-Mihir BhagatDepartment of Dyestuff

TechnologyInstitute of Chemical Technology

Energy Levels in Solids Bands: Valence, Conduction &

Forbidden Conductors, Insulators &

Semiconductors

Every electron in an orbit is at a specific energy level and its given by the Bohr Model of an atom. Its is exact and discrete.

In a lattice due to phenomena like coulombic interaction, rotation, vibration of the electrons & overlap of energy levels, they occupy an energy band rather than a discrete energy level.

The energy band is thus, a continuous set of energy levels and can have a range of energies.

There are primarily three types of bands, namely Valence, Conduction and Forbidden bands.

The valence band describes the energy levels possessed by the valence shell of an atom. Electrons excite from the valence band to the conduction band.

The conduction band is the set of energy levels where electrons form a diffused electron sea. Thus, the electrons aren’t bound to a single nucleus and rather, are spread over the whole crystal.

The electron sea is responsible for electrical conduction.

The gap between the Valence and Conduction bands constitutes the forbidden band. It is named so because electrons are forbidden to cross it without an external supply of energy.

If the forbidden band is absent i.e. the conduction and valence bands overlap then the solid is said to be conducting.

If the forbidden band is large and electrons cannot excite to the conduction band under normal conditions, then the solid is said to be insulating.

Semiconductors are ones with an intermediate size of the forbidden band.

Thus they, excite to the conduction band at normal temperatures but insulate at lower temperatures.(Since, temperature is just an indicator of energy present in a material)

So, Why do insulators conduct at high voltage?