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8.5.5 BJT Models (NPN/PNP)

NGSPICE provides two BJT device models. The level specifies the model to be used:

The bipolar junction transistor model in NGSPICE is an adaptation of the integral charge control model of Gummel and Poon. This modified Gummel-Poon model extends the original model to include several effects at high bias levels. The model automatically simplifies to the simpler Ebers-Moll model when certain parameters are not specified. The parameter names used in the modified Gummel-Poon model have been chosen to be more easily understood by the program user, and to reflect better both physical and circuit design thinking.

The dc model is defined by the parameters IS, BF, NF, ISE, IKF, amd NE which determine the forward current gain characteristics, IS, BR, NR, ISC, IKR, and NC which determine the reverse current gain characteristics, and VAF and VAR which determine the output conductance for forward and reverse regions. Level 2 model includes substrate saturation current ISS.

Three ohmic resistances RB, RC, and RE are included, where RB can be high current dependent. Base charge storage is modelled by forward and reverse transit times, TF and TR, the forward transit time TF being bias dependent if desired, and nonlinear depletion layer capacitances which are determined by CJE, VJE, and NJE for the B-E junction, CJC, VJC, and NJC for the B-C junction and CJS, VJS, and MJS for the C-S (Collector-Substrate) junction. Level 2 model defines a substrate capacitance that will be connected to device's base or collector, to model lateral or vertical devices.

The temperature dependence of the saturation currents, IS and ISS (for level 2 model), is determined by the energy-gap, EG, and the saturation current temperature exponent, XTI. Additionally base current temperature dependence is modelled by the beta temperature exponent XTB in the new model. The values specified are assumed to have been measured at the temperature TNOM, which can be specified on the .options control line or overridden by a specification on the .model line.

The BJT parameters used in the modified Gummel-Poon model are listed below. The parameter names used in earlier versions of SPICE2 are still accepted.

Modified Gummel-Poon BJT Parameters:

name parameter units default example scale factor
SUBS substrate connection: 1 for vertical geometry, -1 for lateral geometry. (level 2 only) 1 1.0e-15
IS transport saturation current A 1.0e-16 1.0e-15 area
ISS reverse saturation current, substrate-to-collector for vertical device or substrate-to-base for lateral (level 2 only) A 1.0e-16 1.0e-15 area


BF ideal maximum forward beta - 100 100
NF forward current emission coefficient - 1.0 1
VAF forward Early voltage V infinite 200
IKF corner for forward beta current roll-off A infinite 0.01 area
ISE B-E leakage saturation current A 0 1.0e-13 area
NE B-E leakage emission coefficient - 1.5 2
BR ideal maximum reverse beta - 1 0.1
NR reverse current emission coefficient - 1 1
VAR reverse Early voltage V infinite 200
IKR corner for reverse beta high current roll-off A infinite 0.01 area
ISC B-C leakage saturation current (area is "areab" for vertical devices and "areac" for lateral) A 0 1.0e-13 area


NC B-C leakage emission coefficient - 2 1.5
RB zero bias base resistance Z 0 100 area
IRB current where base resistance falls halfway to its min value A infinite 0.1 area
RBM minimum base resistance at high currents Z RB 10 area
RE emitter resistance Z 0 1 area
RC collector resistance Z 0 10 area
CJE B-E zero-bias depletion capacitance F 0 2pF area
VJE B-E built-in potential V 0.75 0.6
MJE B-E junction exponential factor - 0.33 0.33
TF ideal forward transit time sec 0 0.1ns
XTF coefficient for bias dependence of TF - 0
VTF voltage describing VBC dependence of TF V infinite
ITF high-current parameter for effect on TF A 0 - area
PTF excess phase at freq=1.0/(TF*2PI) Hz deg 0
CJC B-C zero-bias depletion capacitance (area is "areab" for vertical devices and "areac" for lateral) F 0 2pF area
VJC B-C built-in potential V 0.75 0.5
MJC B-C junction exponential factor - 0.33 0.5
XCJC fraction of B-C depletion capacitance connected to internal base node - 1
TR ideal reverse transit time sec 0 10ns
CJS zero-bias collector-substrate capacitance (area is "areac" for vertical devices and "areab" for lateral) F 0 2pF area
VJS substrate junction built-in potential V 0.75
MJS substrate junction exponential factor - 0 0.5
XTB forward and reverse beta temperature exponent - 0
EG energy gap for temperature effect on IS eV 1.11
XTI temperature exponent for effect on IS - 3
KF flicker-noise coefficient - 0
AF flicker-noise exponent - 1
FC coefficient for forward-bias depletion capacitance formula - 0.5 o
TNOM Parameter measurement temperature ^oC 27 50
TRE1 1st order temperature coefficient for RE (level 2 only) 1/^oC 0.0 1e-3
TRE2 2nd order temperature coefficient for RE (level 2 only) 1/^oC^2 0.0 1e-5
TRC1 1st order temperature coefficient for RC (level 2 only )1/^oC 0.0 1e-3
TRC2 2nd order temperature coefficient for RC (level 2 only) 1/^oC^2 0.0 1e-5
TRB1 1st order temperature coefficient for RB (level 2 only) 1/^oC 0.0 1e-3
TRB2 2nd order temperature coefficient for RB (level 2 only) 1/^oC^2 0.0 1e-5
TRB1 1st order temperature coefficient for RBM (level 2 only) 1/^oC TRB1 1e-3
TRB2 2nd order temperature coefficient for RBM (level 2 only) 1/^oC^2 TRB2 1e-5