point contact transistor
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The point-contact transistor was the first type of transistor to be successfully demonstrated. It was developed by research scientists
John Bardeen John Bardeen (; May 23, 1908 – January 30, 1991) was an American physicist and engineer. He is the only person to be awarded the Nobel Prize in Physics twice: first in 1956 with William Shockley and Walter Brattain for the invention of the tran ...
and Walter Brattain at Bell Laboratories in December 1947. They worked in a group led by physicist William Shockley. The group had been working together on experiments and theories of
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
effects in
solid state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their u ...
materials, with the aim of replacing vacuum tubes with a smaller device that consumed less power. The critical experiment, carried out on December 16, 1947, consisted of a block of
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
, a semiconductor, with two very closely spaced gold contacts held against it by a spring. Brattain attached a small strip of gold foil over the point of a plastic triangle — a configuration which is essentially a point-contact
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A 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. The piece of germanium used had a surface layer with an excess of electrons. 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 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 bipolar junction transistor invented in 1951 that operates as transistors still do, with the low current input terminal as the base and the two high current output terminals as the emitter and collector. The point-contact transistor was commercialized and sold by
Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
and others but was eventually superseded by the bipolar junction transistor, which was easier to manufacture and more rugged. The point-contact transistor did still remain in production until circa 1966, by which time the silicon planar transistor was dominating the market.


Forming

While point-contact transistors usually worked fine when the metal contacts were simply placed close together on the germanium base crystal, it was desirable to obtain as high an α current gain as possible. To obtain a higher α current gain in a point-contact transistor, a brief high-current pulse was used to modify the properties of the collector point of contact, a technique called 'electrical forming'. Usually this was done by charging a capacitor of a specified value to a specified voltage then discharging it between the collector and the base electrodes. Forming had a significant failure rate, so many commercial encapsulated transistors had to be discarded. While the effects of forming were understood empirically, the exact physics of the process could never be adequately studied and thus no clear theory was ever developed to explain it or provide guidance on improving it. Unlike later semiconductor devices, it was possible for an amateur to make a point-contact transistor, starting with a germanium point-contact diode as a source of material (even a burnt-out diode could be used; and the transistor could be re-formed if damaged, several times if necessary).HOME-MADE TRANSISTORS: P B Helsdon, Wirless World, January 1954
Article starts "It is quite practicable to make point-contact transistors at home which compare quite well with those advertised by professional manufacturers."


Characteristics

Some characteristics of point-contact transistors differ from the slightly later junction transistors: *The common base current gain (or α) of a point-contact transistor is around 2 to 3, whereas α of bipolar junction transistor (BJT) cannot exceed 1 and the common emitter current gain (or β) of a point-contact transistor cannot exceed 1, whereas β of a BJT is typically between 20 and 200. *Differential negative resistance. *Until the development of the
surface barrier transistor The surface-barrier transistor is a type of transistor developed by Philco in 1953 as an improvement to the alloy-junction transistor and the earlier point-contact transistor. Like the modern Schottky transistor, it offered much higher speed than ...
in 1953 point-contact transistors were the fastest transistors available, some operating in the lower part of the
VHF Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF ...
band when the fastest junction transistors could still only barely operate at a few MHz. *Moisture attack was less damaging to point-contact transistors than to junction transistors,Bell Telephone Labs Transistor Technology Volume 1 Bridgers, Staff & Shive Copyright 1958 Van Nostrand Company, Inc. p. 386 because their collector reverse resistance is lower and cutoff collector current higher. *When used in the saturated mode in digital logic, in some circuit designs (but not all) 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.


See also

*
Crystal radio A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio signal to produce sound, needing no external power. It is named for its most impo ...
*
Duodiode A transistor is a semiconductor device with at least three terminals for connection to an electric circuit. In the common case, the third terminal controls the flow of current between the other two terminals. This can be used for amplification, ...
*
Transistron A transistor is a semiconductor device with at least three terminals for connection to an electric circuit. In the common case, the third terminal controls the flow of current between the other two terminals. This can be used for amplification, a ...


References


Further reading

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External links


The Point-contact Transistor


{{Electronic components Transistor types Bipolar transistors Computer-related introductions in 1947