Basic static electricity question

Let's say I walk over the carpet and get charged to 10KV. Then I touch a MOSFET.

Is it fair to assume that the MOSFET won't be damaged unless it has a path to ground?

Reply to
M. Hamed
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No. But the logic may seem sound at first glance.

The mosfet itself is another conductive body with a charge that is different than your own. Ground isn't the only path for electrons.

Reply to
default

Just having difference in charge doesn't mean electrons will flow. Otherwise you would be able to discharge a capacitor by connecting a piece of wire to it.

Reply to
M. Hamed

Connecting a piece of wire across it's terminals _does_ discharge a capacitor.

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www.wescottdesign.com
Reply to
Tim Wescott

I meant connecting just one terminal.

The model I'm imagining is a cap charged to 10kV with one end grounded and the other end touching the gate of a MOSFET. I imagine the cap will get dis charged through the MOSFET, only if the MOSFET source/drain has some path t o ground, to complete the circuit.

If for some reason it's well insulated from its surroundings, I imagine it won't be affected.

Reply to
M. Hamed

It depends on a whole lot of stuff, but basically any object has a certain amount of capacitance to "the universe" (actually to the average charge of whatever is around). If you waltz up to it with a probe that's at 10kV compared to said "universe" and touch a lead, there'll be a discharge into the FET.

All smoky and weird -- but static electricity is smoky and weird.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

If the fet were sitting on an insulated surface, you could pick it up without damage. Or someone could throw it, and you could catch it, again without damage.

When I hand an IC to another person, I touch them first, to equalize our body voltages.

Reply to
John Larkin

Think of the mosfet (and of course the surface it is lying on) as one plate of a cap and you the other. When you touch the mosfet you complete the circuit and discharge the cap.

The major failure mode is the gate insulator breaking down with static. It is very thin and very well-insulated so any charge present has no where to go but through the insulating dielectric (assuming the voltage is high enough)

(a reason you can touch the mosfet gate with a wire biasing it "on," remove the wire and it stays on, for a time - or just sits there toggling on and off with stray RF noise in the area when the charge drops enough)

Reply to
default

Your 'model' of what's happening is slightly flawed. You're thinking in terms of steady state and 'normal voltages/current/conductivitiy.

Think differently when it comes to ESD events. The parasitics and built-in body mass capacitances allow a lot of 'initial' charges to fly around, ...often unexpectedly damaging. Those initial 'adjustments to nature can be devastating.

When it comes to high voltage, and ionic breakdown, better to think in terms of, "Everything conducts."

A litle bit of design/development work in the up to 200kV ranges and you'll understand. With AIR much more 'leaky' than you'd expect.

In answer to your question, you might get away with it, and you might not.

To help gain a bit of understanding [although static only] get a coy of femm 4.2, free finite element analysis program [with a helpful user's group] that has a 'static electricity mode' that enables you to model static charge problems. I've used it to solve discharges at 50kV in a vacuum system. Using the program I gained a lot of insight as to the sources of the problems, and importantly used it to communicate 'quantitative' solutions to the staff.

Reply to
RobertMacy

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