Light Emitting Diode(LED) Video Lecture | Physics Class 12 - NEET

let us discuss about light-emitting
diode or LED light emitting diode which
we also call LED is nothing but like PN
Junction diode nowadays LEDs are most
widely used in semiconductor diodes
these diodes emit light when it is
forward biased normally these diodes are
formed by a PN Junction this is a
semiconductor PN Junction this diode has
two terminals anode and cathode this is
anode or positive terminal this is
cathode or negative terminal the PN
Junction is surrounded by a transparent
hard plastic epoxy resin hemispherical
shaped shell to protect the diode from
external shock this is a transparent
cover of the diode although we have
already said that LED is the same as
ordinary PN Junction diode but it is
also true that ordinary PN Junction
diode does not emit light when it is
forward biased now we will try to
explain what is a speciality of LED so
that it can emit light which the other
cannot when any diode is forward biased
current flows through the diodes in
forward biased condition positive
terminal or battery is connected to
p-type end a negative terminal of
battery is connected to n-type end of
the diode current is caused in the diode
due to flow of holes and free electrons
both here holes are injected by positive
terminals and migrated towards negative
side
and electrons are injected by negative
terminal and migrated towards a positive
site during this process of flow of
holes and free electrons there will be
recombination amongst them that means
free electrons will recombine with empty
holes during this recombination
electrons in the conduction band that is
free electrons will jump to the hole in
valence band during this jump of
electron from conduction band to valence
band electron emits energy in form of
photon this energy is equal to the
forbidden energy gap between conduction
band and valence band here it is to be
noted that condition band is at higher
energy level than valence band we know
that the energy of a photon is nothing
but the product of frequency of
electromagnetic radiation and Planck
constant
now consider the energy gap is eg hence
eg equals HF where H is Planck's
constant and F is the frequency of
electromagnetic radiation now it can be
said that eg is proportional to F this
is what we do in light emitting diode we
have already said that photon will
release in all electron hole
recombination but light only emit in LED
let us explain why velocity of
electromagnetic radiation is fixed and
it is equal to the speed of light that
is C the frequency of radiation F is
related to velocity of light as F equals
C by lambda where lambda is wavelength
of the electromagnetic radiation hence
from Equation e G equals HF we can write
e G equals HC by lambda so we have seen
that wavelength of electromagnetic
radiation is inversely proportional to
the forbidden energy gap in normal
silicon germanium semiconductor this
entire radiation of electromagnetic wave
during recombination is in the form of
inferred radiation the wavelengths of
the inferred are out of our visible
range so we cannot see it inferred
electromagnetic radiation is nothing at
Heat this is because silicon and
germanium semiconductor are not direct
gap semiconductors rather these are
indirect gap semiconductors in indirect
gap semiconductor the maximum energy
level of valence band and minimum energy
level of conduction band do not occur at
the same momentum of electrons hence
during recombination of electrons and
who
that is migration of electrons from
conduction band to valence band the
momentum of electrons would be changed
the photons originated from these
electrons will be mostly utilized for
the electron momentum indirect gap
semiconductor the maximum of valence
band and minimum of conduction band
occur at the same electron momentum
hence there will be no change of
momentum of electrons during migration
from conduction band valence band so the
photons originated due to that migration
have not to provide momentum to
electrons unlike normal diodes which are
made from either germanium or silicon
the indirect gap semiconductor materials
light-emitting diodes are made from
direct gap semiconductor compounded such
as gallium arsenide gallium phosphide
gallium arsenide phosphide silicon
carbide gallium indium nitride also they
may be mixed together at different
ratios to produce more distinct
wavelength of colors this is how an LED
or light emitting diode works hope you
got the idea thank you