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 upon the exact mixture of semiconductors which is used in the manufacture, visible light of any color can be produced. Infrared emitting devices exist. The most common color for the light-emitting diode (LED) is bright red. An infrared-emitting diode (IRED) produces wavelengths which are 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 increases, the brightness increases up to a specific point. If the current continues to increase, no further increase in brilliance takes place. The LED or IRED is then said to be in the state of saturation.

Digital displays

Because LEDs can be made in various different shapes and sizes, they are perfect for use in digital displays. You have probably seen digital clock radios that use them. They are quite 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 In recent years, LED displays have been replaced by liquid-crystal displays. This technology has advantages over LEDs, including lower power consumption and better visibility in the direct sunlight.

Communications

LEDs and IREDs are both useful in communications as their intensity can be modulated to carry the information. When current through the device is enough to produce output, but not sufficient to cause saturation, the LED or IRED output will follow with the rapid current changes. The voices, music, and digital signals can be conveyed over the light beams in this manner. Some modern telephone systems make uses modulated light, transmitted by clear fibers. This is called as fiberoptic technology.

Special LEDs and IREDs produce coherent radiation; these are called as laser diodes. The rays from these diodes are not the intense, parallel beams that you probably imagine when you think about lasers. A laser LED or IRED which generates a cone-shaped beam of low intensity. But it is focused, and resulting rays have some of the same advantages found in the larger lasers.