When an impurity atom occupies its energy level in a pure semiconductor material, it does not have a net charge because it has undergone a process called doping. Doping involves intentionally introducing impurity atoms into a semiconductor crystal to modify its electrical properties.
During the doping process, impurity atoms from elements with either one more or one less valence electron than the atoms in the semiconductor crystal are added. These impurity atoms are known as dopants. The most commonly used dopants in silicon, for example, are elements from Group III (e.g., boron) and Group V (e.g., phosphorus) of the periodic table.
When a Group V element, such as phosphorus, is added as a dopant to a silicon crystal, it introduces an extra electron into the crystal structure. This extra electron is loosely bound and is relatively free to move within the crystal lattice. It is called a donor electron. The presence of this donor electron allows the doped semiconductor to conduct electricity more easily and makes it an n-type semiconductor.
On the other hand, when a Group III element, such as boron, is added as a dopant to silicon, it creates a deficiency of electrons. This deficiency is called a hole. The absence of an electron creates an effective positive charge, and it behaves as if it were a positively charged particle. The doped semiconductor with these holes is known as a p-type semiconductor.
In both cases, the impurity atom itself does not contribute to the net charge because it either provides an extra electron or creates a hole. The net charge is balanced by the overall charge neutrality of the crystal structure, with the positively charged impurity atom counterbalanced by the negatively charged electrons (in the case of n-type doping) or by the positively charged holes (in the case of p-type doping).
Therefore, the impurity atom itself does not have a net charge when it occupies its energy level in a pure semiconductor material due to the charge compensation provided by the surrounding electrons or holes.