1) Band theory explains how materials can be conductors, insulators, or semiconductors based on their electron band structure. 2) Conductors have overlapping or partially filled valence and conduction bands, allowing electrons to move freely. Insulators have a large gap between bands, preventing electron movement. 3) Semiconductors have a small gap that thermal energy can overcome, exciting some electrons into the conduction band and allowing some current flow.

1. ENGINEERING CHEMISTRY
DR FARHAT A ANSARI
ASSISTANT PROFESSOR (AS)
JETGI

2. Band Theory of Solids
In isolated atoms the
electrons are arranged
in energy levels

3. In solids the outer electron energy levels become
smeared out to form bands
The highest occupied band is called the VALENCE band.
This is full.
For conduction of electrical energy there must be
electrons in the CONDUCTION band. Electrons are free
to move in this band.

4. Insulators : There is a big energy gap between the
valence and conduction band. Examples are plastics, paper
…..
Conductors : There is an overlap between the valence and
conduction band hence electrons are free to move about.
Examples are copper, lead ….
Semiconductors : There is a small energy gap between
the two bands. Thermal excitation is sufficient to move
electrons from the valence to conduction band. Examples
are silicon ,germanium….

5. When a conductor is
heated its resistance
increases ; The atoms
vibrate more and the
electrons find it more
difficult to move
through the conductor.
R
T
R
T
But in a semiconductor
the resistance
decreases with an
increase in
temperature. Electrons
can be excited up to
the conduction band.
Conductivity increases

6. Valence band: Band occupied by the outermost electrons
Conduction: Lowest band with unoccupied states
Conductor: Valence band partially filled (half full) Cu.
or Conduction band overlaps the valence band

7. Band Theory and Conductivity
• Band theory helps us understand what makes a conductor,
insulator, or semiconductor.
1) Good conductors like copper can be understood using the free electron
2) It is also possible to make a conductor using a material with its highest
band filled, in which case no electron in that band can be considered free.
3) If this filled band overlaps with the next higher band, however (so that
effectively there is no gap between these two bands) then an applied
electric field can make an electron from the filled band jump to the higher
level.
• This allows conduction to take place, although typically with
slightly higher resistance than in normal metals. Such materials
are known as semimetals.

8. Valence and Conduction Bands
• The band structures of insulators and semiconductors resemble
each other qualitatively. Normally there exists in both insulators
and semiconductors a filled energy band (referred to as the
valence band) separated from the next higher band (referred to
as the conduction band) by an energy gap.
• If this gap is at least several electron volts, the material is an
insulator. It is too difficult for an applied field to overcome that
large an energy gap, and thermal excitations lack the energy to
promote sufficient numbers of electrons to the conduction
band.

9. • For energy gaps smaller than about 1 electron volt, it is
possible for enough electrons to be excited thermally
into the conduction band, so that an applied electric
field can produce a modest current.
The result is a semiconductor.
Smaller energy gaps create semiconductors