N-Type Semiconductors

Description:

When Si or Ge crystal is doped with pentavalent impurity we get n type semiconductor.

Example of pentavalent atom: Phosphorous (P), Arsenic (As), Antimony (Sb).

Pentavalent atom has 5 electrons in its valence shell. Figure shows the structure of n type semiconductor.

Every pentavalent dopant atom finds 4 neighboring Si atoms. It shares its 4 valence electrons with four Si atoms to form octet and Si atoms become stable.

Since valence orbit can hold maximum 8 electrons, the 1 extra electron of dopant atom is not the part of covalent bonding and hence it becomes free electron.

The free electron of phosphorous atom has energy 0.01eV less than the conduction band energy of Silicon. At room temperature these free electrons move to the conduction band and are available for conduction of electricity.

Due to these extra free electrons in the crystal structure, the number of electrons become greater than the number of holes in the crystal.

Note − Number of holes will decrease but will never become zero, there would be a small number of holes present in the crystal.

Relationship between the number of electrons and number of holes is given by -

n_e × n_h = n_i²

Where,

n_e = number of electrons.

n_h = number of holes.

n_i = total number of charge carriers.

Conduction band is crowded by electrons and holes in the valence band are decreased in N type semiconductor.

In N type semiconductor electrons are majority charge carriers and holes are minority charge carriers.

Since every pentavalent dopant atom donates 1 electron for conduction; it is called donor type dopant.

As we can control the number of dopant i.e. we can control the number of free electrons and hence control the conductivity of semiconductor.