Pinch-off: definition · Chip Field Guide

Beginner

When the path of electrons under the gate gets squeezed to nothing at the far end, so more push stops adding current.

Novice

When $V_{\mathrm{DS}}$ reaches $V_{\mathrm{GS}} - V_{\mathrm{t}}$ , the gate-to-drain voltage falls to the threshold and the channel vanishes at the drain end. Carriers still cross the short depleted gap, but the current no longer grows with $V_{\mathrm{DS}}$ .

Expert

Occurs at $V_{\mathrm{GD}} = V_{\mathrm{t}}$ , i.e. $V_{\mathrm{DS}} = V_{\mathrm{GS}} - V_{\mathrm{t}}$ in the long-channel model. The pinch-off point moves toward the source as $V_{\mathrm{DS}}$ rises, which shortens the effective channel: that is channel-length modulation.