As you have seen, an impurity atom releases an electron when it bonds with four silicon atoms.
Since there is still an equal number of protons and electrons (including the free electrons) throughout the material, there is no net charge in the material and so it is neutral.
If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a pn junction forms at the boundary between the two regions and a diode is created, as indicated in Figure 1–19(a).
The p region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers).
The n region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers).