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Why are some MOSFET body diodes drawn as zeners and others as diodes?

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AlfBaz:
Some data sheets show the body diode with as just that, a diode symbol and others use a zener symbol
Is this an actual thing, lack of convention or some contentious topic?

Signal32:
> The zener symbol is often used for MOSFETs where the body diode is "Repetitive Avalanche Rated
> The zener in the symbol represents the avalanche rating of the MOSFET and is of use when switching an unclamped inductive load. You won't see it conduct unless you exceed the voltage rating VDS and get to V(BR)DSS.

https://electronics.stackexchange.com/questions/111485/intrinsic-zener-diode-in-mosfet

AlfBaz:
Thanks for that :-+

After some reading, lets see if I understand this correctly
There are 2 types of "zener" effects. The actual zener effect caused by quantum tunneling and controlled avalanche breakdown with the latter used for higher voltage zeners.

If the body diode is not "Repetitive Avalanche Rated" and dies if it avalanches, then it's typically drawn as a diode as diodes are generally not repetitive avalanche rated.
If the body diode is "Repetitive Avalanche Rated" then it's drawn as a zener simply because like an avalanche type zener, it can handle reverse current without self destructing with Vz equaling BVdss




T3sl4co1l:
That seems customary, yes.

Note that the body diode is already drawn in the symbol, that's the arrow pointing into/out of the center.  (At least, it is for old school lateral MOS, where the substrate and source and distinct elements.  Using it for modern VDMOS is kind of weird.)  So the antiparallel diode is redundant.

And yeah, there is a continuum from zener (tunneling) action to avalanche (impact ionization) breakdown, the terms are used fairly interchangeably as a result.

Note also that, although a transistor might be rated for avalanche, that doesn't mean it's a good idea to use it that way.  It takes time for the avalanche current to dissipate, greatly increasing device dissipation when used at high supply voltages.  Abusing avalanche ratings in a switching converter, for example (where stray inductance causes flyback spikes), will likely lead to quick failure, even if the device is apparently well within ratings.

There's one more "gotcha" about avalanche ratings: it is exclusive of good MOSFET performance.  If you need that one tiny bit of extra performance (particularly in body diode recovery time), you'll have to keep shopping.  (Probably not something a beginner will worry about, at least!)

Tim

EEVblog:

--- Quote from: AlfBaz on September 11, 2018, 07:21:46 am ---Thanks for that :-+

After some reading, lets see if I understand this correctly
There are 2 types of "zener" effects. The actual zener effect caused by quantum tunneling and controlled avalanche breakdown with the latter used for higher voltage zeners.

--- End quote ---

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