Question

Name two pentavalent impurities.

Answer 1

- Hint: Doping of semiconductor depends upon the impurity’s properties. If we are adding pentavalent impurities to the semiconductor, it will be n-type and adding trivalent impurities will create p-type semiconductors. Pentavalent impurities have five valence electrons in their outermost orbit.

Complete step-by-step solution -

Pentavalent impurities are the atoms with five valence electrons used for the doping of semiconductors. i.e. Arsenic (As), Phosphorous (Pi), Antimony (Sb), etc.

Additional information:
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 in 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. If we are adding pentavalent impurities to the semiconductor, which means we are fabricating the 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. 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.
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: It is better to remember elements with 5 electrons in the valence shell for pentavalent impurities and elements with 3 electrons in the valence shell for trivalent impurities. These impurities are not the same and their functions are also different.