Unit – X A

Electrical Properties of Solids

Classification of Solids on the Basis of Electrical Conductivity:

• Solids exhibit a varying range of electrical conductivities, extending of magnitude ranging from 10–20 to 107 ohm–1 m–1.
• Solids can be classified into three types on the basis of their conductivities.

Conductors:

• The solids with conductivities ranging between 104 to 107 ohm–1 m–1 are called conductors.
• Metals have conductivities in the order of 107 ohm–1 m–1 are good conductors.
• Examples: Copper, Aluminium, Silver , Gold, All metals

Insulators :

• These are the solids with very low conductivities ranging between 10–20 to 10–10 ohm–1 m–1.
• Examples: Glass, wood, paper, plastic, mica

Semiconductors :

• These are the solids with conductivities in the intermediate range from 10–6 to 104 ohm–1 m–1.
• Examples: Silicon, Germanium

Conduction in Metallic Solids:

• A metal conductor conduct electricity through movement of electrons. Metals conduct electricity in solid as well as molten state.
• The conductivity of metals depend upon the number of valence electrons available per atom.

Concept of Energy Bands in Solids:

Energy Bands:

• The group of discrete but closely spaced energy levels for the orbital electrons in a particular orbit is called energy band.
• Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands.
• The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band. With no external energy, all the valence electrons will reside in the valence band.

Valence Band:

• The energy band formed by energy levels of valence electrons of atoms in solid is called valence band.
• Very few electrons have energy sufficient to get transferred from valence bond to conduction band. Such electrons with high energy are responsible for conduction.

Conduction Band:

• A band of energy level which is occupied by the conduction electrons of the solids is called conduction band.
• In conductors the gap between valence band and conduction band is very small. In good conductors valence band and conduction band overlap.

Explanation of Conductors on the Basis of Band Theory:

• If the lowest level in the conduction band happens to be lower than the highest level of the valence band, the electrons from the valence band can easily move into the conduction band.
• Normally the conduction band is empty. But in case of metallic conductors it overlaps on the valence band electrons can move freely into it.
• Thus at room temperature large number of free electrons are available for conduction.

Explanation of Insulators on the Basis of Band Theory:

• If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are available in the conduction band. This makes the material an insulator.
• But at higher temperature some of the electrons from the valence band may gain external energy to cross the gap between the conduction band and the valence band. Then these electrons will move into the conduction band. At the same time they will create vacant energy levels in the valence band where other valence electrons can move. Thus the process creates the possibility of conduction due to electrons in conduction band as well as due to vacancies in the valence band.
• In case of insulators the forbidden energy gap between the valence band and conduction band is greater than 3 eV.

Explanation of Semiconductors on the Basis of Band Theory:

• In semiconductors at absolute zero there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are available in the conduction band. This makes the material an bad conductor at absolute zero.
• In case of semiconductors the forbidden energy gap between the valence band and conduction band is less than 3 eV. As the temperature increases many electrons from the valence band may gain external energy to cross the gap between the conduction band and the valence band. Then these electrons will move into the conduction band. At the same time they will create vacant energy levels in the valence band where other valence electrons can move. Thus the process creates the possibility of conduction due to electrons in conduction band as well as due to vacancies in the valence band.
• Thus above absolute zero, semiconductors behave like conductors.

Characteristics of Conductors:

1. The substances which conducts electricity through them in greater extent are called conductors.
2. In conductors the conduction band and valence band overlap with each other or gap between them is very small.
3. There are free electrons in the conduction band.
4. There is no energy gap between conduction band and valence band or very small difference exists.
5. Due to increase in temperature conductance decreases.
6. There is no effect of addition of impurities on conductivity of conductors.
7. Their conductivity ranges between 104 to 107 ohm–1 m–1 are called conductors.
8. Example : Metals, Alloys

Characteristics of Insulators:

1. The substances which can not conducts electricity through them under any conditions are called insulators.
2. In Insulators the conduction band and valence band are widely separated.
3. There are no free electrons in the conduction band.
4. There is energy gap between conduction band and valence band which is more than 3 eV.
5. Therte is no effect of temperature on insulators..
6. There is no effect of addition of impurities on conductivity of conductors.
7. They have very low conductivities ranging between 10–20 to 10–10 ohm–1 m–1.
8. Example : Glass, wood, rubber, plastics

Characteristics of Semiconductors:

1. The substances which conducts very less electricity through them at lower temperature but conducts high electricity through them at higher temperature are called semiconductors.
2. In semiconductors the conduction band and valence band are very close to each other eacho ther or gap between them is very small.
3. The electrons of valence bond can easily be excited to conduction band..
4. There is energy gap between conduction band and valence band which is less than 3 eV.
5. Due to increase in temperature conductance increases.
6. There is effect of addition of impurities on conductivity of semiconductors.
7. Their conductivity ranges from 10–6 to 104 ohm–1 m–1.
8. Example : Silicon, Germanium