introduction to semiconductors

1. SEMICONDUCTORS
By Unsa Shakir

2. Electrically, all materials fall into 1
of 3 classifications:
• Conductors
• Insulators
• Semi-Conductors

3. Conductors:
• In general all metals are good
conductors.
• The purpose of conductors is to allow
electrical current to flow with minimum
resistance.
• Examples:
Copper, gold, silver,iron,bronze

4. Insulators
• Insulators are used to prevent the flow of
electricity.
• Insulating materials such as glass, rubber,
plastic, polyethylene are also called
dielectrics, meaning they can store charges.
• Dielectric materials are used in components
like capacitors which must store electric
charges

5. Semi-Conductors:
• Materials which are neither conductors nor insulators i.e;
whose electrical properties lie between Conductors and
Insulators.
• Used in components like transistors
 Some common semiconductors
 elemental
 Si - Silicon , Ge - Germanium
 compound
 GaAs - Gallium arsenide, GaP - Gallium
phosphide, AlAs - Aluminum arsenide, AlP -
Aluminum phosphide, InP - Indium Phosphide

6. Silicon
Nucleus
Valence Band
Energy Bands
(Shells)
Si has 14 Electrons
Silicon has 4 outer shell /
valence electrons

7. Intrinsic semiconductors:-
Intrinsic semiconductors are pure semiconductors, no impurities
are added in these conductors.
The number of free electrons in the conduction band is equal to
the number of holes in the valence band.
Conductivity of these semiconductors is low
Electrical conductivity is a function of temperature alone
Example: Crystalline form of pure Silicon and Germanium.
TYPES OF SEMICONDUCTOR

8. Intrinsic Semiconductor
Forms into a lattice structure to
share electrons

9. EXTRINSIC SEMICONDUCTORS
 An extrinsic semiconductor is a semiconductor that has
been doped, that is, into which a doping agent has been
introduced, giving it different electrical properties than the
intrinsic semiconductor.
 The number of electrons and holes are not equal. There is
excess of electrons in n-type semi-conductors and excess of
holes in p-type semi-conductors.
 Electrical conductivity is high
 Electrical conductivity depends on temperature as well as on
the amount of impurity doping in the pure semiconductor
 Examples:Impurity like As, Sb, P, In, Bi, Al etc. are dopped
with Germanium and Silicon atom

10. Improving Conduction by Doping
 Doping involves adding dopant atoms to an intrinsic
Semiconductor to make semiconductors better conductors, add
impurities (dopants) to contribute extra electrons or extra holes
 Dominant carrier concentrations in an extrinsic semiconductor
classify it as either an n-type or p-type semiconductor.
 elements with 5 outer electrons contribute an extra electron to
the lattice (donor dopant)
 elements with 3 outer electrons accept an electron from the
silicon (acceptor dopant)

11. What are P-type and N-type ?
 Semiconductors are classified in to P-type and
N-type semiconductor
 P-type: A P-type material is one in which
holes are majority carriers i.e. they are
positively charged materials (++++)
 N-type: A N-type material is one in which
electrons are majority charge carriers i.e. they
are negatively charged materials (-----)

12. THE P-N JUNCTION

13. The Junction

14. Diodes
Electronic devices created by bringing
together a p-type and n-type region within the
same semiconductor lattice. Used for
rectifiers.

15. Diodes
It is represented by the following symbol,
where the arrow indicates the direction of
positive current flow.

16. Characteristics of Diode
 Diode always conducts in one direction.
 Diodes always conduct current when
“Forward Biased” ( Zero resistance)
 Diodes do not conduct when Reverse Biased
(Infinite resistance)

17. Forward Bias and Reverse Bias
 Forward Bias : Connect positive of the Diode
to positive of supply…negative of Diode to
negative of supply
 Reverse Bias: Connect positive of the Diode
to negative of supply…negative of diode to
positive of supply.

18. P-N Junction – Forward Bias
 positive voltage placed on p-
type material
 holes in p-type move away from
positive terminal, electrons in n-
type move further from negative
terminal
 depletion region becomes
smaller - resistance of device
decreases
 voltage increased until critical
voltage is reached, depletion
region disappears, current can
flow freely

19. P-N Junction – Reverse Bias
 positive voltage placed on n-
type material
 electrons in n-type move closer
to positive terminal, holes in p-
type move closer to negative
terminal
 width of depletion region
increases
 allowed current is essentially
zero (small “drift” current)

20. Note:
 The chief difference between germanium and
silicon diodes is the voltage at which electric
current begins to flow freely across the diode.
A germanium diode typically begins to
conduct electric current when voltage
properly applied across the diode reaches 0.3
volts. Silicon diodes require more voltage to
conduct current; it takes 0.7 volts to create a
forward-bias situation in a silicon diode