microwave

They don't. But treating a device as a one-port network with a negative resistance makes a tractable way to design an oscillator. And there are devices that are useful at microwave frequencies that exhibit a negative resistance effect.

I really miss tunnel diodes. Sob.

John

I missed out on it -- to young (except for my Heathkit "Tunnel Dip Meter", which will need a complete rebuild if ever the tunnel diode craps out).

I believe that reflex klystrons and magnetrons also exhibit an apparent negative resistance (or at least negative absorption) at their working frequencies; this is what makes them oscillate.

You got me curious. Taking a look around the net I only found a hand-full of companies who still manufacture them, and the prices are outrageous. Is there a reason why they are not so popular anymore? Were they always so expensive?

I think they were more popular once but I guess that was before my time...

When I was a kid I used to buy low-current (5, 10 mA peak point current) TDs from Allied for a few bucks each. The higher current parts were in the $20s or so, out of range of my allowance. The bigger ones had switching risetimes in the 10s of picoseconds, outrageous for anything in the 1960's.

It's sort of like a zener diode that's so heavily doped that the zener voltage crosses over past zero, so it's still zenering at small forward voltages. There's a gap between this ersatz zener region and when regular forward conduction kicks in, and that gives a negative resistance region.

TDs were fabricated by starting with a chunk of heavily doped germanium, p-type maybe. Then a metal mesh was pressed/welded to the surface such as to form a very abrupt n-type alloy junction. The sudden highly-doped junction allowed carrier tunneling, the Esaki thing.

The next step was to make physically stable connections to the base and the mesh and then liquid etch away germanium until only one tiny mesa of germanium touched one bit of the mesh. The resulting junction area and capacitance were tiny, and current density was huge.

Nobody seems to have come up with a modern planar way to do this, except for the microwave back diode versions, which are planar, wire-bonded things. But they are useless as switches.

TDs are cute, but 2-terminal negative resistance devices are tricky to use.

I found a dusty bin full of TDs at Haltek a while back. They didn't know what they were, so I got a bunch for 10 cents each.

John

There are a few vendors in Russia selling them on ebay. Around a buck each. I've gotten a few and they test ok on a curve tracer. They take a while to ship though.

Tm

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They had problems, and they didn't do much that other devices couldn't do, and soon after tunnel diodes arrived, other devices came along that were better for doing things with.

They got a lot of press, certainly in the hobby publications, and that got the attention of the hobbyists. They were novelties, and people played with them when they were cheap.

But there was virtually nothing that you could do with tunnel diodes that couldn't be done with other devices. Looking at the applications, the one thing that seemed useful at the time was that they did operate at fairly high frequencies, when other semiconductor devices were just moving up.

Once other devices came along to handle those higher frequencies, the tunnel diode was just a novelty. Yes, you could use it as a mixer and oscillator for a receiver, but you could do that with a transistor, and you'd have better results with a single transistor for the mixer and a single transistor for the oscillator. Tunnel diodes appealed to a certain hobbyist mindset at the time, either because they were new so you'd be playing with something that not everyone else was playing with yet, or because they were portrayed in all kinds of simple circuits, for those who wanted simple circuits.

They did not see a lot of commercial use. I have a Sony AM/FM portable radio from the early sixties that makes mention of the Esaki diode (ie the tunnel diode) on the front panel. I've not seen a schematic, and it's not clear what the tunnel diode is in there for, maybe the mixer in the FM section, maybe as an oscillator in the FM section, but it gave no advantage to that radio, other than as a selling point, "hey, we've got this new device that no other radio has".

What did happen is that it showed up in some esoteric test equipment, I seem to recall someone mentioning when this had come up previously that one oscilliscope used a tunnel diode as a trigger for the sweep, and that's the sort of thing you'd need a tunnel diode for now. Not for the function, but because you wanted to keep some commercial equipment alive that happened to use a tunnel diode in that brief window when tunnel diodes were "cool". If you can't get a replacement, then you can no longer use that equipment.

There probably are some esoteric functions where the tunnel diode can still be of value, I can't see why anyone would still make them otherwise, but they just dont' serve a lot of purpose.

Michael

TDs were the fastest parts, by far, for a decade at least. Tek and HP used them extensively in their scopes, especially the multi-GHz samplers. They were used in all sorts of circuits, especially microwave trigger countdowns and TDR step generators.

Tunnel diodes would generate 20 picosecond steps when nothing else got anywhere close.

They sure did. Look at a Tek 547 schematic. Or a 1S2. They're filthy with the little critters.

John

Not exactly true; the bulk of UHF tuners in TVs for a couple of decades were based on tunnel diodes. Most tunnel diodes were Ge (some now are GaAs) though, and the Ge production lines became ... ineconomic.

They were never 'popular' because there weren't any good ways to limit the GHz gain, and lots of folk couldn't debug 'em if/when the oscillations got out of hand.