University of Tripoli - Engineering Faculty

Computer Engineering Department

EC 310: Electronic Material & Devices

Dr. Amna Elhawil
A.elhawil@uot.edu.ly

CHAPTER (1): ENERGY L EVELS AND ENERGY BANDS

 Contents
Chapter (1): Energy levels and energy bands
Chapter (2): Semiconductor physics
Chapter (3):Conduction in semiconductors
Chapter (4): The pn junction
Chapter (5): Diodes
Chapter (6): Diode applications
Chapter (7): Other types of diodes
Chapter (8): Bipolar junction transistors (BJT)
Chapter (9 & 10): Field-effect transistors (JFET & MOSFET)

EC310 - CHAPTER (1): ENERGY LEVELS AND ENERGY BANDS 2

 Main textbooks:
• Semiconductor Physics and Devices: Basic Principles, D. A. Neamen, 4th edition.
• Electronic devices and circuit theory, R. Boylestad, L. Nashelsky, 9th edition.

Other references
• Microelectronics, J. Millman, A. Grabel
• Microelectronic Circuits, A. Sedra and K. Smith, 6th edition

EC310 - CHAPTER (1): ENERGY LEVELS AND ENERGY BANDS 3

Chapter (1)
Energy levels and
energy bands
Read Chapter (3): (3.2) of textbook

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 1.1 Atomic Structure
Atoms are made up of 3 types of particles
electrons, protons and neutrons.
Electrons are tiny, very light particles that have a
negative electrical charge.
Protons are much larger and heavier than
electrons and have the opposite charge, protons
have a positive charge.
Neutrons are large and heavy like protons;
however neutrons have no electrical charge.

Fig. 1.1

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 Energy levels
The electrons are arranged in energy levels or shells around the nucleus and with 'orbits' on
average increasing in distance from the nucleus.
Each electron in an atom is in a particular energy level (or shell) and the electrons must occupy
the lowest available energy level (or shell) available nearest the nucleus.
When the level is full, the next electron goes into the next highest level (shell) available.
The maximum number of electrons allowed in each shell is as following:
◦ The first shell can contain a maximum of 2 electrons.
◦ The second shell can contain a maximum of 8 electrons.
◦ The third shell also has a maximum of 18 electrons.
◦ The forth shell contains 32 electrons.

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 Each shell number n has n subshells, there are four subshells (s, p, d, f ):
 The subshell s contains 2 electrons, p contains 6 electrons, d has 10 electrons and f has 14 electrons
 The first shell has the subshell s. So maximum number of electrons is 2.
 The second shell has the subshells s and p. So maximum number of electrons is (2 + 6 = 8).
 The third shell has the subshells s, p and d. So maximum number of electrons is 18 (2 + 6 + 10).
 The forth shell has s, p, d and f. So maximum number of electrons is 32.

Fig. 1.2

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The maximum number of electrons allowed in each shell is as following:

Shell Subshells Capacity

s
1
2 2
s p
2
2 6 8
s p d
3
2 6 10 18
s p d f
4
2 6 10 14 32

Fig. 1.3

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Atomic Number

Atomic Number = number of electrons in a neutral atom

1. Boron (B, atomic number = 5):
1s2 2s2 2p1.

2. Silicon (Si, atomic number = 14):

1s2 2s2 2p6 3s2 3p2

3. Germanium (Ge, atomic number = 32):

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2.

EC310 - CHAPTER (1): ENERGY LEVELS AND ENERGY BANDS 9

 Fig. 1.4

EC310 - CHAPTER (1): ENERGY LEVELS AND ENERGY BANDS 10

Group No. 1 2 3 4 5 6 7 8

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 The Bohr model
1. Electrons exist in certain stable, circular orbits about the nucleus.
2. These orbits are associated with definite energies and are also called
energy shells or energy levels.
3. The electron may shift to an orbit of higher or lower energy, thereby
gaining or losing energy equal to the difference in the energy levels
(by absorption or emission of a photon of energy hv).

ΔE = E2 - E1= hv

where f (or v) is the frequency of the radiation,

h is a quantity now called Planck's constant (h = 6.63 x 1034 J-s).

Fig. 1.5

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 Bonding forces and energy bands in solids

1. Ionic bond: the electrons are shared between two different atoms, one atom accepts an
electron or more and becomes negative ion and the other donates an electron or more and
becomes positive ion.
 For example, NaCl structure: sodium (Na) (11) is 3s1 and chloride (Cl) (17) is 3s23p5 form
an ionic bond, to make NaCl, or table salt.
 Once the electron exchanges have been made between the Na and CI atoms to form the
Na+ and CI- ions, the outer orbits of all atoms are completely filled. There are no loosely
bound electrons to participate in current flow; as a result, NaCl is a good insulator.

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 Ionic bond is very strong.
 The movement of electrons under applied voltage is not possible  good insulators
 Have very high melting and boiling temperature.

Fig. 1.6 Ionic atom

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2. Covalent bond:
• It is formed when two same atoms share electron(s) .
• each atom in the Ge, Si, or C diamond lattice is surrounded by four nearest neighbors, each with
four electrons in the outer orbit.
• In these crystals each atom shares its valence electrons with its four neighbors
• The bonding forces arise from a quantum mechanical interaction between the shared electrons.
• This is known as covalent bonding; each electron pair constitutes a covalent bond.
• In the sharing process it is no longer relevant to ask which electron belongs to a particular atom—
both belong to the bond. The two electrons are indistinguishable.

Fig. 1.7

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3. Metallic bond:

• Metallic bonding is the main type of chemical bond that forms

between metal atoms.
• Metallic bonding is defined as a force of attraction that exists
between metal ions and valence electrons.
• It is the sharing of many detached electrons between many
positive ions.
• Because the electrons move freely, the metal has some electrical
conductivity. It allows the energy to pass quickly through the
electrons, generating an electric current. Metals conduct heat for the
same reason: the free electrons can transfer the energy at a faster
rate than other substances with electrons that are fixed into
position.

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 Energy bands
1. Valence band: The electrons in the outermost orbit of an atom are known
as valance electrons. In the normal atom, valance band has the electrons
of highest energy.
2. Conduction band: The valance electrons may get detached to become free
electrons. They become responsible for the conduction of current thus
they are called conduction electrons. The range of energies possessed by
conduction band is known as conduction band.

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 Representing the energy bands
1. Valence band: is the lower band of allowed states. In the drawings it is
depicted by a line labeled by Ev which represents the highest energy
state in the valence band. Since electrons have a tendency to fill the
lowest available energy states. The valence band is always nearly
completely filled with electrons
2. Conduction band: is the upper band of allowed states. When it is drawn
it is represented by a line labeled by Ec which represents the lowest
possible energy state in the conduction band.
• This band is usually empty. It contains few or no electrons since energy is required for them to get
there from the valence band. Electrons in the conduction band are free to move about the crystal, thus
the name conduction band.
Fig. 1.8

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3. The Band Gap: The band gap energy is the energy needed to break a bond in the crystal. When a
bond is broken, the electron has absorbed enough energy to leave the valence band and "jump" to
the conduction band. The width of the band gap determines the type of material (conductor,
semiconductor, insulator) you are working with. This is shown pictorially using a band diagram.

• EG of some semiconductors
– Silicon (Si): 1.1 eV
– Germanium (Ge): 0.7 eV
– Gallium arsenide (GaAs): 1.4 eV
– Zinc selenide (ZnSe): 2.7 eV

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Material Classification

Materials can be classified according to their bandgaps:

1. An insulator is a poor conductor since it requires a lot of energy, 5-8 eV.
2. A metal is an excellent conductor because, at room temperature, it has electrons in its conduction band
constantly, with little or no energy being applied to it.
3. A semiconductor: the reason semiconductors are so popular is because they are a medium between a
metal and an insulator. The band gap is wide enough to where current is not going through it at all times,
but narrow enough to where it does not take a lot of energy to have electrons in the conduction band
creating a current.

Fig. 1.9

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Conduction in materials
1. In metals there are empty states just above the Fermi levels, where electrons can be promoted. The
promotion energy is negligibly small so that at any temperature electrons can be found in the conduction
band.
2. In insulators, there is an energy gap between the valence and conduction bands, so energy is needed to
promote an electron to the conduction band. This energy may come from heat, or from energetic radiation,
like light of sufficiently small wavelength.

◦ For example, the bandgap energy Eg of the Carbon is about 6 eV. This large forbidden band separates the valance band
from the conduction band.
◦ Since the electron cannot acquire sufficient applied energy, conduction is impossible, and here Carbon is an insulator.

3. In semiconductors the width of the bandgap is relatively small ( ≈ 1 eV). Energies of these values normally
cannot be acquired from an applied field.
◦ Hence the valance band remains full, the conduction band empty and these materials are insulators at low temperature.
However, the conductivity increases with temperature so some valance electrons acquire thermal energy greater than Eg
and move to the conduction band.

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