Lec 3 - SEMICONDUCTORS.ppsx Lec 3 - SEMICONDUCTORS.ppsx

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1. SEMICONDUCTORS

2. WHAT IS A
SEMICONDUCTOR
?
A semiconductor is a material that is neither a good conductor
nor a good insulator.
In their purest form, semiconductors have few applications in
electronics.
However, when the characteristics of a pure semiconductor
are altered through a process known as doping, many useful
electronic devices can be developed.
Some common semiconductor materials are silicon (Si),
germanium (Ge), and carbon (C).

3. Terms to
Remember
Impurities – Donor or Acceptor atoms
Doping – Process of adding impurities to a pure semiconductor
Dopants – Elements used for doping
Pure Semiconductor – Single element present in structure
Intrinsic Semiconductor - pure semiconductor without any
significant dopant species present
Extrinsic Semiconductor - one that has been doped

4. Short
Review:
THE OCTET
RULE
the tendency of atoms to prefer to have eight electrons in
the valence shell.

5. A Silicon Atom
Structure
Pure Silicon
Semiconductor

6. N-type
Semiconducto
r
These are materials which
have Pentavalent impurity
atoms (Donors) added and
conduct by “electron”
movement and are
therefore called, N-type
Semiconductors.

7. N-type
Semiconducto
r
Created when the dopant is an element that has five electrons in
its valence layer. Phosphorus, Arsenic and Antimony is commonly
used for this purpose.
The antimony atoms join right in the crystal structure of the
silicon, each one bonding with four adjacent silicon atoms just
like a silicon atom would. Because the antimony atom has five
electrons in its valence shell, but only four of them are bonded
to adjacent atoms, the fifth valence electron is left hanging out
with nothing to bond to.
The extra valence electrons in the antimony atoms start to
behave like the single valence electrons in a regular conductor
such as copper. They are free to move about.
Because this type of semiconductor has extra electrons, it’s
called an N-type semiconductor.

8. P-type
Semiconducto
rs
These are materials which
have Trivalent impurity
atoms (Acceptors) added
and conduct by “hole”
movement and are
therefore called, P-type
Semiconductors.

9. P-type
Semiconductor
s
Happens when the dopant (such as boron) has only three electrons in
the valence shell.
When a small amount is incorporated into the crystal, the atom is able
to bond with four silicon atoms, but since it has only three electrons
to offer, a hole is created.
The hole behaves like a positive charge, so semiconductors doped in
this way are called P-type semiconductors.
Like a positive charge, holes attract electrons. But when an electron
moves into a hole, the electron leaves a new hole at its previous
location. Thus, in a P-type semiconductor, holes are constantly moving
around within the crystal as electrons constantly try to fill them up.

10. P-N Junction
Diode
A popular semiconductor device called a diode is
made by joining p - and n –type semiconductor
materials
Notice that the doped regions meet to form a p-n
junction. Diodes are unidirectional devices that
allow current to flow through them in only one
direction.

11. P-N Junction
Diode
Reverse-biased
The polarity of voltage across
a diode that prevents the
diode from conducting any
current.
Leakage Current – current
produced in a reverse-biased
diode
Forward-biased
 Forward-biasing a diode
allows current to flow easily
through the diode.

12. Light
Emitting
Diode
When free electrons from the n
side cross over into the p side, they
fall into a hole. When an electron
falls, it releases energy. This
energy is mainly heat or light.
For the normal silicon diode, the
light cannot escape because the
device is not transparent.
Because LEDs use a
semitransparent material,
however, light can escape to the
surrounding environment.
The color of the light emitted from
the LED depends on the type of
element used in the manufacture
of the LED.

13. Types of
Diodes and
their
Symbols

14. Transistors
NPN Transistors
PNP Transistors

15. Types of
Transistors
and their
symbols

16. Transistors
(sample
circuits with
transistors)

17. Transistors
(sample
circuits with
transistors)

18. Thank you!