Energy Band Diagram :-
The range of energies that an electron may possess in an atom is known as the energy band.
- Valence Band
- Conduction Band
- Forbidden Band
→ 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 ;
→ 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 resistivity (or extremely low conductivity) at room temperatures.
The materials 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.
→ The materials, in which the conduction and valence bands are separated by a small energy gap (<3eV) 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 behave like insulators at 00K, 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.