Light-Emitting Diodes: Physics and Engineering of Semiconductor Electroluminescence

Electroluminescence and Bandgap Fundamentals

Light-emitting diodes operate through electroluminescence, where injected electrons and holes recombine in a semiconductor, releasing photons. The photon energy equals the semiconductor bandgap, determining emission wavelength. Bandgap engineering via alloy composition enables spectral tuning across ultraviolet, visible, and infrared ranges.

Bandgap Engineering and Emission Wavelengths

Material	Bandgap (eV)	Emission Range
GaAs	1.42	Near-infrared
InGaN	2.7–3.4	Blue to green
AlGaInP	1.8–2.3	Red to amber
GaN	3.4	Ultraviolet to blue

Adjusting alloy composition in ternary and quaternary systems shifts bandgap continuously, allowing precise color targeting.

Direct Versus Indirect Bandgap Semiconductors

• Direct bandgap (e.g., GaAs, InGaN): Conduction band minimum and valence band maximum align in momentum space, enabling efficient radiative recombination without phonon assistance.
• Indirect bandgap (e.g., Si, Ge): Momentum mismatch requires phonon participation, drastically reducing emission efficiency. High-performance LEDs exclusively use direct bandgap materials.

Efficiency Metrics

Internal quantum efficiency (IQE) measures radiative recombination fraction. External quantum efficiency (EQE) incorporates light extraction. Typical high-quality LEDs achieve IQE > 90%, while EQE is lower due to total internal reflection and absorption losses.

Parameter	Definition	Typical Range
IQE	Radiative recombinations / total injected carriers	80–95%
EQE	Photons emitted externally / electrons injected	30–70%
Light extraction efficiency	Photons escaping device / photons generated	30–80%

Heterojunction and Quantum Confinement Design

Advanced structures confine carriers to enhance recombination probability.

1. Double heterostructures (DH): A narrow bandgap active layer sandwiched between wider bandgap cladding layers, suppressing carrier leakage.
2. Quantum wells (QWs): Ultrathin active layers (2–10 nm) create discrete energy levels, increasing oscillator strength and spectral purity.
3. Multiple quantum wells (MQWs): Stacked QWs boost total emission intensity, commonly used in InGaN/GaN blue LEDs.
4. Quantum dots (QDs): Zero-dimensional confinement yields size-tunable emission and low threshold currents.

Distributed Bragg reflectors and photonic crystals enhance light extraction by redirecting trapped modes.

Challenges: Efficiency Droop and Thermal Management

• Efficiency droop: At high current densities, Auger recombination and carrier leakage reduce IQE, particularly in GaN-based LEDs.
• Thermal effects: Elevated temperatures increase non-radiative recombination and degrade lifetime. Flip-chip packaging and ceramic substrates improve heat dissipation.
• Material defects: Dislocations and impurities act as non-radiative centers, limiting maximum IQE.

Applications

LEDs dominate solid-state lighting via phosphor-converted white LEDs, achieving luminous efficacies over 200 lm/W. In displays, micro-LED arrays provide high dynamic range and wide color gamut without color filters. Specialized uses include UV sterilization, horticultural lighting, and automotive headlamps. Optical communication benefits from fast modulation speeds.

Future Directions

Research focuses on materials such as AlN and BN to mitigate droop, and on nanostructuring for enhanced extraction. Continued improvements in epitaxy and packaging will push efficiency and reliability further.