Question

The valency of donor impurity in order to be added for doping germanium crystal is
\[\begin{align}
  & A.2 \\
 & B.3 \\
 & C.4 \\
 & D.5 \\
\end{align}\]

Answer 1

Hint: Doping is the act of adding impurities into an intrinsic semiconductor in order to alter their properties. Normally Trivalent and Pentavalent elements are taken to dope Silicon and Germanium. Such a doped crystal is referred to as an extrinsic semiconductor.

Complete step-by-step answer:
First of all let us take a look at what the process of doping is meant to be. Doping is the intentional addition of impurities into an intrinsic semiconductor in order to modulate its electrical, optical and structural properties. The doped material is known as an extrinsic semiconductor. Any impurity deliberately added to a semiconductor for the purpose of modifying its electrical conductivity is called a dopant. The most generally used elemental semiconductors are silicon and germanium, which can form crystalline lattices in which each atom is sharing one electron with each of its four nearest neighbours. As we know the valency of the germanium atom is being four, it can accept, donate and share four electrons with neighbouring atoms for completing its octet. In order to dope with a given donor impurity, we have to dope with an atom having 5 electrons in the outermost orbit so that after sharing four electrons with a germanium atom, it can donate one electron.
So the correct answer is given as option D.

Note:As the dopants are impurities, the chemical composition also changes due to doping. Shallow states are having smaller ionisation energies and when the doping density is higher, the dopant states will create a band. If this band is very much near to the valence or conduction band edge, then the band-gap will decrease.