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Note: Model number 85193D has been discontinued; however, the feature/capability is now included in the W8542EP/ET.
The information below is provided for reference only.
The VBIC vertical BJT model has been developed specifically as a replacement for the SPICE Gummel-Poon model, by representatives of the IC and CAD industries. The SPICE Gummel-Poon model has remained essentially an unchanged industry standard for over 20 years. For devices available until recent times it has been an intuitive and physically consistent formulation that has generally met the requirements of BJT device modeling.
However, improvements in modern device technology have increasingly indicated a need for improvements in BJT modeling techniques. Some of the technology changes are these:
- Reduced base width.
- Smaller device dimensions and changes in device structure.
- Changes in doping profiles.
- Parasitic substrate transistor.
Reduced base width means that higher-order effects have more significance. The Early effect approximation in the Gummel-Poon model is not sufficiently accurate to account for these effects.
Smaller device dimensions increase the importance of parasitic overlap capacitances not accounted for by the Gummel-Poon model. As the base width becomes smaller, the response becomes more nonlinear.
Changes in doping profiles increase the quasi-saturation effects of collector resistance, which is not accounted for in the Gummel Poon model.
The parasitic substrate transistor is not included in the Gummel-Poon model. Therefore both DC and AC modeling are less accurate than needed in today's devices.
To overcome these issues, representatives from the IC and CAD industries have collaborated over the last few years to recommend an improved standard BJT model for the semiconductor industry. The result is VBIC, a model that is now viable and will continue to develop as further improvements are made.
VBIC (vertical bipolar inter-company model) includes improved modeling of the Early effect (output conductance), substrate current, quasi-saturation, and behavior over temperature: information necessary for accurate modeling of current state-of-the-art devices. However, it has additionally been defined so that, with default parameters, the model will simplify to be as similar as possible to the Gummel-Poon model.
Advantages of VBIC over the Gummel-Poon model include the following:
- An Early effect model based on the junction depletion charges.
- A modified Kull model for quasi-saturation valid into the Kirk regime (the high-injection effect at the collector).
- Inclusion of the parasitic substrate transistor.
- An improved single-piece junction capacitance model for all three junction capacitances.
- Improved static temperature scaling.
- First-order modeling of distributed base and emitter AC and DC crowding.
- Overall improved high-level diffusion capacitance modeling (including the quasi-saturation charge).
- Inclusion of parasitic overlap capacitances.
- Inclusion of the onset of weak avalanche current for the base-collector junction.
- High-order continuity (infinite) in equations.
- A noise model similar to that of the Gummel-Poon model, with shot, thermal, and 1/f components.
- A defined self-heating model with hooks included in code: the self-heating model itself will be available in a future release of VBIC.
The Keysight 85193D VBIC model assumes the device under test to be a vertical bipolar NPN transistor, fabricated in a P-type substrate.