Semiconductors

• Semiconductors are materials which have

a conductivity between conductors such as metals
and non-conductors or insulators like ceramics
• Semiconductors can be compounds, such as
gallium arsenide, or pure elements, such as
germanium or silicon
• Gallium arsenide, germanium and silicon are some
of the most commonly used semiconductors
• Silicon is used in electronic circuit fabrication, and
gallium arsenide is used in solar cells, laser diodes,
etc.
Charge carriers in semiconductors
• There are two types of charge carriers accountable for
the flow of current in semiconductors which are holes and
electrons
• Holes (missing electrons) are the positively charged electric
charge carriers, whereas electrons are the negatively
charged particles
• Both electrons and holes are equal in magnitude but
opposite in polarity
Mobility of Electrons and Holes
• In a semiconductor, the mobility of electrons is higher than
that of the holes. It is mainly because of their different band
structures
• Electrons travel in the conduction band, whereas holes travel
in the valence band
• When an electric field is applied, holes cannot move
as freely as electrons due to their restricted
movement
• The elevation of electrons from their inner shells to
higher shells results in the creation of holes in
semiconductors
• Since the holes experience stronger atomic force by
the nucleus than electrons, holes have lower mobility
• For intrinsic silicon at 300 K, the mobility of electrons
is 1500 cm2 (V∙s)-1, and the mobility of holes is 475
cm2 (V∙s)-1
• The bond model of electrons in silicon of valency 4 is shown below.
Here, when one of the free electrons (red dots) leaves the lattice
position, it creates a hole (blue dots)
• This hole thus created takes the opposite charge of the electron and
can be imagined as positive charge carriers moving in the lattice
Energy Band Diagram of Semiconductors
• We know that the electrons in an atom are present at different
energy levels. When we try to assemble a lattice of a solid with N
atoms, each level of an atom must split into N levels in the solid
• This splitting of sharp and tightly packed energy levels forms Energy
Bands. The gap between adjacent bands representing a range of
energies that possess no electron is called a Band Gap.
Fermi Level in Semiconductors
• The Fermi level (EF) is present between the valence and
conduction bands
• It is the highest occupied molecular orbital at absolute zero.
The charge carriers in this state have their own quantum
states and generally do not interact with each other
• When the temperature rises above absolute zero, these
charge carriers will begin to occupy states above the Fermi
level
• The energy of the electrons associated with the fermi level is
called fermi energy denoted by Ef
Some properties of semiconductors
• Semiconductors act like insulators at zero Kelvin. On increasing the
temperature, they work as conductors
• Due to their exceptional electrical properties, semiconductors can be
modified by doping to make semiconductor devices suitable for
energy conversion, switches and amplifiers
• Lesser power losses
• Smaller in size and possess less weight
• Their resistivity is higher than conductors but lesser than insulator
• The resistance of semiconductor materials decreases with an increase
in temperature and vice-versa
Types of Semiconductors
• There are two types of semiconductors
1. Intrinsic Semiconductor
2. Extrinsic Semiconductor
What is Doping ?
• The process in which an impurity is added to a pure type of
the material is known as doping
• Considering semiconductors, if we have pure silicon crystal,
then by adding some atoms of germanium into it makes it
doped or impure.
Negative temperature co-efficient of semiconductors
Why Does the Resistivity of Semiconductors Go Down with
Temperature?
• The resistivity of semiconductors decreases with
temperature because the number of charge carriers
increases rapidly with an increase in temperature, making
the fractional change, i.e., the temperature coefficient
negative
• The difference in resistivity between conductors and
semiconductors is due to their difference in charge carrier
density
Intrinsic Semiconductor
• The semiconductor in its extremely pure form is called
Intrinsic Semiconductor
• It is made up of only a single type of element
• Germanium (Ge) and silicon (Si) are the most common types
of intrinsic semiconductor elements
• They have four valence electrons (tetravalent). They are
bound to the atom by a covalent bond at absolute zero
temperature
Intrinsic Semiconductor
The Lattice of Pure Silicon Semiconductor at Different Temperatures
• At absolute zero Kelvin temperature: At this temperature, the covalent
bonds are very strong, there are no free electrons, and the semiconductor
behaves as a perfect insulator
• Above absolute temperature: With an increase in temperature, a few valence
electrons jump into the conduction band, and hence, it behaves like a poor
conductor
Extrinsic Semiconductor
• The conductivity of semiconductors can be greatly improved
by introducing a small number of suitable replacement
atoms called IMPURITIES
• The process of adding impurity atoms to the pure
semiconductor is called DOPING
• Usually, only 1 atom in 107 is replaced by a dopant atom in
the doped semiconductor
• Hence an impure semiconductor is called extrinsic
semiconductor
• Further, there are two types of extrinsic semiconductor
1. N-type Semiconductor
• When a pure semiconductor (Si or Ge) is doped by
pentavalent impurity (P, As, Sb, Bi), then four electrons out of
five valence electrons bond with the four electrons of Ge or
Si
• The fifth electron of the dopant is set free. Thus, the
impurity atom donates a free electron for conduction in the
lattice and is called a “Donar”
• Since the number of free electrons increases with the
addition of an impurity, the negative charge carriers
increase. Hence, it is called an n-type semiconductor
• Crystal as a whole is neutral, but the donor atom becomes
an immobile positive ion.
• As conduction is due to a large number of free electrons, the
electrons in the n-type semiconductor are the majority
carriers, and holes are the minority carriers.
2. P-Type Semiconductor
• When a pure semiconductor is doped with a trivalent
impurity (B, Al, In, Ga), then the three valence electrons of
the impurity bond with three of the four valence electrons of
the semiconductor
• This leaves an absence of electron (hole) in the impurity.
These impurity atoms which are ready to accept bonded
electrons are called “Acceptors”
• With an increase in the number of impurities, holes (the
positive charge carriers) are increased. Hence, it is called a p-
type semiconductor
• Crystal, as a whole, is neutral, but the acceptors become an
immobile negative ion
• As conduction is due to a large number of holes, the holes in
the p-type semiconductor are majority carriers, and
electrons are minority carriers
Difference between Intrinsic and Extrinsic
Semiconductors
Intrinsic Semiconductor Extrinsic Semiconductor

Pure semiconductor Impure semiconductor

The density of electrons is equal to the density of The density of electrons is not equal to the density
holes of holes

Electrical conductivity is low Electrical conductivity is high

Dependence on temperature only Dependence on temperature, as well as on the

amount of impurity

No impurities Trivalent impurity and pentavalent impurity

Applications of Semiconductors
• Their reliability, compactness, low cost and controlled
conduction of electricity make them ideal to be used for
various purposes in a wide range of components and devices
• Transistors, diodes, photosensors, microcontrollers,
integrated chips and much more are made up of
semiconductors
• Temperature sensors are made with semiconductor devices
• They are used in 3D printing machines
• Used in microchips and self-driving cars
• Used in calculators, solar plates, computers and other
electronic devices
• Transistors and MOSFET used as a switch in electrical circuits
are manufactured using semiconductors
• The physical and chemical properties of semiconductors
make them capable of designing technological wonders like
microchips, transistors, LEDs, solar cells, etc.
• The microprocessor used for controlling the operation of
space vehicles, trains, robots, etc., is made up of transistors
and other controlling devices, which are manufactured by
semiconductor materials
Introduction to diode
PN-Junction
• When a single crystal ( Si or Ge) is grown in such a way that
its one half becomes p-type and the other become n-type,
then the junction formed between them is known as pn-
junction
• The n-type region contains free electrons as majority charge
carriers and the p-type region contains holes as majority
charge carriers
• When the junction is formed, the electrons in the n-type
region diffuse into p-type region and vice versa
• Due to diffusion, a region is formed around the junction
where charge carriers are depleted. This region is known as
depletion region
• Due to charge on these ions, a potential difference develops
across the pn-junction which is known as potential barrier
• The value of potential barrier is 0.7 V for Si and 0.3 V for Ge
• This potential barrier stops further diffusion of electrons in
the p-region
What is a diode?
• A diode is a 2-terminal, basic electronic component, made
up of semiconductor material, which allows a unidirectional
flow of current through it, i.e it only conducts current in one
direction
• A diode is analogous to a uni-directional water flow valve,
which allows the water to flow in one direction but restricts
it to flow backward
• Diode consists of two terminals, named:
• Anode (+).
• Cathode (-)
• In a diode, current flows from Anode to Cathode(diode acts
as a closed switch), but if the current flows in the opposite
direction(i.e. from Cathode to Anode), the diode will block it,
so we can say, the diode is acting as an open switch
Biasing of Diode
What is biasing?
• To apply some potential difference across the terminals of
the diode is called biasing of a diode
• There are two types of biasing for a diode
1. Forward Biasing
2. Reverse Biasing
• The PN Junction created at the center of two regions is very
small but it's powerful enough to stop the free electrons
from passing through it
Forward biasing of a diode
• So, if we could provide some external power to these
electrons, they can break this barrier and can make their
entry into the P-Type region
• When the external potential is applied to a diode in a such a
way that its p-side is positive and the n-side is negative then
it is said to be forward biased
• The energy provided by the external potential to the
electrons and holes is sufficient to overcome the barrier
• So, a current of few mA starts flowing across the junction
and we can study the current & voltage relationship
Forward biasing of a diode
Reverse biasing of a diode
• When the external source of voltage is applied to a diode in
a such a way that its p-side is connected to the negative
terminal and the n-side is connected to positive terminal
then it is said to be reverse biased
• In this way, no current flows across the p-n junction due to
the majority charge carriers
• However, a very little current of few microampere flows due
to the minority charge carriers known as reverse current or
leakage current
• The relationship between reverse voltage and reverse
Voltage can be studied is called reverse characteristics of the
diode
• From the VI characteristics, it can be seen that when reverse
voltage is increased, the reverse current rises very quickly
and then it becomes constant with further increase in
voltage
• At this point resistance offered by the diode is high and is in
the order of several mega-ohm
• If the reverse voltage is further increased, then at a certain it
is sufficient enough to break the covalent bond.
• As a result of this, more electron-hole pairs are created and
the current due to minority charge carriers becomes large
• Due to crossing of large amount of minority charge carriers
across the junction, the value of reverse current is so large
that it can break the junction
• The value of reverse voltage at which the junction breaks
down is called breakdown voltage
• Normally, in reverse biasing the diode acts as an open switch
whereas in forward biasing, it acts as a closed switch.