Transistors are composed of three parts – a base, a collector, and an emitter. The base
is the gate controller device for the larger electrical supply. The collector is the
larger electrical supply, and the emitter is the outlet for that supply. By sending
varying levels of current from the base, the amount of current flowing through the gate
from the collector may be regulated. In this way, a very small amount of current may be
used to control a large amount of current, as in an amplifier. The same process is used to
create the binary code for the digital processors but in this case a voltage threshold of
five volts is needed to open the collector gate. In this way, the transistor is being
used as a switch with a binary function: five volts – ON, less than five volts – OFF.
The layers of an NPN transistor must have the proper voltage connected across them. The
voltage of the base must be more positive than that of the emitter. The voltage of the
collector, in turn, must be more positive than that of the base. The voltages are
supplied by a battery or some other source of direct current.
The emitter supplies electrons. The base pulls these electrons from the emitter because
it has a more positive voltage than does the emitter. This movement of electrons creates
a flow of electricity through the transistor.
The current passes from the emitter to the collector through the base. Changes in the
voltage connected to the base modify the flow of the current by changing the number of
electrons in the base. In this way, small changes in the base voltage can cause large
changes in the current flowing out of the collector.
Manufacturers also make PNP junction transistors. In these devices, the emitter and
collector are both a p-type semiconductor material and the base is n-type. A PNP junction
transistor works on the same principle as an NPN transistor. But it differs in one
respect. The main flow of current in a PNP transistor is controlled by altering the
number of holes rather than the number of electrons in the base. Also, this type of
transistor works properly only if the negative and positive connections to it are the
reverse of those of the NPN transistor.
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