MOSFET Basics
A MOSFET is a voltage-controlled transistor. A small gate-to-source voltage controls a much larger drain current, so MOSFETs are used everywhere from microcontroller load switches to power converters and analog input stages.
Learning Objectives
By the end of this lesson, you should be able to:
- identify gate, drain, source, and body diode behavior;
- distinguish threshold voltage from full enhancement;
- estimate conduction loss from drain current and RDS(on);
- read the important datasheet limits before using a MOSFET;
- avoid beginner failures such as floating gates, missing heat checks, and wrong logic-level assumptions.
Terminals and Device Types
A MOSFET has three user terminals:
- Gate (G): insulated control electrode. It behaves mainly like a small capacitor.
- Drain (D): main current terminal, usually connected toward the positive side in an N-channel low-side switch.
- Source (S): reference terminal for gate voltage and main current return.
An N-channel enhancement MOSFET turns on when VGS is sufficiently positive. A P-channel enhancement MOSFET turns on when VGS is sufficiently negative. N-channel devices usually have lower RDS(on) for the same size and cost.
Every discrete MOSFET also has a body diode. In switching circuits that diode can conduct unexpectedly if the MOSFET is installed backwards or if current reverses.
Threshold Is Not Full Turn-On
The datasheet value VGS(th) is measured at a tiny drain current, often only 250 uA. It only means the channel has just started to form.
For switching, use the RDS(on) table instead. A logic-level MOSFET for a 3.3 V controller should specify low RDS(on) at VGS = 2.5 V or VGS = 3.0 V, not only at 10 V.
Operating Regions
MOSFET behavior depends on both VGS and VDS.
| Region | Practical meaning | Typical use |
|---|---|---|
| Cutoff | Gate drive is too low; drain current is near zero | Off switch |
| Ohmic or triode | Channel acts like a controlled resistor | Fully-on switch, small signal resistance |
| Saturation | Current is mainly controlled by VGS | Analog amplifiers, current sources |
The word saturation can confuse beginners: in MOSFET analog theory it means current-source behavior, while in power switching we usually want the MOSFET driven hard into the low-resistance ohmic region.
Loss and Heating
When a MOSFET is fully on, conduction loss is approximately:
P_\text{cond}=I_D^2 R_{DS(on)}
Example: a MOSFET carrying 3 A with RDS(on) = 40 mohm dissipates:
P = 3^2 \times 0.040 = 0.36\ W
Junction temperature rise is estimated from thermal resistance:
\Delta T_J = P \theta_{JA}
If hetaJA = 60 degC/W, the rise is about 21.6 degC. Real boards vary, so measure case temperature and derate generously.
Gate Drive and Protection
The gate draws little DC current, but it must be charged and discharged every switching event. Large MOSFETs have larger gate charge Qg, so weak microcontroller pins switch them slowly and create heat.
Good beginner practice:
- add a 10 kOhm to 100 kOhm gate-to-source pulldown or pullup so the MOSFET stays off during reset;
- add a small series gate resistor, often 22 Ohm to 220 Ohm, to reduce ringing;
- keep gate wiring short;
- protect the gate from ESD and from VGS(max) violations, commonly plus or minus 20 V.
Datasheet Checks
Before choosing a MOSFET, check:
- VDS(max) exceeds supply and transients with margin.
- ID rating is valid for your board and temperature, not just the headline package limit.
- RDS(on) is specified at your actual gate drive voltage.
- VGS(max) will never be exceeded.
- Qg and switching losses are acceptable at your PWM frequency.
- Package thermal resistance can remove the heat.
- Avalanche, safe operating area, and body diode ratings fit the load.
Common Mistakes
- Selecting by VGS(th) instead of RDS(on) at the real drive voltage.
- Leaving the gate floating during boot.
- Ignoring body diode direction.
- Assuming the current rating applies without PCB copper or heat sinking.
- Switching motors or coils without a flyback path or clamp.
Summary
A MOSFET is a voltage-controlled device, but practical MOSFET design is about charge, resistance, heat, and ratings. Use RDS(on) at the actual gate voltage, control the gate state during reset, check VDS, current, thermal limits, and remember the body diode.
Further Reading
- Infineon, Power MOSFET Basics application notes.
- Texas Instruments, Understanding MOSFET Data Sheets.
- Microchip, MOSFET Gate Drive Considerations.