To check a transistor with a multimeter, set your meter to Diode Test mode and measure the voltage drop across the semiconductor junctions. A healthy silicon Bipolar Junction Transistor (BJT) will display a forward voltage drop between 0.500V and 0.750V across the base-emitter and base-collector junctions, and an "OL" (Open Loop) reading when reverse-biased. For MOSFETs, you will test the intrinsic body diode and the gate capacitance.
While modern digital multimeters (DMMs) like the Fluke 87V or Brymen BM235 often feature a dedicated "hFE" transistor gain socket, that socket only works for small-signal through-hole parts and cannot test power transistors, MOSFETs, or components soldered to a board. The diode test method is the universal, bench-standard technique used by technicians to verify junction integrity.
Multimeter Setup and Bench Safety Requirements
Before probing any semiconductor, configure your meter to supply the correct test current without risking damage to the component or the meter itself.
Meter Configuration
- Dial Position: Set to Diode Test (symbol:
->|). Do not use the Ohms (Ω) range, as the test voltage on some older analog and cheap digital meters in resistance mode may not be high enough to forward-bias a silicon junction (which requires ~0.6V). - Lead Jacks: Black lead in COM, Red lead in V/Ω.
- Range: Auto-ranging is standard. If manual, set to the lowest DC voltage or millivolt range capable of reading up to 3.0V.
Testing Bipolar Junction Transistors (NPN and PNP)
A BJT (such as the common 2N3904 NPN or 2N3906 PNP) functions electrically as two back-to-back diodes sharing a common anode or cathode (the Base). By treating the Base-Emitter (B-E) and Base-Collector (B-C) junctions as discrete diodes, we can map the health of the silicon die.
Step-by-Step Probe Placement
- Identify the Pinout: Consult the datasheet. For a standard TO-92 package held with the flat side facing you and pins pointing down, the pins are typically Emitter (E), Base (B), Collector (C). For TO-220 power packages (tab facing you), it is usually Gate/Base, Drain/Collector, Source/Emitter.
- Forward Bias Test (NPN): Place the Red probe on the Base and the Black probe on the Emitter. Note the reading. Move the Black probe to the Collector. Both readings should be between 0.500V and 0.750V.
- Reverse Bias Test (NPN): Swap the probes. Place the Black probe on the Base and the Red probe on the Emitter, then the Collector. Both must read "OL".
- Collector-Emitter Check: Place probes across C and E in both polarities. Both must read "OL". A reading of 0.000V or a low resistance indicates a shorted, destroyed die.
For PNP transistors, simply reverse the probe polarities: the Black probe goes on the Base for the forward bias test, and the Red probe goes on the Base for the reverse bias test.
Expected Readings: Silicon BJT Junction Matrix
The table below defines the exact numerical thresholds for pass/fail conditions on standard silicon BJTs. Keep this matrix handy at your bench.
| Junction Tested | NPN Forward (Red on Base) |
NPN Reverse (Black on Base) |
PNP Forward (Black on Base) |
PNP Reverse (Red on Base) |
Failure Mode Indicated |
|---|---|---|---|---|---|
| Base to Emitter | 0.550V - 0.750V | OL (Open) | 0.550V - 0.750V | OL (Open) | Short (<0.100V) or Open (OL both ways) |
| Base to Collector | 0.550V - 0.750V | OL (Open) | 0.550V - 0.750V | OL (Open) | Short or Open junction |
| Collector to Emitter | OL (Open) | OL (Open) | OL (Open) | OL (Open) | Short (0.000V) means blown thermal die |
| Darlington B-E (e.g., TIP120) | 1.100V - 1.400V | OL (Open) | 1.100V - 1.400V | OL (Open) | Misdiagnosed as "Open" if expecting 0.6V |
Note: The Collector-Base junction typically reads 10mV to 30mV higher than the Base-Emitter junction due to differences in doping concentrations during manufacturing. This is normal and indicates a healthy, authentic part.
Checking MOSFETs and the Body Diode
Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) like the IRFZ44N or IRF520 do not have a base-emitter diode. Instead, they feature a high-impedance capacitive Gate and an intrinsic body diode connected between the Drain and Source. According to Vishay's application notes on MOSFET handling, the gate oxide layer is highly sensitive to electrostatic discharge, meaning your multimeter's diode test voltage (usually ~3V) is perfectly safe and sufficient to charge the gate capacitance.
N-Channel MOSFET Test Sequence
- Discharge the Gate: Briefly short the Gate (G) to the Source (S) with a probe tip or a 10kΩ resistor. This bleeds off any residual gate charge, ensuring the MOSFET is in the "OFF" state.
- Test the Body Diode (OFF state): Place the Red probe on the Drain and the Black probe on the Source. You should read OL. Swap the probes (Black on Drain, Red on Source). You should read the body diode forward drop: typically 0.400V to 0.600V.
- Charge the Gate (Turn ON): Leave the Black probe on the Source. Touch the Red probe to the Gate for one second. The meter's internal voltage will charge the gate capacitor, turning the channel ON.
- Verify Conduction (ON state): Move the Red probe back to the Drain (Black remains on Source). Because the MOSFET is now turned ON, the channel resistance drops to milliohms. The meter should read 0.000V to 0.050V (or a very low resistance, depending on your meter's diode test resolution).
- Reset: Short Gate to Source again to turn it off and verify the Drain-Source path returns to OL in the forward direction.
For P-Channel MOSFETs, reverse the polarities: charge the gate by applying Black to Gate and Red to Source, and test the body diode with Red on Source and Black on Drain for the forward drop.
Misleading Readings and In-Circuit Pitfalls
The most common reason a technician incorrectly condemns a good transistor—or passes a bad one—is failing to account for the surrounding circuit topology. Fluke's official testing guidelines heavily emphasize that out-of-circuit testing is the only way to guarantee a definitive verdict. If you must test in-circuit, watch for these specific measurement traps:
Parallel Resistance Paths
If a 1kΩ resistor is wired in parallel with the Base-Emitter junction (common in biasing networks or as a pull-down), your multimeter will read the voltage drop across the resistor, not the silicon junction. Since the DMM's diode test outputs roughly 1mA to 2mA, a 1kΩ resistor will yield a reading of ~1.0V to 2.0V, or simply clamp the meter's open-circuit voltage, resulting in a false "OL" or an out-of-spec reading. Rule of thumb: If an in-circuit B-E reading is below 0.500V or perfectly matches a parallel resistor's calculated drop, desolder at least one leg of the transistor to isolate it.
The Darlington Pair Confusion
Power Darlingtons like the TIP120 (NPN) or TIP125 (PNP) contain two transistors wired in series on the same die. Because the current must pass through two base-emitter junctions, the forward voltage drop is additive. A healthy TIP120 will read 1.100V to 1.400V on the Base-Emitter test. Technicians expecting the standard 0.650V often misdiagnose these as "open" or "failed" and throw them away. Always check the datasheet to see if the part number denotes a Darlington configuration.
Germanium vs. Silicon
While rare in modern designs, vintage audio equipment and RF circuits still use germanium transistors (e.g., AC128, 2N1304). Germanium has a much lower bandgap than silicon. A good germanium BJT will show a forward voltage drop of 0.150V to 0.300V. Applying silicon thresholds to germanium parts will lead to false "shorted" diagnoses.
Skin Resistance and MOSFET Gates
The gate of a MOSFET is essentially a capacitor separated by an ultra-thin insulating layer of silicon dioxide. It has nearly infinite DC resistance. If you touch the Gate and Source pins simultaneously with your bare fingers while probing, the moisture and salts on your skin create a high-value resistive path (often 100kΩ to 1MΩ). This will slowly drain the gate charge you just applied with the meter, causing the MOSFET to appear as though it is failing to hold the "ON" state, or yielding erratic, drifting mV readings on the Drain-Source path. Always handle MOSFETs by the package body, not the leads, during testing.
For deeper theoretical background on junction behaviors and doping profiles, Electronics Tutorials provides excellent cross-sections of how these physical layers dictate the multimeter readings you see on the bench.
By strictly following the diode test matrix, accounting for Darlington voltage additions, and isolating components from parallel PCB traces, you can reliably check any BJT or MOSFET with a standard digital multimeter in under sixty seconds.
