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White Light-Emitting Diodes

Explore the structural, operational, and spectral characteristics of white LEDs, including energy band diagrams, schematics, and comparisons with traditional lamps. Learn how these semiconductor devices emit light and conserve energy. Discover the significance of direct gap materials and phonon-assisted transitions.

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White Light-Emitting Diodes

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  1. White Light-Emitting Diodes Robert Nicholas & Daniel Freno Junior Lab Spring 2007

  2. White LEDs • The structural and operational characteristics of white LEDs will be discussed. • Spectra will be taken from both a traditonal lamp and a white LED for comparison http://www.hebeiltd.com.cn/leds/Mr.Led.jpg

  3. Energy Band Diagrams Holes • Electrons are pumped from the valence band to the conduction band. • Upon relaxation, light is emitted to conserve energy. • Direct gap semiconductor materials are needed for optoelectronic devices. • Momentum must be also conserved. • For momentum to be conserved in an indirect gap semiconductor, phonon-assisted transitions must take place. • Three particle processes are very improbable. • Applied voltage across a p-n junction causes electrons to be promoted to the conduction band, leaving holes behind. • The electrons and holes will then be swept towards each other where they recombine, emitting light. • This area is referred to as the depletion region.

  4. Schematics • Physical structure of a white LED. • Primary emission is from an InxGa1-xN LED. • Primary blue light is partially absorbed by a phosphor [Ce:YAG ( Cerium doped Yttrium Aluminum Garnet)] layer. • Results in secondary, redshifted phosphorescence. • Wavelengths are mixed and appear to the human eye as white.

  5. Spectra of phosphor based white LED vs. filament lamp • The intensity of the LED is larger, as expected. • The lower energy peak of the white LED is from the InGaN diode. • The broader redshifted peak is from the secondary emission due to the phosphor. InxGa1-xN Ce:YAG

  6. References • http://micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html • http://en.wikipedia.org/wiki/LED • http://www.hebeiltd.com.cn/leds http://upload.wikimedia.org/wikipedia/commons/c/cb/RBG-LED.jpg

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