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CMOS inputs on microcontrollers and other ICs can be damaged by ESD discharges. Can the gate of a big discrete MOSFET (2N7000, IRF9530, etc.) be damaged by ESD discharges?

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Ordinary BJTs are sensitive to ESD, also, especially high-frequency devices. – Leon Heller Feb 9 '11 at 19:32
The question was about MOSFETs. It's good to know that BJTs are sensitive, but this doesn't answer the question. – Kevin Vermeer Feb 9 '11 at 20:53
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There is no doubt that discrete MOSFETs are very sensitive to ESD. However, an interesting question if big MOSFETs are a lot less sensitive. I'd guess yes, but I have no figures to prove it. – AndreKR Feb 9 '11 at 21:21
I mentioned it in case people thought that MOSFETs were the only discrete devices that could be damaged by ESD. – Leon Heller Feb 9 '11 at 21:26

4 Answers

up vote 13 down vote accepted

Yes. I've used MOSFETs which had a conductive rubber band around the pins to protect the gate(s) by shorting the pins, to be removed after soldering. (TO-39, IIRC)

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Very true. I've seen many BSS84, BSS123 and the like fail. They are much more sensitive than ICs are because ICs usually have protective diodes at the I/Os and discrete MOSFETs don't. Also, damaged small signal MOSFETs often don't fail in an obvious way but get degraded just a bit (although enough to cause trouble later on). I have no doubt the same is true for big MOSFETs since their structure looks like many small MOSFETs in parallel. However, big MOSFETs have higher parasitic capacitance that acts as a somewhat better protection: More (dis-) charge is required to raise the voltage. – zebonaut Feb 10 '11 at 20:53
up vote 7 down vote

Any MOSFET outside of a circuit will be extremely ESD sensitive as one spike on the gate that raises its voltage above the maximum and it will be dead. MOSFETs in circuits very frequently have explicit protection (Zeners on gates or clamping diodes in drivers) and other incidental ESD protections like pulldowns or possibly increased capacitance.

More to the point of "are big (and/or) discrete MOSFETs less sensitive", they are for two reasons:

  1. The gate oxide is likely to be thicker and take more voltage to breakdown (though the input lines on an IC are probably overengineered in this manner as well), and
  2. The gate capacitance will be vastly larger, so it will take much more charge to build up a lethal voltage.

In a circuit, the more common failure modes (in my experience) are inductive spikes on the source pin blowing the gate, or those on the drain which can cause a fatal avalanche breakdown. I don't think I've ever positively identified a dV/dt failure, which is where the voltage rise on the MOSFET is so fast, the parasitic capacitances between drain-gate-source are able to turn on the MOSFET causing bad things to happen.

Nevertheless, if you ground your source well and blast the gate right at the package with an ESD gun on 11, you might be able to kill it. Users shouldn't be able to stick their grubby little hands on your gate lines because they could have just shuffled their wool socks along the polyester carpet, but if they can for some reason (???), a Zener should protect nearly everything.

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up vote 1 down vote

There is a second source of 2n7000 whith " KL " at the end of the reference made from VISHAY and is totally protected.

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Totally protected? The datasheet says (very prominently) 2000V, which is quite a lot for a FET, but that's only class 1C under the human body model. – Kevin Vermeer May 4 '11 at 13:30
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YES absolutely.

I made the mistake of putting 2N7000 in my designs before and worked on them in environments that were not well ESD protected. I have destroyed literally dozens of 2N7000's doing this.

The key issue for me is "how much" protection is necessary in designs. Especially for production when adding protection costs money.

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