Channel-length modulation (CLM): definition

Beginner

After the current levels off, it still creeps up a little as the push across the transistor grows.

Novice

As $V_{\mathrm{DS}}$ rises past saturation, the depleted region at the drain widens and the effective channel gets shorter, so a little more current flows. It is modeled as a factor $(1 + \lambda V_{\mathrm{DS}})$ , where $\lambda$ is fitted to measurements.

Expert

$L_{\mathrm{eff}} = L - L_{\mathrm{d}}(V_{\mathrm{DS}})$ . Modeled as $I_{\mathrm{D}} = (\beta/2)V_{\mathrm{GT}}^2(1 + \lambda V_{\mathrm{DS}})$ , $\lambda = 1/V_{\mathrm{A}}$ by analogy with the BJT Early voltage. It sets the finite output resistance $r_{\mathrm{o}} \approx 1/(\lambda I_{\mathrm{D}})$ and gets worse as L shrinks.