Gate overdrive (VGS − Vt): definition

Beginner

How far the gate voltage is past the tipping point. The farther past, the more current flows.

Novice

The amount by which the gate-to-source voltage exceeds the threshold voltage, written $V_{\mathrm{GS}} - V_{\mathrm{t}}$ or $V_{\mathrm{GT}}$ . Above-threshold current depends on this difference, not on $V_{\mathrm{GS}}$ alone.

Expert

$V_{\mathrm{GT}} = V_{\mathrm{GS}} - V_{\mathrm{t}}$ sets the channel charge per unit area ( $C_{\mathrm{ox}} V_{\mathrm{GT}}$ ) at the source end. Drive current scales as $V_{\mathrm{GT}}^2$ in the square law and roughly $V_{\mathrm{GT}}^{\alpha}$ ( $1 < \alpha < 2$ ) in partly velocity-saturated devices.