Chapter 1
Part I
Semiconductor Materials
SEE 2063 Chapter 1, Part I , Semiconductor Materials 1
Objectives:
j
¾Discuss the basic structure of atoms
¾Discuss properties of insulators, conductors, and
semiconductors
¾Discuss covalent bonding
¾Describe the properties of both p and n type materials
I this
In
thi Part
P t I,
I we will:
ill
• gain a basic understanding of semiconductor material properties
–Two types of charged carriers that exist in a semiconductor
–Two mechanisms that generate currents in a semiconductor
SEE 2063 Chapter 1, Part I , Semiconductor Materials 2
Bohr model of
an atom
t
Electrons circle
th nucleus.
the
l
Atomic structure
of a material
determines its
ability to conduct
or insulate.
i) Conductor
ii) Semiconductor
iii) Insulator
Nucleus
SEE 2063 Chapter 1, Part I , Semiconductor Materials 3
Brief Review : Electric Charge
2 types of electric
charge
h
Positive charge
Negative charge
Electron
Negative charge
Proton
Positive charge
Neutral material
Electron = proton
Negative charged
material
Positive charged
material
Electron > proton
Electron < proton
SEE 2063 Chapter 1, Part I , Semiconductor Materials 4
Conductivity & Resistivity
The higher
g
the conductivity
y level, the lower the resistance level.
Conductor: Any material that will support a generous flow of charge when a
voltage source is applied across its terminals.
Insulator: A material that offers a very low level of conductivity under pressure
from an applied voltage source.
Semiconductor: A material that has a conductivity level somewhere between a
conductor and an insulator.
Resistivity, ρ, is often used when comparing the resistance levels of materials.
Its metric unit is
.
( )
R
ρ
RA (Ω ) cm 2
=
= Ωcm
ρ=
l
cm
1 cm
A=1 cm2
l 1 cm
l=1
SEE 2063 Chapter 1, Part I , Semiconductor Materials 5
Conductor
ρ ≅ 10 −6 Ωcm(copper
pp )
Semiconductor
Insulator
ρ ≅ 50Ωcm( germanium )
ρ ≅ 1012 Ωcm(mica)
ρ ≅ 50 × 10 3 Ωcm( Silicon)
Why semiconductor is used for electronic device?
(i) Th
Their
i material
t i l characteristics
h
t i ti can b
be changed
h
d significantly
i ifi
tl
through the process known as “doping”.
“Doping” is a process of adding impurity into semiconductor
material. That material can be changed from poor conductor
to a good conductor of electricity.
(ii) Their characteristics can be altered significantly through the
application of heat or light- an important consideration in the
development of heat- and light-sensitive devices.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 6
Materials:
C d t
Conductors,
IInsulators,
l t
and
d Semiconductors
S i
d t
¾ The ability of a material to conduct current is
based on its atomic structure.
Shell
¾ The orbit paths of the electrons
surrounding the nucleus are called shells.
Electron valence
N
M
L
K
¾ Each shell has a defined number of
electrons it will hold.
Ex:Copper:2 8 18 1 (1 electron valence)
Ex:Copper:2.8.18.1
¾ The outer shell is called the valence
shell
shell.
Copper Atom
Valence shell
¾ The less complete a shell is filled to capacity the
more conductive the material is
is.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 7
Shell
K
Sub-shell/state 1s
Numbers of electrons
2
L
M
2s
2p
3s
3p
3d
4s
4p
4d
4f
2
6
2
6
10
2
6
10
14
3d10
4s2
4p6 4d10
1s2 2s2 2p6
Symbol
Total numbers
of electrons in shell
N
2
3s2 3p6
8
4f14
32
18
For certain shell, there are sub-shells named as s, p, d and f.
n
Sub-shell
Shell
s
p
d
f
1
K
1s
2
L
2s
2p
3
M
3s
3p
3d
4
N
4s
4p
4d
4f
32
5
O
5s
5p
5d
5f
32
6
P
6s
6p
6d
7
Q
7s
7p
2
8
18
18
8
SEE 2063 Chapter 1, Part I , Semiconductor Materials 8
Conductors, Insulators, and Semiconductors
The valence shell determines the ability of material to
conduct current.
A Copper
C
atom
t
h
has
only
l 1
electron in its valence ring. This
makes it a good conductor.
Cu : 2.8.18.1
A Silicon
Sili
atom
t
h 4 electrons
has
l t
i
in
its valence ring. This makes it a
semiconductor.
Si : 2.8.4
SEE 2063 Chapter 1, Part I , Semiconductor Materials 9
Covalent Bonding
There are 4 electrons in the valence shell.
The potential (ionization potential) required to remove
any one of these 4 valence electrons is lower than that
required
q
for any
y other electron in the structure.
In silicon crystal, these 4 valence electrons are bonded
to 4 adjoining atoms.
Si : 2.8.4
atom
Materials
M
t i l iin group IV are referred
f
d tto as ttetravalent
t
l t atoms
t
because they each have 4 valence electrons.
Covalent bonding
(sharing of electrons)
A bonding of atoms, strengthened
by the sharing of electrons, is
called covalent bonding.
Although the covalent bond will
result in a stronger bond between
the valence electrons and their
parent atom, it is still possible for
the valence electrons to absorb
sufficient kinetic energy from
natural causes to break the
covalent bond and assume the
“free” state.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 10
Free Carriers
The natural causes include;
(i) effects such as light energy in the form of photons
photons.
(ii) thermal energy from the surrounding medium.
The “free”state
free state refers to existence of “free
free carriers
carriers”.
“carriers” refers to “electrons” and “holes”.
At room temperature,
temperature there are approximately 1
1.5
5 x 1010 free carriers in a
cubic centimeter of intrinsic silicon material.
The free carriers are sensitive to applied electric fields such as established by
voltage sources or any difference potential.
Produce the flow of current
SEE 2063 Chapter 1, Part I , Semiconductor Materials 11
Intrinsic Semiconductors
•
Intrinsic materials are those semiconductors that have been carefully refined to
reduce the impurities to a very low level ÆIdeally 100% pure material
•
Semiconductors can be grouped into two categories:
– Elemental semiconductors
• Silicon (Si)
– Most common semiconductor used today
• Germanium (Ge)
– First
Fi semiconductor
i
d
used
d iin p-n diodes
di d
– Compound semiconductors
• Gallium Arsenide (GaAs),
(GaAs) Gallium Nitride(GaN)
• Silicon Carbide (SiC)
SEE 2063 Chapter 1, Part I , Semiconductor Materials 12
Examples of Semiconductor Materials
Elemental
Semiconductors
IV Groups
Si, Ge
VI Groups
Se, Te
III-V Compound
Semiconductors
GaAs, GaP, InAs, InP
II-VI Compound
S i
Semiconductors
d t
ZnS,, ZnSe,, CdS,, CdSe
Oxide Compound
Semiconductors
ZnO, Cu2O
SEE 2063 Chapter 1, Part I , Semiconductor Materials 13
Effect of Temperature
hole
At 0K, no bonds are broken.
Si is an insulator.
As temperature increases, a bond can
break, releasing a valence electron and
leaving a broken bond (hole).
Current can flow.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 14
Effect of Temperature
Temperature
Conductor
Semiconductor
The numbers of carriers in a
conductor will not increase
significantly with temperature, but
their vibration pattern about a
relatively fixed location will make
it increasingly difficult for electrons
to pass through.
Increase in free
carriers
Resistivity
SEE 2063 Chapter 1, Part I , Semiconductor Materials 15
Energy Band Diagram
Isolated Atomic Structure
Energy
Energy gap
Energy gap
etc
etc.
Valence level
(outermost shell)
Second level
(next inner shell)
Third level
Nucleus
The more distant the electron from the nucleus,
the higher the energy state, and any electron that
has left its parent atom has a higher energy state
than any electron in the atomic structure.
Between the discrete energy levels are gaps in
which no electron can appear.
Crystal Lattice Structure
Energy
Conduction band
Energy gap
Valence band
As the atoms of a material are brought closer together to
form the crystal lattice structure, there is an interaction
between atoms that will result in the electrons in the
particular orbit of one atom having slightly different energy
levels from electrons in the same orbit of an adjoining
atom
atom.
The net result is an expansion of the discrete levels of
possible energy states for the valence electrons.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 16
Crystal Lattice Structure
Energy
Conduction band
Energy
gy g
gap
p/
Band gap
There are boundary levels and maximum energy states in
which any electron in the atomic lattice can find itself, and
there remains the forbidden region between the valence
band and the ionization level.
IIonization
i ti iis th
the mechanism
h i
whereby
h b electron
l t
can absorb
b b
sufficient energy to break away from the atomic structure
and enter the conduction band.
Valence band
Energy gap is measured in electron volt (eV)
Room temperature=300K
Energy
I
Insulator
l t
Conduction band
E
Energy
Electrons
Semiconductor
“free” to
establish
Conduction band
conduction
Eg > 5eV
Eg
Valence band
Valence
electrons
bound to
the
atomic
structure
Valence band
Energy Conductor
The bands
overlap
Conduction band
Valence band
Hole
Eg=1.1 eV(Si)
Eg=0.67eV(Ge)
Eg=1.41eV(GaAs)
SEE 2063 Chapter 1, Part I , Semiconductor Materials 17
Energy Band Diagram
Ev – Maximum
a
u e
energy
e gy o
of a valence
a e ce e
electron
ect o o
or hole
oe
Ec – Minimum energy of a free electron
Eg – Energy required to break the covalent bond
SEE 2063 Chapter 1, Part I , Semiconductor Materials 18
Movement of Holes
A valence electron in a
nearby bond can
move to fill the broken
bond making it
bond,
appear as if the ‘hole’
shifted locations.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 19
Intrinsic Carrier Concentration
ni = BT e
32
− Eg
2 kT
B – coefficient
ffi i t related
l t d to
t specific
ifi semiconductor
i
d t
T – temperature in Kelvin
Eg – semiconductor bandgap energy
k – Boltzmann’s constant
ni ( Si,300 K ) = 1.5 x10 cm
10
−3
SEE 2063 Chapter 1, Part I , Semiconductor Materials 20
Extrinsic
t s c Semiconductors
Se co ducto s
The characteristics of semiconductor materials can be altered significantly by the
addition of certain impurity
p y atoms into the relativelyy p
pure semiconductor material.
These impurities, although only added to perhaps 1 part in 10 million, can alter the
band structure sufficiently to totally change the electrical properties of the material.
A semiconductor material that has been subjected to the doping process is called
extrinsic semiconductor.
If certain impurities are added to the intrinsic semiconductor materials, energy
states in the forbidden bands will occur which will cause a net reduction in Eg
for both semiconductor materials – consequently, increased carrier density in
th conduction
the
d ti b
band
d att room ttemperature.
t
There are two types of extrinsic semiconductors : n-type and p-type
SEE 2063 Chapter 1, Part I , Semiconductor Materials 21
Phosphorous – Donor Impurity in Si
Phosphorous (P) replaces a Si atom and forms four covalent
bonds with other Si atoms.
The fifth outer shell electron of P is easily freed to become a
conduction band electron, adding to the number of electrons
available to conduct current.
Thi process will
This
ill create
t n-type
t
Si.
Si
SEE 2063 Chapter 1, Part I , Semiconductor Materials 22
Boron – Acceptor Impurity in Si
Boron (B) replaces a Si atom and forms only three covalent bonds
with other Si atoms.
The missing covalent bond is a hole, which can begin to move
through the crystal when a valence electron from another Si atom
is taken to form the fourth B-Si bond.
This process will create p
p-type
type Si
Si.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 23
Summary:
2 ttype off extrinsic
t i i semiconductors
i
d t
: n-type
t
and
d p-type
t
semiconductors.
i
d t
The process of creating n- and p-type materials is called doping.
How to create N-type
Example: Adding other atoms with 5
valence
l
electrons
l t
such
h as
Antimony(Sb),Arsenic(As) and
Phosporous(P) to Silicon to increase
tthe
e free
ee e
electrons.
ect o s
Donor atom
How to create P-type
Example: Adding other atoms with 3
l
electrons
l t
such
h as B
Boron(B),
(B)
valence
Gallium(Ga) and Indium(In) to Silicon to
create a deficiency of electrons or hole
ccharges.
a ges
Acceptor atom
SEE 2063 Chapter 1, Part I , Semiconductor Materials 24
The effect of this doping process can be described through the use of the energy-band
diagram.
Note that the discrete energy
gy level(donor
(
n-type
n
type
Energy
energy level) appears in the forbidden
gap/band with an Eg significantly less
than that of the intrinsic material. Those
Conduction band
“f ” electrons
l t
due
d to
t the
th added
dd d
Eg=0.05eV(Si), “free”
impurity sit at this energy level and have
0.01eV(Ge)
less difficulty absorbing a sufficient
Donor
energy
gy level
measure of thermal energy
gy to move into
Eg as before
the conduction band at room
temperature.
Valence band
The result is that at room temperature,
there are a large n
number
mber of
p-type
carriers(electrons) in the conduction
Energy
level and the conductivity of the material
increases significantly.
g
y
Conduction band
Eg as before
Acceptor
energy level
Explanation concept is same with the
above.
Valence band
SEE 2063 Chapter 1, Part I , Semiconductor Materials 25
Electron and Hole Concentrations
n = electron concentration
p = hole concentration
n = n⋅ p
2
i
p = n / ND
2
i
n-type:
n = ND, the donor concentration
p-type:
p = NA, the acceptor concentration
n = n / NA
2
i
SEE 2063 Chapter 1, Part I , Semiconductor Materials 26
Electron versus Hole Flow
-
-
Si
-
-
Si
-
-
Si
-
-
-
-
-
+
Si
-
-
-
Si
-
-
-
-
B
-
-
-
-
Si
-
-
The effect of the hole on conduction
is described. If a valence electron
acquires sufficient kinetic energy to
b k itits covalent
break
l tb
bond
d and
d fill
fills th
the
void created by a hole, then a vacancy
or hole, will be created in the covalent
bond that released the electron.
-
-
Si
-
-
There is, therefore, a transfer of holes
to the left and electrons to the right.
Hole flow
Electron flow
SEE 2063 Chapter 1, Part I , Semiconductor Materials 27
Majority and Minority Carriers
In the intrinsic state, the number of free electrons in Ge or Si is due only to those
few electrons in the valence band that have acquired sufficient energy from
thermal or light sources to break the covalent bond or to the few impurities that
could not be removed.
The vacancies left behind in the covalent bonding structure give limited supply of
holes.
In an n-type material, the number of holes has not changed significantly from this
Intrinsic level. The net result, therefore, is that the number of electrons far
outweighs the numbers of holes.
In an n-type material, the electron is called the majority carrier and the hole is
called the minority carrier.
For the p-type material the number of holes far outweighs the number of electrons.
In a p-type material the hole is the majority carrier and the electron is the
minority
i it carrier.
i
SEE 2063 Chapter 1, Part I , Semiconductor Materials 28
Donor ions
- - + - + +
- - - + - - - - - + - - +
+ +
- + - + +- +
- - + - - - - +
n-type
Acceptor ions
Majority
carrier
+ - + +
+
- + + +- + - + +
+
- + + +
- ++ - + - - +
+
+
-
Minority
carrier
p-type
When the fifth electron of a donor atom leaves the parent atom, the atom
remaining acquires a net positive charge: hence the positive sign in the
donor-ion representation.
For the similar reasons, the negative sign appears in the acceptor ion.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 29
Drift Currents
Electrons and hole flow in opposite directions when under the influence
of an electric field at different velocities.
The drift currents associated with the electrons and holes are in the
same direction.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 30
Diffusion Currents
Concentration
Concentration
High
High
Low
Low
Both electrons and holes flow from high concentration to low
low.
The diffusion current associated with the electrons flows in the opposite
direction when compared to that of the holes.
SEE 2063 Chapter 1, Part I , Semiconductor Materials 31