(i) A Semiconductor in its pure form without impurity is called an intrinsic semiconductor.
(ii) Impurity means any other atom in the crystal lattice.

(iii) Each silicon atom has four electrons in the outmost orbit and is covalently bonded with the neighboring atoms to form the lattice. The band diagram is shown in the figure.

(iv) A small increase in temperature is sufficient enough to break some of the covalent bonds and release the electrons free from the lattices as shown in figure.

(v) The vacancies produced in the valence band are called holes. As the holes are dericiency of electrons, they are treated to possess positive charges Hence electrons and holes are the two charge carriers in semiconductors.

(vi) The number of electrons in the conduction band is equal to the number of holes in the valance band. The conduction is done to the electrons in the conduction band and holes in the valence band.
\(\begin{array}{l} I=I_c+I_{\mathrm{h}} \\ I=\text { Total current } \\ I_e=\text { Electron current } \\ I_{\mathrm{h}}=\text { hole current } \end{array}\)
(vii) It behaves like an insulator at 0 K. The increase in temperature increases the number of charge carriers.
(viii) The intrinsic carrier concentration is the number of electron in the conduction band or number of holes in the valence band in an intrinsic semi conductor.

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S.No	Avalanche breakdown	Zener breakdown
(i)	Heavily doped p-n junctions have narrow depletion layers of the order of <10-6 m.	It occurs in lightly doped junctions  Which have wide depletion layers.
(ii)	Electric field produced is strong in nature.	Weak electric field is produced.
(iii)	When a reverse voltage across the junction is increased to the breakdown limit, a very strong electric field is set up. It ruptures the covalent bonds in the lattice and thereby generating electronic-hole pairs. This effect is called Zener effect	Thermally generated minority charge carriers accelerated by the electric field gains sufficient kinetic energy, collide with the semiconductor atoms while passing through the depletion region. This leads to the breaking of covalent bonds and in turn covalent bonds and in turn generates electron-hole pairs
(iv)	Even a small further increases in reverse voltage produces a large number of charge carriers. Hence the junction has very low resistance in the breakdown region.	The newly generated charged carriers are also accelerated by the electric field resulting in more collisions and further production of charged carriers.
(v)	This process of emission of electrons due to the strong electric field is known as internal field emission or field ionization.	This cumulative process leads to an avalanche of charge carriers across the junction and consequently reduces the reverse resistance the diode current increases sharply.

The leakage current is the current that the diode will leak when a reverse bias is applied to it.

S.No Intrinsic semiconductor Extrinsic semiconductor (i)  A semiconductor in its pure form without impurity is called an intrinsic
semiconductor. When small amount of impurity is doped in pure semiconductors it becomes extrinsic semiconductors.
  (ii) The number of free electrons in conduction band is almost equal to number of holes in valence band. The number of free electrons and holes is never equal. (iii) Electrical conductivity is a function of temperature along. Its electrical conductivity depends upon the temperature as well as on the quantity of impurity atoms doped the structure.