(a) What change occurs when AgCI is doped with CdCl2 ? (b) What type of semiconductor produced when silicon is doped with boron ?
(a) Two Ag+ ions are replaced by one Cd2+ to maintain electrical neutrality. Thus, a hole is created at the lattice site for every Cd2+ introduced. (b) Boron has only three electrons in its valence shell. When silicon is doped with boron, three covalent bonds are formed by boron with neighbouring three Si atoms. A hole is created where fourth electron is missing. An electron from neighbouring atom jumps to fill this hole and in turn a hole is created at the site from here this electron jumps. As it continues, the electron holes will move in a direction opposite to the flow of electrons. In an electric field the holes more towards negatively charged plate and electron torwards positive plate. Thus it results in the formation of p-type semiconductor.