Until the mid-90s LEDs had a limited range of colors, and in particular commercial blue and white LEDs did not exist. Examples of III-V materials commonly used to make LEDs are gallium arsenide (GaAs) and gallium phosphide (GaP). LEDs are comprised of compound semiconductor materials, which are made up of elements from group III and group V of the periodic table (these are known as III-V materials). The precise wavelength (color) can be tuned by altering the composition of the light-emitting, or active, region. Different semiconductor materials with different bandgaps produce different colors of light. The photon energy determines the wavelength of the emitted light, and hence its color. the bandgap) determines the energy of the photons (light particles) that are emitted by the LED. Inside the semiconductor material of the LED, the electrons and holes are contained within energy bands. The term solid-state lighting, which also encompasses organic LEDs (OLEDs), distinguishes this lighting technology from other sources that use heated filaments (incandescent and tungsten halogen lamps) or gas discharge (fluorescent lamps). Since light is generated within the solid semiconductor material, LEDs are described as solid-state devices. Light is produced when the particles that carry the current (known as electrons and holes) combine together within the semiconductor material. In the simplest terms, a light-emitting diode (LED) is a semiconductor device that emits light when an electric current is passed through it.
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