How Does the Depletion Zone Affect Current Flow in a P-N Junction?

[mike2349]

I've been reading various explanations for a number of hours, and I'm still completely confused. I'll refer to an explanation from Wikipedia, but do point me to a better one if you know one.

What does the depletion zone have to do with current flow? Looking at Figure A (see link above), there are plenty of charge carriers throughout the depletion layer, mostly holes on the left and mostly electrons on the right. And they are in equilibrium, so there is no net force stopping them. If a potential difference is applied either way, why won't current flow through the depletion zone like in a normal conductor, with the oncoming electrons filling the oncoming holes?

In case it helps with pinpointing my source of confusion, I have (or should have) roughly high-school-level physics and chemistry knowledge (no such claims about understanding), I visualise free electrons as tiny charged balls that obey Newtonian physics and Coulomb's law and nothing more, and I don't understand how non-ionic chemical bonds work.

 

[willem2]

There are no holes or loose electrons in the depletion layer. What you see in figure A, are the negatively charged acceptor atoms, and the positively charged donor atoms, that have created the free electrons and the holes that have diffused to the other side of the junction. The donor/accpetor atoms can't move. The only way of getting the p-n junction to connect is feeding in electrons on the n-side to fill in the positively charged part of the depletion layer, and removing them to create holes to fill in the negatively charged part of the depletion layer.

 

[mike2349]

The negatively charged acceptor atoms and the positively charged donor atoms are the circled minuses and pluses, right? But what about the red and blue lines? I thought they show the free electrons and holes, which can move?

 

[willem2]

The negatively charged acceptor atoms and the positively charged donor atoms are the circled minuses and pluses, right? But what about the red and blue lines? I thought they show the free electrons and holes, which can move?

They show the concentration of holes. Note that it's on a logarithmic scale, so the concentration both of holes and electrons at the point where the red and the blue line meets, is very small

 

[PJC01]

The p-n junction is a fundamental component in semiconductor devices, such as diodes and transistors. It is formed by bringing together two different types of semiconductor materials, one with excess of electrons (n-type) and one with excess of holes (p-type). This creates a junction between the two materials, with a depletion zone in between.

The depletion zone is created due to the difference in the number of free electrons and holes in the two materials. In the n-type material, there are excess of free electrons, while in the p-type material, there are excess of holes. When these two materials come into contact, the free electrons from the n-type material diffuse into the p-type material, and the holes from the p-type material diffuse into the n-type material. This creates a region in the middle where there are no free carriers, hence the name "depletion zone".

Now, let's consider what happens when a potential difference (voltage) is applied across the p-n junction. When a positive voltage is applied to the p-type material and a negative voltage is applied to the n-type material, the depletion zone widens. This is because the positive voltage repels the holes in the p-type material and the negative voltage repels the free electrons in the n-type material. This creates a larger region with no free carriers, increasing the depletion zone.

On the other hand, when a negative voltage is applied to the p-type material and a positive voltage is applied to the n-type material, the depletion zone narrows. This is because the negative voltage attracts the holes in the p-type material and the positive voltage attracts the free electrons in the n-type material. This reduces the region with no free carriers, decreasing the depletion zone.

So, how does this affect current flow? In a normal conductor, current flows due to the movement of free electrons. However, in a p-n junction, the depletion zone acts as a barrier for the free carriers. When the depletion zone is wide, it is difficult for the free carriers to cross it, hence reducing the flow of current. On the other hand, when the depletion zone is narrow, it is easier for the free carriers to cross it, increasing the flow of current.

In summary, the depletion zone plays a crucial role in the functioning of a p-n junction. It helps to control the flow of current by acting as a barrier for free carriers. Understanding the concept of depletion zone and its relationship with voltage is essential in understanding