The sub threshold I-V curve depends exponentially on the threshold voltage, introducing a strong reliance on any manufacturing variation that influences threshold voltage; for instance: differences in oxide thickness, junction depth/body doping which change the degree of drain-induced barrier lowering. The resultant sensitivity to fabricational variations complicates optimization for leakage and performance.
Figure: MOSFET drain current vs. drain-to-source voltage for various values of VGS - Vth; the boundary in between linear (Ohmic) and saturation (active) modes is pointed out by the upward curving parabola.
Figure: Cross section of a MOSFET that is operating in the linear (Ohmic) region; strong inversion region exist even near drain
Figure: Cross section of a MOSFET operating in the saturation (active) region; channel shows pinch-off near drain