Textbook descriptions of the MOS transistor behavior hide a subtlety regarding the channel length modulation, a behavior we usually tend to ignore. A N-channel MOS transistor that does not manifest channel length modulation has a drain current described in the triode and saturation region by the following equations: \(\displaystyle {} \begin {array}{ll} \text{Triode region:} & i_{D}=\frac {1}{2}k_{n}\left [2\left (v_{GS}-V_{t}\right )v_{DS}-v_{DS}^{2}\right ]\\ \text{Saturation region:} & i_{D}=\frac {1}{2}k_{n}\left (v_{GS}-V_{t}\right )^{2}\\ \end {array} \) where \(k_{n}=\mu _{n}C_{ox}\left ({\displaystyle \frac {W}{L}}\right )\) . \(\mu _{n}\) is the electron mobility, \(C_{ox}\) is the gate-to-channel capacitance per unit area, and W and L are the channel width and channel length, respectively. A MOS transistor’s drain current levels out when it saturates when the drain-to-source voltage exceeds the gate-to-source over voltage which is equal to \(v_{GS}-V_{t}\) . \(V_{t}\) is the transistor’s threshold voltage. A MOS transistor described by these equations is an ideal MOS transistor. Beyond the saturation point, that is, \(v_{DS}>v_{GS}-V_{t}\) , a phenomenon called the channel modulation affects the drain current (Fig. 1.1.) The drain current begins to increase with increasing drain-to-source voltage. The effect is known as the Early effect after James Early, who investigated the effect at Bell Laboratories.