Regions of the MOSFET

The three main regions of operation for a MOSFET in simpler language, We can also say as First order effect in MOSFET

Cutoff (Off): In this state, the MOSFET is essentially switched off. There’s no channel for current to flow between the source and drain, regardless of how much voltage you apply between them. Ideally, the resistance between the source and drain should be extremely high, like an open switch. This situation is similar to the accumulation and depletion modes of a MOS capacitor.

Ohmic (Linear or Triode): This is the region where the MOSFET is turned on, and a channel has formed between the source and drain. However, the voltage between the source and drain isn’t high enough to pinch off or restrict the channel at the drain end. Current flow in this mode is controlled by all three terminals of the MOSFET – the gate, source, and drain. It behaves like a regular resistor, and the resistance between the source and drain is relatively low. It’s called “triode” because all terminals influence the current, and it shows a linear relationship between current and voltage for small voltage changes.

Saturation: In this state, the channel has fully formed under the oxide layer, and there’s a significant voltage between the drain and source, enough to pinch off or limit the channel at the drain. When the MOSFET is in saturation, the current flowing between the source and drain reaches its maximum and is mainly determined by the gate and source terminals. The drain terminal has less control over the current compared to the other two terminals. Think of it as the “on” state with the highest current flow, and the drain can’t influence it much.

These three states or modes of operation are crucial for understanding how a MOSFET works in different conditions, whether it’s completely off, partially on and acting like a resistor, or fully on and allowing maximum current flow.

The Current Equations for NMOS


The Current Equations for PMOS


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