A semiconductor is a substrance which has resisitivity (10^-4 to 0.5 ohm) in between conductors and insulators e.g. germanium, silicon, selenium, carbon etc.

However a semiconductor can be defined much more comprehensively on the basis of energy band as under:-
A semiconductor is a substance which has almost filled valence band and nearly empty conduction band with a very small energy gap (1ev) separating the two.
The Forbidden energy gap in semiconductor is very small being 1.1ev for silicon and 0.7ev for germanium. Therefore, relatively small energy is needed by their valence electrons to cross our the conduction band. 
Even at the room temperature, some of the valence electrons may acquire sufficient energy to enter into the conduction band and thus become free electrons. So, at this temperature, the number of free electrons available is very small. Therefore, at room temperature, a semiconductor is neither a good conductor nor an insulator. For this reason, such substances are called semiconductors.

Semiconductor Device

Properties of semiconductors

  1. The resistivity property of a semiconductor is less than an insulator but more than a conductor.                            
  2. Semiconductor have negative temperature coefficient of resistance i.e. the resistance of a semiconductor (material) decrease with the increase in temperature and vice-versa. For example, germanium is actually an insulator at low temperature but it become a good conductor at high temperatures.                                                                                                                     
  3. When a suitable metallic impurity (e.g. gallium, arsenic etc) is added to a semiconductor, its current conducting properties change appreciably. This property is most important.                                   
  4. Semiconductors are formed by covalent bonds                                                                                          

Commonly used semiconductor

We have many semiconductors available but very few of them have a practical application in electronics. The two most frequently used materials for Germanium and silicon it is because the energy required to break their covalent bond example energy required to release an electron from their valence bond is very small being 0.7ev for Germanium and 1.1ev for Silicon

The atomic number of Germanium is 32. So, the number of electrons in the first Orbit, Second Orbit, Third orbit and fourth orbit of a germanium atom is 2, 8 ,18 and 4 respectively. note that Germanium atoms has 4 Valence Electrons.

The atomic number of silicon is 14. Therefore the number of electrons in the first orbit, Second orbit, Third orbit of a silicon atom is 2, 8 and 4 respectively. Again a silicon atom has 4 Valence Electrons.

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