Transistors

Electronics began with the development of the transistor in the 1950s. Transistors are essentially tiny semiconductor amplifiers and/or switches, several thousands of which can be put on a 1mm² piece of silicon.

Transistors have three leads: the emitter, collector and base. The base lead controls the transistor: applying an electrical current to the base lead switches the transistor on. When the transistor is on, current flows from the collector to the emitter - but when it is off no current will flow.

A transistor and its three leads

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.

Darlington pairs

Some transistors can take a very low current flowing in the base and amplify it to give a much higher current in the collector (called gain [gain: the amount of amplification of the input voltage - ie how much bigger the output voltage is compared to the input voltage ]). Other transistors can output a large current at the collector - but without very much gain. Single transistors cannot have both high gain and high collector current.

To overcome this problem, a high-gain transistor is paired up with a high-current transistor in what is called a Darlington pair. The combined transistors allows 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.

Circuit diagram for a Darlington pair

Transistors are often used as interface devices - that is, devices which ensure that the right amount of current is supplied to power another device, such as an output component. Examples of output components that might require transistors are direct current motors, solenoids and meters.