What are Intrinsic semiconductors?
The semiconductors that are chemically pure, that is, free from any impurities are termed as intrinsic semiconductors. This means the holes or vacancies in the valence band are not provided by any “foreign” atom that act as an impurity. They are also termed as undoped semiconductors or i-type semiconductors. Silicon and germanium are the examples of i-type semiconductors. As we know, these elements belong to the IVth Group of the periodic table and their atomic numbers are 14 and 32, and thus the electronic configuration
Silicon – 1s2 2s22p6 3s2 3p2
Germanium – 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2.
We notice here that both the elements have four electrons in their outermost shell or valence shell. As the temperature of the semiconductor is increased, the electrons gain more thermal energy and thus break free from their shell. The process of ionization of the atoms in the crystal lattice creates a vacancy in the bond between the atoms. The position from which the electron gets dislodged has a hole which is equivalent to an effective positive charge. The hole is then occupied by a free electron, as a result of which the latter vacant position becomes a hole and the former becomes a neutral position. This way the hole or the effective positive charge is transferred from one position to another. In an intrinsic semiconductor, the number of free electrons is equal to the number of holes. Mathematically,
Here, the ni gives the number of total intrinsic carrier concentration which is equal to the total number of holes or the total number of electrons.
When the temperature of an intrinsic semiconductor is T=0K, it behaves like an insulator. When the temperature is increased further, (T>0), the electrons get excited and move from the valence band to the conduction band. These electrons occupy the conduction band partially, leaving correspondingly equal number of holes in the valence band.
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