The Quick Answer: Default Transistor Picks for 90% of Projects

If you are building a switching circuit and need a part right now, stop reading and grab one of these three safe defaults. These cover almost every hobbyist, Arduino, and low-voltage DIY application you will encounter on the bench:

  • Under 200mA (LEDs, small sensors, logic level shifting): Use the 2N7000 (N-Channel MOSFET). It is cheap, logic-level, and requires zero continuous gate current.
  • 200mA to 1A (Small relays, motors, solenoids): Use the 2N2222 or 2N3904 (NPN BJT). They are robust, predictable, and easy to bias with a single base resistor.
  • 1A to 30A (Power LEDs, large DC motors, heating elements): Use the IRLZ44N (Logic-Level N-Channel MOSFET). The "L" stands for logic-level, meaning it fully turns on with a 5V Arduino GPIO pin, unlike standard MOSFETs that require 10V+.
Bench Rule: Always use an NPN BJT or N-Channel MOSFET on the low side of your load (between the load and ground). High-side switching requires PNP or P-Channel parts, which complicates your gate/base drive circuitry and invites floating-ground bugs.

BJT vs. MOSFET: The Decision Tree

Choosing between a Bipolar Junction Transistor (BJT) and a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) comes down to your load current and your microcontroller's drive voltage. Use this decision matrix to pick the right topology.

Application Scenario Drive Signal Recommended Type Concrete Part Pick
Switching tiny loads (<200mA), battery-powered devices where quiescent current matters 3.3V or 5V GPIO Small-Signal MOSFET 2N7000 (TO-92)
Driving standard 5V/12V relays, buzzing alarms, small water pumps (200mA - 1A) 5V GPIO NPN BJT 2N2222A or PN2222
High-current loads (1A - 30A), PWM dimming for high-power LEDs or DC motors 5V GPIO (Logic Level) Logic-Level Power MOSFET IRLZ44N (TO-220)
Extreme current (>30A) or high-frequency PWM (>50kHz) 10V-15V via Gate Driver IC Standard Power MOSFET IRFB4110 + TC4420 Driver

Understanding Pinouts, Symbols, and Operation Regions

Before you solder, you must know your pinout. The physical layout of transistor leads is notoriously inconsistent across manufacturers and regions. Always verify with a datasheet, but here are the standard bench defaults when holding the component with the flat side facing you and the leads pointing down:

  • 2N2222 / 2N3904 (NPN BJT, TO-92 package): Emitter (E), Base (B), Collector (C). Note: European equivalents like the BC547 are wired C, B, E. Do not mix them up.
  • 2N7000 (N-Channel MOSFET, TO-92 package): Source (S), Gate (G), Drain (D).
  • IRLZ44N / IRF520 (Power MOSFET, TO-220 package): Gate (G), Drain (D), Source (S). The metal tab on the back is internally connected to the Drain.

Transistors operate in three distinct regions. For switching applications, you only care about the first and the last. For audio or analog amplification, you operate in the middle.

Region BJT State (NPN) MOSFET State (N-Ch) Typical Voltages / Currents Use Case
Cutoff OFF (Vbe < 0.6V) OFF (Vgs < Vth) Ic = 0A, Id = 0A Switch Open
Active / Linear Amplifying (Vbe ≈ 0.7V) Saturation/Linear (Vgs > Vth) Vce > 1V, Ic = β × Ib Audio Amps, Current Sources
Saturation / Ohmic ON (Hard driven) ON (Fully enhanced) Vce(sat) < 0.2V, Rds(on) is minimal Switch Closed (Digital Logic)

How to Bias a Transistor: A Concrete Switching Circuit

Let us build a low-side switch to control a 12V, 50mA automotive-style relay using a 5V Arduino Nano GPIO pin. We will use the 2N2222 NPN BJT. A microcontroller GPIO can typically source 20mA to 40mA, but we want to keep it under 10mA for safety and thermal headroom.

The Math (Biasing for Saturation)

To use a BJT as a switch, we must drive it into saturation. We do not use the datasheet's typical hFE (beta) of 200; we use a forced beta of 10 to guarantee saturation across all temperature ranges.

  • Collector Current (Ic): 50mA (Relay coil requirement)
  • Required Base Current (Ib): Ic / 10 = 50mA / 10 = 5mA
  • GPIO Voltage (Vgpio): 5V
  • Base-Emitter Voltage Drop (Vbe): 0.7V
  • Base Resistor (Rb): (Vgpio - Vbe) / Ib = (5V - 0.7V) / 0.005A = 860Ω

The closest standard E12 resistor value is 1kΩ. Using 1kΩ yields 4.3mA of base current, which is perfectly adequate to saturate the 2N2222 for a 50mA load.

Wiring Steps

  1. Connect the Arduino 5V pin to one side of the relay coil.
  2. Connect the other side of the relay coil to the Collector (C) of the 2N2222.
  3. Connect a 1kΩ resistor between the Arduino GPIO pin and the Base (B) of the 2N2222.
  4. Connect the Emitter (E) of the 2N2222 to the common circuit Ground (GND).
  5. Critical Step: Place a 1N4001 flyback diode in reverse bias across the relay coil (cathode/stripe to 5V, anode to Collector). When the transistor turns off, the collapsing magnetic field in the relay coil generates a massive reverse voltage spike. Without this diode, the spike will punch through the transistor's collector-emitter junction and destroy it instantly.

Failure Modes and Multimeter Testing

Transistors rarely fail gracefully. They usually fail shorted, taking your microcontroller or power supply with them. Understanding how they fail helps you diagnose dead boards.

The IRF520 Trap: Many beginners buy IRF520 MOSFETs for Arduino projects because they are cheap. The datasheet lists a Gate Threshold Voltage (Vgs(th)) of 2.0V to 4.0V. Beginners assume this means it turns on at 5V. It does not. Vgs(th) is the voltage where it barely starts to conduct a tiny fraction of a milliamp. To fully turn on and carry its rated 9A, the IRF520 requires 10V on the gate. If you drive it with 5V, it operates in the linear region, acts like a 2-ohm resistor, overheats, and melts the solder off your board. Always look for the "L" (Logic Level) or check the Rds(on) spec at Vgs = 4.5V.

How to Test a BJT with a Digital Multimeter (DMM)

Set your DMM to Diode Test mode (the symbol with an arrow and a line). A BJT is essentially two diodes back-to-back.

  1. Forward Bias Base-Emitter: Red probe on Base, Black probe on Emitter. You should read a voltage drop between 0.550V and 0.750V.
  2. Forward Bias Base-Collector: Red probe on Base, Black probe on Collector. You should read a similar drop, usually slightly lower (e.g., 0.500V to 0.700V).
  3. Reverse Bias: Swap the probes (Black on Base, Red on Emitter/Collector). The meter must read "OL" (Over Limit) or open circuit.
  4. Collector-Emitter Check: Probes on C and E in either direction. Must read "OL". If it reads near 0.00V, the transistor is shorted and dead.

How to Test a MOSFET with a DMM

MOSFET testing is trickier because the gate is insulated by a microscopic layer of silicon dioxide. Static electricity from your finger can exceed the ±20V Vgs maximum and punch a hole through this oxide layer, permanently shorting the gate.

  1. Discharge the Gate: Touch the Gate and Source leads together with your finger or a resistor to drain any stored charge.
  2. Body Diode Check: Red probe on Source, Black probe on Drain. You should read a diode drop of roughly 0.400V to 0.600V (this is the intrinsic body diode). Swap probes: must read "OL".
  3. Gate Isolation: Measure between Gate and Source, and Gate and Drain. Both must read "OL" in both directions. If you read a short (0.00V), the gate oxide is blown. Throw it away.
  4. Turn-On Test: While keeping the Black probe on the Source, momentarily touch the Red probe to the Gate to charge it via the DMM's internal battery. Then move the Red probe to the Drain. The meter should now read near 0.00V (the channel is turned on). Short Gate to Source again to turn it off.

Safe Defaults and Stocking Your Bench

Stop buying random assortments from unbranded overseas sellers where the silicon dice are undersized and fail at half the rated current. Stock these specific, industry-standard part numbers from reputable distributors like Digi-Key, Mouser, or Arrow. For deeper theory on junction behaviors, refer to the All About Circuits semiconductor guide.

Part Number Type Max Vds / Vce Max Id / Ic Package Approx. Cost (2026)
2N3904 NPN BJT 40V 200mA TO-92 $0.05
PN2222A NPN BJT 40V 600mA TO-92 $0.08
2N7000 N-Ch MOSFET 60V 200mA TO-92 $0.12
IRLZ44N N-Ch MOSFET (Logic) 55V 47A TO-220 $1.20
TIP120 NPN Darlington 60V 5A TO-220 $0.60

Note on the TIP120: While popular in older Arduino tutorials, the TIP120 is a Darlington pair. It has a massive Collector-Emitter saturation voltage (Vce(sat)) of up to 2.0V to 4.0V. If you run 3A through it, it will dissipate 6 to 12 watts of heat and requires a massive heatsink. For modern designs, replace the TIP120 with a logic-level MOSFET like the IRLZ44N, which has an Rds(on) of 0.022Ω. At 3A, the IRLZ44N dissipates just 0.2W and runs cool to the touch without a heatsink.

Keep a dedicated drawer for your TO-92 BJTs and MOSFETs, clearly separated. A quick glance at the flat-side pinout before you apply power will save you from magic smoke and dead microcontrollers. Always verify your specific manufacturer's datasheet—ON Semiconductor's PN2222 datasheet is the gold standard reference for the most common bench BJT—because pinouts are the single most common point of failure for beginners wiring up discrete semiconductors.