What Are Inorganic Light Emitting Diodes?

Inorganic light emitting diodes (LEDs) are solid-state semiconductor devices that convert electrical energy into light through electroluminescence in crystalline inorganic materials. When a forward bias is applied across a p-n junction, electrons and holes recombine within the semiconductor, releasing energy as photons. The wavelength of the emitted light, and therefore its color, is determined by the band gap energy of the semiconductor material, a quantity that can be tuned by choosing different III-V or II-VI compound semiconductor systems or by varying alloy composition within a given material family.

Inorganic LEDs hold a longer development history than organic light-emitting alternatives, tracing commercial origins to visible red gallium arsenide phosphide (GaAsP) devices demonstrated in the early 1960s. The field advanced substantially through the 1990s with the development of high-brightness blue LEDs based on gallium nitride (GaN) by Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura, work recognized with the 2014 Nobel Prize in Physics and enabling the white LED lamps that have largely replaced incandescent and fluorescent sources for general illumination.

Semiconductor Materials and Band Gap Engineering

The emission spectrum of an inorganic LED is determined by the energy gap between the conduction and valence bands of the active semiconductor layer. Different material systems cover different portions of the spectrum: aluminum gallium indium phosphide (AlGaInP) compounds produce red and yellow light; indium gallium nitride (InGaN) alloys cover the blue, green, and, in principle, amber range; and aluminum nitride and aluminum gallium nitride (AlGaN) extend into the ultraviolet. Silicon carbide (SiC) served as an early substrate and active material for early yellow and blue devices before being largely supplanted by GaN-based heterostructures.

Quantum well structures embedded in the active region, in which a thin layer of lower band gap material is sandwiched between wider band gap layers, confine carriers and increase the probability of radiative recombination. Varying the indium fraction in InGaN quantum wells shifts the emission wavelength across the visible spectrum, a principle that underpins full-color LED fabrication. Research on recent advances in GaN-based micro-LEDs summarizes how nitride material engineering drives performance improvements in both efficiency and color range for advanced display applications.

Device Structure and Fabrication

A conventional inorganic LED consists of epitaxially grown semiconductor layers deposited on a substrate such as sapphire, SiC, or silicon. The epitaxial stack includes n-type and p-type cladding layers, one or more quantum well active layers, and electrical contacts. Packaging adds a phosphor layer in white LED devices, where blue photons from the GaN chip are partially converted to yellow by a cerium-doped yttrium aluminum garnet (Ce:YAG) phosphor, and the combined emission appears white to the eye.

Efficiency is characterized by external quantum efficiency (EQE), the ratio of photons emitted from the device to electrons injected. Improvements in EQE have come from better epitaxial growth techniques that reduce threading dislocations, optimized contact geometries, and surface passivation to minimize non-radiative recombination at exposed semiconductor surfaces.

Micro-LED Technology

Miniaturizing inorganic LEDs to pixel pitches below 100 micrometers produces micro-LED arrays that are candidates for high-resolution displays, augmented reality optics, and high-density optical communications. Integration technology reviews for micro-LED displays identify three principal integration approaches: transfer printing, bonding, and monolithic growth integration, each representing a different trade-off between yield, cost, and color gamut. Ultra-high brightness micro-LED pixels on GaN-on-silicon platforms have demonstrated luminances exceeding 10 million candelas per square meter, enabling legible outdoor displays and direct retinal projection systems. The Light: Science and Applications study on micro-LED future trends examines how micro-LED arrays are being adapted for transparent, free-form, and near-eye display configurations.

Applications

Inorganic light emitting diodes have applications in a wide range of fields, including:

• General lighting and solid-state lamp systems
• Large-area and ultra-fine-pitch display screens
• Augmented and virtual reality near-eye displays
• Automotive lighting and signal indicators
• Optical wireless communications and Li-Fi
• Horticultural lighting and UV disinfection