Energy band diagram of n-type
Many new free electrons are
produced by the addition of
pentavalent impurity.
Thermal energy of room temperature
still generates a few electron-hole
pairs. However, the number of free
electrons provided by the
pentavalent impurity far exceeds the
number of holes.
The current conduction in an n-type semiconductor is predominantly by
free electrons i.e. negative charges and is called n-type or electron type
conductivity .
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Energy band diagram of p-type
The addition of trivalent impurity has
produced a large number of holes.
There are a few conduction band
Free electrons due to thermal energy
associated with room temperature.
The number of holes provided by the
trivalent impurity far exceeds the
number of conduction band electron.
The current conduction in p-type semiconductor is predominantly by holes
i.e. positive charges and is called p-type or hole-type conductivity.
As the holes are positively charged, therefore, they are directed towards
the negative terminal, constituting what is known as hole current.
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Conduction in n-type and p-type
Fig : conduction in n-type semiconductor
Due to thermal energy
electrons break their
covalent bond which creates
electrons and holes termed
as electron-hole pair
generation. The opposite
effect is called
recombination
Fig: conduction in p-type semiconductor 3
P-N junction
When a p-type semiconductor
is suitably joined to n-type
semiconductor, the contact
surface is called pn junction.
Charge carriers start moving
from high concentration area
towards low concentration area
to achieve uniform
concentration all over the
material. This process is called
diffusion.
And exist when there is non-uniform concentration of charge carriers in the
material.
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Fabrication of p-n junction
P-n junction is fabricated by special techniques. One of the common
methods is alloying.
A small block of indium (trivalent impurity) is placed on an n-type
germanium slab as shown in figure.
The system is then heated to a temperature about 500 C. The indium and
some of the germanium melt to form a small puddle of molten
germanium-indium mixture.
The temperature is then lowered and puddle begins to solidify.
Under proper conditions, the atoms of indium impurity are suitable
adjusted in the Ge slab to form a single crystal.
The addition of indium overcomes the excess of electrons in the n-type
germanium to such an extent that it creates a p-type region. 5
Fabrication of p-n junction
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Depletion region
At the time of p-n junction formation,
holes enter the n region and recombine
with donor atoms and become positively
charged immobile ions.
The electrons diffusing n side to p side
recombines with acceptor atoms and
become negatively charged immobile ions.
Thus in thermal equilibrium, near the junction region, there exist a wall of
negative immobile charges on p side and positive immobile charges on
n-side. In this region there are no mobile charge carriers.
This region is called depletion region or depletion layer. The term depletion
is due to the region is depleted ( i.e. emptied) of charge carries.
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Before formation of Depletion region
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After formation of Depletion region
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Barrier potential
Once p-n junction is formed and
depletion layer created, the diffusion of
free electrons stops.
In other words, the depletion region acts
as a barrier to the further movement of
free electrons across the junction.
The positive and negative charges set up an electric field across the
junction. This creates a potential difference across the junction which is
called barrier potential (𝑉0 ).
The barrier potential depends on: i) The type of semiconductor material
ii) The donor impurity added, iii) The acceptor impurity added
iv) The temperature
For silicon, 𝑽𝟎 = 0.7 V ; For germanium, 𝑽𝟎 = 0.3 V
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Biasing a p-n junction
Applying external dc voltage to any electronic device is called biasing. In a
p-n junction, there are following two bias conditions:
1. Forward biasing 2. Reverse biasing
1. Forward biasing: When external d.c. voltage applied to the junction is
in such a direction that it cancels the potential barrier, thus permitting
current flow, it is called forward biasing.
2. Reverse biasing: When the external d.c. voltage applied to the
junction is in such a direction that potential barrier is increased, it is
called reverse biasing.
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Forward biasing of a p-n junction
To apply forward bias, connect
positive terminal to p -type and negative
terminal to n -type
1. The potential barrier is reduced and at
some forward voltage (0.1 to 0.3 V), it is
eliminated altogether.
2. The junction offers low resistance
(called forward resistance , 𝑅𝑓 ) to
current flow.
1. Current flows in the circuit due to the
establishment of low resistance path.
The magnitude of current depends upon
the applied forward voltage.
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Forward biasing
For forwarding bias, connect +ve terminal to the battery to p-type
and –ve terminal to n-type as shown in following figure.
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Forward biasing
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Reverse biasing of a p-n junction
To apply reverse bias, connect negative
terminal of the battery to p-type and
positive terminal to n-type
1. The potential barrier is increased.
2. The junction offers very high resistance
(called reverse resistance, 𝑅𝑟 )to current
flow.
3. No current flows in the circuit due to
the establishment of high resistance
path.
Reverse bias to the junction, a high resistance path is established and hence
no current flow occurs. On the other hand, with forward bias to the junction,
a low resistance path is set up and hence current flows in the circuit.
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Reverse biasing
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Reverse biasing
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Current flow in a forward biased p-n junction
The mechanism of current flow in a forward biased p-n junction are as
1. The free electrons from the negative terminal continue to pour into
the n-region while the free electrons in the n-region move towards
the junction.
2. The electrons travel through the n-region as free electrons i.e.
current in n-region is by free electrons.
3. When the electrons reach the junction, they combine with holes and
become valence electrons.
4. The electrons travel through p-region as valence electrons i.e.
current in the p-region is by holes. The valence electrons move
towards left in the p- region which is equal to the holes moving to
right.
5. When these valence electrons reach the left end of crystal, they flow
into the +ve terminal of the battery.
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Any Question?
Thanks to All
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