Electrical Properties

Electrical Conduction
R

Ohm’s law

V = IR

I
Area, A

l
V

where I is current (Ampere), V is voltage (Volts) and R is the resistance (Ohms or ) of the conductor

Resistivity
Resistivity, = RA/l ( -m), where A is the area and l is the length of the conductor.

Electrical conductivity
Conductivity,

= 1/ = l/RA ( -m)-1

Band Theory
Electrons occupy energy states in atomic orbitals
When several atoms are brought close to each other in a solid these energy states split in to a series of energy states
(molecular orbitals).
The spacing between these states are so small that they overlap to form an energy band.

Band Theory
The furthest band from the nucleus is filled with valence electrons and is called the valence band.
The empty band is called the conduction band.
The energy of the highest filled state is called Fermi energy.
There is a certain energy gap, called band gap, between valence and conduction bands.
 Primarily four types of band structure exist in solids.

Band Theory
In metals the valence band is either partially filled (Cu) or the valence and conduction bands overlap (Mg).
Insulators and semiconductors have completely filled valence band and empty conduction band.
It is the magnitude of band gap which separates metals, semiconductors and insulators in terms of their electrical conductivity. The band gap is relatively smaller in semiconductors while it is very large in insulators.

Conduction Mechanism
An electron has to be excited from the filled to the empty states above Fermi level (Ef) for it to become free and a charge carrier. In metals large number of free valence electrons are available and they can be easily excited to the empty states due to their band structure.
On the other hand a large excitation energy is needed to excite electrons in