COURSE NAME: SEMICONDUCTORS
Course Code: PHYS 473
Learning Outcomes

Define conductor, insulator and semiconductor, and state the
resistance or conductance of each.

Name at least three semiconductor materials and state the
most widely used.

Name the basic structure of material and explain how it is
formed with atoms.

Define doping and name the two types of semiconductor
material formed with doping.

Name the current carriers in N and P-type material.

Explain how current flows in semiconductor material.
Electronic Materials
Conductors
Good conductors have low resistance so
electrons flow through them with ease.
Examples:
1. MetalsCopper, silver,
aluminum, & nickel.
2. Alloys- Brass & steel.
3. Liquids- Salt water
gold,
Conductor Atomic Structure
The atomic structure of good
conductors usually includes only one
electron in their outer shell.
1.
It is called a valence electron.
2.
It is easily striped from the
atom, producing current flow.
Insulators
Insulators have a high resistance so current does not flow in
them.
Examples:
• Glass, ceramic, plastics, & wood.
The atoms are tightly bound to one another so electrons are
difficult to strip away for current flow.
Semiconductors
Semiconductors are materials that essentially can be conditioned
to act as good conductors, or good insulators, or any thing in
between.
Examples:
Carbon, Silicon, Germanium.
Silicon is the best and most widely used semiconductor.
• Silicon (Si) is in the IVA Column (4 valence electrons)
• Boron (B) is in the IIIA Column (3 valence electrons)
• Phosphorus (P) is in the VA Column (5 valence electrons)
Semiconductor Valence Orbit
• The main characteristic of a
semiconductor element is that it
has four electrons in its outer or
valence orbit.
Crystal Lattice Structure
• The
unique capability of
semiconductor atoms is their
ability to link together to form a
physical structure called a
crystal lattice.
• The atoms link together with
one another sharing their outer
electrons.
• These links are called covalent
bonds.
2D Crystal Lattice Structure
11
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
S
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
S
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
S
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
S
Si
Si
Si
Si
Si
Si
Si
Si
S
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
S
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
S
Covalent Bonds:
Si
Si
Shared electrons to
fill orbital
Intrinsic Silicon
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Poor conductor: No free electrons to carry current
Need to engineer electrical properties (conduction)
Semiconductors can be Insulators?
• If the material is pure semiconductor material like silicon, the
crystal lattice structure forms an excellent insulator since all the
atoms are bound to one another and are not free for current
flow.
• Good insulating semiconductor material is referred to as
intrinsic.
• Since the outer valence electrons of each atom are tightly bound
together with one another, the electrons are difficult to dislodge
for current flow.
• Silicon in this form is a great insulator.
• Semiconductor material is often used as an insulator.
Doping
Doping is the incorporation of impurities into a semiconductor
according to our requirements.
“Impurities” are different elements.
Examples: Boron, Arsenic
Semiconductors can be Conductors
• An
impurity, or element like
arsenic, has 5 valence electrons.
• Adding arsenic (doping) will allow
four of the arsenic valence
electrons to bond with the
neighboring silicon atoms.
• The ONE electron left over for
each arsenic atom becomes
available to conduct current flow.
N-type Doping
Si
Si
Si
Si
Si
Si
Si
Si
Si
P
Si
Si
Si
Si
Si
Si
Si
Si
Si
P
Si
Si
P
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Each N-type dopant brings an extra electron to the lattice
Resistance Effects of Doping
• If you use lots of arsenic atoms for doping, there will be lots of
extra electrons so the resistance of the material will be low and
current will flow freely.
• If you use only a few boron atoms, there will be fewer free
electrons so the resistance will be high and less current will
flow.
• By controlling the doping amount, virtually any resistance can
be achieved.
N-Type Doping
Conduction
Band
Energy
Energy
Gap
P
P
P
P
P
P
P
P
P
P
P
P
P
Valence
Band
Position
Doping the silicon lattice with atoms with 5 valence electrons (V) create sites in
the band diagram that require little energy to break the bond to the dopant atom
and become free to move in the lattice or in other words move into the
conduction band.
P-type Doping
• You can also dope a semiconductor material with an atom such as
•
•
•
•
boron that has only 3 valence electrons.
The 3 electrons in the outer orbit do form covalent bonds with its
neighboring semiconductor atoms as before. But one electron is
missing from the bond.
This place where a fourth electron should be is referred to as a hole.
The hole assumes a positive charge so it can attract electrons from
some other source.
Holes become a type of current carrier like the electron to support
current flow.
P-type Doping
Si
Si
Si
Si
Si
Si
Si
Si
B
Si
Si
B
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
B
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Each P-type dopant is short an electron, creating a hole in the lattice
P-Type Doping
Conduction
Band
Energy
Energy
Gap
B
B
B
B
B
B
B
B
B
B
B
B
B
Valence
Band
Position
Doping the silicon lattice with atoms with 3 valence electrons (III) create sites
in the band diagram that require little energy to trap an electron into the dopant
atom. Holes are created in the valence band that are free to move.
Types of Semiconductor Materials
• The silicon doped with extra electrons is called an “N type”
semiconductor.
• “N” is for negative, which is the charge of an electron.
• Silicon doped with material missing electrons that produce
locations called holes is called “P type” semiconductor.
• “P” is for positive, which is the charge of a hole.
Current Flow in Semiconductors
For a pure (intrinsic) semiconductor at T= 0° K;
All electrons are associated with
their covalent bonds.
2. The conduction band (above
energy EC) is empty of electrons.
3. The valence band (below energy
EV) is full.
4. Eg (energy-gap) = EC - EV.
1.
5.
Eg (energy-gap) is the forbidden
gap.
Current Flow in Semiconductors (continued……)
For a pure (intrinsic) semiconductor at T > 0° K;
1.
Some electrons gain enough
energy to break their bonds and
jump the forbidden gap (only one
shown).
2.
The conduction band now contains
free electrons, while the valence
band now has free holes.
Current Flow in Semiconductors (continued……)
When we apply potential to a pure (intrinsic) semiconductor;
Electrons in the conduction band
move to the right.
2. Electrons in the valence band also
move to the right, but move by
filling a hole.
3. This process is equivalent to holes
moving to the left.
4. Number of holes in the valence
band = Number of electrons in the
conduction band.
1.
Current Flow in Semiconductors (continued……)
When we apply potential to an n-type semiconductor;
1.
Electrons
carrier.
are
the
majority
2.
There are more free electrons in
the conduction band than holes
in the valence band.
Current Flow in Semiconductors (continued……)
When we apply potential to a p-type semiconductor;
1.
Holes are the majority carrier.
2.
There are more holes in the
valence band than electrons in
the conduction band.
Forming a p-n Junction
Doping one side of a piece of silicon with boron (a p-type
dopant) and the other side with phosphorus (an n-type dopant)
forms a p-n junction.
Doped silicon.
n-type and p-type materials
brought together.
PN-junction model, schematic symbol, physical part.
Diode: A semiconductor device, which conduct the current in one
direction only.
Diffusion establishes “built-in” electric field.
1.
In a process called "diffusion", the
random
motions
of
mobile
electrons and holes over time cause
them to attempt to distribute
equally within the total volume.
2.
As they cross the junction, the
fixed ions they leave behind
establish a "built-in" electric field
at the junction.
Motion of mobile electrons and holes due to
diffusion and the “built-in” electric field.
A mobile electron or hole near the "built-in" electric field will be
attracted and swept back into its original volume. At the junction
there are two effects occurring:
1. Diffusion with electrons moving from n-type to p-type.
2. The "built in" electric field sweeping locally affected electrons
back into the n-type volume. The holes are affected similarly but in
opposite directions.
Depletion Region
Within the depletion region, there are very few mobile electrons
and holes. It is "depleted“ of mobile charges, leaving only the
fixed charges associated with the dopant atoms.
As a result, the depletion region is highly resistive and now
behaves as if it were pure crystalline silicon: as a nearly perfect
insulator.
Depletion Region
Forward Bias of the p-n junction
If a positive voltage is applied to the
p-type side and a negative voltage
to the n-type side, current can flow.
This configuration is called
"Forward Biased“.
At the p-n junction, the "built-in"
electric field and the applied electric
field are in opposite directions.
Reverse bias of the p-n Junction
If a negative voltage is applied to
the p-type side and a positive
voltage to the n-type side, no (or
exceptionally
small)
current
flows. This configuration is called
"Reverse Biased".
At the p-n junction, the "built-in"
electric field and the applied
electric field are in the same
direction.
I-V curve of the silicon p-n junction diode
Applications of diodes
1.
2.
3.
4.
5.
Temperature measuring
Ionizing Radiation detectors
Logic gates
Power conversion
Radio demodulation