• Semiconductors.
Semiconductor are special class of element having conductivity between that of a good conductor and that of an insulator. It has two types of semiconductor.
(A) intrinsic semiconductor ( pure s.c )
(B) extrinsic semiconductor ( impure s.c)
Extrinsic semiconductor is also two types of -
(a) n - type
( b) p - type.
( A) intrinsic semiconductor.
It is also called pure semiconductor or non-degenerate semiconductor ( as basic properties are not changed).
Degenerate means change of basic properties by adding impurity .Hance extensive semiconductor is also known as degenerate semiconductor.
i.e , Available in its purest form.
Ex : Si , Ge , selenium etc.
The properties of this pure semiconductor are as follows.
T = 0k insulator
T = 300 k. Conductor
n = concentration of free electrons / cm^3
P = concentration of holes /cm^3
Ni = concentration of intrinsic charge carrier/ cm^3. i.e it tells about pair.
• condition for intrinsic s.c :
n = p = Ni
In a s.c n = 1.5 × 10 ^10 / cm^3
P = 1.5 × 10 ^10 / cm^3
i.e n= P = intrinsic semiconductor.
• Properties of intrinsic s.c :
• The number of free electrons is equal to the number of holes.
• the conduction capability is small at room temperature.
• Intrinsic semiconductor behaves as a perfect insulator at zero kelvin (0 k).
•intrinsic semiconductor at 0k a is a perfect insulator.
• the sharing of electrons with neighbouring atoms is called covalent bonding.
• At 0k all valence electrons are in perfect covalent bonding.
• Application :
it is used in pin diode.
• Note :
• less conductivity of intrinsic semiconductor.
• Ratio of majority charge carriers to the majority charge carriers in intrinsic semiconductor is equal to 1.
• electrical conductivity of intrinsic semiconductor can be increased by two methods.
(a) by increasing temperature.
( b) by adding impurity ( Dropping).
• Doping :
The process of adding impurities to the semiconductor materials is termed as doping.
The impurities added are generally pentavalent and trivalent impurities impurities.
Other way , the electrical conductivity of intrinsic semiconductor can be increased many times by adding very small amount of impurity to the intrinsic semiconductor , this process is called doping.
(B) extrinsic semiconductor :
An impure semiconductor which is formed by doping a pure semiconductor is called as an extrinsic semiconductor. It is also called impurity semiconductor or dobbed semiconductor or artificial semiconductor or degenerate semiconductor or compensated semiconductor.
There are two types of extrinsic semiconductors depending upon the type of impurity added. They are
(a) N - type extrinsic semiconductor and
( b) p-type extrinsic semiconductor.
• pentavalent impurities:
The pentavalent impurities are the ones which has five valence electron in the outermost orbit.
Ex : bismuth , antimony , phosphorus , arsenic etc.
• pentavalent atom is called as a donor atom because its donates 1 electron to the conduction band of pure semiconductor atom.
•pentavalent impurities is called as n - type semiconductor.
• Trivalent impurities :
The trivalent impurities are the ones which has three valence electron in the outermost orbit.
Ex : Gallium , Indium, aluminium, boron etc.
• The trivalent atom is called as an acceptor atom because it accept one electron from the semiconductor atom.
• Trivalent impurities is called as p - type semiconductor.
