semiconductors electric conductivity

7/29/2019 Semiconductors Electric Conductivity

1/16

Semiconductors &

Electric Conductivity


2/16

Lecture 13

Electric Conductivity

Semiconductors

Diodes, Transistors


3/16

What we learned

Economic Price & Availability

Recyclability

General Physical Density

Mechanical Modulus (Stiffness)

Yield and Tensile

Strength

Hardness

Fracture Toughness

Fatigue Strength

Creep

Damping

Thermal Thermal Conductivity

Specific Heat

Thermal Expansion

Coefficient

Electrical & Magnetic Resistivity

Dielectric Constant

Magnetic Permeability

Environmental

Interaction

Oxidation

Corrosion

Wear

Production Ease of manufacture

Joining

Finishing

Aesthetic Colour

Texture

Feel


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Electrical Conductivity

Electrical conductivity measures a material's ability to conduct electric current.

Conductor is a material which contains movable electric charges (ions, electrons).

Insulators are non-conducting materials with very few or no movable charges.

Metals

Semiconductors

Ceramics

Polymers


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Electrical Resistivity & Conductivity

Transmission of electric current through solid materials: Ohms Law

Vvoltage (V); I current (A); R resistance of the materials ()

R is influenced by the specimen geometry

A cross-sectional area (perpendicular to the direction of the

electric current, m2)

L distance between the 2 points at which the voltage is measured (m)

resistivity (m)

electrical conductivity (-1m-1)

RIV

L

AR

AR

L

1


6/16

ExampleVoltage of 24V is applied to the ends of a pure tin wire with cross-sectional area of

1 10-6 m2 and the length of 1m. The ampere-meter reading was 2.66 amp. Find the

electric conductivity of tin?

17

6

26

101.11

1002.91

10102.9

02.966.2

24

m

mm

m

l

RA

A

V

I

VRIRV

AI

mL

mA

VV

66.2

1

101

24

26


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Band theory

In a single atom, energy of each electron is determined by its position (shell, orbital).In a solid, atoms are connected by bonds and electrons (especially valence ones) are

acted upon by electrons and nuclei of adjacent atoms.

This gives a number of energy levels that are close together and form an electron

energy band. The highest energy level is valence energy band.


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Band theory

The highest energy level is valence energy band.

Conduction occurs when electrons are in the conduction band.

Conductivity is determined by the energy band gap.energy band gap = gap between the valence energy band and the conduction band.


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Energy Band Gaps in Materials

At normal temperature,

the thermal energy of

some electrons is

sufficient for them to

reach the conduction

band, where they becomeconduction electrons.

Conductivity of the

materials depends on the

temperature!


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Semiconductors

Semiconductoris a material with intermediate electrical conductivity, between that of

a conductor and an insulator.

Silicon (Si) is the basic materials for semiconductor production.

Germanium (Ge) has better properties, but more expensive.

(Group IV/14; 4 valence electrons to conduct electricity)


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Semiconductors

The electronic properties of a semiconductor are enhanced by adding very little

quantities of other elements (impurities) - dopants.

In general, dopants have more/less electrons available to conduct electricity than the

basic material, and are classified accordingly either as electron acceptors or as

electron donors.

Two types of semiconductors: n-type & p-type.


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n- semiconductors

Doping: substituting Si atom in the lattice by an impurity atom with the one that has

one extra valence electron or one valence electron fewer

Donor: Impurities that have one extra electron that they donate to Si. That extra

electron that moves freely allows electricity to be conducted through the material.Extra negative charges (electron): n-type. Donors: group 15: P, As, Sb.


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p- semiconductors

Doping: substituting Si atom in the lattice by an impurity atom with the one that has

one extra valence electron or one valence electron fewer

Acceptor: Impurities that have one electron less than Si which creates holes; acceptor

atom has one electron less than the rest of the structure; that initiates movement ofElectrons throughout the lattice; virtually it seems like a positive charge is moving:

p-type. Acceptors: Group 13th: B, Al, In, Ga.


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Diodes

When p-type and n-type materials are placed in contact with each other, the junction

behaves very differently than either type of material alone: p-n junction.


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Diodes

When voltage is applied, current will flow readily in one direction (forward biased),

but not in the other (reverse biased), creating the basic diode.


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Transistors

A transistor is a semiconductor device used to amplify and switch electronic signals.

Transistor has three terminals for connection to an external circuit.

A small current in the centre or base region can be used to control a larger current

flowing between the end regions (emitter and collector).

semiconductors electric conductivity

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