Light Emitting Diodes (LED)

The LED is an optical diode, which emits light when  forward biased. The fig shows
the symbol of LED which is similar to p-n junction diode apart from the two arrows
indicating that the device emits the light energy.



Basic Operation

Whenever a p-n junction is forward biased, the electrons cross the p-n junction from
the  n  type  semiconductor  material  and  recombine  with  the  holes  in  the  p  type
semiconductor  material.  The  free  electrons  are  in  the  conduction  band  while  the
holes are present in the valence band. Thus the free electrons are at higher energy
level  with  respect  to  the  holes  When  a  free  electron  recombines  with  hole,  it  falls
from  conduction  band  to  a  valence  band.  Thus  the  energy  level  associated  with  it
changes   from   higher   value   to   lower   value.   The   energy   corresponding   to   the
difference  between  higher  level  and  lower  level  is  released  by  an  electron  while
traveling  from  the  conduction  band  to  the  valence  band.  In  normal  diodes,  this
energy  released  is  in  the  form  of heat.  But  LED  is  made  up  some  special  material

which releases this energy in the form of photons which emit the light energy. Hence
such diodes are called light emitting diodes.

This process is called electroluminescence.




The fig shows the basic principle of this process. The energy released in the
form  of  light  depends  on  the  energy  corresponding  to  the  forbidden  gap.  This
determines  the  wavelength  of  the  emitted  light.  The  wavelength  determines  the
colour  of  the  light  and  also  determines  whether  the  light  is  visible  or  invisible
(infrared). The various impurities are added during the doping process to control the
wavelength  and colour  of the  emitted  light.  For  normal  silicon  diode, the  forbidden
energy gap is 1.1 eV and wavelength of the emitted light energy corresponds to that
of infrared light spectrum hence in normal diodes the light is not visible. The infrared
light is not visible.



LED Voltage and Current:

Consider a source connected to LED and a resistor as shown in the Fig. The outward
arrows associated with a diode indicate that it is LED. The resistor R is the current
limiting  resistor.  Due  to this  resistor,  the  current  through the  circuit  is  limited and
prevented from exceeding the maximum current rating of the diode






Let
V = drop across LED
Vs = supply voltage

Applying KVL to the circuit we can write,


When  forward  biased,  the  voltage  drop  across  conducting  LED  is  about  2  to  3  V
which is considerably greater than that across a normal silicon or germanium diode.
The  current  range  of  commercially  available  LEDs  is  10  to  80  mA.  Unless  and
otherwise   specified,   while   analyzing   the   LED   circuits,   the   drop   across   LED   is
considered as VD= 2V. The reverse breakdown voltage of LED is much less than the
normal diode, which is about 3V to 1OV



Spectral Output Curves for LED

The  visibility  of  the  light  is  decided  from  its  wavelength  (A). of  LED  against
the  wavelength  gives  the  various  curves  called  LEDs.  These  are  shown  in  the  Fig.
The graph of output light spectral output curves for LED’s.





It  can  be  seen  that  wavelength  is  expressed  in  nm  (nanometres).  The  normalized
output for visible light shows peak at 460 nm for blue, at 540 nm for green, at 590
nm for yellow and at 660 nm for red. The infrared invisible light output shows a peak
at 940 nm.

Advantages of LED

1.The various advantages of LED are,

2.LEDs are small  in size, and hence can be regarded as point  source of light.
Because of their small size, several thousand LEDs can be packed in one sq.
metre  area.  The  brightness  of  light  emitted  by LED  depends  on  the  current
flowing through LED. Hence the brightness of light can be smoothly controlled

by  varying  the  current.  This  makes  possible  to  operate  LED  displays  under
different ambient lighting conditions.

3.LEDs  are fast  operating devices.  They can  be turned  on and off in time less
than 1 microsecond.

4.The LEDs are light in weight.

5.The LEDs are available in various colours.

6.The LEDs have long life.

7.The LEDs are cheap and readily available.

8.The LEDs are easy to interface with various other electronic circuits.

9.Some  LEDs  radiate  infrared  light  which  is  invisible  but  still  useful  in  some
applications like burglar alarm systems.



Disadvantages of LED:

The various disadvantages of LED are,

1.It draws considerable current requiring frequent replacement of battery in low
power battery operated devices.

2.Luminous efficiency of LEDs is low which is about 1.5 lumen/watt.

3.The characteristics are affected by temperature.

4.Need large power for the operation compared to normal p-n junction diode.



Applications of LED:

Due  to  the  advantages  like  low  voltage,  long  life,  cheap,  reliable,  fast  on-off
switching  etc,  the  LEDs  are  used  in  many  applications.  The  various  applications  of
LED are,


1.All  kinds  of  visual  displays  i.e.  seven  segment  displays  and  alpha  numeric
displays. Such displays are commonly used in the watches and calculators.

2.In the optical devices such as optocouplers.

3.As on-off indicator in various types of electronic circuits.

4.Some LEDs radiate infrared light which is invisible. But such LEDs are useful
in remote controls and applications like burglar alarm.


Comparison of LED and p-n Junction Diode




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