What's the chemistry behind a semiconductor?

[Vismutti]

TL;DR Summary

Chemistry behind semiconductor dont make sense. Example adding phosphorous into silicon cause it to lose electron, but at the same time it gains 4, and it ends up being positive. Explanations needed.

This is a confusing subject for me. It's like only getting half of the story. Reading physics but not understanding how it works in chemistry makes it nonsense to me.

Imagine we are creating a silicon-based semiconductor. They have covalent bonds between them, with each silicon atom having four covalent bonds, each containing two electrons.

We add a small amount of phosphor to the other side of the silicon block to make an n-type. Silicon wants to establish the same number of bonds as it does with the other silicon atoms, so it forms them with them. One additional electron in phosphorus will go to the conduction band. This will make it positive because it lost one electron, but how can it be positive while it gains four more shared electrons from silicon? doesn't make sense, does it? Furthermore, phosphor has stronger electronegativity, which means it attracts electrons with greater force, making it even more negative. After all of this, how will it still be positive, rather than some of the silicon atoms?

Now, let's begin to create the p-type on another side of the silicon block with gallium. Gallium tends to lose electrons, but for some reason, it gets one nevertheless, turning it negative and providing room for another electron in silicon's valence shell. The electric band diagram reveals it to be closer to the valence electron, so they may move easily, but it's strange they prefer gallium's shell since silicon is more electronegative and gallium didn't want it to begin with.

Do they teach it this way since behind this explanation there is the fact that there are multiple options to build a semiconductor that has different electronegativities, therefore behaving differently but the same way under electricity? Or does room temperature really excite them so much?

Then, because I don't comprehend p-type and n-type, I can't grasp the combination of them. Somehow, electrons fill some of the holes on the p side, bypassing the positively charged phosphor and its shell and proceeding down to the silicon's shell, and so on.

 

[Borek]

how can it be positive while it gains four more shared electrons from silicon?

It doesn't. Electrons are shared between atoms, so at best you can say it gains 50% of the four shared electrons from the neighboring atoms, at the same time it loses 50% of electrons it "gives" for sharing. The net effect (from the POV of an atom) is zero (not exactly, but that's a good first approximation).

 

[Vismutti]

It doesn't. Electrons are shared between atoms, so at best you can say it gains 50% of the four shared electrons from the neighboring atoms, at the same time it loses 50% of electrons it "gives" for sharing. The net effect (from the POV of an atom) is zero (not exactly, but that's a good first approximation).

A reasonable approximation in terms of added electricity? Otherwise, I would disagree. There most certainly will be at least the same amount of positive charge as that lost electron, but the question is: where?
In every textbook and video that I've seen, people point fingers at phosphor, but it appears to me that the charge should be obtained by silicon atoms.

 

[berkeman]

I've studied a lot about semiconductors (as an EE), but never seen their doping or operation explained in terms of chemistry. Do you understand the traditional approach via Solid State Physics already?

 

[DrClaude]

One additional electron in phosphorus will go to the conduction band. This will make it positive because it lost one electron, but how can it be positive while it gains four more shared electrons from silicon?

By that logic, all silicon atoms should be negative since they also gain four more shared electrons from other silicon atoms.

There most certainly will be at least the same amount of positive charge as that lost electron, but the question is: where?
In every textbook and video that I've seen, people point fingers at phosphor, but it appears to me that the charge should be obtained by silicon atoms.

If the electrons in the conduction band are uniformly distributed among all atoms, then phosphorous will be more positive than silicon since it has one more proton in its nucleus.

 

[Vismutti]

I've studied a lot about semiconductors (as an EE), but never seen their doping or operation explained in terms of chemistry. Do you understand the traditional approach via Solid State Physics already?

There are a few explanations, but not as detailed, as I would like it to be.
I'm trying to understand it, but the more I read more confused I seem to get. Just read that it doesn't produce electricity since the voltage difference adds up to 0, doesn't make sense. It depends on the reference point whether it adds up to zero or not.

By that logic, all silicon atoms should be negative since they also gain four more shared electrons from other silicon atoms.

It's not the same as silicon. Silicon also loses electrons, so the ratio is 50:50, but I'm referring to the phosphor, which is neutral when added. Then it receives four electrons from the silicon. Yes, they are shared, but they weren't there to begin with. As a result, the overall result should be negative. + increased electronegativity, bringing it closer to the phosphor

If the electrons in the conduction band are uniformly distributed among all atoms, then phosphorous will be more positive than silicon since it has one more proton in its nucleus.

That's a great point, why I hadn't thought of that. With the same logic, I could see why gallium has a minus charge but am not really sure why the electrons end up in its electron shell. Maybe they prefer being there since electrons repel each other and the energy difference between the shells is small. I go with this logic until someone comes up with a better solution. Thank you DrClaude

Now I need to figure out why doesn't it produce electricity due to the voltage difference

 

[Borek]

It's not the same as silicon. Silicon also loses electrons, so the ratio is 50:50, but I'm referring to the phosphor, which is neutral when added. Then it receives four electrons from the silicon. Yes, they are shared, but they weren't there to begin with. As a result, the overall result should be negative. + increased electronegativity, bringing it closer to the phosphor

You are ignoring what you are being told. Phosphorus not only gets electrons, it also GIVES electrons. Net effect is mostly zero.

Now I need to figure out why doesn't it produce electricity due to the voltage difference

Sorry to be blunt, but this statement is mostly meaningless and suggests you are trying to explain phenomena using very flawed understanding of the basic concepts.

 

[Vismutti]

You are ignoring what you are being told. Phosphorus not only gets electrons, it also GIVES electrons. Net effect is mostly zero.

Oh yeah, now I see what you meant, sorry and thanks.

Sorry to be blunt, but this statement is mostly meaningless and suggests you are trying to explain phenomena using very flawed understanding of the basic concepts.

Haha, well I'm trying to learn the basics so I can totally understand your point. Ofc it can't produce electricity like a battery but what I meant is if it's a closed circuit why there is no current moving due to voltage difference? Few electrons could just jump to those holes from the wire Or are there? it just happens once so the holes get filled with electrons and no more current can flow

 

[hutchphd]

You will need to understand some basic solid state physics and how the electrons in conductors are really not associated with atoms The trickiest parts are the issues of physical interfaces at equilibrium (i.e the P-N junction) and how that interacts with applied potentials or light.
You will need to understand some quantum mechanics.