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16. (AIEEE 2004)

A piece of copper and another of germanium are cooled from room temperature to 77 K , the resistance of

A. copper increases and germanium decreases

B. each of them decreases

C. each of them increases

D. copper decreases and germanium increases

The Correct Answer is Option (D)

Copper is a conductor, so its resistance decreases on decreasing temperature as thermal agitation decreases; whereas germanium is semiconductor therefore on decreasing temperature resistance increases.

17. (AIEEE 2004)

The manifestation of band structure in solids is due to

A. Bohr's correspondence principle

B. Pauli's exclusion principle

C. Heisenberg's uncertainty principle

D. Boltzmann's law

The Correct Answer is Option (B)

Pauli's exclusion principle.

18. (AIEEE 2003)

A strip of copper and another of germanium are cooled from room temperature to 80 K . The resistance of

A. each of these decreases

B. copper strip increases and that of germanium decreases

C. copper strip decreases and that of germanium increases

D. each of these increases

The Correct Answer is Option (C)

The resistance of metal (like C u ) decreases with decrease in temperature whereas the resistance of a semi-conductor (like G e ) increases with decrease in temperature.

19. (AIEEE 2002)

At absolute zero, Si acts as

A. non-metal

B. metal

C. insulator

D. none of these

The Correct Answer is Option (C)

Pure silicon, at absolute zero, will contain all the electrons in bounded state. The conduction band will be empty. So there will be no free electrons (in conduction band) and holes (in valence band) due to thermal agitation. Pure silicon will act as insulator.

20. (AIEEE 2002)

By increasing the temperature, the specific resistance of a conductor and a semiconductor

A. increases for both

B. decreases for both

C. increases, decreases

D. decreases, increases

The Correct Answer is Option (C)

Specific resistance is resistivity which is given by

ρ = m m e 2 τ

where n = n o . of free electrons per unit volume and τ = average relaxation time

For a conductor with rise in temperature n increases and τ decreases. But decrease in τ is more dominant than increase in n resulting an increase in the value of ρ

For a semiconductor with rise in temperature, n increases and τ decreases. But the increase in n is more dominant than decrease in τ resulting in decrease in the value of ρ .