1. types of semiconductor 2. Doping 3. diffusion 4.Energy band diagram

1. Semiconductor Fundamentals

2. Conductors Insulators Semiconductors
Materials
have low resistance
which allows
electrical current
flow
Ex.:Copper, silver,
gold, aluminum, &
nickel
have high resistance
which suppresses
electrical current
flow
Ex:Glass, ceramic,
plastics, & wood
can allow or
suppress electrical
current flow
Ex: carbon, silicon,
and germanium
2

3. 1. Insulators: suppress flow of electric current (high
resistance).
2. Conductors: allow electrical current flow. Low
resistance, high conductivity.
3. Semiconductors: can allow or suppress electrical
current flow .
Difference in conductivity

4. Energy Band models for solids
Band Theory
An energy band is a range of allowed electron energies.
Useful way to visualize the difference between
conductors, insulators and semiconductors is to plot the
available energies for electrons in the materials.
The last completely filled (at least at T = 0 K) band
is called the Valence Band
• The next band with higher energy is the
Conduction Band
• The Conduction Band can be empty or partially
filed
• The energy difference between the bottom of the
CB and the top of the VB is called the Band Gap
(or Forbidden Gap)

5. Energy Bands for Solids

6. Insulators
 material does not conduct electric current. Example: glass, ceramics, plastics
 valence electron are tightly bound to the atom – less free electron
there is a large forbidden gap between the energies of the valence electrons and the
energy at which the electrons can move freely through the material (the conduction
band).
Semiconductors, Conductors and Insulators

7. Conductors
material that easily conducts electrical current.
 Metals (copper, silver, gold, aluminum) are good conductors of
electricity.
 valence electrons are very loosely bound to the atom
There is an overlap of the valence band and the conduction band
so that at least a fraction of the valence electrons can move through
the material.
Semiconductors, Conductors and Insulators

8. Semiconductor
A semiconductor material is one whose electrical properties lie in
between those of insulators and good conductors.
Examples are: germanium and silicon.
In terms of energy bands, semiconductors can be defined as those
materials which have almost an empty conduction band and
almost filled valence band with a very narrow energy gap (of the
order of 1 eV) separating the two.

9. • Conductors have free electrons and partially filled
valence bands, therefore they are highly conductive .
• Insulators have filled valence bands and empty
conduction bands, separated by a large band gap E
g(typically >4eV), they have high resistivity (c ).
• Semiconductors have smaller band gap. Some
electrons can jump to the empty conduction band by
thermal or optical excitation E g=1.1 eV for Si, 0.67 eV
for Ge and 1.43 eV for GaAs
9
Band gap = the minimum photon energy required to
excite an electron up to the conduction band from
the valence band.

10. Semiconductor Materials (cont.)
Semiconductor
Bandgap
Energy EG (eV)
Carbon (diamond) 5.47
Silicon 1.12
Germanium 0.66
Tin 0.082
Gallium arsenide 1.42
Gallium nitride 3.49
Indium phosphide 1.35
Boron nitride 7.50
Silicon carbide 3.26
Cadmium selenide 1.70

11. INTRINSIC AND EXTRINSIC
SEMICONDUCTORS

12. Intrinsic semiconductors
• A Semiconductor in its
extremely pure form is said
to be an intrinsic
semiconductor.
• Germanium and Silicon (4th
group elements) are the
best examples of intrinsic
semiconductors and they
possess diamond cubic
crystalline structure.

13. Si
Si
Si
Si
Si
Valence Cell
Covalent bonds
Intrinsic Semiconductor

14. Extrinsic semiconductors
• Extrinsic semiconductors are the impure semiconductor
that contains added impurities.
• The conductivity of an extrinsic semiconductor is very
large compared to the intrinsic material, due to the
presence of impurity atoms.
• An impure semiconductor, which is formed by doping a
pure semiconductor is called as an extrinsic
semiconductor
Doping
The process of adding impurities to the semiconductor
materials is termed as doping.
• There are two types of extrinsic semiconductors
depending upon the type of impurity added .They are N-
type extrinsic semiconductor and P-Type extrinsic
semiconductor.

15. N-type semiconductor:
- Pentavalent impurities are added to Si or Ge, the result is an
increase the free electrons
- Extra electrons becomes a conduction electrons because it is not
attached to any atom
- No. of conduction electrons can be controlled by the no. of impurity atoms
- Pentavalent atom gives up an electron -call a donor atom
- Current carries in n-type are electrons – majority carries
- Holes – minority carries
Pentavalent impurity atom in a Si crystal
Sb
impurity
atom
N-types and P-types Semiconductors

16. N-Type
A small amount of pentavalent impurity is added to a pure
semiconductor to result in N type extrinsic semiconductor. The
added impurity has 5 valence electrons.

17. - Trivalent impurities are added to Si or Ge to create a deficiency of
electrons or hole charges
- The holes are created by doping process
- The no. of holes can be controlled by the no. of trivalent impurity atoms
- The trivalent atom can take an electron- acceptor atom
- Current carries in p-type are holes – majority carries
- electrons – minority carries
B
impurity
atom
P-types Semiconductors

18. P-Type
A small amount of trivalent impurity is added to a pure semiconductor to result in P-
type extrinsic semiconductor. The added impurity has 3 valence electrons.

19. P-Type
Atoms from column III (B, Al, Ga, and In) introduce impurity levels
in Ge or Si near the valence band. These levels are empty of electrons
at 0 K.
Since this type of impurity level "accepts" electrons from the valence
band, it is called an acceptor level, and the column III impurities are
acceptor impurities in Ge and Si.
Doping with acceptor impurities can create a semiconductor with a
hole concentration p0 much greater than the conduction band electron
concentration n0 (this is p-type material). Thus holes are majority
carriers and electrons are minority carriers.

20. PN Junction Diode
• A p-n junction diode is two-terminal or two-electrode
semiconductor device, which allows the electric current in
only one direction while blocks the electric current in opposite
or reverse direction.
• A PN junction behaves as ideally short circuit when it is in
forward biased and behaves as ideally open circuit when it is
in the reverse biased

21. • When the junction is first formed, mobile carriers diffuse across
the junction (due to the concentration gradients)
– Holes diffuse from the p side to the n side, leaving
behind negatively charged immobile acceptor ions
– Electrons diffuse from the n side to the p side, leaving
behind positively charged immobile donor ions
A region depleted of mobile carriers is formed at the junction.
• The space charge due to immobile ions in the depletion region
establishes an electric field that opposes carrier diffusion.
Equilibrium pn Junction
+
+
+
+
+
–
–
–
–
–
p n
acceptor ions donor ions

22. Unbiased pn junction
Potential barrier
• the barrier opposes the flow of majority charge carriers
• for an isolated junction net current is zero

23. Diode Forward biased
VD
The process of applying the external voltage to a p-n junction
semiconductor diode is called biasing.

24. • In Forward bias, negative terminal is connected to the n-type
semiconductor and positive terminal is connected to the p-type
semiconductor.
• Holes from the p-side are attracted towards the negative
terminal whereas free electrons from the n-side are attracted
towards the positive terminal.
• The electrons travel from the n-side to the p-side and go to the
positive terminal of the battery. The holes that travel from the
p-side to the n-side combine with the electrons injected into
the n-region from the negative terminal of the battery. This
way the diode conducts when forward-biased.
• This decreases the width of depletion region.
• Hence, the width of the depletion region decreases with
increase in voltage.

25. The p-side or the positive side of the semiconductor has an
excess of holes and the n-side or the negative side has an excess
of electrons.
In a semiconductor, the p-n junction is created by the method of
doping.

26. Diode Reverse Biased

27. In Reverse bias, negative terminal is connected to the p-type
semiconductor and positive terminal is connected to the n-type
semiconductor,
holes from the p-side are attracted towards the negative terminal
whereas free electrons from the n-side are attracted towards the
positive terminal.
If the reverse biased voltage applied on the p-n junction diode is
further increased, then even more number of free electrons and holes
are pulled away from the p-n junction.
This increases the width of depletion region.
Hence, the width of the depletion region increases with increase in
voltage.
The wide depletion region of the p-n junction diode completely
blocks the majority charge carriers.
Hence, majority charge carriers cannot carry the electric current.

29. Junction Diode Static I-V Characteristics
• The forward voltage at which the flow of current through the PN Junction starts
increasing quickly is known as knee voltage. This voltage is also known as cut-in voltage
• The maximum reverse bias voltage that can be applied to a p-n diode is limited by
breakdown. Breakdown is characterized by the rapid increase of the current under
reverse bias. The corresponding applied voltage is referred to as the breakdown voltage.

30. Ideal Diode Equation









 1
nkT
qV
S
D
D
e
I
I

31. Diode Applications
• Rectifying a voltage, such as turning the AC
into DC voltages.
• Isolating signals from a supply.
• Voltage Reference.
• Wave shaping
• Detection signals.
• Lighting systems (LED)
• LASER diodes.
31

32. Diode Breakdown Voltage
• When a reverse bias is applied no conduction should take place.
• But due to the presence of minority charge carriers, a small reverse
current flows through the diode known as leakage current.
• Due to the flow of reverse current the width of the junction barrier
increases.
• When this applied reverse bias voltage is increased gradually at a
certain point a rapid increase in the reverse current can be
observed. This is known as Junction breakdown.
• The corresponding applied reverse voltage at this point is known
as Breakdown Voltage of the PN junction diode. This is also known
as Reverse Breakdown Voltage.
• Exceeding this voltage level causes the exponential increase in the
leakage current of the diode. When a diode breakdown’s,
overheating can be observed.

33. Zener vs. Avalanche Breakdown
• Zener breakdown is a result of the large electric field
inside the depletion region that breaks electrons or
holes off their covalent bonds.
• Avalanche breakdown is a result of electrons or holes
colliding with the fixed ions inside the depletion region.