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