26. Recombination of electrons and holes takes place when :
(a) An electron falls into a hole
(b) A positive ion and a negative ion bond together
(c) Avalanche electron becomes a conduction electron
(d) An atom is formed
27. Semiconductors are :
(I) Ohmic as they obey Ohm’s law
(a) Only I is true
(b) Only II is true
(c) Both I & II are true
(d) Both I & II are false
28. Which of the following statements are correct ?
1. In semiconductor, the mobility of electrons is more than that of holes.
2. In semiconductor, when temperature increases, the resistivity also increases.
3. Metal has positive TCR.
4. In metals, thermal conductivity is inversely proportional to electrical conductivity at constant temperature.
(a) 1, 2, 3
(b) 1, 2, 4
(c) 2, 3, 4
(d) 1, 3, 4
29. The carrier mobility in a semiconductor is found to be 0.4 m2/VS. Its diffusion constant at 300 K will be :
(a) 0.43 m2/s
(b) 0.16 m2/s
(c) 0.04 m2/s
(d) 0.01 m2/s
30. The Hall coefficient of an intrinsic semiconductor is :
(a) Positive under all conditions
(b) Negative under all conditions
(c) Zero under all conditions
(d) Zero at 0°K
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