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I was looking for some MOSFETs and would like to know when should I choose a MOSFET with a gate-source diode (or add one externally.)

I also noticed that they are always seems to be bidirectional like the DMP3007. It would seem to be protecting from ESD, but where does this ESD come from? Plugging in the power supply? When connecting the load? What if the load is soldered to the PCB the ESD protection is not needed?

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  • \$\begingroup\$ When you are in a situation with considerable (parasitic) inductance-to-capacitance ratio at the source or gate, the impedance of these nodes can be rather high at higher frequencies. They can ring and attain voltages beyond the gate-source-breakdown. \$\endgroup\$
    – tobalt
    Jul 1 '21 at 10:20
  • \$\begingroup\$ @tobalt can you please me a an example of this situation ? all i can think of is the parasitic inductance of the wire and traces, are they really that susceptible at high frequencies? typically how high is high ? \$\endgroup\$
    – Jake quin
    Jul 1 '21 at 10:35
  • \$\begingroup\$ A typical example is a long narrow gate drive trace, with no gate resistor and powerful gate driver. Another is, as @MarcusMüller mentions, inductive loads at the source, e.g. in H-bridges with 4 NMOS. And yes, due to the rise times, inductances in the nH range can be already too much. That is why some new fast MOSFET products come in flat packages or even with integrated drivers. \$\endgroup\$
    – tobalt
    Jul 1 '21 at 10:52
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The source-gate diode in the DMP3007 you reference is purely for protection from electrostatic discharge (ESD):

enter image description here

This paper from Infineon implies that gate ESD protection is mostly intended to prevent damage before the device is installed on the PCB.

Equipment that is exposed to ESD often may require gate protection while operating, though in that case there should be other protection involved as well (shielding, closed housing protecting the circuitry, etc.)

This article from TI mentions that gate protection often comes at the cost of higher gate leakage and that you may need to avoid gate protection diodes in low power equipment (smart phones and other battery powered devices.)

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  • \$\begingroup\$ from your last paragraph, it seems to be implying that you would always want a gate-source diode unless gate leakage is a concern am i correct? \$\endgroup\$
    – Jake quin
    Jul 1 '21 at 12:34
  • \$\begingroup\$ @Jakequin: Not necessarily. Read the TI link. Most (modern) MOSFETs have some intrinisic ability to withstand ESD. If you store them properly and onyl handle them at an ESD safe worksplace, they'll be fine. In fact, TI says that most MOSFETs have no extra protection at all - just their intrinsic ability. I'd say the rule is "use MOSFETs with extra ESD protection when you know you must have it." \$\endgroup\$
    – JRE
    Jul 1 '21 at 12:38
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Yes that ESD protection is really neccesary and essential.

A MOSFET is generally constructed like this:

enter image description here

Note how there is an Oxide layer between the Gate and the rest of the transistor. This Oxide layer is a very good isolator so the Gate is completely floating.

This means that if an electrical charge gets trapped on the Gate, it has nowhere to go. If that charge difference between the Gate and the rest of the transistor is large enough, the voltage across that Oxide could reach a critical level and destroy the oxide.

Since this Oxide layer is quite thin (that is needed to make a "good" MOSFET) it is also easy to damage. Some MOSFETs are damaged by only a few volts.

An ESD event can inject a lot of charge into the MOSFET causing charge differences resulting in a voltage across the Oxide. So that voltage across the Oxide has to be somehow limited and that is what the (ESD protection) diodes between gate and source do.

Read more about ESD here.

Even if you would take all precautions needed to prevent an ESD event, it can still happen and cause damage. Worst of all, the damage can be "imminent failure" meaning, there's damage but you don't notice it yet, the MOSFET works fine.

But after some useage (hours, days, weeks or even years) that damage might eventually become fatal resulting in a broken MOSFET. So it is really important to have at least some protection against ESD.

The diode has a certain breakdown voltage (similar to a Zener diode or TVS) that will equalize the charge differences in the MOSFET so that the voltage acorss the Oxide doesn't become too high to cause damage.

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  • \$\begingroup\$ if its essential why arent manufacturers design their mosfet with in built gate-source diodes? Is there a practical reason why you must let the circuit designer choose the diode? \$\endgroup\$
    – Jake quin
    Jul 1 '21 at 12:36
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    \$\begingroup\$ why arent manufacturers design their mosfet with in built gate-source diodes? I think you misunderstand. Manufacturers ARE selling MOSFETs with included (build-in) diodes. The diode can only protect when it it there. A MOSFET that does not have protection will be very vulnerable and will be damaged very easily. The circuit designer cannot choose the diode, the diode is present inside the MOSFET. \$\endgroup\$ Jul 1 '21 at 12:45
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where does this ESD come from

The gate may be controlled by external signals, or by circuits/networks that pass ESD from external sources. Button-controlled electronic switches are notoriously susceptible to ESD.

Also, remember that the protection isn't really "at the gate". It's between two terminals, and will protect from any situation where an overvoltage condition would exist between them. The source and the gate are thus equally susceptible: it doesn't matter where the hit comes, what matters is that the gate-to-source voltage is too high. Holding gate "steady" and hitting the source is equivalent to holding the source "steady" and hitting the gate!

Thus, the protection extends to source and drain pins. Reverse polarity ESD pulses will be shunted across the channel by the body diode. Positive ESD pulses may cause trouble still, but the charge and energy needed to destroy the device while it's avalanching in forward breakdown is much higher than what the gate can take - potentially orders of magnitude higher.

As for "why bundle the protection"? Just think about costs: do you really want to have to add an ESD protector? You're already paying for a semiconductor device, and most of its cost is in packaging and testing. The tiny semiconductor die inside is almost free (<10% of the cost for a discrete device like this, IIRC). Throwing in an ESD protection for the fragile gate is a no-brainer. It'd be almost ridiculous not to.

So why it's not done in all discrete MOSFETs? The protection device has some non-ideal characteristics, i.e. adds capacitance and nonlinearities that cause distortion. This is of no concern in many applications, but not all. The protection diode is not suitable when lowest gate leakage is desired, or fastest possible switching. Distortion may be problematic in some RF applications. But it's a very good thing most of the time!

Curiously enough, ESD can be a hit from body through an "insulating" case. This happens often enough, especially with cheap cases. All it takes is there to be a tiny crack in the plastic - not hard to come by when dealing with hand-held stuff, or stuff kept in pockets. Or just the natural gaps between separate plastic parts. Over time the crack/gap accumulates some wet salt and other junk from sweat and it's like having your own very special ESD test setup :)

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MOSFETs are commonly used as switches in switch-mode power supplies and in motor control.

In both cases, they switch an inductive load. Switching off means that the \$\frac{\mathrm di(t)}{\mathrm dt}\$ is very high. Combine that with what you know about inductors:

The quick turn-off will lead to high voltages over the load. That will in turn make it rather likely there'd be some unwanted discharges, and damaging the gate-drain or -source isolation can lead to quite dramatic failures, when the MOSFET then becomes always on, and connect the power rail to the logic part.

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    \$\begingroup\$ For an answer, I suggest you omit phrases like "Combine that with what you know about inductors!" and just directly mention the consequence. \$\endgroup\$
    – tobalt
    Jul 1 '21 at 10:11
  • \$\begingroup\$ What is di/dt of load content has to do with antistatic gate protection? May be that things just protect the device before soldering to PCB? \$\endgroup\$
    – user263983
    Jul 1 '21 at 10:29
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    \$\begingroup\$ for inductive loads isnt the tvs diode in parellel with the load and not on the gate source? \$\endgroup\$
    – Jake quin
    Jul 1 '21 at 10:29

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