When you crack open the black epoxy TO-92 or TO-220 package and look at the inside of a transistor, you will not find moving parts or microscopic gears. Instead, you will find precisely doped silicon crystals engineered to control electron flow. Inside a standard Bipolar Junction Transistor (BJT), there are three alternating layers of doped silicon (N-P-N or P-N-P) forming two PN junctions. Inside a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), you will find a metal gate insulated by a microscopic layer of silicon dioxide, sitting above a semiconductor channel. Understanding these physical differences is the key to knowing why a BJT requires continuous base current to stay on, while a MOSFET only needs a voltage pulse to charge its gate capacitance.
This guide bridges the gap between semiconductor physics and the workbench. We will map the internal structures to their schematic symbols, define the biasing voltages required to turn them on, select safe default part numbers for your next build, and walk through exactly how to test them when they inevitably fail.
The Physical Anatomy: What is Actually Inside of a Transistor?
To use a transistor effectively, you must map its internal physical layers to the pins on your breadboard. For a BJT, the three internal layers correspond to the Emitter (E), Base (B), and Collector (C). The schematic symbol uses an arrow on the emitter leg to indicate conventional current direction (pointing out for NPN, pointing in for PNP). For a MOSFET, the internal structure maps to the Source (S), Gate (G), and Drain (D), with a broken line in the symbol representing the insulated gate that prevents DC current from flowing into the control terminal.
| Feature | NPN BJT (e.g., 2N2222) | N-Channel MOSFET (e.g., IRLZ44N) |
|---|---|---|
| Internal Structure | Three doped silicon layers (N-P-N) forming two back-to-back diode junctions. | P-type substrate with N-type source/drain wells; metal gate isolated by SiO2 dielectric. |
| Primary Charge Carriers | Both electrons and holes (minority carrier injection). | Electrons only (majority carriers in the N-channel inversion layer). |
| Control Mechanism | Current-controlled: Base current ($I_B$) dictates Collector current ($I_C$). | Voltage-controlled: Gate-Source voltage ($V_{GS}$) creates an electric field to form a channel. |
| Input Impedance | Low to Moderate (k$\Omega$ range); draws continuous DC current. | Extremely High (G$\Omega$ range); draws virtually zero steady-state DC current. |
| Switching Speed Limit | Storage time delay (minority carriers must recombine before turn-off). | Gate capacitance charging/discharging (Miller effect plateau). |
If you are switching high currents at low frequencies (like a PWM heater), the MOSFET’s near-zero steady-state gate draw makes it vastly superior. If you are building linear amplifiers or high-frequency RF oscillators, the BJT’s predictable transconductance and lack of gate capacitance issues often make it the better choice.
Operation Regions and Biasing Voltages
Knowing what is inside of a transistor is useless if you do not know how to bias it. Biasing means applying the correct DC voltages to the pins to force the internal silicon into a specific operational state. Below are the exact voltage thresholds you need to design your biasing networks.
| Region | BJT Bias Condition (NPN) | MOSFET Bias Condition (N-Ch) | Primary Circuit Use |
|---|---|---|---|
| Cutoff | $V_{BE} < 0.6V$, $I_B = 0$ | $V_{GS} < V_{th}$ (Threshold) | Open switch (OFF state). |
| Active / Linear | $V_{BE} \approx 0.65V$, $V_{CE} > 1V$ | $V_{GS} > V_{th}$, $V_{DS} > V_{GS} - V_{th}$ | Signal amplification, linear regulators, current mirrors. |
| Saturation / Ohmic | $V_{BE} \approx 0.7V$, $V_{CE(sat)} < 0.3V$ | $V_{GS} \gg V_{th}$, $V_{DS}$ is very low | Closed switch (ON state), power switching, motor drivers. |
For a deeper dive into the semiconductor physics governing these junctions, the All About Circuits BJT chapter provides an excellent breakdown of minority carrier injection and the Ebers-Moll model.
Workbench Defaults: Safe Part Numbers and Ratings
When prototyping, you should not have to hunt for obscure silicon. Keep these verified, high-availability defaults in your component drawers. They cover 95% of hobbyist and light-industrial DIY applications.
| Part Number | Type | Max $V_{CE}$ / $V_{DS}$ | Max $I_C$ / $I_D$ | Package | Best Application |
|---|---|---|---|---|---|
| 2N3904 | NPN BJT | 40V | 200mA | TO-92 | Low-power signal switching, LED drivers, logic level translation. |
| 2N2222A | NPN BJT | 40V | 800mA | TO-18 / TO-92 | Medium current loads, relay drivers, general-purpose amplification. |
| TIP120 | NPN Darlington | 60V | 5A | TO-220 | High-current 5V/12V loads (Note: high $V_{CE(sat)}$ of ~2V requires heavy heatsinking). |
| IRF520 | N-Ch MOSFET | 100V | 9.2A | TO-220 | 12V/24V power switching driven by 10V+ gate signals (Not logic-level). |
| IRLZ44N | N-Ch Logic MOSFET | 55V | 47A | TO-220 | Direct drive from 3.3V/5V microcontrollers (ESP32, Arduino, Raspberry Pi). |
A common beginner mistake is buying the IRF520 for an Arduino project because it is cheap and handles high current. However, the IRF520 requires a $V_{GS}$ of at least 10V to fully turn on and achieve its rated $R_{DS(on)}$. An Arduino’s 5V GPIO will leave it partially in the linear region, causing it to overheat and fail. Always choose a logic-level MOSFET (usually denoted by an ‘L’ in the prefix, like IRLZ44N) when driving gates directly from microcontrollers.
Practical Application: 12V Logic-Level Motor Driver
Let’s put the IRLZ44N to work. Below is a complete, robust circuit for driving a 12V, 2A DC cooling fan or small motor using a 3.3V GPIO pin from an ESP32.
Component List
- Q1: IRLZ44N N-Channel Logic-Level MOSFET
- R1 (Gate Resistor): 100Ω (limits inrush current into the gate capacitance, protecting the ESP32 GPIO)
- R2 (Pull-down Resistor): 10kΩ (ensures the gate is pulled to GND during ESP32 boot-up, preventing motor spin-out)
- D1 (Flyback Diode): 1N4007 (cathode to 12V, anode to Drain; clamps inductive voltage spikes when the motor turns off)
- Load: 12V DC Motor / Fan
Wiring Steps
- Gate Network: Connect the ESP32 GPIO pin to one leg of the 100Ω resistor (R1). Connect the other leg of R1 to the Gate of the IRLZ44N.
- Pull-down: Connect the 10kΩ resistor (R2) between the Gate and the Source pins. This bleeds off gate charge when the GPIO goes high-impedance.
- Source to Ground: Connect the Source pin directly to the common system Ground (ESP32 GND and 12V supply GND must be bonded).
- Drain to Load: Connect the Drain pin to the negative terminal of the DC motor.
- Power and Protection: Connect the positive terminal of the motor to the 12V supply. Place the 1N4007 diode in parallel with the motor, with the silver stripe (cathode) pointing toward the 12V positive rail.
When the ESP32 drives the GPIO HIGH (3.3V), the $V_{GS}$ exceeds the IRLZ44N’s threshold (typically 1V to 2V), pulling the drain low and completing the circuit. The 100Ω gate resistor is critical here; without it, the rapid charging of the MOSFET’s internal gate capacitance can draw momentary current spikes exceeding 50mA, which can degrade or destroy the ESP32’s delicate GPIO silicon over time.
Failure Modes and Multimeter Diagnostics
Transistors rarely die of old age; they die from thermal stress, voltage spikes, or improper biasing. According to SparkFun’s transistor guide, the most common failure modes are thermal runaway in BJTs (where heat increases leakage current, which creates more heat) and gate oxide punch-through in MOSFETs (caused by ESD or exceeding the $\pm$20V $V_{GS}$ absolute maximum rating).
When a transistor fails, it almost always fails short (Collector-to-Emitter or Drain-to-Source), leaving your load permanently powered. Occasionally, it fails open. Here is how to diagnose them on the bench using a standard digital multimeter (DMM).
Testing a BJT (Diode Test Mode)
Set your DMM to the diode test setting (the symbol with an arrow and a line). You are essentially testing the two internal PN junctions as if they were standard diodes.
- Base-Emitter Junction: Place the red probe on the Base and the black probe on the Emitter (for an NPN). You should read a forward voltage drop between 0.55V and 0.75V. Reverse the probes; it should read ‘OL’ (Over Limit / Open).
- Base-Collector Junction: Place the red probe on the Base and the black probe on the Collector. Again, expect 0.55V to 0.75V forward, and ‘OL’ in reverse.
- Collector-Emitter: Measure between Collector and Emitter in both directions. Both should read ‘OL’. If you read a short (near 0.00V) or a low resistance, the silicon has melted internally and the part is dead.
Testing a MOSFET (Gate Charge Trick)
MOSFETs cannot be tested simply by measuring diode drops across all pins, because the gate is insulated. However, you can test the internal body diode and the channel switching capability. For a detailed visual walkthrough of DMM testing, the Fluke multimeter testing guide is an excellent bench reference.
- Discharge the Gate: Touch the DMM probes across the Gate and Source pins to bleed off any residual static charge.
- Check the Body Diode: Place the red probe on the Source and the black probe on the Drain (N-Channel). You should read a diode drop of roughly 0.4V to 0.6V. Reverse the probes; it should read ‘OL’.
- Turn it ON: While keeping the black probe on the Drain, touch the red probe to the Gate for a second. This uses the DMM’s internal battery (usually 3V to 9V) to charge the gate capacitance above the threshold voltage.
- Verify Conduction: Move the red probe back to the Source (black probe stays on Drain). The meter should now read a very low voltage drop (near 0.00V), indicating the internal channel has turned on and is bypassing the body diode.
- Turn it OFF: Touch a finger across the Gate and Source pins to discharge the gate. The reading between Drain and Source should revert back to the 0.4V - 0.6V body diode drop.
If the MOSFET reads shorted between Drain and Source in both directions, or if the gate fails to hold a charge and turn the channel on, the silicon is compromised. Desolder it, toss it in the e-waste bin, and grab a fresh IRLZ44N from your drawer.
