Silicon crystals are semiconductors. which of the following is a correct reason for the increase in the conductivity of si crystals when a small fraction of si atoms are replaced with those of a different element? a. p atoms introduce additional mobile negative charges. b. p atoms introduce additional mobile positive charges. c. ge atoms have more electrons than si atoms. d. ge atoms are much smaller than si atoms.

a. P atoms introduce additional mobile negative charges

Explanation:

Silicon atoms have a valency of 4, this means that there are 4 electrons in their outermost orbital. Thus, in silicon crystals, each Si atom is connected to 4 different Si atoms.

When we replace a few of these Si atoms with P atoms, which have a valency of 5, 4 out of these 5 outermost electrons will be bonded with the surrounding Si atoms. The fifth electron would not be bonded, meaning it would be free to move, acting as a mobile negative charge carrier (See the picture below).

Just to be clear, regarding option c), it's true that Ge atoms have more electrons that Si atoms, however that it's not a reason for an increase in conductivity.