# Introduction to Physics of the Solid State

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"Introduction to Physics of the Solid State"

```					II. Concepts of Materials Science
Unit Goals:

1.    Structure
a) Motivation
b) Material properties
c)  Structural types for solids
d) Crystalline materials

2.    Energy bands
a) Insulators, semiconductors and conductors
b) Energy bands and gaps of semiconductors
c)  Optical properties

1
Material properties
• Goal of materials science and engineering – structure,
property, processing relationships

• Materials properties:
–   Mechanical
–   Optical
–   Electrical
–   Thermal
–   Chemical

• Types of materials: Conductors, ceramics, polymers,
semiconductors, and composites

2
Structure of Solids

• Structural types of solids:
– Crystalline: atoms (or molecules) are arranged in a
regular pattern that can extend throughout the crystal
(long range order).
• Examples include diamond and silicon

– Amorphous : atoms (or molecules) lack long range
order, but local order may exist (short range order).
• Examples include glasses and wax

3
Structure of Solids
• Crystalline structures and Bravais lattices
– Crystalline structures are classified according
to their symmetry and basic geometry.

– A unit cell is defined for each lattice type.

– The definition of a unit cell is not unique, but
the duplication of that cell in 3-D produces the
crystal.
4
Structure of Solids
System Name     Number of Bravais
Lattices
Triclinic       1

Monoclinic      2

Orthorhombic    4

Tetragonal      2

Cubic           3

Trigonal        1

Hexagonal       1

5
Structure of Solids

Various structural units can
be repeated to describe the
crystalline structure. The
simplest for is the primitive
unit cell.                              6
Figures from ref. 1
Structure of Solids
Examples of cubic crystals:

FCC – one atom per lattice point             FCC – two atoms per lattice point
e.g., copper and silver                      e.g., Sodium Chloride (NaCl)l

7
Figures from ref. 1
Structure of Solids
Examples of cubic crystals:

FCC/diamond cubic – two atom per            FCC/Zinc blend – two atoms per lattice
lattice point (e.g., silicon, germanium)    point (e.g., GaAs, CdS)

8
Figures from ref. 1
Electrical Properties of Matter
Electrical conductivity is one property in
terms of which materials are
classified.
Electrical conductivity is determined by
the presence of mobile charge
carriers
The categories of materials are: metal,
semiconductor and insulators
• In metals the charge carriers are
electrons.
• In insulators and semiconductors,
electrons and holes (missing
electrons) are considered charge
carriers
The presence of energy gaps helps to
determine whether carriers are
mobile.

9
Figure from ref. 1
Energy bands: insulators,
semiconductors, conductors
Energy diagrams for categories defined by electric conductivity:

Conduction
Band (CB)

Energy                                       Energy Gap
Gap                 Energy Gap

Valence
Band (VB)

Insulator            Intrinsic                 Conductor
semiconductor

10
More on Energy Bands
Conductors:
Example of an energy band diagram for a solid (sodium metal);

1s, 2s, 2p, 3s are electron energy levels. The uppermost energy band, 3s,
is half filled. Electrons can change energy states and are mobile within the
material. Hence, this material conducts electricity – it is a conductor.       11
Figure from ref. 1
Energy Bands in Semiconductors
Semiconductor band diagrams

****
****

Extrinsic (or doped)
Intrinsic       p-type                 Extrinsic (or doped)
semiconductor   (missing electrons     n-type
from the VB –called    (additional electrons in the CB)
holes)

12
Optical Properties of Solids
• Optical properties of solids include ability to
transmit, absorb and emit light (electromagnetic

• Optical properties are linked to the energy band
structure of the solid

• For example, many insulators are transparent
since visible light is not absorbed (electrons are
not given enough energy to move from the
valence to the conduction band)

13
Concepts of materials science
References for Materials Science

1.   “Introduction to Materials Science for Engineers”, sixth edition by J.
Shackelford
2.   “Introduction to Nanotechnology” by C. Poole and F. Owens; Wiley-
Interscience
3.   “Solid State Physics” by H. Stokes; second edition; Brigham Young
University
4.   “The Science and Engineering of Materials”, by Donald Askland
5.   “Principles of Materials Science and Engineering”, by William Smith

14

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