Lecture 2 PN Junction and its Characteristics_Electronic Engineering_Fall 2023.pdf

Lecture slides

1. Electronic
Engineering
EE-2006

2. The PN Junction
A block of Si doped partly with pentavalent impurity
(N-type) and remaining with trivalent impurity (P-
type)
A pn-junction forms at the boundary between two
regions
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3. n-region has majority electrons and minority holes whereas p-
region has majority holes and minority electrons. The free
electrons in n-region diffuse across the region in p-region where
they combine with holes. Similarly, holes from p-region combine
with electrons in n-region forming a depletion region.
Depletion Region
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4. After initial surge of free electrons across the pn-junction, the
depletion region expands to a point where an equilibrium is
established and there is no further diffusion of electrons across the
junction. The depletion region acts as a barrier to the further
movement of electrons.
Depletion Region
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5. Barrier Potential
The forces between opposite charges in the
depletion region forms an electric field
An external energy must be provided to move
electrons across the barrier of the electric field
The amount of voltage required to move electrons
across through the electric field across the depletion
region is known as ‘barrier potential’
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6. Diode
A semiconductor (such as Si) is doped half as p-
region and half as n-region forming a depletion
region and pn-junction in between.
p-region is called anode and n-region is called as
cathode
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7. Biasing a Diode –Forward Bias
Forward Bias:
Anode is connected to positive terminal of battery
whereas cathode is connected to negative terminal
of the battery. Forward bias allows current to flow
through the diode
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8. Forward Biasing a Diode
The negative side of the bias voltage source pushes
free electrons in the n-region, towards the pn-
junction causing electron current to flow
The bias voltage source imparts enough energy to
free electrons to overcome barrier potential and
move on through into the p-region. Once in p-
region, electrons have lost energy to immediately
combine with the holes in p-region.
Since like charges attract, positive terminal of bias
voltage attracts free electrons while holes provide a
pathway to these electrons to move through the p-
region.
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9. Forward Biasing a Diode
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10. Forward Biasing a Diode
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11. Biasing a Diode –Reverse Bias
Positive terminal of bias voltage is connected to n-
region while negative terminal is connected to the
p-region. Reverse bias of diode prevents current
flow through the diode.
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12. Reverse Biasing a Diode
Since unlike charges attract, free electrons in n-
region are attracted towards the positive terminal of
bias voltage creating holes which widens the
depletion region.
The electrons from negative side of voltage source
enter p-region creating additional negative ions at
depletion region. This widens the depletion region.
Reverse Current: An extremely small current flows
through the depletion region during reverse bias
due to minority carriers in the n and p-region.
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13. Reverse Biasing a Diode
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14. Reverse Breakdown
There a very small flow of reverse current through a
diode during reverse bias, however if this bias
voltage is increased to a point called the
breakdown voltage, the reverse current will
drastically increase
The high reverse bias voltage imparts energy to free
minority electrons so that they speed through the p-
region, they collide with atoms with enough energy
to knock valence electrons out of orbit into the
conduction band. The newly created conduction
electrons are also high in energy and repeat the
process. The multiplication of conduction electrons
is known as ‘avalanche effect.’
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15. Quiz
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16. Thank You.
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