Why a contact difference of potential exist in PN junction ?

Dear student,
 The difference helps minority carriers to drift across the junction and discourages the diffusion of any more majority carriers across the junction.The significance of this potential across the junction is that it opposes both the flow of holes and electrons across the junction.
Regards

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We know that a PN junction is formed when we connect a p type semiconductor to an n type semiconductor. The p type layer contains an excess quantity of positive charge carriers. Similarly the n type layer contains an excess quantity of negative charge carriers.

When we bring these two layers into contact, the free charge carriers on both sides combine with each other and are neutralized. So the region where the two layers are in contact end up with no free charge carriers (due to the recombination). This region is called the depletion layer.

So what we have now is a large number of positive charges on one side and a large number of negative charges on the other side, and these charges are separated by a depletion region.

Now, what does a charged capacitor look like? You guessed it - two regions with opposite charges separated a nonconducting region.

What do you observe between the plates of a charged capacitor? You will observe a voltage difference.

Q = CV (the capacitor equation).

So our PN junction behaves like a charged capacitor and the voltage between its terminals is called the contact potential difference
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