General

In the field of bipolar transistor compact modelling the emphasis has always been placed on vertical NPN compact models, with lateral PNP compact models receiving relatively little attention. Most lateral PNP compact models therefore lack a physical basis and are, at best, semi-empirical formulations. Consequently, the integration of these models into process blocks has been problematic and the reliability of circuit simulators involving lateral PNP devices has been quite inadequate. As a solution to these problems Modella was developed as the first truly physics based lateral PNP compact model.

Physics

Obviously a compact model of a lateral PNP bipolar transistor has many things in common with a compact model of a vertical bipolar transistor. The main bipolar device behaviour is the same. For that reason a lot of problems with older vertical models also appear in older lateral models. The bias-dependent Early effect, for instance, is not present in the vertical Spice-Gummel-Poon models and is therefore also not present in lateral versions. Modella takes this bias dependence into account.

As in vertical bipolar models, high injection effects in the base are taken into account using a knee current and appropriate formulations. The substrate current, which is not present in a simple Spice-Gummel-Poon model is also taken into account.

For the formulation of the depletion capacitances the standard piece-wise continuous model is not used, but an improvement that is continuous everywhere. The diffusion charges due to the built-up of charge in the base region are modelled in terms of the minority carrier concentrations, just as in Mextram, but rather unlike most other bipolar compact models.

A large difference between Modella and vertical bipolar compact models is related to the intrinsic 2-dimensional nature of a lateral bipolar transistor. The electrons will not only flow directly from emitter to collector via the shortest route, but a large part of them will go down into the epilayer and make quite a detour before getting collected at the collector. Within Modella both the lateral current as well as the vertical current are modelled. Extra resistance in the lateral current path is also taken into account. Also, the contact between the intrinsic base and the base contact has all kinds of current crowding effects that need to be modelled. Obviously, for all of these currents, appropriate charge contributions are also modelled.

Effects modelled by Modella

• Bias dependent Early effect;
• Fully symmetric model description;
• Lateral and vertical current flow;
• High-injection effects in the base;
• Depletion charges and diffusion charges;
• Series resistances;
• Excess phase shift;
• Low-level non-ideal base current;Low-level non-ideal base current;
• Substrate current, capacitance and resistance;
• Thermal noise, shot noise and flicker noise; and
• Thermal scaling;