ELECTRICAL THROUGHMATERIALS (ELECTRONICS)

TOPIC: - ELECTRICAL CONDUCTION THROUGH MATERIALS (ELECTRONIC)

INSTRUCTIONAL MATERIALS: -         Metals and non-metals

REFERENCES BOOK: -    

1.     FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2.     M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publishers.   

3.     INTERNET

PREVIOUS KNOWLEDGE: -     Students have been familiar with electronics material

INSTRUCTIONAL OBJECTIVES: - At the end of the lesson students should be able to: -

1.       Classified materials from an electrical point of view.

2.       Explain conductor, semi-conductor and insulator in term of band theory.

3.       State and explain types of semi-conductor

4.       Explain forward and reverse biasing.

CONTENT:

ELECTRICAL CONDUCTION THROUGH MATERIALS (ELECTRONIC)

From an electrical point of view, most materials can be classified into three groups; namely:

(a)                 Conductors.

(b)                 Semi-conductors.

(c)                 Insulators.

 Conductors

According to atomic theory, an electron can escape from the parent atom and move about between the atoms. This is called a "free electron”.  Once an electron escapes from an atom, the atom becomes net positively charged. It may then attract a free electron to become a neutral atom again.

Metals are characterized by many free electrons. Materials having many free electrons easily allow an electric current to flow through them and are called conductors. Electrons are the electric charge carriers in solid conductors. Metals are normally good conductors.

Some of the conductors, in order of their conductivity, are silver, copper, gold, aluminium, tungsten and brass. Silver is the best conductor of all metals, but is only rarely used because it is very expensive. It is used in precision instruments and in special switches as a coating but not in wires. The most commonly used conductors are copper and aluminium because of their abundance.

Copper is used in flexible cables such as wiring whereas aluminium is used mostly in overhead transmission cables because it is lighter than copper. It is also cheaper.

When an electric field is applied it sets up an electric force.

Without an electric field applied to a conductor, the electrons move in all directions, so it cannot be said that there is an electric current. However, if a potential is applied across the ends of a conductor, the free electrons will tend to move in the same direction. This is what happens when a conductor is connected between the terminals of an electric cell.

DISTINCTION BETWEEN CONDUCTORS, SEMI-CONDUCTORS AND INSULATORS IN TERMS OF BAND THEORY.

CONDUCTOR

Metals are conductors. There is no band gap between their valence and conduction bands, they overlap. There is a continous  availability of electrons in those closely space orbitals.

INSULATOR

In insulators, the band gap between the valence band, the conduction band is so large that electron cannot make the energy jump from the valence band to the conduction band.

SEMI-CONDUCTORS

Semi-conductors have a small energy gap between the valence band and conduction band. Electrons can make the jump up to the conduction band; but not with the same ease as they do in conductors.

Semi-conductors contains two types of mobile charge carriers:

1.       Holes- positively charged.                        II.  Electrons- negative charged

Semi-conductor materials are Silicon and Germanium

TYPES OF SEMI-CONDUCTORS

1.       Intrinsic semi-conductor

2.       Extrinsic semi-conductor

INTRINSIC SEMI-CONDUCTOR: This is a semi-conductor in its pure state. For every electrons that jumps into the conduction band, the missing electron will generate a hole that can move freely in the valence band. The number of holes will equal the number of electrons that have jumped. i.e. it contains equal number of free electrons and holes.

EXTRINSIC SEMI-CONDUCTOR: In extrinsic semi-conductors, the band gap is controlled by purposefully adding small impurities to the materials. This process is called doping. Doping or adding impurities to the lattice can change the electrical conductivity of the lattice and therefore vary the efficiency of the semi-conductor. These impurities atoms are known as dopants. In extrinsic semi-conductors, the number of holes will not equal the number of electron jumped.

DOPING

Doping is the introduction of impurity atoms into a semi-conductor.

How doping improves the conductivity of a semi-conductor

1.       It reduce the energy gap between the conduction band and the valence band, so that charge of minimum energy can move into the conduction band.

2.       Doping increases the number of charge carriers/electrons or holes thus increasing the conductivity of the semi-conductor.

There are two different kinds of extrinsic semi-conductor:

1.       P- type (positive charge doped )                II. n-type (negative charge doped)

p-type: (Regions of P-type is called ANODE.)

 Group III elements such as Boron, Aluminium, Gallium and Indium are classified as p-type impurities or p-type dopants. These elements have 3 valence electrons. When p-type impurities are doped into silicon crystal, all the 3 valence electrons form 3 strong covalent bonds with  adjacent crystal atoms. There is a deficit of electrons to form the forth covalent bond and this deficiency is termed as holes. Likewise, every p-type impurity atom produces a hole in the valence band which will drift to conduct electric current, if a potential is applied to the material. P-type semi-conductors can also be referred as Acceptors. It contains mobile charges which are mainly holes.

n-type:(Regions of n-type ia called CATHODE.)

 Group V elements such as Phosphorous, Antimony and Arsenic are usually classified as n-type impurities. These elements have five valence electrons. When n-type impurities are doped into silicon crystal four of the five valence electrons form four strong covalent bond with adjacent crystal atoms leaving one free electron in the conductor band which will drift to conduct electric current if a potential is applied to the material. n-type semi-conductor is referred as Donor. It contains mobile charges which are primarily electrons.

JUNCTION DIODE- FORWARD AND REVERSE BIASING

A P-N junction diode is a two-terminal semi-conductor device. It allows the electric current to flow in one direction while oppose current in other direction.

A P-N junction diode is formed when a p-type semi-conductor is fused to a n-type semi-conductor creating a potential barrier voltage across the diode junction.

Biasing  means applying external voltages to the device. Biasing of a diode is of two types:

1.       Forward Biasing: The voltage potential is connected positively, to the p-type material and negative to the n-type materials across the diode which has the effect of decreasing the p-n junction diode width. In this configuration, diode allows the current to flow in uni-direction.

2.       Reverse Biasing: The voltage potential is connected negative to the p-type material and positive to the n-type materials across the diode which has the effect of increasing the p-n junction diode width. In this configuration, diode does not allow the flow of current.

Advantages of p-n diode over diode valve

i. It need low voltage to operate.

Ii. It does not need time to warm up

Iii. It is not bulky

Iv. Cheaper to manufacture.

PRESENTATION

Step I: The teacher explains electrical conduction through materials

Step II: The teacher explain band theory

Step III: The students mention the classification of material from an electrical point of view.

Step IV: The teacher  explains type of semi-conductor

Step V: The teacher explains junction diode.

EVALUATION

The teacher evaluates the students by asking the following questions:

1.       Classified materials from an electrical point of view.

2.       Explain conductor, semi-conductor and insulator in term of band theory.

3.       State and explain types of semi-conductor

4.       Explain forward and reverse biasing.

ASSIGNMENT

Differentiate between an intrinsic semi-conductor and extrinsic semi-conductor. Atleast  2 points.

Ans.

Intrinsic semi-conductor	Extrinsic semi-conductor
1.       In a pure state	Not in pure state
2.       Contain equal number of free electrons and hole	Does not contain equal number of free electron and hole
3.       Impurities are not added	Impurities are added

Differentiate between p-type and n-type semi-conductor. At least 3 points.

p-type	n-type
1.       Has hole as the major carrier of electricity	Has electron as major carriers of electricity
2.       Impurity is trivalent	Impurity is pentavalent
3.       Created by acceptor impurity	Created by donor impurity
4.       Carries a net positive charge	Carries a net negative charge