What Is a MOSFET?
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled semiconductor device widely used in analog and digital circuits. It controls current flow between drain and source terminals using the voltage applied to the gate terminal. MOSFETs are the building blocks of modern integrated circuits and are found in processors, memory chips, power converters, and amplifiers.
The enhancement-mode MOSFET is normally off and requires a gate-source voltage exceeding the threshold voltage to conduct. The square-law model provides a good approximation of MOSFET behavior in the saturation and triode regions for hand calculations and circuit design.
MOSFET Current Formulas
Operating Regions
| Region | Condition | Behavior |
|---|---|---|
| Cutoff | VGS < Vth | No current flows |
| Linear (Triode) | VDS < VGS - Vth | Acts as voltage-controlled resistor |
| Saturation | VDS ≥ VGS - Vth | Current nearly constant |
Key Parameters
- Vth: Threshold voltage - minimum VGS to turn on the MOSFET (typically 1-3V for enhancement mode).
- kn: Transconductance parameter - depends on oxide capacitance, channel geometry (W/L ratio), and carrier mobility.
- gm: Small-signal transconductance - measures the gain of the MOSFET in amplifier circuits.
- RDS(on): On-resistance in the linear region - determines conduction losses in switching applications.
Frequently Asked Questions
How do I determine the operating region?
First check if VGS > Vth (otherwise cutoff). Then compare VDS with VGS - Vth: if VDS is smaller, the MOSFET is in the linear region; if larger or equal, it is in saturation.
What is the overdrive voltage?
The overdrive voltage VOV = VGS - Vth is the voltage above threshold that determines how strongly the channel is turned on. Higher overdrive means more current and lower on-resistance.