microwave
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From: John Larkin on 22 Jul 2010 11:54 On Thu, 22 Jul 2010 06:07:48 -0400, JW <none(a)dev.null> wrote: >On Wed, 21 Jul 2010 09:25:21 -0700 John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in Message id: ><nq7e46h4lum2916sdsra0qe2vcst6n43nb(a)4ax.com>: > >>I really miss tunnel diodes. Sob. > >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
From: tm on 22 Jul 2010 12:54 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:sbpg46lqn9gjom4dmbkkp3nv98qditmchc(a)4ax.com... > On Thu, 22 Jul 2010 06:07:48 -0400, JW <none(a)dev.null> wrote: > >>On Wed, 21 Jul 2010 09:25:21 -0700 John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in Message id: >><nq7e46h4lum2916sdsra0qe2vcst6n43nb(a)4ax.com>: >> >>>I really miss tunnel diodes. Sob. >> >>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. > 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 --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
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