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Newbie confused about diodes reversed bias forward bias like charges repelling etc

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rokuez

New Member
I have an understanding problem with diodes, and current.

I understand that silicon diodes are doped to give part of them P & N.
The P sections having more holes, and being called an anode.
The N sections having more electrons and being called a cathode.
I understand that diodes are not like on/off single pole single throw switches and that they have a exponential V I curve.

part 1

What I don't understand is why a forward based diode allows current to flow?
The reason why I'm not fully understanding this is because you have the positive terminal of the voltage source/battery connected to the anode (P with more holes) & the negative terminal of the voltage source/battery connected to the cathode (N with more electrons in the silicon substance)....

So doesn't the cathode in some sense repel electrons, sense it has more electrons in the silicon?

& likewise wouldn't the anode repel the conventional current flow from the positive terminal

Or at least for a split second, an initial moment, shouldn't this slight and immediate resistance to like forces be happening right before the larger battery voltage eventually overcomes the initial resistance so to speak of the doped N cathode having more electrons in the silicon and having electrons flowing towards the cathode from the negative terminal of the battery?

I do understand that like charges repel, and unlike electrical charge attract so in some sense I do understand how it works. I guess it makes sense to me because I think about when the voltage coming from both directions so to speak meets in the middle of the diode then current flows... But I just think for the first couple of electrons encountering the cathode they would be repelled, maybe for a slight instant??



part 2

also i'm a bit confused about a reversed biased diodes, for pretty much the same reason. the electrons are going towards the anode , which has "holes" i.e more positive charge? Likewise the cathode the silicon section with more electrons having it connected to the positive terminal of the battery shouldn't that attract the conventional current flow...?



I guess in summary I see the foward bias going like positive battery terminal to positive diode silicon part, and then negative battery terminal going to a negative cathode. I guess overall because greater potential difference/voltage pressure of the battery the charge flows through the diodes it makes sense...

But then I think about the reverse bias, and I think well why isn't the anode when facing the negative battery terminal causing some attraction & likewise the cathode facing the positive battery terminal causing some attraction thus allowing current to flow....
 

BrownOut

Banned
Well in a sense, you are right about reverse bias; the positive charge from the anode does flow towards the negative potential, and visa-versa for the negative charge in the cathode. But what you have to think about here is what's happening at the junction of the diode. As the charges flow out of thier respective regions, the junction becomes deviod of any carriers, and so there is nothing to carry the current across it. For foward current flow, the positive charges in the anode flow towards the negative changes in the cathode, and the junction becomes rich in current carriers, as the 'excess' electrons from the cathode flows in the 'holes' of the anode.
 
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Triode

Active Member
Just so you know, I'm not patronizing you, this kind of explaination just helps me,

Imagine a gym with a wall in the middle, people on both sides that can throw basket balls over the wall and a door at each end. And at each door you can either place a bin full of balls, or a basket to throw them into. So lets say these people are really dumb and dont work together very well. They all want to score baskets, and so will take a ball toward a basket or an empty handed team mate, or will run toward a ball if they dont have one, but theyre childish, and the people on one side don't like holding a ball and will only take one if they're near someone they can throw it to or a basket to throw it in, the people on the other side like having a ball, and will only throw theirs if they can see where another one is availible.

So if you put the bin, or the source, on the side with the people who dont want to carry anything, they will run up to it to get one, but they'll look around and see that the people on the other side are all the way at the other end, and they can't get rid of the ball if they pick it up, so they wait there. At the same time the people who like to hold a ball are by the basket, but they wont thow theirs because they're like "aww, where am I gonna get another one?"

But if you put the source on the side with the people who like to have them, and the basket on the other side, the people who don't like to cary balls will happily run up and throw theirs in, the people who do like to carry them will all go grab one, and knowing they can get another, then they'll run up to the people who don't have one, and they'll throw them a ball, which they will accept, knowing that they can get rid of it.
 
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rokuez

New Member
Thanks to both of you two. Both of the explanations really help.

One follow up question. In a reverse biased diode, where does the energy go?

Does the energy get from stopping the current get turned into heat and absorbed by the air like with a resistor?
 

crutschow

Well-Known Member
Most Helpful Member
One follow up question. In a reverse biased diode, where does the energy go?

Does the energy get from stopping the current get turned into heat and absorbed by the air like with a resistor?
Energy is current times voltage times time. Since there is very little current in the reverse direction (a small, very short current spike during the reverse recovery time) then the typical reverse energy is very small, typically negligible compared to the power loss from the forward bias current.
 
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