Here is an excerpt from a work
For example, in the case of compound semiconductors in group IV of the periodic table such as silicon, the main donor impurities are those which, being from group V of the period table (arsenic, phosphorous, etc), are substituted in place of a silicon atom in the crystal structure: since silicon is tetravalence, these atoms naturally form four covalent bonds with the silicon atoms around them, and also easily give up their surplus electron to the crystal structure.
The Attempt at a Solution
What I don't understand is the excerpt.
Okay, silicon is #14, which has 3s2 3p2, total of 4 valence electrons.
But I do not know
(1) first, which and where is this surplus electron referring to / from? from the substitution (group V), or four covalent-bonded silicon?
(2) second, so what about the substitution with an element of group V?
Any comment is appreciated.