How is a diode damaged by exceeding its Vrrm (maximum peak repetitive reverse voltage)? At this point it conducts more current than usual, so is it the current flow that damages it? Why would that be any different from being forward biased when it conducts almost unlimited current? Or is there some other factor?
Electronic – Damage to diodes by exceeding Vrrm
diodesmaximum-ratings
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78K2120 is the Newark product code for that diode.
http://www.newark.com/vishay-general-semiconductor/1n4004-e3/diode/dp/78K2120?Ntt=78K2120
Google FTW.
That diode's temperature range is -50~+150 degrees celsius. Data sheet: http://www.vishay.com/doc?88503; Farnell Australia (au.farnell.com) part number 1421919.
It's just the basic operating principle of a diode. An ideal diode would allow current to flow in one direction but block current flow in the other direction. This is based on how it's made, with a p-type region, an n-type region, and a depletion zone in between. Like the bottom diode in this picture:
When you apply a some voltage, in your case 0.9V then the p-type holes and the n-type electrons move into the depletion region because they are repelled by their respective battery terminal. With enough voltage (0.9V in your case) the free electrons in the depletion region get moving and current begins to flow like this:
Now in the ideal case if you were to reverse that battery the opposite will happen and you'll get no current flowing:
In the real world though you can only apply so much reverse voltage or push before you hit the breakdown voltage and current begins to flow freely in the reverse direction. Zener diodes take advantage of this fact and are constructed to breakdown at lower voltages such as your 3.3V.
Sources:
You can read more about how zeners are made here
Or see the article I got all the pictures from here
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Best Answer
From looking at a diode curve you can see that its reverse current sharply rises at Vr much like the forward current does. From this I would imagine that repetitively hitting the diode with reverse voltage in excess of Vrrm would cause the diode to conduct significant current and the resultant power dissipation to either melt the silicon and fail open, or, if the supply is limited, fail shorted.
A diode will certainly not "conduct almost unlimited current" -- the device has a designed power dissipation and will not conduct more than this amount for very much time. When reverse-biased you are essentially damaging the thin junction and it is for this reason that it's not the same as simply forward-biasing the diode. Avalanche and Zener diodes are designed to operate reverse-biased and have differently-formed junctions which are designed to withstand this mode of operation. Zener diodes especially have a much more heavily doped junction than normal diodes.