How to test a MOSFET


#1

This post will cover how to test a MOSFET transistor using an analog multimeter.

MOSFET (also known as FET) is short for Metal Oxide Semiconductor Field Effect Transistor. FETs are widely used for switching and amplifying electronic signals in the electronic devices. The FET is a three-terminal device such as source, gate, and drain. You will find them in power supplies and many electronic devices. FET failure and leakage are quite high in a circuit and you need to know how to accurately test it.

First you will need to find which lead is the gate, drain and source pinout from semiconductor cross reference book or a datasheet.

One Example:

image

Once you have the cross reference or diagram for each pin of the FET, then you can use your analogue multimeter set to times 10K ohm range to check the FET. Assuming you are testing the n channel FET then put the black probe to the drain pin.

Touch the gate pin with the red probe to discharge any internal capacitance in the FET. Now move the red probe to source pin while the black probe still touching the drain pin. Use your right finger and touch the gate and drain pin together and you will notice the analogue multimeter pointer will move forward to center range of the meter’s scale.

Use your finger to touch on the gate and drain pin.

Lifting the red probe from the source pin and putting it back again to the source pin, the pointer will remain at the middle of the meter’s scale. To discharge the FET you must lift the red probe and touch just one time on the gate pin. This will eventually discharge the internal capacitance again.

At this time, use the red probe to touch on the source pin again, the pointer would not kick at all because you have already discharge it by touching the gate pin. These are the good FET characteristic. You need to practice more by taking some FET from your bench or from your component’s compartment. Once you know the secrets, testing other FET is as simple as testing diode.

If you notice that all the result that you measured kicked towards zero ohms and will not discharge, then the FET is considered shorted and need replacement. Testing the P channel FET field effect transistor is just the same way as when you check N channel FET. What you do is to switch the probe polarity when checking the P channel. Some analog multimeter have the times 100k Ohm range, this type of meter can’t really test FET due to the absent of 9 Volt battery inside the multimeter. This type of meter will not have enough power to trigger the FET. Make sure you use a meter that have the times 10k ohm range selector.


#2

Quite likely destroying it. MOSFETS, almost any semiconductor device MUST be handled at static safe work stations. A human finger can have up to several Kv of static charge. Touching a flying gate lead is asking to damage the gate oxide layer and either break it down, or punch thru it and cause leakage.


#3

True.
I (should) follow proper ESD safety precautions and test them in a circuit with a load.
You, also, get an idea of the “RDS_ON” value.