The transistor is the fundamental building block of modern electronic devices.A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. The vast majority of transistors are now produced in integrated circuits (often shortened to IC, microchips or simply chips) and along with other electronic components. A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, can use as many as billions of transistors
SIMPLIFIED OPERATION
The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. A transistor can control its output in proportion to the input signal; that is, it can act as an amplifier. Alternatively, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.
TRANSISTOR AS A SWITCH
Transistors are commonly used as electronic switches, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates.
In any switching circuit, values of input voltage would be chosen such that the output is either completely off, or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only “on” and “off” values are relevant.
TRANSISTOR AS AN AMPLIFIER
Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both. From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing
The amplifier is designed so that a small change in current through the base of the transistor; (the transistor amplify the current) and produce large output
ADVANTAGES
The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are
No power consumption by a cathode heater.
Small size and minimal weight, allowing the development of miniaturized electronic devices.
Low operating voltages compatible with batteries of only a few cells.
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.
Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.
LIMITATIONS
Silicon transistors can age and fail.
High-power, high-frequency operation, such as that used in over-the-air television broadcasting, is better achieved in vacuum tubes due to improved electron mobility in a vacuum.
Solid-state devices are more vulnerable to Electrostatic discharge in handling and operation
A vacuum tube momentarily overloaded will just get a little hotter; solid-state devices have less mass to absorb the heat due to overloads, in proportion to their rating
Sensitivity to radiation and cosmic rays. Vacuum tubes create a distortion, the so-called tube sound, that some people find to be more tolerable to the ear.
TYPES
Transistors are categorized by
1. Structure: BJT, JFET, IGFET (MOSFET), insulated-gate bipolar transistor.
2. Electrical polarity (positive and negative): n–p–n, p–n–p (BJTs); n-channel, p-channel (FETs)
3. Maximum power rating: low, medium, high
4. Maximum operating frequency: low, medium, high, radio (RF), microwave frequency
5. Application: switch, general purpose, audio, high voltage, super-beta, matched pair
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