Dnk - In most materials, the orbiting electrons are tightly bound to their central nucleus, and it is only with great dificulty (i.e. exceedingly high pressure) is it possible to occasionally pry one away from it's atom. Naturally these materials are poor conductors.
In a few materials (i.e. good conductors) the electrons in the outermost orbit are loosely bound. So much so that they occasionally wander off by themselves, even with no external pressure applied. Thus, we have materials that have some quantity of "free electrons". These electrons can leave their orbits, wander around a bit, end up falling into another atom's orbit, any number of things. They are also readily available for current flow, should the opportunity arise.
Lastly, there are some materials in which the outermost electrons are bound neither too tightly nor too loosely. These materials do not have a lot of free electrons, but if sufficient pressureis applied (and a flow path provided), electrons will be moved from their orbits and form a flow of current. This stuff we refer to as semi-conductor material (not semi-conductor devices).
By the way, the spot in the outer orbit of an atom that is vacated by a renegade electron is called a "hole". Holes have positive charges, since the negative electron is gone, and can be observed as traveling in a direction counter to the direction of electron current flow. The holes don't actually move, or flow, but as electrons move from one orbit to another, then on to another, the holes give the appearance of moving the opposite way. This is important for reasons that I'll pretty much ignore for the moment.
Interestingly, it is possible to chemically alter semi-conductor materials (with neither too strong nor too weak electron binding), and sort of inject the material with an excess of electrons or and excess of holes (electron deficiency). By combining these altered materials in different ways, semi-conductor devices are produced.
Scott mentioned something about anyone seeing an atom to please report - they haven't and they won't. Even relatively big atoms are far far smaller than the wavelength of anything we could use to observe them directly. So we live with having to try to detect the predicted effects, and trying to minimixe the effect of our observation on the results of that observation. Very dificult.
To put this in some small degree of perspective, you'll remember from basic E&E that 1 amp of current is equal to i coulomb of charge per second. So what is that? 1 coulomb = 6.28 x 10 ^18 (10 to the 18th power). That is 6,280,000,000,000,000,000 or 6.28 billion-billion electrons past a given point in 1 second. 2 amps is twice that many, and so on.
Kind of makes you wonder, who counted them? And who came up with the right spin and left spin of electrons (more on that later).
Radar
