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Transistors

The transistor was one of the most important discoveries of the 20th Century. The name transistor was derived from a combination of the words transfer and resistor i.e. transfer resistor. Transistors are today used in nearly all electronic devices, including silicon chips, computers, aircraft and mobile telephones.

The most common types of transistor include a Bipolar Junction Transistor and a Field-Effect Transistor or FET.

A Bipolar Junction transistor can act as a switch or an amplifier. This transistor looks like two diodes back-to-back and is made up of three layers of 'doped' semiconductor material most commonly in 'npn' sandwich.

Some of the basic properties exhibited by Bipolar Transistors are recognisable as being diode-like. When the middle semiconductor material is fairly thin some interesting effects become possible that allow us to use the transistor as an amplifier or a switch.

Field Effect Transistors were developed after bipolar junction transistors. They draw virtually no power from an input signal, overcoming a major disadvantage of the bipolar junction transistor.

There are number of different kinds of FETs, one of the most common is called a Metal-Oxide Semiconductor Field-Effect Transistor or MOSFET.

A MOSFET is made up of layers of silicon each with different doping. The transistor has three electrodes called a source, drain and gate. The current flowing from source to drain is controlled with the charge of the gate, which is normally constructed from a highly conductive material.

To function properly this gate-electrode needs to be insulated from the rest of the transistor. This insulation is made possible by a layer of oxide that surrounds the gate.

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Diodes

Transistors

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Guide Written by Paul Wilson

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