Regarding the tunnel diode branch from this thread - it is indeed possible to see the complete I-V curve of a tunnel diode in low frequency or DC conditions. Normal curve tracers can't do it because their series R is too high, so it appears discontinuous. Specially made, very low resistance test fixtures can suppress the instability caused by the TD's negative R, allowing display of the actual I-V - even that magical negative R region.
You can find such a test circuit in either (or maybe both - I forget) the GE or RCA TD manuals from the 1960s. The circuit displays the I-V on a scope, just like a curve tracer, but it's dedicated to only this function. I had once thought about building one of these, for curiosity, but for general go-no-go testing of TDs, it's not really necessary to see that part anyway. You can trust that the TD is doing its thing if you can see the endpoints, on a regular curve tracer, or in-circuit.
Ed
