When makers and students first ask "how do transistor work", they are often met with dense quantum physics explanations about electron holes and depletion regions. But on the workbench, a transistor is simply an electrically operated valve. It uses a small input signal to control a much larger output current, allowing low-power microcontrollers like an Arduino or ESP32 to switch high-power loads like motors, LED strips, and heating elements.

This guide skips the abstract physics and focuses entirely on practical circuit implementation. We will cover the two main transistor families—Bipolar Junction Transistors (BJTs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)—detailing their pinouts, operation regions, default part numbers, and exactly how to test them when they inevitably fail.

The Short Answer: Pinouts, Symbols, and Basic Operation

Before wiring anything, you must understand the physical terminals and how they map to schematic symbols. The two dominant types operate on fundamentally different principles: BJTs are current-controlled, while MOSFETs are voltage-controlled.

Bipolar Junction Transistors (BJTs)

A BJT uses a small base current to allow a larger collector-to-emitter current to flow. The ratio of output current to input current is called the DC current gain (hFE or β).

  • Pins: Base (B), Collector (C), Emitter (E).
  • Symbol: A circle (often omitted in modern schematics) with a vertical bar and three lines. The Emitter line has an arrow. For an NPN transistor, the arrow points out (Not Pointing iN). For a PNP, it points in.
  • Operation: Current flows from Collector to Emitter (NPN) only when the Base is about 0.6V to 0.7V higher than the Emitter, and a small current is actively injected into the Base.

Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)

A MOSFET uses an electric field generated by a gate voltage to create a conductive channel. Because the gate is insulated by a thin layer of silicon dioxide, it draws virtually zero steady-state current.

  • Pins: Gate (G), Drain (D), Source (S).
  • Symbol: Three parallel lines representing the Gate, Drain, and Source, intersected by a channel line. An arrow on the Source line indicates the body diode direction (pointing in for N-channel).
  • Operation: Current flows from Drain to Source (N-channel) when the Gate-to-Source voltage (VGS) exceeds the threshold voltage (Vth). No continuous gate current is required to hold it open.
Bench Tip: Never confuse the pinouts of TO-92 and TO-220 packages. A 2N3904 (TO-92) is typically E-B-C from left to right (flat side facing you), while a TIP120 (TO-220) is B-C-E. Always verify with the manufacturer's datasheet before soldering.

Operation Regions and Safe Default Part Numbers

To use a transistor as a switch, you must drive it fully into the Saturation (BJT) or Ohmic/Linear (MOSFET) region. If you leave it in the Active region, it acts as a variable resistor, dissipating massive amounts of heat and destroying itself. The table below maps the typical voltages and currents for each region.

Region BJT Condition (NPN) BJT Current (IC) MOSFET Condition (N-Ch) MOSFET Current (ID)
Cutoff VBE < 0.6V, VCE = VCC ≈ 0A VGS < Vth, VDS = VDD ≈ 0A
Active / Saturation* VBE ≈ 0.7V, VCE > 1.0V IC = hFE × IB VGS > Vth, VDS > (VGS - Vth) ID ∝ (VGS - Vth)²
Saturation / Ohmic** VBE ≈ 0.7V, VCE < 0.2V Limited by load VGS >> Vth, VDS < 1.0V Limited by load & RDS(on)

*Note on terminology: BJT "Saturation" means fully ON (switch closed). MOSFET "Saturation" means fully ON as a constant current source (amplifier mode). For MOSFET switching, we use the "Ohmic" or "Linear" region. This naming collision is a notorious source of confusion for beginners.

The "Buy These" Default Transistor Table

Stop guessing which part to use. Stock your bench with these proven, widely available defaults. According to Electronics Tutorials, selecting a component with at least a 20% safety margin on voltage and current ratings is standard engineering practice.

Part Number Type VDS / VCE Max ID / IC Max Key Spec Best Use Case
2N3904 NPN BJT 40V 200mA hFE ≈ 100-300 Low-power logic level shifting, driving small relays.
2N2222 NPN BJT 40V 800mA hFE ≈ 100-300 Medium current switching (up to 500mA continuous).
IRLZ44N N-Ch MOSFET 55V 47A RDS(on) = 22mΩ @ 5V Logic-level (3.3V/5V) switching of high-current loads.
AO3400 N-Ch MOSFET 30V 5.8A RDS(on) = 27mΩ @ 4.5V SMD (SOT-23) space-constrained 3.3V ESP32 projects.
TIP120 NPN Darlington 60V 5A VCE(sat) ≈ 2.0V High current, but avoid if efficiency matters (runs hot).
Warning: The TIP120 is a Darlington pair. While it offers massive current gain, its saturation voltage (VCE(sat)) is around 2.0V. If you switch 3A through a TIP120, it will dissipate 6W of heat (P = 2V × 3A) and requires a large heatsink. An IRLZ44N MOSFET at 3A dissipates only 0.2W (P = I² × R = 9 × 0.022). Always prefer logic-level MOSFETs for high-current DC switching.

Biasing and Building a Real Application Circuit

Let's build a practical circuit: driving a 12V, 0.5A PC cooling fan using a 5V output from an Arduino Uno. Because the fan is an inductive load (a DC motor), we must use a flyback diode to protect the transistor from voltage spikes when the fan spins down. We will use the IRLZ44N logic-level MOSFET.

Component List and Values

  • Q1: IRLZ44N (N-Channel Logic-Level MOSFET)
  • D1: 1N5819 (1A, 40V Schottky Diode) - Flyback protection
  • R1: 100Ω (Gate series resistor) - Limits inrush current to the gate capacitance and dampens high-frequency ringing.
  • R2: 10kΩ (Gate pull-down resistor) - Bleeds off gate charge when the microcontroller pin is floating (e.g., during boot-up), preventing the fan from spinning erratically.

Wiring Steps

  1. Connect the Load: Wire the positive (red) lead of the 12V fan to the +12V power supply. Wire the negative (black) lead of the fan to the Drain (middle pin) of the IRLZ44N.
  2. Install the Flyback Diode: Place the 1N5819 diode in parallel with the fan. The cathode (silver stripe) must point toward the +12V rail, and the anode must point toward the MOSFET Drain. This routes the inductive kickback safely back to the power supply.
  3. Wire the Source: Connect the Source (right pin, tab facing you) of the IRLZ44N directly to the system Ground (GND). Ensure the 12V supply GND and the Arduino GND are tied together to share a common reference.
  4. Bias the Gate: Connect the 10kΩ pull-down resistor (R2) between the Gate (left pin) and GND. This ensures VGS is 0V by default.
  5. Connect the Control Signal: Connect one end of the 100Ω series resistor (R1) to the Arduino PWM pin (e.g., Pin 9). Connect the other end of R1 to the MOSFET Gate.

When the Arduino outputs 5V, VGS becomes 5V. According to the IRLZ44N datasheet, at VGS = 5V, the RDS(on) is roughly 25mΩ. The voltage drop across the MOSFET is V = I × R = 0.5A × 0.025Ω = 0.0125V. The transistor runs completely cool, and the fan receives nearly the full 12V.

Failure Modes and How to Test Transistors with a Multimeter

Transistors rarely fail randomly; they fail because of electrical overstress (EOS) or thermal runaway. According to SparkFun's Transistor Guide, understanding these failure modes is critical for debugging dead prototypes.

Common Failure Mechanisms

  • Gate Punch-Through (MOSFET): The silicon dioxide insulating layer under the gate is incredibly thin. If VGS exceeds the absolute maximum rating (usually ±20V), the dielectric breaks down, permanently shorting the Gate to the Source. This often happens from electrostatic discharge (ESD) or inductive spikes coupling into the gate trace.
  • Secondary Breakdown (BJT): If a BJT experiences high voltage and high current simultaneously (operating in the Active region with a heavy load), localized hot spots form on the silicon die, melting the junction and causing a Collector-Emitter short.
  • Inductive Kickback (Both): Failing to use a flyback diode across a relay coil or motor will generate voltage spikes hundreds of volts high, instantly exceeding the VDS or VCEO rating and destroying the silicon.

How to Test a BJT with a Multimeter

Set your multimeter to Diode Test mode. A healthy BJT behaves like two diodes sharing a common anode (NPN) or cathode (PNP).

  1. Base to Emitter (Forward): Red probe on Base, Black on Emitter (NPN). Expect a reading of 0.55V to 0.75V.
  2. Base to Collector (Forward): Red probe on Base, Black on Collector. Expect 0.55V to 0.75V.
  3. Reverse Bias: Swap the probes for both junctions. The meter must read "OL" (Over Limit / Open).
  4. Collector to Emitter: Test both directions. It must read "OL" both ways. If it reads near 0.00V or beeps, the transistor is shorted and dead.

How to Test a MOSFET with a Multimeter

Testing a MOSFET is trickier because the gate is insulated. You are testing the intrinsic body diode and the gate capacitance.

  1. Discharge the Gate: Briefly touch the Gate and Source pins together with your finger or a resistor to bleed off any stored charge.
  2. Test the Body Diode: Set meter to Diode mode. For an N-channel MOSFET, place the Red probe on the Source and Black on the Drain. You should read the body diode forward voltage (typically 0.4V to 0.6V). Swap probes (Red on Drain, Black on Source); it must read "OL".
  3. The Gate Charge Test: Keep the Black probe on the Source. Use your finger to briefly touch the Gate and Drain pins together (or use a 9V battery to momentarily apply positive voltage to the Gate relative to the Source). This charges the gate capacitor and turns the MOSFET ON.
  4. Verify Conduction: Move the Red probe to the Drain (keep Black on Source). The meter should now read near 0.00V (or beep), indicating the channel is conducting. Touch the Gate and Source together again to discharge it; the meter should revert to "OL".

If the MOSFET reads 0.00V across Drain-Source before you charge the gate, or if the Gate shows continuity to the Source, the component has suffered electrical overstress and must be replaced. Keep a handful of IRLZ44Ns and 2N3904s in your bin—they are the workhorses that will save your prototypes from the magic smoke.