Question

Germanium-based semiconductors can be fabricated by adding the impurities of wither (i) bismuth or phosphorous and (ii) Aluminum or Gallium will produce ........................... and .................................. respectively

a) n-type and n- type

b)p-type and n-type

c) p-type and p-type

d) n-type and p-type

Answer 1

Ans. Option (D). n-type and p-type

Explanation: Since electronic configurations of the germanium atom,aluminium OR Gallium and Phosphorus or bismuth type atoms is:

Ge- [Ar] 3d¹⁰ 4s² 4p² Valence electrons- 4(adding electrons in 4th shell)

Al- [Ne] 3s² 3p¹ , Ga- [Ar] 3d¹⁰ 4s² 4p¹   Valence electrons- 3 ( 2 from 4s and 1 from 4p)

Ph- [Ne] 3s² 3p³ , Bi- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p³Valence electrons- 5 (2 from 3s and 3 from 3p)

As we can see from above, Germanium has 4 valence electrons, these 4 valence electrons are used in forming bonds with adjacent germanium atoms and this leaves a low number of electrons available for conduction.

Now when it is doped with bismuth or phosphorus ,having 5 valence electrons, the 4-4 valence electrons of each atom form bonds with each other , leaving 1 e- from phosphorus unpaired.This electron is free to move across the doped material form and thus in this kind of semiconductor, the conduction gets enhanced and happens through that unpaired electron and thus is called n-type due to net excess negative charge.

Similarly for germanium's doping with aluminium or gallium type material, which is having 3 valence electron, the 3-3 valence electrons of both germanium and aluminium(or gallium) form bonds, but the fourth electron stays unbonded, adjacent to which lies a vacant space called hole, for current to conduct, an electron must move and when it moves from its own position to that adjacent vacant space, similar kind of vacant space called hole forms in its initial position.Thus there is always a net greater number of holes than electrons and thus the type of doped material formed is called P-type semiconductor.