Correct Option: D

When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalent bonds with the germanium atoms, leaving 1 electron relatively free in the crystal structure. Pure germanium may be converted into an N-type semiconductor by “doping” it with any donor impurity having 5 valence electrons in its outer shell. Semiconductors which are doped in this manner - either with N- or P-type impurities - are referred to as EXTRINSIC semiconductors.