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Why p and n type semiconductor are electrically neutral


Asked by abhishekrajkr 3rd November 2018, 7:40 PM
Answered by Expert
p-type and n-type semiconductors are called extrinsic semi conductors, because an element which is different from the
element of given semiconductor material is added as impurity to provide charge carriers.
These added impurity elements are called dopants.

In a tetravalent semiconductors like Si/Ge , if pentavalent materials like As, P are added then we get n-type semiconductor.
In this n-type semiconductor, four electrons of pentavalent element forms bond with the host element.
Hence the excess one electron of dopant remains very weakly bound to the atom of host material.
As a result ionisation energy required to free this electron is very small.
This required ionisation energy is about 0.01 eV for Ge and about 0.05 eV for Si. Once required ionisation energy is supplied,
the electron is free to move and the dopant atom is positively charged.

Similarly in a teravalent Si/Ge , trivalent metal like B is added to get p-type semiconductors.
The dopant has one electron less than Si/Ge. Hence this dopant atom can form covalent bond with neighbouring three Si/Ge atom,
but does not have electron to offer to the fourth Si/Ge atom. So the bond between the fourth neighbour has vacancy or hole.
Since the neighbouring Si atom in the lattice wants an electron to form the bond, an electron in the outer orbit of Si/Ge atom can
jump into this vacancy and create hole on its own atom. Thus conduction is taking place because of holes.
Hence in this type, when dopant receives electon from neighbouring Si/Ge atom, it becomes negatively charged and
create a positive hole in neighbouring Si/Ge atom.

Thus free charged carriers i.e., electron and holes are created for conduction in impurity semiconductors.
But the crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal
in magnitude and opposite in sign to that of the ionised cores in the lattice.
Answered by Expert 5th November 2018, 10:23 PM
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