Band theory describes how electrons are arranged in energy levels, or bands, within solid materials. There are generally three main bands - the conduction band, valence band, and forbidden gap. Insulators have a large forbidden gap, making it difficult for electrons to move between bands. Semiconductors have a smaller gap, allowing electrons to more easily move between bands with a small amount of energy. Conductors have no forbidden gap, allowing electrons to freely move between overlapping valence and conduction bands. The arrangement of bands determines whether a material is a conductor, insulator, or semiconductor, and how easily electrons can flow as electric current.

1. By Unsa Shakir
BAND THEORY

2. ENERGY LEVELS:

4. Arrangementof electronsin anatom
• Range of energy
possessed by an
electron in a
solid is known as
energy band.

5. CONDUCTION BAND
• The set of electron orbitals, generally the outermost shells of
the atoms in a conductor or semiconductor, in which
electrons are free to move and thereby carry an electric
current.
VALENCE BAND
• The outermost electron shell of atoms in an insulator or
semiconductor in which the electrons are too tightly bound
to the atom to carry electric current.
FILLED BAND
• Energy band, each of whose energy levels is occupied by n
electron.

6. ENERGY BANDS

7. Insulators
Valence Band
Conduction Band
FORBIDDENGAP
Energ
y
• In an insulator, the
forbidden gap is very large
and in general is more than
3eV.
• No electron is available
for conduction.
• Large amount of energy
is needed to move
electron from valance
band to conduction
band.
FilledBand

8. SEMICONDUCTORS
• Semiconductorshaveresistivity betweengoodconductorsand
insulators.Theresistivity ofsemi-conductorliesapproximately
inbetween10-5 to 104 ohm-meterascomparedtothe
values ranges from 10 -8 to 10 -6 ohm-meter for
conductors and from 10 7 to 10 8 ohm-meter for
insulators.

9. Semiconductors
Valence Band
Conduction Band
FORBIDDENGAP
Around 0.7eV (Ge)and
1.1 eV(Si)
• In the caseof semiconductorsthe
forbidden gapisvery small.
Energ
y
• At 0K the conduction band is
empty and the valence band is
completelyfilled.
• When a small amount of energy is
supplied, the electrons can easily
jumptheforbiddengap.
• The conductivity of a
semiconductorisoftheorderof
10-6 to 104 (ohm-m)FilledBand

10. Insulators vs. Semiconductors
• There is no difference between Insulators and
Semiconductors at very low temperatures
• In neither material are there any electrons in the
conduction band – and so conductivity vanishes in the
low temperature limit
INSULATOR SEMI-CONDUCTOR

11. Insulators vs. Semiconductors
• Differences arises at high temperatures
– A small fraction of the electrons is thermally excited into the
conduction band. These electrons carry current just as in metals
– The smaller the gap the more electrons in the conduction band at a
given high temperature
– Resistivity decreases with temperature due to higher concentration
of electrons in the conduction band

12. Electronsandholesin semiconductors
• At absolute 0 temperature, in a
pure semiconductor the valence
bandis completelyfilled andthe
conductionbandisvacant.
At0K
Conduction Band
Valence Band
Electron
Energ
y

13. • In solids , the atoms are arranged in a
systematic space lattice and each atom is
influencedbyneighbouring atoms.
• The closeness of atoms results in the
intermixing of electrons of neighbourring
atoms.

14. Conductors
Valence Band
Conduction Band
Energ
y
• In conductors there is no forbidden
gap.
• The valence band and the
conductionband overlap.
• The electrons from valence band
freely enter into the conduction
banddueto overlapping of bands.
• Therefore very low potential
difference can cause continuous
flow of current.
Noforbidden gap

16. • Conductivity, by contrast, is the degree to which a
conductor allows the flow of electricity through itself.
• Think of a pipe and water, the bigger the pipe, the
easier water can flow and thus the lower the resistance.
• The unit of conductivity is the siemens (S). It was
formerly called the mho.
• Conductivity is just the inverse of resistivity, σ = 1/ρ,
where σ is conductivity.
CONDUCTIVITY

17. DRIFT VELOCITY:
It the average velocity with which electrons move
along the conductor under some electric field.
Before applying electric field After applying electric field