My chem textbooks and my chem teacher tell me that silicon is a semiconductor. That means it partly conducts while it partly insulates? Whats that even mean ?? Thats like saying, " turn the light half on " . Its either on or off.......
Recall that there are different energy levels around a molecule, and a cerain number of electrons orbit the molecule in each energy level. The highest energy level that has electrons in it is called the 'valence' level, iirc. Electricity occurs when electrons from this valence layer are excited into a high enough energy state to flow with an electric current. How those electrons are 'lifted' is what separates a conductor, from an insulator, from a semiconductor.
Most conductors need a very small amount of voltage applied to them in order to excite their valence electrons. Because you need such a small amount of voltage, it is very easy to conduct electricity with these elements, so they are classified as conductors.
Insulators are elements that form very tight bonds with one another. Their valence electrons require very large amounts of voltage in order to excite them to an energy level where they can flow with electric current. Typically, applying too much voltage to an insulator such as wood will cause the insulator to burn way before its valence electrons are excited enough to conduct electricity. Since they effectively insulate current, they're called insulators.
Semiconductors are like a combination of both. A semiconductor will conduct electricity, but only if you apply a certain amount of voltage to it. Whereas a conductor will conduct electricity even with a very small voltage, a semiconductor will only conduct electricity if you apply, say, a medium amount of voltage; less than an insulator, but more than a conductor.
I don't feel like looking up exact numbers, but let's say that a typically doped silicon diode requires 1 volt to conduct electricity. You can apply .9V, and it'll just sit there as an insulator. Once you apply 1V or more, its valence electrons will be excited to a high enough energy level that they can join the current band and conduct electricity.You can see how useful semiconductors are for computers.
With a small amount of electricity, nothing happens (the 'off' state) but with a moderate increase in voltage, you have an electric current (the 'on' state). This is why binary is so important: 0 == off (low voltage), 1 == on (high voltage).
Silicon in its natural form is more or less an insulator (it conducts some electricity, but not much), so it is typically doped with other elements to make it a more usable semiconductor. Germanium is also a semiconductor.
Anyways, it gets even more confusing for me. I was reading about CPU fabrication and it said that pure pure silicon crystals are actually insulators ?? ( I thought they were " semi conductors ") and that they needed to make it dirty with germanium to actually let it conduct a little ?
Germanium is a semiconductor just like Silicon. Doping elements for silicon typically include, but are not limited to, things like phosphorus and boron. This is because phosphorus and boron have five valence electrons (silicon and germanium each have four). In a pure silicon or germanium lattice, all four valence electrons will be used to bond to other molecules. After dopoing, however, when the bonds are all formed, there's an extra electron: four valence electrons from the dopant bond to the semiconductor's four electrons, and the dopants last valence electron is free to be picked off.
Ninja edit: Rewritten for clarity.