The document discusses Light Emitting Diodes (LEDs), which are semiconductor devices that emit light through electroluminescence when forward biased. It details the mechanism of light generation, the color variations based on materials used, and the operational characteristics including efficiency, power requirements, and applications in various electronic devices. LEDs are characterized by their long lifespan, fast switching capabilities, and their use in numerous applications such as calculators, digital watches, and optical communication systems.

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20EC001- Electron Devices
Module-5
Special Semiconductor Devices
LED
Ms.J.Shiny Christobel
Assistant Professor
Department of ECE

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LED- Light Emitting Diode
❖ The Light Emitting Diode (LED) is a PN junction device which
emits light when forward biased, by a phenomenon called
electroluminescence.
❖ In all semiconductor PN junctions, some of the energy will be
radiated as heat and some in the form of photons.
❖ In silicon and germanium, greater percentage of energy is given
out in the form of heat and the emitted light is insignificant.
❖ In other materials such as gallium phosphide (GaP) or gallium
arsenide phosphide (GaAsP), the number of photons of light
energy emitted is sufficient to create a visible light source.
❖ Here, the charge carrier recombination takes place when
electrons from the N-side cross the junction and recombine with
the holes on the P-side.

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❖ When an LED is forward biased, the electrons and holes move
towards the junction and recombination takes place.
❖ As a result of recombination, the electrons lying in the
conduction bands of N-region fall into the holes lying in the
valence band of a P-region.
❖ The difference of energy between the conduction band and the
valence band is radiated in the form of light energy.
❖ Each recombination causes radiation of light energy.
❖ Light is generated by recombination of electrons and holes
whereby their excess energy is transferred to an emitted
photon.
❖ The brightness of the emitted light is directly proportional to the
forward-bias current.

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Recombination and Emission of Light

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❖An N-type layer is grown on a substrate and a P-type is
deposited on it by diffusion.
❖Since carrier recombination takes place in the P-layer, it
is kept uppermost.
❖The metal anode connections are made at the outer
edges of the P-layer so as to allow more central surface
area for the light to escape.
❖LEDs are manufactured with domed lenses in order to
reduce the reabsorption problem.
❖ A metal (gold) film is applied to the bottom of the
substrate for reflecting as much light as possible to the
surface of the device and also to provide cathode
connection.
❖LEDs are always encased to protect their delicate wires.

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Efficiency of LED
❖The efficiency of generation of light increases
with the increases in injected current and with
a decrease in temperature.
❖ The light is concentrated near the junction as
the carriers are available within a diffusion
length of the junction.

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LED in different Colours
❖LEDs radiate different colours such as red, green,
yellow, orange, blue and white.
❖Some of the LEDs emit infrared (invisible) light also.
❖The wavelength of emitted light depends on the
energy gap of the material.
❖Hence, the colour of the emitted light depends on
the type of material used is given as follows.
❖Gallium arsenide (GaAs) – infrared radiation
(invisible)
❖Gallium phosphide (GaP) – red or green
❖Gallium arsenide phosphide (GaAsP) – red or
yellow

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❖In order to protect LEDs, resistance of 1 kW or 1.5
kW must be connected in series with the LED.
❖LEDs emit no light when reverse biased. LEDs
operate at voltage levels from 1.5 to 3.3 V, with
the current of some tens of milliamperes.
❖The power requirement is typically from 10 to
150 mW with a lifetime of 1,00,000 + hours.
❖LEDs can be switched ON and OFF at a very fast
speed of 1 ns.

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Applications of LED
They are used in
❖ burglar alarm systems
❖ picture phones
❖ Multimeters
❖ Calculators
❖ Digital meters
❖ Microprocessors
❖ Digital computers
❖ Electronic telephone exchange
❖ Intercoms
❖ Electronic panels
❖ Digital watches
❖ Solid state video displays
❖ optical communication systems.
❖ Also, there are two-lead LED lamps which contain two LEDs, so that a reversal in
biasing will change the colour from green to red, or viceversa.