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