I think the best thing, if you want to find out more,
would be to read a textbook chapter on semiconductors
(could be in solid state chemistry, or in physics, or electronics...)

There you will learn all about the wonderful werld of germanium diodes,
gallium a_rsenide (why won't this board let me type A_RSEnic?!? lol )
the silicon revolution and so forth.

But just remember how much empty space there is between all those nuclei. Venus and Mars are probably closer -)

Strange, when you thank that everything is made of mostly nothing!

Yes, when you consider that the universe is about 99.999999% empty nothing, the fact that you've met anyone else at all is probably a sure sign of madness lol

When you get to see the nobel physics' prize for this year about Quarks, you'll find that your little Silicon atom is very BIG indeed.

auch there's no such thing as quarks,
they're just made up lol

The whole reason silicon is conductive, is because of impurities. WE WANT regions of high conductivity, and regions of high resistivity (transitors and spaces between transitors respectively). If we didn't care about purity, the base silicon would possess all kinds of defects(who knows, it could be doped p type or n type). The conductivity would not be uniform across the sample, and our transitors would short between one another. extremely high purity silicon is used, because specific regions are doped, therebye creating transistors/regions where electrons can flow. The undoped areas are strong resistors, blocking electrons from passing between transistors.

The band gap is very complicated. It predicts the "lowest energy jump" for a semiconductor. Silicon is an indirect gap semiconductor. The electron, before jumping into the conduction band, must change both energy and direction. A direct gap semiconductor, the e- must only jump energies, therefore it is more efficent (think of it as traveling in a straight line is better, vs. zig zagging through different orbitals). GaAs is a direct gap. The efficency is much higher than Si if we were to compare power outputs in a solar cell. Some energy is lost changing the direction in k-space. Lots of complicated things going on.

http://hp737.ceg.uiuc.edu/tutorials/bandstructure/pseudo.html
this is one model to calculate the "band structure". Its very complex math set in k-space/reciprocal space.

The final solution plots out something like this. http://www.ioffe.rssi.ru/SVA/NSM/Semicond/Si/bandstr.html (click on the picture). From the plot, the band gap is Eg. That is, the e- must travel from the maximum of the valence band (very center of plot, where the energy axis crosses) to the left slightly, where the minimum in the conduction band lies. GaAs as a direct gap, would have the minimum and maximums straight above one another without having to chane the x-value. The x-value can be thought of as the direction the e- is traveling.....crystal direction denoted <110> <101> <111> etc.

Here is a link to a dissertation on the band structure of SiC. http://drum.umd.edu:8003/dspace/retrieve/106/dissertation.pdf
page 11 gives properties of Si vs. 3 different forms of SiC. page 40 and on, show the entire band structure. Enjoy

How about the solarcell effect in Silicon?

Solar cells work when a photon of an energy greater than the bandgap is absorbed. The photons energy is transfered to valence band electrons, which then are promoted to the conduction band producing a tiny voltage/current.

Think of a solar cell as a light emitting diode working in reverse.

Voltage applied across a junction = photons emitted (LED)
Photons absorbed in a junction = voltage created (solar cell)

That means every conducting or semiconducting aspect of Silicon are because of inmprities with them?

most of the real properties of ANY bulk solid are due to the defects in the solid structure.

All those nice text book diagrams of lattice shapes are a loud of idealised rubbish ;-)