WHITE LED S Yogesh Kumar 2008ph10643 Group no. 3 Course- EPL335

W ORKING OF AN LED p-n junction diode Forward bias Photon release during recombination process Direct band-gap semiconductors

U SES 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

U NIQUE PROPERTIES High energy efficiency Low power consumption High life-time High switching speed Compatible Intensity independent colors Temperature dependent output

W HITE 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 LEDs 2. Phosphor coating(Phosphor Based LEDs)  High intensity LEDs

RGB C OLOR S PACE M ODEL 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

W HITE 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

P HOSPHOR BASED LED S 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 (Ce 3+ :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

P HOSPHOR BASED LED fig: spectrum broadening due to Stokes shift

L IMITATIONS OF WHITE LED S Complicated packaging Larger energy consumption due to heat produced in Stokes shift Low color rendering index High color temperature

S TRAIN A DJUSTED 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

S TRAIN ADJUSTED WHITE LED S fig: spectrum of a single chip white LED