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Chapter 1
Diode Basics, Application and Special
Diodes
WEEK 1
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Chapter Outline
1.1 Atomic Structure of Semiconductor
1.2 Semiconductor Material and PN Junction
1.3 Diode
1.4 Diode Characteristics
1.5 Special Purposes Diodes
1.6 Diode in DC and AC Circuits
1.7 Diode Applications
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Atomic Structure
All matter is made from atom.
All atoms consists of electrons (negative charge), protons(positive charge) and neurons (uncharged).
An atom is the smallest particle of an element.
Each type of atom has a certain number of electrons andprotons that distinguishes it from the atoms of all otherelements.
The atomic number equals the number protons in the
nucleus, which is the same as the number of electrons in anelectrically balanced (neutral) atom. (the positive chargescancel the negative charges and the atom has a net charge ofzero).
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Atomic Structure
Energy increases as the distance from the nucleus increases
(electrons near nucleus have less energy than those in more
distant orbits).
These energy level known as shells, and each shell has a
maximum number of electrons at permissible energy levels.
There are 7 layer of shell that orbit the nucleus.
The maximum electron, Ne are determine by, 2 x n2, where n is
number of layer of shell.
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Atomic Structure
The Bohr model of an atom showing electrons in orbits around
the nucleus, which consists of protons and neutrons
(Nitrogen configuration)
electron
neuron
proton
http://education.jlab.org/qa/atom_model_03.gif
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Atomic Structure
Electrons that are in orbits farther from nucleus have higher
energy and are less tightly bound to the atom than those
closer to the nucleus.
This is because the force of attraction between the positively
charged nucleus and the negatively charged electron
decreases with increasing distance from the nucleus.
Electrons in the outermost shell have highest energy.
The outermost shell is known as valence shell and electrons in
the shell are called valence electrons.
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Atomic Structure
Atomic number for Aluminium is 13.
i. What is electron configuration for Aluminium?
ii. Draw a diagram of a Aluminium atom.
iii. How many electrons valence for Aluminium atom?
+13
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Atomic Structure
When an atom absorbs energy, the energies of the electronsare raised.
The electron valence posses more energy and are moreloosely bound to the atom. So they can easily jump to higher
orbits within the valence shell when external energy isabsorbed by the atom.
If a valence electrons acquires a sufficient amount of energy,it can actually escape from the outer shell and atomsinfluence.
The process of losing a valence electron is known asionization (the atom become positive charge with moreprotons than electrons).
The escape valence electron is called a free electron.
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Atomic Structure
The chemical symbol for hydrogen is H. When a neutral
hydrogen loses its valence electron and becomes a positive
ion, it is designated H+.
What is happened if the hydrogen atom receive electron?
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Insulators
An insulator is a material that does not conduct electrical
current under normal conditions.
Valence electrons are tightly bound to the atoms, therefore,
there are very few free electrons in an insulator.
Examples of insulators are rubber, glass, mica, etc.
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Conductors
A conductor is a material that easily conducts electrical
current.
The best conductors are single-elements materials (copper,
silver, gold), which are characterized by atoms with only one
valence electron very loosely bound to the atom.
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Semiconductors
A semiconductor is a material that is between conductors and
insulators in its ability to conduct electrical current.
A semiconductor in its pure (intrinsic) state is neither a good
conductor nor a good insulator.
The most common single-element semiconductors are silicon,
germanium and carbon (the single-element semiconductors
are characterized by atoms with four valence electron).
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Energy Bands
When an electron acquires enough additional energy, it can
leave the valence shell, become free electron, and exist in
what is known as the conduction band.
The different in energy between the valence band and the
conduction band is called an energy gap.
This is the amount of energy that a valence electron must
have in order to jump from the valence band to the
conduction band.
Once in the conduction band, the electron is free to move
throughout the material and is not tied to any given atom.
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Energy Bands
Valence band
Conduction band
Energy
Valence band
Conduction band
Energy
Valence band
Conduction band
Energy
Overlap
a) Insulator b) Semiconductor c) Conductor
Energy diagrams for the three types of materials
Energy gap
Energy gap
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Energy Bands
The valence electron of metal are held loosely by all atom and
free to move about. This electron holds the positive ions of
the metal together, forming metallic bonding.
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Comparison Between Conductor,
Semiconductor and Insulator
Characteristic Conductor Insulator Semiconductor
Resistance
Low resistance for
easy current flow.
High resistance so
current cannot
flow.
Between
conductor and
insulator.
Valence
electron
Atom is tend to
release valence
electron and it
flow freely from
one atom to
another.
Atom is tend to
absorb valence
electron to valence
layer to make it
stable and try
avoid electrical of
chemical activity.
Difficult to free or
accept valence
electron from other
atom.
Energy band
Conduction and
valence band
overlap, electron
easily move
The energy gap is
big, so electron
cannot easily
move.
Between
conductor and
insulator.
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Silicon and Germanium
Silicon is the most widely used material in diodes,
transistors, integrated circuits and other semiconductors
devices.
Both silicon and germanium have the characteristic of four
valence electrons.
Silicon atom
Germanium atom
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Silicon and Germanium
The valence electrons in germanium are in fourth shell while
in silicon in the third shell, closer to the nucleus.
Germanium valence electrons are at higher energy level than
those in silicon (require a small amount of additional energy
to escape from the atom).
This property makes germanium more unstable at high
temperature.
This is why silicon is a more widely used as a semi-conductive
material.
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Covalent Bonds
Atom with 4 valence electron is not stable and it try tocomplete it from 4 to 8 by sharing valence electron with other
atom.
The sharing valence electron is called covalent bond.
This will make the atom is stable and the bond is strong. At absolute zero temperature all electron is in valence band
and it will act as insulator.
At room temperature many electron have sufficient energy to
move to conduction band and it will act as conductor. Silicon absorb more heat than germanium before it act as
conductor, so its more popular.
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Silicon Covalent Bonds
Figure below shows how each silicon atom positions itself withfour adjacent silicon atoms to form a silicon crystal.
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Silicon Covalent Bonds
This effectively creates eight shared valence electrons for
each atom and produces a state of chemical stability.
The centre silicon atom shares an electrons with each of four
surrounding silicon atoms, creating a covalent bond with
each. The surrounding atom are in turn bonded to other
atoms and so on.
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Current In Semiconductors-Conduction Electrons and Holes-
When an intrinsic (pure) silicon crystal covalent bondinterrupted by heat, temperature, doping electron will releasefrom the atom bond and jump to conduction band becomingfree electrons.
Free electrons are also called conduction electrons. Electronis negative charge and is called negative current carrier.
A vacancy is left in the valence band and it is called hole(itspositive charge). It also called positive current carrier.
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Hole
Electron
Heat energy
For every electron free, there is one hole left in valence band
and create electron-hole pair. Recombination occurs when a
conduction band lost energy.
Current In Semiconductors-Conduction Electrons and Holes-
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Current In Semiconductors-Electrons and Holes Current-
Electron current in intrinsic silicon is produced by the movement of
thermally generated free electrons
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Current In Semiconductors-Electrons and Holes Current-
When a voltage is applied across a piece of intrinsic silicon,
free electrons are generated in the conduction band (easily
attracted toward the positive end).
The movement of these free electrons is one type of current
in a semiconductive materials, called electron current.
Another type of current occurs in the valence band, where
the holes created by the free electrons exist.
Electrons remaining in the valence band are still attached to
their atoms and are not free to move randomly as the free
electrons.
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Current In Semiconductors-Electrons and Holes Current-
However, a valence electron can move into a nearby hole with
little change in its energy electrons, thus leaving another hole
where it came from.
Effectively the hole has moved from one place to another in
the crystal structure.
This is called hole current even though the current in the
valence band is produced by valence electrons.
When a valence electron moves left to right to fill a hole while
leaving another hole behind, the hole has effectively move
from right to left.
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N-Type and P-Type Semiconductor
Doping is a process which impurity atoms are introduces to
intrinsic semiconductor in order to alter the balance between
holes and electron.
There are two categories of impurities : n-type and p-type
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N-Type semiconductor
To increase the number of conduction-band electrons in
intrinsic silicon, pentavalent atom (atoms with 5 valence
electrons such as arsenic (As), antimony (Sb)) are added.
Pentavalent impurity atom in a silicon crystal structure. An antimony (Sb)
impurity atom is shown in the centre. The extra electron from the Sb atom
becomes a free electron.
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N-Type semiconductor
Each pentavalent atom (antimony) forms covalent bond withfour adjacent silicon atoms.
Four of the antimony atoms valence electron are used toform the covalent bonds with silicon atoms, leaving one extraelectron (this is conduction electron because not involved in
bonding). The pentavalent atom also called as donor atom.
The number of conduction electrons can be controlled by thenumber of pentavalent impurity atoms added to the silicon.
The electrons are called the majority carriers in n-typematerials.
There are also a few holes created when electron-hole pairsare thermally generated and holes in n-type material arecalled minority carriers.
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P-Type semiconductor
To increase the number of holes in intrinsic silicon, trivalent
atom (atoms with 3 valence electrons such as boron (B),
indium (In)) are added.
Trivalent impurity atom in a silicon crystal structure. A boron (B)
impurity atom is shown in the centre.
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P-Type semiconductor
Each trivalent atom (boron) forms covalent bond with fouradjacent silicon atoms.
3 of the boron atoms valence electron are used in thecovalent bonds and since 4 electrons are required, a holeresults when each trivalent atom is added.
The trivalent atom also called as acceptor atom (trivalentatom can take an electron).
The number of holes can be controlled by the number oftrivalent impurity atoms added to the silicon.
The holes are called the majority carriers in p-type materials.
There are also a few conduction-band electrons created whenelectron-hole pairs are thermally generated and electrons inp-type material are called minority carriers.
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