THEORY: The commonly-used semiconductors, silicon and germanium, have the same crystalline structure as diamond. Each atom has 4 electrons in its outer shell, and by sharing an electron with each of its 4 nearest neighbors, the atom attains the magic number 8 outer electrons. If this sounds spooky, go study chemistry.

The pure semiconductors described above are very poor conductors because their electrons are not free. By thermal agitation a few of them get torn loose and thus very small currents flow at reasonable voltages. With the voltages commonly used in electronics, the current is less than a microamp. At a high enough voltage, electrons get loose in a huge avalanche, and as voltage is increased a little, current increases very much. Thus the current vs voltage graph does not look nice to Mr. Ohm.

N type semiconductors: during crystal growth an impurity with 5 outer electrons is introduced. Thus there is an extra electron per impurity atom, and it turns out that these are free. It is now a good conductor.

P type semiconductors: an impurity with 3 outer electrons. There is a vacancy (called a hole) per impurity atom. It turns out that these act like positive charges, and they too are free. This stuff is a good conductor too.

Diodes: an n type bonded to a p type. If we connect + to p and - to n, it conducts well (see the first quadrant in the graph below). Reverse these connections and the free electrons and holes are immediately depleted, and it conducts like a pure semiconductor: hardly at all until the voltage gets high enough to cause the avalanche. Thus the graph looks like this:

The diode is useful for converting AC into DC and a number of other purposes. A diode that is designed to operate in the almost vertical part of the graph on the left is called a Zener diode (rhymes with wiener). These are useful for keeping a voltage constant, among other things.

In the above graph, consider the part on the left with negligible current. It turns out we cannot measure this current with conventional meters, but we can with the oscilloscope. There are two distinct regions to consider: the negligible current region and the high current region. First, as voltage is increased from 0 in the negative direction, the free holes and electrons are not depleted until the V is some critical value. So electrons traveling in the p type and holes in the n type are like swimmers in a sea of sharks, and the ones that don't make it contribute to a potential barrier (- in p, + in n) which opposes the current. After the critical voltage is reached, the sharks are gone and it should behave like pure silicon, with that avalanche of current if the voltage is high enough

Transistors: The simplest type is an NPN or PNP sandwich with a thin slab in the middle called the base (B). The outer layers are called emitter and collector (E & C), and they are generally not identical, so if they were interchanged, the behavior of the transistor would change. (Incidentally, in some circuits interchanging them will not change the operation of the circuit.)

The way it works: the main (conventional) current goes from E to C in the PNP or C to E in the NPN. The following is a discussion of the NPN. The main current is electron flow. If the base is not connected to anything, electrons fill many of the holes in it, and it becomes negative. This blocks electron flow. Now suppose we run a small current from B to E. This restores holes in the base, so the main current can flow again.

It turns out that the voltage of the base is roughly proportional to this current, and this greatly influences the main current. A small increase of I_B causes a big increase in I_C. Thus the transistor can be used to amplify a signal.

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