Light-emitting diodes (LEDs) are semiconductor devices that convert electrical energy into light through electroluminescence. The underlying principle involves the recombination of electrons and holes in a semiconductor material, releasing energy in the form of photons. The wavelength of emitted light is determined by the bandgap energy of the semiconductor, making bandgap engineering a critical aspect of LED design.

Bandgap engineering allows precise control over the emission wavelength, enabling LEDs to produce light across the ultraviolet, visible, and infrared spectra. For example, gallium arsenide (GaAs) emits in the near-infrared, while indium gallium nitride (InGaN) can produce blue or green light, and aluminum gallium indium phosphide (AlGaInP) is used for red and amber emissions. By adjusting the composition of ternary or quaternary alloys, the bandgap can be tuned to achieve desired colors.

A fundamental distinction in semiconductor materials is between direct and indirect bandgap materials. In direct bandgap semiconductors like GaAs and InGaN, the conduction band minimum and valence band maximum occur at the same momentum value, allowing efficient radiative recombination. Indirect bandgap materials, such as silicon or germanium, require phonon assistance for electron-hole recombination, resulting in much lower light emission efficiency. Consequently, most high-performance LEDs are fabricated from direct bandgap semiconductors.

The efficiency of an LED is quantified using internal quantum efficiency (IQE) and external quantum efficiency (EQE). IQE measures the ratio of radiative recombinations to total electron-hole pairs injected, reflecting material quality and defect density. EQE accounts for both IQE and light extraction efficiency, which is influenced by factors like internal reflection and photon absorption. High-quality LEDs achieve IQE values exceeding 90%, while EQE is typically lower due to optical losses.

Heterojunction designs are essential for improving LED performance. A simple p-n junction LED suffers from carrier leakage and poor confinement, leading to inefficiencies. Double heterostructures (DH) confine charge carriers within a narrow active region, enhancing recombination rates. For example, an AlGaN/GaN/AlGaN structure confines electrons and holes in the GaN layer, increasing radiative efficiency. Further refinements include quantum well (QW) and quantum dot (QD) structures, where nanoscale layers or dots create discrete energy levels, improving emission intensity and spectral purity.

Multiple quantum well (MQW) designs are widely used in high-brightness LEDs, particularly in blue and green emitters based on InGaN/GaN. The quantum confinement effect shifts emission wavelengths and reduces non-radiative recombination. Another advanced design is the use of distributed Bragg reflectors (DBRs) or photonic crystals to enhance light extraction by redirecting trapped photons toward the surface.

LEDs have revolutionized lighting and display technologies due to their energy efficiency, longevity, and compact size. In general lighting, white LEDs are produced by combining a blue LED with a phosphor coating that down-converts some blue light into yellow or red, creating a broad spectrum. Phosphor-converted LEDs dominate solid-state lighting, with luminous efficacies exceeding 200 lumens per watt in some cases.

In display applications, LEDs serve as backlights for liquid crystal displays (LCDs) and as individual pixels in micro-LED and mini-LED displays. Red, green, and blue (RGB) micro-LED arrays enable high dynamic range (HDR) and wide color gamut performance, making them suitable for advanced televisions and augmented reality (AR) devices. The absence of color filters in micro-LED displays improves efficiency compared to traditional LCDs.

Specialized LED applications include ultraviolet (UV) LEDs for sterilization, horticultural lighting for optimized plant growth, and automotive lighting for headlamps and indicators. High-power LEDs are also used in optical communication systems, where their modulation speed and reliability are advantageous.

Despite their advantages, LEDs face challenges such as efficiency droop at high current densities, particularly in GaN-based devices. This phenomenon is attributed to Auger recombination and carrier leakage, which reduce IQE under high injection levels. Research continues into novel materials, such as boron nitride (BN) and aluminum nitride (AlN), to mitigate these effects.

Thermal management is another critical consideration, as elevated temperatures degrade LED performance and lifespan. Advanced packaging techniques, including flip-chip designs and ceramic substrates, improve heat dissipation.

In summary, LEDs leverage bandgap engineering, heterojunction design, and quantum confinement to achieve efficient light emission across diverse wavelengths. Their applications span lighting, displays, communications, and beyond, driven by continuous advancements in materials science and device architecture. Future developments may further enhance efficiency, reduce costs, and expand functionality into new domains.