Light-Emitting Diodes (Chapter on Daylight Technologies)
About this book
E. Fred Schubert's Light-Emitting Diodes, published in its second edition by Cambridge University Press in 2006, stands as the definitive graduate-level textbook on the physics, technology, and applications of LED devices. Schubert, a Wellfleet Senior Constellation Professor at Rensselaer Polytechnic Institute who has made pioneering contributions including the first demonstration of the resonant-cavity LED, brings both theoretical depth and practical engineering perspective to a text that encompasses every aspect of modern LED science.
The physical foundation of all LED operation lies in the behaviour of charge carriers within semiconductor p-n junctions. When a forward bias voltage is applied across such a junction, minority carriers are injected across the depletion region: electrons move into the p-type material and holes move into the n-type material. The book provides a rigorous treatment of radiative recombination, the quantum mechanical process by which an electron occupying a conduction-band state drops to recombine with a valence-band hole, releasing the energy difference as a photon.
In direct-bandgap semiconductors such as gallium arsenide (GaAs), gallium nitride (GaN), and indium gallium nitride (InGaN), this transition is highly probable, making them far superior light emitters compared with indirect-bandgap materials like silicon. The emitted photon energy corresponds closely to the semiconductor bandgap, which can be engineered by alloying to produce emission across the visible and ultraviolet spectrum. Competing non-radiative recombination pathways—including Shockley-Read-Hall recombination through crystal defects and Auger recombination at high carrier densities—reduce the internal quantum efficiency, and Schubert discusses strategies for minimizing these losses through material quality improvement, heterostructure design, and doping optimization.
The p-n junction chapter builds from basic diode theory to the double heterostructure and quantum well architectures used in high-brightness LEDs. By confining carriers and photons within thin semiconductor layers sandwiched between higher-bandgap cladding layers, quantum well LEDs achieve far superior radiative efficiency than homojunction devices. The book covers how the epitaxial growth of III-V nitride materials (AlGaInN) on sapphire or silicon carbide substrates has enabled high-power blue and violet LEDs, which are the technological basis for modern white LED sources.
White LEDs through phosphor conversion represent one of the most commercially transformative topics in the book. In this approach, a blue or near-ultraviolet LED chip excites a yellow-emitting cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor coating (or a blend of multiple phosphors), and the mixture of residual blue emission from the chip with the broadband yellow phosphor emission creates an approximation of white light. Schubert addresses the trade-offs involved: the Stokes energy loss inherent in the photon-down-conversion process reduces overall efficiency, while the exact phosphor composition and particle size distribution determine the correlated color temperature and the color rendering index (CRI) of the resulting white source.
Achieving high CRI values—essential for lighting applications where accurate color appearance matters—generally requires adding a red-emitting phosphor component, which introduces additional conversion losses. Thermal management receives dedicated treatment because junction temperature profoundly affects both efficiency and lifetime. As LED drive current increases, heat generated in the active layer must be conducted through the chip, the package, and the heat sink to the ambient environment.
Elevated junction temperatures shift the emission spectrum, reduce quantum efficiency through increased non-radiative recombination, and accelerate degradation mechanisms. Schubert discusses thermal resistance modeling, heat spreading in ceramic and metallic packages, and techniques for measuring junction temperature through forward-voltage or photoluminescence methods. The book's breadth extends to high-brightness and high-power device architectures, optical reflector design for light extraction enhancement, UV LEDs for germicidal and photocuring applications, and an introduction to human photometry and colorimetry that contextualizes LED performance metrics within visual system response.
The treatment of solid-state lighting in the final chapters anticipates developments that were then emerging—including daylight-spectrum designs that tune spectral power distribution to optimize circadian stimulus while maintaining high efficacy—making the book a prescient roadmap for the industry that followed.