How to test a MOSFET with a normal DMM:
1) Turn your meter on to the "ohms" scale; "auto" range if it does that, otherwise start at the highest range for these tests except when you expect a low value, then use the lowest range that gives you a reading. If you choose "auto", you can ignore everything below about switching ranges.
2) With the test leads installed, but disconnected from the device, verify that the reading is really really big. If it doesn't say at least 1MΩ, go buy another DMM, the Chinese ones on eBay are cheap.
3) With a MOSFET out of the circuit, connect both DMM leads to the source terminal on your device (DUT=device under test). A perfect meter will read 0Ω. Yours probably won't. wiggle the leads a bit to get a feel for how well the meter connects to the DUT. Write down the lowest consistent reading you get. This value represents a 0Ω reading (you are really measuring the resistance of the test leads, connections, etc.). You will subtract this value from any low Ω reading you get later.
4) Not really a step - a comment: Get in the habit of doing these sanity (or calibration) checks with all of your measurements. DMMs, Oscilloscopes, Network Analyzers, whatever. You need to know if your instruments are telling the truth, or if you're using them wrong. Occasionally this will save you hours of confusion.
5) Connect the negative lead to the Source terminal, and the positive lead to the Gate terminal and measure the Drain Source to Gate resistance. Verify that it's >1MΩ. Reverse the polarity of the test leads and measure the Drain Source to Gate resistance. Verify that it's >1MΩ. It really should be >10MΩ, but don't worry for now if it's not.
6) Connect the negative lead to the Source terminal, and the positive lead to the Drain terminal (this is for N-Channel MOSFETs, you'll reverse the polarity for P-Channel devices). Connect the Gate terminal to the Source terminal (a short circuit, you can do this with a knife blade, paper clip, whatever is conductive) and measure the Drain to Source resistance. Verify that it's >1MΩ. Disconnect the short from Gate to the Source and verify that the Drain to Source resistance doesn't change much, at least for several seconds.
7) Set your DMM to the highest Ω range. The range is important; you want the DMM to put out the largest test voltage it can, and often the low Ω ranges use low test voltages. Connect the negative lead to the Source terminal, and the positive lead to the Gate terminal for at least 1 second. Now, carefully move the positive lead from the Gate terminal to the Drain terminal, without accidentally connecting the Gate to anything else (like the Source). Change the Ω range to a low setting and and measure the Drain to Source resistance. Verify that this is a low value (let's say <10Ω or so) for at least a few seconds. What you are doing in this step is charging up the Gate capacitance to the DMM test voltage and leaving the charge there while you measure the saturated channel resistance. Any accidental connection to the Gate may allow that charge to escape.
8) If your DMM has a diode test feature you could also test the body diode (anti-parallel to the Drain-Source channel) of the DUT. But it's unnecessary, so I won't describe it unless you ask me to.
- I can't guarantee this process works for all DMMs, but I can't recall a meter I ever used (out of many, many meters) that wouldn't do it properly.
- Be careful how you hold the device (or better yet, don't hold it) you don't want to mistake the resistance between your fingers for the DUT measurement. Heat sink tabs are connected to the transistor.
- This is a basic functional test to spot damaged devices, not a method to verify specifications. That's why the datasheet (THAT YOU NEED TO READ!) has test circuits and footnotes.
- This test is for discrete, single MOSFETs. If your MOSFET is part of an IC then you are measuring whatever circuit is in there, which may not act like a simple MOSFET.
edit: fixed some typos.
