The PN Junction

The PN junction is the most basic junction type. But this is the basis of how diode's work and in a more complicated fashion, the basis of how transistors work.

But before we start with that, maybe a little bit of theory first. I'm going to start with the difference between a conductor, a semiconductor and an insulator. The theory behind all this is very deep and often not necessary to know. Ignoring a lot of the background theory, when looking at the atomic structure of a silicon crystal, energy levels can be detected. There are many of these levels, but as engineers, we're only interested in two, the Valence and the Conductive bands. Now electrons need to move between these bands for current to flow. In a conductor, these electrons move more freely than in an inductor. The difference is measured in the difficulty it takes in moving one electron from the Valence band to the Conduction band.

This is measured in electron Volts (1eV is equal to 1.9 x 10^-19 Joules). Joules is the unit of energy, for example, there are 2000 Joules in a mars bar, which is 478 calories.

Back to the bands, in an inductor, the difficulty in moving the electron from one to the other is typically greater than 5eV. For Semiconductors, this is ranges from 0.5eV to 3eV. Obviously conductors offer very little resistance and are less than 0.5eV. Three common materials used for semiconductor material is:
Silicon (Si): Eg = 1.1eV
Geranium (Ge): Eg = 0.7eV
Gallium Arsenide (GaAs): Eg = 1.3eV
Silicon is of course the most popular of these three as it has a reasonable bandgap and is easy to manufacture in large quantities with sufficient purity. Gallium Arsenide is particularly useful in high speed devices and military applications.

With this theory in mind, I can now explain p-type semiconductors and an-type semiconductors. Put them together, what have you got? That's right, a PN junction.
In a process involving adding impurities to the above materials, called doping, we add either arsenic or phosphorous to the semiconductor material. These parts will be added in the ratio 1:10,000,000 to 1:1,000,000,000, so you can imagine how precise they have to be (think of the lads in the bunny suits in the Intel manufacturing plant, thats what they do!). So an n-type semiconductor is negative as it has extra electrons, a p-type is positive as it has extra electron holes. Since electrons have negative polarity, this should make sense.

Putting these two types together, you will see the free electrons in the n-type flow to the electron holes in the p-type and similarly, electron holes in the p-type will flow to the n-type, causing what's called a depletion region. For current to flow through this depletion region, a minimum of 0.7V (for silicon devices) is needed to bridge the gap. This gives the effect of pushing the electron up a hill into a higher energy level.

In a diode, because of this PN junction, one side being positive and the other negative, this means that current will only flow in one direction. For the current to flow through, the diode must be put in the right way, this is called Forward Biasing. The inverse is called Reverse Biasing, where no current will flow. Now obviously, when it is reverse biased, current will flow, if you apply an excessive amount of voltage, this is called the breakdown voltage, where the device simply melts and becomes a short circuit.

I think I'll leave it at that for the moment, I don't really want to get bogged down in theory here. Will try and write up Transistors (PNP and NPN junctions) next as they follow on from this topic.