Atoms of arsenic (As) are often added to silicon (Si) in a process called doping to change the conductivity of the silicon. How does the addition of arsenic change the conductivity of silicon?

Question

An atom of arsenic has one fewer valence electron and more electron shells than an atom of silicon, so the conductivity decreases because the arsenic atom gains an electron.
An atom of arsenic has one more valence electron and more electron shells than an atom of silicon, so the conductivity decreases because the arsenic atom loses the electron.
An atom of arsenic has one fewer valence electron and fewer electron shells than an atom of silicon, so the conductivity increases because the arsenic atom gains an electron.
An atom of arsenic has one more valence electron and more electron shells than an atom of silicon, so the conductivity increases because the arsenic atom loses the electron.

Answer 1

The correct explanation for how the addition of arsenic affects the conductivity of silicon is as follows:

An atom of arsenic (As) has one more valence electron compared to an atom of silicon (Si). Silicon has four valence electrons, while arsenic has five. When arsenic is added to silicon in the doping process, it acts as a donor impurity. The extra valence electron from arsenic is not needed for bonding in the silicon crystal structure, allowing it to become a free electron.

This additional free electron increases the number of charge carriers in the silicon, leading to an increase in its electrical conductivity. Thus, the correct description is:

"An atom of arsenic has one more valence electron and more electron shells than an atom of silicon, so the conductivity increases because the arsenic atom loses the electron."