Introduction to Semiconductor
Physics and Devices
Arup Neogi
Institute of Fundamental and Frontier Sciences
University of Electronic Science and Technology of China
1
Prerequisites:
To understand this course, you should
have the following prior knowledge:
– Draw the structure of an atom, including
electrons, protons, and neutrons.
– Define resistance and conductance.
– Label an electronic schematic, indicating
current flow.
– Define Ohm’s and Kirchhoff’s laws.
– Describe the characteristics of DC and AC
(sine wave) voltages.
2
Why semiconductors?
3
THE INTEGRATED CIRCUIT (IC)
First IC by Jack
Kilby at Texas
Instruments
An IC can contain the arithmetic,
logic, and memory functions on a
single chip—The primary example of
this type of IC is the microprocessor.
Integration => Miniaturization for use
in satellites, phones, laptop
computers etc.
4
Moore’s Law
Moore’s Law: Number of transistors on an integrated circuit will double every two years
(with minimal rise in cost)
5
Basic elements of integrated circuits - I
Conductors:
Good conductors have low resistance so
electrons flow through them with ease.
• Best element conductors include:
– Copper, silver, gold, aluminum, & nickel
Copper
Atom
• Alloys are also good conductors:
– Brass & steel
• Good conductors can also be liquid:
– Salt water
• The atomic structure of
good conductors usually
includes only one electron
in their outer shell.
• It is called a valence
electron.
• It is easily striped from the
atom, producing current
flow.
6
Basic elements of integrated circuits -II
Insulator
• Insulators have a high resistance so current
does not flow in them.
• Good insulators include:
– Glass, ceramic, plastics, & wood
• Most insulators are compounds of several
elements.
• The atoms are tightly bound to one another so
electrons are difficult to strip away for current
flow.
7
Basic elements of integrated circuits - III
Semiconductors
• Semiconductors are materials that
essentially can be conditioned to act
as good conductors, or good
insulators, or any thing in between.
• Common elements such as carbon,
silicon, and germanium are
semiconductors.
• Silicon is the best and most widely
used semiconductor.
• The main characteristic of
a semiconductor element
is that it has four electrons
in its outer or valence
orbit.
8
Semiconductor Materials
9
The Structure of Crystalline Solids
ISSUES TO ADDRESS...
• How do atoms assemble into solid structures?
• How does the density of a material depend on
its structure?
• When do material properties vary with the
sample (i.e., part) orientation?
10
Crystal Structure
Single Crystal
Amorphous
Polycrystalline
11
Materials and Packing
Crystalline materials...
• atoms pack in periodic, 3D arrays
• typical of: -metals
-many ceramics
-some polymers
crystalline SiO2
Adapted from Fig. 3.23(a),
Callister & Rethwisch 8e.
Noncrystalline materials...
• atoms have no periodic packing
• occurs for: -complex structures
-rapid cooling
"Amorphous" = Noncrystalline
Si
Oxygen
noncrystalline SiO2
Adapted from Fig. 3.23(b),
Callister & Rethwisch 8e.
12
Energy and Packing
• Non dense, random packing
Energy
typical neighbor
bond length
typical neighbor
bond energy
• Dense, ordered packing
r
Energy
typical neighbor
bond length
typical neighbor
bond energy
r
Dense, ordered packed structures tend to have
lower energies.
13
Crystal Structure
Unit cell
Primitive cell
FCC
cubic
BCC
a is the lattice constant
14
Metallic Crystal Structures
• Tend to be densely packed.
• Reasons for dense packing:
- Typically, only one element is present, so all atomic
radii are the same.
- Metallic bonding is not directional.
- Nearest neighbor distances tend to be small in
order to lower bond energy.
- Electron cloud shields cores from each other
• Have the simplest crystal structures.
We will examine three such structures...
15
Metallic Crystal Structures
• How can we stack metal atoms to minimize
empty space?
2-dimensions
vs.
Now stack these 2-D layers to make 3-D structures
16
Simple Cubic Structure (SC)
• Rare due to low packing density (only Po has this structure)
• Close-packed directions are cube edges.
• Coordination # = 6
(# nearest neighbors)
Click once on image to start animation
(Courtesy P.M. Anderson)
17
Atomic Packing Factor (APF)
Volume of atoms in unit cell*
APF =
Volume of unit cell
*assume hard spheres
• APF for a simple cubic structure = 0.52
atoms
unit cell
a
R=0.5a
APF =
volume
atom
4
p (0.5a) 3
1
3
a3
close-packed directions
contains 8 x 1/8 =
1 atom/unit cell
Adapted from Fig. 3.24,
Callister & Rethwisch 8e.
volume
unit cell
18
Body Centered Cubic Structure (BCC)
• Atoms touch each other along cube diagonals.
--Note: All atoms are identical; the center atom is shaded
differently only for ease of viewing.
ex: Cr, W, Fe (), Tantalum, Molybdenum
• Coordination # = 8
Click once on image to start animation
(Courtesy P.M. Anderson)
Adapted from Fig. 3.2,
Callister & Rethwisch 8e.
2 atoms/unit cell: 1 center + 8 corners x 1/8
19
Diamond Structure
• Silicon or Carbon structure is basically a body-centered
cubic with four of the corner atoms missing.
• Every atom in the tetrahedral structure has four nearest
neighbors and it is this structure that is the basic building
block of the diamond lattice.
20
Atomic Packing Factor: BCC
• APF for a body-centered cubic structure = 0.68
3a
a
2a
Adapted from
Fig. 3.2(a), Callister &
Rethwisch 8e.
R
a
Close-packed directions:
length = 4R = 3 a
atoms
volume
4
p ( 3a/4) 3
2
unit cell
atom
3
APF =
volume
3
a
unit cell
21
Face Centered Cubic Structure (FCC)
• Atoms touch each other along face diagonals.
--Note: All atoms are identical; the face-centered atoms are shaded
differently only for ease of viewing.
ex: Al, Cu, Au, Pb, Ni, Pt, Ag
• Coordination # = 12
Adapted from Fig. 3.1, Callister & Rethwisch 8e.
Click once on image to start animation
(Courtesy P.M. Anderson)
4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8
22
Atomic Packing Factor: FCC
• APF for a face-centered cubic structure = 0.74
maximum achievable APF
Close-packed directions:
length = 4R = 2 a
2a
a
Adapted from
Fig. 3.1(a),
Callister &
Rethwisch 8e.
Unit cell contains:
6 x 1/2 + 8 x 1/8
= 4 atoms/unit cell
atoms
volume
4
3
p ( 2a/4)
4
unit cell
atom
3
APF =
volume
3
a
unit cell
23
FCC Stacking Sequence
• ABCABC... Stacking Sequence
• 2D Projection
B
B
C
A
B
B
B
A sites
C
C
B sites
B
B
C sites
• FCC Unit Cell
A
B
C
24
Theoretical Density, r
Density = r =
r =
where
Mass of Atoms in Unit Cell
Total Volume of Unit Cell
nA
VC NA
n = number of atoms/unit cell
A = atomic weight
VC = Volume of unit cell = a3 for cubic
NA = Avogadro’s number
= 6.022 x 1023 atoms/mol
25
Theoretical Density, r
• Ex: Cr (BCC)
A = 52.00 g/mol
R = 0.125 nm
n = 2 atoms/unit cell
Adapted from
Fig. 3.2(a), Callister &
Rethwisch 8e.
atoms
unit cell
r=
volume
unit cell
R
a
a = 4R/ 3 = 0.2887 nm
2 52.00
g
mol
a3 6.022 x 1023
rtheoretical = 7.18 g/cm3
ractual
= 7.19 g/cm3
atoms
mol
26
Miller Indices
1. The plane in Figure 1.8a is parallel to the b and c axes so
the Intercepts are  p =1, q =  and s 
Taking the reciprocal, we obtain the Miller indices as (1, 0, 0)
2.
The intercepts of the plane shown in Figure 1.8b are
p =1, q = 1 and s = 
3.
The intercepts of the plane shown in Figure 1.8c are
p =1, q = 1 and s = 1
27
Defects and Impurities
28
Semiconductor Growth
Molecular Beam Epitaxy
29
SUMMARY
• Atoms may assemble into crystalline or amorphous structures.
• Common metallic crystal structures are FCC, BCC, and
HCP. Coordination number and atomic packing factor
are the same for both FCC and HCP crystal structures.
• We can predict the density of a material, provided we
know the atomic weight, atomic radius, and crystal
geometry (e.g., FCC, BCC, HCP).
• Crystallographic points, directions and planes are
specified in terms of indexing schemes.
Crystallographic directions and planes are related
to atomic linear densities and planar densities.
30