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Course: Modern Physics (Essentials) - Class 12th>Unit 5

Lesson 3: How to create a one way conductor using semiconductors?

Diffusion, drift & barrier voltage

Let's look at the different currents in a PN junction.  Created by Mahesh Shenoy.

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• Ok, so here's how I conclude it. We take a P type semiconductor (Holes-Maj, Electrons-Min) and an N type semiconductor (Holes-Min, Electrons-Maj) and join them together.
Since there is a conc. gradient, Electrons and holes start diffusing from N to P, and P to N respectively.
As they do so, the depletion region becomes more +ve on the N side, and -ve on P side, creating an Electric field from N to P, and simultaneously a potential gradient.
Because of this, the minority charge carriers are attracted and they constitute drift current opposite to diffusion current.
Both the processes reach equilibrium when Idrift=Idiff. So, there's no net current flowing now.
But if there's no net current, how can there be a net potential gradient, called as the Barrier potential in this case?
• What are the directions of diffused and drift current??
• diffused current(carrying maj carriers) moves from p to n and drift current(carrying min carriers) moves from n to p.
• When we join P-type and N-type semiconductors together, diffusion current starts at a max, then decreases, and simultaneously drift current increases. At equilibrium, these effects cancel each other and there's no net current. So, why do we talk of "equilibrium barrier potential"? The barrier potential only offers resistance to diffusion, but helps drift. So, does that mean, drift continues forever? That doesn't make any sense. Please tell me where I'm wrong.
• At equilibrium, for every hole/electron that get diffused, there is a corresponding electron/hole that gets drifted and so they basically cancel each other out. But there is still drift and diffusion current; it's just that in average, they cancel each other out.

So even if there is a barrier potential, it doesn't really mean that no diffusion happens. There is still diffusion happening. It just means that the electrons generally move towards the direction of the barrier potential.

And so, at equilibrium you can even say that diffusion and drift continue forever since they just cancel each other out. The depletion region really just depicts that no electrons (or 'holes') stay in this region.
(1 vote)
• Important point left unexplained--> Where does the the so-called "kick" come from that moves the coulomb of positive charge AGAINST the electric field? His explanation of the exact source of the 0.7V across the depleted region is lacking. Where does this 0.7 actually come from?
(1 vote)
• The "kick" is just some hypothetical force used to demonstrate what a potential actually is.
The 0.7V is something that people solved to be the voltage difference in the depletion region of a PN junction (of a Silicon at room temperature).
(1 vote)
• "How does this help in charging our mobile phone battery using alternating current?"
(1 vote)