Transistors are switches and amplifiers, and on a laptop board they are everywhere: small bipolar transistors and MOSFETs switching signals and enabling rails, and large power MOSFETs in every voltage regulator, in the charger and in the battery path. A multimeter cannot fully characterise a transistor, but its diode mode can reveal the faults that matter in repair — shorted, open or leaking junctions. This guide shows how to test both kinds, out of circuit and on the board.
The two families
| Family | Pins | Controlled by | Where on a laptop board |
|---|---|---|---|
| Bipolar transistor (BJT), NPN or PNP | Base, collector, emitter | Base current | Small signal switching, level shifting, LED drivers |
| MOSFET, N-channel or P-channel | Gate, drain, source | Gate voltage | Power stages, load switches, battery and adapter paths, signal switching |
On modern laptop boards MOSFETs far outnumber BJTs. Both use the designator Q (or PQ in power sections), and both come in small three-pin SOT packages; power MOSFETs use larger flat packages with several drain and source pins, often with two MOSFETs inside one package.
The pinout of a specific part is in its datasheet and is usually shown on the schematic symbol. Never assume the pin order from the package alone.
Testing a bipolar transistor
A BJT behaves like two diodes sharing the base:
- NPN — two diodes pointing away from the base: base-to-collector and base-to-emitter.
- PNP — two diodes pointing towards the base.
Out of circuit, in diode mode:
- NPN: red on the base, black on the collector — a drop of roughly 0.5–0.7 V.
- Red on the base, black on the emitter — a similar drop, usually slightly higher than base-collector.
- Black on the base, red on either other pin — OL.
- Collector to emitter, both directions — OL.
For a PNP transistor, swap the probe colours in steps 1–3. Collector to emitter is still OL both ways.
| Reading | Meaning |
|---|---|
| Two drops from base, OL elsewhere | Good |
| Near zero on any pair | Shorted junction |
| OL where a drop should be | Open junction |
| Any reading collector–emitter | Leakage or short |
Some meters have an hFE socket that measures the gain of a loose BJT. It confirms that the transistor amplifies, but in repair work the junction test is usually enough. More on this family in the glossary entry for bjt.
Testing a MOSFET
A MOSFET has an insulated gate and a built-in body diode between source and drain. That structure gives a recognisable pattern.
N-channel MOSFET, out of circuit
- Discharge the gate — touch gate, drain and source together with a probe tip or tweezers. A MOSFET's gate can hold charge and stay switched on.
- Body diode — red on the source, black on the drain: a drop of roughly 0.4–0.7 V.
- Reverse — red on the drain, black on the source: OL.
- Gate — gate to source and gate to drain, both directions: OL. The gate is insulated.
The body diode is explained in the glossary entry for body diode.
The gate-charge trick
You can switch a loose N-channel MOSFET on with the meter itself:
- Black on the source, red on the gate for a moment — the meter's test voltage charges the gate.
- Now measure drain to source in either direction — it should read low, because the MOSFET is switched on.
- Short the gate to the source to discharge it — the drain-source reading returns to the body-diode pattern.
This works when the meter's diode-mode voltage is above the MOSFET's gate threshold, which is true for most logic-level parts used on laptop boards. If the trick does not work, the MOSFET may still be fine — its threshold may simply be higher than your meter's test voltage.
P-channel MOSFET
Everything is mirrored: the body diode conducts with red on the drain and black on the source, and the gate is charged with the black probe on the gate and the red probe on the source.
Testing MOSFETs in circuit
Removing every MOSFET to test it is impractical. On the board, these checks find most failures:
- Drain to source, both directions — a reading near zero in both directions usually means a shorted MOSFET. Power MOSFETs most often fail this way.
- Gate to source and gate to drain — on a healthy MOSFET these should not be near zero. A low reading means the gate oxide may be damaged, or there is a low-value resistor or a driver output on the gate net; check the schematic.
- Comparison — measure the same pins on an identical MOSFET elsewhere on the board, for example the other phase of a multi-phase regulator. Matching readings are a strong sign both are fine.
The trap: in a buck regulator, the high-side MOSFET's drain sits on the input rail and the low-side MOSFET's source on ground, and they share the switch node. A short between the switch node and ground could be the low-side MOSFET — or anything else on that node. Lift or remove the part to confirm before replacing it.
The power-section side of this, including load switches and the battery path, is covered in detail in MOSFETs on Laptop Motherboards: How to Test Them.
Dual and multi-pin packages
Many power packages hold two MOSFETs, sharing pins or not. Each 8-pin flat package may have four source pins, three gate-and-source pins, and a large drain pad underneath. Use the datasheet pinout: measure each MOSFET in the package separately, and remember that pins with the same function are joined inside.
How transistors fail
- Drain-source or collector-emitter short — the most common failure in power stages, after overcurrent, overheating or a failed controller.
- Gate damage — a gate short to source or drain after static discharge or overvoltage; it often takes the driver with it.
- Open — less common; after a short burned an internal bonding wire.
- Partially on — a damaged MOSFET that conducts slightly when it should be off, creating leakage that shows as a warm part or a rail that does not discharge.
A shorted MOSFET is usually a symptom as much as a cause. If a power-stage MOSFET failed, check the controller, the gate drive and the load before powering the board with a replacement.
Example: a dead system rail regulator
A board has no system rail. With power removed, the switch node of the regulator reads almost zero ohms to ground. The schematic shows a dual MOSFET in one package, with the low-side MOSFET between the switch node and ground. Its drain-to-source reading is near zero in both directions; the identical package on another regulator reads the normal body-diode pattern. After removal, the switch node reads normally and the old part reads shorted on the bench. A new dual MOSFET restores the rail — after confirming the controller's gate outputs are not shorted, which would have destroyed the new part.
Replacing a transistor
For MOSFETs, match the channel type, the package and pinout, the voltage rating, the current rating, and ideally the on-resistance and gate threshold. For power stages, an exact or closely matching part is safest. For small signal BJTs and MOSFETs, a part with the same pinout and similar ratings usually works, but check the gate threshold of a MOSFET if it is driven from a low-voltage logic signal. Testing diodes, whose junctions behave like those in transistors, is covered in How to Test a Diode with a Multimeter: Schottky, Zener, TVS and LED.
FAQ
Why does a good MOSFET sometimes read low between drain and source?
Its gate may be charged, leaving it switched on. Short the gate to the source and measure again. In circuit, other parts on the same nets can also create a low-resistance path.
Can I test a MOSFET without knowing its pinout?
You can guess by finding the body diode and the insulated gate, but it is slow and error-prone. Look up the datasheet or the schematic symbol first.
Does a working body diode mean the MOSFET is good?
Not entirely. The body diode can be fine while the gate is damaged or the channel does not switch on properly. The gate-charge trick checks switching; a working circuit is the final test.
Why did my replacement MOSFET fail immediately?
Usually because the original cause is still present — a shorted load, a faulty controller driving both MOSFETs on, or a gate driver output stuck high. Check those before fitting another part.
What is the hFE setting for?
It measures the current gain of a loose bipolar transistor in a small socket. It confirms that a BJT amplifies, but junction tests are enough for most board repair.
Bottom line
A BJT is two diodes around the base; a MOSFET is an insulated gate plus a body diode. Out of circuit, diode mode and the gate-charge trick confirm both. On the board, look for drain-source shorts, compare with identical parts, and always find out why a power MOSFET failed before replacing it. Keep the MOSFET basics in mind, and most transistor faults on laptop boards become straightforward to find.