measure a mosfet gate with a digital multimeter

[mssong]

How to measure a mosfet gate with a digital multimeter

I measured the resistance by connecting the + terminal to the MOSFET GATE and the - terminal to the GROUND to determine whether the MOSFET will fail.

Some read large values in the tens of ohms and some read small values in the hundreds of ohms.

I know that when measured with a digital multimeter, it draws a small current, but is this current what caused the gate to blow, or is there a path for the leakage current to flow in the first place?

Or is the way I am measuring the gate resistance with my digital multimeter wrong?

 

[betwixt]

MOSFETs do not have a ground so presumably you are measuring the resistance while still connected to other circuitry. Most likely you are measuring the path through that circuitry and not the MOSFET itself.

Brian.

 

[KlausST]

I measured the resistance by connecting the + terminal to the MOSFET GATE and the - terminal to the GROUND to determine whether the MOSFET will fail.

A MOSFET
* has: GATE, DRAIN, SOURCE connections. Not GND, not "-", not "+"
* may be N-Ch or P-Ch
* may have internal Gate protection or not.

***
In most cases the gate should be very high ohmic with respect to SOURCE. In the high Megaohms.
I´d call any value below 1MOhm suspicious.

For sure the measurement voltage (by DVM) need to be within the MOSFET´s specification.

Also mind: Gate is a sensitive input. In worst case an ESD pulse (just by connicting to a DVM) may immediately kill the MOSFET. Thus be sure to keep on standard ESD protection rules.

Klaus

 

[mssong]

sa

MOSFETs do not have a ground so presumably you are measuring the resistance while still connected to other circuitry. Most likely you are measuring the path through that circuitry and not the MOSFET itself.

Brian.

You're right, we measured the gate with the circuit connected. We applied voltage to the gate without applying VDD, so it could blow up.

--- Updated Feb 17, 2025 ---

A MOSFET
* has: GATE, DRAIN, SOURCE connections. Not GND, not "-", not "+"
* may be N-Ch or P-Ch
* may have internal Gate protection or not.

***
In most cases the gate should be very high ohmic with respect to SOURCE. In the high Megaohms.
I´d call any value below 1MOhm suspicious.

For sure the measurement voltage (by DVM) need to be within the MOSFET´s specification.

Also mind: Gate is a sensitive input. In worst case an ESD pulse (just by connicting to a DVM) may immediately kill the MOSFET. Thus be sure to keep on standard ESD protection rules.

Klaus

I connected the + terminal to the gate of the mosfet to be biased and the - terminal to the ground represented by VSS in the circuit.

Should I consider it to have failed then?

 

[KlausST]

and the - terminal to the ground represented by VSS in the circuit.

??? Use the MOSFET connection terminology, please. There is only GATE, SOURCE, DRAIN.

we measured the gate with the circuit connected

Mesurements "in circuit" don´t give reliable / valid results, unless you perfectly know how the circuit behaves AND at the same time this circuitry does not influence the measurement significantly.

Klaus

 

[mssong]

??? Use the MOSFET connection terminology, please. There is only GATE, SOURCE, DRAIN.


Mesurements "in circuit" don´t give reliable / valid results, unless you perfectly know how the circuit behaves AND at the same time this circuitry does not influence the measurement significantly.

Klaus

Simplifying my circuit, it looks like this

The question was about the small resistances of tens to hundreds of kilohms that I see on my multimeter when I connect the + and - probes like this, with no voltage across VDD and GND.

 

[std_match]

With a standard multimeter it should be an open circuit.
Is something else connected to the gate in the circuit?
Do you touch the probes with your fingers?
Edit: if the gate really isn't connected to anything else, it is easy to damage it with overvoltage just by touching it, even with probes. Connect the source probe first!

 

[mssong]

Naturally, I don't touch the probe directly with my hand, and when I do measure with the multimeter probe, I touch the - terminal to GND first.

Is it possible that there is just a small current flowing on the + terminal, causing the gate to fail?

 

[danadakk]

The MOSFET has high input C, so when you did measurement you allowed
input to "settle', EG. that gate C to charge ? Your meter has a very high
impedance so current is low and it takes time to charge the C.

Depends on meter used, its methods for high Z measurements.

 

[dick_freebird]

Simplifying my circuit, it looks like this
View attachment 197370
The question was about the small resistances of tens to hundreds of kilohms that I see on my multimeter when I connect the + and - probes like this, with no voltage across VDD and GND.

--- Updated Feb 17, 2025 ---

You need to understand the specific part. I have seen some which appear to embed some gate protection diodes and these would show gate conduction in at least one direction (G < S for NMOSFET). Many though do not and can show < 100V gate rupture (which is what this test is meant for, though it is not the only way to fail hard).

 

[danadakk]

This might be useful. - https://download.tek.com/document/High_Resistance_Meas_AN.pdf

Keep in mind when doing high Z measurements holding the probes not a good idea. Eg.
body C coupling from power line, environment.

Ap not discusses other issues as well.


Regards, Dana.

 

[mssong]

With a standard multimeter it should be an open circuit.
Is something else connected to the gate in the circuit?
Do you touch the probes with your fingers?
Edit: if the gate really isn't connected to anything else, it is easy to damage it with overvoltage just by touching it, even with probes. Connect the source probe first!

Thanks for the great article. By the way, do you have any information on how much current a normal (or bad) multimeter will flow when measuring large resistances?

 

[D.A.(Tony)Stewart]

Resistance measurements measure voltage with uA level currents up to 1mA or so..

How to Test MOSFETs: A Comprehensive Guide

MOSFET (metal-oxide-semiconductor field-effect transistors) is an incredibly versatile and effective component to add to electronic circuitry.

www.nextpcb.com

 

[FvM]

Are you sure that's nothing connected to MOSFET gate other than the multimeter probe? What's the MOSFET function in the circuit? Unlikely that it involves no gate connection, you didn't report to have cut a connection before measuring.