Types of Materials

TYPES OF MATERIALS

Solids are classified in to three types based on energy band theory such as

(i)Conductors (ii) Insulators (iii) Semiconductors

Conductors

Conducting materials are those in which plenty of free electrons are available for electric conduction. Conductor material that easily conduct (or) pass the current. The best conductors are single element material such as copper, silver and aluminium. These atoms have only one valence electrons.

The conductors energy band diagram have over lapping valence and conduction band. It is shown in fig 3.8. There is no physical distinction between the two bands. Hence the availability of a large number of conduction electrons. The absence of forbidden energy gap in good conductors. The resistance of the conductor is very small compared with semiconductor and insulator.

Properties of conductors

(i) It is rigid, nondirectional and crystalline in nature

(ii) Conductivity is good

(iii) Low melting and boiling temperatures

Insulators

Insulators are those materials in which valence electrons are bound very tightly to their parent atoms thus requiring very large electric field to remove them from the attraction of their nuclei.

Insulators have no free charge carrier available with them under normal condition. The insulators have

(i) Full valence band

(ii) An empty conduction band

(iii) A large energy gap between them

This is shown in fig 3.9 for conduction to take place, electrons must be given sufficient energy to jump from the valence band to the conduction band. Increase in temperature enables some electrons go to the conduction band which fact accounts for the negative temperature co-efficient of resistance of insulator.

Example: paper, mica, etc,

Properties of insulators:

(i) It is rigid unidirectional and crystalline in nature

(ii) Conductivity is poor in the solid form

(iii) It have high melting and boiling temperature

Semiconductor

A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors. It is has 4 valance electrons.

In terms of energy bands, semiconductors can be defined as those materials, which at room temperature have

(i) Partially-filled conduction band

(ii) Partially-filled valence band

(iii) A very narrow energy gap between then (order of 1.1 ev)

Examples: Germanium, silicon, Gallium arsenide etc.

At 0°K, there are no electrons in the conduction band of semiconductors and their valence band is completely filled. It means absolute zero temperature a piece of Ge (or) Si acts like a perfect insulators. The resistivities of semiconductors is in the range of 10^-3 to 10^-6 Ω^-m.

Semiconductor compounds containing covalent bonds hence these compounds are called covalent compounds.

Properties of semiconductors:

(i) It is rigid, directional and crystalline in nature

(ii) Conductivity can increased if proper doping material in added

(iii) Low melting and boiling temperature.