Brief study of Semiconductor for class 12 students

1. SEMICONDUCTOR
DEVICES
I would say that hardware is the bone of the head,
the skull. The semiconductor is the brain within the
head. The software is the wisdom and data is the
knowledge.

2. CRYSTALLINE SOLIDS
Crystalline solids, or crystals, have distinctive
internal structures that in turn lead to
distinctive flat surfaces, or faces. The faces
intersect at angles that are characteristic of
the substance.
⬥ They have regular structure
⬥ They have sharp melting points
⬥ They are anisotropic in nature.
⬥ They are true solids
SOLIDS:
AMORPHOUS SOLIDS
Amorphous solids have two characteristic
properties. When cleaved or broken, they
produce fragments with irregular, often curved
surfaces; and they have poorly defined patterns
when exposed to x-rays because their
components are not arranged in a regular array.
⬥ They do not have a regular structure.
⬥ They do not have sharp melting points
⬥ They are isotropic in naturer.
⬥ They are not true solids
2

3. CLASSIFICATION OF SOLIDS
CONDUCTORS
⬥ A conductor is material usually
a pure metal which conducts
electricity easily.
⬥ Their conductivity depends on
their nature but changes with
temperature.
⬥ Their properties cannot be
altered by adding impurities.
SEMI-CONDUCTORS
⬥ A semiconductor is a
material product usually
comprised of silicon,
which conducts
electricity more than an
insulator, such as glass,
but less than a pure
conductor, such
as copper or aluminium
⬥ Their conductivity and
other properties can be
altered with the
introduction of
impurities, called doping
INSULATORS
⬥ An insulator is material do
not conduct electricity.
⬥ Their conductivity does not
change with temperature.
⬥ Their properties cannot be
altered by adding imputities
3

4. BAND THEORY OF SOLIDS
When two atoms combine with each other to form
bonds then their individual energy levels interact with
each other to form attraction and repulsion energy
levels.

5. An isolated atom possesses discrete energies of different electrons.
Suppose two isolated atoms are brought to very close proximity,
then the electrons in the orbits of two atoms interact with each
other. So, that in the combined system, the energies of electrons
will not be in the same level but changes and the energies will be
slightly lower and larger than the original value. So, at the place of
each energy level, a closely spaced two energy levels exists. If ‘N’
number of atoms are brought together to form a solid and if these
atoms’ electrons interact and give ‘N’ number of closely spaced
energy levels in the place of discrete energy levels, it is known as
bands of allowed energies. Between the bands of allowed energies,
there are empty energy regions, called forbidden band of energies.

7. Valence Band
The electrons move in the atoms in certain energy levels but the energy of the
electrons in the innermost shell is higher than the outermost shell electrons.
The electrons that are present in the outermost shell are called as Valence
Electrons.
These valence electrons, containing a series of energy levels, form an energy
band which is called as Valence Band. The valence band is the band having the
highest occupied energy.
Conduction Band
The valence electrons are so loosely attached to the nucleus that even at room
temperature, few of the valence electrons leave the band to be free. These are
called as free electrons as they tend to move towards the neighboring atoms.
These free electrons are the ones which conduct the current in a conductor and
hence called as Conduction Electrons. The band which contains conduction
electrons is called as Conduction Band. The conduction band is the band having
the lowest occupied energy.

8. Forbidden gap
The gap between valence band and conduction band
is called as forbidden energy gap. As the name
implies, this band is the forbidden one without
energy. Hence no electron stays in this band. The
valence electrons, while going to the conduction
band, pass through this.
The forbidden energy gap if greater, means that
the valence band electrons are tightly bound to the
nucleus. Now, in order to push the electrons out of
the valence band, some external energy is required,
which would be equal to the forbidden energy gap.

9. Insulators
Insulators are such materials in which the conduction cannot take
place, due to the large forbidden gap. Examples: Wood, Rubber.
The structure of energy bands in Insulators is as shown in the
following figure.
Characteristics
The following are the characteristics of Insulators.
 The Forbidden energy gap is very large.
 Valance band electrons are bound tightly to atoms.
 The value of forbidden energy gap for an insulator will be of
10eV.
 For some insulators, as the temperature increases, they might
show some conduction.
The resistivity of an insulator will be in the order of 107 ohm-meter.

10. Semiconductors
Semiconductors are such materials in which the forbidden energy gap is
small and the conduction takes place if some external energy is applied.
Examples: Silicon, Germanium. The following figure shows the structure of
energy bands in semiconductors.
Characteristics
The following are the characteristics of Semiconductors.
 The Forbidden energy gap is very small.
 The forbidden gap for Ge is 0.7eV whereas for Si is 1.1eV.
 A Semiconductor actually is neither an insulator, nor a good conductor.
 As the temperature increases, the conductivity of a semiconductor
increases.
The conductivity of a semiconductor will be in the order of 102 mho/meter

11. Conductors
Conductors are such materials in which the forbidden energy gap disappears as
the valence band and conduction band become very close that they overlap.
Examples: Copper, Aluminium. The following figure shows the structure of
energy bands in conductors.
Characteristics
The following are the characteristics of Conductors.
 There exists no forbidden gap in a conductor.
 The valance band and the conduction band gets overlapped.
 The free electrons available for conduction are plenty.
 A slight increase in voltage, increases the conduction.
There is no concept of hole formation, as a continuous flow of electrons
contribute the current.

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13. “
Intrinsic Semiconductor and Extrinsic Semiconductor
The semiconductor is divided into two types.
One is Intrinsic Semiconductor and other is an Extrinsic semiconductor. The pure form of the
semiconductor is known as the intrinsic semiconductor and the semiconductor in which
intentionally impurities is added for making it conductive is known as the extrinsic
semiconductor. The conductivity of the intrinsic semiconductor become zero at room
temperature while the extrinsic semiconductor is very little conductive at room temperature.

14. Intrinsic Semiconductor
An extremely pure semiconductor is called as Intrinsic
Semiconductor. On the basis of the energy band
phenomenon, an intrinsic semiconductor at absolute zero
temperature is shown below.
Its valence band is completely filled and the conduction
band is completely empty. When the temperature is raised
and some heat energy is supplied to it, some of the valence
electrons are lifted to conduction band leaving behind
holes in the valence band as shown below.
The electrons reaching at the conduction band move
randomly. The holes created in the crystal also free to
move anywhere. This behavior of the semiconductor shows
that they have a negative temperature coefficient of
resistance. This means that with the increase in
temperature, the resistivity of the material decreases and
the conductivity increases.

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Extrinsic Semiconductor
A semiconductor to which an impurity at
controlled rate is added to make it conductive
is known as an extrinsic Semiconductor.
An intrinsic semiconductor is capable to conduct a
little current even at room temperature, but it is
not useful for the preparation of various electronic
devices. Thus, to make it conductive a small amount
of suitable impurity is added to the material.

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Doping
The process by which an impurity is added to a semiconductor is known as Doping.
The amount and type of impurity which is to be added to a material has to be
closely controlled during the preparation of extrinsic semiconductor. Generally,
one impurity atom is added to a 108 atoms of a semiconductor.
The purpose of adding impurity in the semiconductor crystal is to increase the
number of free electrons or holes to make it conductive. If a Pentavalent
impurity, having five valence electrons is added to a pure semiconductor a large
number of free electrons will exist.
If a trivalent impurity having three valence electrons is added, a large number of
holes will exist in the semiconductor.
Depending upon the type of impurity added the extrinsic semiconductor may be
classified as n type semiconductor and p type semiconductor.

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DOPING

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20. p Type Semiconductor
The extrinsic p-Type
Semiconductor is formed when
a trivalent impurity is added to a pure
semiconductor in a small amount, and
as a result, a large number of holes are
created in it. A large number of holes
are provided in the semiconductor
material by the addition of trivalent
impurities
like Gallium and Indium. Such type of
impurities which produces p-type
semiconductor are known as
an Acceptor Impurities because each
atom of them create one hole which
can accept one electron.

21. In the fourth covalent bonds, only the Silicon atom
contributes one valence electron, while Boron atom
has no valence bonds.
Hence, the fourth covalent bond is incomplete,
having one electron short.
This missing electron is known as a Hole.
Thus, each Boron atom provides one hole in the
Silicon crystal.
As an extremely small amount of Boron impurity has
a large number of atoms, therefore, it provides
millions of holes in the semiconductor.
A trivalent impurity like Boron having three valence
electrons is added to Silicon crystal in a small
amount.
Each atom of the impurity fits in the Silicon crystal
in such a way that its three valence electrons form
covalent bonds with the three surrounding Silicon
atoms

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Energy Band Diagram of p-Type Semiconductor
The energy band diagram of a p-Type Semiconductor is shown below.
A large number of holes or vacant space in the covalent bond is
created in the crystal with the addition of the trivalent impurity. A
small or minute quantity of free electrons is also available in the
conduction band.
They are produced when thermal energy at room temperature is
imparted to the germanium crystal forming electron-hole pairs. But
the holes are more in number as compared to the electrons in the
conduction band. It is because of the predominance of holes over
electrons that the material is called as a p-type semiconductor. The
word “p” stands for the positive material.

23. Conduction Through p Type Semiconductor
In p type semiconductor large number of holes are created by the trivalent
impurity. When a potential difference is applied across this type of semiconductor
as shown in the figure below.
The holes are available in the valence band are directed towards the negative
terminal. As the current flow through the crystal is by holes, which are carrier of
positive charge, therefore, this type of conductivity is known as positive or p type
conductivity. In a p type conductivity the valence electrons move from one covalent
to another.
The conductivity of n type semiconductor is nearly double to that of p type
semiconductor. The electrons available in the conduction band of the n type
semiconductor are much more movable than holes available in the valence band in a
p type semiconductor. The mobility of holes is poor as they are more bound to the
nucleus.
Even at the room temperature the electron hole pairs are formed. These free
electrons which are available in minute quantity also carry a little amount of current
in the p type semiconductors.

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n Type Semiconductor
When a small amount of Pentavalent
impurity is added to a pure
semiconductor providing a large number
of free electrons in it, the extrinsic
semiconductor thus formed is known
as n-Type Semiconductor. The
conduction in the n-type semiconductor
is because of the free electrons denoted
by the pentavalent impurity atoms.
These electrons are the excess free
electrons with regards to the number of
free electrons required to fill the
covalent bonds in the semiconductors.

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The addition of Pentavalent impurities such as
arsenic and antimony provides a large number of
free electrons in the semiconductor crystal. Such
impurities which produce n-type semiconductors are
known as Donor Impurities. They are called a donor
impurity because each atom of them donates one
free electron crystal.
When a few Pentavalent impurities such
as Phosphorus whose atomic number is 15, which is
categorised as 2, 8, 5. It has five valence
electrons, which is added to Silicon crystal. Each
atom of the impurity fits in four germanium atoms
as shown in the figure above.
Hence, each Phosphorus atom provides one free
electron in the Silicon crystal. Since an extremely
small amount of Phosphorus, impurity has a large
number of atoms; it provides millions of free
electrons for conduction.

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Energy Diagram of n-Type Semiconductor
The Energy diagram of the n-type
semiconductor is shown in the figure below.A
large number of free electrons are available in
the conduction band because of the addition of
the Pentavalent impurity. These electrons are
free electrons which did not fit in the covalent
bonds of the crystal.However, a minute quantity
of free electrons is available in the conduction
band forming hole- electron pairs.
The following points are important in the n-
type semiconductor.
 The addition of Pentavalent impurity results in a
large number of free electrons.
 When thermal energy at room temperature is
imparted to the semiconductor, a hole-electron
pair is generated and as a result, a minute
quantity of free electrons are available. These
electrons leave behind holes in the valence
band.
 Here n stands for negative material as the
number of free electrons provided by the

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Conduction Through n-Type Semiconductor
In the n-type semiconductor, a large number of free electrons are available
in the conduction band which are donated by the impurity atoms. The figure
below shows the conduction process of an n-type semiconductor.
When a potential difference is applied across this type of semiconductor,
the free electrons are directed towards the positive terminals. It carries an
electric current. As the flow of current through the crystal is constituted
by free electrons which are carriers of negative charge, therefore, this
type of conductivity is known as negative or n-type conductivity.
The electron-hole pairs are formed at room temperature. These holes which
are available in small quantity in valence band also consists of a small amount
of current. For practical purposes, this current is neglected.

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32. A P-N Junction Diode is formed by doping one side of a piece of
silicon with a P-type dopant (Boron) and the other side with a N-type
dopant (phosphorus).Ge can be used instead of Silicon. The P-N
junction diode is a two-terminal device. This is the basic construction
of the P-N junction diode. It is one of the simplest semiconductor
devices as it allows current to flow in only one direction. The diode
does not behave linearly with respect to the applied voltage, and it
has an exponential V-I relationship.
What is a P-N junction Diode?
A P-N junction diode is a piece of silicon that has two terminals. One
of the terminals is doped with P-type material and the other with N-
type material. 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.

33. When the N-type semiconductor and P-type semiconductor materials are
first joined together a very large density gradient exists between both
sides of the PN junction. The result is that some of the free electrons from
the donor impurity atoms begin to migrate across this newly formed junction
to fill up the holes in the P-type material producing negative ions.
However, because the electrons have moved across the PN junction from the
N-type silicon to the P-type silicon, they leave behind positively charged
donor ions ( ND ) on the negative side and now the holes from the acceptor
impurity migrate across the junction in the opposite direction into the region
where there are large numbers of free electrons.
As a result, the charge density of the P-type along the junction is filled with
negatively charged acceptor ions ( NA ), and the charge density of the N-
type along the junction becomes positive. This charge transfer of electrons
and holes across the PN junction is known as diffusion. The width of these P
and N layers depends on how heavily each side is doped with acceptor
density NA, and donor density ND, respectively.

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This process continues back and forth until the
number of electrons which have crossed the
junction have a large enough electrical charge to
repel or prevent any more charge carriers from
crossing over the junction. Eventually a state of
equilibrium (electrically neutral situation) will occur
producing a “potential barrier” zone around the area
of the junction as the donor atoms repel the holes
and the acceptor atoms repel the electrons.
Since no free charge carriers can rest in a position
where there is a potential barrier, the regions on
either sides of the junction now become completely
depleted of any more free carriers in comparison to
the N and P type materials further away from the
junction. This area around the PN Junction is now
called the Depletion Layer.

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Typically at room temperature the voltage across
the depletion layer for silicon is about 0.6 – 0.7 volts
and for germanium is about 0.3 – 0.35 volts. This
potential barrier will always exist even if the device
is not connected to any external power source, as
seen in diodes.
The significance of this built-in potential across the
junction, is that it opposes both the flow of holes
and electrons across the junction and is why it is
called the potential barrier. In practice, a PN
junction is formed within a single crystal of material
rather than just simply joining or fusing together
two separate pieces

36. Junction diode characteristics

37. 37
Forward Biased PN Junction Diode
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-
type material and a positive voltage is applied to the P-type material. If this external voltage
becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3
volts for germanium, the potential barriers opposition will be overcome and current will start to
flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them
the energy to cross over and combine with the holes being pushed in the opposite direction
towards the junction by the positive voltage. This results in a characteristics curve of zero
current flowing up to this voltage point, called the “knee” on the static curves and then a high
current flow through the diode with little increase in the external voltage

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Forward Characteristics Curve for a Junction Diode
The application of a forward biasing voltage on the junction diode results
in the depletion layer becoming very thin and narrow which represents a
low impedance path through the junction thereby allowing high currents
to flow. The point at which this sudden increase in current takes place is
represented on the static I-V characteristics curve above as the “knee”
point.

39. REDUCTION OF POTENTIAL BARRIER
This condition represents the low resistance path through the PN junction allowing very large
currents to flow through the diode with only a small increase in bias voltage. The actual
potential difference across the junction or diode is kept constant by the action of the
depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction
diodes.
Since the diode can conduct “infinite” current above this knee point as it effectively becomes a
short circuit, therefore resistors are used in series with the diode to limit its current flow.
Exceeding its maximum forward current specification causes the device to dissipate more
power in the form of heat than it was designed for resulting in a very quick failure of the
device.

40. Reverse Biased PN Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type
material and a negative voltage is applied to the P-type material.
The positive voltage applied to the N-type material attracts electrons towards the positive
electrode and away from the junction, while the holes in the P-type end are also attracted away
from the junction towards the negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and
presents a high impedance path, almost an insulator and a high potential barrier is created across
the junction thus preventing current from flowing through the semiconductor material.

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Reverse Characteristics Curve for a Junction Diode
Sometimes this avalanche effect has practical applications in
voltage stabilising circuits where a series limiting resistor is used
with the diode to limit this reverse breakdown current to a
preset maximum value thereby producing a fixed voltage output
across the diode.

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Increase in the Depletion Layer due to Reverse Bias
This condition represents a high resistance value to the PN junction and practically zero current
flows through the junction diode with an increase in bias voltage. However, a very small reverse
leakage current does flow through the junction which can normally be measured in micro-amperes,
( μA ).
One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high
enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect
around the junction. This may cause the diode to become shorted and will result in the flow of
maximum circuit current, and this shown as a steep downward slope in the reverse static
characteristics curve .

43. 43
Reverse breakdown
The high reverse-bias voltage gives enough energy to the free
minority electrons, so that as they move through the p region,
they collide with atoms and knock valence electrons out of
orbit and into the conduction band.
Now, these electrons that were knocked out from their orbit
become conduction electrons.
They are also high in energy and so they repeat this process
of colliding with atoms that results into multiplication of
conduction electrons.
Because these electrons possess high energy, after they
cross the depletion region, they don’t combine with the
minority holes but go through the n region as conduction
electrons.
The multiplication of conduction electrons causes the reverse
current to increase drastically. If the reverse current is not
limited, this might cause damage to the diode.

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In electronics, Rectifier circuit is the most used circuit
because almost every electronic appliance operates
on DC (Direct Current) but the availability of the DC
Sources are limited such as electrical outlets in our
homes provide AC (Alternating current). The rectifier is
the perfect candidate for this job in industries & Home
to convert AC into DC. Even our cell phone chargers use
rectifiers to convert the AC from our home outlets
to DC. Different types of Rectifiers are used for
specific applications.
RECTIFIER

45. 45
A p-n junction diode conducts current only when it is forward biased.
The same principle is made use of in a half wave rectifier to convert AC
to DC.
• The input we give here is an alternating current. This input voltage is
stepped down using a transformer.
• The reduced voltage is fed to the diode ‘D’ and load resistance RL.
• During the positive half cycles of the input wave, the diode ‘D’ will be
forward biased and during the negative half cycles of input wave, the
diode ‘D’ will be reverse biased. We take the output across load
resistor RL.
• Since the diode passes current only during one-half cycle of the input
wave, we get an output as shown in the diagram.
• The output is positive and significant during the positive half cycles of
the input wave.
• At the same time output is zero or insignificant during negative half
cycles of the input wave. This is called half wave rectification.

47. Disadvantages of Half wave rectifier
1. The output current in the load contains, in addition to dc component,
ac components of basic frequency equal to that of the input voltage
frequency. Ripple factor is high and an elaborate filtering is, therefore,
required to give steady dc output.
2. The power output and, therefore, rectification efficiency is quite low.
This is due to the fact that power is delivered only during one-half cycle
of the input alternating voltage.
3. Transformer utilization factor is low.
4. DC saturation of the transformer core resulting in magnetizing
current and hysteresis losses and generation of harmonics.

48. 48

49. • We apply an AC voltage to the input transformer.
• During the positive half-cycle of the AC voltage,
terminal 1 will be positive, centre-tap will be at
zero potential and terminal 2 will be negative
potential.
• This will lead to forward bias in diode D1 and
cause current to flow through it. During this
time, diode D2 is in reverse bias and will block
current through it.
• During the negative half-cycle of the input AC
voltage, terminal 2 will become positive with
relative to terminal 2 and centre-tap.
• This will lead to forward bias in diode D2 and
cause current to flow through it. During this
time, diode D1 is in reverse bias and will block
current through it.

51.  Output Waveforms

52. 52

53. 53

54. A photodiode is one type of light
detector, used to convert the light
into current or voltage based on the
mode of operation of the device. It
comprises of optical filters, built-in
lenses and also surface areas. These
diodes have a slow response time
when the surface area of the
photodiode increases. Photodiodes
are alike to regular semiconductor
diodes, but that they may be either
visible to let light reach the delicate
part of the device.

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Dark Resistance of Photodiode
It is true that there are always some minority charge
carriers in the semiconductor crystal even in extreme dark
condition — these minority charge carriers in the
semiconductor crystal present due to unavoidable impurities
and natural thermal excitation of the crystal. So even in
dark condition, there would be a tiny and constant reverse
saturation current in the diode. This current is fixed for a
photodiode, and the current is known as dark current. The
ratio of maximum withstandable reverse voltage to the dark
current of a photodiode is called dark resistance of that
diode.
When we apply light to the diode, the reverse current
increase. This relation is linear. The value of reverse current
is directly proportional to the intensity of incident light
energy.
If we go on increasing the light intensity, after a certain
value of reverse current. The current will not increase
further with increasing light intensity. We call this maximum
value of reverse current as saturation current of the
photodiode.

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57. What is a Light Emitting Diode?
The lighting emitting diode is a p-n junction diode. It
is a specially doped diode and made up of a special
type of semiconductors. When the light emits in the
forward biased, then it is called as a light emitting
diode.
How does the Light Emitting Diode work?
The light emitting diode simply, we know as a diode.
When the diode is forward biased, then the
electrons & holes are moving fast across the
junction and they are combining constantly, removing
one another out. Soon after the electrons are
moving from the n-type to the p-type silicon, it
combines with the holes, then it disappears. Hence it
makes the complete atom & more stable and it gives
the little burst of energy in the form of a tiny
packet or photon of light.

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Types of Light Emitting Diodes
There are different types of light emitting
diodes present and some of them are
mentioned below.
•Gallium Arsenide (GaAs) – infra-red
•Gallium Arsenide Phosphide (GaAsP) – red to
infra-red, orange
•Aluminium Gallium Arsenide Phosphide
(AlGaAsP) – high-brightness red, orange-red,
orange, and yellow
•Gallium Phosphide (GaP) – red, yellow and
green
•Aluminium Gallium Phosphide (AlGaP) – green
•Gallium Nitride (GaN) – green, emerald green
•Gallium Indium Nitride (GaInN) – near
ultraviolet, bluish-green and blue
•Silicon Carbide (SiC) – blue as a substrate
•Zinc Selenide (ZnSe) – blue
•Aluminium Gallium Nitride (AlGaN) –
ultraviolet

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Working Principle of LED
The working principle of the Light emitting diode is
based on the quantum theory. The quantum theory
says that when the electron comes down from the
higher energy level to the lower energy level then,
the energy emits from the photon. The photon
energy is equal to the energy gap between these two
energy levels. If the PN-junction diode is in the
forward biased, then the current flows through the
diode.
There are different types of light emitting diodes
are available in the market and there are different
LED characteristics which include the color light, or
wavelength radiation, light intensity. The important
characteristic of the LED is color. In the starting
use of LED, there is the only red color. As the use
of LED is increased with the help of the
semiconductor process and doing the research on
the new metals for LED, the different colors were
formed.

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What is a Solar Cell?
A solar cell (also known as a photovoltaic cell or PV
cell) is defined as an electrical device that converts
light energy into electrical energy through the
photovoltaic effect. A solar cell is basically a p-n
junction diode. Solar cells are a form of
photoelectric cell, defined as a device whose
electrical characteristics – such as current, voltage,
or resistance – vary when exposed to light.
Individual solar cells can be combined to form
modules commonly known as solar panels. The
common single junction silicon solar cell can produce
a maximum open-circuit voltage of approximately 0.5
to 0.6 volts. By itself this isn’t much – but remember
these solar cells are tiny. When combined into a
large solar panel, considerable amounts of renewable
energy can be generated

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Construction of Solar Cell
A solar cell is basically a junction diode, although its construction it is little bit different
from conventional p-n junction diodes. A very thin layer of p-type semiconductor is grown
on a relatively thicker n-type semiconductor. We then apply a few finer electrodes on the
top of the p-type semiconductor layer.
These electrodes do not obstruct light to reach the thin p-type layer. Just below the p-
type layer there is a p-n junction. We also provide a current collecting electrode at the
bottom of the n-type layer. We encapsulate the entire assembly by thin glass to protect
the solar cell from any mechanical shock.

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Working Principle of Solar Cell
When light reaches the p-n junction, the light photons can
easily enter in the junction, through very thin p-type layer.
The light energy, in the form of photons, supplies sufficient
energy to the junction to create a number of electron-hole
pairs. The incident light breaks the thermal equilibrium
condition of the junction. The free electrons in the depletion
region can quickly come to the n-type side of the junction.
Similarly, the holes in the depletion can quickly come to the
p-type side of the junction. Once, the newly created free
electrons come to the n-type side, cannot further cross the
junction because of barrier potential of the junction.
Similarly, the newly created holes once come to the p-type
side cannot further cross the junction became of same
barrier potential of the junction. As the concentration of
electrons becomes higher in one side, i.e. n-type side of the
junction and concentration of holes becomes more in another
side, i.e. the p-type side of the junction, the p-n junction will
behave like a small battery cell. A voltage is set up which is
known as photo voltage. If we connect a small load across the
junction, there will be a tiny current flowing through it.

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V-I Characteristics of a Photovoltaic Cell

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Materials Used in Solar Cell
The materials which are used for this purpose must have band gap close to 1.5ev. Commonly used
materials are-
1. Silicon.
2. GaAs.
3. CdTe.
4. CuInSe2
Criteria for Materials to be Used in Solar Cell
1. Must have band gap from 1ev to 1.8ev.
2. It must have high optical absorption.
3. It must have high electrical conductivity.
4. The raw material must be available in abundance and the cost of the material must be low.
Advantages of Solar Cell
1. No pollution associated with it.
2. It must last for a long time.
3. No maintenance cost.
Disadvantages of Solar Cell
1. It has high cost of installation.
2. It has low efficiency.
3. During cloudy day or at night the energy cannot be produced.

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Compiled by: Dharmendra Goswami