In practical electrical terms, a diode is a self-adjusting resistance.
Its range of behavior encompasses a narrow volt range (a little to either side of its threshold voltage).
To pass a wide range of current, a diode adopts an enormous range of resistance values.
A particular volt drop goes with a particular current goes with a particular resistance. More or less.
So the diode assumes a value of megohms when dropping 0.3 V at 0.1 uA.
Then it assumes a value below 1 ohm when dropping 0.8 V at 2 A.
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Reverse breakdown does not take place instantaneously. As with forward biasing, reverse biasing develops gradually.
At the 50 volt level, it may span several volts, during which current flow goes from 1 uA to damaging levels.
When is it a damaging reverse current? When it goes above 1mA? 100mA? If it overheats the device? Don't know the answer to that.
And is a 50 V zener diode really different from an ordinary diode which has a reverse rating of 50 V? Don't know the answer to that one.