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BPH‐106: PHYSICS FOR ENGINEERS
Unit‐1 Date: 24.02.2023
Semiconductors
Y K Awasthi, PhD
Professor-ECE
MRIIRS

Topics to be covered:
• Holes and Electrons
• Band Theory
• Properties of Semiconductors
• Types of Semiconductors
• Intrinsic Semiconductor
• Extrinsic Semiconductor
• N‐Type Semiconductor
• P‐Type Semiconductor
• Intrinsic vs Extrinsic
• Applications

Semiconductors
• What are Semiconductors?
• Semiconductors are the materials which have a conductivity between
conductors (generally metals) and non‐conductors or insulators (such as
ceramics). Semiconductors can be compounds such as gallium arsenide or pure
elements, such as germanium or silicon. Physics explains the theories, properties
and mathematical approach governing semiconductors.
• Examples of Semiconductors:
• 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.

Holes and Electrons in Semiconductors:
• Holes and electrons are the types of charge carriers accountable for the flow of
current in semiconductors. Holes (valence electrons) are the positively charged
electric charge carrier 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 and scattering mechanisms.
• 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.

The mobility of a particle in a semiconductor is more if;
• Effective mass of particles is lesser
• Time between scattering events is more
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 (blue dots) leaves the lattice position, it creates a hole
(grey dots). This hole thus created takes the opposite charge of the electron and
can be imagined as positive charge carriers moving in the lattice.

Band Theory of Semiconductors:
• The introduction of band theory happened during the quantum revolution in science. Walter
Heitler and Fritz London discovered the energy bands.
• We know that the electrons in an atom are present in different energy levels. When we try to
assemble a lattice of a solid with N atoms, then each level of an atom must split up into N
levels in the solid. This splitting up 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.
k= 1.380649 × 10-23 m2 kg s-2 K-1

Conduction Band and Valence Band in Semiconductors:
Valence Band:
• The energy band involving the energy levels of valence electrons is known as the
valence band. It is the highest occupied energy band. When compared with
insulators, the bandgap in semiconductors is smaller. It allows the electrons in the
valence band to jump into the conduction band on receiving any external energy.
Conduction Band:
• It is the lowest unoccupied band that includes the energy levels of positive
(holes) or negative (free electrons) charge carriers. It has conducting electrons
resulting in the flow of current. The conduction band possess high energy level
and are generally empty. The conduction band in semiconductors accepts the
electrons from the valence band.

What is Fermi Level in Semiconductors?
• Fermi level (denoted by 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 Fermi level.
• In a p‐type semiconductor, there is an increase in the density of unfilled states.
Thus, accommodating more electrons at the lower energy levels. However, in
an n‐type semiconductor, the density of states increases, therefore,
accommodating more electrons at higher energy levels.

Properties of Semiconductors:
• Semiconductors can conduct electricity under preferable conditions or circumstances.
This unique property makes it an excellent material to conduct electricity in a controlled
manner as required.
• Unlike conductors, the charge carriers in semiconductors arise only because of external
energy (thermal agitation). It causes a certain number of valence electrons to cross the
energy gap and jump into the conduction band, leaving an equal amount of unoccupied
energy states, i.e. holes. Conduction due to electrons and holes are equally important.
• Resistivity: 10‐5 to 106 Ωm
• Conductivity: 105 to 10‐6 mho/m
• Temperature coefficient of resistance: Negative
• Current Flow: Due to electrons and holes

Why does the Resistivity of Semiconductors go down with
Temperature?
• The difference in resistivity between conductors and semiconductors is due to their difference
in charge carrier density.
• The resistivity of semiconductors decreases with temperature because the number of charge
carriers increases rapidly with increase in temperature, making the fractional change i.e. the
temperature coefficient negative.
Some Important Properties of Semiconductors are:
• Semiconductor acts like an insulator at Zero Kelvin. On increasing the temperature, it works as
a conductor.
• 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.
• Semiconductors are smaller in size and possess less weight.
• Their resistivity is higher than conductors but lesser than insulators.
• The resistance of semiconductor materials decreases with the increase in temperature and
vice‐versa.

Types of Semiconductors:
Semiconductors can be classified as:
• Intrinsic Semiconductor
• Extrinsic Semiconductor

Intrinsic Semiconductor:
• An intrinsic type of semiconductor material is made to be very pure chemically.
It is made up of only a single type of element.

• Germanium (Ge) and Silicon (Si) are the most common type of intrinsic
semiconductor elements. They have four valence electrons (tetravalent). They are
bound to the atom by covalent bond at absolute zero temperature.
• When the temperature rises, due to collisions, few electrons are unbounded and
become free to move through the lattice, thus creating an absence in its original
position (hole). These free electrons and holes contribute to the conduction of
electricity in the semiconductor. The negative and positive charge carriers are
equal in number.
• The thermal energy is capable of ionizing a few atoms in the lattice, and hence
their conductivity is less.
The Lattice of Pure Silicon Semiconductor at Different Temperatures:
•At absolute zero Kelvin temperature: At this temperature, the covalent bonds are very
strong and there are no free electrons and the semiconductor behaves as a perfect
insulator.
•Above absolute temperature: With the increase in temperature few valence electrons
jump into the conduction band and hence it behaves like a poor conductor.

Energy Band Diagram of Intrinsic Semiconductor:
• The energy band diagram of an intrinsic semiconductor is shown below:
In intrinsic semiconductors, current flows due to the motion of free electrons as well as
holes. The total current is the sum of the electron current Ie due to thermally generated
electrons and the hole current Ih
Total Current (I) = Ie + Ih

For an intrinsic semiconductor, at finite temperature, the probability of electrons to
exist in conduction band decreases exponentially with increasing bandgap (Eg)
n = n0e‐Eg/2.Kb.T
Where,
Eg = Energy bandgap
Kb = Boltzmann’s constants
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. An extrinsic
semiconductor can be further classified into:
N-type Semiconductor
P-type Semiconductor

N-Type Semiconductor:
•Mainly due to electrons
•Entirely neutral
•I = Ih and nh >> ne
•Majority – Electrons and Minority – Holes

• When a pure semiconductor (Silicon or Germanium) is doped by pentavalent
impurity (P, As, Sb, Bi) then, four electrons out of five valence electrons bonds
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 “Donar“.
• Since the number of free electron increases by the addition of an impurity, the
negative charge carriers increase. Hence, it is called 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.

P‐Type Semiconductor:
• Mainly due to holes
• Entirely neutral
• I = Ih and nh >> ne
• Majority – Holes and Minority – Electrons
• When a pure semiconductor is doped with a trivalent impurity (B, Al, In, Ga ) then, the three
valence electrons of the impurity bonds 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 the increase in the number of impurities, holes (the positive charge carriers) are
increased. Hence, it is called 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
Density of electrons is equal to the density of
holes
Density of electrons is not equal to the density
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, pentavalent impurity

Applications of Semiconductors:
• Let us now understand the uses of semiconductors in daily life. Semiconductors
are used in almost all electronic devices. Without them, our life would be much
different.
• 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.

Uses of Semiconductors in Everyday life:
• 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.
• Transistor and MOSFET used as a switch in Electrical Circuits are manufactured
using the semiconductors.

Industrial Uses of 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.
Importance of Semiconductors:
• Here we have discussed some advantages of semiconductors which makes them
highly useful everywhere.
• They are highly portable due to the smaller size
• They require less input power
• Semiconductor devices are shockproof
• They have a longer lifespan
• They are noise‐free while operating

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