In this technique, the transistor is operated in the linear region (or "ohmic mode"), where V DS is small and I D ∝ ∝ V G S with slope m lin. Then the mobility is: μ μ = m lin L W 1 V D S 1 C i . This equation comes from the approximate equation for a MOSFET in the linear region: I D = μ μ C i W L ( ( V G S − − V t h ) V D S − − V D S 2 2 ) In practice, this technique may overestimate the true mobility, because if V DS is not small enough and V G is not large enough, the MOSFET may not stay in the linear region.