semiconductor hall effect

1. SEMICONDUCTORS
PRESENTED BY
AMRUTHA T.K

2. INTRODUCTION
 Semiconductor are materials whose electrical conductivity lies
between conductor and an insulator.
 Electrical Conductivity: Level to which a material conducts
electricity.
 Conductor: allows the current to flow through it with the
application of voltage like copper.
 Insulator: Do not allows the current to flow through it with the
application of voltage like Glass.
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3. Energy band diagram of materials
 Conductor: have a very small energy gap
Result: current flows easily
 Insulator: have a large energy gap
Result: no current flows
 Semiconductor: have a medium energy gap
Result: only a small amount of current can flow
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4. ENERGY BAND THEORY
 There Important energy bands are,
 Valence Band
 Conduction Band
 Forbidden Band
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5. Valence Band
 Outermost electron orbital of an atom of material that
electrons actually occupy. This is lower band. From this
band electrons can jump out of, moving into the higher
energy level.
 An e- in valence band, expenses strong force of
attraction from nucleolus.And it can’t move freely
when extrenal electric field is applied. It is called
bounded electron.
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6. CONDUCTION BAND
This band is generally empty and high in energy. When
the electrons are in these band, they have enough energy to
move freely in the material. This movement of electrons
creates a current.
An e- in Conduction band has weak influence of
nucleolus and hence it can move free under the effect of
applied electric field and thus it produces current, it is
called free electrons
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7. Forbidden Band / Energy Gap
 In solid-state chemistry, an energy gap or bandgap, is an energy
range in a solid where no electron states can exist.
 It generally refers to the energy difference(in electron volts)
between the top of the valence band and the bottom of the
conduction band in insulators and semiconductors.
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8. Types of Materials
 Materials can be divided into 3 types based on the values of
energy gap
 Insulator
 Conductor
 Semi Conductor
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9. INSULATORS
 It is a material with large energy gap(Eg)
 Eg= large
 ev= 1.6*10-19 joules
 Due to large energy gap an e- from valence
band can’t move into conduction band
remains complete fill.
 Conduction band completly empty.
 Ex: glass, Diamond, Silicon di oxide
 Energy gap of diamond is ~6ev.
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10. CONDUCTORS
 It is a material having zero energy gap.The
materials in which conduction and valence bands.
 Valence electrons can move valence to conduction
band without requiring thermal energy.
 The overlapping indicates a large number of
electrons available for conduction.
 Hence the application of a small amount of
voltage results a large amount of current.
 Ex: All metals.
 Best conducting materials are
 Silver is best, cupper is second best

11. SEMICONDUCTORS
 The material which has electrical conductivity between that of a conductor and an insulator is
called as semiconductor. Silicon, germanium and graphite are some examples of
semiconductors.
 In semiconductors, the forbidden gap between valence band and conduction band is very small.
It has a forbidden gap of about 1 electron volt (eV).
 At low temperature, the valence band is completely occupied with electrons and conduction
band is empty because the electrons in the valence band does not have enough energy to move
in to conduction band. Therefore, semiconductor behaves as an insulator at low temperature.
 However, at room temperature some of the electrons in valence band gains enough energy in
the form of heat and moves in to conduction band.
 When the temperature is goes on increasing, the number of valence band electrons moving in
to conduction band is also increases. This shows that electrical conductivity of the
semiconductor increases with increase in temperature. Ie a semiconductor has negative
temperature co-efficient of resistance. The resistance of semiconductor decreases with increase
in temperature 11

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14. semiconductors
 Semiconductors are materials whose electronic properties are
intermediate between those of Metals and Insulators.
 They have conductivities in the range of 104 to 10+4S/m.
 The interesting feature about semiconductors is that they are bipolar and
current is transported by two charge carriers of opposite sign.

15.  Silicon and Germanium are elemental semiconductors and they have four
valence electrons which are distributed among the outermost S and p orbital's
 Semiconductors are mainly two types
1. Intrinsic (Pure) Semiconductors
2. Extrinsic (Impure) Semiconductors

16. Intrinsic Semiconductor
A Semiconductor which does not have any kind of impurities,
behaves as an Insulator at 0k and behaves as a Conductor at
higher temperature is known as Intrinsic Semiconductor or
Pure Semiconductors.
Germanium and Silicon (4th group elements) are the best
examples of intrinsic semiconductors and they possess diamond
cubic crystalline structure.
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17. Si
Si
Si
Si
Valence Cell
Covalent bonds
Intrinsic Semiconductor

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Ef
Ev
Valence
band
E
c
Conduction band
Ec
E
Electron
energy
KE of Electron
= E - Ec
KE of Hole
=
Ev -E
Fermi energy level

19. Carrier Concentration in Intrinsic Semiconductor
 When a suitable form of Energy is supplied to a Semiconductor then electrons
take transition from Valence band to Conduction band.
 Hence a free electron in Conduction band and simultaneously free hole in
Valence band is formed. This phenomenon is known as Electron - Hole pair
generation.
 In Intrinsic Semiconductor the Number of Conduction electrons will be equal
to the Number of Vacant sites or holes in the valence band.

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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.

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Energy Band Diagram of Intrinsic Semiconductor
The energy band diagram of an intrinsic semiconductor is shown
below

22. Extrinsic Semiconductors
 The Extrinsic Semiconductors are those in which impurities of large
quantity are present. Usually, the impurities can be either 3rd group
elements or 5th group elements.
 Based on the impurities present in the Extrinsic
Semiconductors, they are classified into two categories.
1. n-type semiconductors
2. P-type semiconductors

23. ◈ When any pentavalent element such as Phosphorous,Arsenic or Antimony is
added to the intrinsic Semiconductor(si or Ge) , four electrons are involved in
covalent bonding with four neighboring pure Semiconductor atoms.
◈ The fifth electron is weakly bound to the parent atom. And even for lesser
thermal energy it is released Leaving the parent atom positively ionized.
n type Semiconductors

24. Si
Si
Si
Si
P
free electron
impure atom
(donor)
n type semiconductor

25. n type semiconductors
◈ The Intrinsic Semiconductors doped with pentavalent impurities are called n-
type Semiconductors.
◈ The energy level of fifth electron is called donor level.
◈ The donor level is close to the bottom of the conduction band most of the donor
level electrons are excited in to the conduction band at room temperature and
become the Majority charge carriers.
◈ Hence in N-type Semiconductors electrons are Majority carriers and holes are
Minority carriers.
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E
Ed
Ev
Valence band
Conduction band
Ec
Electron
energy
Donor levels
Eg
Conduction band
Valence band
Eg
Donor level Ed

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28.  At very low temperatures all donor levels are filled with electrons.
 With increase of temperature more and more donor atoms get ionized
and the density of electrons in the conduction band increases.
 the delocalization of electrons thus endow the crystal with enhanced
conduction when external field is applied this is known as n-type
conduction.
 where n denotes negative charge carriers
 n type semi conductors no hole is created in the valance band of the
host ; the impurity provides an excess electrones and the majority
charge carriers are electrons

29.  When a trivalent elements such as Al, Ga or Indium have three electrons in
their outer most orbits , added to the intrinsic semiconductor all the
three electrons of Indium are engaged in covalentbonding with the three
neighboring Si atoms.
 Indium needs one more electron to complete its bond. this electron maybe
supplied by Silicon , there by creating a vacant electron site or hole on the
semiconductor atom.
 Indium accepts one extra electron, the energy level of this impurity atom is
called acceptor level and this acceptor level lies just above the valence band.
 These type of trivalent impurities are called acceptor impurities and
the semiconductors doped the acceptor impurities are called P-type
semiconductors.
P-type semiconductors

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Si
Si
Si
In
Si
Hole
Co-Valent
bonds
Impure atom
(acceptor)
P-type Semiconductor

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 Even at relatively low temperatures, these
acceptor atoms get ionized taking electrons
from valence band and thus giving rise to holes
in valence band for conduction.
 Due to ionization of acceptor atoms only holes
and no electrons are created.
 Thus holes are more in number than electrons
and hence holes are majority carriers and
electros are minority carriers in P-type
semiconductors.

34. In an external external field the migration of positively charged
holes gives the crystal a higher conduction.this is known as p type
conduction
Where p is denoted as positive charge of the carriers
In p type semi conductor, the impurity provides an excess of
holes and majority charge carriers are holes
 similar case Ge doped with Ga or Al

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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

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Hall effect
When a magnetic field is applied perpendicular to a current carrying conductor or
semiconductor, voltage is developed across the specimen in a direction perpendicular
to both the current and the magnetic field. This phenomenon is called the Hall effect
and voltage so developed is called the Hall voltage.
Let us consider,a thin rectangular slab carrying current (i) in thex-
direction.
If we place it in a magnetic field B which is in the y-direction. Potential
difference Vpq will develop between the faces p and q which are
perpendicular to the z-direction.

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