The original foxhole radio used a razor blade with an oxide layer and a makeshift cats whisker to form a diode. How exactly did this makeshift diode work? Is it actually a P/N junction, or does it work through some other principal?
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1\$\begingroup\$ en.wikipedia.org/wiki/Cat%27s-whisker_detector [Edited by a moderator.] \$\endgroup\$– TransistorCommented Aug 14, 2016 at 18:58
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3\$\begingroup\$ @Transistor That just says something like metal oxides have semiconductor properties - pretty unsatisfying. \$\endgroup\$– Scott SeidmanCommented Aug 14, 2016 at 19:04
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2\$\begingroup\$ It's actually an interesting question if this is a junction between two semiconductors, or between a semiconductor and a metal that might be considered a primitive Schottky diode, ie en.wikipedia.org/wiki/Schottky_barrier Given the need to manually adjust in search of a region and contact pressure with the right properties, it's even possible the answer is not consistent. \$\endgroup\$– Chris StrattonCommented Aug 14, 2016 at 19:10
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\$\begingroup\$ @Transistor The most specific that page gets, unless I'm missing something, is "The oxide layers that form on many ordinary metal surfaces have semiconducting properties[...]" Even its reference doesn't go much further in reference to that kind of material. I was hoping for something a bit more specific. So, can I assume that the whisker is the P-type conductor and the oxide layer is the N-type conductor and together can form a poor diode? But like Christ Stratton suggests, I had a feeling the answer might not have necessarily been that simple, especially since the whisker material varies. \$\endgroup\$– Ten BitcombCommented Aug 14, 2016 at 19:17
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2\$\begingroup\$ @Ravenstine: Sorry if I brusque. Your question is actually more nuanced than many of the "please do a web search for me or write me an article to save me looking it up" type of questions and I didn't pick up on it. I think Chris is doing a good job below. \$\endgroup\$– TransistorCommented Aug 14, 2016 at 19:21
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Buried in the Wikipedia Article on Crystal Radio is an asseration that a cat's whisker detector is a form of Schottky Barrier Diode formed by the junction between a metal and a metal oxide semiconductor. This would be in contrast to an "ordinary" diode formed by the junction between two semiconductors being relatively P-type vs N-type with respect to one another.
However, there was a lot of variation in materials used, and getting satisfactory results seemed to require a lot of manual adjustment to find a region with the right properties and suitable contact pressure, so it is at least worth considering if more than one phenomenon has been utilized - one might ask if there might also have been junctions between regions of different oxidation states, for example.
A contrasting explanation is that of a point contact diode, where the migration of metal from the contact wire into the semiconductor serves to locally dope a region to differing properties than the surrounding bulk - apparently this is done in manufacturing by passing a large current through it to cause migration, so it's an interesting question if anyone ever "primed" their cat's whisker setup with a few electrochemical battery cells. Jeri Ellsworth electro-forms such doped regions around a phosphor broze wire by discharging a capacitor through a resistor.
It so happens that if you want to make a crystal-radio like device today (or a diode power detector for measuring RF) a modern packaged Schottky Diode is a frequent choice, typically having a low forward voltage and often being more available and cheaper than a Germanium Diode.
answered Aug 14, 2016 at 19:17