Electronics > Beginners
Diode symbols in MOSFETs
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lordvader88:
Are they a common MOSFET and diode or zener diode combined, so 2 parts instead of 1 ? So in a SMPS it could act as the switcher when it's on, and then the clamping diode for resetting a transformer when it's off ?
james_s:
I believe you're referring to the body diode, which is actually a parasitic diode that exists due to the way the mosfet is constructed.

Occasionally you will see a mosfet that has an internal protection zener diode on the gate, that is intentionally added to limit the Vgs.
lordvader88:
Ok some mosfets have internal protection added, so that's 2 in 1 as well. Some have a zenor symbol so does that mean if they are reversed biassed a predictable current (hopefully) would flow ?

Do any 3 pin mosfets have it such that if on a N-ch modfet, if the source voltage is higher than the drain by a diode amount...then an internal diode would forward conduct ?

Maybe a - +3 pin SMD chip does, I was reading some datasheet and IDR, maybe a combined mosfet/clamp diode.

IDK if a 3 pin device can act as a typical mosfet with Vdd>Vss but as a diode when Vss>Vdd
bson:
I don't know if a body zener is reliably usable for anything other than ESD and transient protection...
Kleinstein:
All normal MOSFETs with 3 pins have an internal diode from source to drain. This is part of the construction, as source is connected to the substrate. Only special MOSFETs with a separate substrate pin can be without that diode. Sometimes the symbol includes that diode, but the diode is always there.

The gate protection is often found, but not always. It is an extra feature to protect the gate from too much voltage, that could damage the FET.#

Many mosfets are more or less capable to operate in avalanche mode, just like a higher voltage zener. So if the drain to source voltage gets too high, it will act like a zener diode. If the power is not to high it can survive this.
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