Intrinsic Semiconductor & Extrinsic Semiconductor

 Intrinsic semiconductor :   A pure semiconductor which is free from impurity  is called an Intrinsic semiconductor.
Crystalline form of Germanium (Ge) and Silicon (Si) are examples of intrinsic semiconductors.

 Covalent Binding. Germanium has 32 electrons distributed as 2, 8, 18 and 4. It has four balance electrons. Each atom is surrounded by four atoms at equal distances from it, lying on the four corners of a regular tetrahedron, the atom itself being at the centre.

Each one of the four valence electrons of an atom enters into covalent bond with one balance electron with each of the four neighbouring atoms.  With these covalent bond, each atom behaves as if it has eight electrons in its outermost orbit, leaving no electron free. The covalent binding thus provides a stable structure.

Electrons & Holes in an Intrinsic semiconductor

Due to covalent binding in Germanium, each atom behaves as if it has eight valence electrons. The valence electrons occupy filled valency band (V) and the conduction band (C) remains completely as shown in figure.
The two bands have an energy gap of 0.75 eV.
At absolute zero, intrinsic semiconductor behaves as an insulator.

Even at room temperature, electrons from valence band gain sufficient thermal energy to break the covalent bonds and enter the conduction band. The conduction band becomes partially filled  .

The number of electrons in conduction band increases with the rise in temperature.
The electrons leaving the valence band to enter conduction band, leave behind an equal number of vacant sites near the top of the valance band as shown  .  These vacant sites are called holes. These holes, representing an electron vacancy, have a positive charge.

In intrinsic semiconductor the number of holes in valence band equals the number of free electrons in conduction band.

Intrinsic Conductivity

When an electric field is applied to an intrinsic semiconductor, electrons in valence band gain energy and they move to fill up the empty energy levels produced by the presence of holes.

On gaining further energy, they enter conduction band. Thus electrons are set in motion and some current flows. This gives some moderate electrical conductivity to the semiconductor. The electrical conductivity of pure semiconductors, is called intrinsic conductivity.
In electrical conductivity of pure semiconductors, electrons and holes have equal contribution. The semiconductor has both types of charge carriers (electrons with negative charge and holes with positive charge) in equal number.

In Intrinsic Semiconductor , ne = nh = ni

Where ne , ne are the number density of electrons in CB , number density of holes in VB and ni is the number density of intrinsic carriers (electrons or holes ) in a pure semiconductors .

Extrinsic Semiconductor

 Definition. A pure semiconductor, made deliberately impure, by adding to it some impurity element, is called an extrinsic semiconductor.  Its electrical conductivity becomes more.

Description. Germanium has 4.52 x 1022 atoms per cm3 . An addition of only one impurity atom per million (106) Ge atoms, is sufficient to give desired conductivity to it.

Doping : The process of deliberately adding suitable impurity atom to the intrinsic semiconductor, is called doping. The impure semiconductor is called doped semiconductor .

n – type Semiconductor :

 Preparation. An n-type Ge is obtained by adding a small quantity (one-millionth part) of a pentavalent impurity like phosphorous (15), Arsenic (33), Antimony (51), Bismuth (83) to a Ge crystal. Generally Arsenic (As) is taken for this purpose.

 Working. Each impurity atom donates one (fifth) free electron and becomes donor impurity atom figure . The energy level of this excess electron is only slightly less (0.01 eV) than the lowest energy level of the conduction band as shown in figure .
These free electrons (1 per impurity atom and 4.52 x 1016 per cm3), considerably increase the extrinsic conductivity.
Since electrons with negative charge help in current conduction, the impure Ge is called n-type.

p – type Semiconductor :

Preparation: A p-type Ge is obtained by adding a small quantity (one millionth part) of a trivalent impurity like Boron (5), Aluminium (13), Gallium (31), Indium (49) or Thalium (81) to a Ge crystal. Generally Indium (In) is taken for this purpose.

Working. Each impurity atom accepts one electron (to make its valence electron number four) and becomes acceptor impurity atom creating one hole as shown in figure .

The energy level of this hole is slightly more than the highest energy level of the valence band as shown in figure .
These holes (1 per impurity atom and 4.52 x 1016 per cm3) considerably increase the extrinsic conductivity.
Since holes with positive charge help in current conduction, the impure Ge is called p-type.

In a doped semiconductors ,

ne nh = ni2

Where ne , ne are the number density of electrons and holes respectively and ni is the number density of intrinsic carriers (electrons or holes ) in a pure semiconductors .

In n-type semiconductor , number density of electrons is nearly equal to number density of donor atoms Nd and is very large as compared to number density of holes .

ne ≈ Nd ≫ nh ; WhereNd = number density of donor atoms

In p-type semiconductor , number density of holes is nearly equal to number density of acceptor atoms Na and is very large as compared to number density of electrons

nh ≈ Na ≫ ne ; Where ; Na = number density of acceptor atoms

Also Read :

∗ Energy Levels & Energy Bands
∗ P-N Junction & P-N Junction Diode
∗ Junction Diode as Rectifier
∗ Zener diode
∗ Junction Triode (Transistor)
∗ Transistor as Amplifier

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