Transconductance (gm): definition

Beginner

How much the current changes when the gate voltage changes a little. Higher means the transistor reacts more strongly.

Novice

The slope of the drain current versus gate voltage: $g_{\mathrm{m}} = \Delta I_{\mathrm{D}}/\Delta V_{\mathrm{GS}}$ , measured in siemens. It tells you how much extra current one more millivolt on the gate buys.

Expert

$g_{\mathrm{m}} = \partial I_{\mathrm{D}}/\partial V_{\mathrm{GS}}$ . In the square law $g_{\mathrm{m}} = \beta V_{\mathrm{GT}} = 2I_{\mathrm{D}}/V_{\mathrm{GT}}$ ; when fully velocity-saturated it tends to $W C_{\mathrm{ox}} v_{\mathrm{sat}}$ , independent of $V_{\mathrm{GS}}$ . The ratio $g_{\mathrm{m}}/I_{\mathrm{D}}$ peaks in subthreshold at $1/(n\,kT/q)$ .