The Solid State

Question 229

Assign reasons the following:
(i) Phosphorus doped silicon is a semi-conductor.
(ii) Schottky defect lowers the density of a solid.
(iii) Some of the very old glass objects appear slightly milky instead of being transparent.


(i) When silicon is doped with phosphorus four valence electrons of phosphorus are involved in bond formation with the neighbouring silicon atoms, while the fifth valence electron is left free to conduct electricity. This type of conduction which arises due to the availability of excess electrons is called n-type conduction.

(ii) Schottky defect arises when equal number of cations and anions are missing from their lattice sites. As the number of ions decreases due to this defect In order to maintain electrical neutrality, the number of missing cations and anions are equal, the mass decreases, but the volume remains the same. As a result, the density of the solid decreases.

(iii) Some of the very old glass objects appear slightly milky instead of being transparentbecause of some crystallisation at that point.

Question 230

What is a semiconductor? Describe two main types of semiconductors, giving examples and their distinctive features.


Semiconductors : These are the solids with conductivities in the intermediate range from 10–6 to 104 ohm–1m–1.

In case of semiconductors, the gap between the valence band and conduction band is small Therefore, some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semiconductors increases with rise in temperature, since
more electrons can jump to the conduction band. Substances like silicon and germanium show this type of behaviour and are called intrinsic semiconductors.
The conductivity of these intrinsic semiconductors is too low to be of practical use. Their conductivity is increased by adding an appropriate amount of suitable impurity. This process is called doping.

There are two types of semiconductor
i) n- type semiconductor
ii) p- type semiconductor

Element of group14 of the periodic table, when doped with a group 15 element like P or As, Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron is extra and becomes delocalised.

These delocalised electrons increase the conductivity of doped silicon (or germanium). Here the increase in conductivity is due to the negatively charged electron, hence silicon doped with electron-rich impurity is called n-type semiconductor.

p-type : 

Silicon or germanium can also be doped with a group 13 element like B, Al or Ga which contains only three valence electrons. The place where the fourth valence electron is missing is called electron hole or electron vacancy. This hole can be act as charge carrier in the lattice. This known as p - type semiconductor.

Question 231

With reference to crystal structure, what is meant by coordination number? 


The coordination number of an atom, ion or molecule is the number of constituent particles which touch that particular atom, atom or molecule.

Question 232

What is the coordination number of atoms
(i) in a cubic close packed structure?
(ii) in a body-centred cubic structure?


i) The coordination number of  cubic close packed structure is 12.

ii) The coordination number of body-centred cubic structure is 8.

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