Conductor, Insulator, and Semiconductor

The electrons that occupy the lower energy bands are of no importance in determining many of the electrical properties of materials.
Instead, the electrons in the high-energy bands of materials are important in determining many of the electrical properties of materials.
Hence we are interested in those two allowed energy bands called valence bands and conduction bands.
These two bands are separated by another region known as the forbidden energy gap in which no electrons can normally exist.
The valence band is defined as the range of energies possessed by valence electrons.
This band may be completely or partially filled. For example, in the case of inert gases, the valence band is full whereas for other materials it is only partially filled.
The conduction band is defined as the range of energies possessed by the conduction band electron. We have already studied that the valence electrons in certain materials are loosely attached to the nucleus.
Even at room temperature, some of the electrons may get detached and flow through the material.
These electrons are known as free electrons which are responsible for conduction. They are also known as conduction electrons.

What is a Conductor?

A conductor may be a material, which easily allows the flow of electrical current.
The best conductors are copper, silver, gold, and aluminum.
In these materials, the atom consists of only one electron, which is very loosely bound to the atom.
These electrons can easily break away from their atoms and become free electrons causing conduction in the material.
In terms of the energy band diagram, the valence band and conduction band overlap with each other.
That is in the case of the conductor there is no forbidden gap.
A very large number of electrons are available for conduction at extremely low temperatures.

Energy band diagram of conductor

An insulator is a material that does not conduct an electric current.
In these materials, the valence electrons are tightly bound to the atoms.
The valence band is full and the conduction band is empty.
The energy gap between the valence band and conduction bands is very large (approximately 6 eV).
Therefore a high electric field is required to cause an electron to go from valence band to conduction band.

Energy band diagram of insulator

A semiconductor is a material that has an electrical conductivity that lies between conductors and insulators.
A semiconductor in its pure state is neither a good conductor nor a good insulator.
The most common semiconductors are silicon, germanium, and carbon.
The energy band diagram of a semiconductor is shown in fig.
At absolute zero (0K) the valence band is usually full and there may be no electrons in the conduction band.
However, the forbidden energy gap(for Si 1.2.eV and for Ge 0.785eV)of a semiconductor is very much narrower than that of an insulator: Therefore a small electric field is required to push the electron from the valence band to the conduction band.

Energy band diagram of semiconductor

Semiconductors are classified as intrinsic and extrinsic semiconductors.
A pure semiconductor is called an intrinsic semiconductor.
We already know that the electrical properties of any material depend on the number of valence electrons.
In the case of copper, the valence electron can be easily detached from the atom.
This created a large number of free electrons and the copper atom becomes a positive ion.
The electrostatic force of attraction that exists between the ions and free electrons is the bonding force that holds the material together in a solid.
This bonding force is termed metallic bonding.
In the case of germanium (32) and silicon (14), the number of valence electrons is four.

Crystal structure of silicon

Hence the atoms are known as tetravalent atoms.
To achieve the state of chemical stability the electrons in the outer shell of each atom form a bond with neighboring electrons in the outer shell of other atoms in such a way that they are orbiting in the valence shells of both atoms.
This type of bonding is known as a covalent bond.
Fig shows the covalent bond among Si atoms. In Fig the four valence electrons of the Si atom share an electron with each of its four neighboring Si atoms.
Since the four valence electrons form a covalent bond with the neighboring atoms the crystal acts as a perfect insulator at 0°K.
In order to provide conduction covalent bonds are to be broken.
The energy required to break such a covalent bond is about 0.72eV for Ge and 1.1eV for Si.
When a covalent bond is broken an electron escapes to the conduction band leaving behind an empty space in the valence band called a hole as shown in Fig.

Silicon with broken covalent bond

Germanium

Silicon

At absolute zero, the conduction band is empty whereas the valence band is filled with electrons.
But at room temperature, some of the valence electrons may acquire sufficient energy to enter into the conduction band and thus become free electrons.
However, the number of electrons available for conduction at room temperature is very small.
Therefore at room temperature germanium or silicon is neither a good conductor nor an insulator.
From Fig we find that the forbidden energy gap for silicon is 1.1.eV and for germanium 0.7eV.
Therefore the valence electrons of germanium require smaller energy to cross over to the conduction band than that of silicon.
Hence at room temperature germanium has free electrons than silicon.
This is the main reason why silicon is mainly used in the fabrication of diode, transistors,and other semiconductor diodes.