BeginnerTopic 14 of 104Electronics & Embedded
MOSFET regions of operation
Cut-off, triode and saturation define switch and amplifier behaviour.
In depth
For NMOS: cutoff (V_GS<V_th), triode/linear (V_DS<V_GS-V_th; I_D≈k[(V_GS-V_th)V_DS-V_DS²/2]) acts like a resistor, saturation (I_D=½k(V_GS-V_th)²) acts like a current source for amplification. Channel-length modulation slightly raises I_D with V_DS.
Key points
- Overdrive voltage V_ov
- Transconductance g_m=2I_D/V_ov
- Digital use is switching between cutoff and triode
ExampleAmplifiers operate in saturation; logic gates swing between cutoff and triode.
What is MOSFET regions of operation?
Cut-off, triode and saturation define switch and amplifier behaviour.
Why is it important to understand MOSFET regions of operation?
The regions of MOSFET operation decide whether a device acts as a switch or an amplifier, so you must know which region your circuit uses.
What is the threshold voltage?
The gate-to-source voltage at which a conducting channel forms, typically between about 1 and 4 V for enhancement devices.
When is a MOSFET in saturation?
When the drain-source voltage is high enough that the channel pinches off, and the drain current is then roughly set by the gate voltage.
How is MOSFET regions of operation used in practice?
Typical applications include confirming that a switching MOSFET is fully enhanced for low on-resistance, biasing a MOSFET amplifier in saturation for linear gain and explaining why a MOSFET cuts off below its threshold voltage.
What should I learn before MOSFET regions of operation?
Study "MOSFET" first (topic 13 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
What are the key points of MOSFET regions of operation?
Overdrive voltage V_ov. Transconductance g_m=2I_D/V_ov. Digital use is switching between cutoff and triode.
Can you give an example of MOSFET regions of operation?
Amplifiers operate in saturation; logic gates swing between cutoff and triode.
What comes after MOSFET regions of operation?
The next topic is "CMOS inverter", which builds directly on this one.
How difficult is MOSFET regions of operation?
It is classed as Beginner in Electronics & Embedded (topic 14 of 104). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.