Question

The addition of pentavalent impurities ____________ of the semiconductor.
A.) brings no change to the conductivity
B.) decreases the conductivity
C.) increase the conductivity
D.) reduces the formation of free electrons

Answer 1

Hint: Understand the doping process to alter the electrical properties of semiconductor for various applications.

Complete step by step answer:
Semiconductors can be classified as; intrinsic semiconductor and extrinsic semiconductor. Extrinsic semiconductors are further classified as n-type and p-type semiconductors based on the doping impurities. Intrinsic semiconductor is the pure form of semiconductor. For these semiconductors, the number of holes will be equal to the number of electrons at room temperature.

Extrinsic semiconductors are designed by adding additional impurities to increase certain charge carriers and that helps for the advancement of conduction properties. In this question we are adding pentavalent impurities to the semiconductor, which means we are fabricating n-type semiconductors. Electrons are rich in these semiconductors, hence called majority carriers and their fermi level is nearby to the conduction band.

Arsenic is an example for pentavalent impurity. They have 5 valence electrons. Four of these electrons are shared between the two atoms and are known as shared electron pairs. Fifth electron will be free and all other electrons will participate in covalent bonds. That is, they are providing electrons to the semiconductor, hence it is known as a donor atom. Due to the increment of free electrons, the conductivity also increases.
Therefore, the correct option is (C).

Additional information:
p-type semiconductors are obtained by introducing trivalent impurity atoms. Examples are Ga, In, Al and B. These elements have 3 valence electrons in their valence shell and which will participate in the covalent bonding with neighbour atoms. The fourth covalent bond will create vacancy due to the deficiency of electrons. This vacancy is treated as a hole. These holes are the majority carriers in the p-type semiconductors.

Note: Pentavalent impurities are used for the fabrication of n-type semiconductors. While p-type semiconductors are fabricated by trivalent impurities.