Transistors and power transistors are semiconductor devices used as switches, and to amplify currents.
There are 2 kinds, npn and pnp. The npn transistor has a p-type semiconductor between 2 n-type semiconductors. A pnp transistor has an n-type, between 2 p-types.
Each of the 3 regions has a terminal. The center region is always called the base. The outer regions are the collector, and the emitter. In the symbol for transistor, the emitter is the terminal with the arrow always pointing to the negative material.
In a circuit, an npn transistor can act as a switch. If the control switch is open, the depletion layer at one pn junction is blocking current from flowing through the transistor and driving the load.
With a closed control switch, a small current flows through the emitter-base pn junction. The base has only a limited number of charge carriers, so extra ones flow across the emitter-collector pn junction, letting current operate the load. The transistor then operates as a low-resistance conductor. A small current through the base lets larger current flow across the emitter-collector junction. The transistor is then said to be turned on.
The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, 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, a transistor 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, stabilized 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.
Importance
The transistor, considered by many to be one of the greatest inventions in modern history, ranks with the printing press and the telephone. 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 289 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.
Types
Broadly speaking, transistors are categorized as follows:
Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN, low power, high frequency switch.
FET Families
FET's are divided into two families: junction FET (JFET) and insulated gate FET (IGFET) also known as metal oxide silicon (or semiconductor) FET (MOSFET). Unlike IGFET's, the JFET gate terminal forms a diode with the channel (semiconductor material between source and drain). Functionally, this makes the N channel JFET the solid state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and cathode. Also, both devices operate in the "depletion mode", they both have a high input impedance, and they both conduct current under the control of an input voltage.
FET's are further divided into enhancement mode and depletion mode types. Mode refers to the polarity of the gate voltage with respect to the source when the device is conducting. Taking N channel FET's: in depletion mode the gate is negative with respect to the source while in enhancement mode the gate is positive. For both modes, if the gate voltage is made more positive the source/drain current will increase. For P channel devices the polarities are reversed. Most IGFET's are enhancement mode types and nearly all JFETS are depletion mode types.
Other types
Source: CDX Global & Wikipedia - en.wikipedia.org
