A transistor is a semiconductor device capable of strengthening a small input signal or switching a larger load in response to a small input signal. In most applications they have replaced vacuum tubes because they are smaller, faster, cheaper, more energy efficient and cooler.

Bipolar Junction Transistor

The junction transistor is formed by sandwiching a layer of one type of semiconductor between two layers of another type. Thus we have both NPN transistors and PNP transistors. The centre layer is known as the base, and the other two layers are called the collector and the emitter. A metal conductor is attached to each layer so the transistor can be connected to other devices.


Bipolar transistors are often sold as a single (discrete) device in a package with three pins. The size of the current carrying capacity of the transistor. For both PNP and NPN configurations, the three leads are called the collector, emitter and base. A part number on the face of the package identifies the manufacturer and the model number of the transistor. The manufacturers publish data books which give the technical specifications for each device. These specifications are now also available on the World Wide Web.