What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.

Semiconductors allow to flow a very small quantity of electric current through them at the room temperature. These are the solids with conductivities in the intermediate range from 10–6 to 104 ohm–1m–1.Semiconductors are perfect insulator at absolute zero. Silicon and Germanium are examples of semiconductor.
There are two types of main semiconductors
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n–type semiconductor:
there semiconductors have Electron – rich impurities. Silicon and germanium belong to group 14 of the periodic table and each has four valence electrons . Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. Extra fifth electron is becomes delocalized. These delocalized electrons increase the conductivity of doped silicon or germanium both . Here the increase in conductivity is due to the negatively charged mobile electron. Hence semiconductors doped with electron–rich impurity are called n–type semiconductor

p–type semiconductor:–
Si or Ge can also be doped with a group 13 element. The elements of group 13 contains only three valence electrons. Elements of 14th group have has 4 valence electron. The place where the fourth valence electron is missing is called electron hole .An electron from a neighbouring atom can come and fill the electron hole, but in doing so it would leave an electron hole at its original position. If it happens, it would appear as if the electron hole has moved in the direction opposite to that of the electron that filled it. Under the influence of electric field, electrons would move towards the positively charged plate through electronic holes, but it would appear as if electron holes are positively charged and are moving towards negatively charged plate. This type of semi conductors are called p–type semiconductors.

A semiconductor is a material which has electrical conductivity between that of a conductor such as copper and that of an insulator such as glass.
Current conduction in a semiconductor occurs through the movement of free electrons and "holes", collectively known as charge carriers. Adding impurity atoms to a semiconducting material, known as "doping", greatly increases the number of charge carriers within it. When a doped semiconductor contains mostly free holes it is called "p-type", and when it contains mostly free electrons it is known as "n-type". The semiconductor materials used in electronic devices are doped under precise conditions to control the location and concentration of p- and n-type dopants. A single semiconductor crystal can have many p- and n-type regions; the p–n junctions between these regions are responsible for the useful electronic behaviour.
Semiconductors can be classified as per their majority carriers or dopant types.

Intrinsic Semiconductors

There are two ways to define an intrinsic semiconductor. In simple words, an intrinsic semiconductor is one which is made up of a very pure semiconductor material. In more technical terminology it can stated that an intrinsic semiconductor is one where the number of holes is equal to the number of electrons in the conduction band.

The forbidden energy gap in case of such semiconductors is very minute and even the energy available at room temperature is sufficient for the valence electrons to jump across to the conduction band.

Extrinsic semiconductors
These are semiconductors in which the pure state of the semiconductor material is deliberately diluted by adding very minute quantities of impurities. To be more specific, the impurities are known as dopants or doping agents.The materials chosen for doping are deliberately chosen in such a manner that either they have 5 electrons in their valence band, or they have just 3 electrons in their valence band. Accordingly such dopants are known as pentavalent or trivalent dopants respectively.

The type of dopant also gives rise to two types of extrinsic semiconductors namely P-type and N-type semiconductors.

A pentavalent dopant such as Antimony are known as donor impurities since they donate an extra electron in the crystal structure which is not required for covalent bonding purposes and is readily available to be shifted to the conduction band. This electron does not give rise to a corresponding hole in the valence band because it is already excess, therefore upon doping with such a material, the base material such as Germanium contains more electrons than holes, hence the nomenclature N-type intrinsic semiconductors.

On the other hand when a trivalent dopant such as Boron is added to Germanium additional or extra holes get formed due to the exactly reverse process of what was described in the upper section. Hence this dopant which is also known as acceptor creates a P-type semiconductor.

Hence electrons are the majority carriers (of current) in N-type while holes are minority carriers. The reverse is true of P-type semiconductors. Another difference is that whereas the Fermi level of intrinsic semiconductors is somewhere midway between the valence band and the conduction band, it shifts upwards in case of N-type while it drifts downward in case of P-type due to obvious reasons.