Semiconductors are those materials whose conductivity lies between insulators and conductors. At 0 Kelvin semiconductors behave as insulators but at room temperature some of the valence electrons move towards the conduction band. This is property by which we can distinguish semiconductors from insulators and conductors. For conductors the valence band and conduction band overlaps and for the insulators the energy gap difference is 7eV but for the semiconductors it is approximately 1.107eV. Therefore at room temperature the valence electrons get enough energy to move from valence band to the conduction band.
Types of semiconductors:
There are two types of semiconductors
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There will be one vacant space named as hole. This hole will also help for the conductivity. An electron from the other silicon atom will come to fill this space and a hole will be generated there. Similarly another electron from any other silicon atom will come to fill this gap and hole will be generated again. In this way this process will continue and the electrons will move towards the conduction band. One thing should be remembered that the electrons and hole move in the opposite directions. During this process temperature (heat) will be provided to the semiconductors for the rupturing (breaking) of covalent bond of silicon to silicon atoms.
Majority charge carriers:
The charge carriers which are in excess are named as majority charge carriers. In p-type material holes will be the minority charge carriers
Minority charge carriers:
The charge carriers which are lesser in amount are called minority charge carriers. In p-type materials electrons are minority charge carriers.
Thermal ionization (ionization by providing heat) results in the formation of free electrons and holes and due to this ionization the number of electrons is equal to the number of holes. These holes and electrons move randomly throughout the silicon lattice and during this process some of the electrons may fill some of the holes. This process is called recombination and due to this processes some if the free electrons and holes may disappear. So the number of holes and the number of free electrons may not remain equal to the free number of electrons. The rate of recombination is directly proportional to the free number of electrons and holes and it depends upon the ionization rate.
N-type semiconductors are that semiconductor in which a pentavalent impurity is added i.e. the impurity belongs to the 5-A group of the periodic table which has five electrons in the valence shell. For example phosphorus is selected.
Four of the valence electrons of the impurity will form bond with the four valence electrons of the silicon atom and the fifth electron will be free. This fifth electron will move towards the conduction band and form a bound positive ion. Remember that there is a difference between hole and a bound positive ion. In the hole there is a free space for the formation of a single bond but in the bound positive ion all the four electrons are bonded to the silicon and this fifth electron was free and has moved to the conduction band. There is no free space for the formation of any other bond. This fifth electron will towards the conduction band. In the mean while temperature will be provided to the silicon structure due to which rupturing of covalent bond will continue and it will provide free electrons to the conduction band. Meanwhile hole will also be generated and the electrons from the other silicon atoms will come to fill this vacant space. Same process will continue hole will move downward and the...