• In order to build a n-type semiconductor from silicon, it has to be doped with electron-rich impurities (element having more valence electrons).

• As group 14 element Si has 4 valence electrons, therefore to get an extra electron carrier for the semiconductor it has to be doped with elements of valency 5 i.e. group 15 elements such as P or As.

• Four out of five electrons of group 15 element (P or As) will be forming four covalent bonds with the four silicon atoms which are adjacent.

• The fifth valence electron of the group 15 element now becomes an extra and gets freed and delocalised. These delocalised electrons enhance the conductivity of doped silicon.

Hence, other 3 options (i) 2 (ii) 1(iii) 3 are incorrect, because their doping would not form any n type semiconductor.