A DIY semiconductor experiment

[nebogeo]

Hi all, first time here - not sure where the right place to post this is, but I've been playing with simple DIY semiconductors a little.

I happen to live in a place (Cornwall) with a lot of naturally occurring minerals - more specifically a lot of non-ferrous metallic ores, which happen to have semiconductor properties. Historically material like galena (lead ore) was used to create the diodes in crystal radios.

I've been messing around a lot with pyrites and galena, but recently tried chalcocite (a secondary copper mineral) and noticed something interesting. Below is a curve trace in the way I think you 'normally' do it, using a sawtooth rising from -2.5V and 2.5V and plotting it against the current that passes through the crystal and a point contact 'cat's whisker':



It's a bit rough and ready but we see (I think) is a rectifying behaviour where reverse voltage is (more or less) blocked and forward voltage starts conducting at a point between 1.5 and 2.5V.

I guess these relatively giant instabilities are to be expected in a few hundred million year old, naturally grown crystal that's been sitting in the rain on a mine dump for a few hundred years!

At some point I made a mistake and passed a sinewave through the crystal instead of a sawtooth, and I've noticed that the crystal does not "turn off" symmetrically, when the crystal is "turned on" and the voltage is dropping, it doesn't exhibit this threshold:



I presume this is a well known effect, I just can't find much about it online - or what it would be called.

[BrokenYugo]

Note you're working this diode about 1000x harder than a crystal set would. I would think a realistic IV plot for such a thing would end at 5-10mA forward current. The behavior right in the corner near zero is what actually makes a good "crystal detector" work

[nebogeo]

This is a very good point, thank you - I'll try this! Similarly, I actually had a thought after posting that I could have the whole thing upside down, and the threshold shown could in fact be the reverse breakdown voltage.