A) boron
B) gallium
C) aluminium
D) all of these
Correct Answer: D
Solution :
If a trivalent impurity is mixed in a pure (intrinsic) semiconductor, then it becomes a \[p-\]type semiconductor. As given in problem, all boron, gallium and aluminium are trivalent impurities so, they must be doped to Ge (or Si) to make it \[p-\]type semiconductor. Note: The added impurity is very small of the order of 1 atom million atoms of the pure semiconductor. When impurity atoms are incorporated in a pure semiconductor crystal, the electric properties are altered and most of the charge carriers originate from impurity atoms.
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