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Question

What is the number density of donor atoms which must be added to a pure germanium semiconductor to produce an N-type semiconductor of conductivity 6.4×102 ohm1m1 ? The mobility of electrons in N-type germanium is 4×101 m2V1s1. Neglect contribution of holes to conductivity.

A
1020 m3
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B
1021 m3
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C
1022 m3
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D
1023 m3
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Solution

The correct option is C 1022 m3
Mobility of the electron is
μ=VEV=μE ...(1)
current density is
J=σE
neV=σE
Putting value of V from equation (1),
neμE=σE
σ=neμ
n=σeμ=6.4×1021.6×1019×4×101
=1022/m3
=1022 m3
Hence, option (C) is correct.

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