During the repair of electronic equipment, it is sometimes required to check the field effect transistors. These semiconductor devices act as powerful key devices in most cases. Sometimes it happens that they fail.
Multimeter or ohmmeter
Checking the field-effect transistor when it is soldered into the electronic circuit will fail, so desolder it before checking. Examine the case. If there is a hole on the case from the crystal melting, then there is no point in checking the transistor. If the body is intact, then you can start checking.
The vast majority of power field effect transistors are MOS-FET and insulated gate n-channel. Less common with the p-channel, mainly in the final stages of audio amplifiers. Different structures of field-effect transistors require different ways of testing them.
After soldering the transistor, let it cool down.
Place the transistor on a dry piece of paper. Insert the red ohmmeter leads into the positive connector and the black one into the negative connector. Set the measurement limit to 1kΩ. The channel resistance of an open transistor depends on the voltage applied to the gate relative to the source, therefore, in the process of working with a transistor, you can set a more convenient measurement limit for you. The connection of the electrodes inside the case is shown in the photo.
Touch the “source” electrode of the transistor with the black probe, and touch the “drain” electrode with the red one. If the meter shows a short circuit, remove the probes and connect all three electrodes with a flat screwdriver. The goal is to discharge the capacitive junction of the gate, it may have been charged. Then repeat the measurement of the channel resistance. If the device still shows a short circuit, then the transistor is faulty and must be replaced.
If the device shows a resistance close to infinity, then check the gate transition. It is checked in the same way as a channel transition. Touch any probe of the electrode "source" of the transistor, and with the other touch the electrode "gate". The resistance must be infinitely great. The insulated gate is not electrically connected to the channel of the transistor and any resistance detected in this circuit indicates a malfunction of the transistor.
The procedure for checking a fully functional transistor looks like this: Touch the black probe of the ohmmeter to the "source" electrode of the transistor, touch the red probe of the "gate" electrode. The resistance should be infinitely high, then, without closing the "gate" to the other electrodes, touch the "drain" electrode with the red probe. The device will show a small resistance in this area. The value of this resistance depends on the voltage between the ohmmeter probes. Now touch the "source" electrode with the red probe, repeat the above procedure. The channel resistance will be very high, close to infinity. The method of testing a MOS-FET transistor with a p-channel differs in that during measurements it is necessary to change the red and black ohmmeter probes between themselves.