Pinch off Voltage:

The current in N-JFET because of a small voltage V_DS is described by:

I_DSS = (2a) W/L (qN_dμ_nV_DS)

In which2a = channel thickness

1. W stands for width
2. L stands for length
3. q stands for electronic charge = 1.6 x 10^-19 C
4. μ_n stands for electron mobility
5. N_d stands for n type doping concentration

In the saturation region:

I_DS =I_DSS [1- V_GS /V_P] ²

In the linear region

Or (in terms of I_DSS):

I_D = 2I_DSS/V²_P (V_GS - V_P - V_DS/2) V_DS