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FUNDAMENTAL SOLID STATE PRINCIPLES : THE ATOMIC THEORY
FIGURE 1 : BOHR model of the atom.
THE BOHR MODEL
An atom consist of 3 basic particles:
-protons
-neutrons
-electrons
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Orbital paths, or shells, are identified using the letters K through
Q.
The innermost shell K shell
The outermost shell Valence shell
The valence shell of an atom determines the conductivity of theatom.
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In general , the nth shell can contain amaximum of 2n electrons, when n is the shellnumber and shell number 1 is the innermost
and closest to the nucleus. Example: Let us consider the structure of the
copper atom, whose atomic number is29.Allocate the number of electrons in each
shell.
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FUNDAMENTAL SOLID STATE PRINCIPLES : THE ATOMIC THEORY
The valence shell of an atom can contain up to 8 electrons.
The conductivity of an atom depends on the number of electrons of the
valence shell.
Conductivity decreases with the increase in the number of valence
electrons
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FUNDAMENTAL SOLID STATE PRINCIPLES : THE ATOMIC THEORY
FIGURE 2 : SEMICONDUCTOR ATOMS
Semiconductors are atoms that contain four valence electrons.
A semiconductor atom is neither a good conductor nor a good
insulator.
Three of the most commonly used semiconductor materials are
silicon (Si), germanium (Ge), and carbon (C).
Silicon and germanium are used in the production of solid-state
components.6
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FUNDAMENTAL SOLID STATE PRINCIPLES : THE ATOMIC THEORY
FIGURE 4 : SILICON COVALENT BONDING
Covalent bonding is the method by
which atoms complete their valence
shells by "sharing" valence electrons
with other atoms.
The results of this bonding are as
follows:1. The atoms forms a solid
substance.
2. The atoms are all electrically stable
3. The completed valence shells
cause the silicon to act as an
insulator. Thus, intrinsic silicon is avery poor conductor.
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The valence shell of an atom represents a bandof energy levels.
The valence electrons are confined to that
band. When an electron acquires enough additional
energy, it can leave the valence shell, become afree electron and exist in what is known as the
conduction band. The difference in energy between the valence
band and conduction band is called an energygap.
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This is the amount of energy that a valenceelectron must have in order to jump fromvalence band to the conduction band.
Once in conduction band, the electron is free tomove throughout the material and is not tied toany given atom.
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Doping is the process of impurity atoms to intrinsic silicon or
germanium to improve the conductivity of the semiconductor.extrinsic semiconductor.
Two element types are used for doping:
(i) trivalent(ii) pentavalent.
A trivalent element has 3 valence electrons.
A Pentavalent element has 5 valence electrons.
When trivalent atoms are added to intrinsic semiconductors, the
resulting material is called a p-type material.
When Pentavalent impurity atoms are used, the resulting material iscalled an n-type material.
FUNDAMENTAL SOLID STATE PRINCIPLES : DOPING
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FUNDAMENTAL SOLID STATE PRINCIPLES : DOPING
FIGURE 5 : NTYPE MATERIAL FIGURE 6 : ENERGY DIAGRAM OF
N-TYPE MATERIAL
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FUNDAMENTAL SOLID STATE PRINCIPLES : DOPING
When pentavalent impurities are added to silicon or germanium, the result is
an excess of electrons in the covalent bonds.
The material is still electrically neutral.
Each arsenic atom has the same number of protons as electrons, just likethe silicon or germanium atoms.
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FUNDAMENTAL SOLID STATE PRINCIPLES : DOPING
Electrons are majority carriers.
Valence band holes are minority carrier.
N-type material implies an excess of electrons.
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FIGURE 7 : P-TYPE MATERIAL FIGURE 8 : ENERGY DIAGRAM OF
P-TYPE MATERIAL
FUNDAMENTAL SOLID STATE PRINCIPLES : DOPING
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FUNDAMENTAL SOLID STATE PRINCIPLES : DOPING
Doping element:
Majority carriers:
Minority carriers:
N-TYPE
Pentavalent (donor atoms)
Conduction band electrons
Valence band holes
P-TYPE
Trivalent (acceptor atoms)
Valence band holes
Conduction band electrons
FIGURE 9 : COMPARISONS16
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FUNDAMENTAL SOLID STATE PRINCIPLES : THE PN JUNCTION
FIGURE 11 : FORMATION OF DEPLETION LAYER
When a free electron wanders from the n-type material across the junction,it will become trapped in one of the valence-band holes in the p-type
material.
As a result, there is one net positive charge in the n-type material and one
net negative charge in the p-type material.
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1. Each electron that diffuses across the junction leaves one positively
charged bond in the n-type material and produces one negatively
charged bond in the p-type material.
2. Both conduction-band electrons and valence shell holes are needed
for conduction through the materials. When an electron diffuses across
the junction, the n-type material has lost a conduction-band electron.
When the electron falls into a hole in the p-type material, that material
has lost a valence-band hole. At this point, both bonds have been
depleted of charge carriers.
FUNDAMENTAL SOLID STATE PRINCIPLES : DEPLETION LAYER
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FUNDAMENTAL SOLID STATE PRINCIPLES : DEPLETION LAYER
With the buildup of (-) charges on the p side and the buildup of (+)charges on the n side of the junction, there is a natural difference of
potential between the two sides of the junction.
This potential is referred to as the BARRIER POTENTIAL.
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FUNDAMENTAL SOLID STATE PRINCIPLES : DEPLETION LAYER
When an n-type material is joined with a p-type material:
1. A small amount of diffusion occurs across the junction. The amount of diffusion is limited by
the difference between the conduction-band energy levels of the two materials.
2. When electrons diffuse into the p region, they give up their energy and "fall" into the holes in
the valence-band covalent bonds.3. Since the Pentavalent atoms (near the junction) in the n region have lost an electron, they
have an overall positive charge.
4. Since the trivalent atoms (near the junction) in the p region have gained an electron, they
have an overall negative charge.
5. The difference in charges on the two sides of the junction is called the barrier potential. The
barrier potential is approximately equal to 0.7 V for silicon and 0.3 V for germanium
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FUNDAMENTAL SOLID STATE PRINCIPLES : BIASING
FIGURE 13 : FORWARD BIAS
1. The conduction band
electrons in the n-type
material are pushed toward
the junction by the negative
terminal potential.
2. The valence band holes in the
p-type material are pushed
toward the junction by thepositive terminal potential.
3. If V is greater than the barrier
potential of the junction, the
electrons in the n-type
material will gain enoughenergy to break through the
depletion layer. the electrons
will be free to recombine with
the holes in the p-type
material and conduction will
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FUNDAMENTAL SOLID STATE PRINCIPLES : BIASING
FIGURE 14 : REVERSE BIAS
When a PN junction is reversed bias,
the depletion layer becomes wider
and junction current is reduced toalmost zero.
The electrons sin the n type matreial
will head towards the positive
terminal
The holes will heading to thenegative source terminal.
Thus the depletion region will grow.
Resistance of the junction has been
drastically increased, and conductiondrops to near zero.
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