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WHITE LEDS
Yogesh Kumar
2008ph10643
Group no. 3
Course- EPL335
WORKING OF AN LED
p-n junction diode Forward bias Photon release during
recombination process Direct band-gap
semiconductors
USES
Automotive lighting(brake lamps, indicators, turn signals)
White LEDs in Anti collision systems Text displays Sensors Communication Technology(high switching
rates) Infra red LEDs in remote controls White LEDs can be used for lighting purposes
UNIQUE PROPERTIES
High energy efficiency Low power consumption High life-time High switching speed Compatible Intensity independent colors Temperature dependent output
WHITE LED
Mainly two basic methods to make White LEDs –
1. Mixing of primary color( Red, Green & Blue) LEDs (RGB white LEDs)
Can be used to make LEDs of different colors Color of LED is determined using RGB color
space model Low intensity LEDs2. Phosphor coating(Phosphor Based LEDs) High intensity LEDs
RGB COLOR SPACE MODEL
Three parameters x, y( Chromaticity co-ordinates) & Y and one more parameter z(=1-x-y)
x & y represents intensity of color using “CIE1931 color space”
Y represents brightness of color
Fig: CIE 1931 color space
WHITE LED USING CIE MODEL The x and y coordinates are determined
using the CIE 1931 color space Any color enclosed in the triangle formed by
the coordinate points representing the three color can be obtained
Ratio of luminance(brightness) of three colors determines the resultant color
PHOSPHOR BASED LEDS Commercial high intensity white LEDs LED of one color(mostly blue LED made of InGaN
or GaN) are coated with layer of phosphor of distinct color
Blue light goes Stokes shift and the spectrum is broadened
Cerium-doped yttrium Aluminium garnet (Ce3+:YAG) is a commonly used phosphor
UV LEDs can be coated with europium-based red and blue emitting phosphors plus green emitting copper and Aluminium doped zinc sulfide (ZnS:Cu, Al)
Simpler and cheaper than RGB system
PHOSPHOR BASED LED
fig: spectrum broadening due to Stokes shift
LIMITATIONS OF WHITE LEDS
Complicated packaging Larger energy consumption due to heat
produced in Stokes shift Low color rendering index High color temperature
STRAIN ADJUSTED WHITE LED Single chip Phosphor free 4-period MQW
(multiple quantum well) active region- 3nm InGaN well and 14 nm GaN barrier
InGaN ultra-layer (strained)
Light is emitted in InGaN well
Indium rich quantum dots emit yellow; In poor QDs emits blue resulting white light
fig: white LED with InGaN ultra layer
STRAIN ADJUSTED WHITE LEDS
fig: spectrum of a single chip white LED