Question

Why are elemental dopants for Silicon for Germanium usually chosen from group 13 or group 15?

Answer 1

Dopant, any impurity purposefully added to a semiconductor to alter its electrical conductivity. Silicon and germanium are the most often utilised elemental semiconductors because they create crystalline lattices in which each atom shares one electron with each of its four nearest neighbours.

Complete answer:
The size of the dopant atom should be comparable to that of $Si$ or $Ge$ . So that the symmetry of pure $Si$ or $Ge$ is not disturbed, and dopants can contribute charge carriers through covalent connections with $Si$ or Germanium atoms. Because silicon and germanium belong to the XIV group, atoms of similar sizes will be found in the XIII and XV groups of the contemporary periodic table.

Additional information: Dopants are important in semiconductors because the band gap is so tiny in semiconductors that doping with small amounts of impurities can drastically boost the material's conductivity. Doping, as a result, enables scientists to use the properties of groups of elements known as "dopants" to regulate the conductivity of a semiconductor.Extrinsic semiconductor refers to the doped material. A degenerate semiconductor is one that has been doped to such high levels that it behaves more like a conductor than a semiconductor.

Note: It should be emphasized that in solid-state electronics, the use of the right types and amounts of dopants in semiconductors is what provides the p-type and n-type semiconductors required for the fabrication of transistors and diodes.