Tunnel diode rectification

Does anyone still make tunnel diodes? I'm interested in having a play with them. Particularly I'm interested in the fact that they pass a (relatively) high current at low forward voltage, and in the forward direction the forward current never goes to zero. Effectively I'd like to use it as a rectifier with no forward voltage - where a Schottky won't do. It doesn't matter that it has a low reverse breakdown voltage or valley point, as long as the valley floor is above zero.

Is there a more modern device that will do this?

Thanks Theo

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Theo Markettos                 theo@markettos.org.uk 
Clare Hall, Cambridge          atm26@cam.ac.uk 
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Reply to
Theo Markettos
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If you're prepared to pay megabucks, yes. Sometimes the turn up at hamfests and swap meets.

I'm interested in having a play with

A few milliamps, usually

The valley floor has to be above zero, otherwise the device would contain a voltage source.

"Reverse breakdown" isn't really a property of tunnel diodes, they conduct in the reverse direction more or less from zero.

Tunnel diodes don't rectify, in the sense that I take you to mean the term.

Fun devices, though. Probably the fastest switching device there is. Quite easy to generate pulses with picosecond rise times. If you're *really* clever, you can amplify with them.

A word of warning: Don't try to test a tunnel diode with an ohmmeter. More of them have been damaged doing that than ever failed in service, and even if the device survives, it'll tell you nothing, since they conduct in both directions.

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Then there's duct tape ...  
              (Garrison Keillor)
Reply to
Fred Abse

True, good point.

Am I misreading something from the graph at:

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which seems to suggest that they're basically like a Zener except the tunnelling current means they have a lower resistance at low applied voltages than at medium ones? I suppose the problem is lack of scale on the left/lower parts of the graph. It does seem to suggest that they're actually better at conducting in the reverse direction than the forward direction - is this true? I can't seem to find any datasheets on these diodes.

My interest is for a crystal radio, where the forward voltage drop of a silicon, Schottky or germanium diode means you lose a lot of signal, but power isn't available. I was wondering whether a tunnel diode might be more efficient? Are there any other types of diode that might do the trick instead?

Thanks Theo

--
Theo Markettos                 theo@markettos.org.uk 
Clare Hall, Cambridge          atm26@cam.ac.uk 
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Reply to
Theo Markettos

I think you're looking at the superimposed "conventional" characteristic, and implying reverse blocking. Tunnel diodes conduct in reverse more or less from zero. It isn't a very good idea to apply reverse bias as more than a few milliamps will destroy them.

Tunnel diodes exhibit useful properties (ie. negative resistance) in the forward direction only.

A Zener has a defined reverse breakdown voltage and is generally used reverse-biased. A tunnel diode has a reverse breakdown voltage of approximately zero.

except the

Yes, but they're never used in the reverse direction, nothing useful there. It's the negative resistance portion between the peak and valley that makes them useful. With a resistive load, increasing forward voltage through the peak point will result in a current step with picosecond rise time. Alternatively, you can use the negative resistance to overcome the losses of a tuned circuit, and oscillation will occur. That technique is usable at many GHz.

There was *some* data in the old GE "Transistor Manual", if you can find a copy. Most information was published in the 1960s, and is probably out of print, now. They were used extensively as trigger switches in high-end oscilloscopes by Tektronix and HP.

voltage drop of a

It would certainly oscillate, but I guess a transmitter isn't what you want :-)

Are there any other types of diode that might do the

There's a thing called the backward diode, which is similar in principle to the tunnel diode, but which is designed as a (microwave) detector. They're probably as difficult to find as pork chops in a synagogue :-)

Is it worth going to all that trouble for a crystal receiver? Selectivity is just too poor for today's crowded spectrum.

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Then there's duct tape ...  
              (Garrison Keillor)
Reply to
Fred Abse
[snip]

but

be

principle

Selectivity

That's a good point. And it takes a decent long wire antenna, which is usually larger than most dwellings can accommodate.

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Watson A.Name - "Watt Sun, th

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