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White Light-Emitting Diodes

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

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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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