Semi conductor

What is a semiconductor ?

 Semiconductors are matter having conductivity in between conductor and insulator.All matters form of atoms.Atom consists of nucleus (a tiny place where all mass and positive charge is included) and electron, the negatively charge particles.In metals ,in general, due to thermal excitation of the room temperature the electrons of outer shells become detached from the atom and moves randomly inside the metal.This electrons can never escape from the metal until energy is incident on them from outside.These electrons are called free electrons and gives rise to current electricity when a potential difference is maintained at its ends.In case of insulator these free electron can never be generated.And in semiconductors these are generated with increase in temperature.

   
   Actually this difference between conductor,semi conductor and insulator can be well explained by band theory in solids.According to the theory electrons in crystalline solid are present in energy bands.In other words the energy levels are not fine, they forms bands.The valence band is full of valence electrons.The free electrons stay in the conduction band.The energy difference of valence and conduction band is called the forbidden gap.This gap is largest for the INSULATOR.In METAL this two bands overlap; as a result electrons are always present in the conduction band. In SEMICONDUCTOR the gap is nearly 6.0 eV i.e in between conductor and insulator.