# How a diode works

Discussion in 'Mathematics and Physics' started by GregJ7, May 21, 2014.

1. ### GregJ7New Member

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I am trying to understand plain diodes at the physics level. Can someone check my thinking about what happens at the p-n junction?

I would love to use electron flow notation, but my Multisim trial doesn’t support that, so I am trying to avoid labels and the word current (and its direction) in my thinking and in this post and just understand which side of the AC Power Source is an electron source and which is an electron sink. (However, I have to translate to conventional flow thinking and notation when referencing Multisim.)

My understanding at the moment:

The diode schematic symbol’s triangular arrow points to the diode’s cathode side which has the N-plate = semiconductor material doped with excess weakly bound electrons.

The other side of the junction is the diode’s anode side which has the P-plate = semiconductor material doped to have excess holes.

When the diode’s cathode is connected to an electron source, there’s more than enough electrons present in the N-plate, and as a result, there are lots of free electrons and charge flows across the junction normally—from the cathode side to the anode side.

When the diode’s cathode is connected to an electron sink, the sink attracts the weakly bound electrons, pulling them away from the N-P junction, and as a result, there are few free electrons and charge (mostly) does not flow.

2. ### RatchitWell-Known Member

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There are many different types of diodes. Are you referring to a silicon junction diode?

Then you should get a good textbook and read up on how diodes they work. After that, come back and ask questions.

Ratch

3. ### GregJ7New Member

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Yes, silicon diode. Sorry, I must have written this in the wrong forum. I already did all that study. I'm looking for my first human contact on the physics level of the subject.

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5. ### RatchitWell-Known Member

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OK, this forum cannot give you a full course in silicon diodes. What particular question did you have about them?

Ratch

6. ### GregJ7New Member

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I got an alert that LvW posted "As a starting point you must try to understand what a depletion regon is - and how it originates (diffusion process and corresponding voltage)." I'm not sure why I don't see his response here.

What is it in my thread that implies that I do not understand these things already? That info would help direct my study. I thought I understood all these points already. I was looking for confirmation that I understood them correctly before asking a question based on that info.

7. ### RatchitWell-Known Member

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GregJ7,

We have no way of knowing how much or how good is your knowledge of silicon junction diodes. That is determined by tests given in instructional classes. Since we do not administer or grade tests, we can only respond to direct questions.

Ratch

8. ### GregJ7New Member

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My question is in bold in my original post..?

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Ratch

10. ### nsaspookWell-Known Member

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What's important is the equilibrium of the depletion zone when the junction is formed. This creates a electric field across the junction that can decrease the depletion zone to allow electron and hole flow in one polarity (forward bias) of an externally applied field and increase the depletion zone against electron and hole flow in the other polarity (reverse bias).

http://www.electronics-tutorials.ws/diode/diode_2.html

Last edited: May 23, 2014
11. ### MicksterWell-Known Member

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AFAIK, LvW isn't a member here.
I've seen his/her posts on All About Circuits Forum, Edaboard Forum and Electronics Point Forum.
As it happens, I saw his reply to your post on P-N diode junctions just now on Electronics Point Forum.

Regards.

12. ### GregJ7New Member

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Thanks, nsaspook. What you didn't say answers my question sufficiently, and what you did say identified something I hadn't given enough attention to.

13. ### nsaspookWell-Known Member

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The AC source shouldn't be thought of as an electron source and the load shouldn't be a electron sink. The total amount of electrons doesn't change and either move very slowly or have no net motion in conductors. The most important concept is that we design structures of matter to control the flow of electromagnetic field energy by making devices that restrict, promote or modify its form. In solid-state devices the composition , type of charge carrier and how quickly they change state in response to fields is very important at the atomic and quantum scale of interaction but in the end the carriers carry little or no energy, it's the fields that do the work.

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14. ### nsaspookWell-Known Member

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Here's some information on the mechanics of how we construct devices. A good thing to remember is that junctions seem static but there are immense forces used to modify the silicon and that energy creates zones that maintain part of that energy in their modified atomic lattice structure.

It's from some market 'Droid' but the content is good.

Last edited: May 24, 2014
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15. ### GregJ7New Member

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Wow, this response was gold, thanks! It rang true to me as soon as I read it, because I had heard the movement of electrons described as "drifting" (slow moving) somewhere. I've spent a fair amount of time studying since your posts. (The videos give me a better understanding of why Intel's 1.4 billion transistor chips cost so much, heh.) I'm shocked at how none of the dozens of diode materials I've read made your point clear. In fact, they very clearly explain why the current (electrons) flows or doesn't flow.

Is the travel of electricity down a wire due to the propagation of EMF (like a traveling wave) along the wire matter? Does quantum mechanics explain how EMF travels down a wire without the electrons moving much? i.e., current isn't flowing because a lot of electrons are filling a lot of holes really fast.

16. ### nsaspookWell-Known Member

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Don't take my statements on conductors as saying electrons don't move quickly in response to fields in a diode because they do and the distance scale of movement is very small at the junctions that control conductance so current flow can be controlled quickly. Current is flowing but it's not like one unique electron on the P terminal lead zipped across the diode to the N terminal lead that originated from the source to the load.

http://en.wikipedia.org/wiki/Electron_mobility

17. ### GregJ7New Member

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Ah, I've mixed electron flow near the junction with current flowing through a wire. My interest is really in what it is exactly that is moving at a good % of the speed of light through a circuit, but that's no longer on the topic of diodes.