Energy emission

Some semiconductor diodes emit radiant energy when a current passes through the P-N junction in a forward direction. This phenomenon occurs as electrons fall from higher to lower energy states within atoms.

LEDs and IREDs

Depending on the exact mixture of semiconductors used in manufacture, visible light of almost any color can be produced. Infrared-emitting devices also exist. The most common color for a light-emitting diode (LED) is bright red. An infrared-emitting diode (IRED) produces wavelengths too long to see.
The intensity of the light or infrared from an LED or IRED depends to some extent on the forward current. As the current rises, the brightness increases up to a certain point. If the current continues to rise, no further increase in brilliance takes place. The LED or IRED is then said to be in a state of saturation.

Digital displays

Because LEDs can be made in various different shapes and sizes, they are ideal for use in digital displays. You’ve probably seen digital clock radios that use them. They are common in car radios. They make good indicators for “on/off,” “a. m. /p. m.,” “battery low,” and other conditions.
In recent years, LED displays have been largely replaced by liquid-crystal displays (LCDs). This technology has advantages over LEDs, including much lower power consumption and better visibility in direct sunlight.

Communications

Both LEDs and IREDs are useful in communications because their intensity can be modulated to carry information. When the current through the device is sufficient to produce output, but not enough to cause saturation, the LED or IRED output will follow along with rapid current changes. Voices, music, and digital signals can be conveyed over light beams in this way. Some modern telephone systems make use of modulated light, transmitted through clear fibers. This is known as fiberoptic technology.
Special LEDs and IREDs produce coherent radiation; these are called laser diodes. The rays from these diodes aren’t the intense, parallel beams that you probably imagine when you think about lasers. A laser LED or IRED generates a cone-shaped beam of low intensity. But it can be focused, and the resulting rays have some of the same advantages found in larger lasers.