What is an LED die (LED chip)?


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An LED die, more commonly known as an LED chip, is a small block of compound semiconductor material that has a p-n junction (positive-negative junction) sandwiched between oppositely doped layers. Under biased conditions the p-n junction can break down causing current to flow from the p-side of the diode to the n-side (anode to cathode). Electrons drop down from the conduction band of the n-doped semiconductor layer and holes from the valence band of the p-doped semiconductor layer recombine in the active region of the diode. The radiative recombination causes electrons to drop into a state of lower energy. The excess energy is released in the form of a photon which is a quantum of electromagnetic radiation. This effect is called electroluminescence. A photon can transport electromagnetic radiation of all wavelengths, including infrared (IR), visible and ultraviolet (UV) light. The color of the light (corresponding to the wavelengths of the emitted photons) is determined by the energy band gap between a conduction band and a valence band of a semiconductor active layer (quantum well).

The majority of LED dies are fabricated from III-V semiconductors such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), gallium phosphide (GaP) and their alloys. A direct band gap has the maximum of the valence band and the minimum of the conduction band located in the same location in k-space (momentum space). Semiconductors with a direct band gap have a higher probability of radiative recombination than those with an indirect band gap. In direct bandgap semiconductors, gallium nitride (GaN) exhibits a high thermal stability and has a broad band gap (ranging from 0.8 to 6.2 eV). GaN-based devices hold a dominant position in the solid state lighting industry. The highest efficiency LEDs today are made from Indium-Gallium Nitride (InGaN) which exhibits good external quantum efficiency in the violet and blue range from UV-A (~365 nm) to deep green (~550 nm). The active region between the p-doped GaN and the n-doped GaN can be grown with different concentrations of InGaN to create quantum wells. The wavelength of the emitted photons can be tuned by varying the concentration of quantum wells.

An LED die yields narrow-band emission with typical bandwidths of a few tens of nanometers. To generate wide-band colored emission or white light from InGaN dies, a phosphor wavelength converter is employed to partially or completely converts the electroluminescence within an LED package.

Blue, UV and green LEDs are generally formed using the AlInGaN material system. Yellow, amber or red LEDs are currently based on aluminum indium gallium phosphide (AlInGaP) materials.
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