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Basic:
A semi conductor is material that contains two types of charge carriers; electrons and holes.
Electrons carry -ve charge while holes carry an equivalent amount of +ve charge.
Elements of groupIV (Si, Ge) in Periodic Table form these types of materials.
A pure Si/Ge is essentially called an intrinsic semiconductor material and has equal amount of electrons and holes.
More:
The balance of electrons/holes can be adjusted to suit an application by adding impurities from groupIII/V of periofic table to form p-type (more number of positive charge carriers) or n-type (more number of negative charge carriers) materials. These types of materials formed after adding impurities are then known as extrinsic semiconductor materials.
Semiconductor is called so because they are bad conductors in pure form at room temperature.
However their forbidden gap of energy between the valance band and conduction band is very narroe unlike insulators which is wide and conductors that overlap.
The electrons can cross this narrow band with little energy from the room temperature. This causes the semiconductor to conduct when heated.
Unlike the conductors, which exibits increase in resistance as the temperature increases, the semiconductors display a decreas in resistance with increase in temperature.
However it may be remembered that the resistance of a conductor is always lower than that of semiconductor.
The doping is done to increase the conductivity at lower temperature.
Also by doping in a different way, as explained by Kaleem, you can take advantage of the potential barrier and depletion regions generated.
semiconductor are materials whose property lies between conductors and insulators which can be shown by band diagram.
When atoms combine to form substances, the outermost shells, subshells, and orbitals merge, providing a greater number of available energy levels for electrons to assume. When large numbers of atoms exist in close proximity to each other, these available energy levels form a nearly continuous band wherein electrons may transition.
conductors:
In metallic substances, empty bands overlap with bands containing electrons, meaning that electrons may move to what would normally be (in the case of a single atom) a higher-level state with little or no additional energy imparted. Thus, the outer electrons are said to be "free," and ready to move at the beckoning of an electric field.
insulators:
In some substances, a substantial gap remains between the highest band containing electrons (the so-called valence band) and the next band, which is empty (the so-called conduction band). As a result, valence electrons are "bound" to their constituent atoms and cannot become mobile within the substance without a significant amount of imparted energy. These substances are electrical insulators.
semiconductors:
Materials that fall within the category of semiconductors have a narrow gap between the valence and conduction bands. Thus, the amount of energy required to motivate a valence electron into the conduction band where it becomes mobile is quite modest. At low temperatures, there is little thermal energy available to push valence electrons across this gap, and the semiconducting material acts as an insulator. At higher temperatures, though, the ambient thermal energy becomes sufficient to force electrons across the gap, and the material will conduct electricity.
So...
Let me summarize how do we define semiconductors.
There are three definitions given here.... as our understanding of these material
improves...
First Premitive Definition: Semiconductors are the materials having electrical
conductivity between conductors and insulators.
Second Quantum Mecahanical : Semiconductor are materials having band gab of
about 1eV.
Third and most current : Semicondutors are the materials in which we can control
electron conductivity of it by doping (or by adding impurity).
To Know more about semiconductor electronics the best book I will suggest is
Semiconductor Electronics by Gibbs MGH. This one is out of print!
Thats all folks!
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