CBSE Class 12-science Answered

Energy Band Diagram :-

          The range of energies that an electron may possess in an atom is known as the energy band.

                   There Important energy bands are,

• Valence Band
• Conduction Band
• Forbidden Band

Valence Band :-

                             The Range of Energy possessed by valence electrons is known as valence Bands.

Conduction Band :-

                             The valence electrons are less tightly bound with the nucleus. So that even an appication of small elctric field some of the valence electrons detached from the nucleus and it becomes free electrons. These fee elctrons are responsible for the conduction of current in good conductors. The electrons are also called conduction electrons.

    ?  The Range of energy possed by these elctrons is known as conduction band.

Forbidden Band (or) Energy Gap :-

                   The energy band in between the condition band and the valence band is called forbidden Band.

Classification of Materials (or) Solids According to Energy Bands :-

          Solids are classified in to there types

1. Insulators
2. Conudctors
3. Semi conductors

Insulators :-

          ? The materials in which the condition band and valence bands are separeated by a wide energy gap (? 15 eV) as shown in figure.

          ? A wide energy gap means that a large amount of energy is required, to free the electrons, by moving them from the valence band into the condition band ;

          ? Since at room temperature, the valence electrons of an insulator do not have enough energy to jump in to the condition, therefore insulator do not have an ability to conduct current. Thus insulators have very high resistively (or extremely low conductivity) at room temperatures.

          ? However if the temperature is raised, some of the valence electrons may acquire energy and jump in to the conduction band. It causes the resistively of insulators to decrease.

          ? Therefore an insulator have negative temperature co-efficient of resistance.

Conductors :-

          The materilas in which conduction and valence bands overlap as shown in figure are called conductors.

          ? The overlapping indicates a large number of electrons available for conduction.

          ? Hence the application of a small amount of voltage results a large amount of current.

Semiconductors :-

          ? The materials, in which the conduction and valence bands are separeated by a small energy gap (1eV) as shown in figure are called semiconductors.

          ? Silicon and germanium are the commonly used semiconductors.

          ? A small energy gap means that a small amount of energy is required to free the elctrons by moving them from the valence band in to the conduction band.

          ? The semiconductors behave4 like insulators at 0K, because no electrons are available in the conduction band.

          ? If the temperature is further increased, more valence elctrons will acquire energy to jump into the conduction band.

          ? Thus like insulators, semiconductors also have negative temperature co-efficient of resistance.

          ? It means that conductivity of semiconductors increases with the increases tempertature.

  Answered by | 07 Nov, 2012, 11:01: AM