1. BEEE Prof. Vatsal Patel
Department of
Electrical Engineering
Prof. Vatsal Patel
Unit no:- 4
Unit title:-
Semiconductor
Diodes
Subject name and
code:- BEEE
(01EE0101)
Ch:4 Semiconductor Diodes
Energy Band Diagram of conductor, semiconductor and
insulator; Crystal Structure of Semiconductor Materials, Intrinsic
and Extrinsic Semiconductor Materials. Symbol and
Construction, Operating Characteristics in Forward and Reverse
Bias, Applications of Diode as Switch, Clipper, Clamper and
Rectifier; Special Purpose Diodes: Zener Diode; Optical Diodes
like LED, Photo Diode, Seven Segment Display
2. Prof. Vatsal Patel
The material can be classified according to their
resistivity range as –
Conductors (1.6 X 10-8 to 10-6 ohm-m)
Semiconductors (10-4 to 106 ohm-m)
Insulators (107 to 1016 ohm-m)
Semiconductors are the materials whose resistivity (and
hence the conductivity) lies between those of conductors
and insulator
The semiconductor devices are highly compact,
efficient, more reliable, low power consuming, free from
mechanical noise and are cheap.
Most commonly used semiconductors are Si, Ge, GaAs,
InP etc.
Introduction
4. Prof. Vatsal Patel
Electron
EnergyStates
of an isolated
Atom
Properties of Semiconductors –
1. Usually high resistivity.
2. Semiconductors are unipolar.
3. They have negative temperature coefficient.
4. They are metallic in nature.
5. At 0K they behaves like insulators.
6. Both electrons and holes can be charge carrier.
5. Prof. Vatsal Patel
Electron
EnergyStates
of an isolated
Atom
Types of Semiconductors
1. Elemental and compound semiconductors
2. Direct and indirect bandgap semiconductors
3. Intrinsic and Extrinsic semiconductors
6. Prof. Vatsal Patel
Intrinsic
semiconductor
Intrinsic or pure semiconductors – Silicon and germanium
have crystalline structure.
Their atoms are arranged in an ordered array known as the
crystal lattice.
There material are tetravalent i.e. with four valence
electrons in the outermost shell.
The neighboring atoms form covalent bonds by sharing
four electrons with each other so as to achieve stable
structure.
7. Prof. Vatsal Patel
Intrinsic
semiconductor
Crystal lattice structure and band
gap diagram of pure germanium
Energy gap – 0.72 eV for Ge.
Valence Band remains full and
conduction band is empty and
material behaves as insulator.
At room temp, valence band
electrons acquire thermal energy
greater than Eg and hence they can
jump to the higher energy
conduction band.
They are free now and can move
under the influence of small
applied field.
8. Prof. Vatsal Patel
Intrinsic
semiconductor
The absence of electrons in valence
band is known as hole. Hence in
semiconductor there are 2 types of
charge carriers.
The total current is the sum of
electron and holes currents. At any
given temperature, no of electrons =
no of holes in valence band are
same.
With the rise in temperature, more and more electrons hole pairs are
formed and more charge carriers are available for conduction.
Hence the conductivity of intrinsic semiconductors increases with rise
in temperature.
The intrinsic semiconductor have low conductivity.
9. Prof. Vatsal Patel
Extrinsic
semiconductor
Doped or Extrinsic Semiconductors – Doping is the
process of adding a controlled quantity of impurity to an
intrinsic semiconductor, so as to increase its conductivity.
A semiconductor doped with impurity atoms is called an
extrinsic semiconductor.
The impurity is added by melting Ge or Si, then the
crystal is grown in which the impurities are incorporated.
The impurity atoms occupy lattice positions which were
occupied by Ge atoms in pure metal.
Doping element is from III and V group elements.
Two types of extrinsic semiconductors are produced
depending upon the group of impurity atom.
10. Prof. Vatsal Patel
Extrinsic
semiconductor
N-type Semiconductors – The pentavalent impurities is
added in Ge crystal lattice. It forms four covalent bonds
with four neighboring Ge atoms.
The 5th electron, not used in bonding, it loosely bound &
with supply little energy, it can be made free leaving
behind a positively charged immobile ion.
The impurity atoms donate free electrons to the crystal
thereby increasing the conductivity of material. Hence
they are called the donor impurities.
The conductivity is due to negatively charged electrons.
Hence the material is called N-type semiconductor.
11. Prof. Vatsal Patel
Extrinsic
semiconductor
Electron-hole pairs are
generated in Ge due to
thermal energy.
For n-type the
concentration of free
electrons is far greater
than concentration of
holes.
Addition of donor impurities generates new energy levels in
the band picture.
The energy levels ED of neutral donor atoms lie very close to
the lower edge of EC of conduction band.
12. Prof. Vatsal Patel
Extrinsic
semiconductor
With the supply of little energy (0.01eV for Ge and
0.04eV for Si) the neutral donor atom loses fifth electron
for conduction and itself gets positively charged.
13. Prof. Vatsal Patel
Extrinsic
semiconductor
P-type Semiconductors – The trivalent impurities is added
in Ge crystal lattice.
Trivalent is one electron short of being able to complete
the stable structure. The absence of electron in one of
these bonds is a hole.
With the small amount of energy, it can accept an electron
from the neighboring Ge atom and vacancy shifts there.
The impurity atom becomes a negative charged ion on
accepting the electron.
Thus impurity atom supply holes which are ready to
accept electrons. Hence it’s a acceptor impurity.
14. Prof. Vatsal Patel
Extrinsic
semiconductor
The holes concentration is much more than the electron
concentration.
The conductivity is due to the positively charged holes.
Hence the semiconductor is called P-type semiconductor.
The addition of impurity introduces additional energy
levels EA, in the band picture, slightly above the top of the
valence band.
With supply of little energy these vacancies can be
occupied by electrons in VB and thus increasing the holes
in VB.
The extrinsic materials are electrically neutral at any
given temperature.
16. Prof. Vatsal Patel
P-N Junction
If a single piece of germanium doped with P-type material
from one side and the other half is doped with N-type
material, then the Ge is dividing into two zones forms a P-
N junction.
the P-N junction is the basic element for semiconductor
diodes. A Semiconductor diode facilitates the flow of
electrons completely in one direction only – which is the
main function of semiconductor diode.
It can also be used as a Rectifier.
17. Prof. Vatsal Patel
P-N Junction
There are two operating regions: P-type and N-type.
And based on the applied voltage, there are three possible
“biasing” conditions for the P-N Junction Diode, which
are as follows:
Zero Bias – No external voltage is applied to the PN
junction diode.
Forward Bias– The voltage potential is connected
positively to the P-type terminal and negatively to the N-
type terminal of the Diode.
Reverse Bias– The voltage potential is connected
negatively to the P-type terminal and positively to the N-
type terminal of the Diode.
18. Prof. Vatsal Patel
P-N Junction –
Zero Bias
There are two operating regions: P-type and N-type.
And based on the applied voltage, there are three possible “biasing”
conditions for the P-N Junction Diode, which are as follows:
Zero Bias – No external voltage is applied to the PN junction diode.
In this case, no external voltage is applied to the P-N junction diode;
and therefore, the electrons diffuse to the P-side and simultaneously
holes diffuse towards the N-side through the junction, and then
combine with each other.
Due to this an electric field is generated by these charge carriers.
19. Prof. Vatsal Patel
P-N Junction –
Zero Bias
When a p-n junction is formed, some of the free electrons
in the n-region diffuse across the junction and combine
with holes to form negative ions. In so doing they leave
behind positive ions at the donor impurity sites.
21. P - Doped N - Doped
Two Semiconductors
Donor impurity Examples: Phosphorus,Antimony, Arsenic
Accepter impurity Examples: Boron, Aluminium
22. Depletion Layer
N - Doped
P - Doped
P-N Junction Unbiased
In Depletion Region, there is no charge carriers
E
E = −
𝑑𝑉
𝑑𝑥
23. P - Doped N - Doped
P-N Junction Forward Biased
24. P - Doped N - Doped
P-N Junction reverse Biased
Depletion Layer
25. Prof. Vatsal Patel
P-N Junction –
Forward Bias
In the forward bias condition, the negative terminal of the
battery is connected to the N-type material and the
positive terminal of the battery is connected to the P-Type
material. This connection is also called as giving positive
voltage.
Electrons from the N-region cross the junction and enters
the P-region. Due to the attractive force that is generated
in the P-region the electrons are attracted and move
towards the positive terminal.
Simultaneously the holes are attracted to the negative
terminal of the battery. By the movement of electrons and
holes current flows.
In this condition, the width of the depletion region
decreases due to the reduction in the number of positive
and negative ions.
26. Prof. Vatsal Patel
P-N Junction
Diode
Forward Bias– The voltage potential is connected positively to the
P-type terminal and negatively to the N-type terminal of the Diode.
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27. Prof. Vatsal Patel
P-N Junction –
Reverse Bias
In the Reverse bias condition, the negative terminal of the
battery is connected to the P-type material and the positive
terminal of the battery is connected to the N-type material.
Hence, the electric field due to both the voltage and
depletion layer is in the same direction. This makes the
electric field stronger than before.
Due to this strong electric field, electrons and holes want
more energy to cross the junction so they cannot diffuse to
the opposite region.
Hence, there is no current flow due to the lack of
movement of electrons and holes.
28. Prof. Vatsal Patel
P-N Junction
Diode
Reverse Bias– The voltage potential is connected positively to the N-
type terminal and negatively to the P-type terminal of the Diode.
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34. Prof. Vatsal Patel
Diode
Approximations
Definition: The approximation technique that helps in analyzing the
various initial criteria of the diode can be defined as Diode
Approximations. Each approximates relates from assuming ideal
conditions to reaching practical ones.
There are three (3) Diodes Approximations:
Ideal Diode (1st Approximation)
Second Approximation
Third Approximation
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35. Prof. Vatsal Patel
Diode
Approximations
• Ideal (1st-Approximation) - For the 1st-approx. assume the diode
drop voltage is zero (Perfect closed switch)
• Second approximation - For the 2nd-approx. assume the diode
drop voltage of 0.7 volts
• Third Approximation - For the 3rd –approx. assume the diode
drop voltage of 0.7 volt and consider the forward bulk resistance
of the diode:
Vd = 0.7 V + Id Rb
Ignore bulk resistance of the diode if Rb < 0.01 Rth
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36. Prof. Vatsal Patel
Ideal Diode
An ideal diode is a diode that acts like a perfect conductor when voltage is
applied forward biased and like a perfect insulator when voltage is applied
reverse biased.
So when positive voltage is applied across the anode to the cathode, the diode
conducts forward current instantly.
When voltage is applied in reverse, the diode conducts no current at all.
This diode operates like a switch.
Forward Bias – Closed switch,
Reverse Bias – Open Switch
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37. Prof. Vatsal Patel
Characteristics
of Ideal Diode -
Forward Biased
1) Zero forward Resistance
2) Infinite Amount of Current
3) Zero Threshold Voltage
4) No Reverse Breakdown Voltage
5) Zero Reverse Leakage Current
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38. Prof. Vatsal Patel
Characteristics
of Ideal Diode -
Forward Biased
1) Zero Resistance
An ideal diode does not offer any resistance to the flow of
current through it when it is in forward biased mode. This
means that the ideal diode will be a perfect conductor when
forward biased. From this property of the ideal diode, one can
infer that the ideal diode does not have any barrier potential.
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39. Prof. Vatsal Patel
Characteristics
of Ideal Diode -
Forward Biased
2) Infinite Amount of Current
This property of the ideal diode can be directly implied from its
previous property which states that the ideal diodes offer zero
resistance when forward biased. The reason can be explained as
follows. In electronic devices, the relationship between the current
(I), voltage (V) and resistance (R) is expressed by Ohm’s law which
is stated as I = V/R. Now, if R = 0, then I = ∞. This indicates that
there is no higher limit for the current which can flow through the
forward-biased ideal diode
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40. Prof. Vatsal Patel
Characteristics
of Ideal Diode -
Forward Biased
3) Zero Threshold Voltage
Even this characteristic of the ideal diode under the forward biased
state can be referred from its first property of possessing zero
resistance. This is because threshold voltage is the minimum voltage
which is required to be provided to the diode to overcome its barrier
potential and to start conducting. Now, if the ideal diode is void of
depletion region itself, then the question of threshold voltage does
not arise at all. This property of the ideal diode makes them conduct
right at the instant of being biased, leading to the green-curve of
diode characteristics
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41. Prof. Vatsal Patel
Characteristics
of Ideal Diode
when Reverse
Biased
Infinite Resistance
An ideal diode is expected to fully inhibit the flow of current
through it under reverse biased condition.
In other words it is expected to mimic the behavior of a perfect
insulator when reverse biased.
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42. Prof. Vatsal Patel
Characteristics
of Ideal Diode
when Reverse
Biased
5) Zero Reverse Leakage Current
This property of the ideal diode can be directly implied from
its previous property which states that the ideal diodes possess
infinite resistance when operating in reverse biased mode. The
reason can be understood by considering the Ohm’s law again
which now takes the form Thus it means that
there will be no current flowing through the ideal diode when
it is reverse biased, no matter how high the reverse voltage
applied be.
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43. Prof. Vatsal Patel
Characteristics
of Ideal Diode
when Reverse
Biased
4) No Reverse Breakdown Voltage
Reverse breakdown voltage is the voltage at which the reverse biased diode
fails and starts to conduct heavy current. Now, from the last two properties
of the ideal diode, one can conclude that it will offer infinite resistance
which completely inhibits the current flow through it. This statement holds
good irrespective of the magnitude of the reverse voltage applied to it.
When the condition is so, the phenomenon of reverse breakdown can never
occur due to which there will be no question of its corresponding voltage,
the reverse breakdown voltage. Due to all these properties, an ideal diode is
seen to behave as a perfect semiconductor switch which will be open when
the reverse biased and closed when forward biased.
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45. Prof. Vatsal Patel
Second
Approximation
In the second approximation, the diode is considered as a
forward-biased diode in series with a battery to turn on the
device.
For a silicon diode to turn on, it needs 0.7V. A voltage of
0.7V or greater is fed to turn on the forward-biased diode.
The diode turns off if the voltage is less than 0.7V.
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47. Prof. Vatsal Patel
Third
Approximation
47
The third approximation of a diode includes voltage across the
diode and voltage across bulk resistance, RB.
The bulk resistance is low, such as less than 1 ohm. almost always
less than 10Ω.
The bulk resistance, RB corresponds to the resistance of p and n
materials.
This resistance changes based on the amount of forwarding voltage
and the current flowing through the diode at any given time.
The voltage drop across the diode is calculated using the formula
𝑉𝑑 = 0.7 + 𝐼𝑑 × 𝑅𝐵
49. Prof. Vatsal Patel
Third
Approximation
49
The Bulk Resistance, RB, of a diode is the approximate resistance
across the terminals of the diode when a forward voltage and
current are applied across the diode.
The bulk resistance represents the resistance of the p and n
materials of the p-n junction of the diode.
Its value is dependent on the doping level and the size of the p and
n materials.
The bulk resistance is not a fixed resistance but a dynamic one. It
changes according to the amount of forward voltage and current
going through the diode at any particular time.
50. Prof. Vatsal Patel
Bulk Resistance
of Diode
50
The bulk resistance of a diode can be calculated at any given time
by ohm's law:
𝑅𝐵 =
∆𝑉𝐹
∆𝐼𝐹
Where, ∆𝑉𝐹 is forward voltage drop and ∆𝐼𝐹is forward current
flowing through diode.
51. Prof. Vatsal Patel
Numerical
related to Diode
Approximation
51
Use the second approximation of diode and find the current
flowing through the diode.
52. Prof. Vatsal Patel
Numerical
related to Diode
Approximation
52
Look at both of the circuits and calculate using the third
approximation method of diode
