What is a Semiconductor

What is a Semiconductor :-

• A semiconductor is a substance which has resistivity in between 

 conductors and insulators.

 E.g. germanium, silicon, selenium, carbon etc.

Properties of Semiconductor :-

• The resistivity of a semiconductor is less than an insulator but 

   more than a conductor.

• It has negative temperature co-efficient of resistance. That 

  means the resistance of a semiconductor decreases with 

  increase in temperature and vice-versa.

• When a suitable metallic impurity is added to a semiconductor, 

  its current conducting properties change appreciably.

Commonly Used Semiconductors :-

• The two most frequently used semiconductors are (i) 

    germanium (Ge) and (ii) silicon (Si).

• It is because the energy required to break their co-valent bonds 

   is very small; being 0.7 eV for Ge and 1.1 eV for Si.

(i) Germanium(Ge) :-

• Atomic number of germanium is 32. So it has 32 protons and 32 

   electrons.

• Two electrons are in the first orbit, eight electrons are in the 

   second, eighteen electrons in the third and four electrons in the 

   outer or valence orbit. This is shown in fig.1(i).

• It is clear that germanium atom has four valence electrons i.e. it 

   is a tetravalent element.

• Fig.1 (ii) shows how the various germanium atoms are held 

    through co-valent bonds.

                                             Fig:-01 Germanium(Ge)

                                 

                                         Fig:-01 Germanium(Ge)

(ii) Silicon(Si) :-

• Atomic number of silicon is 14. So it has 14 protons and 14 

   electrons.

• Two electrons are in the first orbit,eight electrons are in the 

   second orbit and four electrons in the third orbit. This is shown 

   in fig.2(i).

• It is clear that silicon atom has four valence electrons i.e. it is a 

   tetravalent element.

• Fig.2(ii) shows how various silicon atoms are held through co-

   valent bonds.

Silicon(Si) :-

Fig:-01   Silicon(Si)

Fig:-02  Silicon(Si)

N-type Semiconductor :-

• When a small amount of pentavalent impurity is 

  added to a pure semiconductor, it is known as n-

  type semiconductor.

• The addition of pentavalent impurity provides a 

  large number of free electrons in 

  the semiconductor crystal. Typical example of 

  pentavalent impurities are arsenic (As), and 

  antimony (Sb) .

• Such impurities which produce n-type  

semiconductor are known as donor impurities as 

they donate free electrons to the semiconductor 

crystal.

• To explain the formation of n-type 

   semiconductor, consider a pure germanium 

   crystal. We know that germanium atom has 

  four valence electrons. When a small amount 

  of pentavalent impurity like arsenic is added to 

  germanium crystal, a large number of free 

  electrons become available in the crystal. The 

  reason is explained below.

• Arsenic is pentavalent i.e. its atom  has five 

  valence electrons. An arsenic atom fits in the 

  germanium crystal in such a way that its four 

 valence electrons form covalent bonds with 

four germanium atoms.The fifth valence 

electron of arsenic atom finds no place in co-

valent bonds and  thus remains free as shown in 

fig.2.

      P-type  Semiconductor :-

• When a small amount of trivalent impurity is 

added to a pure semiconductor, it is called p-

type semiconductor.

• The addition of trivalent impurity provides a 

large number of holes in the semiconductor. 

Typical example of trivalent impurities are 

gallium (Ga) and indium (In).

• Such impurities which produce p-type 

semiconductor are known as acceptor impurities 

as the holes created can accept electrons.

• To explain the formation of p-type 

semiconductor, consider a pure germanium 

crystal.

• When a small amount of trivalent impurity like 

gallium is added to germanium crystal, there 

exist a large number of holes in the crystal. The 

reason is explained below:

• Gallium is trivalent i.e. its atom as three 

valence electrons. Each atom of gallium fits into 

the germanium crystal but now only three co-

valent bonds can be formed. It is because three 

valence electrons of gallium atom can form 

only three single co-valent bonds with three 

germanium atoms as shown in fig.5.

                 

           P-type  Semiconductor

         n-type Semiconductor