In the case of N-type impurities, the fifth valence electron of the impurity atom is typically involved in the formation of an extra electron (an excess electron) that contributes to the conduction process. This excess electron is free to move within the crystal lattice and is often found in the conduction band of the material.
To understand this, let's consider an example of N-type doping in silicon (Si). Silicon is a semiconductor with four valence electrons. When a small amount of a pentavalent impurity, such as phosphorus (P), is introduced into the silicon lattice, one electron from the impurity atom can easily break free from the impurity and enter the crystal lattice.
The four valence electrons of silicon form covalent bonds with neighboring silicon atoms, creating a stable valence band. The fifth electron from the phosphorus atom, which is the impurity, does not form a covalent bond and becomes a "donor electron." This donor electron occupies an energy level just below the conduction band. When thermal energy is supplied, this electron can be excited into the conduction band, becoming a mobile charge carrier and contributing to electrical conductivity.
In summary, the fifth valence electron of an N-type impurity is typically involved in the formation of an excess electron that resides in the conduction band of the semiconductor material, allowing it to participate in conduction.