Discussion
The valence electron of the impurity atom that is to be added to germanium crystal so as to make it a n-type semiconductor is
Answer: B
The impurity atom must have 5 valence electrons.
Germanium is in group 14 and has four valence electrons, each shared in a covalent bond with a neighbouring atom. A pentavalent dopant such as phosphorus, arsenic or antimony uses four of its five electrons in those bonds, and the fifth is only loosely held, so it is easily freed to conduct. Because the extra carriers are negative electrons, the crystal becomes an n-type semiconductor.
A trivalent impurity (3 valence electrons, such as boron or indium) leaves a hole in one bond and gives a p-type semiconductor. An atom with 4 valence electrons simply fits into the lattice like germanium itself, and 6 is not the valence of any standard dopant for germanium.
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