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### Course: Class 12 Physics (India)>Unit 14

Lesson 3: The PN junction

# The PN junction

A P-type has a lot of holes and an N-type lot of electrons. What if we put them together? In this video, let's explore the famous PN junction. We will see what happens when we have a single crystal with a P-type semiconductor on one side and an N-type on the other.  Created by Mahesh Shenoy.

## Want to join the conversation?

• Why don't minority charges combine with the side they're originally on?
• They can't get recombined.

For example, consider only n-type semiconductor. We dope it with phosphorus and we get majority charge carriers as electrons and minority as holes.

Now, electrons may randomly leave their original spot to occupy holes. The point here is that the space they left acts now as a hole!

The above process continues randomly. So, holes move from one spot to another till it comes close enough near the junction boom it crosses the junction and reaches p side.
• how could P+ ions repel the holes...
holes are not positive?
• Holes are not real positively charged particles like protons, but are instead lack of electrons. So they behave like a +vely charged particle. And thus P+ will repel the holes.(like charges repel)
• Why "Class 12 Physics (India)" is not included in the main "Science" section? Are there any other hidden sections?)
• This section was specifically made for Class 12 under the Indian Board. It's equivalent to AP, I guess. If you want to check out the other videos, I guess you could check out the class 11/12 Physics, Chemistry and Biology (India) sections :)
(1 vote)
• When you are saying that the electrons and holes recombine to destroy each each other, then what exactly happens? Do the electrons just vanish after recombination ?
• From the author:The free electrons and holes are mobile, they are responsible for the conduction of electricity. When the free electrons recombine with holes, the electron is still there, but it is no longer mobile, it no longer conducts electricity.
So in that sense, we have 'lost' charge carriers.
• wont the electron in the p type region (minority charges) be repelled by negatively charged boron ion. and since boron ion is closer, repulsion force will be greater than the attraction force mentioned in the video. so how does the electron go to the n type region??
• If you notice there is a greater repulsion for the electron from the site it is coming. I mean there are more Boron ion on the other side those are repelling the electron with a greater repulsion.
• What exactly happens AFTER recombination across forward based PN junction. When electron hole pairs are diminished, does it mean that the current stops flowing after that (as no electrons are left anymore in conduction band)?
If yes, then what is the advantage we get by supplying current across diode only for a short period of time?
• in the depletion layer since oppositely charges ions are present why don't they interact among themselves or even why the +ve ions on the n side attract more electrons to neutralise them ?why necessarily create an electric field ?
• From the author:The ion's do attract the charge carriers. Imagine an electron in the depletion region. The positive Phosphorus ion will attract (or the negative Boron ion will repel) it towards the N side.
It's this attraction (or repulsion) that impedes the diffusion of electrons from N side to P side.