- The document discusses energy bands in solids, specifically valence bands, conduction bands, and forbidden bands. - It explains that conductors have no forbidden band between the valence and conduction bands, insulators have a large forbidden band that prevents electron excitation, and semiconductors have an intermediate forbidden band size. - Semiconductors can excite electrons to the conduction band at normal temperatures but insulate at lower temperatures, while the size of the forbidden band determines whether a solid is a conductor, insulator, or semiconductor.

1. -Mihir Bhagat
Department of Dyestuff Technology
Institute of Chemical Technology

2.  Energy Levels in Solids
 Bands: Valence, Conduction & Forbidden
 Conductors, Insulators & Semiconductors

3.  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.

4.  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.

5.  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.

6.  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.

7.  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.

8.  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.

9.  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)

10. So, Why do insulators conduct at high
voltage?