Resistors. Semiconductors

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 galliumarsenide, 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 (Fig. 1-6).

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 sky-scraper, 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.

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