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8.5.7 JFET Models (NJF/PJF)

The 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. Charge storage is modelled by nonlinear depletion layer capacitances for both gate junctions which vary as the -1/2 power of junction voltage and are defined by the parameters CGS, CGD, and PB.

Note that in Spice3f and later, a fitting parameter B has been added. For details, see [9].

name parameter units default example area
VTO threshold voltage (V_T0) V -2.0 -2.0
BETA transconductance parameter (B) A/V^2 1.0e-4 1.0e-3 *
LAMBDA channel-length modulation parameter (L) 1/V 0 1.0e-4
RD drain ohmic resistance Z 0 100 *
RS source ohmic resistance Z 0 100 *
CGS zero-bias G-S junction capacitance (C_gs) F 0 5pF *
CGD zero-bias G-D junction capacitance (C_gs) F 0 1pF *
PB gate junction potential V 1 0.6
IS gate junction saturation current (I_S) A 1.0e-14 1.0e-14 *
B doping tail parameter - 1 1.1
KF flicker noise coefficient - 0
AF flicker noise exponent - 1
FC coefficient for forward-bias depletion capacitance formula - 0.5
TNOM parameter measurement temperature ^oC 27 50