TESTING FETs
Testing a field effect transistor is more complicated than testing a normal transistor. The following points must be considered before actually testing an FET:
1. Is the device a JFET or a MOSFET?
2. Is the FET an N-channel device or a P-channel device?
3. With MOSFETs, is the device an enhancement mode device or a depletion mode device?
Before removing an FET from a circuit or handling it, check to see whether it is a JFET or a MOSFET. MOSFETs can be damaged easily unless certain pre- cautions in handling are followed.
1. Keep all of the leads of the MOSFET shorted until ready to use.
2. Make sure the hand used to handle the MOSFET is grounded.
3. Ensure that the power to the circuit is removed before insertion or removal of the MOSFET.
Both JFETs and MOSFETs can be tested using commercial transistor test equipment or an ohmmeter. If using commercial transistor test equipment, refer to the operations manual for proper switch settings.
Testing JFETs with an Ohmmeter
1. Use a low-voltage ohmmeter in the R X 100 range.
2. Determine the polarity of the test leads. Red is positive and black is negative.
3. Determine the forward resistance as follows:
a. N-channel JFETs: Connect the positive lead to the gate and the negative lead to the source or drain. Because a channel connects the source and drain, only one side needs to be tested. The forward resistance should be a low reading.
b. P-channel JFETs: Connect the negative
lead to the gate and the positive lead to the source or drain.
4. Determine the reverse resistance as follows:
a. N-channel JFETs: Connect the negative test lead of the ohmmeter to the gate and the positive test lead to the source or drain. The JFET should indicate an infinite resistance. A lower reading indicates a short or leakage.
b. P-channel JFETs: Connect the positive test
lead of the ohmmeter to the gate and the negative test lead to the source or drain.
![clip_image002[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2016/02/clip_image0021_thumb.gif "clip_image002[1]")
Testing MOSFETs with an Ohmmeter
![clip_image002[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2016/02/clip_image0021.gif "clip_image002[1]"){kind=small}
The forward and reverse resistance should be checked with a low-voltage ohmmeter on its highest scale. MOSFETs have extremely high input resistance because of the insulated gate. The meter should register an infinite resistance in both the forward- and reverse-resistance tests between the gate and source or drain. A lower reading indicates a breakdown of the insulation between the gate and source or drain.
QUESTIONS
1. What questions must be answered before actually testing an FET?
2. Why is it important to know whether a device is a JFET or a MOSFET before removing it from a circuit?
3. Describe how to test a JFET using an ohmmeter.
4. Describe how to test a MOSFET using an ohmmeter.
5. What procedure is used to test either a JFET or MOSFET with a commercial transistor tester?
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