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Light Emitting Diodes (LEDs)
Light can be obtained from various sources like candles, lamp and sunrays etc. Light bulb had invented by Thomas Edison in 1879. An electric current is passed through a filament inside bulb, it gets heated up and emits light when sufficient current is passed through the filament. That means it converts electrical energy into heat energy in the bulb. Where in Light Emitting Diodes (LED), electronic electrical energy can directly converted into light energy.
Light is a energy which is released by atoms. Particles of light are photons which have no mass. Atoms are basic building blocks which are made of small particles such as electrons (negatively charged), protons (positively charges) and neutrons (no charge). These are parts of a nucleus. Electrons revolve around nucleus in different orbits or shells which has different energy level with respect to distance from orbit.
When electrons orbiting the nucleus gains energy from outside source the jump to higher orbit or higher energy level for temporary period then fall back to its lower level energy and releases energy in the form of a photon or light. It cases energy loss in the form of heat.
Light Emitting Diode (LED)
It is most widely used semiconductor which emit either visible light or invisible infrared light when forward biased. Remote controls generate invisible light. A Light emitting diodes (LED) is an optical electrical energy into light energy when voltage is applied.
In forward biased, energy is released in the form of light when free electrons in conduction band recombines with holes in the valence band. This process is called electroluminescence. Like a normal p-n junction diode allows electric current only in direction, LEDs also operates only in forward bias condition. The n-type material should be negatively charged and p-type material should be positively charged to create an LED.
Layers of LED
There are three layers in a light emitting mode (LED). These are
P-type semiconductor
Trivalent impurities are added to the intrinsic or pure semiconductor. Holes are majority charge carriers.
N-types semiconductor
Pentavalent impurities are added to the intrinsic semiconductor. Free electrons are majority charge carriers.
Depletion layer or region
It is a region or layer between p-type and n-type semiconductor where no charge carriers (free electrons or holes) are present. It is a barrier to current. To overcome this barrier, we need to apply voltage which is greater than barrier potential of this layer.
Working of LED
Light Emitting Diode (LED) works only in forward biased condition where free electrons are from n-side and the holes from p-side are pushed towards the junction. The recombination of free electrons and holes takes place in depletion region as well as in p-type and n-type semiconductor. The free electrons in conduction band releases energy in the form of light before they recomine with holes in the valence band.
When external voltage is applied to valence electrons, the valence electrons which breaks bonding with parent atom are called free electrons and that empty space is called a hole. The grouping the range of energy levels of all the valence is called valence band.
Since energy of free electrons is always more than the energy of valence electrons or holes, so it need to lose energy in order to recombine with holes. Hence each recombination of charge emits energy in the form of light. The brightness of the emitted light is depends on the materials used for conducting LED and forward current flow through the LED.
In normal silicon diodes, energy gap between conduction and valence band is less, so low energy photons are released which low frequency and invisible to human eye. Whereas, in LED, energy gap between conduction and valence band is very large so high energy photons released and visible to human eye.
Since LEDs are operate only forward bias condition, so if reverse bias voltage is applied to LED, it may be damaged. LEDs can switched ON and OFF at a very fast speed of 1 ns. The most common colors of LEDs are green, red, orange, and yellow but schematic symbol is same.
Biasing of LED
The safe forward voltage and current of most LED is from 1V to 3V and 200 mA to 100mA respectively. To avoid destruction of device because of more voltage, we need to place a register (Rs) in series with LED which is placed between LED and voltage source (Vs) and this register is called as current limiting resistor. So we can protect LED from damage by using this current limiting register. The current flowing through LED is mathematically written as,
IF = VS - VD/RS
Where, IF is forward current, VS is source or supply voltage, VD is voltage drop across LED, and RS is register or current limiting register.
The amount of output light emitted by LED is directly proportional to the amount of forward current flowing through the LED.
Types of LED
There are two classes of light emitting diodes, these are visible LEDs and invisible LEDs. Also, the wavelength or color of emitting light depends on the forbidden gap or energy gap of the materials. These are various types of light emitting diodes:
Gallium Arsenide (GaAs) – infrared
Gallium Arsenide Phosphide (GaAsP) – red to infrared, orange
Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red, orange-red, orange, and yellow
Gallium Phosphide (GaP) – red, yellow and green
Aluminium Gallium Phosphide (AlGaP) – green
Gallium Nitride (GaN) – green, emerald green
Gallium Indium Nitride (GaInN) – near ultraviolet, bluish-green and blue
Silicon Carbide (SiC) – blue as a substrate
Zinc Selenide (ZnSe) – blue
Advantages of LED
These are following advantages of light emitting diodes:
- Brightness of LED can be easily controlled by varying the current.
- These are smaller in size, light in weight and have longer in lifetime.
- These are very cheap, readily available and consume low energy.
- LEDs operate very fast.
- These can emit different colors of light.
- LEDs do not contain toxic material like mercury which is used in fluorescent lamps.
Disadvantages of LED
Main disadvantages of LEDs are efficiency is low and need more power to operate than normal p-n junction diodes.
Applications of LED
These are various applications of LEDs:
- Digital computers and calculators
- Traffic signals and Burglar alarms systems
- Microprocessors and multiplexers
- Picture phones and digital watches
- Camera flashes and automotive heat lamps
- Aviation lighting
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