More practice questions

4. Q4. In a pure semiconductor at equilibrium, electron-hole pairs are created all the time. Why does n_i not keep growing?

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   Electrons also recombine with holes. At equilibrium the rate of recombination equals the rate of generation, so n_i stays constant at a given temperature (NCERT page 328).

5. Q5. In a pure semiconductor sample the electron current is 2.5 mA and the hole current 1.0 mA. What is the total current?

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   I = I_e + I_h = 2.5 + 1.0 = 3.5 mA (NCERT Eq. (14.2), page 328).

6. Q6. Assertion: in an intrinsic semiconductor the number of free electrons equals the number of holes. Reason: each broken covalent bond releases one electron and leaves one hole. Are both true, and does the reason explain the assertion?

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   Both are true, and the reason explains the assertion (NCERT page 327).

7. Q7. Each atom in a Si crystal has how many nearest neighbours, and how many electrons does it share in covalent bonds?

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   Four nearest neighbours. It shares one of its electrons with each, and takes a share of one electron from each, so eight electrons in four bonds surround it (NCERT page 327).

8. Q8. If n_i of a sample doubles when it is warmed, what happens to its intrinsic conductivity, if the mobilities do not change?

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   σ = en_i(μ_e + μ_h) doubles (beyond the NCERT text).