The Direct Answer: Testing BJTs and MOSFETs in 60 Seconds

To check an NPN bipolar junction transistor (BJT), set your digital multimeter to Diode Test mode. Place the red probe on the Base and the black probe on the Emitter; a healthy silicon transistor reads between 0.55V and 0.75V. Swap the probes (black to Base, red to Emitter), and it should read 'OL' (open loop). Repeat this between the Base and Collector. If any junction reads 0.00V or emits a continuous continuity beep, the transistor is shorted and dead. If it reads 'OL' in both directions, the junction is open and dead.

For an N-channel MOSFET, the test is slightly different because you are checking the intrinsic body diode rather than a PN junction. With the meter in Diode Test mode, place the black probe on the Source and the red probe on the Drain. You should read a forward voltage drop of about 0.4V to 0.6V. Swap the probes, and it should read 'OL'. If it reads near 0V in both directions, the drain-source channel is blown short.

Bench Tip: Always remove the transistor from the circuit before testing. Parallel components like bleeder resistors, flyback diodes, and transformer windings will create false low-resistance readings and mask a dead semiconductor.

Understanding the Pins: Symbol, Pinout, and Safe Default Part Numbers

Before you touch the probes to the metal, you need to know what you are probing. Transistors come in two primary flavors for general DIY and repair work: BJTs and MOSFETs.

BJT Pinout & Symbol: A BJT has three pins: Emitter (E), Base (B), and Collector (C). In the schematic symbol, the Emitter is the leg with the arrow. For an NPN transistor, the arrow points outward (away from the Base). For a PNP, it points inward. The Base is the control terminal, acting like a current-operated valve between the Collector and Emitter.

MOSFET Pinout & Symbol: A MOSFET has three pins: Source (S), Gate (G), and Drain (D). The Gate is completely insulated from the channel by a thin layer of silicon dioxide. It is voltage-operated, drawing virtually zero steady-state current.

Safe Default Part Numbers (Keep These in Your Kit)

When you need a replacement or are prototyping a new circuit, rely on these industry-standard workhorses. Never use a part number without verifying its maximum ratings against your circuit's demands.

Part Number Type Max Vds / Vceo Max Id / Ic Package Best Use Case
2N2222 (or PN2222) NPN BJT 40V 800mA TO-92 / TO-18 Low-power switching, logic level shifting, LED drivers
2N3906 PNP BJT -40V 200mA TO-92 High-side switching, complementary pairs with 2N3904
IRF520 N-Ch MOSFET 100V 9.2A TO-220 Higher voltage DC motor control, 12V/24V solenoid drivers
IRLZ44N N-Ch Logic MOSFET 55V 47A TO-220 Direct drive from 5V/3.3V Arduino/ESP32 GPIO pins
TIP120 NPN Darlington 60V 5A TO-220 High-gain applications where base drive current is limited

How Transistors Fail and How to Test Them Step-by-Step

Transistors rarely fail gracefully. According to All About Circuits, semiconductor failures usually manifest as dead shorts or open circuits due to specific physical abuse.

Common Failure Modes:

  • Thermal Runaway (BJT): As a BJT heats up, its base-emitter voltage drop decreases, causing it to draw more base current, which creates more heat, leading to a catastrophic short between Collector and Emitter.
  • Secondary Breakdown (BJT): Occurs when high voltage and high current exist simultaneously across the device, creating localized hot spots that melt the silicon die.
  • Gate Oxide Puncture (MOSFET): The Gate is insulated by a microscopic layer of silicon dioxide. A static shock (ESD) or a voltage spike exceeding the Vgs max (usually ±20V) will punch a hole through this insulator, permanently shorting the Gate to the Source.

Step-by-Step: Testing an NPN BJT

  1. Isolate the component: Desolder at least two pins to remove parallel circuit influences.
  2. Set the meter: Turn the dial to the Diode Test symbol (looks like an arrow pointing at a line).
  3. Test Base-Emitter (Forward): Red probe to Base, Black probe to Emitter. Expect 0.55V - 0.75V.
  4. Test Base-Emitter (Reverse): Black probe to Base, Red probe to Emitter. Expect 'OL'.
  5. Test Base-Collector (Forward): Red probe to Base, Black probe to Collector. Expect 0.55V - 0.75V.
  6. Test Base-Collector (Reverse): Black probe to Base, Red probe to Collector. Expect 'OL'.
  7. Test Collector-Emitter: Probes in either direction across C and E should read 'OL'. If it reads a low voltage or beeps, the transistor is internally shorted.

Step-by-Step: Testing an N-Channel MOSFET

Testing a MOSFET requires checking the body diode and then actively 'turning on' the channel using the multimeter's internal battery. For a deeper look at MOSFET structures, refer to the Electronics Tutorials MOSFET guide.

  1. Discharge the Gate: Touch the Source and Gate pins together with a piece of wire or your finger to bleed off any residual gate charge.
  2. Check the Body Diode (Forward): Black probe to Source, Red probe to Drain. Expect 0.4V - 0.6V.
  3. Check the Body Diode (Reverse): Red probe to Source, Black probe to Drain. Expect 'OL'.
  4. Charge the Gate: Keep the Black probe on the Source. Briefly touch the Red probe to the Gate to charge the internal capacitance (turning the MOSFET on).
  5. Verify Channel Conduction: Move the Red probe back to the Drain (Black still on Source). The meter should now read a very low voltage drop (near 0.00V to 0.05V), indicating the channel is fully enhanced and conducting.
  6. Reset: Touch Gate to Source again. The channel should turn off, returning to the 'OL' state in the reverse direction.
Safety Warning: When testing high-power MOSFETs in TO-247 packages, ensure your hands are grounded. The gate oxide on large FETs like the IRFP460 is highly susceptible to ESD damage from your body just by holding the part.

Biasing and Operation Regions: Putting it to Work

Knowing how to check a transistor is only half the battle; you must know how to bias it for your specific application. BJTs operate in three distinct regions depending on the voltage applied to the Base and the resulting Collector-Emitter voltage.

Operation Region Base-Emitter Voltage (Vbe) Collector-Emitter Voltage (Vce) State / Application
Cutoff < 0.5V Equals Supply Voltage (Vcc) OFF state (Open switch)
Active / Linear ~0.6V to 0.7V Between 0.2V and Vcc Amplification, audio pre-amps, current mirrors
Saturation > 0.7V (Driven hard) ~0.1V to 0.3V (Vce_sat) ON state (Closed switch), digital logic, relay drivers

Complete Application Circuit: 12V Relay Driver

Here is a complete, bench-tested circuit for using a 2N2222 NPN transistor in saturation to switch a 12V relay using a 5V microcontroller GPIO pin (like an Arduino Uno or ESP32).

  • Q1: 2N2222 NPN BJT
  • K1: 12V DC Relay (e.g., Songle SRD-12VDC-SL-C, coil resistance ~400Ω, draws ~30mA)
  • R1: 1kΩ Base Resistor (Limits base current to ~4.3mA, providing a forced beta of ~7 to guarantee deep saturation)
  • D1: 1N4007 Flyback Diode (Cathode to 12V, Anode to Collector. Protects Q1 from inductive voltage spikes when the relay turns off)

Wiring Steps:

  1. Connect the 5V GPIO pin to one leg of the 1kΩ resistor (R1).
  2. Connect the other leg of R1 to the Base (middle pin) of the 2N2222.
  3. Connect the Emitter (right pin, tab facing you) directly to system Ground.
  4. Connect the Collector (left pin) to one terminal of the relay coil.
  5. Connect the other terminal of the relay coil to the 12V supply.
  6. Place the 1N4007 diode in parallel with the relay coil, with the silver stripe (cathode) pointing toward the 12V supply.

When the GPIO goes HIGH, 4.3mA flows into the base. The transistor saturates, dropping Vce to roughly 0.2V. The relay coil sees 11.8V, pulls 30mA, and clicks on. When the GPIO goes LOW, the base current stops, the transistor cuts off, and the 1N4007 safely dissipates the collapsing magnetic field.

Frequently Asked Questions

How to check a transistor without removing it from the circuit?

In-circuit testing is notoriously unreliable because parallel components (like base pull-down resistors or transformer windings) will skew your multimeter readings. However, you can perform a 'quick check' for dead shorts. Set your meter to continuity or resistance mode. Measure across the Collector and Emitter (or Drain and Source for MOSFETs) with the power completely disconnected. If you read near 0 ohms or hear a continuity beep, the transistor is almost certainly shorted and dead. If it reads high resistance, you cannot confirm it is healthy without desoldering at least the Base/Gate pin to isolate it from the driving circuitry.

How to check a SMD transistor with a standard multimeter?

Surface-mount devices (SMD) like the SOT-23 package are tested using the exact same diode-test principles as through-hole parts, but physical access is harder. Use fine-tipped multimeter probes or solder short lengths of 22 AWG solid wire to the SMD pads to give your probes something to grab. For an NPN SOT-23 (like the MMBT3904), Pin 1 is Base, Pin 2 is Emitter, and Pin 3 is Collector. Always verify the pinout on the specific manufacturer's datasheet, as SMD pinouts can vary between European (Pro Electron) and Japanese (JIS) naming conventions.

Why does my MOSFET test as shorted after soldering?

If a MOSFET tests perfectly on the bench but reads as a dead short across the Drain and Source immediately after soldering it to a PCB, you have likely experienced Gate Oxide Puncture. Soldering irons with ungrounded tips can leak stray AC voltage directly into the Gate pin, exceeding the ±20V Vgs maximum and blowing through the silicon dioxide insulator. To prevent this, always use a grounded-tip soldering station (like a Hakko FX-888D or Weller WE1010), or solder the Source and Drain pins first, then use a grounded wrist strap or a 10kΩ resistor to safely discharge the Gate before soldering it.

How to check a Darlington transistor like the TIP120?

Darlington transistors consist of two BJT dies wired in a single package to achieve massive current gain (hFE > 1000). Because you are testing two base-emitter junctions in series, the forward voltage drop on your multimeter's Diode Test mode will be roughly double that of a standard BJT. When testing the Base to Emitter on a TIP120, expect a reading between 1.1V and 1.5V instead of the standard 0.6V. If your meter reads 'OL' in the forward direction, the internal junction is open. Furthermore, Darlington packages usually include an internal snubber resistor and a flyback diode across the Collector and Emitter, so testing C to E in reverse will show a diode drop rather than 'OL'.