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Written By: 

Nikhil Agnihotri
A simple p-n diode is a junction where p-type and n-type layers are doped on a silicon or germanium wafer. A p-type semiconductor is formed by doping of trivalent or acceptor impurity atoms on a pure silicon or germanium thereby having an excess concentration of holes. An n-type semiconductor is formed by doping of pentavalent or donor impurity atoms on a pure silicon or germanium thereby having an excess concentration of electrons. So, holes are the majority charge carriers in a p-type region whereas electrons in the n-type region. Electron-holes pairs are thermally generated in both types which constitute the minority charge carriers. It is remarkable that a p-type material is not positively charged in spite of having excessive holes while an n-type material is not negatively charged in spite of excessive electrons.  This is because in a p-type material along with holes, the anions are generated and the total number of protons and electrons still remain the same. This is similarly observed for the n-type material.
The junction of a p-type and n-type doping on silicon or germanium wafer produces a small region of the order of micrometers which is depleted from the free charge carriers. This region is formed due to diffusion of holes from a p-type and electrons from an n-type material called as the depletion region or space charge region or transition region. The p-type region to the left of the depletion region is having acceptor negative ion layer and to the right are donor positive ion layer which induces an electric flux or potential difference across the junction. The charge concentration is positive on left of the junction and negative on the right of the junction. This potential barrier stops the holes to migrate into n-type region and electrons to migrate into p-type region as the potential rises for holes and electrons will allow migrating in to n-type and p-type regions. The charge carrier regions around the depletion regions are also called as the uncovered regions. This is shown in graph below.
Graph diagram of Linear Graded diode and Step Graded diode
It is also important that the minority charge currents i.e. electron current in p-type region and hole current in n-type region decreases exponentially across the diode length.  The minority current is due to electron hole pairs generated thermally and dependent upon temperature. These currents are so small in magnitude in the order of microamperes. However in conduction state, the current through the diode crystal remains stable.  The total current is a sum of minority and majority charge currents due to bipolar nature of the diode. The majority charge currents is hole current in p-type and electron current in n-type are reduced as they migrate near junction due recombination. The minority currents is electron current in p-type and hole current in n-type are maximum near junction and reduces as they migrate away from the junction as an exponential function. The majority charge currents in their regions after crossing the junction are the diffusion currents while before junction are drift currents. 
Concept of Ohmic contacts – In addition to PN junction diode, there is a two metal semiconductor junctions originating from the leads in order to connect the device. It is assumed that the resistance of these metal semiconductor contacts remain constant despite of the magnitude and direction of current. During the diode operation, the applied voltage is solely effective for increasing or decreasing the potential barrier height of the PN junction.
Note: The use of a step graded diode can improve the performance of the diode.