How Does a Field Effect Transistor Work? A Practical Circuit & Biasing Guide

The Core Mechanism: Voltage-Controlled Current Flow

A field effect transistor (FET) works by using an electric field applied to the Gate terminal to control the flow of current between the Drain and Source terminals. Unlike Bipolar Junction Transistors (BJTs) that require continuous base current to stay on, a FET is a voltage-controlled device. Once the gate capacitance is charged, it draws virtually zero steady-state current, making it ideal for microcontroller GPIO pins and high-efficiency power switching.

To understand how a field effect transistor works in practice, you must map the physical pins to the schematic symbol. For the most common variant—the N-channel Enhancement MOSFET—the pinout and symbol features are:

• Gate (G): The control terminal. In the schematic symbol, it is drawn as a line separated by a small gap from the rest of the device, representing the insulated silicon dioxide layer.
• Drain (D): Where current enters the channel. In power packages like the TO-220, this is typically the center pin and is often bonded to the metal mounting tab.
• Source (S): Where current exits the channel. The symbol includes an arrow on the Source leg pointing inward toward the channel for an N-channel device.
• Body Diode: The schematic symbol shows a diode pointing from Source to Drain. This is a parasitic structural diode inherent to the silicon layout. It conducts if the Drain voltage drops below the Source voltage, a critical factor in motor and relay switching.

When you apply a positive voltage to the Gate relative to the Source (V_GS), the electric field repels holes in the P-type substrate and attracts electrons, forming an N-type "inversion layer" or channel. This bridges the Drain and Source, allowing current to flow. For a deeper look at the semiconductor physics, All About Circuits provides an excellent breakdown of FET theory.

Operation Regions: The Terminology Trap

One of the most common ways hobbyists and students misunderstand how a field effect transistor works is by confusing MOSFET operation regions with BJT regions. In a BJT, "saturation" means the switch is fully ON. In a MOSFET, "saturation" means the device is acting as a constant current source (partially ON), while the "Ohmic" or "Linear" region is where it acts as a closed switch.

Selecting, Biasing, and a Complete Switching Circuit

Knowing how a field effect transistor works theoretically is useless if you select the wrong part number. You must match the threshold voltage (V_GS(th)) and the on-resistance (R_DS(on)) to your logic levels and load current.

Safe Default Part Numbers

• 2N7000 (TO-92): 60V, 200mA. Standard small-signal FET for level shifting or driving low-current LEDs from 5V logic.
• IRLZ44N (TO-220): 55V, 47A. A logic-level power FET. The "L" indicates it fully enhances at V_GS = 5V, making it perfect for Arduino/Raspberry Pi projects driving motors or high-power relays.
• AO3400 (SOT-23 SMD): 30V, 5.7A. The go-to surface-mount default for compact PCB designs requiring 3.3V logic switching.

Complete Application Circuit: 12V Relay Low-Side Switch

Here is a robust, real-world circuit to drive a 12V, 70mA relay coil using a 5V Arduino GPIO. This design includes the necessary protection components that basic tutorials often omit.

1. The Load: Connect one side of the 12V relay coil to your 12V power supply. Connect the other side to the Drain of the IRLZ44N.
2. The Flyback Diode: Place a 1N4148 or 1N4007 diode in reverse bias across the relay coil (cathode/stripe to 12V, anode to Drain). When the FET turns off, the collapsing magnetic field generates a high-voltage spike; this diode safely recirculates that energy, preventing avalanche breakdown of the FET.
3. Gate Series Resistor: Place a 100Ω resistor between the Arduino GPIO pin and the FET Gate. This dampens high-frequency LC ringing caused by PCB trace inductance and the FET's gate capacitance, protecting both the microcontroller and the gate oxide.
4. Gate Pulldown Resistor: Connect a 10kΩ resistor between the Gate and Source (Ground). Microcontroller pins float during boot-up. Without this pulldown, ambient EMI can partially turn the FET on, pushing it into the high-heat linear region and destroying it before your code even starts.
5. Source Connection: Connect the Source directly to the common Ground shared by the 12V supply and the Arduino.

For further reading on designing reliable MOSFET gate drive circuits, refer to the FET tutorials on Electronics Tutorials.

Failure Modes and Multimeter Testing

Because of the ultra-thin silicon dioxide layer insulating the Gate, FETs are highly susceptible to specific failure modes. Understanding how a field effect transistor works means understanding how it breaks.

• Gate Oxide Puncture: Caused by Electrostatic Discharge (ESD) or exceeding the maximum V_GS rating (typically ±20V). The gate shorts to the source or drain. The FET may fail permanently ON or OFF.
• Thermal Runaway: Caused by under-driving the gate (e.g., using a standard IRF520 with a 5V Arduino). The R_DS(on) remains high, generating heat, which increases resistance further until the silicon melts or the solder reflows.
• Avalanche Breakdown: Caused by inductive kickback exceeding the V_DSS rating without a flyback diode, punching through the Drain-Source junction.

How to Test a MOSFET with a Multimeter

You can verify an N-channel MOSFET's health using the Diode Test mode on a standard digital multimeter. Follow these exact steps:

1. Discharge the Gate: Touch your finger across the Gate and Source pins, or use a jumper wire to short them. This bleeds off any stored gate charge.
2. Check the Body Diode (Reverse): Place the Red probe on the Drain and the Black probe on the Source. The meter should read "OL" (Open Loop). If it reads a short (~0.00V), the FET is blown.
3. Check the Body Diode (Forward): Swap probes: Red on Source, Black on Drain. You should read a standard diode drop, typically between 0.4V and 0.6V.
4. Charge the Gate: Move the Red probe to the Gate while keeping the Black probe on the Source. Hold for 2 seconds. This applies the multimeter's internal ~3V battery to the gate, turning the channel on.
5. Verify the Channel: Move the Red probe back to the Drain (Black remains on Source). The meter should now read a very low voltage drop (typically 0.0V to 0.2V), indicating the internal channel has successfully formed and is conducting.

Frequently Asked Questions

How does a field effect transistor work compared to a BJT?

A BJT (Bipolar Junction Transistor) is a current-controlled device; you must continuously supply base current to maintain collector current, which wastes power and requires current-limiting resistors calculated via hFE (gain). A FET is a voltage-controlled device. It relies on an electric field to form a conductive channel. Once the gate capacitance is charged, it requires virtually zero continuous current to remain ON. This makes FETs vastly superior for power switching and battery-operated circuits, while BJTs often remain better for simple, low-cost linear amplification.

Why does my field effect transistor get hot when switching PWM?

If your FET is hot during Pulse Width Modulation (PWM), it is likely spending too much time in the linear (transition) region between fully OFF and fully ON. Every time the gate voltage crosses the threshold, the FET acts as a resistor with high voltage and high current simultaneously, generating massive heat. To fix this, you need to switch the gate faster. Lower the gate series resistor value (e.g., from 1kΩ down to 10Ω), or use a dedicated MOSFET gate driver IC (like the TC4420) to supply the high peak current (often 2A to 4A) required to charge and discharge the gate capacitance in nanoseconds.

How does a field effect transistor work in high-side switching configurations?

Using an N-channel MOSFET on the high side (between the positive supply and the load) is problematic because the Source voltage rises as the load turns on. To keep the FET in the Ohmic (ON) region, the Gate voltage must be higher than the Source voltage by at least V_GS(th). If your supply is 12V, you would need 15V+ on the Gate to fully turn it on. For high-side switching, you should either use a P-channel MOSFET (which turns on when the Gate is pulled lower than the Source) or use an N-channel FET paired with a "bootstrap" gate driver circuit that generates the required elevated gate voltage.