In a pure semiconductor material, each atom within the crystal lattice has a balanced number of protons and electrons, resulting in a net charge of zero. This balance is necessary for the overall electrical neutrality of the material.
When an impurity atom is introduced into the semiconductor lattice, it can disrupt this balance by having either fewer or additional electrons compared to the host atoms. These impurity atoms are often referred to as dopants.
There are two common types of dopants: donor dopants and acceptor dopants. Donor dopants introduce extra electrons into the semiconductor lattice, while acceptor dopants create "holes" by removing electrons from the lattice.
In the case of a donor dopant, such as phosphorus in silicon, the impurity atom has one extra electron compared to the silicon atoms. This extra electron occupies an energy level within the band structure of the semiconductor but remains relatively loosely bound to the donor atom. The electron is easily excited to a higher energy level, becoming a mobile charge carrier, contributing to the conductivity of the semiconductor. However, the net charge of the donor atom and the surrounding lattice remains neutral since the number of protons and electrons in the atom still cancel each other out.
Similarly, for an acceptor dopant, such as boron in silicon, the impurity atom has one fewer electron compared to the silicon atoms. This creates a "hole" or an absence of an electron in the valence band of the semiconductor. This hole can move through the lattice, effectively behaving as a positive charge carrier. Again, the net charge of the acceptor atom and the lattice remains neutral due to the balanced number of protons and electrons.
In both cases, while the impurity atom itself may have a positive or negative charge, when it occupies its energy level within the semiconductor material, it does not result in a net charge on a macroscopic scale. The overall charge neutrality of the semiconductor is maintained, but the presence of these impurity atoms and their associated charge carriers significantly affects the electrical properties of the material.