So your circuit's acting up. Maybe your LED driver went dim, maybe your buck converter let the magic smoke out, or maybe that motor controller just stopped making that satisfying whirring noise. Before you blame the microcontroller or start swapping out capacitors, there's a very good chance the culprit is that silent workhorse of modern electronics: the MOSFET. It's a three-legged device that can fail in spectacular silence. I can't tell you how many times a simple multimeter test has saved me from redesigning an entire board. So let's stop guessing and start measuring how to know if your MOSFET is broken or shorted.
You can't just eyeball a MOSFET and tell if it's fried, though sometimes they do literally crack or pop. Honestly, most failures are invisible. The most common failure mode is a short circuit between the drain and source. Think of it like a stuck valve that's always open. That means current flows constantly, components get hot, and your circuit tries its best to become a soldering iron. The second most common failure is a gate that's been zapped by static or overvoltage, leaving the device in a partially on or completely off state. My aim here is to give you a rock-solid, practical method to figure out, once and for all, if that MOSFET is broken or shorted.
We're going to rely on the single most powerful tool in your box for this job: the humble digital multimeter. You don't need an oscilloscope or a curve tracer for 90% of these checks. You just need diode mode. If your meter doesn't have a diode test setting (look for the little arrow with a vertical line), you can use resistance mode, but diode mode is far more revealing. Why? Because a MOSFET has a built-in parasitic diode between the source and drain. When the MOSFET is broken or shorted, that diode often disappears or behaves erratically. Let's get into the specific steps.
The Multimeter Method: Your First Line of Defense
This isn't theory. This is how I check parts fresh out of a blown power supply. First, understand the pinout. Most of the time, when looking at the front of a TO-220 package (the most common through-hole type), the left pin is the gate, the middle pin is the drain, and the right pin is the source. But don't trust your memory. Look at the datasheet or Google the part number quickly. I've shorted a few perfectly good FETs by assuming the pinout. Seriously, it happens. Set your meter to diode mode.
Why Diode Mode is Your Best Friend
A healthy N-channel MOSFET will show a specific signature. Place your black probe on the source (middle or right pin, check your datasheet) and your red probe on the drain. You should see a reading between 0.4V and 0.7V. That's your internal body diode acting like a standard silicon diode. Now, reverse the probes: red on source, black on drain. You should read 'OL' or '1', which means open line, or very high resistance. Lookâthis simple test alone catches about 70% of failures. If you see a short (0.000V, or near-zero resistance in both directions), the MOSFET is shorted. It's done. Grab a new one.
But what if you get an 'OL' in both directions? That could mean the internal diode is blown open, or the gate is holding the channel closed. It could also just be a very high voltage device or a different type of FET. Don't toss it yet. There's a deeper check we need to do. A single diode test is fast, but it's not foolproof for every failure mode.
Step-by-Step: Checking the Body Diode
Let's walk through this methodically so you don't miss anything.
1. Remove the MOSFET from the circuit. I know it's a pain, but in-circuit testing is often useless. Your meter is reading the rest of the board, not just the FET. Desolder it or clip it out. It's the only way to be sure. 2. Discharge the gates. Touch the gate pin to the source pin with a screwdriver or a resistor. MOSFET gates are tiny capacitors, and they can hold a charge that will trick your meter. This step is crucial. If you skip this, a stored charge can make a good FET look broken or shorted. 3. Test the diode again. Black probe on source (N-channel), red on drain. You want that 0.4V to 0.7V reading. Reverse the probes. You want OL. If you get short in both directions, it's dead. If you get OL in both directions, the body diode is likely gone. That's a fail. 4. Check for Gate-Source shorts. Set your meter to high resistance (like 200k or 2M ohm range). Put probes on gate and source. You should see infinite resistance (OL). Any resistance below about 10k ohms usually indicates a damaged gate oxide layer. The MOSFET is broken and unreliable.
The Hidden Killers: Gate Leakage and Partial Shorts
Here's where it gets tricky. Sometimes a MOSFET passes the diode test but still fails under load. I've seen gate leakage that reads perfectly fine with a 9V battery in a multimeter but collapses the moment you put 12V on it. We call these 'soft failures' or 'weak gate' failures. They are a nightmare to trace because they only show up when the part is active. The device isn't fully shorted, but it's certainly broken in a way that causes heat and inefficiency.
A classic sign is a circuit that works at low power but dies when you crank it up. The MOSFET gets hot, but not from a dead short. It heats up from being partially on. Its resistance (Rds(on)) has drifted up. You can't reliably measure Rds(on) with a standard multimeter on an unpowered part because it requires turning the gate on first. But you can check for gate leakage, which is a leading cause of this behavior.
How to Test the Gate Without Blowing It Up
The gate oxide layer is incredibly fragile. It's a layer of glass that can be punctured by a static shock as low as 30 volts. When that happens, a tiny hole appears, and the gate starts to leak current. To test this properly, you need the high resistance setting on your meter.
1. Set your meter to the highest ohms range. 20M or 200M if you have it. 2. Place the black probe on the source. 3. Place the red probe on the gate. 4. Wait for the reading to stabilize. It should climb to OL. If it stops at a finite value, like 5M ohms, that gate is compromised. It will likely fail completely soon. 5. Now test the gate to drain. Same procedure. Red on gate, black on drain. You want OL.
It's a big deal when that gate fails. Usually, a shorted gate to drain means the FET is permanently turned on. It'll be shorted from drain to source even without voltage on the gate. In contrast, a leaky gate to source often makes the FET behave like a resistor instead of a switch. It will conduct but poorly. This is a classic sign of a broken MOSFET that is not completely dead.
The 'Wiggling' Trick for Intermittent Failures
Sometimes a MOSFET only fails when it gets hot. You can't always test that at your bench. But here is a trick I learned from an old Avionics tech. Grab a can of freeze spray and a hot air gun. First, heat up the MOSFET to about 80 degrees Celsius (use a thermocouple or just wait until it's too hot to touch comfortably). Then test the diode immediately. If the diode reading drops or disappears, the part has a thermal failure. It's broken. Conversely, cooling it down can sometimes bring a shorted part back temporarily. If you freeze a shorted MOSFET and the short goes away, that part is absolutely toast. It's a ticking time bomb. Toss it.
Another intermittent gremlin is mechanical damage. A tiny crack in the die can create a 'wiggle' short. If you gently push on the package with a non-conductive tool while measuring the drain-source resistance, and you see the reading jump, the internal bond wires or the silicon die itself is physically damaged. That MOSFET is broken. No ifs, ands, or buts. This method has saved me from re-flowing a perfectly good board only to find the problem was a cracked component all along.
To summarize the key failure signatures in a list:
- Shorted (D to S): Near-zero resistance between drain and source in both directions. Diode test reads 0.000V. Dead. Replace immediately.
- Open Body Diode: Diode test reads OL in both directions. No rectification path. Part is broken internally.
- Leaky Gate: Finite resistance (less than 10M ohm) from gate to source or gate to drain. FET will turn on partially or be inconsistent. Unreliable.
- Thermal Failure: Passes cold tests but fails (shorts or opens) when heated. The heat is the trigger. Part is broken.
- Gate Short to Source: Zero resistance from gate to source. No gate control. Typically results in a permanently off or permanently shorted condition depending on design.
When to Throw in the Towel
There is a certain point where the diagnostic time costs more than the part. A $2 MOSFET isn't worth an hour of your weekend. If you've done the diode test, checked for a gate short, and the part is warm to the touch while the circuit is off, just replace it. Seriously. I have a rule: if a MOSFET fails the three basic checks (diode, gate-source resistance, gate-drain resistance), I don't try to 'save' it. I cut the leads and move on.
But there's another scenario I see often: a MOSFET that tests perfectly fine on the bench but fails in the circuit. This almost always points to a driver issue or a voltage spike on the drain. The MOSFET itself isn't broken, but its operating conditions are exceeding its limits. Look at the gate drive voltage. Are you driving a 10V logic-level FET with 3.3V? The FET is barely turning on, it's running hot, and you're blaming the part. The part is fine; the design is broken. Always suspect the environment before you damn the component.
One last tip: use a component tester. You can get a cheap transistor tester for about $20 on any online marketplace. It identifies the pins, tests the capacitance of the gate, and measures the threshold voltage (Vth). If the Vth is way off from the datasheet spec, the part has degraded. It's broken internally even if the diode is intact. These testers are a lifesaver when you have a handful of identical MOSFETs and you need to weed out the one that's slightly shorted. They're not perfect, but they're far better than a multimeter for deep analysis.
Common Questions About Testing If Your MOSFET Is Broken or Shorted
Can I test a MOSFET while it's still soldered to the board?
You can try, but it's unreliable. The multimeter will measure the entire circuit path, not just the MOSFET. If you get a short while in-circuit, the MOSFET is likely shorted. But if you get a good reading, the FET could still be bad. You have to desolder it for a definitive pass/fail verdict. That's just the nature of the job.
What does a shorted MOSFET look like when powered?
It usually gets very hot, very fast. You might see a constant current draw on the power supply. In a switching converter, it will stop regulating and the output voltage might equal the input voltage (if it's a top-side FET). In a motor driver, the motor might run at full speed all the time and you can't turn it off. It's a classic 'stuck on' failure.
Can a MOSFET be broken but not shorted?
Absolutely. That's the 'gate leakage' or 'high Rds(on)' failure mode. The device still switches but it runs inefficiently and overheats. The multimeter diode test will pass, but the gate may have a small puncture. These are the hardest broken parts to find without a dedicated curve tracer or thermal camera.
How do I test a P-channel MOSFET differently?
You reverse the probes on the diode test. For a P-channel FET, place the red probe on the source and the black probe on the drain to see the body diode. A P-channel is just the mirror image of N-channel. All the same principles apply: look for a diode drop one way and high resistance the other. If it shows a short in both directions, it's shorted.
My MOSFET gets hot but doesn't short. Is it broken?
It could be, but first check the gate drive voltage. If the gate isn't being driven to the full recommended voltage (usually 10V for standard FETs, 5V for logic-level), the MOSFET will operate in its linear region, acting more like a resistor than a switch. That heat is normal behavior under those conditions. Fix the driver first. If the driver is good and the MOSFET still overheats at low load, then yes, it is likely broken with elevated Rds(on).