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

Energy Band Diagrams 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. Holes 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.

Schematics Physical structure of a white LED. Primary emission is from an In x Ga 1-x N 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.

Spectra of phosphor based white LED vs. filament lamp In x Ga 1-x N Ce:YAG 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.

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