Diode Flavours

A week or so ago, I had an exciting poster session with my second year solid-state physics class. Their main assignment for the term was to prepare, in small groups, a poster on a particular flavour of diode. My intention here was to get the students to learn about the underlying physics of a solid-state diode. Rather than simply tell them about it, and then set some test questions on it, I thought that posters would be much more interesting and motivating.

But the students didn’t just have to make their own posters. Part of the exercise was to look at other posters and comment on them. That way I hoped they’d learn a bit from each other too. Whether they’ve done that I can’t tell just yet – the students are yet to sit their final exam, but overall I felt that the session was really productive. I was impressed by the amount of work students had put into their posters and the way that they had found out, distilled, and presented information. They seemed to enjoy it too – there was a  certainly a lot of talking in the room while we were looking at the posters.

 I had groups look at five different flavours of diode, namely the tunnel diode, Zener diode, Avalanche diode, Photodiode and Schottky diode. A standard diode allows electrical current to pass one way, but not the other. It can be made very simply from semiconductor material. The other diodes have subtlely different behaviours. For example, the Zener and the Avalanche diodes will breakdown at high enough reverse ‘bias’ (voltage) so they start conducting if you apply a large enough voltage. My ‘favourite’ though is the tunnel diode. At a small range of voltages, quantum mechanical tunnelling allows a higher current to flow than otherwise would be expected. At the upper end of this voltage, this means that current reduces as voltage increases – quite an unusual effect. That leads to a few specialist uses, some of which the students were able to elucidate. Well done to them.

I’m in the UK at the moment, enjoying the cold wet weather. Just 5 degrees on Exmoor at 4pm on a June afternoon a couple of days ago. Nicer than earthquakes, though.


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