Switches, transistors and relays


Electronics began with the development of the transistor in the 1950s. Transistors are tiny electronic switches and amplifiers. Several thousand transistors can be put on a piece of silicon with a surface area of just 1mm2.

Transistors have three main uses:

There are two main types of transistor: bipolar transistors and field effect transistors.

Bipolar transistors

Bipolar transistors have three leads:

The base lead controls the transistor: once it receives a voltage of at least 0.6V, it switches the transistor on and allows (some) current to flow from the collector to the emitter. When turned on it acts as an analogue device, which can be used to amplify the current at the base leg.

Transistors are easily damaged so it is important not to mix up the three leads. To help identify the leads some transistors have a dot near the collector, and/or a tab near the emitter. Each type of transistor is identified by a code printed on the side.

A single bipolar transistor cannot have both high gain and high collector current. ADarlington pair is a special arrangement of bipolar transistors that combines a high-gain transistor with a high-current transistor. The combined transistors allow both a higher gain and a larger amount of current to flow than would be possible with a single transistor. Darlington pairs are often used to drive motors. They can be bought packaged as a single discrete component or packaged as a transistor array, consisting of several transistors in a single integrated circuit (IC) (such as ULN2803 and ULN2003).



Field-effect transistors

Field-effect transistors (FETs) are digital switches that respond to an input voltage to allow an increase in either voltage or current. They have three leads:


When the gate leg receives a voltage of at least 2V it switches on fully, allowing electricity to flow from the source to the drain. The current continues to flow until the voltage at the gate leg falls below 2V.







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