How Light-Emitting Diodes (LEDs) Work: Semiconductor Physics Explained

This section presents advanced LED structures beyond simple homojunctions: (1) Heterojunction structures with quantum wells confine carriers to narrow bandgap active regions while wider bandgap barriers provide optical transparency; (2) Quantum wells enhance radiative recombination through quantum confinement effects; (3) Phosphor-based white LEDs combine blue LEDs with yellow phosphors that down-convert blue light, achieving higher overall efficiency than direct white light emission; (4) Dislocation density critically impacts LED efficiency, with gallium arsenide failing above 10¹⁵ cm⁻² and indium gallium nitride maintaining ~40% efficiency at 10⁹ cm⁻² due to alloy disorder forming natural quantum dots that spatially separate carriers from defects; (5) Modern LEDs achieve ~50-60% wall-plug efficiency, representing a ten-fold improvement over traditional incandescent lighting.

A Double Heterojunction LED contains two heterojunctions with a narrow band gap active layer sandwiched between two wide band gap semiconductor layers. One heterojunction exists between P-type wide band gap material and the narrow band gap active layer, while the second heterojunction exists between N-type wide band gap material and the active layer. Examples include AlGaAs/GaAs/AlGaAs and InP/InGaAsP/InP structures. This design provides superior carrier and optical confinement, concentrating recombination to the narrow band gap active region. Advantages include higher efficiency, better carrier confinement, higher output power, and lower threshold current for light emission, making DH-LEDs the preferred choice for most optical communication applications.

Quantum wells—extremely thin semiconductor layers (4nm) sandwiched between barriers—enhance LED efficiency by confining charge carriers. Indium gallium nitride quantum wells enable blue LEDs, where indium content precisely controls emission wavelength. The invention of blue LEDs earned Akasaki, Amano, and Nakamura the 2014 Nobel Prize in Physics. White LEDs use phosphor conversion: blue LED chips excite yellow phosphor coatings, combining transmitted blue and converted yellow light to appear white. Organic LEDs (OLEDs) use organic molecules instead of inorganic compounds, offering flexibility for displays but facing challenges in longevity and efficiency. Research explores biodegradable OLED materials. Laser diodes share p-n junction bases but produce directional beams through optical feedback, requiring mirrors and reaching threshold currents before lasing action produces coherent, single-wavelength light. These technologies represent the pinnacle of semiconductor optoelectronics, achieving efficiencies up to 60-70% for certain applications.

LED performance depends on structural design: homojunctions use single semiconductor with varying doping (lower efficiency), while heterojunctions employ multiple materials with different band gaps. Double heterojunctions feature two interfaces with narrow active regions and wide barrier layers, providing superior carrier confinement and photon extraction. External quantum efficiency measures optical-to-electrical energy conversion, comprising recombination efficiency and photon extraction efficiency. Different compound semiconductors (GaN, GaP, AlGaAs) enable color-tunable emission (blue, green, yellow, orange) through band gap engineering, supporting diverse optoelectronic applications.

This section covers advanced LED structures and material engineering strategies for performance optimization. Single quantum well LEDs use a narrow band gap active region sandwiched between wider band gap barrier layers. The wider band gap barriers confine electrons and holes to the active region, preventing reabsorption of emitted photons and reducing non-radiative recombination. This design increases carrier lifetime in the active region and improves light extraction efficiency. For GaN-based LEDs, confinement layers must belong to the III-nitride family (AlN, GaN, InGaN) for lattice compatibility. The confinement material must have larger band gap than GaN (EG > 3.4 eV). Aluminum nitride (AlN, EG ≈ 6.1 eV) satisfies this requirement, while AlGaN alloys with varying aluminum composition can also be used. Material selection balances band gap requirements against lattice mismatch-induced stress. The Varshni's law describes how band gap energy varies with temperature. Lattice matching is essential for heteroepitaxial growth, requiring similar lattice constants between materials. Mismatched materials induce stress that can degrade device performance.

When electrons and holes meet in direct band gap materials, their wave functions overlap and the electron drops to emit a photon directly (radiative recombination). The photon energy equals the band gap energy, given by E = hc/λ. However, not every recombination produces a photon. Shockley-Read-Hall recombination occurs when crystal defects create trap states inside the band gap, releasing energy as heat. Auger recombination involves three particles where energy transfers to a third carrier instead of emitting a photon, causing the infamous efficiency droop at high currents. Indirect Auger recombination (involving carrier scattering) is about 1,000 times stronger than direct processes. Surface recombination at crystal edges with dangling bonds also causes non-radiative losses.

When electrons and holes meet, their wave functions overlap. The electron falls from the conduction band to fill the hole in the valence band—this is electron-hole recombination. In direct band gap materials, the electron and hole have the same momentum, allowing direct photon emission. Gallium nitride (3.4 eV band gap) produces ultraviolet light; indium gallium nitride narrows the band gap for visible colors. William Shockley and William Red published Auger recombination theory in 1952—three-particle process where energy transfers to a third carrier instead of emitting a photon. This depends on the cube of carrier concentration and dominates at high currents, causing efficiency droop. A 2023 study confirmed Auger recombination in gallium nitride LEDs, with indirect processes being 10 times stronger than direct ones.

When voltage is applied to an LED, electrons and holes meet in the depletion zone (10-100 nm wide) where quantum effects dominate. Quantum tunneling allows electrons to pass through barriers they cannot classically surmount—their wave functions extend across barriers, and transmission probability depends on barrier height, width, and particle mass. In direct band gap semiconductors like gallium nitride, electrons and holes have matching momenta, allowing direct photon emission without phonon assistance. Three non-radiative mechanisms waste energy: Shockley-Read-Hall recombination through crystal defects, Auger recombination where energy transfers to a third carrier, and surface recombination at crystal boundaries. Auger recombination dominates at high currents, causing efficiency droop—the phenomenon where LEDs become less efficient at higher brightness levels.

Efficiency droop is a critical challenge in LED technology where output power increases sublinearly with current, causing efficiency (output/input power) to decrease at higher currents. LEDs operate through p-n junctions where electrons recombine with holes in active layers. Three recombination mechanisms exist: radiative recombination (desired, creates photons), Shockley-Read-Hall recombination via defects (produces phonons), and Auger recombination (produces hot carriers). Auger recombination increases with the cube of carrier density, eventually dominating at high currents and stealing electrons from photon generation. The ABC model extracts recombination parameters, but different parameter combinations can produce identical efficiency curves, creating fundamental ambiguity in parameter extraction.

When electrons and holes meet, their wave functions overlap. In direct band gap materials, they have matching momentum, allowing direct photon emission. Gallium nitride (3.4 eV) emits ultraviolet at 365 nm; for blue at 450 nm (2.75 eV), manufacturers use indium gallium nitride—adding indium narrows the band gap. Three recombination mechanisms cause efficiency losses: Shockley-Read-Hall (defects create mid-gap trap states, dominant at low currents), Auger (three-particle process where energy transfers to a third carrier, dominant at high currents causing efficiency droop), and surface recombination (dangling bonds at crystal edges). A 2023 study using 4,000 processor cores for one million hours confirmed Auger recombination as the primary cause of efficiency droop in gallium nitride LEDs, with indirect Auger processes approximately 1,000 times stronger than direct processes. Surface recombination occurs at crystal edges where dangling bonds create trap states. Manufacturers minimize this by passivating surfaces with aluminum oxide or silicon dioxide. Internal quantum efficiency for modern blue LEDs reaches 80-90% at moderate currents but decreases at high currents due to Auger recombination. An electron falls from conduction to valence band in approximately 1 nanosecond, creating a photon. The photon moves randomly within the semiconductor (refractive index ~2.4 for gallium nitride). Total internal reflection occurs at angles greater than the critical angle (~25°), limiting extraction to about 4% for bare chips. Manufacturers improve this by shaping chips into pyramids and using encapsulating domes, achieving 60-70% extraction. Combined with 80% internal efficiency, overall external quantum efficiency reaches about 50%. The Purcell effect (1946) describes how spontaneous emission rates depend on electromagnetic mode density—emitters in resonant cavities emit faster, while those in photonic crystals blocking specific frequencies emit slower. A 2020 study proposed that quantum tunneling involves electron-hole annihilation and creation via virtual photons, resolving the paradox of particles passing through barriers with insufficient energy. A 2025 Physical Review Letters study by Park and Kimin at Korea's Institute for Advanced Photonics showed spontaneous emission behaves contrary to predictions in time-crystal photonic materials—decay rates increase rather than disappear at specific frequencies. They predicted 'stimulated spontaneous emission' where atoms gain energy without absorbing photons. A 2024 Nature Physics study by Schneebell's team demonstrated control over spontaneous emission using ultra-cold matter waves, achieving subradiant states where emission is suppressed.

Diodes interact with photons for both detection and emission. Photodiodes generate photocurrent when photons excite electrons in the depletion zone, operating in photovoltaic or photoconductive modes for light sensing. LASCRs (Light Activated SCRs) trigger via photons instead of gate current, conducting until current falls below holding level. Opto-triacs similarly trigger via light for AC switching. LEDs emit light when forward-biased, with color determined by semiconductor band gap. Laser diodes achieve coherent light through stimulated emission, requiring constant current drivers. These optoelectronic devices enable applications from optical communication to precision measurement and illumination.

Optoelectronic devices are electronic devices that use light energy to perform electrical functions. They convert light energy into electrical energy (photodiodes, solar cells) or convert electrical energy into light energy (LEDs). The three main types of special diodes are: (1) Photodiode - detects light energy, (2) Solar cell - converts light to electrical energy, and (3) LED (Light Emitting Diode) - converts electrical energy to light. A photodiode is a semiconductor device that detects light energy by converting it into electrical current.

Photodiodes operate in reverse bias and generate current under light exposure, functioning as optical detectors. LEDs operate in forward bias and emit light when current flows, using materials like gallium arsenide phosphide for visible light emission. Solar cells convert light to electrical energy through the photovoltaic effect, with key parameters including open-circuit voltage and short-circuit current. The wavelength-voltage relationship is inverse (E = hc/λ). These devices are essential for understanding optoelectronics and renewable energy applications.

Laser (Light Amplification by Stimulated Emission of Radiation) produces high-intensity monochromatic light. LED (Light Emitting Diode) produces low-intensity monochromatic light. Solar cells convert light to electrical power. Photodiodes detect light and convert to electrical signals.

When light shines on a diode, photons with sufficient energy can eject electrons from their atomic shells, creating electron-hole pairs. The electric field in the depletion zone separates these pairs, pushing electrons toward the n-type side and holes toward the p-type side. This generates a voltage across the diode, converting light energy into electrical energy.