SEMICONDUCTORS : 1.Classification of solids on the basis of their conductivity
2.Difference between valence band, conduction band and energy band gap
3.Distinction between conductors semiconductors and insulators on the basis of their energy bands. SEMICONDUCTORS
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conductors : conductors These are those solids which have high conductivity and low resistivity
For example Al,Cu,Ag etc.
Its conductivity lies between 102 to108 S/m
Insulators : Insulators These are those solids which have very low conductivity and very high resistivity.
For example –rubber, plastic, putty etc.
The value of conductivity for insulators is less than 10-8S/m
Semiconductors : Semiconductors These are those solids which have conductivity and resistivity in-between metal conductors and insulators.
For examples are Si ,Ge etc.
The value of conductivity lies in-between 10-5 to100 S/m
Types of semiconductors : Types of semiconductors Elemental semiconductors-These are the semiconductors available in natural form.
Examples are silicon(Si),germanium(Ge).
Compound semiconductors-These are the semiconductors which are made by compounding the metals.
Examples are Cds,GaAs,CdSe(Inorganic) ,anthracene,doped pthalocyanines(organic),polypyrole,polyaniline(organic polymers).
Valence band : Valence band In the energy band diagram of semiconductors, the valence band is a lower band belonging to valence electrons of the given crystal. This band may be partially or completely filled with electrons. This band is never empty. In this band electrons are not capable of gaining energy from external electric field. Therefore, the electrons in this band do not contribute to the electric current.
Conduction band : Conduction band In the energy band diagram of semiconductors, the conduction band is an upper band in which the electrons are not present at 0 K.At room temp., this band is either empty or partially filled with electrons .In this band, electrons can gain energy from external electric field and contribute to the electric current.
Energy band gap : Energy band gap In the energy band diagram, energy band gap is the separation between highest energy level of valence band and lowest energy level in conduction band. Electrons are not found in this band. This band is completely empty. The minimum energy required for shifting electrons from valence band to conduction band is called energy band gap (Eg)
Distinction between conductors,semiconductors and insulators on the basis of their energy bands : Distinction between conductors,semiconductors and insulators on the basis of their energy bands Conductors- The energy band diagram for a metal is such that either the conduction band is partially filled with electrons, or the conduction and valence band partly overlap each other and there is no forbidden energy gap in between.
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Insulators : Insulators In insulators the valence band is completely filled, the conduction band is empty and energy gap is quite large i.e. 3eV
In semiconductors the valence band is totally filled and the conduction band if empty but the energy gap between conduction band and valence band is quite small. It is less than 3eV
Ge- 0.72eV
Si- 1.1eV1616