What Are Light Emitting Diodes?

Light emitting diodes (LEDs) are semiconductor devices that convert electrical energy directly into light through the process of electroluminescence. When a forward bias is applied across a p-n junction formed in a direct-bandgap semiconductor material, injected electrons and holes recombine in the active region, releasing the energy difference between conduction and valence bands as photons. The wavelength of the emitted light is determined by the semiconductor's bandgap energy, allowing engineers to select or tune the emission color by choosing or alloying different semiconductor compounds. LEDs draw on the foundations of solid-state physics, semiconductor device theory, and photonics, and have largely displaced incandescent and fluorescent sources in applications where energy efficiency, long operating life, and controllable spectral output are required.

The practical development of LEDs spans decades of materials science. Devices emitting in the infrared and red were commercially available by the mid-1960s using gallium arsenide (GaAs) and gallium arsenide phosphide (GaAsP). High-brightness blue LEDs, achieved in the early 1990s using gallium nitride (GaN) grown on sapphire substrates, are recognized by the IEEE Milestones program as a landmark development that enabled white LEDs and solid-state general illumination.

P-N Junction Operation

The core of an LED is a p-n junction in which excess minority carriers are injected by forward bias and recombine radiatively in the active region. Physics resources from HyperPhysics at Georgia State University describe how in a simple homojunction device, the active region extends across the full depletion width, which limits carrier confinement and radiative efficiency. High-efficiency devices use double heterostructures or quantum wells, where a thin layer of lower-bandgap material is sandwiched between wider-bandgap cladding layers. This configuration spatially confines both electrons and holes in the active region, increasing the probability of radiative recombination and reducing the competing non-radiative losses. The quantum efficiency of a modern III-nitride LED can exceed 80 percent in the blue region before packaging and phosphor conversion losses are considered. Forward voltages typically range from about 1.8 V for red devices to over 3.0 V for blue and ultraviolet emitters.

Microcavity and Heterostructure Design

Placing the active region of an LED inside a resonant optical microcavity formed by distributed Bragg reflectors (DBRs) modifies the spontaneous emission rate and narrows the emission spectrum through the Purcell effect. Microcavity LEDs emit into fewer optical modes than a conventional device, which improves coupling efficiency into optical fibers or detectors and reduces spectral width. Molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) are the primary deposition techniques for growing the multilayer heterostructures required by both quantum-well and microcavity architectures with atomic-layer precision. Organic LEDs (OLEDs), which substitute organic semiconductor layers for their inorganic counterparts, are a parallel technology used extensively in display applications and exploit the same carrier recombination physics within amorphous rather than crystalline material systems.

Visible Light Communication

Because LEDs can be modulated at rates from kilohertz to several hundred megahertz, they serve as the transmitters in visible light communication (VLC) systems. In a VLC link, intensity-modulated light encodes data that a photodetector recovers, operating in spectrum that is unregulated and free from radio-frequency interference. The IEEE 802.11bb-2023 standard, which covers light communication systems operating in the 800 to 1000 nm near-infrared band, enables throughputs up to 9.6 Gb/s and is interoperable with the existing 802.11 Wi-Fi MAC framework.

Applications

Light emitting diodes have applications in a range of fields, including:

• General illumination in residential, commercial, and street lighting
• Display backlighting and full-color screen panels
• Visible light and near-infrared communication links
• Horticultural lighting with tailored spectral output for plant growth
• Automotive headlamps and signal lighting
• Medical phototherapy and optogenetics research