Before touching the main topic of the invention history of the phototransistors, it is important to have a basic understanding on what phototransistors and photodiodes are. In simple words, a phototransistor is a transistor operated by light rather than electric current. In brief, a phototransistor is a bipolar transistor enclosed in a transparent case so that light can reach the base collector junction. This being the case of phototransistors, a photodiode is a kind of photo detector that is capable of converting light into either current or voltage based on the mode of operation. To understand it better, a traditional solar cell that is used to generate electric solar power can be called a large area photodiode.
The electrons that are produced by photons in the base-collector junction are injected in to the base of a phototransistor and this photodiode current is amplified by the transistor’s current gain. The phototransistor becomes a photodiode if the emitter is not connected.
That being said, now who invented the phototransistor? To travel through the invention history, it is vital to know about the inventor of phototransistor John Northrup Shive.
The Brain Behind the Invention – John Northrup Shive
Early Life and Education
John N. Shive was an American physicist and inventor who made remarkable contributions in solid state physics and electronic engineering during the early days of development of transistors at the Bell Telephone Laboratories. Shive was born in Baltimore and grew up in New Jersey. He graduated from the Rutgers University with a graduation in physics and chemistry in the year 1934. He earned his PhD from John Hopkins University by submitting a dissertation Practice and theory of the modulation of Geiger counters in the year 1939.
Transistor development at Bell Telephone Laboratories
Shive joined the Bell Telephone Laboratories in 1939. He initially worked on physical research, device development and later on training and education. Consequently, Shive was working on developing transistors. He discovered that gold-plated tungsten point contacts on a p-type layer of germanium grown on an n-type substrate showed a “terrific triode effect.” This happened on the 30th of January, 1948. On the 13th of February 1948, he also discovered that a transistor that consisted of bronze contacts on the surface of an n-type substrate without a p-layer gave “gains up to 40x power!” He tried to leverage this discovery to construct a point contact transistor with bronze contact on the front and back of a thin wedge of germanium, thus proving that holes could diffuse through bulk germanium and not just along the surface as previously believed. This perhaps confirmed William Shockley’s idea that he had until then kept as a secret from the rest of the team. The secret idea was that it was possible to build a junction transistor. Shockley later announced that the efforts and working of the team were a “mixture of cooperation and competition.” Besides, he also admitted that he had kept some of his own work as secret until his hand was forced by Shive’s 1948 advancements.
The Birth of the Phototransistor
It was in the same year 1948 that Shive used a beam of light instead of a wire as the emitter of a point contact transistor, generating holes that flow to the collector; and this device was later named as phototransistor. However, this invention was not announced by the Bell Labs until the year 1950. The invention embodies the modern day countrywide direct distance dialing system. Thus, Shive invented the phototransistors when he was working on the development of transistor-like devices.
The Significance of the Invention of Phototransistors
As mentioned earlier, Bell Telephone Laboratories announced the invention of the phototransistors by John N. Shive on the 30th of March, 1950. In fact, Bell was proud of the invention and mentioned that an entirely new type of “electric eye” that was smaller and sturdier than the existing photoelectric cells and possibly much cheaper had been invented at the Laboratories. Until the invention of the phototransistors, electric eyes were largely used in electronics for their ability to control electric currents by the action of light. One type of electric eyes that a layman knows is the one that is used for opening and closing doors automatically; however, such devices had a lot of other remarkable uses in sound motion pictures, television, wire photos, and a lot more in the industry.
The key advantage of the electric eye invented by Shive, the phototransistor, was that it delivered very high power for a photoelectric device; in some cases, it generated massive power that was sufficient to operate a switch directly without the initial amplification that was normally required. The whole apparatus of the phototransistor was housed in a tiny cylinder that was as big as a 22 caliber rifle cartridge. Similar to the transistor, it had no glass envelope, no vacuum, no grid, plate or hot cathode.
When Bell Telephone Laboratories announced its invention, the phototransistor was still in the experimental stage; scientists and engineers at the laboratory believed that after the essential development, phototransistors will have far-reaching significance in the field of electronics and electronic communication. They were also clear in the point that a phototransistor is not expected to replace the existing photoelectric cells. However, due to their small size and expected long life as well as their low cost might reasonably result in mass production and eventually phototransistors were expected to find many applications.
During the announcement of the invention itself, consideration was given, for instance, to use them in a machine under development for toll dialing, a plan according to which a telephone operator directly dials a telephone in a distant city.
Cards for Conversation: The First Application of Phototransistors
A telephone dial system needs a lot of information, that too in a very short span of time, to find out how to route long distance calls. In order to provide this facility, in about 1953, engineers at Bell Laboratories developed a new kind of card file – a file that dial systems can read.
This is where the phototransistors came in handy! The punched holes on metal cards tell how each call should be handled. When a call comes, the dial system “asks” the “card file” how to proceed to a specific location. Immediately, the prompt instruction card is displaced so that its pattern of holes is projected by light beams on a bank of phototransistors. In a jiffy, the phototransistors signal switches to set up the best connection! Cards are then quickly changed when new instructions are required.
With the help of phototransistors, the card file was expected to have its widest use in speeding long distance calls that was then dialed by a telephone operator.
However, today, phototransistors have a wide range of applications and are used comprehensively in computer, business, industrial, consumer and medical fields.