P-N junction diode is the most fundamental and the simplest electronics device. When one side of an intrinsic semiconductor is doped with acceptor i.e, one side is made p-type by doping with n-type material, a p-n junction diode is formed. This is a two terminal device. It appeared in 1950’s. P-N junction can be step graded or linearly graded. In step graded the concentration of dopants both, in n - side and in p - side are constant up to the junction. But in linearly graded junction, the doping concentration varies almost linearly with the distance from the junction. When the P-N diode is in unbiased condition that is no voltage is applied across it, electrons will defuse through the junction to p-side and holes will defuse through the junction to n-side and they combine with each other. Thus the acceptor atom near the p-side and donor atom near n-side are left unutilized. An electron field is generated by these uncovered charges. This opposes further diffusion of carriers. So, no movement of region is known as space charge or depletion region.
If, we apply forwards bias to the p-n junction diode. That means if positive side of the battery is connected to the p – side, then the depletion regions width decreases and carriers flow across the junction. If the bias is reversed the depletion width increases and no charge can flow across the junction.