MOSFET Operation: Channel, Pinch-off and I-V Regions

An interactive n-channel enhancement MOSFET using the square-law (level-1) model with a subthreshold exponential tail (Neamen, Semiconductor Physics and Devices, 4th ed., Ch. 10-11; Sze and Ng). Below threshold ($V_{GS} < V_{th}$) the device is off and the drain current is a tiny subthreshold exponential; above threshold the inversion channel forms and, for $V_{DS} < V_{ov} = V_{GS} - V_{th}$, the device is in the triode region with $I_D = k_n[V_{ov} V_{DS} - V_{DS}^2/2]$; at $V_{DS} = V_{ov}$ the channel pinches off at the drain and the device saturates at $I_D = (k_n/2) V_{ov}^2 (1 + \lambda V_{DS})$. The output panel draws the $I_D$-$V_{DS}$ family for several gate voltages with the pinch-off locus $V_{DS} = V_{GS} - V_{th}$, the cross-section panel animates the inversion channel tapering and pinching off as $V_{DS}$ sweeps, and the transfer panel shows $I_D$-$V_{GS}$ with the threshold, so the gate-controlled switch and the saturated current source the device acts as are both visible.