why does diode breakdown into conduction when reverse biased?
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Start here; http://en.wikipedia.org/wiki/P–n_junction
i didn't get, so could you please elaborate my above statement ?
Elaborate on what? You must be more precise in your question.
These links explains the two reverse breakdown modes.
While the above is correction, a handwavy, more pop-science version would be:
If you put enough reverse bias on a diode than you can accelerate electrons hard enough that when they hit a lattice site they can release more electrons and so on, making a chain reaction that causes a large current to flow.
I'll try an explanation. Think of the PN junction as a hill with a steep angle, not 90 say 80 degrees or so. When a bowling ball is sent down the hill, no problem it just rolls down the hill and proceeds on. But try to send one up the hill and you need a very high velocity to achieve this. The PN junction is an energy hill in one direction the voltage "pressure" that pushes it down the hill only takes a little energy to travel that distance. However in the opposite direction much more energy is needed, and the two mechanisms, as pointed out in Baluncore's response are Zener or avalanche breakdown process. Both present a tipping point (particular voltage or pressure) of the necessary energy to achieve useful amounts of current in the reverse direction. Then analogdesign's response elaborates further on the events that occur in the atomic lattice.
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