Transistor in General

Topics: Bipolar junction transistor, Transistor, Diode Pages: 6 (1951 words) Published: September 17, 2010
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 is ubiquitous in modern electronic systems. Following its release in the early 1950s the transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, amongst other things. Advantages

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 50 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. Limitations

* 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 vulnerable 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. Bipolar junction transistor
Bipolar transistors are so named because they conduct by using both majority and minority carriers. The bipolar junction transistor (BJT) is a combination of two junction diodes, and is formed of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (an n-p-n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (a p-n-p transistor). This construction produces two p-n junctions: a base–emitter junction and a base–collector...
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