Testing a MOSFET with a multimeter feels like testing a capacitor, and that feeling is the clue. A good MOSFET holds a charge on its gate like a capacitor, the body diode can restore that charge, and a shorted part reads like a diode in both directions — the fingerprint of the failed device. This article explains why the gate-capacitance test works, maps gate, drain, and source on common packages, shows the body-diode check and its limits, works the short and leak detection the meter can prove, and closes by separating the MOSFET test from the BJT test.
Why a MOSFET Test Feels Like a Capacitor Test
The first surprise of MOSFET testing is that a good part does not read like a diode at all; it reads like a tiny capacitor. The reason is the gate structure. The MOSFET’s gate is a metal layer on a thin insulating oxide above the semiconductor; between gate and body, the structure is literally a capacitor — the oxide is the dielectric, the gate is one plate, and the body is the other.
When a meter touches the gate, it charges that capacitor; the meter’s test voltage deposits charge on the gate, and the part stores it. A good MOSFET therefore shows a charging response — the meter reading rises or falls as the gate capacitor charges, and it discharges back when the probe is removed. This capacitance sign is the healthy fingerprint, and it is the basis of the whole test method. The MOSFET datasheet article and the power MOSFET selection guide develop the structure this article tests; the NPN transistor test article covers the sibling family, and the final section here separates the two meter behaviors.
The practical test habit is to look for the charging sign, not the zero reading. A meter that charges the gate then lets it discharge is seeing a healthy MOSFET; a meter that reads open with no charge or discharge is seeing a broken gate path; a meter that reads like a diode both ways is seeing a shorted part. The capacitance sign is the anchor the rest of the tests read against.
Pin Mapping: Gate, Drain, Source on Common Packages
The gate-capacitance test only works if the pins are identified correctly, and the pin mapping is the first skill. The three terminals — gate, drain, source — appear on common packages in recognizable patterns.
On a typical through-hole or D2PAK package, the gate is the terminal that charges like a capacitor, the drain and source are the terminals of the body. On an axial-flavored package the body terminals map by the package convention, and on the SMD packages the pin layout follows the datasheet drawing. The reliable mapping habit is to identify the gate by its capacitance behavior first — the terminal that charges and discharges — and then identify the drain and source relative to it, using the body diode as the reference. The MOSFET datasheet article and the MOSFET polarity article give the pin conventions and the polarity meaning of the body.
The mapping mistake costs the whole test: a meter touching the wrong terminals reads a capacitor or a diode that is not the part, and the verdict is nonsense. The mapping is the safety rail for everything that follows, and it is worth the thirty seconds it takes to confirm before testing.
The expected reads are compact enough for a table, and the table is the bench reference:
| Test | Healthy N-channel | Broken / shorted | What the meter shows |
|---|---|---|---|
| Gate to body | Charges then discharges | No charge, or low-resistance leak | Capacitance sign appears and decays |
| Drain to source (forward) | Body diode conducts | Conducts both ways | One-way read is healthy |
| Drain to source (reverse) | Body diode blocks | Conducts both ways | Both-way read is a short |
| Gate to drain | Charges like a capacitor | Low resistance | The short is visible on the gate |
The table is the method in one view: each probe placement has a healthy read and a broken read, and the broken read is usually the opposite of healthy rather than a subtler variant. The power MOSFET selection guide and the MOSFET polarity article give the N-channel and P-channel directions that the table’s polarity column assumes.
The Body-Diode Check and Its Limits
The second reading is the body diode. Every MOSFET has a body diode between the drain and the source — the junction that forms between the semiconductor and the base — and it is the most diode-like reading the part gives.
On an N-channel part, the body diode conducts from source to drain: the meter in diode mode reads conduction in one direction and blocking in the other. That one-way reading is a healthy sign. The body-diode check is fast and useful, but it has limits that the test must respect. First, the body diode’s VF is a fraction of a volt to a couple of volts, and the meter’s diode mode must be able to read it. Second, the body diode is not the whole MOSFET: a part with a healthy body diode can still have a broken gate or a failed channel, because the two structures fail independently. Third, the body-diode reading is polarity-sensitive, so the N-channel and P-channel parts read in opposite directions. The MOSFET polarity article and the BJT test article cover the direction and the family distinction.
The body-diode check is a screen, not a verdict. A part with no body diode is definitely broken; a part with a healthy body diode needs the gate-capacitance and short checks to complete the story. The three tests together are the full meter method, and the limits of each keep the verdict honest.
Detecting Shorts and Leaks: What the Meter Can Prove
The short detection is where the meter earns its keep, and the shorted MOSFET has a distinctive fingerprint: it reads like a diode in both directions. The explanation is that a short between the gate and the channel, or between the drain and the source, turns the part into a conductive path that the meter sees as conduction either way.
The leak detection is the subtler read. A leaky gate, a damaged oxide, or a degraded channel shows as a resistive path where a healthy part reads charging or open. The ohms-continuity check on the gate, held against the known behavior, exposes the leak: a gate that should read as a capacitor but reads as a low resistance is leaking. The MOSFET testing and the power discrete package testing article cover the continuity and leak checks in the production-inspection context, and this article applies them to the bench.
The honest boundary of the meter test is that it proves open-circuit, continuity, and diode behavior, not dynamic performance. A part that passes the meter is a structurally intact part; it is not proven to switch at speed or carry its rated current. The meter is the first gate of the diagnosis, and the dynamic checks belong to the bench that can apply load.
The in-circuit caveat completes the bench picture. Testing a MOSFET while it is still soldered to a board reads the parallel path of the surrounding circuit, not the bare part, so the capacitor sign and the body-diode read can be masked by the neighboring components. The practical rule is to lift one lead or desolder the part before trusting a verdict, or to treat the in-circuit read as a screen that points to the suspect part rather than a proof. The power discrete package testing article documents the in-circuit and out-of-circuit distinction in the production-inspection context, and it is the same caution the repair bench follows when a loaded board reads inconclusively.
MOSFET vs BJT Meter Tests: When Ohms Confuses
The final distinction is the one that confuses the most benches: the MOSFET test versus the BJT test. The two families are tested differently, and using the wrong method produces a wrong verdict.
The BJT is tested as a pair of junctions: base, collector, and emitter read in diode mode with the base-collector and base-emitter junctions behaving like diodes. The MOSFET is tested as a capacitor plus a body diode: gate, drain, and source read with the gate as a capacitive terminal and the drain-source as a diode. The confusion arises in ohms mode, where a multimeter’s continuity test can mislead: the BJT’s junctions read conductive in one direction, and the MOSFET’s body diode also reads one-way, but the gate terminal has no diode equivalent in the BJT. The BJT test article and the MOSFET selection guide document the two families; the bench habit is to name the family before the test.
The rule that keeps ohms from confusing: if the part has a terminal that charges like a capacitor, test it as a MOSFET; if it shows two diode junctions and no gate capacitance, test it as a BJT. The meter method follows the structure, and the structure names the verdict.
The MOSFET meter test is a three-part method built on one physical fact: the gate is a capacitor, the body is a diode, and a short reads like conduction both ways. Map the pins, watch the charging sign, check the body diode with its limits, and read the shorts and leaks the meter can prove. Do not confuse the gate-capacitance part with the BJT’s junction pair, and the multimeter becomes a fast, honest first gate for every MOSFET that lands on the bench. The power MOSFET category supplies the parts whose structure this guide tests, and the MOSFET selection guide completes the family context.