What Is a MOSFET?
A MOSFET is a three-terminal (gate, drain, source) transistor controlled by the voltage applied to its gate. Unlike a BJT it's voltage-controlled, not current-controlled, and it's very efficient in power switching.
How Does It Work?
The voltage applied to the gate forms a conductive channel between drain and source. Since the gate draws almost no current, very little energy is needed to drive a MOSFET.
N-Channel and P-Channel
| Type | Gate for conduction | Typical use |
|---|---|---|
| N-Channel | Positive | Low-side |
| P-Channel | Negative | High-side |
Difference From a BJT
A BJT is driven by current, a MOSFET by voltage. The MOSFET offers higher efficiency, faster switching and low drive power.
RDS(on) and Switching
While conducting, the drain-source resistance is RDS(on); the lower it is, the less heat (loss). In high-frequency switching this is a critical selection criterion.
Where Is It Used?
Switch-mode power supplies (SMPS), motor drives, DC-DC converters and high-current load switching.
Common Mistakes
- Half-driving the gate: insufficient gate voltage doesn't fully turn the MOSFET on; it overheats and burns out (use a gate driver if needed).
- Ignoring ESD: the gate is very sensitive to ESD.
- Forgetting a gate pull-down: a floating gate can conduct randomly.
Frequently Asked Questions
Why is a MOSFET preferred in power circuits?+
What is a logic-level MOSFET?+
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