Semiconductors

What is a semiconductor and what is the basis of this classification?
A semiconductor is a material that intermediates between a conductor and an insulator.

Electrical conduction represents the ability of a material to conduct electric charges. Some materials are very good at it and are called conductors; the materials that are hopeless at it are called insulators.
Semiconductors (aka semi-insulators) are not very good at it. But under certain conditions, they can almost be a conductor.

What are the electron shells in an atom?
Electrons ‘revolve’ around the nucleus but they do not have the same orbit. The organization of electrons within an atom is actually a very difficult problem, which is solved by a discipline called quantum physics.
Electrons occupy orbits that are called energy levels. The first orbit closest to the nucleus in the K-shell (or shell 1); the second orbit, further away is the L-shell (or shell 2); the third shell is the M-shell (or shell 3), etc.
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