How to Troubleshoot a MOSFET Transistor

By J.T. Barett
You can check a MOSFET with a multimeter.

MOSFET transistors are used in applications ranging from switching power supplies to computers. Unlike a standard bipolar transistor, which depends on current, a MOSFET depends on voltage. You can troubleshoot a MOSFET by reading its resistance on a multimeter. Before you test it, you first have to determine if it is P- or N-channel and if it’s an enhancement- or depletion-mode device. The transistor’s data sheet has this information.

P-Channel MOSFET

Disconnect the electronic equipment from the AC outlet and allow 10 minutes for the capacitors to discharge. Connect the antistatic wrist strap to a solid electrical ground, such as a cold water pipe. Slip it on your wrist. Turn the multimeter on and set it to read resistance at the x1,000 ohm range.

Connect the meter’s positive, or red, probe tip to the MOSFET’s source lead and the negative tip to the gate lead. You should read a very high resistance. Move the negative probe to the drain lead and clip the source to the gate with one of the alligator jumper wires. For an enhancement-mode device, you should again read a very high resistance. If it’s depletion-mode, you should see a resistance of between about 1 and 1,000 ohms. Disconnect the jumper wire.

Connect a 9-volt battery into the battery clip. Using the jumper wires, connect the battery clip’s red wire to the MOSFET’s source lead. Connect the black wire to the MOSFET’s gate lead. Touch the red multimeter probe to the source lead and the black probe to the drain lead. Read the resistance on the meter. An enhancement-mode transistor should indicate a resistance of a few ohms or less. A depletion-mode device will have a very high resistance, possibly indicating “OF” or an overflow reading on the meter. If the meter does not give these results, double check your connections. The device may be defective. Carefully remove the device from the circuit with a soldering iron and solder wick. Retest the device, performing Steps 2 and 3 as before.

N-Channel MOSFET

Disconnect the electronic equipment from the AC outlet and allow 10 minutes for the capacitors to discharge. Connect the antistatic wrist strap to a solid electrical ground, such as a cold water pipe. Slip it on your wrist. Turn the multimeter on and set it to read resistance at the x1,000 ohm range.

Connect the meter’s negative, or black, probe tip to the MOSFET’s source lead and the positive tip to the gate lead. The meter should show a very high resistance, possibly indicating “OF” or a similar infinite resistance code. Move the positive probe to the drain lead and clip the source to the gate with one of the alligator jumper wires. For an enhancement-mode device, you should again read a very high resistance. If it’s depletion-mode, you should see a resistance of between about 1 and 1,000 ohms. Disconnect the jumper wire.

Connect a 9-volt battery into the battery clip. Using the jumper wires, connect the battery clip’s black wire to the MOSFET’s source lead. Touch the red wire to the MOSFET’s gate lead. Touch the black meter probe to the source lead and the red probe to the drain lead. Read the resistance on the meter. An enhancement-mode transistor should indicate a resistance of a few ohms or less. A depletion-mode device will have a very high resistance. If the meter does not give these results, double check your connections. The device may be defective. Carefully remove the device from the circuit with a soldering iron and solder wick. Retest the device, performing Steps 2 and 3 as before.

Warning

MOSFETs may be damaged by the static electricity on your hands or clothing. Wear a grounded wrist strap to drain the static as you work. Do not use a desoldering pump to remove a MOSFET, as these can build up static charges. Use desoldering wick instead.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."