Classification of Semiconductors 

A semiconductor is a substance with resistivity (10^-4 to 0.5 Ω) between conductors and insulators e.g.germanium, silicon, selenium, carbon, etc.

There are two types of semiconductors

     1. Intrinsic semiconductor /Pure semiconductor

    2. Extrinsic semiconductor /Impure semiconductor

Extrinsic semiconductors are two types

    1. N-type Semiconductor

    2. P-type Semiconductor

Intrinsic semiconductor

A semiconductor, which is in its extremely pure form, is known as an intrinsic semiconductor. Silicon and germanium are the most widely used intrinsic semiconductors. Both silicon and germanium are tetravalent. Each atom shares its four valence electrons with its four immediate neighbors so that each atom is involved in four covalent bonds. Even at room temperature,hole-electron pairs are created. When an electric field is applied across an intrinsic semiconductor, the current conduction takes place by two processes, 

1. Free electrons
2. Holes

The free electrons are produced due to the covalent bonds. Under the influence of the electric field, conduction through the semiconductor is the covalent bond. Under the influence of an electric field, conduction through the semiconductor is by both free electrons and holes. So the total current inside the semiconductor is the sum of current due to free-electron and holes.

                  Figure: Current conduction through semiconductors

We can show the figure, holes being positively charged move towards the negative terminal of supply. As the holes reach the negative terminal, electrons enter the semiconductor crystal near the terminal and combine with the holes. thus canceling them. At the same time, when loosely held electrons near then the positive terminal are attracted away from their atom into the positively terminal 

Extrinsic Semiconductor

A semiconductor material that has been subjected to the doping process is called an extrinsic material.
The intrinsic semiconductor has little current conduction capability at room temperature. This is achieved by adding a small amount of suitable impurity ta a semiconductor. It is then called impurity or extrinsic semiconductor.

The usual dopants are :

(i) Pentavalent atoms have five valence electrons. Arsenic (As) Antimony (Sb), phosphorus (P).

(ii) Trivalent atoms have three valence electrons.   gallium (Ga), indium (In), aluminum (Al), and boron (B).

Pentavalent impurities are called donor impurities because they donate free electrons to the semiconductor.
Trivalent impurities are called acceptor impurities because the holes created can accept the electrons.

Depending upon the type of impurity added, extrinsic semiconductors are classified into;

  1. N-type Semiconductor

   2. P-type Semiconductor

Difference between Pure semiconductor and impure semiconductor

SL
Pure semiconducto
Impure semiconductor
i
 Pure semiconductor. It could be either Ge or Si
These are obtained by adding impurities to
 the pure semiconductor. These may be 
p-type or n-type.
ii
At zero temperature, it becomes an insulator.
Such is not a case here.
iii
Conductivity increases with temperature.
Conduction is possible at room temperature also.
iv
Conductivity is in between metals and insulators; it is less compared to extrinsic semiconductors.
Conductivity is much higher than intrinsic semiconductors (but less than metals).
v
No. of electrons and holes are equal.
No. of electrons and holes are not equal. In n-type, electrons > > holes. In p-type, holes > > electron
vi
Fermi level is exactly midway between the conduction band and valence band.
Fermi level is close to the conduction band in n-type and is close to the valence band in p-type.