PN Junction diodes are made by doping the trivalent element into one side of an intrinsic semiconductor like silicon and doping pentavalent to another side of it. The side of the trivalent element will become P-type material where the side of the pentavalent element will become N-type material. As far as both types of material are at the same substrate, therefore a point of conjunction will exist there, which is called PN Junction.
We have seen previously that the doping of trivalent and pentavalent into germanium or silicon can produce P-Type and N-type materials. In N-type materials, the pentavalent atom makes four covalent bonds with silicon atom and one electron remains un-bonded which can move freely, and are called a free electron. There are more free electrons in N-Type material then holes which are caused by impurities in the intrinsic semiconductor. The electrons referred as majority carriers and hole are referred to as minority carriers N-region of the PN Junction. Similarly, in P-type material, trivalent atom makes three covalent bonds with silicon atom and one room remain left to bond with silicon which is called the hole. There are more holes and are called majority carriers than thermally generated free electrons, which are called minority carriers. Remember that the number of electrons and protons in both N-type and P-type material are same, so they are electrically neutral.
If one side of an intrinsic semiconductor substrate is doped with trivalent and another with a pentavalent element, so the whole substrate will be called PN junction diode. A question may be raised that this new substrate would be unstable, all electron and holes will combine and PN junction substrate will lose its meaning. Let us have a look at the internal situations inside the substrate.
Depletion region Formation:
At the instant of doping, free electrons near the junction from the N-type region pass the joining point toward the P-type region and combine with a hole near the junction. As the N-type region losses electrons and P-type regions losses hole, so they are not electrically neutral. But a small region near the junction in the P-type region become negatively charged and similarly a small region near junction N-type region become positively charged. Both these charged regions are cumulatively called depletion region which contains neither free electrons nor holes. This depletion region is very thin and formed very quickly.
After the first surge, the very thin depletion region expands to some extent and make it impossible for free electrons to pass the region and diffuse into another side. As the electrons diffuse to the P-type region, the negative charge increase inside the P-type after some diffusion, the negatively charged region in P-type repel the diffusion of electrons. Thus after the formation of the depletion layer, no free electrons can pass the depletion region.
After the formation of depletion layer these positive and negative charges are held together across the junction. According to the Coulomb law, these charge region across the junction will be acting force on each other. These charges will form an electric field which opposes the passage of free electron from N-region to P-region. External energy is required for electrons to pass through the electric field. The potential difference required for electrons to pass through the electric field produced by depletion region is called Barrier Potential. After the application of this voltage in proper polarity the electron from N-region can get into the P-region. The amount of barrier potential depends upon doping level, temperature, semiconductor material. Typical barrier potential for silicon is 0.7v and that for germanium is 0.3 v at room temperature.