Modella -PNP Bipolar Model Released in SmartSpice
Introduction
The Modella lateral PNP bipolar model was developed by Philips Electronics N.V. and first released to the public domain in 1990 [1,2]. A release of this model has been implemented within SmartSpice, and can be accessed by setting the LEVEL parameter of the BJT model card to 500.
Description
Modella stands for MODEL-LAteral and is a PNP lateral bipolar model. Since most processes use the conventional lateral PNP as a standard, it was necessary to take its specificity in account with a new model. It is intended to provide physically-based equations, instead of using inaccurate vertical models such as Ebers-Moll or Gummel-Poon with modified parameters to represent the lateral behavior of the device. The physical effects of this lateral transistor lead to a totally new model that accounts for the complex bi-dimensional structure of this device.
Usual models for NPN transistors use the Gummel concept of computing majority charge in the neutral base to express collector current. This one-dimensional concept cannot be applied to Modella, because it needs a bi-dimensional physical description. Therefore, Modella has been developed using another approach, based on a physical analysis of the transistor. Most major modeling equations are derived for the forward active case.
Both lateral and vertical currents flow through the device, modeled by four current sources. The symmetry of the structure is reflected by the model, as well as its vertical and lateral elements (Figure 1).
Figure 1. Large signal equivalent circuit.
Modeled effects are :
- Temperature (without self-heating)
- Charge storage
- Excess phase shift for current and storage charges
- High-injection
- Built-in electric field in base region
- Bias-dependent Early effect
- Low-level non-ideal base currents
- Hard and quasi-saturation l Weak avalanche
- Current crowding (DC, AC and transient) and conductivity modulation for base resistance
- Hot carrier effects in the collector epilayer
- Explicit modeling of inactive regions
- Split base-collector depletion capacitance
Validation
In order to validate SmartSpice results, values were compared with Philips own in-house simulator output. PStar was used with test circuits for operating point, DC and AC analysis. Outputs for DC simulations match between PStar and SmartSpice.
Examples
Modella is more complex than Gummel-Poon models, because it is composed of as much as 6 internal nodes. However its symmetry and the nature of the equations used lead to convergence with the same performance as other models do.
Typical characteristics for the Modella device are presented in Figure 2.
Figure 2. Forward IC vs Vce characteristics.
Modella also accounts for advanced effects, such as excess phase.
The basic Gummel-Poon model is a one-pole model, but in fact a bipolar device has two poles. This results in simulation errors, estimating cutoff frequency and gain too high and also predicting smaller phase shift. The EXPHI parameter of the Modella model allows the designer to add the second pole phase contribution, expressed in radians. Figure 3 presents the small-signal gain hFE for an inverter circuit using one Modella device.
Figure 3. Forward current gain vs. frequency
and excess-phase
References
[1] 'Nat. lab Unclassified Report No. 2001/804, Physically based compact modeling of lateral PNP transistors.' F.G. O'Hara B.E.
[2] 'Nat. lab Unclassified Report No. 6131, A new physical compact model for lateral PNP transistors', F.G. O'Hara, J.J.H. van den Biesen, H.C. de Graaff and J.B. Foley.