronic components and the development of very large integrated circuits, in which large numbers of electronic components are combined on one silicon chip.
Most materials in nature, in contrast, are amorphous, meaning that they have no precise arrangement of atoms or molecules. Most scientists once believed that amorphous materials could not be used in electronic devices because they contained far too large a concentration of defects, impurities and minute spaces between atoms to permit a current to flow.
Scientists also disliked working with amorphous materials because their very disorderliness made it exceptionally difficult to calculate their properties in advance, a critical failing for the electronics applications.
One Man Credited
That physicists have come to change their view of amorphous materials is largely the work of one man, Stanford Ovshinsky, the founder of Energy Conversion Devices.
The son of immigrants, Ovshinsky never went to college, but taught himself through extensive reading. In 1968, Ovshinsky stunned the physics world by announcing the development of a transistor based on amorphous silicon.
The transistor was crude and slow and Ovshinsky was charged with sloppy science--as well as stock manipulation to increase the value of his newly formed company--but critics eventually conceded that the device did work.
The key to that development was Ovshinsky’s discovery that amorphous materials could be subjected to chemical and physical treatments that sharply reduce the number of defects that block current flow. He was ultimately able to produce thin films of electronic-quality amorphous silicon in any size desired.
Energy Conversion Devices and other companies have subsequently developed a range of devices based on amorphous materials, including the imaging devices in photocopiers and inexpensive photovoltaic cells that convert sunlight into electricity.
Transistors based on amorphous films, however, have generally been much slower than those based on crystalline materials, and their use has been very limited. The new development promises to overcome that limitation and, in doing so, open the way for many new applications beyond the simple replacement of conventional transistors.
‘Can Be Made in Any Size’
“Because amorphous films can be made in any size desired,” Ovshinsky told The Times in a recent interview, “we can make integrated circuits as large as we desire.
“Perhaps even more important, we can stack these thin films one on top of another to make the first practical three-dimensional electronic assemblies. This could produce extremely small but nonetheless very powerful computers, for example. Such devices simply aren’t practical using crystalline materials.”
An additional advantage of the DIFET materials is that they can emit light. “One problem of three-dimensional assemblies that had not been solved,” Ovshinsky said, “is communicating between the different layers. The light output of the DIFETs could make that possible. Using these techniques, we think it is possible for the first time to produce computers with an artificial intelligence that can come close to matching man’s own capacity.”
NEW TRANSISTOR TECHNOLOGY
CURRENT: The transistor is an electronic gatekeeper: when it is open, electricity flows; when it is closed, there is no power. Transistor components are produced by two methods:
Those made from crystals generally open an electronic gate quickly and allow a small amount of electricity to flow.
Those produced from amorphous silicon alloys open such an electronic gate more slowly, and allow a small amount of electricity to flow. Such transistors are relatively inexpensive to produce.
THE NEW: Energy Conversion Devices Inc. of Michigan has now announced development of a thin-film amorphous transistor known as DIFET (Double Injection Field Effect Transistor), pictured above. Compared with current transistors, it opens a wider electronic gate much more quickly. Because it is made from thin film, electrons can move from transistor to transistor in three dimensions rather than only linearly, as current technology allows. One slice of interconnected DIFET transistors is seen below.
THE IMPACT: The effect of such new technology should be more compact electronic machines that work more quickly.
