Saturday 29 June 2013

Diode PN Junction Explained

I never felt like I properly understood how the PN junction in a diode works. So I looked it up and came across some explanations that really helped me understand it better. I've put it all into a diagram. But let's introduce some of the concepts first.

A diode is made of a semiconductor (like Silicon) with two regions. The semiconductor makes up a crystal. One region in the crystal has been doped with atoms that have a spare electron when in the crystal lattice. It is known as the N region. The other region in the crystal has been doped with atoms that have spare room for an electron when in the crystal lattice (the spare spots are known as holes). It is known as the P region. That's where the name PN junction comes from.

The N region with its spare electrons conducts via movement of these spare electrons (the electrons are the charge carriers in an N region). The electrons are the charge carriers. Conversely, the P region conducts via the movement of the 'holes' (the holes are the charge carriers in a P region). As an aside, undoped semiconductors don't conduct as well because they don't have mobile charge carriers.

An important note to remember is that while the regions may be called N and P, this does not mean that they are overall positively or negatively charged. The regions have an overall neutral charge. Following on from that, if you were to take away the electrons in an N region, the crystal structure would be positively charged. The opposite would occur if you filled the holes in a P region with electrons.

So, with that explanation, here's the diagram:
So the depletion region gives a potential barrier (from the fixed charges). The width of the depletion region (and thus also the size of the potential barrier) depends on the applied potential (in effect, it depends on the direction of the attempted flow of current).

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