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I have the circuit structure that looks as shown below. An I/O signal, sigA, (active low) is driving the P1 port on unit U1. An NMOS transistor's drain is connected to this signal with the gate/source/bulk connected to ground. How does this NMOS transistor work and what is its purpose?

The gate/source are connected to ground, so I am not sure how current will flow through (i.e., what condition will make M1 turn on). It looks like a pull down device, but since sigA is active low, it doesn't make sense to me.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Reverse voltage protection perhaps. If sigA goes underground the n-channel will tend to turn on at some point. Kinda weird though... the inbuilt diode will kick in long before that. Are you sure its not a p-channel \$\endgroup\$ – Trevor_G Oct 31 '17 at 21:01
  • \$\begingroup\$ Hi, thanks for the help. Yes, it is n-channel. \$\endgroup\$ – user4979733 Oct 31 '17 at 21:07
  • \$\begingroup\$ You say it's an N-channel but is it an enhancement mode device like your schematic shows, or is it a depletion mode device? The latter would make more sense. \$\endgroup\$ – Spehro Pefhany Nov 1 '17 at 2:41
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If this inside an IC it is for ESD protection and referred to as a ggNMOS device.

It is intended to breakdown (without suffering damage itself) before the gate oxide of the IC when it is subjected to ESD due to static electricity.

It relies upon a parasitic bipolar transistor being formed during the normal CMOS fabrication process. This bipolar transistor avalanches when the voltage exceeds a safe level and dissipates the input ESD strike.

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    \$\begingroup\$ Hm, to someone (me) who doesn't know anything about this, there seem to be two equally reasonable, but different, answers. Do you talk about the same thing using different words, or is one of you wrong? :) \$\endgroup\$ – pipe Nov 1 '17 at 13:56
  • \$\begingroup\$ @pipe It is difficult to say without more information from the OP. My answer does describe how some ICs use such a structure. \$\endgroup\$ – Kevin White Nov 1 '17 at 14:18
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There is (or should be) a diode (zener) across drain-source inside the MOSFET. It is called "body diode".

Grounding both gate and source guarantees the off-state. So, that diode will remain active and can be used as an overvoltage protection across the input.

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    \$\begingroup\$ +1.. its still weird though. \$\endgroup\$ – Trevor_G Oct 31 '17 at 21:15

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