1. Class 12 th Physics
GGIC Vijay Nagar
Semiconductor Part 4
2. Application of Junction Diode as a Rectifier
Rectifier is an electronic circuit which converts alternating current into fluctuating direct
current i.e., unidirectional current. This action of rectifier is known as rectification.
The resistance of a p-n junction diode becomes low when forward biased and becomes high
when reverse biased.
The fluctuating direct current can be converted into steady direct current by another circuit
(known as filter).
Rectifiers are of two types
(1) Half wave rectifier
(2) Full wave rectifier
Half wave rectifier
Principle:- converts the AC voltage signal into the DC voltage. The half wave rectifier only passes the
one half of the input sine wave (either positive or negative) and rejects the other half. The output of
the half wave rectifier is pulsating DC.
•Transformer with primary and secondary coils ( step-down transformer) ):-- A step-down transformer
is a type of transformer that converts the high voltage (HV) and low current from the primary side of
the transformer to the low voltage (LV) and high current value on the secondary side of the
transformer. The reverse of this is known as a step up transformer.
3. Diode :-- A diode is a device that acts like a conductor
since. it allows current to pass in one direction (known as
FORWARD BIASING) and it acts as an insulator since it
blocks current passing in the opposite direction (known as
•Load resistance RL :-- Th e load resistance or, more
generally, the load impedance (in AC circuits) is the
equivalent resistance/impedance of the device attached to
the output of a given circuit or system.
•Alternating Current Source :--The voltage source
which can produce or deliver alternating voltage as
output is termed as Alternating Voltage Source. Here,
the polarity gets reversed at regular intervals. This
voltage causes the current to flow in a direction for a
time and after that in a different direction for another
5. Circuit diagram
# During the positive half cycle of the input AC voltage, suppose P1 is negative and P2 is positive
# On account of inductance, S1 becomes positive and S2 becomes negative
# The p-n junction is forward biased and hence the resistance of the p-n junction diode becomes
# Hence, current flows in the circuit and we get output across the load resistance RL
# During the negative half cycle of the input AC voltage, suppose P1 is positive and P2 is negative
# On account of inductance, S1 becomes negative and S2 becomes positive
# The p-n junction is reverse biased and hence the resistance of the p-n junction diode becomes
# Hence, no current flows in the circuit and we do not get any output across the load resistance RL
6. Full wave rectifier
Principle:- Converts both positive and negative half cycles of the input AC signal into output
pulsating DC signal.
# Transformer with primary and secondary coils( step-down transformer):-- A step-
down transformer is a type of transformer that converts the high voltage (HV) and low
current from the primary side of the transformer to the low voltage (LV) and high
current value on the secondary side of the transformer. The reverse of this is known as
a step up transformer.
# Diode( two diodes – D1 and D2):-- A diode is a device that acts like a conductor since. it
allows current to pass in one direction (known as FORWARD BIASING) and it acts as an insulator
since it blocks current passing in the opposite direction (known as REVERSE BIASING).
# Load resistance RL :-- Th e load resistance or, more generally, the load impedance (in AC
circuits) is the equivalent resistance/impedance of the device attached to the output of a given
circuit or system.
7. Alternating Current Source :--The voltage source which can produce or deliver
alternating voltage as output is termed as Alternating Voltage Source. Here, the
polarity gets reversed at regular intervals. This voltage causes the current to flow in a
direction for a time and after that in a different direction for another time.
8. Circuit Diagram:-
# During the positive half of the input cycle of AC voltage, the junction diode D1 is forward biased
as shown in the diagram above.
# Hence, current flows in the above circuit as indicated.
# The diode D2 is reverse biased and hence no current due to D2.
# We get output when the same is measured across the load resistance R due to the diode D1
9. Case 2 The circuit diagram for the negative half of the input cycle of AC voltage:
# During the positive half of the input cycle of AC voltage, the junction diode D2 is forward biased
as shown in the diagram above
# Hence, current flows in the above circuit as indicated
# The diode D1 is reverse biased and hence no current due to D1
# We get output when the same is measured across the load resistance R due to the diode D2
We observe that one of the diode conducts and the flow of current across the load resistance is in
the same direction Also, current flows during both cycles of the input AC voltage. However, the
output though unidirectional has ripple contents. Ripple contents indicate both AC and DC
10. 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.
Individual solar cells can be combined to form modules commonly known as solar
A solar cell is basically a p-n junction which generates emf when solar radiation
falls on the p-n junction.
It works on the same principle (photovoltaic effect) as the photodiode, except that
no external bias is applied and the junction area is kept much larger for solar
radiation to be incident because we are interested in more power.
Solar cell:-It is a device which converts solar energy into electrical
energy. Basically solar cell is a specially designed p-n junction in which
electromotive force is produced if solar radiations fall on it. This
electromotive force is known as photovoltaic e.m.f. and this effect is
known as photovoltaic effect.
Principle :–These photo voltaic devices convert the optical
radiation into electricity.
11. Circuit :-
# Photovoltaic e.m.f.:- When solar light falls on a p-n junction, it generates emf
# PN Junction:-As the solar radiation is incident at the junction, the junction area is kept much
larger for more power generation.
# Back contact:- The other side of the p-layer is coated with a metal. This servers as a back
# Front contact :-On the top of n Si layer, metallic grid is deposited. This is called front contact.
The light is incident on the grid from the top.
12. Working :-
The generation of emf by the solar cell, when light falls on, is
due to the following three basic processes –
(a) generation (b) separation and (c) collection
# The generation of electron-hole paid due to light with energy h>ט Eg close to the junction
# The separation of electrons and holes due to the electric field of the depletion region
# The electrons are swept to the n-side and the holes to the p-side
# The electrons reaching the n-side are collected by the front contact and holes reaching the p-
side are collected by the back contact
# Thus, the p-side becomes positive and the n-side becomes negative giving rise to photo
# When external load is connected, a photo current IL flows through the load
13. 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.
14. 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.Availability of raw material
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, the energy cannot be produced and also at night we will not get solar energy
Uses of Solar Generation Systems
1.It may be used to charge batteries.
2.Used in light meters.
3.It is used to power calculators and wrist watches.
4.It can be used in spacecraft to provide electrical energy.
# The graph showing the VI characteristics, with V along the X-axis and I along the Y-axis is as given above
# The graph is indicated in the fourth quadrant as solar cell does not draw current but supplies the same to
# Solar cells are used in power electronic devices in satellites and space vehicles
# They are also used as power supply in calculators
Note: Although principle of solar cell is similar to photo diode but its construction and uses
are entirely different from photo diode.
16. Light Emitting Diode or L.E.D. : - It is a specially designed p-n junction diode which emits light when
operated in forward bias. A light-emitting diode (LED) is a semiconductor light source that emits
light when current flows through it. Electrons in the semiconductor recombine with electron holes,
releasing energy in the form of photons.
Principle:– convert electrical energy into light energy
# One of the methods used to construct LED is to deposit three semiconductor layers on the substrate.
# The three semiconductor layers deposited on the substrate are n-type semiconductor ,p -type semiconductor
and active region.
# Active region is present in between the n-type and p-type semiconductor layers.
# In LED, most of the charge carriers recombine at active region.
# Therefore, most of the light is emitted by the active region.
# The active region is also called as depletion region.
# Light Emitting Diode (LED) works only in forward bias condition.
# When Light Emitting Diode (LED) is forward biased, the free electrons from n-side and the
holes from p-side are pushed towards the junction.
# When free electrons reach the junction or depletion region, some of the free electrons
recombine with the holes in the positive ions.
# We know that positive ions have less number of electrons than protons.
# Thus, free electrons recombine with holes in the depletion region.
# In the similar way, holes from p-side recombine with electrons in the depletion region.
# some free electrons from n-type semiconductor cross the p-n junction and recombines
with holes in p-type semiconductor.
# In the similar way, holes from p-type semiconductor cross the p-n junction and recombines
with free electrons in the n-type semiconductor.
# Thus, recombination takes place in depletion region as well as in p-type and n-type semiconductor.
# The free electrons in the conduction band releases energy in the form of light before
they recombine with holes in the valence band.
# In silicon and germanium diodes, most of the energy is released in the form
of heat and emitted light is too small.
# However, in materials like gallium arsenide and gallium phosphide the emitted
Photons have sufficient energy to produce intense visible light.
19. The colour of the light depends upon the types of material used in making the semiconductor diode.
(i) Gallium – Arsenide (Ga-As) – Infrared radiation
(ii) Gallium – phosphide (GaP) – Red or green light
(iii) Gallium – Arsenide – phosphide (GaAsP) – Red or yellow light
Output characteristics of LED:-
The amount of output light emitted by the LED is directly proportional to the amount of
forward current flowing through the LED. More the forward current, the greater is the emitted
output light. The graph of forward current vs output light is shown in the figure.
Uses of LED:-
Used for tv backlighting
Uses in displays
Used in automotive
LEDs used in the dimming of lights
Types of LED
Bi and Tri-Colour
Red Green Blue LEDs
Longevity- 50,000 hours or more if properly engineered
They light up faster – LED’s light instantly – in nanoseconds
Cost Effectiveness- LEDs are more cost-effective than normal lights.
Durability- LEDs is that they would last long as they are robust.
Less Risk of Injury- LED Lights are much cooler, and a result hence does not involve unfortunate incidents
Colour - LED Lights can come in various colors
Eco-friendly - LED’s contain no mercury or other hazardous substances
Energy Efficient- LED’s are now capable of outputting 135 lumens/watt
Flexible Design- LED lights, unlike regular bulbs, come in all shapes and sizes, making them exceptionally
Directional – With LED’s you can direct the light where you want it, thus no light is wasted
High initial cost- they are expensive.
Temperature Sensitive- LED Lights are very temperature dependent on their environment.
Blue Light Pollution- LEDs tend to emit proportionately more blue light than the light of any other color.
Consume more power- The LED consume more power as compared to LCD, and their cost is high. Also, it
is not used for making the large display.
Light Distribution- Another drawback to LEDs is that they do not give a spherical
distribution of light
21. Photodiode :--The photodiode is a kind of pn junction semiconductor diode which
works with the intensity of light falling on it at the reverse biased condition.
A special type of PN junction device that generates current when exposed to light is known as
Photodiode. It is also known as photodetector or photosensor. It operates in reverse biased
mode and converts light energy into electrical energy.
Principle :--The working principle of a photodiode is, when a photon of ample energy strikes the
diode, it makes a couple of an electron-hole. This mechanism is also called as the inner
photoelectric effect. It operates in reverse biased mode and converts light energy into
Photodiode basically operates in two modes:
•Photovoltaic mode: It is also known as zero-bias mode because no external
reverse potential is provided to the device. However, the flow of minority carrier
will take place when the device is exposed to light.
•Photoconductive mode: When a certain reverse potential is applied to the device
then it behaves as a photoconductive device. Here, an increase in depletion width
is seen with the corresponding change in reverse voltage.
22. Photodiode Construction
The surface of a layer of N type is bombarded with P type silicon ions to produce a P type layer
about 1 µm (micrometer) thick. During the formation of the diode, electrons from the N type layer
are attracted into the P type material and holes from the P type are attracted into the N type layer,
resulting in the removal of free charge carriers close to the PN junction, so creating a depletion
The (light facing) top of the diode is protected by a layer of Silicon Dioxide (SiO2) in which there is
a window for light to shine on the semiconductor. This window is coated with a thin anti-reflective
layer of Silicon Nitride (SiN) to allow maximum absorption of light and an anode connection of
aluminium (Al) is provided to the P type layer. The N type layer is a more heavily doped N+ layer to
provide a low resistance connection to the cathode.
23. The front area of the diode is divided into two types that are active surface and
non-active surface. The non-active surface is made up of SiO2 (Silicon di
Oxide) and the active surface is coated with anti-reflection material. The active
surface is called so because the light rays are incident on it.
While on the non-active surface the light rays do not strike. The active layer is
coated with anti-reflection material so that the light energy is not lost and the
maximum of it can be converted into current. The entire unit has dimensions of the
order of 2.5 mm.
24. When light falls on PN junction, it is absorbed by the junction. This will generate more electron-
hole pairs. Or we can say, characteristically, the amount of reverse current increases.
In other words, as the intensity of falling light increases, resistance of the PN junction diode
Working of Photodiode
When a diode is in reverse biased condition, there would be a
reverse saturation current flowing through it from positive to
the negative terminal of the diode. The unavoidable minority
charge carriers cause this reverse saturation current in the
semiconductor crystal. The value of this reverse saturation
current does not depend on the applied reverse voltage across
the diode rather it depends on the concentration of minority
charge carriers in the semiconductor crystal. Hence for a
certain range of reverse voltage across the diode, this current
remains almost constant. We can control the reverse saturation
current in a diode by controlling the concentration of minority
charge carriers in the semiconductor crystal. We can change
the concentration of minority charge carriers in .
semiconductor by supplying external energy to the crystal.
26. When the conventional diode is reverse biased, the depletion region starts
expanding and the current starts flowing due to minority charge carriers. With the
increase of reverse voltage, the reverse current also starts increasing. The same
condition can be obtained in Photodiode without applying reverse voltage.
The junction of Photodiode is
illuminated by the light source, the
photons strike the junction surface. The
photons impart their energy in the form
of light to the junction. Due to which
electrons from valence band get the
energy to jump into the conduction band
and contribute to current. In this way,
the photodiode converts light energy
into electrical energy.
The current which flows in photodiode before light rays are incident on it is
called dark current. As leakage current flows in the conventional diode, similarly
the dark current flows in the photodiode.
27. •PN Photodiode
•Schottky Photo Diode
Advantages of Photodiode
•It shows a quick response when exposed to light.
•Photodiode offers high operational speed.
•It provides a linear response.
•It is a low-cost device.
Disadvantages of Photodiode
•It is a temperature-dependent device. And shows poor temperature stability.
•When low illumination is provided, then amplification is necessary.
Applications of Photodiode
1.Photodiodes majorly find its use in counters and switching circuits.
2.Photodiodes are extensively used in an optical communication system.
3.Logic circuits and encoders also make use of photodiode.
4.It is widely used in burglar alarm systems.
Types of Photodiode
28. V-I Characteristics of Photodiode
The characteristics curve of the photodiode can be understood with the help of the
below diagram. The characteristics are shown in the negative region because the
photodiode can be operated in reverse biased mode only.