A semiconductor device known as an LED is made up of an LED die (chip) and other parts that serve as mechanical support, electrical connections, thermal conductors, optical regulators, and wavelength converters. The fundamental structure of an LED chip is a p-n junction device made of compound semiconductor layers with opposing dopants. Gallium nitride (GaN), a compound semiconductor that is often used, has a direct band gap, which increases the likelihood of radiative recombination compared to semiconductors having an indirect band gap. When the p-n junction is biassed forward, electrons from the n-type semiconductor layer's conduction band pass the boundary layer into the p-junction, where they recombine with holes from the p-type semiconductor layer in the diode's active area. The electrons descend into a lower energy state as a result of the electron-hole recombination, and the extra energy is released as photons (packets of light). Electroluminescence is the name of this phenomenon. All wavelengths of electromagnetic energy may be carried by the photon. The energy band gap of the semiconductor determines the precise wavelengths of light emitted by the diode.
The LED chip's electroluminescence produces light with a restricted wavelength range and a typical bandwidth of a few tens of nanometers. Light from narrow-band emissions is only one colour, such red, blue, or green. The width of the spectral power distribution (SPD) of the LED chip must be increased in order to provide a white light source with a broad spectrum. Photoluminescence in phosphors converts the electroluminescence from the LED chip partially or entirely. Most white LEDs mix the re-emitted longer wavelength light from phosphors with the short wavelength emission from InGaN blue chips. The phosphor powder is distributed in a matrix made of silicon, epoxy, or another type of resin. The LED chip has a coating made of a matrix comprising phosphor. By pumping red, green, and blue phosphors with an ultraviolet (UV) or violet LED chip, white light may also be generated. In this situation, the resultant white can portray colours more accurately. However, due to the significant wavelength shift and considerable Stokes energy loss involved in the down-conversion of UV or violet light, this method has a low efficiency.
