What is the number density of donor atoms which must be added to a pure germanium semiconductor to make $n-$ type semiconductor of conductivity $6.4\ {\Omega ^{ - 1}}\,c{m^{ - 1}}$? The mobility of electr ons in $n-$ type germanium is $4 \times {10^3}\ cm^2\ V^{-1}\ s^{-1}$. Neglect the contribution of holes to conductivity
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