chapter 43 molecules and solids. molecular bonds – introduction the bonding mechanisms in a...
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Chapter 43
Molecules and Solids
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Molecular Bonds – Introduction
The bonding mechanisms in a molecule are fundamentally due to electric forces
The forces are related to a potential energy function
A stable molecule would be expected at a configuration for which the potential energy function has its minimum value
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Features of Molecular Bonds
The force between atoms is repulsive at very small separation distances This repulsion is partially electrostatic and partially due to
the exclusion principle Due to the exclusion principle, some electrons in
overlapping shells are forced into higher energy states The energy of the system increases as if a repulsive force
existed between the atoms
The force between the atoms is attractive at larger distances
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Potential Energy Function
The potential energy for a system of two atoms can be expressed in the form
r is the internuclear separation distance m and n are small integers A is associated with the attractive force B is associated with the repulsive force
( )n m
A BU r
r r
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Potential Energy Function, Graph At large separations, the
slope of the curve is positive Corresponds to a net
attractive force At the equilibrium
separation distance, the attractive and repulsive forces just balance At this point the potential
energy is a minimum The slope is zero
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Molecular Bonds – Types
Simplified models of molecular bonding include Ionic Covalent van der Waals Hydrogen
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Ionic Bonding
Ionic bonding occurs when two atoms combine in such a way that one or more outer electrons are transferred from one atom to the other
Ionic bonds are fundamentally caused by the Coulomb attraction between oppositely charged ions
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Ionic Bonding, cont.
When an electron makes a transition from the E = 0 to a negative energy state, energy is released The amount of this energy is called the electron
affinity of the atom The dissociation energy is the amount of
energy needed to break the molecular bonds and produce neutral atoms
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Ionic Bonding, NaCl Example
The graph shows the total energy of the molecule vs the internuclear distance
The minimum energy is at the equilibrium separation distance
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Ionic Bonding,final
The energy of the molecule is lower than the energy of the system of two neutral atoms
It is said that it is energetically favorable for the molecule to form The system of two atoms can reduce its energy
by transferring energy out of the system and forming a molecule
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Covalent Bonding
A covalent bond between two atoms is one in which electrons supplied by either one or both atoms are shared by the two atoms
Covalent bonds can be described in terms of atomic wave functions
The example will be two hydrogen atoms forming H2
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Wave Function – Two Atoms Far Apart
Each atom has a wave function
There is little overlap between the wave functions of the two atoms when they are far away from each other
1 3
1( ) or a
s
o
ψ r eπa
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Wave Function – Molecule
The two atoms are brought close together
The wave functions overlap and form the compound wave shown
The probability amplitude is larger between the atoms than on either side
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Active Figure 43.3
Use the active figure to move the individual wave functions
Observe the composite wave function
PLAYACTIVE FIGURE
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Covalent Bonding, Final
The probability is higher that the electrons associated with the atoms will be located between them
This can be modeled as if there were a fixed negative charge between the atoms, exerting attractive Coulomb forces on both nuclei
The result is an overall attractive force between the atoms, resulting in the covalent bond
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Van der Waals Bonding
Two neutral molecules are attracted to each other by weak electrostatic forces called van der Waals forces Atoms that do not form ionic or covalent bonds
are also attracted to each other by van der Waals forces
The van der Waals force is due to the fact that the molecule has a charge distribution with positive and negative centers at different positions in the molecule
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Van der Waals Bonding, cont.
As a result of this charge distribution, the molecule may act as an electric dipole
Because of the dipole electric fields, two molecules can interact such that there is an attractive force between them Remember, this occurs even though the
molecules are electrically neutral
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Types of Van der Waals Forces
Dipole-dipole force An interaction between two molecules each
having a permanent electric dipole moment Dipole-induced dipole force
A polar molecule having a permanent dipole moment induces a dipole moment in a nonpolar molecule
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Types of Van der Waals Forces, cont.
Dispersion force An attractive force occurs between two nonpolar
molecules The interaction results from the fact that, although
the average dipole moment of a nonpolar molecule is zero, the average of the square of the dipole moment is nonzero because of charge fluctuations
The two nonpolar molecules tend to have dipole moments that are correlated in time so as to produce van der Waals forces
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Hydrogen Bonding
In addition to covalent bonds, a hydrogen atom in a molecule can also form a hydrogen bond
Using water (H2O) as an example There are two covalent bonds in the molecule The electrons from the hydrogen atoms are more
likely to be found near the oxygen atom than the hydrogen atoms
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Hydrogen Bonding – H2O Example, cont.
This leaves essentially bare protons at the positions of the hydrogen atoms
The negative end of another molecule can come very close to the proton
This bond is strong enough to form a solid crystalline structure
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Hydrogen Bonding, Final
The hydrogen bond is relatively weak compared with other electrical bonds
Hydrogen bonding is a critical mechanism for the linking of biological molecules and polymers
DNA is an example
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Energy States of Molecules
The energy of a molecule (assume one in a gaseous phase) can be divided into four categories Electronic energy
Due to the interactions between the molecule’s electrons and nuclei
Translational energy Due to the motion of the molecule’s center of mass
through space
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Energy States of Molecules, 2
Categories, cont. Rotational energy
Due to the rotation of the molecule about its center of mass
Vibrational energy Due to the vibration of the molecule’s constituent
atoms The total energy of the molecule is the sum of
the energies in these categories: E = Eel + Etrans + Erot + Evib
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Spectra of Molecules
The translational energy is unrelated to internal structure and therefore unimportant to the interpretation of the molecule’s spectrum
By analyzing its rotational and vibrational energy states, significant information about molecular spectra can be found
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Rotational Motion of Molecules
A diatomic model will be used, but the same ideas can be extended to polyatomic molecules
A diatomic molecule aligned along an x axis has only two rotational degrees of freedom Corresponding to rotations
about the y and x axes
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Rotational Motion of Molecules, Energy
The rotational energy is given by
I is the moment of inertia of the molecule
µ is called the reduced mass of the molecule
2rot
1
2IE ω
2 21 2
1 2
Im m
r μrm m
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Rotational Motion of Molecules, Angular Momentum
Classically, the value of the molecule’s angular momentum can have any valueL = Iω
Quantum mechanics restricts the values of the angular momentum to
J is an integer called the rotational quantum number
1 0 1 2, , ,L J J J
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Rotational Kinetic Energy of Molecules, Allowed Levels
The allowed values are
The rotational kinetic energy is quantized and depends on its moment of inertia
As J increases, the states become farther apart
2
rot 1 0 1 22
, , ,IJE E J J J
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Allowed Levels, cont. For most molecules,
transitions result in radiation that is in the microwave region
Allowed transitions are given by the condition
J is the number of the higher state
2
rot
2
21 2 3
4
I
, , ,I
photonE E J
hJ J
π
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Active Figure 43.5
Use the active figure to adjust the distance between the atoms
Choose the initial rotational energy state of the molecule
Observe the transition of the molecule to lower energy states
PLAYACTIVE FIGURE
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Vibrational Motion of Molecules
A molecule can be considered to be a flexible structure where the atoms are bonded by “effective springs”
Therefore, the molecule can be modeled as a simple harmonic oscillator
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Vibrational Motion of Molecules, Potential Energy
A plot of the potential energy function
ro is the equilibrium atomic separation
For separations close to ro, the shape closely resembles a parabola
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Vibrational Energy
Classical mechanics describes the frequency of vibration of a simple harmonic oscillator
Quantum mechanics predicts that a molecule will vibrate in quantized states
The vibrational and quantized vibrational energy can be altered if the molecule acquires energy of the proper value to cause a transition between quantized states
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Vibrational Energy, cont.
The allowed vibrational energies are
v is an integer called the vibrational quantum number
When v = 0, the molecule’s ground state energy is ½hƒ The accompanying vibration is always present,
even if the molecule is not excited
vib
10 1 2
2ƒ , , ,E v h v
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Vibrational Energy, Final
The allowed vibrational energies can be expressed as
Selection rule for allowed transitions is Δv = 1
The energy of an absorbed photon is Ephoton = ΔEvib = hƒ
vib
1
2 2
0 1 2, , ,
h kE v
π μ
v
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Molecular Spectra
In general, a molecule vibrates and rotates simultaneously
To a first approximation, these motions are independent of each other
The total energy is the sum of the energies for these two motions:
21
12 2
ƒI
E v h J J
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Molecular Energy-Level Diagram
For each allowed state of v, there is a complete set of levels corresponding to the allowed values of J
The energy separation between successive rotational levels is much smaller than between successive vibrational levels
Most molecules at ordinary temperatures vibrate at v = 0 level
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Molecular Absorption Spectrum
The spectrum consists of two groups of lines One group to the right of center satisfying the selection rules ΔJ
= +1 and Δv = +1 The other group to the left of center satisfying the selection rules
ΔJ = -1 and Δv = +1 Adjacent lines are separated by h/2πI
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Active Figure 43.8
Use the active figure to adjust the spring constant and the moment of inertia of the molecule
Observe the effect on the energy levels and the spectral lines
PLAYACTIVE FIGURE
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Absorption Spectrum of HCl
It fits the predicted pattern very well A peculiarity shows, each line is split into a doublet
Two chlorine isotopes were present in the same sample Because of their different masses, different I’s are present
in the sample
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Intensity of Spectral Lines
The intensity is determined by the product of two functions of J The first function is the number of available states
for a given value of J There are 2J + 1 states available
The second function is the Boltzmann factor
2B( 1)/(2 )IJ J k T
on n e
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Intensity of Spectral Lines, cont
Taking into account both factors by multiplying them,
The 2J + 1 term increases with J The exponential term decreases
This is in good agreement with the observed envelope of the spectral lines
2B( 1)/(2 )2 1 II J J k TJ e
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Bonding in Solids
Bonds in solids can be of the following types Ionic Covalent Metallic
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Ionic Bonds in Solids
The dominant interaction between ions is through the Coulomb force
Many crystals are formed by ionic bonding Multiple interactions occur among nearest-
neighbor atoms
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Ionic Bonds in Solids, 2
The net effect of all the interactions is a negative electric potential energy
α is a dimensionless number known as the Madelung constant
The value of α depends only on the crystalline structure of the solid
2
attractive e
eU αk
r
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Ionic Bonds, NaCl Example
The crystalline structure is shown (a) Each positive sodium ion is surrounded by six negative
chlorine ions (b) Each chlorine ion is surrounded by six sodium ions (c) α = 1.747 6 for the NaCl structure
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Total Energy in a Crystalline Solid
As the constituent ions of a crystal are brought close together, a repulsive force exists
The potential energy term B/rm accounts for this repulsive force This repulsive force is a result of electrostatic
forces and the exclusion principle
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Total Energy in a Crystalline Solid, cont
The total potential energy of the crystal is
The minimum value, Uo, is called the ionic cohesive energy of the solid It represents the energy
needed to separate the solid into a collection of isolated positive and negative ions
2
total e m
e BU αk
r r
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Properties of Ionic Crystals
They form relatively stable, hard crystals They are poor electrical conductors
They contain no free electrons Each electron is bound tightly to one of the ions
They have high melting points
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More Properties of Ionic Crystals
They are transparent to visible radiation, but absorb strongly in the infrared region The shells formed by the electrons are so tightly
bound that visible light does not possess sufficient energy to promote electrons to the next allowed shell
Infrared is absorbed strongly because the vibrations of the ions have natural resonant frequencies in the low-energy infrared region
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Properties of Solids with Covalent Bonds
Properties include Usually very hard
Due to the large atomic cohesive energies High bond energies High melting points Good electrical conductors
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Cohesive Energies for Some Covalent Solids
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Covalent Bond Example – Diamond
Each carbon atom in a diamond crystal is covalently bonded to four other carbon atoms
This forms a tetrahedral structure
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Another Carbon Example -- Buckyballs
Carbon can form many different structures
The large hollow structure is called buckminsterfullerene Also known as a
“buckyball”
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Metallic Solids
Metallic bonds are generally weaker than ionic or covalent bonds
The outer electrons in the atoms of a metal are relatively free to move through the material
The number of such mobile electrons in a metal is large
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Metallic Solids, cont.
The metallic structure can be viewed as a “sea” or “gas” of nearly free electrons surrounding a lattice of positive ions
The bonding mechanism is the attractive force between the entire collection of positive ions and the electron gas
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Properties of Metallic Solids
Light interacts strongly with the free electrons in metals Visible light is absorbed and re-emitted quite
close to the surface This accounts for the shiny nature of metal
surfaces High electrical conductivity
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More Properties of Metallic Solids
The metallic bond is nondirectional This allows many different types of metal atoms to
be dissolved in a host metal in varying amounts The resulting solid solutions, or alloys, may be
designed to have particular properties Metals tend to bend when stressed
Due to the bonding being between all of the electrons and all of the positive ions
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Free-Electron Theory of Metals
The quantum-based free-electron theory of electrical conduction in metals takes into account the wave nature of the electrons
The model is that the outer-shell electrons are free to move through the metal, but are trapped within a three-dimensional box formed by the metal surfaces
Each electron can be represented as a particle in a box
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Fermi-Dirac Distribution Function
Applying statistical physics to a collection of particles can relate microscopic properties to macroscopic properties
For electrons, quantum statistics requires that each state of the system can be occupied by only two electrons
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Fermi-Dirac Distribution Function, cont.
The probability that a particular state having energy E is occupied by one of the electrons in a solid is given by
ƒ(E) is called the Fermi-Dirac distribution function
EF is called the Fermi energy
B( )
1( )
1ƒ
FE E k TEe
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Fermi-Dirac Distribution Function at T = 0
At T = 0, all states having energies less than the Fermi energy are occupied
All states having energies greater than the Fermi energy are vacant
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Fermi-Dirac Distribution Function at T > 0
As T increases, the distribution rounds off slightly
States near and below EF lose population
States near and above EF gain population
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Active Figure 43.15
Use the active figure to adjust the temperature
Observe the effect on the Fermi-Dirac distribution function
PLAYACTIVE FIGURE
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Electrons as a Particle in a Three-Dimensional Box
The energy levels for the electrons are very close together
The density-of-states function gives the number of allowed states per unit volume that have energies between E and E + dE:
3 21 2
3
8 2( ) eπm
g E dE E dEh
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Fermi Energy at T = 0 K
The Fermi energy at T = 0 K is
The order of magnitude of the Fermi energy for metals is about 5 eV
The average energy of a free electron in a metal at 0 K is Eavg = (3/5) EF
2 32
F
3(0)
2 8e
e
h nE
m π
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Fermi Energies for Some Metals
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Wave Functions of Solids
To make the model of a metal more complete, the contributions of the parent atoms that form the crystal must be incorporated
Two wave functions are valid for an atom with atomic number Z and a single s electron outside a closed shell:
( ) ( ) ( ) ( )ƒ ƒo oZr na Zr nas sψ r A r e ψ r A r e
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Combined Wave Functions
The wave functions can combine in the various ways shown ψs
- + ψs- is equivalent to
ψs+ + ψs
+ These two possible
combinations of wave functions represent two possible states of the two-atom system
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Splitting of Energy Levels The states are split into two
energy levels due to the two ways of combining the wave functions
The energy difference is relatively small, so the two states are close together on an energy scale
For large values of r, the electron clouds do not overlap and there is no splitting of the energy level
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Splitting of Energy Levels, cont.
As the number of atoms increases, the number of combinations in which the wave functions combine increases
Each combination corresponds to a different energy level
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Splitting of Energy Levels, final
When this splitting is extended to the large number of atoms present in a solid, there is a large number of levels of varying energy
These levels are so closely spaced they can be thought of as a band of energy levels
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Energy Bands in a Crystal In general, a crystalline
solid will have a large number of allowed energy bands
The white areas represent energy gaps, corresponding to forbidden energies
Some bands exhibit an overlap
Blue represents filled bands and gold represents empty bands in this example of sodium
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Electrical Conduction – Classes of Materials
Good electrical conductors contain a high density of free charge carriers
The density of free charge carriers in an insulator is nearly zero
Semiconductors are materials with a charge density between those of insulators and conductors
These classes can be discussed in terms of a model based on energy bands
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Metals
To be a good conductor, the charge carriers in a material must be free to move in response to an electric field We will consider electrons as the charge carriers
The motion of electrons in response to an electric field represents an increase in the energy of the system
When an electric field is applied to a conductor, the electrons move up to an available higher energy state
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Metals – Energy Bands At T = 0, the Fermi energy
lies in the middle of the band All levels below EF are filled
and those above are empty If a potential difference is
applied to the metal, electrons having energies near EF require only a small amount of additional energy from the applied field to reach nearby empty states above the Fermi energy
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Metals As Good Conductors
The electrons in a metal experiencing only a weak applied electric field are free to move because there are many empty levels available close to the occupied energy level
This shows that metals are excellent electrical conductors
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Insulators
There are no available states that lie close in energy into which electrons can move upward in response to an electric field
Although an insulator has many vacant states in the conduction band, these states are separated from the filled band by a large energy gap
Only a few electrons can occupy the higher states, so the overall electrical conductivity is very small
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Insulator – Energy Bands The valence band is filled
and the conduction band is empty at T = 0
The Fermi energy lies somewhere in the energy gap
At room temperature, very few electrons would be thermally excited into the conduction band
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Semiconductors
The band structure of a semiconductor is like that of an insulator with a smaller energy gap
Typical energy gap values are shown in the table
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Semiconductors – Energy Bands
Appreciable numbers of electrons are thermally excited into the conduction band
A small applied potential difference can easily raise the energy of the electrons into the conduction band
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Semiconductors – Movement of Charges
Charge carriers in a semiconductor can be positive, negative, or both
When an electron moves into the conduction band, it leaves behind a vacant site, called a hole
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Semiconductors – Movement of Charges, cont.
The holes act as charge carriers Electrons can transfer into a hole, leaving another
hole at its original site The net effect can be viewed as the holes
migrating through the material in the direction opposite the direction of the electrons The hole behaves as if it were a particle with
charge +e
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Intrinsic Semiconductors
A pure semiconductor material containing only one element is called an intrinsic semiconductor
It will have equal numbers of conduction electrons and holes Such combinations of charges are called electron-
hole pairs
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Doped Semiconductors
Impurities can be added to a semiconductor This process is called doping Doping
Modifies the band structure of the semiconductor Modifies its resistivity Can be used to control the conductivity of the
semiconductor
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n-Type Semiconductors
An impurity can add an electron to the structure
This impurity would be referred to as a donor atom
Semiconductors doped with donor atoms are called n-type semiconductors
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n-Type Semiconductors, Energy Levels
The energy level of the extra electron is just below the conduction band
The electron of the donor atom can move into the conduction band as a result of a small amount of energy
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p-Type Semiconductors An impurity can add a hole
to the structure This is an electron
deficiency This impurity would be
referred to as a acceptor atom
Semiconductors doped with acceptor atoms are called p-type semiconductors
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p-Type Semiconductors, Energy Levels The energy level of the hole
is just above the valence band
An electron from the valence band can fill the hole with an addition of a small amount of energy
A hole is left behind in the valance band
This hole can carry current in the presence of an electric field
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Extrinsic Semiconductors
When conduction in a semiconductor is the result of acceptor or donor impurities, the material is called an extrinsic semiconductor
Doping densities range from 1013 to 1019 cm-3
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Semiconductor Devices
Many electronic devices are based on semiconductors
These devices include Junction diode Light-emitting and light-absorbing diodes Transistor Integrated Circuit
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The Junction Diode
A p-type semiconductor is joined to an n-type This forms a p-n junction A junction diode is a device based on a
single p-n junction The role of the diode is to pass current in one
direction, but not the other
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The Junction Diode, 2
The junction has three distinct regions a p region an n region a depletion region
The depletion region is caused by the diffusion of electrons to fill holes This can be modeled as if
the holes being filled were diffusing to the n region
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The Junction Diode, 3
Because the two sides of the depletion region each carry a net charge, an internal electric field exists in the depletion region
This internal field creates an internal potential difference that prevents further diffusion and ensures zero current in the junction when no potential difference is applied
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Junction Diode, Biasing
A diode is forward biased when the p side is connected to the positive terminal of a battery This decreases the internal potential difference
which results in a current that increases exponentially
A diode is reverse biased when the n side is connected to the positive terminal of a battery This increases the internal potential difference
and results in a very small current that quickly reaches a saturation value
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Junction Diode: I-V Characteristics
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LEDs and Light Absorption
Light emission and absorption in semiconductors is similar to that in gaseous atoms, with the energy bands of the semiconductor taken into account
An electron in the conduction band can recombine with a hole in the valance band and emit a photon
An electron in the valance band can absorb a photon and be promoted to the conduction band, leaving behind a hole
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Transistors
A junction transistor is formed from two p-n junctions A narrow n region sandwiched between two p
regions or a narrow p region between two n regions
The transistor can be used as An amplifier A switch
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Integrated Circuits
An integrated circuit is a collection of interconnected transistors, diodes, resistors and capacitors fabricated on a single piece of silicon known as a chip
Integrated circuits Solved the interconnectedness problem posed by
transistors Possess the advantages of miniaturization and
fast response
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Superconductivity
A superconductor expels magnetic fields from its interior by forming surface currents
Surface currents induced on the superconductor’s surface produce a magnetic field that exactly cancels the externally applied field
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Superconductivity and Cooper Pairs
Two electrons are bound into a Cooper pair when they interact via distortions in the array of lattice atoms so that there is a net attractive force between them
Cooper pairs act like bosons and do not obey the exclusion principle
The entire collection of Cooper pairs in a metal can be described by a single wave function
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Superconductivity, cont.
Under the action of an applied electric field, the Cooper pairs experience an electric force and move through the metal
There is no resistance to the movement of the Cooper pairs They are in the lowest possible energy state There are no energy states above that of the
Cooper pairs because of the energy gap
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Superconductivity - Critical Temperatures
The critical temperature is the temperature at which the electrical resistance of the material decreases to virtually zero
A new family of compounds was found that was superconducting at “high” temperatures First discovered in 1986 Found materials that are superconductive up to
temperatures of 150 K Currently no widely accepted theory for high-
temperature superconductivity