Page 34 - Teach Yourself Electricity and Electronics

14 Basic physical concepts

Electrical resistance is measured in units called ohms. The higher the value in
ohms, the greater the resistance, and the more difficult it becomes for current to flow.
For wires, the resistance is sometimes specified in terms of ohms per foot or ohms per
kilometer. In an electrical system, it is usually desirable to have as low a resistance, or
ohmic value, as possible. This is because resistance converts electrical energy into heat.
Thick wires and high voltages reduce this resistance loss in long-distance electrical
lines. This is why such gigantic towers, with dangerous voltages, are necessary in large
utility systems.

Semiconductors

In a semiconductor, electrons flow, but not as well as they do in a conductor. You might
imagine the people in the line being lazy and not too eager to pass the balls along. Some
semiconductors carry electrons almost as well as good electrical conductors like copper
or aluminum; others are almost as bad as insulating materials. The people might be just
a little sluggish, or they might be almost asleep.
Semiconductors are not exactly the same as resistors. In a semiconductor, the ma-
terial is treated so that it has very special properties.
The semiconductors include certain substances, such as silicon, selenium, or gal-
lium, that have been “doped” by the addition of impurities like indium or antimony.
Perhaps you have heard of such things as gallium arsenide, metal oxides, or silicon
rectifiers. Electrical conduction in these materials is always a result of the motion
of electrons. However, this can be a quite peculiar movement, and sometimes engi-
neers speak of the movement of holes rather than electrons. A hole is a shortage of an
electron—you might think of it as a positive ion—and it moves along in a direction
opposite to the flow of electrons (Fig. 1-6).

1-6 Holes move in the opposite direction from electrons in a semiconducting material.