A lot of SOT-23 FET gates have built-in back-to-back Zener diodes to source, presumably to protect against over-voltage. However, the datasheets typically say nothing about the characteristics of these diodes. The gate leakage current will be much higher in FET's that have this structure (compared to those that don't), but otherwise the datasheets are silent about it.
One example: Diodes, Inc. p/n DMP10H4D2S-7.
http://www.diodes.com/_files/datasheets/DMP10H4D2S.pdf
I am designing a circuit where I am installing external back-to-back Zener diodes to protect the FET gate under certain conditions. But I am curious if I really need them or not. It seems to me that if I limit the current sufficiently, the built-in Zeners should protect the gate.
If anybody has any thoughts on this topic, I would be very eager to hear them. I mean, the obvious thing to do is just add the external protection to be safe. But I still want to know if anyone has any insight on the built-in protection.
Best Answer
The zener protection on Mosfet gate is against ESD (electrostatic discharge) damage. In this case the Mosfet is tiny and you could damage it just touching the pin, without the Zener.
Please note that specific datasheet says nothing about the ESD protection level, so I assume is not very strong. Unlikely is 2kV HBM, may be just 500V. Usually if the Zener is present the ESD resistance capability is reported on the datasheet; this specific datasheet is unusual on that.
Most mosfets, specially the larger, have no Zener ESD protection. The absence is due to cost, a version with zener needs 1-2 masks more so the die is a bit more expensive (~15%).
ESD pulse are low energy short pulse; but the gate oxide is thin: 500 to 1000 Å and damages are catastrophic. Without the Zener, the capability to sustain the ESD stress depends by thickness and area of the gate oxide. Logic level (Vgs-th ~ 1.5) are more sensible than standard gate (Vgs-th ~ 3) On big Mosfets the gate capacitance is able to handle the ESD pulse without damage.
The Zener voltage is few volts more than the max rated Vgs. It protects the gate oxide but it will be destroyed itself by DC or long pulse overvoltage. The ESD Zener also introduce a current path between gate and source so the Igss leakage is orders of magnitude larger then the gate capacitance itself.
Some reading: http://www.jedec.org/sites/default/files/IndustryCouncil_HBM_January2014_JEDECversionMay2014.pdf
and one of the test methodology http://www.aecouncil.com/Documents/AEC_Q101-001A.pdf. Same than JESD22-A114