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.
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.
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.