Introduction:

Semiconductors are materials that have electrical conductivity between that of a conductor and an insulator. They are of two types, n-type and p-type, depending on whether they contain impurities that donate or accept electrons.

N-type semiconductors:

N-type semiconductors are those in which impurities are added that donate electrons to the material. This increases the number of free electrons in the material, which are responsible for its electrical conductivity. The impurities commonly used to produce n-type semiconductors are those from group V elements of the periodic table, such as phosphorus, arsenic, and antimony.

Impurity to be added:

Out of the given options, the impurity that is added to produce n-type semiconductors is phosphorus. This is because phosphorus is a group V element, which means it has five valence electrons. When it is added as an impurity to a semiconductor, it bonds with the semiconductor atoms, and because it has one extra electron, it donates that electron to the semiconductor material, which increases the number of free electrons in the material. This is what makes the material n-type.

Other options:

Sulphur is not commonly used as an impurity to produce n-type semiconductors. While it is a group VI element and has six valence electrons, which means it has one less electron than required to form a covalent bond with a semiconductor material, it is not as effective as other group V elements like phosphorus.

Aluminium is a group III element, which means it has only three valence electrons. When it is added as an impurity to a semiconductor, it creates a "hole" in the material, which behaves like a positive charge carrier. This makes the material p-type, not n-type.

Conclusion:

In conclusion, the impurity that is added to produce n-type semiconductors is phosphorus, which is a group V element that donates one extra electron to the semiconductor material, increasing the number of free electrons in the material and making it n-type.