A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of 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 flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.
The transistor is the fundamental building block of modern electronic devices, and its presence is ubiquitous in modern electronic systems. Following its release in the early 1950s the transistor revolutionised the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, amongst other things.
Physicist Julius Edgar Lilienfeld filed the first patent for a transistor in Canada in 1925, describing a device similar to a Field Effect Transistor or "FET".However, Lilienfeld did not publish any research articles about his devices, nor did his patent cite any examples of devices actually constructed. In 1934, German inventor Oskar Heil patented a similar device.
In 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in the United States observed that when electrical contacts were applied to a crystal of germanium, the output power was larger than the input. Solid State Physics Group leader William Shockley saw the potential in this, and over the next few months worked to greatly expand the knowledge of semiconductors, and thus could be described as the "father of the transistor". The term was coined by John R. Pierce. According to physicist/historian Robert Arns, legal papers from the Bell Labs patent show that William Shockley and Gerald Pearson had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles.
The first silicon transistor was produced by Texas Instruments in 1954. This was the work of Gordon Teal, an expert in growing crystals of high purity, who had previously worked at Bell Labs.The first MOS transistor actually built was by Kahng and Atalla at Bell Labs in 1960.
The transistor is the key active component in practically all modern electronics, and is considered by many to be one of the greatest inventions of the twentieth century.[Its importance in today's society rests on its ability to be mass produced using a highly automated process (semiconductor device fabrication) that achieves astonishingly low per-transistor costs.
Although several companies each produce over a billion individually-packaged (known as discrete) transistors every year, the vast majority of transistors now produced are in integrated circuits (often shortened to IC, microchips or simply chips), along with diodes, resistors, capacitors and other electronic components, to produce complete electronic circuits. A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, as of 2009, can use as many as 2.3 billion transistors (MOSFETs)."About 60 million transistors were built this year  ... for [each] man, woman, and child on Earth."
The transistor's low cost, flexibility, and reliability have made it a ubiquitous device. Transistorized mechatronic circuits have replaced electromechanical devices in controlling appliances and machinery. It is often easier and cheaper to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function.
The bipolar junction transistor, or BJT, was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of manufacture. Desirable properties of MOSFETs, such as their utility in low-power devices, usually in the CMOS configuration, allowed them to capture nearly all market share for digital circuits; more recently MOSFETs have captured most analog and power applications as well, including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters, motor drivers, etc.
Comparison with vacuum tubes
Prior to the development of transistors, vacuum (electron) tubes (or in the UK "thermionic valves" or just "valves") were the main active components in electronic equipment.
The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are
Small size and minimal weight, allowing the development of miniaturized electronic devices.
Highly automated manufacturing processes, resulting in low per-unit cost.
Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
No warm-up period for cathode heaters required after power application.
Lower power dissipation and generally greater energy efficiency.
Higher reliability and greater physical ruggedness.
Extremely long life. Some transistorized devices have been in service for more than 30 years.
Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.
Silicon transistors do not operate at voltages higher than about 1,000 volts (SiC devices can be operated as high as 3,000 volts). In contrast, electron tubes have been developed that can be operated at tens of thousands of volts.
High power, high frequency operation, such as that used in over-the-air television broadcasting, is better achieved in electron tubes due to improved electron mobility in a vacuum.
Silicon transistors are much more sensitive than electron tubes to an electromagnetic pulse generated by a high-altitude nuclear explosion.
Transistors are categorized by
Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide, etc.
Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
Maximum power rating: low, medium, high
Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain).
Application: switch, general purpose, audio, high voltage, super-beta, matched pair
Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array, power modules
Amplification factor hfe (transistor beta)
Thus, a particular transistor may be described as silicon, surface mount, BJT, NPN, low power, high frequency switch.