Question

In terms of band theory, what is the difference:
                 (A) Between a conductor and an insulator?
                 (B) Between a conductor and a semiconductor?

Answer 1

Hint: The band theory sees the electron behaviour by taking the atomic or molecular orbitals to be various energy levels. It takes the difference in energy levels to be band gaps and differentiates the insulators, conductors and semiconductors on the basis of these bad gaps.

Complete answer:
-First of all let us see what band theory says.
The bond theory explains the electron behaviour in solids by assuming the existence of energy bands. In an isolated atom the electrons are present in atomic orbitals and each orbital forms a discrete energy level.
When atoms combine to form molecules, the atomic orbitals split to form molecular orbitals and each molecular orbital has different energy. As more and more atoms are brought together the number of orbitals also increases and ultimately the energy difference between them becomes smaller.
Thus in solids these energy levels occur in the form of continuous bands of energy and not discrete energy levels (like in isolated atoms). When some intervals of energy have no orbital in them, they form band gaps.
In metals: the conduction band and valence band overlap with each other.
In semiconductors: they have small band gaps and the conduction band and valence band do not overlap. Thus voltage needs to be applied to transfer electrons from valence band to conduction band.
In insulators: they have a large band gap and so a large amount of voltage needs to be applied for electrons to flow.
Now we will talk about the differences.
(A)Between conductor and insulator.

Conductor	Insulator
-It’s valence band is filled partially and it occupies the high energy occupied conduction band. -The electrons can flow easily under any applied electric field or voltage.	-Here the valence band is completely filled and there is a large energy gap between the valence band and conduction band.-A very large amount of voltage needs to be applied.

(B)Between conductor and semiconductor.

Conductor	Semiconductor
- It’s valence band is filled partially and it occupies the high energy occupied conduction band. -The electrons can flow easily under any applied electric field or voltage.	-here the valence band is filled and there is a small energy gap present between the valence band and the conduction band.-The electrons can jump from the valence band to the conduction band very easily and thus conduct electricity.

Note:
The semiconductors have properties that are between both conductors and insulators. They are formed by doping which means by adding small amounts of different elements that increase the charge carriers. They can be of 2 types: intrinsic and extrinsic semiconductors.