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CONDUCTION IN PURE SEMICONDUCTOR


CONDUCTION THROUGH PURE SEMICONDUCTORS
CONCEPT OF HOLE FORMATION

A pure semiconductor is called intrinsic semiconductor. The conductivity through a pure semiconductor at room temperature is zero because it does not contain any free electron at room temperature. The conductivity is produced through a pure semiconductor if its temperature is increased.
A semiconductor like Ge or Si has crystalline structure
Germanium and silicon has 4 unpaired valency electrons. Each atom of Ge or Si has valency equal to 4. 
These four valency electrons form four weak covalent bonds with four nearest electrons of neighborhood atoms. 
At room temperature all the four electrons are complete and no free electron is available. This is continued throughout the crystal lattice. Hence there will be no conduction of charges through a pure semiconductor at room temperature.
When the temperature of substance is increased, a few valency electrons get energized to come out of their orbit. 
Hence that many number of bonds will be broken and the electrons become free. These free electrons move randomly with in the crystal lattice. When an electron comes out as a free electron, there will be a vacancy created at its position in the broken
 (or incomplete) bond. This vacancy is known as HOLE.
The number of free electrons is equal to the number of holes.

Generally a free electron is denoted by a dot and a hole is denoted by a circle. 
When a free electron comes closer to a vacancy due to random motion, it is capture by the hole and the electron-hole pair is recombined. The broken bond is now complete. As a free electron is reduced due to the recombination of electron Hole pair, the hole is considered as a positive charge carrier. 
And the electron is obviously a negative charge carrier.

  There is no conductivity through a pure semiconductor at room temperature. The conductivity increases as the temperature if raised. But it still low conductivity only. Because the concentration of free electrons is equal to that of holes.
To increase the conductivity through the semiconductor either free electrons or holes must be greater in number. For this we add some impurities to the pure semiconductor. Then the semiconductor is called impure or extrinsic semiconductor

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