semiconductor theory

[Gary Goodpaster]

I read the following statement in an electronics book "When exactly 8 electrons are present in the outer shell of an atom, the atom is considered stable and will not give up or accept electrons easily" Is this why when silicon atoms join in a crytal lattice structure, it does not conduct easily, because due too the covalent bond it now has 8 electrons in the outer shell?

 

[subzero349]

I don't know if this will help but....

When any element is in its crystal form it is pure and in it's most natural state. Because it is at its natural state, it is very hard to make it change state (ie: move electrons, conduct, etc).

When an atom has 8 valance (outer) electrons it does not like to accept any new electrons or give any electrons up.

 

[hamfiles]

Actually, an atom of silicon only has 4 electrons in its valence shell. Because of the way a pure silicon crystal is put together, it makes a lousy conductor, that is, each electron is shared by a neighbouring atom.

Good conductors have 1 free electron in its valence shell, and good insulators have 8. Good Semiconductor materials are typically said to have 4, but generally they are made to semiconduct properly by doping.

By adding either arsenic or phosphorus to silicon, an electron is either added making it negatively charged (pentavalent), or taken away making it positively charged (trivalent). After doping the silicon atom, it then has an extra, less than stable electron, or it has an empty space for one, and it will semi-conduct because the electons are no longer all shared/bonded to other silicon atoms.


Consider if you had a suit of ringmail.

oooooo
oooooo
oooooo
oooooo


Each inside ring is looped to 4 other rings on 4 different sides. But the rings at one of the openings/edges are not looped to 4, and it would be easier to pull one of those rings from the suit instead of one of the inside rings. The inside rings are like silicon before doping it, all the electrons are being shared. the outside rings are like silicon, that is they don't have all 4 electrons holding on. Not the best analogy, but its kinda close.

 

[subzero349]

I agree... I'm sorry to mislead... I meant ANY atom with 8 valence electrons is bad at conducting. I didn't mean to imply that silicon has 8 valence electrons...

 

[Gary Goodpaster]

Still confused

What I don't understand is that a semiconductor atom (Si) has 4 electrons in the valence shell. This is just a single atom. But when you have more than 1 atom, the valence electrons of each atom share the valence shells of the adjoining atoms and now you have 8 electrons in the valence shell of all adjoining atoms. Is this now considered an insulator? I am also having trouble with the theory on doping. If Si is doped with an N-type impurity, does this mean the the valence shell now has 1 electron because of the extra electron added and the fact that the valence shell can only hold a maximum of 8 electrons?

Also, what is the defination of a free electron?

 

[subzero349]

To my understanding
When you have more that one atom, the adjoing atoms still have 4 valence electrons, not 8. Silicon is purified and put in it's crystal state. Than it is "doped" with an atom with either 3 valence electrons or 5 valence electrons. Doped means that only a few atom are added to make the silicon impure. If you add an atom with 5 valence electrons to the crystal lattice, it will have 1 more electron than the silicon, this electron is free to move around... the picture below shows the result of adding 1 atom with 3 valence electrons and 1 atom with 5 valence eletrons... (which you would never do in practice... you would only add one or the other)

**broken link removed**

**broken link removed**

 

[abhishek singh]

hi,

yes you are rigt 8 electrons in valance shell makes some thing inert. if you look at the theory conductioon occurs due to free electrons in conduction band which has more energy than valance electron. when doped free electron comes to the conduction band.


abhishek

 

[windozeuser]

Yes Silcon has a total of 14 electrons's, 2 in the first shell, 8 in the second shell and 4 in its outer shell. It is a natural semi-conductor which needs to be bonded with impurities (phospurous and boron i think) to accept and give electron's

Correct me if im wrong