Doubts on Bonding in P-Type Semiconductor

[i.mehrzad]

There is a doubt i have with the bonding in a P type semiconductor.

It is said that for an atom to be stable there are needed 8e's in its valence shell.
In the case of a P type semiconductor there are 7.
What is the cause behind the trivalent atom forming bonds with the neighbouring silicon atoms, eventhough they don't have a stable calence electronic configuration.

 

[dlgoff]

For each impurity atom added, one valence electron disapear. ie one hole is created. That's what makes it a p type material.

 

[i.mehrzad]

I get that point.
A Si atom has 4 electrons and a trivalent atom has 3 electrons in its valence shell.

Why will a Si atom want to bond to a trivalent atom when it knows that it will not attain stable electronic configuration.

That is there should be no bond formed in the first place.

Thanking you,
Mehrzad

 

[dlgoff]

Just like in n type materials, the dopant (added impurities) atoms take the place of the silicon atoms in the crystal lattice.
Take a look at this wikipedia page on http://en.wikipedia.org/wiki/Band_gap" . As you can see, the band gap energy for semiconductors is low.

 

[cepheid]

I get that point.
A Si atom has 4 electrons and a trivalent atom has 3 electrons in its valence shell.

Why will a Si atom want to bond to a trivalent atom when it knows that it will not attain stable electronic configuration.

That is there should be no bond formed in the first place.

Thanking you,
Mehrzad

Atoms are not sentient or even alive, but I understand what you are getting at. All I know about it is that if you look at an 'energy level diagram' for a doped semiconductor, the energy level of the electrons in a donor impurity atom is usually immediately below the bottom of the conduction band...although its exact level varies from element to element. That makes it easier for an electron from that donor atom to break free and enter the conduction band.

Similarly, the energy level of the valence bonding site in an impurity atom is usually just above the top of the valence band, although its exact level varies from element to element.

This suggests that those impurities that work well satisfy these conditions.

 

[cepheid]

Why will a Si atom want to bond to a trivalent atom when it knows that it will not attain stable electronic configuration.

To answer your question more directly, sure, the Si atom will not achieve a stable electronic configuration, but the trivalent atom will (albieit as an ion). So the question is, who wins (or, who gets what they want)?

If we're going to talk about covalent bonding in a qualitative, nebulous sort of way, then we can't really answer these questions. I mean, it makes you wonder, right, WHY 4 electrons in that shell constitute a stable electron configuration in the first place (and to answer that, we must appeal to quantum mechanics). So if we're not going to do that, then, for the time being, we must be satisfied that the answer to most questions like this is "because that's what they do. That's what happens."