2
BJT Models
(Both NPN and PNP)
The bipolar junction transistor model in SPICE 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 will automatically simplify 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, and NE which determine
the forward current gain characteristics, and VAF and VAR which determine the output conductance for
forward and reverse regions.
Three ohmic resistances RB, RC, and RE are included, where RB can be
high current dependent.
Base charge storage is modeled 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 determine by CJE, VJE , and MJE for the B-E junction.
CJC, VJC, and MJC for
the B-C junction and CJS, VJS, and MJS for the C-S (Collector-Substrate) junction.
The temperature
dependence of the saturation current, IS, is determined by the energy-gap, EG, and the saturation current
temperature exponent, XTL.
Additionally base current temperature dependence is modeled by the beta
temperature exponent XTB in the new model.
The BJT parameters used in the modified Gummel-Poon model are listed in the next few
slides.
The parameter names are used in earlier versions of SPICE2 are still accepted.