Lecture on Introduction of Semiconductor at North South University as the undergraduate course (ETE411) ======================= Dr. Mashiur Rahman Assistant Professor Dept. of Electrical Engineering and Computer Science North South University, Dhaka, Bangladesh http://mashiur.biggani.org

1. ETE411 :: Lecture 11
Dr. Mashiur Rahman
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2. Review of last class
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3. • During the formation of junction, 2 phenomena
take place:
1. A thin layer is set up on both sides of the junction.
2. A junction/barrier potential is developed across the
junction
• Formation of depletion layer:
1. Greater concentration of holes in P than N region
2. Greater concentration of electrons in N than P
region
• The difference in concentration establish a density
gradient across the junction resulting in carrier
diffusion and terminate the existence by
recombination
3

4. OBJECTIVES
Up on completion of this topic the student will be able
to know
• How to forward bias the diode, under forward biasing how
the diode works.
• How to reverse bias the diode, under reverse biasing how the
diode works.
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5. PN-JUNCTION FORWARD BIAS
+ -
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6. • As soon as battery connection is made, the
holes in P region are repelled by the positive
battery terminal & electrons in N region are
repelledby the negative battery terminal. Thus
both holes & electrons are driven towards the
junction where they recombine.
• This allow easy current flow through the
semiconductor as the crystal offers low
resistance. FB reduces the thickness of the
depletion layer.
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7. Forward characteristics
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8. • At room temperature, a potential difference is
require before a reasonable amount of
forward current starts flowing in a junction.
This is known as threshold voltage, Vth.For
applied voltage less than Vth, current flow is
negligible. As applied voltage increase beyond
Vth, forward current increase sharply. If
forward voltage increase beyond a certain safe
value, it will produce an extremely large
current which may destroy the junction due to
overheating.
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9. PN-Junction with forward Bias
• Positive terminal of the battery is connected to anode and negative
terminal is connected to the cathode.
• Increasing the applied voltage the potential barrier decreases.
• Potential barrier disappears when applied voltage exceeds barrier
potential.
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10. • The resistance of junction becomes nearly zero and current
starts flowing in the junction.
• Now the current increases sharply with increase in applied
voltage.
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11. PN-JUNCTION REVERSE BIAS
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12. • During the RB, holes are attracted by
the negative battery terminal &
electrons are attracted by the
positive battery terminal. Thus, both
holes & electrons move away from
the junction. Hence, no electron-hole
combination, no current flow and
the junction offers high resistance.
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13. Practically no current flow,
yet there is a small current
due to minority carriers
which is known as reverse
current or reverse current,
Isor Io. It also referred as
leakage current of PN
junction.
If reverse bias applied to a
junction is increase, a
point is reached when the
junction breakdown and
revere current rises to
value limited only by the
resistance connected in
series with junction. This
critical value of voltage is
known as breakdown
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14. PN-JUNCTION WITH REVERSE BIAS
• Negative terminal of the battery is connected to the anode and
positive terminal is connected to the cathode.
• The resistance is very high in the order of mega ohms.
• Current is almost zero in reverse bias.
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15. • Small reverse leakage current flows due to minority carriers present
in P & N regions.
• Reverse voltage exceeds breakdown voltage the conductivity of the
junction rapidly increases causing the diode goes to damage.
• Operate the diode below break down voltage.
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16. Zener breakdown
When reverse bias is increased, the electric
field at the junction also increases.
High electric field causes covalent bonds to break.
Thus a large number of carriers are generated. This causes a
large current to flow.
This mechanism of breakdown is zener breakdown.
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17. Avalanche breakdown
• The increased electric field causes increase in the velocities of
minority carriers.
• These high energy carriers break covalent bonds, thereby
generating more carriers.
• These generated carriers are accelerated by the electric field.
• They break more covalent bonds during their travel.
• Small reverse leakage current flows due to minority carriers
present in P & N regions.
• Reverse voltage exceeds breakdown voltage the conductivity
of the junction rapidly increases causing the diode goes to
damage.
• Operate the diode below break down voltage.
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18. SUMMARY
We have discussed about
• Forward biasing of diode
• Reverse biasing of diode
• Avalanche and zener breakdowns.
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19. QUIZ
When we apply reverse bias to a junction diode, it
(a) Decreases the potential barrier
(b) Increases the potential barrier
(c) Greatly increases the minority carrier current
(d) Greatly increases the majority carrier current
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20. 2. When forward bias is applied to a junction diode, it
(a) Increases the potential barrier
(b) Decreases the potential barrier
(c) Reduces the majority carrier current to zero
(d) Reduces the minority carrier current to zero
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21. 3. The reverse saturation current in a junction diode is
the current that flows when
(a) Only majority carriers are crossing the junction
(b) Only minority carriers are crossing the junction
(c) The junction is unbiased
(d) The potential barrier is zero
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22. Frequently asked questions
• 1) Discuss the behavior of PN junction under forward biasing
• 2) Explain the working of PN junction diode under reverse
biasing
• 3) Define avalanche and zener break down
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