Zeners look like...

Well, at least the avalanche effect, taken for a simple 24V Chekhov circuit inside our Dangerous Box:-

Strong Zener Avalanche effect at 100 µs/div.

Strong Zener Avalanche effect at 100 µs/div.

Notice the peak to peak voltage which is natural and unamplified. And zoomed in horizontally 500 times:-

Strong Zener Avalanche effect at 200 ns/div.

Strong Zener Avalanche effect at 200 ns/div.

These oscilloscope images closely match those from other published sources, such as a SenSys 2016 presentation and Krieger 2017.

Labelled parts of Zener Avalanche breakdown.

Labelled parts of Zener Avalanche breakdown.

Notice here the magnitude of Vpp. It took nearly 30 minutes to catch this large 3.48 V peak as they’re rare. Also notice the Avalanche drop rate (slew). It’s approximately -80 MV/s. Wow, that’s fast! It’s almost a pure short. The slight bend of the charge curve is characteristic of exponentially reducing charge accumulation within a capacitance (with respect to time $t$) as:-

$$ V = V_o \big(1-e^{-t/RC} \big) $$

Capacitance $C$ arises from the Zener’s PN junction (approx. 150 pF) and the gate of the Dangerous Box’s MOSFET 2N7000 voltage follower (approx. 60 pF).