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 http://www.hebeiltd.com.cn/leds/Mr.Led.jpg

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 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.

Spectra of phosphor based white LED vs. filament lamp

InxGa1-xN

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.

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