Chapter 1 - Atomic Interactions

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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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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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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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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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FIGURE 5 : NTYPE MATERIAL FIGURE 6 : ENERGY DIAGRAM OF

N-TYPE MATERIAL

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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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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


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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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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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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

occur.21


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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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Chapter 1 - Atomic Interactions