3. Silicon and Germanium
Commonly used Semiconductors
At absolute zero and Above absolute zero
Effect of temperature on Semiconductor
Current conduction due to holes
Concept of Hole current
Silicon and Germanium
Energy Bands in Semiconductors
Intrinsic and Extrinsic
Types of semiconductors
5. Energy Bands in Silicon and Germanium
Semiconductor is a substance
which has almost filled
valence band and nearly
empty conduction band with
a very small energy gap
(~1eV) separating the two.
Therefore, relatively small
energy is needed by their
valence electrons to cross
over to the conduction band.
6. Effect of temperature on semiconductor
At absolute zero
All the electrons are tightly
held by the semiconductor
atoms. Covalent bonds are very
strong and there are no free
semiconductor behaves as a
Above absolute zero
Covalent bonds break due to
thermal energy supplied. Free
electrons exist in semiconductor
constitute electric current.
7. Types of Semiconductor
Fig: Classification of Semiconductors Fig: Intrinsic Semiconductor
A semiconductor in an extremely pure form is known as intrinsic semiconductors.
A semiconductor formed by adding suitable impurity to a pure semiconductor is k’as extrinsic semiconductor.
Holes being positively charged move towards the negative terminal of supply. As the holes reach the negative terminal B, electron enters the
semiconductor crystal near the terminal and combine with holes, thus cancelling them. At the same time loosely held electrons near the positive
terminal A are attracted away from their atoms into the positive terminal. This creates few holes which again drift towards –ve terminal.
8. Extrinsic Semiconductor : n-type
Fig: Covalent bond in n-type Fig: Energy bands in n-type Fig: Current conduction in n-type
When a small amount of pentavalent impurity is added to a pure semiconductor, it is k’as n-type semiconductor. Fifth valence electron
of arsenic atom finds no place in covalent bond and is thus free. Many new free electrons are produced by the addition of pentavalent
impurity. Thermal energy of room temp still generates a few hole-electron pairs.However, the no. of free electrons provided by
pentavalent impurity far exceeds the holes. It is due to predominance of electrons over holes that is is called n-type semiconductor.
9. Extrinsic Semiconductor : p-type
Fig: Covalent bond in p-type Fig: Energy bands in p-type Fig: Current conduction in n-type
When a small amount of trivalent impurity is added to a pure semiconductor, it is called p-type semiconductor. Fourth bond is
incomplete; being short of one electron. This missing electron is called hole.The addition of trivalent impurity has produced large no. of
holes. However, there are few conduction band electrons due to thermal energy associated with room temp. But the holes far outnumber
the conduction band electrons. It is due to the predominance of holes over free electrons that it is called p-type semiconductor.
10. Charge on n-type & p-type semiconductor
The terms n- and p-type doped do only refer to the majority charge carriers. Each positive or negative charge carrier belongs to a
fixed negative or positive charged dopant. p and n type materials are NOT positively and negatively charged. An n-type material by
itself has mainly negative charge carriers (electrons) which are able to move freely, but it is still neutral because the fixed donor
atoms, having donated electrons, are positive. Similarly p-type material by itself has mainly positive charge carrier (holes) which
are able to move relatively freely, but it is still neutral because the fixed acceptor atoms, having accepted electrons, are negative.
In a tetravalent semiconductors like Si/Ge , if pentavalent materials like As are added then we get n-type semiconductor. In this n-type
semiconductor, four electrons of pentavalent element forms bond with the host element. Hence the excess one electron of dopant remains
very weakly bound to the atom of host material. As a result ionisation energy required to free this electron is very small. This required
ionisation energy is about 0.01 eV for Ge and about 0.05 eV for Si. Once required ionisation energy is supplied, the electron is free to move
and the dopant atom is positively charged. Similarly in p-type, when dopant receives electon from neighbouring Si/Ge atom, it becomes
negatively charged and create a positive hole in neighbouring Si/Ge atom. Thus free charged carriers i.e., electron and holes are created for
conduction in impurity semiconductors.But the crystal maintains an overall charge neutrality as the charge of additional charge carriers is just
equal in magnitude and opposite in sign to that of the ionised cores in the lattice.
12. Difference between n-type & p-type
P-type semiconductor is formed due to the dopping of III group elements i.e. Boron, Aluminium, Thallium.
N-type semi conductor is formed due to dopping of V group elements Nitrogen, Phosporus, Arsenic, Antimony,
In P-type impurity added creates vacancy of electrons (holes) called as Acceptor Atom.
In N-type impurity added provides extra electrons and is known as Donor Atom.
P-type are also known as Trivalent semi conductors.
N-type are also known pentavalent semiconductor.
P-type semiconductors is positive type semiconductor it means it deficiency of 1 electron is required.
N-type semiconductor is negative type semi-conductor it means excess of 1 electron is required.
In P-type semiconductor majority charge carries are holes and minority charge carries are electrons.
In N-type semiconductor majority charge carries are electrons and minority charge carries are hole.
A hole indicates a missing electron. In P-type no. of holes is more than the no. of electrons.
In N-type semiconductor the no. of holes is less than the no. of free electron.