A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled semiconductor device used primarily for switching and amplifying electronic signals. Unlike Bipolar Junction Transistors (BJTs), which require continuous base current to stay on, a MOSFET uses an electric field across an insulating oxide layer to control current flow. Once the gate capacitance is charged, it draws virtually zero steady-state current, making it the undisputed king of modern power electronics, microcontroller interfacing, and high-efficiency switching.
If you are asking what is mosfet transistor in the context of a workbench project: it is your go-to solid-state relay. You apply a voltage to the Gate, and it connects the Drain to the Source, allowing heavy loads like motors, LED strips, and heating elements to run safely from low-voltage logic.
Symbol, Pinout, and Safe Default Part Numbers
Every standard MOSFET has three active terminals and one parasitic component you must respect. The Gate (G) is the control terminal. The Drain (D) is where current enters (for N-channel) or exits (for P-channel). The Source (S) is the common reference point. Crucially, the physical construction of a MOSFET creates an intrinsic body diode between the Drain and Source. In an N-channel MOSFET, this diode points from Source to Drain. If you wire it backward, the diode will conduct regardless of the Gate state, and your switch will fail to turn off the load.
When selecting a part, the two most critical datasheet parameters are VGS(th) (Gate-Source threshold voltage, the minimum voltage to barely start turning it on) and RDS(on) (Drain-Source on-resistance, which dictates heat generation). Below are the safe default part numbers I keep in my bench bins for 95% of hobbyist and prototyping tasks.
| Part Number | Type / Package | VDS (Max) | ID (Continuous) | RDS(on) @ VGS | Best Application |
|---|---|---|---|---|---|
| IRLZ44N | N-Channel / TO-220 | 55V | 47A | 0.022Ω @ 5V | Arduino/ESP32 high-current loads (motors, heaters) |
| IRF520 | N-Channel / TO-220 | 100V | 9.2A | 0.27Ω @ 10V | 12V/24V systems with 10V+ gate drive (not logic-level) |
| AO3400 | N-Channel / SOT-23 | 30V | 5.7A | 0.04Ω @ 4.5V | Compact SMD PCB designs, low-power 3.3V logic switching |
| IRF9540N | P-Channel / TO-220 | 100V | 23A | 0.117Ω @ 10V | High-side switching, reverse polarity protection |
Notice the IRLZ44N specifies RDS(on) at VGS = 5V, while the IRF520 specifies it at 10V. The IRF520 is a "standard" MOSFET; if you drive its gate with 3.3V from an ESP32, it will barely open, acting like a high-value resistor and burning up. Always choose a "logic-level" MOSFET (usually denoted by an 'L' in the prefix, like IRL...) when driving gates directly from 5V or 3.3V microcontrollers. For deeper selection criteria, refer to the All About Circuits guide on MOSFET basics.
Operating Regions: Cutoff, Linear, and Saturation
Understanding how a MOSFET behaves across different voltage thresholds is where most beginners get tripped up, largely because the terminology overlaps with BJTs in confusing ways. In a BJT, "saturation" means the switch is fully ON. In a MOSFET, the "saturation" region is actually where it acts as an amplifier, and the "linear" (or ohmic) region is where it acts as a fully ON switch.
| Region Name | Mathematical Condition | Physical Behavior | Typical VDS / ID State | Practical Use Case |
|---|---|---|---|---|
| Cutoff | VGS < Vth | Channel is pinched off; no current flows. | ID ≈ 0A, VDS = Supply Voltage | Switch OPEN (Load OFF) |
| Linear (Ohmic/Triode) | VGS > Vth AND VDS < (VGS - Vth) | Channel is fully enhanced; acts as a low-value resistor. | ID is high, VDS is very low (mV range) | Switch CLOSED (Load ON), PWM motor control |
| Saturation (Active) | VGS > Vth AND VDS > (VGS - Vth) | Channel pinches off at drain end; ID becomes constant regardless of VDS. | ID is constant, VDS varies widely | Signal amplification, constant current sources |
When using a MOSFET as a switch for a DC motor or LED strip, you want to drive the Gate voltage high enough to push the device deep into the Linear (Ohmic) region. In this state, the RDS(on) is minimized. If you accidentally operate it in the Saturation region while switching high currents, the voltage drop across the Drain-Source junction will be massive, and the silicon will rapidly overheat and fail.
How to Bias and Select a MOSFET for Your Circuit
Let us look at a complete, real-world application circuit: driving a 12V, 5A DC motor using an ESP32 (which outputs 3.3V logic). We will use the IRLZ44N because its RDS(on) is guaranteed to be low at a 4.5V gate drive, and 3.3V is sufficient to fully enhance it for a 5A load.
The Application Circuit Component Values
- Q1 (Switch): IRLZ44N N-Channel MOSFET. Source to GND, Drain to Motor negative terminal.
- R1 (Gate Resistor): 220Ω series resistor between ESP32 GPIO and MOSFET Gate. This limits the inrush current required to charge the gate capacitance, protecting the microcontroller's GPIO pin and dampening high-frequency ringing.
- R2 (Pull-down Resistor): 10kΩ resistor from Gate to Source (GND). During ESP32 boot-up, GPIO pins are high-impedance (floating). Without this resistor, ambient noise can accidentally turn the MOSFET on, spinning your motor unpredictably.
- D1 (Flyback Diode): 1N5819 Schottky diode placed in reverse-bias across the motor terminals (Cathode to 12V, Anode to Drain). When the MOSFET turns off, the motor's inductive kickback will destroy the MOSFET via avalanche breakdown if this diode is not present to recirculate the current.
Selection Framework for Custom Loads
When the default parts do not fit your needs, use this three-step selection checklist:
- Voltage Headroom: Select a VDS(max) rating at least 1.5x your supply voltage. For a 24V system, use a 40V or 60V MOSFET to survive inductive spikes.
- Thermal Check: Calculate power dissipation using P = I² × RDS(on). If you push 10A through an IRLZ44N (RDS(on) = 0.022Ω), it dissipates 2.2W. A standard TO-220 package without a heatsink can safely dissipate about 1.5W to 2W in still air before the junction temperature exceeds safe limits. At 10A, you must bolt it to a small heatsink.
- Gate Drive Matching: Check the datasheet's RDS(on) table. If your microcontroller outputs 3.3V, ensure the datasheet lists an RDS(on) spec at VGS = 2.5V or 4.5V. If the only spec listed is at VGS = 10V, the part is not logic-level and requires a dedicated gate driver IC.
Failure Modes and How to Test a MOSFET with a Multimeter
MOSFETs are rugged, but they have three primary failure modes. Gate punch-through occurs when electrostatic discharge (ESD) or a voltage spike exceeding ±20V breaches the microscopic oxide layer, permanently shorting the Gate to the Source. Avalanche breakdown happens when an inductive load spikes the Drain voltage past the VDS(max) rating, punching a hole through the silicon. Finally, thermal runaway occurs when RDS(on) increases with heat, causing more power dissipation, leading to a catastrophic short circuit between Drain and Source.
When a MOSFET fails, it almost always fails "short" (Drain to Source conducts in both directions). Here is how to verify its health on the bench using a standard digital multimeter (DMM) in Diode Test mode. For reference on component testing, the Vishay IRLZ44N datasheet outlines the internal diode characteristics that make this test possible.
Numbered Steps: Testing an N-Channel MOSFET
- Discharge the Gate: Remove the MOSFET from the circuit. Touch a finger or a 1kΩ resistor across the Gate and Source pins to drain any residual charge in the gate capacitance.
- Test the Body Diode: Set your DMM to Diode mode. Place the Red (positive) probe on the Source and the Black (negative/COM) probe on the Drain. The meter should read a forward voltage drop of roughly 0.4V to 0.6V. Reverse the probes (Red on Drain, Black on Source); the meter should read "OL" (Open Loop), confirming the diode blocks reverse current.
- Charge the Gate: Keep the Black probe on the Source. Move the Red probe to the Gate for about two seconds. This uses the DMM's internal battery to charge the gate capacitance and turn the channel on.
- Test the Channel: Move the Red probe back to the Drain (Black remains on Source). The meter should now read a very low voltage (near 0.00V) or beep for continuity, indicating the internal channel is fully enhanced and conducting.
- Verify Turn-Off: Short the Gate to the Source again to discharge it. Re-test Drain to Source (Red on Drain, Black on Source). It should revert to reading "OL". If it still reads near 0V, the gate oxide is punctured, and the MOSFET is dead.
By keeping a bin of IRLZ44Ns and AO3400s on hand, understanding the difference between the linear and saturation regions, and always using a pull-down resistor and flyback diode, you will eliminate the vast majority of solid-state switching failures in your projects.
