"tunnel diode" oscillator

when i was young, i had a junkbox with a lot of exotic military surplus parts, like microwave cartridge diodes, reflex klystron tubes, etc... but i never got the chance to play around with tunnel diodes, which were expensive, and rarely used. i had a weather sonde transmitter that i think used one, but i never dug it out of the transmitter. i recently came across a circuit using an N-channel and a P-channel jfet to perform the same function as a tunnel diode, with one extra bonus, after the VI curve slopes down to a minimum current value, it levels off at that current, so, unlike a tunnel diode, too much voltage (within reason) won't fry it. growing up in the 60s i saw lots of circuits for tunnel diode oscillators. in case anybody has never heard of a tunnel diode, it's a diode that has a negative resistance region in it's forward bias curve. this means instead of the current going up as the voltage goes up, there's a part of it's curve where the current DECREASES as the voltage goes up. with a tunnel diode, the current begins going up again after you pass the bottom of the "valley", and can easily fry the diode. however, this circuit using jfets doesn't do that, the current settles at a low value and stays there. the negative resistance part of the curve, however is quite useful, because if you add a couple of reactive components like a coil and a couple of caps, it becomes an oscillator. but, it seems they don't make them anymore (unless it's a specialty item made for alphabet soup agencies... a tunnel diode oscillator would be small enough to make a bugging device). somewhere, i stumbled across this circuit that uses two jfets (a P channel and an N-channel) wired kind of similar to using two bipolar transistors to make a SCR or a triac. so, i looked up one of those 1960s vintage tunnel diode oscillator circuits and, except for the actual circuit voltage, it seems to work the same,

While you can get negative resistance effects this way it won't be the same as the classic 60s tunnel diode. What made those devices unique was the physics of a very heavy doped extremely thin PN junction that allowed GHz range quantum effects across the thin depletion region of space.
http://www.ee.sc.edu/personal/faculty/simin/ELCT563/08 Tunnel Diodes.pdf

There are plenty of 'real' tunnel diodes on places like ebay.
http://w140.com/tekwiki/wiki/Russian_tunnel_diodes

it looks like most tunnel diode applications have been migrated to using Gunn diodes. the difference being that a Gunn diode has no PN junction, and operates like a solid state version of a reflex klystron. other klystron-like diodes used for microwave applications are IMPATT, TRAPATT and BARITT diodes that all use bunches of charge carriers propagating with a known transit time across the junction.

yes, the dual jfet device might not be a true tunnel diode, but as a two-terminal device with well defined negative resistance it does present some interesting possibilities. the dual jfet negative resistance region is actually very linear along most of it's extent. the range of forward voltage in the negative resistance region is also very wide (from about 1.2V to 9V) with the peak current at 3mA, and the valley current at 0.3mA.

there's an experimenter that has fabricated tunnel diodes using dissimilar metals http://sparkbangbuzz.com/els/ntype-nr-el.htm

Fascinating circuit Jed, thanks for posting.

unclejed613 said:

it looks like most tunnel diode applications have been migrated to using Gunn diodes. the difference being that a Gunn diode has no PN junction, and operates like a solid state version of a reflex klystron. other klystron-like diodes used for microwave applications are IMPATT, TRAPATT and BARITT diodes that all use bunches of charge carriers propagating with a known transit time across the junction.

yes, the dual jfet device might not be a true tunnel diode, but as a two-terminal device with well defined negative resistance it does present some interesting possibilities. the dual jfet negative resistance region is actually very linear along most of it's extent. the range of forward voltage in the negative resistance region is also very wide (from about 1.2V to 9V) with the peak current at 3mA, and the valley current at 0.3mA.

there's an experimenter that has fabricated tunnel diodes using dissimilar metals http://sparkbangbuzz.com/els/ntype-nr-el.htm

Click to expand...

Cool stuff, Quantum bulk effect devices are very interesting. The Gunn 'diode' uses quantum electron-phonon interactions to create acoustic modes across the bulk semiconductor for negative resistance effects.

after seeing some pics of tunnel diodes, it's possible my milsurp collection of parts had a couple of tunnel diodes in it, but i didn't know that's what they were. i don't think they were listed in most of the semiconductor data books i had at the time.

The two JFET negative resistance device is also known as a Lambda diode. So you can google that term and get lots of hits. I've played around with that circuit a few years ago. It does work very well. I'd been curious about whether a negative resistance device could be used to make an amplifier instead of the usual oscillator. I was able to demonstrate that if set up properly, it will amplify an input signal with true power gain without going into oscillation, but it's really just a curiosity. Practical applications for two terminal active devices are few and far between, due to the difficulty of isolating the input signal from the output signal.