The level 1 JFET model is derived from the FET model of Shichman and Hodges. The dc characteristics are defined by the parameters VTO and BETA, which determine the variation of drain current with gate voltage, LAMBDA, which determines the output conductance, and IS, the saturation current of the two gate junctions. Two ohmic resistances, RD and RS, are included.

[\begin{array}{ll} {vgst = vgs - VTO} & \ \end{array}]

[\begin{array}{ll} {\beta_{p} = BETA\left( {1 + LAMBDA vds} \right)} & \ \end{array}]

[\begin{array}{ll} {bfac = \frac{1 - B}{PB - VTO}} & \ \end{array}]

[\begin{array}{ll} {I_{Drain} = \begin{cases} {vds \cdot GMIN,} & {{if} vgst \leq 0} \ {\beta_{p} vds\left( {vds\left( {bfac vds - B} \right) vgst\left( {2B + 3bfac\left( {vgst - vds} \right)} \right)} \right) + vds \cdot GMIN,} & {{if} vgst \geq vds} \ {\beta_{p} vgst^{2}\left( {B + vgst bfac} \right) + vds \cdot GMIN,} & {{if} vgst < vds} \ \end{cases}} & \ \end{array}]

Note that in Spice3f and later, the fitting parameter B has been added by Parker and Skellern. For details, see [9]. If parameter B is set to 1 equation above simplifies to

[\begin{array}{ll} {I_{Drain} = \begin{cases} {vds \cdot GMIN,} & {{if} vgst \leq 0} \ {\beta_{p} vds\left( {2vgst - vds} \right) + vds \cdot GMIN,} & {{if} vgst \geq vds} \ {\beta_{p} vgst^{2} + vds \cdot GMIN,} & {{if} vgst < vds} \ \end{cases}} & \ \end{array}]

Charge storage is modeled by nonlinear depletion layer capacitances for both gate junctions, which vary as the (- \frac{1}{2}) power of junction voltage and are defined by the parameters CGS, CGD, and PB.

Additional to the standard thermal and flicker noise model an alternative thermal channel noise model is implemented and is selectable by setting NLEV parameter to 3. This follows in a correct channel thermal noise in the linear region.

[\begin{array}{ll} {S_{noise} = \frac{2}{3} 4kT \cdot BETA \cdot Vgst\frac{\left( {1 + \alpha + \alpha^{2}} \right)}{1 + \alpha}GDSNOI} & \ \end{array}]

with

[\begin{array}{ll} {\alpha = \begin{cases} {1 - \frac{vds}{vgs - VTO},} & {{if} vgs - VTO \geq vds} \ {0,} & {else} \ \end{cases}} & \ \end{array}]