Thursday 14 February 2013
BANDS,ELECTRON & HOLE
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Energy Bands
In an isolated atom the valence electrons can exist only in one of the allowed orbitals each of a sharply defined energy called energy levels. But when two atoms are brought nearer to each other, there are alterations in energy levels and they spread in the form of bands.
Energy bands are of following types
Valence band:
The energy band formed by a series of energy levels containing valence electrons is known as valence band. At 0 K, the electrons fills the energy levels in valence band starting from lowest one.
(i) This band is always filled by electrons.
(ii) This is the band of maximum energy.
(3) No flow of current due to such electrons.
(4) The highest energy level which can be occupied by an electron in valence band at 0 K is called Fermi level.
Conduction band:
The higher energy level band is called the conduction band.
(i) It is also called empty band of minimum energy.
(ii) This band is partially filled by the electrons.
(iii) In this band the electrons can gain energy from external electric field.
(iv) The electrons in the conduction band are called the free electrons. They are able to move anywhere within the volume of the solid.
(v) Current flows due to such electrons.
Forbidden energy gap (DEg):
Energy gap between conduction band and valence band
(i) No free electron present in forbidden energy gap.
(ii) Width of forbidden energy gap upon the nature of substance.
(iii) As temperature increases (), forbidden energy gap decreases (¯) very slightly.
It is also called band energy gap
(i) No free electron present in forbidden energy gap.
(ii) Width of forbidden energy gap upon the nature of substance.
(iii) As temperature increases (), forbidden energy gap decreases (¯) very slightly.
It is also called band energy gap
Holes
A semiconductor is a material that has the band gap or energy gap between that of a conductor and an insulator.e.g.Germanium has a energy gap of 1.1 eV.
Holes in semiconductors
At absolute zero temperature (0 K) conduction band of semiconductor is completely empty and the semiconductor behaves as an insulator.
When temperature increases the valence electrons acquires thermal energy to jump to the conduction band (Due to the braking of covalent bond). If they jumps to C.B. they leaves behind the deficiency of electrons in the valence band. This deficiency of electron is known as hole or cotter. A hole is considered as a seat of positive charge, having magnitude of charge equal to that of an electron.
(1) Holes acts as virtual charge, although there is no physical charge on it.
(2) Effective mass of hole is more than electron.
(3) Mobility of hole is less than electron.
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