Transistors are one of the most common
type of semiconductor and have many uses in electronics and can
be broadly grouped into Bipolar and Field Effect types. although
there are many sub-categories. The semiconductor material is
typically silicon although germanium is still sometimes used.
Silicon tends to be the preferred choice as it has much better
characteristics and better thermal stability and repeatability
than germanium. A transistor is made up from two different types
of silicon (or germanium) known as n type and p type. The manufacturing
process begins with a pure piece of silicon which is then treated
with other compounds to either make the resulting molecular structure
either more positive or more negative than the pure material.
This process is usually known as doping. A bipolar transistor
is made from a sandwich of the two types of silicon which results
in either an npn transistor or a pnp transistor depending on
which type of silicon is used for the 'meat' in the sandwich.
Bipolar Transistors The transistor has three terminals,
the outer two terminals are known as the emitter and the collector.
The middle terminal is known as the base terminal. The collector
and emitter terminals are similar in manufacture. The base terminal
between them is much thinner. The bipolar transistor is a current
controlled device. A small current applied to the base terminal
will control a much larger current flowing from the collector
to the emitter terminals. The emitter terminal is usually shown
by an arrow on the component diagram. Notice that the direction
of the arrow indicates whether the transistor is npn or pnp and
indicates the direction of current flow. The principles of operation
of the two types of transistor is the same apart from the direction
of current flow through the device.
Field Effect Transistors (FET) The FET has three terminals which have
similar functions to those of the bipolar transistor. The terminals
are known as the source, gate and drain which are similar to
the emitter, base and collector of the bipolar transistor. The
main operating difference is that the FET is a voltage controlled
device. A control voltage applied to the gate terminal can regulate
the current flow between source and drain terminals. The gate
terminal of the FET usually has an extremely high input impedance
of millions of ohms and doesn't present any loading on the gate
control circuitry. This can be very useful if the FET is being
used as a small signal amplifier but it also has the drawback
of making the FET very susceptible to damage by static electricity.
Fortunately, transistor manufacturers design in various diodes
as part of the manufacturing process to help to limit the effects
of static electricity.