12. "We have been talking about conductors and insulators. What is a semi-conductor? Where does the word 'semi' come from? They use silicon, a semi-conductor, in microchips; if there is conductive material in silicon why don't electrons migrate over the silicon?"
An excellent question! A detailed explanation of semi-conductors would take a long time; one could give a whole semester's course on semi-conductors! But I can give you a short description of what a semi-conductor is, how they differ from conductors and insulators and tell you about some of their useful properties.
Let's review first the difference between conductors and insulators. Our definition is that, in a conductor, electrons can move very easily. What that means is it requires very little energy to get an electron to move, i.e., there is no 'threshold' energy to overcome. (Remember, when we dealt with friction, we saw that a certain minimum force was required to get an object to move over a surface. So, we could say that a certain 'threshold' force is required.) Electrons in a conductor will move when only a minimal potential difference - a fraction of a volt - is applied (although its got nothing to do with friction!). In an insulator, we said that charges do not move. A more correct statement would say that in an insulator the threshold energy required to get electrons to move is very large. In fact, if an increasing potential difference (or electric field) is applied to an insulator, breakdown can occur (usually at electric field strengths in excess 106 V/m, when the electrical forces literally 'rip' the molecules apart). In semi-conductors, the threshold is finite but much smaller than in insulators. In fact, the threshold energy is given a special name ... it's called the energy gap. This energy gap can be overcome by thermal effects, i.e., the temperature may be sufficient to "excite" some of the electrons over the threshold. Then those electrons can move under the influence of a potential difference; not as many electrons as in a conductor but nevertheless more than in an insulator.
Strictly speaking it is this energy gap that gives semi-conductors their important properties. For example, when you heat up a metallic conductor (like a copper wire) its resistance to the passage of electrons increases, see (a) above. What that means is for a given potential difference across the ends of a copper wire, the current (i.e., the flow of charges) decreases as temperature increases. In a semi-conductor, increasing temperature means more electrons overcome the energy gap (the threshold) and so there are more charges available to flow. So, in a semi-conductor, (b), the resistance appears to decrease as temperature increases. Roughly speaking, as the temperature increases a semi-conductor becomes more like a conductor (low resistance); as the temperature decreases it becomes more like an insulator (very high resistance). Hence the description ... 'semi'-conductor.
In some semi-conductors it's possible that the energy gap cannot be overcome to any great extent by temperature (thermal) effects but it can be easily overcome by the energy in a beam of light. (This is called photo-exitation.) So, in the dark, the semi-conductor has few charges available to produce a current, see (a) above. However, when illuminated, the energy in the light excites electrons over the energy gap and so the semi-conductor becomes more like a conductor and a current can flow, as shown in (b) above. What we've done here is given a description of a "light sensitive switch", where the current in a circuit can be switched on/off by light/dark. You can imagine how a device that that could be used in security circuits, for example.
What I have described above corresponds to "pure" or "intrinsic" semi-conductors. It turns out that the properties of semi-conductors are very strongly influenced by impurities (called dopants) that are deliberately introduced into "pure" semi-conductor materials, like silicon (Si) and germanium (Ge). For example, it's possible to design a semi-conductor with a gap that makes it sensitive to infra-red light of a particular frequency. There are two main types of dopants; those that produce "n-type" semi-conductors, for example, P in Si or As in Ge, and "p-type" semiconductors, for example, Al in Si or Ga in Ge. I can't go into the details here, but it is the different properties of the n- and p-type semi-conductors that give semi-conductors their wide range of applications.
For example, the p-n junction shown above acts as a rectifier (or diode). That means it only lets electrical current (charges) flow in one direction only! With a "forward potential difference" a large current can flow; with a "reverse potential difference" very little current flows. A related device is the n-p-n, or p-n-p junction; better known as the transistor.
There are many devices based on the properties of semi-conductors; unfortunately, far too many to describe here! In addition, they have produced a whole new science of fabrication and quality control. The modern era of semi-conductor electronics started with the invention of the junction transistor by Bardeen, Brattain and Shockley in 1949. Since then, semi-conductor devices have had a profound effect on almost every aspect of modern life.
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