Transistor

The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilisation, signal modulation and many other functions. It acts as a variable valve which, based on its input current (BJT) or input voltage (FET), allows a precise amount of current to flow through it from the circuit's voltage supply.

In essence, it has three terminals. A current or voltage applied through/across two terminals controls a larger current through the other terminal and the common terminal. In analog circuits, transistors are used in amplifiers. Analog circuits include audio amplifiers, stabilised power supplies and radio frequency amplifiers. In digital circuits, transistors function essentially as electrical switches. Digital circuits include logic gates, RAM (random access memory) and microprocessors.

Transistor was also the common name in the sixties for a transistor radio, a portable radio that used transistors (rather than vacuum tubes) as its active electronic components. This is still one of the dictionary definitions of transistor.

The transistor was invented at Bell Telephone Laboratories in December 1947 (first demonstrated on December 23) by John Bardeen, Walter Houser Brattain, and William Bradford Shockley, who were awarded the Nobel Prize in physics in 1956. Ironically, they had set out to manufacture a field-effect transistor (FET) predicted by Julius Edgar Lilienfeld as early as 1925 but eventually discovered current amplification in the point-contact transistor that subsequently evolved to become the bipolar junction transistor (BJT).

The transistor, considered by many to be one of the greatest inventions in modern history, ranks with the printing press and banking. It is the key active component in practically all modern electronics. Its importance in today's society rests on its ability to be mass produced using a highly automated process (fabrication) that achieves vanishingly low per-transistor costs.

Although millions of individual (discrete) transistors are still used, the vast majority are fabricated in chips along with diodes, resistors, capacitors and inductors to produce complete electronic functions, either analog or digital. Often both types of function are integrated on the same chip. The cost of designing and developing a complex chip is astronomical, but when spread across millions of production units the individual chip cost is minimized. A logic gate comprises about 20 transistors whereas an advanced processor, as of early 2005, can use as many as 250 million transistors.

The word "chip" is now used rather loosely: originally it referred to the actual piece of semiconductor before packaging. Once the chip had been packaged it was called an "integrated circuit", or just "IC", and sometimes a "bug". Chip, integrated circuit and IC are now used interchangeably while bug has gone out of fashion. The term "solid state" is used to describe a device which does not control charge flow through a vacuum (vacuum tube) or a gas and which does not use moving parts (relay) to control charge flow. In the same vein, a circuit or item of equipment may be described as "solid state".

The transistor's low cost, flexibility and reliability have made it an almost universal device for non-mechanical tasks. Whereas a common item, say a refrigerator, would have used electromechanical devices for control, today it is often less expensive and more effective to simply use a standard integrated circuit (containing a few million transistors) and write a computer program to carry out the same task through logic. Transistors have replaced almost all electromechanical devices, are used in most simple feedback control systems, and appear in huge numbers in everything from traffic lights to washing machines.

Hand-in-hand with low cost has come the trend to digitize information. With transistor-utilizing computers offering the ability to quickly find, sort and process digital information, more and more effort has been put into making information digital. Almost all media today is delivered in digital form, finally being converted and presented in analog form by computers. Familiar areas influenced by the drive to digitize are television, radio and newspapers.

There are two basic types of transistors, the bipolar junction transistor (BJT) and the field-effect transistor (FET), which work differently. Bipolar transistors are so named because the main conduction channel uses both electrons and holes to carry the main electric current. In BJTs the main current must cross a very thin insulating layer called the Depletion zone, and the width of this layer can be electrically varied at very high rates in order to control the main current. Field-effect transistors (also called unipolar transistors) use only one of the two types of carrier (either electrons or holes, depending on the subtype of the FET). In FETs the main current appears in a narrow conducting channel with an insulating Depletion zone at the side, and the width of this insulating zone can be altered by varying its applied voltage, in order to control the width of the conducting channel and therefore control the main current. See the articles on each type of device for more information.