When you open a semiconductor drawer, the sheer volume of three-legged components can be paralyzing. However, for 95% of bench and jobsite applications, the different kinds of transistors you actually need to understand boil down to three distinct families: Bipolar Junction Transistors (BJTs), Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), and Insulated-Gate Bipolar Transistors (IGBTs).

Here is the 10-second selection rule: Use BJTs for low-power signal amplification and simple switching under 1A. Use MOSFETs for high-speed and high-current DC switching (1A to 100A+). Use IGBTs for high-voltage AC/DC motor drives, welders, and inverters operating above 300V. Let's break down exactly how they operate, how to bias them, and how to verify them when they inevitably fail.

The Big Three: Pinouts, Symbols, and Core Specs

Before wiring anything, you must identify the pins. While the physical packages (like the ubiquitous TO-92 or TO-220) might look identical, the internal topology and pin assignments are completely different.

  • BJT (NPN/PNP): Pins are Base, Collector, Emitter. The schematic symbol features an arrow on the Emitter pointing outward for NPN and inward for PNP. It is a current-controlled device.
  • MOSFET (N-Channel/P-Channel): Pins are Gate, Drain, Source. The symbol shows the Gate separated from the channel by a line (the oxide insulator). It is a voltage-controlled device.
  • IGBT: Pins are Gate, Collector, Emitter. The symbol is a hybrid: the input looks like a MOSFET Gate, but the output uses BJT Collector/Emitter terminology. It is a voltage-controlled device designed for high-voltage, high-current environments.
Bench Tip: Never assume the pinout of a TO-220 package based on the tab shape. Always check the specific manufacturer datasheet. A TIP120 (BJT) and an IRF520 (MOSFET) both use TO-220 packages, but their pin 1 is Base vs. Gate, and pin 3 is Emitter vs. Source.
Feature BJT (e.g., 2N2222) MOSFET (e.g., IRLZ44N) IGBT (e.g., FGH40N60)
Control Mechanism Current (Base current) Voltage (Gate-Source) Voltage (Gate-Emitter)
Typical Max Voltage 30V - 100V 30V - 250V 400V - 1200V+
Typical Max Current 100mA - 5A 10A - 150A+ 20A - 100A+
Switching Speed Slow to Medium (MHz range) Very Fast (100MHz+) Medium (20kHz - 50kHz)
Input Impedance Low (draws continuous current) Extremely High (capacitive) Extremely High (capacitive)
On-State Loss Fixed Vce(sat) drop (~0.2V - 1.5V) Resistive Rds(on) (milliohms) Fixed Vce(sat) drop (~1.5V - 3V)

Operation Regions and Biasing for the Job

Selecting the right transistor is only half the battle; you must bias it correctly to operate in the desired region. For switching applications, you want the device fully saturated (or in the ohmic region). For audio amplification, you want it in the active (linear) region.

Region BJT Condition (NPN) MOSFET Condition (N-Ch) Practical Application
Cutoff Vbe < 0.6V, Ib = 0 Vgs < Vth, Ig = 0 Switch is OPEN (Off)
Active / Linear Vbe ≈ 0.7V, Vce > 1V Vgs > Vth, Vds > Vgs - Vth Signal amplification, linear regulators
Saturation / Ohmic Vbe ≈ 0.7V, Vce < 0.2V Vgs >> Vth, Vds is minimal Switch is CLOSED (On)

How to Bias a BJT for Switching

A BJT requires continuous base current to stay on. To saturate an NPN transistor, you must supply enough base current ($I_B$) to support the collector current ($I_C$). Using a conservative forced beta ($\beta$) of 10 for switching:

I_B = I_C / 10

If your load draws 200mA, you need 20mA of base current. If driving from a 5V Arduino GPIO, your base resistor is: R_B = (5V - 0.7V) / 0.020A = 215Ω. Use a standard 220Ω resistor.

How to Bias a MOSFET for Switching

A MOSFET draws virtually zero steady-state gate current, but its gate acts like a capacitor. You must apply a Gate-to-Source voltage ($V_{GS}$) well above the threshold voltage ($V_{GS(th)}$). If a datasheet lists $V_{GS(th)}$ at 2V to 4V, applying 3.3V might barely turn it on, causing massive heat. For 5V or 3.3V logic, always select a logic-level MOSFET guaranteed to have a low $R_{DS(on)}$ at $V_{GS} = 4.5V$.

The Workhorse Application: Switching a 12V DC Motor

Let's build a reliable, real-world switching circuit. We are driving a 12V DC motor with a 2A stall current using an ESP32 (3.3V logic output). We will use an N-channel logic-level MOSFET.

Safety Note: Inductive loads like motors generate massive voltage spikes when switched off. A flyback diode is not optional; omitting it will cause avalanche breakdown and instantly destroy your MOSFET.

Component List and Values

  • Q1: IRLZ44N (N-Channel Logic-Level MOSFET, TO-220)
  • D1: 1N4007 or Schottky SS34 (Flyback diode)
  • R1: 150Ω (Gate series resistor)
  • R2: 10kΩ (Gate-to-Source pull-down resistor)

Wiring Procedure

  1. Load Connection: Connect the motor's positive terminal to the 12V supply. Connect the motor's negative terminal to the MOSFET's Drain pin.
  2. Flyback Diode: Place D1 in reverse bias across the motor. The cathode (stripe) goes to the 12V side, and the anode goes to the Drain side. This clamps the inductive kickback to ~0.7V above the supply rail.
  3. Gate Drive: Connect the ESP32 GPIO pin to the MOSFET's Gate through the 150Ω resistor (R1). This resistor dampens high-frequency ringing caused by the parasitic inductance of the gate trace and the MOSFET's internal capacitance.
  4. Pull-Down: Connect the 10kΩ resistor (R2) directly between the Gate and Source pins. This ensures the MOSFET stays firmly off during ESP32 boot-up when GPIO pins are floating.
  5. Ground: Connect the MOSFET's Source pin to the common ground shared by the 12V supply and the ESP32.

Safe Default Part Numbers for Your Bin

Stop guessing which component to order. Stock these specific, proven part numbers to cover almost any hobbyist or prototyping scenario. Always verify the $V_{DS}$ (Drain-Source voltage) and $I_D$ (Continuous Drain current) ratings against your specific load.

Part Number Type Max Vds / Vce Max Id / Ic Key Metric (Rds(on) or Vce(sat)) Best Use Case
2N3904 NPN BJT 40V 200mA Vce(sat) ≈ 0.2V Low-power signal switching, LED drivers
2N2222 NPN BJT 30V 800mA Vce(sat) ≈ 0.3V General purpose switching, relay drivers
TIP120 NPN Darlington 60V 5A Vce(sat) ≈ 2.0V (High!) High-gain needs, but requires heavy heatsinking
IRLZ44N N-Ch MOSFET 55V 47A Rds(on) = 22mΩ @ Vgs=5V Logic-level switching, 12V/24V motor control
IRF3205 N-Ch MOSFET 55V 110A Rds(on) = 8mΩ @ Vgs=10V High-current DC loads, requires 10V gate drive
FGH40N60 IGBT 600V 40A Vce(sat) ≈ 1.5V Mains voltage inverters, induction heaters

How Transistors Fail and How to Test Them

Transistors rarely fail gracefully. BJTs typically suffer from thermal runaway (as they heat up, they conduct more current, generating more heat until the silicon melts). MOSFETs usually die from avalanche breakdown (overvoltage spikes punch through the die) or ESD damage to the ultra-thin gate oxide. IGBTs often fail due to latch-up or exceeding their short-circuit withstand time.

You can diagnose a dead transistor on the bench without desoldering it entirely using a digital multimeter (DMM) in Diode Test Mode.

Testing a BJT (NPN)

  1. Set DMM to Diode Test mode.
  2. Place the red probe on the Base and the black probe on the Emitter. You should read a forward voltage drop between 0.55V and 0.75V.
  3. Move the black probe to the Collector. You should read a similar 0.55V to 0.75V drop.
  4. Reverse the probes (black on Base, red on Emitter/Collector). The meter should read OL (Open Loop). If it reads near 0.00V or beeps, the junction is shorted and the BJT is dead.

Testing a MOSFET (N-Channel)

  1. Discharge the gate by briefly touching a resistor or your finger across the Gate and Source pins.
  2. Set DMM to Diode Test mode.
  3. Place the red probe on the Source and the black probe on the Drain. You are testing the internal body diode. You should read a forward drop of roughly 0.4V to 0.6V.
  4. Reverse the probes (red on Drain, black on Source). The meter should read OL.
  5. The Gate Charge Trick: While keeping the black probe on the Source, use your finger or a jumper wire to briefly touch the Drain (red probe) to the Gate. This charges the gate capacitance. The meter should now read near 0.00V (the MOSFET has turned on). Short the Gate to Source again, and it should return to the 0.4V-0.6V body diode reading.
Pro Insight: If a MOSFET reads shorted (0.00V) in both directions between Drain and Source, the silicon has melted internally. Desolder it and check the flyback diode in your circuit, as a failed diode is usually the root cause of the MOSFET's death.

For deeper reading on semiconductor topology and gate driving techniques, consult the SparkFun Transistors Tutorial for foundational concepts, and review All About Circuits' guide to IGBTs when stepping up to high-voltage inverter designs.