A diode can be thought of as the electronic version of a one-way check valve. By restricting the direction of movement of charge carriers, it allows an electrical current to flow in one direction, but essentially blocks it in the opposite direction.

FIGURE 36-49
Diode symbols. A. Standard diode symbol. B. Zener diode symbol. C. LED symbol.

A diode is made up of semiconductor material, one half being P material and the other half being N material. Diodes have a single PN junction. If the diode is connected to a current source, with the P region connected to a negative pole and the N region to a positive pole, the holes will be attracted toward the negative pole and the electrons to the positive pole. This movement enlarges the depletion layer, which makes the insulated space larger, stopping current flow across the junction. Such diodes are referred to as reverse biased.

When the voltage source is reversed, lots of holes flow across the junction toward the negative pole, and electrons travel in the opposite direction toward the positive pole. The PN junction floods with charge carriers, the depletion layer disappears, and with it, the insulator effect disappears. In this direction, the diode allows current to flow. Such diodes are referred to as forward biased.

Using conventional current flow, a diode lets a low voltage current flow through it if current flows from its P side to its N side, but stops current flowing through it from its N side to its P side.

A Zener diode is designed to block current flow through it, but if the voltage is large enough, it can force current to flow through the diode. This is called breakdown. As breakdown voltage is reached, the Zener diode’s resistance suddenly collapses. It lets a large current flow through it, without damage. Because Zener diodes respond to certain voltage changes similar to switches, they are used in voltage regulators.

LEDs, another type of diode, are available in a range of colors and are often used as indicator lights on electrical and electronic appliances. High-powered white LEDs are increasingly being used to replace traditional incandescent light bulbs; they emit light when they are connected in a forward direction Figure 36-49.