MOSFET Overview: From Gate Voltage to Drain Current
Move the gate, drain and body voltages of a long-channel nMOS transistor and watch the inversion layer in the cross-section and the I–V curves respond, from weak inversion through saturation.
Cross-section cartoon (schematic: layer thickness follows the inversion charge at each end of the channel)
Output characteristics: ID versus VDS for VGS = 0.4 … 1.8 V (thin blue), your VGS (black)
Transfer characteristic: ID versus VGS at your VDS and VSB
Think of the gate as a valve that sets how much water (inversion charge) is in a pipe, and VDS as the slope that pushes it along. In saturation the layer pinches off at the drain end and the water is swept over the edge like a waterfall: more drop, no more flow.
On the log plot the current is a straight exponential below VT (weak inversion, one decade per S = nφt ln10), then bends into a square law. The two regimes are one smooth curve: the interpolation covers moderate inversion too.
Raising VSB moves VT up as γ(√(ψ₀+VSB) − √ψ₀), so the whole transfer curve slides right: the body effect. The shift per volt shrinks as VSB grows (square-root law), and n falls slightly, so the subthreshold slope improves a little. A larger γ makes the body effect stronger and the swing worse.
Try this
Set VDS = 1.8 V and sweep VGS. Where does the log plot change from a straight line to a bend? Compare with the shaded regions.
Choose “square root of ID” and VDS = 1.8 V. Does the curve follow the dashed line in strong inversion, and where does it cross zero?
Raise VSB from 0 to 1.5 V for γ = 0.6, then γ = 1.2. How far does VT move in each case?
Set VGS = 1.2 V and sweep VDS from 0 to 1.8 V. Where do the cross-section and the ID curve say the channel pinches off?