n-MOSFET

Sweep gate voltage and observe the threshold and drain-current behavior of an enhancement-mode n-channel MOSFET.

V1 sweeps gate voltage from 0 to 5 V in 20 mV steps. The source is grounded and RD = 1 kΩ connects the drain to a 5 V supply. The level-1 model has threshold 2 V, KP = 2 mA/V² and equal channel width and length.

$$I_D\approx\frac{K_P}{2}\frac WL(V_{GS}-V_T)^2\quad\text{(saturation)}$$ $$V_D=5\,\mathrm{V}-I_D\cdot1\,\mathrm{k\Omega}$$

Gate voltage is green, drain voltage red and drain current blue. Below threshold current is nearly zero; above threshold it increases and pulls the drain down. At low drain voltage the MOSFET enters its linear region, so the square-law saturation expression no longer applies.

Read the results. The saved DC sweep chart and table show each bias point. The first voltage probe is green and the second red. The first current probe is blue; a second current probe is purple.

Model limits. The generic level-1 MOSFET model is intended to explain bias and low-frequency behavior. It omits detailed subthreshold conduction, gate charge, body effects and thermal behavior; these are not part-specific design results.

Try it. Set M1 threshold voltage VTO to 3 V; conduction starts at a higher gate voltage. Restore the original values after comparing the results.