White LightEmitting Diodes Robert Nicholas Daniel Freno Junior

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. n Spectra will be taken from both a traditonal lamp and a white LED for comparison n http: //www. hebeiltd. com. cn/leds/Mr. Led. jpg

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

Schematics n n n Physical structure of a white LED. Primary emission is from an Inx. Ga 1 -x. N LED. Primary blue light is partially absorbed by a phosphor [Ce: YAG ( Cerium doped Yttrium Aluminum Garnet)] layer. ¨ n 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 n n n The intensity of the LED is larger, as expected. The lower energy peak of the white LED is from the In. Ga. N diode. The broader redshifted peak is from the secondary emission due to the phosphor. Inx. Ga 1 -x. N Ce: YAG

References n n n 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