# Cant imagine this situation

i have the the electrons which exist in the hightest energetic level the closest to the nucleus
we call the value level electrons

we excite electrons from the value level by heat to go to the conductivity level

and we have the conductivity level which exists in the outer shell
the most distance from the nuclease.

how can they say that silicon has 4 value level electrons in the outer shell
?
value level is not in the outer shell

## Answers and Replies

tiny-tim
Homework Helper
Hi nhrock3!

Do you mean valence level electrons?

See http://en.wikipedia.org/wiki/Valence_electron" [Broken]

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yes valence

but its doesnt explain my question

MATLABdude
Sure it does: look in the chart under (and actually read) the section 'Number of Valence Electrons'.

As to the deeper reasoning, (easy) conduction occurs when you have a sea of electrons available to jostle into one another and transmit current (as an analogy, think water particles transmitting waves or air molecules transmitting sound).

Good conductors (okay, metals) contribute valence electrons very easily to this shared 'electron sea'. If I recall correctly, for pure metals, this sea exists at all temperatures where the metal remains solid.

Good insulators do not easily contribute electrons, which is why current does not (easily, at any rate) flow through them.

Semiconductors are somewhere between conductors and insulators. Thermal energy can excite enough electrons into the valence band (the electron sea) to make reasonable conduction possible (and usually, the hotter, the better the conduction). Doped semiconductors can be made even more conductive (while still capable of being mad insulating), and this ability to control whether a piece of semiconductor is conductive or not is what gives them their great power, and brought about the digital age: electronic switches!

EDIT: Band gap article at Wikipedia:
http://en.wikipedia.org/wiki/Band_gap

Note that metals have overlapping valence and conduction bands, so they'll always be conducting.

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Thermal energy can excite enough electrons into the valence band (the electron sea) to make reasonable conduction possible (and usually, the hotter, the better the conduction).

I think that what you meant to say was that thermal energy can excite electrons so that they jump from the valence band and go up into the conduction band.

nhrock, the energy levels around an atom change when you put it into a crystal.

If an atom is alone by itself, it has very well defined bands. When you put an atom into a tightly packed crystal, the energy levels become blurred into what is called the state density function.

The valence band of a silicon crystal is not the same exact thing as the valence shells of an atom but they are closely related.

It's all very hard to explain without being able to draw lot's of diagrams. Your book must have a section near the beginning that discusses how energy bands are formed inside a crystal.

MATLABdude
nhrock3, I don't mean to be condescending here, but viewing some of your previous posts here, you seem to know some some more advanced math, but not some of the basics. You seem to know of some of the more advanced topics in semiconductor physics, but again, seem to lack many of the fundamentals.

Are you learning from a book? A website or Wikipedia (both fine for introductory purposes or reference, but usually not meant for a really good understanding)?

If from a book, read from the beginning. Also, take this as constructive criticism: use a search engine (e.g. Google) and search for things. It's often a whole lot faster and easier than, for instance, having people respond after a day or two that the chemical symbol for antimony is Sb (the Wikipedia page on antimony wouldve answered thus in 5 seconds), and will make the pace of your progress faster.

Just some suggestions!

"The valence band of a silicon crystal is not the same exact thing as the valence shells of an atom but they are closely related.
"

i do have this diagram
and it does present the level as it should be

i cant see where and what energetic level is the valence band and conductivity band in silicon Cristal

The Wikipedia link does have band diagram. It's this one: