RAS question
Semiconductors are materials with electrical conductivity between:
Correct answer: (D) Insulators and conductors.
Semiconductors are materials whose electrical conductivity lies between that of insulators and conductors.
Explanation
A semiconductor is not defined as a perfect conductor or as an insulator. NCERT classifies solids by conductivity and resistivity: metals have high conductivity, insulators have low conductivity, and semiconductors occupy the intermediate range. This middle position is what makes them useful in electronics. Their behaviour can be changed because the number of mobile charge carriers in a semiconductor can be affected by light, heat or a small applied voltage. Doping, meaning the deliberate addition of a suitable impurity, can modify conductivity. So the correct pair is insulators and conductors, with semiconductors placed between the two.
Why the other options are wrong
- (A) Conductors and superconductors are not the boundary used for semiconductors; NCERT places semiconductors between metals or conductors and insulators by conductivity.
- (B) Semiconductors are not only conductors, because their conductivity is intermediate rather than as high as that of metals or conductors.
- (C) Semiconductors are not only insulators, because they can conduct under suitable conditions and have conductivity above the insulating range.
Concept
This tests the basic Science and Technology concept of classifying materials by electrical conductivity. It recurs in RAS because semiconductor electronics, doping and device behaviour all start from this conductor-semiconductor-insulator distinction.