The expression for the drain current of a nchannel MOSFET in the linear regime $(V_{DS} < V_{GS}  V_T)$ is,
\[ \begin{equation} I_{D}=\frac{\mu WC}{L}\left( V_{GS}V_T\frac{V_{DS}}{2}\right)V_{DS}. \end{equation} \]Here $I_D$ is the drain current, $V_{DS}$ is the drainsource voltage, $V_{GS}$ is the gatesource voltage, $V_T$ is the threshold voltage, $L$ is the length of the transistor (in the direction that the current flows), $W$ is the width of the transistor, $C$ is the specific capacitance of the gate in F/m², and $\mu$ is the mobility.
For $(V_{DS} > V_{GS}  V_T)$, the channel is pinched off and the transistor is in the saturation regime. The expression for the drain current in the saturation regime is,
\[ \begin{equation} I_{D}=\frac{\mu WC}{2L}\left( V_{GS}V_T\right)^2. \end{equation} \]MOSFET charateristics are typically drawn for several gate voltages. In the figure below, the threshold voltage is 1 V and the gate voltages are 2, 3, 4, 5 and 6 V.


The quantities in the right column are used to calculate the threshold voltage $V_T$,
$$V_T = \frac{2t_{ox}}{\epsilon_{ox}}\sqrt{\epsilon_{\text{semi}}N_Ak_BT \ln \left (\frac{N_A}{n_i} \right )} +\frac{2k_BT}{e} \ln \left (\frac{N_A}{n_i} \right ) +V_{fb}.$$Here φ_{m} is the work function of the metal, χ_{s} is the electron affinity of the semiconductor, φ_{s} is the work function of the semiconductor, $V_{fb}=$ φ_{m}  φ_{s} is the flat band voltage, $E_g$ is the band gap (It is possible to include the temperature dependence of the band gap.), $t_{\text{ox}}$ is the thickness of the oxide, $\epsilon_{\text{ox}}$ is the relative dielectric constant of the oxide, $\epsilon_{\text{semi}}$ is the relative dielectric constant of the semiconductor, and $n_i$ is the intrinsic carrier concentration. The temperature dependence of the device enters primarily through the temperature dependence of the threshold voltage.