LEDs: Light Emitting Diodes - LED.pdf

4.13 LLIGHT EMITTING DIODE LED is aheavily doped pn junction of suitable materials that emits light when it is forward biased. In the forward biased state, electrons move from n region to p region and holes move from p region ton region. Electron hole recombination takes lace on either...

4.13 LLIGHT EMITTING DIODE LED is aheavily doped pn junction of suitable materials that emits light when it is forward biased. In the forward biased state, electrons move from n region to p region and holes move from p region ton region. Electron hole recombination takes lace on either side of the pn junction. The loss of energy in these recombination appears band gap energy. as light. The energy of the emitted photons is nearly equal to the current, reaches increase of forward The intensity of the emitted light incréases with  4.30 Phsicsfor Engincers wavelength of the andthen decreases. The colour or light. a maximum value depends on the band gap encrgy. hc Wavelength of emitted light À= E, constant, cis the velocity of light in free space where his the Planck's the band gap energy. Different materials are used to get red, green, semicon blue, yeliow, orange etc light. All ductor diodes produce radiation during electron hole recombination, but the emitted radiation is Anode Cathode absorbed by the semiconductor material. In an LED, the band gap is wide and the junction is Fig. 4.23 Symbol constructed in such a way that the radiation can escape. The semiconductor materials used for LEDs should have a band gap energy of about 2eV. Here the diode is encapsulated with a trans parent cover to emit the light. Usually LEDs emit colored light which is nearly monochromatic. We can pro duce white light from colour LEDs by phosphor conversion, RGB systems or a hybrid method. In phosphor (a luminescent material critical to many lighting applications) conversion, a phosphor is used to convert blue or near ultraviolet light from an LED into white light. RGB systems mix light from multiple monochromatic LEDs (red, green and blue) to get white light. Hybrid method uses both phosphor-converted and monochromatic LEDs. Red and green LEDs had been with us for about three decades and blue remained a challenge for many years. In the year 1994, three scientists, Isamu Akasaki. Hi roshi Amano and Shuji Nakamura had developed the blue LED using Gallium Nitride (GaN). They got the 2014 Nobel Prize in Physics for this great achievement. In ablue LED,a portion of the light can be converted into green, yellow and red light by using aphosphor material. This mixture of coloured light will be perceived by humans as white light and can be used for general illumination. Usually Indium Gallium Nitride (/nGaN) is used for making violet, blue and green LEDs, where as Aluminium Gallium Indium Phosphide (AIGalnP) is used for green, yellow, orange and red LEDs. These two semiconductor materials with slight changes in their composition give different colours to the LEDs. Zine Se  Semiconductor Devices 4.31 lenide (ZnSe), phide (GaAsP), Al AlAluminium uminium Gallium Phosphide Now-a-days Gallium Arsenide (AlGaP),etc.Gallium Arsenide Phos- Organic Light Emitting Diodes(AlGaAs) are also used. ting Diodes (PLEDs) are also available. Anew (OLEDs) and Polymer Light Emit- the discovery of Quantum dot LEDs advancement in LED technology is nano crystals exhibiting quantum (QLEDs). Quantum dots are tiny particles or in placed betweenn-type and mechanical p-type properties. QLEDs quantum dots are In electrons and holes recombine in thesemiconductors. When an electric field is applied, quantum dot layer to produce light. QLEDS are iahle,energy efhcient, low cost and tunable over the entire visible wavelength range. A.14 LED CHARACTERISTICS The junction voltage - current char acteristic of an LED is similar to the V.I characteristics of diodes. How (|) current Forward ever, there is one major difference. The knee voltage of a diode is re lated to the barrier potential of the material used in the device. Silicon diodes and bipolar junction transis whose tors are very commonly used, knee voltage or junction voltage is about 0.7 V. LEDs are made of different chemical substances. In Forward voltage (V) an LED, depending on the mate rial used its junction voltage can be Fig. 4.24 anywhere between 1.5 to 2.2 Volts. greater than the LED's forward from a DC voltage source When operating an LED resistor must be included to prevent full source "dropping" Voltage, a series-connected emitting light as its forward voltage damaging the LED. LED starts the increase voltage particular level and its intensity will increase from further with fact, operat- reaches at a reverse biased. In voltage. LEDs emit no light when voltage is quite applied forward quickly destroythemif the applied ing LEDs in reverse direction will curve of LED is shownin figure 4.24. characteristic  4.32 Physics for Engineers Advantages of LED LED as: There are many advantages of using an 1. It helps in saving energy. 2. Reduction in costs. current are enough to drive the LED. 3. Very low voltage and milliwatts. 4. Total power output will be less than 150 10 nanoseconds. J. The response time is very less, only about 6. The device does not need any heating and warm up ime. 7. Miniature in size and hence light weight... Havearugged construction and hence can withstand shock and vibrations 9. AnLED has a life span of more than 10years Disadvantages 1. Aslight excess in voltage or current can damage the device. 2. High initial price; LEDs are currently more expensive (price/lumen) than conventional light sources. 3. Efficiency of LED decreases as the electriccurrent increases. Applicationsof Light Emitting Diodes There are many applications of the LED and some of them are explained below. 1. LEDs in conjunction with a photodiode or other photosensitive device can used as light sources in optical fibre communication systems 2. It is used in digital displays in allmodern day electronicdeyices 3. LED is used as a bulb in homes and industries 4. The light emitting diodes are used in motorcycles and cars 5. These are used in mobile phones to display the 6. At the traffic light signals LEDsare used message