A PN junction is formed by joining p-type and n-type doped semiconductors together in very close contact. The p- and n-type semiconductors are conducting because of the available free carriers. However, because the carriers diffuse into the adjoining p and n regions by a process called recombination the contact region is non-conducting. This region is called depletion region.
The application of a positive bias on the p-type end will lead a PN junction into the forward biased mode of operation. Positive biases lower the bands in p region. As a result, electrons in an n region and holes in a p region will have smaller barriers to overcome and diffuse to the other side. This leads to a shrinking depletion region and increased conductivity.
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