Saturation region: definition · Chip Field Guide

Beginner

An on state where the flow has leveled off. More push across the transistor barely helps; only the gate can raise it.

Novice

The region where $V_{\mathrm{GS}}$ is above threshold and $V_{\mathrm{DS}}$ is large (at least $V_{\mathrm{GS}} - V_{\mathrm{t}}$ in the simple model). The channel pinches off near the drain, and the current stops growing with $V_{\mathrm{DS}}$ , so the transistor acts like a current source set by the gate.

Expert

$V_{\mathrm{DS}} \ge V_{\mathrm{DSAT}}$ . Not to be confused with velocity saturation, which is a carrier-speed limit and changes where $V_{\mathrm{DSAT}}$ is. In real devices $I_{\mathrm{D}}$ still rises slowly with $V_{\mathrm{DS}}$ through channel-length modulation and DIBL.