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This may be a silly one but I couldn't seem to find an explicit answer: with a 3-pin MOSFET, how do I determine whether it's an NMOS or PMOS?

I'm making some assumptions here:

  • I've already found the Gate pin, which has no conduction to the other two pins (at the voltage level of a DMM diode test)
  • I don't know whether or not an internal protection diode is in use
  • I don't know if the Source is tied to the Bulk (not that it matters, since there is no Bulk/Body pin in a 3-pin MOSFET)

Thanks!

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    \$\begingroup\$ ...look up the part number and find the datasheet. \$\endgroup\$ – Ecnerwal Feb 5 '16 at 3:25
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    \$\begingroup\$ I meant experimentally. I suppose I should've said that when I wrote the question. \$\endgroup\$ – PoGaMi Feb 5 '16 at 3:26
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    \$\begingroup\$ @PoGaMi: All Mosfests have intrinsic body diode, which will be between Source to Drain or vice-versa depending upon PMOS or NMOS. Please put image of your Mosfet \$\endgroup\$ – AKR Feb 5 '16 at 4:20
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    \$\begingroup\$ You are not writing a procedure to identify them. You are enlisting free labor to have the procedure written for you. I am debating whether to downvote this question. The whole thing seems kind of dubious. What would you do with a MOSFET if you don't know what part it is? You can't really use it for anything. And if you do know what part it is, you don't need to test the polarity. Just look it up. \$\endgroup\$ – mkeith Feb 5 '16 at 9:09
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    \$\begingroup\$ I agree with @mkeith - so you identify (by whatever means) the polarity of the device then what? Can you use it in a circuit? Well I wouldn't because I have learnt nothing about its vital characteristics that WOULD make it useful to me. Once last chance - explain why any method of identifying ONLY the polarity of a MOSFET has any practical use. \$\endgroup\$ – Andy aka Feb 5 '16 at 9:58
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Assuming that is is an enhancement MOSFET (most common):

If it becomes conducting if the gate voltage is some volts higher than the source or drain voltage it is a N-MOSFET.

If it becomes conducting if the gate voltage is some volts lower than the source or drain voltage it is a P-MOSFET.

It is very likely that there is an internal protection diode (there are only very very few MOSFETs without them; at least if it is a power MOSFET). You can use it to find out which pin is source and which one is drain:
P-MOSFET: anode is connected to drain, cathode is connected to source
N-MOSFET: anode is connected source, cathode is connected to drain.

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  • \$\begingroup\$ I think I was hung up on the idea that there may not be a body diode, but it sounds like everyone agrees there will be. Therefore, your procedure works. Thanks! \$\endgroup\$ – PoGaMi Feb 6 '16 at 16:50
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Measure the polarity of the body diode (between the two non-gate pins). The cathode is either the drain of an n-channel or the source of a p-channel.

Apply a moderate positive voltage (say 8V) with a series LED and resistor to to the cathode with anode grounded.

Tie the gate to cathode. If the LED turns on it's n-channel, if it is off then it is p-channel.

schematic

simulate this circuit – Schematic created using CircuitLab

(This is assuming it can only be a p or n channel enhancement mode MOSFET, if it can be a JFET or depletion type or other type again, then other tests will be necessary).

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  • \$\begingroup\$ This works only if he knows which of the two non-Gate pins is Drain and which is Source. If he doesn't know such basic facts as the channel type (or the part number) I'd assume that also the pinout isn't known. \$\endgroup\$ – Curd Feb 5 '16 at 13:31
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    \$\begingroup\$ @Curd There is nothing in my answer that requires that knowledge. It only requires the identification of cathode vs. anode of the body diode (assuming the gate has already been identified). \$\endgroup\$ – Spehro Pefhany Feb 5 '16 at 15:44
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    \$\begingroup\$ @Sphero Pefhany: Ok, now I see it ("Tie the gate to cathode"). I missed that first and thought only the direction of the diode was determined and the gate left alone. \$\endgroup\$ – Curd Feb 5 '16 at 15:50
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With the part number you can easily find a data sheet online that will give you more information than you will probably need.

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  • \$\begingroup\$ Sorry, meant experimentally. \$\endgroup\$ – PoGaMi Feb 5 '16 at 3:26
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A n-channel FET turns on as the gate voltage becomes more positive. A p-channel FET turns on as the gate goes more negative.

Many common FETs have an internal diode between the source and drain (body connected to the source), so you may be able to determine the source vs drain by using the polarity that produces less current.

One a small voltage (perhaps a volt) is applied between the source and drain (choose terminals arbitrarily), change the gate voltage to be -1 and then 1 volt. If you don't see a difference, try using slightly higher voltages. The FET will likely be operating in the subthreshold region, so the currents may be VERY small (nanoamps to microamps). Note that at room temperature, the subthrehold current in a good MOSFET should increase by 10 times for every approximately 70 mV of gate voltage change. If the positive gate voltage allows a higher current, then it is a n-FET.

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