This simulator opens up a planar n-channel MOSFET. Gate bias draws electrons into an inversion channel beneath the oxide, and drain bias moves them along the channel, eventually pinching the channel off. Change the voltages, threshold and geometry to move between cutoff, linear and saturation operation.
• A cutaway of the p-type body, n+ source and drain with contacts, gate dielectric and electrode, the inversion channel with its pinch-off point and electron transport markers. • Controls for gate-to-source voltage (0-3 V), drain-to-source voltage (0-3 V), threshold voltage (0.4-1.2 V), width-to-length ratio (2-20) and an automatic gate sweep. • Readouts for applied gate and drain bias, gate overdrive, drain current, transconductance and output conductance. • Experiments for cutoff (zero drain current below threshold) and saturation (1 V overdrive gives 1 mA in the default device).
With β = μnCox W/L and μnCox = 200 µA/V², overdrive is Vov = Vgs - Vth. For Vov ≤ 0 the device is in cutoff and Id = 0. In the linear region Id = β(Vov Vds - Vds²/2), and once Vds ≥ Vov the channel pinches off and Id = β Vov²/2 is constant. The inversion charge along the channel follows Qinv(x) ∝ max(Vov - V(x), 0), which is why the channel thins toward the drain as Vds grows.
The model is a long-channel enhancement nMOS with the body tied to the source, constant mobility and constant capacitance. Subthreshold current, channel-length modulation, velocity saturation, gate leakage and thermal effects are not modeled. The cross-section and carrier motion are illustrative, not atomic-scale trajectories.
It is the gate voltage at which an inversion layer of electrons forms under the oxide and the transistor begins to conduct. In this lab you can adjust it from 0.4 V to 1.2 V and see the overdrive change.
When the drain voltage reaches the gate overdrive, the inversion charge at the drain end falls to zero and the channel pinches off. Beyond that point the ideal drain current stops increasing with drain voltage.
In the square-law model Id = β Vov²/2. Doubling the overdrive therefore quadruples the current; transconductance rises linearly with overdrive.
No. It is a long-channel square-law model without velocity saturation, channel-length modulation, subthreshold conduction or leakage.