Semiconductor Light Source
When bombarded by light, heat, electrons and other form of energy, most semiconductor crystals will emit visible or infrared light. The best semiconductor light source, however, are PN junction diodes.
Light Emitting Diodes
The light emitting diode convert an electrical current directly into light. Therefore the light emitting diode (LED) is more efficient then many other light sources.
The forward voltage across a diode must exceed a threshold level before a current can cross the junction. For silicon, which emits a tiny amount of near-infrared, the threshold is 0.6-volt. For Gallium Arsenide, which emits considerable near-infrared, the threshold is 1.3-volts. This voltage excites the electrons. When the electrons cross the junction and combine with holes, they emit photons.
Kinds of LEDs
Since the LED is a light source, it's helpful to know what's inside the plastic or metal LED case. Shown here is a typical LED. The heavy leads help conduct heat away from the chip. The reflector collects light emitted from the edges of the chip. The epoxy is usually colored when the LED is a visible light emitter. Light scattering particles are often added to the epoxy. This diffuses the light and causes the end of the LED to appear brighter.
Visible light LEDs
These inexpensive LEDs are used as indicator lights. Certain red LEDs are used to transmit information. Most are encapsulated in epoxy.
Many kinds of LED readouts capable of displaying digits and characters are available.They are more rugged than liquid crystal display, but they used much more current.
Infrared LEDs should be called infrared emitting diodes. They are used to transmit information. They are also used in intrusion alarms, remote control devices, etc. A special kind of infrared LED is the diode laser. Some emit several watts!