Point-contact transistor

Point-contact transistor

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A point-contact transistor was the first type of solid-state electronic
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 ever constructed. It was made by researchers John Bardeen
John Bardeen
John Bardeen was an American physicist and electrical engineer, the only person to have won the Nobel Prize in Physics twice: first in 1956 with William Shockley and Walter Brattain for the invention of the transistor; and again in 1972 with Leon Neil Cooper and John Robert Schrieffer for a...

 and Walter Houser Brattain
Walter Houser Brattain
Walter Houser Brattain was an American physicist at Bell Labs who, along with John Bardeen and William Shockley, invented the transistor. They shared the 1956 Nobel Prize in Physics for their invention. He devoted much of his life to research on surface states.- Early life and education :He was...

 at Bell Laboratories in December 1947. They worked in a group led by physicist William Bradford Shockley
William Shockley
William Bradford Shockley Jr. was an American physicist and inventor. Along with John Bardeen and Walter Houser Brattain, Shockley co-invented the transistor, for which all three were awarded the 1956 Nobel Prize in Physics.Shockley's attempts to commercialize a new transistor design in the 1950s...

. The group had been working together on experiments and theories of electric field effects in solid state materials, with the aim of replacing vacuum tubes with a smaller, less power-consuming device.

The critical experiment, carried out on December 16, 1947, consisted of a block of germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

, 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
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

. 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 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
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 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
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 and others but was rather quickly superseded by the junction transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 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
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 and other alloyed materials today, but remains in use in diodes such as those used for high-precision sensors including radiation counter
Semiconductor detector
This article is about particle detectors. For information about semiconductor detectors in radio, see Diode#Semiconductor_diodes, rectifier, detector and cat's-whisker detector....

s.

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
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 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.

Unusual characteristics

  • The common base current gain (or α) of a point-contact transistor is around 2 to 3, whereas the α of a bipolar junction transistor is typically between 0.98 and 0.998.
  • Differential negative resistance
    Negative resistance
    Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...

    .
  • 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.

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