where V D S {\displaystyle V_{DS}} = drain-to-source voltage, I D {\displaystyle I_{D}} = drain current and λ {\displaystyle \lambda } = channel-length modulation parameter. Without channel-length modulation (for λ = 0), the output resistance is infinite. The channel-length modulation parameter usually is taken to be inversely proportional to MOSFET channel length L, as shown in the last form above for rO:[2]