Optoelectronics combines electronics and optics to study and apply electronic devices that interact with light. Major optoelectronic devices include light-emitting diodes (LEDs), laser diodes, photodiodes, and solar cells. A light emitting diode (LED) is an optical diode that emits light when forward biased. LEDs use semiconductors like gallium arsenide to emit light of different colors depending on the material's band gap. They have advantages like small size, long life, and availability in many colors.

2. OPTOELECTRONIC
 Optoelectronics is an interesting branch of electronics that combines both electronics
and optics.
 Optoelectronics is the study and application of electronic devices that interact with
light.
 Optoelectronic devices find varied applications in telecommunications, military services,
medical field, and automatic control systems.
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3. 3
Examples of Optoelectronic Devices

4. Major Optoelectronic Devices
 Light-emitting diodes (LEDs)
(display, lighting,···)
 Laser diodes (LDs)
(data storage, telecommunication, ···)
 Photodiodes (PDs)
(telecommunication, ··· )
 Solar Cells
(energy conversion)

5. LIGHT EMITTING DIODE
 LED is an optical diode == it emits light when FB
 Symbol is similar to PN Junction diode apart from two arrows indicates
- device emits the light energy
 Basic operation :
 Pn junction is in forward bias = electron in N type cross the Jn & recombine with
hole in P type semiconductor material
 Free electrons are in the conduction band and at a higher energy than the holes in
the valence band.
 When free electron combine with holes, it fall from CB to VB === energy level
changed from high value to low value
 The difference in energy between Higher level and lower level is released in the
form of photon( emits light energy)( due to some material) ( normal diode = heat)
 such diodes = LED = process called electroluminescence 5

6. LIGHT EMITTING DIODE
 Energy released in the form of light depends on the energy corresponding to forbidden
gap. Determines the wavelength of the emitted light.
 wavelength determines colour of the light. Also determines light is visible or invisible(IR)
 Various impurities are added during the doping process to control the wavelength &
colour of the emitted light
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7. LIGHT EMITTING DIODE
 Normal Silicon diode, FG = 1.1ev , wavelength of emitted light correspond to IR light
spectrum. == light is not visible.
 Materials used in LED
 GaAs, GaAsP or GaP (mixures of elements gallium, arsenic and phosphorus)
 GaAs - Infrared radiation (Invisible)
 GaAsP - Red or yellow light
 GaP - Red or green light
 Some LEDs- blue & orange light(InGaN)
 LEDs emit no light when reverse biased. Operating LED in reverse direction will quickly
destroy them
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8. LIGHT EMITTING DIODE
Construction of LED
 Deposit three semiconductor layers on the substrate
 Action region = between p type & n type regions = active region
emits light
 During FB, Holes from P type recombine with Electrons from N
type in the active region = light is emitted
 LED emit light all the way around the layered structure. So placed
the entire system in the tiny reflective cup so that the exit light
reflected towards the desired direction.
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9. LIGHT EMITTING DIODE
LED : voltage & current
 Resistor = Rs = current limiting resistor = current is limited ( not
exceeding the maximum current rating of the diode)
 VS = Supply voltage
 VD = voltage drop across LED
 Apply KVL to the circuit
VS = IS RS + VD
IS = VS - VD
RS
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 Voltage drop in LED > Voltage in normal diode
 Voltage drop = 2 to 3 V
 Current range = 10 to 80 mA
 Reverse break down voltage << normal diode
 Reverse break down range = 3V to 10 V

10. LIGHT EMITTING DIODE
Spectral output curves
 Visibility of light = decided by the wavelength
 Graph = output light Vs wavelength(nm) = spectral curve
 peak at 460nm(blue), 540nm(green), 590nm(yellow) and 660nm(red)
 Infrared visible light peak at 940nm
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11. LIGHT EMITTING DIODE
Radiation Pattern of LED
 LED = directional light source
 it emits light in a particular way which has a typical radiation pattern
 most of the energy emitted within 20˚ of the direction of maximum light
 Some LEDs = plastic lens = spread the light to greater angle to increase the
visibility
 Coloured lens are also used = to
enhance the colour
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12. LIGHT EMITTING DIODEAdvantages :
 Small in size
Brightness controlled by varying current
 Fast operating devices
 light in weight
 Available in various colours
 Have long life
Cheap and readily available
 Easy to interface to various electronic devices
 used in Burglar alarm system
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Disadvantages :
 Luminous efficiency is low
 Characteristics affected by temp
 Frequent replacement of battery
 need large power for its
operation
Applications :
 Sensor Applications
 Sign Applications
 LED Signals
Mobile Applications
 Indicators