The "doping" of a semiconductor to improve its performance means
A. slightly raising the temperature of a pure semiconductor to improve the ability of its electrons to flow from atom to atom.
B. fusing layers of two different pure semiconductors together to give the electrons that absorb the solar radiation characteristics of the electrons of both semiconductors.
C. mixing large amounts of a metal with small amounts of a semiconductor so that the metal itself becomes a semiconductor.
D. adding small amounts of other elements to the pure semiconductor to change the freedom of electron motion throughout the crystal.