# Compound semiconductors in group IV

## Homework Statement

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.

A substitution from group V will have five valence electrons instead of four.

So it is from the substitution, since you can't fit the extra one.

Thanks