In this technique, for each fixed gate voltage V GS, the drain-source voltage V DS is increased until the current I D saturates. Next, the square root of this saturated current is plotted against the gate voltage, and the slope m sat is measured. Then the mobility is: μ μ = m sat 2 2 L W 1 C i where L and W are the length and width of the channel and C i is the gate insulator capacitance per unit area. This equation comes from the approximate equation for a MOSFET in saturation mode: I D = μ μ C i 2 W L ( V G S − − V t h ) 2 . where V th is the threshold voltage. This approximation ignores the Early effect (channel length modulation), among other things. In practice, this technique may underestimate the true mobility.