We are making a thesis where we use MOSFETs as an alternative radiation detector.

So to explain it, it works when the MOSFET is irradiated with an external radiation source; its voltage threshold increases, which will be used to determine the radiation dose.

I'm currently asking for help on how we measure the voltage threshold. BTW, we are using an N-channel MOSFET (model: IRFP250NPbF). Also in the datasheet provided by the manufacturer, it says here
VGS(th)/Gate Threshold Voltage Min: 2.0 ––– Max: 4.0 V.
There is a condition here with VDS = VGS, ID = 250µA.

Does this mean that to measure the VGS, we need to first satisfy the conditions?
To measure the voltage threshold, what node will we use to measure the VGS (th)?
Is it at the drain to the source terminal or still at the gate to the source terminal?

Feel free to share your thoughts, if you have any.

I would also like to add that we have already tried to supply a voltage at the gate with respect to the source terminal. We use a 4 V supply voltage, and when we tried to measure the VDS (drain to source voltage), there was a voltage drop, so we've got a 3.5 V. Also, we are using an Arduino to measure its voltage and a multimeter for checking.

  • 2
    \$\begingroup\$ Why you use such powerful mosfet for measurement purpose? Get a signal one with much lower Vgs treshold. They have lower gate capacitance also (advantage). \$\endgroup\$ Jun 19, 2023 at 6:10
  • \$\begingroup\$ Actually we saw it used in an academic paper about using commercial mosfet as radiation detector. Is it okay to know your suggestion or model of a less powerful mosfet? Thanks! much appreciated \$\endgroup\$ Jun 19, 2023 at 6:21
  • \$\begingroup\$ The answers so far prompt me to ask: Are you asking for help on how to bias the FET into a stable condition, or how to measure a voltage? \$\endgroup\$
    – spuck
    Jun 19, 2023 at 16:07
  • \$\begingroup\$ Both, specifically how to measure the voltage threshold in a simple way \$\endgroup\$ Jun 19, 2023 at 22:33

2 Answers 2


Here I use an op-amp to bias the MOSFET with exactly the right gate potential to obtain exactly 2.5V across the source resistor R1:


simulate this circuit – Schematic created using CircuitLab

The result is a constant and exact 250μA of drain/source current, which is the condition you are aiming for.

The op-amp's output will be \$V_{GS}\$ above the source at +2.50V, giving you this relationship:

$$ V_{OUT} = V_{GS} + 2.50V $$

You can use whatever circuitry or process you like to subtract the 2.5V offset, to obtain an accurate measure of \$V_{GS}\$.

  • \$\begingroup\$ Hello, do you have a video on how do you assemble this circuitry, since this is my first doing like this so i'm still confused with wirings \$\endgroup\$ Jun 19, 2023 at 11:41
  • 1
    \$\begingroup\$ @LazyHotdog You can assemble prototype circuits using "solderless breadboards", here's an introduction to breadboarding video on youtube. There's also a Udemy course on breadboarding. \$\endgroup\$ Jun 19, 2023 at 12:02
  • \$\begingroup\$ How do you measure its voltage threshold? \$\endgroup\$ Jun 19, 2023 at 12:27
  • 3
    \$\begingroup\$ @LazyHotdog I think you might be out of your depth. \$\endgroup\$ Jun 19, 2023 at 12:54
  • \$\begingroup\$ Do you think a breadboard is accurate enough for a 250uA drain? I feel like the variability in a breadboard will throw you out of calibration \$\endgroup\$ Jun 19, 2023 at 15:39

Yes, to measure threshold voltage connect gate to drain and measure the source to drain voltage at some chosen current.

If you use a different current you will see a different voltage as the result.

You may also need to control the temperature of the MOSFET, because MOSFETS are also a thermometers.

Unless you have a great many of these parts, pick a different one, one that's not marked OBSOLETE.

  • \$\begingroup\$ So do we still need to fulfill the specified condition before measuring the voltage threshold and is it also normal to have a 0.5 Vdrop in the Vds terminal ? \$\endgroup\$ Jun 19, 2023 at 6:12
  • \$\begingroup\$ @LazyHotdog What do you mean by "the Vds terminal"? There's a drain terminal and a source terminal, and "Vds" means the difference between the potentials of those two terminals. There's no Vds terminal. \$\endgroup\$ Jun 19, 2023 at 21:06
  • \$\begingroup\$ Yes, it looks like connecting drain to gate would be a good way to assure they are equal, as the measurement seems to require. \$\endgroup\$
    – PStechPaul
    Jun 20, 2023 at 6:11

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