Basic Electronics - Conductors,Insulators,Resistors and Semiconductors

Conductors

In some materials, electrons move easily from atom to atom. In others, the electrons
move with difficulty. And in some materials, it is almost impossible to get them to move.
An electrical conductor is a substance in which the electrons are mobile.
The best conductor at room temperature is pure elemental silver. Copper and aluminum
are also excellent electrical conductors. Iron, steel, and various other metals are
fair to good conductors of electricity.
In most electrical circuits and systems, copper or aluminum wire is used. Silver is
impractical because of its high cost.
Some liquids are good electrical conductors. Mercury is one example. Salt water is
a fair conductor.
Gases are, in general, poor conductors of electricity. This is because the atoms or
molecules are usually too far apart to allow a free exchange of electrons. But if a gas becomes
ionized, it is a fair conductor of electricity.
Electrons in a conductor do not move in a steady stream, like molecules of water
through a garden hose. Instead, they are passed from one atom to another right next to
it.This happens to countless atoms all the time. As a result, literally trillions
of electrons pass a given point each second in a typical electrical circuit.
You might imagine a long line of people, each one constantly passing a ball to the
neighbor on the right. If there are plenty of balls all along the line, and if everyone keeps
passing balls along as they come, the result will be a steady stream of balls moving along
the line. This represents a good conductor.
If the people become tired or lazy, and do not feel much like passing the balls along,
the rate of flow will decrease. The conductor is no longer very good.

Insulators
If the people refuse to pass balls along the line in the previous example, the line represents
an electrical insulator. Such substances prevent electrical currents from flowing,
except possibly in very small amounts.
Most gases are good electrical insulators. Glass, dry wood, paper, and plastics are
other examples. Pure water is a good electrical insulator, although it conducts some
current with even the slightest impurity. Metal oxides can be good insulators, even
though the metal in pure form is a good conductor.
Electrical insulators can be forced to carry current. Ionization(
Ions
If an atom has more or less electrons than neutrons, that atom acquires an electrical
charge. A shortage of electrons results in positive charge; an excess of electrons gives a
negative charge. The element’s identity remains the same, no matter how great the excess
or shortage of electrons. In the extreme case, all the electrons might be removed
from an atom, leaving only the nucleus. However it would still represent the same
element as it would if it had all its electrons.
A charged atom is called an ion. When a substance contains many ions, the material
is said to be ionized.) can take place;

when electrons are stripped away from their atoms, they have no choice but to move along.
Sometimes an insulating material gets charred, or melts down, or gets perforated by a
spark. Then its insulating properties are lost, and some electrons flow.
An insulating material is sometimes called a dielectric. This term arises from the
fact that it keeps electrical charges apart, preventing the flow of electrons that would
equalize a charge difference between two places. Excellent insulating materials can be
used to advantage in certain electrical components such as capacitors, where it is important
that electrons not flow.
Porcelain or glass can be used in electrical systems to keep short circuits from occurring.
These devices, called insulators, come in various shapes and sizes for different
applications. You can see them on high-voltage utility poles and towers. They hold the
wire up without running the risk of a short circuit with the tower or a slow discharge
through a wet wooden pole.

Resistors
Some substances, such as carbon, conduct electricity fairly well but not really well. The
conductivity can be changed by adding impurities like clay to a carbon paste, or by winding
a thin wire into a coil. Electrical components made in this way are called resistors. They
are important in electronic circuits because they allow for the control of current flow.
Resistors can be manufactured to have exact characteristics. Imagine telling each
person in the line that they must pass a certain number of balls per minute. This is analogous
to creating a resistor with a certain value of electrical resistance.
The better a resistor conducts, the lower its resistance; the worse it conducts, the
higher the resistance.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 material
is treated so that it has very special properties.
The semiconductors include certain substances, such as silicon, selenium, or gallium,
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 engineers
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

Note:Holes move in the opposite direction from electrons in a semiconducting material

When most of the charge carriers are electrons, the semiconductor is called
N-type, because electrons are negatively charged. When most of the charge carriers are
holes, the semiconducting material is known as P-type because holes have a positive
electric charge. But P-type material does pass some electrons, and N-type material carries
some holes. In a semiconductor, the more abundant type of charge carrier is called
the majority carrier. The less abundant kind is known as the minority carrier.
Semiconductors are used in diodes, transistors, and integrated circuits in almost
limitless variety. These substances are what make it possible for you to have a computer
in a briefcase. That notebook computer, if it used vacuum tubes, would occupy a skyscraper,
because it has billions of electronic components. It would also need its own
power plant, and would cost thousands of dollars in electric bills every day. But the circuits
are etched microscopically onto semiconducting wafers, greatly reducing the size
and power requirements.