Point-contact transistor
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A point-contact transistor was the first type of solid-state electronic transistor ever constructed. It was made by researchers John Bardeen and Walter Houser Brattain at Bell Laboratories in December of 1947. They were working with physicist William Bradford Shockley, who wanted to share credit for the invention. The three had been working together on theories of electric field effects in solid state materials.
The experiment consisted of a block of germanium with two very closely spaced gold contacts held against it by a spring. Brattain had attached a small strip of gold foil over the point of a plastic triangle -- a configuration which is essentially a point-contact diode. He then carefully sliced through the gold at the tip of the triangle. This produced two electrically isolated gold contacts very close to each other.
Germanium is a semiconductor, which means it can either allow current to flow through it or let none through. The piece used had a surface layer with an excess of electrons, called N-type germanium. When an electric signal traveled in through the gold foil it injected holes (points which lack electrons). This created a thin layer which had a scarcity of electrons.
A small positive current applied to one of the two contacts had an influence on the current which flowed between the other contact and the base upon which the block of germanium was mounted. In fact, a small change in the first contact current, caused a greater change in the second contact current, thus it was an amplifier. The first contact is the "emitter" and the second contact is the "collector". Today the terminology for the three terminals of a bipolar transistor are base, emitter and collector. The low current input terminal into the Point Contact Transistor is the Emitter, while the output high current terminals are the Base and Collector. This differs from the later type of junction transistor invented in 1951 that operates as modern transistors do, with the low current input terminal as the Base and the two high current output terminals are the Emitter and Collector.
The point-contact transistor was commercialized and sold by Western Electric and others but was rather quickly superseded by the junction transistor because this later type was easier to manufacture and more rugged. Germanium was employed extensively for two decades in the manufacture of transistors. It has been almost totally replaced by silicon and other alloyed materials today, but remains in use in diodes such as those used for high-precision sensors including radiation counters.
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[edit] Forming
To make a point-contact transistor work, a brief high-current pulse was used to fuse the wires to the germanium and create the P-type material around the point of contact, a technique called 'electrical forming'. Most manufacturers sold their transistors un-formed, requiring the customer to do the forming. Usually this was done by charging a capacitor of a specified value to a specified voltage then discharging it between the emitter and the base electrodes. Forming had a significant failure rate, so many transistors had to be discarded.
[edit] Unusual characteristics
- Unlike bipolar junction transistors (which always have an α less than 1), point-contact transistors always had α much greater than 1.
- Differential negative resistance.
- When used in the saturated mode in digital logic, they latched in the on-state, making it necessary to remove power for a short time in each machine cycle to return them to the off-state.
The exact mechanism that caused these effects was never fully researched and is still not understood.
