Basic static electricity question

[M. Hamed]

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?

 

[M. Hamed]

On Saturday, February 14, 2015 at 12:02:02 PM UTC-7, default wrote:

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.

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.

 

[M. Hamed]

On Saturday, February 14, 2015 at 2:09:40 PM UTC-7, Tim Wescott wrote:

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

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 discharged through the MOSFET, only if the MOSFET source/drain has some path to ground, to complete the circuit.

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

 

[default]

On Sat, 14 Feb 2015 10:34:02 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

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?

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.

 

[Tim Wescott]

On Sat, 14 Feb 2015 12:15:29 -0800, M. Hamed wrote:

On Saturday, February 14, 2015 at 12:02:02 PM UTC-7, default wrote:
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.

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.

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

--
www.wescottdesign.com

 

[Tim Wescott]

On Sat, 14 Feb 2015 13:53:15 -0800, M. Hamed wrote:

On Saturday, February 14, 2015 at 2:09:40 PM UTC-7, Tim Wescott wrote:

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


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 discharged through the MOSFET, only if the MOSFET source/drain has
some path to ground, to complete the circuit.

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

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[John Larkin]

On Sat, 14 Feb 2015 10:34:02 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

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?

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.

 

[default]

On Sat, 14 Feb 2015 12:15:29 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

On Saturday, February 14, 2015 at 12:02:02 PM UTC-7, default wrote:
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.

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.

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)

 

[RobertMacy]

On Sat, 14 Feb 2015 11:34:02 -0700, M. Hamed <mhdpublic@gmail.com> wrote:

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?

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.