Ngspice provides three BJT device models, which are selected by the .model card.

.model QMOD1 BJT level=2

This is the minimal version, further optional parameters listed in the table below may replace the ngspice default parameters. The** level** keyword specifies the model to be used:

• level=1: This is the original SPICE BJT model, and it is the default model if the level keyword is not specified on the .model line.
• level=2: This is a modified version of the original SPICE BJT that models both vertical and lateral devices and includes temperature corrections of collector, emitter and base resistors.
• level=4: Advanced VBIC model (see http://www.designers-guide.org/VBIC/ for details)

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 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, 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.

The level 1 model has among the standard temperature parameters an extension compatible with most foundry provided process design kits (see parameter table below tlev).

The level 1 and 2 models include the substrate saturation current iss. 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, where the forward transit time tf can be bias dependent if desired. Nonlinear depletion layer capacitances are defined with 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.

The level 1 and 2 model support a substrate capacitance that is connected to the device's base or collector, to model lateral or vertical devices dependent on the parameter subs. The temperature dependence of the saturation currents, is and iss (for the level 2 model), is determined by the energy-gap, eg, and the saturation current temperature exponent, xti.

In the new model, additional base current temperature dependence is modeled by the beta temperature exponent xtb. 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 level 4 model (VBIC) has the following improvements beyond the GP models: improved Early effect modeling, quasi-saturation modeling, parasitic substrate transistor modeling, parasitic fixed (oxide) capacitance modeling, includes an avalanche multiplication model, improved temperature modeling, base current is decoupled from collector current, electrothermal modeling, smooth and continuous mode.

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.