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The new photo-electric device, shown here about nine times actual size. ( add
photo here)
An entirely new type of "electric eye" much smaller and sturdier than present photo-electric cells and possibly cheaper-has been invented at the Laboratories. During the past quarter century, electric eyes have found widespread use in electronics because of their ability to control electric currents by the action of light. To the layman, one type is perhaps best known for automatically opening and closing doors, but such devices have many other important uses in television, sound motion pictures, wirephotos, and still many more in industry.
One of the major advantages of the new electric eye is that it delivers very high power for a photo-electric device-in some cases enough to operate a switch directly without the preliminary amplification usually required.
Appropriately, the new device has been named the Phototransistor. The whole apparatus is housed in a tiny cylinder about as big as a 22 calibre rifle cartridge. Like the Transistor, it has no vacuum, no glass envelope, no grid, plate or hot cathode. It was invented by Dr. John N. Shive in the course of development work on Transistor-like devices.
Although the Phototransistor is still in the experimental stage, Laboratories scientists and engineers expect that, after the necessary development, it may have far-reaching significance in electronics and electrical communication. Just as the Transistor is not expected to supplant vacuum tubes, but rather to supplement them, so the Phototransistor is not expected to displace existing photo-electric cells. Because of their small size and expected long life, however, together with economies that might reasonably result from mass-production, Phototransistors should find many applications where it is not now practical to use present-day photoelectric devices.
Consideration is already being given, for example, to using them in a machine under development for toll dialing, a plan whereby a telephone operator directly dials a telephone in a distant city.
The heart of the parent device, the amplifying Transistor, is a tiny chip of germanium, a semiconductor material, against one side of which the points of two hair-thin wires are pressed, hardly two-thousands of an inch apart. The flow of very small electrical currents in one of these wires (the emitter) controls the flow of currents in the other wire (the collector) in such a way as to give signal amplification.
The Phototransistor is similar in operation to the amplifying Transistor, but it is controlled by light rather than by the electric current of the emitter. It also uses a piece of germanium but only a single collector wire. The tip of this wire rests in a small dimple ground into one side of the germanium disk. At this point the germanium disk is only three thousandths of an inch thick.
Light focussed on the opposite, un-dimpled side of the disk can control the flow of current in the wire, thus making a control device similar in function to a photo-electric cell.
The Phototransistor has a high power output for a photo-electric device and gives good response to a rapidly fluctuating light source. It is particularly sensitive to the wavelengths of light given off by ordinary incandescent light bulbs, and is well suited to operate with these easily available sources with good fidelity. Another virtue is the device’s low impedance.
Fig. 2 Longitudinal section of the Phototransistor
With Permission, Bell Laboratories RECORD
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