CONTENTS

i) Introduction

ii) p-Type Semiconductor

iii) n-Type Semiconductor

iv) Formation of p-n Junction

v) Forward biasing in p-n Junction

vi) Reverse Biasing in p-n junction

vii) Uses of p-n Junction

viii) Limitations of p-n Junction

ix) Conclusion
X) Bibliography
 INTRODUCTION
Semiconductor: materials are a class of
materials with electrical conductivity between
conductors and insulators. Key features include a
crystalline structure, a band gap that allows
control of electrical properties, and the ability to
be doped for specific conductivity modifications.
Silicon and germanium are common
semiconductors, and advancements in
semiconductor technology underpin modern
electronics, from transistors and integrated
circuits to solar cells and LEDs.
A p-n junction is a boundary or interface
between two
semiconductor materials—one doped with
positive charge
carriers (p-type) and the other with negative
charge carriers (n-type). This junction forms the
basis of many electronic
 components. Under forward bias, it allows
current flow, while under reverse bias, it acts as
a barrier. P-n junctions are
fundamental to the operation of diodes,
transistors, and other semiconductor devices in
electronic circuits.

P-Type Semiconductor
 Here, a trivalent atom from group III elements
such as Boron,
Aluminium, Gallium and Indium is added to the
Germanium or Silicon substrate.
 As Germanium atom has four valence electrons,
one electron
position of the dopant in the Germanium crystal
lattice will
remain vacant. The missing electron position in
the covalent
bond is denoted as a hole.
 To make complete covalent bonding with all four
neighbouring atoms, the dopant is in need of one
more electron. These dopants can accept
electrons from the neighbouring atoms.
 Therefore, this impurity is called an acceptor
impurity.
 The energy level of the hole created by each
impurity atom is
just above the valence band and is called the
acceptor-energy level.
 For each acceptor atom, there will be a hole in
the valence band in addition to the thermally
generated holes. In such an
extrinsic semiconductor, holes are the majority
carriers and
thermally generated electrons are minority
carriers. The
semiconductor thus formed is called a p – type
semiconductor.

N-Type Semiconductor
 A n-type semiconductor is obtained by doping a
pure
Germanium (or Silicon) crystal with a dopant
from group V
Penta valet elements like Phosphorus, Arsenic,
and Antimony.
 The dopant has five valence electrons while the
Germanium
atom has four valence electrons. During the
process of doping, a few of the Germanium
atoms are replaced by the group V dopants.
 Four of the five valence electrons of the impurity
atom are
bound with the 4 valence electrons of the
neighboring replaced Germanium atom. The fifth
valence electron of the impurity atom will be
loosely attached with the nucleus as it has not
formed the covalent bond.
 The energy level of the loosely attached fifth
electron from the dopant is found just below the
conduction band edge and is called the donor
energy level.
 The group V pentavalent impurity atoms donate
electrons to the conduction band and are called
donor impurities. Therefore, each impurity atom
provides one extra electron to the conduction
band in addition to the thermally generated
electrons.
 These thermally generated electrons leave holes
in valence band. Hence, the majority carriers of
current in an n-type
Semiconductor are electrons and the minority
carriers are holes.
 Such a semiconductor doped with a pentavalent
impurity is
called an n-type semiconductor.

Formation of p-n
Junction
 A p-n junction is a single crystal of Silicon or
Germanium
doped in such a manner that one half of it acts
as a p-type
semiconductor while the other half acts as n-
type
semiconductor.
 Two processes occur during the formation of
semiconductors, namely diffusion and drift.
 As soon as the p-n junction is formed, due to
concentration
 gradient, holes diffuse from p to n side and
electrons diffuse
from n to p side leaving behind acceptor ions on
p-side and
donor ions on n-side which are immobile.
 This diffusion of majority charge carriers across
the junction
gives rise to an electric current from p to n side
known as
diffusion current.
 The small region in the vicinity of the junction
which is depleted of free charge carriers and has
only immobile ions is called depletion layer.
 The accumulation of negative charges in p-region
and positive charges in n-region sets up a
potential difference across the junction which
acts as a barrier and opposes further diffusion of
electrons. This barrier is known as potential
barrier.
 The barrier potential sets up a barrier field from n
to p side
which pushes the electrons towards n-side and
holes towards p-side. This current set up by
barrier field from n to p side is known as drift
current.
 In equilibrium state, diffusion current is equal to
drift current.
 Forward Biasing of p-n Junction
 When we apply the external voltage across the
semiconductor diode in such a way that the p-
side is connected to the positive terminal of the
battery and the n-side is connected to the
negative terminal, then the semiconductor diode
is said to be forward-biased.
 In this case, the built-in potential of the diode
and thus the width of the depletion region
decreases, and the height of the barrier gets
reduced.
 Initially current increases slowly, almost
negligibly, till voltage crosses a certain value
called threshold or cut-in voltage.
 After cut-in voltage current increases rapidly for
small increase in bias voltage. Resistance across
the junction is quite low.
 Reverse Biasing of p-n Junction
 When we apply the external voltage across the
semiconductor diode in such a way that the
positive terminal of the battery is connected to
its n-side and the negative terminal of the
battery is connected to the p-side of the diode,
then it is said to be in the condition of reverse
bias.
 When the diode is reverse biased, the reverse
bias voltage
produces a very small current(in microamperes)
which almost remains constant with bias. This
small current is called reverse saturation current,
which is due to the drift of minority charge
carries.
 When reverse bias voltage reaches a sufficiently
high value,
 reverse current suddenly increases to a large
value. This voltage at which breakdown of diode
occurs is known as breakdown voltage or peak
inverse voltage.

Uses of p-n Junction

1) Rectifiers
The main application of p-n junction diode is in
rectification
circuits. These circuits are used to describe the
conversion of a.c signals to d.c in power supplies.
Diode rectifier gives an
alternating voltage which pulsates in accordance
with time. The filter smooth’s the pulsation in the
voltage and to produce d.c voltage, a regulator is
used which removes the ripples.
a) Half Wave Rectifier
 In a half-wave rectifier, one half of each a.c input
cycle is
rectified. When the p-n junction diode is forward
biased, it
gives little resistance and when it is reversed
biased it
provides high resistance. During one-half cycles,
the diode is
forward biased when the input voltage is applied
and in the
opposite half cycle, it is reverse biased. During
alternate half
cycles, the optimum result can be obtained.
b) Full Wave Rectifier
The full-wave rectifier utilizes both halves of
each a.c input.
When the p-n junction is forward biased, the
diode offers low resistance and when it is reverse
biased it gives high
resistance. The circuit is designed in such a
manner that in the first half cycle if the diode is
forward biased then in the
second half cycle it is reverse biased and so on.
2) Zener Diode
3) A Zener diode is a heavily doped
semiconductor device that is designed to
 4) operate in the reverse direction. A Zener
Diode, also known as a breakdown
5) diode, is a heavily doped semiconductor
device that is designed to operate in
6) A Zener diode is a heavily doped
semiconductor device that is designed to
7) operate in the reverse direction. A Zener
Diode, also known as a breakdown
8) diode, is a heavily doped semiconductor
device that is designed to operate in
A Zener Diode, also referred to as a breakdown
diode, is a
specially doped semiconductor device
engineered to function in the reverse direction.
When the voltage across a Zener diode’s
terminals is reversed and reaches the Zener
Voltage (also known as the knee voltage), the
junction experiences a breakdown, allowing
current to flow in the opposite direction. This
phenomenon,
known as the Zener Effect, is a key
characteristic of Zener
diodes. When the input voltage is higher than
the Zener breakage voltage, the voltage across
the resistor drops resulting in a short circuit, this
can be avoided by using the Zener diode.
2) Photodiodes
 Photodiode is a PN-junction diode that
consumes light energy to produce an electric
current. They are also called a photo-detector, a
light detector, and a photo-sensor. Photodiodes
are designed to work in reverse bias condition.
Typical photodiode materials are Silicon,
Germanium and Indium gallium arsenide.
4)Light Emitting Diodes(LEDs)
A light-emitting diode (LED) is a semiconductor
device that emits light when an electric current
flows through it. When current passes through an
LED, the electrons recombine with holes emitting
light in the process. LEDs allow the current to
flow in the forward direction and blocks the
current in the reverse direction.
Light-emitting diodes are heavily doped p-n
junctions. Based on the semiconductor material
used and the amount of doping, an LED will emit
coloured light at a particular spectral wavelength
when forward biased. As shown in the figure, an
LED is encapsulated with a transparent cover so
that emitted light can come out.
 Limitations Of p-n
Junction
While p-n junctions are fundamental to the
operation of many electronic devices, they do
have certain limitations. Here are some key
limitations associated with p-n junctions:
 Temperature Sensitivity:
The electrical characteristics of p-n junctions are
temperature dependent. Changes in temperature
can affect the mobility of charge carriers and
alter the performance of semiconductor devices.
 Breakdown Voltage:
P-n junctions have a breakdown voltage, beyond
which the
junction can experience a sudden increase in
current flow,
potentially damaging the device. Special
precautions, such as adding protective
components, are necessary to prevent
breakdown
 Reverse Bias Leakage Current:
Even in reverse bias conditions, a small leakage
current may
 exist across the p-n junction. This leakage can
limit the
effectiveness of certain applications, particularly
in high
precision circuits.
 Cost of Manufacturing:
The fabrication of high-quality p-n junctions,
especially in
materials like silicon, can involve complex
processes,
contributing to the cost of manufacturing
semiconductor
devices.

Conclusion
In conclusion, p-n junctions represent a
foundational and versatile element in the realm
of semiconductor physics and electronic device
engineering. Through the intentional combination
of p-type and ntype semiconductors, these
junctions enable the controlled flow of electrical
current, forming the basis for a plethora of
electronic components.
The ability to manipulate the behavior of p-n
junctions through forward and reverse biasing
has paved the way for the development of
essential devices such as diodes, transistors, and
integrated circuits.
These devices, in turn, have become the
building blocks of modern electronics, influencing
various industries and technologies.
While p-n junctions offer remarkable
advantages, including their role in energy-
efficient light-emitting diodes (LEDs), solar cells,
and the precision control of electronic signals,
they are not without limitations. Temperature
sensitivity, breakdown voltage concerns, and
manufacturing challenges are among the factors
that engineers and
scientists continually strive to address.
In practical terms, the applications of p-n
junctions are extensive,
ranging from power supplies and communication
devices to sensors and renewable energy
technologies. As research advances and
technology evolves, the limitations of p-n
junctions are being mitigated, and innovations
are expanding the horizons of their applications.
In summary, the significance of p-n junctions in
the field of
electronics cannot be overstated. Their
fundamental role in shaping the landscape of
 modern technology underscores their
importance,
and ongoing research and development aim to
harness their potential while addressing existing
limitations.

Bibliography
o https://byjus.com/physics/
semiconductor-diode/
o https://en.wikipedia.org/wiki/P–
n_junction
o NCERT Textbook Class – XII
o NCERT Physics Lab Manual
o S.L. Arora

Acknowledgement For
Physics Project
I would like to express my sincere thanks and
gratitude to my physics teacher Mrs.Manju
jaiprakash for her sincere guidance and
advice to complete my project successfully.
Also, I am thankful to him for providing such
an interesting topic for our physics project.

I am also grateful to my parents and friends

for their constant support and help
throughout the project, without their
encouragement and support this project could
not have been completed on time.

Lastly, I would like to thank all the accessories

and every single person who helped me to
complete this physics project successfully.