P-N Junction Diode
When a p-type semiconductor is bought into close contact with n-type semiconductor crystal, the resulting arrangement is called a p-n junction or junction diode. In the p-type semiconductor, the holes are majority carriers and the electrons are minority carriers whereas in the n-type semiconductor, the electrons are majority carriers and the holes are minority carriers.
The n-type region has a high electron concentration and the p-type a high hole concentration, electron diffuse from n-type side to p-type side. Similarly, holes flow by diffusion from p-type side to n-type side. If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same, as happens if two gases come into contact with each other. However, in a p-n junction, when electrons and holes are move to the other side of junction, they leave behind exposed charges on dopant atoms site, which are fixed in the crystal lattice and unable to move.
On the n-type side positive ion cores are exposed and on the p-type side negative ion cores are exposed. An electric field E forms between the positive ion core in n-type material and negative ion core in p-type material. This region is called “Depletion region”, since the electric field quickly sweeps free carriers out, hence the region is depleted of free carrier. A “Built in potential” Vp due to E is formed at the junction.
The width of the depletion layer and the magnitude of the barrier potential depend on the nature of the semiconductor and doping concentration on two sides of the p-n junction. If the doping concentration of n-type and p-type semiconductor forming p-n junction is small the diffusing electrons and holes across the junction can move to quite large distance before suffering a collision with another holes and electrons to be recombined. Due to which the width of the p-n junction is large and junction field is small.
On the other hand, if the doping concentration of n-type and p-type semiconductor forming p-n junction is large, the width of the p-n junction would be small and junction field would be large. It means p-n junction will show different behaviour by changing the doping levels on both the sides.
The p-n junction can be considered to be equivalent to a capacitor with p and n regions will acts as the plates of a capacitor and the depletion medium as the dielectric medium. We cannot measure the potential barrier of p-n junction by putting a sensitive voltmeter across its terminals because there are no free electrons and holes in the depletion layer and in the absence of forward biasing, the depletion layer offers infinite resistance.
