Depletion region
From ArticleWorld
Consider a crystal of a pure semiconductor to be doped so that one half of it is p-type and the other half is n-type. The surface at which both the surfaces meet is known as the p-n junction. Such a diode can be constructed by doping one half region of a single crystal of Germanium or Silicon with acceptor impurity and the other half with donor impurity. On either side of the p-n junction where there are hardly any mobile charges, the region is called the depletion region.
There is a greater concentration of holes in the positive side i.e. the p-region and a greater concentration of electrons in the negative side or the n-region. Thus, holes are majority charge carriers in the p-region and electrons are majority charge carriers in the n-region. In absence of an external electric field, there is charge carrier diffusion from one semiconductor to the other, due to chaotic thermal movement of charge carriers. The charge carriers tend to diffuse from the region of high concentration to the region of low concentration. Thus electrons move from n-type semiconductor to and holes diffuse from the p-type semiconductor in opposite directions.
As a result, a region is formed on both sides around the junction in which there are space charges of opposite polarity. In the n-region, a positive space charge is built up because of diffusion of electrons from n-type to p-type. Similarly, a negative space charge is built up in the p-region, as holes of p-type recombine with electrons which have come from n-type semiconductors.
Thus there are positive and negative ions on either side of the junction which are not free to move as they take part in covalent bonding. These two regions on either side of the p-n junction have practically no mobile charge carriers and hence constitute the depletion layer. The thickness of a depletion layer in a typical junction diode is about a millionth of a metre. This depletion region offers great resistance to the flow of current as it has very little mobile charges.