Unbiased PN junction
In a given material if the doping is not uniform then at one place large number of charge carriers exist while at other place small number of charge carriers exist.
Inside there are large number of electrons while on p side electrons are minority in number. This process is called diffusion and exists when there is nonuniform concentration of charge carriers in the material. So there is high concentration of. In a high charge carrier concentration area, all charge carriers are of similar type, either electrons or holes and hence start repelling each other.
Due to this, charge carriers start moving from high concentration area towards low concentration area, to achieve uniform concentration all over the material.
Formation of Depletion Region
As holes enter the n-region, they find number of donor atoms. The depletion region is also called space-charge region. Practically width of the depletion region is very small of the order of few microns where I micron = 1 x 10 m. The formation of immobile ions near the junction.
As more number of holes diffuse on n side, Formation of immobile ions large positive charge gets accumulated on a side near the junction. And the diffusion of holes stops.
Similarly due to large negative charge accumulated on p side, the diffusing electrons get repelled and eventually the diffusion of electrons also stops.
Thus in thermal equilibrium, in the region near the junction, there exists a wall of negative immobile charges on p side and a wall of positive immobile charges on n side. In this region, there are no mobile charge carriers. As donor atoms accept additional holes, they become positively charged immobile ions. Thus depletion region acts as the barrier.
The physical distance from one side to other side of the depletion region is called width of the depletion region. In equilibrium condition, the depletion region gets widened upto a point where no further electrons or holes can cross the junction. Eventually the diffusing holes which are positively charged, get repelled due to accumulated positive charge on n side. Such a region is depleted of the free mobile charge carriers and hence called depletion region or depletion layer. The electrons diffusing from n side to p side recombine with the acceptor atoms on p side. As acceptor atoms accept additional electrons, they become negatively charged immobile ions: Such large number of negatively charged immobile ions get formed near the junction on p side. This happens immediately when holes cross the junction hence number of positively charged immobile ions get formed near the junction on n side.
Junction
Atoms on p side are acceptor atoms
Drift Current
The applied voltage make the electrons drift towards the positive terminal. Thus drift current means the flow of current due to bouncing of electrons from one atom to another, travelling from negative terminal to positive terminal of the applied voltage. The electrons, moving towards positive terminal collide with the atoms of semiconductor and connecting wires, along its way. When a voltage is applied to a semiconductor, the free electrons try to move in a straight line towards the positive terminal of the battery. Each time the electron strikes an atom, it rebounds in a random direction. This current produced due to drifting of free electrons is called drift current and the velocity with which electrons drift is called drift velocity. This drift causes current to flow in a semiconductor, under the influence of the applied voltage
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