Q: To get a n- type semiconductor, the impurity to be added to silicon should have which of the following number of valence electrons

Explanation:

n-type semiconductor 

To create an n-type semiconductor, impurities with five valence electrons are added to silicon. Silicon itself has four valence electrons, which form strong bonds with neighboring silicon atoms to create a stable crystal structure. When a pentavalent impurity, which has five valence electrons, is introduced into the silicon lattice, four of its electrons bond with the surrounding silicon atoms just like silicon does. The fifth electron, however, does not have a bonding partner and becomes free to move within the material.

This extra free electron increases the number of negative charge carriers in the semiconductor, enhancing its electrical conductivity. Since the added impurity contributes additional electrons, the semiconductor is termed “n-type,” where “n” stands for negative, indicating the charge carriers are negative electrons. Common pentavalent impurities used for this purpose include phosphorus and arsenic.

Therefore, to obtain an n-type semiconductor, an impurity with five valence electrons is required.