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Nanosecond current pulses arise from avalanche breakdown in virtually any diode. Here some experiments exploring these in Zener-diodes, LEDs and finally photon-counting avalanche photodiodes are desribed together with a few tiny circuits useful in this context. Questions like what happens to a LED subjected to a (large) reverse voltage and the issue of their non-linear current intensity relation (at very low current its quadratic!) are answered.

Did you ever wonder what happens if a LED is stressed beyond its specified reverse voltage limits, what the nature of the noise generated using Zener diodes is or in general the nature of junction breakdown? The answer is that electron avalanches form that lead to current surges. Indeed can this be observed with nearly any semiconductor diode.
 
Before avalanches become abundant, they may be initiated by incoming photons and thus allow photon detection using avalanche photodiodes (APD)s.
 
Triggered by the first questions I started a little journey to discover some intricacies beyond the sudden rise of the measured (DC) current if the reverse voltage of a diode is raised above a threshold, the breakdown voltage .
 
Finally this yielded some experience with APDs and insights into the (non-proportional) relation of current and LED light output.
In the following I present the (relatively simple) measuring setup together with results from Zener diodes, simple diodes, various LED’s (mis)used as APDs and a genuine APD.

Please see the attached pdf for the full story!