How to Test Field-Effect Transistors (FETs) with an Analog Multimeter

Using a simulated multimeter to test field-effect transistors (FETs) is a practical skill for engineers and technicians. This guide outlines the steps for identifying and verifying FET functionality, with a focus on junction FET (JFET) and metal oxide semiconductor FET (MOSFET).

Step 1: Identify FET type and pins

Firstly, determine whether the FET is a JFET or a MOSFET. JFET has a PN junction between the gate and channel, while MOSFET has an insulated gate. Use the R × 100 Ω or R × 1k Ω range of a multimeter to identify pins:

JFET： Connect the red bezel to the gate (G) and the black bezel to the source (S). A low resistance (~1k Ω) indicates a good PN junction. For MOSFETs, due to gate insulation, skip this step.

MOSFET： Using the R × 10k Ω range, charge the gate by touching the black chip to G and the red chip to S. This will create a conductive channel.

Step 2: Test the leakage source conductivity

Switch to the R × 1 Ω range, connect the black bezel to the drain (D), and connect the red bezel to S:

JFET： If the FET is an N-channel, the resistance should be very low (~10 Ω to 1k Ω). For P-channel JFET, reverse its polarity.

MOSFET： After charging the gate (step 1), the D-S resistance should significantly decrease (~10 Ω to 100 Ω), indicating that the channel formation is correct. If the resistance is still high, the FET may open or be damaged.

Step 3: Check the gate source insulation

Test whether the gate source (G-S) junction is short circuited or leaking:

JFET： Use R × 10k Ω range. A low resistance (<1M Ω) indicates a fault in the PN junction.

MOSFET： Measure the G-S resistance using R × 10k Ω. A reading below 10M Ω indicates damage to the gate oxide.

Safety precautions

ESD protection: MOSFETs are sensitive to electrostatic discharge (ESD). Always ground yourself and the device before operation.

Power off: Ensure that the circuit is powered off to prevent damage during testing.

Avoid overheating: Long term testing under high current may cause the FET to overheat.

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By following these steps, you can effectively diagnose FET issues using an analog multimeter. For precision measurement, please consider YFW Dior's advanced semiconductor solutions. Visit www.yfwdiode.com to learn about our product line