Solids and Semiconductors

Types of extrinsic semiconductors

In last article I have started discussion on extrinsic semiconductor. In today’s article I will discuss the extrinsic semiconductor and its 2 types in detail. As already discussed a doped semi-conductor is known as an extrinsic semi-conductor. It is of 2 types:

a) n-type semiconductor and b) p-type semiconductor

Let us discuss these two types of extrinsic semiconductors in detail:

(i)                 n-type semi-conductor: – When a pentavalent impurity atom such as arsenic (As) Phosphorous (P) etc. are added to a pure semi-conductor an n-type semi-conductor is formed. The impurity atom contains 5 electrons in their outermost shell 4 out of which will form covalent bonds with the atoms of semiconductor like Si or Ge and 1e is left free. So it is known as a donor impurity. At room temperature some of the covalent bonds may break up due to which electron hole pairs are generated hence the conductivity increases. So at the end the number of electrons are greater than the number of holes. Hence in an n-type semi-conductor electrons are majority carries, whereas the holes are minority carries.

The highest level up top which the electrons are filled in the valence band is known as the Fermi-level and the corresponding energy is known as Fermi-energy. In case of n-type semi-conductor, the Fermi-level is close to the conduction band.

(ii)        P-type Semi-conductor: – When a trivalent impurity atom like B, Aℓ, In etc. are added to a pure semi-conductor a p-type semi-conductor is formed. It contains 3 electrons in its outermost shell which combine with the 3 atoms of Si or Ge but a vacancy is left. This vacancy or deficiency of the electron is termed as a hole. So it is an acceptor impurity ie. It can accept other electrons. Hence, in a p-type and semi-conductor, the no. of holes are greater than the no. of electrons or holes are majority carries but electrons are minority carries. Moreover, the Fermi-level is close to the valence band.

I hope you got the basic idea of extrinsic semiconductor and its 2 types.

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