Optoelectronics Arrives

A pocket-sized electronic calculator that almost instantaneously flashes answers in bright numbers. A tabletop clock that at the press of a button displays with lighted numerals the hour, minute and second in any of the world's 24 time zones. A transistorized depth-finder that tells the Sunday sailor in glowing red numbers exactly how many feet, or fathoms, of water lie under his keel. These futuristic devices, already on the market, are only samples of the dazzling consumer spin-offs from a totally new scientific field called "optoelectronics"--the marriage of modern optics with space-age electronics.

The journal Physics Today devotes most of its current issue to optoelectronics, calling it "without doubt one of the fastest-growing areas in physics." The new technology has already produced miniaturized lasers that are no bigger than a grain of sand. It is turning holography (three-dimensional photography) into an exciting new adjunct of diagnostic medicine, civil engineering and computer technology. It has yielded light-detection devices that can virtually see in the dark, and it offers a promising way to help relieve the jam in cable and radio communications by transmitting messages on beams of light. Yet in terms of everyday impact, optoelectronics has had its greatest visibility in the rapidly proliferating consumer devices that use electronics to display numbers, letters and other changing signals.

Such "alphanumeric" displays are not entirely new. Since the 1960s, cathode ray tubes (CRTs) similar to those in home TV sets have been used to perform such varied chores as giving stock information in brokerage offices, confirming reservations at distant hotels, and even showing air traffic patterns over crowded airports. For simpler tasks, such as those performed by electronic meters--where only numbers are needed on display panels--there are smaller and less expensive devices called gas discharge tubes. The Burroughs Corp.'s popular Nixie tube, for example, contains ten overlapping electrodes that form the digits 0 to 9. If current is sent into one of these electrodes, all of which have their own separate connections at the base of the tube, the electrode will light up (reason: the gas surrounding that electrode quickly begins to glow).

Despite their many uses, CRTs and gas-discharge tubes have certain drawbacks. They require considerable electrical power, are sensitive to vibrations and other stresses and cannot readily be miniaturized. These shortcomings are all the more significant in military and space applications. Aboard a rocket ship, for instance, every part must be as compact as possible and also be capable of surviving the shock of sudden acceleration and deceleration. To fill this need, the Pentagon and NASA began to look for other types of electronic display systems.

One Way. The search quickly led to the same technology that produced that tiny workhorse of modern electronics, the transistor, which owes its success to a class of materials called semiconductors. These are crystalline substances that will readily conduct an electric current only if they are contaminated --or, in technical jargon, "doped" --with other substances that give them either a surplus or deficit of electrons. Moreover, if two dissimilar semiconductors are joined together--one with a shortage of negatively charged electrons (known as a P-type because it has a positive charge), the other with an electron abundance (or N-type because it has a negative charge)--an electrical current applied to this junction will flow in only one direction: from the N side to the P side, much the same as the oneway current flow in old-fashioned radio vacuum tubes called diodes. Even more significant, certain semiconductors, notably those made of gallium phosphide and gallium arsenide phosphide, will glow with a bright red light when current is flowing through them.

These remarkable new optoelectrical components are called light-emitting diodes, or LEDs. Often only 1/32 of an inch wide, they have advantages that many of the older optical displays lacked: a longer lifetime (up to 100 years in the opinion of some scientists), very low power consumption (much less than that needed even by a tiny flashlight bulb) and, like the transistor, a high resistance to shock and other abusive treatment. Most important of all, they can be easily assembled into miniature electronic displays that form numbers in a flash.

In a typical LED display, such as those made by RCA, Monsanto or General Electric, each digit is formed of seven separately wired segments on a single base plate (see diagram). Reminiscent of matchsticks laid out for a parlor game, the segments are so arranged that they can form any digit from 0 to 9. The trick is to send an electric current into the proper combination of segments to form the required number.

Switching. That may require very complex electronics. A depthfinder, for instance, works by bouncing sound waves off the ocean floor and clocking how long it takes them to return. Thus the intervals between the original signals and their echoes are actually measurements of depth. But before such measurements can be visually displayed, they must first be converted into an electric current with fluctuations that precisely mirror those echo intervals. The reason is that the depthfinder is, in effect, a miniature computer or switching system. Only those circuits linked to the appropriate diode segments will be switched on with each fluctuation of current.

In more sophisticated arrangements, where complicated images like letters are required, there may be a larger number of segments and back-up circuitry of greater complexity. Even so, many scientists are convinced that in the future light-emitting diodes will be increasingly used in everything from wristwatches to auto dashboards. As Dr. Henry Kressel, head of semiconductor device research at RCA, puts it: "The LED's day has come."

This file is automatically generated by a robot program, so reader's discretion is required.