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Light-emitting diode physics

A light-emitting diode (LED) is a semiconductor device that emits light via electroluminescence, a process in which forward-biased current across a p-n junction causes electrons from the n-type region to recombine with holes in the p-type region, releasing photons with energy equal to the material's band gap.^[1]^[2] The fundamental physics relies on direct band gap semiconductors, such as gallium arsenide (GaAs) or gallium nitride (GaN), where the recombination occurs efficiently without significant phonon involvement, enabling the conversion of electrical energy to optical energy with high efficiency.^[3]^[4] In operation, applying a forward voltage reduces the depletion region's potential barrier, allowing carrier injection and diffusion into the active region, where spontaneous emission dominates light production.^[2] The emitted light's wavelength, and thus color, is precisely tuned by the band gap energy—ranging from infrared (e.g., ~1.4 eV for GaAs at 900 nm) to ultraviolet (e.g., <400 nm for certain InGaN alloys)—with common materials like AlGaInP for red-orange and InGaN for blue-green emissions.^[1]^[3] This band gap engineering allows LEDs to span the visible spectrum, from violet (~400 nm) to red (~760 nm).^[2] Efficiency in LEDs is characterized by internal quantum efficiency, which measures the fraction of injected carriers that result in photons via radiative recombination, often approaching 0.75–0.95 in optimized III-V semiconductors, though external efficiency is lower (~20–80% as of 2025) due to total internal reflection at the semiconductor-air interface, with modern extraction techniques significantly mitigating these losses.^[4]^[5] Advances in heterostructures and quantum wells enhance carrier confinement and reduce non-radiative losses, enabling luminous efficacies that surpass traditional sources like incandescent bulbs.^[3] White light, essential for general illumination, is achieved not through direct emission but via phosphor conversion of blue LEDs or color mixing of red, green, and blue variants.^[6]

Basic Principles

p-n Junction Operation

A p-n junction is formed by joining a p-type semiconductor, doped with acceptor impurities that create holes as majority carriers, to an n-type semiconductor, doped with donor impurities that provide electrons as majority carriers.^[7] Upon contact, electrons from the n-region diffuse into the p-region, and holes from the p-region diffuse into the n-region, leading to recombination near the interface and the exposure of fixed ionized donors (positive charge) on the n-side and ionized acceptors (negative charge) on the p-side.^[8] This charge separation establishes an electric field that opposes further diffusion, resulting in a depletion region—a carrier-depleted zone around the junction where mobile charges are minimized.^[9] The built-in potential V_{bi} across the junction in equilibrium arises from this charge separation and is given by

V_{bi} = \frac{kT}{q} \ln \left( \frac{N_A N_D}{n_i^2} \right),

where k is Boltzmann's constant, T is temperature, q is the elementary charge, N_A and N_D are the acceptor and donor doping concentrations, and n_i is the intrinsic carrier concentration.^[10] Energy band diagrams illustrate this: in equilibrium, the Fermi levels align across the junction, bending the conduction and valence bands to form a potential barrier of height qV_{bi}, with the depletion region showing a linear band slope due to the electric field.^[7] Under reverse bias, applying a negative voltage to the p-side (relative to the n-side) widens the depletion region and increases the barrier height, enhancing the electric field and suppressing carrier diffusion.^[11] In forward bias, a positive voltage to the p-side reduces the barrier, narrowing the depletion region and allowing majority carriers to overcome it.^[9] Under forward bias exceeding V_{bi}, minority carriers are injected across the junction: electrons from the n-region into the p-region and holes from the p-region into the n-region.^[12] These injected minorities then diffuse into the respective quasi-neutral regions, where they recombine with majority carriers over a characteristic diffusion length, typically on the order of micrometers, governed by the minority carrier lifetime and diffusivity.^[13] The resulting current-voltage characteristics follow the Shockley diode equation:

I = I_s \left( e^{qV / kT} - 1 \right),

where I_s is the reverse saturation current, dependent on material parameters like doping and lifetime, describing the exponential increase in forward current due to enhanced carrier injection and diffusion.^[14] In p-n junctions, carrier dynamics differ between direct and indirect bandgap semiconductors: in direct bandgap materials, momentum conservation allows efficient radiative recombination of injected carriers without phonon involvement, while in indirect bandgap materials, phonon-assisted processes are required for momentum matching, leading to lower recombination probabilities and predominantly non-radiative pathways.^[15] This distinction influences the overall efficiency of carrier transport and recombination in the junction, with direct bandgap semiconductors exhibiting faster radiative dynamics essential for light emission.^[16]

Electroluminescence Mechanism

Electroluminescence in light-emitting diodes (LEDs) arises from the recombination of injected electrons and holes in a semiconductor p-n junction under forward bias, converting electrical energy into photons. This process was first observed in 1907 by Henry Joseph Round, who noted a yellow glow from a silicon carbide (SiC) crystal when applying a voltage, marking the initial discovery of solid-state electroluminescence. The practical realization of visible electroluminescence came in 1962 with Nick Holonyak's demonstration of a red-emitting GaAsP diode, which relied on direct bandgap radiative recombination to achieve visible light emission.^[17] The core mechanism of light emission is radiative recombination, where an electron from the conduction band recombines with a hole in the valence band, releasing a photon with energy approximately equal to the semiconductor's bandgap energy E_g. In direct bandgap materials like gallium arsenide (GaAs), the conduction band minimum and valence band maximum occur at the same wavevector k = 0 in the Brillouin zone, allowing momentum conservation during the transition without additional phonon assistance; thus, the process is efficient, with the emitted photon's energy given by E \approx E_g. In contrast, indirect bandgap semiconductors such as silicon (Si) have band extrema at different k-values, requiring phonon emission or absorption to conserve momentum, which suppresses radiative efficiency and favors non-radiative paths, making them unsuitable for efficient LEDs. Non-radiative recombination competes with the radiative process, reducing light output by dissipating energy as heat. Shockley-Read-Hall (SRH) recombination occurs via defect or impurity states within the bandgap, trapping carriers and enabling thermal re-emission or non-radiative annihilation, with rates dependent on defect density and capture cross-sections.^[18] Auger recombination involves three carriers: an electron-hole pair recombines radiatively, but the energy excites a third carrier to a higher state, which then relaxes non-radiatively via phonons; this process becomes dominant at high carrier densities in narrow-bandgap materials.^[19] Surface recombination further contributes at interfaces, where dangling bonds act as efficient traps, accelerating non-radiative decay and necessitating passivation techniques.^[20] The internal quantum efficiency (IQE) quantifies the fraction of injected carriers that result in radiative recombination, defined as

\text{IQE} = \frac{R_{\text{rad}}}{R_{\text{rad}} + R_{\text{SRH}} + R_{\text{Auger}} + R_{\text{surface}}}

where R_{\text{rad}}, R_{\text{SRH}}, R_{\text{Auger}}, and R_{\text{surface}} are the respective recombination rates; high IQE (>80%) is achievable in direct bandgap III-V semiconductors with low defect densities.^[20] Recombination processes and emission properties exhibit temperature dependence. Radiative rates in direct bandgap materials show weak temperature dependence, while non-radiative SRH rates may decrease if traps become ionized; Auger rates, however, rise strongly with temperature owing to higher carrier energies. The emission wavelength \lambda shifts redward with increasing temperature as the bandgap narrows, following \lambda = \frac{hc}{E_g(T)}, where E_g(T) decreases approximately linearly for many semiconductors, leading to a typical shift of 0.1–0.3 nm/K in GaN-based LEDs.

Device Structure and Materials

Semiconductor Materials

Light-emitting diodes (LEDs) primarily utilize III-V compound semiconductors due to their direct or tunable bandgaps, which facilitate efficient radiative recombination for light emission. Gallium arsenide (GaAs) is a direct-bandgap material with an energy gap of 1.42 eV, enabling infrared emission around 870 nm and serving as a foundational material for early LED devices.^[21] Gallium phosphide (GaP), an indirect-bandgap semiconductor with a bandgap of 2.26 eV, has been employed for green light emission at approximately 550 nm, though its indirect nature results in lower efficiency compared to direct-bandgap alternatives.^[22] Indium gallium nitride (InGaN), a direct-bandgap alloy, offers tunable bandgap energies from approximately 0.7 eV (InN) to 3.4 eV (GaN) by varying the indium composition, enabling emission from near-infrared to ultraviolet wavelengths. High-efficiency visible LEDs are primarily achieved in the blue to ultraviolet range, with recent advances (as of 2024) enabling high-efficiency green and red emissions for full-color displays.^[23]^[24]^[25] Alloying III-V compounds, such as aluminum gallium arsenide (AlGaAs) and indium gallium phosphide (InGaP), is crucial for bandgap engineering while maintaining lattice matching to substrates like GaAs to minimize defects and dislocations that degrade performance. AlGaAs exhibits near-perfect lattice matching to GaAs (mismatch <0.2%), promoting high-quality epitaxial growth with reduced non-radiative recombination centers.^[26] In InGaP alloys, compressive or tensile strain is intentionally introduced to adjust the bandgap for red to yellow emission, but careful control is required to avoid phase separation and strain-induced defects that increase with indium content.^[27] Achieving p-type doping in wide-bandgap III-V materials like gallium nitride (GaN) presents significant challenges, primarily due to the high activation energy of magnesium (Mg) acceptors, which ranges from 200 to 265 meV, limiting hole concentration and conductivity at room temperature.^[28] This requires high-temperature annealing to activate Mg and remove hydrogen passivation, yet solubility limits and compensation by defects often result in suboptimal p-type layers essential for p-n junctions.^[28] The development of LED semiconductors evolved from infrared GaAs devices in the early 1960s to visible-light emitters, culminating in efficient blue InGaN LEDs that enabled white light generation through phosphor conversion.^[29] This breakthrough, recognized by the 2014 Nobel Prize in Physics awarded to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura, overcame longstanding challenges in growing high-quality GaN-based materials. Despite their advantages, LED semiconductors face limitations including toxicity in cadmium-based II-VI compounds like CdSe used historically for green and yellow emission, high production costs for GaN due to complex epitaxial growth on lattice-mismatched substrates, and thermal stability issues where elevated temperatures can accelerate defect formation and reduce efficiency in wide-bandgap materials.^[30]^[31]^[32]

Heterostructures and Quantum Confinement

Heterostructures in light-emitting diodes (LEDs) enhance performance by providing effective confinement of charge carriers and photons within the active region, enabling higher efficiency and brighter emission compared to homojunction devices. The foundational concept of the double heterostructure (DH), proposed in 1963, involves sandwiching a narrower-bandgap active layer between wider-bandgap p-type and n-type cladding layers, which creates discontinuities in the conduction and valence band edges, denoted as ΔE_c and ΔE_v, respectively. These band offsets confine electrons and holes to the active region, reducing non-radiative recombination losses outside it.^[33] A classic example is the GaAs active layer bounded by AlGaAs cladding layers, where the aluminum content in AlGaAs creates sufficient band offsets (typically ΔE_c ≈ 0.3–0.6 eV depending on composition) to achieve efficient carrier injection and radiative recombination at wavelengths around 870 nm. Experimental comparisons of such DH LEDs with single heterostructure counterparts demonstrated internal quantum efficiencies exceeding 20% and reduced operating voltages, highlighting the role of symmetric carrier confinement in minimizing current spreading.^[34]^[35] Building on DH designs, quantum wells (QWs) introduce two-dimensional (2D) carrier confinement by using ultrathin active layers (typically 2–10 nm thick) sandwiched between thicker barrier layers, which quantize the energy states and modify the density of states. In InGaN/GaN multiple quantum well (MQW) structures, for instance, alternating InGaN wells and GaN barriers enable blue emission at ~450 nm with enhanced wavefunction overlap between electrons and holes. This confinement increases the oscillator strength and radiative recombination rate, where the inverse lifetime 1/τ_rad scales with the square of the electron-hole wavefunction overlap |ψ|^2, leading to faster emission and higher efficiency. High-brightness blue QW LEDs achieved external quantum efficiencies up to 9% at 20 mA drive current, a significant improvement over bulk active layers.^[36] These structures offer key advantages, including reduced threshold currents for lasing applications (though LEDs benefit from lower drive currents for equivalent output) and elevated internal quantum efficiency (IQE) due to suppressed carrier leakage. However, in wurtzite III-nitride materials like InGaN/GaN grown along the c-axis, spontaneous and piezoelectric polarization fields induce an internal electric field (~1–3 MV/cm), causing the quantum-confined Stark effect (QCSE). This red-shifts the emission wavelength and reduces |ψ|^2 by separating electron and hole wavefunctions, thereby decreasing IQE at longer wavelengths such as green. To address QCSE, non-polar and semi-polar GaN orientations have been developed, reducing internal fields and enabling higher efficiency in green and yellow LEDs, as demonstrated in recent epitaxial structures (as of 2023).^[37]^[38] Epitaxial growth of these heterostructures and QWs relies primarily on metal-organic chemical vapor deposition (MOCVD), which allows precise control of layer thickness and composition at atmospheric or low pressure, essential for achieving sharp interfaces and low defect densities in materials like GaAs/AlGaAs and InGaN/GaN.^[39] Further dimensionality reduction to quantum wires (1D confinement) and quantum dots (0D confinement) modifies the density of states into step-like or delta functions, potentially enhancing gain and efficiency, though these are explored in detail in advanced nanocrystal-based LEDs.

Optical Physics

Refractive Index Mismatch

In light-emitting diodes (LEDs), the significant difference between the refractive index of the semiconductor material and that of the surrounding medium, typically air, leads to substantial optical losses through total internal reflection (TIR). Semiconductor materials commonly used in LEDs, such as gallium arsenide (GaAs) with a refractive index n \approx 3.5 or gallium nitride (GaN) with n \approx 2.4, contrast sharply with air's n = 1.^[40]^[41] When photons generated by recombination in the active region propagate toward the chip's surface, those incident at angles exceeding the critical angle \theta_c = \sin^{-1}(n_{\text{out}}/n_{\text{in}}) undergo TIR and remain confined within the device. For GaN-based LEDs, this critical angle is approximately 24°, meaning approximately 91% of possible emission directions result in reflection back into the semiconductor (with ~9% within the escape cone).^[40]^[42]^[43] The escape cone defined by this critical angle severely restricts the external quantum efficiency by limiting the solid angle through which light can exit the LED chip. For isotropic emission from the active region, the fraction of light escaping into air without enhancement is given by the geometric factor $1/(2n^2), representing the portion of the hemispherical solid angle within the escape cone.^[43] In high-index materials like GaAs (n \approx 3.5), this yields only about 4% extraction efficiency in the simple model, while for GaN (n \approx 2.4), it is around 9%; however, accounting for chip geometry and multiple interfaces often reduces the effective value to approximately 2% for unoptimized planar devices.^[43]^[44] This confinement arises because recombination in the active region produces photons uniformly in all directions, but only those nearly perpendicular to the surface avoid TIR. Under the ray optics model, emitted photons are traced as rays that bounce multiple times within the LED chip due to TIR at the semiconductor-air interfaces, eventually leading to reabsorption or escape from sidewalls.^[45] These guided rays travel laterally through the high-index layers, undergoing successive reflections that increase the path length and probability of parasitic absorption by free carriers, defects, or metallic contacts.^[45] In typical LED chips, this trapping dominates optical losses, as the rays' trajectories form closed paths or waveguiding modes parallel to the surface, preventing direct extraction. The refractive index mismatch is further influenced by wavelength-dependent dispersion, where n varies with photon energy according to relations like the Sellmeier equation, n^2(\lambda) = 1 + \sum_i B_i \lambda^2 / (\lambda^2 - C_i), capturing resonant contributions from electronic transitions.^[41] In III-V semiconductors, shorter wavelengths (e.g., blue light in GaN LEDs) correspond to higher n near the band edge, resulting in a smaller \theta_c and more pronounced TIR compared to longer red wavelengths.^[46] This dispersion exacerbates light trapping in ultraviolet or blue LEDs, where extraction fractions drop below those in red devices. The mismatch profoundly impacts emission patterns in LED chips, favoring confinement for isotropic sources while permitting higher escape for directed emission aligned normal to the interfaces. In practice, the active region's recombination yields largely isotropic radiation, promoting lateral waveguiding and reducing perpendicular output, which contrasts with ideal directed emitters that could exploit the full escape cone more effectively.^[43]

Light Extraction Techniques

One primary method to enhance light extraction in LEDs involves encapsulation with materials that reduce the refractive index contrast at the semiconductor-air interface. By surrounding the LED chip with an epoxy resin having a refractive index of approximately 1.5, the critical angle for total internal reflection increases, enlarging the escape cone and allowing more light to exit the device. This approach typically improves extraction efficiency by 1.5 to 2 times compared to air-encapsulated devices. Further enhancement is achieved through hemispherical dome shapes in the epoxy, which redirect rays that would otherwise undergo multiple internal reflections, effectively shaping the emission pattern and boosting output power. Surface texturing techniques scatter light to redirect photons beyond the critical angle, significantly increasing extraction. Microroughening the p-GaN surface, for instance, has been shown to double the light output power in InGaN-based LEDs by promoting diffuse reflection and reducing Fresnel losses.^[47] Photonic crystals, formed by periodic nanopatterns on the surface, further amplify this effect through diffraction and bandgap engineering, achieving 2-3 times higher extraction in GaN LEDs by coupling guided modes to free space. Transition coatings employ thin multilayer films to create a graded refractive index profile, minimizing reflections at interfaces. Structures such as SiO₂/TiO₂ multilayers approximate an antireflection coating, where the reflection coefficient R = \left| \frac{n_1 - n_2}{n_1 + n_2} \right|^2 is reduced across layers, enabling up to 30% improvement in light output for GaN devices. These coatings provide a smooth index transition from the high-index semiconductor (n ≈ 2.5) to air (n = 1), suppressing waveguiding losses. Chip shaping modifies the device geometry to optimize ray paths and minimize trapping. Hemispherical or truncated pyramid designs in GaN LEDs redirect light toward the surface, increasing extraction by 20-50% depending on the aspect ratio, as steeper angles reduce the probability of total internal reflection for oblique rays. Post-2010 advances have integrated nanorod arrays and advanced photonic crystals to achieve extraction efficiencies exceeding 50% in GaN LEDs. Nanorod structures, such as ZnO or GaN arrays, scatter light laterally and vertically, enhancing output by over 100% in some green emitters through improved photon escape from sidewalls. Similarly, optimized photonic crystal patterns on ITO or p-GaN layers have demonstrated light extraction up to 70% by precisely controlling mode coupling, representing a key step toward high-brightness applications. As of 2025, experimental demonstrations in advanced GaN microLEDs with nanorod arrays and photonic crystals have achieved light extraction efficiencies exceeding 60%, with simulations suggesting potential up to 80% under optimized conditions.^[48]

Performance Metrics

Efficiency Parameters

The efficiency of light-emitting diodes (LEDs) is quantified through several key parameters that assess the conversion of electrical energy into light, encompassing internal processes within the device and external factors affecting output. These metrics are essential for evaluating overall performance, guiding material and structural optimizations, and comparing devices across applications.^[49] Internal quantum efficiency (IQE) measures the fraction of injected electrons (or holes) that recombine radiatively to generate photons within the active region of the LED, relative to the total number of injected carriers. It is defined as the ratio of the internal optical power emitted from the active region to the electrical power associated with carrier injection, expressed as IQE = \frac{P_\mathrm{int}}{I \cdot (h\nu / q)}, where P_int is the internal optical power, I is the injection current, h\nu is the photon energy, and q is the elementary charge. This parameter fundamentally depends on the balance between radiative and non-radiative recombination processes in the semiconductor.^[49]^[50]^[51] External quantum efficiency (EQE) extends this by accounting for the fraction of internally generated photons that successfully escape the device as useful output light, incorporating carrier injection efficiency (the proportion of injected carriers reaching the active region) and light extraction efficiency (the portion of photons that avoid total internal reflection or absorption). It is given by EQE = IQE × injection efficiency × extraction efficiency, representing the number of emitted photons escaping the LED per injected electron. High EQE requires optimizing all these factors, with light extraction playing a critical role due to refractive index mismatches at interfaces.^[49]^[52]^[53] Wall-plug efficiency (WPE), also known as power efficiency, evaluates the overall energy conversion from electrical input to optical output, defined as WPE = P_opt / (I V) × 100%, where P_opt is the optical power output and V is the forward voltage. This metric relates to EQE through WPE = EQE × (hν / q V), where hν is the photon energy, highlighting the impact of operating voltage on practical efficiency. WPE is particularly important for high-power applications, as it directly indicates electrical-to-optical power utilization.^[49]^[50]^[54] For visible-spectrum LEDs, luminous efficacy provides a human-centric measure of efficiency, quantifying visible light output in lumens per watt (lm/W) while accounting for the eye's spectral sensitivity via the photopic luminosity function V(λ), which peaks at 555 nm with a maximum efficacy of 683 lm/W for monochromatic green light. It is calculated as luminous efficacy = (optical power × 683 × ∫ V(λ) S(λ) dλ / ∫ S(λ) dλ) / electrical power, where S(λ) is the LED's spectral power distribution; typical values for white LEDs range from 100 to 200 lm/W in modern devices. This parameter bridges radiometric efficiencies like EQE with perceptual brightness.^[55]^[56] Efficiency parameters exhibit strong dependence on operating conditions, with IQE typically peaking at low injection currents (around 1-10 mA/cm²) where non-radiative losses are minimized, and decreasing at higher currents due to increased carrier densities. Temperature also influences performance, as rising junction temperatures enhance non-radiative recombination rates, reducing IQE by 5-20% from 25°C to 100°C in GaN-based LEDs depending on device quality, while extraction efficiency may vary due to thermal changes in refractive indices.^[57]^[58]^[59] Measurement of these efficiencies has evolved with standardized techniques, such as temperature-dependent electroluminescence for IQE assessment and integrating sphere photometry for EQE, enabling precise characterization. Advancements since the early 2000s have dramatically improved blue LED performance, with EQE rising from below 5% in 2000 to over 80% by the 2020s through refinements in epitaxial growth and device design.^[60]^[58]^[61]

Efficiency Droop

Efficiency droop is a critical limitation in light-emitting diodes (LEDs), characterized by a decrease in external quantum efficiency (EQE) as the injection current density increases beyond a certain threshold. In typical InGaN/GaN blue LEDs, the EQE peaks at low current densities of approximately 20–100 mA/cm² and can decline by more than 50% at 1 A/cm², hindering the scaling of light output for high-power operation.^[62] This phenomenon primarily affects the internal quantum efficiency (IQE), which is governed by the competition between radiative and non-radiative recombination processes under high carrier injection.^[63] The main mechanisms contributing to efficiency droop include Auger recombination, a non-radiative process where the energy from electron-hole recombination excites another carrier, leading to losses proportional to the cube of the carrier density (∝ n³, where n is the carrier concentration). Direct measurement of hot electrons emitted from operating LEDs has confirmed Auger's dominance, with coefficients C on the order of 10⁻³⁰ cm⁶/s.^[64] Additionally, carrier delocalization in InGaN quantum wells (QWs) at high densities reduces localization in potential fluctuations, promoting spillover and non-radiative pathways, while thermal heating causes a rise in junction temperature that exacerbates recombination losses. These effects are particularly pronounced in c-plane InGaN/GaN structures due to the quantum-confined Stark effect (QCSE), which separates electron and hole wavefunctions. The ABC model provides a framework for quantifying these recombination dynamics, expressing the total recombination rate R as:

R = A n + B n^2 + C n^3

where A represents Shockley-Read-Hall (SRH) non-radiative recombination (defect-related, linear in carrier density), B is the radiative bimolecular coefficient, and C captures the Auger term; at high currents, the C n³ term dominates, driving the IQE downturn.^[63] Experimental fits to this model across various LED designs reveal A values around 10⁸ s⁻¹ and B near 10⁻¹¹ cm³/s, underscoring Auger's role in droop.^[62] Mitigation strategies focus on suppressing these mechanisms, such as employing thinner QWs to enhance carrier confinement and wavefunction overlap against QCSE, alongside polarization engineering using nonpolar or semipolar GaN orientations to eliminate piezoelectric fields. Recent advancements in the 2020s, including ultra-low-defect-density homoepitaxial GaN substrates with threading dislocation densities below 10⁵ cm⁻², have reduced droop to less than 20% at 500 A/cm² in blue micro-LEDs, primarily by minimizing SRH losses.^[65] These improvements enable higher flux in lighting applications, where droop otherwise limits output compared to low-current display uses that operate near peak efficiency.^[62]

Reliability and Advanced Topics

Lifetime and Failure Modes

The lifetime of light-emitting diodes (LEDs) is typically quantified using the L70 metric, which represents the operational time in hours until the luminous flux depreciates to 70% of its initial value under specified conditions.^[66] This metric accounts for gradual degradation rather than abrupt failure, as LEDs rarely cease functioning suddenly but instead exhibit diminishing output over extended periods. Lifetime predictions often employ the Arrhenius equation for extrapolation, incorporating an activation energy E_a (typically 0.1–0.5 eV depending on the LED type) to model thermal acceleration of degradation processes from accelerated test data.^[67]^[68] Key failure modes in LEDs, particularly those based on GaN, involve structural and chemical changes that increase non-radiative recombination. Dislocation climb in GaN layers, driven by recombination-enhanced mechanisms, generates additional defects that serve as non-radiative centers, reducing radiative efficiency over time.^[69] In p-type GaN doped with magnesium (Mg), diffusion of Mg atoms into the active region during operation or high-temperature processing can introduce impurities that form deep-level traps, accelerating carrier leakage and flux decay.^[70] Contact degradation, often at n- or p-type interfaces, arises from electromigration or oxidation under high current densities, leading to increased series resistance and uneven current distribution.^[71] Thermal effects play a central role in degradation, as elevated junction temperatures exacerbate failure modes. The junction temperature \theta_j rises due to power dissipation, approximated by \theta_j = P_\text{elec} \times R_\text{th}, where P_\text{elec} is electrical power input and R_\text{th} is thermal resistance (typically 10–50 K/W for high-power LEDs).^[72] This heating shortens the Shockley-Read-Hall (SRH) recombination lifetime exponentially, promoting defect generation and non-radiative losses that contribute to both gradual degradation and transient efficiency droop under heat.^[73] Acceleration factors amplify these processes in real-world operation. Current crowding, where current density is non-uniform across the active area, creates localized hot spots that intensify thermal and defect-related degradation, particularly at high drive currents above 100 mA/mm².^[74] In packaged LEDs, humidity-induced corrosion at metal interfaces or encapsulants, often involving sulfur or chloride contaminants, erodes electrical contacts and phosphor layers, hastening lumen depreciation in moist environments (e.g., >85% relative humidity).^[75]^[76] Standardized testing under IES LM-80 protocols measures lumen maintenance over at least 6,000 hours at controlled currents (e.g., 350–700 mA) and case temperatures (55°C, 85°C, or 105°C), enabling reliable extrapolation to longer lifetimes.^[77] Post-2010 advancements in GaN epitaxy, thermal management, and packaging have elevated L70 lifetimes for modern white LEDs beyond 50,000 hours at nominal operating conditions, reflecting reduced defect densities and improved heat dissipation.^[78]^[79]

Quantum-Dot LEDs

Quantum-dot light-emitting diodes (QD-LEDs), also known as QLEDs, incorporate colloidal semiconductor nanocrystals known as quantum dots (QDs) as the emissive layer to achieve enhanced color purity and efficiency in electroluminescent devices. These QDs exhibit zero-dimensional (0D) quantum confinement, where electrons and holes are restricted in all three spatial dimensions within nanocrystals typically 2-10 nm in diameter, such as CdSe or InP.^[80] This confinement leads to discrete energy levels, enabling size-tunable bandgaps that span the visible spectrum; the confinement energy shift follows the particle-in-a-box model, with \Delta E \propto 1/r^2, where r is the particle radius.^[80] In QD-LED structures, the QDs form the active emissive layer in hybrid organic-inorganic devices, sandwiched between charge transport layers—typically a hole transport layer (e.g., organic polymers like poly(9,9-dioctylfluorene)) and an electron transport layer (e.g., inorganic ZnO nanoparticles)—to facilitate balanced carrier injection and recombination. The solution-processable nature of colloidal QDs allows for low-cost fabrication via spin-coating or inkjet printing, distinguishing these devices from epitaxial quantum well-based LEDs.^[81] Key advantages of QD-LEDs stem from the physics of confined excitons in QDs, including narrow emission spectra with full width at half maximum (FWHM) below 30 nm, which provides superior color purity compared to organic LEDs. Internal quantum efficiencies (IQE) exceeding 90% are achievable due to suppressed non-radiative Auger recombination, as the 0D confinement reduces multi-exciton interactions that dominate in bulk materials or higher-dimensional structures.^[82] Stability is further enhanced by core-shell architectures, such as CdSe cores overcoated with ZnS shells, which passivate surface traps, boost photoluminescence quantum yields to 70–90%, and protect against environmental degradation.^[83] Despite these benefits, challenges persist from charging effects and blinking, where intermittent fluorescence arises from Auger recombination in charged QDs or trapping of carriers at surface defects, leading to non-radiative decay and reduced device reliability.^[84] Toxicity concerns with cadmium-based QDs, regulated under hazardous substance directives, have driven development of lead-free alternatives like InP QDs, which maintain comparable performance while offering environmental safety for display applications.^[85] Progress in QD-LEDs traces back to the 1990s, with seminal colloidal synthesis of monodisperse CdSe QDs via hot-injection methods enabling controlled size distributions. The first electroluminescent QD-LED was demonstrated in 1994 using CdSe nanocrystals as the emissive layer. By the 2020s, advancements in shell engineering and charge balance have yielded commercial QLED displays with external quantum efficiencies (EQE) over 20%, with records reaching 31% for red devices as of November 2025, approaching the theoretical limits for lighting and next-generation televisions.^[86]^[87]

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