Question

(i)What changes occurs when ${\text{AgCl}}$ is doped with ${\text{CdC}}{{\text{l}}_2}$?
(ii) What type of semiconductor is produced when silicon is doped with boron?

Answer 1

When silver chloride is doped with cadmium chloride, some of the silver ions are replaced with cadmium ions. The carbon has four valence electrons and boron has three valence electrons, so a hole is created when silicon is doped with boron.

Complete answer:
(i) When a compound is doped with another compound this type of defect is known as impurity defects. When silver chloride is doped with cadmium chloride, some of the silver ions are replaced with some of the cadmium ions.

The charge of silver ions is $+ 1$ and the charge of the chloride ion is $+ 2$.
So, each cadmium ion replaces two silver ions, so one site is occupied by a cadmium ion and the cationic site remains left. So, a cationic vacant site is created. These types of defects are known as impurity defects.

(ii) The silicon is a $14$ group element and the boron is a $13$ group element. The valence electrons in silicon are four and the valence electrons in boron are three.

When silicon is doped with boron, each valence electron of silicon forms a covalent bond with a valence electron of boron. So, three electrons of silicon form a bond with three valence electrons of boron and the space of the fourth electron of the boron remains vacant which is known as a hole or electron vacancy. This type of conductor is known as a p-type semiconductor.

Therefore, when ${\text{AgCl}}$ is doped with ${\text{CdC}}{{\text{l}}_2}$, impurity defect occurs and when silicon is doped with boron a p-type semiconductor forms.

Note: The number of cationic sites in the silver chloride crystal is equal to the number of cadmium ions. In a p-type semiconductor, an electron of a neighboring atom moves to a hole so, a hole is created again, so the hole causes the conductivity of the p-type semiconductor.