change the degree of drain-induced barrier lowering. The resulting sensitivity to fabricational variations complicates optimization for leakage and performance. Triode mode or linear region (also known as the ohmic mode When VGS > Vth and VDS < VGS − Vth: The transistor is turned on, and a channel has been created which allows current between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as: where is the charge-carrier effective mobility, is the gate width, is the gate length and is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest. Saturation or active mode When VGS > Vth and VDS ≥ (VGS – Vth): The switch is turned on, and a channel has been created, which allows current between the drain and source. Since the drain voltage is higher than the source voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. Although the channel does not extend the full length of the device, the electric field between the drain and the channel is very high, and conduction continues. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate-source voltage, and modeled approximately as: The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is: where the combination Vov = VGS − Vth is called the overdrive voltage, and where VDSsat = VGS − Vth accounts for a small discontinuity in which would otherwise appear at the transition between the triode and saturation regions. Another key design parameter is the MOSFET output resistance rout given by: . rout is the inverse of gDS where . ID is the expression in saturation region. If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits. As