Intrinsic semiconductor is pure (un-doped) semiconductor whereas extrinsic semiconductor is impure (doped) semiconductor. The intrinsic carrier number density Ni and extrinsic carrier number densities densities Ne and Nh are related by the law of mass action as
Ni^2 = NeNh
Note that Ni is the number density of mobile electrons as well as holes in the pure semiconductor where as Ne and Nh are their number densities in the doped semiconductor. Simple problems based on this simple law often find place in Medical and Engineering entrance test papers. Consider the following M.C.Q. which appeared in Kerala Engineering entrance test paper of 2006:
The number densities of electrons and holes in pure germanium at room temperature are equal and its value is 3×10^16 per m^3. On doping with aluminium the hole density increases to 4.5×10^22 per m^3. Then the electron density in doped germanium is
Ni^2 = NeNh
Note that Ni is the number density of mobile electrons as well as holes in the pure semiconductor where as Ne and Nh are their number densities in the doped semiconductor. Simple problems based on this simple law often find place in Medical and Engineering entrance test papers. Consider the following M.C.Q. which appeared in Kerala Engineering entrance test paper of 2006:
The number densities of electrons and holes in pure germanium at room temperature are equal and its value is 3×10^16 per m^3. On doping with aluminium the hole density increases to 4.5×10^22 per m^3. Then the electron density in doped germanium is
(a) 2×10^10/m^3 (b) 5×10^9/m^3 (c) 4.5×10^9/m^3 (d) 3×10^9/m^3 (e) 4×10^10/m^3
From the law of mass action we have, Ne = Ni^2 /Nh = (9×10^32)/4.5×10^22 = 2×10^10. The correct option therefore is (a).
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