P-N Junction Diodes and Zener Diodes
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A normal
p-n junction diode
allows electric
current
only
in
forward biased condition
. When
forward biased voltage is applied to the p-n
junction diode, it allows large amount of electric
current. Hence,
a
forward biased p-n junction
diode
offer only a small resistance to the electric
current.
When
reverse biased voltage
is applied to the
p-n junction diode, it blocks large amount of
electric current. Hence, a
reverse biased p-n
junction diode
offer large resistance to the
electric current
.
If reverse biased voltage applied to the p-n
junction diode is highly increased, a sudden
rise in current occurs. At this point, a small
increase in
voltage
will rapidly increases the
electric current.
This sudden rise in electric
current causes a junction breakdown called
zener or avalanche breakdown
.
The voltage at
which breakdown occurs is called
zener
voltage
and the sudden increase in current is
called
zener current.
A normal p-n junction diode does not operate in
breakdown region
because the excess current
permanently damages the diode
. A
zener diode
is
a special type of device designed to operate in
the
reverse breakdown region
.
Zener diodes
acts
like normal p-n junction diodes under forward
biased condition (
it allows large amount of
electric current under forward biased voltage
).
Also, allows electric current in the reverse
direction if the applied reverse voltage is greater
than the zener voltage
.
Thus, it is always
connected in reverse direction because it is
specifically designed to work in reverse direction.
The zener breakdown voltage of the zener diode
is depends on
the amount of doping applied
.
If
the diode is heavily doped, zener breakdown
occurs at low reverse voltages
.
On the other
hand, if the diode is lightly doped, the zener
breakdown occurs at high reverse voltages
.
Zener
diode
is heavily doped than the normal p-n
junction diode. Hence, it has very thin
depletion
region
.
Therefore, zener diodes allow more
electric current than the normal p-n junction
diodes
. Zener diodes are available with zener
voltages in the range of 1.8V to 400V.
The symbol of zener diode is shown in below
figure. Zener diode consists of two terminals:
cathode and anode. The symbol of zener
diode is similar to the normal p-n junction
diode, but with bend edges on the vertical
bar
Power dissipation capacity is very high
High accuracy
Small size and Low cost
It is normally used as voltage reference
Zener diodes are used in voltage stabilizers
or shunt regulators.
Zener diodes are used in switching operations
Zener diodes are used in clipping and
clamping circuits.
Diodes only allow a considerable current to flow
when they are connected in forward bias.
Therefore, they can be used to ensure that
current in a circuit flows along a given direction.
For instance, diodes can be used to convert
alternating current to direct current.
However a large reverse voltage can cause
reverse currents to flow. This is referred to as
breakdown
, and can take place either as
Zener breakdown
or as
avalanche breakdown
.
The differences between the two types of
breakdown are outlined below.
The avalanche breakdown
occurs in both
normal diodes and zener diodes
at high
reverse voltage.
When high reverse voltage
is
applied to the p-n junction
diode, the
free electrons
gains
large amount of
energy
and
accelerated to greater
velocities. The free electrons
moving at high speed will
collides with the
atoms
and
knock off more electrons.
These electrons are again accelerated and collide with
other atoms. Because of this continuous collision with
the atoms, a large number of free electrons are
generated. As a result, electric current in the diode
increases rapidly.
This sudden increase in electric current may
permanently destroys the normal diode.
However,
zener diodes may not be destroyed
because they are carefully designed to
operate in avalanche breakdown region
.
Avalanche breakdown occurs in zener diodes
with zener voltage (V
z
) greater than 6V.
In terms of energy bands, the incoming
charge carrier’s kinetic energy must be larger
than the energy gap between conduction and
valence bands
for
impact ionization
to take
place. Then, once the collision takes place
and the electron-hole pair is formed, this
electron and the hole are essentially in the
conduction and valance bands respectively.
For most diodes
, avalanche breakdown
is the
dominant effect which is determined by
:
1-
The material used to construct the
junction.
2-
The level of doping.
In Zener breakdown
,
electrons
tunnel
from the
valence band
of the
p
side
to the
conduction band
on the
n
side
.
In classical physics, electrons should not have been
able to cross over in this way.
The probability for an
electron to tunnel
across is
higher
when
the space
charge region is narrower
,
and when
the electric
field is larger
. Typically, Zener breakdown occurs
where materials used to construct the
p
–
n
junction
are heavily doped. In these junctions, due to heavy
doping, the space charge region is quite narrow
even when the junction is under reverse bias.
When reverse biased voltage
applied to the diode reaches close
to zener voltage, the electric field
in the depletion region is strong
enough to pull electrons from their
valence band. The valence
electrons which gains sufficient
energy from the strong electric
field of depletion region will breaks
bonding with the parent atom and
become free electrons.
These free electrons carry electric current from one place to
another place. At zener breakdown region, a small increase in
voltage will rapidly increases the electric current
Zener breakdown
occurs when the doping levels are high,
and involves electrons tunnelling from the valence band of
the
p
side to the conduction band on the
n
side.
Avalanche breakdown
occurs when charge carriers which
are accelerated by the electric field gain enough kinetic
energy such that, when they collide with lattice atoms, they
ionise the lattice atoms to produce electron-ion pairs.
These pairs, in turn, cause further
ionizations
, leading to
an avalanche effect
Zener breakdown
occurs at
low reverse voltage
whereas
avalanche breakdown
occurs at
high reverse voltage
.
A normal p-n junction diode allows electric current only in forward biased condition, offering small resistance. When reverse biased, it blocks current. If the reverse biased voltage is highly increased, it can lead to zener or avalanche breakdown. Zener diodes are specifically designed for working in reverse bias, allowing current in the reverse direction. The zener breakdown voltage depends on the doping level applied. Zener diodes operate in breakdown region at low reverse voltages due to heavy doping. The symbol of a zener diode consists of two terminals: cathode and anode.
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Presentation Transcript
A normal p-n junction diode allows electric current only forward biased voltage is applied to the p-n junction diode, it allows large amount of electric current. Hence, a forward biased p-n junction diode offer only a small resistance to the electric current. only in forward biased condition forward biased condition. When
When reverse biased voltage p-n junction diode, it blocks large amount of electric current. Hence, a reverse biased p-n junction diode offer large resistance to the electric current. reverse biased voltage is applied to the
If reverse biased voltage applied to the p-n junction diode is highly increased, a sudden rise in current occurs. At this point, a small increase in voltage will rapidly increases the electric current. This sudden rise in electric current causes a junction breakdown called zener which breakdown occurs is called zener voltage called zener zener or avalanche breakdown or avalanche breakdown. The voltage at zener voltage and the sudden increase in current is zener current. current.
A normal p-n junction diode does not operate in breakdown permanently damages the diode. A zener a special type of device designed to operate in the reverse like normal p-n junction diodes under forward biased condition (it allows large amount of electric current under forward biased voltage). Also, direction if the applied reverse voltage is greater than the zener voltage. connected specifically breakdown region region because the excess current zener diode Zener diodes diode is diodes acts reverse breakdown breakdown region region. Zener allows electric current in the reverse always direction. . Thus, it it is is always direction because to work Thus, in reverse connected in specifically designed in reverse designed to reverse direction work in because it it is is reverse direction
The is is depends the diode is heavily doped, zener breakdown occurs at low reverse voltages. On the other hand, if the diode is lightly doped, the zener breakdown occurs at high reverse voltages. Zener diode junction region electric current than the normal p-n junction diodes. Zener diodes are available with zener voltages in the range of 1.8V to 400V. The zener depends on zener breakdown on the amount of doping applied. If breakdown voltage voltage of of the the zener zener diode diode Zener diode is is heavily junction diode region. heavily doped diode. . Hence, Therefore, doped than Hence, it it has zener than the has very diodes the normal very thin allow normal p p- -n n thin depletion depletion more
The symbol of zener diode is shown in below figure. Zener diode consists of two terminals: cathode and anode. The symbol of zener diode is similar to the normal p-n junction diode, but with bend edges on the vertical bar
Power dissipation capacity is very high High accuracy Small size and Low cost
It is normally used as voltage reference Zener diodes are used in voltage stabilizers or shunt regulators. Zener diodes are used in switching operations Zener clamping circuits. diodes are used in clipping and
Diodes only allow a considerable current to flow when they are connected in forward bias. Therefore, they can be used to ensure that current in a circuit flows along a given direction. For For instance, instance, diodes diodes can can be be used used to to convert convert alternating alternating current current to to direct direct current current. .
However a large reverse voltage can cause reverse currents to flow. This is referred to as breakdown, and can take place either as Zener The differences between the two types of breakdown are outlined below. Zener breakdown breakdown or as avalanche avalanche breakdown breakdown.
The avalanche breakdown occurs in both normal diodes and zener diodes at at high high reverse reverse voltage voltage. .
When applied diode, the free electrons gains large accelerated velocities. moving collides knock off more electrons. These electrons are again accelerated and collide with other atoms. Because of this continuous collision with the atoms, a large number of free electrons are generated. As a result, electric current in the diode increases rapidly. When high to high reverse the reverse voltage p-n voltage junction is amount of to free high the energy and greater electrons speed atoms The at with will and
This sudden increase in electric current may permanently However, zener diodes may not be destroyed because operate Avalanche breakdown occurs in zener diodes with zener voltage (Vz) greater than 6V. destroys the normal diode. they in are carefully breakdown designed to avalanche region.
In terms of energy bands, the incoming charge carrier s kinetic energy must be larger than the energy gap between conduction and valence bands for impact place. Then, once the collision takes place and the electron-hole pair is formed, this electron and the hole are essentially in the conduction and valance bands respectively. impact ionization ionization to take
For most diodes, avalanche breakdown , avalanche breakdown is the dominant effect which is determined by: 1 1- - The material used to construct the junction. 2 2- - The level of doping.
In valence on the n n side In Zener valence band Zener breakdown band of the p p side side. breakdown, electrons tunnel side to the conduction tunnel from the conduction band band In classical physics, electrons should not have been able to cross over in this way. The electron charge region is narrower, and when the electric field is larger. Typically, Zener breakdown occurs where materials used to construct the p n junction are heavily doped. In these junctions, due to heavy doping, the space charge region is quite narrow even when the junction is under reverse bias. The probability higher when the space probability for for an an electron to to tunnel tunnel across is higher
When applied to the diode reaches close to zener voltage, the electric field in the depletion region is strong enough to pull electrons from their valence electrons energy field of depletion region will breaks bonding with the parent atom and become free electrons. These free electrons carry electric current from one place to another place. At zener breakdown region, a small increase in voltage will rapidly increases the electric current reverse biased voltage band. which from The gains strong valence sufficient electric the
Zener and involves electrons tunnelling from the valence band of the p side to the conduction band on the n side. Avalanche are accelerated by the electric field gain enough kinetic energy such that, when they collide with lattice atoms, they ionise the lattice atoms to produce electron-ion pairs. These pairs, in turn, cause further ionizations an avalanche effect Zener avalanche Zener breakdown breakdown occurs when the doping levels are high, Avalanche breakdown breakdown occurs when charge carriers which ionizations, leading to Zener breakdown avalanche breakdown breakdown occurs at low reverse voltage whereas breakdown occurs at high reverse voltage.
