Sunday, May 20, 2012

Unbiased P-N Junction

       If 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. 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, to achieve uniform concentration all over the material. This process is called diffusion and exists when there is uniform concentration of charge carriers in the material. In a p-n junction, on n side there are large number of electrons while on p side electrons are minority in number. So there is high concentration of electrons on n side while low concentration of electrons on p side. Hence diffusion starts and electrons start moving from n side towards p side.
       Similarly the holes from p side diffuse across the junction into the n-region.
       The initial diffusion is shown in the Fig. 1.
Fig. 1 Initial diffusion

1.1 Formation of Depletion Region
       As holes enter the n-region, they find number of donor atoms. The holes recombine with the donor atoms. As donor atoms accept additional holes, they become positively charged immobile ions. This happens immediately when holes cross the junction hence number of positively charged immobile ions get formed near the junction on n side.
       Atoms on p side are acceptor atoms. The electrons diffusing from n side to p side recombine with the acceptor atoms on p side. As acceptor atom accept additional electrons, they become negatively charged immobile ions get formed near the junction p side. The formation of immobile ions near the junction is shown in the Fig. 2.
Fig. 2  Formation of immobile ions

       As more number of holes diffuse on n side, large positive charge gets accumulated on n side near the junction. Eventually the diffusing holes which are positively charged. get repelled due to accumulated positive charge on n side. 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. Such a region is depleted of the free mobile charge carriers and hence called depletion region or depletion layer. The depletion region is also called space-charge region. In equilibrium condition, the depletion region gets widened upto a point where no further electrons or holes can cross the junction. The 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.
        Practically width of the depletion region is very small of the order of few microns where 1 micron = 1 x  10-6 m.
1.2 Barrier Potential 
       Due to immobile positive charges on n side and negative charges on p side, there exists an electric field across the junction. This creates potential difference across the junction which is called barrier, junction potential, built-in potential or cut-in voltage of p-n junction.
Fig. 3 Open circuited p-n junction

       The barrier potential depends on,
1. Types of semiconductor                 2. The donor impurity added
3. The acceptor impurity added          4. The temperature     5. Intrinsic concentration

The barrier potential is called height of the depletion region and expressed in volts. Symbolically it is denoted as VJ , Vo or Vγ.

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